CAUSES OF PAVEMENT FAILUREINTRODUCTIONThere are certain basics with respect to pavement failure that have existed since the first pavements were laid. Pavements crack, pavements slip, water damages them, and pavements rut. Irrespective of the tests used to evaluate pavements, failures have the same basic causes.CRACKINGNo matter where the cracking occurs, it is caused by the inability of the asphalt to relax the stresses, and must rupture.Fatigue Cracking. Stress and strain are what are called tensors, which means that a pavement can be under compression and tension at the same time, but in different directions. While a tire compresses a pavement downward, it forms a deflection basin which causes the pavement to go into tension in both horizontal directions. Many years ago we used data from deflection testing and, assuming a parabola, did a line integral to calculate strain. If the pavement is not strong enough, the asphalt is stretched too far, separates and a crack forms in the wheel track. Also a crack may form between the wheel tracks.Longitudinal Cracking on Joints. The joint between two passes are especially week. Inside any one pass of the paver, some aggregate will be on both sides of any plane or slice inside of the pavement. In fact, when sample undergoes an indirect tensile test such as is done in stripping tests, rocks actually fracture. A joint, however, is held together only by the asphalt layer, which has a tensile strength of about 200-1000 psi, depending on the temperature and shear rate. If the asphalt in the mix can flow vertically in response to thermal stresses, the crack wont form. However, if the stresses exceed that at the joint, a crack forms. As a result the pavement on either side of the crack can shrink or expand independently. Often what happens then is that the pavement sections shrink away from each other in the cold, but do not expand completely back together in the heat. For that reason it is crucial to follow proper technology of forming a joint.Thermal Cracking. The mechanism of formation of thermal or non-load associated cracks is again the lack of the asphalt to be able to relieve thermal stresses by flowing vertically up when the pavement is hot and vertically down when the pavement is cold.PAVEMENT SLIPPAGEFrom time to time the pavement will shift. In one project I has on at the LAX airport, a 2 lift was slipping on a 4 lift from landing of air traffic. A core was made of the section so it was possible to observe a daily slippage. Two sources of the problem. First, it was supposed to be 4 over 2. Secondly, if there was a tack coat, it had been ruined as a result of a dust storm. To prevent slippage a prime needs to be used between the base and pavement, and a tack coat between two lifts.RUTTINGThere are two causes of rutting, improper aggregate gradation and studded tires.Gradation. Asphalt itself is too weak to stop the flow of the mix by itself. If the coarse aggregate in the mix cannot interlock the mix has to rely on a mastic composed of the fines and asphalt, which cannot carry the load. The solution is a coarse gradation with no humps in the fine mastic area.Studded Tires. Research is under way on how to solve this problem. Harder aggregate has helped, but no solution is available now.

WATER DAMAGEIf the pavement is not protected from water damage, all of the above is blowing in the wind. There are data that suggest that even pavement protected by amine or lime antistrips will lose much of its strength thus cannot complete its design life. Many aggregates are wetted by water better than asphalt so that if the surface cannot be permanently altered to prefer wetting by asphalt, eventually water will replace the asphalt.Why Does Pavement Crack? Installing asphalt pavement requires it to be placed in strips or paver passes. This means there is a construction joint / seam every 12 to 18 feet across a typical parking lot. A crack will form along these seams within 1 year after installation, depending on the weather conditions. Cracks also occur due to failures in the surface layer of asphalt, lack of bonding between the asphalt layers, and structural failures due to inadequate design. Here are a few examples of the various type of cracks.Seam Cracks As the pavement ages it becomes dry and brittle causing cracking. Low temperatures winter freeze and thaw cycles causing the asphalt to expand and contract. Cracks form where each paver pass starts and stops.Seam Cracks

These type of cracks can be cleaned of debris and then sealed with hot rubber.Which prevents water from getting into the base creating potholes.Bond Failure: Slippage Cracks Slippage cracks develop when there is not a good bond between the surface layer and the binder layer of asphalt. Lack of tack coat used between asphalt layers. Vehicles turning the wheels when the vehicle is not moving.Bond Failure: Slippage Crack

A full depth asphalt patch is required to repair this type of defect.

