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Faculty of Engineering and Information Sciences

2018

Performance of marine clay stabilised with vacuum pressure: Performance of marine clay stabilised with vacuum pressure:

Based on Queensland experience Based on Queensland experience

Buddhima Indraratna University of Wollongong, indra@uow.edu.au

Cholachat Rujikiatkamjorn University of Wollongong, cholacha@uow.edu.au

Pankaj Baral University of Wollongong, pbaral@uow.edu.au

Jayantha Ameratunga Coffey Geotechnics, Golder Associates Pty Ltd

Follow this and additional works at: https://ro.uow.edu.au/eispapers1

Part of the Engineering Commons, and the Science and Technology Studies Commons

Recommended Citation Recommended Citation Indraratna, Buddhima; Rujikiatkamjorn, Cholachat; Baral, Pankaj; and Ameratunga, Jayantha, "Performance of marine clay stabilised with vacuum pressure: Based on Queensland experience" (2018). Faculty of Engineering and Information Sciences - Papers: Part B. 2321. https://ro.uow.edu.au/eispapers1/2321

Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au

Performance of marine clay stabilised with vacuum pressure: Based on Performance of marine clay stabilised with vacuum pressure: Based on Queensland experience Queensland experience

Abstract Abstract Stabilising soft marine clay and estuarine soils via vacuum preloading has become very popular in Australasia over the past decades because it is a cost-effective and time-efficient approach. In recent times, new land on areas outside but adjacent to existing port amenities, the Fisherman Islands at the Port of Brisbane (POB), was reclaimed to cater for an increase in trade activities. A vacuum preloading method combined with surcharge to stabilise the deep layers of soil was used to enhance the application of prefabricated vertical drains (PVDs). This paper describes the performance of this combined surcharge fill and vacuum system under the embankment and also compares it with a surcharge loading system to demonstrate the benefits of vacuum pressure over conventional fill. The performance of this embankment is also presented in terms of field monitoring data, and the relative performance of the vacuum together with non-vacuum systems is evaluated. An analytical solution to radial consolidation with time-dependent surcharge loading and vacuum pressure is also presented in order to predict the settlement and associated excess pore water pressure (EPWP) of deposits of thick soft clay.

Disciplines Disciplines Engineering | Science and Technology Studies

Publication Details Publication Details Indraratna, B., Rujikiatkamjorn, C., Baral, P. & Ameratunga, J. (2019). Performance of marine clay stabilised with vacuum pressure: Based on Queensland experience. Journal of Rock Mechanics and Geotechnical Engineering, 11 (3), 598-611.

This journal article is available at Research Online: https://ro.uow.edu.au/eispapers1/2321

Full Length Article

Performance of marine clay stabilised with vacuum pressure: Based onQueensland experience

Buddhima Indraratna a,*, Cholachat Rujikiatkamjorn b, Pankaj Baral c, Jayantha Ameratunga d

a Industrial Transformation Training Centre (ITTC) for Advanced Technologies in Rail Track Infrastructure & Centre for Geomechanics and Railway Engineering(CGRE), School of Civil, Mining and Environmental Engineering, University of Wollongong, Wollongong City, NSW 2522, Australiab School of Civil, Mining and Environmental Engineering, University of Wollongong, Wollongong City, NSW 2522, AustraliacCentre for Geomechanics and Railway Engineering, University of Wollongong, Wollongong City, NSW 2522, AustraliadGolder Associates Pty Ltd., Milton, Queensland 4064, Australia

a r t i c l e i n f o

Article history:Received 12 July 2018Received in revised form22 October 2018Accepted 26 November 2018Available online 5 December 2018

Keywords:Soft clayPrefabricated vertical drains (PVDs)Vacuum consolidationMembrane-less systemMembrane system

a b s t r a c t

Stabilising soft marine clay and estuarine soils via vacuum preloading has become very popular inAustralasia over the past decades because it is a cost-effective and time-efficient approach. In recenttimes, new land on areas outside but adjacent to existing port amenities, the Fisherman Islands at thePort of Brisbane (POB), was reclaimed to cater for an increase in trade activities. A vacuum preloadingmethod combined with surcharge to stabilise the deep layers of soil was used to enhance the applicationof prefabricated vertical drains (PVDs). This paper describes the performance of this combined surchargefill and vacuum system under the embankment and also compares it with a surcharge loading system todemonstrate the benefits of vacuum pressure over conventional fill. The performance of this embank-ment is also presented in terms of field monitoring data, and the relative performance of the vacuumtogether with non-vacuum systems is evaluated. An analytical solution to radial consolidation withtime-dependent surcharge loading and vacuum pressure is also presented in order to predict thesettlement and associated excess pore water pressure (EPWP) of deposits of thick soft clay.� 2018 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

licenses/by-nc-nd/4.0/).

1. Introduction

The thick soft clays with undesirable geotechnical propertiessuch as high compressibility, low permeability and shear strengththat are presented in the coastal regions of Australia have a seriouseffect on the stability of superstructure due to excessive differentialsettlement and intolerable lateral deformation (Holtz et al., 1991;Indraratna and Redana, 2000). Therefore, a proper groundimprovement technique is needed to address these problems, ofwhich prefabricated vertical drains (PVDs) combined with vacuumand surcharge preloading are a cost-effective and time-efficienttechnique that promotes radial flow and accelerates soft soilconsolidation. Over the last decades, several analytical and nu-merical analyses have been used to predict the behaviour of soft soilthat is treated by vertical drains in combinationwith surcharge andvacuum pressures. For instance, Mohamedelhassan and Shang

(2002) proposed an analytical solution to one-dimensional (1D)consolidation with vacuum pressure using the principle of super-position; while Indraratna et al. (2005) derived a radial consoli-dation theory for a vacuum application under instantaneousloading that includes the effect of a loss of vacuum along the lengthof the drain.

