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Author: Arulrajah, A., Bo, M. W., Leong, M. & Disfani, M. M. Title: Piezometer measurements of prefabricated vertical drains improvement of soft soils under land reclamation fill

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PIEZOMETER MEASUREMENTS OF PREFABRICATED

VERTICAL DRAINS IMPROVEMENT OF SOFT SOILS

UNDER LAND RECLAMATION FILL

A. ARULRAJAH 1 B.Eng, MEngSc, Ph.D, MIEM, FIEAust, CPEng Associate Professor, Swinburne University of Technology, Melbourne, AUSTRALIA.

M. W. BO 2 B.Sc, DUC, M.Sc, Ph.D, FGS, FICE, IntPE, P.Eng, P.Geo, C.Geol, C.Sci, C.Eng, C.Env, Eur Geol, Eur Eng Director, DST Consulting Engineers Inc., Thunder Bay, Ontario, CANADA

M. LEONG 3 BEng Principal, Geofrontiers Group Pty Ltd, Melbourne, AUSTRALIA

M. M. Disfani 4 PhD, MSc, BSc, MIEAustACCEPTED MANUSCRIPT Lecturer, Swinburne University of Technology, Melbourne, AUSTRALIA

Corresponding author: A/Prof Arul Arulrajah, Swinburne University of Technology, Faculty of Engineering (H38), PO Box 218, Hawthorn, VIC 3122, AUSTRALIA Email: [email protected] Tel: +613-92145741

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Abstract

Piezometers are commonly used for monitoring the dissipation of pore water pressure and therefore determination of degree of consolidation of soft soils after ground improvement in land reclamation projects. This paper compares the degree of consolidation obtained using pore pressure monitoring data from vibrating wire piezometers and pneumatic piezometers installed in a test site in the Changi East Reclamation project in Singapore. The test site consisted of a vertical drain sub-area at which prefabricated vertical drains were installed at

2.0 m × 2.0 m square spacing as well as an adjacent control sub-area where no prefabricated vertical drains were installed. Pneumatic piezometers were installed at the same elevations as the vibrating-wire piezometers in both sub-areas for comparison purposes. During the surcharge period of 32 months, the piezometer monitoring data were analysed at various periods to determine the degree of consolidation of the underlying soft marine clay. The degree of consolidation values interpreted from both types of piezometers was compared and with predictions from neighbouring settlement plates to evaluate their performance. The findings of the comparison between pneumatic and electric vibrating-wire piezometers indicate that both types of piezometer are suitable for monitoring the consolidation behaviour of soft soil under ACCEPTEDland reclamation fills. MANUSCRIPT

Keywords: soft soil; field instrumentation; piezometer; ground improvement; prefabricated vertical drains.

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1. Introduction

Land reclamation on soft soils deposits is becoming increasingly common at the nearshore or offshore locations in many land scarce countries in the world. These soft soil deposits often have to be improved using a variety of ground improvement techniques. Ground improvement techniques that are commonly used in the treatment of soft soils in land reclamation projects include Prefabricated Vertical Drains (PVD) in combination with surcharging (Holtz 1991;

Bergado et al 1996; Indraratna and Bamunawita 2002; Bo et al. 2007 ; Chu et al. 2009a ; Chu et al. 2009b), stone columns (Arulrajah et al. 2009a), deep soil mixing (Tan et al. 2002;

Lorenzo and Bergado 2003 ; Larsson et al 2005) or other suitable techniques. Of these ground improvement techniques, the prefabricated vertical drain with surcharge is arguably the most widely used and economical technique in land reclamation works on soft soil deposits (Bo et al. 1999 ; Bo et al. 2003; Chu et al. 2009a).

Following the completion of appropriate ground improvement works, the soft soil improvement has to be monitored by field instrumentation observational methods (Mesri and

Choi 1985; Chu et al. 2009b; Arulrajah et al. 2009b) or in-situ testing methods (Arulrajah et al. 2005 ; Arulrajah et al. 2006) to ascertain that the required degree of improvement of the soft soil has beenACCEPTED achieved. Field instrumentation MANUSCRIPT monitoring for this purpose involves the usage of settlement gauges or piezometers.

Piezometers are increasingly used for monitoring the dissipation of pore water pressure of soft soils after ground improvement in land reclamation projects (Arulrajah et al. 2004a). Through this evaluation, the degree of consolidation of the improved soft soil can be determined.