Structural Failure: Fatigue Cracking Fatigue or alligator cracking can occur due to inadequate pavement design. The surface, base, or subgrades are not appropriate for the amount of weight / traffic. As pavement ages the amount of cracking increases each year. These shattered areas will need to be cut out and replaced. This can occur when proper asphalt maintenance is not completed. Poor drainage can also cause this type of cracking due to standing water.Structural Failure: Alligator Cracking

Complete removal and replacement is needed to repair this area.

How Do Potholes Form?

Water enters the cracks in the pavement and seeps into thestone basebeneath the asphalt. Whenthe temperatures drop and the moisture freezes it causes the ground to expand and push the pavement up. As a thaw cycle occurs, the ground returns to its normal level but the pavement can remain raised making a gap. When traffic drives over this area the surface of the pavement cracks and leads to the beginning of a pothole. Plows also speed up the process.Cold mix asphalt is available now if you need emergency pothole repairs. This is a quick but temporary fix to a potentially hazardous situation. Hot Mix asphalt will become available in April.In a future post we will talk about why your asphalt CRACKS and what you can do to help slow the formation of the potholes by taking care of the CRACKS in your parking lot.

EFFECTS OF PAVEMENT FAILUREThe effect of road failure or pavement failure is that it may lead to an accident and the maintenance cost will be bigger than the construction cost.

ENGINEERING SOLUTIONPavement EngineeringOur highly skilled pavement engineering specialists are able to advising on new build construction as well as rehabilitation and maintenance of existing pavements, using engineering judgment to determine maintenance solutions maximising long-term benefit and minimizing cost. We are able to undertake pavement assessment and evaluation and to design and determine maintenance solutions for flexible and rigid pavements.

Technical adviceTRL Appia provides assistance to a number of key clients and their advisors in the UK including contractors, consultants, banks, public and private companies, central and local government. We have assisted in legal matters where strong technical skills were paramount. Our team responded quickly and professionally to our clients needs resulting in production of technical reports to be submitted as evidence. The outcome of recent cases has been positive for our clients. Our service to support legal matters includes: Investigation of problems, defect and failures Design and construction advice Specification Advice Evidence based analysis of current failures Preparation of advice notes and expert reportsPavement AdviceOur teams of skilled engineers are fully conversant with current pavement evaluation techniques and pavement design and have over 25 years experience in highways maintenance practices from local authority to local government projects. To support your pavement maintenance design/ evaluation needs, we can offer the following services.Value Engineering Material Specification Scheme Appraisal Whole Life Costing

GPR Procurement and AnalysisDetermining Material thickness Specifying Core locations etc. Moisture Utility detection etc.LWD Procurement and Analysis (Prima Testing) Ensuring reinstatement compaction with LWD Site Supervision Testing Sub-grade (Unbound material)Visual Survey Procurement and Analysis HAPMS Visual Surveys including input into HAPMS UKPMS Visual Surveys Engineering Surveys Concrete Surveys Bespoke SurveysFWD Procurement and Analysis Structural Properties of pavement Load transfer efficiency Back-analysis Evaluation of performance Overlay designs

Other advice and analysis: Dynamic Cone Penetrometer (DCP) Deflectograph Coring including core log specificationRecycling AdviceAppias engineers have substantial knowledge of recycling and materials and can help highway authorities to reduce the amount of waste that is going to landfill and to meet statutory targets set by the Government and the EU Landfill Directive. We have helped recycling solutions using bituminous emulsions, foamed bitumen, and hydraulically bound products to stabilise and modify WRAP, crushed concrete and other secondary aggregates used in both rigid and flexible materials to promote: Sustainability In both construction and maintenance of highways, new products (virgin materials) have predominated; in consuming these we deplete the resources available to us on the planet. By minimising this consumption and recovering and reusing materials we prolong the life of the planetalong witheverything and everyone on it. Energy Efficiency removing minerals from the ground, processing them and then transporting them to the point of manufacture has a major impact upon the environment. Recycling significantly reduces the amount of CO (reducing your carbon footprint) released into the atmosphere. Cost efficient and well-programmed recycling can realise significant savings (typically 30%).

