Our lab in Liverpool operates a fully automated direct shear apparatus that accommodates 60 mm and 100 mm square shear boxes, allowing us to test both undisturbed tube samples and compacted remoulded specimens under consolidated drained conditions. The load frame applies normal stresses up to 800 kPa while the horizontal shear displacement rate is controlled via a stepper motor, typically set between 0.002 and 2.4 mm/min. For cohesive soils from the Triassic mudstones and glacial tills that dominate the Liverpool subsoil profile, we run multistage tests at three normal loads to define the Mohr-Coulomb failure envelope. The system logs force, displacement, and pore pressure (when using the saturated option) at 1-second intervals, and we process the data to extract cohesion intercept and friction angle in accordance with BS 1377-7:1990. This test is routinely requested before designing shallow foundations along the Mersey waterfront where soft alluvial deposits overlie bedrock.
Peak strength can overestimate stability by up to 50% in Liverpool's glacial tills — direct shear quantifies the real residual envelope for safe design.
Process overview
A common mistake among contractors in Liverpool is assuming that peak strength from a quick undrained triaxial test is sufficient for long-term stability analyses of embankments and cuttings. For clay fills and weathered Keuper Marl, the residual shear strength can be 30–50% lower than the peak value, which directly impacts design of stability of slopes and the required passive resistance for retaining walls. The direct shear test addresses this by allowing us to shear the specimen beyond peak to large displacements — typically 10–15% of the sample length — recording the constant or slowly decreasing post-peak strength. We prepare specimens at field moisture content and remould them to target densities when evaluating compacted engineered fill. The test provides:
Peak cohesion (c') and friction angle (φ') under drained conditions
Residual shear strength parameters for reactivated slip surfaces
Evaluation of dilation angle and critical state behaviour
These parameters feed directly into limit equilibrium and finite element models used for Liverpool's ongoing brownfield redevelopments.
Local context
During the excavation for a five-storey residential block on Duke Street in Liverpool city centre, the contractor hit a buried glacial channel filled with soft laminated clay that was not identified in the preliminary desk study. The original foundation design assumed a friction angle of 32 degrees for the underlying sandstone, but the actual clay stratum exhibited a residual friction angle of only 16 degrees. Without a direct shear test on undisturbed samples from that layer, the factor of safety against sliding dropped below 1.2 — unacceptable under Eurocode 7. We ran three multistage direct shear tests on thin-walled tube samples recovered from the clay, and the results allowed the structural engineer to extend the pile lengths through the channel into the competent Bunter Sandstone. The final design achieved a factor of safety of 1.6, avoiding a potential foundation failure that could have delayed the project by four months.
For cohesive soils such as Liverpool's glacial tills and laminated clays, we perform three-stage tests at normal stresses of 100, 200, and 400 kPa on a single 60 mm specimen. This minimises sample disturbance and provides the full peak and residual envelope from one tube sample. The test includes specimen extrusion, trimming, saturation under back-pressure, and shearing at 0.024 mm/min. Results are delivered as a Mohr-Coulomb plot with c' and φ' values.
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Large-Scale Direct Shear on Granular Fill
When evaluating imported granular fills for Liverpool's dockland redevelopments, we use the 100 mm shear box to test compacted sand and gravel specimens at relative densities of 50%, 70%, and 90%. The larger box reduces the influence of particle size on shear behaviour. Normal stresses range from 50 to 600 kPa, and the test quantifies both peak and constant-volume friction angles, essential for calculating bearing capacity of shallow foundations on compacted fills.
Relevant standards
BS 1377-7:1990 — Methods of test for soils: shear strength (direct shear), Eurocode 7 (EN 1997-1:2004) — Geotechnical design (limit state approach), BS 5930:2015 — Code of practice for ground investigations
Common questions
What is the difference between peak and residual shear strength in the direct shear test, and why does it matter for Liverpool soils?
Peak strength is the maximum shear resistance mobilised at small displacements (typically 2–5% of sample length), while residual strength is the constant or slowly decreasing value reached after large displacements (8–15%). In Liverpool's glacial tills and Keuper Marl, the residual friction angle can be 10–15 degrees lower than the peak value. This matters because many existing slopes and cuttings in the Liverpool area are already at residual conditions along pre-existing shear surfaces. Designing based on peak parameters alone would overestimate stability and could lead to unsafe factors of safety under Eurocode 7.
How much does a direct shear test cost in Liverpool?
Our standard multistage direct shear test on an undisturbed sample costs between £440 and £620, depending on the number of stages, saturation requirements, and whether pore pressure monitoring is requested. Large-scale tests on the 100 mm box with compacted granular fill typically fall in the upper part of this range. These prices include specimen preparation, testing, data analysis, and a full report with Mohr-Coulomb parameters. Volume discounts may apply for projects requiring 10 or more tests.
When should I request a direct shear test instead of a triaxial test for a project in Liverpool?
The direct shear test is preferred when you need the residual shear strength along a pre-existing failure surface, such as in landslide investigations or reactivated slip planes in Liverpool's glacial till slopes. It is also more practical for granular soils where undisturbed sampling is difficult — the test can be run on compacted remoulded specimens. Triaxial tests are better for anisotropic stress paths or when you require effective stress parameters under undrained conditions. For routine foundation design in Liverpool's stiff clays, we often recommend both: triaxial for short-term undrained conditions and direct shear for long-term drained stability.
