Miscalculation of earth pressures from buildings may lead to errors of the order of 25 per cent. While debilitating, these do not generally lead to structural failure. On the other hand, poor judgement of water pressures may predict lateral loads on a structure half those actually experienced. So accurate measurement of water pressures during a ground investigation and consideration of possible variations over the design life of the building are essential.
'To measure ground water pressures accurately, it is generally necessary to install special measuring devices called piezometers [standpipes are a special type of piezometer]. The ground water pressure may vary with time owing to seasonal, tidal, or other causes and it may be necessary to take measurements over an extended period of time in order that such variations may be investigated.' (bs 5930: 19812)
But instead, undue reliance is sometimes placed on observations made during excavation of boreholes or trial pits. The bs warns: 'Such methods should be treated with great caution, since it is seldom that sufficient time is allowed for the water level to stabilise. Moreover, the levels from which the water is entering the borehole may not always be known . . . For most conditions it is preferable to use a standpipe piezometer.'
The extra capital cost of installing them is very small compared to the initial cost of excavating the borehole, although the cost of visiting site to monitor water levels after the ground-investigation contractor has left site can be relatively high, particularly as several visits are required. Even so, these costs are tiny compared to the effort that can be spent in sorting out problems caused by poor identification of the water table and its effects, both for the building and the construction process.
Frequently the ground investigation is geared only to the finished building and does not provide data to help the contractor in the design of his temporary works. During construction the contractor often finds it necessary to excavate below the water table for pile caps, drainage trenches and lift pits, even for buildings which are otherwise above the water table. These short-term excavations are seldom scrutinised, and he has a financial incentive to minimise their extent. As a first attempt the contractor may try to make these excavations either by using a sump pump in the hole or by some local dewatering. If these fail, grouting may be attempted. Failing that, more positive measures such as sheet piling will be used.
By the time problems have been resolved, there will have been a large delay, the contractor will attempt to recoup his costs by claiming for his losses, and in some situations the surrounding ground will have been so badly disturbed that the structure under construction or its neighbour may need repair.
Guidance on alternative dewatering systems is given by ice3 and ciria4, illustrated in the first box overleaf.
Piling is also susceptible to problems from groundwater. Examples of the problems are given in another ciria5 publication. Where piles need to be installed in permeable ground with a high water table, driven piling systems are most robust but are often rejected where nearby neighbours could be disturbed by the resulting noise and vibration. Bored piling systems can be used in conjunction with temporary casings, bentonite support fluid (fluid injection that stiffens the ground locally) or continuous- flight augers (where concrete is pumped down the auger core as it is withdrawn).
Light structures with deep basements such as light-rail stations or pumping chambers are vulnerable to flotation. Their design normally entails the provision of extra ballast or permanent dewatering beneath the structure or the use of tension piles, or in some cases allowing extreme external water levels to overtop the structure so that the ensuing flood-water adds to their weight.
In nearly all soils the build-up of water pressures on the sides of the structure adds to the total horizontal forces. Normally the structure needs to be designed to accommodate a relatively uniform extra pressure. However, in some situations the new structure acts as a dam to water flow, and water pressures build up unequally so that the water force on one side is much higher. It can be difficult to design for this situation as the structure itself has to resist the unbalanced lateral force.
Maintaining a dry ground formation is necessary for quality control in placing structural materials and helps in foundation design. A wet formation leads to an under-strength layer in the founding soil which contributes to additional settlement of the structure. It can also lead to inadequate resistance to sliding, important where the structure needs to resist unbalanced forces. High water pressures and especially flows of water can contribute to a loss in 'effective strength' of the soil, which in turn can lead to downgrading of pile capacities and a need for extra embedment on embedded retaining walls. This is very important in buildings with basements which penetrate the water table.
Basement water resistance
bs 81026 and ciria Report 1397 cover methods for providing particular levels of water resistance in basements. They give guidance on the design water levels that should be used, the various grades of basement space depending on intended end-use and the means of attaining a satisfactory amount of water proofing. These grades of space are summarised in the second box (based on bs 8102).
Problems of poor waterproofing are unfortunately common and are usually due to the following (avoidable) factors:
failure to anticipate water levels over the design life of the building
failure to understand the building owner's requirements for the basement space or to explain the possible consequences of the designed performance on the selected grade of use poor design or detailing bad workmanship.
A broader view
In addition to measuring water levels, it is important for the geotechnical design to take into account the factors which cause those water levels (hydrogeology) and possible long-term changes that could occur. For instance, in the Victoria area of London, water levels have been drawn down locally through dewatering by London Underground and the Royal Parks8. Water levels are up to 3m lower than would otherwise be the case. While water measurements on a site there would only indicate the current situation, this broader understanding suggests designing for higher water levels in the future since continued pumping by others cannot be relied on.
In London and a number of other cities here and abroad, water levels are rising. This phenomenon is sometimes incorrectly linked to climate change or wrongly blamed for water leaks into basements. In London, rising water levels are explained by a fall-off in private water abstraction from the deep chalk aquifer9. This leads to a recovery of water levels, in some areas rising by up to 70m.
In most areas the shallow sand and gravel layers (Thames Terrace Gravels) are insulated from the effects of this rise by the thick deposits of clay found as part of the London Clay and the Lambeth Group Clay (formerly known as the Woolwich and Reading Beds). Thus only some critical structures in particular parts of London will be affected.Clearly, the broader view can be as important as current on-site conditions.
Tim Chapman is an associate with Arup Geotechnics