QUALITY MANAGEMENT ON PREFABRICATED VERTICAL DRAIN WORKS IN LAND RECLAMATION PROJECT Bo Myint Win 1, J. Chu2 and V. Choa3 ABSTRACT For the Changi East Reclamation Projects in Singapore, about 140 million metres of prefabricated vertical drain (PVD) are being installed. Due to the requirement of large quantity of materials, several types of prefabricated vertical drains are being used. Quality control system was established to check on the quality of vertical drains. The methods and procedure of quality control tests of both hydraulic and mechanical parameters of prefabricated vertical drain are described in this paper. The tests results and the factors affecting the resultant parameters of PVD are discussed. INTRODUCTION

The Changi land reclamation projects in Singapore cover more than 2000 hectares area and are being carried out in four phases over 7 years. Several types and more than 140 million metres of PVD are being used for soil improvement works. These include Colbond CX 1000, Mebra MD7007 produced in Holland , Korea and Malaysia and Flexi FD767 drains. As different types and huge quantity of drains are being used, quality control of the drain materials and installation procedures become an important task. To serve this purpose, requirements for the specific properties of vertical drains are specified and a quality control system has been established to check the properties of the drains and to supervise the construction process. For vertical drains, both the hydraulic and mechanical properties were checked before installation. Laboratory vertical drain testing devices were developed to measure the discharge capacity of the drains, the permeability of the filter, the apparent opening size (AOS) of the filter, and the tensile strength of both the drain and the filter. The hydraulic properties affect the performance of PVD whereas the tensile strengths of both the drain and the filter are required to ensure the integrity of the drain after installation. The details of the quality control tests are described and the hydraulic and mechanical properties of different PVD are compared and discussed in this paper. PROCEDURE OF QUALITY CONTROL To control the quality, first supplied material has to meet required specification. In addition to that consistency of parameters of supplied material is also essential. In the following chapter the quality control system for each parameter will be explained and discussed on the test results. Generally drains are delivered in 200 to 300 metres length of roll. One time delivery may be about one million metre length of drains which could be delivered in ten containers. Therefore specification called for full scale testing of drain material by third party accredited laboratory for every one million metre length of drains which generally from same batch of manufacturing process. One research laboratory was also set up to check on the discharge capacity on every 100,000 meter length of vertical drain to be used for installation as well as to carry out the other tests such as AOS, permeability etc. and special test under various conditions. ___________________________ Executive Geotechnical Engineer, # 08-00, PSA Building, 460 Alexandra Road, Singapore 119963 Assistant Professor, School of Civil & Structural Engineering, Nanyang Technological University, Blk. 1, 1A-29, Nanyang Avenue, Singapore 639798 Professor and Dean of Students, Nanyang Technological University, Office of Dean of Students, Administrative Building # 05-10, Singapore 639798

One on-site laboratory is also needed to be set up by vertical drain installation contractor to carry out discharge capacity and tensile strength tests on every 20,000 linear meter length of vertical drains. Consistency of quality of drain was monitored and controlled based on those three laboratories results. CONTROL PARAMETERS AND TEST RESULTS Dimension of the drain.

Dimension of drain supplied by various vertical drain manufacturers are now standardized to be about 100 mm width and 4 mm thick except some large dimension vertical drains produced for special purposes by some manufacturers. The basic design of vertical drain is dependent upon spacing of vertical drain as well as equivalent diameter of drain itself. Therefore effect of dimension of drain is also a controlling factor of degree of consolidation even same spacing is used. The specification for the project requires the drain dimension to be 100 ± 2 mm width and 3 to 4 mm thickness and most drains comply with the specified values although some are found to be slightly lower than the specified margin. The effect of drain dimension on the time required for consolidation is shown in Fig. 1. It can be seen from Fig. 1 that variation of time required for 90 % degree of consolidation with specified spacing due to dimension variation of maximum 6 mm in width and 2 mm in thickness in certain type of soil is found to be only 12 days. It may be necessary to check the thickness of vertical drain under pressure. At site laboratory the measurement of width was carried out with vernier scale and variation of thicknesses under pressures were measured with the help of direct simple shear equipment in which vertical displacement and load are able to measure accurately. Fig. 2 shows variation in the thickness of vertical drains versus normal pressure.

