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Geotextile tube dewatering of contaminated sediments, Tianjin Eco-City, China
Geotextiles and Geomembranes 31 (2012) 39e50
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Geotextile tube dewatering of contaminated sediments, Tianjin Eco-City, China T.W. Yee a, C.R. Lawson a, *, Z.Y. Wang b, L. Ding b, Y. Liu b a b
TenCate Geosynthetics Asia Sdn Bhd, 14, Jalan Sementa 27/91, 40400 Shah Alam, Malaysia TenCate Industrial Zhuhai Co. Ltd., Gaolan Port Economic Zone, Zhuhai, Guangdong Province, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 March 2011 Received in revised form 17 July 2011 Accepted 19 July 2011 Available online 11 August 2011
An account is given of the use of geotextile tubes to dewater dredged contaminated sediments at the Tianjin Eco-City site in China. Approximately 5 million m3 of contaminated sediments from the bed of a lake were dredged and dewatered in this way with the effluent water returned to the lake. The dewatered solids were utilized within the project site, or were disposed of in a landfill, depending on their degree of contamination. The paper details the tube dewatering evaluation process undertaken and presents the results on which the dewatering facility was designed. To enable an assessment of the fullscale dewatering performance various relationships were derived based on a conservation of mass of the dewatering process. The design, construction and operation of the dewatering tube facility for the treatment of the moderately contaminated sediment waste stream is also presented. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Geotextile tubes Dewatering Contaminated sediments
1. Introduction Tianjin Eco-City is a new development in China being undertaken in an ecologically sound and environmentally sustainable manner. The Tianjin Eco-City project is a strategic cooperation between the China and Singapore National Governments aiming to improve the living environment and to build an eco-culture that will serve as a role model for future developments in the country. The project site is located on an existing wetland area adjacent to the coastline of Bohai Bay in Hangu District, about 40 km East of Tianjin city centre. Tianjin is located approximately 150 km Southeast of Beijing the capital. The new city will have an area of 30 km2 with a planned population of 350,000. Prior to the development of Tianjin Eco-City, the land consisted of either natural salt marshes or salt fields and ponds for shrimp and crab farming. The general site layout is shown in Fig. 1. It is bounded by the Ji Canal to the Northwest and West, and Bohai Bay to the Southeast. An important part of the development of this area is the provision of a central lake around which will be constructed some of the housing developments. The lake is also required to provide leisure and recreational facilities for the local population in a clean and healthy environment. However, in its existing state, this lake has been used as a wastewater impoundment containing domestic and industrial wastewater. An extensive programme of remediation and
* Corresponding author. E-mail address: [email protected] (C.R. Lawson). 0266-1144/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2011.07.005
development of the lake was required before it could be utilised in an environmentally safe manner. 2. Existing wastewater impoundment lake 2.1. Background In early 1976 the Hangu District Council converted a low lying area into a wastewater impoundment lake. The wastewater lake was created by the construction of a 3 m high perimeter earth dyke. The lake covers an area of about 3.0 km2 with an impoundment capacity of 5.6 million m3. The lake bottom slopes from the Western side to the Eastern side of the lake. At the Southwest edge of the lake are located two pipe sluice gates, each 2 m in diameter. These pipes discharge into a culvert that drains directly to the sea. The Ji Canal flows adjacent to the wastewater impoundment lake on its way to the sea. During the rainy season when the Ji Canal is prone to overflowing, the sluice gates are closed to prevent river and canal water backflow into the wastewater impoundment lake. From mid1976 this wastewater impoundment lake began receiving untreated domestic and industrial wastewater from Hangu District, pumped in through underground pipes. 2.2. Lake bed soil layers Investigations of the lake bed revealed a generally consistent layering of soil types. A black silty clay layer of thickness between 1.3 m and 2.5 m overlies a brown clay layer. This black silty clay layer with high organic content, odour and contaminants is the
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N Eco-City development area
represents the alluvial deposits formed from the Ji Canal overflows. Typical soil particle sizes range between 6 and 9 mm. The demarcation of the two layers is quite distinct with no significant existence of a contamination transition zone. Below these two layers is marine clay.
2.3. Contaminated sediments
Ji Canal The upper black silty clay layer is contaminated with mercury, arsenic, copper, cadmium, hexachlorobenzene and DDT along with high levels of raw sewage. The degree of contamination tends to be higher at the wastewater entry point and around the shores of the wastewater lake. Based on current practice adopted in Tianjin for mercury contamination, contaminated sediments are classified into three levels of contamination e lightly contaminated (mercury concentration less than 10 mg/kg dry weight), moderately contaminated (mercury concentration greater than or equal to 10 mg/kg but less than 20 mg/kg dry weight) and heavily contaminated (mercury concentration greater than or equal to 20 mg/kg dry weight). In the lake lightly contaminated sediments accounted for 38% of the area while moderately contaminated sediments accounted for 48% and heavily contaminated sediments accounted for 14%.
Ji Canal
Central lake
2.4. Remediation options
Discharge pipes
Yongding river Bohai Bay
Integral to the development of Tianjin Eco-City is the plan to remediate the wastewater impoundment lake that is now laden with contaminated sediments. The project to turn the wastewater impoundment area into an ecologically friendly recreational lake and surroundings was projected to cost USD 230 million. A total of about 5 million m3 of contaminated sediments with an average solids concentration of 10% was required to be dredged and disposed of as part of the lake remediation programme. As these sediments consisted predominantly of fine materials with contaminants they were not suitable fill material in the conventional sense for use in land reclamation or for other beneficial uses; and disposal at sea was not a viable option. Consequently, the onshore disposal options evaluated included the beneficial use of the solids following dewatering and stabilization or disposal at a secured landfill following dewatering. 2.5. Dewatering options
Fig. 1. General layout of the Tianjin Eco-City development site.
result of sediments discharged along with the untreated wastewater over the last 35 years. This top sediment layer can be further divided into three sub-layers according to differing solids concentration. The 0.2 me0.8 m thick top slurry sub-layer has an estimated average solids concentration of 2% while the 0.2 me0.6 m thick middle sub-layer has an estimated average solids concentration of 10%. These two sub-layers are unstable and can be easily dispersed into the water column when disturbed and can be carried away by currents. The 0.3 me1.2 m thick bottom sub-layer consists of a combination of clay, silt and fine sand with an estimated average solids concentration of 15%. Typical soil particle sizes in this black silty clay layer range between 5 and 11 mm. The lower brown clay layer represents the original ground surface prior to construction of the wastewater impoundment lake and is not contaminated. This layer is 0.2 me0.5 m in thickness and
Environmental dredging produces slurries with low solids concentrations, i.e. very high moisture contents, and these required dewatering for the entire onshore disposal options considered. Dewatering increases the solids concentration of the dredged sediments thereby changing their consistency to a solid or semisolid form which makes their handling and disposal easier. Dewatering also results in a large reduction in the volume of contaminated sediments for disposal. The options considered for dewatering included mechanical dewatering devices, such as belt filter presses and chamber filter presses, and geotextile tube dewatering technology. Geotextile tube dewatering technology has been successfully applied to dewater and dispose of contaminated sediments in projects internationally, e.g. Lawson (2008), Yee et al. (2006). Geotextile tube dewatering technology has the advantages of being able to handle large slurry volumes along with very high solids capture rates and low capital investment costs. It was for these reasons that geotextile tube dewatering technology was chosen as the dewatering option for this project.
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2.6. Strategy for managing the dredged contaminated sediments In keeping with its “Eco-City” name and culture, the strategy was to maximise the beneficial use of the contaminated dredged sediments in an environmentally permissible way within the Tianjin Eco-City development project. Fig. 2 shows the strategy flowchart for managing the dredged contaminated sediments according to the degree of contamination. Once dewatered, lightly and moderately contaminated sediments were to be put to beneficial use within the project site while heavily contaminated sediments were to be deposited in a secured landfill. The dewatered, lightly contaminated sediments were stabilized (with cement) and used for road subgrade fill. The dewatered, moderately contaminated sediments were used to construct a lakeside landscaped mound. Dewatering the three different categories of contaminated sediments required the construction of three different dewatering platforms around the edge of the lake. Effluent water from the geotextile tube dewatering process was returned to the wastewater impoundment lake. Later, a wastewater treatment plant will be built to service Tianjin Eco-City and this treatment plant will also be used to treat and clean up the water in the impoundment lake. 2.7. Lakeside landscaped mound Almost half (48%) of the contaminated sediments in the lake were classified as moderately contaminated, which accounted for some 2.4 million m3. Subsequent sections of this paper deal with the design and construction of a landscaped mound beside the central recreational lake to dewater and contain these moderately contaminated sediments. This mound consisted of the dewatered moderately contaminated sediments which were left in place inside the tubes following dewatering, with the facility later capped and landscaped. This innovative disposal solution made a lot of sense for a number of reasons. First, the dewatering tube facility was constructed in close proximity on a reclaimed platform extending from the Western side of the lake. Second, the close proximity of the dewatering facility to the lake minimised the piping and pumping energy required. Third, the dewatered solids remain secured within the dewatering tubes during and after the dewatering process. Fourth, the proposed dewatering facility being
Wastewater impoundment lake Dredged slurry Lightly
Heavily Degree of contamination
Geotextile tube dewatering
Moderately Stabilised solids Road subgrade use
Stabilised solids
Geotextile tube dewatering + landscaped capping
conceptually designed as a landfill with a geomembrane base liner and eventually a capping means the dewatered contaminated solids do not need to be transported to an off-site landfill. 3. Small-scale dewatering tube evaluation The principles by which geotextile tubes dewater slurry wastes have been described by Lawson (2008). Waste behaviour varies considerably, not only between different waste streams, but also from site to site for the same waste stream. Consequently, it is normal for each new dewatering application to undergo an evaluation programme in order to arrive at the optimal dewatering solution. This is normally accomplished in three stages. First, initial scoping tests are carried out to screen the waste stream/chemical accelerant combinations. Second, semi-performance tests are performed to determine likely dewatering rates and effluent quality. Third, full-scale prototype tests are carried out to confirm, or modify, the design assumptions obtained from semi-performance testing and to allow the new assumptions to be incorporated into the main project design. Depending on the scale of the project the full-scale prototype tests may be separate to, or part of, the main project. Dewatering effectiveness is normally measured by percentage solids capture, dewatering rate and achievable solids concentration of the dewatered material. Effluent water quality is also a measured success criteria, especially if there is no further water treatment planned prior to the effluent water being released back into the environment or reused. 3.1. Initial scoping tests Rapid Dewatering Tests (RDT) are where the slurry waste/ chemical accelerant combinations are poured into a container housing the proposed dewatering tube fabric (see Fig. 3(b)). Visual observation is made of the solids retention on the geotextile and the clarity of the effluent that has passed through the geotextile. From this, the chemical accelerant type and its approximate dosage rate can be determined. RDT evaluations were carried out to pre-qualify candidate geotextile and chemical accelerant types and dosage rates for the (later) GDT evaluations. This pre-qualification was based on the relative performance of achieving effluent quality and the rate of drying of the flocked slurry solids. 3.2. Semi-performance tests
Effluent
Effluent
Geotextile tube dewatering
41
External landfill Effluent
Fig. 2. Strategy flowchart for managing the contaminated sediments dredged from the wastewater impoundment lake.
