Physical modelling of consolidation behaviour of a composite foundation consisting of a cement-mixed soil column and untreated soft marine clay

Yin, J.-H. & Fang, Z. (). Géotechnique 5, No. 1, 3 TECHNICAL NOTE Physical modelling of consolidation behaviour of a composite foundation consisting of a cement-mixed soil column and untreated soft marine clay J.-H. YIN* and Z. FANG* KEYWORDS: clays; consolidation; ground improvement; model tests; pore pressures; settlement INTRODUCTION Nowadays, the deep cement mixing (DCM) method is widely used in many countries and regions. Although much research work has been done on DCM in previous decades, most of the work has been confined to study of the strength and stiffness of the cement-mixed soil columns (e.g. Broms, 1979; Kawasaki et al., 191; Terashi & Tanaka, 191a, 191b, 193; Kamon & Bergado, 199; Walker, 199; Kamaluddin & Balasubramaniam, 1995; Schaefer et al., 1997; Yin, 199, 1, ; Yin & Lai, 199; Lin & Wong, 1999; Fang et al., 1; Porbaha et al., 1; Porbaha, ; Tan et al., ; Horpibulsuk et al., ). Although there have been some investigations (Terashi & Tanaka, 191a, 191b, 193; Kitazume & Yamamoto, 199; Kitazume et al., ), research work on the consolidation behaviour of a DCM composite foundation is still limited, and insufficient experimental data are available. In this study, a physical model test of a composite foundation in a steel cylindrical mould was carried out in order to understand the consolidation behaviour of the composite foundation treated by the DCM method. The pore water pressures at different locations in the untreated soft clay (USC) and the earth pressure and load acting on the USC and the DCM column in the composite foundation model were measured throughout the testing. The results from the test are presented, interpreted and discussed in the paper. Finally, the failure mode of the composite foundation is examined. PHYSICAL MODEL TESTING Apparatus A cylindrical stainless steel mould 3 mm in diameter and 5 mm high was used for the present experimental research, as shown in Fig. 1. A rigid platen was placed on the top of the composite foundation to apply the same displacement to the DCM soil column and the USC (Fig. 1). This physical modelling test aimed at simulating a composite foundation where DCM columns were installed vertically in a triangular/square pattern at the same spacing into a horizontal clay layer and were subjected to uniform vertical fill pressure loading over an extensive area. This case was similar to the case using vertical wick drains. The Manuscript received June ; revised manuscript accepted 1 October 5. Discussion on this paper closes on 3 July 5, for further details see p. ii. * Department of Civil and Structural Engineering, The Hong Kong Polytechnic University. consolidation/compression around one DCM column was approximately axisymmetrical with an equivalent diameter, at the boundary of which lateral displacement and drainage were not permitted. The vertical load on the composite foundation was applied by dead weights on a level hanger. The arrangement of the experimental apparatus and set-up is shown in Fig. 1. Composite foundation preparation Hong Kong marine clay (HKMC), currently used in laboratory investigations in a remoulded state, was taken from a coastal area near Tai Kowk Tsui Harbour in Hong Kong. The basic physical and mechanical properties of in situ HKMC are shown in Table 1. The remoulded HKMC slurry was placed in the cylindrical stainless steel mould and preconsolidated under a vertical pressure of kpa with drainage from the top and bottom sides. The average water content of the marine clay slurry was 97. 7%, which was about 1.9 times the liquid limit. The initial height of slurry was estimated to be 9 mm. During the preconsolidation process, the pore water pressure was monitored using pore pressure transducers (PPTs) inserted at various positions in advance of the following consolidation process. When the measured pore water pressures reduced to about kpa, showing that the degree of consolidation had increased up to 9%, the loading was released. The final height of the USC after consolidation was about 1 mm. Similar to the preceding preparation of USC, another part of the soft marine clay slurry was also preconsolidated under a pressure of kpa. After completion of the preconsolidation process, a specified weight of dry Portland cement power was added to the HKMC at a cement/clay ratio of 1% by dry mass and mixed thoroughly by means of a concrete mixer for 1 min. The cement-mixed clay paste was placed and compacted in three layers into a cylindrical plastic PVC tube with an internal diameter of 5 mm and height of mm to form a DCM column. The DCM column was taken out from the PVC tube on the next day and placed into a water tank to cure for 3 days. After this, the DCM column was inserted into a pre-drilled hole in the middle of the untreated soft clay (USC) in the cylindrical mould to form a composite foundation model, as shown in Fig.. Note that this method of forming a DCM column is different from that in the field. The advantage of the present approach was that a uniform and quality DCM column was obtained and installed in the clay. This was good for studying the basic failure mechanism and consolidation behaviour of the soft clay installed with a quality DCM column. The quality control of the DCM columns was not the goal of this project. The final height of the composite foundation model was mm, which includes a mm DCM column in the middle, a 1 mm thick earth pressure cell on the DCM 3

