Drilled Displacement (DD) Piles or Auger Screw Piles : Features & Construction

Auger screw piles are a special type of full-displacement pile (DP), which are also called augered displacement or drilled displacement piles (DD) in the USA.

Auger screw piles are piles formed by drilling into the ground by displacing the soil until it reaches the desired pile depth and filling the void created using grout or concrete.

Fig.1. Construction Steps of a Screw or Drilled Displacement Pile - Types: Atlas Pile

 
Screw piles involve specially designed helical segments drilled into the ground in a rotary direction. The helical segment is drilled with both vertical force and torque.

Note: Helical piles or Helical screw piles are single or multiplex-helix steel auger piles, that are screwed into the ground to form a pile. It must not be confused with screw piles.

Features of Auger Screw Piles or Drilled Displacement (DD) Piles

Auger screw piles are a type derived from auger piling technology. Hence, it can be considered a classification of auger piles. For more information, the nomenclature used for auger piles employed in Europe and the USA is given in Figure 2.

Fig.2. Nomenclature for Auger Piles in Europe and The USA

The screw piles are hence mentioned as augered displacement piles. The soil displacement during the installation of the piles is dependent on the type of auger piling equipment. Based on the type of auger flight taken, it can be partial-displacement or full-displacement.

Auger screw Piles are full-displacement Piles. Compared to partial displacement or no-displacement piles, full-displacement piles carry larger loads.

Components of Screw Piling or Drilled Displacement (DD) Technology

Auger screw piles essentially follow DD technology that primarily consists of a drilling tool.

Fig.3. Components of a Screw Pile or Drilled Displacement Pile

In general, the drilling tool for the DD pile has the following components:

1. A soil displacement body: This is an enlarged diameter section of the tool that facilitates the lateral movement of the soil.
2. A helical auger segment
3. A sacrificial tip at the bottom of the drilling tool
4. A casing/mandrel: This is connected to the drilling tool; having a diameter smaller or equal to the pile diameter as shown in Figure 3.

The degree of soil displacement during the DD pile installation can be partial to full-displacement, depending on the:

1. Type of drilling tool
2. Type of piling rig technology

Construction of Drilled Displacement Piles/ Auger Screw Piles

1. The soil is displaced using the DD piling rigs with high torque in the range of 150 kNm to 500 kNm. They also provide vertical thrust during the drilling process.
2. The drilling process proceeds due to the rotation of the drilling tools and axial force / "crowd force" typically applied in hydraulic rams.
3. Once the drilling tool reaches the desired pile depth, the sacrificial is released.
4. Now, concrete or grout is placed in the pile hole through the casing as the drilling tool and casing is extracted from the ground.
5. A reinforcement cage can be placed before placing the concrete, as per the requirement.

Types of Auger Screw Piles / Drilled Displacement Piles

Based on the different drilling tools used, different DD piles can be obtained. They are:

• APGD
• Atlas
• De Waal
• Fundex
• Olivier
• Omega
• SVB
• SVV

Fig.4. Drilling Tools for Screw Piles or DD Piles

From the above types, a Fundex pile gives a nearly smooth pile shaft, as the casing is withdrawn with alternating 180 degrees clockwise and counter-clockwise rotations. APGD, De Wall, and Omega Piles give a nearly smooth pile shaft, as they rotate the drilling tool clockwise while withdrawing the casing from the ground.

Atlas and Olivier piles give a screw-shaped shaft, as they rotate the drilling tool counter-clockwise during the withdrawal.

Benefits of Auger Screw Piles or Drilled Displacement Piles

The main benefits of screw piles are:

1. Enable pile construction with minimal vibration or noise, compared to driven piles (which involves hammer piling or driven piling).
2. Minimal spoil during construction; contaminated site construction
3. High-load carrying capacity, as the soil around the pile is partial/fully displaced.

Also Read: What are Displacement Piles?