Geotextile tube Dewatering Tests (GDT) utilise the geotextile tube fabric formed into a pillow-shape (see Fig. 3(c)) into which is poured the waste slurry/chemical accelerant combination (which is recorded). The rate of effluent outflow and its quality is measured over time. The final solids concentration and effluent quality are also measured. The results of the GDT evaluations confirmed the chemical accelerant type as well as its appropriate dosage rate from the RDT programme. The evaluation programme also confirmed the most appropriate geotextile hydraulic properties for the dewatering process. The final solids concentration achieved in the tests was 30% after 13 days. This was established as the initial target for the (later) full-scale trials. The effluent water quality was very good with <1 mg/kg of contaminants in the effluent in the first half hour of testing and then insignificant levels after that. The difference in clarity between the waste slurry and the effluent water from the GDT evaluations is shown in Fig. 3(d).
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Fig. 3. Small-scale tube dewatering evaluation (a) waste slurry sample (b) RDT evaluation (c) GDT evaluation (d) comparison between waste slurry and effluent water.
4. Full-scale prototype dewatering tube evaluation 4.1. Establishment of the prototype trial Because of the large quantities of contaminated sediments involved in this project it was decided to carry out full-scale prototype testing to fully assess the performance of the dewatering tube system and to arrive at accurate design parameters. The prototype testing was carried out on the (already constructed) dewatering platform for the treatment of the highly contaminated sediments located near the wastewater impoundment lake (one of the three platforms constructed for the treatment of the lightly contaminated, moderately contaminated and highly contaminated sediments). Fig. 4 shows the work procedure for the full-scale prototype trial beginning with the constructed dewatering platform (Fig. 4(a)), then the laying out of the dewatering tube (Fig. 4(b)), then the filling of the tube with slurry waste (Fig. 4(c)), and the monitoring of the dewatering tube (Fig. 4(d)). Finally, several of the tubes were cut open at the completion of the dewatering evaluation to determine the final solids concentration achieved and to observe the quality and consistency of the dewatered solids (Fig. 4(e)). Dewatering tubes were subsequently placed side by side and stacked according to the supplier’s recommendations to determine the likely performance of the final full-scale facility. Because of the nature of the full-scale prototype trial (the geotextile tube was installed on the extensive full-scale dewatering platform) only certain aspects were controllable during the filling and dewatering evaluation stages. Slurry volume and solids concentration entering the prototype tube were controlled and measured. Also, the change in height of the tube over time was controlled and recorded. Very little else could be accurately controlled or recorded. In order to accurately assess the performance of the full-scale prototype trials a series of relationships were derived in order to calculate the various dewatering parameters. The derivations of these relationships are contained in Appendix A and are based on the conservation of mass between slurry inflow, water outflow and amount contained within the
dewatering tube. These relationships enabled the important dewatering performance parameters to be determined based on the simple measurement of slurry inflow, solids concentration and tube fill height with time. 4.2. Tube fill height, hT The dewatering tubes were filled up to a specified control height with waste slurry and then filling was halted to allow the contained slurry to continue dewatering for a few days. As the slurry in the tube was dewatered, the tube height reduced, and subsequently, another cycle of filling and dewatering was carried out. This sequence of tube filling and dewatering was repeated over five to eight cycles, depending on the performance of the individual dewatering tube being tested. A number of filling and dewatering trials were carried out in this way. Data from one trial is listed in Table 1. This data is used for illustrative purposes below to demonstrate how the overall dewatering performance was assessed for the full-scale prototype trials. The dewatering tube used for this recorded trial had a circumference of 27.5 m (theoretical diameter DT ¼ 8.75 m) and length LT ¼ 30.6 m. Fig. 5(a) shows the detailed tube fill height profile over time for the full-scale prototype trial listed in Table 1. The maximum tube fill height was limited to hT ¼ 2.65 m which was considered the maximum safe filling height for the trials (giving hT/ DT ratio ¼ 0.3). 4.3. Dewatering rate, Qout Because of the large extent of the drainage platform (constructed full-scale to house the dewatering of the heavily contaminated sediments) and the size of the full-scale prototype tube it was not possible to physically measure the dewatering rate from the tube during the dewatering trial. Consequently, the relationships derived in Appendix A were used to calculate the dewatering rate based on the filling/dewatering data contained in Table 1 and the tube fill height profile shown in Fig. 5(a).
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Fig. 4. Full-scale prototype geotextile tube evaluation (a) dewatering platform (b) rolling out dewatering tube (c) filling dewatering tube (d) monitoring dewatering tube (e) opening tube following dewatering.
The calculated dewatering rate Qout versus time is shown in Fig. 5(b). The trial showed a marked difference between the dewatering rate during the filling process (tube inflation phase) and the dewatering rate when the filling process was stopped. Based on the first three cycles, the calculated dewatering rate of effluent water from the dewatering tube during the filling process ranged from 100 m3/h to 220 m3/h, with most of the values falling within the band of 150 m3/h to 200 m3/h. The calculated dewatering rate of
effluent water during the initial 100 h after filling had stopped ranged from 10 m3/h to 30 m3/h. After the initial 100 h however, the calculated dewatering rate of effluent water reduced to less than 5 m3/h. The reasons why the dewatering rates were significantly higher during the times that the tube was being filled was due to the slurry pumping creating a greater internal pressure within the tube leading to a greater dewatering rate; the slurry pumping breaking
Table 1 Overall data from slurry filling and dewatering for one of the full-scale prototype trials. Slurry filling 3
Cycle Cycle Cycle Cycle Cycle Cycle Cycle Cycle a
1 2 3 4 5 6 7 8
Dewatering
Rate (m /hr) Qin
Duration (hr) tin
Pumped in solids concentration (%) Sin
Tube height at end of filling (m) hT
a
275 275 275 275 275 275 275 275
1.5 5.3 2 3 4.5 1.3 1.7 3.3
6 6 6 6 6.5 5 8.5 6.5
0.45 1.38 1.35 1.80 2.65 1.90 2.65 2.65
4.1 17.6 12.2 13 38 3.8 37 76
Dewatering duration includes filling duration.
Duration (hr) tout
Tube height at end of dewatering (m) hT 0.25 0.70 0.70 1.50 1.30 1.78 1.34 1.57
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a
b
c
d
Fig. 5. Results from the full-scale prototype dewatering trial (a) tube height profile versus time (b) calculated dewatering rates (c) cumulative volume profile versus time (d) solids concentration in the tube versus time.
down any filter cake formed against the geotextile skin; the slurry pumping supplying additional water into the tube; and the slurry pumping disturbing and redistributing the solids inside the tube. 4.4. Volumes, Vin, Vout and VT Fig. 5(c) shows the cumulative volume profiles of the various dewatering tube components. The measured volume of slurry waste entering the tube Vin has been taken from Table 1 and is plotted in Fig. 5(c). Using the relationships derived in Appendix A the calculated volume of water exiting the tube Vout and the calculated volume contained within the tube VT is also plotted in Fig. 5(c). At the end of the dewatering trial (after 200 h) 6,180 m3 of waste slurry had been pumped into the dewatering tube, 5,600 m3 of water had exited the tube, and 580 m3 of dewatered sediments had been contained in the tube. 4.5. Solids concentration, ST Table 1 lists the solids concentration of the waste slurry entering the tube Sin over the period of the dewatering trial. For the first 4 filling/dewatering cycles Sin ¼ 6%. Thereafter, Sin ranged from 5% to 8.5%.
