Environmental Dredging Using Geotextile Tubes (Lessons Learned).

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ENVIRONMENTAL DREDGING USING GEOTEXTILE TUBES; LESSONS LEARNED
B. J. Mastin 1, PhD
ABSTRACT
Prolime was tasked to hydraulically dredge 2294( m3 3,000 yd3) of residuals (25% solids) from two primary surge
basins at a petroleum coke and coal-fire power plant in Jacksonville, FL. US AquaVac was tasked to hydraulically
dredge 1529 m3 (2,000 yd3) of settling basin residuals (10% solids) from a manufacturing facility in Calumet City,
IL. In both cases, the settled solids were a heterogeneous mixture of unspecified products, tank and pipeline washout, storm water and truck-wash runoff, and other facility process waters. Geotextile tubes and a site-specific
chemical conditioning program were identified and implemented by both contractors as the preferred dewatering
option for these contaminated residuals. Geotextile tube technology is a high volume, high flow containment option
that provides dredging and environmental services contractors efficient, cost effective, and environmentally friendly
dewatering. The objective of this study was to evaluate two environmental dredging projects that used chemical
conditioning programs to expedite dewatering of contaminated residuals in geotextile tubes for additional
operational parameters that influenced overall performance but were not originally considered during project
conception and/or feasibility phases. In both cases, tube filtrate water was returned to the basin being dredged or
another basin within the wastewater management system. Operational parameters not evaluated prior to start-up
included potential polymer residuals in the tube filtrate water, suspended solids due to breakthrough and resuspension within the receiving basins, potential for polymer overfeed, and daily pH fluctuations due to additional
inputs during dredging operations. Geotextile tube capacity and stability due to slope within the containment area,
erosion control, tube filtrate volume and flow rate back to the basins, inline mixing energy and contact time between
the polymer introduction point(s) and the tubes, and operational tube capacity versus dewatered tube capacity were
also not evaluated prior to mobilization and influenced the profitability of both projects for the general contractors.
Keywords: Polymer, contaminated residual, chemical conditioning, polymer residuals, operational capacity.
INTRODUCTION
Geotextile tubes have been used for containment and consolidation of coarse-grain materials (e.g., sand and shells)
for over 50 years. In many of these applications the tubes are left in place to form coastal protection or hydraulic
management structures, including berms, levees, groyns, breakwaters, and other beach stabilization and protection
structures. Containment and consolidation of fine-grained materials in geotextile tubes is a developing field but has
had success in the municipal, industrial, and environmental dredging markets with recent innovations in chemical
conditioning products and activation equipment (Mastin and Lebster, 2006; Mastin and Lebster, 2007; Mastin et al.,
2008a; Mastin et al., 2008b; Mastin and Lebster, 2008c; Mastin and Lebster, 2009a). This new and innovative
technology has been successfully used to dewater fine-grained, contaminated materials that contained dioxins,
polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), pesticides, metals (with a lithic
biogeochemical cycle), and other hydrophobic materials (Fowler et al., 1996; Mastin and Lebster, 2006; Mastin and
Lebster, 2007; Mastin et al., 2008a; Mastin et al., 2008b; Mastin and Lebster, 2008c; Mastin and Lebster, 2009a;
Taylor et al., 2000).
It has only been in the last five years that the chemical and geotextile industries have combined their research and
experience to improve the use of geotextile tubes for management of fine-grain sediments and residuals. Benchscale testing and identification of a site-specific chemical conditioning program and corresponding geotextile fabric
are the first steps to a successful application. In order to exceed project objectives and analogous facility
expectations, demonstration-scale testing (e.g., hanging bag tests) is typically required prior to mobilization to refine
operational assumptions for both time and consolidation rates during full-scale application. Technological advances
in the bench-scale evaluation and online application of polymers and other chemical conditioning agents for the
11
President, MAST Environmental Solutions, Inc., 7825 Ashwood Dr. SE, Ada, MI 49301, USA, T: 616-881-1081,
Fax: 616-676-9388, bmastin@mastenvironmental.com.
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expedient separation of solids from water has facilitated an increased use of geotextile tubes and other passive
technologies for containment, dewatering, and consolidation of contaminated residuals. Although not a new
technology, geotextile tubes are proving to be a useful tool for both environmental services contractors and dredgers
due to simplified online chemical conditioning programs, turn-key chemical activation equipment, and economical
start-up costs as well as operational flexibility to streamline the transition from current management techniques to
use of geotextile tubes. With most environmental dewatering applications using geotextile tubes resulting in
economical success, researchers are currently investigating onsite operational techniques to further optimize
chemical conditioning and overall dewatering performance.
