Influence of Vertical Drains on Improving Dredged Mud by Vacuum Consolidation Method

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,∗

aFalcuty 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

1. 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, Thai- land, Japan and Vietnam. Literature and engineering projects both indicate that the vacuum preloading

∗Corresponding author. E-mail address: [email protected] (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.

2. 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.1 and2). 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

Vacuum pumps Drainage system Compensation fill Airtight sheets Vacuum pumps Drainage system Compensation fill Airtight sheets

Sealing wall PVDs

Figure 1. Schematic chart of vacuum FigureFigure 1. Schematic1. Schematic chart of vacuum chart consolidation of vacuum FigureFigureFigure 2 2..2 An. An An application application of ofVCM VCM for of an for VCMindustrial for consolidation systemsystem. project in Vietnam consolidation system. anan industrial project project in Vietnam. in Vietnam. So far, numerousSo far, numerousbooks, papers, books, papers, reports reports and and documents documents about about mechanism, mechanism, methodology methodology and practice of vacuum consolidation method have64 been published over the world. A lot and practiceof of experimental vacuum consolidation or monitoring data method has been have presented been among published these literals over [4,the 7]. world. All A lot of experimentalthese effortsor monitoring or achievement datas provide has been a better presented understanding among on quality these control literals of this[4, 7]. All these effortstechni or achievementque. s provide a better understanding on quality control of this technique. 3. Case history for application of VCM for improving dredged mud 3. Case historyIt has for been application demonstrated thatof VCMkey challenges for improving of VCM for dredged improving themud dredged mud are: (1) How to create of a construction platform on the top surface of disposal site; (2) It has been demonstratedHow to enhance thateffectiveness key challenges vertical drainage of VCM against for the improving clogging and the bending dredged mud are: (1) Howproblems to creat duee of to a large construction settlement. In plat orderform to provideon the a top reference, surface the ofapplication disposal of site; (2) VCM for improving the dredged mud at Dalian Zhuanghe port industry and logistic How to enhanestatece project effectiveness in China [6] isvertical subsequently drainage summarized. against The project the cloggingsite covers an and area bending problems dueof about to large 170 hectares settlement and is. dividedIn order into several to provide blocks with a reference, every 30 to the40 hectaresapplication. of VCM for improvingEach block was the filled dredged by dredging mud slurry at Dalian with thickness Zhuanghe from 7 port to 8 m. industry The dredging and logistic estate projectslurry in Chinis completela [6]y is clayey subsequently silt or silty with summarized. extremely high The initial project water content, site covers nearly an area zero bearing capacity, very soft and flowable to soft plastic. of about 170 hectares and is divided into several blocks with every 30 to 40 hectares. Each block Aswas illustrated filled by in Fig.dredging 3, the slurry construction with work thickness at the projectfrom was7 to conducted8 m. The into dredging following steps: slurry is completel- Surroundy clayey dike siltconst orruction: silty Dikeswith wereextremely built along high the initialboundary water or interface content, of nearly zero bearingthe capacity, blocks by veryfilling softof compacted and flowable mountain to soil. soft plastic. - Dredged mud filling: the dredged mud was filled by hydraulic pumping method. It As illustratedis noted in in Fig. the dredged 3, the mud construction that its silty content work (parti at cle the size project varies from was 0.005 conducted to 0.05 into following steps: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 - Surroundgrid anddike 0.4 const m hydraulicruction: sand Dikes filling was were developed built toalong build workingthe boundary platform. or interface of the blocks by filling- Installation of compacted of vertical andmountain horizontal soil. drains: Conventional PVDs were installed - Dredgedusing mud lightweight filling: installationthe dredged machine. mud Horizontalwas filled perforated by hydraulic pipes wrapped pumping by non method.- It is noted in thewoven dredged geotextile mud were that buried its into silty the contentsand blanket (parti layer.cle size varies from 0.005 to 0.05 - Installation of airtight sheets: Two airtight sheets protected by two non-woven mm) is aboutgeotextile 18 %, layersand its were clay covered content on the (particle surface. 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 wrapped3 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.

3 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 monitoring- Operation data has of vacuumbeen presented system: among vacuum these literals pumps [3 , system4,9]. 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working workingcapacity, load load very is is soft less less and than than flowable 3030 cm. cm.cm. to soft plastic.

