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Soil Consultants' Alan Watson offers his observations from ground investigations on the influence of groundwater

The influence of groundwater seems to be the primary contributing factor to many construction difficulties. It is a moving target as conditions encountered during ground investigation do not always provide a reliable indication for early stages of construction.

Restricted headroom piling

Soil Consultants found shallow groundwater during its ground investigation for a project using segmental flight auger (SFA) piling to install grout mini-piles in restricted headroom.

SFA piling requires careful control of the grout head within the hollow drill string to avoid overbreak in uncased holes. This increases with the presence of shallow groundwater because the grout head should exceed the groundwater head to avoid disturbance of the pile.

On this site, gravel overlay sandstone with clay lenses and groundwater was just below basement level. After attempts at piling, integrity tests detected anomalies.

This technique needs even more control in this ground profile, which included lenses of cohesive soils within otherwise more permeable strata, particularly when grouting and when removing auger segments.

Groundwater pressure is reduced within substantial clay beds because it is less permeable than surrounding weathered sandstone. This means that once lower clay is penetrated, groundwater pressure comes to bear around the flight auger.

Where a cohesive low permeability lens lies below the water table, there is an upward pore pressure on the underside of the lens. Drawing back the base of the auger flight through the lower clay lens is a critical stage because there is a tendency for groundwater below to want to find its way up the augered hole. If the head of grout is not maintained in excess of that of the groundwater the pile grout can be disturbed.

Whenever possible, splitting augers should be avoided while the lower point of the flight is at, or around, the lower part of a cohesive stratum below the water table. Where this is unavoidable, the level of the top of the remaining string should be well above ground level while the uppermost segment is removed. This is so the head of grout in the hollow stem can be maintained.

In these ground conditions, upward pressure will exist even when groundwater above is in hydraulic conductivity with the groundwater below the lens. This situation would not be identified as a sub-artesian pressure in a ground investigation – hence the need for careful attention where substantial clay beds are present.

Piling into aquifers
In some circumstances, the Environment Agency expects foundation risk assessments, for example when a major aquifer on a contaminated site will be penetrated. Potable water supplies must be protected and there are genuine concerns at some contaminated sites where new pathways for downward percolation may be caused.

Consider a site where contamination has been found. The primary factor when assessing the risks of piling into an aquifer is whether an intervening natural low permeability layer, or aquitard, is present beneath contaminated soil or groundwater.

A substantial layer of low permeability natural clay should provide a barrier to downward percolation of contaminants.

Such a stratum may be perceived as a prime asset by regulators when it is known to effectively impede the downward percolation of contaminants.

Where contamination is at unacceptable levels over an aquifer not protected by a natural protective barrier, remediation is required. However, if such a barrier is present, and risks are shown to be acceptable, there may be a case for leaving low levels of contamination insitu, particularly if the barrier's integrity is reliable. In this scenario piling or ground improvement is likely to attract close scrutiny.

Deep basement floor slabs resisting groundwater pressure
Where perched groundwater overlies an aquitard, as is the case with groundwater within River Terrace Deposits overlying London Clay, then pore pressure profile is hydrostatic within underlying saturated London Clay. At greater depths, and if the clay is underdrained by Basal Sands, pore pressure can lessen to a profile below hydrostatic.

Within the upper hydrostatic zone, boreholes may not necessarily show the saturated condition of the clay because the permeability is so low, inflows are very slow, if any.

Medium to long-term monitoring of conventional piezometers is unlikely to give a reliable indication of pore pressure.

For these reasons, observations from standard ground investigation techniques can give rise to incorrect assumptions about the magnitude of upward pressures from groundwater on a deep basement floor slab.

The worst-case design assumption in British Standard 8102 for water standing 1m below ground level behind a basement retaining wall, may not give a full hydrostatic pressure beneath the whole area of a basement slab due to the transient nature of potential causes of the high water level, such as burst water mains or short-term storm flows.

One approach is to allow for worst-case hydrostatic pressure, from the assumed high water level, over a narrow zone around the edges of the floor slab. This allows theoretical effects to be kept within tolerable proportions.

Moderating effects of an aquitard can be seen as a good thing, but in other examples the combination of groundwater and clay horizons within otherwise permeable granular strata can give rise to construction problems.

Assessments of groundwater influence should be set in the context of the particular construction technique in the short term, the potential influences of groundwater in the long term and the impact of the development proposals.

Alan Watson is senior engineer at Soil Consultants

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