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Geotextiles and Geomembranes 27 (2009) 152–155
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Geotextiles and Geomembranes
journal homepage: www.elsevier .com/locate/geotexmem
Technical Note
A new type of prefabricated vertical drain with improved properties
Han-long Liu a, Jian Chu b,*
a Geotechnical Research Institute, Hohai University, Nanjing 210098, Chinab School of Civil and Environmental Engineering, Nanyang Technological University, Block N1, 50 Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e i n f o
Article history:Received 31 May 2008Received in revised form20 September 2008Accepted 23 September 2008Available online 11 November 2008
Keywords:Prefabricated vertical drainsSoft soilSoil improvement
* Corresponding author. Tel.: þ65 67904563; fax: þE-mail addresses: [email protected] (H.-long Liu), c
0266-1144/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.geotexmem.2008.09.006
a b s t r a c t
Conventionally prefabricated vertical drain (PVD) consists of a core and a filter sleeve as two separablecomponents. A new type of PVD is now available in which the filter and the core are bound together byheat melting to become an integrated body. This new type of integrated PVD has offered a new methodin improving the properties and performance of PVD. Laboratory test results are presented in this note toshow that both the tensile strength and the discharge capacity of the integrated PVD have increased ascompared with those of the separable type of the same materials.
� 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Prefabricated vertical drain (PVD) has been used widely for theimprovement of soft soil by preloading (Holtz, 1987; Bergado et al.,1990, 1993a,b, 1996, 2002; Li and Rowe, 2001; Arulrajah et al.,2004; Bo, 2004; Chai et al., 2004, 2008; Chu et al., 2004, 2006;Rowe and Li, 2005; Indraratna and Chu, 2005; Shen et al., 2005;Abuel-Naga et al., 2006; Sinha et al., 2007; Rowe and Taechakum-thorn, 2008.) and PVD techniques have been well established (Holtzet al., 1991; Bo et al., 2003). As discussed by a number ofresearchers, the effectiveness of PVD in soil improvement isaffected by the quality of the PVDs (Chu et al., 2004). A PVD mustprovide sufficient tensile strength and discharge capacity amongother factors to ensure its effective use in soil improvement.
As million meters of PVDs can be used for a large scale soilimprovement project, the unit cost of PVD becomes an importantconsideration. One way to improve the cost-effectiveness of PVD isto improve the quality of PVD with little or no increase in materialcosts.
PVD consists of a core and a filter sleeve. The core is normallyplaced loosely inside the filter as two separate components, asshown in Fig. 1(a). There are two major shortcomings with thisdesign. Firstly, the tensile strengths of the core and the filter arenormally not compatible and sometimes differ a lot. As a result, onecomponent, either the core or the filter, will break first. Once onecomponent breaks, the PVDs will no longer function properly.Secondly, as the filter is fitted loosely to the core, the filter will
65 [email protected] (J. Chu).
All rights reserved.
indent into the drainage channels of the core under earth pressureas shown in Fig. 2. As a result, the discharge capacity of the PVD willbe reduced, as discussed in detail by Broms et al. (1994).
The above two problems can be overcome by a new type of PVDin which the filter and the core are adhered together by heatmelting along the contacts between the core and the filter to forman inseparable body, as shown in Fig. 1(b). A picture of the inte-grated PVD is also shown in Fig. 1(c). The new design possesses thefollowing advantages.
(1) The tensile strength of the integrated PVD will be higher thanthat of the separable type. This is because the core and filterwill deform as one body and the combined tensile strength ishigher than the weaker element between the core and the filterdue to the interaction between the two components.
(2) The discharge capacity of the integrated PVD will be greater.This is mainly because the indentation of filter into thedrainage channels of the core is considerably reduced for theintegrated PVD and the vertical wings of the core are also lesseasy to be bent under pressure as the overall stiffness of thePVD has increased after the core and filter are adheredtogether.
(3) The integrated PVD becomes more resistant to cloggingbecause the drainage channels in the integrated type of PVDare not connected. Therefore, when one channel is blocked, itwill not affect the rest.
(4) Saving in the filter material. The width of the filter is nowexactly 100 mm in the integrated PVD, whereas in the sepa-rable PVD, the filter warps the core so it has to be at least 20–40 mm longer in circumference.
100±2mm
5±1 mm
a
b
c
Fig. 1. Types of PVD. (a) Separable type; (b) integrated type; and (c) a picture of theintegrated drain.
Valve
Allan Screw
'O' Rings
Base
Top Cap
Valve
Marine ClayDrain 100mm x 100mm
Marine Clay
HollowExtension Plate
a
b
Fig. 3. Discharge capacity testing devices. (a) Straight drain tester; and (b) bucklingdrain tester.
