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Before starting any construction project, ensuring that the foundation is properly designed in accordance with the relevant building design code is essential. Foundations are a critical part of the construction process as they transfer the building loads safely into the ground.

Some of most important building design codes for piled foundations in the UK are:

• BS 8004:2015 + A1:2020
• BS 8002:2015 – TC
• Eurocode 7 – Geotechnical Design
• Eurocode 2 – Design of Concrete Structures

These are augmented by a large number of best practice ‘special execution documents’, industry position papers & supplementary guidance as well as requirements for designing foundations This is to support sensitive structures such as roads or railway lines.

Another important document to be considered when designing or installing piled foundations is the recognised specification document for the UK – ICE Specification for Piling and Embedded Retaining Walls (SPERWall)

The Importance of Foundation Design Codes

The design codes in common use in the UK adopt a system of partial (safety) factors, applied to actions – building loads etc. and resistances – soil strength & resistances.

The partial factors adopted for use in the UK range from 1.0 to 2.0, and have been selected to ensure that, through a use of a prescribed combination of factors, the completed foundation is suitable remote from risk of failure.

The application of the Eurocodes, and the partial factors adopted, vary by country and typically cover a range of design considerations, including –

• Building Safety: Ensuring that the completed foundation is remote from the risk of failure, defined by –

Risk of collapse – the partial factors adopted within the design have been selected to ensure that, in so far as reasonably practical, the completed foundations will not suffer from a catastrophic failure.

Excessive settlement – the Eurocodes require consideration of foundation settlements under the applied loading conditions, to ensure that excessive movement doesn’t result in building defects.

• Structural Durability: Ensuring that the completed foundations are suitably robust to ensure consistent performance throughout their intended design life – design life in the UK typically tends to range between 50 & 120 years.

Types of Piling & Ground Improvement

Understanding the types of piling solutions and ground improvement solutions, and when they are used is the first step in understating and complying with the design code requirements – many of the commonly used piling solutions have their own special execution documents For example the special execution document for driven piles is –

BS EN 12699:2015 – TC, Execution of special geotechnical works. Displacement piles

Here are some of the piling and ground improvement types you will see in construction:

1. CFA Piles: CFA Piles have become one of the more popular methods of piling in the UK because of their versatility. Modern CFA rigs have the capacity to form piles in Very Soft Soils through to Weak Rock formations.
2. Driven Piles: Driven Piles are most commonly used in Soft Soil conditions, and are particularly advantageous on sites with contaminated soils as displacement piling methods avoid the generation of spoil arisings.
3. ODEX Piles: ODEX Piles are used when the ground situation is a lot more challenging, with shallow competent rock, existing unreinforced foundations, or the presence of boulders for example.
4. Vibro Stone Columns: using Vibro Stone Columns in lieu of traditional piling methods can offer significant cost, time and environmental benefits. The system can be used in a wide range of ground conditions, and across a wide range of structural types.

Key Requirements for Successful Foundation Construction

1. Site Investigation

Before construction begins, a thorough site investigation is required. This will determine the ground and ground water conditions to a suitable depth below site level, with a range of test methods available to characterise the soil strength and stiffness.

A specialist site investigation contractor will carry out this site investigation, typically comprising an initial desk study followed by a site based intrusive investigation including trial pits and boreholes.

The site investigation specialist will recommend a suitable scope of investigation, based on their prior knowledge of the ground conditions in the area. We would also recommend getting a piling and ground improvement contractor involved in specifying the scope of site investigation, to ensure the information gathered is sufficient to meet the design requirements.

A note of caution – opting for the cheapest possible site investigation, resulting in poor characterisation of the ground conditions can result in significant costs and delays during the foundation construction work. If you are unsure, please get in touch with us for free, no obligation advice.

2. Design Requirements

The design of the foundation must be in accordance with the Eurocode requirements, and any other relevant design codes and guidance documents.

The starting point is to employ a Structural Engineer who can accurately assess the loads transferred from the building to the foundations. The loads calculated by the Structural Engineer can then be used to develop the foundation design, resulting ultimately in a schedule of loads, or bearing pressure requirements that can be passed on to the Piling Contractor or Ground Improvement Contractor.

The design prepared by the contractor should ensure that the piles or improved soil can safely support the loads, without risk of collapse or excessive settlement.

3. Material Specifications

The various Eurocodes, special execution documents and SPERWall specify the types of materials that can be used for building foundations. Common materials used on GeMech Foundations Limited projects include:

• Ready Mixed Concrete
• Non-shrink Construction Grout
• High Strength Reinforcement Steel
• Structural Steel Sections
• Graded Aggregates

The relevant documents for each material outline the minimum strength requirements and standards for mixing, curing, and testing of these materials to ensure they meet the necessary specifications for strength and durability.

4. Construction Practices

The construction of piled foundations, retaining walls and ground improvement work must be carried out in accordance with the requirements of the Eurocode special execution documents, SPERWall, and any site specific requirements, including the Structural Engineers Specification document.

The piling or ground improvement contractor should prepare a design document, a quality method statement and an inspection and test plan, all combining to identify how they propose to ensure the work is completed in accordance with the specification requirements.

6. Inspections and Testing

Inspection and testing methods for piling and ground improvement work vary significantly across methods, and the requirements often change on a site by site basis.

Guidance on appropriate regimes of testing are provided in the various Eurocode documents and SPERWall. Industry organisations such as the Federation of Piling Specialists (FPS) have issued a position paper on behalf of their members, entitled the ‘Handbook on Pile Load Testing’, detailing appropriate test methods and regimes of testing based on project size and piling method.

A critical consideration when determining the inspection and testing regime for piling or ground improvement works are the specific requirements of any building warranty provider with an active involvement in the scheme.

The testing and inspection requirements of building warranty providers are often extremely onerous with respect to piling and ground improvement work and can occasionally outweigh the building design code or Structural Engineers own specification requirements.

We would recommend that the foundation design package, containing details of the proposed testing regime, is presented to the building warranty provider at least two weeks prior to any work starting on site, to ensure that the proposals meet with their needs.

GeMech specialists and our fleet were down on site in Weston-Super-Mare for repeat client Curo Group, installing over 400nr. 300mm diameter CFA augured piles for housing plots on the former RAF Locking site. Using the versatile GEAX EK110, we are installing piles in to Mercia Mudstone from 4.5m depth and terminating at a maximum depth of 6.0m. Whilst the geology around Weston-Super-Mare generally is suited for a driven piled solution to be used, the ground conditions on this site called for an augured piling method.

We are pleased to be working again with Curo Group, providing a robust and cost effective solution.

We’re installing steel cased bottom driven piles inside an existing warehouse in Avonmouth. With restricted headroom of between 6.0m – 5.5m, the mast on our Vermeer HL-2500 is able to reduce down to allow it to work in tight access sites without compromise on performance. A total of 107nr. 220mm steel casings driven to 12.0m with refusal on the Mercia Mudstone.