Standard Test Procedures Manual

Transcription

1 STP Standard Test Procedures Manual Section: 1. SCOPE 1.1. Description of Test This method describes the procedure for determining the liquid limit, plastic limit and the plasticity index of coarse-grained soils. This includes all materials having more than 50% of its particles retained on a No. 400 µm Canadian Metric sieve Application of Test The liquid limit, plastic limit, and plasticity index of soils are used extensively, either individually or together with other soil properties to correlate with engineering behaviour such as compressibility, permeability, compactibility, shrink-swell and shear strength Units of Measure and definitions The liquid limit and plastic limit are called the Atterberg limits and represent the water content at which defined level of consistencies are achieved. The limits vary according to the percent of clay in the mixture. The plasticity index of a soil is the numerical difference between the liquid and plastic limits, the range of water content over which the soil behaves plastically. It is an indication of the clayeyness or plasticity of a clay. The liquid limit is the water content, in percent, at which the soil passes from a plastic state to a liquid state. At the liquid limit, the soil becomes a semifluid like softened butter. The plastic limit is the water content, in percent, at which the soil passes from a plastic state to a brittle state. At the plastic limit, the soil begins to crumble while being rolled into a 3 mm diameter thread. 2. APPARATUS AND MATERIALS 2.1. Equipment Required -Pans, a supply of suitable pans or pails. Date: Page: 1 of 9

2 -Drying Apparatus - oven or stove suitable for drying samples. -Sieve - a No. 400 µm sieve with pan. -Evaporating Dish - a plastic dish about 100 mm in diameter. -Spatula - a spatula having a blade approximately 75 mm in length and 19 mm in width. -Grooving Tool - a combination grooving tool and gauge as shown in Figure Surface for Rolling - a ground glass plate or piece of smooth unglazed paper used for rolling plastic limit determinations. -Containers - suitable containers such as aluminum tares with covers to prevent the loss of moisture during the weighing of the sample. -Balance - a balance sensitive to 0.02 g. -Siphon - rubber tubing about 5 mm in diameter. -Pulverizing Apparatus Materials Required A supply of clean water Samples To Be Tested Obtain a sample as described in Standard Test Method STP 105. Mix the sample well and use a quartering method or sample splitter to reduce the sample to sufficient size to yield approximately 115 g of material passing the No. 400 µm sieve. The final sample size should be approximately 1500 g to provide representative and sufficient material for both the liquid and plastic limit determinations Data Required Sample information such as contract number, pit file or land location, date sampled, operation that aggregate is used for, test number and aggregate type. Page: 2 of 9 Date:

3 3. PROCEDURE 3.1. Description of Equipment Preparation Adjust the liquid limit device by means of the adjustment plate on the device and the gauge on the handle of the grooving tool. Adjust so the centre of the wear point on the bottom of the cup is lifted exactly 1 cm above the base. Secure adjustment plate by adjusting the screws Sample Preparation Place the aggregate sample in a pan and cover with tap water to soften and break down particle aggregations. Break up clay lumps with the fingers. Agitate sample to bring fine material into suspension and pour the suspension through the 400 µm sieve into a pail. Repeat washings until the wash water is clear and the material in the pan is clean. Dry the washed material and dry-sieve on the 400 µm sieve. Allow the wash water in the pail to settle until the water above the soil is clear. This will take several hours. Carefully siphon off as much water as possible from the pail without losing any soil. Transfer the soil from the pail to a pudding pan and dry the soil at a temperature not exceeding 110 o C. Carefully scrape and brush the material (the fines) from the pail and pulverize with a rubber tipped mortar or in some other way that does not cause the breakdown of individual grains and sieve the material through the 400 µm sieve. Return any material retained on the 400 µm sieve to the pulverizing apparatus and repeat the pulverizing and sieving operations as many times as necessary to ensure that all the material has passed through the 400 µm sieve. Date: Page: 3 of 9

4 Mix thoroughly the pulverized material (the fines) and add to the material that has been previously dried and sieved on the 400 µm sieve. The combined sample will be used for the determination of the liquid and plastic limits Test Procedure Liquid Limit Determination Place approximately 100 grams of prepared soil in the evaporating dish and add 15 to 20 ml of demineralized water and mix water thoroughly with the soil. Then add sufficient water and thoroughly mix to form a uniform mass of stiff consistency. The material will be used to perform two liquid limit tests and one plastic limit test. Place a portion of the prepared mixture in the cup over the spot where the cup rests on the base and spread into place with as few strokes as possible to a depth of 10 mm. Take care to prevent the entrapment of air bubbles within the mass. Keep the unused soil in the evaporating dish and cover with a wet towel or other means to retain moisture in the sample. Divide the soil in the cup with a firm stroke of the grooving tool along the diameter through the centerline of the cam follower. Sandy materials may require several strokes of the grooving tool to make satisfactory groove without tearing the soil. For these soils, increase the depth of the groove with each stroke and only the last stroke should scrape the bottom of the cup. Turn the crank to raise and drop the cup twice per second until the two sides of the grooved sample come in contact at the bottom of the groove along a distance of 12.5 mm. Record the number of blows. If the number of blows is between 15 and 40, take a representative sample of the soil in the cup for moisture content. If the number of blows is outside the range of 15 to 40, transfer the soil from the cup to the evaporating dish. Adjust the moisture content by mixing, with or without the addition of water and repeat the test until the blows fall within the range of 15 to 40 blows. Page: 4 of 9 Date:

