Development of Rapid Consolidation Equipment for Cohesive Soil

Geotech Geol Eng (2015) 33:167–174 DOI 10.1007/s10706-014-9819-7 TECHNICAL NOTE Development of Rapid Consolidation Equipment for Cohesive Soil Khairul Anuar Kassim • Ahmad Safuan A. Rashid • Ahmad Beng Hong Kueh • Chong Siaw Yah • Lam Chee Siang Norhazilan Mohd Noor • Hossein Moayedi • Received: 24 January 2014 / Accepted: 9 September 2014 / Published online: 1 October 2014 Ó Springer International Publishing Switzerland 2014 Abstract In this study, rapid consolidation cell equipment (RACE) was developed as an alternative device to the conventional consolidation test using Oedometer to determine the consolidation characteristic of cohesive soil. RACE operates based on the constant rate of strain (CRS) consolidation theory, which is a continuous loading method of testing and could accelerate the consolidation process for cohesive soil, shortening the time consumption from 1 week (when using Oedometer and Rowe cell tests) to only a few hours. A slightly modification has been made on the normal CRS test by proposing a direct back pressure system to the specimen using a tube to saturate the soil sample. Four types of sample were tested with different rates of strain using the RACE equipment and their results were compared with those K. A. Kassim  A. S. A. Rashid (&)  H. Moayedi Department of Geotechnics and Transportation, Universiti Teknologi Malaysia, 81310 Johor Baharu, Johor, Malaysia e-mail: [email protected] A. B. H. Kueh Construction Research Centre, Universiti Teknologi Malaysia, Johor Baharu, Johor, Malaysia conducted using the Oedometer on the same soil type, from which fairly good agreements were evident in many specimens. It was found that, the RACE equipment is capable to determine the consolidation characteristic of the cohesive soil. In this study, the acceptable strain rates were proposed as compared with the Liquidity Indices for cohesive soil. It was found that the range of strain rate of CRS test for LI \ 15 % was between 0.01 and 0.3, while for LI closed to 25 %, the range was between 0.01 and 0.1. Keywords Consolidation  Cohesive soil  Constant rate of strain  Oedometer  Strain rate  Liquidity index Notation cc Compression index cv Coefficient of consolidation Ho Sample height r Rate of strain ua Excess pore water pressure rv Applied pressure b Normalized strain rate C. S. Yah  L. C. Siang Universiti Teknologi Malaysia, Johor Baharu, Johor, Malaysia 1 Introduction N. M. Noor Department of Structure and Materials, Universiti Teknologi Malaysia, Johor Baharu, Johor, Malaysia Constant rate of strain (CRS) theory was implement in consolidation test to accelerate the consolidation process for cohesive soil, shortening the time 123 168 consumption from one week (when using Oedometer and Rowe cell tests) to only a few hours (Kassim and Clarke 1999). Several studies have been conducted previously to determine the consolidation characteristic of different types of soil (Smith and Wahls 1969; Wissa et al. 1971; Sallfors 1975; Gorman et al. 1978; Lee 1981; Kassim and Clarke 1999; Larsson and Sallfors 1985; Lee et al. 1993; Sheahan and Watters 1997; Ahmadi et al. 2011; Ozer et al. 2012; Raftari et al. 2014). Kassim and Clarke (1999) have used the CRS equipment and proposed a procedure in order to determine the consolidation characteristic of stabilised soil under different amount of stabiliser agents. They conducted the tests within 2 h to represent the stiffness at that age of curing. Thus, it is possible to use the CRS test to predict the variation of stiffness with age and stress rather than using the increment loading (IL) system. Sample and Shackelford (2011) fabricated a new system of CRS test by varies the height of the testing chamber to observe the consolidation of slurry mixed soil. The similar finding was discovered by Sample and Shackelford (2011) where the CRS testing apparatus offered a convenient, rapid, and economical approach for evaluating the consolidation behavior of the bentonite-ZVI slurry mixed sand. Ozer et al. (2012) reported that the main problem with continuous loading consolidation is to determine a proper strain rate for the consolidation test. Many recommendations had been offered from the previous researchers for the selection of practically acceptable test rate, based on several criteria of acceptance (the relationship of the void ratio, e, against effective stress, r0 , coefficient of consolidation, cv, liquid limit value, normalized strain rate, b, and ratio of excess pore pressure to applied total stress, ua/rv) (Smith and Wahls 1969; Wissa et al. 1971; Sallfors 1975; Gorman et al. 1978; Lee 1981; ASTM 1982, 1991, 2001, 2008; Larsson and Sallfors 1985; Lee et al. 1993; Sheahan and Watters 1997; Ahmadi et al. 2011; Ozer et al. 2012). However, no attempt has been made to study the acceptable test rate with soil Liquidity Indices (LI), which is obtained by dividing the difference of in situ water content and Plastic Limit by the difference of Liquid Limit and Plastic Limit. This relationship is important because water is an influencing factor in the saturation process and the in situ water content keeps changing due to environmental effects in practice (Ishak et al. 2012; Rashid et al. 2014; Shahminan et al. 2014). 