Using the relationships derived in Appendix A the solids concentration profile in the dewatering tube was calculated over the filling/dewatering period. This profile is shown in Fig. 5(d). At the end of the dewatering trial the solids concentration in the tube was calculated to be ST ¼ 53%. At the end of the trial the dewatering tube was cut open and the contained material was measured to have a solids concentration averaging 55% which is in excellent agreement to the calculated value. This final solids concentration was significantly greater than the solids concentrations achieved in the (earlier) GDT evaluations of 30% after 13 days (see section 3.2), which indicates significantly improved dewatering performance. It is to be expected that the full-scale evaluation gives the more realistic results. The dewatered solids are shown in Fig. 4(e) and appear quite dry and easily handled. 4.6. Effluent quality At points in time during the dewatering trial water samples were taken to test effluent quality. Generally, within the first hour of the dewatering trial beginning contaminant levels in the effluent water were less than 1 mg/kg (dry weight) and this reduced to negligible levels at later times. This demonstrated that the hydraulic properties of the dewatering tube in combination with
T.W. Yee et al. / Geotextiles and Geomembranes 31 (2012) 39e50 Table 2 Quantities of dewatering tubes used in the moderately contaminated dewatering facility. Dewatering tube layer
Tube circumference
Total length of tubes
1st layer 2nd layer 3rd layer a 4th layer
27.5 29.0 30.5 29.0
7,013 5,373 3,861 2,452
a
m m m m
45
the chemical accelerant were very effective in trapping the contaminants within the dewatering tube.
5. Final dewatering tube design parameters
m m m m
5.1. Tube filling control Based on the results of the full-scale trial where the tubes were filled to a maximum height of 2.65 m it was decided that in the final project the tubes would be filled to a maximum height of 3.0 m.
Additional 4th layer added during the operation phase e see section 6.3.
a N
Reclaimed for dewatering facility
Wastewater impoundment lake
Perimeter dyke of wastewater impoundment lake
b
400 kN/m woven polyester geotextile reinforcement
HDPE geomembrane + nonwoven geotextile cushion RL+3.65m
RL+4.0m
Perimeter drain
Drainage gravel RL+2.5m Compacted earthfill
RL+2.0m
Perimeter dyke
Sandy gravel RL+1.0m Lake foundation
RL0.0m
Fig. 6. Area location and cross-section of the dewatering platform (a) location (b) cross-section.
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This decision was made on the basis that several tubes had been successfully filled to this height (and greater) during the evaluation trials. At this maximum filled height the dewatered height would be expected to be 2.3 m. This was used as the basis for designing the final height of the dewatering tubes in the mound platform. 5.2. Tube target solids concentration The contaminated sediments in the lake bed are heterogeneous over its area and its depth. It was estimated that the average insitu solids concentration approximated 10%. Based on the full-scale dewatering evaluations, the minimum target solids concentration for the project dewatering tubes was set at 50%, which was slightly lower than that achieved in the full-scale trials (53% and 55% e see section 4.5).
and allowed heavy machinery to be deployed immediately for construction works. A second layer of geotextile reinforcement was then laid on top of the sandy gravel layer. The minimum design factor of safety adopted against instability through the backfilled mound and the soft foundation underneath was 1.25. The dewatering facility was designed according to the principles of a landfill; having a geomembrane liner for liquid isolation and a drainage system above this liner for drainage, collection, and removal of the effluent water. Earthfill was placed above the top geotextile reinforcement layer and compacted to form the base for the laying of the HDPE geomembrane liner. Above the geomembrane liner a nonwoven geotextile cushion layer was laid before drainage aggregate was placed on top. The top of the dewatering facility platform corresponds to the top of the gravel drainage blanket at RL þ 3.65 m. Effluent water from the dewatering process was designed to pass into the drainage blanket
5.3. Volumes, tube numbers and tube stacking One critical aspect of design concerns the question of the slurry volume reduction achievable using the dewatering tube solution and the time needed to achieve it. When the insitu lake bed contaminated sediments with an initial solids concentration of 10% are reduced to a final solids concentration of 50% (see section 5.2), the volume reduction will be about 83% (approximately 6 times) based on the assumption of 100% solids capture within the dewatering tubes. Based on this volume reduction ratio, the 2,400,000 m3 of moderately contaminated lake bed sediments would reduce to about 400,000 m3 of dewatered solids remaining in the dewatering tubes. This volume reduction ratio was used to engineer the sizes, numbers and stacking of the dewatering tubes to form the core of the landscaped mound. Based on the containment of 400,000 m3 of dewatered solids, and the use of 27.5 m circumference dewatering tubes, the total length of tubes required to dewater the moderately contaminated sediments was 15.6 km. Due to area limitations at the dewatering platform site it was necessary to stack the tubes into multiple layers. Consequently, the tubes were resized to meet the optimal stacking geometry and three planned layers were necessary to meet the dewatered volume requirements. Table 2 details the dewatering tubes used in the facility. 6. Design, construction and operation of the dewatering tube facility
N Drainage platform 1st layer dewatering tubes 2nd layer dewatering tubes
3rd layer dewatering tubes
Land
Impoundment lake
6.1. Dewatering platform The dewatering tube facility for the treatment of the moderately contaminated sediments was located on a site reclaimed from the Western side of the impoundment lake (see Fig. 6(a)). The reclaimed area approximates 120,000 m2 with a maximum length about 760 m and a maximum width about 230 m. Initially, a perimeter dyke with crest elevation at RL þ 4.0 m was formed to enclose the reclamation area. The enclosed area was then pumped dry to allow ground stabilization works to begin. The cross-section details of the dewatering platform are shown in Fig. 6(b). To stabilize the very soft lake foundation and improve the bearing capacity of the foundation for the dewatering facility two layers of 400 kN/m woven polyester geotextile reinforcement were used as basal reinforcement across the base of the facility. One layer was laid at the lake bottom which had been pumped to remove as much ponding water as possible. A sandy gravel layer 1 m in thickness was then laid above the bottom geotextile reinforcement layer. The combination of sandy gravel and geotextile reinforcement acted as a stabilized working platform over the very soft foundation
Reclamation dyke
Fig. 7. Plan layout of the dewatering tubes.
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47
quickly. The bottom of the drainage blanket slopes both Eastward and Westward on a North-South alignment to facilitate water collection. Two lines of perimeter drains with grilled covers, one on the Eastern side and one on the Western side of the dewatering facility, were designed to collect the effluent water from the drainage blanket and transfer it to collection sumps. Pumps were used to remove and return the effluent water to the wastewater impoundment lake.
79.0 m. The use of dewatering tubes with marginally larger circumferences for the upper layers is to ensure that a standard layer thickness of 2.3 m is achievable for each layer of dewatered tubes. The planned top of the three layer stack of dewatering tubes corresponds approximately to RL þ 10.55 m.
6.2. Dewatering tubes layout
Three dredges were deployed for the supply of sediment slurry to the dewatering platform e see Fig. 8(a). These dredges were dismantled for transportation by land to site and reassembled for use on the impoundment lake. One dredge had a pumping capacity of 1,600 m3/h while the remaining two dredges each had a pumping capacity of 700 m3/h. Although the total pumping capacity worked out at 3,000 m3/h, at most times usually only two dredges were operating simultaneously on a 24 h basis. Therefore, the pumping capacity averaged between 1,500 and 2,000 m3/h. A lifting crane was used to lift the roll of geotextile tube into position on the platform. The dewatering tubes were rolled out on the drainage platform according to the layout plan e see Fig. 8(b). About six tubes were laid out at any one time at an average layout rate of 20 min per roll.
It was planned to stack the dewatering tubes in three layers; each layer having a dewatered thickness of approximately 2.3 m. Fig. 7 shows the plan dewatering tube layout on the drainage platform. The first layer of dewatering tubes was standardized using a circumference of 27.5 m e see Table 2. This layer consisted of 135 units of dewatering tubes with lengths ranging from 17.0 m to 79.0 m. The second layer of dewatering tubes was standardized using a circumference of 29.0 m e see Table 2. This layer consisted of 86 units of dewatering tubes with lengths ranging from 17.0 m to 79.0 m. The third layer of dewatering tubes was standardized using a circumference of 30.5 m e see Table 2. This layer consisted of 77 units of dewatering tubes with lengths ranging from 13.0 m to
6.3. Operation of the dewatering tube facility
Fig. 8. Operating the dewatering tube facility (a) dredging contaminated sediments (b) laying out dewatering tubes (c) slurry pipeline to dewatering tubes (d) first two layers of dewatering tubes (e) extent of the dewatering facility.
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Fig. 9. Details of the capping for the dewatered contaminated sediments.
