Influence of vertical drains on improving dredged mud by vacuum consolidation method

Vacuum consolidation preloading method (VCM) has been widely adopted as an effective solution for soft soil improvement over the world. Recently, VCM has been successfully applied for improving the geotechnical properties of dredged mud, which is normally dumped at reclamation area by hydraulic pumping.

Journal of Science and Technology in Civil Engineering NUCE 2018 12 (5): 63–72 INFLUENCE OF VERTICAL DRAINS ON IMPROVING DREDGED MUD BY VACUUM CONSOLIDATION METHOD Phan Huy Donga,∗ a Falcuty of Bridge and Highway Engineering, National University of Civil Engineering, 55 Giai Phong road, Hai Ba Trung district, Hanoi, Vietnam Article history: Received 30 July 2018, Revised 27 August 2018, Accepted 30 August 2018 Abstract Vacuum consolidation preloading method (VCM) has been widely adopted as an effective solution for soft soil improvement over the world Recently, VCM has been successfully applied for improving the geotechnical properties of dredged mud, which is normally dumped at reclamation area by hydraulic pumping However, it has been also reported that application of VCM for treatment of the dredged mud has been failed in some particular cases The failures are mainly caused by clogging problem in vertical drains due to fine-grained soils that reduces the drainage efficiency of drainage system To address this issue, a series of model tests have been conducted to investigate the performances of vertical drains among prefabricated vertical drain, sand drain and filter pipe As the goal, the performances of types of the vertical drain solutions are analyzed based on the monitoring data of settlement, influencing zone surrounding the vertical drains The test results reveal that sand drain shows the best performance among the others In addition, the clogging problem is clearly shown in case of PVD Keywords: dredging slurry; vacuum consolidation method; model test; PVD; filter pipe; sand drain https://doi.org/10.31814/stce.nuce2018-12(5)-07 c 2018 National University of Civil Engineering Introduction Along with rapid development of infrastructures, Vietnam has high demand for construction of mega projects along the coastal line area, where number of dredging projects has been increasing significantly The mud generated from dredging work has been mainly dumped at the disposal sites by hydraulic pumping method, only small amount has been utilized for reclamation The dredged mud is normally in fluid state and it has very low engineering properties, almost no bearing capacity After hydraulic reclamation, hundreds of hectares landfill area has been formed Since the dredged mud needs a long period to be settled down and solidified, the disposal sites cause serious environmental and ecological issues at its surrounding area On the other hand, construction activities at the coastal area normally needs huge amount of sand filling material, whereas a shortage of sand materials raises it prices rapidly, especially when regulations on preservation of natural sand resource from Vietnam national law has become more strictly Consequently, treatment of dumping disposal sites to create landfill reclamation has been strongly demanded Vacuum consolidation method (VCM) was initially proposed by [1] Currently, the VCM has been widely applied in wide areas of infrastructure development in many countries, such as China, Thailand, Japan and Vietnam Literature and engineering projects both indicate that the vacuum preloading ∗ Corresponding author E-mail address: dongph@nuce.edu.vn (Dong, P H.) 63 Dong, P H / Journal of Science and Technology in Civil Engineering method is an effective technique for the treatment of soft soil deposits [2–4] In recent years, VCM has been successfully applied to treat the newly reclaimed mud However, big challenges has been remained due to the extremely high water content and nearly zero bearing capacity condition of the dredged mud Besides success applications of VCM for treatment of dredged muds [5–7], several failures have been also observed For an example, the failure of VCM for improving newly hydraulic reclaimed site at Tianjin province, China as reported by Bao et al [8], where the effective depth is extended to very shallow depth Shear strength of soil obtained by Vane shear test ranges from 4.0 kPa to 9.0 kPa after 30 days of vacuum loading In addition, many engineering practices have also proved that after a short period time of improvement, vertical drains have larger bending and very serious clogging problem, resulting in the poor drainage performance of vertical drains as well as low effective improvement In this study, a series of model tests have been conducted to investigate the performance of vertical drainage among prefabricated vertical drain, sanddrain and filter pipes As the goal, performances of types of drainage solution are analyzed based on the monitoring data of settlement, influencing zone surrounding the vertical drains Methodology of Vacuum consolidation method The methodology of vacuum consolidation method is applying a vacuum pressure into an isolated soil mass to reduce the atmosphere pressure and pore water pressure in the soil, resulting soil consolidation and effective stress enhance Therefore, instead of increasing the effective stress in the soil mass by increasing the total stress by means of conventional surcharge filling, the vacuum preloads the soil by reducing the pore water pressure while maintaining a constant total stress Basically, vacuum consolidation system consists of drainage system, isolation system and vacuum pumps (Figs and 2) Once generated in vacuum pumps, vacuum suction rapidly spreads into soils along horizontal and vertical drainages system, reducing atmosphere and pore water pressure and forming pressure difference between vertical drains and pore water in soils This pressure difference causes the pore water flows toward vertical drain, which means soil consolidation happens Vacuum suction keeps taking out water and air, accelerating soil consolidation Drainage system is a connected network of the vertical drains, horizontal filter pipes and sand layer, forming a complete path for um pumps Drainage system Compensation fill Airtight sheets Vacuum pumps Drainage system Compensation fill Airtight sheets PVDs Sealing wall Sealing wall PVDs ure Figure Schematic chart chart of consolidation Schematic ofvacuum