There are several literatures discussing the efficiency of a vac-uum preloading system combined with PVDs (e.g. Chu et al., 2000;Chai et al., 2005) to minimise the lengthy consolidation time withhelp from staged construction; this has also been discussed byIndraratna et al. (2005) and Sathananthan et al. (2008), who foundthat a vacuum can reduce the surcharge height by several metreswhen the atmospheric pressure is sustained by at least 70%(Rujikiatkamjorn et al., 2008). Yan and Chu (2003) also found thatthe rate at which an embankment is constructed can be increasedby reducing the number of construction stages. Reducing the risk interms of differential settlement by lessening post-constructionsettlement is possible only after the stiffness and shear strengthof soil are increased via consolidation, as reported by Shang et al.(1998). Several other analytical models for vacuum consolidationwhich incorporate soil destructuration and others factors (elastic

* Corresponding author.E-mail address: indra@uow.edu.au (B. Indraratna).Peer review under responsibility of Institute of Rock and Soil Mechanics, Chi-

nese Academy of Sciences.

Contents lists available at ScienceDirect

Journal of Rock Mechanics andGeotechnical Engineering

journal homepage: www.rockgeotech.org

Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 598e611

https://doi.org/10.1016/j.jrmge.2018.11.0021674-7755 � 2018 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

visco-plastic properties, and smeared zone, etc.) as well as labo-ratory large-scale specimen testing considering vacuum pressureare also available from other researchers (e.g. Indraratna et al.,2015; Perera et al., 2017; Baral et al., 2018). In addition, severalClass A and C predictions have been performed by the first authorand his team to investigate the behaviour of embankment in termsof settlement and excess pore water pressure (EPWP) dissipationon soft soil considering radial flow, facilitated with surcharge andvacuum preloading (Indraratna et al., 2010, 2016, 2018).

The rapid increase in trading activities at the Port of Brisbane(POB, Australia’s third largest container port) has resulted in thereclamation of 235 ha (1 ha ¼ 10,000 m2) of new land adjacent tocurrent port facilities; this reclamation was located between Fish-erman Island and the mouth of the Brisbane River. The soft clay inthis area had undrained shear strength of less than 15 kPa as well ashigh compressibility and low permeability. This means that consol-idation with surcharge alone would take more than 50 years andwould result in vertical settlements between 2.5 m and 4 m underservice loading in absence of any ground improvement technique.This is why vacuum consolidation combinedwith PVDswas selectedto accelerate the process and limit lateral deformation as the sitewasimmediately adjacent to the Moreton Bay, Marine Park.

Despite the rapid advancement of vacuum consolidation facili-tated with PVDs, there is no case history of modern vacuum tech-nology and conventional surcharge preloading being practised inthe same area where different drains were installed with differentdrain spacings. This paper describes the performance of non-vacuum and vacuum areas in terms of settlement, EPWP, andlateral deformations, as well as the effects that the type and spacingof drains has on the degree of consolidation (DOC, U%). This paperalso presents analytical solutions to radial consolidation, whichconsiders the effect of time-dependent surcharge loading.

2. System of vacuum preloading

There are two types of vacuum preloading systems: (a) amembrane-less system, and (b) a membrane system.

2.1. Membrane system

Once the PVDs have been installed, a network of horizontalperforated pipes is connected to the PVDs to form a dischargesystem, and then a sand blanket is installed. A membrane is thenlaid over the top of the sand blanket, its edges are buried in a trenchfilled with bentonite slurry (see Fig. 1a), and then a vacuum pumpis connected to the discharge system. The vacuum pressure in thissystem can easily be circulated within the sand platform and thesoil surface and then propagate down the PVDs. The radialconsolidation still occurs in shallow soil layer under vacuumpressure as the ratio of PVD length to spacing is more than 10 with

minimum vertical consolidation effect. The efficiency of suchsystem depends entirely on the damage caused within the entiremembrane over a long period of time.

The vacuum pressure propagates from the horizontal drainthrough the layer of sand, the PVDs, and the clay layer in amembranesystem as shown in Fig. 2a. This three-dimensional (3D) flow in asand blanket beneath the membrane (0� z � Lw, Lw is the thicknessof the layer of sand (m)) can be expressed as (Geng et al., 2012):

vεv1vt

¼ �mv1

�vu1vt

� dqdt

�(1)

�kh1gw

1rvu1vr

þ v2u1vr2

!� kv1

gw

v2u1vz2

¼ vεv1vt

ðrw � r � reÞ (2)

v2uw1

vz2¼ � 2kh1

rwkv1

vu1vr

�����r¼rw

(3)

u1 ¼ 1p�r2e � r2w

� Zrerw

2pru1dr (4)

where εvi (i ¼ 1,2) is the vertical strain; r and z are the radial andvertical co-ordinates, respectively (m); t is the time (s); gw is thewater density; kvi (i¼ 1,2) is the coefficient of permeability of the soilin vertical direction (m/s); khi (i ¼ 1,2) is the coefficient of perme-ability of soil in horizontal direction (m/s); rw is the radius of drainwell; re is the influenced zone radius (m); q is the surcharge pre-loading (time-dependent, kPa); mvi (i ¼ 1,2) is the volumecompressibility of soil (m2/kN); ui (i¼ 1,2) is the porewater pressure(PWP) (kPa);ui (i¼ 1,2) is the average porewater pressure (kPa); anduwi (i ¼ 1,2) is the EPWP within the vertical drain (kPa). It is notedthat subscripts 1 and 2 refer to the layer beneath the membrane(i.e. sand blanket), and underlying soil layer, respectively.

