Generally, displacement piles are load-bearing columns designed to be installed without spoil material (soil that’s removed from the ground). Instead, special equipment is used to displace soil laterally, compacting it into the surrounding terrain. While not ideal for every situation, displacement piles can sometimes support higher capacities than alternative helical piles or bored piles, as more of the load is placed on the grout and dissipated evenly into the earth.
Over the years, commercial construction engineers have created dozens of displacement pile styles, all of which are either pre-cast or cast-in-place, and most are either driven, drilled or screwed into the ground.
The most common and generally accepted types of displacement piles fall into the following 4 categories:
Precast Driven Displacement Piles
Preformed concrete piles have wide application in a variety of soil conditions. Concrete pillars are hammered into the ground until enough friction or an end-bearing point can support the required capacity. Because of the enormous force and violent vibration on the pile during installation, pre-cast driven concrete piles are almost always pre-stressed or reinforced in some way.
Cast-in-place Driven Displacement Piles
Another form of driven pile requires large, hollow steel tubes to be driven into the ground, creating a void to be filled on-site with concrete. These tubes are capped on the nose to force the soil outwards in the same way a nail is driven into wood. For uncased piles, the tube is removed while the concrete is being poured, and reused to form each new grout column. In other situations, the tube is left underground, as a permanent casing for the pile.
Cast-in-place Drilled Displacement Piles
Drilled displacement piles are similar to fully bored piles, except without the spoils. Specially designed displacement tools are drilled into the ground and force soil outward, creating a more compact and stable column for the concrete. Once the drill has reached the appropriate depth, grout is pumped in continuously as the drill is extracted. This ensures the column stays intact and the grout has a chance to fill every available crevice. A rebar cage is also typically used to reinforce the remaining concrete column.
Cast-in-place Screw Displacement Piles
As the name suggests, these piles are installed similarly to the very popular screw pile. Permanent steel pipes are twisted into the ground with specially designed screw flight attachments which also displace the soil laterally. As the pile spirals its way down, concrete is continuously gravity fed into the voided column. Once the piles has achieved its required depth, it is left there as reinforcement to the concrete column. This method is particularly useful in areas with limited accessibility, as only small machinery is needed to install screw piles.
Need advice on the right types of piles for your job?
Displacement piles are a valuable tool in an engineer’s toolbox when devising structurally sound foundations. Which style of displacement pile will depend on the project, soil conditions, load bearing requirements and accessibility for installation.