iiiCONTENTS 1 DETERMINATION OF LIQUID LIMIT – CASAGRANDE METHOD ................................................... 1 1.1 Objective ..................................................................................................................................... 1 1.2 Concepts and Significance .......................................................................................................... 1 1.3 Apparatus and Accessories ......................................................................................................... 1 1.4 Procedure .................................................................................................................................... 2 1.5 Calculation and Test Report ........................................................................................................ 3 1.6 Precautions ................................................................................................................................. 3 1.7 Discussion ................................................................................................................................... 4 2 DETERMINATION OF PLASTIC LIMIT .............................................................................................. 5 2.1 Objective ..................................................................................................................................... 5 2.2 Concepts and Significance .......................................................................................................... 5 2.3 Apparatus and Accessories ......................................................................................................... 5 2.4 Procedure .................................................................................................................................... 5 2.5 Calculation and Test Report ........................................................................................................ 6 2.6 Precautions ................................................................................................................................. 6 2.7 Discussion ................................................................................................................................... 7 3 CALCULATION OF THE PLASTICITY INDEX .................................................................................. 8 3.1 Objective ..................................................................................................................................... 8 3.2 Concepts and Significance .......................................................................................................... 8 3.3 Procedure .................................................................................................................................... 8 3.4 Calculation and Test Report ........................................................................................................ 9 3.5 Precautions ................................................................................................................................. 9 3.6 Discussion ................................................................................................................................... 9 4 REFERENCES ................................................................................................................................. 10 1 1 DETERMINATION OF LIQUID LIMIT – CASAGRANDE METHOD 1.1 Objective To determine the liquid limit of a given soil by the Casagrande method according to the Australian Standard AS1289.3.1.1-2009 (Australian Standard for determination of the liquid limit of a soil – four point Casagrande method). 1.2 Concepts and Significance Soils with clay minerals content can be remoulded without crumbling in presence of humidity due to the adsorbed water surrounding the clay particles. At a very low moisture content, soil behaves more like a brittle solid. When the moisture content is very high, the soil and water may flow like a liquid. Hence, on an arbitrary basis, depending on the moisture content, the nature of soil behavior can be broken down into four basic states: solid, semisolid, plastic, and liquid, as shown in Figure 1.1. (Das, 2013). Moisture content corresponding to the points of transition from one state to another are known as Atterberg limits. The liquid limit is defined as the point of transition from plastic state to liquid state. Its importance is that knowing this value and the plastic limit it is possible to determine the plasticity index of a given soil, which with its grain size distribution are used to classify the soil under the AASHTO and Unified soil classification systems. Figure 1.1 Atterberg limits (Das, 2013) 1.3 Apparatus and Accessories The following apparatus and accessories were used during the test: a) Mixing plate. b) Mixing bowl. c) Palette knives. d) Liquid limit apparatus (see Figure 1.2). e) Grooving