Civil Engineering Project Management (4th Edition)
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142 <strong>Civil</strong> <strong>Engineering</strong> <strong>Project</strong> <strong>Management</strong><br />
contractor to approach the appropriate drainage authority (the Environment<br />
Agency in England and Wales) to seek advice on the best course of action to<br />
alleviate the problem, such as arranging some form of stank to pond the runoff<br />
and allow the heaviest suspended solids to settle out. It is the contractor’s<br />
responsibility to dewater the site, and this includes the obligation to do so without<br />
causing harm or damage to others.<br />
Dewatering can range from simple diversion or piping to ditches, to fullscale<br />
24 h pumping and groundwater table lowering. It is usual to cut perimeter<br />
drains on high ground around all extensive excavations. In dry weather<br />
this may seem a waste of time, but once wet weather ensues and the ground<br />
becomes saturated, further rain may bring a storm runoff of surprising magnitude.<br />
If no protection exists for these occasions extensive damage can be<br />
caused to both temporary and permanent works. The resident engineer<br />
should assist the contractor to appreciate the danger of flood damage by providing<br />
him with data showing possible flood magnitudes. A frequently used<br />
precaution is to assume that a flood of magnitude 1 year in 10 (i.e. 10 per cent<br />
probability) will occur during the course of construction.<br />
The need to dewater an excavation in the British Isles is the rule rather than<br />
the exception. Once dewatered an excavation should be kept dewatered. To<br />
repeatedly dewater an excavation during the day and let it fill up overnight<br />
can cause ground instability, and timbering to excavations may be rendered<br />
unsafe. The need for 24 h pumping should be insisted upon by the resident<br />
engineer if he thinks damage or danger could occur from intermittent dewatering.<br />
The electric self-priming centrifugal pump is the most reliable for continuous<br />
dewatering, having the advantage that it is relatively silent for night<br />
operation as compared with petrol or diesel engine driven pumps.<br />
For groundwater lowering, pointed and screened suction pipes are jetted<br />
into the ground at intervals around a proposed excavation and are connected to<br />
a common header suction pipe leading to a vacuum pump. It may take a week<br />
or more before the groundwater is lowered sufficiently, but when the process<br />
works well (as in silt or running sand) the effect is quite remarkable. It permits<br />
excavation to proceed with ease in ground that, prior to dewatering, may be<br />
semi-liquid. However, it can be difficult to get the well points jetted down into<br />
ground containing cobbles and boulders; and in clays the well points need to be<br />
protected by carefully graded filters, or the withdrawal of water may eventually<br />
diminish because the well point screens become sealed by clay.<br />
Special precautions must be taken to avoid damage to any adjacent structures<br />
when dewatering any excavation or groundwater lowering. In some soils<br />
groundwater lowering may cause building foundations to settle, causing considerable<br />
damage. The contractor may have to provide an impermeable barrier<br />
between the pumped area and nearby structures, monitor water levels and perhaps<br />
provide for re-charge of groundwater under structures. A vital precaution<br />
is for the resident engineer to record in detail all signs of distress (cracks, tilts,<br />
etc.) in adjacent structures and take photographs of them, dated and sized,<br />
before work starts, in order to provide evidence of the extent of any damage<br />
which may occur.