Why ground investigation is just as important for driven piling as it is for bored methods
There is sometimes an assumption that because driven piling is a more robust method in challenging ground conditions, the ground investigation matters less. This is not the case. A thorough ground investigation is just as important for SCD driven piling as it is for any other foundation method, and in some respects it is more important because the design decisions that depend on it, including the set criterion, the casing specification, and the driving resistance profile, are all calculated before installation begins and need to be based on reliable data. Getting those calculations wrong because the investigation was inadequate creates problems during installation that are more disruptive and costly to resolve than the investigation itself would have been to commission properly.
What the ground investigation needs to establish for SCD driven piling
The investigation needs to identify the soil types present at the site, their depths and thicknesses, their strength and density characteristics, and the groundwater conditions. For driven piling specifically it also needs to identify any layers that could cause driving refusal before the design depth is reached, any layers where relaxation after driving is a known risk, and any chemical conditions that could affect the concrete specification or the steel casing over its design life. The investigation should extend to a depth of at least one and a half times the anticipated pile length to ensure the founding conditions are properly characterised and that no unexpected layers are present below the design depth that could affect pile performance.
How ground investigation data is used in SCD driven pile design
The ground investigation produces a factual record of the ground conditions at the site, including soil descriptions, laboratory test results, in-situ test data, and groundwater observations. This factual data is interpreted by a geotechnical engineer, whose findings provide the soil parameters the structural engineer needs to calculate pile capacity and derive the set criterion.
In granular soils, SPT N values and CPT cone resistance values from the investigation are used to estimate shaft friction and end bearing capacity and to predict the driving resistance profile the casing should encounter as it advances through each soil layer. This predicted profile is compared to the actual driving resistance recorded during installation, and significant deviations from the prediction are investigated before the programme continues.
In cohesive soils, undrained shear strength values are used to calculate shaft friction capacity along the pile length. Where the investigation reveals interbedded layers of cohesive and granular material, the design needs to account for the contribution of each layer to the overall pile capacity and the driving resistance profile needs to reflect the varying resistance the casing will encounter as it passes through different materials.
When ground investigation is most critical for driven piling projects
Ground investigation is critical on every driven piling project, but it is particularly important on sites where the ground conditions are known or suspected to be variable or where the below ground history of the site introduces uncertainty about what is present. Brownfield sites, former industrial land, riverside and waterside development plots, and sites with a history of multiple phases of development are all locations where a thorough and well designed investigation is essential before any driven piling design can be reliably produced.
On sites where previous development has left buried structures, old foundations, or other obstructions below ground, the investigation needs to be designed to identify these as far as possible before piling begins. Capital Piling recommends that clients commission their ground investigation before any piling design work starts, and we are happy to advise on the scope and specification of the investigation required for a specific site if that support is needed at the early stages of a project.
The specific investigation methods and outputs most relevant to SCD driven piling
Cable percussion boreholes are the most commonly used investigation method for driven piling projects in the UK, providing continuous soil sampling and SPT N value data throughout the borehole depth. Rotary boreholes are used where harder materials are encountered or where rock needs to be characterised below the expected pile tip level. CPT testing, in which a cone is pushed into the ground at a controlled rate and the resistance is measured continuously, provides a higher resolution profile of ground resistance than SPT testing and is particularly useful in granular soils where the variation in density with depth is important to the pile design.
Chemical testing of soil and groundwater samples is required to assess the aggressiveness of the ground towards concrete and steel. Where elevated sulphate levels are identified, the concrete mix must be specified accordingly. Where the steel casing is to remain in the ground permanently, the potential for corrosion needs to be assessed and the casing specification adjusted if necessary to ensure adequate performance over the design life of the pile.
What happens when the ground investigation is inadequate for driven piling
Inadequate ground investigation for driven piling tends to manifest as problems during installation rather than in the design office. Unexpected hard layers cause driving refusal before the design depth is reached, requiring a reassessment of the pile design and in some cases a change of method mid programme. Unidentified soft layers below the design depth result in piles that reach the set criterion at a shallower depth than expected, raising questions about whether the founding conditions are as assumed. Uncharacterised obstructions cause casing damage or programme delays that could have been avoided with a more thorough investigation.
Each of these outcomes costs more to deal with on site than a properly scoped investigation would have cost upfront. The ground investigation is not an optional preliminary. It is the document that makes a reliable driven pile design possible, and its quality directly determines the reliability of the design that is built from it. If you are unsure whether your ground investigation is sufficient for a driven piling design, it is a question worth raising with your geotechnical engineer and piling contractor before any design work is committed to.
