Laboratory 2 Hydrometer Analysis Atterberg Limits Sand Equivalent Test

Transcription

1 Laboratory 2 Hydrometer Analysis Atterberg Limits Sand Equivalent Test INTRODUCTION Grain size analysis is widely used for the classification of soils and for specifications of soil for airfields, roads, earth dams, and other soil embankment construction. The hydrometer analysis determines the relative proportions of fine sand, silt and clay contained in a given soil sample. A knowledge of the range of moisture content over which a soil will exhibit a certain consistency is beneficial to the understanding of how a soil might behave when used as a construction material. The Atterberg limits, which include the liquid limit and plastic limit, are readily accepted in the engineering community as an objective measure of consistency. When coarse soil particles (sand and gravel) are used as a construction material, their suitability and behavior is influenced by the amount of clay fines that may be present after processing. The Sand equivalent test, developed to provide and indication of the clay content of a coarse aggregate, may be used as an indicator for specification compliance. HYDROMETER ANALYSIS A hydrometer analysis is required to determine the particle size distribution for that portion of the soil which passes through a No. 200 sieve (0.075 mm). The test is conducted on that fraction of a soil sample which passes through a No. 10 sieve (2 mm); however the sand particles in excess of mm settle almost immediately and thus little information about their size and relative proportion is obtained during this test. Mechanical sieve analyses are commonly used to determine the relative distribution of soil particles greater than mm. When both the mechanical and hydrometer methods are performed on the same soil, the analysis is said to be a combined analysis. The hydrometer method depends on Stoke s equation for the terminal velocity of a falling sphere. Stoke s equation was developed for perfect spheres whereas most silt and clay particles are platey shaped. Furthermore, clay particles have a tightly bound layer of adsorbed water which remains on the particle as it falls through the water column, resulting in a greater resisting surface than that of the clay particle alone. Notwithstanding these discrepancies, the hydrometer method is accepted as being of value in attempting to learn the diameter and proportion of the smallest soil particles. 1

2 Prior to the conduct of the hydrometer test, the hydrometer bulb (151 H) is calibrated to the dispersing solution and prevalent test temperatures. This is simply accomplished by obtaining hydrometer readings in a 5g/l sodium hexametaphosphate solution at two or more temperatures. The 151 H hydrometer bulb is manufactured to provide a reading of when placed in pure distilled water at 21 o C. Because the sodium hexametaphosphate solution has a specific gravity greater than 1, a hydrometer reading in excess of will be obtained. The difference between this reading and unity is considered as a composite correction factor which is applied to all subsequent hydrometer readings of the soil-water suspension. To provide reasonably accurate results, a soil sample must be completely broken down into individual soil grain prior to testing. This is accomplished by thorough wetting and mixing of the soil in a dispersing agent. A concentrated solution of water and sodium hexametaphosphate (40g/l) is used for this purpose. After complete dispersion, the soil-water suspension is introduced into a 1 litre settlement tube and diluted with distilled water such that the resulting sodium hexametaphosphate solution a concentration of 5g/l. Successive, timed measurements of the specific gravity of the soil-water suspension, using a calibrated hydrometer bulb, provides an indication of the maximum size of a soil particle still in suspension and the proportion of soil fines still in suspension. These values are then used to compute the percent of soil by weight finer than a given diameter. ATTERBERG LIMITS When clay minerals are present in fine grained soil, the soil can be remolded in the presence of some moisture without crumbling. In the early 1900's, a Swedish soil scientist named Albert Atterberg proposed a set of six rather arbitrary states of soil moisture content to assist agriculturists in determining field agricultural conditions. He termed the divisions between these six states as limits, known as the shrinkage, cohesive, sticky, plastic and liquid limits. The methods suggested by Atterberg to determine the moisture contents associated with each limit were highly empirical and not very applicable to engineering. In 1942, Arthur Casagrande revised the original agricultural definitions, dropped the cohesive and sticky limits, and developed procedures that could be adopted for engineering applications. It has been found that the water contents corresponding to the transitions from one state to another usually differ for clays having different physical properties in the remolded state, and are approximately equal for clays having similar physical properties. Therefore, the limiting water contents, or limits, may serve as index properties useful in the classification of clays. Actually, as the soil-water mixture passes from one state to another, there is no abrupt change in the physical properties. The Atterberg limit tests, therefore, are arbitrary tests that have been adopted to define the limiting values. The Atterberg limits vary with the amount of clay present, the type of clay mineral, and the nature of the ions adorbed on the clay surface. 2