The sediment slurry was supplied by a floating pipeline across the surface of the impoundment lake that was then connected into an onshore pipeline network. The onshore pipeline network branched out with one line of 450 mm diameter steel pipes running along the Western edge of the dewatering platform and another line, also of 450 mm diameter steel pipes, running along the Eastern edge of the dewatering platform. Tap off steel pipe outlets of 160 mm diameter with gate control valves were positioned in sets of six at convenient locations along the onshore incoming slurry pipelines e see Fig. 8(c), (d) and (e). Generally, at any time six tubes were filled simultaneously, keeping the filling rate at between 250 and 300 m3/h, consistent with the filling rate in the full-scale prototype trials. When these tubes were filled to the control height, the control valves were shut while sediment slurry was diverted to the adjacent battery of six tubes laid out ahead of time. When the filled tubes had dewatered for some time and reduced in height, they were filled again to the control height. This repeated filling and dewatering was controlled by the adjustment of the manifold valve system until the target solids concentration of 50% in the tube was achieved. Once this was achieved, the next layer of tubes was then deployed. The process was then repeated for this new layer of dewatering tubes. However, the rate of work progress was comparatively slower for the upper tube layers than that experienced for work on the bottom tube layer due to difficulties in traversing over the lower dewatering tube layers. For example, the time taken to unroll six tubes for the upper layers was about 40 min per roll (twice that for the bottom layer). The chemical accelerant was added to the sediment slurry before it was pumped into the dewatering tubes. Downstream of the accelerant injection points, sampling points were provided to allow extraction of samples to check quality of flocculation. Samples were taken every morning and evening. The solids concentration of the sediment slurry was also checked several times per week. Based on experience on site, the solids concentration was maintained within a range of 5%e8% for optimum solids delivery and flocculation quality. Five water pump units, each with pumping capacity of 350 m3/h, situated at five sump locations, were used to pump the water effluent into steel pipes of 820 mm diameter. These pipes, which were laid beside the incoming slurry pipes, discharged the water effluent back into the impoundment lake e see Fig. 8(c) and (e). In general terms the dewatering tube work progressed very well. In an attempt to catch up on the overall project schedule the dewatering work was sped up which meant each layer of dewatering tubes were allowed less time than was required to fill with adequate solids. Consequently, the design dewatered height of 2.3 m for each layer of tubes was not achieved. The contract allowed for a 10% variation of quantity for the tubes and the client decided to utilize this provisional quantity to add a fourth layer of
dewatering tubes to achieve the design platform height e see Fig. 9. Table 2 lists the quantities of dewatering tubes used in the four layers. 6.4. Mound final capping Fig. 9 shows the typical East-West alignment cross-section of the artificial mound. Once the tubes had dewatered, general soil fill was used to cover over the geotextile tubes and form a smooth, graded surface for the mound core. The bermed side slopes were graded to an inclination of about 1V:5H. The final capping consisted of a HDPE geomembrane liner laid over the prepared soil surface with a two-layered soil cover placed on top of the geomembrane liner. The lower layer of soil cover consisted of 1 m thickness of coarse grained fill while the upper layer of soil cover consisted of 1.5 m thickness of nutrient rich top soil to support vegetation. The finished level of the mound top platform averaged RL þ 13.0 m, shaped with a North-South aligned mid ridge that slopes both Eastward and Westward at a gentle downward inclination of 5% to promote surface runoff. 7. Conclusions The use of geotextile tubes to dewater a large volume of contaminated sediments at the Tianjin Eco-City site in China has proven to be a highly successful project. The contaminated sediments were divided into three different waste streams according to the degree of contamination e lightly contaminated, moderately contaminated and highly contaminated. All three waste streams were dewatered using geotextile tubes with the contained dewatered solids subsequently used on site, or disposed of, depending on the degree of contamination. For the dewatered, lightly contaminated sediment stream, the contained solids were stabilized with cement and used as part of the pavement earthworks on site. For the dewatered, highly contaminated sediment stream, the contained solids were stabilized and deposited in a specially constructed, secured, landfill. The dewatered, moderately contaminated sediment stream was left in place to form a landscaped mound within the project site. In assessing the performance of the geotextile tube dewatering and generating the final design criteria a conventional approach was followed beginning with small-scale evaluations leading to full-scale prototype testing. To supplement the measured data obtained from the full-scale tests a series of dewatering relationships were developed which enabled the relevant performance parameters to be determined. These were used as the basis for establishing the design criteria for the overall dewatering project. The dewatering platform for the moderately contaminated sediment stream was located on a reclaimed area on the Western side of the water impoundment lake. The site was designed similar
T.W. Yee et al. / Geotextiles and Geomembranes 31 (2012) 39e50
to a landfill with a base barrier system with drainage, and a capping barrier system with an earth covering following completion of the dewatering process. Dewatering the volume of moderately contaminated sediments required four layers of geotextile tubes spread over the dewatering facility. The filling/dewatering cycles were managed in a planned, progressive manner to ensure operational efficiency.
Appendix A. Derivation of dewatering tube parameters Conservation of mass throughout the dewatering process gives:
Min ¼ MT þ Mout
(A1)
where, Min ¼ mass of slurry entering the dewatering tube, MT ¼ mass of slurry contained in the dewatering tube, and Mout ¼ mass exiting the dewatering tube. These variables are shown in Fig. A1. Over any time interval t-1 to t the conservation of mass equation becomes:
½Min tt1 ¼ ½Mout tt1 þ ½DMT tt1
(A2)
where, DMT ¼ change in dewatering tube mass. Mass entering the dewatering tube, Min: Here it is assumed that slurry enters the tube with a solids concentration ¼ Sin, where Sin ¼ ratio of mass of solids to total mass of slurry inflow. Associated relationships are as follows:
Msin ¼ Min Sin and Mwin ¼ Min ð1 Sin Þ
(A3)
therefore,
Vsin
VT ¼ VsT þ VwT
(A4)
VT ¼ D2T LT
"
hT DT
0:815 8:6 # h T DT
Vin ¼ Qin tin and Vin ¼ Vsin þ Vwin
(A5)
(A9)
where, VT ¼ tube volume, LT ¼ tube length, DT ¼ theoretical diameter of the tube, hT ¼ filling height of the tube. The relationship between VT/LT and hT/DT for various tube theoretical diameters DT is plotted graphically in Fig. A2. Mass exiting the dewatering tube, Mout: Here, it is assumed that all solids are contained within the tube and only water exits the tube. Thus,
Mwout ¼ Mout
(A10) Mout
gw
(A11)
and,
Vout ¼ Qout tout
and,
(A8)
where, Ms-T ¼ mass of solids in the tube, MT ¼ total mass of slurry in the tube, ST ¼ solids concentration in the tube, Mw-T ¼ mass of water in the tube, Vs-T ¼ volume of solids in the tube, Vw-T ¼ volume of water in the tube, G ¼ specific gravity of the solids, gw ¼ density of water, VT ¼ volume of slurry in the tube. The volume of slurry in the dewatering tube VT is regulated by its filling height. This also provides the required level of safety against tube rupture. To develop a relationship between filling height and contained volume for various sized tubes the computer programs GeoCops (Leshchinsky et al., 1996) and SoftWin (Palmerton, 2002) were used. Both gave the same results. The relationship derived is as follows:
Vwout ¼ Vout ¼
M S M ð1 Sin Þ ¼ in in and Vwin ¼ in gw G gw
49
(A12)
where, Mw-out ¼ mass of water exiting the tube, Mout ¼ total mass exiting the tube, Vw-out ¼ volume of water exiting the tube,
where, Ms-in ¼ mass of solids entering the tube, Min ¼ total mass of slurry entering the tube, Mw-in ¼ mass of water entering the tube, Vs-in ¼ volume of solids entering the tube, Vw-in ¼ volume of water entering the tube, G ¼ specific gravity of the solids, gw ¼ density of water, Vin ¼ slurry volume entering the tube, Qin ¼ volume pumping rate entering the tube, tin ¼ time period. Mass contained in the dewatering tube, MT: The mass and volume of the solids and water components inside the dewatering tube is:
MsT ¼ MT ST and MwT ¼ MT ð1 ST Þ
(A6)
therefore,
VsT ¼
MT ST M ð1 ST Þ and VwT ¼ T gw Ggw
(A7)
and, Mass, Min
Mass, MT Dewatering tube
Mass, Mout
Fig. A1. Description of the overall dewatering tube mass components.
Fig. A2. Relationship between VT/LT and hT/DT for various tube theoretical diameters DT.
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Vout ¼ total volume exiting the tube, Qout ¼ volume dewatering rate, tout ¼ time period for dewatering. There is also a conservation of mass of the water phase of the dewatering process,
Mwout ¼ Mwin MwT
(A14)
The volume of water flowing out of the tube during any time interval t-1 to t will be:
½Vwout tt1 ¼ ½Vout tt1 ¼ ½Vwin tt1 ½DVwT tt1
Glossary
(A13)
therefore,
Vwout ¼ Vwin VwT
Yee, T.W., Lim, L.K., Choi, J.C., 2006. Geotextile containment, dewatering and disposal of contaminated dredged material. Proceedings International Conference on New Developments in Geoenvironmental and Geotechnical Engineering, Incheon, Korea, 428e439.
(A15)
where, DVw-T ¼ change in volume of water in the tube during time interval t-1 to t. The above equations can be entered into a spreadsheet program to analyse the various dewatering parameters over time. References Lawson, C.R., 2008. Geotextile containment for hydraulic and environmental engineering. J. Geosynthetics International 15 (6), 384e427. Leshchinsky, D., Leshchinsky, O., Ling, H.I., Gilbert, P.A., 1996. Geosynthetic tubes for confining pressurized slurry: some design aspects. J. Geotechnical Eng. Division, Am. Soc. Civil Engineers 122 (8), 682e690. Palmerton, J. B., 2002. Distinct element modelling of geosynthetic fabric containers. Proceedings of the 7th International Conference on Geosynthetics, Nice, France, vol. 3, 1021e1024.
