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International Conference on Case Histories in (2008) - Sixth International Conference on Case Histories in Geotechnical Engineering

14 Aug 2008, 7:00 pm - 8:30 pm

Case Study of the Changi East Reclamation Project,

A. Arulrajah Swinburne University of Technology, Melbourne, Australia

M. W. Bo DST Consulting Engineers, Ontario, Canada

H. Nikraz Curtin University of Technology, Perth, Australia

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Recommended Citation Arulrajah, A.; Bo, M. W.; and Nikraz, H., "Case Study of the Changi East Land Reclamation Project, Singapore" (2008). International Conference on Case Histories in Geotechnical Engineering. 2. https://scholarsmine.mst.edu/icchge/6icchge/session09/2

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CASE STUDY OF THE CHANGI EAST LAND RECLAMATION PROJECT, SINGAPORE

A. Arulrajah M. W. Bo H. Nikraz Swinburne University of Technology DST Consulting Engineers Curtin University of Technology Melbourne, Australia Ontario, Canada Perth, Australia

ABSTRACT

Since the early 1990’s till early 2000s, the Changi East Reclamation Project in the Republic of Singapore involved the filling of approximately 200 million cubic meters of for the reclamation of a total land area of about 2500 hectares. The land reclamation works were carried out in 5 phases. The edges of the newly reclaimed land in the project were either retained by vertical retaining structure or coastal protection rock bund with suitable slopes and berms. Land reclamation was carried out using fill materials derived from granular material from the seabed at the borrow source. Prefabricated vertical drains with surcharge were used extensively in the project to accelerate the consolidation process. In addition, deep sand compaction of the hydraulically placed sandfill was carried out by various deep compaction methods. Geotechnical instruments were required to monitor the settlement and pore pressure dissipation of the improved soft . During the implementation of the 5 phases of land reclamation and soil improvement projects, several thousand geotechnical instruments of various types were installed. In-situ testing of the marine was carried out prior to reclamation a as after soil improvement. In-situ testing of the marine clay was carried out by means of field vane shear, self boring pressuremeter, and dilatometer tests. This paper provides a case study into the land reclamation, ground improvement, field instrumentation, in-situ testing and deep sand compaction works that were carried out in the Changi East Reclamation Project.

INTRODUCTION In the entire project, a total of 142 million linear meters of vertical drains were installed making this one of the largest From 1992 till mid 2004, the Changi East Reclamation Project projects in in which prefabricated vertical drains in the Republic of Singapore involved the filling of were used. In order to monitor the performance of ground approximately 200 million cubic meters of sand for the improvement and to validate the efficiency of the reclamation of a total land area of about 2500 hectares. The prefabricated vertical drain system several geotechnical land reclamation works were carried out in 5 phases. Land instruments were installed to monitor the degree of reclamation was carried out using fill materials obtained from consolidation at both area with PVD and area without PVD as dredging granular material from the seabed at the borrow control area. source. This land reclamation project required areas that are currently submerged to be raised to levels permanently above Settlement gauges including deep settlement gauges were the sea level. The fill material chosen was well-graded, free installed at the top of each sub layers whereas draining granular soil with fines contents of less than 10%. were installed at the centre of each compressible sub layer in When the fill was placed by pumping, some fines in the fill order to monitor the settlement and pore pressure dissipation. was further removed, a process described by Choa (1985). Fig. Settlement and pore pressure were monitored with close 1 shows the location and various phases of the Changi East interval in the first three months and wider interval at the later Reclamation Project in Singapore. part of monitoring. Ultimate settlements were predicted using the field settlement results applying the Asaoka and The combination of prefabricated vertical drain (PVD) with hyperbolic methods. preloading ground improvement technique was successfully applied in this project to improve the underlying compressible This paper also presents a case study of the ground . The project comprises the installation of prefabricated improvement works carried out with prefabricated vertical vertical drains and the subsequent placement of surcharge to drains at a Pilot Test Site at the Changi East Reclamation accelerate the consolidation of the underlying marine clay. Project.

Paper No. 9.03 1

Fig. 1. Location of the Changi East Reclamation Project in the Republic of Singapore.

PREFABRICATED VERTICAL DRAINS SINGAPORE MARINE CLAY AT CHANGI

The prefabricated vertical drain (PVD) with preloading Singapore marine clay at Changi is a quartenary deposit that method was considered the most feasible and this method was lies within valleys in the Old Alluvium. The Case Study used in the project. The objective of using the vertical drains Area comprises of two distinct layers of marine clay which are with preloading technique is to accelerate the rate of the “Upper Marine Clay” layer and the “Lower Marine Clay” consolidation and to minimize future settlement of the treated layer. The “Intermediate Stiff Clay” layer separates these two area under the future dead and live loads. Soil improvement distinct marine clay layers. works is carried out in such a way that a specified degree of primary consolidation is designed to be attained within the The upper marine clay is soft with undrained desired time frame by improving the soil system. values ranging from 10 to 30 kPa. The upper marine clay has a liquid limit of between 80-95%, plastic limit of between 20- The primary use of prefabricated vertical drains is to 28% and of 70-88%. The upper marine clay is accelerate consolidation to greatly decrease the duration of generally overconsolidated with overconsolidation ratio consolidation process caused by built over soft (OCR) of about 1.5-2.5. The coefficient of consolidation due soils. This will ensure that the final construction to vertical flow (cv) of the upper marine clay is between 0.47- can be completed in a reasonable time and with minimal post 0.6 m2/year while the coefficient of consolidation due to 2 construction settlement. Preloading increases the effective horizontal flow (ch) is between 2-3 m /year. stress and reduces the compressibility of weak ground by forcing soft soils to consolidate. By doing so, the The lower marine clay has a liquid limit of 65-90%, plastic consolidation process also improves the strength of in-situ soft limit of 20-30% and water content of 40-60%. The lower soils. marine clay is lightly overconsolidated with OCR of 2. The coefficient of consolidation due to vertical flow (cv) of the lower marine clay is between 0.8-1.5 m2/year while the coefficient of consolidation due to horizontal flow (ch) is between 3-5 m2/year.

