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Geotextiles and Geomembranes 27 (2009) 493–496

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Geotextiles and Geomembranes

journal homepage: www.elsevier .com/locate/geotexmem

Technical Note

New methods for measuring the installation depth of prefabricated vertical drains

Han-Long Liu a, Jian Chu b,*, Zaiyong Ren c

a Geotechnical Research Institute, Hohai University, Nanjing 210098, Chinab School of Civil and Environmental Engineering, Nanyang Technological University, Block N1, 50 Nanyang Ave, Singapore 639798, Singaporec Zhejiang Binwang Engineering Materials Co. Ltd, Taizhou, Zhejiang Province 318020, China

a r t i c l e i n f o

Article history:Received 22 September 2008Received in revised form4 May 2009Accepted 6 May 2009Available online 7 June 2009

Keywords:Prefabricated vertical drainsQuality controlSoil improvement

* Corresponding author. Tel.: þ65 67904563; fax: þE-mail addresses: hliu@hhu.edu.cn (H.-L. Liu), cjch

0266-1144/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.geotexmem.2009.05.001

a b s t r a c t

The installation depth of prefabricated vertical drain (PVD) is one of the key factors affecting the outcomeof soil improvement when PVDs are used to accelerate the consolidation of soft soil. As PVDs are installedunderground, it is difficult to verify the installation depth of PVDs for quality control purpose.In this note, three new methods that have been used in China for measuring the installation depth of PVDare introduced. The working principles of each method are described. The advantages and disadvantagesof each method are discussed. An example is also given to illustrate the use of the methods in soilimprovement projects.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Prefabricated vertical drain (PVD) has been used widely in soilimprovement projects around the world (Holtz, 1987; Holtz et al.,1991; Bergado et al., 1990, 1993a,b, 1996, 2002; Li and Rowe, 2001;Arulrajah et al., 2004; Bo, 2004; Bo et al., 2003; Chai et al., 2004,2008; Chu et al., 2004, 2006, 2009; Indraratna and Chu, 2005; Shenet al., 2005; Abuel-Naga et al., 2006; Abuel-Naga and Bouazza,2009; Rowe and Taechakumthorn, 2008; Liu and Chu, 2009; Huangand Han, 2009). PVDs are normally used to reduce substantially thedrainage path so as to accelerate the dissipation of excess porepressures generated by the application of surcharge load. In most ofthe cases, PVDs are installed through the entire compressible soillayer. If a full penetration of PVDs is assumed in the design and yetthe PVDs are not installed to the entire depth of soft clay, thepredicted rate of consolidation will be incorrect. Therefore, it isimportant to measure the real penetration length of the PVDinstalled on site. Another reason for measuring the installationdepth of PVD is to gain a more specific knowledge of the depth ofsoft soil at the PVD installation locations. The installation depth ofPVD is normally specified by the designer. However, when erraticsoil profiles are encountered, contractors are allowed to terminatethe PVD only when the stiff or hard formation below the soft soilformation is encountered which can be gauged based on the effortsrequired to penetrate the mandrel. In this case, the thicknesses ofthe soft soil layer at different PVD installation points can be known

65 67910676.u@ntu.edu.sg (J. Chu).

All rights reserved.

more precisely. This in turn will result in a more accurate estima-tion of the ground settlement and rate of consolidation.

At the present, the following three methods have been adopted inmeasuring the penetration depth of PVD as described by Bo et al.(2003): (1) using a meter on the mast; (2) using a dial gauge; and (3)using an automatic digital counter. However, all the three methodsmeasure only the length of the PVDs that pass through the pointwhere the counter or the dial gauge is located, not the real length ofthe PVDs that has been installed into the soft clay. For this reason,none of the three methods can provide a direct measurement of thePVD installed in the ground. Therefore, none of the three methods issuitable to be used for independent checking or auditing purposes.Without measuring the penetration depth of the PVD directly, it willbe impossible to check whether there is any mistake or cheating inthe PVD installation records. There were cases where PVDs were notinstalled deliberately to the required depths. Therefore, a methodthat can measure directly the installation depth of PVD is required. Inthe followings, three new methods that can measure the penetra-tion depth of PVDs directly after the PVD has been installed areintroduced. These three new methods are digitised PVD, PVD withtwo wires and PVD with one wire, respectively.

2. New methods

2.1. Digitised PVD

The first method is to print a meter scale on the surface of PVD atan interval of 20 or 25 cm so its linear length can be read directly.

