The 1986 edition of IS 2720 Part 13 lays down the prescribed methodology for performing the direct shear test on soils with particles up to 4.75 mm. It outlines procedures to evaluate soil shear strength parameters under undrained, consolidated undrained, and consolidated drained scenarios, which are vital for geotechnical assessments. Engineers rely on this standard to analyze soil stability and strength properties relevant to foundations, slopes, and earth structures.
Overview
The 1986 edition of IS 2720 Part 13 lays down the prescribed methodology for performing the direct shear test on soils with particles up to 4.75 mm. It outlines procedures to evaluate soil shear strength parameters under undrained, consolidated undrained, and consolidated drained scenarios, which are vital for geotechnical assessments. Engineers rely on this standard to analyze soil stability and strength properties relevant to foundations, slopes, and earth structures.
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Contents
Structure
This section defines the extent of the direct shear test for soil shear strength determination using a shear box apparatus. It covers the range of normal stresses to be applied, which should simulate field stress conditions and design demands, typically spanning from low to high values depending on soil characteristics. The application of weights to exert these normal stresses is described, alongside instructions on rounding test results according to IS 2-1960 standards. Terminology adheres to IS 2809-1972 soil engineering glossary. Key shear strength parameters such as normal stress (σ), shear stress (τ), cohesion (c), and internal friction angle (φ) are introduced, with the Mohr-Coulomb shear strength equation τ = c + σ tan φ provided. A flowchart illustrates the process from applying normal load to determining shear strength parameters.
This chapter details important references and formulas related to weights and normal load application, emphasizing that normal stresses during shear tests must reflect actual field conditions. It presents the standardized proforma for documenting test outcomes, including project and sample details, strain rates, calibration information, specimen measurements, dial readings, and calculated shear stresses. Essential calculations such as shear stress (τ = F/A) and determination of cohesion and friction angle from stress plots are outlined. A summary table for recording test results is introduced, alongside guidance on analyzing consolidation times and plotting shear stress against displacement and normal stress.
Terminology used in this standard follows the glossary in IS 2809-1972. The section specifies that weights apply the necessary normal load to the soil sample during testing to mimic in-situ stresses. Selected normal stress values must correspond to field and design conditions to ensure accurate test relevance. Rounding off of test results must adhere to IS 2-1960 criteria to maintain consistency and precision in reporting.
Specimens should be undisturbed and prepared to standard sizes, typically cylindrical or rectangular, following sampling procedures outlined in IS 2720 Part 1 (1983). Trimming and leveling are carried out using tools such as spatulas and straight edges. Calibrated weights are used to apply normal loads during testing. Final test values are rounded in line with IS 2-1960. Typical specimen dimensions are provided, and a flowchart summarizes the steps from sampling to testing and data recording.
This section refers to IS 11229:1985 for detailed apparatus specifications for the direct shear test. The main components include the shear box, loading devices to apply normal and shear loads, calibrated weights for normal stress, and dial gauges or displacement transducers for measuring shear displacement. Test results must be recorded using the proforma in Appendix B and rounded as per IS 2:1960. A summary table lists apparatus components and their purposes, accompanied by a flowchart depicting the testing setup.
Normal stresses applied during the test must be selected based on actual site conditions and design considerations. Test results should be recorded meticulously using the proforma recommended in Appendix B. Final values are rounded according to IS 2-1960. The typical procedure involves preparing the sample, applying normal stress, measuring deformation or pore pressure, recording data systematically, and calculating shear strength parameters. An example table illustrates rounding rules.
Normal stresses chosen should replicate real soil loading conditions to ensure applicable results. Test data must be documented in the consistent format of Appendix B, capturing all relevant parameters such as loads, displacements, and failure points. Rounding of values follows IS 2-1960. The Mohr-Coulomb equation τ = c + σ tan φ is used to calculate cohesion and internal friction angle from plotted shear versus normal stress data. Reports should include sample details, applied stresses, calculated parameters, observations, and a compliance declaration with IS 2720 Part 13.
Final test results require rounding as per IS 2-1960, preserving significant digits to maintain consistency. Micrometer dial gauges with 0.01 mm accuracy are used to measure horizontal and vertical specimen movements, ensuring precise data capture. Test outcomes are recorded on the recommended proforma for clarity. The rounding rules dictate rounding down if the dropped digit is less than 5, rounding up if more than 5, or rounding to the nearest even number if the digit is 5. A flowchart shows the process from measurement to final recording.
This appendix describes the procedure for consolidated drained direct shear tests using a shear box equipped with perforated plates and saturated porous stones. Normal stress is applied incrementally to allow full consolidation, guided by IS 2720 Part 15. Shearing is performed slowly to ensure at least 95% pore pressure dissipation. The time factor (T_v) is calculated from consolidation theory (T_v = C_v t / H²), with T_v approximately 0.848 for 95% consolidation. Multiple tests on specimens with the same density at different normal stresses are advised. Final moisture content is measured post-test. A flowchart illustrates the steps, emphasizing consolidation before shearing.
The recommended proforma includes sections for project and sample identification, strain rate determination, load calibration, specimen dimensions and moisture content, dial gauge readings, and shearing data such as load and displacement. Calculations for shear stress and plotting of results to determine cohesion and internal friction angle are included. Notes emphasize matching strain rates to field conditions, adherence to IS 2-1960 rounding, and accurate calibration records. A flowchart outlines the entire recording and reporting process from test initiation to final report.
Frequently Asked
IS 2720 Part 13 applies to the direct shear testing of a variety of soils with particle sizes up to 4.75 mm. Suitable soils include cohesive types such as clays and silty clays; cohesionless soils like sands, silts, and fine gravels; and mixed soils including sandy clays and silty sands. It is intended for soils where a distinct failure plane can be assumed. Soils containing large particles such as cobbles or boulders that cannot fit into the shear box are not appropriate for this test.
For cohesive soils, specimens can be compacted to the desired density and moisture content, then trimmed to size or compacted directly within the assembled shear box. Cohesionless soils are typically tamped inside the shear box with base plates or porous stones placed beneath. Plain grid plates without porous stones are used for undrained tests. Serrations on grid plates should be perpendicular to the shear direction. Moisture is maintained using water jackets, and a small gap (approximately 1 mm) is left between shear box halves before shearing to ensure consistent shear plane location.
Sandy soils are generally sheared at a rate around 0.2 mm per minute, allowing relatively rapid testing. For clayey or cohesive soils, a slow strain rate of approximately 0.01 mm per minute or less is recommended to permit adequate pore pressure dissipation. The strain rate for cohesive soils is calculated based on consolidation parameters, targeting a 95% pore pressure dissipation before failure is induced at about 5% strain.
Shear strength parameters are obtained by plotting the maximum shear stress against the applied normal stress and fitting a straight line to the data points. The slope of this line represents the angle of internal friction (φ), and the intercept on the shear stress axis corresponds to cohesion (c). In cases of nonlinear plots, tangents at expected field stresses are used to estimate these parameters. The fundamental Mohr-Coulomb equation τ = c + σ tan φ guides interpretation. Controlled strain tests yield more reliable parameters, especially for soils with particles smaller than 4.75 mm.
The testing apparatus must include a shear box conforming to IS 11229-1985 standards, a loading frame capable of applying vertical normal stresses without vibrations or loss of shear force, and devices to impose shear forces at controlled rates. Micrometer dial gauges with an accuracy of 0.01 mm are required to measure horizontal displacement and vertical compression. The equipment must enable precise control of stress application rates and maintain constant vertical stress during testing.
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