H.-long Liu, J. Chu / Geotextiles and Geomembranes 27 (2009) 152–155 153
(5) Increase in production. As the filter is no longer required towrap the core, the PVD can now be produced by adheringtogether core and filter materials of any width greater than100 mm first and then cutting the integrated core and filterinto 100 mm wide PVD. In this way, the PVD production can bemade much easier and productive.
Soil
Filter
Core
Fig. 2. Reduction of flow area caused by the deformation of filter.
Some laboratory test results are presented in this paper to showthat both the tensile strength and the discharge capacity of theintegrated PVD are higher than those of the separable PVD made ofthe same materials.
2. Experimental setup and PVD tested
2.1. Tensile strength test
The tensile strength tests were conducted using a materialtesting machine. A clamp which had a width of more than 100 mmwas used to fix a 100 mm wide drain specimen at the two ends. Thegauged length of the drain was 100 mm. The pulling rate was25 mm/min. The specimen was tested until the drain (or the core orfilter) broke. The drain specimens at both dry and wet conditionsare tested.
2.2. Discharge capacity test
The discharge capacity of the PVD was measured using a straightdrain tester (Chu et al., 2004) and a buckled drain tester as shown inFig. 3. In the former, a 100 mm long drain specimen was embeddedin marine clay and the vertical pressures were applied using deadweight via an oedometer loading frame (Chu et al., 2004). In thelatter, the PVD specimen was enclosed in a rubber membrane. The
Table 1Tensile strength tests of integrated and separable PVD samples at dry state
Specimen Tensile strength at10% axial strain (N)
Tensile strengthat failure (N)
Tensile strainat failure (%)
Integrated-1 3140 3480 20Integrated-2 3260 3460 16Integrated-3 3100 3460 22Separable-1 2800 2920 12Separable-2 2620 2820 14Separable-3 2680 2840 14
H.-long Liu, J. Chu / Geotextiles and Geomembranes 27 (2009) 152–155154
effective length of the PVD was 400 mm. To simulate the effect ofbuckling on the discharge capacity of PVD, the PVD specimen wasbent in the middle in a way as shown in Fig. 3(b). The pressureswere applied using the hydrostatic pressure of the cell fluid.Chinese PVD quality inspection standard, JTJ/T257-96 (1996), wasadopted in conducting both tests in which the hydraulic gradientused for the discharge capacity test was taken as 0.5. The use ofa hydraulic gradient of 0.5 for discharge capacity test is reasonable,as explained in detail in Chu et al. (2004).
2.3. PVD used
The separable and integrated PVDs used in the experimentswere made of the same materials. The core was made of 100%transparent polypropylene plastic and the filter was made ofDuPont grey colour, pure long-fibre hot-bonded non-woven filterfabric. It had a width of 100� 0.2 mm and a thickness of4.5� 0.2 mm. The shape of the drainage channel is shown in Fig. 1.The effective pore opening size was 0.075 mm and the weight toarea ratio of the filter was larger than 110 g/m2.
3. Test results
3.1. Tensile strength tests
For each specimen, 3 identical tests were conducted. The resultsfor both integrated and separable type of PVD are shown in Fig. 4(a)and (b) for dry and wet conditions, respectively. The tensilestrengths measured from these tests are given in Tables 1 and 2 fordry and wet conditions, respectively. A comparison of the averagetensile force versus tensile strain curves between the integratedand separable types of PVDs is shown in Fig. 5 for both dry and wet
0
1000
2000
3000
4000
0 5 10 15 20
Elongation (%)
0 5 10 15 20Elongation (%)
Ten
sile fo
rce (N
)T
en
sile fo
rce (N
)
Separable -1Separable -2Separable -3Integrated -1Integrated -2Integrated -3
Separable -1Separable -2Separable -3Integrated -1Integrated -2Integrated -3
Dry samples
0
1000
2000
3000
4000Wet samples
a
b
Fig. 4. Tensile strength tests on integrated and separable PVDs at (a) dry state; and (b)wet state.
conditions, respectively. The data show that the average axial strainat failure has increased by 52% from 12.7% for the separable to 19.3%for the integrated PVD. The average failure tensile strength has alsoincreased by 18.7% from 2903 N for the separable to 3447 N for theintegrated PVD. The average tensile strength at 10% axial strain hasalso increased by 14.5% from 2747 N for the separable to 3146 N forthe integrated PVD. Therefore, the tensile strength properties of thePVDs have greatly improved by using the integrated type of PVD.
