Section 1: Preliminary considerations
4 Primary objectives
COMMENTARY ON CLAUSE 4
The amount and types of investigation can vary from the very simple, e.g. a visual inspection, to simple probing or boring to establish the level of an easily recognized rock-head, to the highly complex, involving sophisticated techniques for boring, sampling and testing; the selection of which methods to adopt are controlled by the primary objectives identified.
It has been assumed that in the selection of construction sites due regard has been paid to the wider environmental and economic considerations affecting the community generally. More than one site might need detailed investigation before the final choice is made.
The identification of the objectives of the investigation should follow the requirements specified in BS EN 1997-2.
The primary objectives when carrying out ground investigations should include the following, as appropriate.
- a) Choice of site — where alternatives exist, to advise on the relative suitability of different sites, or different parts of the same site.
- b) Suitability — to assess the general suitability of the site and environs for the proposed works, including, where applicable, the implications of any previous use or contamination of the site.
- c) Design — to enable an adequate and economic design to be prepared, including the design of temporary works.
- d) Existing works — unless the contrary can be demonstrated, it should be assumed that ground investigations are necessary when reporting upon the existing works (see 15.2), and for investigating cases where failure has occurred (see 15.3). The objectives of such investigations should be directly related to the particular problems involved.
- e) Effect of changes — to determine the changes that might arise in the ground and surrounding environment, either naturally or as a result of the works, and the effect of such changes on the works and on adjacent works.
- f) Construction — to plan the best method of construction; to foresee and provide against difficulties and delays that could arise during construction due to ground, groundwater and other local conditions; in appropriate cases, to explore sources of indigenous materials for use in construction; and to select sites for the disposal of waste or surplus materials. (See 15.4 for recommendations for investigation of materials for construction purposes.)
A ground model for the site should be constructed at the very beginning of the investigation process; this model is used to identify the known conditions and, importantly, uncertainties in the knowledge of the ground, and so identify the investigations required. The model should be continuously updated in the light of the investigation findings. A register identifying the risks posed by the ground should be prepared and maintained in parallel with the ground model.
NOTE The register of risks to the investigation and the engineering works referred to here is separate from, and additional to, the site safety plan which identifies the health and safety risks associated with the ground investigation that might affect workers on the site, the public and the environment (see Clause 5 and Section 2).
Ground investigations should obtain reliable information to enable an economic and safe design, to assess any hazards (physical or chemical) associated with the ground, and to meet design and construction requirements. At each stage, the investigation should be designed to verify and expand the information previously collected.
An objective of the ground investigation should be to obtain a clear understanding of the geomorphology, geology and hydrogeology of the site through appropriate desk study, site reconnaissance, mapping and intrusive field investigations.
The geological profile should be established by visual inspection and systematic description of the ground using the methods and terminology given in Section 3 to Section 8. The understanding of the profile should be supplemented by field and laboratory testing to determine the engineering properties of the geological materials encountered in detail (see Clause 15 for the scope of field investigations).
In order to evaluate fully the nature of the ground and the groundwater and so as to achieve the objectives of the ground investigation, the work should be planned, undertaken and supervised by personnel who have suitable qualifications, skills and experience relevant to the particular aspects of geotechnical work on the site (see Clause 6).
The investigation should cover all ground in which significant temporary or permanent changes might occur as a result of the works (these include changes in stress and associated strain; changes in water content and associated volume changes; changes in groundwater level and flow pattern; and changes in properties of the ground, such as strength and compressibility). Materials placed in the ground might deteriorate, especially in landfill and contaminated former industrial sites; therefore, information should be provided from which an estimate of the corrosivity and aggressiveness of the ground can be made (see Clause 21 and Clause 61).
Wherever appropriate, measures should be taken to locate natural and man-made underground cavities, such as old mine entries, mine workings or swallow holes, which could collapse and cause damage. Other hazards could also arise on account of earlier uses of the site (see Clause 12).