Attachment B - Geotechnical Analysis

The normal structural arrangement is that the inner leaf of
brickwork 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 doubt. In any case, externally visible cracking
is important as a puide 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 ome 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 sail:

© 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 of even ignored. The table
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 /Cure

Plumbing

Where building movernent 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 of 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 ran 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

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.

Ie 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 BT F 19 and
may properly be regarded as an area for an expert consultant.

Protection of the building perimeter

Ie 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

 

 

 

 

 

 

 

 

Approximate crack width Damage

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