Due to the significant size effect in the direct shear test of coarse grained soil

and the incapability of the general indoor direct shear apparatus to adjust the size of the sample

it is difficult to obtain the shear strength parameters of coarse grained soil. Therefore

an assembled direct shear apparatus which can be employed to change the size of the sample is introduced. This direct shear apparatus is composed of an output device

a displacement measuring device

a reaction frame

and a shear box. By changing the number of steel rings nested in each shear box

the diameter of the sample can be set to be 6.18

10

15 and 20 cm. By changing layers of shear boxes

the height of the sample can be set to be 5

10

15 and 20 cm. The values of diameter and height of the sample can form 16 combinations in total. On this basis

abandoned slags from the diversion tunnel of Xuecheng Hydropower Station were selected as samples

and then 5 groups of parallel direct shear tests were carried out to verify repeatability of the direct shear apparatus and reliability of the test results. By changing the diameter D and height H

a series of direct shear tests was conducted to study the size effect of the sample

and to determine the reasonable size of the sample.