Figure 1 : Time required to complete 90% Degree of Con- Figure 2 : Variation of Thickness of Vertical Drain solidation with various dimensions Vertical Drains. under various Normal Pressure Apparent Opining Size (AOS) The (AOS) is specified for filter material. Since the filter of the drain is to retain the fine grain soil not to enter into the filter pores and yet offer sufficient permeability, an AOS of O95 less than or equal to 75 µm is required. This means 95% of opening size are smaller than 75 µm. For a woven or non-woven geotextile, if AOS is smaller than D85 of surrounding soil pipping will not occur. Generally D85 of natural soft clay is greater than 75 µm. Therefore specifying O95 of 75 µm is sufficient to retain the surrounding soil. On the other hand AOS of O95 or O15 should also be large enough to prevent the clogging potential. Following criteria are suggested to prevent clogging potential.

AOS = O95 ≥ 3 D15 (1)

O15 = 2 - 3 D15 (2) The typical grain size distribution of Singapore Marine Clay at Changi is shown in Fig. 3. The AOS of O95

of 75 µm is much greater than D15 of natural clay at Changi. Therefore for Singapore marine clay, the above criteria are suitable to prevent clogging.

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The AOS tests were carried out using standard glass bead with grain size of diameter covering 40 to 170 µm. the percentage of glass beads retained on the filter cloth is calculated from following equation. AOS is obtained from the grain size distribution curve provided by glass beads manufacturer as shown in Fig 4. The test method follows ASTM D4751-87.

B = 100 P/T (3)

Where : B = beads passing through specimen, (%) T = Total mass of glass beads used, (gm) P = mass of glass beads in the pan (gm)

Figure 3 : Grain size distribution of Singapore Marine Clay. Figure 4 : Cumulative Size Distribution Chart. Tensile Strength The drain should be able to withstand the stress occurred due to penetration resistance caused by friction between vertical drain roll. Due to the fact that elongation could be occurred. Therefore vertical drain should have sufficient tensile strength with certain elongation which could more or less maintain the dimension of drain without major deformation. For the Changi East Reclamation project the tensile strength of vertical drain has been specified as 100 N/cm or 1 kN/10 cm at 10 % elongation for both dry and wet condition. However the actual elongation test carried out with vertical drain installation rig shows that elongation of Mebra MD 7007 is as low as 1 %. It indicates that the stress occured due to friction between roll of vertical drain during penetration is insignificant. Typical tensile strength tests results of Mebra MD 7007, Colbond CX 1000 and Flexi FD 767 under wet and dry conditions are shown in Figures 5 and 6. It can be seen from Figures 5 and 6 that the strain of drain at specified tensile strength was normally lower than specified strain of 10 % for most drains. The tensile strength tests at site were carried out with a modified tri-axial compression machine. Vertical drain was gripped across the whole section at the two ends. The size of the jaw face is 140mm in width and 50mm in height.

Figure 5 : Tensile Strength test results on various Figure 6 : Tensile Strength test results on various drains drains under dry condition. under wet condition.

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Vertical displacement was measured during extension with linear vertical displacement transducer and stress incurred was measured using extension proving ring. The tensile strength tests were carried out under a strain rate of 7 % strain/min on PVD sample with a gauged length of 200 mm. The method of testing was basically followed ASTM, D459D-86. However it was found that the effect of test strain rate on strength of geotextile is insignificant.

Figure 7 : Permeability of various vertical drains. Permeability To meet the permeability requirement, Holtz et. al.(1991) has suggested permeability of geotextile should be at least 10 times more permeable than the surrounding soil. For the Changi East reclamation project the specified permeability of filter is greater than 5 X 10-6 m/s which is much greater than the permeability of the soil which is in the order of 10-9 m/s (Bo Myint Win 1998). Permeability tests were carried out with simple constant head permeability apparatus. The apparatus used is similar to the apparatus specified in the ASTM D4491-85. The device consists of an upper and lower unit which could be fasten together. The sample could be position in between the two units. There are manometers connected to the upper and lower unit for water supply and head measurement. Permeability tests were carried out under various head differences. The Darcy permeability measured for different drains are shown in the Fig. 7. Discharge Capacity of Drain Discharge capacity is most important parameter for vertical drain. The discharge capacity of drain is affected by factors such as lateral stress, buckling and siltation. As such discharge capacity of drains is measured under straight and buckled condition. The specification requires the discharge capacity to be 25 x 10-6 m3/s under straight condition and 10 x 10-6 m3/s under buckled condition with a axial strain of 25 % for the Changi East reclamation project. Discharge capacity, tests were performed under various lateral stresses and under various hydraulic gradients for straight and buckled condition and some of the results are shown in Figures 8 & 9.