Piezometers are utilized to measure the piezometric head in the soil (Hanna 1985). If regular monitoring is carried out to measure the piezometric head together with the static water level,

ACCEPTED MANUSCRIPT changes of excess pore pressure due to additional load as well as dissipation of pore pressure with time during the process of consolidation can be ascertained. The degree of consolidation of the improved soft soil can thus be computed. Piezometers types commonly used in land reclamation projects include pneumatic, vibrating-wire and open-type piezometers (Arulrajah et al. 2004a; Bo et al. 2003 ; Chu et al. 2009a).

A test site was carried out in the Changi East Reclamation Project in the Republic of

Singapore consisting of a prefabricated vertical drain sub-area at which prefabricated vertical drains were installed at 2.0 m × 2.0 m square spacing as well as an adjacent control sub-area where no prefabricated vertical drains were installed. The geological and geotechnical characteristics of the soft Singapore marine clay found in the project site have been described by Arulrajah et al. (2008). Ground improvement works in the project comprises of the installation of prefabricated vertical drains and the subsequent placement of surcharge to accelerate the consolidation of the underlying soft marine clay (Bo et al. 2012). Prior to the installation of prefabricated vertical drains at an elevation of +4 m CD (Admiralty Chart

Datum, where mean sea level is +1.6 m CD), instruments comprising settlement plates, deep settlement gauges, pneumatic piezometers, vibrating-wire piezometers and water stand-pipes were installed in ACCEPTEDboth sub-areas of the test siteMANUSCRIPT. The deep settlement gauges and piezometers were installed at various elevations in the underlying marine clay. The pneumatic piezometers were installed at close to the same elevations as the electric vibrating-wire piezometers, each in separate boreholes in both sub-areas so that a comparison could be made between the pore ware pressure measurements.

The piezometer data was compared for the A2S-71 (2.0 m × 2.0 m) and A2S-74 (No Drain) sub-areas where the two types of piezometers were both installed at almost similar elevations

ACCEPTED MANUSCRIPT in the marine clay for comparison purposes. A distance of 30 m was considered between the two sub-areas to negate the influence of fill on the adjacent sub-area. The sub-areas were subsequently surcharged to the surcharge elevation of +7 mCD. The piezometer monitoring data in this paper were analysed at various surcharging periods of 12, 24 and 32 months to determine the degree of consolidation of the underlying soft marine clay. The results of both types of piezometers were compared to study their performance in marine clay treated with and without prefabricated vertical drains.

2. Types of Piezometers

Various types of piezometers were installed in the test site including vibrating-wire piezometers, pneumatic piezometers and open type piezometers. Water stand-pipes are often installed in piezometer clusters so as to measure the hydrostatic water level.

Each piezometer is installed in an individual borehole at a predetermined elevation.

Pneumatic and vibrating-wire piezometers were calibrated for the local environment prior to installation in a large diameter tube well and pressure measured against the actual water column pressure on the piezometer. Piezometers are packed in a sand bag and saturated in water for 24 hoursACCEPTED prior to installation. FollowingMANUSCRIPT installation of the piezometer in the borehole, a sand filter layer of 0.5 to 1 m is placed followed by placement of a bentonite seal, suitable for marine conditions. The borehole is then backfilled with sand on top of the bentonite plug to the original level. The installed piezometers are then measured at regular intervals to determine pressure or water head above the measured level as well as to monitor the dissipation of excess pore pressure. These measured values are translated into piezometric

ACCEPTED MANUSCRIPT head or excess pore pressures (Bo et al. 2004). A brief description of each of these piezometers is provided in this section.

2.1. Pneumatic Piezometer

The pneumatic piezometer consists of a pneumatic transducer which has been permanently installed in a borehole (Bo et al. 2003). Tubing runs from the transducer to a terminal on the surface. Readings for pneumatic piezometers are obtained with a pneumatic indicator. To obtain a pressure reading, the operator connects the transducer tubing to the indicator and directs a flow of compressed nitrogen gas to the transducer. When the transducer tubing brings a flow of gas back to the surface, the operator knows the transducer has been activated and shuts off the flow of gas. Gas pressure inside the transducer now balances water pressure outside and the reading is recorded (Bo et al. 2003).

