HDPE Geomembrane Reservoir Lining System for Hydroelectric Project, China

Vince Zipparro, Chief Civil Engineer, MWHPeter A. Dickson, Principal Geological Engineer, MWH

This paper describes experience at the Shandong Taishan Hydroelectric Project in China where a horizontally installed geomembrane liner system has been successfully used for water retention at the bottom of the upper reservoir of this pumped-storage project. This project, which was finished in early 2006, is the first time in China that geosynthetics have been used for such a purpose.

The Taishan upper reservoir geomembrane perimeter connections involve three different interfaces: with the dam concrete facing, with concrete faced rock surfaces on one side slope of the reservoir, and with a horizontal excavated rock surface (and grouting gallery). Concerns and issues included design for stresses on the liner system imposed by constant fluctuations in water velocities and pressures as well as the fact that the liner on the bottom of the reservoir is placed on top of backfill materials that could settle during the lifetime of the project. The reservoir liner will be subject to daily fluctuating reservoir heads of up to 35 m but will always be covered by at least 2 m water. The designers carried out extensive testing at research laboratories to examine these conditions and to determine the most appropriate connection details for use on this project to ensure compatibility with the properties of the selected liner material and expected loads that will be taken by the liner at these interfaces.

The designers considered two alternative material options (PVC and HDPE) for a geomembrane liner system and finally selected HDPE (1.5 mm) with underlying non-woven geotextile (600 g/m2). Photographs and sketches are provided that illustrate installation of the liner system and connection details.

To mitigate excessive differential settlements in the reservoir bottom, importance was given to very careful excavation, fill placement, and compaction at the junction between fill and rigid structures. The design of the bedding layers was intended to reduce the potential for differential settlements and provide sufficient protection from puncture of the liner from large fill fragments. The design also permits reduction in flow velocity of seepage to facilitate repair by dumping of fines on top of the liner, should it be ever needed in case of a major leak or tear.

Introduction

The Shandong Taian Pumped Storage Project is located in Shandong Province in eastern China and was developed by the Shandong Taishan Pumped Storage Power Station Co., Ltd. The design of the project was done largely by the East China Investigation and Design Institute (ECIDI). Other design institutes and Chinese experts were also involved in the engineering design and review process. The owner also retained foreign consultants (J-Power of Japan, in association with MWH of USA) to review the detail

civil design and to provide advisory services during construction. Specifically, these consultants provided assistance during the project preparation stage and advice on detailed design of the upper reservoir, the water conveyance system, and the underground caverns. They also performed review of the project construction schedule and implementation plan and quality checks during construction. One important aspect was to investigation of case histories of projects in other countries where geomembrane systems have been adopted for reservoir liners on pumped-storage projects.

Upper Reservoir

The upper reservoir is formed by:

A 100-m-high concrete face rockfill dam, A reinforced concrete slab on the right flank of the reservoir, and A geomembrane liner system placed over backfill within the reservoir bottom

below elevation 375 m (see Figure 1).

Mass balance construction involving excavation and backfill were used to create a reservoir of volume 10.65 million m3. Quarrying operations to provide construction materials contributed to the required volume. The project design entailed a backfilled reservoir bottom at EL. 375 m and excavated reservoir floor at El. 386 m as indicated on Figure 1.

The 540 m long dam crest is 10m wide, common for high dams of this type. However, it is understood the volume excavated from the reservoir calls for bulkier than needed dam. The maximum dam height is 100 m above the downstream toe elevation, but only 38.8 m above the reservoir bottom (backfill). The upstream slope is set at 1 V:1.5 H. The downstream slope is set at 1:1.4, with wide berms, resulting in an unduly flat average downstream slope of 1:1.8. The rationale for this was reported to be to accommodate the excavation from the reservoir enlargement, to provide an access road of sufficient width up the downstream face, and to permit aesthetic and environment improvements, including vegetation. The reinforced concrete facing slab of the dam is 30 cm thick, with 12 m wide panels. At the upstream dam toe, the face slab ends at the chosen top of fill elevation of 375 m, where it is connected to a geomembrane liner by means of a connection slab that is 6 m wide and 0.6 m thick.

1.1.1 Reservoir Seepage

The right and upstream reservoir flanks are underlain by sound rocks, with groundwater table normally higher than the future reservoir level. Accordingly, for this zone there is no danger of reservoir seepage. On the right bank the groundwater table may be locally (close to faults) lower than the FSL. Accordingly, the designer has suggested a concrete lining on the portion of the right reservoir flank upstream of the dam, covering an area of 700 m. The need for this lining is not obvious as low permeability may be provided by grouting, see Figure below.

1.1.2 Reservoir Treatment and Grouting Gallery

According to the present design, a perimeter grouting gallery and vertical grout curtain are provided at the upstream end of the geomembrane liner. The geomembrane would be connected to this gallery, and the grouting curtain extends to 5 m below the relatively impermeable layer, with the grouting holes in rows at 1.5 m intervals and 3 m apart. Around fault F1 three grouting rows are foreseen, while for the rest two grouting rows are planned. Possibly this may be reduced to one row for low take areas and two rows for the more permeable zones, including the fault zone.

The reinforced concrete slab on the right bank is 30 cm thick, with a slope of 1:1.5. A 80 cm thick crushed rock layer is placed underneath. It is suggested to replace this by porous (none fines) concrete.

1.2 Background on Use of Geomembranes

A major and growing application of geosynthetic materials is that of providing an impermeable or leak-proof barrier for water retention, or liner systems, in ponds and reservoirs.

In the hydroelectric industry, geomembrane materials can be found in many anti-seepage or water –proofing applications, including:

Facings on dams, Water barriers within fill dam structures, Reservoir or pond liners, Canal liners, Water or vapor barriers for underground works.

Typically, a "geomembrane facing" for a dam or a “geomembranes liner” for a reservoir application involves a whole system of superposed layers necessary for the construction, placement, and the preservation of the impervious barrier. Multiple-layer, composite systems employed as reservoir liners and watertight facings to dams have to be designed to meet the unique circumstances and conditions for each job. Such a system may be described schematically as follows, from bottom to top (i.e. progressively towards the water body):

Base layer, forming the transition from the material in reservoir floor or in the main body of the dam to the watertight system;

Watertight facing proper, comprising:o Supporting layer, which may act as a filter and drain for seepage control,

and/or a stress distribution material; o Geomembrane, for watertightness (one or two layers separated by a

drainage layer); Protective layer over the geomembranes - optional.

Geomembranes can be either left exposed or externally protected. Initially, the state of the practice involved a covered membrane system, but increasingly the protective layer is being omitted for various reasons including improvement in the durability of the geosynthetic materials.

In the case of specific use on pumped-storage projects, there are few case histories involving application of geomembranes for reservoir liners though there are a larger number of pumped-storage dams that have been equipped with geomembrane facings – mostly as repairs or as part of rehabilitation.

There has been general trend in Europe towards applications involving PVC membranes in contrast to North American practice that has seen proportionately more HDPE use.

1.3 Geomembrane Liners for Reservoirs on Pumped-Storage Projects

Upper Pond at Seawater Pumped Storage Power Plant, Okinawa, Japan

The following criteria were established for the impervious element of the upper pond:

Virtually zero-permeability to provide a barrier preventing seawater leakage into ground.

Performance with repetitive water level fluctuation. Easy maintenance. Geomembrane was considered superior to concrete lining and asphaltic-concrete

lining in terms of lower cost and lower permeability. An exposed geomembrane without a protective cover layer was selected as it would permit easy repair in the event of damage while assuring high reliability subject to appropriate maintenance inspection.

The requirements for the geomembrane bedding, or transition layer included:

A sufficiently stable and compact interface to support the geomembrane and to ensure sufficient slope stability,

Drainage of groundwater and surface water from the surrounding ground, thereby dissipating the pore water pressure on the back of geomembrane,

Diversion of leakage water to an inspection gallery built around the pond bottom, in the event of geomembrane failure, thereby preventing infiltration of seawater into the surrounding ground,

Venting of air behind geomembrane in order to prevent geomembrane buoyancy, , Crusher run with a maximum size of 20 mm was selected among a number of

materials meeting the above requirements in terms of water permeability and air porosity, with due consideration given to economy and seismic stability. The thickness of transition layer was designed at 50 cm.

A geotextile was laid between the geomembrane and transition layer. Cushion fabric prevents irregularities and protrusion on the surface of transition layer from

affecting geomembrane. It also prevents failure of geomembrane due to local deformation. Long-fiber non-woven spun bonded fabric is generally used as cushion material. Polyester was selected for raw materials, because of its more excellent heat resistance and waterproof performance compared with polypropylene. A minimum of 800 g/m2 was specified for cushion fabric based on test results.