DT: theoretical diameter of the tube G: specific gravity of solids hT: filling height of the tube LT: tube length Min: mass of slurry entering the tube Mout: mass exiting the tube Ms-in: mass of solids entering the tube Ms-T: mass of solids in the tube MT: mass of slurry in the tube Mw-in: mass of water entering the tube Mw-out: mass of water exiting the tube Mw-T: mass of water in the tube Qin: volume pumping rate entering the tube Qout: volume dewatering rate Sin: solids concentration of slurry entering the tube ST: solids concentration of slurry in the tube t: time tin: time period entering the tube tout: time period for dewatering Vin: slurry volume entering the tube Vout: volume exiting the tube Vs-in: volume of solids entering the tube Vs-T: volume of solids in the tube VT: volume of slurry in the tube Vw-in: volume of water entering the tube Vw-out: volume of water exiting the tube Vw-T: volume of water in the tube gw: density of water
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Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem
Geotextile tube dewatering of contaminated sediments, Tianjin Eco-City, China T.W. Yee a, C.R. Lawson a, *, Z.Y. Wang b, L. Ding b, Y. Liu b a b
TenCate Geosynthetics Asia Sdn Bhd, 14, Jalan Sementa 27/91, 40400 Shah Alam, Malaysia TenCate Industrial Zhuhai Co. Ltd., Gaolan Port Economic Zone, Zhuhai, Guangdong Province, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 March 2011 Received in revised form 17 July 2011 Accepted 19 July 2011 Available online 11 August 2011
An account is given of the use of geotextile tubes to dewater dredged contaminated sediments at the Tianjin Eco-City site in China. Approximately 5 million m3 of contaminated sediments from the bed of a lake were dredged and dewatered in this way with the effluent water returned to the lake. The dewatered solids were utilized within the project site, or were disposed of in a landfill, depending on their degree of contamination. The paper details the tube dewatering evaluation process undertaken and presents the results on which the dewatering facility was designed. To enable an assessment of the fullscale dewatering performance various relationships were derived based on a conservation of mass of the dewatering process. The design, construction and operation of the dewatering tube facility for the treatment of the moderately contaminated sediment waste stream is also presented. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Geotextile tubes Dewatering Contaminated sediments
1. Introduction Tianjin Eco-City is a new development in China being undertaken in an ecologically sound and environmentally sustainable manner. The Tianjin Eco-City project is a strategic cooperation between the China and Singapore National Governments aiming to improve the living environment and to build an eco-culture that will serve as a role model for future developments in the country. The project site is located on an existing wetland area adjacent to the coastline of Bohai Bay in Hangu District, about 40 km East of Tianjin city centre. Tianjin is located approximately 150 km Southeast of Beijing the capital. The new city will have an area of 30 km2 with a planned population of 350,000. Prior to the development of Tianjin Eco-City, the land consisted of either natural salt marshes or salt fields and ponds for shrimp and crab farming. The general site layout is shown in Fig. 1. It is bounded by the Ji Canal to the Northwest and West, and Bohai Bay to the Southeast. An important part of the development of this area is the provision of a central lake around which will be constructed some of the housing developments. The lake is also required to provide leisure and recreational facilities for the local population in a clean and healthy environment. However, in its existing state, this lake has been used as a wastewater impoundment containing domestic and industrial wastewater. An extensive programme of remediation and
* Corresponding author. E-mail address: [email protected] (C.R. Lawson). 0266-1144/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2011.07.005
development of the lake was required before it could be utilised in an environmentally safe manner. 2. Existing wastewater impoundment lake 2.1. Background In early 1976 the Hangu District Council converted a low lying area into a wastewater impoundment lake. The wastewater lake was created by the construction of a 3 m high perimeter earth dyke. The lake covers an area of about 3.0 km2 with an impoundment capacity of 5.6 million m3. The lake bottom slopes from the Western side to the Eastern side of the lake. At the Southwest edge of the lake are located two pipe sluice gates, each 2 m in diameter. These pipes discharge into a culvert that drains directly to the sea. The Ji Canal flows adjacent to the wastewater impoundment lake on its way to the sea. During the rainy season when the Ji Canal is prone to overflowing, the sluice gates are closed to prevent river and canal water backflow into the wastewater impoundment lake. From mid1976 this wastewater impoundment lake began receiving untreated domestic and industrial wastewater from Hangu District, pumped in through underground pipes. 2.2. Lake bed soil layers Investigations of the lake bed revealed a generally consistent layering of soil types. A black silty clay layer of thickness between 1.3 m and 2.5 m overlies a brown clay layer. This black silty clay layer with high organic content, odour and contaminants is the
40
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N Eco-City development area
represents the alluvial deposits formed from the Ji Canal overflows. Typical soil particle sizes range between 6 and 9 mm. The demarcation of the two layers is quite distinct with no significant existence of a contamination transition zone. Below these two layers is marine clay.
2.3. Contaminated sediments
Ji Canal The upper black silty clay layer is contaminated with mercury, arsenic, copper, cadmium, hexachlorobenzene and DDT along with high levels of raw sewage. The degree of contamination tends to be higher at the wastewater entry point and around the shores of the wastewater lake. Based on current practice adopted in Tianjin for mercury contamination, contaminated sediments are classified into three levels of contamination e lightly contaminated (mercury concentration less than 10 mg/kg dry weight), moderately contaminated (mercury concentration greater than or equal to 10 mg/kg but less than 20 mg/kg dry weight) and heavily contaminated (mercury concentration greater than or equal to 20 mg/kg dry weight). In the lake lightly contaminated sediments accounted for 38% of the area while moderately contaminated sediments accounted for 48% and heavily contaminated sediments accounted for 14%.
Ji Canal
Central lake
2.4. Remediation options
Discharge pipes
Yongding river Bohai Bay
Integral to the development of Tianjin Eco-City is the plan to remediate the wastewater impoundment lake that is now laden with contaminated sediments. The project to turn the wastewater impoundment area into an ecologically friendly recreational lake and surroundings was projected to cost USD 230 million. A total of about 5 million m3 of contaminated sediments with an average solids concentration of 10% was required to be dredged and disposed of as part of the lake remediation programme. As these sediments consisted predominantly of fine materials with contaminants they were not suitable fill material in the conventional sense for use in land reclamation or for other beneficial uses; and disposal at sea was not a viable option. Consequently, the onshore disposal options evaluated included the beneficial use of the solids following dewatering and stabilization or disposal at a secured landfill following dewatering. 2.5. Dewatering options
Fig. 1. General layout of the Tianjin Eco-City development site.
result of sediments discharged along with the untreated wastewater over the last 35 years. This top sediment layer can be further divided into three sub-layers according to differing solids concentration. The 0.2 me0.8 m thick top slurry sub-layer has an estimated average solids concentration of 2% while the 0.2 me0.6 m thick middle sub-layer has an estimated average solids concentration of 10%. These two sub-layers are unstable and can be easily dispersed into the water column when disturbed and can be carried away by currents. The 0.3 me1.2 m thick bottom sub-layer consists of a combination of clay, silt and fine sand with an estimated average solids concentration of 15%. Typical soil particle sizes in this black silty clay layer range between 5 and 11 mm. The lower brown clay layer represents the original ground surface prior to construction of the wastewater impoundment lake and is not contaminated. This layer is 0.2 me0.5 m in thickness and
Environmental dredging produces slurries with low solids concentrations, i.e. very high moisture contents, and these required dewatering for the entire onshore disposal options considered. Dewatering increases the solids concentration of the dredged sediments thereby changing their consistency to a solid or semisolid form which makes their handling and disposal easier. Dewatering also results in a large reduction in the volume of contaminated sediments for disposal. The options considered for dewatering included mechanical dewatering devices, such as belt filter presses and chamber filter presses, and geotextile tube dewatering technology. Geotextile tube dewatering technology has been successfully applied to dewater and dispose of contaminated sediments in projects internationally, e.g. Lawson (2008), Yee et al. (2006). Geotextile tube dewatering technology has the advantages of being able to handle large slurry volumes along with very high solids capture rates and low capital investment costs. It was for these reasons that geotextile tube dewatering technology was chosen as the dewatering option for this project.
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2.6. Strategy for managing the dredged contaminated sediments In keeping with its “Eco-City” name and culture, the strategy was to maximise the beneficial use of the contaminated dredged sediments in an environmentally permissible way within the Tianjin Eco-City development project. Fig. 2 shows the strategy flowchart for managing the dredged contaminated sediments according to the degree of contamination. Once dewatered, lightly and moderately contaminated sediments were to be put to beneficial use within the project site while heavily contaminated sediments were to be deposited in a secured landfill. The dewatered, lightly contaminated sediments were stabilized (with cement) and used for road subgrade fill. The dewatered, moderately contaminated sediments were used to construct a lakeside landscaped mound. Dewatering the three different categories of contaminated sediments required the construction of three different dewatering platforms around the edge of the lake. Effluent water from the geotextile tube dewatering process was returned to the wastewater impoundment lake. Later, a wastewater treatment plant will be built to service Tianjin Eco-City and this treatment plant will also be used to treat and clean up the water in the impoundment lake. 2.7. Lakeside landscaped mound Almost half (48%) of the contaminated sediments in the lake were classified as moderately contaminated, which accounted for some 2.4 million m3. Subsequent sections of this paper deal with the design and construction of a landscaped mound beside the central recreational lake to dewater and contain these moderately contaminated sediments. This mound consisted of the dewatered moderately contaminated sediments which were left in place inside the tubes following dewatering, with the facility later capped and landscaped. This innovative disposal solution made a lot of sense for a number of reasons. First, the dewatering tube facility was constructed in close proximity on a reclaimed platform extending from the Western side of the lake. Second, the close proximity of the dewatering facility to the lake minimised the piping and pumping energy required. Third, the dewatered solids remain secured within the dewatering tubes during and after the dewatering process. Fourth, the proposed dewatering facility being
Wastewater impoundment lake Dredged slurry Lightly
Heavily Degree of contamination
Geotextile tube dewatering
Moderately Stabilised solids Road subgrade use
Stabilised solids
Geotextile tube dewatering + landscaped capping
conceptually designed as a landfill with a geomembrane base liner and eventually a capping means the dewatered contaminated solids do not need to be transported to an off-site landfill. 3. Small-scale dewatering tube evaluation The principles by which geotextile tubes dewater slurry wastes have been described by Lawson (2008). Waste behaviour varies considerably, not only between different waste streams, but also from site to site for the same waste stream. Consequently, it is normal for each new dewatering application to undergo an evaluation programme in order to arrive at the optimal dewatering solution. This is normally accomplished in three stages. First, initial scoping tests are carried out to screen the waste stream/chemical accelerant combinations. Second, semi-performance tests are performed to determine likely dewatering rates and effluent quality. Third, full-scale prototype tests are carried out to confirm, or modify, the design assumptions obtained from semi-performance testing and to allow the new assumptions to be incorporated into the main project design. Depending on the scale of the project the full-scale prototype tests may be separate to, or part of, the main project. Dewatering effectiveness is normally measured by percentage solids capture, dewatering rate and achievable solids concentration of the dewatered material. Effluent water quality is also a measured success criteria, especially if there is no further water treatment planned prior to the effluent water being released back into the environment or reused. 3.1. Initial scoping tests Rapid Dewatering Tests (RDT) are where the slurry waste/ chemical accelerant combinations are poured into a container housing the proposed dewatering tube fabric (see Fig. 3(b)). Visual observation is made of the solids retention on the geotextile and the clarity of the effluent that has passed through the geotextile. From this, the chemical accelerant type and its approximate dosage rate can be determined. RDT evaluations were carried out to pre-qualify candidate geotextile and chemical accelerant types and dosage rates for the (later) GDT evaluations. This pre-qualification was based on the relative performance of achieving effluent quality and the rate of drying of the flocked slurry solids. 3.2. Semi-performance tests
Effluent
Effluent
Geotextile tube dewatering
41
External landfill Effluent
Fig. 2. Strategy flowchart for managing the contaminated sediments dredged from the wastewater impoundment lake.
