FOUNDATION ENGINEERING

Why is site investigation important for pile design?

At Dawson Foundation Engineering, we are often asked what level of site investigation is required before a piled foundation can be designed.

A site investigation is a crucial part of pile design because it provides information about the ground beneath the proposed development. This allows the engineer and piling contractor to select a foundation solution that is safe, practical and appropriate for the site.

Without reliable ground information, it can be difficult to determine the correct pile type, diameter, depth or installation method. Unexpected ground conditions may also result in delays, design changes and additional costs once work has started.

What Is a Site Investigation?

A site investigation is carried out to establish the ground conditions beneath a proposed development.

Depending on the project, the investigation may provide information about:

Soil types and the sequence of ground layers
The strength and consistency of the ground
The depth of suitable bearing material
Groundwater levels
Made ground or previously disturbed material
Buried foundations, concrete, boulders or other obstructions
Weak, compressible or variable soils

This information helps establish whether conventional shallow foundations are suitable or whether a piled foundation is required.

1. Understanding the Ground Conditions

The first purpose of a site investigation is to understand what lies beneath the proposed structure.

Ground conditions can vary considerably, even across a relatively small site. One part of a development may contain firm natural ground, while another area may contain fill, disturbed material or weaker soils.

A site investigation can help determine:

The soil type, strength and bearing capacity
The thickness of weak or unsuitable ground
The depth of stronger load-bearing strata
The location of the groundwater table
The presence of buried obstructions
Whether the ground conditions vary across the proposed building footprint

Where a site contains fill, disturbed material or buried obstructions, a specialist foundation solution may be required. Read our guide to piling in made ground to learn how these conditions can affect pile selection, depth and design.

2. Selecting the Right Pile Type

There is no single piling method that is suitable for every site.

The ground investigation findings help the engineer determine which systems can safely and practically achieve the required foundation performance.

Potential piling methods may include:

For example, a site with suitable cohesive soils and sufficient access may be appropriate for CFA piling. A restricted-access or low-headroom site may require a sectional or mini-piling system. Ground containing cobbles, boulders, buried concrete or other obstructions may require a specialist drill-and-case method.

Choosing the correct pile type at the design stage helps avoid specifying an installation method that is unsuitable for the ground or impractical for the site.

3. Determining Pile Depth and Diameter

Pile depth and diameter cannot be selected using a standard specification for every development.

The required design will depend on:

The depth of suitable bearing strata
The strength and consistency of the soils
The structural loads being supported
The selected pile type
The required compressive, tensile or lateral capacity
The foundation layout and spacing of the piles

Where weak or unsuitable soils extend to greater depth, longer piles may be required to transfer the loads into stronger ground below.

Reliable ground information allows the pile length and diameter to be designed more accurately, reducing the likelihood of unnecessary material use or changes during installation.

4. Identifying Groundwater and Installation Risks

Groundwater can influence both pile selection and the installation process.

The investigation may identify:

The depth of the groundwater table
Water-bearing granular layers
Ground that may become unstable during drilling
Conditions that affect concreting or casing requirements

Understanding groundwater conditions in advance allows the piling method and installation sequence to be planned appropriately.

This is particularly important for bored piling systems, where bore stability and concrete placement must be carefully controlled.

5. Identifying Buried Obstructions

Previously developed sites may contain old foundations, buried slabs, concrete, masonry, cobbles, boulders or other obstructions.

These materials can prevent driven or augered piles from reaching the required depth and may result in refusal or damage to the installation equipment.

If the investigation identifies difficult or obstructed ground, a method such as overburden drill and case piling may be considered.

Our guide to ODEX piling explains how steel casing can be installed through difficult overburden before forming a pile within more competent material beneath.

6. Ensuring Structural Stability and Safety

The purpose of a piled foundation is to transfer the loads from the proposed structure safely into suitable ground.

A site investigation helps identify conditions that could affect the long-term performance of the foundation, including:

Weak or compressible soils
Variable ground conditions
Settlement risks
Ground heave
Made ground and fill
Groundwater
Changes in ground level across the site

This information allows the foundation to be designed around the actual site conditions rather than assumptions.

Correct pile selection and design help reduce the risk of excessive movement, differential settlement or foundation failure after construction.

7. Reducing Project Risks and Unexpected Costs

A suitable site investigation can reduce uncertainty before piling begins.

Without reliable ground information, the project may encounter:

Unexpected changes in pile length
Obstructions that prevent installation
The need to mobilise different piling equipment
Redesign of the foundation system
Additional concrete, steel or pile sections
Programme delays
Increased spoil-removal or disposal costs

Site investigation does involve an initial project cost, but it can help prevent significantly more expensive changes after construction work has started.

It also supports more accurate budgeting by allowing the piling contractor to prepare a quotation based on the likely pile type, quantity, depth and installation requirements.

Read more about the factors that influence piling costs.

8. Supporting Planning, Building Control and Warranty Requirements

Ground investigation information may also be required by structural engineers, building control teams and third-party warranty providers.

The available information helps demonstrate that the foundation design is based on the site conditions and that the proposed piling system is appropriate for the development.

Providing the relevant ground and structural information early can reduce the risk of queries, design revisions or approval delays later in the project.

What Information Does a Piling Contractor Need?

The information required will vary according to the project, but useful documents can include:

Ground investigation reports
Borehole or trial-pit information
Structural drawings
Foundation layouts
Structural load information
Existing and proposed site plans
Access and working-area details
Information about nearby buildings, trees or services

The more complete the available information, the easier it is to assess the likely piling requirements and prepare an appropriate proposal.

What Happens if There Is No Site Investigation?

Where no site investigation information is available, it may not be possible to design or accurately price a piled foundation with confidence.

Assumptions about the ground can lead to an unsuitable pile specification, inaccurate quantities or unexpected changes during installation.

In some cases, an initial site visit can help identify practical access and installation considerations. However, a visual inspection cannot establish the full ground profile or replace appropriate investigation.

The level of investigation should be proportionate to the size, complexity and risk of the project.

Site Investigation for Residential Projects

Ground investigation is not limited to large commercial developments.

Residential extensions and new-build properties can also encounter:

Made ground
Weak or variable soils
Historic foundations
Tree-related ground movement
Restricted access
Unexpected groundwater

Where traditional foundations are unsuitable, mini piling, screw piling or bottom driven piling may be considered.

Site Investigation for Commercial Developments

Commercial projects often involve larger structural loads, wider building footprints and more complex ground conditions.

The site investigation can help inform:

The selected piling method
Pile lengths and diameters
Pile layout and spacing
Working-platform requirements
Groundwater and bore-stability considerations
Obstruction risk
The construction programme and budget

Visit our Commercial Piling page for more information about our capabilities.

Discuss Your Ground Investigation and Piling Requirements

A suitable site investigation provides the information required to select, design and price a piled foundation with greater confidence.

At Dawson Foundation Engineering, we can review the available ground investigation information, structural drawings, load requirements and access constraints before recommending an appropriate piling solution.

If you are unsure whether the available information is sufficient, our team can advise on what may be required before the foundation design is finalised.

To discuss your project, call 0333 224 5196 or contact our team to arrange a free site visit and no-obligation quotation.