H.-L. Liu et al. / Geotextiles and Geomembranes 27 (2009) 493–496494

An example is shown in Fig. 1. The meter scale can be printed ontothe surface of PVD automatically using the computer scale-spat-tering digital metering technology. When the PVDs are installed byfollowing a given sequence, the differences in the meters printed ontop of the current and the last PVD installed will be the installationlength of the current PVD. The total length of PVDs used can also becalculated easily. This method is simple and incurs almost no extracost. However, this method has the following shortcomings. 1) Thenumbers printed on the filter of PVD become illegible when thePVDs are stored or exposed outside for too long; 2) the readings areaffected by the expansion or contraction of PVDs due to tempera-ture variation or wetting; 3) PVDs may be stretched duringinstallation; and 4) the scale printed may not be accurate.Furthermore, it is still not a direct measurement as the length of thePVDs is not measured directly.

Fig. 2. PVD with two copper wires embedded for PVD penetration depthmeasurement.

2.2. PVD with two wires

The second method is to embed two shielded thin copper wiresin the PVD, as shown in Fig. 2. This method has been patented(Ren, 2004). The length of the PVD can be calculated by measuringthe resistance of the wires. The two wires are embedded along theoverlapping joint of the filter as shown in Fig. 2. Before installa-tion, the two wires at the bottom end of the PVD need to beconnected together. At the top end of the PVD, the wires areconnected to a meter to measure the electrical resistance of thetwo wires as one loop. A readout unit as shown in Fig. 3 has beenspecially designed for this purpose. This readout unit can measurethe resistance of the wires, convert it directly into length, displayand store the readings. This is probably the most direct and reli-able method available so far. However, this method also has someshortcomings. Firstly, it incurs extra costs to the PVDs to use twowires. Secondly, the connection of two wires before each instal-lation of PVD is troublesome. Furthermore, the connection has tobe done properly otherwise the method will not work. Disputesmay rise sometimes on whether the PVD is not installed properlyor simply because the wires at the end of the PVD are not con-nected properly. For this reason, a standard procedure should beadopted. It is suggested to use a minimum connection length of20 mm. The insulation at the connection must be removed byburning or scratching. The connected portion should be put backinto the filter.

Fig. 1. PVD with scale printed on it for PVD penetration depth measurement.

2.3. PVD with one wire

To overcome the problems associated with the two-wire PVDmethod, a third method has been developed. This method is similarto the two-wire PVD method, but uses only a single wire. This thincopper wire is embedded along the overlapping joint of the filter asshown in Fig. 4. This method is based on the principle of microwaveimpedance measurement as explained in detail by Somlo andHunter (1985). When PVD is installed into the ground, the wire inthe PVD and another wire connecting to the ground as provided bythe readout unit form a two-wire system. The impedance, Z, at theentrance of this two-wire system is given by the followingapproximate formula:

Z ¼ 0:5hrðRþ jXÞ (1)

where: hr is a parameter dependent upon the characteristic of thesoil where the wire is embedded, and R and X are given by:

R ¼ 60fC þ lnðklÞ � CiðklÞ þ 0:5 sinðklÞ½Sið2klÞ � 2SiðklÞ�þ 0:5 cosðklÞ½C þ lnðkl=2Þ þ Cið2klÞ � 2CiðklÞ�g (2)

Fig. 3. Readout unit for the measurement of PVD penetration depth.

Fig. 4. PVD with one copper wire embedded for PVD penetration depth measurement.

Table 1Installation depth measured using the one-wire PVD.

# Real installation depth (m) Measured installation depth (m) Error (m)

1 18 17.6 0.42 18 17.2 0.83 18 17.5 0.54 18 17.0 1.05 18 17.3 0.76 18 17.4 0.67 18 17.7 0.38 18 17.3 0.79 18 17.6 0.410 18 17.4 0.611 18 17.1 0.912 18 16.8 1.213 18 17.5 0.5

H.-L. Liu et al. / Geotextiles and Geomembranes 27 (2009) 493–496 495

X ¼ 30n

2SiðklÞ þ cosðklÞ½2SiðklÞ � Sið2klÞ� � sinðklÞh2CiðklÞ

� Cið2klÞ � Ci

�2ka2=l

�io(3)

where: C¼ 0.5772, Ci(x) and Si(x) are the cosine and sine integrals,k¼ 2p/l, l¼ c/f is the wavelength, c¼ 3�108 m/s, f is thefrequency, a is the radius of the wire, l is the length of the wireembedded in the soil.

It can be seen from Eqs. (1)–(3) that the impedance Z is relatedto the length of the wire embedded in the soil, that is, the length ofPVD. In using this method, the relationship between the impedanceand the length of a PVD installed for a given site should be cali-brated. This is done by measuring the impedance of several PVDsinstalled with known depths. The impedance of the wire ismeasured by recording the emitting and receiving time of themicrowave. The length of the PVD installed can be estimated basedon the impedance measured. A measuring device similar to thatshown in Fig. 3 has been specially designed to measure theimpedance and convert it directly into length. The measuring erroris normally within 20 cm and the maximum error is within 50 cmwhich is good enough for many projects.

