Quality of the lubrication layer used in element tests on granular materials

In order to find the proper composition of lubrication layer to be used for element tests of granular materials in the laboratory, a series of direct shear tests between a granular material specimen and a lubrication layer was performed. The granular materials used ranged from a fine sand to a gravel with D50 = 0.16 mm approx. 1.85 mm. The lubrication layer consisted of a layer of silicone grease and a latex rubber disk, and its variations. Based on the test results, the following points are shown: (1) For Toyoura Sand (D50 = 0.16 mm), when a properly designed lubricated layer is used , the apparent frictional angle Φμ can be as small as around 0.14 approx. 0.16 degrees when a normal stress is 2.0 approx. 6.0 kgf/cm2 (196 approx. 588 kN/m2). These values are independent of void ratio. (2) For Hime Gravel (D50 = 1.85 mm) and Silver Leighton Buzzard Sand (D50 = 0.62 mm), a proper amount of powder should be added to the original grease in order to improve the quality of lubrication by preventing the squeezing out of grease from the places of stress concentration in the proximity of particles indenting the lubrication layer. At the same time, the amount of powder should not be too much in order not to increase the shear resistance of a mixture of grease and powder. Fly ash is suggested as a proper type of powder. By this method, even for Hime gravel, a low value of Φμ of around 0.8 degrees at normal stresses of 1.5 approx. 7.0 kgf/cm2 (150 approx. 700 kN/m2) can be obtained.