Paper No. 9.03 2 The intermediate stiff clay is sandwiched between the upper Degree of consolidation for settlement gauges can be marine clay and lower marine clay. This layer comprises of computed based on the field settlement. Degree of predominantly stiff sandy or sandy clay. The intermediate consolidation is defined as percentage of magnitude of stiff clay has a liquid limit of about 50%, plastic limit of 18- settlement that occurred at time “t” upon ultimate primary 20% and water content of 10-35%. The intermediate stiff clay consolidation settlement as indicated in Equation 1. From is moderately overconsolidated due to dessication, with OCR measured field settlement and predicted ultimate settlement, of 3-4. The coefficient of consolidation due to vertical flow degree of consolidation can be estimated. Ultimate settlement 2 (cv) of the intermediate marine clay is between 1-4.5 m /year can be predicted for marine clays treated with vertical drains while the coefficient of consolidation due to horizontal flow and preload by the Asaoka (Asaoka, 1978) or Hyperbolic 2 (ch) is between 5-10 m /year. The of the Singapore (Tan, 1995) methods. marine clay at Changi have been discussed previously by Bo et al. (1997, 1998) and Arulrajah et al (2004a, 2004b). Us (%) = St / Sα (1)

where St = field settlement at any time t; Sα = ultimate GEOTECHNICAL FIELD INSTRUMENTATION settlement; and Us (%) = average degree of consolidation.

Field instruments suitable for the study of consolidation Piezometers are utilized to measure the pore pressure in the behavior of underlying soils and monitoring of land soil. If regular monitoring is carried out to measure the reclamation works include surface settlement plates, deep piezometric head together with static water level, dissipation settlement gauges, multi-level settlement gauges, liquid of excess pore pressure can be detected and thus degree of settlement gauges, pneumatic piezometers, electric consolidation can be assessed. Average residual excess pore piezometers, open-type piezometers, water standpipes, pressure is defined as ratio of excess pore pressure at time “t” , deep reference points and total earth pressure upon initial excess pore pressure. Therefore degree of cells. A total of 7246 geotechnical instruments were installed consolidation for a soil element, Uu can be defined as shown at the Changi East Reclamation Projects. 1 shows the in Equation 2. detailed breakdown of instruments installed and monitored at the Changi East Reclamation Projects. Uu (%) = 1- (Ut / Ui) (2)

Assessment of degree of consolidation could be carried out by where Uu (%) = degree of consolidation for a soil element; Ut means of field instrument monitoring at regular time intervals. = the excess pore pressure at time t; and Ui = initial excess Details on assessment of degree of consolidation have been pore pressure which is equal to the additional load (∆σ’). discussed by Bo et. al (1997) and Arulrajah et al. (2004a, 2004b).

Table 1: Detailed breakdown of the number of geotechnical instruments installed at the Changi East Reclamation Projects. Phase Phase Phase Area "A" Area "A" Asean Soil Instruments Total 1A 1B 1C North South Aerospace Pneumatic 26 781 458 122 227 12 1626 Open Type Piezometer 9 70 84 29 54 246 Electric Piezometer 122 150 272 Settlement Plate 125 778 1092 173 707 16 2891 Settlement Gauge 72 72 Deep Settlement Gauge 23 790 450 131 266 12 1672 Multi-Level 3 17 11 25 56 Settlement Gauge Deep Reference Point 8 10 3 12 1 34 Water Standpipe 29 59 14 49 3 154 25 52 47 23 24 3 174 Inclinometer with Measurement of 9 2 11 Vertical Displacement Earth Pressure Cell 3 12 7 5 11 38 Total 214 2740 2370 500 1375 47 7246

Paper No. 9.03 3 PILOT TEST SITE: A CASE STUDY include period of assessment, hydrogeologic boundary condition, settlement of piezometer tip and reduction of initial The Pilot Test Site consisted of 4 sub-areas, three of which imposed load due to submergence effect (Bo et al. 1999, were installed with vertical drains at various spacings. Long 2003). duration field settlement monitoring was carried out at regular intervals at these sub-areas. The seabed elevation is about -6 Field instrument monitoring was carried out at regular mCD (Admiralty Chart Datum, where mean sea level is +1.6 intervals so that the degree of improvement could be mCD) while the thickness of the soft marine clay in the monitored and assessed throughout the period of the soil location was up to 45 meters thick. improvement works for the project. Instruments were monitored at close intervals of up to 3 times a week during Land reclamation was first carried out to the vertical drain sandfilling and surcharge placement operations. At other times platform elevation of +4 mCD. Field instruments comprising the instrument was monitored usually at a frequency of once a of surface settlement plates, deep settlement gauges, week. pneumatic piezometers, electric piezometers and water stand- pipes were installed from the platform level where vertical Table 2. Summary of Pilot Test Site sub-area vertical drain drain were installed. Instruments were installed prior to spacings. vertical drain installation. Pilot Test Site Vertical Drain Square Spacing Following the installation of vertical drains, surcharge was Sub-Areas next placed by hydraulic filling to an elevation of +7 mCD A2S-71 2.0 meter x 2.0 meter simultaneously for all the sub-areas. As such, an assessment could be carried out and compared between the sub-areas A2S-72 2.5 meter x 2.5 meter treated with vertical drains at various spacings when subjected A2S-73 3.0 meter x 3.0 meter to the same surcharge preload. The analysis of the instrumentation results for the various sub-areas was carried A2S-74 No Drain out 32 months after surcharge placement which equates to a total monitoring duration of about 42 months.