Geotextile tube Dewatering Tests (GDT) utilise the geotextile tube fabric formed into a pillow-shape (see Fig. 3(c)) into which is poured the waste slurry/chemical accelerant combination (which is recorded). The rate of effluent outflow and its quality is measured over time. The final solids concentration and effluent quality are also measured. The results of the GDT evaluations confirmed the chemical accelerant type as well as its appropriate dosage rate from the RDT programme. The evaluation programme also confirmed the most appropriate geotextile hydraulic properties for the dewatering process. The final solids concentration achieved in the tests was 30% after 13 days. This was established as the initial target for the (later) full-scale trials. The effluent water quality was very good with <1 mg/kg of contaminants in the effluent in the first half hour of testing and then insignificant levels after that. The difference in clarity between the waste slurry and the effluent water from the GDT evaluations is shown in Fig. 3(d).
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Fig. 3. Small-scale tube dewatering evaluation (a) waste slurry sample (b) RDT evaluation (c) GDT evaluation (d) comparison between waste slurry and effluent water.
4. Full-scale prototype dewatering tube evaluation 4.1. Establishment of the prototype trial Because of the large quantities of contaminated sediments involved in this project it was decided to carry out full-scale prototype testing to fully assess the performance of the dewatering tube system and to arrive at accurate design parameters. The prototype testing was carried out on the (already constructed) dewatering platform for the treatment of the highly contaminated sediments located near the wastewater impoundment lake (one of the three platforms constructed for the treatment of the lightly contaminated, moderately contaminated and highly contaminated sediments). Fig. 4 shows the work procedure for the full-scale prototype trial beginning with the constructed dewatering platform (Fig. 4(a)), then the laying out of the dewatering tube (Fig. 4(b)), then the filling of the tube with slurry waste (Fig. 4(c)), and the monitoring of the dewatering tube (Fig. 4(d)). Finally, several of the tubes were cut open at the completion of the dewatering evaluation to determine the final solids concentration achieved and to observe the quality and consistency of the dewatered solids (Fig. 4(e)). Dewatering tubes were subsequently placed side by side and stacked according to the supplier’s recommendations to determine the likely performance of the final full-scale facility. Because of the nature of the full-scale prototype trial (the geotextile tube was installed on the extensive full-scale dewatering platform) only certain aspects were controllable during the filling and dewatering evaluation stages. Slurry volume and solids concentration entering the prototype tube were controlled and measured. Also, the change in height of the tube over time was controlled and recorded. Very little else could be accurately controlled or recorded. In order to accurately assess the performance of the full-scale prototype trials a series of relationships were derived in order to calculate the various dewatering parameters. The derivations of these relationships are contained in Appendix A and are based on the conservation of mass between slurry inflow, water outflow and amount contained within the
dewatering tube. These relationships enabled the important dewatering performance parameters to be determined based on the simple measurement of slurry inflow, solids concentration and tube fill height with time. 4.2. Tube fill height, hT The dewatering tubes were filled up to a specified control height with waste slurry and then filling was halted to allow the contained slurry to continue dewatering for a few days. As the slurry in the tube was dewatered, the tube height reduced, and subsequently, another cycle of filling and dewatering was carried out. This sequence of tube filling and dewatering was repeated over five to eight cycles, depending on the performance of the individual dewatering tube being tested. A number of filling and dewatering trials were carried out in this way. Data from one trial is listed in Table 1. This data is used for illustrative purposes below to demonstrate how the overall dewatering performance was assessed for the full-scale prototype trials. The dewatering tube used for this recorded trial had a circumference of 27.5 m (theoretical diameter DT ¼ 8.75 m) and length LT ¼ 30.6 m. Fig. 5(a) shows the detailed tube fill height profile over time for the full-scale prototype trial listed in Table 1. The maximum tube fill height was limited to hT ¼ 2.65 m which was considered the maximum safe filling height for the trials (giving hT/ DT ratio ¼ 0.3). 4.3. Dewatering rate, Qout Because of the large extent of the drainage platform (constructed full-scale to house the dewatering of the heavily contaminated sediments) and the size of the full-scale prototype tube it was not possible to physically measure the dewatering rate from the tube during the dewatering trial. Consequently, the relationships derived in Appendix A were used to calculate the dewatering rate based on the filling/dewatering data contained in Table 1 and the tube fill height profile shown in Fig. 5(a).
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43
Fig. 4. Full-scale prototype geotextile tube evaluation (a) dewatering platform (b) rolling out dewatering tube (c) filling dewatering tube (d) monitoring dewatering tube (e) opening tube following dewatering.
The calculated dewatering rate Qout versus time is shown in Fig. 5(b). The trial showed a marked difference between the dewatering rate during the filling process (tube inflation phase) and the dewatering rate when the filling process was stopped. Based on the first three cycles, the calculated dewatering rate of effluent water from the dewatering tube during the filling process ranged from 100 m3/h to 220 m3/h, with most of the values falling within the band of 150 m3/h to 200 m3/h. The calculated dewatering rate of
effluent water during the initial 100 h after filling had stopped ranged from 10 m3/h to 30 m3/h. After the initial 100 h however, the calculated dewatering rate of effluent water reduced to less than 5 m3/h. The reasons why the dewatering rates were significantly higher during the times that the tube was being filled was due to the slurry pumping creating a greater internal pressure within the tube leading to a greater dewatering rate; the slurry pumping breaking
Table 1 Overall data from slurry filling and dewatering for one of the full-scale prototype trials. Slurry filling 3
Cycle Cycle Cycle Cycle Cycle Cycle Cycle Cycle a
1 2 3 4 5 6 7 8
Dewatering
Rate (m /hr) Qin
Duration (hr) tin
Pumped in solids concentration (%) Sin
Tube height at end of filling (m) hT
a
275 275 275 275 275 275 275 275
1.5 5.3 2 3 4.5 1.3 1.7 3.3
6 6 6 6 6.5 5 8.5 6.5
0.45 1.38 1.35 1.80 2.65 1.90 2.65 2.65
4.1 17.6 12.2 13 38 3.8 37 76
Dewatering duration includes filling duration.
Duration (hr) tout
Tube height at end of dewatering (m) hT 0.25 0.70 0.70 1.50 1.30 1.78 1.34 1.57
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a
b
c
d
Fig. 5. Results from the full-scale prototype dewatering trial (a) tube height profile versus time (b) calculated dewatering rates (c) cumulative volume profile versus time (d) solids concentration in the tube versus time.
down any filter cake formed against the geotextile skin; the slurry pumping supplying additional water into the tube; and the slurry pumping disturbing and redistributing the solids inside the tube. 4.4. Volumes, Vin, Vout and VT Fig. 5(c) shows the cumulative volume profiles of the various dewatering tube components. The measured volume of slurry waste entering the tube Vin has been taken from Table 1 and is plotted in Fig. 5(c). Using the relationships derived in Appendix A the calculated volume of water exiting the tube Vout and the calculated volume contained within the tube VT is also plotted in Fig. 5(c). At the end of the dewatering trial (after 200 h) 6,180 m3 of waste slurry had been pumped into the dewatering tube, 5,600 m3 of water had exited the tube, and 580 m3 of dewatered sediments had been contained in the tube. 4.5. Solids concentration, ST Table 1 lists the solids concentration of the waste slurry entering the tube Sin over the period of the dewatering trial. For the first 4 filling/dewatering cycles Sin ¼ 6%. Thereafter, Sin ranged from 5% to 8.5%.