The governing equations for the underlying soil (Lw � z � H),where H (m) is the thickness of the entire layer (i.e. for the mem-brane system, the sand blanket plus the layer of clay; and for themembrane-less system, only the layer of clay), can be expressed as

vεv2vt

¼ �mv2

�vu2vt

� dqdt

�(5)

�ks2gw

1rvus2vr

þv2us2vr2

!�kv2gw

v2u2vz2

¼vεv2vt

ðrw�r�rsÞ (6)

Fig. 1. Vacuum preloading systems: (a) membrane system, and (b) membrane-less system (Baral, 2017).

B. Indraratna et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 598e611 599

�kh2gw

1rvun2vr

þv2un2vr2

!�kv2gw

v2u2vz2

¼vεv2vt

ðrs�r�reÞ (7)

v2uw2

vz2¼ � 2ks2

rwkw

vus2vr

�����r¼rw

(8)

u2 ¼ 1p�r2e � r2w

�0@ Zrs

rw

2prus2dr þZrers

2prun2dr

1A (9)

where usi (i ¼ 1,2) is the pore water pressure within the smearedzone at any point (kPa); un2 is the PWP in the natural soil zone atany point (kPa); us2 is the PWP in the smeared zone at any point; rsis the smeared zone (m); and ksi (i ¼ 1,2) is the permeability insmeared zone.

The boundary conditions for the vertical and radial directionsare

vun2vr

¼ 0

vu1vr

¼ 0

9>>=>>; ðr ¼ reÞ (10a)

ks2vus2vr

¼ kh2vun2vr

ðr ¼ rsÞ (10b)

us2 ¼ un2 ðr ¼ rsÞ (10c)

us2 ¼ uw2u1 ¼ uw1

�ðr ¼ rwÞ (10d)

uw1 ¼ pu1 ¼ p

�ðz ¼ 0Þ (10e)

vuw2

vz¼ 0

vu2vz

¼ 0

9>>=>>; ðz ¼ H

�(10f)

where p is the vacuum pressure (kPa).Continuity at the interface between the underlying soil layer

(z ¼ Lw) and the sand blanket can then be written as

uw1 ¼ uw2 ðz ¼ LwÞ (10g)

u1 ¼ u2 ðz ¼ LwÞ (10h)

kv1vuw1

vz¼ kw

vuw2

vzðz ¼ LwÞ (10i)

kv1vu1vz

¼ kv2vu2vz

ðz ¼ LwÞ (10j)

where kw is the drain permeability (m/s).The initial condition is:

u1 ¼ u2 ¼ u0ðzÞ ¼ q0 ðt ¼ 0Þ (10k)

where q0 is the initial value of preloading (kPa).

2.2. Membrane-less system

In this system, vacuum pipes are connected to each PVD via atubing system, and the connections are shown in Fig.1b. This systemis very efficientwhen an area is to be sub-divided into different partsand improved individually because all the tubing system must beindividually fitted to hundreds of drains, which is a time-consumingand cumbersome process. The efficiency of the vacuum depends oneach drain, unlike a membrane system where vacuum efficiencydepends on minimizing any leaks in the entire setup.

Fig. 2. Unit cells schemes with vertical drains: (a) membrane system, and (b) membrane-less system (Geng et al., 2012, with permission from ASCE).

B. Indraratna et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 598e611600

The boundary conditions are the only difference between amembrane and a membrane-less vacuum consolidation technique.With membrane-less vacuum consolidation, a vacuum pump isconnected to individual PVDs with horizontal pipes (see Fig. 2b).The governing equations and initial conditions for amembrane-lesssystem are the same as for a membrane system, as given by the setof Eqs. (10a)e(10d) and (10k), where the only difference is thedistribution of vacuum pressure which is assumed as p at the topsurface and then as hp where it varies linearly from top to bottom.The term h is the ratio of the magnitude of vacuum pressure at thetop to the bottom andwith values that vary between 0 and 1. Thus ifthere is no vacuum pressure, the value of h becomes zero and ifthere is no loss of vacuum at the bottom of the PVDs, the value of hbecomes 1 (Geng et al., 2012).

The boundary conditions for a membrane-less system are asfollows:

uw ¼ p

vuvz

¼ 0

9=; ðz ¼ 0Þ (10l)

vuwvz

¼ h� 1H

p

vuvz

¼ 0

9>>=>>; ðz ¼ HÞ (10m)

Further details of analytical solutions based on these governingequations and boundary conditions for both types of systems canbe found in Appendix.

The efficiency of the vacuum systems varies from site to site. Theinfluential factors are not just related to soil properties but also thetechnical know-how and experience of contractors that offer variedtechniques of vacuum application. Where the membrane can beproperly protected from damage caused by sharp aggregatesand where leaks can be eliminated by effective sealing and addi-tional protection at the embankment boundaries (e.g. bentonitetrenches), the membrane-type vacuum application can be effectivecompared to membrane-less type and with comparable costs. Thiswas the authors’ experience at the POB. In essence, the choice be-tween membrane and membrane-less systems depends on projectcriteria and budget, contractor choices, past experiences, andamong others.

3. Characteristics and site conditions

Reclamation at the POB commenced in 2003 at the FishermanIsland adjacent to the mouth of Brisbane River, as shown in Fig. 3. Aseries of trial areas (see Fig. 4) was selected to compare the per-formance of a non-vacuum system with a vacuum system. Threecontractors (A, B and C) were chosen to carry out these trials, witheach contractor being assigned a trial area of 3 ha. The main aimwas to compare their performances based on construction anddesignwork. Contractor A had 8 trial areas (S3A) to carry out 6 trialswith surcharge only (WD1-4, WD5A, and WD5B), and 2 trials withsurcharge and vacuum consolidation (VC1 and VC2). The area setaside for the vacuum consolidated trials had a membrane system asdescribed in the previous section. Contractor B had seven trial areas(T11), five of which had a surcharge with different types of drains;while two of them had surcharge combined with a membrane-lessvacuum consolidation system. Contractor C had three subdividedareas labelled Areas 4, 5 and 6 (all of themwere in T11). A surchargepreloading was applied for up to one year to the sub-areas 4 and 5with vertical drains being spaced at 1.4 m, while sub-area 6 had asurcharge preloading applied for almost six months; it wasequipped with vertical drains at a spacing of 1 m.