The reason for the increase in the tensile strength is mainlybecause of the better deformation compatibility between the coreand the filter in the integrated PVD. When the filter is adhered tothe core, the tensile strength of the PVD will be controlled by thecore. For separable PVD, the filter and the core are fitted loosely. Thefilter is normally 10–20 mm wider in width. When the drain ispulled, the filter and core deform separately. When the filter fabricis pulled, the central portion will tend to neck more than the twoends to form an hourglass pattern. As a result, the central portion ofthe filter will undergo a deformation larger than the core and breakfirst. Therefore, for separable PVD, the tensile strength is controlledby the filter at least for the PVD tested.
3.2. Discharge capacity
The discharge capacity of the integrated and separable types ofdrains as measured at a hydraulic gradient of i¼ 0.5 by both thestraight drain tester and the bulked drain tester are compared inFig. 6. It can be seen that in both tests, the discharge capacity of theintegrated PVD is higher than that of the separable one. At anapplied pressure of 350 kPa, the discharge capacity of the inte-grated PVD is 38% and 49% higher than that of the separable one,respectively. This was mainly due to the following two reasons. (1)The indentation of filter into the core (as shown in Fig. 2) is muchsmaller for the integrated drain as discussed in Section 1. (2) As canbe felt by hand, the integrated PVD is stiffer against bending thanthe separable PVD. This is because the filter fabric is bound to thecore and thus limits the stretching of the core when bent. There-fore, the resistance to bending is better for the integrated drain.
It should be noted that the two types of tests impose two differentconditions. In the straight tests, the drain specimens were straightand embedded in soil, whereas in the buckling tests, the drainspecimens were sealed in a membrane and artificially bent. Whetherthe specimen is tested in soil or not will affect the discharge capacitymeasurement, as discussed in Chu et al. (2004). As the membranewas slightly oversized, the flows between the PVD and themembrane could not be completely eliminated under low pressure.
Table 2Tensile strength tests of integrated and separable PVD samples at wet state
Specimen Tensile strength at10% axial strain (N)
Tensile strengthat failure (N)
Tensile strainat failure (%)
Integrated-1 3120 3480 20Integrated-2 3140 3380 22Integrated-3 3120 3420 16Separable-1 2680 2860 12Separable-2 2820 2960 12Separable-3 2880 3020 12
Integrated -drySeparable -dryIntegrated -wetSeparable -wet
0 5 10 15 20
Elongation (%)
Ten
sile fo
rce (N
)
1000
2000
3000
4000
0
Fig. 5. Comparison of tensile strength tests on integrated and separable PVDs at bothdry and wet states.
0
25
50
75
100
125
150
0 50 100 150 200 250 300 350
Applied pressure (kPa)
IntegratedSeparable
0
25
50
75
100
125
150
100 200 300150 250 350
Applied pressure (kPa)
Dis
ch
arg
e c
ap
ac
ity
(m
l/s
)D
is
ch
arg
e c
ap
ac
ity
(m
l/s
)
IntegratedSeparable
a
b
Fig. 6. Comparison of discharge capacity tests on integrated and separable PVDs using(a) straight drain tester; and (b) buckled drain tester.
H.-long Liu, J. Chu / Geotextiles and Geomembranes 27 (2009) 152–155 155
As a result, the discharge capacity would be overestimated underlow pressure. Therefore, the results shown in Fig. 6(a) and (b) maynot be compared directly. However, this does not affect thecomparison of the two types of PVD when the same testing methodis adopted. For the buckled drain, the discharge capacity at a lowpressure may also be affected by how the membrane fits the PVD atthe bending point. This explained why the discharge capacity of thebuckled PVDs did not reduce gradually with the increase in appliedpressure as was observed from the tests on straight PVD.
4. Conclusions
A new type of PVD, the so called integrated PVD in which the coreand filter are adhered together by heat melting, is introduced in this
paper. The integrated PVD offers a number of advantages over theseparable PVD. Both the tensile strength and the discharge capacity ofthe integrated PVD are higher than those of the separable one of thesame material. The data presented in this note show that the averageaxial strain at failure has increased by 52% from 12.7% for the separableto 19.3% for the integrated PVD. The average failure tensile strength hasalso increased by 18.7% from 2903 N for the separable to 3447 N for theintegrated PVD. The integrated PVD has a stronger resistance tobending. The integrated PVD also uses less filter materials and is moresuitable for massive production. Therefore, the integrated PVD is animportant improvement to the conventional design of PVD.
Acknowledgements
The authors would like to thank Zhejiang Binwang EngineeringMaterials, Co., Ltd. for providing the PVD materials used in this study,and to Mr Bo Zhang for conducting some of the discharge capacity tests.
