Reference Document - Geotechnical Site Classification Repor (As Published)t · MCU25/0022

                             Gardens for a reactive site                                         • Water that is transmitted into masonry, metal or timber building
                                                                                                   elements causes damage and/or decay to those elements.
                                                                                                 • High subfloor humidity and moisture content create an ideal
                                                                                                   environment for various pests, including termites and
                                                                                                   spiders.
                                                                                                 • Where high moisture levels are transmitted to the flooring and
                                                                                                   walls, an increase in the dust mite count can ensue within the
                                                                                                   living areas. Dust mites, as well as dampness in general, can be
                                                                                                   a health hazard to inhabitants, particularly those who are
                                                                                                   abnormally susceptible to respiratory ailments.

                                                                                              The garden
                                                                                               The ideal vegetation layout is to have lawn or plants that require only light
                                                                                               watering immediately adjacent to the drainage or paving edge, then more
                                                                                               demanding plants, shrubs and trees spread out in that order.
                                                                                               Overwatering due to misuse of automatic watering systems is a common
                                                                                               cause of saturation and water migration under footings. If it is necessary
                                                                                               to use these systems, it is important to remove garden beds to a
                                                                                               completely safe distance from buildings.
                                                                                              Existing trees
                                                                                               Where a tree is causing a problem of soil drying or there is the existence
                                                                                               or threat of upheaval of footings, if the offending roots are subsidiary
                                                                                               and their removal will not significantly damage the tree, they should be
                                                                                               severed and a concrete or metal barrier placed vertically in the soil to
                                                                                               prevent future root growth in the direction of the building. If it is not
                                                                                               possible to remove the relevant roots without damage to the tree, an
                                                                                               application to remove the tree should be made to the local authority. A
                                                                                               prudent plan is to transplant likely offenders before they become a
                                                                                               problem.

                                                                                              Information on trees, plants and shrubs
                                                                                               State departments overseeing agriculture can give information regarding
                                                                                               root patterns, volume of water needed and safe distance from buildings
                                                                                               of most species. Botanic gardens are also sources of information. For
                                                                                               information on plant roots and drains, see Building Technology File 17.

                                                                                              Excavation
                                                                                               Excavation around footings must be properly engineered. Soil
      extend outwards a minimum of 900 mm (more in highly reactive soil) and                   supporting footings can only be safely excavated at an angle that allows
      should have a minimum fall away from the building of 1:60. The finished                  the soil under the footing to remain stable. This angle is called the angle
      paving should be no less than 100 mm below brick vent bases.                             of repose (or friction) and varies significantly between soil types and
      It is prudent to relocate drainage pipes away from this paving, if possible,             conditions. Removal of soil within the angle of repose will cause
      to avoid complications from future leakage. If this is not practical,                    subsidence.
      earthenware pipes should be replaced by PVC and backfilling should be                   Remediation
      of the same soil type as the surrounding soil and compacted to the same                    Where erosion has occurred that has washed away soil adjacent to
      density.                                                                                   footings, soil of the same classification should be introduced and
      Except in areas where freezing of water is an issue, it is wise to remove                  compacted to the same density. Where footings have been undermined,
      taps in the building area and relocate them well away from the building –                  augmentation or other specialist work may be required. Remediation of
      preferably not uphill from it (see BTF 19).                                                footings and foundations is generally the realm of a specialist consultant.
      It may be desirable to install a grated drain at the outside edge of the                   Where isolated footings rise and fall because of swell/shrink effect, the
      paving on the uphill side of the building. If subsoil drainage is needed this              homeowner may be tempted to alleviate floor bounce by filling the gap
      can be installed under the surface drain.                                                  that has appeared between the bearer and the pier with blocking. The
                                                                                                 danger here is that when the next swell segment of the cycle occurs, the
     Condensation                                                                                extra blocking will push the floor up into an accentuated dome and may
      In buildings with a subfloor void such as where bearers and joists support                 also cause local shear failure in the soil. If it is necessary to use blocking,
      flooring, insufficient ventilation creates ideal conditions for                            it should be by a pair of fine wedges and monitoring should be carried
      condensation, particularly where there is little clearance between the floor               out fortnightly.
      and the ground. Condensation adds to the moisture already present in the                   This BTF was prepared by John Lewer FAIB, MIAMA, Partner,
      subfloor and significantly slows the process of drying out. Installation of                Construction Diagnosis.
      an adequate subfloor ventilation system, either natural or mechanical, is
      desirable.
      Warning: Although this Building Technology File deals with cracking in
      buildings, it should be said that subfloor moisture can result in the
      development of other problems, notably:

                          The information in this and other issues in the series was derived from various sources and was believed to be correct when published.

                              The information is advisory. It is provided in good faith and not claimed to be an exhaustive treatment of the relevant subject.

                                      Further professional advice needs to be obtained before taking any action based on the information provided.

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Version: 1, Version Date: 18/12/2025
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