Eurocode 7 and BS 5930 both stress the importance of measuring in-situ deformation parameters, especially in overconsolidated clays. That is why the Flat Dilatometer Test (DMT) has become our go-to method for projects across London. The city sits on the London Clay Formation, a stiff, fissured stratum that can behave unpredictably under load. We have run DMT soundings on sites near the Thames, where the clay is softer, and on the higher ground of Hampstead, where it is heavily overconsolidated. The test gives us the horizontal stress index, dilatometer modulus, and material index in one pass. In our experience, combining this data with a presurometro reading provides a complete picture of the soil stiffness profile.
A single DMT sounding gives us horizontal stress, modulus, and material index: three parameters from one 20 cm push interval.
Scope of work
Area-specific notes
London has not seen a damaging earthquake since the 1580 Dover Straits event, but seismic risk is not zero. The London Clay can amplify low-frequency waves, and the city has thousands of Victorian terraces built on shallow foundations. If a magnitude 5 event struck near the city, the soft alluvium in the Thames floodplain would liquefy in places. The DMT helps us map the stiffness contrast between the clay crust and the deeper softer layers. That data feeds directly into site class assessments per Eurocode 8. For a recent development in Greenwich, we used the test to justify a reduction in seismic design forces by proving the clay was stiffer than assumed.
Standards used
BS EN ISO 22476-11 (Standard Test Method for Performing the Flat Plate Dilatometer), Eurocode 7 (EN 1997-1:2004) – Geotechnical Design, BS 5930:2015 – Code of Practice for Ground Investigations, ISSMGE TC16 – Suggested Method for DMT
Linked services
DMT Sounding for Foundation Design
Full-depth DMT profiles for pad, strip, and raft foundations in London Clay. We provide K0, OCR, and constrained modulus at 20 cm intervals.
DMT for Retaining Wall Design
Lateral stress profiles for basement walls and secant pile walls. The test measures the at-rest pressure that governs wall deflection.
DMT for Embankment and Fill Control
Monitor stiffness gain in compacted fills and assess the overconsolidation ratio of existing embankments along rail corridors.
Seismic DMT (SDMT)
Combined DMT with vs30/" data-interlink="1">shear wave velocity measurement for site class assessment per Eurocode 8. Ideal for schools, hospitals, and high-rise towers.
Typical parameters
Q&A
What is the difference between DMT and CPT in London Clay?
The DMT measures the lateral stress directly by inflating a membrane, while the CPT measures cone tip resistance and sleeve friction. In overconsolidated London Clay, the DMT gives a more reliable K0 value because it captures the locked-in horizontal stress. We often run both tests on the same site and compare the profiles.
How deep can the DMT reach in London?
We routinely push the blade to 30 metres in London Clay using a 20-tonne CPT truck. In the alluvium of the Thames floodplain, the depth is limited by gravel layers; we have reached 18 metres in Bermondsey before hitting sand that stopped the push.
How much does a DMT survey cost in London?
A typical DMT sounding for a residential site costs between £570 and £890, depending on depth, access, and number of test points. The price includes on-site testing, data reduction, and a summary report with K0 and modulus profiles.
Do you provide DMT for basement impact assessments?
Yes. We regularly carry out DMT soundings for basement design in conservation areas like Kensington and Bloomsbury. The test data supports the lateral earth pressure assumptions needed for temporary works and permanent retaining walls.