STANDARD OPERATING PROCEDURES

Transcription

1 Date: July 27, 2014 Author: Emily Barasa STANDARD OPERATING PROCEDURES METHOD FOR DETERMINATION OF ATTERBERG LIMITS AND LINEAR SHRINKAGE IN SOILS Contact Details: Telephone: +254 (20) /4235/4279/ : Laboratory Manager, Website: www. Address Details: World Agroforestry Centre, Mailing: P.O Box Nairobi, Kenya Physical address: United Nations Avenue Off Limuru Road, Gigiri, Nairobi, Kenya

2 METHOD DOCUMENT CONTROL LOG Name and position Signature Author(s) Emily Mwake Barasa_Senior Laboratory Technician [signature for completeness and correctness of document] Verifiers Josephine Muteti_Laboratory Technician [signature for completeness and correctness of document] [signature for completeness and correctness of document] Authorizer Mercy Nyambura_Laboratory Manager Elvis Weullow_Deputy Laboratory Manager. Distribution of valid copies of this document: D102B In transition [signature for completeness and correctness of document] Revise before: Historical data Date retrieval: [date] [dates of previous issues] [to be filled in once the document has become obsolete and is archived] Changes in this version compared to previous version: [Describe what has changed in the text of this version compared to the previous version] 2 A t t e r b e r g S O P

3 SCOPE AND APPLICATION This procedure describes the determination of the bulk density, Atterberg limits (liquid and plastic limits) using cone penetrometer and linear shrinkage. The Atterberg limits are a basic measure of the nature of a fine-grained soil such as its shrinkage limit, plastic limit, and liquid limit. Depending on the water content, soil- may appear in four states; solid, semi-solid, plastic and liquid. In each state the consistency and behavior of the soil is different and thus so are its engineering properties. Therefore, the boundary in each state can be defined based on a change in the soil s behavior. The Atterberg limits can be used to distinguish between silt and clay and it can distinguish between different types of silts and clay. PRINCIPLE Bulk density is a measure of the weight of the soil per unit volume (g/cc), usually given on an oven-dry (110 C) basis. Variation in bulk density is attributable to the relative proportion and specific gravity of solid organic and inorganic particles and to the porosity of the soil. Bulk density is important in quantitative soil studies and is generally used in calculating soil moisture movement within a profile and to compare soils qualitatively by converting percent to weight per unit volume. The determination usually consists of drying and weighing a soil sample, the volume of which is known. Liquid or plastic limit of soils is a measure of their consistency (compressibility, permeability, to compact, shrink-swell and shear strength). The method is based on the relationship between the moisture content and the penetration of a cone into the moist soil. The liquid limit (LL) is the moisture content at which the soil passes from the plastic, ductile behavior to the liquid, flowing behavior as determined by the liquid limit test. It is also defined as the water content at which the cone penetrates the moist soil to a depth of 20mm when dropped from a standardized height. The plastic limit (PL) is the soil water content as the soil passes from the plastic, ductile behavior to the brittle, cracking behavior. Plasticity index (PI) is the range of the water content over which a soil behaves plastically. Soil shrinkage (LS) is the limit to which a soil material contracts as it loses water on drying. ABBREVIATIONS AND DEFINITIONS LL Liquid Limit PL Plastic Limit P10 Plasticity index LS Soil shrinkage L Length of the mound (mm) L s Longitudinal length of the oven dry soil bar (mm) 3 A t t e r b e r g S O P