Prolime was tasked to hydraulically dredge 2294 m3 (3,000 yd3) of residuals (25-percent solids) from two primary
surge basins at a petroleum coke and coal-fired power plant in Jacksonville, FL. The settled solids in these basins
were a heterogeneous mixture of unspecified products, including ash leachate, tank and pipeline wash-out, storm
water and truck-wash runoff, and other facility process waters. Geotextile tubes and a site-specific chemical
conditioning program were identified by Geo-synthetics, LLC (GSI) and MAST Environmental Solutions, Inc.
(MAST) and implemented by Prolime and power plant managers as the preferred dewatering option for these metalcontaminated residuals. The objective of this project was to effectively contain, dewater, and consolidate surge basin
residuals to greater than 50-percent solids prior to tube excavation, hauling, and disposal. Due to the limited
footprint available for tube deployment, this project was designed in multi-phases, 1147 m3 (1,500 yd3) dredged and
contained in three geotextile tubes per phase.
US AquaVac was tasked to hydraulically dredge 1529 m3 (2,000 yd3) of settling basin residuals (10-percent solids)
from a chemical manufacturing facility in Calumet City, IL. The settled solids in this basin were a heterogeneous
mixture of unspecified adhesive products, tank and pipeline wash-out, storm water and truck-wash runoff, and other
facility process waters. Geotextile tubes and a site-specific chemical conditioning program were identified by
Integrated Water Solutions, LLC (IWS) and MAST and implemented by US AquaVac as the preferred dewatering
option for these contaminated residuals. The objective of this project was to effectively contain, dewater, and
consolidate settling basin residuals to greater than 25-percent solids prior to tube excavation, hauling, and disposal.
In order to facilitate a flock suitable for geotextile tube capture, dewatering, and consolidation, the chemical
conditioning program for this site was an inorganic coagulant followed by an anionic flocculent. The geotextile tube
lay-down area was constructed in the parking lot along the settling basin berg, which allowed for easy collection of
the filtrate water and pumping back to the basin. However, placement of the tubes in the parking lot blinded-off the
bottom of the tubes limiting the available surface area for water to escape. With the limited dewatering rate, the
dredge was able to overcome the geotextile tubes and dredging was delayed for several days in order for the tubes to
“catch-up” and capacity to become available again.
Operational adjustments during a full-scale dewatering application will typically provide the largest opportunity for
chemical optimization and improved performance. For example, placement of the injection manifold and sample
ports relative to the geotextile tubes can either provide too much, too little, or optimal contact time between the
chemistry and inline solids concentration. These types of operational adjustments are not easily evaluated in the
laboratory and thus flexibility in the full-scale design will offer contractors better opportunities to refine their
application. The objective of this study was to evaluate two environmental dredging projects that used chemical
conditioning programs to expedite dewatering of contaminated residuals in geotextile tubes for additional
operational parameters that influenced overall performance but were not originally considered during project
conception and/or feasibility phases.
MATERIALS AND METHODS
Chemical Conditioning and Geotextile Tube Performance Evaluation
The objectives of a chemical conditioning evaluation and geotextile tube dewatering performance trial were to
develop a chemical conditioning program and/or geotextile tube baseline of performance for a “representative”
residual sample(s). Bench-scale evaluations were used to scale-up, develop conservative proposals, develop
operating and project objectives, and propose goodness-of-fit of a potential dewatering application with precision
and accuracy. However, bench-scale analyses were only as accurate as the residual samples used to perform the
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evaluations and poor sample collection may result in feasibility evaluations that under- or over-estimate the
performance of a technology.
Questions considered while planning chemical conditioning evaluation and/or geotextile tube dewatering
performance include:
• What were the overall project objectives?
• What were the bench-scale feasibility study (i.e., chemical conditioning evaluation and dewatering performance
tests) objectives?
• Was the in situ material to be dredged/pumped homogenous or heterogeneous?
• What was the residual vertical profile depth? Were the residuals across the vertical profile homogenous or
heterogeneous?
• Were the residuals stored or deposited in the area to be dredged/pumped across a sufficient timeline that the
treatment/stabilization processes have changed with time?
• Were there material character differences across both the vertical and lateral profiles due to flow in the system
(i.e., Stokes law)? Sand concentration?