In the initial stages, the pneumatic piezometers in the project site were subject to a high damage rate resulted by pinching of the tubing caused by large strain settlements which led to the loss of valuable data until they were replaced. This was however corrected by installing a protective casing throughout the length of the cables and by housing the piezometer in a guard shell to overcomeACCEPTED gripping and pinching ofMANUSCRIPT the cable due to lateral stress and settlement. Figure 1 shows a schematic of the pneumatic piezometer in a protective casing.

2.2. Vibrating-Wire Piezometer

Electric vibrating-wire piezometers were installed in the on-land field instrumentation clusters as well as the long-term field instrumentation clusters. The vibrating-wire piezometer consists of a transducer which converts water pressure to tensional load on a steel strip that is fixed at both ends. When excited by a magnetic coil, the steel strip vibrates at its natural frequency, 6

ACCEPTED MANUSCRIPT generating voltage pulses that are transmitted to the readout device. The readout device counts a set number of pulses and computes a natural period, the inverse of the natural frequency.

The square of the natural frequency is proportional to the tension in the steel strip and hence, the pressure exerting the load on the strip (Bo et al. 2003). Figure 2 shows a schematic of a vibrating-wire piezometer.

2.3. Open-Type Piezometers

Open type piezometers are installed in the more permeable formation where drainage condition required to be checked. Open type piezometers are installed in the same method as pneumatic piezometers. Instead of pneumatic cable and water pressure cable, it has extruding open pipe for water to be floated in the pipe. Water depths are measured with the help of the water level indicator. Sometimes water could overflow through the pipe due to extremely high artesian pressure at aquifer below compressible layer. As such, pressure gauges should be installed to measure the water head (Bo et al. 2003). Figure 3 shows a schematic of an open- type piezometer.

2.4. Water Stand-Pipe

Water stand-pipesACCEPTED were installed at the sand MANUSCRIPT formation within piezometer clusters so as to measure the hydrostatic water level at these locations. This enabled the evaluation of the excess pore water pressures for the piezometers by determining the piezometric elevation and subsequently the excess pore water pressures. The water stand-pipe consists of water intake opening slots that are small enough to prevent the ingress of the surrounding soil into the stand-pipe. A geofabric is often wrapped around the slotted portion of the water stand-pipe. A

ACCEPTED MANUSCRIPT water-level indicator which emits a buzzing sound on contact with water is used to determine the water level (Bo et al. 2003). Figure 4 shows a schematic of a water stand-pipe.

3. Determination of Degree of Consolidation from Piezometers

Piezometer generally measure water pressure or water head above the measured level. The measured values are generally translated into piezometric head or excess pore pressure. Data are usually presented together with construction stages and activities. However care should be taken in analyzing piezometer results as piezometer readings should be corrected to take piezometer tip settlement into account (Arulrajah et al. 2004b). Uncorrected piezometer monitoring data would lead to underestimation of degree of consolidation.

Average residual excess pore pressure is defined as ratio of excess pore pressure at time “t” upon initial excess pore pressure. Therefore degree of consolidation for a soil element, Uu, can be defined as shown in Equation 1. It is to be noted that Equation 1 defines the degree of consolidation for a soil element only.

Uu(%) = 1- (Ut / Ui) Eq. 1 ACCEPTED MANUSCRIPT Where Ut = the excess pore pressure at time t; and Ui = initial excess pore pressure which is equal to the placed surcharge.

4. Factors affecting prediction assessment using piezometers data

Based on the ratio of the excess pore water pressure reading of the piezometer and the initial excess pore water pressure, the degree of consolidation of the piezometer can be established.

Factors that affect piezometer analyses include period of assessment after surcharge

ACCEPTED MANUSCRIPT placement, hydrogeologic boundary phenomenon, correction for settlement of the piezometer tip and reduction of initial imposed load due to submergence effect (Arulrajah et al., 2004b;

Bo et al. 2003).

4.1. Period of assessment after surcharge placement

Pore water pressure dissipates with increasing periods of assessment and as such there is a lower remaining excess pore water pressure with increasing periods of assessment due to change of additional loads due to submergence effects. Correspondingly, the degree of consolidation will increase with increasing period of assessment. The isochrones of the excess pore water pressures is interpreted to obtain the average degree of consolidation of the various sub-areas (Arulrajah et al. 2004a; Bo et al. 2003). In this paper, the period of assessment for the test site is analysed at surcharging periods of 12, 24 and 32 months.

