A portable shear vane and a handheld penetrometer are the first tools our technician sets up on a London site. These instruments give an immediate indication of undrained shear strength in the capital's stiff London Clay, which underlies most of the inner boroughs. We then combine these field readings with laboratory triaxial tests run under UKAS-accredited conditions. The whole workflow feeds into a rigorous factor of safety (FS) calculation that follows Eurocode 7 partial factors. Before finalising the analysis, we often run a borehole SPT to correlate N-values with shear strength parameters, especially where the clay transitions into the Lambeth Group sands. The FS number we deliver is not a generic figure — it is calibrated to London's specific stratigraphy and groundwater regime.
London Clay's high plasticity means the FS can drop by over 30% between summer and winter if the pore-pressure regime is not correctly modelled.
Scope of work
Area-specific notes
In Westminster, where the London Clay is thickest and relatively uniform, the main risk is long-term swelling behind basement walls if the FS against heave is underestimated. In contrast, sites near the Thames in Bermondsey deal with alluvial silts and peat layers that can trigger differential settlement if the FS against bearing failure is too close to unity. We have seen cases where a contractor relied on a single borehole and missed a 3 m thick compressible peat band, leading to a FS value that looked safe on paper but was invalid for the actual ground. That is why we always recommend at least three test pits or boreholes per foundation footprint in London's variable alluvial zones.
Standards used
Eurocode 7 (EN 1997-1:2004) – partial factor approach for FS, BS 5930:2015 – code of practice for ground investigations, CIRIA C760 – guidance on excavations in London Clay, BS EN ISO 22476-2:2005 – field vane test standard
Linked services
Undisturbed sampling & triaxial testing
100 mm diameter rotary core samples from London Clay, tested under UU, CIU, and CID conditions to obtain cu, φ', and E' for FS inputs.
Slope stability analysis
Limit-equilibrium and finite-element modelling using Bishop, Spencer, and Morgenstern-Price methods. We output FS for both short-term undrained and long-term drained conditions.
Foundation bearing capacity check
Calculation of allowable bearing pressure against Eurocode 7 DA1b, including partial factors for vertical and eccentric loads on pad, strip, and raft foundations.
Retaining wall & excavation support
Rankine and Coulomb earth pressure analysis with FS against sliding, overturning, and base heave. We incorporate London's groundwater control measures into each model.
Typical parameters
Q&A
What is the typical factor of safety used for shallow foundations in London Clay?
For pad and strip foundations on London Clay, Eurocode 7 DA1b typically requires a FS of 1.25 against ultimate limit state under persistent conditions. For serviceability limit states, we often use FS ≥ 2.0 to limit settlements to 25 mm. The exact value depends on the variability of shear strength across the site.
How much does a factor of safety (FS) calculation cost for a residential project in London?
A standard FS calculation for a single-family dwelling, including one borehole, triaxial testing, and slope stability modelling, ranges from £460 to £1.480 depending on the number of scenarios analysed and the depth of the borehole. Larger multi-storey projects with several boreholes and 3D modelling fall at the upper end of that range.
Why is FS lower in summer than in winter for London Clay slopes?
London Clay develops high negative pore pressures during dry summers, which increase effective stress and therefore the FS. In winter, rainfall recharges the groundwater and can reduce suction to zero, dropping the undrained FS by 25–35% if the drainage is poor. We always model both seasonal extremes in our FS reports.