Geotechnical design report
An interpretation of ground investigation results, giving foundation recommendations and design parameters.
About this service
A geotechnical design report turns ground investigation data into decisions. It takes the borehole logs, trial pit records and laboratory results and explains what they mean for your project: which foundation types are suitable, what bearing capacity and settlement to design for, how much heave or shrinkage to allow for, what to do about groundwater and which construction risks need managing. It is the bridge between the fieldwork and the structural design.
In London, the interpretation is often where the real value lies. Two sites on London Clay can need very different solutions depending on how deep trees have dried the soil, whether made ground is present and how groundwater behaves in any overlying gravels. Basement schemes in Camden, Islington or Kensington and Chelsea need careful parameters for retaining walls and heave. Getting these right avoids both over-designed foundations and costly failures.
Our report sets out a clear ground model, recommended foundation solutions with reasons, characteristic soil parameters for design to Eurocodes, and the key risks for the contractor. Like all our design work, it goes through our three-step quality control process, with final sign-off by our Head of Engineering. Fees start from £1,250 + VAT, and the report is written to be used by the structural engineer and submitted to Building Control.
When you need it
- You have ground investigation results and need design recommendations
- A larger scheme needs geotechnical design parameters
- Planning or Building Control require a geotechnical report
What's included
- Interpretation of soil and test data
- Foundation options and recommendations
- Design parameters
- Groundwater and construction risks
How it works
- Review the dataWe check the factual report, desk study and proposed scheme, and flag any gaps, such as holes too shallow for the foundation type or missing groundwater readings, before interpretation begins.
- Build the ground modelWe define the soil layers across the site, their thicknesses and variation, groundwater levels, the root-affected zone and any made ground or soft deposits that will influence design.
- Derive parametersWe select characteristic values for strength, stiffness, density and earth pressure, with reasoning, so the structural engineer can design foundations and retaining walls with confidence.
- Recommend foundationsWe compare options such as deepened strips, trench fill, rafts, mini piles or bored piles, giving bearing values, settlement estimates and heave precautions for the preferred solution.
- Construction risksWe set out what the contractor must manage, including excavation stability, groundwater control, effects on neighbouring foundations, concrete class for sulphates and any contamination handling.
What you receive
Choosing between strips, rafts and piles on London ground
On London Clay, the foundation decision often comes down to depth. Near trees, current guidance can push strip foundations down to depths where trench fill becomes awkward and expensive, and mini piles with a suspended ground beam become the more economic answer. The design report explains that trade-off using real site data rather than assumptions. Where trees are the main driver, a tree influence and foundation depth assessment may be all a smaller project needs.
For basements, the report also gives the parameters for retaining wall design, heave on the basement slab and predicted ground movement for neighbours. Those parameters feed into a ground movement and damage assessment where the council requires one. Our guide to London Clay and trees gives background on why clay behaves the way it does.
Projects that used this service
Frequently asked questions
What is the difference between a factual and interpretive ground report?
A factual report records what was found: logs, test results and groundwater readings. An interpretive or geotechnical design report explains what those findings mean for design, including foundation recommendations, soil parameters and construction risks. Many projects need both, with the factual report produced first and the design report built on it.
Do I need a geotechnical design report for an extension?
Usually not for a typical single or double-storey extension, where a trial pit and standard foundation guidance are enough for Building Control. A design report is more valuable for new buildings, basements, piled foundations, sloping or variable sites, and where the council or a warranty provider specifically asks for geotechnical parameters.
Can you write a geotechnical report from someone else's site investigation?
Yes, provided the factual data is complete and reliable. We review it first and tell you if anything important is missing, such as holes not deep enough for the proposed foundations or no laboratory plasticity tests on clay. Sometimes a small amount of extra fieldwork is needed to fill the gaps.
What bearing capacity does London Clay have?
It varies with depth, how weathered the clay is and the foundation size, so there is no single figure. Upper layers are often softer and affected by drying and roots, while deeper clay is stiffer. A design value should come from site-specific testing and interpretation, which is exactly what a geotechnical design report provides.