Avocet Pde OPW - App E Slope · OPW20/0092

Foundation Maintenance
and Footing Performance:
A Homeowner’s Guide

cairo

SIF 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 Dulicings can usually be related to one or more problems in the foundation sow. & 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 against Duliding movement.
This Building Technology File is Gesigned to Identify causes of soil-related buliding movement, and to suggest

methods of prevention of resultant cracking in buildings.

‘The rypes of sotls usually presepr under the topsall in land saned for
residential batidings can be split Into Two approximare groups —
granidar and clay, Quite often, foundation sof Is a milsvure of borh
types. The general problems assoclaned with sells having grarailar
contenr are usually caused by erosion Clay souks are qubject to
saturation and swedll/shrink problenes:

Classifications for a given area can generally be obralmed by
sppiicarion to the local authortry, bur these are sornetines unreliable
and if there ls dowhrt, a georechnlcal report should he commissioned.
As mos bulldings suffering movement problems are founded on chy
sol, there & 2n emphasis on classification of snlls according to the
aenount of swell and shrinkage they experience whh warlations of
wares content. The table below ts Table 2.1 from AS 2870, the
Residermiat Stab and Footing Code.

Settlement due to construction

Thore are two types of sertiement that occur as a resulr of

construction:

* Imediare settiement occurs when a bulding & first placed on tts
foundation soll, 25 a result of compaction af the soll under the
weight of the structure, The cohestve qualiry of clay sou mitigates
agatast this, bur granular (particularly sandy) sall ts suxscepribie.

© Consotidation setriement Is a fearure of clay satl and may rake
pilace becaise of the expulsion of motsture froen the soll or because
of the poll's lack of restetance to local compressive or shear stresses.
This will usually take plane during the firs few months after
construction, bat has been known to take macy years tn
exceptional cases.

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

Erosion

AI sols are prone ta erosion, bur sandy sot) ts particularly suscepethie
to belng washed away. Even clay with a sand component of sty 10%
of more can suffers from erosion,

Saturation

‘This ts particularly a problem tn clay safls. Senurarion creates a bog
Dike suspension of the soll that causes ir to ioae virmually af] of ts
hearing capacity. To a lesser degren, sand 1s affected by saruration
Decente saturated sand may undergo a reduction in volume
particularly Imported sand fil for bedding and blinding layers.
However, this usually occurs as Immediate setriement and should
formally be the prowlnce of the tullder.

‘Seasonal swelling and shrinkage of soli

All clays react to the presence of water by sowty absorbing ir, making
the soil Increase In volune (see table below). The degree of Increase
varles considerably hetwnen different clays, 25 dees the degree af
decrease during the swhsequent drying out caused by fair weather
periods. Because of the low absorprion and expulsion rate, rhis
phenomenon will not wsually he norkceable unless there are
Prolonged rainy or dry periods, usually of weeks or months,
depending on the land and sol) charactertsrics.

‘The swelling of so8 creates an upward force on the foatings of the
building, and shrinkage creares subeidence that takes away the
support needed by the fooring to retain equilibrium.
Shear fathare
Tht phenomenon occurs when the foundation soll does not have
suffictenr strengrh to support the weight of the fooring, There are
Two major past-comstruction causes

* Significart load increase.
* Reduction of lateral support of rhe soll under the fporing due to

 

 

 

 

 

 

 

 

 

fon, BuBting Tecbactagy Fae I J 10) cosie vech =e . Guan eee Racin be caused by saturation of the soll
problems. adjacene to or under the toortng.
GENERAL DEFINITIONS OF SITE CLASSES
Chass Foundation
A Most sand and rock sites with little or na ground movement fram moisture changes
s Stightly reactive clay sites with only sight ground movement from moisture changes
M Moxtoratety reactive clay or stk sites, which can experience moderame ground movement from maisture changes
H Highly reactive clay sites, which can experience high ground movement from motsture changes
E Extremely reactive sites, which can experience extreme ground movement from matsture changes
AwP Filed sites
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