Understanding your ground
Working in the ground is generally more risky than above-ground construction. The materials involved are naturally occurring and hence more variable than the man-made. Construction work is generally out of sight and not easily inspected. No wonder that a large proportion of construction projects that go wrong do so in the ground.
Clearly some technical errors will lie entirely within the province of the geotechnical specialist. But experience shows that these tend to be in the minority. The two most common problems are usually brought about by a wider section of the project team. They are misunderstanding the ground conditions, and dividing up the responsibility for ground investigation, geotechnical design and construction.
The most effective risk-control measure is to include professional advice in your project team. Usually, ground-related issues will be dealt with via the consulting engineer from within his own firm, or by engaging specialists, or by direct appointment.
This article, the fourth in Arup Geotechnics' occasional series1, discusses these common causes of ground-related constructional problems and how they might be mitigated or avoided altogether.
Digging for data
Getting the ground conditions wrong can, of course, result from simple misinterpretation of the ground data. But it is more likely to result from insufficient data or from lack of a proper desk study.
For construction contracts which provide reimbursement of the contractor's additional costs in the case of 'unforeseen ground conditions' (ice and jct standard forms, for example), the site investigation is the yardstick against which the contract is judged. A poor site investigation will often lead to claims and hence poor control of costs.
The adequacy of the site investigation is as much a matter for the architect and client as for the geotechnical engineer. They can all have an input into the scope of the work, if only by placing cost restraints on the investigation. This is not to say that the investigation has to be planned with an open cheque-book. It simply needs an eye to cost-effectiveness and sensible risk control rather than an arbitrary upper cost limit.
Whose job is it anyway?
Divided and/or unclear responsibilities not only lead to errors or omissions, but can also add significantly to the time and cost involved in sorting out problems, since the responsibility for tackling them tends to get debated at the same time. Ideally, one designer should control the site investigation, design foundations and supervise their construction. This is often not the case. If these are done separately, clear definition of the interfaces between designers, and management of these, is the key to success.
Ground-related problems will still occur. The project team's approach to resolving them should bear in mind that:
in many cases, the construction work in question is on the critical path - time is of the essence
the technical cause of problems can be exceedingly difficult to determine within practical cost and time limits - a pragmatic approach is often needed.
Clearly, forms of contract which favour rapid resolution of problems yield best results, whereas those forms which tend to require apportionment of blame before a solution can be sought are more difficult to operate. Costs of delay often overshadow costs of remedial works. Dealing quickly with problems minimises costs and restores confidence.
Typical ground problems
The most frequent cause of geotechnical problems is water. The project team needs good understanding, not just of a site's groundwater conditions, but also of the effect that the construction may have on them. Water is key to stability of slopes and retaining walls, and to the buildability of foundations. Don't forget that the permeability of the soil is equally important. It is also the case that water is often neglected or only treated cursorily in the site investigation. In many cases it needs more attention.
Ways to avoid water problems include:
installing standpipes or piezometers during the site investigation, and making sure they are measured beyond the end of the field work so as to get a set of readings over time
taking account of possible seasonal variations in the ground water level(s) in design and in planning the construction
considering the effect that construction may have on the groundwater conditions, for example will the cutting of a slope expose a spring line leading to instability, or will a retaining wall impound the groundwater?
Other common ground problems include underestimating the depth of made- up ground (and hence the depth to a satisfactory founding stratum); founding on peat, leading to large and continuing settlements; and inadequately compacting backfill below a foundation or slab.
Points to remember
Unfortunately, construction projects go wrong in the ground fairly frequently. Sometimes the underlying reason is complex
To reduce the risk, get help from qualified specialists
Site investigations are too often inadequate
Lack of clarity of design responsibility for investigations, foundation design and construction supervision can lead to problems
In resolving problems, speed is usually important. A technically sound, pragmatic, solution is needed rather than researching precise causes
Non-adversarial contracts usually deal better with ground-related problems, especially when solutions can be reached without attributing blame.
Bill Grose is a director of Arup Geotechnics
Previous articles were on ground investigations, aj 24.9.98 p64, piles and retaining walls, aj 24.9.98 p65, and on gravity retaining walls, aj 22.10.98 p70