586 ENGINEERING GEOLOGY/Site and Ground Investigation penetrometer probes, or geophysical surveys to provide similar or complimentary information Cost is always important, but the designer should prepare the ‘right’ investigation for the project rather than working to a fixed budget provided by the client Teamwork is important and involves pooling ideas and suggestions and taking account of restraints imposed by the client, structural designers, auditors, or environmental specialists, who may have valuable input that can be used to support the geotechnical engineer in preparing the investigation Again, the Procedural Statement is a useful vehicle for conveying proposals to all interested parties Time should be allowed for innovative design and review of proposals ‘Sleeping’ on ideas for a day or two can often lead to the development of better alternatives Checklists are useful to remind the designer of aspects that should be covered Guidance on spacing and depth of and sampling in exploratory holes may be found in the publications of the Site Investigation Steering Group and other key texts There may be a temptation to believe that everything about a site should be discovered at the investigation stage It may be more realistic to accept that certain local features (for example swallow holes in chalk, mine workings beneath a highway cutting, or precise founding levels in variable strata) are best picked up and reviewed during the construction or remediation phase This could be more cost-effective than attempting to detect every void and strata variation during the investigation, but it does require an element of flexibility and the application of appropriate expertise during the construction work Such flexibility is more readily available in the ‘design and construct’ type of construction contract and in the use of ‘observation methods’, which are gaining popularity with clients Ground Investigation Fieldwork The various approaches that might be considered for gaining information on ground conditions are briefly reviewed in this section Trial Pitting Trial pitting is a relatively cheap and efficient method of exploring the ground Techniques vary from digging by hand – useful where services may be present or access by machine is difficult – and using mini excavators – where access is restricted – to using large-tracked back-hoe type excavators, which can reach depths of m or more (Figure 2A) Trial pits enable the stratification and the true nature of each soil horizon to be logged and representative samples selected Block samples can be cut for subsequent laboratory testing, and localized samples can be taken from shear surfaces However, access to the pit for close visual inspection is permitted only where temporary side support (shoring) is provided and a careful assessment of the risk of instability and the presence of gas has been made When inspecting the ground, a small wedge of soil is frequently sheared away from the side of the pit to reveal the true nature of the soil behind the zone of smear resulting from excavation A moisture-content profile using close centres (typically 75 mm) can provide much information about the consistency of the soil This is easily and accurately undertaken using the ‘moisture in the bag’ technique The moisture Figure Two common methods of exploration (A) The trial pit, with shoring and ladder for safe access and egress (B) The percussion boring rig