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Structural Foundation Support Utilizing Helical Piers

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Jack Post Piles · Sectional Concrete Piers · Helical Piles · Mini Piles

Jack Post Piles
Installed by hand beneath an exposed foundation, this affordable approach to foundation support works best at sites with shallow bedrock or boulders and a narrow contamination zone. 2½ diameter jack piles can be spaced as closely as required to accommodate foundation irregularities and they are typically left in place inside the replacement foundation as it is formed and poured. Spans exceeding 3’ are not advisable, therefore where undermining an entire wall, work will typically involve two to four or more seg ments. Jack post piles will accommodate the pneumatic or hydraulic power assisted hand excavation with conveyors approach to cleaning up small residential sites.

As with the sectional concrete pier approach to building underpinning, jack post piles are simple and relatively lower cost, as the foundation is replaced in sections, immediately following the contamination dig out from a limited area. Where contamination has passed deeply beneath a structure, a channel is left in the foun dation for conveyor placement. For this reason it is advisable to have a detailed contamination delineation plan in place before implementation. Cost for a 4’ high by 6’ long jack post pile segment, including foundation replacement typically runs about $5,000.

Sectional Concrete Piers
This practical excavation based approach to structural undermining is viable for sites with limited access and where soil type and condition enable vertical excavation to a depth exceeding the contaminated zone and non-liquid footing soils are attainable. Sectional concrete piers work with any foundation type, including rock, slate or no foundation situations. As in the jack post pile approach, excavated 2-4’ wide sections are immediately filled in with concrete, therefore new permanent foundation installation and contaminated soil removal are done simultaneously.

This simple, lower cost approach makes sense for small and mid sized residential sites. If contamination has passed deeply beneath a structure, then a channel can be left in the foundation for the placement of conveying equipment.

Cost for this approach depends closely on soil type, water table depth and cement truck access. For budgeting purposes a 15’ wall can be undermined and the foundation replaced to 6’ beneath a 6’ deep footing with 36” wide sections for about $15,000.

Helical Piles

Helical Piers

 View 		Installed with a torque drive mounted on a mini hydraulic excavator, this viable approach to foundation support requires pre excavation to the bot tom of a structure’s existing footings. The approach is best suited for one to 2½ story structures. As in the case of mini piles, if no foundation exists, one must be installed. Helical anchors are advanced to refusal, the depth of which will vary greatly from site to site. The 7” to 12" diameter anchors are best suited for sites with silty to gravelly soil, with relatively shallow excavation depths. As with mini piles, a bracket is fitted to the top of anchor 2.75” diameter shaft. This bracket grabs the edge of the footing or angle iron (if required) and is affixed to the footing. The bracket has two tensioning bolts which are tightened to transfer the structural load onto the piles.

Pile spans are typically 5-8’ and allow adequate width to pass conveyors and mini excavators. On completion of contaminated soil dig out, the foundation must be replaced. This approach is priced at $1,800 - $2,000 per helical pile plus the $3,000 – $8,000 cost of pre excavation work depending on site specific details.

Design Capacities

48,000 lbs

Diameter (inside) Nominal

3 in

Wall Thickness

0.216 in

Diameter (outside) Nominal

3.432 in

Weight/foot

7.58 lbs

R - Radius (outside) Nominal

1.716 in

r - Radius of Gyration

1.16 in

I - Moment of Inertia

3.02 in4

B - Bending Factor

1.29

K - Effective Length Factor (theoretical)

0.7

K - Effective Length Factor (recommended design value)

0.8

International Building Code Allowable Capacity

Fully Supported Column - 48,000 lbs
Soft Soil 5' Unbraced Column - 48,000 lbs
Liquid Soil 10' Unbraced Column - 28,000 lba

Mini Piles
Installed with an air rotary drill rig this premium approach to foundation support requires level ground, clear access and minimally 12’ clearance. With a level site no excavation preparation work is necessary. 10” diameter boreholes are advanced to bedrock, or to a depth sufficient to provide adequate frictional support for 6” diameter steel casing. Mini piles enable removing thick zones of contaminated soil from beneath multi floored residential and commercial structures. Note, this is the same approach used to provide earthquake protection to buildings. A 3” diameter steel post is centered into the casing, and then the casing is filled with grout. This provides a firm base onto which the foundation bracket can be mounted. The bracket is snugged up to pre mounted 8” angle iron, which span from pile to pile and are anchored to the existing footing. Note if the structure has no footing, then a 12” high x 18” wide reinforced concrete footing needs to be constructed beneath the existing wall prior to the mini pile installation. This provides a clean edge to which the angle iron is attached and add significantly to the project cost.

Piles spans are typically 6-9’ and allow enough width to adequately pass conveyors or excava tors. This method enables deep excavating beneath buildings where the structural load is carried by perimeter foundations. There remains significant reconstruction work following contaminated soil dig out as the undermined foundation must be replaced. This will add two to five days of work at the end of the contamination dig out project. This approach is understandably the most costly, with individual mini piles priced at about $2,500 each plus excavation crew support of about $6,000 to $8,000. Pre mini pile installation foundation support, where required, will be priced on request.

 MORE IMAGES 

· Jack Post Piles

· Jack Post Piles (additional view)

· Mini Piles

· Foundation Bracket

· Foundation Support Comparrison Table

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