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e) PVD installation f) Horizontal pipes g) Air-tight sheet h) Vacuum

e)e)e)e) P P VD(e)PVDVD PVD installation installation installation installation f)f)f)f) Horizontal Horizontal(f)Horizontal Horizontal Horizontal pipespipespipes pipes g)g)g)(g) Air Air Air Air-tight-tight--tighttight sheetsheetsheet sheet sheet (h)h)h) Vacuumh)h) VacuumVacuum operationVacuum Vacuum operation e)e) P PVDVD installation installation f)f)f) Horizontal HorizontalHorizontal pipes pipespipes g)g)g) Air AirAir--tight-tighttight sheet sheetsheet h)h)operationh)operationoperation operationVacuumVacuum Vacuum operationoperationoperation

(i) Dredging mud before and after improvement i) Dredging mud before and after improvement i)i) i)Dredging Dredging Dredging mud mud mud before before before and and and after after after improvement improvement improvement Figure 3. ApplicationFigure 3. Applicationi)i) of Dredging VCM of for VCM mudmud improving for beforebefore improving andand the the afterafter newly newly improvementimprovement hydraulichydraulic reclaimation reclaimation mud mud at i) Dredging mud before and after improvement FigureFigureFigure 3 3 .3 .Application .Application Application ofi) i)of of Dredging VCMDredging VCM VCMDalian for atfor for Dalianmud improvingZhuanghemud improving improving before Zhuanghebefore port,theand the andthe port, newly after newly afternewlyChina China improvement hydraulicimprovement hydraulic hydraulic [[7].4] reclaimation reclaimation reclaimation mud mud mud at at at Figure 3. Application ofof VCMVCM forfor improvingimproving thethe newlynewly hydraulichydraulic reclaimationreclaimation mudmud atat Figure 3. Application of DalianVCMDalianDalian for Zhuanghe Zhuanghe Zhuanghe improving port, port, port, the China newlyChina China hydraulic[7]. [7]. [7]. reclaimation mud at 4. FigureModelFigure 3 3tests. .Application Application for evaluating of of VCM VCMDalian performance for for ZhuangheZhuanghe improving improving port,port, of the the vertical newlyChina Chinanewly hydraulic [7].[7].hydraulicdrains reclaimation reclaimation mud mud at at As illustrated in Fig.3, theDalian construction Zhuanghe work atport, the China project [7]. was conducted into following steps: 44. .Model Model tests tests for for evaluating evaluatingDalianDalian performance performance Zhuanghe Zhuanghe of port,of port, vertical vertical China China drains drains[7]. [7]. 4.1.44.. 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44 4 44 444 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.

4. 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). Table1 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].

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 1 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 canDong, be referred P. H. / Journal to Dong of Science PH. [3]. andTechnology in Civil Engineering

FigureFigure 4. Mud 4. Mud samples samples and and laboratory laboratory test test forforevaluating evaluating physical physical properties properties. Table 1. Physical properties of the mud samples. Table 1. Physical properties of the mud samples Unit Organic Silt Clay content, PH UnitweightWater WaterSpecific SpecificVoid Void Organiccontent Siltcontent content,, % % Clay(d < 0.005 content, gravity, ratio, SampleSampleweight (kN/mcontent3) contentgravity, ratio, content(%) %(d(d= = 0.005 %(mmd <) 0.005 PH (kN/m3) (%) (%) ∆  e e (%) - 0.050.005 mm)- mm) 0.05 mm) 1 1.24 210.9 2.52 5.3 18.6 22.4 19.4 7.19 21 1.271.24 202.5 210.9 2.502.52 5.05.3 22.118.6 22.4 23.6 19.4 18.6 7.19 7.18 32 1.311.27 216.3 202.5 2.532.50 5.15.0 11.422.1 23.6 25.7 18.6 17.7 7.18 6.96 3 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 1 m; (3) One layer of geomembrane was covered on the top and sealing5 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 × 3 mm. The SDs with diameter of 5 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 5 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

4.2. Equipment development The test equipment mainly consists of several components as numbered as shown in Figs. 5 and 6. In Fig. 6, (1) indicates the vacuum system includes a vacuum pump with a power output of 7.5 kW which can generate a vacuum pressure of 95 kPa; (2) is the model test box with inner dimensions of 200 cm x 150 cm x 150 cm in length, width and height. The dredged mud was then freely poured in the box with thickness of 1 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 0.2m 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