4.2. Hydrogeologic Boundary Phenomenon

If the piezometer is installed in offshore condition prior to reclamation, the initial excess pore water pressure can be obtained during the monitoring as the initial static pore pressure is known. Otherwise, the initial excess pore pressure has to be calculated from the assumed bulk density of the fillACCEPTED material (Bo et al., 2003 ).MANUSCRIPT For the case of land reclamation projects, it is common to assume a bulk density of 17 to 19 kN/m3 for the sand fill material. Bo et al.

(2003) has measured the density of sand in this land reclamation project and found it to vary from 15 kN/m3 to 19 kN/m3. As such, the calculated excess pore pressure based on assumed bulk density of the fill material could lead to an over-estimation of excess pore pressure for land fill cases and an underestimation for hydraulic filling.

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Initial excess pore pressure is usually assumed to be equal to the applied additional load.

However, initial pore pressure after loading could be different in circumstances where clay layer is underlain by the hydrogeologic boundary. This phenomenon has been explained by

Schiffman et al. (1994). In these circumstances, the profile of pore pressure after additional load could be lower than that calculated. Overestimation of degree of consolidation would occur if the initial lower pore pressure is not taken into consideration. Situations like this will arise when the clay layer is underlain by a water aquifer which is being extracted for water supply. However, study of aquifer location and water tables suggest that the hydrogeologic boundary phenomenon does not arise in this test site.

4.3. Correction for Settlement of Piezometer Tip

Due to the large strain settlements at site, all piezometer raw readings taken have to be corrected to account for the new elevation of the piezometer due to the settlement of the piezometer tip. Without correction, the calculated piezometric elevation would be higher than the actual elevation leading to the underestimation of the degree of consolidation. Correction is essential and if not made will lead to an underestimation of the degree of dissipation of the excess pore water pressure (Arulrajah et al. 2004b; Bo et al. 2003). ACCEPTED MANUSCRIPT

4.4. Reduction of Initial Imposed Load

For marine clay when land reclamation fill is placed, the soft soil rarely gains the effective stress equivalent to the initial applied load due to reduction of load caused by sinking of fill below groundwater level and rise in groundwater level resulted by seasonal recharge. This behaviour has been reported by Mesri and Choi (1985). As such, degree of consolidation

ACCEPTED MANUSCRIPT based on the initial applied load is likely to be underestimated since the available effective additional load at assessed time is smaller than the initial load (Bo et al. 2003).

5. Installation of Piezometers at the Test Site

In the project, piezometers were installed in off-shore protection platforms prior to reclamation as well as in the on-land field instrumentation clusters. Pneumatic and vibrating- wire piezometers were installed in the test site to monitor the dissipation of excess pore pressures of the marine clay under the reclaimed fill load. Water stand-pipes were installed at the sand formation within piezometer clusters in order to measure the hydrostatic water level at these locations. Each piezometer was installed in individual boreholes at various predetermined elevations in the marine clay. The piezometers were installed in the same instrument clusters as the water stand-pipes and settlement gauges. Piezometers were installed close to the same elevations as the deep settlement gauges to provide the ability for the correction of the piezometer tip elevation caused by the large strain settlements of the marine clay under the reclaimed fill.

Prior to the piezometer installation, a site calibration was conducted in a large diameter water well to check on the manufacturer’s calibration. As such, a site calibration chart is produced for each piezometerACCEPTED prior to their installation MANUSCRIPT plotting measured pressure against pressure of water on the piezometer. Piezometers are packed in a sand bag and saturated in the water at least twenty-four hours before installation. After installation in a borehole, sand should be placed again to the certain limit and bentonite seal should be placed on top of sand column.

The bentonite should be suitable for marine condition and upon reaction with seawater sufficient swelling and reduction of permeability must be achieved. On top of the bentonite plug, the borehole should be backfilled up to the original seabed preferably with original soil.

If not it should be backfilled with a good mixture of bentonite cement with a permeability 11

ACCEPTED MANUSCRIPT value equivalent to or lower than natural soil. This is because backfilling with sand will lead to a lower measurement of the excess pore pressure at the location due to the rapid dissipation of pore pressure along the sand fill column above the piezometer (Bo et al., 2003).

The pneumatic and vibrating-wire piezometers were installed in the same clusters as the settlement gauges; close to the same elevation as the settlement gauges to enable for correction of the piezometer tip caused by large strain settlement. Water stand-pipes were installed in the clusters in order to measure the static water level at these locations and to ascertain the excess pore water pressures of the piezometers. The excess pore water pressure is computed from the difference between the total pore water pressure and the hydrostatic pore water pressure.

