Geotechnical Investigation Report

          Tree root growth
                                                                                                   Trees can cause shrinkage and damage
          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                            Wall cracking
          unevenly throughout the building’s foundation soil. Settlement due                    due to uneven
                                                                                                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 cracks
          create the conditions for shear failure by eroding soil adjacent to a         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, water
          a dam that makes water pond. It can also occur wherever there is a            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         underlying propensity is toward dishing.
          the building first, then gradually spreads to the interior. The swelling
          process will usually begin at the uphill extreme of the building, or on       Movement caused by tree roots
          the weather side where the land is flat. Swelling gradually reaches the       In general, growing roots will exert an upward pressure on footings,
          interior soil as absorption continues. Shrinkage usually begins where         whereas soil subject to drying because of tree or shrub roots will tend
          the sun’s heat is greatest.                                                   to remove support from under footings by inducing shrinkage.

          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 the
          mortar bedding fail. Older masonry has little resistance. Evidence of         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 above/          vertical member of the frame.
            below openings such as doors or windows.
          • Vertical cracking in the bricks (usually but not necessarily in line        Effects on full masonry structures
            with the vertical beds or perpends).                                        Brickwork will resist cracking where it can. It will attempt to span
                                                                                        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.
          Seasonal swelling/shrinkage in clay                                           With local shear or erosion, cracking will usually continue to develop
          Swelling foundation soil due to rainy periods first lifts the most exposed    until the original cause has been remedied, or until the subsidence
          extremities of the footing system, then the remainder of the perimeter        has completely neutralised the affected portion of footing and the
          footings while gradually permeating inside the building footprint to lift     structure has stabilised on other footings that remain effective.
          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
          The first noticeable symptom may be that the floor appears slightly           is more likely that the rotational effect will not be exactly reversed,
          dished. This is often accompanied by some doors binding on the                and it is also usual that brickwork will settle in its new position and
          floor or the door head, together with some cracking of cornice                will resist the forces trying to return it to its original position. This
          mitres. In buildings with timber flooring supported by bearers and            means that in a case where swelling takes place after construction
          joists, the floor can be bouncy. Externally there may be visible              and cracking occurs, the cracking is likely to at least partly remain
          dishing of the hip or ridge lines.                                            after the shrink segment of the cycle is complete. Thus, each time the
          As the moisture absorption process completes its journey to the               cycle is repeated, the likelihood is that the cracking will become
          innermost areas of the building, the internal footings will rise. If the      wider until the sections of brickwork become virtually independent.
          spread of moisture is roughly even, it may be that the symptoms will          With repeated cycles, once the cracking is established, if there is no
          temporarily disappear, but it is more likely that swelling will be            other complication, it is normal for the incidence of cracking to
          uneven, creating a difference rather than a disappearance in                  stabilise, as the building has the articulation it needs to cope with the
          symptoms. In buildings with timber flooring supported by bearers              problem. This is by no means always the case, however, and monitoring
          and joists, the isolated piers will rise more easily than the strip           of cracks in walls and floors should always be treated seriously.
          footings or 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.




081203 BTF 18 3pp.indd 2                                                                                                                                     25/10/12 12:40:49