Bag Tests are a Good Field Measurement of Geotextile Tube Performance
Trial using geotextile tubes to dewater mine wastewater and tailings at a lead and zinc mine in northeast Greece, 2001 and 2002. The polyester tubes passed water containing fewer that 100 parts per million heavy metals. The test was done in a landfill to collect effluent from tubes but that proved unneccessary, as the water was 'clean'.
Doug Gaffney demonstrates the small hanging bag test that can be done at the job site. The bag is three feet deep and 2.5 feet wide, and uses a five gallon sample of slurry.
Closeup of water draining from tubes in the trial project for dewatering 50 megaliters per day of semi-contaminated geomaterials at the zinc and lead mine in Greece. The decant water contained less than 100 ppm heavy metals.
The Buchner funnel is used for bench-scale testing of slurry.
A hanging bag test at Fire Pond, Rhode Island in 2001, where 3500 cubic yards of material containing total petroleum hydrocarbons (TPS) and low levels of PCB’s were removed.
Beakers of decant water show the obvious differences in effluent from eight different geotextiles tested with contaminated canal sludge in Asia. When the water samples were tested, analytical results corresponded to the visual results.
Jonathan R. Wynn, Associate Geologist
Hart Crowser, Inc, Gloucestershire, England
The use of geotextile tubes is a cost effective method for dewatering dredged materials.
Early in the design phase of a dredging project, the engineer must determine the client's requirements and priorities, such as reduced turbidity of the dredge discharge to comply with environmental regulations. Other requirements on the same job may be less obvious because they are purely financial - reduced pumping, disposal, storage and regulatory costs. Because geotextile tubes are flexible in their adaptation and use, they can offer benefits in cost reduction far beyond more traditional methods available to a dredge operation.
Not every job can use tubes for dewatering, and a bag test can provide the design engineer with information needed to assess which dewatering method will meet the needs of the client.
Requirements vary from job to job. Effluent cleanliness is critical in some applications, while the dewatering rate is paramount in others. There is often a trade-off between the time required for a slurry to become "dry" and the amount of fine-grained solids that pass through the pores of the geotextile. Optimizing this trade-off has big implications for the dredging contractor, and in the case of geotextile tubes, requires matching the dredge's flow rate with the tube's discharge rate and volumetric capacity.
Tests that determine parameters such as grain size and moisture content (in percentage by weight) are rudimentary to the design of a dredge dewatering project. Environmental dredging projects often include contaminants, and require chemical analyses of both the sediment and decant water. Another crucial piece of information is the specific gravity of the solids. Specific gravity and grain size affect the settling rate and give a good indication of what to expect during the project. Since a slurry's percent solids is usually defined by weight (not volume), the weight of the solid in relation to the weight of water will define the reasonable final percent solids that can be obtained. This is why a heavy steel mill sludge with a specific gravity of 2.9 can be dripping wet at 60 percent solids while it is very difficult to cost effectively dewater biosolids with a specific gravity of 1.1 to as high as 50 percent.
Tests have been devised in the geotextile industry to characterize the physical and hydraulic properties of fabrics used in geotechnical and filtration applications. Tests such as tensile strength, apparent opening size (AOS) and water flow rate provide an index to aid the designer in selecting the proper fabric for the job. These tests are conducted using standard ASTM methods, and are not necessarily correlated to the wide variety of conditions found in the field. For example, the dry, glass beads used to determine AOS in the lab bears very little resemblance to viscous, organic sediment in a lake.
BRIDGING THE GAP
To bridge the gap between the lab and the field, and between geotextile index tests and actual tube dewatering, several tests have been devised.
Moo-Young, et al. (2002) describes pressure filtration testing that can be performed in the lab that provides comparative results for various geotextiles and dredged materials.