Geomembrane Materials: The following criteria were set up for geomembrane materials:

Minimize temperature sensitivity (as the project is located in sub-tropics) Superior elongation and flexibility

Materials considered included:

Synthetic rubber such as Ethylene-Propylene Diene Monomer (EPDM) Soft Polyvinyl Chloride (PVC) High Density Soft Polyvinyl Chloride (HDPVC) Specially developed PVC for improvement of the physical properties and feeling

with increase of PVC molecular weight and addition of plasticizer High Density Polyethylene (HDPE)

EPDM was finally selected after comparative studies of these materials based on the criteria together with workability. A geomembrane thickness of 2.0 mm was selected in consideration of strength, resistance against irregularities and workability. EPDM content at a minimum of 70% was specified for the geomembrane of the exposed structure to improve ozone resistance. In the field test at the project site, wrinkles were found on the lower part of the sloped surface. In order to remove the wrinkles, higher tensile strain of the geomembrane was required. As no specification on tensile strain was mentioned in JIS A 6008, higher tensile strength than JIS was specified for the geomembrane instead.

Tests in addition to JIS A 6008, such as outdoor exposure test at the site, bacteria proof test, seawater resistance test, and marine organism deposit test, were carried out on EPDM geomembrane and long term durability was confirmed.

Jointing and anchoring method: A principle of eliminating field jointing of geomembrane was first established. Considering the restrictions in handling and fabricating limitations at factory and in transportation, the lateral interval of anchoring works was determined as 8.5 m for slopes and 17.0 m at bottom.

U-shaped concrete blocks were adopted for the anchoring and jointing method. The joints of geomembrane were embedded into specially designed U-shaped pre-cast concrete blocks with filling concrete and cover sheet on the joint.

Structural Stability Analysis of Geomembrane: Finite element method (FEM) analysis was applied to the typical cross section of upper pond with a water pressure corresponding to high water level. The analysis showed that the strain at the bottom and

on the slope due to deformation was minimal, being in the order of 10 -2 %, which did not affect the geomembrane.

Site condition is one of the most important and influential factors for selection of type and material of structures.

Selection for impervious element of upper pond was emphasized to prevent seawater from leaking out into surrounding ground. Seawater leakage is an environmental impact to the flora and fauna of the area. Another factor was climatic condition. The site is located on an island of sub-tropical weather and on the route of typhoon frequently.

Watertight geomembrane of EPDM was finally selected based on the criteria predetermined mainly in consideration of site condition.

1.3.1.1 Upper Reservoir at Imaichi Pumped Storage Power Plant (TEPCO, 1987)

Selection of Impervious Element: Criteria for selection of impervious element concerned cost-effectiveness for leakage prevention. Comparative studies were made on the following cases:

Facing work Rim curtain grouting work Combined work of the above

As a result of further study, geomembrane, concrete facing, and rubber asphalt were applied for the appropriate area. Reinforced concrete facing of 20 cm thick was selected for slopes of approximately 1 to 1.5. Rubber asphalt of 4 mm in thickness was sprayed onto the slopes of quarry. Geomembrane of 1.5 mm thick PCV was applied for the slopes flatter than 1 to 3 and bottom of the reservoir. Curtain grouting and cast-in-situ diaphragm of blast furnace slag, gypsum and cement mixture were also provided for leakage prevention measure.

FEM analyses were made for the prediction of seepage on each case for H.W.L and L.W.L. respectively. The effectiveness of the applied leakage prevention measure was estimated as 62~76 %.

Structural Features of Geomembrane:

1. Base structure: Requirements for base structure (bedding) was as follows:

Enough bearing strength to the load onto geomembrane Smooth surface without protruded gravels in order not to damage

geomembrane when hydraulic pressure acts

Buffering effect to geomembrane against the load due to traffic of construction equipment and passing of working personnel

Appropriate impermeability to prevent leakage in case of failure of geomembrane

Stabilized surface to avoid erosion due to running water

Geomembrane Materials: PVC was selected from the viewpoint of higher elongation, strength and flexibility in addition to assured jointing. Tensile strength was approximately 17 MPa.

A single layer product was employed as multiple layer products have a risk of exfoliation in the long use. Virgin materials were specified and recycled materials were banned. Diethyl- hexa-phthalate was used as plasticizer.

Geomembrane of 1.5 mm thick was adopted after hydraulic pressure test, pin hole test and traveling test of heavy equipment in comparison with geomembrane of 1.0 mm thick. Thicker geomembrane raised resistivity against irregularity of supporting or protective layer and impermeability.

Jointing and Anchoring Method: A thermal welding method was employed for jointing of the geomembrane because of higher reliability and workability.

Reinforced concrete anchorages were provided for geomembrane anchoring. Edges of geomembrane were embedded into anchorages. Additional sheets of geomembrane and geotextile with rubber sheets were attached to the geomembrane near anchorages.

Leakage Monitoring System: Monitoring systems have been applied at many landfill projects in Japan. Most geosynthetic materials from which geomembranes are made do not conduct electricity. When leakage occurs, an electric current flows up and down through the geomembrane on the spot of the leakage. Electrodes are installed onto both sides of geomembrane and electricity is applied. More electric current flows in the electrode at and near the leakage point. Location of leakage is detected by measuring the electric current. Monitoring system is controlled by electronic computer. Many firms have developed their own leakage monitoring system.

At Imaichi, no monitoring system was installed for the geomembrane

Refurbishment of upper reservoir seems to be scheduled in near future. Detailed information on the present situation of the upper reservoir is not disclosed.

1.3.2 Pumped-Storage Projects in Europe and North America

The following section discusses experience in the use of geomembranes on pumped-storage projects in Europe and North America.

1.3.2.1 Historical Background

In Europe and North America, there is little experience in the use of geomembranes for lining of reservoirs on pumped-storage projects. The main reason for this is because most pumped-storage projects in Europe and in North America pre-date geosynthetics and were designed and constructed before geosynthetic materials came into common practice in the hydropower engineering industry. There are more than 80 operating pumped-storage projects in Europe and the Americas (about 40 projects in Europe, and 40 in United States and Canada). In the United States, the last of these to go into commercial operation was the Rocky Mountain Project in 1995 and the design of this project took place mostly during the 1980’s. In other words, most projects pre-date common usage or acceptance of geomembrane liners.

Project development typically involved the engineering and use during construction of locally available materials – usually natural and easily used geotechnical materials (clay, sand, gravel, rock) as well as concrete. Water retention structures usually depended on use of low permeability materials, such as clay.

Alternative lining solutions were adopted where required because of special conditions or unavailability of suitable sources of impervious materials. In addition to conventional concrete or shotcrete, these have included asphalt, asphaltic concrete, and rubberized concrete. For facing of dams (including water retention), many dams have been faced with geosynthetic materials and even with metal.

Geosynthetic materials, as they became available, were not quickly adopted for reservoir liners in the hydroelectric industry because they often provided more expensive solutions or did not have proven reliability for hydroelectric project applications. In some countries, regulatory agencies (such as the Federal Energy Regulatory Commission, FERC, in USA) were slow to approve use of geomembranes as a primary water retention feature for new projects.

However, geomembranes are now becoming more attractive for water retaining structures because:

Costs of geosynthetic materials are becoming more competitive in relation to conventional solutions.

Improved manufacturing and installation procedures, as well better general construction know-how, are increasing confidence in the technology.

Improved reliability and considerable experience in their use is being translated from environmental and hazardous waste applications (e.g. landfills).

Sites for hydropower projects are becoming more marginal and harder to develop, therefore innovative use of new materials is being used.

In the USA, the earliest experience with geomembranes on pumped-storage projects involved their use for patching and repairs to existing projects lined with other materials. They have been used for limited areas within reservoirs (such as at Mt. Elbert Pumped-Storage Project, described below) and now geomembrane systems are being designed as reservoir liners for new pumped-storage projects and for complete replacement of liners at existing projects.

Several studies have been conducted in the last 5-10 years involving technical and cost comparison of different reservoir lining options for pumped-storage projects - including geomembranes, geocomposite clay liners (GCLs), asphaltic concrete, conventional concrete and shotcrete, clay liners, etc. Locations have included diverse climatic environments – Israel, Thailand, Morocco, USA. Each location demanded a unique solution that was decided partly on technical merits of the selected liner type but largely on the basis of life-cycle cost analysis.

1.3.2.2 Mt. Elbert Pumped-Storage Project (Colorado, USA).

A geomembrane liner system was used at the Mt Elbert Forebay Reservoir – upper reservoir of this 200 MW pumped storage project of the United States Bureau of Reclamation (USBR). Only part of the reservoir is lined to counteract seepage. The selected geomembrane material was CPE (chloro-polyethylene) and was fabricated in two blanket shapes 61 m by 21 m and 30 m by 43 m, each 1300 m2 in size. Installation was done in 1986 and is the first documented use of geomembrane materials for reservoir water retention for a pumped-storage project in the USA other than for repairs.

