RETAINING SYSTEMS (SHORING)
Retaining walls are structures that hold back soil or rock from falling due to landslides caused by gravity or erosion. Watertight retaining walls (cofferdams) are employed in conjunction with an appropriate De-watering scheme to hold back water and clay in wet areas. In deep excavations, these walls have to withstand great lateral earth pressures and hydrostatic pressures caused by ground water. Depending on design requirements, several pile types and configurations are used to construct retention systems.
The variables, unknowns and constraints involved in retaining wall design are many, making it a complex and challenging task. A retaining wall design has to account for a number of factors, foremost being the stability of the wall itself. A thorough analysis of the geotechnical investigation report, project requirements, construction sequence, type of proposed structure and other factors is initially performed by design engineers.
This analysis, coupled with the experience of the designers, generates the first input for computer software (such as ReWaRD® from Geocentrix and STAAD.pro® from Bentley) to further analyze the complex loads and perform the calculations. Typically a retaining wall designer has to consider the following:
After a satisfactory stability design, structural calculations are made on the wall itself to determine the proper sizing of longitudinal and shear reinforcement.
RETAINING WALL SUPPORT
Designers have to also consider if the retaining wall itself requires further support (such as anchoring or bracing).
An appropriate factor of safety is incorporated into the design calculations for each element of the design criteria.
Earth and water pressures that are expected to act on the wall
The overall external stability of the retaining wall acting as one whole body (sliding and overturning) Sliding instability is caused by horizontal forces while overturning is caused by the moments generated from these forces.
Overturning increases stresses at the wall toe and decreases them at the heel; thus the bearing capacity of the foundation soil has to be considered to ensure that wall deflections are kept within acceptable values.
Even when external and bearing stabilities are acceptable, retaining walls may still be susceptible to an overall rotational type failure. This type of failure originates well below the retaining wall itself due to weaker soil zones.