Foundation design
Every load in a building ends up in the ground, and the foundation is what turns that concentrated load into a pressure the soil can carry without settling. We design strip, trench-fill, raft and piled foundations to suit the ground, for clients across the UK.
How deep do my foundations need to be?
It depends on the ground. On firm gravel a strip may only need to reach below the frost line at around 450 to 600mm, but on shrinkable clay near trees a trench-fill foundation can need to go well over two metres. We set the depth from the soil type, the tree influence and the loads rather than a standard figure.
How a foundation is sized
A foundation does one simple thing: it spreads the weight of the building over enough ground that the soil is not overstressed. The starting point is the bearing capacity of the soil, the pressure it can safely carry before it fails in shear or settles too far. Dense gravel might take 300 kilonewtons per square metre, firm clay perhaps 100, and soft silt far less. We take the load gathered at the base of each wall or column, add the self weight of the foundation, and set the width so the pressure under it stays below the allowable value with a margin.
The second check is settlement. A soil can be strong enough not to fail and still compress under sustained load, and it is differential settlement, one part of the building sinking more than another, that cracks walls and jams doors. We assess the compressibility of the strata, estimate how much the ground will consolidate, and keep the difference across the structure within limits that the superstructure can tolerate. On layered or made ground that often decides the foundation type before bearing capacity does.
Ground conditions, clay and trees
Cohesive clay soils shrink as they dry and swell as they wet, and that seasonal movement reaches deeper where trees are drawing moisture out of the ground. The NHBC guidance ties foundation depth to the plasticity of the clay and the species, height and proximity of nearby trees, and on a high shrinkage clay near a mature oak a trench-fill foundation can need to go two and a half metres or more. We assess the volume change potential, apply the correct zone of influence, and specify depth and any compressible or slip membrane protection to the sides where heave is a risk.
Not every problem is clay. Made ground, old fill, high water tables, running sand, mine workings and existing drains all change what the foundation has to be and how it is built. Where a desk study and the ground investigation show variable or poor strata, we look past a deeper strip to rafts and piles, and we design so the foundation spans or bypasses the weak material rather than sitting on it and hoping.
Choosing the right foundation type
There is no single correct foundation, only the one that suits the loads, the ground and the buildability on that site. Lightly loaded housing on good ground is usually a strip or trench-fill. Poor or variable ground under a low structure often points to a raft, which floats the whole footprint on one stiffened slab. Heavy point loads, deep soft strata or restricted access frequently call for piles that carry load down to a firm layer or mobilise friction along their length. We weigh these against cost, programme and the plant that can actually get to the plot.
We prepare the design to Eurocode 7 with the UK National Annex and Approved Document A, sizing concrete and reinforcement to Eurocode 2 where the foundation is reinforced. The output is a set of calculations and detailed drawings your groundworker can set out and Building Control will accept, whether the project is a single extension in the UK or a new build elsewhere in Europe, since the calculations and reports are delivered wherever you are.
What we check.
The points our calculations resolve for a project like this.
- Allowable bearing pressure from the soil type and ground investigation
- Total and differential settlement kept within tolerable limits
- Foundation depth set for clay shrinkage, heave and nearby trees
- Made ground, water table, drains and existing services on the plot
- Selection between strip, raft and piled solutions on cost and buildability
- Eurocode 7 geotechnical checks and Eurocode 2 reinforcement where needed
Issued drawings, redacted.
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
How deep do my foundations need to be?
It depends on the ground. On firm gravel a strip may only need to reach below the frost line at around 450 to 600mm, but on shrinkable clay near trees a trench-fill foundation can need to go well over two metres. We set the depth from the soil type, the tree influence and the loads rather than a standard figure.
Do I need a ground investigation?
For most extensions a trial hole to confirm the soil and bearing stratum is enough, and the groundworker often digs one on the day. For larger or heavily loaded structures, or poor and variable ground, a proper site investigation with boreholes and lab testing lets us design efficiently rather than assuming the worst case.
What is the difference between strip and trench-fill foundations?
A strip foundation is a shallow band of concrete under the wall with masonry built up off it inside the trench. Trench-fill pours concrete almost to the surface, which is faster and safer to build in deep or unstable ground even though it uses more concrete. We advise which is cheaper overall for your ground.
Can trees affect my foundations?
Yes, especially on clay. Trees draw moisture from the soil, which shrinks the clay and can pull it away from the foundation, while a felled tree lets the clay swell and heave. We apply the recognised zone of influence for the species and distance and design the depth and any heave protection accordingly.
Related in structural design & elements.
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