6. Results and Discussion

Piezometers were installed at different elevations and as such, the average degree of consolidation for the entire compressible unit as well as the degree of consolidation of the sub-layers was determined. Figure 5 presents the cross-sectional profile showing piezometer locations at the A2S-71 (2.0 m ×2.0 m) and A2S-74 (no drain) sub-areas. In Figure 5, the vibrating-wire piezometers are denoted as PZ while the Pneumatic Piezometers are denoted as ACCEPTED MANUSCRIPT PP.

The results obtained from the two types of piezometers were compared to ascertain the performance of the piezometers installed in the marine clay subject to the reclaimed fill load and surcharge load. The piezometers data were compared for the A2S-71 (2.0 m × 2.0 m) and

A2S-74 (no drain) sub-areas where the two types of piezometers were both installed at close to the same elevations in the marine clay for comparison purposes.

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The excess pore water pressures for the A2S-71 (2.0 m × 2.0 m) sub-area is shown in Figure 6 and Figure 7 for the pneumatic and vibrating-wire piezometer respectively. The excess pore water pressures for the A2S-74 (no drain) sub-area is shown in Figure 8 and Figure 9 for the pneumatic and vibrating-wire piezometer respectively. The piezometers in A2S71 indicate a marked increase in excess pore water pressures during the surcharge placement which reaches a peak at 180 to 270 days’ time period. This is followed by a gradual dissipation of the excess pore water pressures during the surcharge period in the sub-areas with prefabricated vertical drains which indicates gaining degree of consolidation of the marine clay over time. The observation of piezometers for A2S-74 (no-drain) sub-area indicates that there is an increase in the excess pore pressure during surcharge placement, which peaks at 270 days, followed by a much slower rate of dissipation of excess pore water pressures.

After approximately 1170 days’ time period some piezometers picked up a slight rise in piezometric elevation and excess pore water pressures which can be attributed to the surcharge placement at areas adjacent and close to these sub-areas. As such the piezometers are noted to be sensitive to the surcharge placement operations and the loading pressure bulbs of these adjacent areas. Damage to piezometers is indicated by the extreme sudden increase in the piezometric elevationACCEPTED and excess pore wate MANUSCRIPTr pressures readings such as damage to PP-438 in A2S-74 (no drain) during the surcharge placement operation at approximately the 1170 days period. Damage can also be indicated by a sudden loss of signal which could be attributed to the damage caused by moving machinery.

Figure 10, 11 and 12 indicate the comparison of the excess pore pressure isochrones from vibrating-wire and pneumatic piezometers monitoring data among the sub-areas at 12, 24 and

32 months’ time intervals after surcharge. In Figure 10, after 12 months of surcharge, it is

ACCEPTED MANUSCRIPT evident that there some difference between the vibrating-wire and pneumatic piezometer readings for A2S71 (2.0 m × 2.0 m) at elevations of -10 to -15 mCD with the pneumatic piezometer giving higher readings. The opposite trend is noted for A2S-74 (no drain) with the vibrating-wire piezometer giving higher readings at most elevations. Similar differences in excess pore water pressure are noted in Figure 11 (24 months after surcharge) and Figure 12

(32 months after surcharge) with variations in the excess pore water pressures noted between the two types of piezometer with inconclusive trends. These differences can be attributed to effects such as slight variations in marine clay profile between piezometer installation locations, disturbance effects during the piezometer installation, presence of sand seams in the marine clay as well pinching of the pneumatic piezometer cables during the settlement process. It is noted that the excess pore water pressures are recorded to be significantly lower at the location of the relatively higher permeability intermediate stiff clay layer for both types of piezometers and this is particularly significant for the pneumatic piezometers at A2S-74

(no drain). For A2S-74 (no drain), the vibrating-wire piezometers were only installed after

270 days and the loss of earlier information on pore pressure dissipation prior to their installation could also have affected the comparisons.

Figure 13 indicatesACCEPTED the comparison of degree MANUSCRIPT of consolidation between pneumatic and vibrating-wire piezometers for the various piezometer elevations 32 months after surcharge. The comparison of the excess pore pressure isochrones indicate that in the vertical drain treated sub-area A2S-71 (2.0 m × 2.0 m), vibrating-wire piezometers consistently provide a lower excess pore pressure than the pneumatic piezometers installed at close to the same elevations. The shape of the excess pore pressure isochrones of the two types of piezometers are of the same trend in the various sub-layers thus indicating that the measurement of the

ACCEPTED MANUSCRIPT excess pore pressures by the piezometers are consistent for both types of piezometers at the same corresponding elevation.