3 Unlike finer soil particles, gravels and sands do not possess the required cohesiveness which permits the Atterberg limits tests to be performed. However, the finer sands and silts often contain sufficient clay coatings to permit the tests to be successfully completed. Thus the Atterberg tests are performed on only that soil fraction which passes through a No. 40 sieve (0.425 mm). Shrinkage Limit The shrinkage limit is defined as the moisture content at which no further volume change (reduction) occurs with a further reduction in moisture content. An alternative definition defines the shrinkage limit as the moisture content representing the amount of water required to fill the voids in a given cohesive soil at its minimum void ratio obtained by drying. Plastic Limit The plastic limit is defined as the moisture content at which a soil thread just begins to crack and crumble when rolled to a diameter of 1/8" (3 mm). Liquid Limit The liquid limit is defined as the moisture content at which a 2-mm-wide groove in a soil pat will close for a distance of ½" (12.5 mm) when dropped 25 times in a standard brass cup, falling 1 cm each time at a rate of 2 drops per second. SAND EQUIVALENT TEST Most granular soils and fine aggregates are mixtures of desirable coarse partiles, sand, and undesirable clay or plastic fines. The sand equivalent test is intended as a rapid field correlation test to indicate the relative proportions of clay-like or plastic fines and dust in granular soils and fine aggregates that pass the No. 4 (4.75 mm) sieve size. The test assigns an empirical value to the relative amount, fineness, and character of clay-like material present in a test specimen. A minimum sand equivalent value may be specified to limit the permissible quantity of clay-like fines in an aggregate. 3

4 CEEN Geotechnical Engineering - Laboratory Session No. 2 Grain Size Determination (Hydrometer Method), Atterberg Limits, Sand Equivalent Test OBJECTIVE: To obtain data necessary for the classification of a soil. EQUIPMENT: 151H Hydrometer bulb, scale, sodium hexametaphosphate dispersion solution, mixing apparatus, beaker, sedimentation cylinder, thermometer, liquid limit device, porcelain dish, spatula, balance, moisture content cans, glass plate, distilled water, drying oven, sand equivalent apparatus, working calcium chloride solution. REFERENCE SPECIFICATIONS: ASTM D , D LAB PROCEDURES: Part 1 - Hydrometer Calibration (Data Sheet 1) 1. Select and clean a 151H hydrometer bulb and record the identifier number. 2. Obtain hydrometer calibration readings in each of the the 1 L graduated cylinders filled with a 5g/L solution of sodium hexametaphosphate in distilled water. Record the temperature of the solutions to 0.5 o C. Part 2 - Sedimentation Test (Data Sheet 2) 1. Obtain a 100 g sample of air-dried soil (minus #10 soil from Lab 1) and place in a 400-mL beaker. Cover with 125 ml of concentrated sodium hexametaphosphate solution (40g/L). Stir until the soil is thoroughly wetted and allow to soak for at least 15 minutes. After soaking transfer the soil-water slurry from the beaker into the dispersion cup, washing any residue from the beaker with distilled water. Add distilled water, if necessary, to fill the dispersion cup approximately half full. Mix the suspension in the mixer for 1 min. 2. Immediately after mixing, wash the specimen into a 1 L graduated cylinder and add enough distilled water to bring level to the 1 L mark. 3. Mix soil and water in cylinder by placing a rubber stopper over the open end and turning the graduate upside down and back for 1 min. The number of turns during this minute should be approx. 60, counting the turn upside down and back as two turns. Any soil remaining in the bottom of the cylinder during the first few turns should be loosened by vigorous shaking of the cylinder while it is in the inverted position. 4. After shaking, replace the cylinder on the table, start the timer, and insert the hydrometer in the suspension. Record the hydrometer readings ( top of the meniscus formed by the suspension around the stem) at elapsed times of ½, 1, 1-½ 2, 5, 10, 20, 30, 40, 60 and 80 minutes. Part 3- Liquid Limit Test (Data Sheet 3) 1. Dry sieve the soil remaining from Lab 1 through a No. 40 sieve. Obtain a 200 g sample of the soil which passes the No. 40 sieve, either by discarding excess soil or by adding additional soil from the control jar. 2. Check the fall height of the liquid limit cup using the end of the grooving tool and adjust as necessary. Record the mass of 5 marked moisture cans to the nearest 0.01g. Three will be available for the liquid limit test and two for the plastic limit test. 4