tool (see Figure 1.2). f) Wash bottle with potable water. g) Digital weighing scale with ±0.01 g tolerance. h) Heat resistant containers. i) Oven. 2 Figure 1.2 (a) liquid limit apparatus; (b) grooving tool (Das, 2013) 1.4 Procedure In accordance with AS1289.3.1.1-2009 the procedure of the test was as follow: a) A sample of approximately 250 g was taken from the material disposed by the laboratory instructor for the test, which was a brown fine-grained soil that according to the instructor passed a 425 µm sieve. b) This sample was placed in a mixing bowl and thoroughly mixed with water until the soil become a thick homogeneous paste. c) A portion of the soil paste was placed in the cup of the liquid limit apparatus, with the cup resting on the base and using a palette knife the mixture was levelled parallel to the base, applying sufficient downward pressure to prevent voids formation, until reaching a depth of soil of about 10 mm. Then, the soil in the cup was divided by drawing the grooving tool along the diameter of the cup. d) The crank was turned at a rate of 2 r/s so that the cup was lifted and dropped until the two parts of the soil come into contact along the bottom of the groove for a distance of 10 mm. As the number of blows at the first test was between 30 and 40 (40 blows) was unnecessary to add water or to dry the sample and repeat the procedure, and therefore that was the first point of the test. e) In order to determine its moisture content, a quantity of soil about 30 g was removed using the spatula from the two portions in the cup, then it was placed in a container with known weight (WC) and weighed together to obtain the weight of the moist sample and the container (WCMS). Finally the container and the sample were placed in the oven and dried at 105°C to 110°C for 24 hours, after which the sample was removed from the oven and weighed to obtain the weight of the dry sample and the container (WCDS). f) The steps (c) to (e) were repeated further four times, using the same sample and increasing the water content by adding water and mixing for at least 1 minute after each addition of water, so that the results were evenly distributed over a range between 40 and 15 blows. The respective weights and number of blows for each test are shown in Table 1.1. 3 1.5 Calculation and Test Report The water content w for each test is shown in Table 1.1, it was calculated using the following equation and the recorded weights: = = − − (1.1) Table 1.1 Number of blows (N) and moisture content (w) obtained on each test Those results and its corresponding number of blows were plotted on a semi-logarithmic chart, with the former as ordinates on the linear scale and the latter as abscissae on the logarithmic scale. Then, a line of best fit was drawn to obtain the flow curve shown in Figure 1.3. The water content corresponding to the intersection of the said line with the intersection of the point corresponding to 25 blows on the abscissa is the liquid limit of the soil, which in this case resulted being LL = 47% (see Figure 1.3). Figure 1.3 Flow curve to determine liquid limit 1.6 Precautions In accordance with AS1289.3.1.1-2009 the following precautions should be considered during test: W C W CMS W T W CDS W S W W w (g) (g) (g) (g) (g) (g) (%) 1 42.85 72.87 30.02 64.00 21.15 8.87 40 41.94 2 42.40 67.38 24.98 60.00 17.60 7.38 33 41.93 3 42.89 76.58 33.69 66.00 23.11 10.58 26 45.78 4 42.43 72.83 30.40 63.00 20.57 9.83 25 47.79 5 43.19 64.22 21.03 57.00 13.81 7.22 15 52.28 N TEST W C = Weight of container W CMS = Weight of container and moist soil W T = Total weight of the soil sample = W CMS - W C W CDS = Weight of container and dry soil (those values have been supposed in order the show the calculations and will be updated with the real values obtained in the laboratory). W S = Weight of solids of the soil sample = W CDS - W C W W = Weight of water = W T - W S 40 30 25 20 LL=47% 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 10 M o i s t u r e C o n t e n t , w ( % ) Number of Blows, N 4 a) Mixing process must allow the necessary time for the water to penetrate to absorptive particles and into the internal structure of the soil, as inadequate mixing may result in an erroneous value of liquid limit. b) Some soils tend to slide on the cup instead of flowing so that the groove closes at a low number of blows, in that case the results should be discarded and the test repeated adding some more water. c) Drying of the sample should be prevented between tests as the number of blows for closure will increase gradually as the sample become dryer. d) According to AS1289.2.1.1 (Determination of the moisture content of a soil) the recommended weigh of the sample should be greater than 100 g, however due to the homogeneity of the sample it is admissible for liquid limit test using samples of about 10 g to determine the moisture content. 