MCU14/0024 - Response to Information Request - Attachment 4

Foundation Maintenance
and Footing Performance- =

reptaces

A Homeowner's Guide Saane

Buildings can and often do move. This movement can be up, down, lateral or rotational. The fundamental cause
ngs can usually be related to one or more probiems in the foundation soil. It 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 building movement.

This Building Technology File is designed to identify causes of soitrelated building movement, and to suggest
methods of prevention of resultant cracking in buildings.

 

 

 

‘So Types Erosion

All soils are prone to erosion, but sandy soil is particularly susceptible
The types of soils usually present under the topsoil in land zoned for to being washed away. Even clay with a sand component of say 10%

 

residential buildings can be split into two approximate groups ~ or more can suffer from erosion.

granular and clay. Quite often, foundation soil 1s a mixture of both

types. The genera problems associated with soils having granular Saturation

content are usually caused by erosion. Clay sails are subject to This is particularly a problem in clay soils. Saturation creates a bog-

like suspension oe soil that causes it to lose virtually ail of its
: bearing capacity. To a lesser de; , San

Classifications for a given area can generally be obtained by bee es a ae A a d aa eee ora Py. eee
application to the local authority, but these are sometimes unreliable particularly imported san ‘d fill for bedding and blinding layers.
and if there is doubt, a geotechnical report should be commissioned. However, this usually occurs as immediate settlement a ae Id
As most buildings suffering movement problems are founded on clay nor aly) be the ones of the builder. i
soils, there is an emphasis on classification of soils according te the :

saturation and swell/shrink problems.

 

amount of swell and shrinkage they experience with variations of Seasonal swelling and shrinkage of soil
water content, The table below is Table 2.1 from AS 2870, the All clays react to the presence of water by slowly absorbing it, making
Residential Slab and Footing Code. the soil increase in volume {see table below). The degree of increase
J varies considerably between different clays, as does the degree of
Causes of Movement decrease during the subsequent drying out caused by fair weather,
Sateen ae ae eae periods. Because of the low absorption and expulsion rate, this
Th Raat RInR phenomenon will not usually be noticeable untess there are
ere are two types of settlement that occur as a result 0! prolonged rainy or dry periods, usually of weeks or months,
construction: depending on the land and soil characteristics.
© Immediate settlement occurs when a building is first placed on its
foundation soil, as a result of compaction of the soil under the The swelling of soil creates an upward focce on the footings of the
weight of the structure. The cohesive quality of clay soil mitigates building, and shrinkage creates subsidence that takes away the
against this, but granular (particularly sandy) soil is susceptible. support needed by the footing to retain equilibrium.

» Consolidation settlement is a feature of clay soil and may take

place because of the expulsion of moisture from the soil or because Shear failure

of the soil’s lack of resistance to local compressive oF shear stresses. This phenomenon occurs when the foundation soll does not have
This will usually take place during the first few months after sufficient strength to support the weight of the footing. ‘There are
construction, but has been known to take many years in two major post-construction causes:

exceptional cases. * Significant load increase.

These problems are the province of the builder and should be taken + Reduction of lateral support of the soil under the footing due to

into consideration as part of the preparation of the site for construc- erosion or excavation.
tion. Building Technology File 19 (BTF 19) deals with these + Inclay soil, shear failure can be caused by saturation of the soil
problems. adjacent to or under the footing.

 

 

Tae "GENERAL DEFINITIONS OF SITE CLASSES

 

Bee ee ea eet G2 Foundation -

 

 

 

A Most sand and rock sites ii little Or no ground movement from moisture changes 5
Soe x Slightly ceactive clay sites with only slight ground movement from moisture changes
M:: ‘Moderately reactive clay of silt sites, Which can experience moderate ground movement from moistuce changes *

He ; Highly reactive clay sites, which can experience high ground imayement fromm mdisttire changes

“| Bxtremely reactive sites, which can experience extreme ground irioverent fram moisture charges

 

 

AtoP. |. © Filled sites

x Sites which include soft soils, such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils} soils subject
td erosion, reactive Sites subject to abnormal moisture conditions or ‘sites which canriot be classified otherwise "