5 CEEN Geotechnical Engineering - Laboratory Session No. 2 Grain Size Determination (Hydrometer Method), Atterberg Limits, Sand Equivalent Test 3. Place the 200 g of soil on a glass plate and add about 15 ml of distilled water. Mix soil and water thoroughly using an alternate of repeated stirring, kneading and chopping action with the spatula. Continue adding water at the rate of 1 to 3 milliliter increments and thoroughly mix each increment into the soil before adding the next. Enough water should be thoroughly mixed to produce a consistency that will require 25 to 35 drops of the cup to cause the groove to close. 4. Place a portion of the prepared soil mixture in the cup of the liquid limit device at the point where the cup rests on the base, squeeze it down, and spread it into the cup to a depth of about 10 mm at its deepest point, tapering it to form an approximately horizontal surface. Take care to eliminate air bubbles from the soil pat but form the pat with as few strokes as possible. Heap the unused soil on the glass plate and cover with an inverted storage dish or wet towel. 5. Form a groove in the soil pat by drawing the tool through the soil on a line joining the highest point to the lowest point on the rim of the cup. Hold the grooving toll against the surface of the cup and draw in an arc, maintaining the tool perpendicular to the surface of the cup. Avoid tearing the sides of the soil groove and do not permit the soil pat to slide in the cup. Up to six strokes are permitted to form the groove. 6. Using a continual motion of the crank, lift and drop the cup at the rate of two drops per second. Record the number of drops of the cup required to cause the two halves of the soil pat to flow together for a distance of 13 mm (1/2 in). 7. Remove a slice of soil approximately the width of the spatula, extending from edge to edge of the soil cake at right angles to the groove and including that portion of the groove in which the soil flowed together. Record the mass of the moist soil and moisture tin to the nearest 0.01g. Place the tin in a drying oven. 8. Return the soil remaining in the cup to the glass plate. Wash and dry the cup and grooving tool and reattach the cup to the carriage. Remix the entire soil specimen on the glass plate adding distilled water to increase the water content of the soil and decrease the number of blows required to close the groove to between 20 to 30 blows. 9. Repeat Steps 4 through 8 for at least two additional trials producing successively lower blow counts to close the groove. One of the trials shall be for closure requiring 20 and 30 blows and one for closure between 15 and 25 blows. 10. Record the mass of the oven dried soil and moisture tin to the nearest 0.01g. Part 4 - Plastic Limit Test (Date Sheet 3) 1. Select a 20 g portion of the soil from the material remaining after the liquid limit test. Reduce the water content of the soil to a consistency at which it can be rolled without sticking to the hands by spreading and mixing continuously on the glass plate. The drying process may be accelerated by blotting with paper that does not add any fiber to the soil, such as hard surface paper toweling. 5

6 CEEN Geotechnical Engineering - Laboratory Session No. 2 Grain Size Determination (Hydrometer Method), Atterberg Limits, Sand Equivalent Test 2. From the 20 g mass, select a portion of 1.5 to 2.0 g and form into an ellipsoid. Cover the remaining soil with a moist towel. Roll this mass between the palm or fingers and the glass plate with just sufficient pressure to roll the mass into a thread of uniform diameter throughout its length. When the diameter of the thread becomes 3 mm, break the thread into several pieces. Squeeze the pieces together, knead between the thumb and first finger of each hand, reform into an ellipsoid, an re-roll. Continue to alternate rolling, gathering, kneading, and re-rolling until the thread crumbles under the pressure required for rolling and the soil can no longer be rolled into a 3 mm diameter thread. 3. Gather the portions of the crumbled thread together and place in a moisture tin and immediately cover. 4. Repeat steps 2 and 3 until the moisture tin contains at least 6 g of moist soil. Record the mass of the moist soil and tin (without cover) to the nearest 0.01g. Place the moist soil and tin in a drying oven. 5. Repeat steps 2 through 4 to produce another moisture tin containing at least 6 g of soil. Record the mass of the moist soil and tin (without cover) to the nearest 0.01g. Place the moist soil and tin in a drying oven. 6. Record the mass of the oven dried soil and moisture tin to the nearest 0.01g. Part 5 - Sand Equivalent Test (Data Sheet 4) 1. Obtain a 500 g sample of soil passing the No. 4 sieve. Fill one tin measure to the brim or slightly rounded above the brim. 2. Siphon approximately 4 in of working calcium chloride solution into the plastic cylinder. Pour the soil sample into the cylinder using the funnel to avoid spillage. Allow the wetted specimen and cylinder to stand for approximately 10 min. 3. After the 10 min soaking period, hold the cylinder in a horizontal position and shake vigorously in a horizontal linear motion from end to end. Shake the cylinder 90 cycles (back and forth motion) in approximately 30 seconds using a throw of 9 inches. 4. Irrigate the sample using the working calcium chloride solution by forcing the irrigator tip through the material to the bottom of the cylinder while the solution is flowing. Continue stabbing and twisting the irrigator tip until the cylinder is filled to the 15 inch graduation mark. Raise the irrigator tube slowly without stopping the flow so that the liquid level is maintained at about the 15 inch mark. Regulate the flow just before complete removal of the irrigator tip so that the final fluid level is at the 15 inch mark. 5. Allow the cylinder and contents to stand undisturbed for 20 minutes after the removal of the irrigator tube. 6. Record the level of the top of the clay suspension after the 20 minute rest period. Place the weighted foot assembly over the cylinder and gently lower the assembly until it comes to rest on top of the sand. Tilt the assembly towards the gradations on the cylinder until the indicator touches the inside of the cylinder. Subtract 10 inches for the level indicated by the extreme top edge of the indicator and record this value as the sand reading. 6

