Slope Stability Report (rec. 16.04.25) · MCU25/0042

 

 

 

 

Wall cracking
due to uneven
footing settlement

 

FIGURE 1 Trees can cause shrinkage and damage.

» 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 to construction tends to be uneven because of:

> Differing compaction of foundation soil prior to construction.

> Differing moisture content of foundation soil priorto construction.
Movement due to non-construction causes is usually more uneven
still. Erosion can undermine a footing that traverses the flow or can
create the conditions for shear failure by eroding soil 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 is a source of water near footings in clay soil. This leads to a
severe reduction in the strength of the soil 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 through
absorption. The swelling process will usually begin at the uphill
extreme of the building, or on the weather side where the land is
flat. Shrinkage usually begins on the side of the building where the
sun’s heat is greatest.

EFFECTS OF UNEVEN SOIL MOVEMENT ON STRUCTURES

EROSION AND SATURATION

Erosion removes the support from under footings, tending to
create subsidence of the part of the structure under which it 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 soil 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 is 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. Externally there may be visible
dishing of the hip or ridge lines.

As the moisture absorption process completes its journey to the
innermost areas of the building, the internal footings will rise. If the
spread of moisture is roughly even, it may be that the symptoms
will temporarily disappear, but it is 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 joists, the isolated piers will rise more easily than the strip
footings or piers under walls, creating noticeable doming of flooring.

As the weather pattern changes and the soil begins to dry out, the
external footings will be first affected, beginning with the locations
where the sun's effect is strongest. This has the effect of lowering
the external footings. The doming is 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 interior 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 is 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 soil causes to be exerted on structures are
vertical — i.e. either up or down. However, because these forces
are seldom spread evenly around the footings, and because the
building resists uneven movement because of its rigidity, forces
are exerted from one part of the building to another. The net result
of all these forces is usually rotational. This resultant force often
complicates the diagnosis because the visible symptoms do not
simply reflect the original cause. A common symptom is 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 raised
points. It is 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
is more likely that the rotational effect will not be exactly reversed,
and it is 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 is likely to at least partly remain
after the shrink segment of the cycle is complete. Thus, each time
the cycle is repeated, the likelihood is that the cracking will become
wider until the sections of brickwork become virtually independent.

With repeated cycles, once the cracking is established, if there is
no other complication, it is normal for the incidence of cracking to
stabilise, as the building has the articulation it needs to cope with the
problem. This is 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 is not a
simple vertical shear stress. There is a tendency for the root to also

© CSIRO 2021