Civil Engineering Reference
In-Depth Information
the knowledge of the building owner until signs of settlement
or heave are observed, usually in the form of cracking in the
building fabric.
The leaking of drains adjacent to buildings tends to occur at
joint positions rather than as a result of the failure of the pipes
themselves and leaks are commonly found at pipe junctions
such as gullies or soil pipes. Tree roots can also break through
house drainage, thereby causing the escape of water and sub-
sequent loss of fines and soil strength.
ground-bearing slabs and supported walls as well as a lateral
force to buried masonry walls and concrete trench foundations.
It should be noted that where trees have been removed prior
to construction, or even heavily pruned, the effects of soil heave
are still possible as the desiccated clay rehydrates as the tree
roots die away. The effects of this rehydration are well known
and there are numerous design guides from sources such as the
National House Building Council (NHBC) and the Building
Research Establishment (BRE) where extensive investigations
have been undertaken and effective details produced to combat
the effects of these soil actions.
Foundations on unconsolidated made ground (fill) can also
lead to ground movement over time and this can be exacer-
bated by vibrations from traffic or nearby construction works
such as piling. This 'vibration induced' consolidation occurs in
granular soils; therefore it is unlikely to cause damage in clay
soils unless the clay has a high sand content.
9.3.1 Mining
Building in mining areas is of particular concern and exten-
sive desktop studies are required when undertaking projects in
these areas. Records are available from various organisations
including the Coal Authority, Brine Boards and the British
Geological Society which may indicate the age, depth and
extent of mine workings in the area, the shaft positions and
any remediation measures undertaken in the past. However, it
is essential that adequate site investigation including rock drill-
ing is undertaken as part of any proposed development.
The type of mine workings may be dependent upon the min-
eral being extracted and these vary around the country. Of par-
ticular concern to buildings is the use of the pillar and stall
method in shallow workings as used from the mid-fifteenth
century to the nineteenth century, where the term 'shallow'
refers to coal seam working within ten metres of the ground
surface. Here, the mineral (usually coal) was extracted leav-
ing pillars of coal to support the roof of the working which
over time can deteriorate and cause a collapse of the roof and
ground above.
9.3.3 Site investigation
As discussed previously, it is important when commencing a pro-
ject that a desktop study is undertaken prior to setting the parame-
ters of any geotechnical site investigation work. This will establish
the history of the area and give adequate clues as to any potential
problems such as historic ponds, clay pits, mining or industrial
works on the proposed site. The need for adequate investigatory
works is necessary for all projects and is part of the engineer's
duty of care towards the client and also extends to the public. The
client should be aware of all the material facts that could increase
future costs during design, construction and beyond.
On smaller projects this element of advance investigative
work may be costly and, regardless of the aforementioned
duty of care, the client is often unhappy to pay for the engin-
eer's time and that of a site investigation team. However, it is
imperative that the designer should gain as much relevant site
information as the budget allows in order that both the designer
and client are protected against future costs or claims. Many
local authorities hold information such as survey maps, aerial
photographs and previous site investigations around the area
and many of these sources can be viewed at little or low cost.
The basis of the investigations should be to the relevant Code
of Practice for Site Investigations (BS 5930:1999) or meet the
requirements of the relevant foundation design code.
This has now been superseded by the National Annexe
to Eurocode 7: Geotechnical Design, Ground Investigation
& Testing (BS EN1997-2:2007, published in 2009. See
Chapter 13:
Soil-structure interaction
for a more detailed dis-
cussion of site investigation and geotechnical design.
9.3.2 Tree action
Buildings with trees located in close proximity are at risk of
damage due to the action of tree roots or more commonly,
water absorption. The shrinkage and swelling of clay soils is
the most common cause of damage to older buildings within
the UK costing the insurance industry £350m per year (Driscol
and Skinner, 2007).
Trees extract varying amounts of water through their roots
during the main growth in spring and summer. In clays, this
results in shrinkage in both the horizontal and vertical direc-
tions, which if adjacent to a building, will cause damage to the
foundations and building fabric. The extent of water absorp-
tion is wholly dependent upon the tree type/size and the clay
soil properties as clays with a liquid limit greater than 50% are
liable to have a high shrinkage capability.
Research indicates that evaporation may reach 0.5 m in
depth with grass and short vegetation extending this drying
range to 1.5 m; however, some of the larger trees can extract
moisture to depths of 5 m or more (IStructE, 2000).
Following times of high or long-term water absorption and
evaporation during summer, a wet winter can cause a swelling
in the clay which can cause as much structural damage as the
shrinking clay. This clay heave can apply vertical pressure to
9.3.4 Poor interpretation of results
In cases where a site investigation and study has been under-
taken, the results are to be reviewed by a competent engineer
with sufficient relevant experience dealing with soils and
geotechnical engineering. This interpretation should provide
at least a summary of the ground conditions on the site, the
