Geopier GP3® System
- Geopier GP3 system Construction Process
- How GP3 Technology Works
- Cement Treated Aggregate (CTA) Geopier Elements
- Applications for GP3 Technology
Developed in the late 1980’s by Dr. Nathaniel Fox, the original Geopier® technology (known today as the GP3 system) has been used to support 1000’s of foundations around the World and over 600 structures in the Mid Atlantic Region since 1997.
“I was working as a geotechnical consultant in Florida and I was disturbed how the state of the practice for dealing with random fill and loose soil sites was to perform over excavation and replacement. It seemed too costly and took too long. There had to be a better way!
So I decided to come up with a way that was less expensive, quicker and better…..and that was Geopier”
Geopier technology has been used to support various mid-level structures up to 20 stories tall. The GP3 elements are constructed of stable, dense pillars of aggregate that can support spread footings with bearing pressures up to 10,000 pounds per square foot. Also known as the Rammed Aggregate Pier® system, it offers reliable settlement control, fast installation and lower cost than traditional foundation-support methods such as over excavation and replacement, piles or drilled shaft foundations. The GP3 system has become the standard for ground improvement technologies. GeoStructures is the exclusive licensee for this technology in New York, New Jersey, Pennsylvania, Delaware, Maryland, Washington, D.C., Virginia and North Carolina.
The construction process is simple and fast:
- Drill a cavity to depths ranging from 7 to 30 feet deep and 24-36 inches in diameter.
- Place a 24-inch layer of open-graded aggregate at the bottom of the cavity.
- Compact the aggregate using a patented tamper that delivers a high-energy impact ramming action. The ramming action compacts the aggregate and pre-stresses the surrounding soil. Successive lifts of well-graded aggregate are then rammed in place.
The high-energy compaction process produces significant lateral pre-straining and pre-stressing of the adjacent matrix soils so much that the lateral stress in the soil surrounding the Geopier element approaches, Kp, the coefficient of passive earth pressure. This means that the lateral stresses of the soil on the Geopier element can be as much as 2 to 3 times greater than originally imparted by the soil.
Stiffness – When a vertical column load is imposed on the GP3 element, it is the lateral stresses that build up in the soil that provides confinement to the GP3 element making it stiffer and able to resist lateral movement and bulging. It is this high stiffness that allows a Geopier supported foundation to control settlement to very close tolerances.
Skin Friction – It is also that same lateral stress that provides excellent coupling of the GP3 element with the soil so that vertical loads can be shed into the soil efficiently. In many applications, the elements essentially shed the entire vertical load over the length of the pier such that the vertical stress at the bottom of the pier is zero. This has been confirmed by load cell instrumentation of GP3 elements.
Settlement Control – Where footing stresses are the highest, GP3 elements provide the needed stiffness. For a rigid footing the stresses are highest on the soil in the zone directly under the footing as well as a depth of 1 to 2 times the footing width. These elements control settlement by providing a very stiff element that will attract stress into a soft soil or variable loose fill material to a depth needed to control settlement and minimize settlement of the soils below it.
Patented CTA GP3 elements consist of well graded aggregate mixed dry with cement to create a pier with 3 to 5 times the stiffness and capacities up to 50% greater than piers built with aggregate alone. For projects with close settlement tolerances, CTA GP3 elements provide excellent value.
Dry Cement can be mixed with well graded aggregate on site or if available dry ready mix concrete can be used to create a CTA GP3 element.
Modulus testing of a CTA GP3 element to a top of pier stress of 34 ksf with less than 0.15 inches of deflection
Advantages/Uses of CTA GP3 Elements
- Brownfields Projects – CTA GP3 elements create an impermeable element so a conduit for cross contamination of aquifers is not formed while also providing high stiffness and capacity.
- Karst Limestone Areas – CTA GP3 elements create an impermeable element which will not be a conduit for water flow.
- Peat and Organic Soil Layers – CTA GP3 elements provide the ability to span across weak compressible layers which might bulge and cause settlement if piers were built out of aggregate alone.
- Increased Capacity/Stiffness – CTA GP3 elements are 3 to 10 times stiffer and up to 50% higher capacity over aggregate piers.
