Site investigation and ground risk

One of the biggest risks to programme and costs of a construction project is unforeseen conditions and those are usually in the ground. Identifying, managing and mitigating those risks is an important step in protecting your business. This article sets out to provide some guidance on achieving this.


  • Except the Major Project Construction Contract, they do not contain specific clauses for dealing with ground conditions in their unamended form


  • Commonly used phase is that it should be allocated to the party best able to manage or control it


  • Risk of unforeseen conditions sits with the contractor unless the contract makes specific provision in respect of additional time and/or money


  • It is normal for the tender documents to include site information that can be used in helping to assess the risk in unforeseen/ground conditions.
  • No obligation for the Employer to provide site information (they do not warrant the site).
  • If information is provided to you as the Contractor and it you are advised that you may rely on it, then the Employer will have warranted, expressly or by implication, the accuracy of the information. If that information turns out to be wrong, you may have a claim for damages.
  • More often the site information will be made available, but with a health warning to the contractor that in effect says “here’s some information, but the risk in the ground is still yours”.
  • Accordingly, it will be up to you as the contractor to understand and interpret the site information you have been provided with or obtain your own. Either way, you must assess any risks and manage them accordingly.


Generally broken down into phases…

Phase 1: Desktop Study

  • Intended to review
    • Contamination
    • Gas risk
    • Groundwater risks
    • Geology/geotechnical properties (natural/man-made)
    • Existing site services
    • UXO’s (Unexploded Ordnances i.e. bombs!)
  • Uses a variety of techniques
  • Acquisition of environmental and geological data that may already exist
  • Review of historical maps (looking at previous uses as indicators of potential contamination)
  • May include a site visit/walk by an experienced/qualified engineer
  • Key part of the Desktop Study is the Conceptual Site Model (CSM), basically a Risk Assessment geared primarily to contaminated land, gas, and groundwater risks. This model is carried through the whole process right through to Stage 4 and is updated at every stage.

Phase 2: Site Investigation

The Desktop Study is a theoretical exercise, and the Site Investigation (SI) work builds on the information already established by the Desk Top Study.

The scope of the SI is dependent on the outcome of the Desktop Study and can range from simple trial pits with chemical sampling of soils only to installation of boreholes to monitor for gas and groundwater contamination. The results from the SI are then collated into a report (including the data from the Desktop Study and the initial Risk Assessment) which builds on the CSM (Conceptual Site Model) further and provides either an assessment of further works required (remediation) or concludes that the pollutant pathway has been broken or doesn’t exist in the first place.

The works may include the below (depending on the specific risks identified as requiring further review in the Desktop Study).

Utilities mapping

Shows the positioning and identification of buried pipes and cables beneath the ground. The procedure involves detecting things like sewers, electric cables, telecoms cables, gas, and water mains.

Contamination testing

Contamination testing is generally required as part of the planning approvals process when a site is being redeveloped, as part of the due diligence process during a land purchase, following a pollution incident, to characterise materials for waste disposal purposes or as part of a site condition or verification report.

Several types of contamination can be tested for including…

  • Asbestos – Presence and Identification, and Quantification analysis
  • Metals and Metalloids
  • Biological oxygen demand
  • Chemical oxygen demand
  • Phenols
  • Cyanides
  • Thiocyanates
  • Sulphates / Sulphides / Sulphur
  • Total organic carbon / Soil organic matter content
  • Mineral oils
  • Petroleum hydrocarbons: total, extractable, or speciated.
  • Polynuclear aromatic hydrocarbons (PAH’s)
  • Semi volatile organic compounds (SVOC’s)
  • Volatile organic compounds (VOC’s)
  • Polychlorinated Biphenyls (PCB’s)
  • Dioxin and Furan Analysis
  • NRA Leaching test
  • Calorific value
  • Nitrates, Nitrites
  • Pesticides
  • Waste Acceptance Criteria testing (WAC testing – see below)
  • Land Gas analysis
  • Topsoil analysis
  • Water Quality
  • Major and Minor Ions

Gas & Groundwater monitoring

Utilising data-loggers installed within boreholes to carry out continuous monitoring of groundwater levels.