vacuum Figure Figure Schematic chart of vacuum Figure2.2 AnAn application of of VCM forofan industrialfor 2.An application Figure application VCM forVCM system project in Vietnam consolidation system nsolidation system an industrial project in Vietnam an industrial project in Vietnam So far, numerous books, papers, reports and documents about mechanism, methodology far, numerous books, papers, reports and documents about mechanism, methodology and practice of vacuum consolidation method have 64 been published over the world A lot d practiceofofexperimental vacuum consolidation method beenamong published over [4, the7].world A lot or monitoring data has beenhave presented these literals All these efforts or achievements a better understanding on quality of this experimental or monitoring dataprovide has been presented among thesecontrol literals [4, 7] All se effortstechnique or achievements provide a better understanding on quality control of this Dong, P H / Journal of Science and Technology in Civil Engineering spreading of vacuum suction and water flow The conventional VCM normally combined with PVD, which comprised a plastic core wrapped by a geotextile layer for filter function So far, numerous books, papers, reports and documents about mechanism, methodology and practice of vacuum consolidation method have been published over the world A lot of experimental or - Operation of vacuum system: vacuum pumps system was executed and kept - Operation of vacuum system: vacuum pumps was executed monitoring has been presented among these literals [3,system 4, 9] All these efforts orand achievements Operation of vacuum system: vacuum pumps system was executed andkept kept running untildata theof average consolidation degree reach 90% After vacuum loading was Operation of vacuum system: vacuum pumps system was executed and kept - Operation vacuum system: vacuum pumps system was executed and kept running until the average consolidation degree reach 90% After vacuum loading was provide a better understanding on quality control of this technique running until the average consolidation degree reach 90% After vacuum loading was - Operation Operation system: vacuum pumps system was executed and kept kept removed, the the compensation fill by means of sand was90% conducted of vacuum system: vacuum pumps system was executed and running until theaverage average consolidation degree reach 90% After vacuum loading was of vacuum system: vacuum pumps system was executed and running until consolidation degree reach After vacuum loading was Operation of vacuum system: vacuum pumps system was executed and kept removed, the compensation fill by means of sand was conducted removed, the compensation fill by means of sand was conducted running until the average consolidation degree reach 90% After vacuum loading was running until the average consolidation degree reach 90% After vacuum loading was removed, the compensation fill by means of sand was conducted consolidation degree reach 90%.90 After vacuum loading was removed, the compensation fill by meanspressure of sandreach was running vacuum until the loading, average consolidation degree 90% After loading During a vacuum of conducted over kPavacuum was observed The Casevacuum history for application of VCM forof improving dredged mud removed, fill by means of sand was conducted During loading, a vacuum pressure of over 90 kPa was observed The removed, the compensation fill by means sand was conducted the compensation fill by means of sand was conducted During vacuum loading, a vacuum pressure of over 90 kPa was observed The removed, the compensation fill by means of sand was conducted During vacuum loading, a vacuum pressure of over 90 kPa was observed The consolidation settlement measured at surface ranges in a relatively wide scatter and During vacuum loading, a vacuum pressure of over 90 kPa was observed The consolidation settlement measured at surface ranges in a relatively wide scatter and consolidation settlement measured at surface ranges in a relatively wide scatter and During vacuum pressure of over over kPa was was observed The During vacuum loading, vacuum pressure of kPa observed It has been demonstrated keym, challenges of VCM for improving the dredged mud The are: vacuum loading, vacuum pressure of over 90 kPa was observed consolidation settlement measured surface ranges in a90 a90 relatively wide scatter and(1) average settlement reaches 0.65 the equivalent volumetric strain is scatter about 9.3% consolidation settlement measured atat surface ranges in relatively wide and During vacuum loading, aaathat vacuum pressure of over 90 kPa was observed The average settlement reaches 0.65 m, the equivalent volumetric strain is about 9.3% average settlement reaches 0.65 m, the equivalent volumetric strain is about 9.3% How to create of a construction platform on the top surface of disposal site; (2) How to enhance consolidation settlement measured at surface ranges in a relatively wide scatter and consolidation settlement measured at surface ranges in a relatively wide scatter and measured atthe surface rangesvolumetric insettlement, relatively wide scatter average settlement reaches 0.65 m, m,at the equivalent volumetric strain about 9.3% Based on monitoring datameasured (vacuum pressure, surface sub-layer settlement) average settlement reaches 0.65 equivalent strain isis about 9.3% consolidation settlement surface ranges in aa relatively wide scatter and Based on monitoring data (vacuum pressure, surface settlement, sub-layer settlement) effectiveness vertical drainage against the clogging and bending problems due to large settlement Based on monitoring data (vacuum pressure, surface settlement, sub-layer settlement) average 0.65 m, the equivalent volumetric strain is about 9.3% average settlement reaches 0.65 m, the equivalent volumetric strain is about 9.3% settlement reaches 0.65 m, the equivalent volumetric strain is about 9.3% Based on monitoring data (vacuum pressure, surface settlement, sub-layer settlement) and in-situ tests (VST, CPT, SPT), following specifications were archived: (1)9.3% surface Based on settlement monitoring data (vacuum surface settlement, sub-layer settlement) average reaches 0.65 m,pressure, the equivalent volumetric strain is about and in-situ tests (VST, CPT, SPT), following specifications were archived: (1) surface In in-situ order tomonitoring provide a reference, the100 application of