Using the relationships derived in Appendix A the solids concentration profile in the dewatering tube was calculated over the filling/dewatering period. This profile is shown in Fig. 5(d). At the end of the dewatering trial the solids concentration in the tube was calculated to be ST ¼ 53%. At the end of the trial the dewatering tube was cut open and the contained material was measured to have a solids concentration averaging 55% which is in excellent agreement to the calculated value. This final solids concentration was significantly greater than the solids concentrations achieved in the (earlier) GDT evaluations of 30% after 13 days (see section 3.2), which indicates significantly improved dewatering performance. It is to be expected that the full-scale evaluation gives the more realistic results. The dewatered solids are shown in Fig. 4(e) and appear quite dry and easily handled. 4.6. Effluent quality At points in time during the dewatering trial water samples were taken to test effluent quality. Generally, within the first hour of the dewatering trial beginning contaminant levels in the effluent water were less than 1 mg/kg (dry weight) and this reduced to negligible levels at later times. This demonstrated that the hydraulic properties of the dewatering tube in combination with
T.W. Yee et al. / Geotextiles and Geomembranes 31 (2012) 39e50 Table 2 Quantities of dewatering tubes used in the moderately contaminated dewatering facility. Dewatering tube layer
Tube circumference
Total length of tubes
1st layer 2nd layer 3rd layer a 4th layer
27.5 29.0 30.5 29.0
7,013 5,373 3,861 2,452
a
m m m m
45
the chemical accelerant were very effective in trapping the contaminants within the dewatering tube.
5. Final dewatering tube design parameters
m m m m
5.1. Tube filling control Based on the results of the full-scale trial where the tubes were filled to a maximum height of 2.65 m it was decided that in the final project the tubes would be filled to a maximum height of 3.0 m.
Additional 4th layer added during the operation phase e see section 6.3.
a N
Reclaimed for dewatering facility
Wastewater impoundment lake
Perimeter dyke of wastewater impoundment lake
b
400 kN/m woven polyester geotextile reinforcement
HDPE geomembrane + nonwoven geotextile cushion RL+3.65m
RL+4.0m
Perimeter drain
Drainage gravel RL+2.5m Compacted earthfill
RL+2.0m
Perimeter dyke
Sandy gravel RL+1.0m Lake foundation
RL0.0m
Fig. 6. Area location and cross-section of the dewatering platform (a) location (b) cross-section.
46
T.W. Yee et al. / Geotextiles and Geomembranes 31 (2012) 39e50
This decision was made on the basis that several tubes had been successfully filled to this height (and greater) during the evaluation trials. At this maximum filled height the dewatered height would be expected to be 2.3 m. This was used as the basis for designing the final height of the dewatering tubes in the mound platform. 5.2. Tube target solids concentration The contaminated sediments in the lake bed are heterogeneous over its area and its depth. It was estimated that the average insitu solids concentration approximated 10%. Based on the full-scale dewatering evaluations, the minimum target solids concentration for the project dewatering tubes was set at 50%, which was slightly lower than that achieved in the full-scale trials (53% and 55% e see section 4.5).
and allowed heavy machinery to be deployed immediately for construction works. A second layer of geotextile reinforcement was then laid on top of the sandy gravel layer. The minimum design factor of safety adopted against instability through the backfilled mound and the soft foundation underneath was 1.25. The dewatering facility was designed according to the principles of a landfill; having a geomembrane liner for liquid isolation and a drainage system above this liner for drainage, collection, and removal of the effluent water. Earthfill was placed above the top geotextile reinforcement layer and compacted to form the base for the laying of the HDPE geomembrane liner. Above the geomembrane liner a nonwoven geotextile cushion layer was laid before drainage aggregate was placed on top. The top of the dewatering facility platform corresponds to the top of the gravel drainage blanket at RL þ 3.65 m. Effluent water from the dewatering process was designed to pass into the drainage blanket
5.3. Volumes, tube numbers and tube stacking One critical aspect of design concerns the question of the slurry volume reduction achievable using the dewatering tube solution and the time needed to achieve it. When the insitu lake bed contaminated sediments with an initial solids concentration of 10% are reduced to a final solids concentration of 50% (see section 5.2), the volume reduction will be about 83% (approximately 6 times) based on the assumption of 100% solids capture within the dewatering tubes. Based on this volume reduction ratio, the 2,400,000 m3 of moderately contaminated lake bed sediments would reduce to about 400,000 m3 of dewatered solids remaining in the dewatering tubes. This volume reduction ratio was used to engineer the sizes, numbers and stacking of the dewatering tubes to form the core of the landscaped mound. Based on the containment of 400,000 m3 of dewatered solids, and the use of 27.5 m circumference dewatering tubes, the total length of tubes required to dewater the moderately contaminated sediments was 15.6 km. Due to area limitations at the dewatering platform site it was necessary to stack the tubes into multiple layers. Consequently, the tubes were resized to meet the optimal stacking geometry and three planned layers were necessary to meet the dewatered volume requirements. Table 2 details the dewatering tubes used in the facility. 6. Design, construction and operation of the dewatering tube facility
N Drainage platform 1st layer dewatering tubes 2nd layer dewatering tubes
3rd layer dewatering tubes
Land
Impoundment lake
6.1. Dewatering platform The dewatering tube facility for the treatment of the moderately contaminated sediments was located on a site reclaimed from the Western side of the impoundment lake (see Fig. 6(a)). The reclaimed area approximates 120,000 m2 with a maximum length about 760 m and a maximum width about 230 m. Initially, a perimeter dyke with crest elevation at RL þ 4.0 m was formed to enclose the reclamation area. The enclosed area was then pumped dry to allow ground stabilization works to begin. The cross-section details of the dewatering platform are shown in Fig. 6(b). To stabilize the very soft lake foundation and improve the bearing capacity of the foundation for the dewatering facility two layers of 400 kN/m woven polyester geotextile reinforcement were used as basal reinforcement across the base of the facility. One layer was laid at the lake bottom which had been pumped to remove as much ponding water as possible. A sandy gravel layer 1 m in thickness was then laid above the bottom geotextile reinforcement layer. The combination of sandy gravel and geotextile reinforcement acted as a stabilized working platform over the very soft foundation
Reclamation dyke
Fig. 7. Plan layout of the dewatering tubes.
T.W. Yee et al. / Geotextiles and Geomembranes 31 (2012) 39e50
47
quickly. The bottom of the drainage blanket slopes both Eastward and Westward on a North-South alignment to facilitate water collection. Two lines of perimeter drains with grilled covers, one on the Eastern side and one on the Western side of the dewatering facility, were designed to collect the effluent water from the drainage blanket and transfer it to collection sumps. Pumps were used to remove and return the effluent water to the wastewater impoundment lake.
79.0 m. The use of dewatering tubes with marginally larger circumferences for the upper layers is to ensure that a standard layer thickness of 2.3 m is achievable for each layer of dewatered tubes. The planned top of the three layer stack of dewatering tubes corresponds approximately to RL þ 10.55 m.
6.2. Dewatering tubes layout
Three dredges were deployed for the supply of sediment slurry to the dewatering platform e see Fig. 8(a). These dredges were dismantled for transportation by land to site and reassembled for use on the impoundment lake. One dredge had a pumping capacity of 1,600 m3/h while the remaining two dredges each had a pumping capacity of 700 m3/h. Although the total pumping capacity worked out at 3,000 m3/h, at most times usually only two dredges were operating simultaneously on a 24 h basis. Therefore, the pumping capacity averaged between 1,500 and 2,000 m3/h. A lifting crane was used to lift the roll of geotextile tube into position on the platform. The dewatering tubes were rolled out on the drainage platform according to the layout plan e see Fig. 8(b). About six tubes were laid out at any one time at an average layout rate of 20 min per roll.
It was planned to stack the dewatering tubes in three layers; each layer having a dewatered thickness of approximately 2.3 m. Fig. 7 shows the plan dewatering tube layout on the drainage platform. The first layer of dewatering tubes was standardized using a circumference of 27.5 m e see Table 2. This layer consisted of 135 units of dewatering tubes with lengths ranging from 17.0 m to 79.0 m. The second layer of dewatering tubes was standardized using a circumference of 29.0 m e see Table 2. This layer consisted of 86 units of dewatering tubes with lengths ranging from 17.0 m to 79.0 m. The third layer of dewatering tubes was standardized using a circumference of 30.5 m e see Table 2. This layer consisted of 77 units of dewatering tubes with lengths ranging from 13.0 m to
6.3. Operation of the dewatering tube facility
Fig. 8. Operating the dewatering tube facility (a) dredging contaminated sediments (b) laying out dewatering tubes (c) slurry pipeline to dewatering tubes (d) first two layers of dewatering tubes (e) extent of the dewatering facility.
48
T.W. Yee et al. / Geotextiles and Geomembranes 31 (2012) 39e50
Fig. 9. Details of the capping for the dewatered contaminated sediments.
