Ground Improvement Techniques

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GSM-IEM Oktoberforum 2006 on Engineering Geology and Geotechnical Engineering, 31st Oct. 2006, Petaling Jaya.

PERFORMANCE OF A HIGH REINFORCED SOIL WALL-SUPPORTED ON VIBRO STONE COLUMNS

Hari Krishna Yandamuri, Sr. Geotechnical Engineer,

Ir. Yee Yew Weng, General Manager

Keller (M) Sdn. Bhd.Kuala Lumpur, Malaysia (ywyee@keller.com.my)

ABSTRACT: This paper will describe a case history in Kuala Lumpur where high reinforced soil walls up to 13m height were supported on Vibro Stone Column treated ground. The performance of the treated ground to support high reinforced soil walls is evaluated based on long-term instrumentation monitoring for more than 3 years, both during construction of embankment and operational stages.

1.0 INTRODUCTION

The New Pantai Expressway (NPE) is one of the modern completed highways in Kuala Lumpur with dual three-lane carriageway which was opened to the traffic in April 2004, on a Build, Operate and Transfer (BOT) basis. The tolled expressway covers a total stretch of approximately 20km; starting from Subang Jaya to Kampung Pasir Dalam (Pantai Dalam Interchange) and diverts from there to Jalan Bangsar and Jalan Ku-chai Lama in two separate directions. The overall plan layout of NPE is shown in Figure 1.

Figure 1: Overall Plan Layout of New Pantai Expressway (Courtesy of New Pantai Expressway Sdn. Bhd.)

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The expressway forms the main interchange at Kampung Pasir Dalam (referred as Pantai Dalam Inter-change) to connect three distinct routes to Subang Jaya, Jalan Bangsar and Jalan Kuchai Lama. The inter-change comprises of Alignment A, B and C; Ramps A, B, C and D and toll plaza. The bridge (BR 13) over the toll plaza connects to Ramp B, Ramp D and Ramp C on one side and Alignment C and Ramp A on other side which subsequently crosses over the existing KTM railway track (BR 14). Due to site constraints at the interchange, high reinforced soil walls were constructed to form the bridge approaches and other ramps to the required design heights (maximum up to 13m). The following Figure 2 represents the detailed plan layout of Pantai Dalam Interchange including instrumentation monitoring scheme.

Figure 2: Plan Layout of Pantai Dalam Interchange

Ground improvement using Vibro Replacement (Vibro Stone Columns) technique has been used since 1950s to support many infrastructure facilities worldwide. Its performance is proven in many types of soil and structures (ref. Raju V.R. et. al., 2004a and Raju V.R. et. al., 2004b). Increasingly, stone columns are used to support reinforced soil walls. The combination has proven economy and has intrinsic technical advantages, i.e. the stone columns ensures relatively quick consolidation as the embankment is built; while the wall is constructed in stages (lifts) with the wall panels placed progressively and adjusted for any movement.

The long-term performance of the treated ground to support high reinforced soil walls at Kampung Pasir Dalam (Pantai Dalam Interchange) is evaluated based on the results of instrumentation monitoring (see Figure 2 for locations of instrumentation) for more than 3 years, both during construction of embankment and operational stages.

2.0 SUBSOIL CONDITIONS

The subsoil conditions at Pantai Dalam Interchange varied from very soft silts to soft sandy silts down to a depth between 5m and 12m followed by hard sandy silts. Typical plots showing results of cone penetration tests are shown in Figure 3a and Figure 3b.

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Figure 3a: Result of Typical Cone Penetration Test at Ramp A

Figure 3b: Result of Typical Cone Penetration Test at Ramp C

3.0 GROUND IMPROVEMENT SCHEME

3.1 Design

The design analyses of Vibro Stone Columns were carried out according to Priebes design methodology (ref. Priebe H.J., 1995) by the project consultants. The ground improvement scheme is designed to treat very soft to soft sandy silt deposits. The following summarises the scheme of ground improvement and typical results of design analyses:

Diameter of columns =1.0m Spacing of columns = 1.5m to 1.8m c/c under reinforced soil walls and 2.0m to 2.3m c/c under

earth fill embankments Area replacement ratio =15% to 35% Estimated settlement =about 200mm FOS against stability =above 1.4 Predicted load factor =about 0.5 (load on column area / load on grid area)

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3.2 Execution Ground improvement using 1.0m diameter Vibro Stone Columns were successfully installed to a depth between 5m and 12m to treat very soft silts and soft sandy silt deposits. The following Figure 4a shows the site conditions during installation of Vibro Stone Columns, whereas Figure 4b shows completed reinforced soil wall (about 13m high) at the same location.

Figure 4a: During Construction Figure 4b: After Completion

In total, an area of approximately 23,000m2 was treated (with proper quality control measures to ensure design diameter and compaction effort throughout the construction process) using custom-built Vibro Re-placement rigs. The installation works were successfully carried out even adjacent to existing dwellings and very close to the constructed bridge abutments (see Figure 5).

Vibration monitoring was carried for such locations and the measured vibration levels in terms of peak particle velocity were less than 20mm/s even when Kellers Mono vibrator was working 1.0m away from the monitoring point (see Figure 6). The British and Australian standards (BS5228 Part 4 and AS2187) ac-cept vibration levels between 20mm/s and 50mm/s for normal structurally sound structures.

