Question

What all factors an engineer must consider for designing a retaining wall to retain a soil of height 10m? How those factors are different for a cohesionless soil, a cohesive soil and a saturated soil?

Answer 1

There are a lot of factors that an engineer needs to consider while designing a retaining wall to retain 10 m soild.

1) The engineer needs to know the type and propoerties of backfill(cohesive,non cohesive). The shear strength parameters of the backfill (c and )are critical. c is cohesion and is angle of shear friction. This dictates the lateral earth pressure coefficient of the backfill to calculate the load on the retainig wall.

2) The engineer needs to know the unit weight of the backfill. This also dicttates the load demand on the retaining wall

3) The engineer needs to know the elevation of water table. This affects the lateral earth pressure on the wall. If the water table is present within the backfill, then the wall experiences hydrostatic pressure as well.

4) The engineer needs to understand the support condition of the wall. This impacts whether the lateral earth coefficient should be at rest or active pressure. Typically, when the wall is cantilever, the pressure coefficient used is active earth pressure. On the other hand, if the wall has intermediate braces, then the pressure coefficient is corresponding to at rest.

5) The engineer needs to know the frcition coefficient between the wall footing to soil to determine the sliding resistance.

6) The engineer needs to know if there is any surcharge load on the backfil because this adds to the ;lateral earth pressure loads on the wall

7) The engineer needs to know the allowable bearing pressure on the soil. This dictates the size of footing required.

8) The engineer needs to know how much factor of safety needs to be ensured for these failure modes : sliding and overturning (these limi states apply to cantilvere walls)

9) The engineer needs to know the grade of concrete and steel to be used for designing the retaining wall and the footing.

10) The engineer needs to know if there is net uplift in the footing due to overturning of the wall (this applied to cantilver walls). Net uplift means there is a loss of contact between the footing and the soil. This is not preferred.