PERTEMUAN ILMIAH TAHUNAN XIV HATTI Development of Geotechnical Engineering in Civil Works and Geo-Environment, Yogyakarta, 2-3 Desember 2010

Combination of Stone Column and Prefabricated Vertical Drain for Shore Protection Foundation

Nurharnani Hassan Technical Engineer, Menard Geosystems Sdn. Bhd. Malaysia

E-mail: nanie@menard-asia.com

Kenny Yee

Regional Director, Menard Geosystems Sdn. Bhd. Malaysia E-mail: kenny@menard-asia.com

1 INTRODUCTION

1.1 General

Over a past decade, structural support solution such as pile embankment, raft foundation, shallow founda-tion and etc. are the only option for geotechnical engi-neer to solve the problematic soil condition. The main purpose of structural support is to transfer safely the structure load into the ground and keep settlement as small as possible. However, this solution may involves higher cost and sometimes not practical to be adopt in certain project.

Due to this reason, ground improvement methods were introduced. In Malaysia, ground improvement methods can be classified into 3 categories; (i) consoli-dation method (i.e., prefabricated vertical drain & vac-uum consolidation), (ii) compaction method (i.e., dy-namic compaction & vibro compaction) and reinforcement method (i.e., stone column, dynamic re-placement & control modulus column). Every method has different application. For example, consolidation method is suitable to treat soft compressible soil while compaction method is utilized to improve bearing ca-pacity in loose granular soil only. Reinforcement

method will covers both type of soil condition (K.Yee and T.A.Ooi, 2008). Therefore, the selection of one or combination of techniques can be applied to fulfill the project requirement if the soil condition is varying much on site. Each method of ground improvement has its own merits and limitation.

ABSTRACT: Conventional structural support involved higher cost and not very practical to utilize in certain ground condition. Due to this reason, ground improvement works has been chosen as alternative solution to replace the conventional structural support solution. In most cases, it proves to be more economical and viable solution to be adopted. Application of ground improvement works can be applied to support railways and road embankments, retaining wall, industrial warehouse and commercial buildings, reclaimed platform (harbour, container terminal), sewerage plant, tank foundation and shore protection foundation. The success of the ground improvement works depends on many factors from planning, investiga-tion, analysis, design, specification of works, construction and closed supervision by design consultants. Flaws in any of the above stages would compromise the effectiveness of a ground improvement. The main function of ground improvement are (1) to reduce settlement (2) accelerate the rate of consolidation (3) increase bearing capacity (4) improve stability and (5) resist liq-uefaction. Due to certain performance criteria and site condition, combination of ground improvement methods shall be done to achieve the effectiveness of this technique. This paper presents the case study of combination of stone column (SC) and prefabricated vertical drain (PVD) for shore protection foundation at A3 Island, Danga Bay, Malaysia. In this paper it demonstrates the de-sign aspects of the combination of SC and PVD treatment as composite and consolidation treatment for the unfavorable ground condition, installation process and the verification of the performance via a comprehensive instrumentation scheme. Keyword: stone column; prefabricated vertical drain; instrumentation work

1.2 Case Study

The proposed development consists of one (1) nos. of club house and 150 nos. of luxurious bungalow houses on an area of about 18.60 hectare of an island. The location of treatment area is within Parcel A3 Is-land which located at Danga Bay, Johor, Malaysia. Figure 1 shows the current and future development in Danga Bay Island. A 4 m height of shore protection work has been constructed along the A3 Island, Danga Bay. Since this development is covering the whole part of A3 Island, the shore protection works need to be carried along the island to overcome scour, erosion and water impact problems.

For this project, the designer had proposed rubble pitching wall for the shore protection works. Figure 2 show the cross section of rubble pitching wall utilize in this project.

Combination of Stone Column and Prefabricated Vertical Drain For Shore Protection Foundation

Figure 1. Development in Danga Bay, Johor Rubble pitching wall consists of:-

0.75 m base slab of rubble wall 4 m height of rubble pitching with 6” x 9” to

be laid single layer with cement mortar (20 N/mm2)

rip rap were installed in front of rubble wall structure

The function of rip rap is to prevent rubble wall

from scouring impact of sea water. The length of the proposed shore protection works is approximately 1,680 m with width of 8 m. Total treatment area is ap-proximately 13,440 m2.

