An Alternative Testing Tool for Plastic Limit

Transcription

1 An Alternative Testing Tool for Plastic Limit Kamil Kayabali Geological Engineering Department, Ankara University, Ankara 06100, Turkey ABSTRACT Atterberg limits are the water contents at critical states of soil consistency. Because the conventional methods employed to determine particularly the liquid and plastic limits involved a number of operator-induced uncertainties, a number of attempts have been made to devise new tools to measure liquid and plastic limits with smaller degree of uncertainties. In this regard, a commercial motorized plastic limit tool was introduced recently. To investigate the reliability of the newly devised tool, 120 soil samples were subjected to plastic limit tests using the rolling device and this motorized device. The comparison between the results of those two methods reveals that the motorized device yielded systematically lower plastic limits than those from the rolling device. The level of underprediction is approximately 20%, suggesting that the plastic limit of fine-grained soils can be determined by simply multiplying the plastic limit obtained from the motorized plastic limit method by KEYWORDS: Plastic limit, bead rolling, rolling device, motorized plastic limit INTRODUCTION Classification of fine-grained soils has been carried out traditionally by employing so called Atterberg limits tests, the results of which are plotted as plasticity index (PI) versus liquid limit (LL) on the Casagrande plasticity chart (Casagrande, 1932, 1948). Procedures for determining LL and PL are incorporated in various standards, BS 1377 Part 1 (BSI, 1990) and ASTM D4318 (2001) being the most prominent ones. The laboratory standard for determining the plastic limit still adopts the method suggested by Casagrande (1958) in which the water content at the plastic limit is determined when the soil specimen on a glass plate starts crumbling at a diameter of approximately 3 mm by hand rolling. The uncertainties involved in the determination of plastic limit through the bead-rolling test have been discussed by many researchers (e.g., Wroth and Wood, 1987; Whyte, 1982; Sivakumar et al., 2009, Kayabali and Tufenkci, 2010). These uncertainties may be collectively listed as the variations in the amount of pressure applied during rolling, the speed of rolling, the geometry of the bead, and the friction between hand, soil and glass. The bead-rolling test is highly subjective in nature and involves over-reliance on operator judgment (Sivakumar et al., 2009)

2 Vol. 17 [2012], Bund. O 2108 Several attempts have been made for developing an alternative approach for determining plastic limit of soils to overcome with limitations related to the bead-rolling method. Lee and Freeman (2007) reviewed ten non-astm test methods; seven of them combined LL and PL tests into a single procedure, and compared them to the bead-rolling test. These investigators recommended the use of fall cone method, which has also been shown by other researchers to be an important alternative to the rolled-bead test. For instance, Sivakumar et al. (2009) modified the standard falling cone apparatus by increasing the force to be externally applied to the existing cone used in LL investigation to 54 N. By examining some 16 different soils, Sivakumar et al. (2009) showed that the method they proposed can be used to evaluate PL with reasonable confidence. However, Prakash and Sridharan (2006) proclaimed that the soil plasticity is due only to soil cohesion and, because the cone penetration method is also affected by undrained friction, the cone penetration method does not represent soil plasticity. Kayabali and Tufenkci (2010), by following the principles outlined by Whyte (1982), carried out a series of reverse extrusion tests and made a new definition for plastic limit. By employing a container of 38 mm in diameter and a ram with a die orifice of 6 mm, they defined the plastic limit as the water content corresponding to an extrusion pressure of about 3000 kpa. ASTM D4318 (2001) standard defines a rolling device method for determining the plastic limit. This tool appears to reduce some of the drawbacks involved in hand rolling of a soil thread by providing a uniform diameter, nearly uniform pressure distribution, and less operator judgment. A commercial motorized plastic limit device was recently brought into use to eliminate all the uncertainties and drawbacks experienced in bead-rolling by hand. This novel tool allows for the adjustment of the thread diameter and rolling speed and eventually produces soil threads totally discarding the operator-induced effects. The scope of this investigation is to check the reliability of the motorized plastic limit method by making a comparison between the plastic limit data obtained using the motorized plastic limit tool and the rolling device. MATERIALS One hundred twenty soil samples were used for the investigation. Sixty locations visited in Ankara clay, a lacustrine formation of Pliocene age, and large bag samples of several kilograms in mass were collected. They were pulverized after drying in an oven and sieved by using a #40 mesh. The soil samples used for this investigation were collected from the outcrops, foundation excavations, and roadcuts in different sites. Their characteristics such as liquid limits (determined using the Casagrande cup method), plastic limits (determined using the rolling device method), and the soil classes according to Unified Soil Classification System (USCS) are summarized in Table 1.

