Attachment C - Geotechnical investigation

Foundation Maintenance
and Footing Performance:
A Homeowner’s Guide

cairo

BIF 18
replaces
Information
Sheet 10/91

Bulidings can and often do move. This movement can be up, down, lateral or rotational. The fundamental cause
of movement in bullcings 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 buliding movement.
This Building Technology File is Gesigned to Identify causes of soll-related buliding movement, and to suqgest

methods of prevention of resultant cracking in bulldings-

‘The rypes of setts usually presepr under the topsoil in land zoned for
residential bulidings can be split Into reo approximate groupa —
granidar and clay, Quite often, foundation soit ts a mlsnure of borh
types. The general protilems associated with solls having gransilar
contenr are usually caused by erosion Clay sotls are qubject ta
saturation and swedll/shrink probleme.

Classifications for a given area can generally be obrained by
application te the local authortry, bur these are sometimes unreliable
and If there Is dowhr, a geotechnical report should be commissioned.
As mar buildings suffering movement problems are founded on chy
sols, there & 2n emphasis om classification of snills according te the
arnount of swell and shrinkage they experience whh warlations of
water content. The table below ts Table 2.1 from AS 2870, the
Residerriat Sab and Footing Code.

Sertlement due to coustruction

There are two types of sertiemenr that occur as a nepulr of

construction.

* lenmediore settiement occur when a bulding & first placed on its
foundation soll, 23 a result of compaction of the soll uncer the
weight of the sructure, The cohesive qualiry of clay sou miriganes
agatast this, bur granular (particulary sandy) sall ts suxscepribhe.

* Consotidation semiement Isa fearure of clay soll and may rake
Pilace becaime of the expulsion of motmare fron the soll or because
of the soil's lack of resterance to bocal compressive or shear stresses.
This will usually take place during the firs few months after
construction, but har been known to take macy years in
exceptional cases.

These problems are the province of the bullder and should be taken

Eraston

All solls are prone to erosion, but sindy soll Gs particularly suscepethle
tm belng washed away. Even clay with a sand component of say 109
of more can suffer from erosion,

Samuration

‘This ts particularly a problem tn clay sotls. Seruration creates a bog-
Dike suspension of the soll thar causes ir to inae virmually afl of ts
heartng capacity. To a lesser degree, sand 1s affected by saruration
because saturated sand may undergo a reduction in volume
particularly Imported sand fill for bedding and blinding Layers.
However, this usually occurs as immediate seriement and should
formally be the prowince of the tutlder.

‘Seasonal swelling and shrinkage of soli

All clays react to the presence of water by sowty absorbing ir, making
the soll Increase In volurne (see rable below). ‘The degree of increase
varies considerably hetwnen different cays, 25 dines the degree af
decrease during the swhsequent drying out caused by fair weather
periods. Because of the low absorpeion and expulsion rate. this
Phenomenon will not asually he norkeable unless there are
prolonged rainy or dry periods, usually of weeks or months,
depending on the land and sol) charactertsrics.

‘The swelling of s08 creates an upward force on the footings of the
building, and shrinkage creates subgidence that taker away the
support needed by the footing ro retain equilibrium.

Shear fathare

Tht phenomenon occurs when the foundation soll dors not have
sufficient strength to suppor the weight of the fooring. There are
Two Major pest-corsvruction causes

+ Significart load increase.

* Reduction of lateral support of rhe soll under the fpoting due to

 

 

 

 

 

 

 

 

 

don. Buacheg Tacrctngy Fae d 181 dese oa ee . Senses bar vee Rebate inset a saturation of the soll
problems adjacene to or under the footing.
GENERAL DEFINITIONS OF SITE CLASSES
Class Foundation
A Most sand and rock sites with little or no ground movement tram matsture changes
s Rightly reactive clay sites with only slight ground movement from moisture changes
M Moderately reactive clay or stk sites, which can experience moderae ground movement from maisture changes
H Highly reactive clay sites, which can experience high ground movement from mokture changes
E Extremely reactive sites, which can experionce extreme ground movement from moisture changes
AwP Pied sires
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