GEOPIER® FAQ     Geopier    Impact Pier

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    Impact Pier
      uplift RAP
                    Impact RAP
                  • 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. Do ICC, DSA and CALTRANS approve Geopier?
                  • 5. What soil conditions do Geopier systems apply?
                  • 6. How is corrosion of uplift steel accounted for in design?
                  • 7. How much load can a Geopier resist?
                  • 8. Can you build Geopier 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 the soil?
                  • 11. Can Geopier RAPs control liquefaction?
                  • 12. Does Farrell have California registered engineers?
                  • 13. How do Geopier products accelerate the construction schedule?
                  • 14. What cities and counties have approved Geopier projects in California?
                  • 15. How is Geopier better than a concrete pier or pile?
                  • 16. How does Geopier compare to soil cement piers?
                  • 17. How are Geopier RAPs different than stone columns?
                  • 18. What is the sequence of a Geopier job complete?
                  • 19. How can I get an estimate for Geopier ground improvement?



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

                    Rammed Aggregate Pier® (RAP) is a revolutionary soil engineering concept and ground improvement construction method using crushed aggregate to improve poor soil. RAP refers to both methods of replacement Geopier and displacement Impact pier.

                    Vertical ramming forces expand the aggregate pier into the adjacent soil increasing the soil density and compaction. By virtue of soil improvement by the ramming process, poor soils are improved to support shallow footings and mats, soil compressibility is controlled, and liquefaction potential is reduced. The beveled rammer head design increases the horizontal "pre-strain effect" in the matrix soil to increase bearing capacity and strength.


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

                    Geopier RAPs reinforce poor soil for heavy foundation loads. 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 strength to support heavy loads.

                    Impact piers are commonly used to reduce liquefaction, increase density of loose sand, increase drainage of saturated soil, and increase the global stiffness of weak soil. The Impact pier is a fully cased, vibro-rod, displacement method of Rammed Aggregate Pier construction.


                    3. Can RAPs resist uplift forces?      ^top

                    YES. A Rammed Aggregate Pier can be equipped with a 2-bar or 4-bar structurally designed uplift anchor. The uplift RAP can resist seismic and wind tension forces from 25 to 100 kips depending on soil conditions.

                    2008 Uplift RAP technical paper (pdf)


                    4. Does ICC, DSA and CALTRANs approve Geopier systems?      ^top

                    YES. Geopier Rammed Aggregate Piers have been evaluated by ICC which is summarized in the ICC Evaluation Report ESR-1685 (pdf)?

                    YES. DSA approves Geopier for the ARC ITC Building (1997), Oakridge High School Stadium Seats (2003), Sacramento High School of Engineering Sciences (2008), Education Services Center Folsom Cordova (2010), ARC Parking Structure (2012).

                    DSA approves Impact pier liquefaction mitigation at Amistad Wilson High School (2010) and East Palo Alto High School (2010).

                    YES. CALTRANs approves Geopier. Geopier RAPs were successfully used at the 880-92 interchange project to support northbound and southbound MSE walls, 880-92 (pdf)


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

                    The Impact Pier system applies best in loose sandy soil and fill above and below the ground water. Impact applies so-so in clay soil as well with the addition of grout.

                    The Geopier system applies 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. In highly corrosive soil, a triple protection method is used for the uplift bars consisting of 1) oversized bars, 2) galvanization, 3) asphalt coating, and 4) poly coating. In low to medium corrosions sites, galvanized and oversized bars provide over 100 years design life.


                    7. How much load can a Geopier support?      ^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 low deflections of less than 1/2 inches.

                    Bearing capacity is often improved upwards of 5,000 to 9,000 psf (ASD).


                    8. Can you build Geopier 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 occasionally sandy, the drilled hole can be temporarily cased to complete the Geopier.

                    When the soil is predominately sand below ground water, the Impact pier system is much more effective with full length casing and vibratory displacement.


                    9. How does Geopier resist lateral forces?      ^top

                    Geopier increases the allowable friction below 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 the soil?      ^top

                    Impact Piers are effective in treating to maximum depths of 45 feet.

                    Geopier RAPs are most effective to maximum depths of 25 feet. Geopier RAPs are commonly 8 to 16 foot deep for most building foundations.


                    11. Can Impact and Geopier RAPs control liquefaction of sandy soil?      ^top

                    Yes. Impact piers can reduce and control liquefaction potential of loose sand, silty and clayey sand, and non-plastic silts. Results are measured by pre and post CPT testing to measure increased density of the soil.


                    12. Does Farrell have California registered engineers?      ^top

                    Yes. Farrell has California registered engineers CE and GE. Farrell has consultant relationships and works with structural, geotechnical and seismic engineering experts in California and the west coast.


                    13. 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 with 300+ RAPs. Production is a function of soil conditions and layout of the building structure.


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

                    Geopier foundations have been approved in San Francisco, San Luis Obispo, Santa Clara, San Jose, Belmont, Palo Alto, Mountain View, Redwood City, Newark, Milpitas, Berkeley, Oakland, Emeryville, Hayward, Dublin, Walnut Creek, Pleasant Hill, Fairfield, Napa, Richmond, Pinole, Davis, West Sacramento, Sacramento, Chico, El Dorado, Stockton, Oakley, Truckee, Tracy, Modesto, Woodland, San Diego, Irvine, Anaheim, Westminster, Oxnard, Santa Barbara, and several more cities and counties in CA.


                    15. 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.


                    16. How does Geopier compare to 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.


                    17. 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 20 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.


                    18. 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.


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

                    Please call 530-621-4867 and ask for Sally, she can get you to the right engineer OR Fax over soil boring logs, foundation loads, and foundation plan to 530.621.4837.

                    Farrell can turn around a bid in a few days to meet your schedule.


                  Geopier® and Impact® and Rammed Aggregate Pier® are registered trademarks of Geopier Foundation Company, Inc.