Site drainage

Difficulty often occurs in draining a site where large scale earthmoving is taking place. The excavations disturb the natural drainage of the land and large quantities of mud may be discharged to local watercourses during wet weather.
Complaints then arise from riparian owners and water abstractors downstream.
If this possibility should occur the resident engineer should advise the  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 contractors responsibility to dewater the site, and this includes the obligation to do so without causing harm or damage to others.

Dewatering can range from simple diversion or piping to ditches, to fullscale 24 h pumping and groundwater table lowering. It is usual to cut perimeter 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 magnitude.
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 providing 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 24 h pumping should be insisted upon by the resident engineer if he thinks damage or danger could occur from intermittent dewatering.
The electric self-priming centrifugal pump is the most reliable for continuous 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 eventually diminish because the well point screens become sealed by clay.
Special precautions must be taken to avoid damage to any adjacent structures when dewatering any excavation or groundwater lowering. In some soils groundwater lowering may cause building foundations to settle, causing considerable damage. The contractor may have to provide an impermeable barrier between the pumped area and nearby structures, monitor water levels and perhaps provide for re-charge of groundwater under structures. Avital 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.

The drainage of clay or clay and silt can present difficulty. The problem is not so much that it cannot be done, but that it can take a long time, perhaps many weeks. Sand drains (i.e. bored holes filled with fine sand), can be satisfactory as part of the permanent design of the works, but they usually operate too slowly to be of use during construction. If ground is too soft, any attempt to start excavating it by machine may make matters considerably worse, and end with the machine having to be hauled out. The act of removing overburden may make a soft area even softer as springs and streams, otherwise restrained by the overburden material, break out and change the area to a semi-liquid state. If the resident engineer sees the contractor moving towards these difficulties he should advise him of the possible consequences, and endeavour to give assistance in devising a better approach. Aparamount need may be to call in an experienced geotechnical engineer to investigate the problem and give advice as to the best policy to handle the situation.

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