On the Construction of Soil Bentonite Radon Barriers

Proceedings of the Second International Conference
on Tailings & Mine Waste '95

Fort Collins, Colorado, USA
January 17-20, 1995

Bill Frame Jr.
Gears, Inc., Crested Butte, CO., USA
Tom Webber
Pyramid Environmental, Inc., Greensboro, NC, USA


ABSTRACT:  This paper will present case histories on two separate soil bentonite radon barrier cover projects. The paper will discuss the constraints, specific demands, problems and solutions involved in the construction of each project. Since the paper is being presented by two contractors, it will detail the hands on approach used to successfully complete these projects.

  • I  Introduction

Gears. Inc. (Gears) and its' sister company Pyramid Environmental, Inc. (PEI) are construction companies based in Colorado and North Carolina. Both companies specialize in the mixing of materials that require a high degree of accuracy.

As part of the U.S. Department of Energy's (DOE) Uranium Mill Tailings Remedial Action (UMTRA) Project, work is being done at two sites in Colorado. These two sites are located in Gunnison and Rifle. These two projects involved the construction ofa cell in which the mill tailings are stored. A part of the construction of these cells involves the mixing and placing of bentonite amended soil radon barriers.

Gears was contracted by Ames Construction (Ames) to mix approximately 41,000 CY of radon barrier material for the Gunnison site. This project was completed in July of 1995. PEI was contracted by Green International, Inc. (Green) to mix approximately 125,000 CY of radon barrier material for the Rifle site. This project is anticipated to be completed by November 15, 1995.

  • II  Components and Specifications

Soil, Bentonite and water are the three components of soil-bentonite. Soil is the major building block of the soil-bentonite. One of the first quality control items necessary to obtain is the consistency of gradation. If the material is not consistent, it will be more difficult to determine the percentage of bentonite necessary to obtain the required permeability. Other important items requiring consideration are: moisture content, percentage of silt and percentage of clay.

Bentonite is a naturally occurring clay comprised primarily of the mineral montmorillonite, whose unique crystalline structure is responsible for the clay's properties. The clay structure, when dry, resembles a negatively-charged stack of plates. When the plates come in contact with a polar liquid such as water, the plates absorb the liquid forcing them apart and this is the mechanism by which bentonite seals soil.

Water is the component that makes the bentonite swell and although very important, is often taken for granted. As a general rule only potable water should be used.

MX-Ferguson Company (MK) was the DOE's contractor for both the Gunnison and Rifle sites. MK prepared the specifications for the radon barrier for both projects. The Gunnison project specified a soil bentonite utilizing 5% dry bentonite to dry soil. The bentonite specified was an untreated sodium bentonite, Envirogel 200 as manufactured by Wyo-Ben, Inc., or approved equal.

The soil was obtained from an on site borrow source. This soil had to meet the following specifications, soil with 95% finer than 1 inch, a minimum of 50% passing the No.200 sieve and contain not more than one percent of organic material. The water was obtained from an on site well.

The Rifle project specified a soil bentonite utilizing 4% dry bentonite to dry soil. The bentonite specified was a Wyoming sodium bentonite as obtained from H&H Bentonite, Grand Junction, Colorado, or an approved equal.

The soil was obtained during the excavation of the cell. This material had to meet the following specifications, soil with 95% finer than 1 inch with a maximum size of 3 inches, a minimum of 60% passing the No.200 sieve and contain not more than one percent by volume of organic matter. The water was obtained from a local municipal source and trucked to the site.

  • III  Mixing Process

Both Gears and PEI utilize Aran Mixing Plants which are continuous mix, volumetric, self erecting and self contained units which are state of the art. The Aran Plants are an Australian made pugmill having manufacturer's rated capacities between 75 and 1,000 Tons Per Hour (TPH) of continuous operation contingent upon the engineering properties of the soil material being used.

During calibration, a relationship between belt speed and throughput, in tons per hour, is established. This relationship is linear and is calculated and recorded by the Contrec model 405 flow computer. This microprocessor based instrument is flilly programmable. All calculation constants are set via the front panel switches and are stored permanently in a non-volatile memory.

The soil is loaded into the plant's aggregate hopper, the soil is then fed to the twin shaft mixing chamber on a belt. The flow of soil is controlled by the operator who programs the Contrec Flow Computer. The bentonite is stored in the plant's 45 ton silo and fed to the mixing chamber via a cleated belt which is hydraulic driven via a chain reduction. Water is metered by means of hydraulically driven constant displacement pumps which have speed variation via a flow control valve.

