Phase 2 Ground Investigation: Understanding This Critical Step in Development

Key Takeaways

• Phase 2 Ground Investigations are essential for identifying and addressing contamination and geotechnical risks, ensuring safe and compliant development.

• A variety of investigative methods, including trial pits and borehole drilling, gather data on soil and groundwater conditions to inform remediation strategies and development design.

• Early investigations help minimise construction costs and delays by proactively identifying issues, facilitating smoother project execution and compliance with regulations.

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The Importance of Phase 2 Ground Investigation

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Phase 2 Ground Investigations:

• Identify and confirm site contamination risks, ensuring they can be addressed before construction and allowing safe development.

• Uncover geotechnical risks and hidden dangers beneath the ground, ensuring these are factored into the development design.

• Help developers avoid costly delays and potential hazards that could compromise the project’s success.

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Phase 2 site investigations are often necessary to comply with Local Planning Authority conditions related to contamination. Site investigations should also consider geotechnical risks to ensure an appropriately robust design. Each site presents unique environmental and geotechnical conditions, necessitating a tailored investigation to assess and mitigate risks effectively.

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Phase 2 Ground Investigations provide the information needed to make informed decisions, allowing construction projects to proceed smoothly and safely. Without suitable investigations, developers risk encountering unidentified issues that could have significant negative impacts for the project

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Overview of Phase 2 Ground Investigation Methods

A variety of methods may be utilised during Phase 2 site investigations to gather data about the ground conditions. Trial pits, for example, are particularly effective for assessing shallow contamination. These pits can be created using a mechanical excavator and can reach depths of up to 4 meters, while shallow hand-dug pits may be useful on sites with limited accessibility.

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For deeper soil investigations, borehole drilling techniques are commonly used, with the type of borehole dependent on the investigation objectives and ground conditions. Small diameter boreholes drilled using a dynamic sampling rig (often referred to as window sampling) can reach up to 5 meters or more in depth. Such rigs are usually tracked, but demountable rigs and hand-held assemblies are also available where access is limited. When greater depths are required, cable percussion rigs can drill beyond 30 meters in suitable ground conditions.

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In challenging ground conditions, for example where rock or dense gravel is present, rotary drilling can be used to advance boreholes to depths of 100 meters or more.

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Both targeted and untargeted soil and water sampling are important components of a Phase 2 Intrusive Site Investigation. Targeted samples are required to assess specific sources of contamination, while untargeted sampling is used to characterise more extensive features, such as Made Ground or certain geological strata.

Careful consideration needs to be given to the sampling and analysis programme to allows areas of concern to be located and determine the extent of contamination.

 

Appropriate investigation methods collectively paint a detailed picture of the site’s condition. The information gathered informs the development design and the scope of any remediation strategy, ensuring the proposed development is safe, robust and compliant with relevant standards and regulations.

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Laboratory Analysis

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​​Laboratory testing is a key element of the Phase 2 site investigation process, used to supplement data obtained in the field. Laboratory analysis results should compliment site observations, providing quantification of contaminant levels in soils, groundwater and surface water to help identify the magnitude and extent of contamination. From this information, the significance of potential risks to human health and the environment can be determined.

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Laboratory testing is also key to determining the physical properties of the soil to inform the design of foundations, floor slabs, roads and drainage.

Samples collected during the investigation should be sent to laboratories with appropriate UKAS and / or MCERTS accreditation. Such laboratories have the expertise, equipment and quality control procedures necessary to provide accurate quantification of specific contaminants such as heavy metals, asbestos and organic compounds, or to determine physical characteristics such as plasticity, shear strength, moisture content and density.

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The results of chemical analyses are compared against appropriate assessment criteria to determine the level of risk. Understanding the nature and extent of actual contamination levels is the basis of effective contaminated land assessment and land contamination risk management.

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Physical test results are used to determine parameters such as bearing pressures, volume change potential and settlement characteristics, that are important factors in development design.

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Siteworks

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Siteworks are the foundation of intrusive ground investigations. Their design is based of the findings of the Phase 1 Desk Study and considers the anticipated ground conditions, potential contamination sources, current site layout, proposed development layout and physical constraints.

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Once on site, engineers gather physical evidence through an appropriate combination of investigation elements that may include trial pits, boreholes and standpipe installation, and facilitate the monitoring and sampling of soil, water, gas and vapour, as appropriate.

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Ensuring the scope of the intrusive investigations is appropriate to the specific site risks and the proposed development is essential to provide a robust foundation for later stages of evaluation and assessment. These in turn will result in recommendations to ensure, for example, the design of an appropriate remediation strategy, the incorporation of suitable mitigation measures, an appropriate foundation design and a viable drainage strategy.

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Developing a Conceptual Site Model

Developing a Conceptual Site Model (CSM) is a key element in the investigation process. This model visually and / or textually represents the site’s characteristics, illustrating potential interactions between contaminants, pathways, and receptors. It helps stakeholders understand the site’s condition and the risks it poses.

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The CSM is first developed during the Phase 1 Desk Study (Preliminary Risk Assessment) stage and is an iterative process, updated as new information is gathered. Sample results and detailed geological and hydrogeological data refine the model, addressing uncertainties related to ground conditions and contamination. This approach ensures the model remains relevant throughout the process.

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Ground Gas Monitoring and Assessment

Ground gas monitoring can be an important element of a Phase 2 site investigation on sites where a gas risk exists, such as those on or near a landfill. This process assesses the risk of potential gas emissions like methane and carbon dioxide, which can present human health risks and in some cases the risk of explosion.

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Monitoring standpipes are used to measure gas concentrations (primarily methane and carbon dioxide, but also hydrogen sulphide, carbon monoxide and organic vapours) and flow rates. Measurements may be in the form of 'spot' monitoring undertaken at a specific time, or continuous monitoring whereby changes in concentrations and flow rates are recorded over an extended period.

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Ground gas investigations make take anything from 6 weeks to several months, depending on site complexity and atmospheric conditions. The aim is generally to identify worst-case conditions, which are often associated with falling atmospheric pressure or rising groundwater.

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Reporting and Regulatory Compliance

The Phase 2 Site Investigation report provides a comprehensive summary of the site works, an assessment of the significance of contamination and design parameters based on geotechnical testing.

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Detailed descriptions of the investigation findings, clear conclusions, and suitable recommendations are all key elements of the report. Depending on the findings of the ground investigation, further testing and targeted investigation works (Phase 3) may be required to better delineate or quantify identified risks, or enough information may be available to recommend remediation works or ground improvement options (earthworks).

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Remediation Strategy

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