Foundation Engineering / module 11
Rafts and mat foundations
One big foundation instead of many small ones — and why that is not automatically better.
What you will understand
- Say when a raft is the right answer and when it is an expensive reflex
- Compute the net pressure under a compensated raft and its degree of compensation
- Explain why a raft settles far more than a pad at the same bearing pressure
- Describe how raft stiffness controls differential settlement, and what a piled raft adds
Lessons
When a raft, and the compensated raft
Three good reasons, and one very good one.
Stiffness, and the piled raft
What a raft is actually good at.
Module summary
- A raft is chosen when pads would cover most of the plan area, when the ground is variable, or when compensation is needed
- Excavating 3 m removed 57 kPa, cutting a 150 kPa gross pressure to 93 kPa net
- Full compensation for that building needed 7.9 m of excavation
- The raft still settled 67.8 mm against the pad's 15.3 mm, because it stresses far deeper ground
- A raft's value is mostly in evening out differential settlement, not in reducing the total
Teaching basis and review status
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 notesCheck your understanding
Practice
A building applies a gross pressure of 150 kPa. In soil weighing 19 kN/m³, how deep must the excavation be for full compensation, in metres?
Check yourself
In the stated flexible-centre half-space comparison, a 12 m × 30 m loaded area at 93 kPa gives 67.8 mm, while a 2.5 m square at 150 kPa gives 15.3 mm. Why is the larger area associated with more movement here?