The CPE lining is buried (i.e. has an overlying protection layer of fill materials). Tests have shown that the lining continues to perform well after 16 years.

1.3.2.3 Taum Sauk Pumped-Storage Project (Missouri, USA)

The upper reservoir of Taum Sauk Pumped-Storage Project (440 MW) has a storage capacity of 4,350 ac-ft, an area of 50 acres, and is over 90 ft deep. Completed in 1963, the Taum Sauk Project was one of the earliest pumped-storage projects in the USA. It is owned and operated by Ameren (formerly Union Electric).

The dam forming the reservoir was constructed from dumped rock fill with 1V:1.3H slopes. The dam is faced with reinforced concrete and shotcrete, while the bedrock forming the reservoir bottom was lined partly with asphalt or shotcrete and elsewhere left exposed, Figure 2.6. Nearly 3 million cubic yards of rock excavation were involved in forming the upper reservoir, which is located on the top of a small mountain.

Unfortunately, from the start of operation, the upper reservoir has experienced considerable leakage, mostly through the dam facing system, but also through the fractured bedrock exposed in the reservoir bottom that was not properly treated. Leakage has fluctuated but generally the maximum has averaged about 80-100 cfs (2,265 - 2,830

l/s), much of which is collected in a dam toe drain system and returned to the upper reservoir. The remainder of the leakage returns to the lower reservoir.

Several attempts at repairs and patching have been undertaken over the years, including application of more shotcrete in areas of obvious damage to the dam facing, but the effectiveness of such repairs have proven to be only temporary and the leakage has always returned. Continuous monitoring and engineering analyses have indicated that there is no danger to the stability or safety of the dam or reservoir due to the constant leakage. It has only been in recent years that a complete reservoir re-lining has been economically justified (i.e. cost of loss of operation during repair period versus benefits gained from sealing leaks). Options for complete reservoir re-lining included:

Asphaltic concrete, Conventional concrete or fibrous shotcrete, or Geomembrane system

The selected option is to provide a complete re-lining with a geomembrane system. The contract for the supply and installation has been made but the actual work has not yet started (expected mid-2002). The selected type of material is HDPE and its thickness is 80 mil (2 mm) on the reservoir slopes and 60 mil (1.5 mm) on the reservoir bottom. The geomembrane will be equipped with a 16 oz geotextile layer underneath, which will be to provide protection and smoothing. The specified HDPE geomembrane is the textured type in order to improve friction between the membrane and geotextile on the steep slopes (samples provided by the Consultants during recent visit May-June, 2002). The total area of the lining system is 867,000 sq.ft. for the reservoir sides and bottom, and 53,000 sq.ft. for lining the parapet wall (see photograph above).

Drainage details are in the process of being established. Details on the method of attachment for the geomembrane to the parapet wall at the top of the dam, as well as at the bottom, are also being developed by the engineer for the liner supplier. Most likely they will involve a combination of stainless steel batten strips and use of concrete embedment /attachment strips (such as the GSE Polylock system – examples provided by the Consultants to the Employer during recent visit to Taian). A conventional quality control program during installation is envisaged combined with selected seam weld testing. Electronic leak detection is not anticipated.

Since this re-lining took place, the upper reservoir dam and containment system failed catastrophically on December 14, 2005. The cause of the failure has not been attributed to the new lining, but could have been influenced by improper instrumentation set up after the re-lining was completed.

1.3.2.4 Kinzua (Seneca) Pumped-Storage Project (Pennsylvania, USA)

The Harza-designed Kinzua Pumped-Storage Project (435 MW) is equipped with a circular upper reservoir that is lined with asphaltic concrete – see detailed information in

Appendix 1-2-2. The project, owned by First Energy, has been in operation since 1970 during which time various repairs have been needed for the liner and shaft. Such periodic maintenance has been normal but in 1997 after 30 years service, it was decided to completely rehabilitate the upper reservoir and provide a new lining system. Two alternatives were identified that met the requirements for rehabilitation:

Replacement of the asphaltic concrete, or PVC geomembrane overlay system (geocomposite Sibelon)

Figure 2.1: Kinzua (Seneca) Pumped-Storage Project

The selected option was to reline the reservoir with another layer of hydraulic asphaltic concrete, largely on the basis of cost – see details of PVC alternative in Appendix 1-2-1 of this report. Construction of the new liner was completed in a 10-week period in 2000.

1.3.2.5 Blue Diamond Pumped-Storage Project (Nevada, USA)

This 400-MW pumped-storage project is in the detailed design stage and ready to go into construction. It has received required licenses and approvals to be constructed and operate from the federal agencies, including environmental permits. The project is located in a desert area just outside Las Vegas, Nevada. Water retention has a very high priority because of the hot, arid climate on the one hand and because highly soluble and weak foundation materials are present, including gypsum and anhydrite. These conditions required the specification of special design and mitigation features, including construction of a seepage barrier system for both the upper and lower reservoirs as well as a reservoir under-drainage system and installation of floating covers to mitigate losses

due to evaporation. Reference materials on the project are provided in Appendix 1-2-2 of this report.

The seepage barrier for the floor and side slopes of the reservoirs will be a membrane system consisting of a geosynthetic membrane supported by a geotextile fabric. Beneath these would be filter and drain layers of processed fill placed on top of transition layers. The floating cover system will also be a similar geomembrane.

The project is being developed by an independent power producer (IPP) and constructed on a turnkey design-build basis. Following common practice in the USA, a specialty contractor will be required to supply, install, and guarantee the geomembrane system for the reservoirs. Detailed design and selection of appropriate construction procedures for the liners will be the responsibility of this contractor. The contractor has to meet certain basic Owner’s (Employer’s) requirements and technical design requirements that include the following aspects.

Reservoir Liners and Covers: The inner slopes and floor of the upper and lower reservoirs have to be fully lined with a double-barrier impermeable membrane lining system consisting of the following:

Primary membrane lining. Leakage collection layer between the primary and secondary linings. Secondary membrane lining.

The basic design involves a 60 mil (1.5 mm) primary liner, a geonet drainage layer, a 40 mil (1 mm) secondary lining, and a geotextile cushion layer. This is considerably more conservative than the Consultants suggest for the Taian Project.

One of the reasons for the double membrane system on this project is because of the presence of gypsum and other soluble or weak materials in the foundation and in the construction materials that will be used for the dikes around the reservoirs. No leakage water should be allowed to enter these materials. [Note: At Taian, this is not the case, and therefore a simpler lining system, as recommended by the Consultants, can be allowed.]

The primary lining is to be provided with a means for electronic testing of the installed lining integrity, or other approved means of testing.

In the event of damage to the primary lining, the embankment will be protected by the secondary lining. Leakage through the primary lining into the leakage collection layer is to be collected and recirculated back into the reservoirs.

The subgrade for the double membrane lining system is to be well-compacted and free from large protrusions and large and/or sharp stones that have the potential for damaging the liner. A sufficiently designed bedding layer is to be placed and compacted on the

prepared subgrade to provide a suitable support and cushion the double membrane lining system.

Material Type Selection: Selection of the geomembrane type is to be left to the contractor but it seems to be quite certain that HDPE will be selected as opposed to PVC, the only other material being considered. GSE has proposed 80-mil thick HDPE with white conductive layer for spark testing for primary lining. Leakage detection layer will either be 2-plane or 3-plane geonet. The secondary lining will consist of 60-mil thick HDPE with white conductive layer.

The primary membrane layer will most likely have an additional white co-extruded layer (5 mils, 0.13 mm), the purpose of which would be to:

Minimize radiant heat build-up Reduce thermal expansion, including wrinkles Reduce problems associated with wrinkles Provide damage detectionAlthough historically HDPE has not demonstrated as

good flexibility, thermal properties, and multiaxial tension capabilities compared to PVC, new product lines (such as white coated and more flexible HDPE) have tended to close the perceived performance difference between PVC and HDPE. Experience in hot climates (such as Nevada, Utah, California, New Mexico, or Arizona in the USA, or in Australia, Africa, and the Middle East) has shown that installation of HDPE geomembranes is not only feasible but is reliable and meets technical requirements. In the United States at least, the preference seems to going towards HDPE. Interfaces: The Contractor is required to design and construct suitable interfaces to maintain the integrity of the double membrane lining system and the floating cover system around penetrations for drainage piping, filling system piping, intake-outlet structures, and all other ancillary structures.Reservoir Covers: The upper and lower reservoirs are to be provided with a floating cover system designed to prevent evaporation of water from the reservoir pools (not relevant at Taian, therefore details are omitted).Performance: The double membrane lining system and the floating cover system for each reservoir are to be guaranteed by the Contractor for performance and service life. Materials and workmanship shall be warranteed for a period of at least 20 years. Performance is to be guaranteed in accordance with the conditions of the Contract. The lining system and floating cover system for each reservoir have to meet performance criteria for the project. These are established based on the Owner’s assessment of cost of water and risk of project interruptions caused by repairs. Design and Construction Standards: Materials, design details, fabrication and installation procedures, and quality assurance testing for the double membrane lining and floating cover system are to be based on current state of the art procedures and precedent in similar environments for similar functions, and in accordance with the following standards: ICOLD Bulletin 78, Watertight Geomembranes for Dams, 1991;

AWWA Manual 25, Flexible-Membrane Covers and Linings for Potable-Water Reservoirs;

AWWA D130, AWWA Standard for Flexible-Membrane-Lining and Floating-Cover Materials for Potable Water Storage;

AWWA Reservoir Floating Cover Guidelines, California and Nevada Section, 1999;

ASTM Standards on Geosynthetics (ASTM Publication Code PCN 03-435091-38, sponsored by ASTM COMMITTEE D-35 on Geosynthetics); and

Geomembrane material and installation specifications of the International Association of Geosynthetic Installers and other geosynthetic standards organizations.

