Direct Shear Test

The direct shear test in accordance with DIN EN ISO 17892-10 is performed on a square or circular specimen placed in a rigid steel frame consisting of two sliding halves lying on top of each other. First, it is consolidated vertically and then sheared horizontally at constant speed forcing a predetermined shear plane. The required force, shear distance and vertical deformation of the specimen are measured. The results are presented in a stress-displacement curve and a τ-σ-diagram. At least three tests need to be conducted at different characteristic stresses to determine the effective shear parameters. 

Application and Potential

Since the pore pressure cannot be measured, a sufficiently low shear rate is selected so that drained conditions can be established. The resulting parameters are the effective friction angle φ'  and the effective cohesion c'. When determined at the point of failure, these are relevant to evaluate the maximum strength of soils. By interpreting the data after long, sometimes multiple shear processes, the residual shear strength φ'r can also be determined which can be used, for example, to assess the stability of slopes. When shearing at higher velocities and thus impeding drainage, it is also possible to determine the undrained shear strength cU. This is used to calculate the initial strength of a soil. 

Specimen Requirements 

The minimum of three test specimens can be made from undisturbed, disturbed, recompacted or processed samples. They can be installed fully saturated, partially saturated, or dry. The specimen dimensions dictaded by the frame size are 60x60x20 mm or 100x100x20 mm (WxDxH) or 70/100 mm in diameter. The maximum permissible grain size is 1/6 of the height. 

To accurately interpretable parameters, information on the in-situ stress environment is required. For preparation of disturbed samples, information on the desired density and moisture content is required. For cohesive, glacial soils, information on the desired overconsolidation ratio (OCR) is necessary.

Technical Equipment

The tests are conducted using either an electromechanical shearing device with an adjustable maximum shear gap or a high-capacity shearing device with free adjustment of the shear gap (min/max specification). Complex controlled shear tests for preventing dilatancy of the specimen during the shearing process. Preconsolidation loads up to 50 kN are possible with a horizontal shear force up to 20 kN. The test can be performed on square or cylindrical (recommended) specimen.

Further Information

  • DIN EN ISO 17892-10 (2019): Direkte Scherversuche 

  • Schwiteilo, Erik; Herle, Ivo (2018): Bewertung von Scherversuchen aus Vergleichsuntersuchungen an feinkörnigem Boden. Aktuelle Themen der Geotechnik (Ohde-Kolloquium 2018). Technische Univesität Dresden

  • Kolymbas, Dimitrios (2019): Geotechnik. Bodenmechanik, Grundbau und Tunnelbau. 5th ed. Chapter 7.7