VCM for improving the dredged mud atatDalian and in-situ tests (VST, CPT, SPT), following specifications were archived: (1) surface Based on (vacuum pressure, surface settlement, sub-layer settlement) Based on data (vacuum pressure, surface settlement, sub-layer settlement) monitoring data (vacuum pressure, surface settlement, sub-layer settlement) and tests (VST, CPT, SPT), following specifications were archived: (1) surface bearing capacity is greater than kPa by plate loading test; (2) shear strength any and in-situ (VST, data CPT,(vacuum SPT), following were sub-layer archived: settlement) (1) surface Based on tests monitoring pressure, specifications surface settlement, Zhuanghe port industry and logistic estate project in China [5] is subsequently summarized The bearing capacity is greater than 100 kPa by plate loading test; (2) shear strength at any bearing capacity is greater than 100 kPa by plate loading test; (2) shear strength at any andin-situ in-situ SPT), following specifications were archived: (1)surface surface and in-situ tests (VST, CPT, SPT), following specifications were archived: (1) tests (VST, CPT, SPT), following specifications bearing capacity is greater than 100then kPa byplate plate loading test; (2) shear strength at any point along 13 mis(VST, depth isthan not less 50 kPa; (3) longtest; term settlement under 50 kPa bearing capacity greater 100 kPa by loading (2) shear strength at any and tests CPT, SPT), following specifications were archived: (1) surface project site 13 covers an areaisofnot about 170 hectares and(3) is divided into settlement blocks with 50 every 30 to point along depth less then 50 kPa; long under point along 13 mdepth depth not less then 50 kPa; (3) longterm termseveral settlement under 50 kPa bearing capacity greater than 100 kPa by plate loading test; (2) shearstrength strength atkPa any bearing capacity isis greater than 100 kPa by plate loading test; (2) shear at any than 100 kPa by plate loading test; (2) shear strength at point along 13 m depth isis not less then 50 kPa; (3) long term settlement under 50 kPa working load ismm less than 30 cm bearing capacity is greater than 100 kPa any point along 13 is not less then 50 kPa; (3) long term settlement under 50 kPa 40 hectares Each block was filled by dredging slurry with thickness from to m The dredging working load isisless less than cm working load less than30 30 cm point along less then 50kPa; kPa;(3) (3) long term settlement underzero 50kPa kPa point 13 m depth is30 not long term settlement under 50 13 m depth is not less then 50 kPa; (3) long term settlement under 50 working is less than 30 cm kPa working load is than cm slurryalong isload completely clayey silt orless siltythen with 50 extremely high initial water content, nearly bearing working cm load less than 30cm cm working less than 30 capacity,load veryisis soft and flowable to soft plastic a) Dike construction b) Dredged mud a) b) Dredged mud a)Dike Dikeconstruction construction b) b)Dredged Dredgedmud mud Dike construction b) Dredged mud a)a) filling (a) Dike construction (b) Dredged mud filling a)a)Dike b) Dredged filling filling construction filling b) Dredged mud Dikeconstruction Dredgedmud mud filling filling filling filling e) PVD installation f) Horizontal pipes (e) PVD installation (f) Horizontal pipes pipes e)e) PVD installation f)f)Horizontal Horizontal pipes e)PVD PVDinstallation installation f)f) Horizontal pipes e) Horizontal pipes e) PVD installation f) Horizontal pipes e) PVD installation f) Horizontal pipes Horizontalpipes pipes e) PVD installation f) Horizontal c) Construction c)c) c)Construction Construction Construction platform (c) Construction platform platform c) Construction c) Construction platform c) Construction platform platform platform platform platform g) Air-tight sheet (g) Air-tight sheet g) Air-tight sheet g) Air-tight sheet g) Air-tight sheet sheet g) Air-tight sheet g) Air-tight sheet Air-tightsheet sheet g)g)Air-tight d) Sand blanket d) Sand blanket d) Sand blanket d) Sand blanket d)filling Sand blanket (d) Sand blanket filling blanket filling filling d) Sand d) Sandblanket blanket filling d) Sand filling filling filling filling h) Vacuum h) Vacuum h) Vacuum h) Vacuum operation operation operation h) Vacuum h) Vacuum operation operation operation operation operation (h) h) Vacuum operation h) Vacuum h)Vacuum Vacuum operation (i) Dredging before and and after improvement i) Dredging mudmud before after improvement i)i) mud before after improvement i)i)Dredging Dredging mud before and after improvement Dredging mud beforeand and afterimprovement improvement mud before and after Figure Application VCMof for improving the newly hydraulic reclaimation Figure Application VCM for improving the newly hydraulic reclaimation mud mud at i)ofDredging Dredging mud before and after improvement Dredging mud before and after improvement Dredging mud beforeport, and after improvement i)i)of Dredging mud before and after improvement at Dalian Zhuanghe port, China [4] Figure Application of VCM for improving the newly hydraulic reclaimation mud Figure Application VCM for improving the newly hydraulic reclaimation mud at Dalian Zhuanghe China [7] Figure Application of VCM for improving the newly hydraulic reclaimation mudat at Figure Figure 3 Application Application of of VCM VCM for for improving improving the the newly newly hydraulic hydraulic reclaimation reclaimation mud mud at at Dalian Zhuanghe port, China [7] 3.Application Applicationof ofVCM VCM for improving the newly hydraulic reclaimation mud atat Dalian Zhuanghe port, China [7] Dalian Zhuanghe port, China [7] Figure3 VCM for improving the newly hydraulic reclaimationmud mudat for improving the newly hydraulic reclaimation Dalian Zhuanghe port, China [7] Figure Model tests forinevaluating performance of vertical drains Dalian Zhuanghe port, China [7] As illustrated Fig 3, the construction work at the project was conducted into following steps: Dalian Zhuanghe port, China [7] Dalian Zhuangheof port, China [7] Dalian Zhuanghe port, China [7] 4.4 tests for performance vertical drains Model tests for evaluating performance of vertical drains 4.Model Model tests forevaluating evaluating performance vertical drains 4.1 Sample preparation Model tests for evaluating performance of drains performance ofofvertical vertical drains tests forevaluating evaluating performance performance of vertical drains 4.Model Model