Module 1 · Lesson 1.2

Gross, net and effective pressure

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

Why this matters

The ground beneath a foundation has been carrying the weight of the soil above it for thousands of years without complaint. What it has not carried is your building. Almost every mistake in foundation pressure comes from confusing the two.

Three pressures, and it is worth being pedantic about which is which.

Gross pressure is the total pressure at founding level: the structure, the foundation itself, and any soil resting on it. It is what a pressure cell under the foundation would read.

Net pressure is the gross pressure less the pressure that was already there before construction — the weight of the soil you excavated. It is the change, and it is what causes settlement.

Effective pressure is the pressure carried by the soil skeleton rather than the porewater. It is what governs strength and compression, and it is the reason the water table matters so much.

Net bearing pressure

q_{n e t} = q_{g r oss} - σ_{v 0}

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

$q_{n e t}$: Net bearing pressure (kPa)
$q_{g r oss}$: Total pressure at founding level (kPa)
$σ_{v 0}$: 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. Widen it and watch the two checks move in opposite directions.

Footing width B2.5 m

Column load V900 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 settlement limit is taken as 25 mm purely so the two checks can be compared on one scale.

Applied pressure q: 144 kPa
Net ultimate q_nu: 386 kPa
Allowable q_a: 129 kPa
Capacity utilisation: 112 %
Settlement s_i: 13.6 mm
Settlement utilisation: 55 %
Settlement if the PRESSURE were held at 150 kPa: 14.2 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

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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