a case history of coastal land reclamation project

7/27/2019 A Case History of Coastal Land Reclamation Project

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GSM-IEM FORUM ON ENGINEERING GEOLOGY & GEOTECHNICS IN COASTAL DEVELOPMENT, 23rd OCTOBER 2002, BANGUNAN IEM, P.J.

A case history of a coastal land reclamation project

C.S. Chen & S.M. TanSSP Geotechnics Sdn Bhd, Malaysia

ABSTRACT: Rapid industrial and commercial expansion in recent years have created the needfor more land. One of the options to create more land is to reclaim coastal land. This paperpresents a case history of a coastal land reclamation project where the site was partly on landfilland partly of soft tidal land. The landfill consists of variety of materials inclusive of domesticrefuse, construction debris, organic substance etc. Subsoil at the site mainly composed of verysoft clay layer overlying firm silty clay or medium dense silty sand layers. Hard or very dense soillayer was encountered at 40 to 50m below the seabed. Potential problem of long termconsolidation settlement of the soft compressible soil was expected. Biodegradation of the landfillresulting unexpected ground settlement was also a concern. Ground treatments were carried out.Surcharge method with and without vertical drains were used to treat the soft clay layerdepending on time available for treatment. Dynamic compaction method was adopted to treat thelandfill. Geotechnical instruments were installed to monitor the subsoil behavior. Settlementmonitoring results are presented in this paper.

1 INTRODUCTIONThe blooming development in recent years has created the need of more land especially landnears to the developed areas. As most of the developed areas are located near to the coastline, oneof the options to create more land is to reclaim coastal areas. However, the subsoil along coastlines mostly compose of soft silty and clayey soils. From the engineering point of view, suchlands generally are not suitable sites for civil construction. Reclamation of coastal land usuallycomes along with problems such as instability of the reclaimed platform and long term excessivesettlement.

A development was planned at a site of about 14.5 hectares along the coastline in Pulau Pinang.The site was mostly tidal land below seawater level during high tide and partly on a landfillground. It was believed that the whole site originally was submersed but later because of humanactivities, part of it was gradually filled up by rubbish and becoming a landfill site. The landfillhad not been properly controlled and was believed had been in place for more than 15 years.Figure 1 shows the plan view of the proposed site. About 60% to 70% of the site are submersed

with ground levels generally vary from Reduced Level (RL) 0.5m to RL -0.5m. Landfill areasoccupied about 30% to 40% of the site area with ground levels generally higher than RL1.0m.Figure 2 shows the existing site. To reclaim this piece of land, it is necessary to fill up the tidalland into a platform higher than seawater levels at all time. For area that had been raised up bylandfill, it should be trimmed down and covered with soil to form a platform.

Site investigation consisted of boreholes, piezocones and trial pits was carried out prior to thereclamation work. The subsoil profiles and the engineering properties are presented. Potentialproblems associated with the reclamation work are discussed. Ground treatment to overcome theanticipated settlement of soft ground is presented. Areas with landfill was treated by usingdynamic compaction. Geotechnical instruments were installed for the monitoring of the subsoilbehavior. At the time of preparing this paper, monitoring works are still going on at site. The up-to-date monitoring results are presented.

2 SUBSOIL CONDITIONSPrior to the reclamation work, soil investigation was carried out to gather subsoil information andengineering properties. Boreholes were sunk at both tidal land and landfill areas whereaspiezocones were mainly carried out at the tidal land. Trial pits were carried out at landfill area sothat more continuous visual information on the landfill materials can be obtained. Figures 3 and 4shows typical subsoil information from boreholes and piezocones carried out at the tidal landzone. Typical subsoil profiles are as follows:


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Figure 1. Existing site topography

Figure 2. Existing site conditions before reclamation

Landfill

The landfill was only found at localized area with thickness varied from 3m to 6m in general. Itwas believed that this landfill had been at site for more than 15 years. From the site investigation,

the landfill consists of variety of materials inclusive of domestic refuse, construction waste,

organic substance etc. The landfill had not been proper controlled and as the results it posed

highly heterogeneous characteristic.

Soft clay layer

Soft silty clay was found at the tidal land as well as below the landfill area. The average thickness

is about 5m. Liquid limit and plastic index of the soft clay were in the ranges of 50% to 80% and

35% to 45% respectively. Figure 5 shows the properties of the soft silty clay.


