Geotechnical Investigation Report

          The normal structural arrangement is that the inner leaf of                   • Corroded guttering or downpipes can spill water to ground.
          brickwork in the external walls and at least some of the internal walls       • Downpipes not positively connected to a proper stormwater
          (depending on the roof type) comprise the load-bearing structure on             collection system will direct a concentration of water to soil that is
          which any upper floors, ceilings and the roof are supported. In these           directly adjacent to footings, sometimes causing large-scale
          cases, it is internally visible cracking that should be the main focus of       problems such as erosion, saturation and migration of water under
          attention, however there are a few examples of dwellings whose                  the building.
          external leaf of masonry plays some supporting role, so this should be
          checked if there is any doubt. In any case, externally visible cracking       Seriousness of Cracking
          is important as a guide to stresses on the structure generally, and it
          should also be remembered that the external walls must be capable of          In general, most cracking found in masonry walls is a cosmetic
          supporting themselves.                                                        nuisance only and can be kept in repair or even ignored. The table
                                                                                        below is a reproduction of Table C1 of AS 2870-2011.
          Effects on framed structures                                                  AS 2870-2011 also publishes figures relating to cracking in concrete
          Timber or steel framed buildings are less likely to exhibit cracking due      floors, however because wall cracking will usually reach the critical
          to swell/shrink than masonry buildings because of their flexibility.          point significantly earlier than cracking in slabs, this table is not
          Also, the doming/dishing effects tend to be lower because of the              reproduced here.
          lighter weight of walls. The main risks to framed buildings are
          encountered because of the isolated pier footings used under walls.           Prevention/Cure
          Where erosion or saturation causes a footing to fall away, this can
          double the span which a wall must bridge. This additional stress can          Plumbing
          create cracking in wall linings, particularly where there is a weak           Where building movement is caused by water service, roof
          point in the structure caused by a door or window opening. It is,             plumbing, sewer or stormwater failure, the remedy is to repair the
          however, unlikely that framed structures will be so stressed as to suffer     problem. It is prudent, however, to consider also rerouting pipes
          serious damage without first exhibiting some or all of the above              away from the building where possible, and relocating taps to
          symptoms for a considerable period. The same warning period should            positions where any leakage will not direct water to the building
          apply in the case of upheaval. It should be noted, however, that where        vicinity. Even where gully traps are present, there is sometimes
          framed buildings are supported by strip footings there is only one leaf       sufficient spill to create erosion or saturation, particularly in modern
          of brickwork and therefore the externally visible walls are the               installations using smaller diameter PVC fixtures. Indeed, some
          supporting structure for the building. In this case, the subfloor             gully traps are not situated directly under the taps that are installed
          masonry walls can be expected to behave as full brickwork walls.              to charge them, with the result that water from the tap may enter
                                                                                        the backfilled trench that houses the sewer piping. If the trench has
          Effects on brick veneer structures                                            been poorly backfilled, the water will either pond or flow along the
          Because the load-bearing structure of a brick veneer building is the          bottom of the trench. As these trenches usually run alongside the
          frame that makes up the interior leaf of the external walls plus              footings and can be at a similar depth, it is not hard to see how any
          perhaps the internal walls, depending on the type of roof, the                water that is thus directed into a trench can easily affect the
          building can be expected to behave as a framed structure, except that         foundation’s ability to support footings or even gain entry to the
          the external masonry will behave in a similar way to the external leaf        subfloor area.
          of a full masonry structure.
                                                                                        Ground drainage
          Water Service and Drainage                                                    In all soils there is the capacity for water to travel on the surface and
                                                                                        below it. Surface water flows can be established by inspection during
          Where a water service pipe, a sewer or stormwater drainage pipe is in         and after heavy or prolonged rain. If necessary, a grated drain system
          the vicinity of a building, a water leak can cause erosion, swelling or       connected to the stormwater collection system is usually an easy
          saturation of susceptible soil. Even a minuscule leak can be enough to        solution.
          saturate a clay foundation. A leaking tap near a building can have the
          same effect. In addition, trenches containing pipes can become                It is, however, sometimes necessary when attempting to prevent water
          watercourses even though backfilled, particularly where broken                migration that testing be carried out to establish watertable height
          rubble is used as fill. Water that runs along these trenches can be           and subsoil water flows. This subject is referred to in BTF 19 and
          responsible for serious erosion, interstrata seepage into subfloor areas      may properly be regarded as an area for an expert consultant.
          and saturation.
                                                                                        Protection of the building perimeter
          Pipe leakage and trench water flows also encourage tree and shrub             It is essential to remember that the soil that affects footings extends
          roots to the source of water, complicating and exacerbating the               well beyond the actual building line. Watering of garden plants,
          problem. Poor roof plumbing can result in large volumes of rainwater          shrubs and trees causes some of the most serious water problems.
          being concentrated in a small area of soil:
                                                                                        For this reason, particularly where problems exist or are likely to
          • Incorrect falls in roof guttering may result in overflows, as may           occur, it is recommended that an apron of paving be installed around
            gutters blocked with leaves etc.                                            as much of the building perimeter as necessary. This paving should


                                             CLASSIFICATION OF DAMAGE WITH REFERENCE TO WALLS
                                                                                                          Approximate crack width               Damage
                             Description of typical damage and required repair                               limit (see Note 3)                 category
               Hairline cracks                                                                                      <0.1 mm                         0
               Fine cracks which do not need repair                                                                  <1 mm                          1
               Cracks noticeable but easily filled. Doors and windows stick slightly.                                <5 mm                          2
               Cracks can be repaired and possibly a small amount of wall will need to be              5–15 mm (or a number of cracks               3
               replaced. Doors and windows stick. Service pipes can fracture. Weathertightness           3 mm or more in one group)
               often impaired.
               Extensive repair work involving breaking-out and replacing sections of walls,            15–25 mm but also depends on                4
               especially over doors and windows. Window and door frames distort. Walls lean                  number of cracks
               or bulge noticeably, some loss of bearing in beams. Service pipes disrupted.




081203 BTF 18 3pp.indd 3                                                                                                                                    25/10/12 12:41:09