The impedance method is new to geotechnical engineers andsome experiences may need to be built up before the method canbe used with confidence. In this case, the PVD with one wiremethod can be combined with the digitised PVD, as shown in Fig. 4.In this way, the one wire method can be verified directly using themeter scale.

3. Example

All the three methods have been used in soil improvementprojects in China. A case study on the third method using one-wirePVD is presented here as an example. The case was a roadconstruction project in Nanjing, China. PVDs were used as part ofthe vacuum preloading method to improve soft ground of an area13,155 m2. The soil profile at the site consisted of 3 layers. The firstlayer was miscellaneous fill of 3 m thick in average. It was mainlyclay with some gravel and crushed bricks. The second layer was softclay. The average thickness of this layer was 15 m. The undrainedshear strength of the clay varied from 9 to 23 kPa. The soil belowthe clay layer was silty sand. The ground water table was 0.2–1.2 mbelow the ground surface.

PVDs were installed through the entire depth of the soft clay ina triangle pattern at a spacing of 1.2 m. The nominal depth of theinstallation was 18 m. One-wire type of PVDs was used for one partof this project. PVDs were randomly selected to measure theinstallation depth using the microwave impedance measurementtechnique. The measured installation depths for 13 PVDs are givenin Table 1. During installation, the depths of PVDs were alsorecorded using a dial gauge installed on the drain installation mask.These values were also given in Table 1 for comparison. The error inthe microwave impedance method is taken as the differencebetween the lengths measured by the dial gauge and that by theone wire method. The maximum error was 1.2 m or 6.7% for aninstallation depth of 18 m. This amount of error is negligible for thepurpose of checking the installation depth of PVD.

4. Summary

Three new methods that can be used to measure the penetrationdepth of PVDs directly are introduced. These are: digitised PVD,PVD with two wires and PVD with one wire. The digitised PVD is toprint meter scale onto the PVD so installation depth can be calcu-lated as the difference between the readings at the top of twoadjacent PVDs. The PVD with two wires or one wire methods are toembed two or one thin copper wires along the overlapping joint ofthe filter. The depth of the PVDs is then estimated by measuring theelectrical resistance of the wires embedded in the PVD with twowires method or the microwave impedance of the wire in the PVDwith one wire method, respectively. All the three methods havebeen used for soil improvement projects in China.

Acknowledgments

The writers would like to acknowledge the supports of theNational Science Foundation of China No. 50639010, and ProvincialScience Foundation of Jiangsu, China No. BK2008040.

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.

Abuel-Naga, H., Bouazza, A., 2009. Equivalent diameter of a prefabricated verticaldrain. Geotextiles and Geomembranes Available online 18 January.

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., 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.

H.-L. Liu et al. / Geotextiles and Geomembranes 27 (2009) 493–496496

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.

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., 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.

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, 341 pp.

Chai, J.-C., Miura, N., Nomura, T., 2004. Effect of hydraulic radius 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.

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.

Chu, J.,Bo,M.W.,Arulrajah,A.,2009.ReclamationofaslurrypondinSingapore.Proceedingsof the Institution of Civil Engineers, Geotechnical Engineering 162 (GE1), 13–20.

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 & Performance, CIRIA Ground Engineering Report: GroundImprovement. Butterworth-Heinemann Ltd., London.

Huang, J., Han, J., 2009. 3D coupled mechanical and hydraulic modeling of a geo-synthetic-reinforced deep mixed column-supported embankment. Geotextilesand Geomembranes Available online 25 February.

Indraratna, B., Chu, J. (Eds.), 2005. Ground Improvement Case Histories. Elsevier,Oxford, 1115 pp.

Li, A.L., Rowe, R.K., 2001. Combined effects of reinforcement and prefabricatedvertical drains on embankment performance. Canadian Geotechnical Journal 38(6), 1266–1282.

Liu, H.L., Chu, J., 2009. A new type of prefabricated vertical drain with improvedproperties. Geotextiles and Geomembranes 27 (2), 152–155.

Ren, Z.Y., 2004. A Prefabricated Vertical Drain with Capability for Penetration DepthMeasurement, China Patent No. 0119469.0.

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.

Somlo, P.I., Hunter, J.D., 1985. Microwave impedance measurement. In: IEEE Elec-trical Measurement Series No. 2. Institution of Electrical Engineers.