Module 2 · Lesson 2.1
How deep does a foundation reach?
The stressed volume decides which layers you have to care about.
Why 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.
The working rule is that the depth of significant influence is about 1.5 to 2 times the foundation width below founding level. Beneath that, added stress is typically under about 10–15% of the existing overburden, and layers there rarely govern.
The critical word is width. The stressed depth is set by the size of the loaded area, not by how heavy it is.
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
- Ground of broadly uniform stiffness — a very soft layer draws stress into itself and extends the zone
- 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?
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint