7 Investigation strategy
The extent of the investigation should reflect the magnitude and nature of the proposed works and the nature of the site in terms of any risks that the ground or groundwater pose to the construction. The former use of a site and the presence of contamination of the ground or groundwater should also be considered as they can have a significant impact on the extent and nature of the investigation.
A risk register should be started at the earliest phase of any investigation. This risk register should identify the risks that the ground conditions could pose to the proposed construction project and should be continually maintained, updated and reviewed as the results of the investigation become available.
A ground investigation should proceed in phases as follows:
- Phase 1: Desk study and field reconnaissance (see Section 2);
- Phase 2: Preliminary investigation;
- Phase 3: Detailed (design) investigation;
- Phase 4: Control investigation; construction review, including any follow-up investigations during construction, and the appraisal of performances (see Section 11).
NOTE 1 Phases 2 and 3 can be conducted together and can comprise one or more phases of investigation including sampling and testing, topographic and hydrographic surveying and any special studies (see Section 3 to Section 9).
NOTE 2 It is important to recognize that the terminology used in other contexts and by other professionals might differ from that used in this British Standard. For example, BS 10175 and BS 8576 use "preliminary investigation" to encompass "desk study and field reconnaissance" and "exploratory investigation" for what is described here as "preliminary investigation".
Phase 1 should be undertaken at the start of every investigation. The desk study should take into account the possible existence of contaminated ground (where additional site safety procedures need to be established in advance of any field reconnaissance or intrusive investigation). The desk study should also include archeological, environmental and ecological considerations, which might impose constraints on the execution of Phase 2 and Phase 3. As far as possible, the assembly of the desk study information should be complete before the ground investigation (Phase 2 and Phase 3) begins. The field reconnaissance should also be carried out towards the end of the desk study and before any investigation activity on the site. The field reconnaissance should be carried out by suitably experienced personnel who are aware of the findings of the desk study and of the proposed works.
A preliminary ground investigation might often be desirable to determine the extent and nature of the main ground investigation; the scope of this investigation should follow the guidance in Clause 17.
NOTE 3 The costs of a ground investigation are low in relation to the overall cost of a project and can be further reduced or can improve project outcomes by intelligent forward planning. Discussion at all phases with a geotechnical advisor or appropriate specialist can be used to formulate an efficient and economic plan for the investigations.
In view of the possibility that the construction of new works could affect or be affected by adjacent property or interests, investigations for new works should consider all factors that might affect adjacent land or existing works and that, where possible and expedient, records of ground levels, groundwater levels and relevant particulars of adjacent properties should be made before, during and after the construction of the new works. Where damage to existing structures or the environment is a possibility, adequate photographic records should be obtained.
8 Planning and control of investigations
All relevant information collected from the sources discussed in Section 2 should be considered to form a preliminary model of the ground conditions and the engineering problems that might be involved before commencing the ground investigation.
A ground investigation should be conducted as an operation of discovery. Planning should be flexible so that the work can be varied as necessary in the light of new information. On many occasions, especially on large or extended sites, a preliminary investigation should be carried out, in order that the design and other investigations might be planned to best advantage. It might also be necessary to carry out phased investigations after the main work to gather more detailed information related to specific matters. These additional investigations could be carried out before or during the construction works and should be guided by the uncertainties identified within the current ground model.
The ground investigation should be completed to the extent of allowing economic design to be made and with the risks of unknown ground conditions having been reduced to an acceptable level before design is completed and the works start; the level of acceptability is a matter for discussion between the geotechnical advisor, the designer and the client. Sufficient time for ground investigation, including testing, reporting, interpretation and monitoring, should be allowed in the overall programme for any scheme. Changes in the project occurring after completion of the main investigation might require additional ground investigation; the programme should be adjusted to allow for this.
The need for additional investigation after the works commence should also be taken into account. In tunnelling, for example, probing ahead of the face might be required to give warning of hazards or changes in ground conditions. The properties of the ground and also the groundwater levels can vary with the seasons; in planning the investigation, the ground conditions at other times of the year should be taken into account.
NOTE 1 The imposition of limitations (for reasons of cost and time) on the amount of ground investigation to be undertaken might result in insufficient information being obtained to enable the works to be designed, tendered for and constructed adequately, economically and on time. Additional investigations carried out at a later stage can prove more costly and result in delays.
Adequate direction and supervision of the work should be provided by a competent person who has suitable knowledge, training and experience and the authority to decide on variations to the ground investigation (see Clause 6).
NOTE 2 Further guidance on planning a ground investigation is found in Effective site investigation .
9 Quality management
A quality management plan should be prepared and implemented to ensure that the results of the ground investigation are submitted within the required standards for accuracy and presentation, as defined in national standards, the contract and the specification.
The quality management plan should include written procedures covering some or all of the following components of a ground investigation:
- a) equipment: standard of maintenance, frequency of checks on performance, frequency of calibrations;
- b) testing: procedures including checking of observations and calculations;
- c) personnel: all personnel should have qualifications, training and experience appropriate to their duties;
- d) audits to check that the quality management plan is being fully implemented.
NOTE These checks can be specific to a particular ground investigation or could be covered by the annual third party audit carried out in accordance with BS 22475-3.
Management of geotechnical data produced from the ground investigation should be as described in BS 8574.