The traditional rigid bearing plate load test (PLT) mainly determines the bearing capacity of foundation

and can not be used to invert the shear strength parameters

. In this study a flexible bearing plate has been invented through the installation of an air bag under the rigid bearing plate. Two self-made inductance displacement sensor are respectively set at the edge of two inner sides of the plate

in order to measure the variation of the thickness of the air bag at this area. When the flexible bearing plate is used to carried out the loading test

the compressive stress p under the plate is in uniform distribution. During the test

a constant uniform surface load q is applied on one side of the bearing plate by a flexible water pressure plate. So that two curves can be got

and two critical edge load values can be judged from the curves’ changing feature corresponding to zero surface load and q respectively. Then the c and can be inversed if we put the critical edge load values and q into the theoretical formula. The cohesive soil and dry sand have been tested respectively during the laboratory experiments; and the feasibility of the inversion method is proved. The applicability and the progressiveness of the inversion method are also approved through analysis.