Foundation Engineering / Rafts and mat foundations
When a raft, and the compensated raft
Three good reasons, and one very good one.
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.
Corrections checked in this pass
- Corrected the flexible-centre reference values. Negative net pressure now identifies an unloading case requiring a separate movement model.
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.
- Embedded rigid-raft settlement and unloading/reloading are not established by the surface reference calculation; raft/pile/structure interaction needs separate analysis.
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
A raft can reduce contact pressure while loading a larger and deeper volume of ground. Assess its total and differential settlement against the alternatives before selecting it.
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
Three reasons to consider a raft are overlapping pad footprints, variable ground and excavation compensation.
Where pads occupy much of the building footprint, compare the construction and structural demands of a raft. A suitably stiff raft can redistribute loads over variable ground, but its bending and settlement still require analysis.
Excavation reduces the net stress increase below the foundation. Full compensation balances the removed soil weight against the building load; it does not eliminate movement caused by unloading, reloading, groundwater change or unequal loading.
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 a flexible-centre elastic comparison
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?