123 Geotech Geol Eng (2015) 33:167–174 Fig. 1 Schematic diagram and photograph of the Constant Rate of Strain Consolidation test equipment (Rapid Consolidation Cell Equipment, RACE) In this study, Rapid Consolidation Equipment (RACE) is developed as alternative equipment and testing to conventional consolidation test, the Oedometer. The objective of this paper is to introduce the RACE and capability of this equipment to determine the consolidation of cohesive material. The RACE has several advantages compared to the conventional cohesive soil consolidation methods, namely a faster process time, whereby the invention reduces the time needed to perform the task, is able to be incorporated with other standard pieces of equipment in soil laboratories, standard loading frame, fully automated and greatly reduces the risk of losing soil samples due to electrical failure, as a result from the reduced preparation time. Some modification has been made based on the standard CRS equipment, allowing for a back pressure system to directly saturate the sample before the test is conducted. A series of laboratory Geotech Geol Eng (2015) 33:167–174 works was conducted employing RACE to determine the consolidation characteristic of various types of clay obtained in Malaysia. This study only focussed on the relationship of the void ratio, e, against effective stress, r0 which contribute to cc value between Oedometer and CRS tests and normalized strain rate, b in order to determine the acceptable test rate of the CRS test. Based on the obtained results, the acceptable strain rates of CRS test were proposed as compared with the Liquidity Indices for cohesive soil. 2 Design of CRS Equipment Constant rate of strain consolidation test equipment was designed and named as RACE. The major components of RACE are base, cell top, cell chamber and the stainless steel ring. Figure 1 shows the general arrangement of the RACE cell. The RACE equipment had to operate within a Triaxial load frame using the pressure systems available in the laboratory. The cell chamber made from a transparent Perspex cylinder which allowed observing the specimen during a test. 25 mm thick aluminium end caps held in place by four bolts. O-rings were used to seal the cell by placing at the top and bottom of the cylinder. The top cap has a guide built into ensure that the loading platen remains perpendicular to the specimen surface. The loading piston is guided by two O-rings, which also act as seals. The 100 mm diameter specimen is contained within a steel ring that sits within the perspex cell designed to withstand pressures of up to 500 kPa with a 25 mm height. The maximum contact pressure with a 10 kN load frame is 1250 kPa, allowing comparisons to be made with results from Oedometer tests on specimens consolidated to 1,250 kPa. Porous stones are placed on the top and bottom of the specimen within the steel ring. Since the steel ring is 25 mm height, a specimen thickness of 23.5 mm is produced. A perforated loading platen sits on top of the top porous disk through which the back pressure is applied. The steel ring is clamped in place by the cell, thus providing the necessary external seal between the top and bottom of the specimen. This means that flow can occur only within the specimen and the pore pressures between the loading piston and the top cap is taken into account at the top and bottom of a specimen can be different. 