To many’s surprise, water is everywhere underground, even in the desert. Therefore groundwater is a thorny problem for almost all underground constructions. Groundwater is among the top 5 challenges for underground construction. This post explains the hazards of groundwater and outlines common solutions.
Hazards of groundwater:
During construction:
Flood the excavation, preventing work from proceeding. A swimming pool is obviously unsuitable for most men or machinery to work in.
De-stabilise the ground by ‘washing out’. Granules of soil, as well as highly weathered rock may be loosened up (reducing effective ground pressure close to zero) and washed out gradually by the flow of water, creating a larger and larger cavity which eventually will cause collapse.
Compromise quality/safety of construction. Ground water flowing into the excavation is caused by water pressure build-up in the surrounding ground. Some construction methods such as sprayed concrete is sensitive to groundwater pressure (it will make the sprayed concrete not ‘stick’ to the ground). Ground water pressure can also compromise the quality of some crucial installations against the substrate such as waterproofing membrane (leading to blistering or rupture).
During use of the structure:
Defeat the functionality of the structure. For example, spaces habitable by human normally have to be dry (enough). Minor water entry brings discomfort (stain, damp, damage to installation, etc.) and a major one may put the structure completely out of use.
Deteriorate materials sensitive to water/moisture, reducing asset life. Amongst commonly used construction materials, steel is most vulnerable to contact with water. Water entry will speed up the corrosion of bar reinforcement encased in concrete.
Solutions:
Active de-watering – pumping out groundwater around the outside of the excavation, to lower the local ground water table to a sufficiently low level. Since the groundwater hazard is cut at the source, the obvious advantage of this option is that no water whatsoever will enter the excavation, creating a completely dry environment. This offers much better flexibility in the choice of construction sequence and form of structures. The disadvantage of this option is that a large amount of water has to be pumped out and discharged, which obviously comes at significant cost. Moreover, lowering groundwater results in ground settlement and movement, and in some extreme cases can create ‘sinkholes’, which are local shear failure of the ground. Therefore this option is normally less attractive in the urban environment where a large number of sensitive existing structures are present.
Passive de-watering – pumping out only whatever ground water that enters the excavation, rather than cutting it at the source, hence the name. In terms of temporary works, this can take the form of ‘pressure relief wells’ or ‘weepholes’, which prevent local water pressure from building up. For large scale construction, these usually take the form of an array of pressure relief points. The frequency or spacing of relief points depends on the permeability of the ground and structural design. For permanent works, passive groundwater relief can take the form of ‘cavity drain’ – see extract from BS8102 ‘Type C’. Given the same project, passive de-watering is likely to pump out less volume of water because the structure itself and the surrounding ground can serve as a barrier/filter to exclude water to certain extent. Depending on the permeability of the ground, the ground water table local to the excavation may still drop, but to a relatively minor extent compared to active de-watering. Sometimes, when the permeability of the ground is high, passive de-watering has to be complemented by a water-barrier (explained later) to reduce the flow of water first, otherwise the running cost of pumping may be too high, or the flow may simply overwhelm any pump available. Balance between draining and exclusion is sought in order to achieve the best quality and economics.
Counter-pressure – applying a pressure in the reverse direction of the groundwater pressure, to reduce/eliminate the pressure difference in and out of the excavation, so that no flow of groundwater will happen. This category is normally used in temporary works. The way of applying the counter-pressure could be compressed air, bentonite suspension fluid (in diaphragm walling and piling), closed face TBM (either EPB or slurry).
Water-barrier – instead of letting the water in, preventing ground water from entering in the first place. In terms of temporary works, this could take the form of ground treatment, such as permeation grouting, deep soil mixing, jet grouting, ground freezing etc. In terms of permanent works, water-barrier can be achieved by means of waterproofing membrane, or ‘structural integrity’ design (i.e. designing the structure itself to be water-resistant, such as controlling cracking of concrete to a sufficiently small level). Note that most of the time such water-barriers are not intended to be ‘perfect’, and water may still leak through to certain extent. Even so, they still serve their purpose, as long as whatever leaked through is within the expected range and manageable.
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