The sub-soil profile shown in Fig. 5 consists of an almost 3 mthick layer of upper Holocene sand beneath dredgedmud, followedby a 20e25 m thick layer of soft Holocene clay that overlies Pleis-tocene deposits of highly over-consolidated clays. The Holoceneclay in this area (VC2) had very low shear strength and lowpermeability, and according to Ameratunga et al. (2010), it wasreferred to as POB clay. The groundwater table was located at 3.5 mRL (below the ground surface) and the water content of the sub-soillayers was higher than the soil liquid limit. Several site in-vestigations, including cone penetration testing (CPT)/piezocone,boreholes, field vane shear, dissipation, and oedometer, were car-ried out to evaluate the design consolidation and stability param-eters. The undrained shear strength of these Holocene clays variedfrom 15 kPa to 60 kPa and the compression indices were between0.4 and 1. The ratio between the coefficient of horizontal consoli-dation and vertical consolidation for soft Holocene clay (cv/ch) was2, whereas for dredged mud, this value was assumed to be 1 as itwas totally remoulded. The specifications imposed during designand construction was stringent, as was the vacuum applicationphase over soft clay deposits. The service load was limited to 15e25 kPa and the maximum residual settlement under this serviceload was restricted to 250 mm over a period of 20 years (criteriafrom POB cooperation). Another unique feature of this vacuum trialwas the design as well as deep cut-off wall for the first time inAustralia (up to 15m depth) along the periphery of trial area. This isnecessitated by the specific soil conditions which were encoun-tered on site. Due to the unfavourable site conditions, Contractor Adesigned 15 m deep cut-off wall with soil-bentonite slurry withpermeability less than 1 � 10�9 m/s.

Fig. 3. Proposed extension area at the POB (Indraratna et al., 2011, with kindpermission from ASCE, Courtesy of Port of Brisbane Corporation (2009)).

B. Indraratna et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 598e611 601

Fig. 4. General site layout (Courtesy of Port of Brisbane Corporation (2009)).

B. Indraratna et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 598e611602

Fig. 5. Soil properties and profile (S3A), Port of Brisbane (Indraratna et al., 2011, with kind permission from ASCE). cc is the compression index; su is the undrained shear strength; chand cv are the coefficients of horizontal and vertical consolidations, respectively.

Fig. 6. Analytically computed DOC with time for (a) non-vacuum in S3A and T11, (b) treatment in S3A only, and (c) vacuum areas in S3A and T11.

B. Indraratna et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 598e611 603

4. Assessing the relative efficiency of the trial systems

4.1. Degree of consolidation (U%) with time

The DOC (U%) at a given time based on settlement is defined asthe ratio of settlement at that specific time to settlement at the end

of consolidation; in these trial schemes, it came from measure-ments from an array of locations (see Fig. 6). All of these mea-surements indicated similar behaviour, irrespective of the type ofimprovement and location of the treatment site. In fact, this entiresite ended up with a relatively high DOC (U%), especially aftera year. Moreover, all the measurements converged when the DOC

(a) (b)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0001001011

Con

solid

atio

n In

dex

(U/ β

) TSP1 (MD7007)

TSP4 (MD 7007)

TSP6 (MD88H)

TSP9 (MD88H)

TC2 (MD7007)

RD1(MD88H)

TC5 (MD88HD)

WD1-1 (MCD34)

WD5B (FD767)

Menard sections

Boskalis & Van Oord sections

Van Oord sections

Boskalis sections

Menard sections Non-vacuum areas

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

001001011 0

Con

solid

atio

n In

dex

(U /

β) VC1-2 (MCD34)

VC2-1 (MCD34)VC1-5 (MCD34)MS 18-1 (MD88)WD5B(FD767)WD1-1 (MCD34)

Menard sections (with vaccum)

Menard sections (without vacuum)

vacuum

no vacuum

Vacuum and non-vacuum in S3A

(a)

Contractor A

Contractor A

Contractor B

Contractor C

Contractors B and C

Contractor A

Contractor A

(c)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0001001011

Con

solid

atio

n In

dex

(U /

β )

VC1-2 (MCD34)

VC2-1 (MCD34)

VC1-5 (MCD34)

RC1 (MD88H)

TA8 (MD88H)

BoskalisMenard sections

Boskalissections

Menard

Vaccum areas in S3A and T11

Contractor A

Contractor A

Contractor B

Contractor B

Fig. 7. Computed DOC (U%)/b with time for (a) non-vacuum in S3A and T11, (b) treatment in S3A only, and (c) vacuum areas in S3A and T11.

0 100 200 300 400Time (d)

-80

-60

-40

-20

0

Tota

l cha

nge

in p

ore

p res

sure

from

the

final

sta

ge o

f con

stru

ctio

n (Δ

u, k

Pa)

WD1WD2WD5VC1VC2RC1RD1TA8TC5TC3Section IV (VWP1)Section V (VWP2)Section VI (VWP3)

(a)

S3A-Contractor A

T11-Contractor B

T11-Contractor C

Fig. 8. Reduction in EPWP with time in areas S3A and T11.