5 Clean the cup of the liquid limit device after each trial test. Determine the moisture content by placing the sample in a tared aluminum can and weighing. Remix the soil in the evaporating dish and repeat the test and obtain a second liquid limit sample for a duplicate result. Oven dry the samples at 110 o C to a constant weight and weigh after cooling Plastic Limit Determination Use approximately 20 g of the prepared soil for the plastic limit determination. Reduce the water content of the soil to a consistency at which it can be rolled without sticking to the hands by spreading or mixing continuously on a glass plate or in a storage dish. The drying process can be accelerated by exposing the soil to the air current from an electric fan or by blotting with paper that does not add fiber to the soil. Take approximately 8 to 12 g of the moistened soil and form into a uniform mass roughly ellipsoidal in shape. Roll the ball of soil by hand on the rolling surface with just enough pressure to form an elongated thread as rolling proceeds. If the soil can be rolled to a thread 3 mm thick without crumbling, amass it and re-roll it. Repeat rolling and amassing until the soil crumbles under slight pressure required for rolling. Crumbling may occur when the soil has a diameter greater than 3 mm, however, this is considered a satisfactory end point, provided the soil has been previously rolled into a 3 mm thread. Gather portions of the crumbled soil and place in a tared aluminum can and weigh, then oven dry at 110 o C to a constant weight and weigh again after cooling. 4. RESULTS AND CALCULATIONS 4.1. Collection of Test Results The collection of test results should be recorded on the required Department form. Date: Page: 5 of 9

6 4.2. Calculations Calculations For Liquid Limit Record the numerical difference between the wet and dry weight to determine the weight of moisture. Determine the percent moisture using the formula: The percent moisture = Weight of Dry weight of Moisture sample X 100 The percent moisture and corresponding number of blows are used to calculate the liquid limit (W L ) at 25 blows. Adjust the percent moisture of the sample by dividing by a denominator (correction factor) selected from the following table: No. of Blows Denominator of Formula (Correction Factor) No. of Blows Denominator of Formula (Correction Factor) Calculate the liquid limit for each test and average the results as shown in the following example. Liquid Limit W L (25 blows) = Percent Moisture Correction Factor Page: 6 of 9 Date:

7 Test 1 2 Blows Percent Moisture W L (25 blows) = = Calculations for Plastic Limit Record the difference between the wet and dry weight as the weight of moisture. Calculate the plastic limit W P by dividing the "weight of moisture" by the "dry weight of sample" and multiplying by 100. Plastic Limit (W P ) = Weight of moisture Weight of oven dry soil x Calculation for Plasticity Index Plasticity Index (Ip) of a soil is the numerical difference between the liquid limit and the plastic limit. Plasticity Index (Ip) = W L - W P 4.3. Report of Results The results should be reported on the required Department form. Report as non-plastic if the liquid limit or plastic limit cannot be determined. Also report as non-plastic if the plastic limit is equal or greater than the liquid limit. 5. CALIBRATIONS, CORRECTIONS, REPEATABILITY 5.1. Equipment Calibrations It is important to ensure that the proper drop height is set for the Liquid Limit Device. Also it is very important to inspect wear of the Liquid Limit Device. The device should be clean and in good working order. The following points should be checked as described in ASTM D4318 Standard Test Method for Liquid Limit, Plastic Limit and Plasticity Index of Soils: Date: Page: 7 of 9

8 1) Wear of the base 2) Wear of the cup 3) Wear of the cup hanger 4) Wear of the cam 5) Grooving tools 5.2. Tolerances and Repeatability Tolerances for equipment are as described in this procedure and shown in Figure The duplicate liquid limit results should be repeated if they do not fall within the limits as shown below. W L < 30 Repeatability within 1.0% 30 < W L < 60 Repeatability within 2.0% W L > 60 Repeatability within 3.0% 5.3. Sources of Error It is important to take the correct amount of soil for the plastic limit test. The usual error is to take too large a sample which may give an incorrect answer. For the liquid limit test, it is important to thoroughly mix soil and water. Avoid adding dry soil to a mixture which is too wet; dry it by stirring instead. The washings in the pail must be allowed to settle completely without disturbance. This will require setting aside for several hours and in some cases even overnight. To ensure complete pulverized of the wasted and dried minus 400 µm material, it is advisable to continue pulverization until all the material passes the 400 µm sieve. 6. ADDED INFORMATION 6.1. References ASTM D4318 For plasticity index of fine grained soils refer to STP Page: 8 of 9 Date:

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