performanceof ofvertical verticaldrains drains tests for 4.1 Sample preparation 4.1 Sample preparation 4.1 Sample preparation 65 4.1 Sample preparation The model tests were conducted using a dredged soil, which was taken from West Lake, 4.1 Sample preparation 4.1 Sample preparation 4.1 Sample preparation The model tests were aaadredged soil, was taken from West Lake, The model tests were conducted using dredged soil, which was taken from West Lake, Hanoi Since Hanoi City plansusing to launch a dredging workwas fortaken West Lake, in Lake, order The model tests wereconducted conducted using dredged soil,which which was taken from West Lake,to dredged soil, which was taken from West Lake, The model tests were conducted using a dredged soil, which from West Hanoi Since Hanoi City plans to launch a dredging work for West Lake, in order to Thewith model tests were conducted using dredged soil, which was taken from West Lake, Hanoi Since Hanoi City plans to launch a dredging dredging work for West Lake, in order to deal environmental pollution, making West Lake area becomes a West future tourist Hanoi Since Hanoi City plans to to launch dredging workfor forWest West Lake, inorder order The model tests wereCity conducted using dredged soil,which which was takenLake, from West Lake, The model tests were conducted using aaadredged soil, was taken from Lake, Hanoi Since Hanoi plans launch work for West Lake, in order toto launch aaathe dredging work in to deal with environmental pollution, making the West Lake area becomes a future tourist Hanoi Since Hanoi City plans to launch dredging work for West Lake, ininorder order totobe deal with environmental making West becomes tourist area of the city for City sustainable The Lake dredging work is aproposed to deal with environmental pollution, makingaaathe the West Lake area becomes a future future tourist Hanoi Since Hanoi Citypollution, plans to todevelopment launch dredging workarea forWest WestLake, Lake, order Hanoi Since Hanoi plans launch dredging work for in to Dong, P H / Journal of Science and Technology in Civil Engineering - Surround dike construction: Dikes were built along the boundary or interface of the blocks by filling of compacted mountain soil - Dredged mud filling: the dredged mud was filled by hydraulic pumping method It is noted in the dredged mud that its silty content (particle size varies from 0.005 to 0.05 mm) is about 18%, and its clay content (particle size less than 0.005 mm is round 12%) - Setting up a construction platform: A combination of woven geotextile, bamboo grid and 0.4 m hydraulic sand filling was developed to build working platform - Installation of vertical and horizontal drains: Conventional PVDs were installed using lightweight installation machine Horizontal perforated pipes wrapped by non-woven geotextile were buried into the sand blanket layer - Installation of airtight sheets: Two airtight sheets protected by two non-woven geotextile layers were covered on the surface - Operation of vacuum system: vacuum pumps system was executed and kept running until the average consolidation degree reach 90% After vacuum loading was removed, the compensation fill by means of sand was conducted During vacuum loading, a vacuum pressure of over 90 kPa was observed The consolidation settlement measured at surface ranges in a relatively wide scatter and average settlement reaches 0.65 m, the equivalent volumetric strain is about 9.3% Based on monitoring data (vacuum pressure, surface settlement, sub-layer settlement) and in-situ tests (VST, CPT, SPT), following specifications were archived: (1) surface bearing capacity is greater than 100 kPa by plate loading test; (2) shear strength at any point along 13 m depth is not less then 50 kPa; (3) long term settlement under 50 kPa working load is less than 30 cm Model tests for evaluating performance of vertical drains 4.1 Sample preparation The model tests were conducted using a dredged soil, which was taken from West Lake, Hanoi Since Hanoi City plans to launch a dredging work for West Lake, in order to deal with environmental pollution, making the West Lake area becomes a future tourist area of the city for sustainable development The dredging work is proposed to be conducted by cutter suction dredger, the mud is then collected at a designate area within the lake, it subsequently will be transported outside the lake by trucks The project will generate approximate 1.3 million cubic meters of dredged mud An area of about 30 hectares would be required for dumping such huge amount of dredging mud, while the land fund of Hanoi city for disposal site has been very limited In addition, the disposal of that huge dredged mud has many consequences impacts for the ecological environment Therefore, possibility to treat the disposal site by VCM will be a good reference for the manager units in order to select a proper method for dredged mud treatment Several physical properties of mud sample were carried out in laboratory (Fig 4) Table summaries physical properties of 03 typical mud samples Notably, the mud has been generated from decay of aquatic plants and animals for many years, so the soil is very soft with high water content, high void ratio, fine particles and high organic content Detail discussion on the project as well as geotechnical engineering of the dredged mud can be referred to Dong P H [10] 4.2 Equipment development The test equipment mainly consists of several components as numbered as shown in Figs and In Fig 6, (1) indicates the vacuum system includes a vacuum pump with a power output of 7.5 66 4) Table summaries physical properties of 03 typical mud samples Notably, the mud has been generated from decay of aquatic plants and animals for many years, so the soil is very soft with high water content, high void ratio, fine particles and high organic content Detail discussion on the project as well as geotechnical engineering of the P H / Journal of Science and Technology in Civil Engineering dredged mud canDong, be referred to Dong PH [3] FigureFigure Mud samples andand laboratory forevaluating evaluating physical properties Mud samples laboratory test test for physical properties Table Physical properties of the mud samples Table Physical properties of the mud samples Unit Organic Silt Clay content, PH SpecificVoid Void Organic content Silt content, % % Clay (d < 0.005 content, content, Specific WaterWater Unitweight gravity, ratio, Sample