B. Indraratna et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 598e611604

(U%) exceeded 80% so a dimensionless factor (b) was incorporatedto separate “clustering”, especially towards the end of one year(Indraratna et al., 2011). This dimensionless factorwas independentof the properties of soil and represented the drain as well as theloading condition; it mainly depends on:

(a) Increasing the length of the drains (ld);(b) Decreasing the spacing between drains (sd);(c) Drain pattern (a ¼ 1.13 for square and 1.05 for triangular

spacing); and(d) Normalised surcharge height (H) with clay thickness (hc), i.e.

(H/hc).

With these factors used, the dimensionless parameters can bedefined as

b ¼ ldasd

ðH=hcÞ (11)

The three trial paddocks can be differentiated into 3 distinctparts based on the magnitude of the dimensionless factor b, asdetermined at the location of each settlement plate for Areas S3Aand T11. They are as follows:

(a) Low b impact: Magnitude of 2e6 for Area S3A (Contractor A),short drains and low surcharge;

(b) Moderate b impact: Magnitude of 8e12 for Area T11(Contractor B), and

(c) High b impact: Magnitude of 12e18 for Area T11 (ContractorC), long drains and high surcharge.

Please note that, during the calculation of settlement, the flow inthe radial direction is regarded as most predominant compared tothe vertical direction, as the length of drain is relatively longcompared to its spacing. Therefore, the settlement of an individual

soil layer using a single layer theory can be applied for eachindividual soil stratum and subsequently integrated with depth toobtain the total settlement with little error as per Indraratna et al.(2015).

Dividing the DOC (U%) by a dimensionless factor enables therelative performance of all paddocks in Areas S3A and T11 to befiltered, although there is no specific relationship between b andthe DOC (U%). The relation DOC/b is plotted versus time in Fig. 7with a clear division between the vacuum and non-vacuum areas.Moreover, this plot also differentiates between the effect of vacuumconsolidation by Contractors A and B. When all three plots (Fig. 7aec) are considered, consolidation in treatment S3A is greater thanthe other locations due to the use of vacuum consolidation.

0 100 200 300 400Time (t, days)

-0.6

-0.4

-0.2

0

Δu/ Δ

t (kP

a/da

y)

(b)

(a) 0 100 200 300 400

Time (t, days)

-0.25

-0.2

-0.15

-0.1

-0.05

0

Δu/Δ

t/β (k

Pa/d

ay)

(c)

(b)

(c)

0 2 4 6 8 10 12 14 16 18 20

WD1

WD2

WD3

WD4

WD5A

VC1

VC2

∆DOC (%)

∆DOC = DOC(strain based)

- DOC(pore pressure based)

Fig. 9. Comparison of EPWP dissipation between S3A and T11. (a) Rate of EPWP dissipation, (b) EPWP dissipation rate normalized by b, and (c) Comparison of measured DOC basedon strain and EPWP. Note: the legend box for Fig. 9a and b is the same as that in Fig. 8.

Fig. 10. Effect of vacuum consolidation on lateral displacement.

B. Indraratna et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 598e611 605

4.2. Dissipation of excess pore water pressure (EPWP)

The reduction in EPWP versus time for all three paddocks isshown in Fig. 8, with the largest reduction in S3A (for VC2), fol-lowed by VWP3 in T11. Due to the variations in the fill heights andthe thickness of clay in S3A and T11, these comparisons cannotbe made directly because the figure also shows no significant

differences during the first three months. The rates at which EPWPchanges in the same locations with VC2, VC1, and WD1 are shownin Fig. 9a. Here, WD1 has the highest initial rate of dissipationwhereas VC1 sustains a steady state over a long period of time.Unlike the membrane systems (VC1 and VC2), the membrane-lesssystem could not indicate a high rate of EPWP dissipation, butwhen these plots are normalised with the dimensionless factor b

Fig. 11. Critical b values for permissible residual settlement (RS) in S3A and T11.

RS = 14.3hc + 34R = 0.96

RS= 10.8hc - 4R = 0.9

RS = 8.1hc - 14R = 0.99

RS = 3.8hc - 27R = 0.91

0

50

100

150

200

250

300

0 5 10 15 20 25 30

Clay thickness,hc (m)

Res

idua

l set

tlem

ent (

mm

)

OCR = 1.1

OCR = 1.2

OCR = 1.3

OCR = 1.4

VC1-5

VC 2-3VC 2-2

VC 1-2

TA 8

Fig. 12. Effect of over consolidation ratio (OCR) and clay thickness (hc) on RS.

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(see Fig. 9b), in terms of EPWP dissipation, the areas VC1 and VC2provide better treatment than the other areas. While the surchargeheight decreased in the VC areas of S3A and hence involved lessmucking operations, the amount of suction pressure applied to thesystem (i.e. �70 kPa) more compensated for the increased rate ofEPWP dissipation, and also it confirmed the performance of themembrane-type vacuum consolidation technique.

Based on the array of field data from both settlement plates andpiezometers, the difference between strain based DOC (U%) andpore pressure based DOC (U%) was also calculated after 1 year ofdrain installation for all trial sites of Contractor A. It was found thatthe use of wick drains at WD3 site indicated insignificant differencebetween the strain based and pore pressure based DOC (U%). Itimplies that the wick drain dissipates EPWP most effectively.Similarly, the use of circular drain to the trial sites VC1 and VC2 alsodissipated EPWP very effectively, compared to the same drains inthe absence of vacuum (WD1 and WD2). This further suggests thatthe circular drains have no any additional advantages over wickdrains if used only under surcharge fill loading condition. A plot ofdifference in DOC (U%) based on strain and EPWPwith different sitelocations (WD1-4, WD5A and VC1-2) is shown in Fig. 9c.