The summary of the vertical drain spacing in the various sub- areas is indicated in Table 2. Figure 2 shows the layout of the Pilot Test Site. The profile of field instrumentation and the soil profile at the Pilot Test Site is shown in Figure 3.

The field settlement data can be analyzed by the Asaoka (Asaoka, 1978) and Hyperbolic (Sridharan and Sreepada, 1981) methods to predict the ultimate settlement of the reclaimed land under the surcharge fill. Back-analysis of the field settlement data will also enable the coefficient of consolidation for horizontal flow to be closely estimated. Factors that affect prediction by the Asaoka method are the period of assessment after surcharge placement as well as the time interval used for the analysis (Arulrajah et al. 2004a, Bo et al. 1999). Factors that affect prediction by the Hyperbolic method are the period of assessment after surcharge placement Fig. 2. Layout plan and vertical drain spacing of sub-areas at (Arulrajah et al. 2004a, Bo et al. 1999). the Pilot Test Site

Pneumatic and vibrating-wire electric piezometers were installed to monitor the dissipation of excess pore pressures of Settlement Measurements the marine clay under the reclaimed fill load. The piezometers were installed in individual at various Fig. 4 presents the construction sequence of the Pilot Test Site. predetermined elevations in the marine clay. The piezometers Fig. 5 compares the settlement plate results between the were installed in the same instrument clusters as the water various sub-areas in the Pilot Test Site. The A2S-71 (2.0m x stand-pipes and settlement gauges. The pneumatic piezometer 2.0m) sub-area records the highest magnitude and rate of and electric piezometer indicate similar measurements for settlement as compared to the other sub-areas due to its closer piezometric elevation and excess pore water pressures. drain spacing. The A2S-74 (no drain) sub-area on the other Installation of piezometers at the same elevations as the deep hand records the least magnitude and rate of settlement. settlement gauges enabled for the correction of the piezometer tip due to large strain settlements of the marine clay under the reclaimed fill. Factors that affect the analysis of piezometers

Paper No. 9.03 4

Fig. 3. Cross sectional soil profile showing instrument elevations at the Pilot Test Site.

The vast improvement of the vertical drain treated areas Normally, for the same surcharge and the same thickness of compared to the No Drain sub-area is clearly evident in the clay, the same amount of ultimate settlement is obtained after figure. It can be observed that the closer the vertical drain a long time. However, in the Pilot Test Site, variations in the spacing, the higher the corresponding magnitude of settlement. final predicted settlements is due to various reasons. Sub-area A2S-71 with the closest drain spacing indicates the highest settlement readings while the untreated sub-area A2S- It is apparent that the A2S-71 (2.0m x 2.0 m) sub-area has the 74 indicates the least. This indicates that the vertical drain is thickest layer of intermediate marine clay than the other functioning as per their requirements. vertical drain sub-areas. In addition higher excess pore presure were recorded in the A2S-71 (2.0m x 2.0 m) and A2S- Fig. 6 indicates the magnitudes of settlements of the A2S-71 74 (no drain) sub-areas which indicates comparatively lower sub-area. Fig. 7 indicates the magnitudes of settlements of the than the other sub-areas. Furthermore, A2S-74 sub-area. settlement of the sub-areas prior to the installation of instruments will also result in variations in the settlement Fig. 8 compares the field settlement profiles between the measured after installation of instruments. various sub-areas of the Pilot Test Site at various durations 8 after surcharge. The settlement gauges which was installed 0.5 meters beneath the vertical drain platform level in the 7 reclamation sand and the deep settlement gauges which was 6 installed at the top surface of the compressible marine clay gave similar readings for magnitude and time rate of ) 5 CD Install PVD (m settlement. This indicates that the settlement contribution of n io 4 vat e l

the sandfill layer is minimal as would be expected. E

3 Fig. 9 shows the Asaoka plot predictions for the settlement plate at the A2S-71 sub-area of the Pilot Test Site at time 2 intervals of 28 and 56 days. Fig. 10 and Fig. 11 shows the 1 combined and typical Hyperbolic plots for the settlement Ground Elevation (SP-346) Water Level (WS-99) gauges at A2S-71 (2.0m x 2.0 m). 0 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 1440 Time (days) Fig 4. Construction sequence at Pilot Test Site.

Paper No. 9.03 5 0.0 5

0 0.2 SANDFILL

-5 0.4

A2S-74 -10 0.6 UPPER -15 MARINE CLAY

0.8 CD) m t (m) ( n

n -20 io

eme A2S-73 ttl

1.0 evat e l A2S-71: 12 months after surcharge S E -25 A2S-71: 24 months after surcharge A2S-71: 32 months after surcharge INTERMEDIATE 1.2 A2S-72: 12 months after surcharge STIFF CLAY A2S-72 -30 A2S-72: 24 months after surcharge A2S-72: 32 months after surcharge LOWER 1.4 A2S-73: 12 months after surcharge -35 MARINE A2S-73: 24 months after surcharge CLAY A2S-71 (2.0m x 2.0m): SP-346 A2S-71: 2.0m x 2.0m drain spacing A2S-73: 32 months after surcharge A2S-72 (2.5m x 2.5m): SP-396 A2S-72: 2.5m x 2.5m drain spacing A2S-74: 12 months after surcharge 1.6 -40 A2S-73 (3.0m x 3.0m): SP-415 A2S-73: 3.0m x 3.0m drain spacing A2S-74: 24 months after surcharge A2S-71 A2S-74: No drain A2S-74 (No Drain): SP-416 A2S-74: 32 months after surcharge OLD ALLUVIUM

1.8 -45 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 1440 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Settlement (m) Time (days) Fig. 5. Comparison of field settlement between sub-areas at Fig. 8. Comparison of field settlement isochrones between the Pilot Test Site. sub-areas at the Pilot Test Site.