content (%) (d = mm) gravity, ratio, content % (d = 0.005 % (d < 0.005 PH content Sample weight (kN/m ) 0.005mm) - 0.05 mm) (%) e e (%) (%) ∆  (kN/m3 ) 0.05 mm) 1.24 210.9 2.52 5.3 18.6 22.4 19.4 7.19 18.6 22.4 19.4 7.19 1.27 1.24202.5210.9 2.502.52 5.05.3 22.1 23.6 18.6 7.18 1.31 1.27216.3202.5 2.532.50 5.15.0 11.4 25.7 17.7 6.96 22.1 23.6 18.6 7.18 1.31 216.3 2.53 5.1 11.4 25.7 17.7 6.96 kW which can generate a vacuum pressure of 95 kPa; (2) is the model test box with inner dimensions of 200 cm × 150 cm × 150 cm in length, width and height The dredged mud was then freely poured in the box with thickness of m; (3) One layer of geomembrane was covered on the top and sealing in the mud to keep airtight condition; (4) A vacuum gauge is set up right under the geomembrane to monitor the vacuum pressure during loading; (5) A set of 02 vacuum gauges was also set up at different depths of 0.2 m and 0.8 m from the surface in order to observe the transmission of vacuum pressure along the depth Location of the vacuum gauges is presented in Fig 5; (6) The horizontal perforated pipes wrapped by filter geotextile to connect all the vertical drains (7) and (8) shows the vertical drains To evaluate the effectiveness of different types of vertical drains, this study performed three sets of model tests using either prefabricated vertical drain (PVD), Sanddrain (SD) or filter pipes (FP) Detail arrangement of the vertical drains is shown in Fig 5, where the PVD is conventionally comprised a plastic core wrapped by geotextile filter Permeability coefficient of the filter is 0.014 cm/s The size of PVD is 100 mm × mm The SDs with diameter of cm using medium sand were placed in the mud by using a PVC pipe casing, the casing was then carefully removed from the soil The FP is comprised by a slotted PVC pipe with diameter of cm and wrapped by a geotextile layer for filter The FPs were also inserted at the same positions with those of PVD and SD In addition, network of 67 set up at different depths of 0.2m and 0.8 m from the surface in order to observe the transmission of vacuum pressure along the depth Location of the vacuum gauges is presented in Fig 5; (6) The horizontal perforated pipes wrapped by filter geotextile to connect all the vertical drains (7) and (8) shows the vertical drains Dong, P H / Journal of Science and Technology in Civil Engineering 11 22 36 77 Figure Schematic chart chart ofof thethe model test test Figure Schematic model Figure6 Equipment of the tests tests Figure Equipment of model the model the vertical drains and horizontal pipes was placed into a 0.20 m sandmat layer To evaluate the effectiveness of different types of vertical drains, this study performed three of model tests using either prefabricated vertical drain (PVD), Sanddrain (SD) 4.3.sets Monitoring work or filter pipes (FP) Detail arrangement of the vertical drains is shown in Figure 5, Along with 02 vacuum gauges located at 0.2 m and 0.8 m from the surface to observe the transwhere theofPVD is conventionally comprised plastic wrapped by geotextile filter mission vacuum pressure, the surface settlementa was also core observed at positions near the vertical Permeability coefficient of the filter is 0.014 cm/s The size of PVD isare 100 mminxFig mm drain and between the vertical drains Detail locations of monitoring equipment shown The SDs with diameter of cm using medium sand were placed in the mud by using a PVC pipe casing, the casing was then carefully removed from the soil The FP is Results and discussions 5.1 Settlement and rate of volume reduction The vacuum loading in case of PVD was removal after 15 days, until the settlement rates has reduced and get stable value under mm/day in consecutive days For a comparison purpose, the vacuum loading time in other cases of the vertical drains was also performed at same duration The surface settlement measured at vertical drain’s position and that between the vertical drains was plotted in Fig Since the test box has very high stiffness, the horizontal deformation is ignored Therefore, vertical strain is equivalent to volumetric strain or rate of volume reduction as shown in Fig At the end of vacuum loading time, the maximum settlement in case of PVD is only 62 mm with equivalent volumetric strain is 6% This amount seems be lower than the value reported by Liu and Marcello [5], as it was 9.3% However, advantages of SD and FP are revealed again with corresponding maximum settlement of 119 mm and 92.5 mm The equivalent volumetric strain in case of SD and FP is 11.5% and 9.1%, respectively It is here noted that the volumetric strain or volume reduction rate is a significant parameter for the mentioned project, because the mud is dredged and collected at the lake before transporting to dumping landfill, so the transportation cost can reduce with higher volume reduction In addition, the average settlement rate at last 05 days in case of PVD, FP and SD is 1.17, 2.23 and 3.75 mm/day, alternatively This means that consolidation process as well as workability of FP and SD has still significantly undergone until end of vacuum loading 68 Dong, P H / Journal of Science and Technology in Civil Engineering (mm) Settlement (mm) Settlement 0 0 -20 24 48 72 Vacuum loading time (h) 96 120 144 168loading 192 216 Vacuum time 240 (h) 264 288 312 336 360 384 408 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 At drains position between the drains SD At drains position SD the drains between -20 -40 PVD SD FP PVD FP -40 -60 PVD SD FP PVD FP -60 -80 -80 -100 -100 -120 -120 strain v(%) Volumetric strain v(%) Volumetric Figure time Figure 7 Settlement Settlement versus versus loading loading time Figure Settlement versus loading time 0% 0% 2% 2% 4% 4% 6% 6% 8% 8% At drains position Between the drains 10% At drains SD position Between SD the drains 10% SD SD PVD PVD 12% PVD PVD 12% FP FP 14% FP FP 10 14%0.1 Vacuum loading 0.1 10time (h) Vacuum loading time (h) 100 100 1000 1000 Figure Volumetric strain versus loading time Figure strain versus loading loading time time Figure Volumetric Volumetric strain versus 5.2 Influenced zone and clogging phenomenon 5.2 Influenced zone and clogging phenomenon The influenced zone, which clearly shows transmission area of vacuum pressure, can be The influenced zone, clearly showssurround transmission area ofdrains vacuum pressure, can beit observed based on thewhich soil core appeared the vertical For more detail, 5.2 Influenced