YIN AND FANG Datalogger LVDT1 LVDT Rigid plate Rigid permeable plate EPCs PPT Sand PPT1, PPT USC Cemented mixed soil column PPT PPT3 PPT5 Outlet of cable Drainage outlet Rigid plate Rigid base Geotextile Hole 5 Hole PPT(75,) PPT1(,11) Hole PPT(75,11) PPT5(13,11) PPT3(75,3) EPC(,) Hole 1 PPT(,11) Hole 3 EPC1(1,) (b) Hole Fig. 1. Locations of instruments in the model: vertical cross-section view of positions of various transducers; (b) horizontal view (note: the first number in brackets means the radial distance and the second means distance from the bottom in mm) Table 1. Basic properties of Hong Kong marine clay used in this test Specific gravity Water content: % Unit weight: kn/m 3 Liquid limit: % Plastic limit: % ph Particle size distribution: % Clay Silt Sand.5 5. 1. 51.1.1. 7. 7.5 5.5

CONSOLIDATION BEHAVIOUR OF A COMPOSITE FOUNDATION 5 Untreated soft clay (USC) Cement mixed soil column Fig.. Composite foundation model consisting of preconsolidated untreated soft clay (USC) and a cement-mixed soil column at the centre in a cylinder mould Vertical pressure: kpa Settlement: mm 7 5 3 1 1 1 1 1 5 1 15 5 3 1 kpa kpa kpa kpa Failure of DCM column column, and a 1 mm sand layer covering both the USC and the DCM/cell column. Fig. 3. Curves of vertical pressure and settlement against time, measured from the test Instrumentation In this physical modelling test, six miniature pore water pressure transducers (PPTs) were placed at certain positions and used to detect the pore water pressure in the USC (Fig. 1). All PPTs were placed in the clay inside the cylinder model before preconsolidation. PPT3 and PPT5 were fixed on the bottom of the container; others were suspended in the clay. One small earth pressure cell (EPC) (1 mm thick and 5 mm in diameter) was placed on the surface of the USC, and the other was placed on the top of the DCM column. The two EPCs were used to detect pressures acting on the USC and DCM column. In addition, two linear variable differential transformers (LVDTs) were used to measure the settlement of the model foundation. A CR1X datalogger was used to collect data. 1 1 PPT1 PPT PPT3 PPT PPT5 PPT 1 1 1 1 1 Fig.. Variation of pore water pressure with log (time) under a total vertical pressure of kpa TEST RESULTS AND DISCUSSIONS Settlement The vertical pressure loading and settlement over time in four stages are plotted in Fig. 3. After local failure of the DCM column occurred, the settlement was found to increase rapidly to an eventual value of approximately 1 mm. Dissipation of excess pore water pressure in USC The typical relationships between pore water pressure and log (time) in all four stages of loading have been obtained. Because of space limitations, shown in Figs and 5 are the relationships under pressure of kpa (Stage ) and kpa (Stage ). From the test results, the following features are observed for the excess pore water pressure dissipation. Under loading of 1 kpa, the initial pore water pressure increase was lower than the vertical total pressure increase. This was probably caused by some air trapped in the soil during installation of the DCM column. Under the following loading stage, the incremental pore water pressures at PPT1, PPT, PPT and PPT5, which were located at the middle height, were approximately equal to the external vertical total pressure increase. PPT3 and PPT were close to the boundary, and the PPT1 PPT 1 1 1 1 PPT3 PPT PPT5 PPT 1 1 1 1 1 1 1 Fig. 5. Variation of pore water pressure with log (time) under a total vertical pressure of kpa (pore water pressure increase due to local failure of deep-cement-mixed soil column DCM column) pore water pressure increase here was less than the external pressure increase. (b) In this test, the permeability of the DCM column was much higher than that of the USC. The DCM column seemed to speed up the consolidation of the USC. (c) Transducers PPT1 to PPT5 had the same pore water