2.0

0.0 DS-449 DS-474 0.2 DS-448 1.8

DS-447

0.4 DS-446

0.6

) 1.6

DS-473 m ) (

0.8 ) i t (m ( n t e n m e ttl 1.0 me e e l S tt

SP-346 (+3.5 mCD) Se 1.4 1.2 DS-444 (-6 mCD) DS-445 (-12 mCD) DS-473 (-16 mCD) DS-445 1.4 DS-446 (-20 mCD) DS-447 (-26 mCD) DS-444 DS-448 (-28.5 mCD) 1.6 DS-474 (-33 mCD) 1.2 Surcharge Duration = 32 months DS-449 (-37.5 mCD) SP-346 1.8 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 1440 SP-346(+3.5 mCD):56 day intervals Time (days) 45 degree line Ultimate Settlement = 1.838 m Fig 6. Field settlement results of settlement gauges at A2S-71 1.0 (2.0m x 2.0m). 1.01.21.41.61.82.0 Settlement (i - 1) (m)

Fig. 9. Asaoka plot for A2S-71 (2.0m x 2.0m) at time interval

of 28 and 56 days.

0.0

8500 DS-472 0.1 SP-346 (+3.5 mCD) DS-471 8000 DS-444 (-6 mCD) 7500 DS-445 (-12 mCD) DS-473 (-16 mCD) 7000 0.2 DS-470 DS-446 (-20 mCD) DS-469 6500 DS-447 (-26 mCD) DS-468 6000 DS-448 (-28.5 mCD) ) s t (m)

DS-467 r n e t e 5500

0.3 e m e s/m ttl 5000 e

DS-466 ay S d (

DS-465 t 4500 n

SP-416 (+3.5 mCD) e 0.4 4000

DS-465 (-7 mCD) lem t t DS-466 (-10 mCD) e 3500 / S

DS-467 (-15 mCD) e m

i 3000 DS-468 (-20 mCD) T 0.5 DS-469 (-23 mCD) SP-416 2500 DS-470 (-30 mCD) DS-471 (-35 mCD) 2000 DS-472 (-42 mCD) 1500 0.6 1000 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 Time (days) 500 0 Fig. 7. Field settlement results of settlement gauges at 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 A2S-71 (2.0m x 2.0m). Time (days) Fig. 10. Combined Hyperbolic plot of settlement gauges at

A2S-71 (2.0m x 2.0m).

Paper No. 9.03 6 1000 120 PP-245 (-8 mCD) PP-246 (-10 mCD) 900 PP-247 (-12 mCD) PP-248 (-20 mCD) PP-249 (-27 mCD) PP-369 (-33 mCD) 100 800 PZ-35 (-8 mCD) PZ-36 (-12 mCD) PZ-37 (-16 mCD) PZ-38 (-20 mCD)

) PZ-39 (-27 mCD) PZ-40 (-33 mCD) rs

e 700 t me

/ 80 ) s a y P

600 k (

(da e nt e ssur m e e r l 500 t P t 60

e e r S o /

400 s P me i PP-248 T

xces PZ-38 E 40 300 Slope of first linear segment, Si = 0.543 PP-247 Theoretical value of initial linear slope, α = 0.752 PP-246 Predicted Settlement = α / Si = 1.386 m PZ-36 200 Ultimate Settlement = Settlement before surcharge + Predicted Settlement PZ-37 = 0.415 m+ 1.386 m PP-249 = 1.801 m 20 PP-245 100 PP-369 SP-346 (+3.5 mCD): 32 months after surcharge data PZ-40 PZ-35 0 0 PZ-39 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 Time (days) Time (days) Fig. 11. Hyperbolic plot at A2S-71 (2.0m x 2.0m) after Fig. 13. Excess pore water pressures at A2S-71 (2.0m x surcharge duration of 32 months. 2.0m).

26 PP-349 (-8.5 mCD) PP-350 (-12 mCD) Pore Pressure Measurements PP-435 (-15 mCD) PP-436 (-17.5 mCD) 24 PP-437 (-21.5 mCD) PP-438 (-24 mCD) PP-439 (-27 mCD) PP-440 (-30 mCD) 22 PP-441 (-38.5 mCD) PZ-41 (-12 mCD) PZ-42 (-15 mCD) PZ-43 (-17.5 mCD) Pneumatic piezometers were installed in the same clusters as PZ-44 (-24 mCD) PZ-45 (-27 mCD) 20 the settlement gauges, close to the same elevation as the PZ-46 (-38.5 mCD) Water Level 18 PP-437 PP-349 PP-438

settlement gauges to enable for correction of the piezometer PZ-42 PZ-43 CD) 16 PP-436 m PP-350 (

n PZ-41 tip due to large strain settlement. Water stand-pipes were PP-435 io PZ-44

at 14 v PZ-45 installed in the clusters so as to measure the static water level PP-440 c Ele

i 12 r

at these locations and hence to ascertain the excess pore water et m

o 10 ez i

pressures of the piezometers. Due to the large strain P 8 PP-439 settlements at the site, all raw piezometer readings taken were PP-441 PZ-46 corrected to account for the new elevation of the piezometer at 6 4 each monitoring due to the settlement of the piezometer tip. Water Level Correction is essential and if not made will lead to an 2