Where differences in standards occur with respect to minimum required material properties, details, performance and fabrication and installation quality assurance and quality control occur, the most conservative interpretation is to control.

Loads and physical effects to be considered in design of the lining and cover systems include:

Sliding Subgrade differential deformations Punctures Stretching Impact Wind Reservoir waves Uplift from water, air or other gases from the reservoirs Expansion

Physical, chemical and biological effects to be considered include:

Heat UV Radiation Water Chemistry Biological Action (Microorganisms) Vegetation Rodents, birds and other wildlife

Leakage Collection: A leakage collection and re-circulation system is to be provided to monitor, collect, and re-circulate any leakage through the primary lining, as well as monitor the performance of the double membrane lining system. The collection system is integral with the reservoir lining.

The leakage collection system is designed to the standards listed above as well as the lining manufacturer’s recommendations. The leakage collection and re-circulation system for each reservoir shall include, at a minimum, the following items:

A leakage collection layer between the primary and secondary membrane linings to collect any water that may leak through the primary lining. The leakage collection layer is to be designed to safely accept and convey the anticipated leakage from a concentrated leak or tear in the primary lining under full reservoir head to the reservoir under-drainage system.

An under-drainage system of perimeter and cross-drains and collection headers to collect leakage water from the leakage collection layer and convey it safely to a leakage and surface water collection basin located at the toe of the outer reservoir slope. The under-drainage system is to be designed in a manner that divides the reservoir into collection zones such that the source of any leakage through the primary lining can be isolated within each zone. Each zone is to be provided with separate piping such that leakage from each zone can be individually measured. The perimeter drains and collection headers are to be designed to safely convey twice the anticipated maximum leakage to a leakage and surface water collection basin located at the toe of the outer reservoir slope.

A leakage and surface water collection basin to collect all surface and leakage water. The basin is to be located to collect leakage and surface water from all points under the double membrane lining system and from the floating cover system, respectively, and shall be sized to hold all leakage collected for a period of not less than 36-hours. The collection basin is to be designed to remove any particulate material and foreign matter that may harm the performance of the double membrane lining system, water conveyance facilities, or powerhouse equipment. Collection headers emptying into the basin are to be equipped with calibrated weir plates or other suitable equipment to allow for rapid determination of leakage rates from each leakage collection zone without the use of a calculator or computer. The collection basin is to be fitted with a suitably designed pump-back system to re-circulate collected surface and leakage water back into the reservoir. The collection basin is to be provided with an automated water level sensing and alarm system to monitor water levels in the basin.

Alternative means for collecting and re-circulating leakage and surface water into each reservoir can be proposed by the Contractor provided that they meet the minimum requirements set forth in the Employer’s Requirements and other pertinent standards, codes and regulations.

Slope Protection: The outer slopes of each reservoir shall be protected from erosion and damage due to rainfall, surface runoff, wind, vehicle or animal traffic, wildlife and vegetation by a layer of riprap or other protective medium placed over a suitable bedding layer.

Instrumentation: The Contractor is to provide the following minimum instrumentation for long term monitoring of the project:

Surface settlement monuments installed in the embankment crest to monitor settlement.

Weirs at the outlets of all collection headers at the collection basin for each reservoir.

Water level monitoring equipment complete with an alarm system at the leakage collection basins to automatically monitor water levels.

Reservoir water level monitoring equipment and sensing system at each reservoir for monitoring reservoir water levels.

Open standpipe piezometers around the perimeter of the upper and lower reservoir embankments for monitoring of potential seepage in the event of a breach in the reservoir lining system.

Performance Test: Upon completion of each reservoir, and prior to the start-up and commissioning of the Project, the water-tightness of each reservoir is to be tested by a filling test, which includes:

1. The lower (upper) reservoir shall be filled to the minimum operating pool elevation in a controlled manner, and held at this elevation for a minimum of 24 hours to obtain and evaluate readings of all piezometers, and collection header weirs.

2. Leakage rates and water levels in the lower (upper) reservoir piezometers shall be measured and recorded during this period. If the total leakage rate from all collection headers combined exceeds 2 gpm at any time during this period, or if water is detected in any of the piezometers, the reservoir shall be inspected and fully repaired prior to restarting the filling of the reservoir.

3. The reservoirs shall be filled to the maximum operating water elevation in increments of no more than 20 feet in a controlled manner and held at each increment for a period not less than 24 hours. The reservoir shall be held at the maximum operating water elevation until the readings stabilize for at least 6 hours, but for a minimum of three (3) days, to obtain and evaluate readings of all piezometers, and collection header weirs.

Intake-Outlet Structures: The intake-outlet structures will protrude through the membrane lining system for the reservoirs. The Contractor is to be responsible for designing and constructing a lining/structure interface between the intake-outlet structures to maintain the lining integrity, prevent leakage at the lining/structure interface, and prevent distress to the reservoir lining system due to differential movements between the lining and the intake-outlet structures.

1.4 Design Principles for Geomembrane Liners for Reservoirs

1.4.1 Background

According to the Contract 3.1.2.1.c, Appendix A, the Consultants are requested to provide and discuss:

“Design principles for geomembrane liners in regard to material, type, and thickness; assessment on type selection for the Taian Upper Reservoir.”

1.4.2 Liner Type Selection

1.4.2.1 Principles

The subject is covered in several key technical papers including:

ICOLD Bulletin 78, 1991, “Watertight geomembranes for dams, State of the Art” Peggs, I.D., and R. Thiel, 2000. “Selecting a Geomembrane Material”,

Geosynthetica, 2000

Basically, steps involved are:

Identify factors and performance parameters that must be met Identify relative importance of parameters Preliminary ranking of candidate material types Carry out Performance Ranking Develop a preliminary technical ranking Carry out next level by including other factors such as:

o Cost, o Availability, o Construction Issueso Construction/Contractor experience in region

1.4.2.2 Liner Selection Recommendations

It is suggested that in the case of the Taian Project, it is not necessary to go through with a complex ranking exercise since, in the opinion of the Consultants, only two candidate material types are worth following, i.e. HDPE or PVC as illustrated in Figures 2.11 and 2.12 Both types are equally suitable for the purpose intended on this project. Differences in the two material types can be dealt with through appropriate details in regard to installation, seam joining, anchoring, and testing. There are known to be pros and cons of each type that for this project are probably fairly equally weighted. The final selection between these will come down to other non-technical factors, as indicated above such as cost, availability, and available construction know-how.

Discussion on the recommended geomembrane system for the Taian Upper Reservoir is presented later in this report.

1.4.3 Liner Thickness Selection

The basic performance parameters involved in selection are:

Strength (resistance to puncture, burst, tear, impact, seam strength, rodents/animals)

Flexibility Seaming, patching

The performance requirements have to be evaluated, including:

During installation During operation, service life Settlement (i.e. considered in the design of other features/structures, such as the

random fill and bedding materials at Taian upper reservoir)

A similar ranking process can be carried out as described above for material type selection.

Design: Mitigation of punctures and holes (and other strength performance parameters) can be dealt with ways other than by increasing membrane thickness – i.e. through detailed design (e.g. thickness and gradation of bedding materials, selection of suitable geotextile cushion layer, elimination of protection layer), appropriate specifications for installation and testing, manufacturing Quality Control, and construction/installation Quality Control.

Carrying out in situ testing (such as spark or conductivity testing) and careful monitoring during construction can also locate holes/punctures and therefore help mitigate uncertainties with respect to thickness. Design aspects that help to mitigate factors listed above include focus on selection of suitable bedding layers and/or geotextile underlay. There are ASTM puncture tests designed to determine if a given liner is thick enough for a given application.