3 Vol. 17 [2012], Bund. O 2109 Table 1: The liquid limits, plastic limits and soil classes of 120 samples used in the investigation (LL and PL values are in %). No. LL PL USCS No. LL PL USCS No. LL PL USCS CH MH CH CH CH CH CH MH CH CH CH MH CH MH CH CH CL MH CH CH CH CH MH CH CH CH MH CH CH CH ML MH MH CH MH CH CH MH CH MH CH CH MH MH CH MH MH ML CH CH CH MH MH MH MH MH CH CH MH MH CH MH MH CH CH CH CH MH MH ML CH MH CH MH CH ML MH MH CH CH CH CH MH MH MH CH MH CH CH CH MH MH MH CH CH MH CH CH CH CH MH CH CH MH CH CL MH MH CH MH CH CH MH CH CH MH MH CL CH CH

4 Vol. 17 [2012], Bund. O 2110 METHODS Basically, two methods were employed in this research. The first one is the bead-rolling device (Figure 1), which was used by following the principles quoted in the ASTM D4318 (2001) standards. The rolling device was chosen to eliminate some of the uncertainties involved in bead rolling by hand (e.g., non-uniform diameter and the pressure applied by hand). The second tool was a modern tool, which is the motorized plastic limit test device (Figure 2). The device has three rotating grooved rods of 15 mm diameter. A small motor housed by the thicker leg of the motorized unit transmits the revolution action from the top cap of the instrument to the grooved rollers which are supported by ball-bearings at their other ends on the lower table. A knob on this table (B in Figure 2a) permits to adjust the clearance between three rollers (3.2 mm as required by most common standards). The revolution speed of grooved rods ranges from RPM. The reason for providing such an adjustable range is to accelerate the loss of moisture when needed. A fan, mounted on one of the three legs of the motorized unit, is utilized for this purpose as well. When the water content is higher than plastic limit, the roller speed is kept low as the fan is switched on. Otherwise, a moderate level of revolution speed is preferred. Figure 1: The rolling device utilized in running the plastic limit tests.

5 Vol. 17 [2012], Bund. O 2111 Figure 2: (a) The motorized device utilized in running the plastic limit tests. Diameter: 200 mm, Height: 430 mm, Weight: 7,5 kg, A: rollers, B: diameter adjustment knob, C: fan, D: power switch, E: fan switch, F: column hosting the motor unit inside, G: cross section line. (b) Cross section along G on left. A: grooved rollers, B: rolled soil specimen. The material preparation for the motorized plastic limit tool is same as the conventional plastic limit test. A dry sample of about 10g in mass is mixed with water homogeneously so that the ultimate product is a plastic paste. One to two grams of the mixture is picked and rolled between the palms. Then, it is dropped from a hole located in the middle of the top plate of the motorized unit. Because the diameter of the dropped piece of soil specimen is larger than the clearance between the grooved rods, it tends to be squeezed and dragged downwards. The plastic limit is the water content at the moment when the length of threads dropping from the bottom release of the device is somewhere between 1 and 2 cm. As the water content falls below plastic limit, the device produces an irregularly shaped thread instead of a cylindrical one. RESULTS AND DISCUSSION Ten specimens were prepared to determine the plastic limit using both methods. The averages of ten plastic limit experiments are presented in Table 2. The plastic limits obtained from the motorized plastic limit device were somewhat lower than those obtained from the rolling device and therefore a new set of ten specimens were employed for the motorized method. The plastic limits obtained from the motorized plastic limit tool were compared with those obtained from the rolling device in Figure 3. The level of agreement between the results of those two techniques is remarkably good with a regression coefficient (R 2 ) of However, a close visual inspection of Table 2 or Figure 3 unveils that the plastic limits obtained from the motorized technique are systematically lower than those determined via the rolling device, that is, the motorized limit device yields plastic limit, in general, approximately 20% lower than that obtained from the rolling device technique. The most likely reason for the lower plastic limits using the motorized device than those from the rolling device is the compression effect introduced