Due to unfavorable soil condition and economic reasons, the Engineer has proposed to adopt ground improvement method (stone column) as the foundation of shore protection works rather than conventional structural support (pile foundation).

Stone column and prefabricated vertical drain are the most common techniques applied in Malaysia for improvement of poor soil condition. The main function of stone column is:-

to improve bearing capacity in weaker soil increase and improve stability problem as a drainage for dissipation of excess pore wa-

ter pressure and accelerate the rate of consoli-dation

act as reinforcement ‘column’ elements with carrying higher imposed load

Figure 2. Cross section of rubble pitching wall

2 SOIL CONDITION

2.1 Overview

In the preliminary stage, soil investigation (SI) was carried out to identify the existing subsoil ground con-dition for Island A3, Danga Bay.

Seven (7) nos. of boreholes were conducted on Au-gust 2000 within the proposed treatment area. Based on the borehole result, especially under the shore protec-tion area (refer to as BH5, BH8 and BH11), the soil condition can be categorized as unfavorable subsoil ground condition. Generally, the ground condition is heterogeneous some area had critical loose sand and others had very thick soft compressible soil.

In situ pre engineering cone penetration test (CPT) and pressuremeter test (PMT) were conducted along the treatment area. A total of thirteen (13) nos. of CPT and seven (7) nos. of PMT were performed on May 2008. Figure 3 show the location of boreholes and in situ test of CPT and PMT tests conducted.

Figure 3. Location of SI tests conducted at Danga Island

2.2 Pre-Treatment Test

Results from the in-situ tests were interpreted in conjunction with the borehole available along the treatment area. After comparing borehole and in-situ tests, results of in-situ test were utilized in design stage. Based on the Cone Penetration test (CPT) and

PERTEMUAN ILMIAH TAHUNAN XIV HATTI Development of Geotechnical Engineering in Civil Works and Geo-Environment

Pressuremeter test (PMT), the treatment area can be di-vided into 2 different zones. Zone 1 covers 5 nos. of CPT test (i.e., CPT5, CPT6, CPT7, CPT8, and CPT9) and 3 nos. of PMT test (refer to PMT3, PMT4 and PMT5). Summary of the in-situ tests shows the subsoil consist mainly sand and clay. In the technical analysis, the high tide level is located about RL+1.9 m and low tide level is located at RL-1.7 m (0 m CD).

Summary of the in-situ test shows that the upper 1.5 m material is very loose sand with cone resistance value, qc of 1.5 MPa and the harmonic mean pressure-meter modulus, Ep of 12 bar. This is the filled sand platform. At the depth of 1.5 m to 4 m, the subsoil can be considered as soft to firm clay with cone resistance, qc of 0.5 MPa and the pressuremeter modulus, Ep of 15 bar. This is followed by very loose silty sand with cone resistance of 1.5 MPa at 4 m to 10 m deep before firm clay is detected at depth of 10 m to 19 m with cone re-sistance, qc of 1.3 MPa and pressuremeter modulus, Ep of 36 bar. Summary of subsoil profile for Zone 1 is il-lustrated in Figure 4.

Figure 4. Generalised subsoil profile for Zone 1 The ground condition for Zone 2 is worse than Zone

1. It can be summarized that Zone 2 area consists a very thick soft compressible soil layer. Thickness of soft soil is estimated to be about 10-13 m. A total of 8 nos. of CPT were conducted (i.e., namely CPT1, CPT2, CPT3, CPT4, CPT10, CPT11, CPT12 and CPT13) and 4 nos. of PMT (i.e., refer to PMT1, PMT2, PMT6 and PMT7).

Taking the results of all tests, the upper 2 m of sub-soil can be identified as the sand blanket (very loose sand with cone resistance, qc of 1.5 MPa) placed as working platform for ground improvement works. Sub-soil is considered as soft clay from depth of 2 m to 15 m with cone resistance value, qc of 0.25 MPa. Finally, the firm layer is detected beyond the soft clay layer with cone resistance, qc of 1.5 MPa before test stopped. Figure 5 show the summary of generalized soil profile for Zone 2.

