Helical Pulldown Micropiles (Combination Piles)
Tuesday, May 23rd, 2017 by Thrasher
Our clients searching for foundation solutions can expect the most cost-effective methods with Thrasher Foundation Repair. Higher load capacities than ever are now achievable by the patented helical pulldown(R) micropile technology. It continues to overcome obstacles to traditional foundation methods for a growing number of applications.
The helical pulldown(R) micropile is a system for constructing a grout column around the shaft of a standard helical pier foundation system pile.
To begin the process, a screw anchor is placed into the soil by applying torque to the shaft. The helical shape of the bearing plates creates a tremendous pulling force that keeps the anchor advancing downward into the soil. After the lead section with the helical plates penetrates the soil, a lead displacement plate and extension are bolted onto the shaft. Resuming torque on the assembled shaft advances the helical plates and pulls the displacement plate downward, forcing soil outward to create a cylindrical void around the shaft. From a reservoir at the surface, a flowable grout immediately fills this void, surrounding and encapsulating the shaft. Additional extensions and displacement plates are added until the helical bearing plates reach competent load-bearing soil. This displacement pile system does not require removing spoils from the site. It also has the added benefits of resistance to buckling in weak surface soils, stiffer pile and additional corrosion protection against aggressive soils.
The helical pulldown is often referred to as Helicast. Helicast pile systems generate compression capacity as a combination of helix plate end bearing and frictional resistance of the grout column or outer casing. Helicast piles consist of a central steel shaft with helix plates near the tip, soil displacement plates, casing (optional), and grout. The central steel shaft consists of solid square shaft lead and extension sections. Helicast piles are installed into the ground similar to traditional helical piles by applying torque and axial compressive force (crowd). A grout reservoir is constructed at the surface either before or after installation of the lead section. A lead displacement plate is generally located at the first coupler between the lead and first extension. Extension displacement plates are situated at each coupler between successive pile extension sections. As the pile is advanced, the lead displacement plate pushed soil outward and away from the central shaft and allows specialty micropile grout to flow by gravity from the reservoir into the created void. Extension displacement plates help maintain the grout column's size and shape as the pile is advanced. Helicast piles may have temporary or permanent steel or PVC outer casings and a load transfer device that allows attachment to structures.
The central steel shaft consists of a solid square shaft lead section with one or more helix plates and solid square shaft extension sections without helix plates. Lead sections are typically fabricated in 5,7- and 10-foot lengths and extension section lengths are within 5 or 7-foot lengths. Five-foot extension section lengths are most commonly used; however, 7-foot lengths may be acceptable in soil profiles where the grout column has less potential to collapse or neck down, such as stiff clays above the groundwater table. Central steel shafts typically range from 1.5 to 1.75-inch solid square bar.
The lead soil displacement plate is a round steel plate with soil cutting blades or paddles and central square openings sized for the associated square bar being utilized. The lead displacement plates are generally located at the lead section's connection and the first extension section. The lead displacement plate creates the annular space for grout placement. Lead displacement plates typically have a diameter of 5-9 inches. Case lead displacement plates are a lead displacement plate with a factory welded steel collar on one side to facilitate attachment of the casing.
Extension soil displacement plates are round steel plates with two grout holes and a central square opening sized for the square bar being utilized. The extension displacement plates are generally used at each extension coupling to maintain the grout column annular space during the continued advancement of the central steel shaft. Soil displacement plates have a diameter of 5-9 inches. Extension displacement plates may also occasionally include soil cutting blades or paddles to maintain consistent grout column dimensions.
Cased extension displacement plates are used to attach steel or PVC outer casing. The cased extension displacement plates are an extension displacement plate with a factory welded steel collar on each side to facilitate the casing attachment.
Placed as composite steel and concrete pilings, Helicast has advantages of:
- no vibration to adjacent buildings;
- all components can be individually designed;
- pre-engineered system;
- dedicated equipment is not required;
- predictable and reliable capacities;
- cost-effectiveness (especially viewed as dollars per completed job).
To provide an ultimate capacity of Helicast, it is recommended to perform a sacrificial full-scale load test.