4.3. Controlling lateral displacement

Vacuum pressure in conjunction with vertical drains is veryeffective at reducing the lateral yield of soil and increasingembankment stability because it allows for lateral inward move-ment rather than outward movement. This incident has alreadybeen reported by Indraratna et al. (1997, 2005). Controlling thelateral displacement in sensitive areas is imperative, and since theboundary of the POB site is a marine environment, it is important tobalance the environment of marine aquatic lives, not exerting sig-nificant disturbances induced by outward lateral deformation tothe environment. To control this, a vacuum pressure was applied atcertain locations and then the lateral movement of selectedvacuum and non-vacuum areas was compared using limited fielddata from inclinometers installed at certain locations. To make thiscomparison easy, the lateral displacement was normalised byapplying effective stress at the same depth to avoid any confusiondue to different soil profiles and surcharge loads. The plots forlateral deformation with normalised effective stress shown inFig. 10 indicate that the vacuum consolidation effectively controlledthe lateral deformation, and the membrane type consolidation

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technique (VC1-MS28) with 70 kPa vacuum pressure was the bestat the controlling lateral deformation. Similarly, with a membrane-less vacuum consolidation technique, a systemwith 50 kPa suction(MS24) reduced the major portion of lateral deformation, but notas much as the membrane system. By examining the lateraldisplacement profiles, it can be concluded that a suction head waspropagated in all vacuum areas and all the layers associated (lowerand upper Holocene layers) were influenced.

4.4. Residual settlements (RS)

Residual settlement (RS) must also be controlled within anallowable limit (150 mm or 250 mm, based on the thickness of clayand the service load at different areas, which was determined byPOB cooperation). Therefore, all the contractors had to comply.Based on the methods provided by Terzaghi et al. (1996) and Yinand Graham (1994), the RSs are calculated and plotted after nor-malising with b, as shown in Fig. 11, where the values of RS occurbetween 4 and 16 (i.e. 4 < b < 16). Within the POB, the RS for everycontractor is close to the tolerable limits of 250 mm, whereas theRSs are much smaller, with values of b being less than 4 mainly due

to vacuum consolidation. While at high values of b (greater than16), RS tends to decrease due to a relatively high H/hc ratio (seeFig. 12). As shown in Fig. 12, the RS can be favourably controlled byover-consolidation ratio (OCR) after removing the surcharge andvacuum pressure. The lateral displacement can be effectivelyreduced using an appropriate combination between the surchargefill height and the applied vacuum head in relation to the propertiesof the stabilized soft clay layer.

Eqs. (1e10) are used in conjunction with Tables A1 and A2 inAppendix to predict the EPWP and associated settlement for eachsection. Tables A1 and A2 mainly summarise the properties andthickness of individual layers of soil. The compression index (cc)used in this analysis is derived from the oedometer and is related tothe actual stress state within a given range of foundation loading.The coefficients of horizontal (kh) and vertical (ks) compressibilitywere measured using a Rowe cell and oedometer, respectively. Interms of permeability, the kh/ks ratio was assumed to be unity for acompletely remoulded mud dredged seabed and the upper Holo-cene layer of sand, whereas this ratio was assumed to be 2 for theupper and lower Holocene clays. The reason behind this assump-tion is due to the fact that for remoulded (dredged) clays, the

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Fig. 14. VC1 area: (a) stages of loading, (b) surface settlements under the centreline of the embankment, and (c) EPWP dissipation.

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permeability in both directions is considered isotropic. However,for Holocene clay subjected to layered deposition (genesis), thehorizontal permeability is often higher than the vertical perme-ability. Based on the laboratory testing, the magnitude of horizontalpermeability was twice that of the vertical permeability. Similar tothe permeability ratio, the ds/dw ratio (ds and dw are diameter ofsmeared zone and drain well, respectively) was taken as 3, inaccordance with previous literature by Indraratna and Redana(2000).

The unit weight of compacted fill was assumed to be 20 kN/m3

and the embankment load was simulated using stage construction.Settlement and EPWP are predicted using the proposed analyticalmodel. In this case, computation at the centreline of the embank-ment followed 1D consolidation and was straightforward with zerolateral deformation, and in addition, the MATLAB spreadsheetproved to be very convenient. Note that the initial in situ effectivestress is calculated based on the final DOC (U%) of the previousstage for each subsequent stage for surcharge preloaded embank-ments, whereas a suction of 65 kPa is used in the vacuum cases tocompute the settlement and EPWP of the embankment.

The settlement and EPWP dissipation are predicted andcompared with the data measured at WD4 and VC1, and are shownin Figs. 13 and 14. These figures show that the analytical modelpredicted the field data very well in terms of settlement and EPWPdissipation, whereas in the vacuum areas, the DOC (U%) exceeded90% after 400 d and was only 85% of the non-vacuum area for thesame time. This proves that combined vacuum preloading at agiven time is more efficient than surcharge preloading alone due toaccelerated consolidation and the fact that the embankment innon-vacuum areas has been constructed slowly to avoid any po-tential undrained failure in the remoulded layer of dredged mud.

5. Conclusions

PVDs combined with surcharge and vacuum preloading accel-erate the consolidation of soft soil. In this paper, the performance ofsoft soil treatment options in terms of settlements, associated EPWP,and lateral deformation has been analysed and discussed usingmuddredged from the seabed of channels and berths that will be used forshipping. The behaviour of surcharge and vacuum consolidationwasstudied at several trial areas chosen at the POB, and their perfor-mances were compared using the DOC (U%) approach. Whilecomparing on the basis of DOC (U%), the relative treatment in areasS3A and T11 could not be compared because they all achieved highDOC (U%) irrespective of the types of drains and their pattern ofspacing, as well as the clay thickness, and the nature of loading.However, to make the comparison easier, a dimensionless factor b isused because it is totally independent of the consolidation proper-ties of the soil, and it can represent the drain and site factors. Thebeta variable (b) is empirical and used to assess the relative effi-ciency of different trial systems at the POB considering the DOCachieved at a given site of known clay depth and soil properties. Theproposed beta factor is a tool to normalize DOC as well as EPWPtrends. It captures the drain length and drain spacing, clay thickness,and the surcharge height in a dimensionless quantity.