7 CEEN Geotechnical Engineering - Laboratory Session No. 2 Grain Size Determination (Hydrometer Method), Atterberg Limits, Sand Equivalent Test CALCULATIONS: 1. Using the hydrometer calibration data from Data Sheet 1, prepare a linear plot of the composite correction factor vs temperature and develop an equation to predict the composite correction factor for any intermediate temperature. Using the equation below, complete Data Sheet 2 to determine the grain size distribution of the soil sample. Plot these results in combination with Lab 1 dry sieve data and develop a single, smooth grain size distribution curve for the soil sample. The percentage (P) of soil remaining in suspension and the largest diameter (D) of soil in suspension at the level of the hydrometer are calculated as: P 1000 G S P 10 M S R & 1 G S & 1 D K L T where: P = percentage of soil in suspension, % G S = specific gravity of soil particles (Lab 1) P 10 = percent of original soil sample which passes No.10 sieve (Lab 1) M s = dry mass of soil, g R = corrected hydrometer reading (hydrometer reading - composite correction factor) D = diameter of soil particle, mm K = constant depending temperature and specific gravity of the soil (Table 1) L = effective depth, equal to the distance from the surface of the suspension to the level at which the density of the suspension is being measured, cm (Table 2). T = time of hydrometer reading, min. 2. Using the liquid limit data from Data Sheet 3, determine the moisture content of each container of soil after oven drying. Plot the results of the liquid limit tests as discrete data points, each with corresponding blow count and moisture content. Data should be plotted on semi-logarithmic paper with the moisture content as ordinates on the arithmetic scale and blow counts as abscissas on the log scale. Draw the best fit straight line through each set of data to obtain the flow curve. Report the liquid limit (LL) of the soil as the water content corresponding to the intersection of the flow curve with the 25 blows abscissa, rounded to the nearest whole number. 3. Using data from Data Sheet 3, compute the moisture content for each of the plastic limit trials. Report the plastic limit (PL) of the soil as the average of these two values, rounded to the nearest whole number. 4. Compute the sand equivalent (SE) for each sample to the nearest 0.1%. If the calculated SE is not a whole number, report the SE to the next higher whole number (i.e., 41.2 = 42). Prepare a plot of the sand equivalent vs % clay. Comment on the computed SE values based on the % clay in the soil sample. 7

8 CEEN Geotechnical Engineering - Laboratory Session No. 2 Grain Size Determination (Hydrometer Method), Atterberg Limits, Sand Equivalent Test HYDROMETER CALIBRATION DATA DATA SHEET 1 Hydrometer Reading Temperature of 5g/L Sodium Hexametaphosphate Solution C Composite Correction Factor SOIL DATA Weight of Beaker, g Weight of Beaker + Dried Soil, g Weight of Dried Soil, g (Ms) P 10 (Lab 1) Specific Gravity of Soil Solids, G S, (Lab 1) 8

9 CEEN Geotechnical Engineering - Laboratory Session No. 2 Grain Size Determination (Hydrometer Method), Atterberg Limits, Sand Equivalent Test DATA SHEET 2 Time Elapsed Time min Hydrom. Reading Temp o C Comp. Corr. Factor Corr. Hydrom. Reading R K Factor (Table 1) Effective Depth (Table 2) L Percent of Soil in Suspension Particle Diameter mm 9

10 CEEN Geotechnical Engineering Laboratory Session No. 3 - Liquid Limit and Plastic Limit Tests LAB DATA SHEET 3 LIQUID LIMIT TESTS Trial 1 Trial 2 Trial 3 Moisture Tin Number Moisture Tin Wt, g Number of Drops Wt. Wet Soil + Tin, g Wt. Oven-Dry Soil + Tin, g Calculations Wt. Water, g Wt. Oven-Dry Soil, g Moisture Content, w,% PLASTIC LIMIT TESTS Trial 1 Trial 2 Moisture Tin Number Moisture Tin Wt, g Wt. Wet Soil + Tin, g Wt. Oven-Dry Soil + Tin, g Calculations Wt. Water, g Wt Oven-Dry Soil, g Moisture Content, w, % 10

