What is Active Earth Pressure for Partially submerged Sand with Surcharge?
In Figure 1 active earth pressure for partially submerged sand and surcharge is shown. The height of the wall is Z and the position of the water table is at a depth of Z1 and the surcharge acting at the top of horizontal surface of the wall is q. The height of the wall below water table is Z2. The unit weight of the sand above water table is  and that below water table is .
CASE 1 |Â Active Earth Pressure for Partially submerged Sand with Surcharge
Consider a depth z, such that z is less than Z1. Then the earth pressure at depth z is given by:
When,
Case 2 for Partially submerged Sand with Surcharge
Consider a depth z, such that z is greater than Z1. Then the earth pressure at depth z is given by:
When,
When,
As shear strength of water is zero, then value of Ka in the third term of the above equation is unity. Hence,
The following condition can exist.
Soil is dry or moist and no surcharge
In this case, depth Z2 = 0, Z1 = Zand q = 0. Hence,
Soil is completely submerged and no surcharge
In this case, depth Z1 = 0, Z2 = Z and q = 0. Hence,
Soil is dry or moist and carries surcharge
In this case, depth Z2 = 0 and Z1 = Z. Hence,
Soil is completely submerged and no surcharge
In this case, depth Z1 = 0, Z2 = Z and q = 0. Hence,
Effect of soil stratification
If the backfill comprises two or more soil strata, then the top strata acts as a surcharge for the subsequent lower strata.