Page 147 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
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should always be warned when greater accuracy than this must be obtained
with concrete. Usually discrepancies of 5mm can be taken up in the floor screed
of granolithic or terrazzo ground down to the desired smooth finish. To get
tiling accurately laid, small pieces of tile are mortared onto the floor base at
intervals across it, their level being fixed precisely to the correct finished level
by use of the instrument level. A straight edge is then used to keep the finished
tiling at the right level between tile pieces, which are cut off as the work pro-
ceeds. There are, however, some experienced tradesmen who exhibit astonishing
skill in tiling an area perfectly level given only one level point.
12.6 Site investigations

Site investigations taken at an early, feasibility stage of a project will seldom
be adequate for construction. More site tests will be necessary for individual
foundations, etc. British Standard BS 5930: 1999, Code of practice for site investi-
gations, acts as a general guide for further site tests, but this needs to be sup-
plemented by information contained in other publications as suggested at the
end of this chapter. The resident engineer will be expected to have an under-
standing of the major principles and techniques of soil mechanics so that he
can direct work intelligently. But for specifying tests and interpreting their
results, an experienced geotechnical engineer is essential, otherwise mislead-
ing assumptions can be made which later lead to serious trouble on a job.
There is an ‘art’ as well as a science in deciding what additional site investi-
gations should take place when construction is started. Advice from a geotech-
nical engineer or engineering geologist should always be sought, but when
choosing where to site extra boreholes or trial pits ‘hunch’ and ‘suspicion’ can
play a part. A hunch should not be dismissed as unscientific; it can arise from
studying the known facts and an apprehension that more needs to be known
about some aspect of a situation than is currently known at the time. An exper-
ienced engineer will always worry more about what he does not know about
below-ground conditions, than what he does know. Thus investigating some
suspicion there might be a possible unconformity in conditions below ground
can sometimes prove more revealing than gridding an area with boreholes
at regular intervals – but not always!

12.7 Trial pits

Hand-dug trial pits are expensive, take time to excavate and are not always
as informative as expected. They do, however, expose a formation so that it
can be examined in detail. This may be important if thin layers of weak clay
or pre-existing shear zones are suspected below ground. The starting size for
a pit depends on the depth it is to be sunk. If required to a depth of 5m for