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“Earth Piers” for Aboveground Tanks

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.



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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.

Once the bottom bulb is in place, layers of the well-graded aggregate stone are compacted one atop the other. Ramming with the tamper head creates a dense, stiff undulated-sided foundation shaft on top of the bottom bulb. This pre-strains and pre-stresses the soil laterally around the foundation element. The claimed benefits of the rammed aggregate system include the following:

• Increase in the soil’s ability to support high vertical and lateral stress. The pre-stressing within the soil makes it more effective in providing lateral support once the foundation element is loaded by the structure above the foundations.

• Improvement in stability in areas with seismic activity. For example, in tests that subjected rammed aggregate piers to shaking forces equivalent to an earthquake measuring 7.5 on the Richter scale, downward settlement was measured at less than 1/2 inch, and the elements were measured to be 20 to 45 times stiffer than soft matrix clay soil at the site.

• Significant reductions in costs, compared to the cost of deep foundation systems or massive soil overexcavation and replacement.

• Reduced noise levels at the construction site compared to pile driving. The construction apparatus’ energy range is from 250,000 to 1.7 million foot-pounds per minute, while the ramming frequency varies from 300 to 600 cycles per minute.

 
Special tamping equipment used at the Houston Fuel Oil Terminal Company project. The inset shows the tamper head’s 45-degree sides, which help create the bulb at the bottom of the hole. Red flags “mark the spots” where additional pier elements will be installed. Photos by Dr. Nathaniel Fox, Geopier Foundation Company

Performance case studies
From its prototype project in 1988 to date, the Geopier foundation system has been used on construction projects in 30 states and two countries. The projects involve many different kinds of construction. Some structures are in earthquake-affected areas. Others are moderately loaded structures on previously unbuildable sites with very poor subsoil, and still others are at such locations as solid-waste landfills, debris laden fill soil areas and high-groundwater areas.

The systems are being used to increase soil slope stability on active landslides and under retaining walls built over weak soils. The systems have controlled settlement while supporting single column loads as high as 1,100 tons for individual footings and 3,000 tons for multiple columns within composite strip-mats. They have met stringent one-half-inch settlement design for some building additions. Hundreds of full-scale load tests have been performed on the pier elements, with tests monitored by independent geotechnical consulting firms. Following are brief summaries of some of the projects and how the system performed.

• Maddox Park Greenhouse, Atlanta, GA:Although this is a relatively light, one-story, steel framed building with glass walls, it was constructed on a 31-foot thick zone of organic, silt-fill soil with many tree parts mixed in. In three days of work, 168 foundation pier elements were installed to support 168 footings. Three years later, maximum settlement of the structure was 0.25 inches. This was the prototype project.

• First Baptist Church Administration Building, Columbia, SC:
The first major project to use the system, this was a five-story building and auditorium almost a
city-block long. The soil was mainly low-consistency silt in a seismic zone. The pier system was used in lieu of 70-foot-deep piles (with caps tied together with horizontal beams) which otherwise would have been required by code because of potential seismic activity. One year after construction, the foundation had settled an eighth of an inch.

• Key Field Airplane Hangar, Meridian, MS:
The engineering challenge was anchoring the hangar in loose sand and soft clay soil in a high-wind area.Geopier uplift anchors were used instead of traditional helical anchors—i.e., single-helix bars that look like highway augers for utility poles. This was the first such use of the pier system. Each anchor was designed with a 20-ton capacity and passed tests with 30 tons of uplift load. The hangar had instruments that could measure movements as small as 0.01 inches. Six months after completion, the hangar was hit by 70-mph winds but the instruments showed no measurable movement.

• Grady Memorial Hospital Addition, Atlanta, GA:
This was the first mat-supported structure to be supported by the pier system. It is a 16-story structure requiring a 22-foot basement excavation which, with the restricted space at the site, prohibited ramped access for traditional drilled caissons. Crews worked around the clock for about 72 hours to install the pier foundation mat between heavy rainstorms. After completion, the mat settled from three eighths to three fourths of an inch.

• Manufacturing Plant, Vermillion, SD:
This is a 250,000 square-foot structure on soft clay subsoil with a shallow water table. The pier system was used in lieu of overexcavation and replacement, the cost of which was estimated at $2 million. The subsoil was so soft that trucks hauling aggregate sank in to their axles and had to be restricted from the site. In all, 3,400 pier elements were installed within eight weeks, after which the site supported heavy-wheeled trucks and construction equipment. Settlement of the plant’s walls, columns and slabs was less than one inch. The cost of the foundation work was under $1 million.

