Foundation Engineering / Groundwater and layered ground
Layered ground
When the mechanism cannot form the way the equation assumes.
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.
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.
- Verify layered-ground punching/general-shear selection and investigation depth. A one-footing-width groundwater or crust cutoff is an approximation, not a universal boundary.
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
The homogeneous-ground equations need care when the ground is layered. A strong crust may overlie a weaker layer within the stressed zone, so the investigation and resistance model must cover both.
Two cases matter, and they behave quite differently.
Strong over weak. A footing on a firm crust over soft clay. The general shear mechanism cannot develop, because the failure surface would have to pass through the soft layer. Instead the footing punches through the crust, pushing a block of it down into the soft material below. Capacity is governed by the shear resistance around the perimeter of that block plus the bearing capacity of the soft layer — not by the crust's strength.
Weak over strong. A soft layer over dense material. If the soft layer is thicker than the mechanism, the strong layer is irrelevant. If it is thinner, the mechanism is confined and capacity is somewhere between the two, higher than the soft layer alone would give.
The punching check is a straightforward equilibrium statement. The block of crust being pushed down is resisted by:
- shear on its vertical sides, over the crust thickness, and
- the bearing capacity of the soft layer beneath it.
If the crust is thick relative to the footing — more than about one width — the mechanism stays inside it and ordinary bearing capacity applies. If it is thin, punching governs, and the answer can be a small fraction of what the crust alone would suggest.
Check yourself
A 2 m wide footing is founded on 1 m of firm gravel over soft clay. What governs its capacity?
Try it
Where the water table bites
A 3 m strip footing at 1.5 m in sand. Move the water table from the surface down past the failure mechanism.
The two mechanisms
- Above founding level, water reduces the SURCHARGE — the soil beside the footing is buoyant.
- Within about one width below the base, it reduces the SELF-WEIGHT term.
- Below D + B, this simplified water-table correction no longer changes the result. This is a model cut-off, not a general limit on groundwater influence.
- Effective q′_ult, water table as set
- 845 kPa
- Effective q′_ult, deep water table
- 1746 kPa
- Capacity lost
- 52 %
- Depth at which it stops mattering D + B
- 4.5 m
Water above founding level: BOTH terms are reduced, and the footing has lost 52% of its capacity.
Summary
- Water reduces the surcharge term if above founding level, and the self-weight term if within the mechanism
- A water table deeper than about D + B can be ignored for capacity, but not for settlement
- Strong crust over soft ground fails by punching, not general shear
- The reassuring profile is the dangerous one, because shallow investigation confirms the crust and misses what governs