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3 6 3 2 2 7

6 6 7 7

8 8 FigureFigure 5. Schematic 5. Schematic chart chart of of the the model model test test FigureFigure 6 6.. EquipmentEquipment of theof modelthe model tests tests

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 three4.3. sets Monitoring of model work tests using either prefabricated vertical drain (PVD),8 Sanddrain (SD) 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 trans- wheremission the ofPVD vacuum is conventionally pressure, the surface comprised settlement a wasplastic also core observed wrapped at positions by geotextile near the vertical filter. Permeabilitydrain and between coefﬁcient the vertical of the drains. ﬁlter Detail is 0.014 locations cm/s. of monitoringThe size of equipment PVD is are 100 shown mm in x Fig.3 mm.5. The SDs with diameter of 5 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 5. Results and discussions 5.1. Settlement and rate of volume reduction 6 The vacuum loading in case of PVD was removal after 15 days, until the settlement rates has reduced and get stable value under 2 mm/day in 5 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 plot- ted in Fig.7. Since the test box has very high stiffness, the horizontal deformation is ignored. There- fore, vertical strain is equivalent to volumetric strain or rate of volume reduction as shown in Fig.8. 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

Vacuum loading time (h) 0 24 48 72 96 120 144Vacuum168 loading192 216 time 240(h) 264 288 312 336 360 384 408 0 0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 0 At drains position between the drains -20 At drains positionSD betweenSD the drains -20 PVDSD PVDSD -40 FPPVD FPPVD -40 FP FP -60 -60

-80 Settlement (mm) Settlement

-80 Settlement (mm) Settlement -100 -100 -120

-120

FigureFigure 7 7.. Settlement versus versus loading time 0% Figure 7. Settlement versus loading time 0% 2% 2% 4%

(%) 4% v

 6%

(%) v

 6% 8% 8% At drains position Between the drains 10% At drains position Between the drains 10% SD SD

