ABSTRACT
An adequate ground investigation is an essential preliminary to the execution of a civil engineering project. Sufficient information must be obtained to enable a safe and economic design to be made and to avoid any difficulties during construction. The principal objects of the investigation are: (1) to determine the sequence, thicknesses and lateral extent of the soil strata and, where appropriate, the level of bedrock; (2) to obtain representative samples of the soils (and rock) for identification and classification, and, if necessary, for use in laboratory tests to determine relevant soil parameters; (3) to identify the ground water conditions. The investigation may also include the performance of in-situ tests to assess appropri ate soil characteristics. In situ testing will be discussed in Chapter 7. Additional considerations arise if it is suspected that the ground may be contaminated. The results of a ground investigation should provide adequate information, for example, to enable the most suitable type of foundation for a proposed struc ture to be selected, and to indicate if special problems are likely to arise during construction. Before any ground investigation work is started on site, a desk study should be conducted. This involves collating available relevant information about the site to assist in planning the subsequent field work. A study of geological maps and memoirs, if available, should give an indication of the probable soil conditions of the site in question. If the site is extensive and if no existing information is available, the use of aerial photographs, topographical maps or satellite imagery can be useful in identifying exist ing features of geological significance. Existing borehole or other site investigation data may have been
collected for previous uses of the site; in the UK, for example, the National Geological Records Centre may be a useful source of such information. Links to online sources of desk study materials are provided on the Companion Website. Particular care must be taken for sites that have been used previously where additional ground hazards may exist, including buried foundations, services, mine workings etc. Such previous uses may be obtained by examining historical mapping data. Before the start of fieldwork an inspection of the site and the surrounding area should be made on foot. River banks, existing excavations, quarries and road or railway cuttings, for example, can yield valu able information regarding the nature of the strata and groundwater conditions; existing structures should be examined for signs of settlement damage. Previous experience of conditions in the area may have been obtained by adjacent owners or local authorities. Consideration of all of the information obtained in the desk study enables the most suitable type of investigation to be selected, and allows the fieldwork to be targeted to best characterise the site. This will ultimately result in a more effective site investigation. The actual investigation procedure depends on the nature of the strata and the type of project, but will normally involve the excavation of boreholes or trial pits. The number and location of boreholes, trial pits and CPT soundings (Section 6.5) should be planned to enable the basic geological structure of the site to be determined and significant irregularities in the subsurface conditions to be detected. Approximate guidance on the spacing of these investigation points is given in Table 6.1. The greater the degree of variability of the ground conditions, the greater the number of boreholes or pits required. The locations should be offset from areas on which it is known that foundations are to be sited. A pre liminary investigation on a modest scale may be carried out to obtain the general characteristics of the strata, followed by a more extensive and carefully planned investigation including sampling and in situ testing. It is essential that the investigation is taken to an adequate depth. This depth depends on the type and size of the project, but must include all strata liable to be significantly affected by the structure and its construction. The investigation must extend below all strata which might have inadequate shear strength for the support of foundations, or which would give rise to significant settlement. If the use of deep foun dations (Chapter 9) is anticipated the investigation will thus have to extend to a considerable depth below the surface. If rock is encountered, it should be penetrated by at least 3 m in more than one location to confirm that bedrock (and not a large boulder) has been reached, unless geological knowledge indicates otherwise. The investigation may have to be taken to depths greater than normal in areas of old mine workings or other underground cavities. Boreholes and trial pits should be backfilled after use. Backfill ing with compacted soil may be adequate in many cases, but if the groundwater conditions are altered by a borehole and the resultant flow could produce adverse effects then it is necessary to use a cement based grout to seal the hole.