  • IV Case Histories

For the Gunnison project, Ames elected to stage the processing area adjacent to the cell. Ames excavated and hauled the soil material from the on site borrow using bellydumps. Ames selected a Powerscreen MK II unit and a Powerscreen Commander unit in order to try to meet the anticipated production levels of the two Alan plants. Gears provided an Alan ASR 400 E and an Alan ASR 280 B with respective anticipated production levels of 300 and 200 TPH each. In addition to the two plants' silos, two horizontal storage units with 150 ton capacities were brought on site.

The bentonite for this project was provided by Wyo-Ben, Inc. of Billings, MT. The bentonite was railed from Greybull, WY to Commerce City, CO. by Burlington Northern. The bentonite was then transloaded and trucked to the Gunnison site by Don Ward Trucking. Due to scheduling problems, Ames elected to truck direct the last 500 tons of bentonite. During peak production, the two units produced approximately 500 TPH and used 225-250 tons of bentonite per day. On site storage capacity was a problem that limited production on several occasions.

The soil material had been pre-wet at the borrow site during the fall of 1994. The material was close to optimum which was advantageous to the mixing process but did cause the screening operation problems. In hopes of decreasing processing quantities, Ames brought in a CMI rototiller to work the oversize clods prior to screening. This piece of equipment definitely aided the screening operation but it was an unforeseen expense.

The soil was screened and then fed directly to the Alan plants where the soil, bentonite and water were mixed to a homogenous product with 5.1% to 5.4% bentonite and 20-22% moisture. The mixed soil bentonite was then loaded in to belly dumps and hauled and spread on the cell in three six inch lifts. Ames utilized a sheepsfoot roller to spread and compact the soil bentonite. A laser guided motor grader was utilized to blue top the final lift.

The Rifle project's soil was stockpiled at the processing area during the excavation of the cell. PEI was responsible for screening and mixing, Green hauled, placed and compacted the soil bentonite. PEI selected an El Jay 6' X 20' screen deck to screen the soil for both the 400 and 280 plants which fed to a single surge bin, this setup enabled PEI to screen and mix up to 750 TPH.

The bentonite for this project was provided by H&H Bentonite of Laramie, WY. and was trucked direct by Trimac, Inc.. PEI and Green procured 5 horizontal storage units with a total on site capacity of 750-800 tons. During peak production, PEI was using 200-225 tons per day six days per week. Due to loading and trucking restrictions deliveries occurred 24 hours per day while mixing was done 10 hours per day.

The soil material had been stockpiled using scrapers approximately 2 years prior. The soil had a moisture content of between 6 and 9% which aided the screening process considerably. The soil was screened and fed directly to the Alan plants where the soil, bentonite and water were mixed to a homogenous product with 4.5% bentonite and 18-19.5% moisture. The mixed soil bentonite was then loaded into scrapers which hauled and placed the material on the cell in two six inch lifts. Green utilized a D8 dozer to spread and a sheepsfoot roller to compact the soil bentonite.

  • V  Summary

Problems and challenges encountered on both projects were the scheduling of bentonite deliveries, matching screening capacities with mixing capacities, coordinating mixing operations to match placement capacities and meeting DOE and MK safety and quality control specifications.

The Gunnison project did not have sufficient on site bentonite storage but the two independent screening and mixing units was a plus. The Rifle project had adequate on site storage and the larger screen and surge bin enabled higher productions but the units operating as one caused problems. Both projects utilized a single shift for both mixing and placement and scheduled required downtime around lunch breaks.

A key challenge on both projects was the ability to train and equip personnel to meet the strict safety guidelines enforced by MK. Some safety regulations although serving a critically important purpose, safety, made some necessary operating procedures nearly impossible to accomplish. A few regulations had to be interpreted and implemented as policy in a manner that provided a safe work place but also enabled the work to be completed in a timely and cost efficient way.

Another challenge was the DOE's request that MK's Quality Control (QC) verify' the bentonite application rate on a weekly basis. Since the Alan plant is a continuous mix plant, there was no simple way to verify' the application rate using a small sample. The method that was selected had a degree of inaccuracy but it enabled QC to verify the application rate. A small sample of bentonite was run out, timed and weighed, then a sample of soil was run out, timed and weighed. QC then backed out the moisture percentages and converted the weights to TPH and determined the application rate. The problems with these tests were the small sample size, possible human error in operating the stop watch and the manual controls and the basic mechanics of the Alan plant. Gears, PEI and QC were able to verify' the Alan plants accuracy during the calibration process, when larger samples were run out and compared. During construction the use of run out tests, the close recording of physical inventories and the use of the Ramsey scale on the outgoing conveyor reinforced a high level of confidence in the Alan metering system.

Due to the Alan plants consistent accuracy and the experience of both Gears' and PEI's employees and the cooperation of all those involved, both projects were completed successfully. Teamwork and constant communication was the key to both projects.

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