CAUSES OF GROUND FAILUREGround failure is used here to describe zones of ground cracking, fissuring, and localized horizontal and vertical permanent ground displacement that can form by a variety of mechanisms on gently sloping valley floors. Landslides and rock falls that occur on steep hillside slopes are discussed separately beginning on p.44. In general, ground failure may be caused by (1) surface rupture along faults, either as a primary rupture on the seismogenic fault or as a sympathetic rupture; (2) secondary movement on shallow faults; (3) shaking-induced compaction of natural deposits in sedimentary basins and river valleys, or artificial fills; and (4) liquefaction of loose sandy sediment.

Earthquakes and Ground FailuresWhen large faults rupture and produce earthquakes, they generally deform the ground surface. Primary surface faulting, such as the 22-kilometer-long surface rupture associated with the 1971 San Fernando earthquake, is the direct effect of movement on a seismogenic, or earthquake-producing fault. Rupture on nearby faults induced by the primary event (sympathetic rupture) may also produce surface faulting. Earthquakes can also produce secondary features that look similar to primary surface rupture. Primary features related to known or suspected faults can be readily studied by geologists and directly linked with earthquake activity on those faults, while secondary features may be difficult to link to activity on a particular fault. However, studies of secondary features can provide information on the effects of earthquake shaking at selected sitesextremely important information for seismic-hazards evaluation that cannot be directly obtained from studying seismogenic faults alone. How Does Ground Failure Occur?USGS scientists evaluated two alternate mechanisms of localized soil failure and secondary tectonic deformation. Extensive investigations of geotechnical properties of the soils were carried out at three sites at Balboa Blvd., Malden St. and Wynne Ave. These consisted of borings to determine the geologic structure and cone-penetration tests to estimate the soil strengths. All three sites were also in areas of gently sloping ground, and scientists found that they were underlain by saturated soils that could be expected to fail when subjected to high levels of ground shaking. Therefore, localized failure in a buried layer was thought to be the mechanism causing the failure at the surface. However, it was not clear that the failures could have been anticipated even if detailed subsurface investigations had been conducted before the earthquake. Two of the sites, Balboa Blvd. and Wynne Ave., were underlain by saturated sands that were predicted to liquefy at the levels of ground shaking recorded in the epicentral region. However, the cracking at Malden St. is suspected to have been caused by a different mechanism than liquefaction, probably dynamic shear in weak clay. This mechanism should be more seriously considered in areas underlain by weak soils which may be subjected to high levels of ground shaking.Secondary tectonic deformation could possibly explain some of the 1994 ground failures at Balboa Blvd. and Wynne Ave., but USGS scientists consider it unlikely. The stratigraphic complexity and an abrupt change in the depth to ground water at the south end of the Balboa Blvd. study area suggests the presence of a tectonic fault. However, the fault does not appear to have significant recent movement, and the 1994 ground-failure zone extends almost 300 meters to the north where no stratigraphic evidence exists for faulting. The 2-meter step on the top of the sediment observed at Wynne Ave. may have contributed to the location of the ground failure there.EFFECTS OF GROUND FAILUREIn a comprehensive design approach, it should be recognized that damage to structures and facilities may result from different seismic effects. These effects can be classified as Direct and Indirect (or Consequential) as follows:Direct Effects: 1. Ground failures (or instabilities due to ground failures) Surface faulting surface or fault rupture) Vibration of soil (or effects of seismic waves) Ground cracking Liquefaction Ground lurching Differential settlement Lateral spreading Landslides2. Vibrations transmitted from the ground to the structure.Indirect Effects (or Consequential Phenomena):Tsunamis SeichesLandslidesFloodsFiresThe seismic effect or damage that usually concerns the structural engineer, and which is taken into account by code seismic-resistant design provisions, is the vibration of the structure in response to ground shaking at its foundation. Although damage due to other effects may exceed that due to vibration, procedures for gauging the probability of these effects and for coping with them are outside the scope of the structural engineering discipline and so are usually not included in seismic-resistant codes. Nonetheless, the structural engineer should be aware of the different seismic hazards and should advise the client of potential damage involved in locating structures at certain sites. Thus the first step in the design procedure of a future structure should be the analysis of the suitability of the site selected with proper consideration for the potential of any one of the above types of damage.ENGINEERING SOLUTIONRoad design and construction over soft ground especially over very soft and soft marine deposits are interesting engineering challenges to engineers especially at the approaches to bridges and culverts. Many geotechnical options are available for engineers consideration. Of course, the one that uses the local materials and resources that are cheaper and easier to construct would no doubt be the choice. This concept is also common to foundations for buildings and factories. GEOTECHNICAL SOLUTIONS FOR ROADS

