The Benefit of Proper Site Investigations

Site investigations are a crucial component of any civil engineering project. Yet all too often, companies spend insufficient time and effort on carrying them out: it is estimated that around 80% of problems discovered on construction projects are attributable to unexpected ground conditions that would have been spotted and avoided had an adequate site investigation been carried out.

So what value can proper site investigations offer? What stages are involved? And which areas should you pay particular attention to in order to maximise your return on investment? Discover expert answers to these questions and many more in this guide written by our Global Application Technology Manager, Andrew Lees.

Read on to learn more about:

• What is a site investigation?
• What are the stages of site investigation?
• Types of site investigation
• The benefits of a proper site investigation
• What does a basic site investigation involve?
• What are the three main considerations in site investigation?
• Why are site investigations important for Tensar applications?
• What happens after the site investigation?

Tensar offers a range of geotechnical engineering solutions that can help you reduce costs and make sustainability gains on your next project, with diverse applications across the construction of roads, railways, airports, and buildings. Contact our team to find more about the return on investment our solutions could deliver for you.

What is a site investigation?

A site investigation, also known as a geotechnical investigation, is the process of collecting and assessing data relating to potential hazards below a construction site. It will typically involve soil analysis and a review of geological survey maps to inform the construction design.

What are the stages of a site investigation?

The full procedure for a thorough site investigation includes the following stages:

Phase 1: Desk study

The first phase is a preliminary risk assessment report for the site. This step includes geological survey maps, rainfall data, historical information, and previous construction plans. The goal is to identify potential pollutants or risks that can be prioritised for the next stage. 

Phase 2: Intrusive site investigation

Phase 2 includes quantitative risk assessment. Intrusive techniques are being carried out, such as Rock Coring, Rotary Boreholes, Rotary Core Drilling, ground condition assessment, above-ground pipework, and evidence of infilling or subsidence. 

Phase 3: Post site investigations

In this phase, the results from the preliminary site investigation are analysed. Further site investigations might be required, such as:

• Detailed site investigation
• Supplementary investigation and construction control
• Soil investigation
• Environmental monitoring
• Slope stability assessment.

Phase 4: Site investigations report

From all the information collected, an interpretive report will be compiled. 

Types of site investigation

There are four main types of site investigations:

• Desk studies: This is an initial research that includes using data, maps, historical records, and photos to gather information. 
• Field investigation: This type includes geotechnical, geophysical surveys and groundwater monitoring.
• Laboratory testing: Using soil, rock and water samples from the site, investigators then evaluate the properties of strength, density and so on. 
• Environmental assessments: This includes examining the influence on nearby locations, evaluating ecosystems, and looking for contaminants.

The benefits of a proper site investigation

Prof. Stuart Littlejohn’s 1991 quotation, 'You pay for a site investigation whether you have one or not,' is still often cited. His words remain equally true and insightful today. They generally emphasise the inevitable cost and consequences to a client from an inadequate site investigation. 

He was, however, making the point that the costs of ‘unforeseen circumstances’ were initially being paid by insurance companies, resulting in higher premiums for all. Thus everyone paid for those who had made the decision not to have a thorough site investigation.

Littlejohn was commenting on a report by the Institution of Civil Engineers titled ‘Inadequate Site Investigation’. In the thirty years that have followed, much has been written about the need to mitigate risks and associated costs by conducting appropriate site investigations in a timely manner. A thorough and expert site investigation makes unexpected ground problems during the building phase less likely. But the advantages of site investigation extend beyond this.

Minimising project costs and time

Let’s take a realistic example. If a more detailed site investigation had been carried out, which defined the locations of the weaker soil areas, the project designer could vary the design profile according to the actual subgrade strength. The stabilised layer thickness would then vary from as little as 250 mm to up to 550 mm, with only limited areas at up to 550mm. By reducing the stabilised layer thickness, the volume of soil excavated and removed, the volume of quarried aggregate, and the volume of asphalt would all be reduced. 

The savings in imported aggregate would be considerable, reducing construction costs and time and easily covering the additional site investigation costs. Additionally, the reduction in imported aggregate has other cost and environmental benefits: transportation costs are reduced, disruption and damage to local roads are reduced, and energy and construction carbon costs are significantly reduced.

Adequate site investigations increase sustainability

With detailed and reliable information on ground conditions, engineers can be more nuanced in their designs, eliminating any unnecessary conservatism that would inflate construction costs and waste energy and natural resources. The road to Net Zero and other sustainability imperatives means that, politically, it is essential to consider these elements and minimise the global impact of construction. 

Every engineer is responsible for addressing the environmental and economic costs of construction, so designs need to be smarter and more refined. To do this, we need to start with an adequate site investigation.

What does a basic site investigation involve?

Let us consider a project on a site deemed to be low risk, geotechnically: an access road to enable the construction and future servicing of a wind farm. The loads are well-defined, and it’s a greenfield site with no previous development so that no complications would be expected. A geotechnical site investigation could be carried out, focused on the turbine locations with just one or two trial pits along the road alignment. 

Subgrade would probably be variable, let’s say we obtained strengths of 1.5% and 5.0% CBR. It would be possible to design an access road using this information, but with this basic detail, the designer would need to take a conservative view on the subgrade. The resulting design might be a 900mm thick granular layer, or perhaps 550mm layer with stabilisation geogrid incorporated.

What are the three main considerations in site investigation?

Safety, cost efficiency and legal compliance are the main considerations in the process:

• Safety: A site investigation must identify necessary information and point out potential hazards that can affect the stability of the structure. 
• Cost: The information obtained from the report can be used to make informed decisions and effective project plans, preventing any redesign or delays during construction time. 
• Legal compliance: Environmental guidelines, construction standards, local regulations must be aligned. Examples are: Construction, Design and Management Regulations (CDM), BS 1377 (1990) accreditation for soil testing and BS 5930 (1999) standards.

British Standards for site investigations

Other standards that need to be adhered to for the site works and reporting include:

• BSI (2018) BS 1377-3:2018+A1:2021 – Methods of test for Soils for Civil Engineering Purposes – Chemical and electro-chemical testing – British Standard Institution
• BSI (2015) BS 5930:2015+A1:2020 – Code of Practice for Ground Investigation
• BSI (2017) BS 10175:2011+A2:2017 – Investigation of Potentially Contaminated Sites- Code of Practice 
• BSI (2007) BS EN1997-2:2007 – Geotechnical Design: Part 2 Ground Investigation and Testing 
• BS 8004:2015+A1:2020 – Code of practice for foundations.

Why are site investigations important for Tensar applications?

An adequate site investigation allows designers to accurately calculate subgrade strength, with defined locations of weaker areas. This is essential to ensure that a safe, efficient, cost-effective design is achieved. A Tensar geogrid can be incorporated to improve soil strength and further reduce design and maintenance costs.

What happens after a site investigation?

After the four phases of site investigation, with a detailed report written up, the project team will meet to discuss the results and take action. Remediation design, foundation design, surface water drainage planning and earthwork modelling could all be potential next steps. 

Visit our geotechnical engineering solutions page to learn more about our product offering, and see our applications page for information on how we can support your construction project. With the help of our subgrade stabilisation software Tensar+, a Tensar geogrid can be incorporated to improve soil strength and further reduce design and maintenance costs.

Get in touch to discover more about how we can help on your next project.