6 Vol. 17 [2012], Bund. O 2112 by three grooved rollers. Such a compression and/or squeezing would be considered to hold the soil thread uncrumbled at a lower water content than that measured in the conventional method. Regarding the variation of the results plotted in Figure 3, it can be concluded that the results of neither method is exact; that is, while the motorized technique is considered to have somewhat lower level of uncertainties than that involved in the rolling device method, both methods do involve some amount of uncertainty and thus the yield of the comparison obtained from this investigation that shown in Figure 3 can be evaluated as acceptable from the viewpoint of engineering judgment. Figure 3: A comparison between plastic limits obtained using two different tools. Table 2: Plastic limits for 120 soils obtained from the rolling device method (RDM) and the motorized device method (MDM) (PL values are in %). Plastic Limit Plastic Limit Plastic Limit No. RDM MDM No. RDM MDM No. RDM MDM

7 Vol. 17 [2012], Bund. O It should be noted that the rolling device is superior to the bead rolling by hand in reducing the uncertainties such as the uneven pressure applied by hand and the resulting soil thread of nonuniform diameter introduced by the operator. The motorized device is considered to further reduce the remaining uncertainties, if any, involved in the rolling device. In conclusion, based on the plastic limit tests on 120 soils, whose plastic limits ranged from 24-44, the motorized limit technique resulted in a relationship in the form of y = 0.80x 0.53 (1) where y and x are the plastic limits for the motorized and rolling device techniques, respectively. Conversely, the plastic limit of any fine-grained soil (PL) can be determined upon the results of the motorized technique by employing the following equation: PL = 1.25x (2) where is the plastic limit determined through the motorized device. Considering the negligibly small quantity of the y intercept of equation (2) (i.e., 0.66), it can be concluded that the plastic limit of any soil can be determined by multiplying the plastic limit obtained using the motorized technique by CONCLUSION Determination of plastic limit by the same operator using the common standards is highly speculative in repeatability. In this regard, there have been numerous attempts to device alternative tools to improve the reliability of its results. This investigation is one of them. A motorized tool was employed along with a rolling device. One hundred twenty soil samples were utilized for the investigation. Ten tests were performed on all soil samples by using each tool, namely the motorized plastic limit device and the rolling device. A reasonably good relationship was observed between the results of two different tools. It was also observed that the motorized tool yield systematically lower results than those of the rolling device. It was concluded that the plastic limit values from the motorized is systematically 20% lower than those of the rolling device method.

8 Vol. 17 [2012], Bund. O 2114 ACKNOWLEDGMENTS The funding for this research came from the University of Ankara with the Grant No. 09B REFERENCES 1. American Society for Testing Materials, Standard test methods for liquid limit, plastic limit, and plasticity index of soils: ASTM D , West Conshohocken, PA. 2. BSI, 1990, British standard methods of test for soils for engineering purposes, BS 1377: Milton Keynes, British Standards Institution. 3. Casagrande, A., 1932, Research on the Atterberg limits of soils: Public Roads, 13, Casagrande, A., 1948, Classification and identification of soils: Transcations, ASCE, 113, Casagrande, A., 1958, Note on the design of the liquid limit device: Geotechnique, 8(2), Kayabali K. and Tufenkci, O. O., 2010, Determination of plastic and liquid limits using the reverse extrusion technique: Geotechnical Testing Journal 33(1), Lee, L. T. and Freeman, R. B., 2007, An alternative test method for assessing consistency limits: Geotechnical Testing Journal, 30(4), Prakash, K. & Sridharan, A. (2006). Critical appraisal of the cone penetration method of determining soil plasticity. Canadian Geotechnical Journal 43, Sivakumar, V., Glynn, D., Cairns, P., and Black, J. A., 2009, A new method of measuring plastic limit of fine materials: Geotechnique, 59(10), Whyte, I. L., 1982, Soil plasticity and strength a new approach for using extrusion: Ground Engineering, 15(1), Wroth, C. P. and Wood, D. M., 1978, The correlation of index properties with some basic engineering properties of soils: Canadian Geotechnical Journal, 15(2), ejge