The Consultants draw attention to the GSE Geomembrane Protection Design Guide which has been provided during the May, 2002, visit to the Project. This reference will provide useful guidelines in the selection and design of an appropriate support system for the Taian upper reservoir liner.

Cost, Constructibility: The final factors involved in selection of liner thickness concern Cost and Constructibility. As indicated earlier, there are trade-offs between thickness and other performance factors. Thicker geomembranes are more difficult to handle and weld. These aspects can introduce significant potential for more damage during installation and/or leakage at seams due to imperfect welds.

1.4.4 Geomembrane Liner Protection

The need for protection of a geomembrane system depends on several factors:

o Resistance to ageing and UV degradation of the particular polymer compound(s) from which the geomembrane is manufactured,

o Expected stresses and hazards to which the liner will be exposed during project operation, including loads and stresses due to high winds, settlement and deformation of underlying materials, thermal affects, equipment and traffic passing over during maintenance, etc.

o Risk of vandalism,o Puncture or damage during installation caused by personnel, equipment, and

handling, and o Puncture or damage from underlying materials.

Geomembranes can be either left exposed or externally protected. Initially, the state of the practice involved covering membrane systems. At present, both exposed and covered systems are used. Increasingly, the protective layer is being omitted for various reasons including improvement in the durability of the geosynthetic materials and improved handling and installation procedures.

Conventionally, protective layers provided on top a geomembrane system have involved use of soil materials, stone or rock, or prefabricated concrete blocks or slabs. Geotextile layers have also been used, reinforced or weighted as needed.

The need for protection below the geomembrane is a function of the design of the underlay or bedding materials, as indicated above. Typically, the thickness and gradation of the bedding layer is developed such that there is no hazard to the membrane say from underlying coarse rockfill. Provision of a geotextile layer also is standard practice to assist in this regard and to help distribute stresses.

1.4.4.1 Discussion

Information provided to the Consultants indicates that a protection system has been identified for the Taian Upper Reservoir liner. Drawings indicate a rather complex protection layer system as shown on Figure 2.9, with a geotextile placed on top of the geomembrane followed by a 30-cm-thick layer of coarse sand and topped off by a 50-cm-thick layer of crushed rock. An alternative and simplified version was presented to the Consultants during discussions as shown on Figure 2.10 with only a geotextile protection layer and no sand or rock materials.

The reasons for the overlying protection layer at Taian are not clear. Typically on other projects they would include protection against physical damage (natural and vandalism) and degradation by UV and environmental factors. Material placed on top of the geomembrane is sometimes also required to prevent the material from floating (bouancy) and to resist uplift by wind during installation and upon dewatering. Based upon present understanding of the reservoir liner requirements at Taian, such reasons are not really relevant to this project, because:

UV protection is not needed. The reservoir will only be exposed above water for relatively short periods. HDPE, if it is selected, is comparatively UV resistant and PVC can be manufactured to include UV resistance.

Damage during operation (such as from floating objects) is not an issue (also presumably damage due to vandalism).

The liner does not require weighting to counteract buoyancy, because with a free-draining material below it, it will never have balanced water pressures on both sides of the geomembrane.

The Consultants consider that there is good evidence that construction of an overlying protection layer can in fact introduce more harm than good and increase the likelihood of damage to the liner system. According to Geosynthetica, studies by Nosko have shown that most holes occur when the liner is covered with a soil layer and that most holes are not on seams as is commonly thought, Table 2.8.

Table 2.1: Frequency of Holes at Monitored Sites

Damage Occurrence Percent %During Installation 24

While Covering 73After Covering 2

However, it is pointed out that a large proportion of the damage that occurs during placement of the geomembrane is on seams, Table 2.9.

Table 2.2: Causes of Geomembrane Damage

Cause Percent %During Installation

Seaming faults 61Seam melt-through 18

Stone punctures 17Cuts 4

While CoveringStone punctures 68

Survey/depth stakes 16Heavy construction equipment 16

After CoveringHeavy equipment 64

Component installation 27Severe weather 9

It is also pointed out that with protection layer(s) covering a liner, it is much more difficult to locate and repair any such damage to the liner system. Exposed geomembranes can be readily and quickly patched with minimal interruption to plant operation.

It is therefore concluded that there is no strong reason for providing an overlying protection layer for the Taian Project. Further, for this project, introduction of a covering

protection system might be more of a problem than a solution. The Consultants therefore recommend elimination of the any protection materials placed on top of the liner system.

1.4.5 Current Design for Geomembrane Liner System for Taian Upper Reservoir

The present design for water retention involves use of a HDPE geomembrane, with 600g/m2 geotextile on both sides, placed on support sand layer, and then covered by sand and stone-armor, as indicated on Figure 2.9. The liner system is composed of several sub-layers, including (from down up):

10 cm thick crushed rock leveling sub-layer 500 g/m2 dacron non-woven cloth 30 cm thick coarse sand lower cushion sub-layer 300 g/m2 dacron non-woven cloth, 1 mm thick high density polyethylene film 300 g/m2 dacron non-woven cloth 3 cm thick coarse sand upper cushion layer and 50 cm thick crushed rock backfill.

This design is quite complex and vulnerable:

Layers between liner system and random fill are too thin – potential for large differential settlements.

Fill protection layers on top of liner not desirable (discussed later) Repairs will be costly and require a lot of time. Expensive and vulnerable design.

In regard to the geomembrane itself, the 1.0 mm HDPE lining is too thin in the Consultants’ judgment.

Figure 2.2: Current Design for Geomembrane Liner System

50 cm crushed rock

30 cm coarse sand

30 cm coarse sand

10 cm crushed rock

Random fill

1 mm HDPE

GeotextileLayers

Cover System

1.4.6 Recommended Geomembrane Liner System for Taian Upper Reservoir

Two alternatives design options for a geomembrane liner system are presented and recommended by the Consultants for consideration by the Employer (STPC) and the Designer (ECIDI).

HDPE Liner PVC Liner

PVC liner HDPE

2 mm PVC, exposed (UV treated)Geotextile layer

Zone 3, in 40 cm layers, 1.5 m thickZone 4, in 40 cm layers, thickness 2.5 m

1.5 mm HDPE, exposed600-800 g/m2 geotextile

Zone 3, in 40 cm layers, 1.5 m thickZone 4, in 40 cm layers, thickness 2.5 m

Sketches of these are provided in Figures 2.11 and 2.12. Both designs would incorporate a geomembrane of 1.5 to 2.0 mm thickness (final thickness to be determined by final design procedures and contractor), an underlying geotextile, and appropriate bedding layers. If HDPE is selected, it is recommended that specification of white reflective materials be considered as discussed later in the next section of this report.

The alternative designs, as shown on these figures, have several advantages, including:

They are easier to construct, repair, and maintain Elimination of fill protection layers Less potential for damage to the lining during construction Visual inspection and weld seam testing capability Easier to make detect leaks and repairsIt is recommended that final selection of

geomembrane liner thickness, selection of geotextile cushion layer thickness (weight), and design of the bedding material, should follow suggestions in the GSE Geomembrane Protection Design Manual (found on a compact disk [CD] provided by the Consultants), or equivalent. The final solution is to be taken in collaboration with potential suppliers based onCost

possibility to manufacture locally other contractual aspects.

The improved design of the bedding layers, as suggested by the Consultants, would reduce the potential for differential settlements and provide sufficient protection from puncture of the liner from large angular boulders. The suggested design would also permit reduction in flow velocity of seepage to facilitate repair by dumping of fines on top of the liner, should it be ever needed.

Figure 2.3: HDPE Geomembrane Alternative

1.60 m Zone 3 – 40 cm lift Random fill 1.5 mm HDPE (white)

Geotextile 540 g/m2

2.40 m Zone 4 – 40 cm lift No Cover

Compacted in 0.80 m layers to limit differential settlements

Figure 2.4: PVC Geomembrane Alternative

1.60 m Zone 3 – 40 cm lift

Random fill

2.0 mm PVC

Geotextile

2.40 m Zone 4 – 40 cm lift

No Cover

Compacted in 0.80 m layers to limit differential settlements

1.4.7 Recommended Liner Supply and Installation Requirements for Taian ProjectContractor

It is recommended that the supply and installation of the reservoir liner system for the Taian Upper Reservoir should be performed by a specialist contractor. This contractor should demonstrate evidence of ability and experience to supply and install the materials to the standards expected by the Employer.

It is suggested that the Taian Project could follow common practice in the USA and other countries, whereby a specialty contractor is required to supply, install, and guarantee the geomembrane system for the reservoir. Detailed design and selection of appropriate construction procedures for the liner system would be the responsibility of this contractor. The contractor would have to meet certain basic Employer’s requirements and technical design requirements as described in this report.