Soakaway testing

A conventional soakaway comprises a partially perforated cylindrical chamber, which allows the drainage of water into the surrounding soil.

CBR testing

The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of natural ground, subgrades and base courses beneath new carriageway construction. It was developed by the California Department of Transportation before World War II.

WAC testing

WAC testing is used to determine how a waste will behave once it’s buried in a landfill. This is carried out primarily through analysis of leachate derived from that waste during laboratory analysis. It cannot be used to determine whether a waste is hazardous or not.

Plate Bearing testing

The Plate Bearing Test (or Plate Loading Test) is an insitu load bearing test of soil used for determining the ultimate bearing capacity of the ground and the likely settlement under a given load.

Dynamic probing

Dynamic probing involves driving a steel cone vertically into the ground using a sliding hammer and recording the number of blows for each 100mm of penetration. The results obtained from the dynamic probe tests can be correlated to the standard penetration test (SPT).

Used to characterise the strength of the ground and in the location of solution features, areas of made ground, soft deposits, tunnels and other underground voids and cavities. Light (DPL), Heavy (DPH) and Super Heavy (DPSH-B) testing.

Soakaway testing

The soak away test involves excavating a trial pit, filing the trial pit with water, recording the water level at specific time intervals in to establish the drainage rate, then repeating a total of three times in accordance with BRE 365 Soak away Design.

Phase 3: Remediation

If the Site Investigation report identifies unacceptable geotechnical, environmental or contamination risks which would make the development unsuitable for the proposed end usage, then remedial actions or control will be necessary to meet legislative requirements and satisfy planning conditions. The Remediation Statement is a site-specific document and represents a formal agreement between the developer and the regulatory authorities through the remediation process.

Remediation options include…

  • Bioremediation
  • Capping and encapsulation of contamination
  • Contamination treatment (such as soil washing)
  • Excavation, segregation of materials and validation for potential reuse
  • Formation of earth mounds or bunds
  • Ground gas protection methods
  • Installation of underground barriers (such as geotextile membranes)
  • Removal, disposal, and replacement of contaminated soils
  • Removal and treatment of contaminated water
  • Soil stabilisation

After the selection of remediation techniques, detailed plans for the verification of the remedial works are produced. Remedial target criteria state the levels of contamination that can remain on-site without posing an unacceptable risk to any receptors. Depending on the project, contingency plans are produced if the remediation proves unsuccessful or if unexpected contamination is encountered.

It is beneficial to consult the regulatory authorities at the early design stages of the Remediation Statement, as their involvement can streamline the acceptance of proposals and the following validation process.

Phase 4: Validation and verification

Validation works are carried out post-remediation to ensure that the works have been carried out to the satisfaction of the regulator and that no contamination is left on site or located beneath a capping layer as agreed with the local authority. Samples, photographs, drawings, and lorry tickets can be included as part of a report which allows the regulator to sign the remediation off, thereby ensuring that your project can progress as quickly as possible.


  • To reduce the risk you take with unforeseen ground conditions, the best solution would be to exclude this as a Contractor’s risk entirely.
  • Alternatively, stating that “only those conditions that can be reasonably foreseen from a visual inspection of the site have been included”.
  • Taking on board the full risk of unforeseen ground conditions would be unwise.
  • If it is necessary, then ensure that a thorough Site Investigation has taken place, that you understand the contents, its impact on risk and manage that risk accordingly.
  • Would be advisable, regardless of the level of information provided, to undertake your own site visit and risk assessment.
  • Use your supply chain to help review the any Site Information (e.g. any piling or groundworks contractor would be interested in and be able to comment upon the geotechnical information helping you understand the level of risk).
  • If reviewing the Site Information is outside of your comfort zone, engage the services of an expert to assist.

If you need further input/support in respect of Site investigations and ground risk, VOLOCO can assist with our team of Consultants and specialist advisors.

Site Investigation and ground risk header

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