Saint Louis University

School of Engineering and Architecture

Department of Civil Engineering

Review of Related Literature

An Assessment of the Structural Integrity of Existing Government Designed Retaining Structures

Exposed Slopes

An exposed ground surface that stands at an angle with the horizontal is called an unrestrained slope. The slope can be natural or man-made slope. Slope can collapse easily by shear if the soil is nearly saturated and high pore pressure can be built up rapidly. The falling debris carried by its potential and momentum can rush down the slope at very high speed and travel a long distance causing huge damages. (Das, B., Principles of Geotechnical Engineering 7th Edition, 2010 pg. 512)

There are many ways on how to improve the stability of slopes:

Introducing an appropriate drainage system a surface drain system that is capable to discharge all the storm water within the rain water catchment area affecting the slope in a designated period of time and a subsoil drain system that is laid below surface for the discharging of ground water and to maintain the water pressure be kept in a safe level.

For rocky slopes, many measures can be done to help aid its stability. One example is Dentition which is done by trimming soft materials and replacing them with concrete. Another method is the use of high tensile dowel bars, rock bolts or nails. Application of concrete on highly fractured rock faces is also done.

The use of rigid and soft surfaces is also one way of stabilising exposed slopes. Rigid materials such as plasters, shotcrete, masonry or stone pitching is laid out on the exposed slope to prevent infiltration of water that could lead to further movement of the soil. Crops could also be used to stabilise slopes through hydroseeding or turfing. These method could take longer time to be made but is a better solution other than rigid surfaces which creates a very awkward appearance.

The use of retaining walls. Retaining wall are structures usually provided at the toe of a slope to stabilize it from slide, overturn or collapse. A slope will be relatively stable when its profile (section angle) is kept below its angle of repose. Angle of repose is an angle that maintains naturally to a safe equilibrium by the composing material of a slope. This angle deviates from differing materials depending on their compaction, particle size and the nature of the material itself.

Commonly Used Retaining Walls in the Philippines

There are various types of retaining walls found in the Philippines, gabions and ripraps are the types of retaining walls commonly used in the area. The following is a brief description of these two types of retaining walls.

Gabions or Reno Mattresses are wire baskets filled with stones. They are often used as slope protection because they can change shape and settle a lot without any damage - gabions are good for protecting slopes. The following are steps of assembly and construction from the DPWH-Blue Book:

1. Gabions shall be installed in a workmanlike manner. The gabions shall be placed on a smooth foundation. Final line and grade shall be approved by the Engineer. Each gabion unit shall be assembled by binding together all vertical edges with wire ties on approximately 152 mm (6 inches) spacing or by a continuous piece of connecting wire stitched around the vertical edges with a coil every 102 mm ( 4 inches). Empty gabion units shall be set to line and grade as shown on the Plans or as described by the Engineer. Wire ties or connecting wires shall be used to join the units together in the same manner as described above for assembling. Internal tie wires shall be uniformly spaced and securely fastened in each cell of the structure. A standard fence stretcher, chain fall, or iron rod may be used to stretch the wire baskets and hold alignment.

2. When possible the subgrade of the mattress and gabion shall be properly compacted to a depth of 150 mm. The Contractor shall consider the cost of subgrade preparation in the unit prices. Filter fabric as beds of gabions and mattresses forming the structure shall be suitably levelled and shall be securely connected along the complete length of all contact edges by means of the above specified tying and connecting wire.

3. Before the filling material is placed, the gabions and mattresses shall be carefully selected for uniformity of size, and the pieces shall be hand placed to provide a neat appearance as approved by the Engineer. The gabions shall be filled with stone carefully placed by hand or machine to assure alignment and avoid bulges with a minimum voids. Alternate placing of rock and connection wires shall be performed until the gabion is filled. After a gabion has been filled, the lid shall be bent over until it meets the sides and edges. The lid shall then be secured to the sides, ends and diaphragms with the wire ties or connecting wire in the manner described for assembling. The vertical joints of gabions and mattress baskets shall be staggered as in running bond in brickwork.

4. The cells in any row shall be filled in stage so that local deformation may be avoided. That is at no time shall the cell be filled to a depth exceeding 30 cm more than the adjoining cell.

5. Filter fabric shall be placed between earth surface and gabion or mattress structures. Filter fabric shall be rolled out into a flat non-rutted surface free from sharp objects, weighing down the edges. Construction equipment shall not be allowed into unprotected

fabric. Jointing is normally affected by overlapping not less than 300 mm, but it is preferable to joint by sewing or industrial stapling. Joint edges should be facing downwards to avoid protruding through the surface material.

Rip-raps are large rocks or blocks of concrete place against the slope. For rip-rap to work, the rocks must be heavy enough not to be washed away by river water. Stones found in the river close to the bridge cannot be used as rip-rap. They will not be heavy enough. If the rip-rap is being washed away, it must be replaced with larger rocks. The following are requirements for the construction of a rip-rap wall from the DPWH-Blue Book:

1. The bed for riprap shall be excavated to the required depths and properly compacted, trimmed and shaped. The riprap shall be founded in a toe trench dug below the depth of scour as shown on the Plans or as ordered by the Engineer. The toe trench shall be filled with stone of the same class as that specified for the riprap, unless otherwise specified.

