Transmission And Substation Foundations - Technical Design Manual (TD06088E)

CHANCE® HELICAL PULLDOWN® MICROPILES

GroutReservoir

The CHANCE Helical Pulldown Micropile (HPM) is a patented (U.S. patent 5,707,180) method used to form a grout column around the shaft of a standard square shaft or pipe shaft helical pile/anchor. The installation process can employ grout only (see Figure 6-39) or grout in combination with either steel or PVC casing (see Figure 6-40). The result is a helical pile with grouted shaft similar, in terms of installation, to drilled and grouted anchors or auger cast-in-place piles using gravity grouting. The initial reason for developing the HPM was to design a helical pile with sufficient shaft size to resist buckling. However, since its inception, the method has demonstrated more advantages than simply buckling resistance. The advantages and limitations, based on the results of field tests, are summarized herein: 1. Increase buckling capacity of a helical pile shaft in soft/loose overburden soils to the point that end- bearing controls failure. 2. Increased compression capacity due to the mobilization of skin friction at the grout/soil interface. Total capacity is a function of both skin friction and end-bearing. 3. Provides additional corrosion protection to anchor shaft in aggressive soils. The grout column provides additional corrosion protection to the steel pile shaft from naturally occurring aggressive soils with high metal loss rates, organic soils such as peat or other corrosive environments like slag, ash, swamp, chemical waste, or other man-made material. 4. Stiffens the load/deflection response of helical piles. Axial deflection per unit load is typically less than with un-grouted shafts.

NeatCementGrout (VeryFlowable)

Square (SS)or Round (RS) ShaftExtension

Extension DisplacementPlate

CasedExtension Displacement Plate

Square (SS)or Round (RS) ShaftExtension

SteelorPVCPipe

DRAWINGS AND RATINGS

CasedLead DisplacementPlate

Lead Displacement Plate

CasedLead DisplacementPlate

STD. Lead Section

STD. Lead Section

Figure 6-40

Figure 6-39

The installation procedure for CHANCE Helical Pulldown Micropiles is rather unique in that the soil along the sides of the shaft is displaced laterally and then replaced and continuously supported by the flowable grout as the pile is installed. To begin the installation process, a helical pile/anchor is placed into the soil by applying torque to the shaft. The helical shape of the bearing plates creates a significant downward force that keeps the pile advancing into the soil. After the lead section with the helical plates penetrates the soil, a lead displacement plate and extension are placed onto the shaft. Resuming torque on the assembly 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 is gravity fed and immediately fills the void surrounding the shaft. Additional extensions and displacement plates are added until the helical bearing plates reach the minimum depth required or competent load-bearing soil. This displacement pile system does not require removing spoils from the site.

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