Foundation Engineering / What a foundation has to do

Gross, net and effective pressure

What the ground actually feels, and why excavation is worth so much.

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

Replaced the settlement influence approximation with the flexible rectangular solution and a separately identified rigid-square factor. Corrected the net-load comparison.

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.
Account for excavation and foundation weight when establishing actual net load; surface half-space comparison excludes embedment and soil layering.

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 notes

Why this matters

The net foundation pressure measures the change in loading at founding level. Compare the completed structure’s pressure with the pressure removed by excavation, and distinguish this total-stress change from the effective stresses carried by the soil skeleton.

The following pressure definitions refer to different quantities. Keep them distinct when selecting inputs for a calculation.

Net bearing pressure

q_{net} = q_{gross} - \sigma_{v0}

What it calculates: The additional pressure the ground feels because of construction

$q_{net}$: Net bearing pressure (kPa)
$q_{gross}$: Total pressure at founding level (kPa)
$\sigma_{v0}$: Total vertical stress that existed at founding level before excavation (kPa)

In plain terms: Settlement is driven by the change in stress, not by its absolute value. Removing soil is worth exactly as much as not adding load.

Worked example

A basement that adds almost nothing

Given

Building of 6 storeys, gross pressure at founding level 180 kPa
Two basement levels, founding depth 7.0 m below existing ground
Soil unit weight 19 kN/m³ throughout
Water table well below founding level

Find

The net bearing pressure, and what it implies for settlement

Assumptions

Ground is uniform to founding level, so the removed weight is simply γ times depth
The excavation is dry, so no water pressure is removed with the soil

Practice

A warehouse applies a gross pressure of 145 kPa at a founding depth of 1.8 m. The soil above founding level has a unit weight of 20 kN/m³ and the water table is deep. What is the net bearing pressure, in kPa?

Check yourself

A basement is excavated so deep that the weight of soil removed exceeds the weight of the finished building. What should the designer be checking for?

Try it

Which requirement governs?

A square pad on firm clay, 1.2 m deep. Vary the width and compare fixed-load settlement with the separate fixed-pressure estimate.

Footing width B2.5 m

Net additional vertical load V_net900 kN

Soil modulus E25 MPa

What is fixed

Undrained clay, s_u = 75 kPa, γ = 19 kN/m³, founded at 1.2 m.
Allowable pressure is net ultimate over a global factor of 3 — permissible-stress practice, not a Eurocode check.
The load is the net addition after excavation and foundation weight are accounted for. The 25 mm movement threshold is only a teaching comparison.

Applied pressure q: 144 kPa
Net ultimate q_nu: 386 kPa
Allowable q_a: 129 kPa
Capacity utilisation: 112 %
Settlement s_i: 13.0 mm
Settlement utilisation: 52 %
Settlement if the PRESSURE were held at 150 kPa: 13.5 mm

Capacity

Settlement

Bearing capacity is the binding check. Widening relieves it fast — the pressure falls as 1/B² — so a small increase in width goes a long way here.

Summary

Gross pressure is everything at founding level; net pressure is gross less what was already there
Settlement is driven by net pressure, so excavation is worth as much as load reduction
A fully compensated foundation applies zero net pressure, and its risks are heave and uplift rather than bearing failure
Effective pressure — carried by the soil skeleton, not the water — is what governs strength and compression