Slope Stability AG-220616-RPT-G-01_[Rev A]

 

Foundation Maintenance
and Footing Performance:
A Homeowner's Guide

ings can and often do move. This movement can be up, down, lateral or rotational, The fundamental couse

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csiro

STF Is
replaces
Information
Sheet 10/91

of movement in bu lly be reloted to one or more problems in the foundation soil. It is important for
re ates ee nat ie rete an triodi nian

ane

foundation soil can be prevented, thus protecting ogainst building movement.

This Building Technology File is designed to identity causes of soil-related building movement, and to suggest

of prevention of resultant cracking in buildings.

Soil Types
‘The types of sails 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 2 mixture of both
Ee ee ee ear eee
caused by erosion. Clay soils are subject to
focentinn cack onic gecblonas
Classifications for a given arca can generally be obtained by

 

il

soils are peone to erosion, but samy 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 parti in clay soils. Saturation creates a bog-
like suspension iss so dea: cones cb las Vetualiy oll of io

bearing capacity. To a lesser degree, sand is affected by saturation
because saturated sand may ul a reduction in volume —

 

to the local but these are Feabl : “ee
cod there doubt» yoechical report dhol be commioned, Fact par oe Oa cud
As most buildings suffering movement problems are founded on clay ally be the of the builder.
soils, there is an i on claescaion of tals econding vse 7

of swell phage they experience with vari ‘Sitiaiiad ail iad
water content. chapirtinen aie Aneerebirr by rol All clays react to the = —— dips diccee™
Residential Slab and Footung Code. tes mele ee i ow Th tee i
varies considerably between different clays, as docs

Causes of Movement {rhe te er rng ay fo ee
. aie — petiods. Because of the low absorption and expulsion rate, this

There are two types of sextlement thar occur as a result of

construction:
* Immediate settlement occurs when a building Go Seas 0 Be
foundation soil, asa result of vasa hesd onde
weight of the structure. Ths coheloe neoity of Gey soso ten
against his, bur granular (particularly sandy) soil is
* Consolidation sectlement is a feature of clay soil and may take
place because of the expulsion of moisture from the soil or because

Phenomenon will not usually be noticeable unless there ate

rainy or dry periods, usually of weeks or months,

ing on the land and wil characteristics,

‘The swelling of soil creates an upward force on the footings of the
building, and shrinkage creates subsidence thet takes away the
support needed by the footing to retain equilibrium.
Shear failure
This occurs when the foundation sod does not have

 

ji penne papa undnepe sw sig! rol
two major post-construction causes:

* Reduction of lateral support of the sail under the fonting duc 10

of the soil’s lack of to focal comp or shear

This will ueually take during the first few months after

construction, bur has known to take many years in

exceptional cases, * Significant load increase.
Tiers pects be We Peete ead Se en ead Sees

of excavation.

 

into corsich as part off of the sie for
tion. Building nee tie ib eT 19) deals with these
problems,

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

 

 

Foundation

 

Most sand and cock sites with litte or no ground movement from moisture changes

 

Slightly reactive clay sites with only slight ground movement from moisture changes

 

Moderately reactive clay oF silt sites, which can experience moderate ground movement from moisture changes

 

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Highly reactive clay sites, which can experience high ground mavement from moisture changes
Extremely reactive sites, which can experience extreme ground movement from moisture changes

 

AwP Filled sites

 

to erosion; reactive sites subject co

 

 

r Sicca which inclade soft soi, such a sft cay of sit ot foour sands: lands mine wbpidence; collapsing soil: soils subject

whach cannot be classified otherwise