Attachment C - Geotechnical investigation

Tree root growth
Trees and shrubs that are allowed te grow in the vicinity of footings
can cause foundation soil movement in two ways:

© Roots that prow under footings may increase in cross-sectional
size, exerting upward pressure on footings.

* Roots in the vicinity of footings will absorb mach of the moisture
in the foundation soil, causing shrinkage or subsidence.

Unevenness of Movement
The types of ground movernent 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 comstruction_
© Differing moisture content of foundation soil prior to
construction.
Movement due to non-constraction causes is usually more uneven
still Erosion can undermine a footing that traverses the flow or can
create the conditions for shear Gilure by eroding soil adjacent to 2
footing that rans in the same direction as the flow.
Saturation of clay foundation soil may occur where subfloor walls create
adam thar makes water pond. ft can also ocour wherever there isa
source af 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 af
the building first, then gradually spreads eo the interior. The swelling
process will usually begin at the uphill extreme of the building, or on
the weather side where the land is flat. Swelling gradually reaches the
interior soil as absorption continues. Shrinkage usually begins where
the sun's heat is greatest_

Effects of Uneven Soll 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 pap or cantilevering until the bricks or the
mortar bedding fail. Older masonry has little resistance. Evidence of
failure varies according tw circumstances and symptoms may include
© Step cracking im the mortar beds in the body of the wall ar 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).
Lsolated piers affected by erosion or saturation of foundations will
eventually lose contact with the bearers they support and may tilt or
fall ower. The floors that have los this support will become bouncy,
sometimes rattling omaments etc

Seasonal swelling/shrinkage in day

Swelling foundation soil due te 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 footprin w 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 symptoxns 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 Mooring.
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 af lowering the

 

external footings. The doming is accentuated and cracking reduces
or disappears where it ocaurred because of dishing, but other cracks
open up. The roof lines may become convex.

Daring and dishing az 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 i 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 sirub roots will rend
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 — ie. 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 sympeoms 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. Ir will atrempt mm 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 che original cause has been remedied, or until the subsidence
has completely neutralised the affected portion of footing and the
structure has stabilised on ceher 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 mvt 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 w 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 chat the cracking will become
wider until the sections of brickwork became virtually independent.
With repeated cydes, once the cracking is eszablished, if there is no
other complication, it is normal for the incidence of cracking
stabilise, as the building has the articulation it needs to cope with the
problem. This is by no means always the case, however, and monigoring
of cracks in walls and floors should always be treated seriously.
Upheaval caused by growth of tree roots under foceings is not a
simple vertical shear stress. There is a tendency for the mot to also
exert lateral forces that attempt to separate sections of brickwork
after initial cracking has occurred.