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Medium stiff layerUnderlying the soft silty clay is a thick layer of medium to stiff silty clay and medium dense silty

sand layers.

Hard layer and bedrock

Hard or very dense soil layer could only be encountered at about 45m to 55m below the existing

ground level. Granite bedrock were encountered in some boreholes at depth of 50m to 65m.

Figure 3 Typical subsoil profile from boreholes at tidal land

Figure 4 Typical results of piezocones carried out at tidal land

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Figure 5 Physical properties of the silty clay layer

Summary of the subsoil profile is as shown in Table 1.

Table 1 Typical subsoil profileReduced Level (m) Soil Description SPT-N valuesAbove 0m Heterogeneous landfill 0 to 300m to 5m Very soft to soft silty clay 0 to 4-5m to 50m Medium stiff or medium dense

silty clay or silty sand6 to 30

Below 50m Very dense or hard soil layer >50

3

THE POTENTIAL PROBLEMS OF THE RECLAMATION

3.1 The reclamation workTo develop the site, the tidal land will need to be reclaimed to a higher platform level. After studyof the tidal conditions at site, platform level of RL 3.1m was designed. According to thedevelopment schedule, half of the site (section A as shown in Figure 6) will be developed soonafter the reclamation. However, the development of the other parts of the reclaimed land (SectionB) had not been decided and therefore time will not be a concern for this part. In addition, thereare some existing houses in Section B, the reclamation can only be carried out at areas withouthouses. However, materials required for the reclamation of the housing area will be stockpiled inSection B for future use. Figure 6 shows the layout of the reclamation work.

3.2 Potential problemsThe soft clay layer at site has low shear strength and high compressibility characteristic.Additional loading would be imposed to this soft clay layer due to the reclamation work. Twomajor potential problems were anticipated. These problems were (1) stability and (2) long termsettlement of the reclaimed platform.

Stability of the reclaimed platformStability analysis were carried out and it was found that with a gentle reclaimed platform sideslope and proper control on the backfilling rate, the stability of the reclaimed platform will beunder control and this should not be a major concern. The platform will become more stable inthe long term as the subsoil especially the soft clay layer gains strength with time.

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GSM-IEM FORUM ON ENGINEERING GEOLOGY & GEOTECHNICS IN COASTAL DEVELOPMENT, 23 rd OCTOBER 2002, BANGUNAN IEM, P.J.

Figure 6 Reclamation layout plan

Settlement of the reclaimed platformSettlement analysis indicated that long term settlement will be excessive due to the highlycompressible soft silty clay underneath the reclaimed platform. The settlement will take long timeto complete. In addition, the highly heterogeneous landfill at site may also cause unexpectedsettlement when subjected to fill load which causes the collapse of large voids within the landfill.The potential biodegradation process of some organic materials may also contribute groundsettlement.

4 DESIGN OF GROUND TREATMENTExcessive long term settlement was the major concern for the reclamation work. In order tominimize the long term settlement, ground treatment would be required. There were two types ofproblematic soils at site namely the soft clay layer and the landfill. Various ground treatmentmethods had been assessed and it was decided to use surcharge method for the soft soil treatmentwhile the landfill will be treated using dynamic compaction method.

4.1 Surcharge methodSurcharge method is one of the oldest and efficient methods for the treatment of compressiblesubsoil. The use of this method became popular in 1940s (Johnson, 1970). In 1949, US Corps ofEngineers had successfully eliminated about 700 to 800mm settlement for a hydraulic structureusing surcharge method. Since then, many successful cases had been reported. The basic principleof the surcharge method is simple as illustrated in Figure 7. The permanent loading from thereclaimed platform will cause the consolidation settlement of soft compressible soil. With theapplication of surcharge, more consolidation settlement will occur at any given time. Thesurcharge can be removed when sufficient settlement has achieved.

Earth fills are the most commonly used as surcharge. Other alternatives such as lowering theground water level and vacuum method to increase the effective stress of subsoil had also beenused when the stability of the platform is of concern. For this reclamation work, earth fillsurcharge was adopted due to its cost effective compared with other methods

Surcharge method is effective when sufficient time is available for the treatment of compressiblesoil. When the soft soil is thick and required surcharge time is not available, the consolidationprocess can be accelerated by introducing vertical drains. Vertical drains are generally installed at1.2m to 4m spacing. With the help of the vertical drains, water within the soft layer not onlyflows vertically but also flows horizontally to the vertical drains. Thus the drainage path issignificantly shortened. In addition, the horizontal permeability in most soil is usually greater


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than the vertical permeability. With shorten drainage path and higher horizontal permeability, theconsolidation process can be accelerated tremendously.