The hanging bag test was first envisioned by Fowler, et al. (1994) to replicate dewatering in a tube, and to estimate total suspended solids (TSS) in the effluent and the consolidation rate. The test, developed by Fowler and described in the draft Geosynthetic Research Institute (GRI) test method, uses approximately 40 gallons of slurry, a large cylindrical geotextile bag (five feet deep by 12 inches diameter) and a frame. This test is used to investigate the relationship of a geotextile's published AOS with the geotechnical properties of the solids to be retained, and can also be used to assess filter cake formation on the inside of the bag.
It is generally understood that AOS by itself is a very poor indicator of solids retention and tube performance (Gaffney, et al, 1999). Other factors, such as the type of yarns used in the geotextile, the weave, and the properties of the slurry come into play. Koerner and Koerner (2004) concluded that drainage flow rate was directly proportional to the permeability of the soil contained within the tube, and that the hanging bag test adequately predicted this performance. They confirmed that performance was not dependant on geotextile parameters, including the fabric AOS.
One of the primary advantages to the hanging bag test is that the test can be performed in the field. Smaller hanging bags (three feet deep by 2.5 feet across) with smaller sample sizes (five gallons) have been used for observing dewatering characteristics, including the formation of the filter cake. In fact, the hanging bag test is quite informative when simply observed by an engineer experienced in geotextile tube dewatering. An experienced eye can detect subtle differences and relate them to full-scale tube dewatering. The smaller bags have also proven to be effective at supplying quantitative data for estimating full-scale dewatering times, flow rates during tube filling, and effluent characteristics. The photo on page 16 shows the obvious differences in effluent from eight different geotextiles tested with contaminated canal sludge in Asia. When the water samples were tested, analytical results corresponded to the visual results.
The hanging bag test is adaptable, and is conducted in a manner to obtain whatever data are required to design the project Approximately five gallons of representative slurry is obtained at the site and poured into a bag to observe and measure the interaction between the slurry and the bag, the dewatering rate and effluent. The smaller bags, minimized framing requirements and less volume of slurry make the test simpler and faster than the draft GRI test method, and can give valid analytical test results if the test is set up before hand to measure flow rate and effluent. Coupled with bench-scale testing of the slurry using a Buchner funnel, sample geotextiles and polymers, an entire suite of testing has been developed for use in the field. As the material dewaters in the bag, samples can be taken to determine changes in percent solids, suspended solids (TSS) in the effluent and other parameters over time.
The results of a hanging bag test can provide key pieces of information for designing the job, including the number of tubes needed, the optimum size of the tubes, whether polymers are needed, the optimum fabric, how to manage the effluent, optimizing the dredging efficiency, and the estimated time to dewater. Effluent management is typically of primary importance for obtaining permits. The question most often asked of the design engineer is "how long will it take for the tubes to dewater?" The answer to this question still requires a mixture of science and experience, but a well-designed hanging bag test will give a good estimate. Drainage data taken during the hanging bag test can result in curves that can be extrapolated to full size tubes.
Based on many hanging bag tests and full-scale tube dewatering projects, the author has developed a general rule of thumb for commonly-encountered dredged slurries pumped into tubes without chemical conditioning. There is a one order of magnitude decrease in the published flow rate of the geotextile during filling, and two orders of magnitude decrease during passive dewatering. For example, if a geotextile has a water flow rate of 20 gallons per minute per square foot, the flow rate of water passing through the geotextile during filling will be approximately two gallons per minute per square foot, and 0.2 gallons per minute per square foot while the tube is at rest. As noted by Koerner and Koerner (2002) however, flow rate was more directly related to the permeability of the soil than the geotextile. Chemical conditioning using polymers dramatically increases the flow rate through the tube due to increased permeability and reduced filter cake formation.
A properly designed dredging project can save time and money, as well as a great deal of anguish at the end of the project. The hanging bag test is an important part of a properly designed geotextile tube dewatering project. This test, coupled with geotextile index tests and geotechnical tests, can result in an efficiently designed tube dewatering project. Whether the dredged material is waste activated sludge or fine-grained, inorganic industrial solids, the project can be designed so that surprises are minimized, and the project is completed to everyone's satisfaction. Edit Module