Comparison with existing practice

The following sub-sections compare the Ground Investigation and Geotechnical Design Reports with traditional reports on these subjects.

16.5.1 British Standard BS 5930

Current UK practice is specified in Section 7 of BS 5930,5 which defines the series of reports illustrated in Figure 16.7.

Standard Penetration Test

Field reports cover all the information that needs to be obtained while working on site, e.g. the recording of in situ tests (such as the standard penetration test, cone penetration test, pressuremeter, etc.) and production of drillers' logs. They also provide outline Figure 16 J Rcports dcjined by BS 5930 guidance on the generation of the necessary forms for recording such data.

The Descriptive Report provides a factual account of all that was carried out in the field. BS 5930 recommends this report is written under the general headings shown in Figure 16.8. Detailed guidance is given on suitable content for each section, including examples of good practice in the production of borehole logs (for both cable percussion boring and rotary coring).

Figure 16.8. BS 5930]s Descriptive Report

The summary of ground conditions and parameters is developed from data given in the Descriptive Report. Usually the summary is included either in the Descriptive Report itself or in the Interpretative Report (rather than being presented as a separate report in its own right). Where it appears is normally specified in the contract documents and depends on who takes legal responsibility for corresponding aspects of the work. The contents of the

Figure 16.9. BS 5930]s 'summary of ground conditions and parameters'

summary, as specified in BS 5930, are illustrated in Figure 16.9.

The engineering interpretation must include clear statements about the data on which it is based, the nature of the structures to be built (including their dimensions and loadings), and the ground parameters to be used. The typical content of an Interpretative Report to BS 5930 is shown in Figure 16.10.

Outline design information should be given to inform the design and construction of typical geotechnical structures. BS 5930 identifies the following subjects that may require particular attention, if appropriate: spread foundations; piles; retaining walls; basements; ground anchorages; chemical attack; pavement design; slope stability; mining subsidence; tunnels and underground works; safety of neighbouring structures; monitoring of movements; embankments; and drainage.

Potential difficulties for Figure 16.10. BS 5930's Interpretative Report

construction should also be addressed, where relevant, including issues related to: open excavations; underground excavations; groundwater; driven piles, bored piles and ground anchors; grouting; mechanical improvement; and contamination. The report may also include sections on sources of materials for fill and aggregates and failures where these are relevant. Calculations should be included in an appendix.

16.5.2 AGS Guidelines for preparing the Ground Report

The Association of Geotechnical and Geoenvironmental Specialists (AGS) has produced guidance on the preparation of a Ground Report.6 These guidelines identify the following documents, which together form the Ground Report, and refer to BS 5930 as a key source of detail for their contents:

• factual report

• interpretive report

• design report

• validation report.

The Guidelines provide section headings for each report together with outline content for each section.

It is common in UK practice for a project to have a desk study, factual report, and interpretative report prepared according to BS 5930. The design report brings together all the factual and interpretative information together with other design information such as: assumptions; description; design values; calculations; plan of supervision and monitoring; maintenance requirements; and geo-environmental considerations.

AGS recommends that a validation report is written on completion of the works, to provide as-built records of the project's geotechnical elements and to discuss the results of construction monitoring, ongoing maintenance requirements, decommissioning the structure, and re-use of the foundation.

16.5.3 Geotechnical Baseline Reports

The American Society of Civil Engineers recommends7 the use of Geotechnical Baseline Reports (GBRs) for underground construction in the United States of America.

Geotechnical Baseline Reports, which should be produced by suitably experienced and qualified engineers, recognize the variability and unpredictable nature of the ground. They define the baseline conditions that will affect the successful completion of the construction works. When conditions turn out to be better than the baseline, the contractor benefits; when conditions are demonstrably worse than the baseline, the contractor is suitably reimbursed for the adverse consequences.

Baseline statements need to be carefully prepared to reduce ambiguity and ensure that all parties to a contract are clear about the level of risk they are taking. One of the principal aims of a GBR is to reduce the necessity for the contractor to resort to 'unforeseen ground conditions' clauses in order to recover costs when things go awry.

16.5.4 Report checklists

Various government agencies throughout the world provide checklists of information required for geotechnical reports.8 For example, the US Federal Highway Administration (FHWA) has produced guidelines and checklists9 to help ensure that the basic features of a geotechnical report are covered, including:

• site investigation information

• centreline cuts and embankments

• embankments on soft ground

• landslide corrections

• retaining structures

• structure foundations - spread footings

• structure foundations - driven piles

• structure foundations - drilled shafts

• ground improvement techniques

The aim of these checklists is to ensure that all relevant information is provided in the reports that are prepared on behalf of the FHWA.

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