By Antonio Marinucci and Marco Chiarabelli
Displacement piles (DP) are performed without soil removal, thanks to drilling tools that displace the soil laterally toward the bore walls. On completion of boring operation, the drilling string and the tool are extracted while concrete is poured throughout the internal hollow pipe. The steel reinforcement cage is finally lowered into the fresh concrete.
There are different methods for performing displacement piles: De Waal, Fundex, Omega, Berkel (APGD), and Trevi Discrepile.
De Waal Pile
Figure 1 — Installation of the De Waal pile
During the drilling phase, the De Waal displacement tool (Figure 1) is advanced downward into the formation using clockwise rotation and crowd force. Once the desired depth is reached, concrete is placed into the hollow stem of the drill string to a prescribed distance above the ground surface (i.e., head), and then the sacrificial tip at the bottom of the tool is released. The tool and drill string are extracted using clockwise rotation while maintaining a constant head of concrete within the hollow stem, resulting in a relatively smooth surface. The steel reinforcement is typically inserted into the borehole after the concrete has been placed but while it is still fresh. In some instances, the steel reinforcement can be inserted into the hollow stem prior to the placement of concrete. Typical diameters achievable with this technique range from about 300 to 600 mm (12- to 24-inches), and to a maximum depth of about 25 m (82 ft).
Figure 2 — Installation of the Fundex pile
For Fundex displacement piles, the tool with a conical auger tip (Figure 2) is advanced downward into the formation with clockwise rotation and crowd force, thereby displacing the soil radially outward. Once the desired depth is reached, steel reinforcement is inserted into the hollow stem, the sacrificial drilling tip, which forms the enlarged pile bottom, is released, and concrete is placed into the hollow stem. The tool/drill string are then extracted using an oscillating up-and-down motion along with a 180° clockwise-counterclockwise rotation, while ensuring the concrete is maintained at desired level within casing. The withdrawal process produces a nearly smooth shaft. The possible diameters and lengths for Fundex piles range from about 450 to 675 mm (17.5- to 26.5-inches) and to a maximum depth of about 35 m (115 ft).
Figure 3 — Installation of the Fundex pile
As shown in Figure 3, the diameter of the flange along the length of the Omega tool and partial auger flights increases gradually and similarly from both ends of the tool toward the displacement body (with maximum diameter). For displacement piles constructed using this method, the tool (with a varying diameter) and drill string are advanced downward into the formation using clockwise rotation and crowd force. Once the desired depth has been reached, concrete is injected under pressure and the sacrificial tip is released. During the extraction of the tool while maintaining a clockwise rotation, concrete is injected under pressure until the tool and drill string are fully extracted from the borehole. The reinforcement cage is inserted (assisted by vibratory means and/or downward force) down into the fresh concrete. For some Omega piles, it is possible to place the steel reinforcement (e.g., cage or bar) into the drilling stem prior to the placement of concrete.
Berkel (APGD) Pile
Figure 4 — Installation of the Berkel’s APGD pile
For the Berkel Auger Pressure Grouted Displacement (APGD) pile, the tool (Figure 4) is advanced downward into the formation with clockwise rotation and crowd force. Once the desired depth is reached, high-strength grout is injected under pressure through the hollow stem of the drill string. Once the initial target pressure is achieved, the extraction of the tool maintaining a clockwise rotation and grouting of the borehole is initiated. Pressurized grouting is continued along clockwise rotation of the tool until the tool and drill string are fully extracted from the borehole. After the tool is removed from the borehole and while the grout is still fresh, the steel reinforcement is inserted in the grouted hole. Essentially, two types of APGD piles can be constructed: (1) partial displacement piles in loose to dense sands (with N<50) where the diameter ranges from 300 to 500 mm (12- to 20-inches) and to about 17 m (80 ft) in length, and (2) full displacement piles in loose-to-medium dense sands (corresponding to SPT blow count N<25) where the diameter can range from 300 to 450 mm (12- to 18-inches) and to about 24 m (79 ft) in length (NeSmith, 2002).
Figure 5: Different types of Soilmec Discrepile Tooling
The cylindrical (“Cilindrico”) displacement tool shown in Figure 5A is well suited for soft ground conditions: loose to medium-dense sands, soft clays, and organic soils. The conical (“Conico”) displacement tool is shown in Figure 5C, and is well suited for stronger ground conditions: medium-dense to dense sands and stiff clays. The conical tool is modular, and is composed of four primary sections:
(1) a drilling tip fitted with teeth appropriate for the soil conditions being drilled.
(2) a lower section with right-handed partial auger flights that move the soil upward toward the displacement body.
(3) a central cylindrical body that stabilizes and displaces the soil radially outward thereby producing the actual pile diameter.
(4) an upper section with left-hand partial auger flights that move the soil above the tool downward toward the displacement body.
Different pitch lengths can be used on the auger flighting depending upon the anticipated soil conditions:
(1) a short pitch is preferable for very loose and fine sands, soft clays, and organic soils
(2) a medium pitch is suitable for medium-dense sands and medium clays.
(3) a long pitch is preferable for medium-dense to dense cohesionless soils, medium to stiff clay, and sandy gravel.
Cylindrical and conical displacement tools are able to form boreholes with diameters ranging from about 350 to 600 mm (13.5- to 26.5-inches).
The eccentric roller (“Pirucca”) displacement tool shown in Figure 5c is ideally suited for soft to medium ground conditions: loose to medium-dense sands, soft clays, and organic soils. Given the eccentric nature of the tool, only a portion of the roller is in contact with the soil at a time, while the remaining main portion of the tool/drill string is away from the wall, thereby decreasing the frictional resistance acting on the tool/drill string. As a result, less torque is needed to rotate and advance the tooling/drill string, which allows the use of smaller/lighter drill rigs (lower operational and transport costs). Soil that is not compacted into the borehole by the eccentric roller is moved downward toward the displacement body by the downward spiraling auger flights. Eccentric-type roller tools are able to form boreholes with diameters ranging from about 450 to 600 mm (17.5- to 26.5-inches).
The requirements for proper selection of a drill rig capable of constructing displacement piles using the Discrepile tools include:
(a) a rotary head capable of delivering rotation at about 20 to 25 rpm.
(b) a rotary head capable of delivering at least 200 to 250 kN-m (147,000 to 185,000 ft-lbs) of torque.
(c) a pull down system with a crowd force of at least 200 kN (45,000 lb).
(d) a pull-up system capable of providing an extraction force of at least 200 kN (45,000 lbs).
The concrete pump should be sized according to the expected extraction rate of the drill string, the volume of the void created by the displacement tool as it is extracted, and the required pressure range that will be used during the injection of the concrete.
To learn about Soilmec’s multipurpose drill rigs and patent displacement piling technique using Traction Compaction Tool (TCT) contact us today .