Geopier^® and Impact^® Pier systems

Frequently Asked Questions - FAQ

When improved and stiffened soil conditions for foundation support are required, Rammed Aggregate Pier Foundation Systems are the answer.

1. What is a Rammed Aggregate Pier or RAP?
2. What are typical applications of Geopier RAPs and Impact piers?
3. Can Geopier RAPs resist seismic uplift loads?
4. Does ICC approve Geopier foundations?
5. What soil conditions do Geopier RAPs apply to?
6. How is corrosion of uplift steel accounted for in design?
7. How much load can a Geopier element resist?
8. Can you build RAPs below the ground water table?
9. How do RAPs resist building foundation seismic lateral forces?
10. How deep can Geopier and Impact RAPs improve?
11. Can Geopier RAPs control liquefaction of sandy soil?
12. How much do Geopier RAPs cost?
13. Does Farrell have California registered engineers?
14. How do Geopier products accelerate the construction schedule?
15. What cities and counties have approved Geopier projects in California?
16. Which National customers use Geopier products across USA?
17. How is Geopier better than a concrete pier or pile?
18. How does Geopier compare to overexcavation and soil cement piers?
19. How are Geopier RAPs different than stone columns?
20. What is the sequence of a Geopier job complete?
21. How can I get an estimate for Geopier ground improvement?

Contact an engineer with Farrell at (530) 621-4867 for detailed questions about your project soil conditions.

1. What is a Rammed Aggregate Pier^® or RAP?   What is a Geopier? (pdf)    ^top

A Rammed Aggregate Pier is a unique and patented ground improvement method for controlling liquefaction and to stiffen and strengthen loose and soft soil conditions. The process of hydraulic hammering with heavy vertical crowd force increases the stiffness of the soil and densifies sand soil. The beveled rammer head design increases the horizontal "pre-strain effect" in the matrix soil. more detailed answer

2. What are typical applications of Geopier RAPs and Impact piers?      ^top

Geopier RAPs reinforce soil. Typical applications include spread footing or mat support for buildings up to 18 stories, water and fuel storage tank support, support of MSE walls, stiff aggregate reinforcement in landslides, and general applications for sites requiring higher ground strength to support industrial production facilities, heavy industrial slabs, and warehouses. Geopier info flyer (pdf)

Impact piers treat liquefaction ... densify and reinforce loose soil. The Impact method provides both densification of sand with less than 12% fines and reinforces sand with greater fines. Impact pier construction also stiffens and strengthens silt and clay soi. Impact pier info flyer (pdf)

3. Can Geopier RAPs resist uplift forces?      ^top

Yes with Uplift Rammed Aggregate Piers. Rammed Aggregate Piers equipped with a 2-bar or 4-bar structurally designed uplift anchor can resist seismic and wind tension forces from 25 to 100 kips depending on soil conditions, see graphic detail.

Typical Geopier uplift bar sizes include No. 6, 7, 8, 9, and 10 from Williams Form Engineering.

4. Does ICC approve Geopier foundations?      ^top

Yes. Geopier Rammed Aggregate Piers have been evaluated by ICBO which is summarized in the ICBO Evaluation Report ER-5916?

5. What soil conditions do Geopier RAPs apply to?      ^top

Geopier RAPs apply best in soft clays, loose silty sands, soft silts, undocumented fills, and cut-to-fill transitions. If you can drill the soil, Geopier will be a highly cost effective and sound engineering solution.

6. How is corrosion of uplift steel accounted for in design?      ^top

The corrosion potential is always accounted for in Geopier Uplift design. The key to corrosion protection of Geopier uplift elements is to ISOLATE the steel from all rebar in the footings and contact with the soil. We do this in the soil by using schedule 40 pvc pipe over the anchor bars and cement mixed aggregate around the uplift plate at the bottom of the Geopier shaft. In highly corrosive soil, a triple protection method is used for the uplift bars. Typically oversized bars provide over 100 years design life.

7. How much load can a Geopier element resist?      ^top

A typical 30-inch diameter by 12 foot deep Geopier element can resist from 50 kips in soft bay mud to 120 kips in medium clay and loose silty sand. Geopier RAPs will resist these loads with minimal deflections of less than 1/2 inches.

8. Can you build RAPs under the ground water?      ^top

Yes. Large open graded crushed rock is used to stiffen the soil below the water. If the soil is too sandy, the drilled hole can be temporarily cased to complete the Geopier element.

