Evaluation of Causes ofRetaining Wall FailureMu’azu Mohammed ABDULLAHICivilEngineering, F.U.T., P.M.B. 65, Minna, NigerState, Nigeria.E-mail:AbstractRetaining structures are vital geotechnical structure,because the topography of the earth surface is a combination of plain, sloppyand undulating terrain. The retaining wall resists thrust of a bank of earth aswell as providing soil stability of a change of ground elevation. Earthpressures on retaining wall are designed from theories of Soil Mechanics, butunfortunately the engineers using them do not always realize the significanceof the assumption in their development. This is usually accompanied by withfailure and partial failures because of designed based on rules and formulaethat fit only limited conditions. In addition there are also problems of usingbad backfill materials without taking precautionary measures against built–upof hydrostatic pressure by provision of drainage and also poor workmanship.KeywordsRetaining wall, earth pressure, hydrostatic pressure,backfillIntroductionThe term retaining wall has traditionally been appliedto free – standing walls whose purpose is to resist the thrust of a bank ofearth or other materials. Some of these walls are well designed and arefunctional, while others may have the appearance of strength, but other may bemore a show piece.
Retaining wall may also provide soil stability at a changeground elevation.A retaining wall is a permanent, relatively rigidstructure of cribbing masonry or concrete that supports a mass of soil. Itsubstitutes the steep face of the wall for the gentle natural slope of theearth to provide useable space in highway and rail road cut, in and aroundbuilding and also in structures below ground level. The design philosophy dealswith the magnitude and distribution of lateral pressure between a soil mass andan adjoining earth retaining structure. This involves prediction of lateral pressureand deformation by considering the initial stress condition in the soil, thestress – strain relationship for the soil and the boundary condition describingsoil structure interaction. Such a solution would be extremely complex and inpractice simplified methods are used.Real problems in lateral earth pressure involve morethan simple loads produces by soil against a retaining wall.
Earth pressure isnot a unique property of the soil or rock, but it is a function of the materialthat the retaining structure must support, of the loads that the soil behindthe structure must carry, the groundwater condition and the amount ofdeflection the retaining structure undergoes. The pressure exerted by the soilon these structures is known as earth pressure and must be determined before asatisfactory design can be made 3.Theories of soil mechanics dealt with earth pressure onretaining walls, unfortunately, the engineer using these theories have notalways realized the significance of the assumption made in their development.Therefore, the wide use of retaining walls is accompanied by many failures andpartial failures because of designs based on rules and formulas that fit onlylimited conditions. For example, the design of walls backfilled with soft clay isoften based on analysis that apply only to sand and the design of walls thatsupport structures that will be cracked by foundation movement are often basedon active earth pressure that requires the wall to tilt out ward. The onlymargin between success and failure in many cases has been an overgeneroussafety factor.A satisfactory retaining wall must meet the followingrequirements.a. The wall is structurally capable of withstanding theearth pressure applied to it.b.
The foundation of thewall is capable of supporting both the weight of the wall and the forceresulting from the earth pressure acting upon it without:- Overturning or soil failure- Sliding of the wall and foundation- Undue settlementThe earth pressure against a retaining wall depends onthe deformation condition or tilt of the wall, the properties of the soil andthe water condition. For greatest economy, retaining walls are ordinarilydesigned for active pressure as developed by a dry cohesionless backfill, butif necessary, a design can be developed for any condition of yield, soil andwater.Physical investigation of some retaining walls in Minnashows that there were cracks, sliding or even collapse of these walls. Thesedeformations may be due to the following:i.
The sliding of soil onsoil well below the retaining wall which may constitute the most common kind ofsliding failure.ii. By slip of thesurrounding soil which is most common in cohesive soils and can be analyzed fora slope stability problem.iii.
Overturning which should allows for the resultant to bewithin the baseiv. By sliding forward thisis caused by insufficient base friction or lack of passive resistance in frontof the wall.Other may include rotation about a point near the top ofthe wall as may occur in shell piled walls and structural failure caused byfaulty design poor workmanship and deterioration of material etc.In the course of this research work attempt was made toinvestigate some of these cause of failure highlighted above.The greatest proportion of failure of wall higher than3m is caused by water pressure in a backfill that was assumed by the designerto remain dry. The must important single consideration in wall design isinsuring good drainage.
Bearing Capacity Failure Retaining Wall Panels
There are two approaches- Remove water from backfill- Keep water out of backfillThe best backfill is rigid, free – drainage and with ahigh angle of internal friction, so as to develop minimum. Lightweightartificial material such as expanded shale and crushed slag often make goodbackfill. CharacteristicsQuantitySITEASITEBSITECSITEDLiquidlimit LL%58.1240.0435.0951.23PlasticLimit PL%34.0025.6719.3325.76PlasticityIndex PI%24.1214.3715.7625.47ShrinkageLimit SL%22.8018.2016.8019.50MDD(Mg/m 3)1.691.711.731.65OMC%14.2012.0813.2715.41HydraulicConductivity m/s8.17x10 -78.65x 10 71.05x10 67.23x10 7%Passing Sieve No 20060.154.340.756.2Cohesion(C) KN/m 250383568Angleof Internal Friction (°)810129USCSClassificationCHCLCLCHThe amount of yield does not apply to walls that retainbackfills of cohesive soil and especially those that retain soil with a highdegree of plasticity.
In cohesive soils the amount of yield required to givethe thrust on a wall is not usually large but because of plastic flow withinthe clay, the pressure behind the wall continuously tend to increase unless thewall is permitted to yield continuously. The continuous yield, although it isat a slow rate may lead to a large movement over a period of years.Nevertheless, if a wall is capable of withstanding the movement with noundesirable effect it may be designed on the basis of the active pressure. Sucha basis of design is common, but the probable life of a wall with a cohesivebackfill may be relatively short. A wall that is capable of yielding withoutdetrimental result may be designed on the basis of the active case. However thepressure which will act on such a wall will in general be larger than theactive pressure. The pressure distribution diagram may not be triangular eventhough the wall is designed on the basis of the total active case.The fact is that any wall must have a margin of safetythat is the wall must be capable of staying in place under a thrust some what largerthan the active thrust. This margin of strength prevents the wall from reachingthe amount of yield required to give active conditions 7.The factors of safety used in conventional geotechnicalpractice are based on experience which is logical.
However it is common to usethe same value of factor of safety for a given type of application, such aslong – term slope stability, without regard to the degree of uncertaintyinvolved in its calculation. Through regulation or tradition, the same value ofsafety factor is often applied to condition that involve widely varying degreesof uncertainty.