Site Investigations: The Lowdown on Scope, Methods, and Risk Reduction

Key Points to Remember

• A site investigation gives you a clear picture of the ground conditions and any potential risks for a particular site.

• Most of the time you'll need an investigation for foundations, drainage, earthworks and getting planning permission.

• The process usually involves a desk study, a site walkover, and some form of intrusive investigation if needed.

• You've got to be aware of the risks; these include weak ground, water rising up from the ground, contamination, gas risk and slope instability.

• Clearer data from investigations helps with budget certainty and reduces the risk of delays.

• Designers can use the results of the tests to pick a foundation design and earthworks strategy that will work.

• Be aware that all investigations have some limitations because they can only cover so much and you have to make some assumptions.

What Does a Site Investigation Typically Involve

1. What you find from the desk study and the site walkover\

2. Identifying the soil types and made ground\

3. The risk of water rising up from the ground and how that might change with the seasons\

4. Making a judgement on bearing capacity and settlement\

5. A contamination assessment, including getting a sampling plan sorted\

6. Any signs of gas - like methane or carbon dioxide\

7. Whether the earthworks are suitable, including any cut and fill considerations\

8. The risks related to slope stability and any implications for retaining walls\

9. Recommendations for foundation design and drainage strategy\

10. Any construction constraints and ideas on how to get around them

What Affects the Scope of the Investigation

• The kind of building, how much load it's going to bear and what it's laid out like\

• The design options for the foundations and how sensitive they are to settlement\

• If there's any made ground or if the land has been used for something else in the past\

• How deep the water table is and how much drainage you need\

• Whether there's any need for a contamination assessment and what the risk level is\

• The programme risk and how that affects the construction sequencing\

• Any nearby buildings, services or limitations with access to the site

Site Investigation Techniques

Non-Intrusive Methods

• Desk study – getting a history of the site from past records and maps and so on.\

• Site walkover – taking a good look at the surface conditions to see if there are any issues\

• Geophysical surveys – can give you an idea of what's going on under the ground, like if there are any hidden voids or underground structures.

Intrusive Methods

• Boreholes – you can use these to get a deep look at the soil and see how it's structured\

• Trial pits – if you can't use boreholes, trial pits are a good way to get a look at the soil and shallow foundations\

• Window sampling – a way of getting small samples of the soil if you can't access the whole area\

• In-situ testing – getting measurements in the ground to support your assumptions about strength and stiffness\

• Laboratory testing – getting a closer look at properties like moisture content, plasticity and contamination levels

Types of Ground Risks You Can Identify Through Investigation

Typical ground risks and what can go wrong

• Weak ground = increased settlement and a need to redesign the foundations\

• Made ground = unpredictable bearing capacity and potential obstructions\

• Water rising up from the ground = need for dewatering, excavation risks and drainage issues\

• Contamination = need for remediation, disposal controls and programme delays\

• Gas risk = need for protective measures and ventilation\

• Slope instability = need for retaining solutions and higher construction costs.

When to Get a Site Investigation Done

• Before you choose a foundation design strategy\

• Before you finalise your budget and cost plans\

• Before you commit to earthworks and drainage concepts\

• If you're looking to buy a site that has a higher risk of being contaminated\

• If there's a major refurbishment or change of use planned\

• When pricing the works and ground risk is a big factor

Things That Are Often Misunderstood

• Just because the ground next door is safe, that doesn't mean the ground on your site is too\

• A site investigation can't guarantee that there won't be any problems - it just reduces the uncertainty and makes things less of a risk.

The Bottom Line

A site investigation is an important risk management step because it helps you understand the ground conditions and pinpoints any issues that might mess with design, cost and programme. Doing it early on, and not overdoing it, helps reduce the risk of problems during construction and avoids the need for costly redesigns.

Timing and Phasing

Investigations are often phased because early-stage information guides later work. A preliminary investigation may support feasibility and planning. A detailed investigation may support final foundation design and construction sequencing. Phasing helps avoid over-investigation early while still controlling risk.

Early investigations can identify “red flag” risks such as contamination, high groundwater, or thick made ground. Detailed investigations then refine parameters for structural calculations and earthworks design. This sequence supports better programme control.

When to commission a site investigation

• before selecting a foundation design strategy
• before finalising budget cost plans
• before committing to earthworks and drainage concepts
• before purchase on higher-risk or brownfield sites
• before major refurbishment or change of use
• before pricing works where ground risk drives cost

Phasing works best when teams understand common misconceptions.

Common Misconceptions

“Nearby sites are the same” is a common misconception. Ground conditions can change over short distances, especially in made ground or variable geology. A desk study cannot confirm bearing capacity or contamination levels on its own. A site can look flat and still contain weak layers, high groundwater, or buried obstructions.

“An investigation guarantees no problems” is another misconception. A site investigation reduces uncertainty, but it cannot remove all risk because sampling coverage is limited. The correct goal is risk identification and proportionate mitigation.

These clarifications support better decision-making and more realistic expectations.

Conclusion

A site investigation is a key risk management step because it confirms ground conditions and identifies issues that can affect design, cost, and programme. The findings support foundation design, drainage planning, earthworks strategy, and contamination remediation where needed. Early and proportionate investigations reduce construction phase risk and avoid late redesign. Related topics often include geotechnical engineering, foundation design, and drainage strategy, which build on the evidence produced by a site investigation.