Civil Engineering Reference
In-Depth Information
capital cost. An incorrectly installed foundation system can be
costly to remedy, so the project client, local authorities and
other project team stakeholders should be clear on how the
building is likely to behave once constructed.
The term 'behave' is used because
all
buildings move, yet
it can be difficult to fully quantify the level of settlement, its
timescale, the risks and life‑cycle costs that this could provoke.
Moreover, it is also difficult to fully quantify the term 'failure'
as the term is highly subjective; minor structural cracking and
load redistribution of a structural frame might be perfectly ac‑
ceptable to the structural engineer, but if the facade rotates and
cracks the architect and client are less likely to share the same
opinion.
For this reason it is imperative that the structural engineer,
in collaboration with a geotechnical engineer, fully considers
how a foundation is likely to react to load and its environment
over time. Once this is understood, this information can be
communicated to the remainder of the design and client team
so that the building can be cost‑effectively designed and con‑
structed to accommodate this movement within an agreed set
of risk and tolerance boundaries.
16.5.1.2 Slow soil creep in expansive soil
Swelling rates can vary across a complete foundation due to
seasonal or long‑term changes in water levels - often pro‑
longed periods of wet or dry weather - or due to localised
vegetation removing moisture. It is an unavoidable issue when
constructing on clay soils, so understanding how changes in
moisture content could affect the soil is of vital importance as
foundation distortion can result in structural movements that
must either be mitigated or accommodated.
16.5.1.3 Inadequate site drainage underneath or around the
structure, or behind its retaining walls
Uncontrolled water flow from leaking drains can have two
principal effects. In cohesive or expansive soil such as clay
it can change the moisture equilibrium within the soil, which
could cause heave. In a granular soil, the water can wash out
fine particles leading to gradual and sometimes highly local‑
ised consolidation of the remaining soil.
16.5.1.4 Groundwater movement and flooding
Groundwater levels are often seasonal, those near to the coast
are tidal, lunar and seasonal, and in some areas they could
trend upwards or downwards depending on local groundwater
extraction or charging trends. Additionally, flooding can be a
risk in many low‑lying areas. These issues must be fully quan‑
tified by the structural engineer so that the design can be pre‑
pared appropriately.
16.5.1 The causes of foundation movement
Since all foundations settle, an engineering assessment of the
site geotechnical condition and its potential risks should be
undertaken. There are four main risk categories:
inherent geological risks that can be exacerbated by construction
■
activity;
water‑induced risks that can affect soil behaviour and/or building
16.5.1.5 Improper or inadequate site preparation, grading or
compaction
This is a particular problem where shallow foundations are sup‑
ported in the uppermost bearing strata. Insufficient or inadequate
site preparation can leave a building or structure susceptible to
uncontrolled settlement as the supporting soil consolidates.
■
loading;
industrial activity or interferences within the local bearing strata;
■
the construction process and its interaction with the ground and its
■
surrounding buildings.
These risk categories can be broken down into the following
broad reasons for foundation‑related movement.
16.5.1.6 Inadequate pile depth
Older buildings supported by wood or early steel pile foun‑
dations may encounter problems when these do not extend
below the zone of expansive soil that is affected by the climate
or changes in water level. Their shallowness may not provide
sufficient restraint to foundation settlement or heave.
16.5.1.1 Bearing capacity
In common with other construction materials, soils deform
under loading. Soil bearing capacity is the measure of its cap‑
acity to support loads, with ultimate bearing capacity used as
the theoretical maximum pressure that can be supported without
failure; either general shear failure, local shear failure or punch‑
ing shear failure. Weaker soils tend to settle more under loading,
often without shear failure. In such cases, a working stress al‑
lowable bearing capacity is determined by the geotechnical and
structural engineer to ascertain a maximum allowable settle‑
ment. This value is often governed by external constraints, such
as tolerable structural frame movements or facade tolerances.
Soils also deform at differing rates; in general terms granular
soils experience most of their deflection during construc‑
tion, whilst cohesive soils can be prone to slow acting creep
effects.
16.5.1.7 Degradation of foundations
Old timber foundations, and in some cases RC or steel piles,
may be prone to corrosion. All types of foundation are very
resilient if kept below the groundwater level as oxygen to feed
the degradation process is generally lacking despite ample
quantities of water. Confirmation of wetting and drying cycles
is therefore required to predict potential long‑term degradation
and unforeseen building movements.
Degradation can also be initiated by high levels of sul‑
phates, chlorides and other deleterious ions in groundwater,
which can attack reinforced concrete and masonry structures
(see
Figure 16.3
). Analysis of site groundwater contamination