Figure 5. Summary soil profile for Zone 2

3 SELECTION OF GROUND IMPROVEMENT

3.1 Engineer’s Design Criteria

In this project, the Engineer has proposed design criteria to be achieved under rubble wall construction. Settlement criteria for ground improvement works are listed below.

Maximum settlement from commencement of construction of retaining wall to completion shall not exceed 150 mm.

Maximum settlement post retaining wall con-struction shall not exceed 50 mm.

During the preliminary design, the first choice of ground improvement method is stone column. Unfor-tunately, the maximum settlement exceeded 150 mm. Hence, in order to achieve the Engineer’s settlement criteria, a combination stone column and prefabricated vertical drains (PVD) was adopted. However, PVD does not influences the magnitude of settlement but PVD accelerates the rate of consolidation and hence, improves the undrained shear strength Cu of soft soil.

3.2 Stone Column Method

The principle is to replace loose or cohesive soil by compacted stone, together with densification and re-duction in compressibility of surrounding ground to form a composite material. Stone column is commonly used in weak soil to increase bearing capacity, reduce settlement, accelerate the rate of consolidation, im-prove stability and resist liquefaction. It involves re-placing 15%-30% of the cohesive soil with stones in the form of columns in most applications (K.Yee, 2010). Typically, stone column diameters are 800 mm to 1100 mm. The column diameter will naturally vary with the equipment and the soil conditions. Generally, the weaker the soils the larger the diameter of the stone column.

Subject to the availability of adequate water supply, installation of stone columns can be either top feed wet method or bottom feed dry method. In top feed wet

Combination of Stone Column and Prefabricated Vertical Drain For Shore Protection Foundation

method, the vibroflot penetrates to specified depth un-der its own weight with water jets removing fines to make an annulus. With the water flow reduce, stone is introduce down the annulus and compacted in short lifts to build up the column. The process is continued to the surfaces. The complete process of wet method stone columns is illustrated in Figure 6.

Figure 6. Stone column using installation of top feed wet method (replacement method)

While in the bottom feed dry method, the vibroflot

penetrates to specified depth under self-weight with the aid of compressed air and vibration to form a ‘hole’. Stones are placed in the feed hopper and after reaching the required depth, stones are discharged and com-pacted by surging movement of the vibroflot. By the continuous addition of stones, a dense column of stones is constructed, which is tightly interlocked with the surrounding soil. Figure 7 shows the complete process stone column using bottom feed dry method.

Figure 7. Stone column installation using bottom feed dry method (displacement method)

3.3 Prefabricated Vertical Drain

Vertical drain is normally used to accelerate con-solidation by facilitating discharge of excess pore wa-ter pressure by reducing the drainage path length. The objective of using vertical drain with preloading tech-nique is to accelerate the rate of consolidation and to minimize the future settlement of the treated area under the future rubble wall load. Soil improvement works is carried out in such a way that a degree of primary con-solidation is designed to be attained within the desired time period by improving the soil drainage system

(A.Aruljah et al., 2007). Figure 8 show the installation process of PVD works.

Figure 8. Complete process of prefabricated vertical drain works

4 DESIGN ASPECT

4.1 Soil Properties

Based on the in-situ CPT and PMT tests, a summary of interpreted soil properties is shown in Table 1 below. These values are used for the settlement analysis. Fig-ure 9 shows the typical embankment geometry and rubble pitching wall which is adopted in the analysis. T able 1. Soil properties adopted in SC design Soil type γ c’ φ’ Eoed ____________ _____________ kN/m3 kPa ° kPa ______________________________________________ Embankment fill 20 5 30 20,000 Very loose sand 18 1 32 4,500 Soft sandy clay 16 4 25 1,400 Firm sandy clay 19 9 28 8,250 _____________________________________________

Figure 9. Typical embankment geometry and rubble pitching wall to be constructed on treatment area

4.2 Settlement Analysis

Settlement analysis for stone column is carried out using Priebe’s Method (1995). Further analysis was carried out using finite element method, namely Plaxis. The purpose was to recheck the settlement value of the subsoil treated with stone column. Based on the avail-able subsoil profile, stone columns of 1m diameter with 2 m center to center spacing have been proposed and designed as the supporting foundation with stone col-

PERTEMUAN ILMIAH TAHUNAN XIV HATTI Development of Geotechnical Engineering in Civil Works and Geo-Environment

umn length varies from 8 m to 15 m depends on the soil condition. Both analyses shows similar result. The total settlement of the ground treated with stone col-umn for the worse case (water level at low tide level) is estimated to be about 40 cm. In order to achieve the design criteria, prefabricated vertical drain was intro-duced.

