Slope Stability - lindsays

The normal structural arrangement is that the inner leaf of brick-
work in 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 doubr. 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 cause 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. The table
below is a reproduction of Table C1 of AS 2870.

AS 2870 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/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 ata 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 is referred to in BTF 19
and may properly be regarded as an area for an expert consultant.

Protection of the building perimeter

Ic 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

 

CLASSIFICATION OF DAMAGE WITH REFERENCE TO WALLS

 

 

 

 

 

 

 

 

Description of typical damage and required repair Approximate crack width Damage
limit (see Note 3) category

Hairline cracks <0.1 mm 0
Fine cracks which do not need repair <l mm 1
Cracks noticeable but easily filled. Doors and windows stick slightly <S mm 2
Cracks can be repaired and possibly a small amount of wall will need 5-15 mm (or a number of cracks 3.
to be replaced. Doors and windows stick. Service pipes can fracture. 3 mm or more in one group)
‘Weathertightness often impaired
Extensive repair work involving breaking-out and replacing sections of walls, 15-25 mm but also depend 4
especially over doors and windows. Window and door frames distort. Walls lean on number of cracks
or bulge noticeably, some loss of bearing in beams. Service pipes disrupted