Foundation Engineering / Groundwater and layered ground
Water
It attacks twice, and people usually remember once.
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.
Corrections checked in this pass
- Identified the groundwater cut-off as a model assumption and the drawing as schematic.
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.
- Layered ground, transient groundwater and actual failure geometry need site-specific assessment.
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
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
The drained sand calculation here uses effective stress. An undrained clay check instead uses a consistent total-stress formulation. 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?