When groundwater is likely to be encountered within and below an excavation, this generally means bad news for the designer at least due to a number of undesirable effects, including increased loads on supports, flow of material, boiling (see photo opposite) or heave of the formation etc. Many of these problems can be removed or at least alleviated by some form of external dewatering system in order to reduce the level of the water table.
We will often receive design requests which state that the predicted high water table level (as shown via the borehole information and subsequently confirmed by the contractor) is intended to be reduced to a more manageable level by some means. In our experience this may simply mean sump pumping from within the excavation. This is not really a de-watering system as such as the water pressure will still remain largely undiminished outside the excavation, acting on the back of the sheets or piles.
So what are the options for dewatering and what are the limitations or each type?
Well Point Dewatering
This is a simple, relatively cheap and consequently the most common form of external de-watering. It is also a very versatile system in that a matrix of perforated suction tubes can be installed, usually by water jetting, around an excavation of any shape and to various depths (often being relocated as the excavation proceeds deeper). The tubes are then connected via a much larger header pipe at ground level or reduced ground level to a pump which simply sucks or draws the water out of the ground at a sufficient rate so as to draw down the water level in the ground. The waste water is usually processed to remove solids before being stored or discharged to a sewer.
Well pointing is only generally effective to a practical depth of around 5-6m and often much less due to suction pressures and permeability of the surrounding ground. Deeper drawdown can be achieved by staging the excavation.
This method can be used in varying soil conditions but is most effective in non-cohesive soils.
Deep Well Dewatering
This method works on the same principal as well point dewatering but on a larger scale. A borehole, fitted with a permeable liner, is installed and a submersible pump inserted which sends water to the surface. The theory is that a hydraulic gradient is formed which results in a cone shaped area of drained soil around the well or borehole.
In practice the results will depend on soil conditions (as per Darcy’s law, v = ki where v = velocity of flow, i = hydraulic gradient and k = soil permeability) and the rate may also be limited by the pump capacity.
This is relatively uncommon method certainly for smaller short term excavations. This method as you might expect from the name, requires horizontal rather than vertical drainage pipes to be installed.
Where traditional vertical well points can be installed immediately adjacent to the excavation, horizontal drains will require a significantly larger plan area and draining to the desired level may prove very difficult without significant excavation.
The Effects of Dewatering
[Figure 3 - The consequence of soil removal as well as water!] Apart from the beneficial effect the dewatering process has on the soils and pressures immediately surrounding the excavation, de-watering changes can have wider implications for nearby structures such as buildings, roads and embankments.
The possible extraction of fines in addition to the water can cause serious local settlement, erosion and instability and if carelessly (and illegally) discharged, the associated suspended solids cause damage to rivers and sewer systems if not correctly processed.
In the design office, in the absence of definitive data (which is usually the case), we are obliged to design for the worst case scenario, which is understandable considering the risks involved with the increased loadings and instability that comes with high groundwater levels. However taking the overall view it is usually more economic in the long term to use a dewatering system therefore reducing the bracing requirement which can have a knock on effect; smaller lifting requirement for example.
It will of course depend on the size of the works and the particulars of the situation and the ground conditions primarily as to whether dewatering is applicable but with design and installation services readily available it may be worth exploring the de-watering option. More on groundwater next month.
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