Slope Stability Report (rec. 16.04.25) · MCU25/0042

BUILDING ‘Techotocy|nesounces

FOUNDATION MAINTENANCE AND
FOOTING PERFORMANCE

Understanding and preventing soil-related building movement

This Building Technology Resource is designed to identify causes of soil-related
building movement, and to suggest methods of prevention of resultant cracking.

Buildings can and often do move. This movement can be up,
down, lateral or rotational. The fundamental cause of movement
in buildings can usually be related to one or more problems in the
foundation soil. It is important for the home owner to identify the
soil type in order to ascertain the measures that should be put in
place in order to ensure that problems in the foundation soil can
be prevented, thus protecting against building movement.

SOIL TYPES

The types of soils usually present under the topsoil in land zoned
for residential buildings can be split into two approximate groups —
granular and clay. Quite often, foundation soil is a mixture of both
types. The general problems associated with soils having granular
content are usually caused by erosion. Clay soils are subject to
saturation and swell/shrink problems.

Classifications for a given area can generally be obtained by
application to the local authority, but these are sometimes unreliable
andif there is doubt, a geotechnical report should be commissioned.
As most buildings suffering movement problems are founded on
clay soils, there is an emphasis on classification of soils according to
the amount of swell and shrinkage they experience with variations
of water content. Table 1 below is a reproduction of Table 2.1 from
Australian Standard AS 2870-2011, Residential slabs and footings.

CAUSES OF MOVEMENT

SETTLEMENT DUE TO CONSTRUCTION
There are two types of settlement that occur as a result of construction:

> Immediate settlement occurs when a building is first placed on
its foundation soil, as a result of compaction of the soil under the
weight of the structure. The cohesive quality of clay soil mitigates
against this, but granular (particularly sandy) soil is susceptible.

> Consolidation settlement is a feature of clay soil and may take
place because of the expulsion of moisture from the soil or
because of the soil’s lack of resistance to local compressive or
shear stresses. This will usually take place during the first few
months after construction but has been known to take many
years in exceptional cases.
These problems may be the province of the builder and should be
taken into consideration as part of the preparation of the site for
construction.

EROSION

Allsoils are prone to erosion, but sandy soil is particularly susceptible
to being washed away. Even clay with a sand component of say
10% or more can suffer from erosion.

SATURATION

This is particularly a problem in clay soils. Saturation creates a bog-
like suspension of the soil that causes it to lose virtually all of its
bearing capacity. To a lesser degree, sand is affected by saturation
because saturated sand may undergo a reduction in volume,

particularly imported sand fill for bedding and blinding layers.
However, this usually occurs as immediate settlement and should
normally be the province of the builder.

SEASONAL SWELLING AND SHRINKAGE OF SOIL

All clays react to the presence of water by slowly absorbing it,
making the soil increase in volume (see table below, from AS 2870).
The degree of increase varies considerably between different clays,
as does the degree of decrease during the subsequent drying out
caused by fair weather periods. Because of the low absorption and
expulsion rate, this phenomenon will not usually be noticeable
unless there are prolonged rainy or dry periods, usually of weeks
or months, depending on the land and soil characteristics.

The swelling of soil creates an upward force on the footings of the
building, and shrinkage creates subsidence that takes away the
support needed by the footing to retain equilibrium.

SHEAR FAILURE
This phenomenon occurs when the foundation soil does not have
sufficient strength to support the weight of the footing. There are
two major post-construction causes:

» Significant load increase.

» Reduction of lateral support of the soil under the footing due to
erosion or excavation.

In clay soil, shear failure can be caused by saturation of the soil
adjacent to or under the footing.

TREE ROOT GROWTH

Trees and shrubs that are allowed to grow in the vicinity of footings
can cause foundation soil movement in two ways:

» Roots that grow under footings may increase in cross-sectional
size, exerting upward pressure on footings.

TABLE 1. GENERAL DEFINITIONS OF SITE CLASSES.

Class Foundation

A Most sand and rock sites with little or no ground movement from
moisture changes

5 Slightly reactive clay sites, which may experience only slight
ground movement from moisture changes

M Moderately reactive clay or silt sites, which may experience
moderate ground movement from moisture changes

1 Highly reactive clay sites, which may experience high ground
Movement from moisture changes

42 Highly reactive clay sites, which may experience very high ground
Movement from moisture changes

E Extremely reactive sites, which may experience extreme ground

Movement from moisture changes

Source: Reproduced with the permission of Standards Australia Limited © 2011. Copyright
in AS 2870-2011 Residential slabs and footings vests in Standards Australia Limited.

© CSIRO 2021