Foundation Engineering / module 7
Groundwater and layered ground
Two things the textbook equation assumes away, and both of them matter.
What you will understand
- Explain the two separate ways a high water table reduces bearing capacity
- Locate the water table correctly relative to the failure mechanism
- Recognise when a strong crust over soft ground invalidates the bearing capacity equation
- Describe punching-through failure and how it is checked
Lessons
Water
It attacks twice, and people usually remember once.
Layered ground
When the mechanism cannot form the way the equation assumes.
Module summary
- Water reduces the surcharge term and the self-weight term, and both must be taken
- Below roughly one footing width beneath the base, the water table stops mattering
- A footing on a strong crust over soft clay can punch through rather than fail in general shear
- Investigation depth must cover the mechanism, or the layer that governs will be missed
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 footing has a dry ultimate capacity of 1746 kPa. With the water table at the surface it falls to 845 kPa. What is the percentage reduction, to the nearest whole number?
Check yourself
Below roughly what depth does the water table stop affecting bearing capacity?