2. Stones placed below the water line shall be distributed so that the minimum thickness of the riprap is not less than that specified. Stones above the water line shall be placed by hand or individually by machines. They shall be laid with close, broken joints and shall be firmly bedded into the slope and against the adjoining stones. Each stone shall be laid with its longest axis perpendicular to the slope in close contact with each adjacent stone. The riprap shall be thoroughly rammed into place as construction progresses and the finished surface shall present an even, tight surface. Interstices between stones shall be filled with small broken fragments firmly rammed into place.

Unless otherwise provided, riprap shall have the following minimum thickness, measured perpendicular to the slope:

Class A – 300 mm Class B – 500 mm Class C – 600 mm Class D – 800 mm

3. The spaces between the stones shall then be filled with cement mortar throughout the thickness of the riprap. Sufficient mortar shall be used to completely fill all voids; except that the face surface of the stones shall be left exposed. Grout shall be placed from bottom to top of the surface swept with a staff broom. After grouting is completed, the surface shall be cured as Structural Concrete for a period of at least three days. The stones shall also be laid in a manner that the vertical and horizontal alignments of the exposed face shall, as possible be maintained in a straight line.4. All walls and abutments shall be provided with weep holes. Unless otherwise shown on the Plans or directed by the Engineer, the weep holes shall be placed horizontally at the lowest points where free outlets for water can be obtained and shall be spaced at not more than 2 m center to center in a staggered manner. The length of the weep holes shall not be less than the thickness of the walls of the abutment and shall be at least 50 mm diameter PVC or other pipe materials accepted by the Engineer. Weep holes must be provided with filter bags as specified in special provision or as directed by the Engineer.

Retaining structures analysis considers; sliding, bearing capacity, and overturning forces. On sites with slopes, a global stability check will be necessary.

SLIDING is the ability of the retaining wall structure to overcome the horizontal force applied to the wall. Factor of safety= 1.5

OVERTURNING considers the ability of the retaining wall structure to overcome the overturning moment created by the rotational forces applied to the wall. Factor of safety= 1.5

BEARING CAPACITY considers the ability of the underlying soil to support the weight of the retaining wall structure. Factor of safety= 2

GLOBAL STABILITY-Ability of the internal strength of the soil to support the complete soil mass. Contact local design specialist for help in evaluating your site.

Failure of Retaining Walls

Retaining structures are vital geotechnical structure, because the topography of the earth’s surface is a combination of plain, sloppy and undulating terrain. The retaining wall resists thrust of a bank of earth as well as providing soil stability of a change of ground elevation. Retaining wall is a wall to prevent the material of an embankment or cut from the sliding.(Webster’s Comprehensive Dictionary 1999 Edition).

A satisfactory retaining wall must meet the following requirements, the wall is structurally capable of withstanding the earth’s pressure applied to it and that the foundation of the wall is capable of supporting both the weight of the wall and the force resulting from the earth’s pressure acting upon it without overturning or soil failure, and sliding of the wall and foundation (Abdullahi, 2009).

Failure of retaining walls is attributed to many factors. One of the main cause of retaining wall failure is poor drainage. Without proper drainage, hydrostatic pressure builds up behind the retaining wall. Saturated soil is substantially heavier than dry soil, and the retaining wall may not be deigned to handle such a load. Retaining walls should have adequate drainage that will act as a funnel for the water behind the retaining wall, leading such water out and away from the structure to minimize hydrostatic pressure build up.

Another cause of failure is in its construction phase. It is recommended that you make your retaining wall footing deep enough to resist the weight of saturated soil.A retaining wall with a shallow footing has a lower capacity to resist the lateral pressure of the soil and water behind it than a wall with a deeper footing. The depth of the footing becomes even more crucial in gravity walls, which depend mainly on their own weight to be effective. The use of a poor concrete mix, the lack of supports or the lack of reinforcing bars are also causes of retaining wall failure. Remember that even a 4-feet-high, 15-feet-long retaining wall could be holding back as much as 20 tons of soil.

Putting an extra (that is, unaccounted for) load 3 feet from the top of the retaining wall – say a car or a shed – can cause a blowout failure. In this case, the retaining wall leans over and subsequently topples from the extra load. To prevent failure due to extra load, account for all the load the retaining wall must bear before construction. Depending on the amount of load calculated, your contractor may have to widen and/or deepen the footing of the wall, increase its thickness, or install anchors or tiebacks for extra strength.

Retaining wall failures are also related to slope failure. If the dynamics of the slope the wall is holding back suddenly changes, the wall will be exposed to stresses it was not designed to handle. Read how slope failure can induce retaining wall failure. Slope failure is seen as one of the main causes of failures in retaining walls, thus it is one of the main focus of this study.