Geopier technology was originally developed for the support of shallow foundations as an alternative to costly, massive over-excavation and recompaction. Over the past decade, Rammed Aggregate Pier® systems have most often been used as a replacement for costly deep foundations (driven piles and drilled piers), and in the following applications:
- Building Foundations
- MSE walls & embankment support
- Slope Stabilization and Landslide Repairs
- Floor Slab Support
- Tank Support
- Liquefaction Mitigation
- Uplift Control
For conventional spread footing foundations for building from 1 to 20 stories GP3 elements offer the following advantages:
Eliminate Over Excavation and Replacement – The GP3 system is most often used to support buildings on projects with variable fill soils or soft or loose strata that would normally settle excessively or deferentially on a site without over excavation and replacement or use of a deep foundation system.
Eliminate Costly Deep Foundation Systems – GP3 elements are often used to replace piles or drilled shafts at a cost savings on the order of 30% to 50% and can usually be installed quicker than deep foundations often shaving weeks off a construction schedule.
Reduced Footing Sizes – Placement of GP3 elements under conventional spread footings can also benefit a project by increasing the bearing capacity by a factor of 2 to 4 times that the bearing for the capacity soil of the soil by itself, thus reducing the size of footings needed for a project.
MSE Walls and Embankment Support
Constructing MSE walls and embankments on weak or soft foundation soils can result in excessive settlement or inadequate factors of safety for global stability or bearing capacity.
High Friction Angle for Global Stability – GP3 elements exhibit friction angles from 49 degrees to 52 degrees, which increase the composite shear strength of soft foundation soils which increases the factors of safety for bearing capacity and global stability. The use of CTA GP3 Elements, Grouted Impact Pier Elements or GeoConcrete™ Columns can also be used to increase shear strength even more when needed.
Settlement Control – The high stiffness of the GP3 elements significantly reduces embankment and retaining wall settlement. The reduction in settlement magnitude and duration eliminates the need for staged embankment construction and reduces overall construction schedules. The use of stiffer elements such as CTA GP3 Elements, Grouted Impact Pier Elements or GeoConcrete Columns can reduce settlement magnitudes even further.
Slope Stabilization and Landslide Repairs
GP3 technology, exhibiting friction angles from 49 degrees to 52 degrees, can be installed along unstable or marginally stable slopes to intersect failure planes and provide additional shear reinforcement to increase factor of safety for slope stability.
Floor Slab Support
When conventional slab-on-grade solutions are inadequate because of compressible soils or undocumented fill, the GP3 system provides an economical alternative to pile supported structural slabs and or thickened slabs for conventional light to heavily loaded industrial warehouse floor slabs.
Eliminate Need for Structural Slab – Use of GP3 technology provides substantial cost savings by eliminating the need for a structural slab supported on deep foundations. Pier type and spacing is governed by capacity needs, fill thicknesses between the piers and the slab and joint spacing. Savings can be substantial.
Reduce Floor Slab Thickness – As part of the Geopier analysis for foundation support, a Finite Element Analysis of the stresses in the slab can be performed to evaluate the minimum slab thickness required. This can result in substantially reducing slab thickness.
GP3 technology provides cost effective solutions for the support of heavily loaded storage tanks such as those used by municipalities, petrochemical facilities, industrial and agricultural facilities. Installing GP3 elements not only reduces total settlement magnitudes, but also provides a reinforced composite zone that reduces the potential for damaging differential settlement.
Geopier provides economical liquefaction mitigation solutions for many projects.
The installation of a Geopier ground improvement solution reduces the potential for liquefaction by:
Stiff Inclusions – When GP3 elements are installed they are 2 to 10 times or more stiffer than the matrix soil, so their inclusion in the soil matrix will help reduce the liquefaction potential.
Increasing Stiffness of Matrix Soil – The system installation process helps to densify adjacent soil by building up lateral stresses in the soil. By increasing the stiffness of the matrix soil, liquefaction potential is reduced.
Lateral loads induced by seismic and wind forces often result in uplift forces at a building shear walls. GP3 elements with uplift anchors provide allowable uplift resistance up to 75 kips per anchor.
The uplift anchor assembly extends from the bottom of the pier to the ground surface where it is structurally connected to the shallow foundation. The uplift resistance provided by the systems eliminates the need for deep foundations or oversizing of shallow foundations. Construction of the technology using vertical ramming energy results in friction angles of 50 degrees. This provides significantly improved design bearing pressures of 5 to 10 ksf or more depending on soil conditions. The improved bearing pressure reduces footing sizes saving time and money on your project.