IS 2720:1986 Part 13 is the Indian Standard (BIS) for methods of test for soils - direct shear test. This standard specifies the laboratory method for determining the shear strength parameters of soils (cohesion and angle of internal friction) using a direct shear apparatus. It provides step-by-step procedures for unconsolidated-undrained, consolidated-undrained, and consolidated-drained tests.
Specifies the method for conducting direct shear tests on soil samples to determine shear strength parameters (cohesion and angle of internal friction).
Overview
- Status
- Current
- Usage level
- Frequently Used
- Domain
- Geotechnical — Soil and Foundation
- Type
- Testing Method
Also on InfraLens for IS 2720
Practical Notes
! Direct shear test forces the failure plane to be predetermined and horizontal, which may not always be the weakest plane of the soil specimen.
! Pore water pressures cannot be measured during this standard test, making effective stress parameters difficult to reliably obtain for saturated clays.
! Ensure the gap between the upper and lower halves of the shear box is slightly larger than the maximum particle size to prevent friction between the metal box halves during shearing.
Frequently referenced clauses
soilsandgravel
International Equivalents
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Key Values4
Quick Reference Values
typical small shear box size60 mm x 60 mm
typical minimum specimen thickness20 mm
minimum thickness to maximum particle size ratio6
maximum particle size for 60mm box4.75 mm
Key Formulas
Shear Stress (τ) = Applied Shear Force / Area of cross-section
Normal Stress (σ) = Applied Normal Load / Area of cross-section
τ = c + σ * tan(φ) — Mohr-Coulomb failure criterion
Tables & Referenced Sections
Key Clauses
Clause 4 - Preparation of Specimen
Clause 5 - Procedure for testing
Clause 5.1 - Unconsolidated Undrained Test
Clause 5.2 - Consolidated Undrained Test
Clause 5.3 - Consolidated Drained Test
Clause 6 - Calculations
Frequently Asked Questions3
What shear strength parameters are determined from this test?+
The cohesion intercept (c) and the angle of internal friction (φ).
Can pore water pressure be measured in a direct shear test?+
No, standard direct shear apparatus does not have the capability to measure pore water pressure. Triaxial tests are preferred for strictly measuring effective stress in clays.
What is the minimum thickness required for the soil specimen?+
The thickness of the specimen must be at least 6 times the maximum particle diameter of the soil being tested.
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