4 RELATED DOCUMENT Method for operating penetrometer RELATED FORMS Logging form Recording form SAFETY AND ENVIRONMENT Personal protection: Heavy-duty gloves should always be used when removing samples from the oven to avoid burns. Care should be taken when handling the penetrometer to avoid injury. Electrical hazards: Electrical systems must conform to the ICRAF standards. Shock hazards exist inside the instruments. Only an authorized service representative or an individual with training in electronic repair should remove panels or circuit boards where voltages are greater than 20 V. The instruments require a third-wire protective grounding conductor. Three-to-two wire adapters are unsafe for these instruments. REQUIREMENTS Equipment a. Standard cone penetrometer b. Penetration cone assembly, a stainless steel cone with a cone angle of 30±1 O and total moving mass of 80±0.1g, automatic zeroing device, displacement device c. Penetration container (cups) approximately 55mm in diameter and 40mm deep with a rim parallel to the flat base d. Balance of 2kg capacity and 0.001g accuracy e. Automatic controller with a built in 5±1 seconds timer: provides automatic control of the penetration period and also used as a timing device f. Oven capable of maintaining a temperatures of C g. Vernier caliper graduated in millimeters h. Brass steel molds in the form of semi-cylindrical troughs; 140mm long and 21mm internal diameter i. Mixing bowl approximately 150mm diameter, with an air-tight lid 4 A t t e r b e r g S O P

5 j. Two spatula or palette knives with blade approximately 100mm long and 20mm wide k. Test gauge: for checking the excessive wear to the point of the cone l. A flat glass plate m. Moisture boxes n. A metal straight edge o. Flat rubber mat p. Mortar and pestle q. Standard 425µm test sieve Materials & Supplies a. Petroleum jelly b. De-ionized water c. A damp cloth d. String 250mm long e. A scoop of 50 ml volume f. Wash bottle PROCEDURE Sample processing and preparation a. Air-dried Approximately 200g of soil grind and sieve through a 425µm sieve. NB: all portions of the sample must be ground with a pestle and motor to pass through the 425µm sieve. The history of the sample should also be noted. b. Place 80% of the testing portion of the soil in a mixing bowl, wet the soil with an increment of de-ionized water and mix thoroughly with a palette knife. Continue adding increments of water until the test portion becomes a thick homogeneous paste. Add water such that the range of penetration values between approximately 15mm to 25mm are obtained. c. After mixing the soil and before performing the test, the test portion should settle for an adequate period (As a preferred option the test portion can be cured at room temperature in an air tight container for at least 12 hours, and up to 24 hours for moist soils but the test can be done immediately after mixing for soils with low clay content.) to allow the water to permeate through the soil mass 5 A t t e r b e r g S O P

6 d. Weigh 45mg of Soil sample into a 5ml centrifuge tube and add 2.5ml of an aqueous Triton X100 solution Add 40 µl of 1000 mg/l Se internal standard solution d into the suspension using a calibrated pipette. Mix the Suspension thoroughly and sonicate it. Transfer 10µl of the suspension into siliconized quartz disc and dry. e. In the case of Plants, the sample is dried and milled through a 1-mm sieve. Plant sample (45mg) is weighed into a 5ml centrifuge tube and 2.5ml of an aqueous Triton X100 solution is added. 40 µl of 1000 mg/l Sc internal standard solution and 10 µl of 1000 mg/l Y are added into the suspension using calibrated pipettes. Suspension is mixed thoroughly and sonicated. 10µl of suspension is transferred onto a siliconized quartz disc and dried. f. Liquid samples are a diluted appropriately and the different dilutions are tested to find an optimal peak to scattering background ratio. The Liquid specimen (1000 µl) is then sampled into a labeled vial. For accurate determination of light elements only, add 40 µl of 100 mg/l Sc single element standard into the liquid sample and. For determination of heavier elements, transfer 50 µl of 100 mg/l Y single element internal standard using a calibrated pipette into the liquid sample. Note the 10 times dilution of the liquid standards as compared with the soil and plant methods. Liquid limit test a. After overnight curing, place the test portion on a flat glass plate and mix it with the palette knives for 10 to 40 minutes depending on the soil type. If necessary water or soil can be added so that the first cone penetration reading is approximately 16mm. b. Remove approximately 20g of the mixed soil with a palette knife and put aside for plastic limit determinations c. Place the mixed soil into the penetration cup palette knife and level it to a smooth top with the straight edge, taking care not to trap air. Tap the test cup on the rubber mat to remove air voids and refill it. Repeat this procedure until air voids are completely displaced and the cup is level filled. d. Assemble the drop cone apparatus and place the cone in the cone holder and raise the cone assembly to the highest position possible and level the penetrometer. Raise the penetration head as required. e. Place the cup on the base of the drop cone apparatus approximately central under the point of the penetration cone f. By adjusting, lower the penetration head until the tip of the cone makes contact with the surface of the soil such that a slight movement of the cup will just mark the surface of the soil. 6 A t t e r b e r g S O P

