Module 7 · Lesson 7.2
Layered ground
When the mechanism cannot form the way the equation assumes.
Why this matters
The bearing capacity equation assumes one soil, uniform to the bottom of the mechanism. Real ground is layered, and the layering is frequently the whole problem — a firm crust over soft clay is exactly the profile that produces confident calculations and unhappy buildings.
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 it is outside the mechanism and stops mattering — for capacity, though not for settlement.
- qult, water table as set
- 845 kPa
- qult, 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
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