• Was there a MSDS(s) for the in situ residual(s) and did we understand the fate and effect of any or all potential
contaminants?
• What industrial hygiene issues and associated PPE were required for residual sample testing?
• What type of containers were the residual samples shipped in to the laboratory? Preservative(s)?
• How much material was collected and/or shipped in order to answer my objectives?
• How much overlying water was collected and shipped in order to answer my objectives?
• What analytical data points were necessary to answer the objectives? Sample containers? Preservatives?
Holding times? Timeline?
• Did bench-scale evaluation facilitate additional questions and the need for additional evaluation (e.g., hanging
bag evaluations)? Did I have enough sample material to answer these additional questions?
Sample Volume and Packaging
Typically two to three gallons of representative material and two to three gallons of overlying water from each
dredge/pump reach and/or profile were required for preliminary bench-scale evaluation(s). These samples were
shipped in appropriate containers including five-gallon buckets, one-gallon paint cans, 55-gallon drums, etc.
depending on the potential contaminants. Double containment was required for potentially contaminated materials.
Residual samples that were high in organics and/or volatiles were shipped in coolers, kept at less than 4oC, and
transported overnight to the MAST testing facility. Appropriate labeling, MSDS(s), client contact information, and
chain of custodies accompanied all shipments.
Sample Preparation and Chemical Conditioning Evaluation
1.
2.
3.
4.
5.
6.
7.
8.
Evaluated the sampling effort, the sampling methods employed, and which equipment and supplies were
used.
Verified chains-of-custody and acknowledged sample receipt to appropriate sender(s) via receipt and/or
email.
Opened packaging and label containers with appropriate receipt date, test date, and initials on sample
containers.
Once opened: 1) poured off overlying water (if applicable) or 2) homogenized overlying water and solids
and collected 50 to 100 g of residual for TS analysis.
If option #1, homogenized a diluted batch of test residuals with an appropriate ratio of overlying water to
residual in a clean container for bench-testing purposes. Collected 50 to 100 g of diluted, homogenized
residual for TS analysis.
Made-down chemical conditioning products (i.e., organic polymers) at 0.5-percent concentration by adding
1-mL “neat” polymer into 200-mL potable water (or site water) and high-speed mixed for 30 seconds.
Added 150-mL of test residual to a graduated, glass test jar (Figure 1). Added made-down polymer to test
sample with a 1.0 to 10-mL syringe and mixed appropriately (Figure 2).
Recorded observations on appropriate test worksheets including water release rate, water release volume,
water clarity, and flocculent appearance.
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9.
Noted additional dewatering observations that may affect dredging, pumping, and dewatering operations
and potential conditioning alternatives if the optimal chemistry is not available for production.
10. Additional parameters that were evaluated depending on the test objectives:
• Dual product chemical conditioning programs
• Pre-dilution
• Post-dilution
• Shear strength
• Mixing energy
• Range of polymer dose for optimal performance
• Range of inline solids for optimal conditioning and subsequently dewatering performance
• Underfeed characteristics
• Overfeed characteristics
11. Took several pictures of the testing procedures and results.
Figures 1. Homogenized residuals as received
Figure 2. Homogenized residuals after chemical conditioning with anionic flocculent
Sample Preparation and Geotextile Tube Dewatering Performance
1.
Reviewed the chemical conditioning results and revisited the dewatering performance objectives and
overall project objectives.
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2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Determined equipment and supplies that may be specified for the project (e.g., horizontal auger dredge,
cutter/suction dredge, booster pumps, distance to geotextile tubes, discharge pipe diameter, production
rates, flow rates, dewatering and consolidation timeline, etc.).
Added 150-mL of representative test residual to a graduated, glass test jar. Added made-down chemical
conditioning agent(s) to test sample as previously recommended (i.e., dose, formulation, and sequence) and
mixed appropriately.
If percent solids were extremely low, more than one 150-mL sample was chemically conditioned and used
for simultaneous or subsequent filter tests.
Recorded observations on appropriate test worksheets including water release rate, water release volume,
water clarity, and flocculent appearance.
Took several pictures of the testing procedures and results.
Prepared a 8.9-cm (3.5-inch) diameter filter of geotextile fabric(s) and positioned fabric in dewatering test
filter apparatus.
Placed filter apparatus over a 500-mL graduated cylinder.
Poured conditioned residual sample into filter apparatus (Figure 3 and 6). Noted start time on worksheets.
Took pictures of the procedures as they occurred.