7 g. Depress the indicator rod of the dial gauge until it lightly touches the top of the shaft. Note and record the reading taken on the dial gauge to the nearest 0.1mm h. Set the automatic controllers and release the penetrometer shaft and allow the cone to penetrate the soil for 5 seconds and then cramp or restrain the penetrometer shaft. i. Depress the indicator rod of the dial gauge until it lightly touches the top of the shaft. Note and record the reading taken on the dial gauge to the nearest 0.1mm j. Calculate the difference between the two dial gauge readings and record as the penetration value(depth) k. Raise the penetration head to remove the cone from the soil and clean the cone thoroughly with a damp cloth l. Remove approximately 10g, of the test soil portion with a palette knife, from near the area penetrated by the cone and determine the water content of the test sample by oven drying at C.This is best facilitated by placing the penetration cup at a slight angle on the work bench and then drawing the palette knife in a downward and outward movement against the inside lip of the cup m. Remove the remaining soil from the cup and put it on the flat glass plate, remix it and if necessary (incase the first reading is not satisfactory) add water or soil and perform the next cone penetration. n. Repeat Procedures 3 to 12 is until three more determinations (18, 20 and 22) of soil water content and the penetration depth are obtained. Note: The test should always proceed from the drier to the wetter condition of the soil. Clean the cup and the cone thoroughly each time you remove the soil from th cup to add water. o. Reporting results Plot the moisture contents against the corresponding penetration depths for the four determinations on a linear graph with the percent moisture on the horizontal axis and the penetration depth on the vertical axis. Draw a straight line of best fit through the plotted points. Determine the moisture content corresponding to the intersection of the line of best fit and the 20mm penetration ordinate (interpolation). This moisture content is the liquid limit(ll)of the soil Linear Shrinkage test a. Clean and lightly grease the inner walls of the shrinkage mold to prevent the soil from adhering to the mold. Remove the wet soil from the cup after the 7 A t t e r b e r g S O P

8 determination of penetration 20mm, and put in the greased mold with the palette knife. Remove Air bubbles or voids by lightly tapping the base of the mold on a flat rubber mat. Slightly overfill the mold and then level off the excess soil with a palette knife. Remove all soil adhering to the rim and edge of the mold by wiping with a damp cloth to stop friction between the soil in the mold and any adhering to the edges of the mold. b. Air dry The soil water mixture in the mold slowly at room temperature for 12 to 24 hours and then transfer it to an oven and dry to constant weight at 105 o C. c. Allow the mold and the dry soil to cool and measure the mean length of the soil bar to the nearest millimeter. If the soil bar has cracked into pieces during drying, carefully reposition the cracked edges together, then firmly hold the separate parts together and measure the bar s length. If the soil bar curls or is curved, remove it carefully and measure the length of the top and bottom surfaces with a piece of string. The mean of the two lengths is the length of the soil bar. Note: Curling and curving can be prevented by slow drying of the mound and soil- water mixture. This can be done by air drying for periods of hours the oven drying of 60 o C 65 o C until shrinkage has largely ceased, and then at 105 o C to 110 o C to complete the drying. d. Reporting results - The linear shrinkage of the soil is calculated as a percentage of the original length of the soil bar in the mound from the equation: Percentage of the linear shrinkage LS = (L-L s /L) * 100 to the nearest 0.5% Plastic limit test a. Mold the 20g of cured soil put aside for plastic limits test into a ball and roll between the palms until the heat of the hands has dried the soil sufficiently for slight cracks to appear on its surface. Then divide sample into two subsamples for separate determination of each sub sample. b. Divide each subsample into four approximately equal parts and form each part into a thread of about 6 mm in diameter, between the first finger and thump of each hand. Roll the thread between the tips of the fingers of one hand and the surface of the glass plate. Apply sufficient pressure to reduce the diameter of the thread to about 3mm. Note: 5 to 15 complete forward and back movement of the hand is sufficient to reduce the thread to 3mm. Uniform rolling pressure should be maintained throughout the test. 8 A t t e r b e r g S O P

