Direct Shear Strength Test
(IS 2720 Part 13 — Shear Parameters)
Laboratory determination of the shear strength parameters (cohesion c and angle of internal friction φ) of soils using the direct shear test — essential for slope stability analysis, retaining wall design and bearing capacity calculations.
What is the Direct Shear Strength Test?
The Direct Shear Test measures the shear strength of a soil specimen by applying a normal load and inducing shear failure along a predetermined horizontal plane. A 60×60mm (or 100×100mm) specimen is confined in a split shear box; a vertical (normal) load is applied and a horizontal shear force is applied at a constant rate of strain until failure. Tests at 3 different normal stresses yield the Mohr-Coulomb failure envelope.
Conducted as per IS 2720 Part 13, three specimens are tested at different normal stresses (typically 0.5, 1.0 and 2.0 kg/cm²). The shear stress at failure vs normal stress is plotted on a shear stress vs normal stress graph and the best-fit line gives cohesion (c) — intercept, and angle of internal friction (φ) — slope. Global Lab conducts consolidated drained (CD) tests for drained shear parameters.
Direct Shear Strength Test — Site Images
How the Direct Shear Strength Test is Conducted
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1Specimen Preparation3 specimens prepared at same density and water content. Placed in direct shear box in 2 halves.
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2Normal Load ApplicationNormal loads equivalent to 3 different vertical stresses applied (e.g., 0.5, 1.0, 2.0 kg/cm²). Specimens consolidated under normal load.
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3Shear TestHorizontal shear force applied at 1.25mm/min. Shear force and horizontal displacement recorded continuously.
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4Shear Stress at FailurePeak shear stress identified from shear force vs displacement curve for each normal stress.
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5Mohr-Coulomb Plot & Reportτ vs σ plotted. Best-fit line gives c (y-intercept) and φ (angle). NABL report within 5 days.
Applicable Test Standards
| Standard | Title | Scope | Type |
|---|---|---|---|
| IS 2720 Part 13 | Methods of Test for Soils — Direct Shear Test | Direct Shear | Indian Standard |
| IS 1498 | Classification and Identification of Soils for General Engineering Purposes | Soil Classification | Indian Standard |
| IS 6403 | Code of Practice for Determination of Bearing Capacity of Shallow Foundations | Bearing Capacity | Indian Standard |
| ASTM D3080 | Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions | Direct Shear | ASTM |