After 5-min, 15-min, 30-min, etc. noted volume of filtrate in the graduated cylinder as well as water quality
observations (Figure 4).
If dewatering performance objectives require, collected filtrate water for subsequent total suspended solids
(TSS) analysis and filter cake samples (from the geotextile filter) for total solids (TS) analysis according to
recommended collection and preservation protocols (Figures 5, 7, and 8).
Percent solids (TS) of the initial sample(s), diluted test sample(s), and the dewatered geotextile filter cake
sample(s) were measured according to U.S. EPA Method 160.3 (ASTM Method E1756).
TSS concentration of the filtrate was measured according to U.S. EPA Method 160.2 (Standard Method
2540D).
Figure 3. Wastewater residuals (150 mL) conditioned with anionic flocculent were filtered through geotextile
tube fabric
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Figure 4. 60 mL of water was captured in a graduated cylinder after 30 minutes.
Figure 5. Residuals remaining on a geotextile filter after 30-minutes drying time were 10-percent dry weight
solids.
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Figure 6. Alternative filter tests were performed for potable water backwash residuals with low solids
concentrations in order to collect enough mass for subsequent analyses. A water treatment plant residual
(300-mL) conditioned with anionic flocculent was filtered through geotextile tube fabric.
Figure 7. Solids retention, dewatering rate, and filtrate quality were excellent for conditioned samples (right)
compared to unconditioned samples (left). Total suspended solids concentration of the geotextile tube filtrate
from conditioned samples was 39 mg/L for this water treatment plant backwash residual.
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Figure 8. Conditioned residuals remaining on the geotextile filter after 30-minutes drying time were
measured at 34-percent dry weight solids.
RESULTS AND DISCUSSION
Power Plant
MAST 630 (175 ppm) was made-down with facility fire-hydrant water, activated with a Velodyne 114 liters/hour
(30-gph) progressive cavity make-down system, and delivered into the dredge discharge line mid-way between the
dredge and the geotextile tubes in order to allow for sufficient mixing and contact time between the polymer and
sediments (Figure 9). Conditioned sediment samples were collected from a sample port prior to the geotextile tubes
in order to evaluate inline performance and adjust chemical feed dose as necessary. Three 18.3-m (60-ft)
circumference by 30.5-m (100-ft) long geotextile tubes were used for each phase of this project. A 12-millimeter
visqueen liner bermed with 0.61-m (2-feet) of sand was used to direct tube filtrate water back into the wastewater
lagoons.
Geotextile tube filtrate water was returned to the basin being dredged or pumped to an adjacent basin within the
wastewater management system in order to equalize water volume. Dredging was performed and managed with an
IMS 4010 Versi-Dredge outfitted with a dual star-wheel propulsion and 25.4-cm (10-inch) diameter discharge pipe
(Figure 10). A maximum inline sediment slurry discharge rate of 5,678 liters/min (1,500 gpm) of less than 15percent solids was recommended and maintained in order to facilitate efficient chemical conditioning and inline
separation prior to the geotextile tubes. Each three-tube phase took three to four days to complete and a subsequent
three weeks for the contained solids to meet the project goal of greater than 50-percent solids for excavation.
Onsite dewatering management included mobilization and start-up of chemical conditioning equipment, site-specific
daily and weekly reports, and two weeks of onsite support for both chemical optimization and geotextile tube
management. Due to the short timeline for completion of each phase and the sensitivity of the active facility to
ongoing operations, Power Plant managers and Prolime agreed to have a MAST representative onsite throughout
dredging/dewatering operations. This application met project objectives including timeline, solids drying to greater
than 50-percent, and allowed the plant to stay in operation during dredging activities. For their first geotextile tube
dewatering application, Prolime realized the full potential of this technology including the ease of use, overall
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dewatering performance, and decreased capital expense to use the technology in this environment for this type of
contaminated industrial residual.
Figure 9. A Velodyne 114 liters/hr () progressive cavity make-down system was used to activate and deliver
MAST 630 into the dredge discharge line. Filtrate water sheeting off geotextile tubes returned to wastewater
system with TSS concentrations less than 40 mg/L.
Figure 10. IMS Versi-Dredge 4010 was used to remove 2,294 m3 (3,000 yd3) of sediments from the power
plant’s primary surge basins.