The liner deployment and field welding should be performed by a firm experienced in this type of work. The contractor should provided all necessary details regarding experience of key personnel and subcontractor staff in all areas of liner installation, including subgrade preparation, inspection and acceptance, membrane deployment, welding, and management of QA programs.

1.4.7.1 Scope of Work for Supply and Installation

Work items would include:

Preparation, inspection, and acceptance of subgrade surfaces prior to placement of the geomembrane system. In the case of Taian, this could be part of another contract (e.g. dam construction contractor).

Supply of all membrane liner materials including provision of manufacturing QA/QC details.

Supply and installation of all associated geotextiles, leak detection systems, anchors, pipe penetrations, instrumentation, and other ancillary materials and services necessary to complete the geomembrane system.

Supply of all membrane welding/seaming and QA/QC test equipment. Provision of all geomembrane system layout drawings and completion of “As-

Constructed” drawings upon completion. Deployment, welding, testing, and commissioning of geomembrane liner system

(geomembrane, geotextile, instrumentation, etc.).

The work should be performed according to applicable standards, as listed in Appendix 1.4.

1.4.7.2 Quality Management

The work should be subject to a QA/QC system, though this would not have to as stringent as required for an environmental engineering or hazardous waste application of geosynthetic technology. The QA/QC system should include:

Manufacturer’s QA program, Fabrication QA program that includes geomembrane inspection, welding process

control, and testing in accordance with applicable standards, and Field QA program that includes deployed geomembrane panel inspection in the

field, welding process control, and testing in accordance with applicable standards.

1.4.7.3 Comments on Installation

Installation specifications and requirements should address procedures specific to the properties of the type of liner that is to be installed at Taian. For example, differences in the coefficient of thermal expansion between HDPE and PVC will impact respective installation procedures. This is particularly important with respect to controlling smoothness and tautness of the geomembrane.

The geomembrane should lie flat on the underlying geotextile and bedding layers. Shrinkage and expansion of the sheeting, due to changes in temperature during installation, can result in excessive wrinkling or tension in the geomembrane unless measures are taken to control these. Wrinkles on the geomembrane surface will affect the uniformity of the soil-geomembrane interface and can complicate seam welding. Excessive tautness of the geomembrane may affect its ability to resist rupture from localized stresses on the seams or at the edges of the liner system where bridging over the subgrade may occur during installation. In addition to thermal expansion and contraction of the geomembrane, residual stresses from manufacturing remain in some geomembranes and can cause non-uniform expansion and contraction during construction. Some flexibility is needed in the specifications for geomembrane selection to allow for anticipated dimensional changes resulting from thermal expansion and contraction.

Recommendations. The following measures are recommended to help in the control of smoothness and tautness during installation of the geomembrane on the Taian Project:

HDPE White geomembrane. If HDPE material is selected, use of reflective white material should be considered. This would help to:

Minimize radiant heat build up during installation, Reduce thermal expansion, Minimize risk of liner damage due to wrinkles and facilitate seam

welding,

Improve detection of damage (by visual check).

Such a product has been selected for use as a liner for the upper and lower reservoirs of a pumped storage project in Nevada (Blue Diamond Pumped Storage Project, described elsewhere in this report) where desert heat will have a tremendous impact on installation. The materials has also been successfully used on other hydroelectric and water supply reservoir projects in hot climates within North America, Australia, and Africa (Libya).

Time of day. Installation during the cooler morning hours or even at night can greatly help alleviate problems caused by thermal expansion. On a Harza project in Honduras, HDPE installation took place at night and during the early morning hours before it became too hot.

Camber. The designer may investigate the benefits of introducing a slight camber in the random fill zone under the reservoir. This could have two purposes:

To help offset the affects of settlement, and To provide a means of positive drainage of the surface of the reservoir

bottom in case of precipitation during the installation period.

At Taian, the maximum depth of the fill is about 35 m and the maximum width is about 350 m. It is estimated that about 40 cm of settlement might occur at the center of reservoir. Even if the geomembrane surface is flat, the resulting distributed elongation around the periphery would be rather small and a camber might be hardly needed. Nevertheless, the designer could still investigate this aspect since it could only help. The designer should also evaluate the benefit provided with respect to draining rainwater that otherwise would pond on the surface and complicate installation.

Orientation of Panels. The optimum orientation of the geomembrane panels should be assessed in order to minimize stresses on seams and to minimize wrinkling during construction. The geomembrane supplier should also be consulted on this aspect.

1.4.7.4 Specification Requirements

Technical specifications for geomembranes should include:

Information for protection of the material during shipping, storage and handling; Quality control certifications provided by the manufacturer or fabricator (if panels

are constructed); and Quality control testing by the contractor, installer, or a construction quality

assurance (CQA) agent.

Installation procedures addressed by the technical specifications typically include:

Geomembrane layout plan and deployment of the geomembrane at the construction site,

Seam preparation and seaming methods, Seaming temperature constraints, Detailed procedures for repairing and documenting construction defects, and Sealing of the geomembrane to appurtenances, both adjoining and penetrating the

liner.

The performance of inspection activities, including both non-destructive and destructive quality control field testing of the sheets and seams during installation of the geomembrane, is normally addressed in the technical specifications.

Shipping, Storage and Handling. If the geomembrane is not actually manufactured at site, it is shipped in rolls or panels from the supplier, manufacturer, or fabricator to the construction site. Each roll or panel is usually labeled according to its position on the geomembrane layout plan to facilitate installation. Upon delivery, the geomembrane sheeting is inspected to check for damage that may have occurred during shipping.

Proper storage of the rolls or panels prior to installation is essential to the final performance of the geomembrane. For the Taian Project, the Consultants are recommending either PVC or HDPE materials that are less sensitive to environmental factors prior to installation. Some other types of material, such as CSPE and CPE, are sensitive to moisture and heat and can partially crosslink or block (stick together) under improper storage conditions. Adhesives or welding materials, which are used to join geomembrane panels, also should be stored appropriately.

Inspection of Subgrade. Visual inspection and acceptance of the soil liner subgrade should be conducted prior to installing the geomembrane liner system (geotextile and geomembrane). The surface of the subgrade should meet design specifications with regard to lack of protruding objects, grades, and thickness. Once these inspections are conducted and complete, the geomembrane liner system may be installed on top of the bedding materials (Zones 3 and 4).

Placement of Geomembrane System. Deployment, or placement, of the geomembrane panels or rolls should be described in the geomembrane layout plan. Rolls of sheeting, such as HDPE, generally can be deployed by placing a shaft through the core of the roll, which is supported and deployed using a front-end loader or a winch. Panels composed of flexible liner material such as PVC are often folded on pallets, requiring workers to manually unfold and place the geomembrane.

Placement of the geomembrane is closely coordinated and simultaneous with the seaming process. No more than the amount of sheeting that can be seamed during a shift or work day should be deployed at any one time. Panels should be weighted with sand bags if wind uplift of the membrane or excessive movement from thermal expansion is a

potential problem. Proper stormwater control measurements should be employed during construction to prevent puddling of water, erosion of the Zone 3 material (unlikely), or other problems such as washing of fines, debris, and dirt onto the geomembrane or its under-surface.

Seaming. Once deployment of a section of the geomembrane is complete and each section has been visually inspected for imperfections, seaming of the geomembrane may begin. Quality control/quality assurance monitoring of the seaming process would be implemented to detect inferior seams. Seaming can be conducted either in the factory (for PVC) or in the field. Factory seams are made in a controlled environment and are generally of high quality. Field testing would include both non-destructive and destructive tests done at regular intervals along the seams.

Consistent quality in fabricating field seams is critical to liner performance, and conditions that may affect seaming would be monitored and controlled during installation. Factors affecting the seaming process include:

Ambient temperature at which the seams are made; Relative humidity; Control of panel lift-up by wind; The effect of clouds on the geomembrane temperature; Water content of the subsurface beneath the geomembrane; The supporting surface on which the seaming is bonded; The skill of the seaming crew; Quality and consistency of the chemical or welding material; Proper preparation of the liner surfaces to be joined; Moisture on the seam interface; and Cleanliness of the seam interface (e.g., the amount of dust and debris present).

Depending on the type of geomembrane, several bonding systems are available for the construction of both factory and field seams. Bonding methods include solvents, heat seals, heat guns, dielectric seaming, extrusion welding, and hot wedge techniques. To ensure the integrity of the seams, a geomembrane should be seamed using the bonding system recommended by the manufacturer.

Thermal methods of seaming require cleanliness of the bonding surfaces, heat, pressure, and dwell time to produce high quality seams. The requirements for adhesive systems are the same as those for thermal systems, except that the adhesive takes the place of the heat. Sealing the geomembrane to structures around the reservoir and appurtenances will require special care and should be performed in accordance with detailed drawings included in the design plans and specifications.

