Managing Geotechnical Risk

Details:

An essential part of the project lifecycle is Managing Geotechnical Risk. All parties have a role in ensuring that risks are identified and avoided or reduced to tolerable levels. Ground conditions are often uncertain and present risks which have to be managed proactively (after CD622:2020).

The ground is often a major area of risk for a building project and it is important to determine the nature and behaviour of those aspects of a site that could have a significant effect on the project.

Ground investigation involves acquiring all types of relevant information about the ground, particularly identifying adverse ground conditions that may cause problems. The main purpose is to control and reduce ground-related risks; the investigation should provide the engineering parameters needed to produce a practical and economic foundation design and, where appropriate, a ground improvement strategy.

The Design Manual for Roads and Bridges document CD662 Managing Geotechnical Risk, provides some useful definitions – a selection of which are presented below:

• Hazard – An object, feature, event or activity that has the potential to have adverse effects or undesirable consequences.
• Risk – A measure of the likelihood of a hazard occurring and the resulting possible consequences.
• Geotechnical risk – The risk to asset or the project, the public, environment, construction and operational activities created by the site ground conditions.
• Geotechnical Risk Register – The essential tool for documenting risks and actions to manage each risk and used to drive the geotechnical risk management process.

As summarised in BS6031 Code of Practice for Earthworks, to establish the geotechnical design requirements, BS EN 1997-1:2004: Eurocode 7 Geotechnical Design recommends the classification of geotechnical structures into three geotechnical categories according to the complexity of the structure, the ground conditions, the loading and the level of risk that is acceptable. The Geotechnical Categories are used to establish the extent of site investigation required and the amount of input to the design. The Geotechnical Category should be checked throughout the design and constructions process and the Category amended if necessary as information becomes available.

Geotechnical Category 1 – for small and relatively simple structures for which it is possible to ensure that the fundamental requirements will be satisfied on the basis of experience and qualitative geotechnical investigations. Category 1 structures carry negligible risk.

Geotechnical Category 2 – encompasses conventional geotechnical structures with no exceptional risk or difficult ground or soil loading conditions, most earthworks will fall into this category.

Geotechnical Category 3 – includes very large or unusual structures which are not included in Geotechnical Categories 1 and 2. Examples of such Category 3 structures are given as: structures involving abnormal risks or unusual or exceptionally difficult ground or loading conditions; structures in highly seismic areas; structures in areas of ground instability or persistent ground movements that require separate investigation or special measures.

An example of the categories used in a Geotechnical Risk Register is reproduced below (from CD622 Fig B1).

The process for reviewing and documenting the Geotechnical Risk aspects of a project have some key reporting stages, summarised as follows:

• Statement of Intent (SOI);
• Preliminary sources study report (PSSR);
• Ground investigation scoping report (GISR);
• Ground Investigation Report (GIR);
• Geotechnical Design Report (GDR);
• Geotechnical Feedback Report (GFR).

Our Matt Eynon co-authored a paper on Managing Geotechnical Risk. It is featured in Urban Geology in Wales Vol 4 (ISBN 978-0-7200-0632-2).

The publication series gives a comprehensive account of a wide variety of geo-engineering projects carried out throughout the nation and is of considerable interest to geologists, geo-engineers, planners, civil engineers and environmental scientists who require an overview of the applied geology of Wales.

Examples of recent ESP experience:

• Northern Access Road, St Athan – provision of PSSR, GIR, and GDR to major new highway scheme which included embankment crossings of watercourses and associated stability checks.
• M5:A38 Junction – provision of PSSR, GIR, and GDR to highways improvement undertaken to widen an approach to the motorway junction.