Slope Analysis 18 Hay St

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Site Address: 18 Hay Street, Tewan n I   Report No. 2487


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‘Tree root growth

‘Trees and shrubs that are allowed to grow tn the vicinity of footings

can cause foundation soll 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 motsture
tn the foundation soll, causing shrinkage or subsidence.

“Unevenness of Movement

 

The types of ground movement described above usually occur
unevenly throughout the building's foundation soll. Settlement due
to construction tends to be uneven because of:

* Differing compaction of foundation soll prior to construction.
¢ Differing motsture content of foundation soil prior to construction.

Movement due to non-construction causes ts usually more uneven

still. Erosion can undermine a footing that traverses the flow or can
create the conditions for shear failure by eroding soll adjacent to a

footing that runs in the same direction as the flow.

Saturation of clay foundation soil may occur where subfloor walls
create a dam that makes water pond. It can also occur wherever there
1s a source of water near footings in clay soil. This leads to a severe
reduction in the strength of the soll which may create local shear
failure.

Seasonal swelling and shrinkage of clay soil affects the perimeter of
the building first, then gradually spreads to the interior. The swelling
process will usually begin at the uphill extreme of the building, or on
the weather side where the land ts flat. Swelling gradually reaches the
interior soll as absorption continues. Shrinkage usually begins where
the sun's heat ts greatest.

    

Effects of Uneven Soil Movement on Structures

Erosion and saturation

Eraston removes the support from under footings, tending to create
subsidence of the part of the structure under which tt occurs.
Brickwork walls will resist the stress created by this removal of
support by bridging the gap or cantilevering until the bricks or the
mortar bedding fail. Older masonry has little resistance. Evidence of
failure varies according to circumstances and symptoms may include:

+ Step cracking in the mortar beds in the body of the wall or
above/below openings such as doors or windows.

© Vertical cracking in the bricks (usually but not necessarily in line
with the vertical beds or perpends).

Isolated piers affected by erosion or saturation of foundations will
eventually lose contact with the bearers they support and may tilt or
fall over. The floors that have lost this support will become bouncy,
sometimes rattling ornaments etc.

Seasonal swelling/shrinkage in clay

Swelling foundation soll due to rainy periods first lifts the most
exposed extremities of the footing system, then the remainder of the
perimeter footings while gradually permeating inside the building
footprint to lift internal footings. This swelling first tends to create a
dish effect, because the external footings are pushed higher than the
internal ones.

The first noticeable symptom may be that the floor appears slightly
dished. This 1s often accompanied by some doors binding on the
floor or the door head, together with some cracking of cornice
mitres. In buildings with timber flooring supported by bearers and
Joists, the floor can be bouncy. there may be visible
dishing of the hip or ridge lines.

As the motsture absorption process completes its journey to the
innermost areas of the building, the internal footings will rise. If the
spread of moisture ts roughly even, it may be that the symptoms will
temporarily disappear, but it ts more likely that swelling will be
uneven, creating a difference rather than a disappearance in
symptoms. In buildings with timber flooring supported by bearers
and Jotsts, the tsolated plers will rise more easily than the strip
footings or piers under walls, creating noticeable doming of flooring.

Site Address: 18 Hay Street, Tewantin | Report No. 2487

 

As the weather pattern changes and the soll begins to dry out, the
external footings will be first affected, beginning with the locations
where the sun's effect ts strongest. Thts has the effect of lowering the
external footings. The doming fs accentuated and cracking reduces
or disappears where it occurred because of dishing, but other cracks
open up. The roof lines may become convex.

Doming and dishing are also affected by weather in other ways. In
areas where warm, wet summers and cooler dry winters prevail,
water migration tends to be toward the intertor and doming will be
accentuated, whereas where summers are dry and winters are cold
and wet, migration tends to be toward the exterior and the
underlying propensity ts toward dishing.

Movement caused by tree roots

In general, growing roots will exert an upward pressure on footings,
whereas soil subject to drying because of tree or shrub roots will tend
to remove support from under footings by inducing shrinkage.

Complications caused by the structure itself

Most forces that the soll causes to be exerted on structures are
vertical — 1.e. etther up or down. However, because these forces are
seldom spread evenly around the footings, and because the building
resists uneven movernent because of its rigidity, forces are exerted
from one part of the building to another. The net result of all these
forces 1s usually rotational. This resultant force often complicates the
diagnosts because the visible symptoms do not simply reflect the
original cause. A common symptom ts binding of doors on the
vertical member of the frame.

Effects on full masonry structures

Brickwork will resist cracking where it can. It will attempt to span
areas that lose support because of subsided foundations or ralsed
points. It ts therefore usual to see cracking at weak points, such as
openings for windows or doors.

In the event of construction settlement, cracking will usually remain
unchanged after the process of settlement has ceased.

With local shear or erosion, cracking will usually continue to develop
until the original cause has been remedied, or until the subsidence
has completely neutralised the affected portion of footing and the
structure has stabilised on other footings that remain effective.

In the case of swell/shrink effects, the brickwork will in some cases
return to its original position after completion of a cycle, however it
1s more likely that the rotational effect will not be exactly reversed,
and it ts also usual that brickwork will settle in its new position and
will resist the forces trying to return it to its original position. This
means that in a case where swelling takes place after construction
and cracking occurs, the cracking 1s likely to at least partly remain
after the shrink segment of the cycle ts complete. Thus, each time
the cycle ts repeated, the likelthood ts that the cracking will become
wider until the sections of brickwork become virtually independent.

With repeated cycles, once the cracking 1s established, if there ts no
other complication, it 1s normal for the incidence of cracking to
stabilise, as the building has the articulation it needs to cope with
the problem. This ts by no means always the case, however, and
monitoring of cracks in walls and floors should always be treated
seriously.

Upheaval caused by growth of tree roots under footings ts not a
simple vertical shear stress. There ts a tendency for the root to also
exert lateral forces that attempt to separate sections of brickwork
after initial cracking has occurred.

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