Anchorage. An anchor trench along the perimeter of the cell generally is used to secure the geomembrane during construction. Alternative methods might be adopted at Taian, depending on Contractor preferences or particular needs for this project. Run out calculations (e.g. Koerner, 1990) are available to determine the depth of burial at a trench

necessary to hold a specified length of membrane, or combination of membrane and geofabric or geotextile. If forces larger than the tensile strength of the membrane are inadvertently developed, then the membrane could tear. For this reason, the geomembrane should be allowed to slip or give in the trench after construction to prevent such tearing. However, during construction, the geomembrane should be anchored according to the detailed drawings provided in the design plans and specifications.

Surface Protection. For the Taian Upper Reservoir, the Consultants have recommended an exposed geomembrane system, i.e. without a layer or layers of soil on top of the geomembrane. No activities should be permitted on top of the geomembrane liner after the system has been placed, tested, and accepted. As described elsewhere in this report by the Consultants, it is possible to provide access around the perimeter of the geomembrane (for inspection prior to filling and during periodic dewatering inspections) without passage on top of the liner.

It is suggested that a small catch fence system be placed at the bottom of the reservoir slopes (Hunling mountain side) in order to stop any large materials rolling or falling down onto the liner and damaging it after installation but prior to reservoir filling.

If for some reason or other it is decided to place a layer of soil on top of the geomembrane (e.g. in a local area to permit vehicular traffic), then special procedures must be followed for the placement of such fill. Soil must be placed without driving construction vehicles directly on the geomembrane. Light ground pressure bulldozers may be used to push material out in front over the liner, but the operator must not attempt to push a large pile of soil forward in a continuous manner over the membrane. Such methods can cause damage or localized wrinkles to develop and overturn in the direction of movement. Overturned wrinkles create sharp creases and localized stresses in the geomembrane that could lead to premature failure. Instead, the operator should continually place smaller amounts of soil or drainage material working outward over the toe of the previously placed material.

1.5 Leakage Detection, Monitoring, Seepage Control for Geomembrane-Lined Reservoirs

1.5.1 Liner Leakage

For a project such as the Taian Pumped Storage Project, the purpose or objective for a leakage detection system should be defined in such as way that it is clearly understood why it is needed, how it is to used, and how its performance can be measured.

The most obvious reason for a leak detection system associated with a geomembrane reservoir liner is to provide a means to detect any problems with the liner before serious consequences develop. Why should this be needed? Usually for two main reasons:

1. Safety assurance - to provide protection of other features, such as erodible materials, clay core, or other sensitive project elements that could be damaged by leakage – i.e. to help maintain project safety.

2. Value of water – to monitor the potential loss of water on projects where the water budget is critical, such as for a project located in a desert area.

For the Taian project, neither of these issues is of exceptional importance – unlike say for the Blue Diamond Project described above. At Taian, materials behind the liner are not erodible and leakage should not result in risk to project safety (provided drainage provisions are followed as recommended by the Consultants). Also, it does not seem that the project water budget is very critical (unless there is information that the Consultants are not aware of)

The principal types of liner problems comprise:

Seepage and minor leakage (defects, holes, bad seam welds) Leakage leading to piping and major failure Major failure,rip or tear due to failure or settlement of supporting materials

In the United States, it is generally considered that all single geomembrane liner systems can and will leak. Only double membrane systems can provide a dependable leak-proof system. Electric leak detection surveys performed on several hundred geomembrane systems around the world have shown that there are 2 to 12 leaks per hectare in newly installed lining systems, even if good installation and state-of-the-art CQA has been performed. The size of the leaks vary from pinholes to holes. The leakage rate per pinhole (or hole) is a function of the head of water above the geomembrane and the size of the pinhole (see example estimates for Blue Diamond Project in Appendix 1.2.2).

Seepage and minor leaks due to defects, holes, damage during installation, bad seam welds, etc. can be controlled by application of stringent construction quality control and post-installation inspection. Installation damage (where most leaks originate) can be mitigated by use of white co-extruded liner and appropriate QC, as suggested by the Consultants. Testing of welds should also be included. Electric spark or conductivity testing can also be applied.

Piping or internal erosion can be avoided by appropriate design of drainage, filter, and transition materials behind the liner to control the potential for erosion in the event of a hole. In the case of Taian, the liner is supported by suitably thick zones 3 and 4 material on top of non-erodible random rock fill. Major failure by piping should not take place. In Appendix 1.2.3, some photographs are provided of a water supply reservoir (larger than the Taian Upper Reservoir) where the liner failed as a result of piping failure that developed from leakage occurring at a defective seam. The liner system had inadequate drainage and improper design of bedding material. The design for the Taian Upper Reservoir that is recommended by the Consultants would prevent this from occurring.

Excessive settlement of material below the liner system could result in extreme stresses, and rips or tears in the liner. However, in the case of Taian, this would be unlikely provided that appropriate measures are taken for compaction of the random fill and construction of bedding layers as recommended by the Consultants.

For the Taian Project, the Consultants consider that major leakage (large scale damage) can be mitigated by proper design and construction of bedding materials. Dam safety, including stability, would not be threatened by leakage. Piping or damage to other structures is not a significant concern.

1.5.2 Leakage Detection, Monitoring, Collection, and Drainage Provisions

Various detection systems can be used to detect and locate leaks, both large (e.g. tears in the geomembrane liner) and small (e.g. pin holes). Monitoring systems can consist of the

same types of instrumentation and monitoring devises as seen on conventional geotechnical structures, including:

Collection systems for drainage that are sufficiently independent so that seepage points can be located;

Placing of seepage weirs at appropriate points downstream, e.g. dam toe drainage; The geomembrane system could incorporate an advanced electronic detections

system, or it could be a double membrane system with compartmented seepage sensors (however, this adds to cost);

Consideration of a certain degree of automation of the instrumentation linked with alarm systems to alert when/if leakage occurs or exceeds and certain amount.Dam Toe Drainage System

The dam toe drainage system will be monitored on a continuous basis and will provide an index of the amount of water leaking from the upper reservoir. However, monitoring of drainage at the toe cannot be used to identify or distinguish the source of the leakage, e.g. leakage from concrete face vs. leakage from holes in the liner vs. seepage from the foundation.

It will be possible however to interpret if there has been a major failure of the liner by monitoring the volume of drainage over time. Major failures of the lining will be visible upon drawdown of the reservoir.

Small leaks (pin holes) in the liner will not be distinguished from dam toe drainage. However, small leaks will not compromise the safety or the functionality of the Project, as discussed above and, therefore, are acceptable and not required to be repaired immediately.

1.5.2.1 Leak Detection Systems

Specialized leak detection systems are required for landfills and hazardous waste applications in many countries by regulatory requirements (e.g. Federal law in US). The consequences of leakage for water supply or hydroelectric project reservoirs are not necessarily as dire and therefore are not generally as complex or advanced as those used for landfills.

In addition to monitoring executed by measuring the drained water at the dam toe, other more accurate methods are also available, which can provide continuous and permanent monitoring of the efficiency of the liner system.

Types of seepage monitoring that can be considered for reservoir projects with liners include:

‘Smart’ geotextiles and geomembranes that incorporate conductive materials within them (used for continuity testing and/or determination of spontaneous potential)

Double liner systems with collector system (as recommended for the Blue Diamond Project, described earlier)

Geophysical methods: acoustic emission and spontaneous potential (SP) TDR (time domain reflectometry) Fiber optic temperature (time domain optical reflectometry)

Articles on some of these are provided in Appendix 1.4.5, as well as a sample specification for a leak detection survey system (from Geosynthetica, 2002, courtesy of I-Corp International). A recently completed project by the Consultants (MWH) involved the installation of a leak detection system (fiber optic temperature) behind a PVC liner at Windscar Dam in England. For the Blue Diamond Project, GSE proposed that an electrical leakage detection system could be installed in the leakage detection layer that can pinpoint leaks to within several square feet.

A double geomembrane liner equipped with a drain collector system (as recommended for the Blue Diamond Project, described earlier) can have the ability to locate and quantify leakage occurrences and is often considered the simplest and most reliable, though not necessarily the cheapest. However, such a system may be impractical for the Taian Upper Reservoir since the floor, where the liner is to be placed, is flat and drainage by gravity would not be developed without a complex camber arrangement.

Incorporation of a leakage detection system in the floor of the upper reservoir underneath the geomembrane system might be impractical. The bedding materials (zones 3 and 4) and the random fill beneath are relatively free-draining. Unless there is an incredibly close spacing of detection elements or cables, it would be difficult or questionable to detect and locate leakage before it is dissipated into the fill.