Embankment Design

Embankment design of roads needs to satisfy two important requirements among others; the stability and settlement. The short term stability for embankment over soft clay is always more critical than long term simply because the subsoil consolidates with time under loading and the strength increases. In design, it is very important to check for the stability of the embankment with consideration for different potential failure surfaces namely circular and non-circular. supporting the embankment when designing the embankment so that the settlement in the long term will not influence the serviceability and safety of the embankment. The details of the embankment design can be obtained from papers by Tan & Gue (2000).

Design considerations include numerous issues such as those outlined by Tan & Gue (2000). The height or thickness of embankment is often dictated by the flood level. As the height of embankment is most critical over marine deposits, hence decision on height after consideration of settlement of the subsoil is critical to the cost and time of the project. When the height exceeds the maximum one stage construction (loading), multistage construction will be required. Of course more detailed analyses are required when more refined soil layers and properties are obtained. In the detailed analyses, both circular and non-circular slope stability analyses should be carried out. Very often, the non-circular failure is more critical than circular slip failure for layered soil especially with very soft subsoil at top few meters. Long term stability of embankment is usually not an issue for embankment over soft marine deposits because the subsoil would gain strength with time after the excess pore water pressure in the subsoil dissipates during consolidation. When the analyses based on subsoil and thickness of embankment indicate multistage construction is required, the construction of the embankment usually take substantially longer time especially when the cohesive subsoil does not have sand lenses. If time permits with early planning, multistage construction could be reduced by geometry change in the embankment as described by Tan & Gue (2000). However, geometry change requires wide road reserve due to flatter slope and stabilizing berms. Prefabricated vertical drains could also be installed to reduce the time required for consolidation. Other techniques such as piled embankment, stone columns, and vacuum preloading with prefabricated vertical drains could be considered. However, the technique would increase the cost of the embankment. Piled embankment in particular using local wood piles for shallow subsoil could be competitive. The details on the design and construction control for embankment over soft clay are also described in the above reference.

Ground Treatment MethodsIn order to identify suitable ground treatment to be adopted, the design engineer needs to carry out both technical and cost analyses. Some of the embankment construction methods commonly used in Malaysia are as follows : (a)Modification of Embankment Geometry (b)Excavation and Replacement of Soft Soils (c)Surcharging (with or without vertical drains) (d)Staged Construction (e)Lightweight Fills using Expanded Polystyrene (EPS) (f)Geosynthetics Reinforcement (g)Stone Columns (h)Piled Embankment Modification of embankment geometry through reduction of slope angle or construction of counterweight berms can be cost effective option if there is sufficient cut earth or abundant suitable materials nearby. Although excavation and replacement of soft soil (either partial or total) is an old method but still viable and popular where the very soft compressible is not very deep. The experience on highway construction in West Malaysia indicates that the excavation and replacement depth of up to a maximum depth of 4.5m in soft clay is still viable in terms of cost and practicability. The excavation should extend up to the toe of the embankment and beyond to increase the stability of the embankment.