The design for prefabricated vertical drains (PVD) is following the Barron’s solution and Carillo’s equa-tion for the vertical consolidation drains. Prefabricated vertical drains (PVD) with spacing of 1 m square grid are proposed where soft cohesive deposit is encoun-tered. The treatment length for PVD varies from 12 m to 15 m. The PVDs are installed to speed up the con-solidation process of the cohesive deposit. In order to reduce the magnitude of settlement during rubble pitching wall construction and post rubble pitching wall construction, a surcharge of 1 m was proposed. The target degree of consolidation for PVD at 1 m spacing is U = 90%. Based on 90% of average degree of consolidation and 1 m surcharge over a 5-month consolidation period, the induced settlement is about 36 cm. This will leave behind a residual settlement of about 4 cm. Figure 10 and Figure 11 show the cross section and plan view of combination of stone column and prefabricated vertical drain method.

Since presence of soft clay is quite thick which is about 13 m depth, PVD is really necessary to speed up the consolidation process.

Figure 10. Cross section of combination SC & PVD

Figure 11. Plan view of combination SC & PVD

4.3 Slope Stability Analysis

Stability of the rubble pitching wall was analysed using Plaxis.

The soil properties adopted in the slope stability analysis using Plaxis are tabulated in Table 2.

Table 2: Soil properties used in Plaxis Soil type γ c’ φ’ Eoed ____________ _____________ kN/m kPa ° kPa ______________________________________________

3

Embankment fill 20 5 30 20,000 Very loose sand 18 1 32 4,500 Soft sandy clay 16 4 25 1,400 Firm sandy clay 19 9 28 8,250 Concrete base 20 - - 39,000,000 Rubble pitching 22 1 38 250,000 Rip rap 22 1 38 60,000 Stone Column 20 10.5 38.5 16,530_____________________________________________

Based on the analysis with 1 m surcharge but without PVD, the calculated factor of safety (FOS) for short term and long term for the rubble pitching wall is ap-proximately 1.40 and 1.70 respectively as shown in Figure 12(a) and (b). Hence, satisfying the requirement of FOS = 1.25 for short term and 1.50 for long term.

Figure 12 (a). FOS short term for SC treatment

Figure 12 (b). FOS long term for SC treatment

5 SITE IMPLEMENTATION

5.1 Method of Construction

A total of 41,300 li.m of stone column and 165,200 lin.m of PVD were installed at Danga Bay Island. The stone column were installed to varying depth from 8 m to 15 m depending on the soil profile using top feed wet method.

The basic tool is a depth vibrator. It consists of an eccentric weight of steel tubular casing, driven by an electric / hydraulic motor. Depth vibrators range in weight from 10 kN to 15 kN (see Figure 13).

Combination of Stone Column and Prefabricated Vertical Drain For Shore Protection Foundation

Figure 13. Vibroflot for vibro replacement method In this method, water jetting is used to assist pene-

tration and stone is fed from the top of the vibrator. A 60 ton crawler crane is used to support the assembly and penetration to the required depth is assisted by the combined action of vibrations and high pressure water jets placed at the tip vibrator. After reaching the re-quired depth, the water jetting is reduced until a small upward flow around the annulus of the vibroflot is ob-served. At this point, a small quantity of stone aggre-gate (normally 25 to 75 mm) is introduced using exca-vator and the vibroflot is lifted and lowered until the stone charge is fully compacted at the tip of the vibro-flot. The action is repeated in 0.5 m - 1.0 m lifts until the stone column is formed to the surface. Compacted stone columns are constructed to effect stabilization of the treated ground.

Figure 14. Installation SC during high tide level

Figure 15. Installation SC during low tide level

Figure 16. PVD installation during low tide level

Figure 17. After completion rubble pitching wall

6 QUALITY CONTROL

6.1 General

In order to obtain a good quality stone column, the following need to be considered:

Grading of durable stone aggregates Verification test (plate load test) Termination criteria of stone column installa-

tion

6.2 Termination depth using Automated Data Logger

The depth of stone column installed is measured us-ing an automated data logger system (as shown in 18). This system is capable of providing exact depth of penetration (i.e., length of stone column) and vibroflot power consumption during penetration and compac-tion.

