Attachment B - Geotechnical Analysis

Foundation Maintenance
and Footing Performance:
A Homeowner’s Guide

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Information
Sheet 10/91

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. & is important for
the homeowner 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 agains building movement.
This Building Technology File is designed to identify causes of soil-related building movement, and to suggest

methods of prevention of resultant cracking in buildings.

The types of solls asually present under the topsoil In land zoned for
residential buildings can be split Into two approximate groups ~
granular and clay. Quite often, foundation soll Is 2 mixture of both
types. The general problems associated with solls having granular
content are usually caused by eroston Clay sols are subject to
saturation and swell/shrink problems

Classifications for 2 given area can generally be obtained by
application to the local authority, bur these are sometimes unreflable
and if there Is dowhrt, a geotechnical report should be commissioned.
As most bulldings suffering movement problems are founded on clay
solls, there & an emphasis on classification of soils according to the
aenqunt of swell and shrinkage they experience with variations of
water content. The table below ts Table 2.1 fram AS 2870, the
Residential Stab and Footing Code.

Settlement due to construction

‘There are two types of settlement that occur as a result of

construction.

© Immediate settiement occurs when a building & first placed on Its
foundation soll, 23 a result of compaction of the soll under the
weight of the structure, The cohesive quality of clay soll mitigates
against this, but granular (particularly sandy) soll ts susceptible.

* Consolidation setriement sa fearure of clay soll and may take
Place because of the expulsion of motsture from the soll or because
of the soll's lack of resistance to local compmressive or shear stresies.
This will usually take place during the first few months after
construction, but has been known to take mary years in
exceptional cases.

‘These problems are the province of the butider and should be taken

Eraston

All sols are prone to erosion, bur sandy sotl ts particularly suscepethle
to being washed away. Even clay with 2 sand component of say 10%
of more can suffer from erosion,

Saturation

‘This & particularly a problem In clay sols. Saturation creates a bog-
Nike suspension of the soll that causes if to lose virmuaily af] of ts
bearing capactty. To a lesser degree, sand ts 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
norenally be the prowince of the builder.

Seasonal swelling and shrinkage of soll

All days react to the presence af water by slowly absorbing tr, making
the soll Increase In volume (see table below). 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 norkceable unless there are
Prolonged rainy or dry periods, usually of weeks or months,
depending on the land and soll characteristics.

‘The swelling of sol creates an upward force on the footings of the
bullding. and shrinkage creares subsidence that takes away the
support needed by the footing to retain equilibrium.

Shear failure

‘Tht phenomenon occurs when the foundation soll does not have
sufficient strength to support the weight of the footing. There are
two major post-comstruction causes:

* Significant load increase.

* Reduction of lateral support of the soll under the footing due to

 

 

 

 

 

 

 

 

 

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problems. adjacent to or under the footing.
GENERAL DEFINITIONS OF SITE CLASSES
Class Foundation
A Mast sand and rock sites with little or no ground movement fram matsture changes
5 Slightly reactive clay sites with only slight ground movement from malisture changes
M Moderately reactive clay or sii sites, which can experience moderate ground movement from matsture changes
H Highly reactive clay sites, which can experience high ground movement from molsture changes
E Extremely reactive sites, which can experience extreme ground movement from matsture changes
AwP Filted sites
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