Civil Engineering Reference
In-Depth Information
However, in the absence of definitive
limits, the maximum differential
movement may be taken to be about
25 mm for buildings on sands and
40 mm for buildings on clays.
The ground movements that
occur as a result of the loads applied
by the external walls of a building
can normally be reduced to an
acceptable level if the bearing
pressure exerted by the foundations
is limited to one third of that r
equired to cause the ground to fail.
Maximum permissible bearing
pressures for a range of soil types,
including rocks, sands and gravels,
and clays and silts are given in
standards, though no figures are
given for very soft clays, peat and
organic soils or fill.
Soft clays, peat and organic soils
are, generally speaking, too
compressible to be suitable for
bearing foundation loads, and
foundations should be carried down
through such layers to a more
reliable stratum. Made ground or fill
has to be treated with special care
because of its potential for large
irregular settlement, and the
possibility of chemical
contamination. There is also a danger
that any fill with a high organic
content may degrade, generating
methane and causing settlement.
winter is unable to replenish the
moisture extracted by the roots
during the summer and a zone of drier
soil develops under the tree which
can be up to 6 m deep. This
desiccation causes the surrounding
ground to shrink, resulting in possible
subsidence damage to any nearby
foundations. The depth of desiccation
at a particular site can only be
established unequivocally by a site
investigation, and the foundation
designer must decide whether the
extra cost of such an investigation is
merited. On the other hand, if an
existing tree is removed, the moisture
will return slowly to the ground
causing it to swell. In some clay soils
this rehydration can take 20 to 30
years.
The magnitude of the movements
associated with a new tree growing or
an existing tree being removed
depends mainly on the type and size
of the tree, the shrinkage potential of
the soil, and the prevailing climate.
The recommended minimum
distances which trees should be
allowed to grow near buildings if the
effects of root systems are to be
minimised, and unless special
precautions have been taken with
foundation design in shrinkable
clays, can be summarised as follows.
For poplar, oak, willow, whitebeam
and cherry, the minimum distance
should be not less than the mature
height of the tree (Figure 1.19), which
can be up to 24 m in the case of
poplars whose roots run mainly
horizontally. For species such as
plane, lime, ash, beech and birch the
minimum distance should be half
the mature height of the tree. Apple
and pear trees should be at least
5-6 m away.
Height of mature tree
Not less than height of mature tree
Figure 1.19
For poplar, oak, willow, whitebeam and
cherry, the minimum distance should be not
less than the mature height of the tree
what movement is allowable will
depend on visual appearance and
serviceability rather than the stability
of the structure. It is to be expected
that foundations will move to some
degree in service, and this will affect,
for example, the formation of and the
size of cracks in masonry. The
English house condition survey
(2)
records that around 1 in 20 dwellings
suffer from some degree of
settlement, and around
1 in 40 differential movement.
Serviceability is very subjective
and depends on the function of the
building, the reaction of the users and
the owners; also economic factors
such as value, insurance cover and the
cost of making good any damage. In
many cases, the level of acceptable
movement will be dictated by a
particular function of the building or
one of its services (eg overhead
cranes, lifts, precision machinery,
drains etc).
Assessment of visual damage is
often very subjective, although it is
usually desirable wherever possible
to avoid cracking of walls and
cladding materials, and noticeable
deviations from the horizontal or
vertical (ie slopes of more than
1/250). However, it is impossible to
specify general limits on the
movements required to cause
cracking in a wall, because these
values depend on such factors as:
●
Frost heave
In the UK, frost heave can normally
be avoided if the foundations are
taken to a depth of at least 500 mm.
Moisture movement
Seasonal volume changes in firm
clays are likely to influence the
ground to a depth of at least 1 m,
which is the minimum
recommended foundation depth in
this type of soil. Where there are
adjacent trees, the zone of influence
can be considerably deeper.
Creep
There is a possibility that creep may
be involved in displacements caused
by eccentric loads on tall buildings,
although it is more likely that
displacements are mainly caused by
cyclic temperature variations coupled
with ratcheting.
Effect of trees
Trees can also have a long term effect
on clay soils. As trees grow, their
demand for moisture increases and
the root systems extend both
laterally and downwards affecting an
increasingly large volume of soil. In
many clay soils, rainfall during the
the nature of the building material
●
the relative size of the wall, in
particular its length to height ratio
●
the mode of deformation (ie
sagging or hogging of the ground).
