Module 11 · Lesson 11.1
When a raft, and the compensated raft
Three good reasons, and one very good one.
Why this matters
A raft looks like the safe option: spread the load over everything, and the pressure must come down. It does — and the settlement usually goes up. Understanding why is the difference between choosing a raft and defaulting to one.
By the end of this lesson you should be able to
- Recognise the three situations where a raft is genuinely indicated
- Compute net pressure and degree of compensation
- Explain why lower pressure does not mean less settlement
There are three respectable reasons to choose a raft.
The pads have merged. If individual footings would cover more than roughly half the plan area, the excavation, formwork and reinforcement of a raft costs little more and is far simpler to build. This is an arithmetic decision, and it is usually made on a sketch.
The ground is variable. A raft spans across soft spots, old fill, and buried features. It converts a differential settlement problem into a total settlement problem, which is much easier to live with — Module 10's point, in structural form.
Compensation. Excavating soil removes stress that was already there. If a basement removes as much weight as the building adds, the ground below never sees a stress increase at all. This is the strongest reason of the three, and the only one that can eliminate settlement rather than redistribute it.
Worked example
A partially compensated basement raft
Given
- Building 12 m × 30 m, gross bearing pressure at foundation level 150 kPa
- One basement storey, formation 3.0 m below existing ground
- Soil γ = 19 kN/m³, E = 25 MPa, ν = 0.3
- Water table well below formation
Find
The net pressure, the degree of compensation, and the settlement
Check yourself
What is the essential idea behind a compensated raft?
Practice
A raft applies a gross pressure of 180 kPa. The basement formation is 4.0 m below ground in soil weighing 19 kN/m³. What is the net pressure, in kPa?
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