169 Two O-ring are installed between the stainless steel ring and cell chamber to avoid any leakage from the bottom of the specimen to the top side. High loading pressure will be applied to the soil sample in the CRS test. This may cause the stainless steel ring in the RACE cell being lifted up, therefore PVC holder is placed on the steel ring to hold down the steel ring. Loading piston is used to transfer the load to loading platen and sample. The friction between the loading piston and the top cell is reduced using ball bearing. Sealing is achieved by O-rings at the junctions of the cell top and the bottom of the chamber. The cell top, cell chamber and the cell base is hold together by screws and nuts. Drainage is permitted from both end of the sample where first drainage outlet is used for drainage purposes, and the second drainage outlet is used to measure pore pressure of the specimen. A modification has been made on the back pressure system where the back pressure is applied directly through the sample by using a tube. In order to ensure accurate measurement on the back pressure applied on the sample, two O-rings were used between load platen and stainless steel ring to seal the specimen. It is also possible to either apply the same back pressure to the base of the specimen or prevent drainage from the base and measure the pore pressure at the base. During the saturation stage the back pressure is applied to both top and bottom of the specimen; during the consolidation stage it is applied only to the top of the specimen. RACE is mounted on the loading frame platform. The loading frame with multi speed drive unit is the main loading machine used in the CRS test. It can provide constant motor drive speed ranging from 0.0001 to 9.0 mm/min. Three types of measuring devices were used in the CRS test for data measurement. These measuring devices were linear variable displacement transducer (LVDT), pressure transducer and the load cell. A 50 mm LVDT with an accuracy of 0.001 mm was used to measure vertical displacement of the soil sample in the CRS test. This LVDT was attached to the loading piston during the CRS test. 1,500 kPa pressure transducers with an accuracy of 0.1 kPa were used to measure back pressure and the pore pressure from the top and the bottom of the specimen. All tubings connecting to pore pressure and back pressure must be saturated to ensure accurate readings of pressures. A 907 kilogram capacity S type load cell was used for load measurement which can provide a maximum 123 170 Geotech Geol Eng (2015) 33:167–174 pressure of 1,100 kPa on the 100 mm diameter soil specimen. The load cell was attached between the loading frame and the load piston that transfer the load to the load platen and subsequently to the soil sample. The load cell can give to the nearest 0.001 kN. 3 System Calibration The load cell and displacement transducers are calibrated against a dead weight system and micrometer gauge respectively. The transducers are connected to the Data Acquisition Unit (ADU) during the calibration so that the output includes the signal processing of the ADU. These calibrations proved to be linear and repeatable with accuracies of less than 0.1 % over the full working range. System calibration of the equipment was essential to get the accuracy of test results which is based on the compression and the load-pressure measurement. Frictional error between the specimen ring and the load platen could be minimised by applying the silicon grease to the internal surface of the specimen ring. Setting up of the system calibration was similar to the CRS test except the soil specimen inside the ring was change to the uncompressible solid steel within the range up to 10 kN. Then the loading frame was started and the load and displacement were recorded by transducers with ADU. The load calibration was continued until the maximum load of the load cell was achieved. Figure 2 shows the displacement of the loading system expressed in terms applied load and the dimensions of the specimen. The measured displacement during consolidation is corrected for this displacement. Fig. 2 Displacement calibration curve for the RACE testing system 123 Fig. 3 Loading pressure calibration curve for the RACE testing system For the RACE cell loading calibration, data needed to be collected were load and the pore pressure at the bottom of the cell. Soil specimen in the ring was changed to the water to let the load applied to the water act as pore water pressure at the bottom of the cell. Loading applied to the water was measured by the load cell and the pore pressure was measured by the transducer. Figure 3 shows the relationship between the applied load and the pore pressure. The main purpose of this calibration was to find out the corrected pressure applied on the soil specimen. 