Appendix E - Geotechnical site classification report · MCU25/0022

Foundation Maintenance
and Footing Performance:                                                                                                                                               BTF 18-2011
                                                                                                                                                                         replaces

A Homeowner’s Guide                                                                                                                                                    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                                                                                     Erosion
The types of soils usually present under the topsoil in land zoned for                         All soils are prone to erosion, but sandy soil is particularly susceptible
residential buildings can be split into two approximate groups –                               to being washed away. Even clay with a sand component of say 10%
granular and clay. Quite often, foundation soil is a mixture of both                           or more can suffer from erosion.
types. The general problems associated with soils having granular
content are usually caused by erosion. Clay soils are subject to                               Saturation
saturation and swell/shrink problems.                                                          This is particularly a problem in clay soils. Saturation creates a bog-
Classifications for a given area can generally be obtained by                                  like suspension of the soil that causes it to lose virtually all of its
application to the local authority, but these are sometimes unreliable                         bearing capacity. To a lesser degree, sand is affected by saturation
and if there is doubt, a geotechnical report should be commissioned.                           because saturated sand may undergo a reduction in volume,
As most buildings suffering movement problems are founded on clay                              particularly imported sand fill for bedding and blinding layers.
soils, there is an emphasis on classification of soils according to the                        However, this usually occurs as immediate settlement and should
amount of swell and shrinkage they experience with variations of                               normally be the province of the builder.
water content. The table below is Table 2.1 from AS 2870-2011, the
Residential Slab and Footing Code.                                                             Seasonal swelling and shrinkage of soil
                                                                                               All clays react to the presence of water by slowly absorbing it,
Causes of Movement                                                                             making the soil increase in volume (see table below). The degree of
                                                                                               increase varies considerably between different clays, as does the
Settlement due to construction                                                                 degree of decrease during the subsequent drying out caused by fair
There are two types of settlement that occur as a result of                                    weather periods. Because of the low absorption and expulsion rate,
construction:                                                                                  this phenomenon will not usually be noticeable unless there are
                                                                                               prolonged rainy or dry periods, usually of weeks or months,
• Immediate settlement occurs when a building is first placed                                  depending on the land and soil characteristics.
   on its foundation soil, as a result of compaction of the soil under
   the weight of the structure. The cohesive quality of clay soil                              The swelling of soil creates an upward force on the footings of the
   mitigates against this, but granular (particularly sandy) soil is                           building, and shrinkage creates subsidence that takes away the
                                                                                               support needed by the footing to retain equilibrium.
   susceptible.
• Consolidation settlement is a feature of clay soil and may take                              Shear failure
   place because of the expulsion of moisture from the soil or because
                                                                                               This phenomenon occurs when the foundation soil does not have
   of the soil’s lack of resistance to local compressive or shear                              sufficient strength to support the weight of the footing. There are
   stresses. This will usually take place during the first few months                          two major post-construction causes:
   after construction, but has been known to take many years in
   exceptional cases.                                                                          • Significant load increase.
                                                                                               • Reduction of lateral support of the soil under the footing due to
These problems are the province of the builder and should be taken
into consideration as part of the preparation of the site for                                    erosion or excavation.
construction. Building Technology File 19 (BTF 19) deals with these                            In clay soil, shear failure can be caused by saturation of the soil
problems.                                                                                      adjacent to or under the footing.


                                                                  GENERAL DEFINITIONS OF SITE CLASSES
       Class                                                                               Foundation
          A         Most sand and rock sites with little or no ground movement from moisture changes
          S         Slightly reactive clay sites, which may experience only slight ground movement from moisture changes
         M          Moderately reactive clay or silt sites, which may experience moderate ground movement from moisture changes
         H1         Highly reactive clay sites, which may experience high ground movement from moisture changes
         H2         Highly reactive clay sites, which may experience very high ground movement from moisture changes
          E         Extremely reactive sites, which may experience extreme ground movement from moisture changes
Notes
1. Where controlled fill has been used, the site may be classified A to E according to the type of fill used.
2. Filled sites. Class P is used for sites which include soft fills, such as clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soil subject to erosion;
   reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise.
3. Where deep-seated moisture changes exist on sites at depths of 3 m or greater, further classification is needed for Classes M to E (M-D, H1-D, H2-D and E-D).


              8894-193 Sankeys Rd-FEDERAL                                          Page 12 of 15                                                            19/07/2023