17 Scope of the ground investigation
COMMENTARY ON 17.1
A range of methods is available for use in ground investigations; these include non-intrusive methods, such as geophysical surveying (see Section 5) and intrusive methods, such as excavations and boreholes (see Section 4 and BS EN ISO 22475-1) and probing (see Section 7). The factors determining the selection of which method or methods is to be used in a particular investigation are discussed in Clause 19 and Clause 20. In general, the recommendations in 17.2 to 17.7 apply irrespective of the method adopted, and the term investigation point is used to describe a position where the ground is to be explored by any particular method.
The scope of the ground investigation should be determined by the character and variability of the ground and groundwater, the type of project (geotechnical category) and the amount and quality of existing information. The general character and variability of the ground should be established during the preliminary investigation before deciding on the basic principles of the design of the works undertaken for the design investigation; the desk study of existing information might provide sufficient high quality data for a preliminary design to be started with confidence.
NOTE 1 BS EN 1997-2:2007, Annex B outlines the geotechnical investigation process through to use of the structure as well as the selection of different investigation methods. It also provides guidance on the spacing of investigation points for a variety of structures and the depths to which investigation are to be taken.
NOTE 2 Certain methods of investigation are more suitable than others on contaminated land (see BS 10175 and BS ISO 10381-2).
Many ground investigations comprise both geotechnical and geoenvironmental aspects. When designing the scope of integrated investigations, the recommendations given in BS 10175 should be followed (see also 17.8 and 17.9).
17.2 Phased investigation
Following the desk study, a ground investigation should normally be performed in phases in accordance with BS EN 1997-2:2007, Section 2. These phases are preliminary investigation, design investigation, controlling and monitoring during construction; the first two investigation phases may be carried out in a single site visit.
The initial phase (preliminary investigation), which might involve widely spaced boreholes, probing or trial pits, should be designed to establish the general geological conditions, the suitability of different methods of investigation and the groundwater conditions.
NOTE The preliminary investigation assists in the design of an effective programme for the detailed investigation. It might also provide an opportunity to take samples for chemical analysis to determine whether they are contaminated. However, such testing is not a substitute for a properly planned investigation for contamination (see 17.8 and 17.9).
For more complex structures, the preliminary investigation should be followed by a more detailed (design) investigation.
Regardless of the number of phases of investigation, the designers should satisfy themselves that the scope of the investigation(s) provides sufficient data for economic and safe design and construction. Even so, further investigation and/or monitoring might be warranted during and after construction (see Section 11).
17.3 Geotechnical category
The investigation should be appropriate to the geotechnical category and should yield sufficient information on which to base a safe and economical design of the project; the categorization of structures is given in BS EN 1997-1 as follows:
- Category 1 — small simple structures with negligible risk;
- Category 2 — conventional structures with no exceptional risk; and
- Category 3 — large or unusual structures with abnormal risks.
The information from the investigation should be used to decide which of the various possible methods of construction would be desirable and, where appropriate, to suggest sources of construction materials. The lateral and vertical extent of the investigation should cover all ground that might be affected by the new works or their construction.
The scope of the investigation should be in accordance with BS EN 1997-2 as a minimum.
NOTE Other investigation methods over and above these documents might be required as given in this British Standard and other sources.
17.4 Character and variability of the ground
COMMENTARY ON 17.4
Irrespective of the geotechnical category, the greater the natural variability of the ground, the greater the extent of the ground investigation required to obtain an indication of the character of the ground.
The depth of exploration should usually be determined by the nature of the works proposed, but it might be necessary to explore to greater depths at a number of points to establish the overall geological structure and where hazards or changes in ground character can affect the structure during its design life. The technical development of the project should be kept under continuous review since decisions on the design influence the extent of the investigation.
NOTE The detailed geology of a site can only be inferred from aerial photography, from surface outcrops and from subsurface information at the positions of the investigation points. The possibility remains that significant undetected variations or discontinuities can exist, including lateral or vertical variations within a given stratum. Even an intensive investigation can only reduce uncertainties and risks; complete excavation is the only way to reveal the actual nature of the ground. The use of angled drill holes can in certain cases greatly assist interpreting variations between vertical boreholes. In some circumstances, additional information between investigation points can be obtained from geophysical methods (see Section 5).
17.5 Positioning of investigation points
The points of investigation, e.g. geophysical measurements, boreholes, probings, pits, sampling or measurements, should be so located that a general geological view of the whole site can be obtained, providing details of the engineering properties of the ground and of groundwater conditions. More detailed information should be obtained at positions of significant structures and earthworks, at positions of special engineering difficulty or importance and where ground conditions are complicated, e.g. suspected buried valleys, karstic features or landslide areas.
In the absence of other criteria, a regular array of investigation points may be used in the design of an investigation. However, sufficiently close supervision and flexibility (in the programme and contract) should be provided to allow amendments to be made to this pattern as the work proceeds. Sometimes it is not possible to locate structures until much of the ground investigation data has been obtained; in such cases, the programme of work should be kept under review accordingly. The use of geophysical mapping should be considered to interpolate between intrusive investigation points.
When investigating for proposed tunnels and shafts, boreholes should be offset so as not to interfere with subsequent construction or create pathways for groundwater. For other structures, the need to offset boreholes and trial excavations from critical points should be taken into account. Trial excavations should be outside proposed foundation areas.
For linear structures, such as highways, some investigation points should be arranged at offsets to the centre-line of the proposed road, so that lateral variation in ground conditions are revealed and material obtained to test suitability for use as fill material.
17.6 Investigation point spacing
Investigation points should be spaced in accordance with BS EN 1997-2:2007, Annex B, unless there is good reason not to do so, which might include the structure, experience and prior knowledge of the ground conditions.
NOTE Additional rules for low rise buildings are given in BS 8004.
Where a structure consists of a number of adjacent units, one investigation point per unit might suffice. Certain engineering works, such as dams, tunnels and major excavations, are particularly sensitive to geological conditions, and the spacing and location of investigation points should be related more closely to the detailed geology of the area than is usual for other works.