Criteria of Acceptance for Constant Rate of Strain Consolidation Test for Tropical Cohesive Soil
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Geotech Geol Eng DOI 10.1007/s10706-016-0016-8 ORIGINAL PAPER Criteria of Acceptance for Constant Rate of Strain Consolidation Test for Tropical Cohesive Soil Khairul Anuar Kassim . Ahmad Safuan A. Rashid . Ahmad Beng Hong Kueh . Chong Siaw Yah . Lam Chee Siang Received: 7 October 2015 / Accepted: 15 April 2016 Ó Springer International Publishing Switzerland 2016 Abstract In this study, the rapid consolidation This research has produced a set of criteria for equipment (RACE) was developed as an alternative determining the suitable rate for the rapid consolida- device to the conventional consolidation test using tion test based on the ratio of normalized strain rate, b, Oedometer, consuming merely a few hours for the and proposed a new coefficient in terms of a ratio of b whole precedure to determine the consolidation char- to clay fraction (CF), as a part of new criteria for acteristics of cohesive soil. RACE operates based on testing a fine soil. Four types of sample were tested the constant rate of strain (CRS) consolidation theory, with different rates of strain using the RACE and their which is a continuous loading method of testing, results were compared with those conducted using the requiring a good estimation of the loading rate such Oedometer on the same soil type, from which fairly that it is ideal for the achievement of steady state good agreements were evident in many specimens. It condition during testing. The steady state condition is was found from the study that the minimum value of achieved when the cv values from drained and normalized strain rate, b, for the CRS test is 0.005 and undrained face of CRS converged with the cv from for the ua /rv ratio is suggested as 0.01. Also, the Oedometer test. A slightly modification has been made maximum b/CF for soils with clay friction lower and on the normal constant rate of strain (CRS) test by higher than 50 % are 0.008 and 0.001, respectively. proposing a direct back pressure system to the The minimum b/CF value for both conditions is specimen using a tube to saturate the soil sample. 0.0001. Keywords Consolidation Á Soft clay Á Strain rate Á Compression characteristics K. A. Kassim Á A. S. A. Rashid (&) List of symbols Department of Geotechnics and Transportation, Universiti C Compression index Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia c e-mail: [email protected] cv Coefficient of consolidation CF Clay fraction A. B. H. Kueh Ho Sample height Construction Research Centre, Universiti Teknologi r Rate of strain Malaysia, Johor Bahru, Johor, Malaysia ua Excess pore water pressure C. S. Yah Á L. C. Siang rv Applied pressure Universiti Teknologi Malaysia, Johor Bahru, Johor, b Normalized strain rate Malaysia 123 Geotech Geol Eng 1 Introduction The Oedometer test is commonly used to determine the soil consolidation characteristics in which a step loading is applied based on Terzaghi’s theory, the procedure of which that normally takes around 1 week for the completion of one test (loading and unloading stages) (Znidarcic et al. 1986; Head 1986). In addition to such a time-consuming procedure, the test is limited to low and medium loading for a sample size of 75 mm diameter. Besides that, the pore pressure at the bottom of the soil sample cannot be measured. To remedy these issues, the hydraulic consolidation equipment, which is capable of measuring pore pressure, the Rowe cell was therefore introduced, noting however that the testing procedure is also based on a step loading method (Head 1986). Due to several limitations given above, a rapid consolidation equipment abbreviated simply as RACE, as shown in Fig. 1, has been introduced and developed in this paper to conduct the consolidation test based on the constant rate of strain (CRS) theory, which 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. 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 (Head 1986). It is well known that the main problem with Fig. 1 a Schematic and b Photogrammetric of the Rapid continuous loading consolidation is to determine a Consolidation Cell Equipment (RACE) proper strain rate for the consolidation test (Ozer et al. 2012). The selection of the test rate remains as a major (1981), and the maximum ua /rv ratio, which is varied hurdle in the conduct of the CRS test although many between 0.03 and 0.7, as reported in previous studies attempts had been carried out to address this issue. (Gorman et al. 1978; Larsson and Sallfors 1985; Many recommendations had been offered from the Sandbaekken et al. 1986; Sheahan and Watters 1997; previous researchers for the selection of