Volumetric strain Volumetric 12% PVDSD PVDSD Volumetric strain Volumetric 12% FPPVD FPPVD 14% FP FP 14%0.1 1 10 100 1000 0.1 1 Vacuum loading10 time (h) 100 1000 Vacuum loading time (h) Figure 8. Volumetric strain versus loading time Figure 8. Volumetric strain versus loading time 5.2. Influenced zone andFigure clogging 8. Volumetric phenomenon strain versus loading time 5.2. Influenced zone and clogging phenomenon The influenced zone, which clearly shows transmission area of vacuum pressure, can be 5.2. InfluencedobservedThe influenced zone based and onzone, clogging the whichsoil core phenomenonclearly appeared shows surround transmission the vertical area of drains. vacuum For pressure, more detail can ,be it couldobserved be analyzed based on from the soil the core variation appeared of water surround content the alongvertical the drains. depth Forand more radius detail range., it The influenced zone, which clearly shows transmission area of vacuum pressure, can be observed Again,could be it analyzed is in this from project the variationnoted that of water rate of content water along content the reductiondepth and willradius evaluate range. based onpossibil theAgain, soil ity coreit iswhether appearedin this the project surround mud noted can the be that vertical treated rate of drains. by water premixing For content more withreduction detail, cement it could will for evaluate be filling analyzed from the variationmaterial.possibil of waterity whether content the along mud the can depth be treatedand radius by premixingrange. Again, with it cement is in this for project filling noted that rate of watermaterial. content reduction will evaluate possibility whether the mud can be treated by premixing After the vacuum loading, mud samples were collected at different depths and distances with cementAfter for the filling vacuum material. loading, mud samples were collected at different depths and distances Afterfrom the vacuumthe vertical loading, drains mudfor checking samples water were content. collected The at variation different of depths water andcontent distances along from the thefrom depth the verticalat different drains range for checkingfrom the watervertical content. drains The (r =variation 15 cm, ofr = water 30 cm content and r a =long 45 vertical drainsthe depth for checkingat different water range content. from the The vertical variation drains of ( waterr = 15 contentcm, r = along30 cm the and depth r = 45 at different cm) is presented in Figure . The initial water content (W0) is also shown for referent range fromcm) the is vertical presented drains in Figure (r = 15. T cm,he initialr = 30 water cm and contentr = 45(W cm)0) is is also presented shown for in Fig.referent9. The initial 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.8 However, in case of PVD, water content reduces only 20% on the top and even almost 10% at the8 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 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, value.value. It It can can be be seen seen that that the the water water content content increases increases with with the the increasing increasing depth depth and and water contentvalue. reduceIt can s be only seen 20% that on the the water top content and even increases almost with 10 % the at increasing the depth depth of 0.8 and m . radiusradius distance distance from from the the vertical vertical drain drain in in all all three three cases. cases. However, However, in in case case of of PVD, PVD, In general,radius the distance distribution from the of vertical water drain content in all in three case cases. of PVD However, (Figure in casea) is of relative PVD, waterwater content content reduce reduces sonly only 20% 20% on on the the top top and and even even almost almost 10 1 %0 % at atthe the depth depth of of0.8 0.8 m .m . differencewaterIn from general, content that treducehein cases distributions only of 20%FP of (onFigure wate the rtop b) content and and even SD in almost case(Figure of10 PVD%c) ,at since the (Figure depth the aofvalues) 0.8 is relativem of. InIn general, general, the distribution of of wate waterr content content in in case case of of PVD PVD ( Figure (Figure a )a ) is is relative relative water contentdifferencedifference at from differentfrom that that in radiusin cases cases of distance of FP FP ( Figure( Figurenear b) theb) and and surfaceSD SD (Figure ( Figureare c)not ,c) since, since close the the togethervalues values of. of Althoughdifferencewater more content efforts from that at should different in cases be of radius investigated, FP (Figure distance b) this nearand manner SD the ( Figure surface could c) ,are since be not explained the close values together thatof . waterwater content at different radius radius distance distance nearnear the the surface surface are are not not close close together together. . exceed poreAlthough water pressure more efforts was shouldgenerated be investigated,due to the clogging this manner problem, could after be removal explained of that AlthoughAlthough more effortsDong, should shouldP. H. / Journal be be of investigated, investigated, Science and Technology this this manner in manner Civil Engineering could could be be explained explained that that exceed pore water pressure was generated due to the clogging problem, after removal of vacuum exceedexceedloading, pore pore water water pressure could waswas be generatedgenerated rearranged due due andto to the the rebind clogging clogging to problem,surface. problem, afterIn after addition, removal removal ofthe of varadiuscuum distance loading, near pore the surface water are could not close be rearranged together. Although and rebind more efforts to surface. should beIn investigated, addition, the samplingvava workcuumcuum forloading, water pore content waterwater test couldcould was bebe conducted rearrangedrearranged and afterand rebind rebindfew daysto to surface. surface. from Inthe In addition, addition,removal the theof samplingthis manner work could for be explainedwater content that exceed test was pore conducted water pressure after was few generated days from due to the the removal clogging of vacuum samplingsampling loading.problem, afterwork Figure removal for waterd ofshows vacuum contentcontent the loading, test test variation waswas pore conducted conducted water of couldwater after after be content rearrangedfew few days days with andfrom from rebind radius the the removal to removal surface.distance of Inof from thevacuumvacuum vacuumverticaladdition, loading. the loading.drains sampling atFigure Figure two work depthsdd for dshowsshows shows water of the content the 0.2 the variation variation m variation testand was 0.8 ofof conducted of waterm. water water I t content is content after again content few with withclearly days with radius from radius radiusshown thedistance distance removal distance that from the vertical drains at two depths of 0.2 m and 0.8 m. It is again clearly shown that SD casefrom fromgivesof vacuum thethe the vertical loading.best transmissiondrains Fig. 9(d)atat twotwo shows depths depthseffect the variationofof 0.2 0.2vacuum m m ofand and water 0.8pressure, 0.8 contentm. m. I tI tis withwhichis again again radius clearlyresults clearly distance shown in shown a from better that that the SDSDSDvertical casecase case drainsgives gives atthe twothe best depthsbest transmissiontransmission transmission of 0.2 m and effecteffect 0.8 effect m. of of It ofvacuum isvacuum againvacuum clearly pressure, pressure, pressure, shown which which that which SD results results case results givesin in a inbetter thea bettera best better preloadingtransmission effect. effect of vacuum pressure, which results in a better preloading effect. preloadingpreloadingpreloading effect. effect.