zone and clogging phenomenon observed based onfrom the soil appeared surround the along vertical detail, it could be analyzed thecore variation of water content thedrains depthFor andmore radius range The influenced zone, which clearly showsoftransmission area ofthe vacuum pressure, can be observed could be analyzed from the variation water content along depth and radius range Again, it is in this project noted that rate of water content reduction will evaluate based on possibility the soil core appeared surround thethat vertical drains For morewith detail, it could befilling analyzed from Again, it iswhether in this the project rate of water content reduction willforevaluate mudnoted can be treated by premixing cement the variation of water content along the depth and radius range Again, it is in this project noted that possibility whether the mud can be treated by premixing with cement for filling material material rate of water content reduction will evaluate possibility whether the mud can be treated by premixing Afterfor thefilling vacuum loading, mud samples were collected at different depths and distances with cement material After the vacuum loading, samples were collected differentofdepths and distances from the vertical drains for mud checking water content The at variation water content along After from the vacuum loading, mud sampleswater were content collected at variation differentofdepths and distances from the the vertical drains for checking The water content the depth at different range from the vertical drains (r = 15 cm, r = 30 cm and ralong = 45 vertical drains for checking water content The variation of(rwater content at different the depth at different range the vertical = 15 cm, r = along 30 cmthe anddepth r = 45 cm) is presented in Figure from The initial waterdrains content (W 0) is also shown for referent range from theisvertical drains (r = 15 cm, initial r = 30water cm and r = 45 presented in Fig The initial cm) presented in Figure The content (Wcm) also shown for referent 0) is is water content (W0 ) is also shown for referent value It can be seen that the water content increases with the increasing depth and radius distance from the vertical drain in all three cases However, in case of PVD, water content reduces only 20% on the top and even almost 10% at the depth of 0.8 m In general, the distribution of water content in case of PVD (Fig 9(a)) is relative difference from that in cases of FP (Fig 9(b)) and SD (Fig 9(c)), since the values of water content at different 69 water content reduces only onwater the top and almost % the depth of 0.8 m.of difference from thatthe in cases of 20% FP of(Figure b) andeven SD (Figure c),atsince the a) values In general, general, distribution content incase case of10 PVD (Figure isrelative relative In the distribution of water content in of PVD (Figure a) is In general, the distribution ofdistance water content case of PVD a) isthe relative water content at from different nearb)in the surface are (Figure not close together difference from thatinradius incases cases of FP(Figure (Figure b)and and SD (Figure c),since since values of difference that of FP SD (Figure c), the values difference from that in cases of FP (Figure b) and SD (Figure c), since the values ofof Although morecontent effortsatatshould beradius investigated, this could be explained that water different radius distancenear nearmanner surface closetogether together water different distance thethe surface areare notnot close water content content at different radius distance near the surface are not close together Although moreefforts efforts should investigated, thismanner manner couldafter beexplained explainedthat exceed pore water pressure wasshould generated due to the clogging problem, removal ofthat Although bebe investigated, this could Dong, should P H / Journal of Science and Technology in Civil Engineering Although more more efforts be investigated, this manner could bebeexplained that exceedpore pore water pressure was generateddue duetotothe theclogging clogging problem, after removal vacuum exceed loading, pore water could begenerated rearranged to problem, surface Inafter addition, the water pressure was generated problem, removal of of exceed pore water pressure was due and to Although therebind clogging after removal of radius distance near the surface are not close together more efforts should be investigated, vacuum loading, pore water could rearranged andrebind rebind tosurface surface In addition, samplingvacuum work for water content test was conducted after few days from the removal ofthe loading, pore water could bebe rearranged and tosurface Inaddition, addition, vacuum loading, water could be rearranged rebind In thethe this manner could pore be explained that exceed pore waterand pressure wastogenerated due to the clogging samplingwork work forwater content testwas wasconducted conducted afterfew few days from the removal of for content test after days from theremoval vacuum sampling loading Figure dwater shows the variation of could water with radius distance sampling work for water content test was conducted after few days from the ofInof problem, after removal of vacuum loading, pore water becontent rearranged and rebind toremoval surface vacuum loading Figure dshows shows the of water content with radius distance Figure shows the variation water with radius distance addition,loading the sampling workdepths water content test conducted after fewwith days from the removal vacuum loading Figure ddfor variation ofof water radius distance from thevacuum vertical drains at two ofthe 0.2 mvariation andwas 0.8 m It content iscontent again clearly shown that from the vertical drains at two depths of 0.2 m and 0.8 m It is again clearly shown of vacuum loading Fig 9(d) shows the variation of water content with radius distance from thethat from the vertical drains at two depths of 0.2 m and 0.8 m It is again clearly shown that vertical drains at two depths m and 0.8 m It iswhich again clearly that SD case from givesthethe best transmission effectofof0.2 vacuum pressure, resultsshown in a better vertical drains at two depths of 0.2 m and 0.8 m It is again clearly shown that SD case gives the best SDcase casegives givesthe thebest besttransmission transmission effect vacuum pressure, which results a better SD best transmission effect vacuum pressure, which results a better SD effect ofofof vacuum