After normalization, with help from this dimensionless factor b,the DOC (U%), settlement, and lateral displacement/settlementrepresent performances more clearly and precisely so themembrane-type vacuum consolidation in the area S3A achieved byContractor A seems to be the best. Based on the comparison inbetween strain based and EPWP based DOC (U%), it can beconcluded that the circular drains have no any additional advan-tages over wick drains if only used under surcharge fill loadingcondition. Furthermore, while the membrane-less vacuum systemhelped to control the lateral displacement, there was not enough

field data for the inclinometer, so the lateral deformation profiles ofthese two systems could not be compared. It was also clear thatcontrolling the lateral deformation in sensitive areas such as amarine environment will be assisted if a vacuum pressure is appliedto reduce the heights of the surcharge fill.

Determining the relationship between the DOC (U%) and RS for agiven condition is always difficult, but there is no doubt that adecreasing RS is almost linear as the OCR ratio increases, so the RSalways tends to move closer to the prescribed settlement of150 mm for a range of the dimensionless factor b between 4 and 16.There is a minimum value of b in S3A for the vacuum consolidatedareas when the OCR is greater than 1.3, and the value of RS becomescritical when the OCR is less than or equal to 1.1. In fact, a typicalsituation occurs under surcharge preloading with a thick layer ofclay, and this treatment is not as effective as a vacuum. It impliesthat a sufficient surcharge fill is needed to keep the RS withinpermissible limits when there is no vacuum. Moreover, the higherthe service load, the greater the advantage of applying a vacuum toreduce excessive fill heights and control lateral displacement.Therefore, by keeping in mind the excessive RS and lateraldisplacement criteria, applying a vacuum pressure and surchargeloading to achieve a relatively high DOC (U%), and a subsequentunloading for attaining an OCR of less than 1.3, would be the bestchoice for the site with the loading conditions encountered at thePOB. The novelty of this paper stems from the performances incomparison of the different ground improvement methods withinthe same site (POB) using the new dimensionless factor b. Suchcomparisons of field-based ground improvement practices in theAustralian continent do not exist. In particular, we have also lookedat the difference between 2 distinct vacuum systems, i.e. themembrane type andmembrane-less vacuum preloading. This is notonly novel, but also beneficial to the practitioners. The relationshipbetween the RS and the OCR is proposed to ensure that the long-term deformation is within the desired criteria.

A unit cell theory that considers a time-dependent surchargeload and vacuum preloading has been developed to predict thesettlement and associated EPWP dissipation, and it agrees with thefield measurements. For the same amount of applied total stress,the DOC (U%) at 400 d for the vacuum areas was more than that forthe non-vacuum areas. A system of vertical drains combined withvacuum and surcharge preloading is a very useful method foraccelerating radial consolidation and controlling lateral deforma-tion, while the analytical model described in this paper is veryuseful for predicting the performance of soft clay embankmentsstabilised by PVDs. Field observations are needed to model thevacuum pressure accurately enough to determine the distributionof vacuum pressure along the depth of the drain because the majorproblem reported in previous case histories in a marine environ-ment is that the suction pressure varies with time and depth.

Conflicts of interest

The authors wish to confirm that there are no known conflicts ofinterest associated with this publication and there has been nosignificant financial support for this work that could have influ-enced its outcome.

Acknowledgement

The authors acknowledge the support of the Port of Brisbane PtyLtd and Coffey Geotechnics, and research funding from theAustralia Research Council is also acknowledged. More elaboratedetails of some of the contents discussed in this paper can also befound in previous publications by the first author and his co-workers in ASCE, and the Canadian Geotechnical Journal and

B. Indraratna et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 598e611 609

Australia Geomechanics Journal; they are reproduced herein withtheir kind permission. The writers further acknowledge the assis-tance from Prof. A.S. Balasubramaniam (University of Griffith), Prof.Harry Poulos (Coffey), Prof. Hadi Khabbaz (University of TechnologySydney), and Dr. Xueyu Geng (Warwick) for their valuable com-ments and inputs for the POB ground improvement projects.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.jrmge.2018.11.002.

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Buddhima Indraratna is a Civil Engineering graduatefrom Imperial College, London, and obtained his PhD fromthe University of Alberta in 1987. He has worked in in-dustry in several countries before becoming an academic,and has been a United Nations Expert and foreign advisorto numerous overseas projects. He holds the position ofDistinguished Professor at University of Wollongong,Australia, Distinguished Adjunct Professor at Asian Insti-tute of Technology, Thailand, Honorary Professor at Uni-versity of Shanghai for Science and Technology, and BeijingJiaotong University, China. Prof. Indraratna’s pioneeringcontributions to railway geotechnology and various as-pects of geotechnical engineering have been acknowl-edged through numerous national and international