1.7 Discussion This experiment uses trial and error method. In order to obtain a soil sample with adequate moisture, it took multiple tries and repetitive work. The first couple of experiments were not satisfying and it was necessary to repeat it couple of times. There were some precautions needed to be taken while performing the experiment. First of all, distilled water was to be used in order to minimize the possible ion exchange between the soil and any impurities in the water. The groove should close due to the flow of soil and not due to the slippage between the soil and the cup. After mixing distilled water into the soil, enough time should be given to permeate the water through the soil mass. For each observation, the brass cup and the grooving tool should be cleaned. 5 2 DETERMINATION OF PLASTIC LIMIT 2.1 Objective To determine the plastic limit of a given soil according to the Australian Standard AS1289.3.2.1-2009 (Australian Standard for determination of the plastic limit of a soil). 2.2 Concepts and Significance The plastic limit is defined as the moisture content, in percentage, at which the soil when rolled into threads of 3.2 mm in diameter, crumbles (Das, 2013). Which represents the lower limit of the plastic stage of the soil and therefore the point of transition from semisolid to plastic state (see Figure 1.1). Its importance is that knowing this value and the liquid limit it is possible to determine the plasticity index of a given soil, which with its grain size distribution are used to classify the soil under the AASHTO and Unified soil classification systems. 2.3 Apparatus and Accessories The following apparatus and accessories were used during the test: a) Flat area made of glass. b) Mixing bowl. c) Palette knives. d) A rod 3 mm in diameter. e) Digital weighing scale with ±0.01 g tolerance. f) Heat resistant container. g) Oven. 2.4 Procedure a) A sample about 8 g was taken from the paste used in the liquid limit test, then it was roundly shaped by the fingers and rolled on the flat glass area until the whole sample reached an equal diameter in comparison to the rod. The amount of rolling between the hand and the flat area was 80 to 90 times rolling per minute, where every rolling time is the total movement forward and backward. Since the thread became to 3 mm diameter without crumbling, the sample was successively kneaded and re-rolled between the hands and the flat area until the thread crumbled at 3 mm diameter (see Figure 2.1), whose moisture content corresponds to the plastic limit. b) That sample was placed in a container with known weight (WC) and weighed together to obtain the weight of the moist sample and the container (WCMS). Finally the container and the sample were placed in the oven and dried at 105°C to 110°C for 24 hours, after which the sample was removed from the oven and weighed to obtain the weight of the dry sample and the container (WCDS). The respective values are shown in Table 2.1. 6 Figure 2.1 Crumbled sample during plastic limit test 2.5 Calculation and Test Report The recorded weights corresponding to the plastic limit test are shown in Table 2.1, and it was calculated using the following equation: = = = − − (2.1) Table 2.1 Determination of plastic limit The resultant plastic limit of the soil under analysis was PL = 19%. 2.6 Precautions In accordance with AS1289.3.2.1-2009 the following precautions should be considered during test: a) Distilled, demineralised or deionized water should be used where available for initial testing for reference purposes, as some clay types may be affected by the content of potable water. b) The test applies to material passing a 425 µm sieve, though near passing material will have a negligible effect on the obtained plastic limit. c) Curing time will be necessary to allow for the water to penetrate to absorptive particles and into the internal structure of the soil to a representative degree, with some soils needing to stand up to 24 hours to allow for thorough saturation. W C W CMS W T W CDS W S W W w = PL (g) (g) (g) (g) (g) (g) (%) PL 42.56 48.05 5.49 47.17 4.61 0.88 19 TEST W C = Weight of container W CMS = Weight of container and moist soil W T = Total weight of the soil sample = W CMS - W C W CDS = Weight of container and dry soil (this value has been supposed in order the show the calculations and will be updated with the real value obtained in the laboratory). W S = Weight of solids of the soil sample = W CDS - W C W W = Weight of water = W T - W S 7 d) Some soils may be too tough to kneed split portions back together into a uniform mass. In this case rolling the crumbs may signify whether a true plastic limit has been reached, as they should break up immediately at this point. e) According to AS1289.2.1.1 (Determination of the moisture content of a soil) the recommended weigh of the sample should be greater than 100 g, however due to the homogeneity of the sample it is admissible for plastic limit test using samples of about 5 to 20 g to determine the moisture content. 