Module 7 · Lesson 7.1
Water
It attacks twice, and people usually remember once.
Why this matters
Raising the water table to the surface can halve a footing's bearing capacity. That is a bigger effect than most of the corrections in Module 5 put together, and unlike them it is not optional — the water table moves with the seasons whether or not the design accounted for it.
By the end of this lesson you should be able to
- Identify both mechanisms by which water reduces capacity
- Decide which terms a given water table position affects
- Choose a design water level defensibly
Bearing capacity is an effective stress problem. Water pressure carries load but has no shear strength, so anything it carries is subtracted from what the soil skeleton carries. It attacks the equation in two separate places.
The surcharge term. If the water table is above founding level, the surcharge q beside the footing is an effective stress — the total weight of soil less the water pressure at that level. Less surcharge, less q·Nq.
The self-weight term. If the water table is within the failure mechanism below the footing, the soil there is buoyant, and γ′ = γ − γw. For a typical soil that is roughly 19 − 9.81 ≈ 9.2 kN/m³, so the term almost halves.
Forgetting the second is the common error, and it is the larger of the two for a wide footing.
Worked example
The same footing, dry and submerged
Given
- Strip footing, B = 3.0 m, founded at D = 1.5 m
- Sand, φ′ = 33°, c′ = 0, γ = 19 kN/m³
- Vesic Nγ
- Case A: water table 30 m down. Case B: water table at ground surface
Find
The loss of capacity, and how it splits between the two terms
Check yourself
A footing is founded at 1.5 m on sand, and the water table sits at 1.5 m — exactly at founding level. Which terms are affected?
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