The sediment slurry was supplied by a floating pipeline across the surface of the impoundment lake that was then connected into an onshore pipeline network. The onshore pipeline network branched out with one line of 450 mm diameter steel pipes running along the Western edge of the dewatering platform and another line, also of 450 mm diameter steel pipes, running along the Eastern edge of the dewatering platform. Tap off steel pipe outlets of 160 mm diameter with gate control valves were positioned in sets of six at convenient locations along the onshore incoming slurry pipelines e see Fig. 8(c), (d) and (e). Generally, at any time six tubes were filled simultaneously, keeping the filling rate at between 250 and 300 m3/h, consistent with the filling rate in the full-scale prototype trials. When these tubes were filled to the control height, the control valves were shut while sediment slurry was diverted to the adjacent battery of six tubes laid out ahead of time. When the filled tubes had dewatered for some time and reduced in height, they were filled again to the control height. This repeated filling and dewatering was controlled by the adjustment of the manifold valve system until the target solids concentration of 50% in the tube was achieved. Once this was achieved, the next layer of tubes was then deployed. The process was then repeated for this new layer of dewatering tubes. However, the rate of work progress was comparatively slower for the upper tube layers than that experienced for work on the bottom tube layer due to difficulties in traversing over the lower dewatering tube layers. For example, the time taken to unroll six tubes for the upper layers was about 40 min per roll (twice that for the bottom layer). The chemical accelerant was added to the sediment slurry before it was pumped into the dewatering tubes. Downstream of the accelerant injection points, sampling points were provided to allow extraction of samples to check quality of flocculation. Samples were taken every morning and evening. The solids concentration of the sediment slurry was also checked several times per week. Based on experience on site, the solids concentration was maintained within a range of 5%e8% for optimum solids delivery and flocculation quality. Five water pump units, each with pumping capacity of 350 m3/h, situated at five sump locations, were used to pump the water effluent into steel pipes of 820 mm diameter. These pipes, which were laid beside the incoming slurry pipes, discharged the water effluent back into the impoundment lake e see Fig. 8(c) and (e). In general terms the dewatering tube work progressed very well. In an attempt to catch up on the overall project schedule the dewatering work was sped up which meant each layer of dewatering tubes were allowed less time than was required to fill with adequate solids. Consequently, the design dewatered height of 2.3 m for each layer of tubes was not achieved. The contract allowed for a 10% variation of quantity for the tubes and the client decided to utilize this provisional quantity to add a fourth layer of
dewatering tubes to achieve the design platform height e see Fig. 9. Table 2 lists the quantities of dewatering tubes used in the four layers. 6.4. Mound final capping Fig. 9 shows the typical East-West alignment cross-section of the artificial mound. Once the tubes had dewatered, general soil fill was used to cover over the geotextile tubes and form a smooth, graded surface for the mound core. The bermed side slopes were graded to an inclination of about 1V:5H. The final capping consisted of a HDPE geomembrane liner laid over the prepared soil surface with a two-layered soil cover placed on top of the geomembrane liner. The lower layer of soil cover consisted of 1 m thickness of coarse grained fill while the upper layer of soil cover consisted of 1.5 m thickness of nutrient rich top soil to support vegetation. The finished level of the mound top platform averaged RL þ 13.0 m, shaped with a North-South aligned mid ridge that slopes both Eastward and Westward at a gentle downward inclination of 5% to promote surface runoff. 7. Conclusions The use of geotextile tubes to dewater a large volume of contaminated sediments at the Tianjin Eco-City site in China has proven to be a highly successful project. The contaminated sediments were divided into three different waste streams according to the degree of contamination e lightly contaminated, moderately contaminated and highly contaminated. All three waste streams were dewatered using geotextile tubes with the contained dewatered solids subsequently used on site, or disposed of, depending on the degree of contamination. For the dewatered, lightly contaminated sediment stream, the contained solids were stabilized with cement and used as part of the pavement earthworks on site. For the dewatered, highly contaminated sediment stream, the contained solids were stabilized and deposited in a specially constructed, secured, landfill. The dewatered, moderately contaminated sediment stream was left in place to form a landscaped mound within the project site. In assessing the performance of the geotextile tube dewatering and generating the final design criteria a conventional approach was followed beginning with small-scale evaluations leading to full-scale prototype testing. To supplement the measured data obtained from the full-scale tests a series of dewatering relationships were developed which enabled the relevant performance parameters to be determined. These were used as the basis for establishing the design criteria for the overall dewatering project. The dewatering platform for the moderately contaminated sediment stream was located on a reclaimed area on the Western side of the water impoundment lake. The site was designed similar
T.W. Yee et al. / Geotextiles and Geomembranes 31 (2012) 39e50
to a landfill with a base barrier system with drainage, and a capping barrier system with an earth covering following completion of the dewatering process. Dewatering the volume of moderately contaminated sediments required four layers of geotextile tubes spread over the dewatering facility. The filling/dewatering cycles were managed in a planned, progressive manner to ensure operational efficiency.
Appendix A. Derivation of dewatering tube parameters Conservation of mass throughout the dewatering process gives:
Min ¼ MT þ Mout
(A1)
where, Min ¼ mass of slurry entering the dewatering tube, MT ¼ mass of slurry contained in the dewatering tube, and Mout ¼ mass exiting the dewatering tube. These variables are shown in Fig. A1. Over any time interval t-1 to t the conservation of mass equation becomes:
½Min tt1 ¼ ½Mout tt1 þ ½DMT tt1
(A2)
where, DMT ¼ change in dewatering tube mass. Mass entering the dewatering tube, Min: Here it is assumed that slurry enters the tube with a solids concentration ¼ Sin, where Sin ¼ ratio of mass of solids to total mass of slurry inflow. Associated relationships are as follows:
Msin ¼ Min Sin and Mwin ¼ Min ð1 Sin Þ
(A3)
therefore,
Vsin
VT ¼ VsT þ VwT
(A4)
VT ¼ D2T LT
"
hT DT
0:815 8:6 # h T DT
Vin ¼ Qin tin and Vin ¼ Vsin þ Vwin
(A5)
(A9)
where, VT ¼ tube volume, LT ¼ tube length, DT ¼ theoretical diameter of the tube, hT ¼ filling height of the tube. The relationship between VT/LT and hT/DT for various tube theoretical diameters DT is plotted graphically in Fig. A2. Mass exiting the dewatering tube, Mout: Here, it is assumed that all solids are contained within the tube and only water exits the tube. Thus,
Mwout ¼ Mout
(A10) Mout
gw
(A11)
and,
Vout ¼ Qout tout
and,
(A8)
where, Ms-T ¼ mass of solids in the tube, MT ¼ total mass of slurry in the tube, ST ¼ solids concentration in the tube, Mw-T ¼ mass of water in the tube, Vs-T ¼ volume of solids in the tube, Vw-T ¼ volume of water in the tube, G ¼ specific gravity of the solids, gw ¼ density of water, VT ¼ volume of slurry in the tube. The volume of slurry in the dewatering tube VT is regulated by its filling height. This also provides the required level of safety against tube rupture. To develop a relationship between filling height and contained volume for various sized tubes the computer programs GeoCops (Leshchinsky et al., 1996) and SoftWin (Palmerton, 2002) were used. Both gave the same results. The relationship derived is as follows:
Vwout ¼ Vout ¼
M S M ð1 Sin Þ ¼ in in and Vwin ¼ in gw G gw
49
(A12)
where, Mw-out ¼ mass of water exiting the tube, Mout ¼ total mass exiting the tube, Vw-out ¼ volume of water exiting the tube,
where, Ms-in ¼ mass of solids entering the tube, Min ¼ total mass of slurry entering the tube, Mw-in ¼ mass of water entering the tube, Vs-in ¼ volume of solids entering the tube, Vw-in ¼ volume of water entering the tube, G ¼ specific gravity of the solids, gw ¼ density of water, Vin ¼ slurry volume entering the tube, Qin ¼ volume pumping rate entering the tube, tin ¼ time period. Mass contained in the dewatering tube, MT: The mass and volume of the solids and water components inside the dewatering tube is:
MsT ¼ MT ST and MwT ¼ MT ð1 ST Þ
(A6)
therefore,
VsT ¼
MT ST M ð1 ST Þ and VwT ¼ T gw Ggw
(A7)
and, Mass, Min
Mass, MT Dewatering tube
Mass, Mout
Fig. A1. Description of the overall dewatering tube mass components.
Fig. A2. Relationship between VT/LT and hT/DT for various tube theoretical diameters DT.
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T.W. Yee et al. / Geotextiles and Geomembranes 31 (2012) 39e50
Vout ¼ total volume exiting the tube, Qout ¼ volume dewatering rate, tout ¼ time period for dewatering. There is also a conservation of mass of the water phase of the dewatering process,
Mwout ¼ Mwin MwT
(A14)
The volume of water flowing out of the tube during any time interval t-1 to t will be:
½Vwout tt1 ¼ ½Vout tt1 ¼ ½Vwin tt1 ½DVwT tt1
Glossary
(A13)
therefore,
Vwout ¼ Vwin VwT
Yee, T.W., Lim, L.K., Choi, J.C., 2006. Geotextile containment, dewatering and disposal of contaminated dredged material. Proceedings International Conference on New Developments in Geoenvironmental and Geotechnical Engineering, Incheon, Korea, 428e439.
(A15)
where, DVw-T ¼ change in volume of water in the tube during time interval t-1 to t. The above equations can be entered into a spreadsheet program to analyse the various dewatering parameters over time. References Lawson, C.R., 2008. Geotextile containment for hydraulic and environmental engineering. J. Geosynthetics International 15 (6), 384e427. Leshchinsky, D., Leshchinsky, O., Ling, H.I., Gilbert, P.A., 1996. Geosynthetic tubes for confining pressurized slurry: some design aspects. J. Geotechnical Eng. Division, Am. Soc. Civil Engineers 122 (8), 682e690. Palmerton, J. B., 2002. Distinct element modelling of geosynthetic fabric containers. Proceedings of the 7th International Conference on Geosynthetics, Nice, France, vol. 3, 1021e1024.
DT: theoretical diameter of the tube G: specific gravity of solids hT: filling height of the tube LT: tube length Min: mass of slurry entering the tube Mout: mass exiting the tube Ms-in: mass of solids entering the tube Ms-T: mass of solids in the tube MT: mass of slurry in the tube Mw-in: mass of water entering the tube Mw-out: mass of water exiting the tube Mw-T: mass of water in the tube Qin: volume pumping rate entering the tube Qout: volume dewatering rate Sin: solids concentration of slurry entering the tube ST: solids concentration of slurry in the tube t: time tin: time period entering the tube tout: time period for dewatering Vin: slurry volume entering the tube Vout: volume exiting the tube Vs-in: volume of solids entering the tube Vs-T: volume of solids in the tube VT: volume of slurry in the tube Vw-in: volume of water entering the tube Vw-out: volume of water exiting the tube Vw-T: volume of water in the tube gw: density of water