Sand drains and prefrabricated band shape drains are common types of vertical drains. Theformer was mainly employed until early 1970s. The latter became more popular after 1970s dueto economic reason and is most widely used today. For this reclamation work, prefrabricatedvertical drain was adopted.

Figure 7 Illustration of surcharge method4.2 Design of soft soil treatmentPreliminary estimation of platform settlement was based on Terzaghis one dimensionalconsolidation theory. The estimated settlement was about 700mm to 800mm and it might takeabout 3 years to achieve 90% of the settlement. In order to suit the development program wherethe construction at Section A was targeted to commence one and a half years from thecommencement of reclamation work, it was decided to expedite the settlement. It was decided toadopt surcharge method and expedite the consolidation settlement with vertical drain. Thesurcharge height should be designed to compensate the anticipated settlement so that removal ofsurplus material can be minimized. Based on these requirements, analysis was carried out and itwas decided to use 1m height surcharge with vertical drains of 2m grid spacing. Most of theprimary consolidation settlement due to the permanent load is expected to be completed withinthe scheduled time frame.

For Section B, as the time for development had not been decided, ground treatment will not benecessary. However, as there will be surplus fill material for future reclamation at the housingarea in Section B, this surplus material was utilized as surcharge in Section B.

4.3 Dynamic compactionDynamic compaction (DC) method is a method to improve weak soil by repeated dropping of aheavy mass from certain height onto the soil. Usually dropping points are in a pre-determinedgrid pattern. The loose or weak soil becomes dense after subjected to the high energy impacts.The voids in soil are reduced significantly and this will minimize the potential excessivesettlement as well as differential settlement in the future.

Time

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Loading

P e r m a n e n t L o a d

Pe r ma ne n t + Su r c ha r ge Loa d

Se t t le me n t o f Pe r m a ne n t Loa d

Se t t le me n t o f Pe r ma ne n t + S u r c ha r ge Loa d

Re m ova l o f Su r c ha r ge


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The landfill at site poses many uncertainties due to its highly heterogeneous characteristics. Thebest option to treat the landfill is to remove all landfill from site and replaced with suitablematerial. However, this option was ruled out as the local authority did not allow removal of anylandfill from the site. Dynamic compaction method which had been proven suitable for treatmentof landfill (Varaksin et. al. 1994) was adopted. The main purpose of dynamic compaction is todensify the landfill and to improve the bearing capacity thereby decreasing potential settlement infuture. As the planing of future development had not been finalized, densification of the entirelandfill area is needed. Preliminary design was carried out for the determination of pounderweight, drop height and grid spacing. A trial dynamic compaction test was performed prior to thecommencement of work to verify the preliminary design and to select the most suitable designparameters for the dynamic compaction. Table 1 shows the details of trial dynamic compactioncarried out at site.

Table 1 Details of trial dynamic compaction

Trial No. 1 Trial No. 2 Trial No. 3 Trial No. 4

Grid spacing 5m x 5m 5m x 5m 5.5m x 5.5m 5.5m x 5.5m

No. of Blows 8 12 8 12

Energy (ton.m/m2) 96 144 80 120

Ave. print volume 12.2 m3 16.0m3 9.2m3 12.7m3

Note: Pounder weight is 15 ton and drop height is 20m

In-situ tests inclusive of Standard Penetration Test (SPT) and Pressuremeter Test (PMT) werecarried out before and after the trial dynamic compaction. Grid spacing of 5.5m was selected afterreviewing the results of in-situ tests. In addition, heave and penetration tests were also performedto evaluate the most suitable numbers of blows. It was found that the volume of penetrationincreased with the numbers of blows and reached an obvious stabilization from about 9 to 11blows. It was then decided to carry out the dynamic compaction with 5.5m grid spacing and 10blows per print for the entire landfill area.

5 THE RECLAMATION WORKThe reclamation work commenced in January 2002 with placement of an initial layer of fill overthe soft clay subsoil at Section A to form a working platform. The thickness of this workingplatform varied from about 1m to 2m. Vertical drains were installed from this platform by a staticcable-pulled rig. The vertical drain was threaded through the mast into the mandrel and pushed tothe designed depth and secured at the bottom by a disposable anchor plate. The anchor plate alsoserved to prevent soil from entering and clogging the mandrel while pushing down into thesubsoil. After full insertion to the required depth, the mandrel was withdrawn leaving the anchorplate and the vertical drain in place. The drain was cut off approximately 10cm to 30cm above theworking platform. Backfilling of the platform continues after installation of vertical drains. Figure8 shows the installation of vertical drains in progress.

