The art of ground engineering: Piling expert reveals buildings' hidden strength

Ground engineers (as well as soil engineers) use various techniques and methods to investigate a site's soil and rock conditions and design foundation structures appropriate for the site's specific geological and environmental conditions. One such structure, a pile foundation, involves driving long, slender columns (called piles) into the ground to support structures by transferring loads from the structure to the ground. But other foundation types can be used - depending on conditions and the type of building.

To dig deeper, Interesting Engineering (IE) reached out to Steve Hadley— Chair of the Federation of Piling Specialists and Managing Director of Central Piling.

Buildings with large loads require deep foundations 

Hadley clarified that understanding how soils behave is critical to practicing ground engineering. It involves developing solutions to retain soils as well as maintaining the structures built within and on top of them. 

"A good quality ground investigation is crucial for any successful ground engineering project," he said. 

To elaborate, a ground investigation is a thorough and accurate analysis of a site's soil, water, and rock conditions, which provides the data and information necessary for designing and building safe, stable, and durable structures. 

It involves comprehensive data collection methods, such as soil sampling, drilling, geophysical surveys, and laboratory testing, followed by data analysis and data interpretation by qualified professionals.

"Where the ground is strong close to the surface level, then for conventional low-rise structures such as a house, it may be possible to have a trench foundation as shallow as one meter deep," he said.

Trench-fill foundations typically involve excavating a narrow strip of soil, which is then filled with concrete to create a level base for the building. The width and depth of the trench depend on factors such as the size and weight of the building, the soil conditions, and the local building codes.

"A pile is effectively a column of timber, steel or concrete"

"However, for more complex conditions such as large vertical loads from a tower block, horizontal forces from wind, or tensile forces from clay heave (swelling), deeper foundation depths will be needed," Hadley explained. 

"The most efficient way to do this is via a pile. A pile is effectively a column of timber, steel or concrete in the ground created through driving of pre-formed units or the boring and filling of holes."

Hadley's company, Central Piling, is home to a fleet of piling rigs that drive piles in the ground in different ways, depending on factors such as the soil conditions, the size and weight of the pile, and the type of equipment used. 

One common approach is Continuous Flight Auger Piling (CFA). The CFA pile is created by rotating a hollow stem continuous flight auger into the soil to the necessary depth. Concrete or grout is pumped through the hollow center of the auger, maintaining static head pressure. The concrete fills the cylindrical cavity created as the auger is slowly removed, forming a column. A reinforcement cage is then inserted through the fresh concrete.

According to the Central Piling's site, CFA piling is a particularly economical choice of piled foundation in sands, gravels, and low-grade rock as this process requires no casings or drilling fluids.

Another advantage of CFA piles is that they create very little noise and vibration during installation, which makes them suitable for use in urban areas where noise and disruption are concerns.

After any pile is set in the ground, it is tested to ensure it meets the required load-bearing capacity. This is typically done using a static load test. This test applies a weight to the pile, and measurements are taken to determine its deflection and load-bearing capacity.

Weaker soils necessitate larger diameters or deeper piles

"In weaker soils, it will be necessary to have larger diameter or deeper piles to accommodate the pile loads when compared with stronger soils," Hadley said. 

IE then asked Hadley if he could provide insight on what would be considered a challenging piling project. His response was the development of retaining walls, commonly needed for temporary and permanent support for basement excavations. There are two common types of retaining walls: secant (where piles overlap) and contiguous (where there is a gap between piles).

"Retaining walls tend to be more complex, particularly where groundwater needs to be retained. This requires a high degree of precision with the pile installation to ensure overlap," Hadley told IE. 

"It's important to look at ways to mitigate risks in these circumstances," Hadley added.  

"Can the basement depth be reduced so it doesn't go below the water table? Can the groundwater table be lowered? How can the construction methodology be designed to ensure the overlap is maintained?" 

Groundwater investigation is critical for ground engineering

Before determining the ground's ability to support a given load, Hadley also emphasized the importance of understanding its strength and the state of the groundwater. 

Groundwater is the water found beneath the ground's surface in the spaces between soil particles, rocks, and other geological formations. Significantly, it can affect ground engineering projects in several ways.

For example, if the groundwater level is high, it can saturate the soil or rock and weaken its strength, leading to instability or settlement of foundations. This can cause structural damage, such as cracks, tilting, or even collapse.

High groundwater levels can also increase the risk of soil liquefaction during earthquakes in areas with loose or sandy soils. Soil liquefaction occurs when the soil loses strength and stiffness due to rapid shaking, causing it to behave like a liquid rather than a solid. This can lead to foundation failure and structural damage.

Conversely, suppose the groundwater level is too low. In that case, it can cause soil shrinkage, leading to foundation cracking and settling. This can often occur in areas with clay soils, which shrink and swell depending on the moisture content. 

Engineers typically perform a groundwater investigation as part of the site investigation process and consider groundwater conditions in a ground engineering project. This involves measuring the depth and flow of groundwater, analyzing its properties (such as pH, salinity, and temperature), and assessing its impact on soil and rock stability.

By doing this, engineers hope to ensure the structure's safety, stability, and durability over its lifespan.

The future of piling: how will it rise to sustainable heights?

"So long as we're building structures, there will be foundations. However, how they're built and what they're built from will change," expressed Hadley. 

He revealed that we should expect a future where foundations behave the same way and are designed similarly to today but installed using more sustainable materials and methods.

"On-site, machines will look quite similar but be powered differently, probably by hydrogen or e-fuels for larger equipment and electric for smaller machinery."

Additionally, the materials used will have a smaller carbon footprint with increasingly lower percentages of 'Ordinary Portland Cement' (OPC) used in concrete.

To clarify, OPC is the most commonly used type of cement, accounting for more than 80 percent of global cement production. It is the main component of concrete and is well known for its strength and durability, making it suitable for various construction applications, including buildings, bridges, dams, and roads. 

Manufacturing OPC involves heating a mixture of limestone, clay, and other materials in a kiln to form a hard, clinker-like material. This process can have negative environmental impacts due to the high energy consumption required and associated carbon emissions.

"Larger quantities of replacement products are being adopted," Hadley said. While he did not detail the nature of these products, sustainable efforts include using blended types of cement or alternative materials such as fly ash, slag, and silica fume. 

Additionally, there are efforts to improve cement production's energy efficiency and reduce carbon emissions by using renewable energy sources to power the kilns used, as well as adopting carbon capture and storage technologies.