pressure, which results inina in better preloading effect transmission effect of vacuum pressure, which results in a better preloading effect preloadingeffect effect preloading preloading 0.5 0.5 0.5 0.6 0.6 0.6 0.7 0.7 0.7 0.7 0.8 0.8 0.8 0.8 0.9 0.5 0.6 0.2 0.2 0.20.2 0.10.1 0.1 0.3 0.3 0.30.3 0.4 0.40.4 0.4 0.4 0.40.4 0.5 0.50.5 0.6 0.60.6 0.7 0.70.7 0.8 0.8 0.80.8 180% 180% 180% 180% 0.5 0.5 0.50.5 160% 160% 160% 160% 0.60.6 0.6 0.6 0.70.7 0.7 0.7 0.8 0.9 0.90.9 Radial distance (cm) Radial distance Radial distance Radial distance (cm)(cm)(cm) 25 30 35 40 45 20 25 20 15 25 20 30 30 35 35 40 40 45 45 200% 15 15 200% 200% 200% 15 20 25 30 35 40 45 0.2 0.20.2 0.3 0.30.3 0.7 0.9 0.9 0.9 0.9 11 100% 150% 200% 250% 100% 100% 150% 150% 200% 200% 250% 250% 0.1 0.10.1 0.1 Water content (%) Water content Water content (%)(%) Water content (%) 00 Water content (%) 0.4 0.4 0.4 0.4 Water content (%) Water content (%) 0.6 0.3 0.3 0.3 0.3 Water content Water content (%)(%) Water content (%) Water content (%) 0.5 0.2 0.2 0.2 00 Depth (m ) Depth (m ) Depth (m ) 0.4 0.2 Water content (%) Depth (m ) Depth (m ) 0.3 0.1 0.1 0.1 0.1 Depth (m ) 0.2 Water content (%) Water content (%) (%) Depth (m )) Depth (m Depth (m ) 0.1 00 (m )) Depth (m Depth Depth (m ) Water content (%) 140% 140% 140% 140% 0.8 0.80.8 z = 0.2 m z = 0.8m z =m z = 0.8m = 0.2 = 0.8m z = z0.2 m 0.2 z =zm0.8m 0.9 0.90.9 PVD PVD PVDPVD PVDPVD 120% 120% PVD 120% PVD 11 FP FP FP FP FP FP 11 120% 100% 150% 200% 250% FP FP 100% 150% 200% 250% SD SD 100% 100%150% 150%200% 200%250% 250% 100% 100%150% 150%200% 200%250% 250% SD SD SD SD 100% 150% r=200% 250%r=30cm 100% 150% 200% 250% 15cm 100% r=15cm 150% 200%r=30cm 250% r=15cm r=30cm SD SD r= r= 15cm r=30cm 15cm r=30cm 100% r=15cm r=30cm r=15cm r=30cm r=15cm r=30cm r=15cm r=30cm 100% 100% r= 15cm r=30cm r=45cm Initial W0 r=30cm Initial W0 r=30cm r=45cm Initial W0W0 r=15cm r=45cm r=45cm Initial 100% r=45cm Initial W0 r=45cm Initial W0W0 r=15cm r=45cm Initial r=45cm Initial W0 W0 r=45cm Initial r=45cm Initial W0 r=45cm Initial W0 a)(a) PVD case PVD case a) case a)PVD PVD case a) PVD case 0.9 b) FP case (b) FP case b)b) FPFP case case b) FP case r=45cm Initial W0 c)c)SD SD case (c) case SD case c) SD case c) SD case d)d)Water (d) Water content vs radial Water d) Water rangevs d) Water content content vs.vs content radial range content vs.range radial range radial Figure Variation of water content with depth and radius radial range Figure Variation of water content with depth and radius Figure depth and radius Figure9.9.Variation Variationofofwater watercontent contentwith with depth and radius Fig 10 shows the vertical drains after removal of the vacuum loading In case of PVD, clogging Figure Variation ofdrains waterafter content withofof depth and radius Figure shows the vertical removal the vacuum loading In case of Figure 99shows the vertical after the vacuum loading In In case of of phenomenon visually observed Adrains tiny layer of removal fine particlesof appeared on surface of filter layer, Figurewas shows the vertical drains after removal the vacuum loading case PVD, clogging phenomenon was visually observed A FP tiny layer of fine particles whereas no soil particle is seen in plastic core In both cases of the and SD vertical drains methods, PVD, phenomenon was visually observed ofoffine Figure shows the vertical drains removal of theAvacuum loading In particles case of PVD,9clogging clogging phenomenon wasafter visually observed Atiny tinylayer layer fine particles appeared on surface of filter layer, whereas nodrains soil Radius particleofissoilseen in plastic core In a soil core is clearly created surrounding the vertical core in case of FP is about appeared on surface ofoffilter layer, nonosoil is plastic core In In PVD, clogging phenomenon was visually observed Aparticle tiny layer ofin in fine particles appeared on filter layer,whereas whereas soil particle isseen seen plastic core both cases of surface the FPofand SD vertical drains a soil iscore is clearly created 12 cm, and that in case SD is approximately 25 cm.methods, This observation coincided with the change cases ofofof thefilter FP SD vertical drains methods, aisasoil core isplastic created both cases FPand and SDwhereas vertical drains soil isclearly clearly created appearedboth surface layer, particle inabout core In ofon water content described in Fig Consequently, itcore ismethods, suggested the drain case has a better surrounding theasthe vertical drains Radius ofno soilsoil in casethat of seen FP SD iscore 12 cm, and surrounding the vertical drains Radius of soil core in case of FP is about 12 cm, and surrounding vertical drains ofcontent soil core casecore ofisthe FP isclearly about 12 cm, treatment effect both inisSD decreasing theRadius moisture and inainincreasing uniformity both cases of FP and vertical drains methods, soil isprocess created that in the case ofthe SD approximately 25 cm This observation coincided with theatand that in case of SD is approximately 25 cm This observation is coincided with thethe different depths that the inofcase of content SD is as approximately 25 core cm Consequently, This observation is coincided surrounding vertical drains Radius ofinsoil in case of FPitisis about 12 cm, and change water described Figure suggested thatwith the change of water content as described in Figure Consequently, it is suggested that the change water content described in Figure decreasing Consequently, it is suggested that SD drain case acore better as treatment effect the content that in case of of SD ishas approximately 25Filter cm Thisinobservation is moisture coincided withand thethe Plastic Sand drain pipe both Soil core SD drain case has a better treatment effect both in decreasing the moisture content and draincontent case better treatment effect both in decreasing thesuggested moisture content inSD increasing thehas process uniformity different depths change of water asa described in atFigure Consequently, it is that theand Filter in increasing depths Soil core increasingthe theprocess processuniformity uniformityatatdifferent different depths SD drainincase has a better treatment effect both in decreasing the moisture content and in increasing the process uniformity at different depths Figure 10 The vertical drains after removal of vacuum pressure Figure The vertical drains after removal of vacuum pressure 5.2 Distribution of vacuum pressure 70 Vacuum pressure was monitored