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Retaining wall design

Retaining wall design can be a tedious simple task to carry out. A retaining wall design has to account for a number of factors, foremost being the stability of the wall itself. Last, the design has to account for the specific retaining wall type that is used. In simple terms, different retaining wall types might require some additional design checks. Typically a retaining wall design has to consider the following:

Retaining wall design example of a soldier pile retaining wall with tiebacks in New York.

a) Earth - Water pressures in retaining wall design

Before all, a designer has to appropriately select the type of lateral earth pressures that are expected to act on the wall. For most retaining walls active or at-rest earth pressures are appropriate. Passive soil resistance should be used with caution. The possibility of including water pressures has to be considered if sufficient drainage is not provided. In the USA, depending on the design approach, some design codes (LRFD) apply safety factors that multiply each pressure by a safety factor. In Europe, a strength design approach is applied where soil strength is divided by safety factors and loads are multiplied according to their nature (temporary and permanent). Each method has its benefits and its shortcomings.

b) External Stability checks in retaining wall design

External stability checks refer to calculations that represent the overall stability of the retaining wall as if the retaining wall structure acts as a whole single body. Two calculations are typically performed:

b1) Sliding stability of retaining walls: This calculation considers the retaining wall stability in the horizontal direction. The horizontal components of forces are calculated and separated into driving and resisting forces. Soil and retaining wall weights are calculated and then the horizontal shear resistance at the base can be computed as Base Shear Resistance= Sum of Vertical forces x tan (soil friction angle) + Base Length x Soil-Wall Adhesion. Then the overall sliding stability if given by:

Factor of safety sliding = Resisting horizontal forces / driving horizontal forces

Under normal conditions a safety factor of atleast 1.5 is required.

b2) Overturning stability of retaining walls: This type of calculations considers the stability of the wall againgst toppling (i.e. turning over). This calculation is performed by calculating the moment each force component is generating about a given point in the wall. The toe of the wall is usually taken as the point of rotation. Moments are then subdivided into resisting and driving moments and the overturning safety factor is calculated as:

Factor of safety overturning = Resisting moments / driving moments

Under normal conditions a safety factor of atleast 1.5 is required.

c) Bearing Stability in retaining wall design: In all cases a retaining wall has to be founded in some kind of base material (be that rock or soil). When a retaining wall is based on soil the bearing stability tends to be more critical. The first task in this check is to properly compute bearing stresses on the toe and heel of the wall. The reason why bearining stresses have to be computed on both sides is because the overturning causes increased stresses in the toe and reduced stresses on the heel base. The bearing stresses have to be examined againgst the permissible bearing stresses and a minimum safety factor of 3.0 is typically specified. Using such a high safety factor typically ensures that wall settlements are kept within acceptable levels. Otherwise detailed settlement alculations are required if settlement control is critical.

d) Global stability in retaining wall design: Another item of concern is the overall global stability of a retaining wall. In some cases, while the overturning and sliding resistance as well as the bearing checks yield acceptable factors the wall might be succeptible to an overall rotational type failure that extends well below the retaining wall itself. Such a failure mode is most commonly accounted in hillsides where weaker soil zones exist or when a soft geomaterial is found below the wall base.

e) Structural checks in retaining wall design: Once a stability checks are satisfactory then one can design the actual retaining wall structure itself. For concrete retaining walls this involves the proper sizing of longitudinal and shear reinforcement if required. Limited wall bending is generated in most gravity walls that solely rely on their own weight for stability. Hence, in many cases the provided reinforcement is the minimum required for thermal and shrinkage effects.

Deep excavation has developed the DeepXcav software dedicated to deep excavation retaining wall design. DeepXcav has more than 1000 users worldwide. Please sign up for a free trial below!

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