Table 1 summarises the degree of consolidation of the sub-areas based on the isochrones of the piezometers at A2S-71 (2.0 m × 2.0 m) at various periods after surcharge placement. The results from the piezometers were also compared to that obtained from settlement plates by means of assessing the degree of consolidation by the Asaoka (1978) and hyperbolic

(Sridharan and Sreepada 1981; Sridharan et al. 1991; Tan 1995) methods. At the end of the surcharge period of 32 months, the sub-area with the prefabricated vertical drains (A2S-71:

2.0 m × 2.0 m) has achieved 86.2% degree of consolidation according to the vibrating-wire piezometers and 79.6% according to the pneumatic piezometers. The piezometer results in the sub-area with the prefabricated vertical drains (A2S-71: 2.0 m × 2.0 m) were found to be lower when compared to the settlement assessment at the end of the surcharging period of 32 months though they were higher than the settlement assessment at the earlier periods of assessment of 12 months. This is consistent with the findings reported by Arulrajah et al.

(2004a) where it was reported that assessment by piezometer results were less than that of the settlement gauges as the period of assessment increases. ACCEPTED MANUSCRIPT Table 2 summarises the degree of consolidation of the sub-areas based on the isochrones of the piezometers at A2S-74 (no drain) at various periods after surcharge placement. At the end of the surcharging period of 32 months, the control area without prefabricated vertical drains

(A2S-74: no drains) has achieved a 37% degree of consolidation according to the vibrating- wire piezometers and 49.9% according to the pneumatic piezometers. The results of the piezometers could not be compared to the settlement assessment results as a minimum degree

ACCEPTED MANUSCRIPT of consolidation of 60% is required to assess the degree of consolidation of settlement plates by the Asaoka (Asaoka 1978) and Hyperbolic methods (Tan 1995).

The comparisons of the degree of consolidation indicate that in the vertical drain treated sub- area A2S-71 (2.0 m × 2.0 m), vibrating-wire piezometers consistently provide a slightly higher degree of consolidation than the pneumatic piezometers installed at almost the same elevations. The shape of the degree of consolidation isochrones of the two types of piezometers are of the same geometry of the isochrones in the various sub-layers, thus indicating that the measurement of the degree of consolidation are consistent for both types of piezometers at the same corresponding elevation.

In the untreated sub-area A2S-74 (no drain), the opposite trend is found with the pneumatic piezometers registering higher degree of consolidation than the vibrating-wire piezometers installed at close to the same corresponding elevation. It is noted that two of the pneumatic piezometers in the untreated sub-area A2S-74 (no drain) (PP439: installation elevation -27 mCD and PP438: installation elevation -24 mCD) has evidently been installed in a sand seam or a high permeability intermediate layer which contributes to the very low excess pore water pressure and subsequentlyACCEPTED high degree of consolidation MANUSCRIPT at this elevation.

The shape of the degree of consolidation isochrones of the two types of piezometers are generally of the same trend in the various sub-layers indicating that the measurement of the degree of consolidation are consistent for both types of piezometers at the same corresponding elevation.

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Dissipation of excess pore water pressure readings as expected is faster in the A2S-71 vertical drain treated sub-area when compared to the untreated A2S-74 control area. This indicates that the prefabricated vertical drains installed in the project are performing to improve the soil drainage system.

7. Conclusions

Comparing results obtained from pneumatic with those of vibrating-wire piezometers indicates that there is little conclusive difference in reading between the two types of piezometers. As such, either type of piezometer can be used for the monitoring of marine clay behaviour under reclaimed fills. The slight variations observed in this study could be due to differences in the soil stratification or the sensitivity of the piezometer. Reasonably consistent readings were obtained from both the pneumatic and vibrating-wire piezometers and as such either type of piezometer can be used for the monitoring of the marine clay behaviour under reclaimed fill. Dissipation of excess pore water pressure readings is evidently faster in the prefabricated vertical drain treated sub-area. This indicates that the prefabricated vertical drains installed inACCEPTED the project are performing MANUSCRIPT to improve the soil drainage system.

