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

     Tree root growth
      Trees and shrubs that are allowed to grow in the vicinity of footings
      can cause foundation soil movement in two ways:
      • Roots that grow under footings may increase in cross-
        sectional size, exerting upward pressure on footings.
      • Roots in the vicinity of footings will absorb much of the
        moisture in the foundation soil, causing shrinkage or
        subsidence.

     Unevenness of Movement
      The types of ground movement described above usually occur
      unevenly throughout the building’s foundation soil. Settlement due                            looting settlement
      to construction tends to be uneven because of:
      • Differing compaction of foundation soil prior to construction.
      • Differing moisture content of foundation soil prior
          to construction.
      Movement due to non-construction causes is usually more uneven                       external footings. The doming is accentuated and cracking reduces
      still. Erosion can undermine a footing that traverses the flow or can                or disappears where it occurred because of dishing, but other
      create the conditions for shear failure by eroding soil adjacent to a                cracks open up. The roof lines may become convex.
      footing that runs in the same direction as the flow.                                 Doming and dishing are also affected by weather in other ways. In
      Saturation of clay foundation soil may occur where subfloor walls create             areas where warm, wet summers and cooler dry winters prevail,
      a dam that makes water pond. It can also occur wherever there is a                   water migration tends to be toward the interior and doming will be
      source of water near footings in clay soil. This leads to a severe                   accentuated, whereas where summers are dry and winters are cold
      reduction in the strength of the soil which may create local shear failure.          and wet, migration tends to be toward the exterior and the
      Seasonal swelling and shrinkage of clay soil affects the perimeter of the            underlying propensity is toward dishing.
      building first, then gradually spreads to the interior. The swelling                Movement caused by tree roots
      process will usually begin at the uphill extreme of the building, or on              In general, growing roots will exert an upward pressure on footings,
      the weather side where the land is flat. Swelling gradually reaches the              whereas soil subject to drying because of tree or shrub roots will tend
      interior soil as absorption continues. Shrinkage usually begins where                to remove support from under footings by inducing shrinkage.
      the sun’s heat is greatest.
     Effects of Uneven Soil Movement on Structures                                        Complications caused by the structure itself
                                                                                           Most forces that the soil causes to be exerted on structures are
     Erosion and saturation                                                                vertical – i.e. either up or down. However, because these forces are
      Erosion removes the support from under footings, tending to create                   seldom spread evenly around the footings, and because the building
      subsidence of the part of the structure under which it occurs.                       resists uneven movement because of its rigidity, forces are exerted
      Brickwork walls will resist the stress created by this removal of                    from one part of the building to another. The net result of all these
      support by bridging the gap or cantilevering until the bricks or the                 forces is usually rotational. This resultant force often complicates
      mortar bedding fail. Older masonry has little resistance. Evidence of                the diagnosis because the visible symptoms do not simply reflect the
      failure varies according to circumstances and symptoms may include:                  original cause. A common symptom is binding of doors on the
      • Step cracking in the mortar beds in the body of the wall or                        vertical member of the frame.
         above/ below openings such as doors or windows.                                  Effects on full masonry structures
      • Vertical cracking in the bricks (usually but not necessarily in                    Brickwork will resist cracking where it can. It will attempt to span
         line with the vertical beds or perpends).                                         areas that lose support because of subsided foundations or raised
      Isolated piers affected by erosion or saturation of foundations will                 points. It is therefore usual to see cracking at weak points, such as
      eventually lose contact with the bearers they support and may tilt or                openings for windows or doors.
      fall over. The floors that have lost this support will become bouncy,                In the event of construction settlement, cracking will usually remain
      sometimes rattling ornaments etc.                                                    unchanged after the process of settlement has ceased.
                                                                                           With local shear or erosion, cracking will usually continue to develop
     Seasonal swelling/shrinkage in clay                                                   until the original cause has been remedied, or until the subsidence
      Swelling foundation soil due to rainy periods first lifts the most exposed           has completely neutralised the affected portion of footing and the
      extremities of the footing system, then the remainder of the perimeter               structure has stabilised on other footings that remain effective.
      footings while gradually permeating inside the building footprint to lift
      internal footings. This swelling first tends to create a dish effect,                In the case of swell/shrink effects, the brickwork will in some cases
      because the external footings are pushed higher than the internal ones.              return to its original position after completion of a cycle, however it
                                                                                           is more likely that the rotational effect will not be exactly reversed,
      The first noticeable symptom may be that the floor appears slightly                  and it is also usual that brickwork will settle in its new position and
      dished. This is often accompanied by some doors binding on the                       will resist the forces trying to return it to its original position. This
      floor or the door head, together with some cracking of cornice                       means that in a case where swelling takes place after construction
      mitres. In buildings with timber flooring supported by bearers and                   and cracking occurs, the cracking is likely to at least partly remain
      joists, the floor can be bouncy. Externally there may be visible                     after the shrink segment of the cycle is complete. Thus, each time the
      dishing of the hip or ridge lines.                                                   cycle is repeated, the likelihood is that the cracking will become
      As the moisture absorption process completes its journey to the                      wider until the sections of brickwork become virtually independent.
      innermost areas of the building, the internal footings will rise. If the             With repeated cycles, once the cracking is established, if there is no
      spread of moisture is roughly even, it may be that the symptoms will                 other complication, it is normal for the incidence of cracking to
      temporarily disappear, but it is more likely that swelling will be                   stabilise, as the building has the articulation it needs to cope with the
      uneven, creating a difference rather than a disappearance in                         problem. This is by no means always the case, however, and
      symptoms. In buildings with timber flooring supported by bearers and                 monitoring of cracks in walls and floors should always be treated
      joists, the isolated piers will rise more easily than the strip footings or          seriously.
      piers under walls, creating noticeable doming of flooring.
                                                                                           Upheaval caused by growth of tree roots under footings is not a
      As the weather pattern changes and the soil begins to dry out, the                   simple vertical shear stress. There is a tendency for the root to also
      external footings will be first affected, beginning with the locations               exert lateral forces that attempt to separate sections of brickwork
      where the sun’s effect is strongest. This has the effect of lowering the             after initial cracking has occurred.




                  8894-193 Sankeys Rd-FEDERAL                                  Page 13 of 15                                               19/07/2023
Document Set ID: 24390806
                  23591411
Version: 1, Version Date: 18/12/2025
                          28/02/2025