At center, from right to left, are the three major tools used in installing the many elements that make up the pier-like foundations for new tanks at the Houston Fuel Oil Terminal Company: The drill drills the hole, the front loader puts in the aggregate stone, and the tamping mechanism rams the aggregate to first create a bottom bulb and then to compact layers of aggregate until the hole is filled. Photo by Bill Millstead, Millstead Photography

• Office Building, Memphis, TN:
This 15-story building adjoining an eight-level parking garage was the first major
strip-mat-supported structure to be supported by the pier foundation system. The system was used in lieu of the typical 50- to 70-foot-deep drilled caissons used in heavy structures in Memphis for more than 80 years. The pier foundation system was designed for seven- to nine-foot pier elements. Settlement was a quarter of an inch, 18 months after completion.

• Recreation Facility, Hackensak, NJ:
The site of this facility had been used as a solid waste landfill for 20 years. The foundation pier elements penetrated the waste, which averaged 16 feet in thickness. After construction, and before accepting the foundation system, the city engaged a geotechnical firm to test two selected pier elements to 150 percent of the foundation’s maximum design stress. One element settled an eighth of an inch and the second settled three sixteenths of an inch.

• Retaining Wall for State Route 5, Clinton, MD: This was the first major highway project for the foundation pier system. Sixteen wall sections, up to 13-feet tall, were supported in subsoil ranging from very soft clay to moderately strong silt and sand. The wall sections had to resist settling more than one inch, with differential settlement of less than one-half inch between sections. The wall also had to resist the tendency to tilt, which was accomplished with uplift anchors. The foundation pier system included 30-inch-diameter uplift anchors and
36-inch-diameter compression pier elements.

• Commercial Structure, Portland, OR:
This four-story structure near the Portland International Airport is on floodplain deposits of sand and silt as much as 100 feet deep. The City had designated the area as seismically sensitive, with a “high liquefication” potential. The City chose the foundation pier system because of its reliability in earthquakes and its low cost. The foundation pier technology is now making its way into the petroleum storage and distribution industry, as discussed in the next section.

Supporting large ASTs
The Houston Fuel Oil Terminal Company is using the foundation pier system to support 10 new 80,000-barrel, cone-roof tanks at its Area 14 Tank Farm. At this time, the piers are installed and the tanks are under construction. The tanks are located on a 12-acre area at the northwest corner of the company’s plant site along Jacintoport Boulevard, about a half-mile from the Houston Ship Channel.

Each tank has a footprint of about 7,850 square feet and exerts about 3,700 pounds per square foot of pressure. Tommy Williamson, president of the Geopier Foundation Company-Houston, says that 315 pier elements were installed beneath each of the ten tanks. Each pier is 14.5 feet deep and 30 inches in diameter. Each pier element will support 50 tons, while the pier and adjacent soil will support 60 tons.

Tommy Williamson says that highly variable fill soil was encountered from 5 feet to about 12.5 feet below the surface in all borings drilled for geotechnical investigation, showing potentially unstable soil. (Generally, fill soil is soil placed by people rather than by rivers; it is usually compacted, which can make it more stable than natural soils. Uncompacted fill or fill that contains deleterious material, such as trees, grass and refrigerators, can play havoc on foundations).

The fill encountered on this project is generally composed of erratically compacted low- to high-plasticity clay and sandy clay soil. The fill tends to become more granular with depth. Wood fragments and organic matter were generally encountered in the lower, more granular sandy portion of the fill. The geotechnical engineering firm for the project was Fugro South Inc. of Houston; the chief engineer was Robert P. Ringholz, PE.

Williamson says the company installed 100 piers a day and 500 piers a week. Installation was started on January 17 and was completed on February 18. He says an average of two crews were used to install the piers. Some of the piers were installed inside casings to avoid caving problems caused by groundwater. A total of 3,354 piers were installed; 3,150 were installed to support fuel tanks and 204 were installed to support the pipe racks. Pier aggregates were composed of recycled crushed concrete.

Clarence L. Jean Jr., Vice President of the Houston Fuel Oil Terminal Company, says that concern over construction on the site’s unusual soil conditions was the catalyst thadrove him to choose the foundation pier system. He says “I had some difficult soils to deal with. I began looking at alternative foundation construction methods, such as auger-cast piles, vibrated concrete columns and conventional pile systems. I was not too happy with any of them because of the expense. I like the Geopier installation methodology much better than any of the alternatives. It’s a cleaner operation. It’s a very efficient system, in terms of daily production. And the methodology itself is not tied to outside contractors for poured concrete, which can sometimes become a scheduling issue.”

Less time and money
Tommy Williamson says that the Houston tank farm project cost about $700,000 less than it would have cost with other foundation systems, not including further savings from cutting the construction schedule by about 40 working days. Dr. Fox, founder and president of the Geopier Foundation Company of Scottsdale, Arizona, explains that these estimated savings are based on evaluating three alternative soil reinforcement solutions. These included overexcavation and two deep-foundation solutions.

Joe E. Totten was Director of the Office of Internal Evaluation for the U.S. General Accounting Office (GAO) from 1990 through 1994. In this capacity, he directed and managed internal audits of GAO operations. Joe now works as editor/quality contral manager for Petroleum Equipment & Technology.

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