References
Abuel-Naga, H.M., Bergado, D.T., Chaiprakaikeow, S., 2006. Innovative thermaltechnique for enhancing the performance of prefabricated vertical drain duringthe preloading process. Geotextiles and Geomembranes 24 (6), 359–370.
Arulrajah, A., Nikraz, H., Bo, M.W., 2004. Factors affecting field instrumentationassessment of marine clay treated with prefabricated vertical drains. Geo-textiles and Geomembranes 22 (5), 415–437.
Bergado, D.T., Singh, N., Sim, S.H., Panichayatum, B., Sampaco, C.L.,Balasubramaniam, A.S., 1990. Improvement of soft Bangkok clay using verticalgeotextile band drains compared with granular piles. Geotextiles and Geo-membranes 9 (3), 203–231.
Bergado, D.T., Alfaro, M.C., Balasubramaniam, A.S.,1993a. Improvement of soft Bangkokclay using vertical drains. Geotextiles and Geomembranes 12 (7), 615–663.
Bergado, D.T., Mukherjee, K., Alfaro, M.C., Balasubramaniam, A.S., 1993b. Predictionof vertical-band-drain performance by the finite-element method. Geotextilesand Geomembranes 12 (6), 567–586.
Bergado, D.T., Manivannan, R., Balasubramaniam, A.S., 1996. Proposed criteria fordischarge capacity of prefabricated vertical drains. Geotextiles and Geo-membranes 14, 481–505.
Bergado, D.T., Balasubramaniam, A.S., Fannin, R.J., Holtz, R.D., 2002. Prefabricatedvertical drains (PVDs) in soft Bangkok clay: a case study of the New BangkokInternational Airport project. Canadian Geotechnical Journal 39, 304–315.
Bo, M.W., 2004. Discharge capacity of prefabricated vertical drain and their fieldmeasurements. Geotextiles and Geomembranes 22 (1–2), 37–48.
Bo, M.W., Chu, J., Low, B.K., Choa, V., 2003. Soil Improvement: Prefabricated VerticalDrain Technique. Thomson Learning, Singapore, ISBN 981-243-044-X, 341 pp.
Broms, Chu, J., Choa, V., 1994. Measuring the discharge capacity of band drains bya new drain tester. In: Proceedings of the Fifth International Conference onGeotextiles, Geomembranes and Related Products, Singapore, 5–9 September,vol. 3. pp. 803–806.
Chu, J., Bo, M.W., Choa, V., 2004. Practical consideration for using vertical drains insoil improvement projects. Geotextiles and Geomembranes 22, 101–117.
Chu, J., Bo, M.W., Choa, V., 2006. Improvement of ultra-soft soil using prefabricatedvertical drains. Geotextiles and Geomembranes 24, 339–348.
Chai, J.-C., Miura, N., Nomura, T., 2004. Effect of hydraulic radiu on long-term drainagecapacity of geosynthetics drains. Geotextiles and Geomembranes 22, 3–16.
Chai, J.-C., Miura, N., Bergado, D.T., 2008. Preloading clayey deposit by vacuumpressure with cap-drain: analyses versus performance. Geotextiles and Geo-membranes 26 (3), 220–230.
Holtz, R.D., 1987. Preloading with prefabricated vertical strip drains. Geotextiles andGeomembranes 6 (1–3), 109–131.
Holtz, R.D., Jamiolkowski, Lancellotta, R., Pedroni, R., 1991. Prefabricated VerticalDrains: Design and Performance, CIRIA Ground Engineering Report: GroundImprovement. Butterworth-Heinemann Ltd., London.
Indraratna, Chu, 2005. Ground Improvement Case Histories. Elsevier.JTJ/T257-96, 1996. Quality Inspection Standard for Prefabricated Drains. Ministry of
Communications, China.Li, A.L., Rowe, R.K., 2001. Combined effects of reinforcement and prefabricated
vertical drains on embankment performance. Canadian Geotechnical Journal 38(6), 1266–1282.
Rowe, R.K., Li, A.L., 2005. Geosynthetic-reinforced embankments over soft foun-dations. Geosynthetics International 12 (1), 50–85.
Rowe, R.K., Taechakumthorn, C., 2008. Combined effect of PVDs and reinforcementon embankments over rate-sensitive soils. Geotextiles and Geomembranes 26(3), 239–249.
Shen, S.-L., Chai, J.-C., Hong, Z.S., Cai, F.X., 2005. Analysis of field performance ofembankments on soft clay deposit with and without PVD-improvement. Geo-textiles and Geomembranes 23, 463–485.
Sinha, A.K., Havanagi, V.G., Mathur, S., 2007. Inflection point method for predictingsettlement of PVD improved soft clay under embankments. Geotextiles andGeomembranes 25 (6), 336–345.