Figure 8 : Discharge Capacity of vertical drains under straight condition tested with 100 x 100 mm tester.

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Figure 9 : Discharge Capacity of vertical drains under buckled condition. It can be seen in the results that all types of vertical drain meet specified requirement both in straight and buckled conditions. Vertical drain with corrugated core provides comparatively high discharge capacity. However it was found that variation of discharge capacities were measured for same type of vertical drain manufactured in different countries. The discharge capacity under straight condition was carried out with a 100 mm x 100 mm tester (Chu and Choa 1995) in which sample could be surrounded by soil and tested under various pressures, and various hydraulic gradients. The tester is connected to the inflow and outflow water pipes which provide the discharge water and measured the outflow discharge rate of water. Manometers are also connected to the tester to measure hydraulic gradient. The discharge capacity test under buckled conditions test was carried out in a cylinder in which vertical drain was installed and compressed with surrounding soil. It should be mentioned that the measurement of discharge capacity may be affected by the surrounding soil used in the test. It was observed that the higher the moisture content of soil, the lower discharge capacity was measured as shown in Fig. 11. Therefore discharge capacity test should be carried out with soil which is similar to the in-situ soil. Discharge capacity tests were also carried out under long term condition. The discharge capacity decreases with duration of test as shown in Figure 12 since surrounding soil is creeping with time.

Figure 10 : Comparison of discharge capacity of drains with various types surrounding soils.

In order to find out the reduction of discharge capacity due to the siltation, long term consolidation of fine grained soil with vertical drain were carried out in a large diameter cell. The tests were carried out with very soft soil with D50 of about 3 µm and 85% of soil are finer than 75 µm. Vertical drain was made to buckle up to 46% strain under 100 KPa vertical pressure. After the compression, discharge capacity test was carried out on the deformed vertical drain surrounded with consolidated soil. It was found that reduction of discharge capacity was noted and, however it still has discharge capacity of 11 x 10-6 m3/s which is still higher than specified discharge capacity for buckled condition. In addition to that the core was still very clean from silt deposit.

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Figure 11 : Discharge capacity of drains with various Figure 12 : Variation of discharge capacity with time. moisture content surrounding soils.

CONCLUSION

In order to ensure the quality of PVD to meet the required specification, a quality management system was adopted in Changi east reclamation projects. Quality control system consists of third part laboratory, on – site laboratory and research laboratory. One of the major tasks in quality control is to monitor the consistency of PVD supplied. AOS, permeability, dimension and discharge capacity under both straight and buckled conditions, and tensile strength of PVD were tested through out the vertical drain installation works. Quality of supplied prefabricated vertical drains and consistency of quality of materials were monitored and control based on those three laboratories test results. Generally all types of vertical drains meet required specification. Slight variation of dimension were measured in some types of vertical drains. However, slight variation in dimension may not significantly affect the consolidation time. AOS of 75 µm specified for filter is suitable to retain the surrounding soil as well as prevent the clogging potential. Permeabilities of all types of filters are sufficiently higher than surrounding soil. Tensile strengths of entire drain both wet and dry condition at 10 % strain rate are greater than 1 kN/ 10 cm. Discharge capacity of all types of vertical drains are above specified requirement both in straight and buckled condition. Prefabricated drain with corrugated core generally provides comparatively higher discharge capacity. Based on the special test results carried out at changi, it was found that discharge capacity of drain is affected by the surrounding soil used in the test. The higher the moisture content of soil the lower is the discharge capacity. Discharge capacity is also reducing with time due to creeping of surrounding soil. Therefore, discharge capacity test is required to carry out with surrounding soil similar to in-situ soil and also required to carry out long term test. Discharge capacity reduces due to siltation and buckling, however reduced discharge capacity still higher than specified discharge capacity of 10 x 10-6 m3/s. REFERENCES

Bo Myint Win, Arulrajah, A. and Choa, V. (1998). “The Hydraulic Conductivity of Singapore Marine Clay

at Changi”. Quarterly journal of Engineering Geology, November 1998, Vol. 31, Part 4, pp. 291-300 Chu, J. and Choa, V. (1995). “Quality Control tests of Vertical drains for a land reclamation project”.

Compression and Consolidation of clayey soils, Belkema, Rotterdam. Vol. 1, P.P. 43-48 Holtz., R.D., Jamiolkowski, .M.B., Lancellotta, R. and Pedroni, R.(1991). “Prefabricated Vertical Drains

Design and Performance”. CIRIA Ground Engineering Report , Ground Improvement, Butterworth Heinemann, Oxford.

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