1.5.2.2 Recommendations on Leak Detection System

For the Taian Upper Reservoir, the need for an advanced system for the detection of leaks has to be based on determination of:

Technical necessity, Practicality, and Cost

In the opinion of the Consultants, the need for such a Leakage Detection System does not have a sufficiently important technical necessity as discussed above. It will complicate construction and it is uncertain whether it would be practical or will function as intended. Such a system could also be costly, estimated to range from about $1.2 million for a ‘Smart’ geotextile system to about $2.4 million for a fiber optic temperature system, or another $1.0 million to $1.5 million for a double liner collector system.

In summary, therefore, the value of such a Leakage Detection System is questionable for the recommended designs and it is the Consultants’ opinion that it might not be justified.

The Consultants recommend that, provided there is sufficient time in the project schedule, a reservoir filling and holding test be performed prior to wet testing of the units. On other projects, MWH has found this to be a important test of the upper reservoir integrity, the instrumentation system, and waterways (if they can be evaluated separately).

1.6 Method of Connection between Geomembrane and Adjoining Structures

1.6.1 Types of Connections at Taian

The Taian upper reservoir geomembrane connections will involve three different interfaces:

Dam facing Rock surface (concrete faced) on the southwest side Horizontal excavated rock surface (and instrumentation/grouting gallery)

The location for the attachment at the inlet-outlet structure as shown on the drawings is probably correct. On the Blue Diamond Pumped-Storage Project, there was a concern about stresses on the liner system imposed by constant fluctuations in water velocities and pressures. The geomembrane suppliers suggested that a concrete apron be extended out from the bottom of the structure, and that the lining be attached to this apron at some distance from the entrance to the power shaft.

Generally, there is preference to avoid vertical/horizontal lining interfaces with concrete structures. At the Blue Diamond Pumped-Storage Project, the lining is to be attached horizontally to the concrete apron. The distance from the entrance to the edge of the concrete apron will be dependent on flow velocities, etc. In this case, the lining suppliers were interested in seeing the hydraulics of the intake-outlet structure to determine the potential impacts on the lining.

On a water supply reservoir project in the USA, excessive differential settlements occurred at a similar location (see photograph in Appendix 1.2.3). This illustrates the importance at Taian for very careful excavation, fill placement, and compaction at the junction between fill and rigid structures as discussed by the Consultants during their recent visit.

In order to simplify field seaming, it is desirable to consider the following:

Orient the seams in a direction that will minimize stresses and alleviate wrinkles (e.g. parallel to line of slope, down and not across slope).

Minimize number of field seams in corners, odd-shaped geometric locations and outside corners.

Slope seams (panels) should if possible extend a minimum of 1.5 – 2.0 m -feet beyond the grade break into flat areas.

1.6.2 Details

The method of attachment of the reservoir lining to adjacent structures has to be compatible with the properties of the selected liner material and expected loads that will be taken by the liner at these interfaces.

The actual details for attachment of the geomembrane to the structures surrounding the Taian upper reservoir should be developed by the lining supplier/installer based upon his experience, knowledge of the particular materials he is working with, and capabilities of his work force. Each major supplier has patented details that they have found to work best for them. Examples of typical attachment materials and drawing details from GSE were been provided by the Consultants during their recent visit.

The types of anchorage attachments fall into two broad categories: metal batten strip fastenings that hold the geomembrane down onto the concrete surface, and cast-in geosynthetic attachment strip fastenings that are fixed into the concrete and to which the geomembrane is welded (e.g. GSE’s Polylock system).

There is a potential for increased strain in the geomembrane at the connections with adjacent structures due to various factors (differential settlement, movement, etc.). This could result in localized problems with the liner, including weld/seam failure or tearing at the connection. Various solutions could be adopted, some of which are illustrated in the sketches provided.

1.6.2.1 Liner Connection to Concrete Slab

A suggested detail for connecting the concrete slab at the foot of the dam and the geomembrane liner is shown on the review drawings. It may be adequate to use only one waterstop. Since a geomembrane is used as the reservoir liner it makes sense to have this waterstop as PVC or rubber rather than as a bottom copper waterstop as used on many concrete faced rockfill dams. This imbedded waterstop is placed in the joint wave wall/ slab, at vertical slab joints, in the horizontal joint concrete slab/ inclined dam slab and along the upstream toe of the concrete lining, for connection to the geo-membrane, as shown on review drawings.

Only if the entire lining, on the down slope, on the right flank and on the reservoir bottom consists of reinforced concrete, then double waterstops are suggested for all joints, i.e. bottom copper waterstop and embedded PVC/ rubber waterstop. This provides a safe and low maintenance liner.

At the critical connection between the 300 mm reinforced concrete slab and a 1.5 to 2.0 mm geomembrane it is suggested to provide a second line of defense, i.e. a double geomembrane system, or another flexible geomembrane in this critical zone. This top geomembrane should be a flexible high yield material (e.g. 1.5 mm PVC, Hypalon, or HDPE Ultraflex) fixed to the concrete and to the reservoir geomembrane as indicated on

review drawings and/ or as suggested by the manufacturer/installer of the liner. The steel profile (batten) should compress the liner against the concrete, with a prior application of regularizing polymer paste and a Hypalon (soft rubber) strip, by nuts tightened to a specified torque with a dynamometric wrench.

At the dam connection, geomembrane/ concrete slab, a third line of defense may be considered , by covering the connection geocomposite liner/ concrete with low cohesion fines (silty fine sand), acting as a crack stopper in the case of puncture of the waterstop (the fines will migrate into the supporting filter below and clog the seepage path).

An alternative that would be preferred by many USA manufacturers/installers would be to place a geocomposite clay liner (GCL, e.g. Bentofix) strip in this area.

1.7 Seam Welding Technology

1.7.1 Japanese Welding Experience

1.7.1.1 Imaichi Pumped Storage Project

Number of seams Single seam Double seamEquipment Self-propelled

welderHand welder Self-propelled

welder

ApplicationField shop welding Field shop welding

and field welding for lateral joint

Field shop welding and field welding for longitudinal joint

Width of lapping > 3 cm > 8 cmWidth of welding > 2 cm > 5 cm including

slit for air proof test

Welding speed 1.7 m ~ 3.5 m 1.2 m ~1.6 mWelding temperature

590oC ~620oC 580oC ~ 610oC

Performance test Vacuum test Air proof test

1.7.1.2 Adhesive Tape

Adhesive tape of self-curing isobutylene-isoprene rubber was applied to paste cover sheet to EPDM sheet.

1.7.1.3 Welding Seam Quality

Welding seam strength of 0.8 MPa was specified at Imaichi Pumped Storage Project (JIS K 6301)

1.7.2 Taian Upper Reservoir

Two basic field seam welding types are expected at Taian:

Extrusion welds, and Wedge welds.

Welding equipment and accessories have to meet the requirements of the type(s) of material being installed and the design philosophy for the particular liner. Typical specification requirements are included in the sample specifications provided by the Consultants; including factors such as temperature gauges and monitoring, provision of sufficient equipment and electrical power to perform the work efficiently and without interruption.

Details are provided in the sample specifications on the procedures for Extrusion and Hot Wedge welding. Requirements are also included for Trial Welds.

1.8 Repair of Geomembrane Liners

1.8.1 Principles

The recommended arrangement for the Taian Upper Reservoir liner system has it exposed, i.e. without a cover of fill or geotextile fabric. One distinct advantage of this is to facilitate inspection and detection of defects, damaged areas, etc. that could be the location of leaks. Repairs, if required, could then be readily performed without having to remove and then replace overlying layers. Exposed geomembranes can be readily and quickly patched with minimal interruption to plant operation.

A thorough inspection of the liner must be carried out after its installation and prior to first operation. This inspection would be repeated periodically during routine maintenance. All seams and non-seam areas of the geomembrane have to be examined for defects, holes, blisters, undispersed raw materials, and any sign of contamination by foreign matter. The installer needs to repair and non-destructively test each suspect location in both seam and non-seam areas. Repairs need to be done in accordance with the procedures indicated in the sample specifications provided by the Consultants for any one of the following repair methods:

Patching- Used to repair large holes, tears, undispersed raw materials and contamination by foreign matter.

Abrading and Re-welding- Used to repair short section of a seam. Spot Welding- Used to repair pinholes or other minor, localized flaws or where

geomembrane thickness has been reduced. Capping- Used to repair long lengths of failed seams. Flap Welding- Used to extrusion weld the flap (excess outer portion) of a fusion

weld in lieu of a full cap.

References

Takimoto, J. and Onoi, Y. (1997). A Rubber Surface Pond on Strongly Weathered Rock Foundations for the Seawater Pumped-storage Power Plant, ICOLD Congress, 1997 Q.73, R.32, pp 479~498.

H. Shimizu and Y. Ikeguchi (1998). Use of Synthetic Rubber Sheet for Surface Lining of Upper Pond at Seawater Pumped-storage Power Plant. 1998 Sixth International Conference on Geosynthetics