4 Preparation of Soil Sample The soil samples were collected from Air Papan, Gemas and Kluang, which are located in the southern part of West Malaysia. Also, Kaolin clay was used as the control material in the investigation. The classification properties of the soil samples are presented in Table 1 based on Unified Soil Classification System. Remoulded sampler preparation equipment with an internal diameter of 150 mm was used to prepare the sample under different maximum pre-consolidation pressures (100, 200 and 300 kPa) as shown in Fig. 4. One kilogram of oven dried soil sample was mixed with distilled water at 1.4 times the liquid limit to form into slurry before putting it into the remoulded sampler equipment. Porous stone was placed at the bottom of the sampler to drain water from the sample. The soil sample was then loaded using steel load platen. Pressure was applied on the steel load platen using Geotech Geol Eng (2015) 33:167–174 171 Table 1 Classification properties of soil samples Soil Characteristics Soil types Kaolin clay Gemas clay Air papan clay Kluang clay Liquid limit (%) 51.40 47.02 40.47 53.19 Plastic limit (%) 28.40 24.53 19.53 26.87 Plastic index (%) 23.00 22.49 20.95 26.32 Water content (%) 33.96 27.72 24.71 33.26 Liquidity index (%) 24.17 14.18 24.74 24.28 Specific gravity Gs 2.64 2.60 2.59 2.55 Soil classification CH CI CI CH 150 mm diameter slurry cake. Steel ring of diameter 50 and 100 mm were used to press on the compressed slurry cake for Oedometer and CRS tests respectively. Each sample was then trimmed and placed inside the cell. RACE tests were conducted such that the resulting compression curves can be compared with those from Oedometer tests. The result from the RACE tests is considered acceptable if a similar shape of curve is obtained. In this study, the Oedemeter tests were conducted in 7 stages of loading (maximum 1,200 kPa) and 4 stages of unloading (minimum 25 kPa). Moisture content of the samples were determined after the CRS and Oedometer tests were completed. 5 Test Procedure Fig. 4 Schematic diagram of remoulded sampler preparation equipment water pressurised by compressed air. Two O-rings were used to seal the load platen to prevent the water from seeping through into the soil sample and disturb the properties of the remoulded sample. Another two O-rings were put between the cell top and the load platen to avoid the water draining out from the top of the cell, which cause reduce pressure applied to the soil sample. The air pressure applied to the soil sample was based on the maximum applied pressure needed for remoulded sample preparation. Settlement of the remoulded sample was taken from the dial gauge attached on the top of the load platen. For each maximum pressure, a step loading method was applied to ensure the sample was uniformly consolidated. For each level, the air pressure applied was maintained for 24 h. The slurry will form into a In this study 12 major tests had been conducted for 4 samples of soil under 3 different intensities of preconsolidation pressures. A simple notation was used to label the soil samples under different pressures as shown in Table 2 e.g. Air Papan 100 denotes Air Papan soil with a pre-consolidation pressure of 100 kPa. Two Oedometer tests were conducted on each sample to provide confidence as to the repeatability of the test preparation methods. Meanwhile, for the CRS test, the undrained and drained tests were employed. Equation 1 proposed by Lee (1981) is used in this study to determine the normalized strain rate, b, where the b should be less than 0.1 based on Lee (1981) suggestion. b¼ rH0 cv ð1Þ where Ho is the sample’s height, r is the rate of strain and cv is the coefficient of consolidation from the Oedometer test. The values of the normalized strain rate, b, and strain rate for all samples are listed in Table 2 based on Eq. 1. 6 Validation of the CRS Test Figure 5 shows the curve of e/eo against effective stress for Gemas 100 sample from Oedometer and CRS tests. The void ratio had been normalized with that of initial, e/eo due to inconsistency of the initial void ratio. Two rates of strain, which are 0.03 mm/min 123 172 Geotech Geol Eng (2015) 33:167–174 Table 2 Summary of measured consolidation characteristic from CRS and Oedometer tests Soil types with different pre- consolidation pressures Average cv from Oedometer test cc from Oedometer test ß value Strain rate for CRS test (mm/min) Air Papan 100 12.09 0.2345 0.025 0.0125 0.2329 0.05 0.025 0.2348 0.05 0.02125 0.1914 0.075 0.0325 0.1923 0.025 0.015 0.1873 0.05 0.0325 0.1884 0.025 0.03 0.2108 0.05 0.061 0.2134 0.01 0.01 0.2076 0.025 0.0275 0.2081 Air Papan 200 Air Papan 300 Gemas 100 Gemas 200 10.62 16.08 30.44 27.72 0.20800 0.2160 0.01 0.025 0.0125 0.0325 0.2063 0.2063 Kaolin 100 45.00 0.2850 0.01 0.0175 0.3159 Kaolin 200 47.16 0.3050 0.025 0.047 0.3068 0.05 0.094 0.3071 0.025 0.05 0.2549 0.05 0.1 0.2583 50.22 0.2700 Kluang 100 3.05 0.3586 0.10 0.01225 0.3584 Kluang 200 3.59 0.2877 0.10 0.01425 0.2867 Kluang 300 3.09 0.2325 0.10 0.01225 0.2327 0.9 0.8 e/eo 0.2090 32.41 1.0 0.7 0.6 Oedometer 1 Oedometer 2 CRS 0.03mm/min CRS 