Figure 5: Installation of Vibro Stone Columns using Dry Bottom Feed Method next to Constructed Bridge Abutment and Completed Bridge Approach

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Figure 6: Vibration Monitoring during Installation Works using Kellers Mono Vibrator

The following Figure 7 shows successfully completed high reinforced soil walls up to a maximum height of 13m forming bridge approaches which were founded on Vibro Stone Column treated ground.

Figure 7: Completed High Reinforced Soil Walls (up to13m height)

4.0 INSTRUMENTATION MONITORING SCHEME

After completion of ground improvement works using Vibro Stone Columns in June 2003, construction of reinforced soil walls and embankments were commenced. The Figure 2 shows the plan layout of instru-mentation monitoring scheme comprising of settlement markers, inclinometers and pressure cells which were monitored for more than 3 year period to assess the performance of treated ground, both during construction of the embankment and operational stages.

4.1 Results of Settlement Measurements

The data of settlement measurements for more than 3 years showed that the Vibro Stone Columns has provided effective drainage paths to dissipate excess pore water pressures under the newly placed fill loads by means of radial and vertical consolidation processes. The time rate of consolidation was also relatively quick; 90% degree of consolidation was achieved within construction period of embankment itself. The embankments and reinforced soil walls were constructed with a rate of filling of about 1m per week and the highest sections (about 13m high) were completed in about 90 days period. Most of the predicted settlements occurred during the construction period (see Figure 8) leaving minimal residual settlements for the post construction stage. The treated ground settled to a maximum of about 100mm only even under 13m high reinforced soil wall.

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Figure 8: Summary of Results of Settlement Markers The magnitude of measured settlement was lower than the estimated settlement of about 200mm in the design analyses. The difference between the estimated and measured settlements may be attributed to the significant improvement of soil between the columns during installation works, which was not considered in the design analyses. The settlement measurements during operational stage (for more than 3 year period) clearly demonstrated that the stabilised trends are achieved without further secondary consolidation / creep settlements (see Figure 8) and well within the requirements of Malaysian Highway Authority (MHA).

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4.2 Results of Inclinometer Measurements

Inclinometers were installed at highest reinforced soil wall locations to monitor lateral displacements below original ground level, both during construction of the wall and post construction stages. Figure 9 below shows the results of inclinometer measurements at two different locations (near bridge approaches), the results of which indicated less than 30mm lateral displacement (i.e. less than 1/3rd of vertical displacement):

Figure 9: Results of Inclinometers (Lateral Displacements)

4.3 Results of Pressure Cell Measurements

Following the ground improvement works, pressure cells (2 nos.) were installed on top of the treated ground near one of the bridge approach location (BR 13) before embankment filling works (see Figure 2). One pressure cell was installed at the centre of the column (PC-1) and another one at the centre of the grid (i.e. on soil, PC-2) as shown in the following Figure 10.

Figure 10: Schematic Showing Locations of Pressure Cells

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The results of pressure cell measurements indicated a maximum column pressure of 310kPa (PC-1) and maximum soil pressure of 230kPa (PC-2) under 11.5m high embankment fill load as shown in Figure 11. This clearly demonstrates the load sharing between columns and surrounding soil as expected in sandy silt soils.

Figure 11: Results of Pressure Cells

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5.0 CONCLUSIONS

In Malaysia, ground improvement using Vibro Replacement (Vibro Stone Columns) technique has been util-ised extensively in several infrastructure projects, especially in highway sector. The past case histories with proven performance and advanced custom-built equipment enabled the technique to support high rein-forced soil walls up to 13m height. The technique also provided effective drainage paths to accelerate the consolidation process.

The results of instrumentation monitoring scheme for more than 3 year period has demonstrated that high reinforced soils walls performed with adequate stability and tolerable settlement limits, both during con-struction and operational stages. The treated ground vertically settled less than 100mm and the lateral dis-placement was less than 30mm. The stabilised trends of settlement data for the past 3 years without fur-ther secondary consolidation / creep settlements confirms good long-term performance of the treated soil.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the management and staff of Road Builder (M) Sdn. Bhd., Perunding ZKR Sdn. Bhd. and Maunsell Sharma & Zakaria Sdn. Bhd. for their valuable contribution in the implementation of the ground improvement works. The authors also wish to acknowledge colleagues in Keller: Mr. Saw Hong Seik and Mr. P. Sreenivas for their contribution in design and execution, respectively.

REFERENCES

Australian Standard: AS2187-1993, Explosive Code.

British Standard: BS 5228-4:1992, Noise and Vibration Control on Construction and Open Sites, Code of Practice for Noise and Vibration Control applicable to Piling Operations.

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

Raju, V.R., Wegner, R. and Hari Krishna, Y. (2004a), Ground Improvement Using Vibro th

Replacement in Asia 1994 to 2004 -A 10 Year Review, 5International Conference on Ground Improve-ment Techniques, Kuala Lumpur, March 2004.

Raju, V.R., Yee, Y.W., Tam, E. and Sreenivas, P. (2004b), Vibro Replacement for the Construction of a 15m High Highway Embankment over a Mining Pond, Malaysian Geotechnical Conference, Kuala Lumpur, 2004.