Development Application Part 2 of 2 Planning report Geotechnical report and plans MCU11/0168 & OPW11/0257

    Foundation Maintenance
                                                                                                                                                 CSIRO

    and Footing Performance:                                                                                                                      BTF 18
                                                                                                                                                 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
                                                                                   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 band (i,mpnrnent of sav 10%
    residential buildings can be split into two approximate groups                 or inure can suffer front
    granular and clay. Quite often, foundation soil is a mixture of both
    types. The general problems associated with soils having granular              Saturation
    content are usually caused by erosion. Clay soils aresubject to                This is particularly a problem in clay soils. Saturation creates a bog-
    saturation and swell/shrink problems.                                          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
    Classifications for a given area can genet-ally be obtained by
                                                                                   because saturated sand may undergo a reduction in volume -
    application to the local authority, but these are sometimes unreliable
                                                                                   particularly imported sand fill for bedding and blinding layers.
    and if there is doubt, a geotechnical report should be commissioned.
                                                                                   However. this usually occurs as Immediate settlerent and should
    As most buildings suffering movement problems are founded on clay
                                                                                   normally be the province of the builder.
    soils, there is an emphasis on classification of soils according to the
    amount of swell and shrinkage they experience with variations of               Seasonal swelling and shrinkage of soil
    water content. The table below is "lable 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
                                                                                   varies considerably between different clays, as does the degree of
     Causes of Movement                                                            decrease during the subsequent drying out caused by fair weather
                                                                                   periods. Because of the low absorption and expulsion rate, this
    Settlement due to construction
                                                                                   phenomenon will not usually be noticeable unless there are
    There are two types of settlement that occur as a result of
                                                                                   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 force 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 or shear stresses.    This phenomenon occurs when the foundation soil 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                    •   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



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


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


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


          E           Extremely reactive sites, which can experience extreme ground movement from moisture changes


       A to P         Filled sites


          P           Sites which include soft soils, such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soils subject
                      to erosion; reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise




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