Importance of Collecting Soil Samples in Hard Strata for Soil Classification and Determination of Shear Strength Parameter-Paper

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The paper examines how sampling methods affect soil classification and shear-strength estimates in hard strata, using investigations for three railway bridges across the Saraswati, Umardasi, and Pushpavati rivers in Gujarat.<br /><br />Initial investigations (Stage I) drilled boreholes to depths of 40–50 m and used standard penetration tests (SPT). Where SPT N-values exceeded 100, split-spoon sampler penetration was very limited, and undisturbed samples were not obtained. Soil classification therefore relied on disturbed SPT samples, which indicated hard silty clay. Shear-strength parameters at depth were estimated using correlations with extrapolated SPT values.<br /><br />Follow-up investigations (Stage II) used Mazier samplers to obtain higher-quality samples from the hard strata for laboratory testing. SPT values and Atterberg limits were broadly consistent between the two investigation stages, but the soil classification changed substantially: material previously classified as silty clay (CL/CI/CH) was identified as very dense clayey sand (SC). The authors attribute the discrepancy to the limited penetration of the split-spoon sampler, which may have preferentially collected fine particles.<br /><br />Laboratory results from the Mazier samples were used to establish design parameters, including internal friction angles of approximately 31–32 degrees for the deeper clayey sand. The findings show that disturbed SPT samples can produce misleading classifications in very hard soils, potentially affecting pile design and project schedules. The paper recommends using appropriate sampling methods, such as Mazier sampling, to verify soil classification and shear-strength parameters in hard strata.

Keywords

soil classification

shear strength

hard strata

sampling methods

SPT

Mazier sampler

clayey sand

silty clay

railway bridges

Gujarat