The University of Toledo Soil Mechanics Laboratory

Transcription

1 1 Atterberg Limits Liquid and Plastic Limits The University of Toledo Soil Mechanics Laboratory Introduction Fine-grained soils, or coarse-grained soils with a significant amount of fines, are affected by water. The engineering properties of undisturbed or compacted soils vary depending on the in situ water content. In a highly remolded state in the laboratory, soils behave as liquid, plastic or solid materials depending on the amount of water in the soil. Soils consist of different types of minerals and different combinations of the mineral types. Therefore, soils are affected differently by water. The Atterberg limits tests are simple standardized tests that were developed to determine the water contents that will induce particular behavior. The Atterberg limits determined in this laboratory are the liquid and plastic limits. The liquid and plastic limits are defined as the water content above which the soil behaves as a liquid or plastic, respectively. In the laboratory test, the liquid limit is defined as the water content that will result in closure for a distance of 13 mm (1/2 inch) of a standard groove in a remolded soil sample using 25 blows (drops of 10 mm) in the liquid limit device. The plastic limit is defined as water content at which a thread of soil 3 mm (1/8 inch) in diameter begins to crumble. The plasticity index is the difference between the liquid limit and the plastic limit. The limits and the plasticity index are used to classify soils and for correlations with engineering properties. Apparatus 1. No. 40 sieve 2. Mortar and rubber-tipped pestle 3. Balance capable of measuring to 0.1 g 4. Spatula 5. Porcelain dish 6. Wash Bottle 7. Liquid limit device 8. Grooving tool 9. Ground glass plate 10. Water content containers and drying oven Procedure A. Preparation (one week before test) 1) Using the mortar and rubber-tipped pestle, pulverize 250 ± g. of air-dried soil passing the No. 40 sieve. 2) Add 40 ± ml of water to the soil and mix as thoroughly as possible. Keep the moist soil in a plastic bag for a period of 1 week to allow the soil moisture to become uniform and to recover the soil s natural plasticity. 1 ASTM D Atterberg Limits 1

2 B. Laboratory Test Liquid Limit 1) Check to make sure that the height of fall of the liquid limit cup is 10±0.2 mm using the end of the grooving tool to gage the height of fall. 2) Weigh the water content tares. 3) Place the soil in the porcelain dish and mix thoroughly with the spatula until the soil is uniform in color. Test the soil by rolling a small amount between the palms of the hands. If the soil cannot be rolled between the hands without crumbling, add more water and mix thoroughly. Repeat testing the soil as necessary. 4) Place a portion of the soil in the liquid limit cup and spread it into the cup to a depth of approximately 10 mm at its deepest point, tapering the soil to form a horizontal surface. Return any unused soil to the dish. 5) Form a groove in the soil by drawing the grooving tool from the back to the front of the cup in a straight line. To make the groove, hold the grooving tool against the cup so that the tool remains approximately perpendicular to the surface of the cup. Be careful that the soil does not slide forward in the cup. 6) Make sure that there is no excess soil in or on the outside of the cup. Lift and drop the cup by turning the handle of the crank at the rate of 2 drops per second until the two sides of the groove come together at the bottom of the groove over a length of 13 mm (1/2 inch). If the blow count is greater than 35, return the soil in the cup to the porcelain dish, add water and mix thoroughly and repeat the liquid limit procedure. If the blow count is between 25 and 35, record the number of drops to close the groove. 7) Obtain a water content sample by removing a slice of soil the width of the spatula extending from edge to edge across the soil perpendicular to the groove form the area where the groove flowed together. Obtain the wet weight of the soil immediately. 8) Remove the soil from the cup and place it in the dish. Set aside about 20 to 30 grams of soil on the glass plate to use later for plastic limit testing. Add a small amount of water to the soil and mix thoroughly. Repeat steps 4 to 7 until 3 trials are completed with the following number of blows for closure: 25 to 35; 20 to 30; and 15 to 25. 9) Determine the water content of the samples after drying the soil in the oven for a minimum of 16 hours. The liquid limit, defined as the water content that will result in closure of a standard groove in a remolded soil sample using 25 blows (drops of 10 mm) in the standard liquid limit device, is obtained by plotting water content versus the log of the number of drops (see Figure 1), and determining the water content corresponding to 25 blows using the best fit line. Atterberg Limits 2

3 Results Laboratory Test Plastic Limit 1) Select about two grams of the soil set aside in Step 8 above. Roll the soil between the palms of the hand to form an ellipsoidal mass. 2) Roll the soil between the palm or fingers and the plate glass with sufficient pressure to cause the soil to form into a thread of uniform diameter across its length. The thread of soil should be reduced in diameter with each successive stroke until the diameter reaches 3.2 mm (1/8 inch), taking no more than 2 minutes. 3) When the soil reaches 3.2 mm, take the soil and knead it into an ellipsoidal mass again and roll it. Continue this process until the soil crumbles under the pressure of the hand or until it is no longer possible to roll the soil into a thread 3.2 mm in diameter. Crumbling is evidenced when the soil breaks into short barrel-shaped segments or when the soils forms an outside tubular layer that starts splitting at the ends. 4) Place the crumbled threads in a water content container and obtain the wet weight immediately. 5) Repeat steps 1 through 4 two more times. Determine the water content of the samples after drying the soil in the oven for a minimum of 16 hours and calculate the average water content. Complete Tables 1 and 2 and Figure 1. The flow index is the slope of the best-fit line from the plot of water content versus the log of the number of blows. The toughness index is equal to the plasticity index divided by the absolute value of the flow index. Table 1 Atterberg Limits Calculations Atterberg Limits Tests Soil Description Tare no. Tare mass (g) Mass of wet soil + tare (g) Mass of dry soil + tare (g) Mass of dry soil (g) Mass of moisture (g) Water content, % Number of blows Group Date Liquid Limit Test Plastic Limit Test Average water content: Atterberg Limits 3

4 30 Moisture Content, % Number of Blows, N Figure 1 Moisture Content versus the Log of the Number of Blows Sample Calculations: Table 2 Soil Classification Percent Passing No. 200 > 50% Liquid Limit (w L ) Flow Index (I F ) Plastic Limit (w p ) Plasticity Index (I P ) Toughness Index (I T ) AASHTO Classification USCS Classification Conclusions Are the Atterberg limits representative of the type of soil tested in the laboratory? Are the resulting classifications representative of the type of soil tested in the laboratory? If the water content of an in situ soil (subsurface soil) were equal to the liquid limit, would it flow like a liquid? Atterberg Limits 4

5 Wash Bottle Liquid Limit Device Grooving Tools Mortar and Pestle Picture 1- Liquid Limit Apparatus Soil Threads 3.2mm (1/8 inch) Rods Ground Plate Glass Picture 2- Plastic Limit Apparatus Atterberg Limits 5