Unit 2: Soil classification

Unit 2: Soil classificationGeomechanicsHarinder

1. Introduction: Earth, or soil, in the engineering sense, is defined as any unconsolidated material that is the product of weathering and mechanical disintegration of rocks, composed of discrete particles with gases and liquids interspersed between them.

2. Aim:Over a period of several weeks we have to determine several elements in order to classify a soil. In the first prac session, soil samples must be prepared for testing. This essentially involves dividing the soil sample into a coarse fraction and a fine fraction. The next task is to perform a mechanical (sieve) analysis on the coarse fraction and plot its particle size distribution. The last week is to perform several soil index tests on the fine fraction to determine the Atterberg limits. Together, these results can be used to classify the soil according to AS1726.

3. Equipment/ materials:

Spatula Grooving tool Soil Sample Casagrande Liquid Limit apparatus Empty Tins Glass plate

4. Prac Procedure:

4.1 Determination of Liquid LimitThe liquid limit (LL) is essentially the moisture content of a soil when it starts to behave like a liquid. Sift the soil sample through a 425m sieve to remove coarse particles(sand and gravel) Separate Silt and clay particles by observing its behaviour with varying moisture content( liquid and plastic limit) Take rough amount of soil and place on a glass plate Add water to the particles thus making it moist and knead appropriately to form a uniform smooth paste texture similar to a cake icing. The quantity of water added is varied depending on the required consistency in texture required. Weigh the mass of some empty moisture tins and record the respective data ie the tin weight and numbers on the data sheet Using the grooving tool carefully cut a clean straight groove down the centre of the cup. Ensure the tool is perpendicular to the surface of the cup as the groove is made Place the wet soil sample in the cup raised to a standard height say 10mm (see fig 1 below) and divide into two halves by a grooving tool Tap the cup by turning the handle ,twice a second and ensure the number of taps required to bring the 2 halves together until the open groove is closed is recorded usually the 25th tap and the respective moisture content determined The soil sample attains its liquid limit on the 25th blow/tap (drop) since at this point the two soil halves flow together and the gap closes by a length of 0.5inch or 12mm If the soil is unable to be brought to its exact plastic limit, the procedure can be repeated for different water contents and a semi-logarithmic graph is plotted of water content against the number of taps. Hence the water content corresponding to 25taps on the graph is taken as the liquid limit of the soil Using the spatula, take the soil sample from the edge of the soil pat. This soil sample should include soils on both sides where the soil came into contact. Place the soil into the moisture tin and cover it. Weigh the mass immediately and record the data prior to placing it into the oven to determine the shrinkage limit Depending on the nature of the soil sample , leave the moisture tin for roughly 16hrs Wash up and dry all apparatus used

4.2 Determination of Plastic LimitThe plastic limit (PL) is the moisture content of a soil when it starts to behave like a plastic material or when it starts to become easily mouldable. Weigh the empty moisture tins and record the data appropriately Add appropriate quantity of water to of the original soil sample while ensuring you roll the ball of wet soil between the palm of your hand and a glass plate to produce a thread of 3.2 mm / 1/8 inch thick before it starts to crumble. This is done by applying sufficient pressure of about 90 strokes per minute. Estimated time may range to about 2-3 mins Break the thread into several pieces. Knead and reform these pieces into sphere masses and re-roll them. This series of alternate rolling, gathering together, kneading and re-rolling is continued until the thread crumble on the required pressure for rolling & can be no longer rolled into a 3.2mm diameter thread Gather these portions of the crumbled thread together and transfer the contents to moisture tin. Immediately weigh the moisture containing the soil sample and record the appropriate data. Then place into an oven and heat for roughly about 16hrs The water content /Moisture content of the soil in this state is taken as its plastic limit. Compute the plastic index = Liquid Limit Plastic Limit

4.2 Determination of Shrinkage Limit

Place the remaining soil sample close to its liquid limit into a lightly greased half cylindrical mould (Length = 250mm and internal diameter 25mm) Depending on the type of soil sample allow it to dry at room temperature for 12 -24 hrs(clayey soil) before placing in the mould to allow for complete permeation of water through the soil Transfer into a drying oven (105C-110C). Once Shrinkage has stopped simply measure the shrinkage.

5. Sieve Analysis

Shake for roughly 10 minutes at a slight angle Remove each individual sieve and record the mass retained on the respective sieve size. Calculate the % passing for a given sieve size by adding together amounts retained on sieves above the given size, and to subtract that from the total weight of the sample. This is the mass passing sieve Express this figure obtained as a % of the sample weight to obtain the % passing sieve or the Cumulative Passing %

6. RESULTS/ CALCULATIONS (sample)Liquid Limit (LL)Mass of wet soil + tin =21.65gMass of dry soil + tin=13gMass of tin = 14.05gMass of moisture = (Mass of wet soil + tin)(Mass of dry soil + tin)=21.65-13=8.65gMass of dry soil = Mass of dry soil + tin Mass of tin= 18.5-14.05=4.45gMoisture Content= (mass of dry soil)/ (mass of moisture) *100=4.45/8.65*100=51%Therefore the liquid limit is 51%

7. DETERMINATION OF % PASSING SIEVE / CUMULATIVE PASSING % For 13.2mm sieve size, Mass retained on each sieve (g) =24.47g% retained on each sieve= =Cumulative Passing (%) 13.2mm

8. Determination of Shrinkage Limit(Ls)=

9. Classification of soil

% of fine grain particles (silt, clay){particles < 75 mm} = 55.76% % of coarse grain particles (Gravel, Sand) {particles between 75 mm and 63mm) =100%55.76% = 44.24% Gravel component of Coarse fraction (particles between 2.36mm and 63mm)=100%81.2 %( Cumulative passing at sieve size 2.36mm) = 18.8% Sand component of Coarse fraction (particles between 75 mm and 2.36mm)=100% 18.8%)= 25.44%

Soil 18.8% Gravel 25.44% Sand 55.76% Fines (Silt and Clay) Therefore the soil sample is Clay with Gravel, Sand present.Monier Efat 6335357Group: Thursday 2:30