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

exert lateral forces that attempt to separate sections of brickwork
after initial cracking has occurred.

The normal structural arrangement is that the inner leaf of
brickworkin the external walls and at least some of the internal walls
(depending on the roof type) comprise the load-bearing structure
on which any upper floors, ceilings and the roof are supported. In
these cases, it is internally visible cracking that should be the main
focus of attention, however there are a few examples of dwellings
whose external leaf of masonry plays some supporting role, so
this should be checked if there is any doubt. In any case, externally
visible 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 supporting themselves.

EFFECTS ON FRAMED STRUCTURES

Timber or steel framed buildings are less likely to exhibit cracking
due to swell/shrink than masonry buildings because of their
flexibility. Also, the doming/dishing effects tend to be lower
because of the lighter weight of walls. The main risks to framed
buildings are encountered because of the isolated pier footings
used under walls. Where erosion or saturation causes a footing to
fall away, this can double the span which a wall must bridge. This
additional stress can create cracking in wall linings, particularly
where there is a weak point in the structure caused by a door or
window opening. It is, however, unlikely that framed structures will
be so stressed as to suffer serious damage without first exhibiting
some or all of the above symptoms for a considerable period.
The same warning period should apply in the case of upheaval.
It should be noted, however, that where framed buildings are
supported by strip footings there is only one leaf of brickwork and
therefore the externally visible walls are the supporting structure
for the building. In this case, the subfloor masonry walls can be
expected to behave as full brickwork walls.

EFFECTS ON BRICK VENEER STRUCTURES

Because the load-bearing structure of a brick veneer building
is the frame that makes up the interior leaf of the external walls
plus perhaps the internal walls, depending on the type of roof,
the building can be expected to behave as a framed structure,
except that the external masonry will behave in a similar way to
the external leaf of a full masonry structure.

WATER SERVICE AND DRAINAGE

Where a water service pipe, a sewer or stormwater drainage pipe is
in the vicinity of a building, a water leak can cause erosion, swelling
or saturation of susceptible soil. Even a minuscule leak can be
enough to saturate a clay foundation. A leaking tap near a building
can have the same effect. In addition, trenches containing pipes
can become watercourses even though backfilled, particularly
where broken rubble is used as fill. Water that runs along these
trenches can be responsible for serious erosion, interstrata
seepage into subfloor areas and saturation.

Pipe leakage and trench water flows also encourage tree and
shrub roots to the source of water, complicating and exacerbating
the problem. Poor roof plumbing can result in large volumes of
rainwater being concentrated in a small area of soil:

> Incorrect falls in roof guttering may result in overflows, as may
gutters blocked with leaves etc.

» Corroded guttering or downpipes can spill water to ground.

» Downpipes not positively connected to a proper stormwater
collection system will direct a concentration of water to soil
that is directly adjacent to footings, sometimes causing large-
scale problems such as erosion, saturation and migration of
water under the building.

SERIOUSNESS OF CRACKING

In general, most cracking found in masonry walls is a cosmetic
nuisance only and can be kept in repair or even ignored. Table 2
below is a reproduction of Table C1 of AS 2870-2011.

AS 2870-2011 also publishes figures relating to cracking in
concrete floors, however because wall cracking will usually reach
the critical point significantly earlier than cracking in slabs, this
table is not reproduced here.

PREVENTION AND CURE
PLUMBING

Where building movement is caused by water service, roof
plumbing, sewer or stormwater failure, the remedy is to repair the
problem. It is prudent, however, to consider also rerouting pipes
away from the building where possible and relocating taps to
positions where any leakage will not direct water to the building
vicinity. Even where gully traps are present, there is sometimes
sufficient spill to create erosion or saturation, particularly in
modern installations using smaller diameter PVC fixtures. Indeed,
some gully traps are not situated directly under the taps that are
installed 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 been poorly backfilled, the water will either pond
or flow along the bottom of the trench. As these trenches usually
run alongside the footings and can be at a similar depth, it is not
hard to see how any water that is thus directed into a trench can
easily affect the foundation's ability to support footings or even
gain entry to the subfloor area.

GROUND 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 and after heavy or prolonged rain. If necessary, a grated
drain system connected to the stormwater collection system is
usually an easy solution.

It is, however, sometimes necessary when attempting to prevent
water migration that testing be carried out to establish watertable
height and subsoil water flows. This subject may be regarded as an
area for an expert consultant.

PROTECTION OF THE BUILDING PERIMETER

It is essential to remember that the soil that affects footings extends
well beyond the actual building line. Watering of garden plants,
shrubs and trees causes some of the most serious water problems.

For this reason, particularly where problems exist or are likely to
occur, it is recommended that an apron of paving be installed
around as much of the building perimeter as necessary. This
paving should extend outwards a minimum of 900 mm (more in
highly reactive soil) and should have a minimum fall away from
the building of 1:60. The finished paving should be no less than
100 mm below brick vent bases.

It is prudent to relocate drainage pipes away from this paving,
if possible, to avoid complications from future leakage. If this is
not practical, earthenware pipes should be replaced by PVC and
backfilling should be of the same soil type as the surrounding soil
and compacted to the same density.

Except in areas where freezing of water is an issue, it is wise to
remove taps in the building area and relocate them well away
from the building — preferably not uphill.

It may be desirable to install a grated drain at the outside edge of
the paving on the uphill side of the building. If subsoil drainage is
needed this can be installed under the surface drain.

CONDENSATION

In buildings with a subfloor void, such as where bearers and joists
support flooring, insufficient ventilation creates ideal conditions
for condensation, particularly where there is little clearance
between the floor and the ground. Condensation adds to the
moisture already present in the subfloor and significantly slows
the process of drying out. Installation of an adequate subfloor
ventilation system, either natural or mechanical, is desirable.

© CSIRO 2021