awards, including the 1st Ralph Proctor Lecture, and 4th Louis Menard Lecture of theInternational Society of Soil Mechanics and Geotechnical Engineering, ISSMGE (20,000members; 90 þ nations), 2015 Thomas Telford Premium Award (ICE, UK), 2009 EHDavis Memorial Lecture of Australian Geomechanics Society, and 2014 CS Desai Medalfor his substantial and sustained contributions to Transport Geotechnics and GroundImprovement, respectively. He is one of Australia’s highly-cited geotechnical aca-demics with over 6500 ISI/Scopus citations, and an H-Index of 42 (Scopus), and 55(Google) with >10,000 citations. He has successfully supervised over 50 PhD and 25Masters, and over 30 Postdoctoral Fellows, and 1/3 of them have won prestigiousawards, e.g. ATA’s David Sudgen Award, AGS0 Hugh Trollope Award, Int. GeosyntheticsAward, Churchill Award, AGS/ANZ Young Professional awards and numerous Best Pa-per awards. He was a Program Leader of the CRC for Railway Engineering in 2000,and also a Program Leader of the ARC Centre of Excellence for Geotechnical Scienceand Engineering funded by ARC in July 2010. Recently, he became the Founding Direc-tor of ARC Training Centre for Advanced Technologies in Rail Track Infrastructure(ITTC-Rail). He has contributed directly to revision of national standards AS8700 andAS 2758.7 through Standards Committees on the Execution of Prefabricated VerticalDrains, and Aggregates and Rock for Engineering Purposes: Part 7 Railway Ballast.He is the Editorial Chair of the Journal of Ground Improvement, ICE, and an AssociateEditor of two prestigious journals (ASCE Journal of Geotechnical and Geoenvironmen-tal Engineering, and Canadian Geotechnical Journal) and on the editorial boards of 6other international geotechnical and railway journals. He represents Australia on 4ISSMGE Technical Committees: Transport Geomechanics (TC202), Ground Improve-ment (TC 211), Soft Soil Foundations (TC 214), Natural Hazards mitigation (TC303);Of these, he is on the Executive Committee of TC202 and TC211. He has authoredmore than 700 scholarly publications including 300 þ top ranked, peer-reviewed jour-nals, 10 research based books, 400þ peer-reviewed national and international confer-ence papers including 55 invited Keynote papers and Special Guest Lectures. Hisresearch interests include: (1) Ground Improvement including sub-surface drainageand soft clay stabilization, (2) Large scale Geotechnical testing and process simulation,(3) Railway foundations, (4) Jointed Rock Engineering, (5) Geoenvironmental engineer-ing including remediation of acid sulphate soils, (6) Flow through porous and jointedmedia including dam filters, (7) Dams and embankment engineering, and (8) Numer-ical and analytical modelling and ground instrumentation.

Cholachat Rujikiatkamjorn obtained Bachelor in CivilEngineering with Honours from Khonkaen University,Thailand, MEng from Asian Institute of Technology (AIT)and PhD from University of Wollongong (UOW), Australia.He is an Associate Professor in School of Civil, Mining andEnvironmental Engineering at University of Wollongong.He is honoured with several reputed awards includingISSMGE young member award, VC Award for excellence inresearch partnership, Robert M. Quigley Award, D H Trol-lope Medal, VC research excellence award for emergingresearchers, Outstanding paper award by an early careerresearchers by IACMAG, Wollongong Trailblazer Award,AGS thesis award, and AGS young geotechnical engineeraward etc. He is charted Professional Engineer of Australia

and Fellow of Engineers Australia. He is a committee member of Australia Geo-mechanics Journal, Sydney Chapter and editorial board member of ISSMGE Bulletin. Hehas published more than 300 papers in reputed journal and conferences and is areviewer of Canadian Geotechnical Journal, JGGE, ASCE and Computer and Geo-technics. His research interests include: (1) Soft ground improvement, and (2) Tech-nical development and application of prefabricated vertical drains, and (3) Innovativesolutions and numerical applications to problems in soil mechanics and geotechnicalengineering.

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Pankaj Baral obtained his Bachelor’s degree in Civil Engi-neering in2010 from Institute of Engineering (IOE), PulchowkCampus, Tribhuvan University, Nepal. Pankaj then went toAsian Institute of Technology (AIT), Thailand to pursue hismaster’s degree in Geotechnical and Geo-environmental En-gineering (GGE) and completed hisM.Eng degree in 2012. Af-ter working as a Research Assistant at AIT, he then joinedCentre for Geomechanics and Railway Engineering (GRE) atUniversity of Wollongong (UOW) for PhD and graduated in2017 with examiners commendation for outstanding thesis.He is currently working as an Associate Research Fellowwithin the same centre at University of Wollongong. Pankajis a member of Nepal Engineering Council (NEC), a memberof Australian Geomechanics Society (AGS) and a member of

Engineers Australia (EA). He also served Nepal Engineers Association as a president ofBangkok Chapter during 2010e2012. His research interests include: (1) Prefabricated ver-tical drains combined with vacuum and surcharge preloading (2) Elasto-visco-plasticbehaviour of soft soil, (3) Analytical and numerical modelling in geotechnical engineering,and (4) Strain rate dependency of pre-consolidation pressure. He has already published 25peer reviewed journal and conference papers till the date and he is also a recipient of “HisMajesty King Scholarship of Thailand”, “Best paper Award; ISLT 2014” and prestigious“Young Geotechnical Engineer Award; ISGTI 2018”.

Jay Ameratunga graduated from the University of Cey-lon, and an MSc at AIT in Bangkok, followed by a PhDfrom Monash University in Melbourne. He is a practicingengineer and, with over 35 years of experience ingeotechnical engineering, Jay has a strong project port-folio that includes major highways, ports and tunnels.His work has been mostly with the Contractor, fromtender to detailed design stage, to provide simple,constructible solutions. He is a soft soil/groundimprovement specialist and his special interests includereclamation. He had been assisting the Port of Brisbaneas a reclamation/ground improvement specialist for theport expansion works over the last decade. He has co-authored about 50 technical papers on soft clay, reclama-

tion and ground improvement. Recently, Springer Publishing Company published hisfirst book on geotechnical engineering titled “Correlations of soil and rock propertiesin geotechnical engineering”. Jay is a Past Chair, Australian Geomechanics Society,Queensland Division and is a member of the Australian Standards Committee forgeosynthetics.

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