0 underestimation of the degree of dissipation of the excess pore 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 water pressure. The piezometer elevations and excess pore Time (days) water pressures for the A2S-71 (2.0m x 2.0m) sub-area are Fig. 14. Piezometric elevations and excess pore water shown in Fig. 12 and Fig. 13. The piezometer elevations and pressures at A2S-74 (No Drain). excess pore water pressures for the A2S-74 (No Drain) sub- area are shown in Fig. 14 and Fig. 15. Higher excess pore 240 PP-349 (-8.5 mCD) PP-350 (-12 mCD) presure was recorded in the A2S-71 (2.0m x 2.0 m) and A2S- PP-435 (-15 mCD) PP-436 (-17.5 mCD) 220 PP-437 (-21.5 mCD) PP-438 (-24 mCD) 74 (no drain) sub-areas which indicates comparatively lower PP-439 (-27 mCD) PP-440 (-30 mCD) PP-441 (-38.5 mCD) PZ-41 (-12 mCD) 200 PZ-42 (-15 mCD) PZ-43 (-17.5 mCD) effective stress than the other sub-areas. PZ-44 (-24 mCD) PZ-45 (-27 mCD) 180 PZ-46 (-38.5 mCD) 16 PP-437 PP-245 (-8 mCD) PP-246 (-10 mCD) PP-349 PP-438 160 PP-247 (-12 mCD) PP-248 (-20 mCD) ) Pa 14 PP-249 (-27 mCD) PP-369 (-33 mCD) k

re ( 140 PZ-35 (-8 mCD) PZ-36 (-12 mCD) PZ-37 (-16 mCD) PZ-38 (-20 mCD) PZ-43 essu

r PZ-42 PZ-39 (-27 mCD) PZ-40 (-33 mCD) 120 PP-436 12 PP-350 Water Level re P PZ-41 PP-435 PZ-44 s Po 100 PZ-45

CD) 10 PP-440 Exces m 80 (

n PP-248 o i PZ-38 at 60

ev PP-247 l 8 PP-246 c E i

r PZ-36 40 et PZ-37 PP-439

m PP-441

o PP-249 6 PZ-46 ez i PP-245 20 P PP-369 PZ-40 PZ-35 PZ-39 0 4 Water Level 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 Time (days) 2 Fig. 15. Excess pore water pressures at A2S-74 (No Drain).

0 0 90 180 270 360 450 540 630 720 810 900 990 1080 1170 1260 1350 Fig. 16 indicates the comparison of excess pore pressure Time (days) Fig. 12. Piezometric elevations and excess pore water isochrones between the sub-areas 32 months after surcharge. pressures at A2S-71 (2.0m x 2.0m). Non-uniform variation of the excess pore pressure regarding elevation is due to slight difference in the installed location of

Paper No. 9.03 7 the piezometer from the vertical drains as well as the presence Comparison of Observational Methods of thin sand lenses (so-called microlayers). Table 2 summarises the comparison of degree of consolidation 0 A2S-71 PZ: 32 months after surcharge A2S-71: Total Additional Load =160.31 kPa and back-analysed ch between the settlement plates and A2S-71: Submerged Additional Load (32 mths)=134.75 kPa A2S-72 PP: 32 months after surcharge A2S-72: Total Additional Load = 147.08 kPa piezometers at the various sub-areas 32 months after surcharge -5 A2S-72: Submerged Additional Load (32 mths)=126.16 kPa A2S-73 PP: 32 months after surcharge A2S-73: Total Additional Load = 146.5 kPa A2S-73: Submerged Additional Load (32 mths)=127.23 kPa obtained by the observational methods. The degree of A2S-74 PZ: 32 months after surcharge -10 A2S-74: Total Additional Load = 162.35 kPa A2S-74: Submerged Additional Load (32 mths)=140.67 kPa consolidation of the vertical drain treated sub-areas is observed to be far greater than that of the No Drain sub-area. -15

) CD m (

n -20 Table 2. Comparison of observational methods 32 months o A2S-71: 2.0m x 2.0m drain spacing i

at A2S-72: 2.5m x 2.5m drain spacing A2S-73: 3.0m x 3.0m drain spacing after surcharge (41.9 months of monitoring) Elev A2S-74: No drain -25 Sub-Area Comparison Asaoka Hyperbolic Piezometer

-30 A2S-71 Ultimate settlement (m) 1.838 1.801 -

-35 2.0 x 2.0 m Settlement to date (m) 1.687 1.687 - Degree of Consolidation, U (%) 91.8 93.7 86.2 -40 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 A2S-72 Ultimate settlement (m) 1.412 1.408 - Excess Pore Pressure (kPa) Fig. 16. Comparison of piezometer excess pore pressure 2.5 x 2.5 m Settlement to date (m) 1.264 1.264 - isochrones between sub-areas 32 months after surcharge. Degree of Consolidation, U (%) 89.5 89.8 82.5

