Soil Basics - #1

Classification of Soil is based on 3 Methods
MIT System of soil classification

AASHTO classifications of soils

Unified soil classification system (USCS)

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ATTERBERG LIMITS

Liquid limit: The boundary between the liquid and plastic states; Plastic limit: The boundary between the plastic and semi-solid states; Shrinkage limit: The boundary between the semi-solid and solid states. These limits have since been more definitely defined by A. Casagrande as the water contents which exist under the following conditions: 1. Liquid limit The water content at which the soil has such a small shear strength that it flows to close a groove of standard width when jarred in a specified manner. The Liquid Limit, also known as the upper plastic limit, is the water content at which soil changes from the liquid state to a plastic state. OR It is the minimum moisture content at which a soil flows upon application of very small shear force. Liquid Limit (LL or wL) ‑ the water content, in percent, of a soil at the arbitrarily defined boundary between the semi‑liquid and plastic states.
Liquid limit is defined as “ the moisture content at which soil changes from liquid state to plastic state” Or According to Casagrande Liquid Limit Test it is also defined as “ the moisture content at which two sides of a groove come close together for a distance of 12.7 mm under the impact of 25 number of blows” Or According to fall cone test method Liquid Limit is also defined as “the moisture content at which the cone( fall cone test) penetrates with in the soil for 1 cm when falls freely for 5 seconds. 2. Plastic limit The water content at which the soil begins to crumble when rolled into threads of specified size. The Plastic Limit, also known as the lower plastic limit, is the water content at which a soil changes from the plastic state to a semisolid state. Plastic Limit (PL or wP) ‑ the water content, in percent, of a soil at the boundary between the plastic and semi‑solid states. It is defined as “The moisture content at which the soil behaves like a plastic material is called plastic limit” Or It may also be defined as “The moisture content at which the soil changes from plastic state to semi solid state" Or “The moisture content at which the soil begins to crumble when rolled up into a thread of 3 mm in diameter” 3. Shrinkage limit: Shrinkage limit is defined as “the moisture content at which the soil change from a semi solid state to a solid state” Or “The maximum water content at which the reduction in water content will not cause decrease in total volume of soil but the increase in moisture content will cause an increase in moisture content” Or It is also defined as “the lowest water content at which the soil are still completely saturated” 4. Plasticity Index (PI) ‑ the range of water content over which a soil behaves plastically. It is defined as “the range of consistency with in which the soil exhibit plastic properties”. Or It is also defined as “the numerical difference between the liquid limit and plastic limit”. Mathematically, Plasticity index = Liquid Limit – Plastic Limit It is denoted by Ip and Ip = LL – PL 5. Liquidity index: Its advantage is that The liquidity index (LI) is used for scaling the natural water content of a soil sample to the limits. How to find Bearing Capacity of Soil Methods of bearing capacity determination §  Analytic method i.e. through bearing capacity equations like using Terzaghi equation, Meyerhof equation, Hansen equation etc §  Correlation with field test data e.g. Standard penetration test (SPT), Cone penetration test (CPT) etc §  On site determination of bearing capacity e.g Plate load test, Pile load test Presumptive bearing capacity (recommended bearing capacity, in various codes) Following are the methods: Analytical Method of Bearing capacity determination Analytical Method Lower Bound Failure Lower bound failure states that “If an equilibrium distribution of stress can be found which balances the applied load and nowhere violates the yield criterion, the soil mass will not fail or will just be at a point of failure i.e. it will be a lower bound estimate of capacity. Consider the equilibrium conditions in soil under the footing load. When the foundation pushes into the ground, stress block 1 has principal stresses, as shown. The push into the ground however, displaces the soil on the right side of the line OY laterally, resulting in the major principal stress on block 2 being horizontal as shown. When the two blocks are adjacent to each other at the vertical line OY, then Some Formulae Upper Bound theorem Upper bound theorem states that “If a solution is kinematically admissible and simultaneously satisfies equilibrium failure must result” i.e. it will be an upper bound estimate of capacity. For a possible upper bound, consider failure surface as semicircle. Taking moment about O Terzaghi’s bearing capacity equation (1943) Terzaghi developed a general formula for ultimate bearing capacity of spread footing foundation under the following assumptions: The depth of the footing is less than or equal to its width (D, B) The foundation is rigid and has a rough bottom The soil beneath the foundation is homogeneous semi-infinite mass Strip foundation with a horizontal base and level ground surface under vertical loads. The general shear mode of failure governs and no consolidation if the soil occurs (settlement is due only to shearing and lateral movements of the soil) The shear strength of the soil is described by s = c + σ tan