Design details of sheet pile

Earth - Water pressures in sheet pile design

Before all, a designer has to appropriately select the type of lateral earth pressures that are expected to act on the sheet pile. For most sheet pile 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.

External Stability checks in retaining wall design

External stability checks refer to calculations that represent the overall stability of the shoring system. Two calculations are typically performed:

Passive resistance of shoring systems: This calculation considers the available earth resistance in the horizontal direction below the excavation.

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

For temporary conditions a safety factor of atleast 1.2 is required.

Moment - rotational stability: This type of calculation considers the stability of the shoring for rotational failure of the wall.

Factor of safety overturning = Resisting moments / driving moments

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

Bearing Stability in sheet pile 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.

Global stability in for sheet pile design Another item of concern is the overall global stability of the excavation. In some cases, while the other checks yield acceptable factors the sheet pile 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.

Structural checks in a sheet pile wall: Once a stability checks are satisfactory then one can design the actual individual shoring components. For concrete retaining walls this involves the proper sizing of longitudinal and shear reinforcement if required.

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