9. How does Geopier resist lateral forces?      ^top

Geopier RAPs increase the allowable friction below spread footings. With the addition of Geopier RAPs in the soil, spread footings can resist up to 50% of the gravity building loads. Geopier RAPs exhibit tested angles of internal friction of 48 to 52 degrees.

10. How deep can Geopier RAPs and Impact piers treat?      ^top

Impact RAPs are effective in treating to maximum depths of 45 to 50 feet. Geopier RAPs are most effective to maximum depths of 25 to 30 feet. Most foundation applications can use 8 to 16 foot deep RAP ground improvement.

Can Geopier RAPs control liquefaction of sandy soil?      ^top

Yes. Geopier RAPs and Impact piers can reduce and control liquefaction potential of deep clean sand, silty and clayey sand, and non-plastic silts. Impact and Geopier construction results in a non-liquefiable rock pier that has been measured to improve the soil more than a diameter from the edge.

How much do Geopier RAP elements cost?      ^top

Geopier foundation system costs are highly dependent on the soil conditions and foundation loads. Geopier RAPs typically cost between $3.50 to $5.00 per kip or $30 to $45 per foot of total Geopier length. The cost difference compared to piles is that Geopier RAPs deliver 7 to 10 kips per foot of depth (at $40/ft ave.), where piles deliver 1 to 4 kips per foot of depth (at $45/ft ave.). With Geopier, owners get more for their money, higher capacity per foot of depth for an overall lower cost.

Does Farrell have California registered engineers?      ^top

Yes. Farrell has 3 California registered engineers CE and GE. Farrell has consultant relationships and works with some of the best regarded structural, geotechnical and seismic experts in the world.

How do Geopier products accelerate the construction schedule?      ^top

Geopier construction progresses rapidly on a clean and well organized site. Farrell's crews typically produce 40 piers per day and can peak at 50 and 60 on large jobs (300+ piers).

What cities and counties have approved Geopier projects in California?      ^top

Geopier foundations have been approved in San Francisco, Santa Clara, San Jose, Belmont, Palo Alto, Newark, Milpitas, Oakland, Emeryville, Hayward, Dublin, Walnut Creek, Pleasant Hill, Fairfield, Napa, Richmond, Pinole, Davis, West Sacramento, Sacramento, Chico, El Dorado, Stockton, Tracy, Modesto, and several more cities and counties in Northern CA.

Do you have any national customers that use Geopier products across the USA?      ^top

Yes. Geopier RAPs support Target Stores, Wal Mart Stores, Home Depot, COSTCO, and Sams Clubs and many many more.

How is Geopier better than a concrete pier or pile?      ^top

Geopier construction results in a rough sided very dense aggregate pier. As a result, higher strengths are developed at the edge of the Geopier element when compared to a pile. Also, Geopier RAPs improve the strength the soil around the drilled hole up to a diameter from the edge.

How does Geopier compare to overexcavation and soil cement columns?      ^top

Geopier construction is fast and clean. That means no muddy cement slurry all over the site and safe site conditions. With Geopier RAPs, contractors can keep schedules on-time avoiding the risks of deeper excavation due to a soft bottom. With Geopier, engineers can be assured of quality control by observing every Geopier BORIING drilled on the site, unlike ground mixing methods where they only get to observed a very small section of the completed column.

How are Geopier RAPs different than stone columns?      ^top

Geopier RAPs are similar to stone columns in that they are both composed of aggregate replacement columns. Except that Geopier RAPs are 10 to 25 times stiffer than stone columns. This means that Geopier RAPs can carry much higher loads than stone columns and they also improve the surrounding soil much better. Stone columns create a weak mudded zone at the perimeter. Geopier RAPs create a well compacted rough perimeter with aggregate that has been rammed and embedded into the adjacent soil.

What is the sequence of a Geopier job complete?      ^top

At Farrell we start with feasibility letters, estimating, and preliminary design. Once budgets are agreed on, Farrell prepares final stamped plans for submittal with structural sets. After plans are approved, Farrell signs a contract with the General Contractor to install load tests and the Geopier RAPs. Farrell closes the project with as-built plans and quality control records.

How can I get an estimate for Geopier ground improvement?      ^top

Fax over soil boring logs, foundation loads, and foundation plan. Farrell can turn around a bid in a few days if necessary.