In the first of an occasional series on sites and foundations, Arup Geotechnics explains site investigation and its procurement Beneath the sod
Approximately half of project delays were found to be caused by unforeseen ground conditions, following analyses of projects by organisations such as the National Economic Development Office, the Public Accounts Committee and the Transport and Road Research Laboratory. Considering that approximately half of these projects were delayed by more than a month, this suggests 25 per cent of construction projects are significantly delayed by unforeseen ground conditions. Yet for building projects, site investigation costs seldom exceed 0.3 per cent of the building cost, and are usually much lower.
When there are site investigations they are often inadequate. They are considered a commodity, to be bought in at minimum cost, rather than an important element to be incorporated into the flow of the design process. This can lead to inappropriate site investigations where the total amount expended might have been sufficient, but the information obtained does not address the specific problems of the development on the site.
Inevitably, even properly planned site investigations sometimes fail to detect features in the ground that will lead to problems during construction. It is seldom practicable to sample more than 0.001 per cent of the ground beneath a site. However, problems which arise following an appropriate ground investigation are usually easier to solve, and blame for the problem is often easier to apportion, which speeds up problem resolution.
The first stage of a site investigation should be the desk study, allowing the costs of the subsequent ground investigation to be optimised. (A desk study is also a requirement of bs5930:1981 Code of Practice for Site Investigations, and Eurocode 7 Geotechnical Design: Part 1 General Rules.) The desk study should investigate at least the following areas:
topography - to reveal risks such as incipient slope stability
geology - to reveal risks such as major geological features, eg scour hollows or alluvium-filled former river courses
local stratigraphy and ground conditions - to investigate the particular soils beneath site and to warn of problems for their use in foundations, eg swelling clays, soils of very low strength and stiffness, or permeable gravels which could allow water to inundate excavations
site history - to warn of former industries in the vicinity of the site which could have contaminated the ground, or groundwater or other historical features such as back-filled ponds, brick pits or old structures which could affect foundations
buried services - to detect services which could be affected by the ground investigation or by the building foundations.
The desk study can also provide data of benefit to the developer, say by warning of surviving archaeology which could affect the granting of planning permission or by identifying a source of good quality soil which might allow relatively cheap filling operations.
A good desk study will:
provide data for foundation design, allowing more economical foundation types to be identified
give useful background information on the site and allow a complete scheme design to be developed, including the need for ancillary structures such as retaining walls
identify the ground-related hazards that need to be investigated in greater detail during the ground investigation. Without a desk study, the ground investigation becomes a hit-and-miss affair. What are the chances of identifying a former pond 10m across when your boreholes and trial pits are at 20- 30m centres?
Procuring a ground investigation
There are a myriad different techniques for investigating the ground. Choosing which ones to use on a particular site, how many exploration points to investigate, and how deep, are matters of judgement. Site Investigation in Construction, commissioned by sisg,1 gives outline guidance on why a ground investigation is required and examples of what could otherwise go wrong. A second sisg booklet, Planning, Procurement and Quality Management2gives detailed advice on which professionals should be engaged, how the ground investigation and its interpretation should be incorporated into the design process, and how the ground investigation should be procured. These documents identify the following roles:
the client's principal technical advisor (pta), who will often be the architect. The pta will be in charge of the design team and control the design process and is the most important decision-maker in regard to the quality of site investigation. The pta will normally recommend the appointment of a geotechnical advisor unless the structural design team already includes such a person
the geotechnical advisor (ga) has sufficient experience to control the site investigation process
the geotechnical contractor is the body engaged to carry out the ground investigation.
Most ground investigations should use the third booklet in the sisg series, Specification for Ground Investigation,3 as this has been broadly adopted as the national uk specification and people throughout the industry are familiar with its content.
After completion of the ground investigation, the data is interpreted by someone with geotechnical experience. Some of the information is often contradictory. Over-reliance on statistical techniques can be misleading in solving geotechnical problems; often it is the apparently spurious data that turn out to be the most significant.
Tim Chapman is an associate with ArupGeotechnics.