Z40770_Response to InformationRequest_NC_final

Foundation Maintenance
and Footing Performance:
A Homeowner's Guide

2

BIF 18
replaces
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. 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 soil-related building movement, and to suggest

methods of prevention of resultant cracking in buildings.

' Soil Types

The types of soils 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 a mixture of both
types. The general problems associated with soils having granular
content are usually caused by erosion. Clay soils are subject to
saturation and swell/shrink problems.

Classifications for a given area can generally be obtained by
application to the local authority, but these are sometimes unreliable
and if there is doubt, a geotechnical report should be commissioned.
As most buildings suffering movement problems are founded on clay
soils, there is an emphasis on classification of soils according to the
amount of swell and shrinkage they experience with variations of
water content. The table below is Table 2.1 from AS 2870, the
Residential Slab and Footing Code.

  

auses of Movement

Settlement due to construction

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

construction:

¢ Immediate settlement occurs when a building is first placed on its
foundation soil, as a result of compaction of the soil under the
weight of the structure. The cohesive quality of clay soil mitigates
against this, but granular (particularly sandy) soil is susceptible.

* Consolidation settlement is a feature of clay soil and may take
place because of the expulsion of moisture from the soil or because
of the soil’s lack of resistance to local compressive or shear stresses.
This will usually take place during the first few months after
construction, but has been known to take many years in
exceptional cases.

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

into consideration as part of the preparation of the site for construc-

tion. Building Technology File 19 (BTF 19) deals with these
problems.

Erosion

All soils are prone to erosion, but sandy 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 particularly a problem in clay soils. Saturation creates a bog-
like suspension of the soil that causes it to lose virtually all of its

_ bearing capacity. To a lesser degree, sand is 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
normally be the province of the builder.

Seasonal swelling and shrinkage of soil

All clays react to the presence of water by slowly absorbing it, making
the soil 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 noticeable unless there are
prolonged rainy or dry periods, usually of weeks or months,
depending on the land and soil characteristics.

The swelling of soil creates an upward force on the footings of the
building, and shrinkage creates subsidence that takes away the
support needed by the footing to retain equilibrium.

Shear failure

This phenomenon occurs when the foundation soil does not have

sufficient strength to support the weight of the footing. There are

two major post-construction causes:

° Significant load increase.

¢ Reduction of lateral support of the soil under the footing due to
erosion or excavation.

¢ In clay soil, shear failure can be caused by saturation of the soil
adjacent to or under the footing.

 

 

   

 

GENERAL DEFINITIONS OF SITE CLASSES

 

 

 

 
 

 

 

 

Slightly reactive clay sites with only slight ground movement from moistut ¢

  

 

hanes

 

 

 

~ Moderately reactive clay or silt sites, which can experience moderate ground movernent from moisture changes

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

 

"Filled sites Be ie

 

 

 

E | ' |) Extremely reactive sites, which can experience extreme ground movement from moisture changes...

   

Pee Sites which include soft soils, such as soft clay ‘or silt or loose sands: landslip; mine subsidence, collapsing soils} soils subject
fo erosion’ reactive sites subject to abnormal moisture conditions or sites which cannot: be classified otherwise \o 8