To provide the maximum value to the Client (in terms of overall total ground related costs) every site investigation should start with a desk study of available data. This is a low cost element (generally less than £1,000) but provides an early indication of possible ground hazards, and is the recommended first step in all UK guidance (BS59301, BS101752, CLR113, Eurocode 74 etc.). One of the main objectives of the desk study is to allow a ‘Conceptual Ground Model’ to be prepared, along with a preliminary assessment of contamination risks (a requirement of any assessment of potentially contaminated land) and the collation of a geotechnical risk register (a requirement of BS5930).
Development of the Conceptual Ground Model
Our experience has shows that the Conceptual Ground Model is the most critical element in understanding the ground and reducing the risk of subsequently encountering ‘unforeseen ground conditions’. Only when the preliminary Conceptual Ground Model has been prepared can a suitable and appropriately scaled intrusive investigation be designed which, in true scientific method, should test and refine the ground model. This approach has been demonstrated time and time again to be the most successful and cost effective method of managing ground hazards on development sites.
If the ground conditions beneath a site are anticipated to be straightforward, a simple investigation (say trial pitting) may be all that is required to confirm the Conceptual Ground Model and adequately characterise the ground. However, if the site has a significant industrial history or is located in a high risk geotechnical setting, a more comprehensive investigation, involving more boreholes, trial pits and sampling may be required. It must always be remembered that the investigation of the ground and the design methods to mitigate risks to overcome ground hazards always costs less at this initial stage than during construction.
Another common fallacy we have heard is that ‘the site is greenfield, so does not need an site investigation’. The risks from soil contamination at a true greenfield site may be low, however, geotechnical hazards may still be present (e.g. old mine entries, solution features, landslips, the potential for soil shrinkage). In addition, at ESP we are aware of a number of sites which, at first glance, would appear to be ‘greenfield’, but the gently sloping, beautiful grassed surface hides significant ground hazards.
The intrusive investigation techniques used at a site may include:
- Machine excavated trial pits – commonly the cheapest investigation method, and provides good information on shallow soils, but it is very disruptive of the site surface;
- Cable percussion boreholes – a ‘traditional’ and relatively cost effective method which is not very disruptive to the site surface, and is very useful as it can penetrate most soils to depths in excess of 30m;
- Dry sampling/windowless sample boreholes – a very useful technique for sites with limited access or where damage to the site surface is to be minimised (e.g. in private gardens or around existing retail developments). However, the technique does have limitations due to lack of power in penetrating through some soils. Not to be confused with ‘window sampling’ which is a poor quality investigation technique and should only be used ‘as a last resort’;
- Rotary drillholes – used to penetrate rock, and can be ‘open hole’ whereby information can be obtained quickly, or ‘cored’, allowing the maximum amount of information on the bedrock to be obtained;
- Cone penetration testing/ continuous sampling – a very useful technique particularly where a piezocone is used, as it is the only method by which the structure of soft, compressible soils can be identified, but it is more expensive;
- Sonic drilling, geophysical surveying, dilatometer, pressuremeter etc. – alternative and more costly investigation techniques, but will be appropriate for some development sites.
Choosing the right technique
The choice of which of the above methods to use on any site depends on a large number of factors which need to be considered carefully by the designer. The main considerations would be the main objectives of the investigation (e.g. is it purely to understand any physical constraints and how the ground will react following development, or are the levels of soil/water contamination or ground gas to be assessed) and the type and complexity of the proposed development, but other aspects including access limitations may also need to be considered. If contamination is suspected, the exploratory holes should not create new pollution pathways. At ESP we work closely with others in the design team to ensure that any investigation we design is ‘fit for purpose’.
We have considerable experience in the design and implementation of investigations for sites of all complexities and scale, with four senior members of staff qualified at the top two levels of the UK Register of Ground Engineering Professionals (ROGEP). We are therefore able to design appropriate investigations bespoke to individual projects. We do not just turn up with the same borehole rig we used at the last site. The combination of ground hazards (both man-made and natural) present on every site is different, so each site should be treated differently.
Site investigation is also interdisciplinary in nature. At ESP, we have specialists in both geotechnical engineering and contaminated land, who work together to ensure that the appropriate information is obtained on the site to minimise the risks of ‘unforeseen ground conditions’ later in the project life. It is also iterative, and developers should appreciate that sometimes, where complex ground conditions are present, a number of phases of intrusive investigation may be required – but again, the costs of these are likely to be considerably less than only identifying an unforeseen ground hazard when the contractor has commenced on site.
We also have a range of assessment reports available to provide you with the scale of information required, from a low-cost MySite report at feasibility stage (limiting your financial exposure should ground hazards be identified), through to full intrusive investigations and assessments to inform design or satisfy planning conditions.
We also have considerable experience of working with Clients and regulators to get those awkward planning conditions discharged.
If you would like any further information, or would like to discuss any development projects you may have, please do not hesitate to contact us.
- BS5930:2015. Code of Practice for Ground Investigations. BSI.
- BS10175:2013. Investigation of Potentially Contaminated Sites. Code of Practice. BSI.
- Environment Agency. 2004. Model Procedures for the Management of Land Contamination. CLR11.
- BS EN 1997:2007. Eurocode 7: Geotechnical Design – Part 2: Ground Investigation and Testing. HMSO.