9 c. Pick and mold the 3mm thread between the fingers to further dry the soil and then formed into a thread and rolled out again. This procedure is repeated until the thread shears both longitudinally and transversely. This first crumbling point is the plastic limit. The portions of the crumbled soil thread are gathered together and transferred to a moisture container. The other three parts of the subsample is treated similarly and placed in the same moisture container. The moisture content of the soil is determined by oven-drying at C. This soil water content is the plastic limit. d. Reporting results; the average of the water contents obtained from the two plastic limit tests. The plastic limit (PL) is the average of the two water contents. If no shrinkage is observed on the sample the sample has no plastic limit and is reported as non-plastic. Note: If the two results of the moisture content differ by more than 0.5% the test shall be repeated. Plasticity Index (PI) Plasticity index is calculated as follows: PI = LL PL Soil water (moisture) Content determination a. Weigh and label a 50-mL beaker.is weighed and Record the weight. b. Collect subsamples of the wet soil and add to the beaker. Approximately 10g of wet should be sufficient c. Immediately weigh the beaker with wet soil and record the weight d. Dry soil in oven at 105 C for at least 24 hours. e. Weigh beaker with dry soil and record the weight f. Report Sample identification and water content data information as follows: Sample No. Mass of cup (g0 Water content data Mass of cup and wet soil (g) Mass of cup and dry soil mass of wet soil (g) mass of dry soil (g) % water content 9 A t t e r b e r g S O P

10 Hence %Water content (W) = M wet - M dry /M dry Report the liquid limit, plastic limit, and plasticity index to the nearest 0.1 percent or whole number Note: All forms of liquid and plastic test limits are subject to errors due to either operator error or variations in the equipment used. For the liquid limits the consistency of the test portion should be such that when tested, a penetration in the range of 9 to 12 mm is obtained. The plastic limit in particular requires practice and cross-checking to identify the correct point, as it is easy to make the rolled thread crack and crumble prematurely 10 A t t e r b e r g S O P

11 REFERENCES ASTM D Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. 11 A t t e r b e r g S O P

12 ANNEX 1: Workflow ATTERBERG LIMITS WORKFLOW This section determines the atterberg limits (liquid and plastic limits) using cone penetrometer and linear shrinkage. Objective: This helps in determining compressibility, permeability, compatibility, shrink swell and shear strength of the soil LEGEND Stored data Action point From Soil Preparation Factory Grind the sample to 425µm Sample Grinding Decision point Preparation Tape notes Soak samples overnight using de-ionized water Mix using palette knife till soil is thoroughly mixed Sample Curing Process route Liquid Limit Test Liquid limit Plastic limit/liquid limit/linear shrinkage? Linear shrinkage Linear Shrinkage Test Plastic limit Add soil or water to get the right thickness Penetrate first at 16mm then scoop the sample at the centre and get its wet weight Repeat the process at 18, 20 and 22mm and put in respective beakers Sample Wetting Sample Penetration Mould the soil into a ball Roll between your palms till it dries and cracks appear Repeat the process of rolling until the thread crumbles Divide the threads into two moisture beakers Get the wet weight Plastic Limit Roll the Threads Grease the shrinkage mould Fill using pallette knife to level Dry at room temp for 12-24hrs Shrinkage Mould Filling Transfer the scooped sample, the mould and the threads to the oven for 24hrs at 105 o C Get the dry weight Oven Drying Record Results To Database 12 A t t e r b e r g S O P

13 Annex 2. QUALITY CONTROL Process Procedure Penetration Penetration range should be between 9-12 mm Identification Ensure you get the correct points Consistency Ensure consistency to avoid errors Report should include Results of the determination of the limits Possible deviations observed during analysis Other operations that have affected the results 13 A t t e r b e r g S O P