Pile foundation considerations for clay and sand

A pile foundation is made up of long columns or posts, which can be constructed using concrete, steel, or wood. These columns are either bored or driven into the ground to provide support for buildings. They are placed at a depth below the ground surface and are specifically designed to bear the load on solid ground, typically found beneath weaker soils.

Pile foundations can be used in a variety of applications, including buildings, bridges, wharfs, retaining walls, solar farms, and many other types of structures (for instance wind turbines of wind farms or large tanks).

Bridge pile foundations

With a thorough understanding of the soil conditions at a construction site, an informed decision on the appropriate type, design and construction of a piled foundation system can be made. This is particularly evident when looking at the differences in pile capacities in clay and sand – as these two types of soil can have significant impacts on the performance and stability of the foundation.

Differences between clay and sand

Clay and sand have very different physical properties. Clay is a fine-grained cohesive soil, which means that its small particles comprise minerals that mostly chemically interact between each other. For instance, clay softens with increased moisture content and hardens when drying. Stiff and stronger clay ground typically deforms slowly over time under a load. Strong clay soil can serve as a suitable base for placing piles, depending on the desired load they need to support. However, when the clay is wet and soft, it offers minimal resistance.

Clay soil

Sand, on the other hand, is a non-cohesive soil with notably coarser particles, compared to clay, that mostly interact using physical processes (rather than chemical) by friction and interlocking between sand particles. In a dense condition, following compaction if required, sand provides a strong bearing ground to support structures, and therefore would form a suitable bearing ground to end piles into.

Sand soil

What this means for pile foundations

A pile develops its capacity from two main components:

  • The ‘pile end-bearing capacity’ that develops beneath the base area of the pile during compression and shearing of the underlying soils under the pile load; and
  • the ‘pile shaft friction’ that develops along the pile shaft between the pile and the surrounding soils between ground surface and the toe (i.e., base) of the pile.

There are significant implications for the design and construction of a pile foundation system given the differences in physical properties between clay and sand. In general, sand has a greater bearing capacity than clay, both in ‘end bearing’ and ‘shaft friction’, resulting in greater pile capacities to support the load of a structure.

It is important to note, however, that the bearing capacity of both clay and sand can widely vary depending on the conditions of the sand and the clay. Pile performance also follows complex mechanisms that depend on the pile displacement relative to the surrounding ground, under the load applied by the structure to the pile.

Hand drawing Building foundation technical details

Why geotechnical consultation is necessary

It is important to understand the soil and ground conditions for pile design and construction, including any other factors that can affect the performance and stability of the foundation. By working with a geotechnical engineer and conducting thorough soil investigations and analyses, you can ensure that the foundation is designed and constructed to meet the specific needs and conditions of the site and proposed building or structure.

Get in contact with your local Douglas Partners team to discuss your project’s geotechnical needs here.