YIN AND FANG pressure dissipation trend. However, PPT experienced much quicker dissipation, because it was located close to the top drainage boundary. PPT3 showed obvious delay of pore pressure dissipation, as observed in all loading stages. As shown in Fig. 5, at the time of min, the pore water pressure dissipation was reversed to exhibit an increase. This was due to the local failure of the DCM column. The increase was due to two factors. (b) Part of the vertical loading pressure on the DCM column in the top boundary was transferred to the surface of USC, because the stiffness of the DCM was decreased because of local failure. Local failure of the DCM column inside the USC caused expansion of the DCM column laterally, which caused an increase of the lateral stress and pore water pressure of the USC. Edge of cylindrical tank Before local failure of column 1 1 1 1 Column 1 After local failure of column 1 1 1 1 Radial distance from column axis: mm h 5 h h h 5 3 h h 1 h h h 7 h 9 h 1 h 155 h 1 1 Column Edge of cylindrical tank 1 1 1 1 Radial distance from column axis: mm h 5 h 1 h h h h 1 h 1 h h h Fig.. Radial distribution of pore pressure at different times under total vertical pressure of kpa Fig. 7. Radial distribution of pore pressure at different times under total vertical pressure of kpa (pore water pressure increase due to local failure of DCM column) Radial drainage behaviour Figures and 7 show the pore pressure changes with respect to time at three different radial locations for the same distance from the tank bottom. Note that the positions of the PPTs in these figures were those measured after completion of the test. It is evident that larger pore pressure dissipation occurred at the location of PPT1/, nearer the DCM soil column, than at the other two locations (PPT and PPT5), which were further away from the DCM column. At the same time, dissipation of pore pressure at the position near the DCM column was faster for larger applied loading. It can be seen that the DCM column in the foundation behaved similarly to a vertical drain (sand drain or PVD: prefabricated vertical drain) and speeded up the consolidation of the foundation soil. Oedometer tests also show that the permeability of cemented soil is about 1 to times that of the original uncemented soil. The same findings on the Bangkok clays have been reported by other investigators (Uddin et al., 1997). However, there are some differences between a DCM column and a good vertical drain (sand drain or PVD). The permeability of the DCM column (Uddin et al., 1997; Yu et al., 1999) is far less than that of a PVD or sand drain. The DCM column is much stiffer than a sand drain and a PVD can share more vertical load, and can increase the overall stiffness of the composite foundation. When local failure of the cement-mixed soil column happened under a vertical pressure of kpa, the pore pressure in USC increased suddenly and then began to dissipate, as shown in Fig. 7. The excess pore water pressure near the DCM column increased faster and decreased quicker than those points away from the DCM column after the local failure of the column. Loading distribution and transfer In general, a stress ratio n defined as n ¼ ó t /ó u is used to describe the distribution of the total external pressure loading, where ó t is the vertical stress taken by the DCM column and ó u is the stress carried by the USC. The instant stress ratio and final stress ratio for each load stage are shown in Fig.. The instant stress ratio can be fitted well using the dashed curve of an exponent function. According to the laboratory unconfined compression (UC) test, the ratio between values of the secant Young s modulus E 5 of the deepcement-mixed soil column and USC was.1, which is very close to the computed ratio n from the observed results at the failure of the DCM soil column in the foundation model, as shown in Fig.. The value of E 5 is defined as the ratio of the vertical stress equal to 5% of the peak stress to the (cement-mixed soil column/usc) 7 5 3 Instant stress ratio Final stress ratio at column failure Ratio of secant modulus E 5 1 y 5 71e 3x R 5 993 1 3 5 7 Applied loading: kpa Fig.. Variation of stress ratio with respect to total vertical pressure (loading)

corresponding vertical strain. The relationship between stress ratio and corresponding degree of consolidation at the middle height of the USC is shown in Figs 9 and 1. The stress ratio n increases with time, which means progressively more and more loading was transferred from the USC to the DCM column. Failure mode of the composite foundation After test, the DCM column was extruded from the soil. It was found that the column was compressed to failure. Local crushing occurred near the top of the DCM column, with a slip crack of 75 mm from the column top downward. A longitudinal split failure occurred in the lower part of the DCM column. This suggests that the failure of the composite foundation is due mainly to the local failure of the DCM column. SUMMARY AND REMARKS The experimental arrangement and results of a small-scale physical composite foundation model test with instrumentation have been presented in this paper. Based on the discussion and analysis, the main observations in this physical modelling test are summarised as follows. From the pore water pressure dissipation data, partial radial drainage was observed along the DCM column. The DCM column may be regarded as a partial or full vertical drainage, similar to a PVD or a sand drain, depending on the relative permeability of the DCM and the USC. 3 5 15 1 5 Average degree of consolidation (PPT1,) Degree of consolidation (PPT) 1 3 5 Fig. 9. Relationships of stress ratio and degree of consolidation against time under total vertical pressure of kpa CONSOLIDATION BEHAVIOUR OF A COMPOSITE FOUNDATION 7 1 5 1 5 Degree of consolidation (PPT) 9 35 7 3 Average degree of consolidation 5 (PPT1,) 5 15 3 1 5 1 1 1 1 1 Fig. 1. Relationships of stress ratio and degree of consolidation against time under total vertical pressure of kpa 9 7 5 3 1 Degree of consolidation: % Degree of consolidation: % (b) (c) Under the approximately rigid loading condition, the pressure carried by the DCM soil column and USC changed with time and external loading. For a specified loading stage, with the progressive increase of degree of consolidation of USC, the DCM soil column carried a progressively greater proportion of loading than the USC. The failure of the composite foundation was caused mainly by the local failure of the DCM soil column. The scale of this physical model test is smaller than that in the field. There were also differences in other testing conditions, such as installation of the DCM column, the slender ratio of the DCM column, and the base resistance of the model foundation. 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