Page 157 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
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contractor to approach the appropriate drainage authority (the Environment
Agency in England and Wales) to seek advice on the best course of action to
alleviate the problem, such as arranging some form of stank to pond the runoff
and allow the heaviest suspended solids to settle out. It is the contractor’s
responsibility to dewater the site, and this includes the obligation to do so with-
out causing harm or damage to others.
Dewatering can range from simple diversion or piping to ditches, to full-
scale 24h pumping and groundwater table lowering. It is usual to cut peri-
meter drains on high ground around all extensive excavations. In dry weather
this may seem a waste of time, but once wet weather ensues and the ground
becomes saturated, further rain may bring a storm runoff of surprising mag-
nitude. If no protection exists for these occasions extensive damage can be
caused to both temporary and permanent works. The resident engineer
should assist the contractor to appreciate the danger of flood damage by pro-
viding him with data showing possible flood magnitudes. A frequently used
precaution is to assume that a flood of magnitude 1 year in 10 (i.e. 10 per cent
probability) will occur during the course of construction.
The need to dewater an excavation in the British Isles is the rule rather than
the exception. Once dewatered an excavation should be kept dewatered. To
repeatedly dewater an excavation during the day and let it fill up overnight
can cause ground instability, and timbering to excavations may be rendered
unsafe. The need for 24h pumping should be insisted upon by the resident
engineer if he thinks damage or danger could occur from intermittent dewater-
ing. The electric self-priming centrifugal pump is the most reliable for con-
tinuous dewatering, having the advantage that it is relatively silent for night
operation as compared with petrol or diesel engine driven pumps.
For groundwater lowering, pointed and screened suction pipes are jetted
into the ground at intervals around a proposed excavation and are connected to
a common header suction pipe leading to a vacuum pump. It may take a week
or more before the groundwater is lowered sufficiently, but when the process
works well (as in silt or running sand) the effect is quite remarkable. It permits
excavation to proceed with ease in ground that, prior to dewatering, may be
semi-liquid. However, it can be difficult to get the well points jetted down into
ground containing cobbles and boulders; and in clays the well points need to be
protected by carefully graded filters, or the withdrawal of water may eventu-
ally diminish because the well point screens become sealed by clay.
Special precautions must be taken to avoid damage to any adjacent struc-
tures when dewatering any excavation or groundwater lowering. In some soils
groundwater lowering may cause building foundations to settle, causing con-
siderable damage. The contractor may have to provide an impermeable barrier
between the pumped area and nearby structures, monitor water levels and per-
haps provide for re-charge of groundwater under structures. A vital precaution
is for the resident engineer to record in detail all signs of distress (cracks, tilts,
etc.) in adjacent structures and take photographs of them, dated and sized,
before work starts, in order to provide evidence of the extent of any damage
which may occur.