Chance Technical Design Manual

CHANCE HELICAL PULLDOWN MICROPILES

DRAWINGS & RATINGS

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 round shaft heli cal pile. The installation process can employ grout only (see Figure 7-47) or grout in combination with either steel or PVC casing (see Figure 7-48). The result is a helical pile with a grout ed 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 heli cal pile with sufficient shaft size to resist buckling. However, since its inception, the method has demonstrated more ad vantages than simply buckling resistance. The advantages and limitations, based on the results of field tests, are summarized below: • Increases buckling capacity of a helical pile shaft in soft/ loose overburden soils to the point that end bearing con trols failure. • 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. • 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. • Stiffens the load/deflection response of helical piles. Ax ial deflection per unit load is typically less than with un grouted shafts. The installation procedure for Chance Helical Pulldown Micro piles is rather unique in that the soil along the sides of the shaft is displaced laterally and then replaced and continuously sup ported by the flowable grout as the pile is installed. To begin the installation process, a helical pile 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 heli cal 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 displace ment plate downward, forcing soil outward to create a cylin drical 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 mini mum depth required or competent load-bearing soil. This dis placement pile system does not require removing spoils from the site.

Grout Reservoir

Neat Cement Grout (Very Flowable)

Square (SS) or Round (RS) Shaft Extension

Extension Displacement Plate

Cased Extension Displacement Plate

Square (SS) or Round (RS) Shaft Extension

Steel or PVC Pipe

Cased Lead Displacement Plate

Lead Displacement Plate

Cased Lead Displacement Plate

STD. Lead Section

STD. Lead Section

FIGURE 7-48

FIGURE 7-47

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