Dynamic compaction was carried out simultaneously at landfill areas in Section B. The landfillwas trimmed and covered by 1m thick sand blanket to create a working platform. Dynamiccompaction was then carried out. The craters formed were filled up by sand. At Section A,dynamic compaction began after the completion of vertical drains installation. Figure 9 shows thedynamic compaction work is in progress.

6 GEOTECHNICAL INSTRUMENTSDue to the inherent uncertainty of subsoil, it is very difficult to estimate precisely the magnitudeof settlement and the rate of settlement of the reclaimed platform especially when only limitednumbers of boreholes were made and these boreholes were distributed over large area.Monitoring of the subsoil performance during construction is essential. Geotechnical instrumentsinclusive of inclinometers, piezometers and settlement markers were installed during the processof reclamation to monitor the subsoil performance. Four numbers of inclinometers were installedat the edge of the platform slope. Six numbers of pneumetic type Piezometers were installed atdifferent levels mainly in the clayey soil layer. Total 31 numbers of settlement markers wereinstalled immediately after the platform had been formed before placing of additional fill.


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Figure 8 Installation of vertical drain

Figure 9 Dynamic compaction work in progress


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7 SETTLEMENT MONITORING RESULTS AND DISCUSSIONTypical records of fill height and monitored settlement at Sections A and B are shown in Figures10 to 13. At Section A, the fill height reached the designed platform level (inclusive ofsurcharge) of RL 4.0m in May 2002. The monitored settlements as of end of August 2002, about3 months surcharge period, are about 250mm to 500mm. At Section B, the fill height reached thedesigned level (inclusive of stockpiled material as surcharge) in July 2002. The measuredsettlements are about 400mm to 500mm.

Based on the measured field data, back analysis to verify the design assumptions and to predictthe total primary consolidation settlement can be carried out. Although back analysis alwaysinvolves a number of simplifying assumptions which may compromise the reliability of thecompute values, Asaoka (1978) had applied his back analysis method successfully based on fieldsettlement observation using simple graphical procedures. Figure 14 shows the estimation of totalconsolidation settlement using Asaokas method. The predicted final settlement is about 500mmto 750mm.

8 CONCLUSIONSA coastal land reclamation work was commenced in January 2002. The site was on very softsubsoil and partially on a landfill area. Reclamation work was completed in July 2002 as shownin Figure 15. Surcharge method with and without vertical drains was adopted at Section A and B

respectively. Dynamic compaction was carried out to densify the landfill thus minimized thepotential settlement in future. Geotechnical instrument were installed to monitor the performanceof subsoil. Back analysis based on settlement monitoring results predicted that the final settlementis in the ranges of 500mm to 750mm.

Figure 10 Settlement monitoring result at Section A RSG No. 3

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Figure 11 Settlement monitoring result at Section A RSG No.13

Figure 12 Settlement monitoring result at Section B RSG No.19

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Figure 13 Settlement monitoring result at Section B RSG No.20

Figure 14 Back analysis of monitoring results using Asaokas method

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Figure 15 Completion of the reclamation work

9 REFERENCEAsaoka, A. 1978. Observational procedure of settlement prediction. Soils and Foundation, No.4Hansbo, S 1979. Consolidation of clay by band-shaped prefrabricated drains. Ground

Engineering, July 1979.Johnson, S.J. 1970. Precompression for improving foundation soils. Journal of Soil Mechanics

and Foundations Division, ASCE, Vol.96, SM1, pp111-144Menard, L. and Broise, Y. 1975. Theoretical and practical aspects of dynamic consolidation.

Geotechnique 25, No. 1, pp. 3-18.Varaksin, S., Liausu, P., Berger, P. and Spaulding, C. 1994. Optimisation of dynamic

consolidation and dynamic replacement pillars to limit surface deformations of man made fillsoverlaying heterogeneous soft subsoil. Ground Improvement Method, Proceedings of theSeminar by geotechnical division of Hong Kong Institute of Engineers, May 1994.

a case history of coastal land reclamation project

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