Site Investigation For Low-Rise Building
Site Investigation For Low-Rise Building
Site Investigation For Low-Rise Building
uwic
On
08/01/2016
June 1987
procurement
Site investigation must be carefully procured if reliable results are to be
obtained. It should provide information on local site features and, in
particular, ground conditions to ensure that safe structures are built
economically. The usefulness and reliability of site investigation data
depend very much on how well the site investigation work is planned and
carried out.
This digest discusses the value of site investigation and the steps that
should be involved, and provides guidance on contractual methods.
Site investigation can produce unreliable results for a slope instability require a very high level of geotechnical
number of reasons. The photograph below shows a skill. A much more basic problem, not unknown in ground
severely damaged house on deep fill. Damage was investigation, is shown on page 2. A general lack of
caused by landslide movement which occurred during supervision of fieldwork and laboratory testing by
construction. During ground investigation, the extent of experienced geotechnical engineers has meant that in
the fill had been correctly established but geotechnical some cases work has been carried out by unacceptable
analysis based upon the resulting parameters wrongly methods; in other cases, as the cartoon suggests, work
indicated that the site was stable. Problems such as has been deliberately falsified to achieve faster progress.
preparation of report.
Site investigation must be undertaken for every site. It is the first step in the
construction process and is absolutely essential if development work is to be
carried out safely, economically and to schedule. Without a properly
procured ground investigation, the hidden dangers which lie beneath the site
cannot be known. Once preliminary desk studies have been used to identify
the probable risks presented by a site, these risks can be avoided in a
number of ways: by carrying out further site work to allow a proper
engineering design to be prepared, by adopting conservative designs, or by
designing in accordance with general good practice based upon previous
construction experience in the area. If the risks are thought to be small, it
may be possible to insure against them.
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Since the direct cost of site investigation is normally borne initially by the
developer, rather than by the owner or user, the primary considerations
should be the identification of risk and the reduction, as far as reasonable,
of construction costs. Excessive expenditure on drilling and testing soil and
rock is not likely to serve these purposes because of the very variable nature
of most near-surface soils. On the other hand, money spent on desk studies,
air-photograph interpretation and planning of ground investigation will be
cost-effective. Experience has shown that, in most cases, desk studies give
very much better value for money in the investigation of low-rise building
sites than do activities such as the drilling of deep boreholes and extensive
programmes of laboratory testing. However, following a desk study, it is
normally necessary to carry out some soil exploration and classification.
Where possible this is done by the excavation of trial pits, so that the soil can
be examined in-situ, and by tests for particle size distribution, plasticity and
chemicals.
The majority of this sum should be spent on activities which bring the
greatest returns in terms of risk appreciation, reduction in construction
costs and increases in the effectiveness of ground investigation
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In this system the desk study, the planning and supervision of any fieldwork
(such as boring, drilling, trial pitting or in situ testing) and laboratory testing
work that may be necessary is carried out by the geotechnical adviser. He
will often be a member of a firm of civil engineering consultants but may also
be a specialist geotechnical consultant.
The geotechnical adviser is expected to carry out a thorough desk study and
plan an investigation appropriate to the needs of the developer. This is then
used to prepare a specification and bill of quantities which, together with the
conditions of contract, form the basis of the tender for the field and
laboratory work to be carried out by a specialist contractor. Generally
between three and four companies should be selected by the geotechnical
adviser to tender for the field and laboratory work, on the basis of their
previous experience of this type of work, the skills of their staff and the
amount and quality of their equipment. The lowest submitted tender price is
generally accepted but the contract is subject to remeasurement as the work
proceeds. The final cost to the developer of the entire ground investigation
will be the sum of the final contract price after measurement and the
professional fees paid to the consulting engineer.
System I has the advantage of using forms of contract that are well known
in the civil engineering construction field and it can be used to demonstrate
cost-accountability through the tendering process. This is the most
commonly used form of procurement for larger ground investigations and is
therefore well understood. Its difficulties lie in the complexity of its
contractual arrangements, the need to ensure that sufficient expertise and
supervision are provided by the geotechnical adviser and the division of
responsibility for the satisfactory outcome of the investigation between the
geotechnical adviser and the contractor. It has frequently been said that the
method of competitive tendering commonly associated with this system, and
the consequent low prices paid to contractors for investigation work, is a
major cause of low-quality investigation. This problem, however, is a
consequence of too large tender lists prepared without detailed selection of
tenderers. It is not necessarily a result of using the system.
System II Package deal contract, with desk study, planning and execution of
field and laboratory work and reporting being carried out by one company or
a consortium.
Act both for the quality of work and for any recommendations that are made
in the report of the investigation.
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FURTHER READING
UFF, J F and CLAYTON, C R I. Recommendations for the procurement
of ground investigation.
Special Publication 45. CIRIA. 1986.
ADDRESSES
The Institution of Civil Engineers
Great George Street, London SW1P 3AA
071 222 7722
Acknowledgement
The Building Research Establishment gratefully acknowledges
the assistance of Dr C R I Clayton of the University of Surrey
in the preparation of this Digest.
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