Slope Stability

In dealing with retaining walls, we consider the slope that is retained by retaining walls. First we discuss the five major categories of the modes of slope failure by Cruden and Varnes (1996).

1. Fall. This is the detachment of soil and/or rock fragments that fall down a slope. Figure 15.1 shows a fall in which a large amount of soil mass has slid down a slope.

2. Topple. This is a forward rotation of soil and/or rock mass about an axis below the center of gravity of mass being displaced (Figure 15.3).

3. Slide. This is the downward movement of a soil mass occurring on a surface of rupture (Figure 15.4).

4. Spread. This is a form of slide (Figure 15.5) by translation. It occurs by “sudden movement of water-bearing seams of sands or silts overlain by clays or loaded by fills”

5. Flow. This is a downward movement of soil mass similar to a viscous fluid (Figure 15.6). This chapter primarily relates to the quantitative analysis that fall under the category of slide.

In dealing with soil (Earth), we analyze the slope stability by determining the factors of safety. The factor of safety can be calculated as: (Das, B., Principles of Geotechnical Engineering 7th Edition, 2010 pg. 514)

Methods of Slope Stability Analysis

Slope stability analysis should be used to determine whether a proposed slope meets the required safety and performance criteria during design. This type of analysis is also utilized to determine stability conditions of existing natural or constructed slopes and evaluate the influence of proposed remediation methods if required. Slope stability analysis is used in a wide variety of problems including, but not limited to:

Evaluation of existing natural slopes

Determination of stable cut/fill slopesAssessment of overall stability of retaining walls and foundations located on slopesAssessments of landslides, remediation methods, and back analysis

An investigation should be made of all proposed fills supporting highway facilities and cuts that support important adjacent lands or structures. It is also essential that locations of existing landslides be identified before new highway alignment is fixed. Once slope geometry has been modeled and subsurface conditions have been determined, the stability of a slope may be assessed using a limit-equilibrium analysis, with appropriate drainage conditions and shear strengths. It is expected that when performing slope stability analysis the following data should be gathered: Soil Profile, Slope Geometry, Soil Shear Strength and Pore Water Pressure.

There are many methods on how a soil stability analysis is done. One of which is the Limit Equilibrium, the soil slope is commonly analysed based on Limit (or force or moment) Equilibrium methods that measure its stability by a factor of safety (FS) (ie. the ratio of resisting force to driving force). A slope with a FS =1 is theoretically marginally stable, or just ready to move. The shear strength (limit) required along a failure surface is calculated to just maintain stability and is then compared with the available shear strength to provide the overall FS for the slope.

Another way of doing soil stability analysis is by the Method of Slices. If mobilized strength for a soil is to be calculated, the distribution of the effective normal stresses along the failure must be known. This condition is usually analyzed by sectioning the potential failure mass into small vertical slices and treating each as a unique sliding block. This analysis is known as the Method of Slices.

A number of limit equilibrium methods are based on the Method of Slices. While originally calculated by hand, these more rigorous methods were converted to computerized versions.

The methods are generally divided into three categories, based on the number of equilibrium equations to be satisfied:

1. Overall moment equilibrium methods, 2. Force equilibrium methods, and 3. Moment and force equilibrium methods.

A three-dimensional (3D) slope stability analysis method, based on its two-dimensional approaches proposed by Donald and Chen starts from establishing a compatible velocity field and obtains the factor of safety by the energy and work balance equation. Optimization is followed to approach the critical failure mode that offers the minimum factor of safety. The method is demonstrated to be identical to Sarma's limit equilibrium method (1979) that employs inclined slices, if it is extended to the 3D area. However, it has been established on a sound theoretical background supported by the upper bound theorem of plasticity. Test problems have demonstrated its feasibility. A feature of the method is its very simple way to obtain the factor of safety without complicated 3D force equilibrium evaluations. Limited assumptions are involved in this method and their applicability has been justified. (Chen, D., 1997)

The majority of slope stability analyses performed in practice still use traditional limit equilibrium approaches involving methods of slices that have remained essentially unchanged for decades. This was not the outcome envisaged when Whitman & Bailey (1967) set criteria for the then emerging methods to become readily accessible to all engineers. The finite element method represents a powerful alternative approach for slope stability analysis which is accurate, versatile and requires fewer a priori assumptions, especially, regarding the failure mechanism. Slope failure in the finite element model occurs 'naturally' through the zones in which the shear strength of the soil is insufficient to resist the shear stresses. The paper describes several examples of finite element slope stability analysis with comparison against other solution methods, including the influence of a free surface on slope and dam stability. Graphical output is

included to illustrate deformations and mechanisms of failure. It is argued that the finite element method of slope stability analysis is a more powerful alternative to traditional limit equilibrium methods and its widespread use should now be standard in geotechnical practice. (Griffiths, D., 1999)

Slope stability problems are usually statically indeterminate. Assumptions are generally involved to render the problem determinate, with each method of analysis using a different assumption. As a result, the computed factor of safety varies between the different methods.