“Earth Piers” for Aboveground TanksAudio version
Publication: Petroleum Equipment & Technology Archive
Issued: May 2000
Author: Totten Joe E.
New solution to an old problem:
Poor soil at construction sites can be expensive to strengthen and time consuming to correct. An aggregate pier foundation is a quick and less expensive alternative to support 80,000-barrel aboveground petroleum storage tanks in poor soil. PE&T’s Joe Totten examines how these “earth piers” work and the history behind them.
Note: This article is based primarily on information provided by Clarence L. Jean Jr., Vice President of Houston Fuel Oil Terminal Company and by representatives of the Geopier Foundation Company (Dr. Nathaniel Fox, President of the company’s Scottsdale, AZ office, and Tommy Williamson, President of its Houston office.) Some of the information on the older foundation technology was provided by Doug Schwarm of GeoEngineers, Inc., who also is on the American Petroleum Institute’s Pressure Vessels & Tanks Subcommittee.
Naturally, when I see or hear the word “pier,” I envision a structure extending from a shoreline out over the water, supported by pilings or pillars that will withstand current, waves and other forces of nature. Never would I have associated the word with supporting large land-locked structures, such as high-rise buildings, airplane hangars, sports stadiums, highway retaining walls, manufacturing plants or, of all things, large aboveground petroleum product storage tanks.
But today, thanks to the development and patenting (in 1993) of innovative technology called “the aggregate pier foundation system,” pier-like support systems are being used as foundations for many types of superstructures. Among these are ten 80,000-barrel, cone-roof tanks at the Houston Fuel Oil Terminal Company’s tank farm, which is about a half-mile from the Houston Ship Channel. This tank farm marks the first industrial use of the Geopier™ system to support large aboveground petroleum storage tanks in Texas.
In the interest of keeping PE&T readers up to speed on technology affecting their industry, this article will describe the history of the technology, how it works, what it does and some of the major uses made of it to date, after which the design and progression of the Houston tank farm project will be portrayed.
Ancient technology’s frustrations
For thousands of years, the solution to poor soil at construction sites has been either driven piles to carry loads below the unsuitable soils or removing the compressible layer by a method called overexcavation and replacement. As the name implies, the latter involves removing the unsuitable soils to expose competent foundation materials and then replacing them with compacted soil or stone aggregate.
Driven piles are typically avoided because of the cost. While innovative installation methods have been able to reduce costs slightly and reduce vibrations where necessary, pile foundations have always cost more and taken longer than conventional shallow foundations. Piles can also be too effective for some types of equipment (tanks in particular) that cannot tolerate abrupt differential settlement caused by discrete foundation points under the distributed foundation area. A thick crushed rock pad, or even more expensive pile cap, are necessary to provide uniform support.
The overexcavation and replacement method has its faults, too. It costs a lot, mainly because of the large volumes of soil that have to be taken out and replaced. In high groundwater areas, excavations can be unstable and can undermine nearby structures, the underpinning of which adds to the cost. On some projects, water seepage can delay work and otherwise cause excavation and construction costs to escalate. Dewatering associated with deep excavations can also cause unexpected migration of contaminants that flow with the water toward the excavation.
Finished rock pad for one of the 10 new 80,000-gallon tanks being constructed at the Houston Fuel Oil Terminal Company, Houston, TX. Beneath the pad are 315 rammed aggregate pier elements, each of which is 14.5 feet deep and 30 inches in diameter.
Inset: Road grader puts finishing touches on the rock pad. Photos by Bill Millstead, Millstead Photography
In 1984, driven mainly by growing frustration with this ancient technology, Dr. Nathaniel S. Fox (then a geotechnical engineering consultant with a PhD in that field from Iowa State University) began developing an improved method of providing soil reinforcement to support shallow foundations. A couple of years later, Dr. Fox began collaborating with Dr. Evert Lawton, an associate professor at the University of Miami (FL). Together, Fox and Lawton developed a new technology called the Rammed Aggregate Pier™ soil-reinforcement system, for which they were subsequently granted a US patent (No. 5,249,892) and foreign patents. Available from the Geopier Foundation Company, Inc., the system is referred to as the Geopier Intermediate Foundation. It is being marketed as an alternative to deep foundations and as an alternative to overexcavation and replacement.
The alternative technology
The Geopier Intermediate Foundation does two things at once. First, it supports structures built on top of it by distributing the foundation loads over a larger area of foundation soil. Second, it densifies the soil around the foundation pier. In other words, it actually makes the soil around the pier stronger and better able to withstand vertical and lateral pressures.
The foundation system consists of a number of elements. An element is constructed by drilling a hole in the soil. If the soil is too loose, the shaft might be drilled inside a casing to prevent the soil from caving in. The depth of an element is determined on the basis of the geotechnical requirements of the project. Most piers are from 10 to 15 feet deep. Hence the name, “intermediate foundation system” is used to distinguish it from deep foundations (which usually are at least 20 feet and often are from 50 to 100 feet deep) and from shallow foundations (no deeper than 4 or 5 feet).
Diagram of a completed rammed aggregate pier element
Once a hole is drilled, select aggregate stone is rammed into the bottom with a specially designed tamper head. The tamper head has 45-degree sides. Tamping with this tool creates a bottom bulb that vertically pre-strains and pre-stresses the soil surrounding the hole. Stress is the force that is applied by the hydraulic piston divided by the area of the tamper head. Strain is the displacement caused by the stress. Pre-stressing and pre-straining causes the soil matrix to tighten around the bottom bulb, so the soil will be better able to withstand vertical pressure.