Soil Classification and Identification Procedure
The unified soil classification system or USCS soil classification provides a chart for descriptions and identification of soils. AASHTO soil classification follows the soil classification chat for the field identification and description of soils. There are two major soil types, fine-grained and coarse-grained. A fine-grained soil contains fines of 50% and above, while coarse-grained soil has less than 50% fines. The description and identification of soils have broken-down procedures for identifying and testing these two soil types, as elaborated below.
Fine-Grained Soils Identification Procedure
The first step is the selection of a sample representative of the examinable material. Larger particles of medium and large sands get taken out till you get an adequate size for the test. This specimen becomes the subject sample for dilatancy, dry strength, and toughness tests.
- Dry Strength Soil Test – Select sufficient material from the specimen to mold into a ball of about 25 mm / 1 in. in diameter. The molded material should give a consistency of putty. Mold out a minimum of three test specimens, then let them dry in the sun, air, or by artificial means caring not to exceed a temperature of 60°C / 140°F. Where natural dry lumps exist, use the ones with a diameter of 12 mm / 0.5 in instead of the molded balls. Subject the lumps or dry balls to a strength test via conducting an in-between finger crushing. Indicate the noted strength as low, medium, none, high, or very high. Ignore natural lumps with coarse sand particles. High dry strengths are likely due to calcium carbonate presence, which is a high-strength water-soluble cementing material.
- Dilatancy – Select sufficient specimen to mold into balls of 12 mm / 0.5 in. in diameter. Add water where necessary when molding to get a soft but not sticky consistency. Smooth the ball in your palm using a knife’s blade and shake horizontally while striking the hand’s side vigorously severally. Take note of water’s appearance on the soil’s surface. Squeeze the sample while noting the results as done in the strength test. Check for the speed of water appearance when shaking and disappearance when squeezing.
- Toughness – Use the specimen utilized in the dilatancy test, shaping and elongating it to a pat roll, using your hands on a flat surface. The desired diameter for the par roll is 3 mm / 0.1 in. folded and rerolled till they crumble. Take note of the required rolling pressure to near the soil plastic limit, noting the thread’s strength. When the thread crumbles, lump the pieces and knead till the entire lump crumbles. Note the material toughness during kneading. Describe results as low, medium, or high.
These tests should account for organic and inorganic fine-grained soils. The inorganic soils should cover lean clay, fat clay, and silt.
USCS Soil Classification Chart
This classification focuses on the gravel or sand state of the soil. The soil is sand if gravel estimation is less or equal to sand percentage and vice versa is true. The sod is either clean sand or gravel if the proportion of the fines is 5% or less. Further, identify the soil as poorly or well-graded sand or gravel if it has a wide range of particle sizes. The sod can have classifications of either fine sand or gravel, clayey gravel or sand, and silty sand or gravel. Utilize relevant grading symbols to mark the gotten test results appropriately, caring to note boulders or cobbles.
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