At the end of the surcharging period of 32 months, the sub-area with the prefabricated vertical drains (A2S-71: 2.0 m × 2.0 m) has achieved a 86.2% degree of consolidation according to the vibrating-wire piezometers and 79.6% according to the pneumatic piezometers. At the end of the surcharging period of 32 months, the control area without prefabricated vertical drains

(A2S-74: no drains) has achieved a 37.0% degree of consolidation according to the vibrating- wire piezometers and 49.9% according to the pneumatic piezometers.

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It is noted that two of the pneumatic piezometers in the untreated sub-area A2S-74 (no drain)

(PP439: installation elevation -27 mCD and PP438: installation elevation -24 mCD) has evidently been installed in a sand seam or a high permeability intermediate layer which contributes to an inconsistent reading of very low excess pore water pressure and subsequently high degree of consolidation at this elevation.

The findings of the comparison between pneumatic and vibrating-wire piezometers indicate that both types of piezometer are suitable for monitoring the consolidation behaviour of soft soil under land reclamation fills.

Acknowledgements ACCEPTED MANUSCRIPT This research was supported under Australian Research Council's Linkage Projects funding scheme (project number LP0989164).

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List of Tables

Table 1. Comparison of average degree of consolidation at A2S-71 (2.0 m × 2.0 m) at various periods after surcharge placement.

Table 2. Comparison of average degree of consolidation at A2S-74 (no drain) at various periods after surcharge placement.

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Table 1. Comparison of average degree of consolidation at A2S-71 (2.0 m × 2.0 m) at various

periods after surcharge placement.

Degree of Consolidation, U (%) Instrument

12 mths. 24 mths. 32 mths.

Vibrating-wire Piezometer (PZ) 79.7 83.0 86.2

Pneumatic Piezometer (PP) 75.1 82.8 79.6

Settlement Plate (SP) – Hyberbolic method 76.3 89.7 93.7

Settlement Plate (SP) – Asaoka method 71.3 85.3 91.8

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Table 2. Comparison of average degree of consolidation at A2S-74 (no drain) at various periods after surcharge placement.

Degree of Consolidation, U (%) Instrument

12 mths. 24 mths. 32 mths.

Vibrating-wire Piezometer (PZ) 35.3 35.5 37.0

Pneumatic Piezometer (PP) 49.3 44.3 49.9

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List of Figures

Figure 1. Typical installation schematic of a pneumatic piezometer with protective casing (Bo et al. 2003)

Figure 2. Typical installation schematic of a vibrating wire piezometer (Bo et al. 2003)

Figure 3. Typical installation schematic of an open-type piezometer (Bo et al. 2003)

Figure 4. Typical installation schematic of a water stand-pipe (Bo et al. 2003)

Figure 5. Cross-sectional profile showing piezometer locations at the A2S-71 (2.0 m × 2.0) and A2S-74 (No Drain) sub-areas

Figure 6. Excess pore water pressures at A2S-71 (2.0 m × 2.0 m) for pneumatic piezometers

Figure 7. Excess pore water pressures at A2S-71 (2.0 m × 2.0 m) for vibrating wire piezometers

Figure 8. Excess pore water pressures at A2S-74 (no drain) for pneumatic piezometers

Figure 9. Excess pore water pressures at A2S-74 (no drain) for vibrating wire piezometers ACCEPTED MANUSCRIPT Figure 10. Comparison between vibrating-wire and pneumatic piezometer excess pore pressure isochrones at 12 months after surcharge

Figure 11. Comparison between vibrating-wire and pneumatic piezometer excess pore pressure isochrones at 24 months after surcharge

Figure 12. Comparison between vibrating-wire and pneumatic piezometer excess pore pressure isochrones at 32 months after surcharge

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Figure 13. Comparison of degree of consolidation between vibrating-wire and pneumatic piezometer 32 months after surcharge (A2S-71 and A2S-74).

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Fig. 1

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Fig. 2

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Fig. 3 ACCEPTED MANUSCRIPT

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7

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Fig. 8

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Fig. 9

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Fig. 10

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Fig. 11

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Fig. 12

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Fig. 13

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Graphical Abstract

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Research Highlights

 Land reclamation site on soft soils treated with prefabricated vertical drains.  Pore pressure monitoring data from piezometers were analysed for the test site.  Vibrating wire and pneumatic piezometer performance were compared.  Comparisons were made with predictions from neighbouring settlement plates.  Piezometers found to be suitable for monitoring ground improvement of soft soils.

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