A2S-73 Ultimate settlement (m) 1.200 1.169 - Fig. 17 indicates the comparison of degree of consolidation 3.0 x 3.0 m Settlement to date (m) 0.948 0.948 - between piezometers at the sub-areas 32 months after surcharge. Rapid dissipation of excess Degree of Consolidation, U (%) 79.0 81.1 73.1 with time is clearly evident in the vertical drain treated areas A2S-74 as compared to the No Drain sub-area. The sub-area with the No Drain Degree of Consolidation, U (%) - - 37.0 closer vertical drain spacing is found to generally register the higher degree of consolidation at a particular elevation. Some exceptions to this is found at certain elevation which could be IN-SITU TESTING due to the slightly varying soil profiles that exist between the various sub-areas. Furthermore, the presence of sand seams in In-situ testing works in this research study comprises the use the marine clay will increase the permeability of the marine of field vane shear, piezocone, dilatometer and self-boring clay and enable the excess pore water pressure in it to drain pressuremeter. The types of equipment, testing procedure and relatively rapidly. Evidently from the findings of the figures, methods of analysis of these in-situ testing penetration and the degree of consolidation is highest at the sub-area with the dissipation tests applicable for Singapore marine clay at closest vertical drain spacing and lowest for the No Drain sub- Changi have been discussed by Arulrajah et al. (2004b) and area. Bo et al. (2003). In-situ dissipation tests by means of piezocone, dilatometer, self-boring pressuremeter and BAT 0 A2S-71: 2.0m x 2.0m drain spacing A2S-71: Electric Piezometers A2S-72: 2.5m x 2.5m drain spacing permeameter have been used in the characterization of the A2S-73: 3.0m x 3.0m drain spacing A2S-72: Pneumatic Piezometers -5 A2S-74: No drain coefficient of horizontal consolidation and horizontal A2S-73: Pneumatic Piezometers

A2S-74: Electric Piezometers of the Singapore marine clay at Changi. -10

-15

) IN-SITU TEST SITE: A CASE STUDY CD m (

n -20 io The In-Situ Test Site is located in an area where the thickest Elevat -25 compressible layers existed and a portion of where the future airport runway would be located. The original seabed level in -30 the In-Situ Test Site was 3.29 meters below Admiralty Chart

-35 Datum (–3.29 mCD). Various in-situ tests were carried out

Surcharge Period = 32 months prior to the commencement of land reclamation works to -40 0 102030405060708090100 characterize the marine clay properties. Prior to reclamation a Degree of Consolidation (%) series of in-situ tests were carried out at the In-Situ Test Site Fig. 17. Comparison of degree of consolidation between by the field vane shear test (FVT), cone penetration test piezometers at sub-areas 32 months after surcharge. (CPT), dilatometer test (DMT) and self-boring pressuremeter test (SBPT). The results of the in-situ tests carried out prior to reclamation in the In-Situ Test Site have been discussed by Arulrajah et al. (2004b). Bo et al. (2003) and Chang et al. (1986) have also previously reported on the prior to

Paper No. 9.03 8 reclamation in-situ testing of Singapore marine clay at Changi. 0 FVT8: Vertical Drain Area The variation of undrained shear strength with depth by CPT8: Vertical Drain Area DMT8: Vertical Drain Area -5 various in-situ methods prior to reclamation is presented in SBPT8: Vertical Drain Area FVT9: Control area Figure 18. CPT9: Control area -10 DMT9: Control area SBPT9: Control area Following the completion of the pre-reclamation in-situ tests, -15 CD) land reclamation was carried out by hydraulic placement of m ( n sand until the vertical drain platform level of 4 meters above io -20 Admiralty Chart Datum (+4 mCD). Vertical drains were Elevat installed at this elevation at 1.5 meter square spacing, to -25 depths of up to 35 meters in the Vertical Drain Area.

Surcharge was placed until the design elevation of 10 meters -30 above Admiralty Chart Datum (+10 mCD) for both areas.

-35 0 102030405060708090100 Following the completion of ground improvement works with Shear Strength (kN/m2) vertical drains and preloading, another series of in-situ tests Figure 19. Comparison of shear strengths from in-situ testing were carried out in the In-Situ Test Site. The post- between Vertical Drain Area and Control Area, after 23 improvement in-situ tests were carried out after a surcharge months of surcharge loading. period of about 23 months in the Vertical Drain Area where vertical drains were installed at 1.5 meter square spacing as well as an adjacent Control Area where no drains were 0 installed. The locations of these in-situ tests were done close to each other so as to enable a good comparison of the degree -5 of consolidation of the areas treated with and without vertical drains when subjected to the same magnitude of preloading. -10

-15

Comparison of In-Situ Test Results -20

The degree of consolidation by the various in-situ testing -25 methods was calculated by the method of Bo et al. (2003). The CPTU2: Prior to reclamation (after Arulrajah et al., 2004) comparison of undrained shear strength with depth results at -30 Laboratory results: Prior to reclamation (after Arulrajah et al., 2004) the Vertical Drain Area (1.5 x 1.5 m square spacing) and the CPTU8: Vertical Drain Area CPTU9: Control Area Control Area (No drains), 23 months after surcharge loading is -35 024681012 2 compared in Fig. 19. Fig. 20 compares the coefficient of c h (m /yr) horizontal consolidation from CPT dissipation test between Fig. 20 Comparison of coefficient of horizontal consolidation Vertical Drain Area and Control Area after 23 months of from CPTU dissipation test between Vertical Drain Area and surcharge loading. Fig 21 compares the horizontal hydraulic Control Area after 23 months of surcharge loading. conductivity from BAT permeameter between Vertical Drain Area and Control Area after 23 months of surcharge loading. 0

0 -5

FVT2: Prior to reclamation CPT2: Prior to reclamation 5 -10 DMT2: Prior to reclamation SBPT2: Prior to reclamation

10 -15 ) m (

ed 15 -20 seab w lo e

b -25

h 20 t p e

D BAT2: Prior to reclamation (after Arulrajah et al., 2004)

Laboratory Result: Prior to reclamation (after Arulrajah et al., 2004) 25 -30 BAT8: Vertical Drain Area