Although geotextile tube technology was an effective management technique for this application, several operational
parameters were not evaluated and/or considered prior to start-up. Filtrate management was considered for erosion
and volume control, but the polymer residual and total suspended solids concentrations in the water returning to the
wastewater system was not evaluated prior to mobilization. Typically, return of filtrate water back to the facility
head-works or the targeted dredge area is desirable because residual polymer helps settle suspended solids in the
system and maintain clean overlying water. Conversely, suspended solids due to filtrate water return and/or
breakthrough can interfere with ongoing plant operations. Suspended particles may contain contaminants,
breakthrough treatment system containment, interfere with pumps and ongoing treatment, and/or exceed discharge
permit limits. Although filtrate quality was not a project objective and polymer residual and TSS concentrations did
not influence the performance of the technology, return of the water to the operational system could have had
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negative impacts on management of the system. Along the same lines, return of the tube filtrate water to the online
system lends itself to receipt of a polymer overfeed. In many cases, a polymer overfeed could clog online
instrumentation and filters, and polymer residual could breakthrough containment. However, with a polymer
overfeed in this system, enough solids and carbon remained in the system to attract and absorb excess polymer.
Slope of the lay-down area towards the wastewater lagoon was not considered an issue until after mobilization.
Placing a polypropylene geotextile tube on a wet plastic liner requires a stable slope and/or appropriate management
practices (e.g., internal berms, chocks) in order to prevent the tube from sliding with the filtrate water towards the
down-slope. Another operational parameter not considered prior to mobilization was the effect of pH changes during
dredging. With an active wastewater system receiving pulses of leachate and wash water during dredging operations,
the pH of the targeted surge basins fluctuated from 4 to 10 daily. pH may affect dewatering performance as polymer
structure and charge strength are affected by pH greater than 10 and less than 4. In addition, a pulse of low pH
waters to the surge basin containing the dredge corroded through several steel cables of the cutter head winches and
propulsion system forcing a 48-hour delay for repairs.
Operational capacity of the geotextile tubes versus the dewatered tube capacity was considered during bench-scale
testing but was not evaluated as a project objective. With each phase reduced to three tubes, the dredge was able to
overcome the operational capacity of the tubes. In other words, the tubes were filled with both conditioned
sediments and release water and flow to the tubes was stopped in order for water to release from the tubes. During
start-up of flow to a geotextile tube the largest surface area for dewatering is the bottom. In this case, the tubes were
blinded-off against the visqueen liner and with a specific gravity greater than 2.0, the consolidated sediments filled
the tube and quickly restricted the surface area available for dewatering. During start-up of flow to a geotextile tube
the largest surface area for dewatering is the bottom. Overall, restrictions to dewatering rate limited dredge up time
and in order to fill tubes to capacity with solids, several short pulses of dredge material to the tubes was required
over several days. This factor would have had the most influence on the profitability of the project if five days for
each phase were exceeded. Although pulse filling was required to maximize the solids contained within each tube,
dewatering for the first two days of each phase exceeded expectations for containment, consolidation, and
dewatering rate (Figure 11).
Figure 11. Three 18.3-m (60-ft) circumference by 30.5-m (100-ft) long geotextile tubes were deployed end-toend along the settling basin for return of filtrate water to the wastewater management system.
Manufacturing Facility
Aluminum sulfate (7.6 liters/min or 2 gal/min of dredge residual) was fed “neat” (i.e., without additional
manipulation) followed by MAST 126 (100 ppm) made-down with facility fire-hydrant water and activated with a
Velodyne 114-liters/h (30-gph) progressive cavity make-down system. The dual chemical conditioning program was
delivered into a six-inch diameter mixing manifold in the dredge discharge line mid-way between the dredge and the
geotextile tubes in order to allow for sufficient mixing and contact time between the polymer and sediments (Figure
12). Conditioned sediment samples were collected from a sample port prior to the geotextile tubes in order to
evaluate inline performance and adjust chemical feed dose as necessary. One 18.3-m (60-ft) circumference by 30.5m (100-ft) long geotextile tube and one 13.7-m (45-ft) by 30.5-m (100-ft) long geotextile tube were used to contain
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the dredge slurry for this project. A four-inch valved manifold system was used to control the flow to the tubes and
allow for efficient dewatering time between fill cycles. Geotextile tube filtrate water was returned to the basin being
dredged by a 5.1 cm (2 inch) diaphragm pump placed in a collection-sump within the tube lay-down area. Dredging
was performed and managed with a Dino-Six Sediment Removal Dredge System outfitted with a auger cutter-head
and six-inch diameter discharge pipe (Figure 13). A maximum inline sediment slurry discharge of 3,785 liters/min
(1,000 gpm) of less than 5-percent solids was recommended and maintained in order to facilitate efficient chemical
conditioning and inline separation prior to the geotextile tubes. This application met project objectives including
timeline, solids drying to greater than 20-percent, and allowed the plant to stay in operation during dredging
activities. For their first geotextile tube dewatering application using chemical conditioning, US AquaVac did not
realize the full potential of this technology including the ease of use, overall dewatering performance, and decreased
capital expense to use the technology in this environment for this type of contaminated industrial residual because
residual samples were not collected and evaluated by project managers prior to submittal of a proposal,
mobilization, and start-up.
