Geotechnical Investigation Report

                                   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
          extend outwards a minimum of 900 mm (more in highly reactive                       information. For information on plant roots and drains, see Building
          soil) and should have a minimum fall away from the building of                     Technology File 17.
          1:60. The finished paving should be no less than 100 mm below
          brick vent bases.                                                                  Excavation
                                                                                             Excavation around footings must be properly engineered. Soil
          It is prudent to relocate drainage pipes away from this paving, if                 supporting footings can only be safely excavated at an angle that
          possible, to avoid complications from future leakage. If this is not               allows the soil under the footing to remain stable. This angle is called
          practical, earthenware pipes should be replaced by PVC and                         the angle of repose (or friction) and varies significantly between soil
          backfilling should be of the same soil type as the surrounding soil                types and conditions. Removal of soil within the angle of repose will
          and compacted to the same density.                                                 cause subsidence.
          Except in areas where freezing of water is an issue, it is wise to
          remove taps in the building area and relocate them well away from                  Remediation
          the building – preferably not uphill from it (see BTF 19).                         Where erosion has occurred that has washed away soil adjacent to
          It may be desirable to install a grated drain at the outside edge of the           footings, soil of the same classification should be introduced and
          paving on the uphill side of the building. If subsoil drainage is                  compacted to the same density. Where footings have been
          needed this can be installed under the surface drain.                              undermined, augmentation or other specialist work may be required.
                                                                                             Remediation of footings and foundations is generally the realm of a
          Condensation                                                                       specialist consultant.
          In buildings with a subfloor void such as where bearers and joists                 Where isolated footings rise and fall because of swell/shrink effect,
          support flooring, insufficient ventilation creates ideal conditions for            the homeowner may be tempted to alleviate floor bounce by filling
          condensation, particularly where there is little clearance between the             the gap that has appeared between the bearer and the pier with
          floor and the ground. Condensation adds to the moisture already                    blocking. The danger here is that when the next swell segment of the
          present in the subfloor and significantly slows the process of drying              cycle occurs, the extra blocking will push the floor up into an
          out. Installation of an adequate subfloor ventilation system, either               accentuated dome and may also cause local shear failure in the soil. If
          natural or mechanical, is desirable.                                               it is necessary to use blocking, it should be by a pair of fine wedges
          Warning: Although this Building Technology File deals with                         and monitoring should be carried out fortnightly.
          cracking in buildings, it should be said that subfloor moisture can                This BTF was prepared by John Lewer FAIB, MIAMA, Partner,
          result in the development of other problems, notably:                              Construction Diagnosis.


                       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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081203 BTF 18 3pp.indd 4                                                                                                                                         25/10/12 12:41:26