Foundation Engineering / Choosing a foundation
How deep does a foundation reach?
The stressed volume decides which layers you have to care about.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Checks needed for project use
- A geotechnical reviewer must confirm drainage and stress basis, bearing-factor formulation, correlations and settlement influence factors. Partial factors and project-specific ground conditions are outside these teaching checks.
- Investigation-depth and failure-mode rules are indicative. Specialist must check geometry, layering, load, soft layers outside initial influence estimate and any asserted strength-parameter reductions.
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesWhy this matters
Two engineers look at the same borehole log. One says the soft layer at 6 m is irrelevant; the other says it governs the design. Both can be right, because they are designing different foundations — and the thing that separates them is how deep each foundation reaches into the ground.
By the end of this lesson you should be able to
- Estimate the depth of significant influence beneath a foundation
- Use it to decide which layers in a borehole log matter
- Explain why the same soil profile suits one foundation and not another
A foundation does not load the ground uniformly forever downwards. The stress it adds spreads sideways with depth and dies away, and below a certain point the addition is small compared with the stress the soil already carries from its own weight.
For a preliminary review of a roughly square footing, 1.5 to 2 times its width is sometimes used as an initial influence-depth estimate. It is not a cut-off for settlement or investigation. The relevant depth depends on load, plan shape, stress distribution and the thickness and compressibility of deeper layers; strip footings can influence greater depths.
What it calculates: How far below founding level the added stress still matters
- Depth of significant influence below founding level (m)
- B
- Foundation width — the shorter plan dimension (m)
This assumes
- The influence estimate is only preliminary; layered-ground stiffness contrasts require a suitable stress and settlement model
- A vertically loaded, roughly square or strip foundation
In plain terms: A rule of thumb about stress decay, not a boundary. A soft layer just below it still deserves a settlement check; it is a guide to where to look, not permission to stop looking.
Worked example
The same soft layer, two different foundations
Given
- Borehole: stiff clay from 0 to 6 m, soft clay 6 to 9 m, dense gravel below 9 m
- Option A: 2.0 m square pad founded at 1.0 m depth
- Option B: 12 m square raft founded at 1.0 m depth
Find
Whether the soft layer at 6 m matters to each foundation
Practice
A 3.5 m wide strip foundation is founded 1.5 m below ground level. Taking the depth of significant influence as 1.5B, to what depth below EXISTING GROUND LEVEL should the ground investigation reach, in metres?
Check yourself
A pad foundation is performing well on a site. A raft of the same bearing pressure is proposed nearby, on the same ground profile. What should be expected?