BAT9: Control Area Cu = 7.06 + 1.7 (Depth) 30 -35 1.00E-11 1.00E-10 1.00E-09 1.00E-08 1.00E-07 k h (m/s) 35 0 102030405060708090100 Fig. 21. Comparison of horizontal hydraulic conductivity from Shear Strength (kN/m2) BAT permeameter between Vertical Drain Area and Control Fig. 18. Variation of undrained shear strength with depth by Area after 23 months of surcharge loading. various in-situ methods prior to reclamation

Paper No. 9.03 9 The shear strength, degree of consolidation and OCR DEEP SAND COMPACTION comparisons of the post-improvement values obtained from various in-situ tests after a surcharge period of 23 months are Loose granular soil is susceptible to liquefaction upon the found to be agreeable with each other. The post improvement impartation of dynamic forces. Under static conditions, loose in-situ tests indicated clear increases in the soil strength and granular soil may be subjected to failure due degree of improvement which is as expected. The results also to its low angle and for large settlements because of its indicate the expected higher increases in shear strength, degree high compressibility and low deformation modulus. Various of consolidation and overconsolidation ratio between the densification methods are used to enhance the friction angle Vertical Drain Area as compared to the untreated Control and the elastic modulus of granular soils for improving Area. performance.

The in-situ tests indicate that the degree of consolidation of At reclaimed sites, the granular soil mass is commonly placed the Vertical Drain Area had attained a degree of consolidation by hydraulic filling and it cannot be densified by surface of about 70-80% while the Control Area had attained a degree compaction. Deep densification is therefore often required. In of consolidation of only 30-40%. This is based on the results the Changi East Reclamation projects, an area of about 114 of the CPT test which is the authors recommended method for hectares was improved by deep compaction methods and the use based on results obtained in this particular case study area. average thickness of granular fill profile was 7 to 10m. The in-situ tests confirm that the vertical drains used in the study are performing as expected. Three types of deep compaction methods namely, dynamic compaction, vibroflotation and Muller Resonance Compaction The pre-reclamation CPTU holding tests indicate that the ch (MRC), were deployed in the Changi East Reclamation values vary between 2-6 m2/yr. The post-improvement results Project. The areas where the three different types of in the upper and lower marine clay layers indicate ch of 3-6 compaction methods were used are shown in Fig. 22. m2/yr in the Vertical Drain Area and 3-5 m2/yr in the Control Area. ch value is seen to be higher in the Vertical Drain Area The dynamic compaction method was deployed in the area as compared to the Control Area. Despite the kh being lower where the required depth of compaction was 5 to 7 metres. in the Vertical Drain Area, the ch could be higher due to The vibroflotation and MRC methods were adopted in the greater ratio of reduction in the coefficient of volume change, areas where the required thickness of compaction was 7 to 10 mv. The CPTU results were found to be the closest to the metres. Each of the three compaction methods has its own laboratory testing results. advantages and disadvantages depending on the site and soil conditions in various areas. The horizontal hydraulic conductivity results from the BAT tests can be used as the baseline data since the system measures horizontal hydraulic conductivity directly whereas Dynamic Compaction the other in-situ tests required the introduction of additional parameters to evaluate the hydraulic conductivity indirectly Dynamic compaction (DC) is a technique for improving the from ch values. Generally the permeability decreases with mechanical properties of granular soil to relatively great depth. The horizontal hydraulic conductivity measured by the depths by repeatedly lifting and dropping a heavy weight BAT permeameter is generally lower than that of the other in- (pounder) onto the ground surface. Impact energy from situ tests, possibly due to the smear effect when the repeated impacts over split-second durations are imparted on permeameter is pushed into the soil. It is clearly evident that the granular soil when the heavy weight hits the ground the horizontal hydraulic conductivity decreases in the Vertical surface causing the soil particles to rearrange into a denser Drain Area as compared to the prior to reclamation and the state. The selection of spacing and the number of drops per Control Area within the marine clay layer. This is as expected print point is essential for achieving the specified density in due to the smearing effect of the vertical drain treated area and DC at a specified site. confirms that there is a reduction of horizontal permeability from time to time during consolidation. The horizontal The compaction process is usually repeated in several passes hydraulic conductivity of Singapore marine clay prior to until the required post treatment relative density has been reclamation at the In-Situ Test Site is in the order of between achieved. The initial spacing between impact points is usually 10-9 to 10-10 m/s based on the BAT readings. The horizontal equal to the thickness of the densifiable layer in order to allow hydraulic conductivity is in the order of 10-9 to 10-10 m/s in the the impact energy to reach the lower part of the layer. Vertical Drain Area and the Control Area after 23 months of Subsequent passes tend to have progressively closer spacing. surcharge loading. After each pass, the craters created by the dropping pounder are usually backfilled with surrounding materials before the next pass. Finally an “ironing” pass with a low energy impact with reduced drop height is carried out to compact the surface layer. There is usually no further benefit from continued tamping on the same spot after closure of the voids in the treated soil mass.

Paper No. 9.03 10

Fig. 22. Diagram showing deep compaction works carried out by three different methods at the Changi Reclamation Project.

Vibroflotation ground level, leaving on completion, a column of well compacted dense material surrounded by material of enhanced Another method used for densification of hydraulically placed density. granular soil at the Changi East Reclamation project was vibroflotation. It is a technique designed to induce compaction of granular materials at depth. The basic principle behind the process is that particles of non-cohesive soils will be rearranged into denser configuration by means of horizontal vibrations induced by the depth vibrator. For non-cohesive soils with natural dry densities less than their maximum dry density, the influence of vibrations will result in a rearrangement of their grain structure. A schematic of vibroflotation technique is shown in Figure 23.