Figure 12. Onsite confirmation of the recommended chemical conditioning program from samples collected
during mobilization (left) and inline confirmation from the sample port located prior to the geotextile tubes
(right).
Figure 13. A Dino Six Sediment Removal Dredge System was used to hydraulically excavate 3,785 liters/min
(1,000 gpm) of less than five-percent residuals from this industrial lagoon.
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Geotextile tube dewatering technology could have been an effective management technique for this application,
except several feasibility and operational parameters were not evaluated and/or considered prior to start-up. Prior to
using geotextile tubes or any conventional technology for dewatering of hydraulically dredged materials,
representative samples should be collected from the dredge profile and a goodness-of-fit and/or feasibility testing
should be performed. Feasibility testing was not performed for this application prior to submitting a sales proposal
because the US AquaVac project manager assumed the dredge residual would dewater and consolidate in the
geotextile tubes without chemical conditioning. Once dredging was initiated, release of water from the geotextile
tubes and consolidation were not observed (Figure 14). In addition, breakthrough of suspended solids was observed
and the project was suspended until representative samples could be collected and forwarded to MAST for testing.
After bench testing, MAST recommended the dual product conditioning program previously described in order to
contain, dewater, and consolidate the lagoon residuals at this manufacturing facility. Bench testing confirmed that
without chemical conditioning the dredge residual would not be contained and consolidated in the geotextile tubes
for subsequent excavation.
Filtrate management was considered for volume control, but the polymer residual and total suspended solids
concentrations in the water returning to the wastewater system was not evaluated prior to mobilization. Typically,
return of filtrate water back to the dredge area is desirable because residual polymer helps settle suspended solids in
the system and maintain clean overlying water. Conversely, suspended solids due to filtrate water return and/or
breakthrough can interfere with ongoing plant operations. Although filtrate quality was not a project objective and
polymer residual and TSS concentrations did not influence the performance of the technology, return of the water to
the operational system could have had negative impacts on management of the system. At one point during the
operation of the dual conditioning system by US AquaVac personnel, an overfeed of flocculent occurred and
foaming was observed in the filtrate water and in the discharge of the sump pump back to the lagoon. A polymer
overfeed could clog online instrumentation and filters, and polymer residual could breakthrough containment. The
overfeed in this application was large enough to result in “sloppy” residual in the geotextile tubes and solids that did
not dewater and consolidate within 30-d after dredging and pumping were completed.
Operational capacity of the geotextile tubes versus the dewatered tube capacity was considered during bench-scale
testing but was not evaluated as a project objective. With only 757,082 liters (200,000 gallons) of tube capacity
onsite and inefficient dewatering due to no chemical addition, the dredge was able to overcome the operational
capacity of the tubes (Figure 15). In other words, the tubes were filled with both conditioned sediments and release
water and flow to the tubes was stopped in order for water to release from the tubes over time. In this case, the tubes
were blinded-off against the parking lot. Overall, restrictions to dewatering rate limited dredge up time and in order
to fill tubes to capacity with solids, several short pulses of dredge material to the tubes was required over several
days. This factor and the extended timeline for dewatering, consolidation, and excavation influenced the profitability
of the project. US AquaVac spent time and money outside the original project budget for expediting chemical
conditioning feasibility testing, expediting MAST mobilization and technical service, addition of a second geotextile
tube, and the timeline for completion and excavation being pushed back 30-d from projected completion.
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Figure 14. A 18.3-m (60-ft) circumference geotextile tube was filled without chemical conditioning for two
days until dewatering was not observed and TSS was observed in the filtrate water. The recommended dual
chemical conditioning program was implemented in order to facilitate an increase in consolidation rate and
operational tube capacity.
Figure 15. A 18.3-m (60-ft) circumference by 30.5-m (100-ft) long geotextile container filled to a maximum
height of 2.3 meter (7.5 feet).
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