PERTEMUAN ILMIAH TAHUNAN XIV HATTI Development of Geotechnical Engineering in Civil Works and Geo-Environment

Figure 18. Automated data logger of SC rig

6.3 Post-Treatment Test

As part of the quality control plate load tests on group of stone columns were carried out. The purpose of plate load test is to measure the settlement and de-termine the allowable bearing capacity on stone col-umn.

Group plate load test consists of 4 nos. of stone col-umns. The maximum test load per group of columns is 320 tons. Figure 19 shows the plate load test conducted on a 4m x 4m square RC slab. Thickness of the slab is 250 mm. Based on plate load test result under maxi-mum load of 320 tons, the settlement is 9.7 mm (refer Figure 20).

Figure 19. Plate load test conducted on a group of 4 columns

Figure 20. Load –Settlement curve for group test

7 INSTRUMENTATION AND MONITORING

7.1 Settlement Marker

Instrumentation and monitoring works are carried out to monitor the performance of the ground im-provement works carried out.

Settlement markers were introduced to monitor the settlement after completion of rubble pitching wall. Settlement monitoring was conducted over a period of 8 months. A total of 42 nos. of settlement markers were installed along the shore protection work. Summary of result from monitoring works shows that the measured maximum settlement after construction is about 30 mm as compared with the design criteria of 50 mm. Aver-age rate of settlement is about 0.2 mm/day. Typical monitoring results for both zones are shown in Figure 21 (a) and (b).

Figure 21 (a). Typical settlement marker result at Zone 1

Figure 21 (b). Typical settlement marker results at Zone 2

-50

5

0

5

0

5

0

5

0

0

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300Day

Settl

emen

t (m

m)

-4

-4

-3

-3

-2

-2

-1

-1

-5

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0-0.8

-0.6

-0.4

-0.2

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300Day

Rat

e of

Set

tlem

ent (

mm

/day

)

0.0

-50

5

0

5

0

5

0

5

0

0

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300Day

Settl

emen

t (m

m)

-4

-4

-3

-3

-2

-2

-1

-1

-5

-1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

0

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300Day

Rat

e of

Set

tlem

ent (

mm

/day

)

-0

-0

-0

-0

-0-0

-0

-0

-0

0.

Combination of Stone Column and Prefabricated Vertical Drain For Shore Protection Foundation

8 CONCLUSION

A combined ground improvement works using stone columns and prefabricated vertical drains has been used to treat the problematic soil at A3 Island, Danga Bay. From the monitoring results the measured settlement for rubble pitching wall after construction meets the design criteria. The slope stability analysis also shows that the factors of safety for both short and long terms are within the allowable limit.

In this project, a combination of stone column and prefabricated vertical drains has proven to be more cost effective and cheaper than the conventional piling solu-tion. It eliminates the need of a continuous RC slab or individual pile cap, cutting and lengthening of pile, re-inforcement for pile cap and other related works to a structural solution. The construction of the rubble pitching wall commenced immediately after comple-tion of the ground improvement works without any stage construction.

9 PREFERENCES

− Priebe, H.J.(1995). Design of Vibro Replacement. Ground Engineering, December 1995. p.31-37

− Yee.K & Ooi T.A. (2008). Sustainability Ground Improvement for Housing, Infrastructure and Utili-ties Developments in Malaysia – From 1978 to 2006. Seminar on Soil Improvement Techniques for Highways Road Engineering Association of Malay-sia, Kulala Lumpur, August.

- Arulrajah A., Hashim. R, Bo. M.W and Nikraj H. (2007). Prefabricated Vertical Drain Design for a land Reclamation Project. 16th South-East Asian Geotechnical Conference, Kuala Lumpur, May. p.529 – 532

- Yee. K & Ooi T.A. (2010). A Green Technology to-

wards a Sustainable Housing, Infrastructure and Utilities Development in Malaysia. Geotechnical Engineering Journal of the SEAGS & AGSSEA Vol 41 No.3 ISSN 0046-5828, September.