Water content (%) Water content (%) Water content (%) Radial distance (cm) Water content (%) 0 Water content (%) 0 Water content (%) Radial distance (cm) 0 Water content (%) 0 WaterWater content content (%) (%) 0WaterWater content content (%) (%) RadialRadial distance distance (cm) (cm) 00 0 00 0 0 15 20 25 30 35 40 45 0.1 0.1 15 1520 152025 202530 302535 353040 403545 4540 45 0.1 0.1 0.1 200% 0.10.10.1 0.10.1 0.10.1 200%200%200% 0.2 0.2 0.2 0.20.20.2 0.20.20.2 0.20.20.2 0.3 0.3 0.30.3 0.3 0.3 0.3

0.30.30.3 0.3 0.3 180%180%180%180%

)

) ) ) 0.4 0.4 ) 0.40.40.4 0.40.40.4 0.4 0.40.40.4 0.5 0.50.50.5 0.5 0.50.50.5 0.5 0.50.50.5

160%160%160%160%

Depth (m (m Depth

Depth (m (m Depth Depth (m (m Depth

Depth (m (m Depth 0.6 Depth (m (m Depth

0.6 (m Depth Depth (m (m Depth 0.6 0.60.6 0.6 0.60.6 (m Depth 0.6 0.60.60.6

0.7 0.7 0.7 (%) content Water 0.7 0.7 (%) content Water 0.7 0.7 0.70.7 (%) content Water 0.7 0.7 0.7 (%) content Water 140%140%140%140% 0.8 0.8 0.8 0.8 0.80.8 0.8 0.80.8 0.8 0.80.8 z = z0.2 = 0.2zm = m0.2z = z0.8m = m 0.8z =m 0.8m 0.9 0.9 z = 0.2 m z = 0.8m 0.90.9 0.90.9 0.90.90.9 PVD PVD 0.9 0.9 0.9 120% PVDPVD PVDPVD 1 120%120% PVD PVD 1 1 11 1 1 120% FP FP 1 100% 150% 200% 250% 1 1 FP FP FP FP 100%100% 150%150% 200%200% 250%250%1 100%100% 150%150% 200%200% 250%250% 1 100% 150% 200% 250% SDFP SDFP 100% 150% 200% 250% 100%100%150%150%200%200%250%250% SD SD SD SD 100% 150% r=200% 15cm 250%r=30cm 100% 150%r=15cm200% 250%r=30cm 100% 150% 200% 250% r= r=15cm 15cm r=30cmr=30cm r=15cm r=30cm r=15cm r=30cm 100% SD SD r=15cm r=30cm r=15cmr=15cm r=30cmr=30cm 100%100% r= 15cm r=45cmr=30cm Initial W0 r=15cm r=45cm r=30cm Initial W0 r=15cm r=30cm r=45cmr=45cm InitialInitial W0 W0 r=45cmr=45cm InitialInitial W0 W0 r=45cm Initial W0 100% r=45cm Initial W0 r=45cmr=45cm InitialInitial W0 W0 r=45cm Initial W0 r=45cm Initial W0 a)(a)a) PVD PVD casec asecase b)(b)b)b) FP FPFP FP casecase case case (c)c)c) SDSDc) SD caseSD case case case (d) Waterd)d) Water d) contentWater Water vs. radial a) PVD case b) FP case c) SD case d)contentcontent Watercontentrange vs vs. .vs . contentradial range vs. Figure 9. Variation of water content with depth and radius radialradial range range Figure 9. Variation of water content with depth and radiusradial range FigureFigure 9 .9 Variation. Variation of of water water content content with with depth depth and and radius radius Fig. Figure 10Figure shows 9 shows the9. verticalVariation the vertical drains of afterdrains water removal after content removal of the with vacuum of depth the loading. vacuum and In radius caseloading. of PVD, In case clogging of phenomenon Figure Figure was9 9shows shows visually the the observed. vertical vertical A drains tinydrains layer after after of removal fine removal particles of of the appeared the vacuum vacuum on surface loading. loading. of filterIn Incase layer,case of of PVD, clogging phenomenon was visually observed. A tiny layer of fine particles FigurePVD,PVD,whereas 9 clo shows clo nogginggging soil particlethe phenomenon phenomenon vertical is seen indrains plastic was was after core. visually visually Inremoval both observed. casesobserved. of of the the A FPvacuum A tiny and tiny SD layer layer verticalloading. of of drains fine fineIn methods, particlescase particles of appeared on surface of filter layer, whereas no soil particle is seen in plastic core. In PVD, cloappearedagging soil core phenomenonon is clearlysurface created of filter wassurrounding layer, visually thewhereas vertical observed. no drains. soil Radius Aparticle tiny of soilis layer seen core ofinin case fineplastic of FP particles core. is about In bothappeared cases on of surface the FP of and filter SD verticallayer, whereas drains methods,no soil particle a soil coreis seen is clearlyin plastic created core. In both12 cm, cases and that of thein case FP of and SD is SD approximately vertical drains 25 cm. methods, This observation a soil is coincidedcore is clearly with the created change appearedsurrounding bothofon water surface cases content the of of as thevertical describedfilter FP layer, anddrains. in Fig. SD whereas 9Radius . verticalConsequently, of no drainssoil soil itcore is methods,suggestedparticle in case that isof a seen thesoilFP SD is corein drainabout plastic is case 12 clearly has cm,core. a betterand created In surrounding the vertical drains. Radius of soil core in case of FP is about 12 cm, and both casesthatsurroundingtreatment of in the case effect FP ofthe both SDand vertical in is SD decreasing approximately verticaldrains. the Radius moisture drains 25 cm.of content methods, soil This core and observation in ain increasing soilcase coreof is theFP coincided is processis clearly about uniformity with 12 created cm, the at and that in case of SD is approximately 25 cm. This observation is coincided with the surroundingchangethatdifferent the in of depths.casevertical water of content SDdrains. is as approximately Radiusdescribed of in soil Figure 25 core cm. . Consequein This case observation ofntly, FP it isis about issuggested coincided 12 cm,that with theand the changechange of of water water content content as as described described in in Figure Figure . Conseque. Consequently,ntly, it itis issuggested suggested that that the the that in caseSD drain of SDcase ishasPlastic approximately a core better treatment 25Filter effect cm. pipe both This in observation decreasingSand drainthe is moisture coincided content with and the SD drain case has a better treatment effect both in decreasing the moistureSoil corecontent and change ofinSD increasingwater drain content case the has process as a describedbetter uniformity treatment in atFigure different effect . bothConseque depths. in decreasing ntly, it isthe suggested moisture contentthat the and inin increasing increasingFilter the the process process uniformity uniformity at at different differentSoil depths. depths.core SD drain case 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 9 9 Figure 9. The vertical drains after removal of vacuum pressure 5.2. Distribution of vacuum pressure 70 9 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. The delayed time and rising rate increase by sequence of SD, FP and PVD. It can be explained that during a short period of vacuum loading, the whole vacuum loading is focused on the formation of tiny ripples and building of some larger channels. After formation of tiny ripples and building of larger channels inside the soil, the vacuum pressure reaches to stable value because of the silt clogging around the PVDs, thus no longer development of vacuum pressure can be observed.

