Shear Box (Direct Shear) Testing: Strength Parameters for Safer Design

A Shear Box test—also known as the Direct Shear test—is used to determine soil shear strength for both cohesive soils (clays/silts) and granular materials such as sand, gravel and aggregates. It provides the key design parameters of cohesion (c) and angle of internal friction (φ), which are widely used in stability and earthworks calculations. The test setup is selected to suit the material:

• Small shear box (60 mm or 100 mm) for finer-grained soils
• Large shear box (typically 300 mm square) for coarse or granular materials

Shear box testing can be carried out in dry or submerged conditions to suit drained/undrained requirements. A controlled normal (vertical) load is applied through a steel plate, and the test is typically performed in triplicate, using three different normal stresses to define peak shear behaviour.

Following consolidation, the specimen is sheared along a mid-plane while load, vertical displacement, and horizontal displacement are recorded. Where required, the test can be repeated along the same plane to assess residual shear strength. Direct shear can also be undertaken as an interface test, assessing soil interaction with materials such as geomembranes, geogrids, geotextiles, or steel.

Results are used by geotechnical engineers for foundation design, retaining walls, slope and embankment stability, landslip assessment, piling mats, and backfill selection. Testing can be undertaken in accordance with BS EN ISO 17892-10, and is referenced in SHW Series 600 (including interface testing clauses).

Why choose The Testing Lab PLC?

At The Testing Lab PLC, we provide shear box testing with careful sample handling and clear, decision-ready reporting—helping you move from data to design with confidence.

Need direct shear testing? Contact The Testing Lab PLC to discuss sample type, box size, and test conditions.