Geotechnical Investigation Report

          Foundation Maintenance
          and Footing Performance:                                                                                                                                              BTF 18-2011


          A Homeowner’s Guide
                                                                                                                                                                                  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                                                                                    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-
                                                                                                        like suspension of the soil that causes it to lose virtually all of its
          Classifications for a given area can generally be obtained by                                 bearing capacity. To a lesser degree, sand is affected by saturation
          application to the local authority, but these are sometimes unreliable                        because saturated sand may undergo a reduction in volume,
          and if there is doubt, a geotechnical report should be commissioned.                          particularly imported sand fill for bedding and blinding layers.
          As most buildings suffering movement problems are founded on clay                             However, this usually occurs as immediate settlement and should
          soils, there is an emphasis on classification of soils according to the                       normally be the province of the builder.
          amount of swell and shrinkage they experience with variations of
          water content. The table below is Table 2.1 from AS 2870-2011, the                            Seasonal swelling and shrinkage of soil
          Residential Slab and Footing Code.                                                            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
          Causes of Movement                                                                            varies considerably between different clays, as does the degree of
          Settlement due to construction                                                                decrease during the subsequent drying out caused by fair weather
          There are two types of settlement that occur as a result of                                   periods. Because of the low absorption and expulsion rate, this
          construction:                                                                                 phenomenon will not usually be noticeable unless there are
          • Immediate settlement occurs when a building is first placed                                 prolonged rainy or dry periods, usually of weeks or months,
                                                                                                        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
            susceptible.                                                                                support needed by the footing to retain equilibrium.
          • 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 stresses.
                                                                                                        sufficient strength to support the weight of the footing. There are
            This will usually take place during the first few months after                              two major post-construction causes:
            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
                                                                                                          erosion or excavation.
          into consideration as part of the preparation of the site for
          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).




081203 BTF 18 3pp.indd 1                                                                                                                                                                    25/10/12 12:40:29