Effective Angle of Shearing Resistance for Soft Clay

A method is proposed to correct the consolidated-undrained test results for the prestress effect, and its underlying principle is described. According to the "true shear strength" concept, the shear strength of clay can be divided into two components: one is the true cohesion Ce which is a function of the water content or void ratio of the clay; and the other is the true internal friction σnf’tgφe where (?)e is the true angle of internal friction which is a constant independent of the water content of the clay. In the consolidated-undrained test, the specimen has been consolidated to a certain water content under the consolidation pressure and then sheared with constant water content. Hence, regarding to the shear strength determined in this test, there should be a true internal friction corresponding to the effective normal stress on the failure plane and a true cohesion corresponding to the consolidation pressure which is larger than the effective normal stress on the failure plane due to the development of pore pressure in the shearing process. As to the shear strength mobilized in drained test, both of these two components are related with the effective normal stress on the failure plane. Therefore, in order to compare the results of consolidated-undrained test with those of drained test, the true cohesion mobilized in the consolidated-undrained test should be corrected corresponding to the effective normal stress on the failure plane.It is shown by the proposed method that in normally consolidated soft claytg(?)d’ = ξ + tg(?)e andtg(?)d’=cos(?)e+(2Af-1+sin(?)e)tg(?)e/cos(?)e+(2Af-1+sin(?)e)tg(?)’tg(?)’where Af is the Skempton’s pore pressure coefficient; ξ is the coefficient of true cohesion; and (?)d’ is the corrected angle of shearing resistance in terms of effective stress obtained from the consolidated-undrained test which is identical with pd obtained from the drained test.