0.061mm/min 0.5 100 0.1875 Gemas 300 Kaolin 300 0.4 10 0.1875 cc from CRS test 1000 recommended by Leonards (1985) to use a slow rate of strain. Table 2 summarizes all measured consolidation properties obtained from the Oedometer and CRS tests. The compression indices, cc, obtained from the compression curve based on the normalized void ratio for all four types of soil under different pre-consolidation pressures, match closely those obtained from the conventional Oedometer test, ensuring therefore the acceptability of cc produced by the CRS test. 10000 Effective Stress (kPa) Fig. 5 e/eo versus effective stress relationship for the Gemas 100 sample (b = 0.025) and 0.061 mm/min (b = 0.05), were applied in the CRS test for the Gemas 100. The relationships of e/eo versus log r0 v produced from both the CRS test and the standard Oedometer test are in good agreement. It was found that a slower strain rate of CRS test produces a better result with respect to that of Oedometer. This finding was similar as 123 7 Discussion In this study, the rate of the CRS test used was based on the normalized strain rates, b method and relationship of the void ratio, e, against effective stress, r0 which produce the cc results. In general, normalized strain rates, b used in this study which is range from 0.01 to 0.1 have produce an acceptable cc values between the CRS and Oedometer tests. In addition, based on the regression analysis on the relationship between cc value of CRS and Oedometer tests, good Geotech Geol Eng (2015) 33:167–174 173 0.4 y = 1.0008x R² = 0.9668 cc obtained from CRS test 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 cc obtained from Oedometer test Fig. 6 Comparison between cc values obtained from Oedometer and CRS tests where LI is Liquidity Index, Wc is final water content determine after the CRS test listed in Table 1, and PL and LL are plastic and liquid limit of the soil respectively (Gofar and Kassim 2007). The degree of saturation based on the Wc, Gs and final void ratio was approximately 100 % for all tested soils which means the sample were fully saturated. Based on the results, it can concluded that the range of strain rate of CRS test for LI \ 15 % was between 0.01 and 0.3, while for LI closed to 25 %, the range was between 0.01 and 0.1. However, this results only applicable for the soil with PI range between 20 to 27 %. Further investigation is required in order to cover a bigger range of soil PI especially for the soil with PI less than 10 % and different range of strain rate. 30 Liquidity Index (%) Air Papan 25 Gemas 8 Conclusions Kaolin From the current study, several conclusions based on four investigated soil types using Oedometer and CRS tests are listed below. Kluang 20 15 1. 10 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Strain rate (mm/min) Fig. 7 Strain rate range with liquidity index for cohesive soils agreement between both parameter was obtained where the coefficient of determination, R2 is greater than 0.96 as shown in Fig. 6. The strain rate used during the test was range from 0.01 to 0.09 mm/min where it was found that the soil with a lower cv value (Kluang) used a lower rate of strain. Therefore, it is important that the acceptable normalized strain rates, b, should be determined from the compatibility of cv values with the conventional Oedometer test results (Fig. 6). As mentioned in the introduction section, the in situ water content keeps changing due to environment effect in practice. Therefore, an acceptable strain rate range of CRS test was introduced based on Liquidity Index value of cohesive soils. Figure 7 shows the range of strain rate with LI for cohesive soil, whereby LI is obtained by the following Eq. 2. LI ¼ ðWc  PLÞ=ðLL  PLÞ ð2Þ 2. 3. 4. A new RACE has been developed adopting CRS method for cohesive soil consolidation test, reducing testing time from 1 week to merely a few hours. It can be observed that the relationships of e/eo versus log r0 v produced from both the CRS test and the standard Oedometer test are in good agreement. The cc values produced by the CRS test are within the maximum and minimum limits of the standard Oedometer test results. The range of strain rate of CRS test for LI \ 15 % was between 0.01 and 0.3, while for LI closed to 25 %, the range was between 0.01 and 0.1. References Ahmadi H, Rahimi H, Soroush A (2011) Investigation on the characteristics of pore water flow during CRS consolidation test. Geotech Geol Eng 29:989–997 ASTM (1982) Standard test method for one-dimensional consolidation properties of soils using controlled-strain loading. ASTM standard D4186-82. 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