What standards does your soil mechanics laboratory follow?

Our laboratory operates in accordance with British Standards (BS 1377) and Eurocode 7. All testing procedures are accredited under ISO 17025, ensuring compliance with UK regulatory requirements for geotechnical investigations.

How long does a typical soil testing program take?

Standard classification tests such as granulometry and Atterberg limits are completed within 5–7 working days. Advanced triaxial and consolidation tests may require 2–4 weeks, depending on the number of samples and drainage conditions.

Do you provide services for small-scale projects?

Yes, our firm supports projects of all sizes, from residential extensions to major infrastructure. We offer flexible sampling kits and remote consultation to accommodate smaller budgets while maintaining full technical rigor.

Direct Shear Calculator — Cohesion c and Angle φ

The direct shear test in a shear box (BS 1377-7 + BS EN ISO 17892-10) provides the Mohr-Coulomb shear strength parameters: cohesion c and friction angle φ. This calculator fits the linear envelope τ = c + σ·tan φ by least-squares regression to the (σn, τ) pairs measured in the laboratory, typically 3-4 points with different normal pressures. It also calculates residual (post-peak) parameters useful for slope stability analysis with prior movements and large deformation diagnostics.

What is it and when to apply it?

Direct shear is the most widely used test for characterising shear strength in granular and cohesive-frictional soils: sands, silts and clays without fines. It is fast (1-3 days) and economical, but imposes the failure plane and does not measure pore pressures, which limits its accuracy in saturated clays (where triaxial CU with u measurements is preferred). It is used for foundation design, retaining walls, slopes and fills. In rocks it is applied to contacts and discontinuities (barrera-barrera Roger Hoek 1977, ISRM).

Applied formulas

Mohr-Coulomb criterion:

τ = c + σn · tan φ

Linear regression of n points:

tan φ = [n·Σ(σn·τ) − Σσn·Στ] / [n·Σσn² − (Σσn)²]

c = (Στ − tan φ · Σσn) / n

Coefficient of determination R²:

R² = 1 − SSres/SStot, where SSres = Σ(τobs − τpred)²

Residual parameters: cr ≈ 0 (cohesion is lost after rupture); φr is obtained from the descending branch or a large displacement test (reverse direct shear)

Acceptance criteria BS 1377-7 + BS EN ISO 17892-10: 3-4 points with σn covering the project range; shear rate 0.1-1 mm/min (drained); R² > 0.95

Calculation example

Input data — direct shear on silty sand SM, embankment Los Angeles
σn (kPa)	τpeak (kPa)	τresidual (kPa)
50	46	32
100	76	61
200	138	121
300	198	181

Calculation for peak envelope with 4 points: Σσn = 650, Στ = 458, Σσn² = 140,000, Σ(σn·τ) = 100,600. tan φ = (4·100,600 − 650·458)/(4·140,000 − 650²) = (402,400 − 297,700)/(560,000 − 422,500) = 104,700/137,500 = 0.7614. φ = arctan 0.7614 = 37.3°. c = (458 − 0.7614·650)/4 = (458 − 495)/4 = −9.25 kPa. Since c is negative (small), it is interpreted as sand with no real cohesion and reported as c = 0, φ = 37.3° (standard practice BS 1377-7 + BS EN ISO 17892-10). Residual envelope: Σσn = 650, Στr = 395, Σσn·τr = 87,350. tan φr = (4·87,350 − 650·395)/137,500 = (349,400 − 256,750)/137,500 = 92,650/137,500 = 0.6738. φr = 33.9°, cr ≈ 0. R² peak = 0.999 (very good). Loss due to rupture: Δφ = 37.3 − 33.9 = 3.4°, typical behaviour of dense silty sand.

Result: Peak: c = 0, φ = 37.3° · Residual: cr = 0, φr = 33.9° · R² = 0.999.

Interpretation of results

The peak parameters (c = 0, φ = 37.3°) correspond to a dense silty sand in undisturbed state and are suitable for new foundation design on virgin ground. The residual parameters (φ = 33.9°) apply to slopes with a history of movement, reconsolidating compacted fills, or post-failure analysis. A peak-residual difference of 3° is moderate; in sensitive clays or loessial soils the difference can exceed 20° and long-term stability requires residual parameters mandatorily.

Reference standards

BS 1377-7 + BS EN ISO 17892-10 — Methods of test for soils for civil engineering purposes — Part 7: Shear strength tests (total stress) + Geotechnical investigation and testing — Laboratory testing of soil — Part 10: Direct shear tests
BS 1377-7 + BS EN ISO 17892-10 — Consolidated undrained direct simple shear testing
Skempton, A.W. (1964). Long-term stability of clay slopes — residual parameters
BS 1377-1 — Methods of test for soils for civil engineering purposes — Part 1: General requirements and sample preparation
ISRM — Suggested methods for direct shear testing of discontinuities

Frequently asked questions

When do I use peak and when residual?

Peak: design of new structures, foundations, slopes excavated in virgin ground. Residual: analysis of slopes with prior sliding (reactivation), dams with detected stability problems, back-analysis of failures. If in doubt, use residual (conservative).

Why direct shear vs triaxial?

Direct shear is fast and cheap but imposes the failure plane and does not measure u. Triaxial CU in saturated clay measures u and allows obtaining effective c' and φ' separated from total ones. In clean sands both give similar results; in clays triaxial is more accurate.

What shear rate do I use?

For sands 0.5-1.0 mm/min. For silts 0.1-0.3 mm/min. For clays 0.005-0.05 mm/min (fully drained). BS 1377-7 + BS EN ISO 17892-10 specifies t50 from oedometer × 20-50 to estimate minimum rate. Very high rate underestimates c in clays due to u generation without dissipation.

Square or circular box?

Square is the standard BS 1377-7 + BS EN ISO 17892-10 (60×60 mm or 100×100 mm). Circular (63.5 mm) used in old equipment; same area is assumed. For samples with coarse particles a 100×100 mm box is used to minimise size effect. Never particles > 1/10 of box side.