Experimental Study on Variation of Soil Friction and Cohesion Due to Tamping Load on Disturbed Soil Sample.

The reliability of shear strength parameters obtained from the Direct Shear Test is highly dependent on specimen preparation techniques. Existing standards, including IS 2720 Part 13, provide general guidelines for sample preparation but do not clearly define tamping load requirements for disturbed soil specimens. This study investigates the influence of tamping load on the shear strength characteristics of two disturbed soils, namely Sandy Clay and Silty Sand. Laboratory investigations included grain size analysis, specific gravity, Atterberg limits, free swell index, compaction tests, and direct shear tests. Direct shear test specimens were prepared using the standard laboratory method specified in IS 2720 Part 13, with two additional tamping conditions using tampers weighing 0.5 kg and 1.0 kg, respectively. The specimens were prepared in three layers with 25 tamping blows per layer. Direct shear tests were conducted under normal stresses of 0.5, 1.0, and 1.5 kg/cm², and the resulting cohesion (c) and angle of internal friction (φ) were evaluated. The results showed that increasing the tamping load resulted in an increase in both cohesion and angle of internal friction. For Soil 1, cohesion increased from 0.193 kg/cm² to 0.222 kg/cm², and the angle of internal friction increased from 14.98° to 17.02°, while for Soil 2, the corresponding values increased from 0.118 kg/cm² to 0.132 kg/cm² and from 20.29° to 23.31°, respectively. The observed increase in shear strength parameters may be associated with changes in particle arrangement and interparticle contact resulting from the specimen preparation process. Comparative assessment of the two soil samples showed similar trends in the measured shear strength parameters under the investigated normal stress conditions. The findings indicate that variations in specimen preparation and tamping effort can produce noticeable differences in the measured shear strength parameters, which may influence geotechnical analysis and design. The study highlights the sensitivity of direct shear test results to specimen preparation and indicates the need for further investigation of tamping procedures using a wider range of soil types and preparation conditions.