11 CEEN Geotechnical Engineering - Laboratory Session No. 2 Grain Size Determination (Hydrometer Method), Atterberg Limits, Sand Equivalent Test SAND EQUIVALENT DATA DATA SHEET 4 Soil Sample Clay Reading inch Sand Reading inch Sand Equivalent (1) 100% S 95%S - 5%C 90%S - 10%C Lab Sample (1) Sand Equivalent = 100% x (Sand Reading / Clay Reading) 11

12 Table 1: Values of K for Computing Particle Diameter in Suspension Temperature o C Gs

13 Table 2: Effective Depth vs 151 H Hydrometer Reading Corrected Hydrometer Reading Effective Depth, L (cm) Corrected Hydrometer Reading Effective Depth, L (cm)

14 CEEN Lab 2 Hydrometer Calibration Data Composite Correction Factor Temperature, C 14

15 CEEN Hydrometer Test Results % Passing Grain Size, mm 15

16 CEEN Liquid Limit Test Results Water Content, % Drops 25 16

17 CEEN Geotechnical Engineering - Laboratory Session No. 2 Grain Size Determination (Hydrometer Method), Atterberg Limits, Sand Equivalent Test HYDROMETER CALIBRATION EXAMPLE DATA SHEET 1 Hydrometer Reading Temperature of Sodium Hexametaphosphate Solution (5g/L) Composite Correction Factor SOIL DATA Weight of Beaker, g Weight of Beaker + Dried Soil, g Weight of Dried Soil, g 99.9 P 10 (Lab 1) 89.5 Specific Gravity of Soil Solids, G S, (Lab 1)

18 CEEN Geotechnical Engineering - Laboratory Session No. 2 Grain Size Determination (Hydrometer Method), Atterberg Limits, Sand Equivalent Test EXAMPLE DATA SHEET 2 Time Elapsed Time min Hydrom. Reading Temp o C Comp Corr Factor (1) Corr Hydrom Reading R (2) K Factor (Table 1) (3) Effective Depth (Table 2) L (4) Percent of Soil in Suspension (5) Particle Diameter mm (6) 9:04:00 0 9:05: :06: :06: :07: :10: :14: :24: :34: :44: :54: :04: (1) Determined from equation developed from hydrometer calibration data (2) Hydrometer reading - composite correction factor (3) Determined from Table 1 based on temperature and specific gravity of soil solids (4) Determined from Table 2 based on corrected hydrometer reading (5) Calculated based on equation provided; P = fn {Gs, P 10, Ms, R} (6) Calculated based on equation provided; D = fn {K, L, T} CEEN Geotechnical Engineering Laboratory Session No. 2 - Liquid Limit and Plastic Limit Tests 18

19 EXAMPLE DATA SHEET 3 LIQUID LIMIT TESTS Trial 1 Trial 2 Trial 3 Moisture Tin Number A1 D1 E4 Moisture Tin Wt, g Number of Drops Wt. Wet Soil + Tin, g Wt. Oven-Dry Soil + Tin, g Calculations Wt. Water, g Wt. Oven-Dry Soil, g Moisture Content, w,% PLASTIC LIMIT TESTS Trial 1 Trial 5 Moisture Tin Number A2 J2 Moisture Tin Wt, g Wt. Wet Soil + Tin, g Wt. Oven-Dry Soil + Tin, g Calculations Wt. Water, g Wt Oven-Dry Soil, g Moisture Content, w, %

20 CEEN Geotechnical Engineering - Laboratory Session No. 2 Grain Size Determination (Hydrometer Method), Atterberg Limits, Sand Equivalent Test SAND EQUIVALENT DATA DATA SHEET 4 Soil Sample Clay Reading inch Sand Reading inch Sand Equivalent (1) 100% S %S - 5%C %S - 10%C Lab Sample (1) Sand Equivalent = 100% x (Sand Reading / Clay Reading) 20

21 CEEN Lab 2 Hydrometer Calibration Data Y = X Composite Correction Factor (x10^-3) Temperature, C 21

22 CEEN Hydrometer Test Results % Passing Grain Size, mm 22

23 CEEN Liquid Limit Test Results LL = 21.6 = 22 Water Content, % Drops 25 23

24 CEEN Sand Equivalent Test Results Sand Equivalent % Clay 24