practically ASTM 2008). The value of cv can be determined from acceptable test rate, based on several criteria of several equations and charts as demonstrated in acceptance (Gorman et al. 1978; ASTM 2008; Larsson numerous studies done previously for a normally and Sallfors 1985; Sandbaekken et al. 1986; Lee et al. consolidated clay (Carrier 1985; Raju et al. 1995; 1993; Sheahan and Watters 1997; Ozer et al. 2012). Sridharan and Nagaraj 2004). It is however somewhat These criteria of acceptance were determined based on practically unsatisfactory that so far the minimum the following characteristics; the relationship of the limits for both values have not been suggested by void ratio, e, against effective stress, r0, coefficient of previous researchers, to produce a better and accept- consolidation, cv, liquid limit value, normalized strain able CRS test result. rate, b, and ratio of excess pore pressure to applied Ozer et al. (2012) reported that a problem may arise total stress, ua/rv. Well received criteria include the in determining the consolidation properties if the maximum limit for b, which is 0.1 as proposed by Lee strain rate test is conducted at a very small rate. Lee 123 Geotech Geol Eng et al. (1993) also confirmed that the lowest feasible only to the top of the specimen. By proposing this rate depends on the accuracy of the pore pressure modification, the saturation process will be faster and transducer and suggested that further research is could be monitor precisely. needed to address this. Although Ozer et al. (2012) This equipment allows also the pore water pressure proposed a new semi-empirical method to calculate to be observed during the test. O-rings are used to the proper strain rate for CRS testing of soils based on avoid any leakage in the system. The friction between the changes of strain and void ratio, e, of sample the loading piston and the top cell is reduced using ball during the test, this method requires the user to bearings. The main loading machine for the CRS test conduct the incremental loading consolidation proce- consists of a loading frame with a multi-speed drive dure to determine the void ratio of the sample. unit. In this study, the CRS tests were conducted in Therefore, in this study, a series of laboratory works various strain rates. 50 mm linear variable displace- was conducted employing RACE to determine a ment transducer (LVDT), 1500 kPa pressure trans- suitable criterion for the strain rate used in the CRS ducer and 1 kN S type load cell were used to measure test for various types of clay obtained in Malaysia. The the deformation, pore water pressure and the loading, strain rate, r was determined based on Lee (1981) respectively. All the inputs were read and stored approach of normalized strain rate, b where the systemically using the data acquisition unit. maximum limit for b is 0.1. Modification on the available strain rate selection method and hence a set 2.2 Sample Preparation of lower bound values for CRS test was then recommended. The soil samples were collected from Air Papan, Gemas and Kluang, which are located in the southern part of West Malaysia. The locations of the site were 2 Experimental Description displayed in the Fig. 2. Also, Kaolin clay was used as the control material in the investigation. The classi- 2.1 RACE Equipment fication properties of the soil samples are presented in Table 1. This study is conducted on the typical For convenience, a description of RACE is given tropical soil within a range of the Liquid Limit (LL) herein (see Fig. 1). The major components of RACE and Plasticity Index (PI) was between 40–53 and are the base, cell top, cell chamber and the stainless 23–26 respectively. Further investigation is required steel ring (Kassim et al. 2014). The soil specimen for in order to different range of LL and PI of soil. For this equipment comes in a cylindrical form with a each test, a remoulded sample was prepared in a slurry diameter of 100 mm that is placed at the bottom of condition using 1 kg of oven dried soil sample that stainless steel ring with a height of 25 mm. In addition, was mixed with the distilled water at 1.4 times the porous stones are placed at the top and bottom of the liquid limit to produce a homogeneous sample (Rashid soil specimen. A modification has been made on the et al. 2015a, b). In addition, a specially designed back pressure system where the back pressure is remould sampler with an internal diameter of 150 mm applied directly through the sample by using a tube. In was used to prepare the sample under different order to ensure accurate measurement on the back maximum pre-consolidation pressures (100, 200 and pressure applied on the sample, two rubber O-rings 300 kPa).