2.7 Discussion The liquid and plastic limits of a soil depend on the amount and type of clay in the soil and this forms the bases for soil classification system. The plastic limit however, is defined as the water content at which the soil just begins to crumble when rolled into a thread, approximately 3 mm in diameter. Based on the result of our experiment, the Plastic limit of the soil sample is 19%, meaning that the water content at which the soil crumbled when rolled is 19%. In the final discussion, the soil will be classified based on the results of these limits (plastic and liquid limits). 8 3 CALCULATION OF THE PLASTICITY INDEX 3.1 Objective To calculate the plasticity index of a given soil according to the Australian Standard AS1289.3.3.1-2009 (Australian Standard for calculation of the plasticity index of a soil). 3.2 Concepts and Significance Casagrande (1932) studied the relationship of the plasticity index to the liquid limit of a wide variety of natural soils. On the basis of the test results, he proposed a plasticity chart as shown in Figure 3.1. Figure 3.1 Plasticity chart (Das, 2013) The important factor of this chart is the empirical A-line that is given by the equation: PI = 0.73(LL-20). The A-line separates the inorganic clays from the inorganic silts. Plots of plasticity indexes against liquid limits for inorganic clays lie above the A-line, and those for inorganic silts lie below the A-line. The line which lies above the A-line is called the U-line. The U-line is approximately the upper limit of the relationship of the plasticity index to the liquid limit for any soil found so far. The equation for the U- line can be given as: PI = 0.9(LL-8). The information provided in the plasticity chart is of great value and is the basis for the classification of fine-grained soils in the Unified Soil Classification System (Das, 2013). 3.3 Procedure In accordance with AS1289.3.3.1-2009 the procedure of the test was as follow: a) Determine liquid limit in accordance with AS 1289.3.1.1. b) Perform the plastic limit in accordance with AS 1289.3.1.2. 9 c) Ensuring that the same method of preparation of soil for the liquid limit and plastic limit has been followed. 3.4 Calculation and Test Report Using the obtained values of liquid limit (LL=47%) and plastic limit (PL=19%) the plasticity index PI was calculated from the following equation: = − (3.1) The resultant plasticity index of the soil under analysis was PI = 28%. 3.5 Precautions In accordance with AS1289.3.3.1-2009 the following precautions should be considered during calculation: a) If either the liquid limit or the plastic limit cannot be determined due to the soil, the plasticity index may be considered to be non-plastic. b) Given the experimental nature of these results it is possible for the plastic limit to come back as equal to, or greater than, the liquid limit. Provided the experimental results are dependable, the plasticity index may be taken as practicably zero in this instance. 3.6 Discussion A correlation may be found between the Atterberg limits of the air-drying modelling clay which was tested. Looking at Figure 3.1 the properties of the soil are consistent with an inorganic clay of medium-high plasticity. This position gives a good idea of the materials general engineering properties and forms the basis of Casagrande’s classification method, which suggests the clay is of low to medium compressibility and medium to high strength when dry and minimal reaction to shaking. These properties are the result of multiple factors such as the particle size range and distribution of the modelling clay, as well as mineralogical properties of the sample. The nature and influence of these factors on the soil properties may only be reached following a fuller investigation of the soil. As stated above, the plasticity index is the difference between the liquid limit and plastic limit. From the experiment, the plasticity index of the soil sample is 28%, and this will be used to classify the soil using the Plasticity Chart in Figure 3.1 above. From the Plasticity Chart, the point of intersection of Plasticity index (28%) and Liquid limit (47%) is in- between the diagonal lines, U-line and A-line and also between 30% and 50% boundary on the Liquid limit axis. Therefore the soil is classified as INORGANIC CLAY OF MEDIUM PLASTICITY. 10 4 REFERENCES Das, B. M. (2013). Fundamentals of geotechnical engineering. Stanford: CENGAGE Learning. Standards Australia. (2009). Australian Standard for calculation of the plasticity index of a soil (AS1289.3.3.1-2009). Sydney: Standards Australia. Standards Australia. (2009). Australian Standard for determination of the liquid limit of a soil – four point Casagrande method (AS1289.3.1.1-2009). Sydney: Standards Australia. Standards Australia. (2009). Australian Standard for determination of the plastic limit of a soil (AS1289.3.2.1-2009). Sydney: Standards Australia.