Hi,MasterrrrBlasterrrr, Bro's theory is applicable for pile under the lateral loads. This is not the theory for analysing the retaining wall structure. Passive and active earth pressure theories are more appropriate for retaining wall analysis.
Please refer to some soil mechanics books such as...
For any soil sample, you should have the same effective shear strength no matter you shear the soil quickly or slowly. This has been approved by testing the same clay in CD and CU Triaxial tests.
The undrained shear strength is not a fundamental material property of soil. Undrained shear...
BigH made a good point. Please check the variation of water table levels.
You should use effective shear strength for the retaining wall design.
By the way, there is no undrained shear strength for clay if it is above the water table.