during loading Figure 10 presents the vacuum pressure at depth of 0.2 m and 0.8 m for 03 cases among the vertical drains The vacuum pressure at shallow depth was quickly reached stable value right after operation of vacuum system It however needs a specific time to be transmitted at depth of 0.8 m 9 Figure The vertical drains after removal of vacuum pressure 5.2 Distribution of vacuum pressure Dong, P H / Journal of Science and Technology in Civil Engineering Vacuum pressure was monitored during loading Figure 10 presents the vacuum 5.3 Distribution of vacuum pressure pressure at depth of 0.2 m and 0.8 m for 03 cases among the vertical drains The Vacuum monitored during 11 presents the vacuum pressure at depth of vacuumpressure pressurewas at shallow depth wasloading quicklyFig reached stable value right after operation 0.2 mofand 0.8 msystem for 03 cases amongneeds the vertical drains The pressure shallow depth vacuum It however a specific time to vacuum be transmitted at at depth of 0.8 m was quickly reached stable value right after operation of vacuum system It however needs a specific The delayed time and rising rate increase by sequence of SD, FP and PVD It can be time to beexplained transmittedthat at depth m The delayed time and rising rate by sequence of SD, duringofa0.8 short period of vacuum loading, the increase whole vacuum loading is FP and PVD It can explained thatof during short period of vacuum the whole vacuum loading focused onbethe formation tiny aripples and building of loading, some larger channels After is focused on the formation of tiny ripples and building of some larger channels After formation formation of tiny ripples and building of larger channels inside the soil, the vacuum of tiny ripples and building of larger channels inside the soil, the vacuum pressure reaches to stable value pressure reaches to stable value because of the silt clogging around the PVDs, thus no because of the silt clogging around the PVDs, thus no longer development of vacuum pressure can be longer development of vacuum pressure can be observed observed 120 SD FP PVD Vaccum pressure (kPa) 100 SD FP PVD 80 60 40 20 Vacuum loading time (h) 0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 Figure 10.11.Vacuum duringvacuum vacuum loading Figure Vacuumpressure pressure during loading Conclusions In most cases, dredged mud has been believed to be unsuitable material for reclamation Conclusions The advanced VCM creates more possibility to utilize dredging slurry for reclamation, which shall bring benefit in overall cost and environment protection Although more In most cases, dredged mud has been believed to be unsuitable material for reclamation The advanced VCM creates more possibility to utilize dredging slurry for reclamation, which shall bring benefit in overall cost and environment protection Although more detail study on both technical 10 and economic aspects shall be conducted, following conclusions can be withdrawn from this study: - The performance of VCM for treatment of dredged mud is strongly governed by workability of vertical drains The conventional VCM combined with PVD may encounter serious clogging problem in vertical drains of high content fine-grained soils It commonly reduces the drainage efficiency of drainage system, weaken vacuum pressure in soils, and then weaken the soil improvement effect - This study recommends SD as a proper countermeasure for vertical drains, since SD shows suitable stiffness, good drainage and anti-clogging phenomenon In addition, the sand volume inserted in the mud also increase overall stiffness and bearing capacity of the reclamation area For practical use, a consideration on both cost and construction feasibility should be detail investigated - Success of the combination of woven geotextile, bamboo grid and hydraulic sand filling for working platform and horizontal drainage in case history presented in this paper is a good reference for solving difficulty of establishment of working platform 71 Dong, P H / Journal of Science and Technology in Civil Engineering References [1] Kjellman, W (1952) Consolidation of clay soil by means of atmospheric pressure In Proceedings of Conference on Soil Stabilization, MIT, 258–263 [2] Dong, P H (2017) Design and construction experiences on soil improvement work by vacuum consolidation method Vietnam Journal of Construction, 8:239–242 [3] Indraratna, B (2010) Recent advances in the application of vertical drains and vacuum preloading in soft clay stabilization Australian Geomechanics Journal, 45(2):1–43 [4] Loan, T K., Sandanbata, I., Kimura, M (2006) Vacuum consolidation method - Worldwide practice and the latest improvement in Japan Hazama Research Annual Report [5] Liu, Y., Marcello, D (2015) A vacuum consolidation method application case for improving dredging slurry [6] Shang, J Q., Tang, M., Miao, Z (1998) Vacuum preloading consolidation of reclaimed land: a case study Canadian Geotechnical Journal, 35(5):740–749 [7] Wang, J., Cai, Y., Ma, J., Chu, J., Fu, H., Wang, P., Jin, Y (2016) Improved vacuum preloading method for consolidation of dredged clay-slurry fill Journal of Geotechnical and Geoenvironmental Engineering, 142(11):286–299 [8] Bao, S., Dong, Z., Chen, P (2013) Causal analysis and countermeasures of vacuum consolidation failure to newly hydraulic reclamation mud Electronic Journal of Geotechnical Engineering, 18:5573–5579 [9] Long, P V., Nguyen, L V., Bergado, D T., Balasubramaniam, A S (2015) Performance of PVD improved soft ground using vacuum consolidation methods with and without airtight membrane Geotextiles and Geomembranes, 43(6):473–483 [10] Dong, P H (2017) Study on physical and chemical characteristics of dredging muds in Hanoi city and proposal for recycling as land reclamation filling material Vietnam Journal of Construction, 8:263–266 72 ... solution are analyzed based on the monitoring data of settlement, influencing zone surrounding the vertical drains Methodology of Vacuum consolidation method The methodology of vacuum consolidation. .. veryisis soft and flowable to soft plastic a) Dike construction b) Dredged mud a) b) Dredged mud a)Dike Dikeconstruction construction b) b )Dredged Dredgedmud mud Dike construction b) Dredged mud a)a)... filling h) Vacuum h) Vacuum h) Vacuum h) Vacuum operation operation operation h) Vacuum h) Vacuum operation operation operation operation operation (h) h) Vacuum operation h) Vacuum h )Vacuum Vacuum