120 SD SD 100 FP FP PVD PVD 80

20 Vaccum pressure (kPa) Vaccum Vacuum loading time (h) 0 0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 Figure 10. Vacuum pressure during vacuum loading 6. Conclusions 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

Plastic core Filter pipe Sand drain Soil core

Filter Soil core

Figure 9. 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 Vacuumvacuum pressure pressure was at shallow monitored depth during was loading. quickly Fig. reached 11 presents stable value the vacuum right after pressure operation at depth of 0.2 mof and vacuum 0.8 m system. for 03 casesIt however among needs the vertical a specific drains. time The to vacuumbe transmitted pressure at atdepth shallow of 0.8 depth m. was quicklyThe reached delayed stable time value and rising right after rate operationincrease by of vacuumsequence system. of SD, It FP however and PVD. needs aIt specific can be time to beexplained transmitted that at depthduring of a 0.8 short m. Theperiod delayed of vacuum time and loading, rising ratethe increasewhole vacuum by sequence loading of SD,is FP and PVD.focused It can on be the explained formation that of during tiny aripples short period and building of vacuum of loading, some larger the whole channels. vacuum After loading is focusedformation on the of formation tiny ripples of tiny and ripples building and of building larger ofchannels some larger inside channels. the soil, After the v formationacuum of tiny ripplespressure and reaches building to ofstable larger value channels because inside of the the soil, silt the clogging vacuum around pressure the reaches PVDs, to thus stable no value 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 SD 100 FP FP PVD PVD 80

20 Vaccum pressure (kPa) Vaccum Vacuum loading time (h) 0 0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 FigureFigure 10 11.. Vacuum Vacuum pressure pressure duringduring vacuum vacuum loading loading 6. Conclusions In most cases, dredged mud has been believed to be unsuitable material for reclamation. 6. ConclusionsThe 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 technical10 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.