Abstract: The existing shear boxes of large pullout direct instruments cannot meet the experimental requirements of the geosynthetics-geosynthetics interface and the interface between geosynthetics and the granular material. To solve the problem, both upper and lower shear boxes are modified based on existing large instruments, test requirements, and factors affecting experiments. A grillage of type II is put into the lower shear box to fasten the geosynthetics. The upper shear box can realize a flexible bearing load by fixing the geotextiles to the bottom of inner plate, and a rigid bearing load with a rigid plate. After the modification, direct shear tests of smooth geomembrane and sand interface, smooth geomembrane and geotextile interface are carried out. The results agree with those obtained by other researchers, showing that the equipment modification is reasonable and feasible. A series of direct shear tests of the reinforced geomembrane and sand interface, and there inforced geomembrane and geotextile interface have been performed, verifying stability of the instrument after modification.

Key words: direct shear test, geosynthetics, modification, shear box