As a result of vibroflotation process, the and compressibility of the treated soil will be decreased and the angle of shearing resistance increased. The treated compacted soil is capable of sustaining higher bearing pressures compared to the untreated soil. Fig. 23 Schematic of vibroflotation technique.

The essential equipment for vibroflotation process is the vibrator, a long heavy tube enclosed with eccentric weight and Muller Resonance Compaction either electrically or hydraulically driven. The vibrator is MRC does not require water for penetration. In this method, a connected to a power source and a high-pressure water pump. steady-state vibrator is used to densify the soil. As a result of Extension tubes are added as necessary, depending on the vibratory excitation, the friction between the soil particles is treatment depth, and the whole assembly is suspended from a temporarily reduced. This facilitates rearrangement of crane of suitable capacity. With the power source and water particles, resulting in densification of the soil. A specially supply switch on, the vibrator is lowered into the ground. The designed steel probe is attached to a vibrator, which has combination of vibration and high-pressure water jetting variable operating frequencies. The frequency is adjusted to causes liquefaction of the soils surrounding the vibrator, which the resonance frequency of the soil, resulting in strongly assists in the penetration process. When the required depth is amplified ground vibrations and thereby efficient soil reached the water pressure is reduced and the vibrator pulled densification is achieved. back in short steps. With the inter-particle friction temporarily reduced, the surrounding soils then fall back below the Two main types of MRC vibrators normally used for Muller vibrator and, assisted by vibration, are rearranged into a denser Resonance Compaction are the MS-100 and MS-200 vibrators. state of configuration. This process is repeated back up to the The MS-100 vibrator has a maximum static moment of

Paper No. 9.03 11 1000Nm while the MS-200 vibrator has a maximum static the vertical drain treated sub-areas is observed to be far greater moment of 1900 Nm. The probe profile is a “wing” of double than that of the No Drain sub-area. Y-shaped flexible plates with openings. The usual shape of the probe is shown in Figure 24. The length size of the “wing” as The in-situ tests carried out in the In-Situ Test Site indicate well as the size of the opening can be varied depending upon that the degree of consolidation of the Vertical Drain Area had the soil condition. attained a degree of consolidation of about 70-80% while the Control Area had attained a degree of consolidation of only 30-40%.

In addition, deep compaction by the dynamic compaction, vibroflotation and Muller Resonance Compaction techniques are also briefly described in this paper.

m m 450 mm 0 0 7 m m 0 0 REFERENCES 7

Arulrajah, A., Nikraz, H. & Bo, M.W. [2004a]. “Factors Affecting Field Instrumentation Assessment of Marine Clay Treated With Prefabricated Vertical Drains”, and , Vol. 22, No. 5, October, pp. 415-437.

Arulrajah, A., Nikraz, H., Bo, M.W. [2004b]. In-situ testing of Fig. 24. Shape of MRC probe. Singapore marine clay at Changi, Geotechnical and Geological Engineering, Kluwer, Vol. 23, pp. 111-130. The procedure of compaction is such that the probe is inserted into the ground at a high frequency in order to reduce the soil Asaoka, A. [1978], “Observational Procedure of Settlement resistance along the shaft and the toe. Usually during Prediction”, Soil and Foundations, Vol. 18, No. 4, pp. 87-101. penetration the frequency of 23 to 25 Hz is used. When the probe reaches the required depth, the frequency is adjusted to Bo, M.W., Chu, J, Low, B. K. & Choa, V. [2003] Soil the resonance frequency of the soil layers thereby amplifying Improvement–Prefabricated Vertical Drain Techniques, the ground response. Spectral analysis was carried out at the Thomson Learning, Singapore. Changi site and the soil natural frequency was found to be about 12 Hz for uncompacted sand at Changi. Bo Myint Win, A. Arulrajah & V. Choa. [1997]. “Assessment of degree of consolidation in soil improvement project.” The MRC probe is executed in the vertical direction and the Proceedings of the International Conference on Ground vibration energy is transmitted to the surrounding soil along Improvement Techniques. , pp.71-80. the entire length of the probe. When resonance is achieved, the whole soil layer will oscillate simultaneously and this is an Bo Myint Win, A. Arulrajah & V. Choa. [1998]. important advantage compared to that of other vibratory “Instrumentation and monitoring of soil improvement work in methods. The compaction duration depends on the soil Land reclamation projects” 8th International IAEU Congress, properties and on the required degree of densification to be Balkema, Rotterdam. achieved. Normally it is required to vibrate for an average duration of two minutes per meter. Compaction is usually Chang, M.F. [1986]. The flat dilatometer and its application to carried out in a square grid pattern of two or more passes. The Singapore clays, In: Proceedings of the 4th International square grid spacing typically ranges between 3 to 5.5 meters. Seminar Field Instrumentation and In-situ Measurements, In subsequent passes the compaction is carried out in-between Nanyang Technological Institute, Singapore. the first pass compaction points. Choa, V., Bo Myint Win, Arulrajah, A., and Na, Y. M. [1997], “Overview of Densification of Granular Soil by Deep CONCLUSIONS Compaction Methods”, Proceeding of the International Conference on Ground Improvement Techniques, Macau, pp. This paper has provides a case study into the land reclamation, 131-140. ground improvement, field instrumentation, in-situ testing and deep sand compaction works that were carried out in the Sridharan, A. & Sreepada Rao, A. [1981], “Rectangular Changi East Reclamation Project. Hyperbola Fitting Method for One-dimensional Consolidation”, Geotechnical Testing Journal, Vol. 4, No. 4, The assessment of degree of consolidation is found to be in pp. 161-168. good agreement for the Asaoka, Hyperbolic and piezometer methods in the Pilot Test Site. The degree of consolidation of

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