Soil Bearing Capacity - Characteristics and Classification PDF

Soil bearing capacity Characteristics and classification  What is soil? In a construction context, soil is “ that part of the earth which lies below the top soil and above the rock, having been formed by erosion and the earth’s crust by water, atmospheric conditions, and intense pre...

Soil bearing capacity Characteristics and classification  What is soil? In a construction context, soil is “ that part of the earth which lies below the top soil and above the rock, having been formed by erosion and the earth’s crust by water, atmospheric conditions, and intense pressure over many thousands of years.  Classification of soils If organic matter, which is present in only small quantities in most locations, is ignored, the earth’s crust consists mainly of mineral ( inorganic, noncombustible) matter. The earth’s mineral matter is generally classified as rocks or soils. In rocks the mineral particles are firmly bonded together. Soil particles exist either as individual particles or as a conglomerate of several easily separable particles. The top layer of the ground consists of soil; rocks are generally deep beneath the earth's surface. The foundation off most buildings consists of soil supported. Soils can be classified as cohesive and non-cohesive soils  Non-cohesive soils Gravel: a natural deposit consisting of rock fragments in a matrix of finer and usually sandy material. Many of the particles are larger than 2mm in size. Sand: A natural sediment consisting of the granular and mainly siliceous products of rock weathering. It is gritty with no real plasticity. The particles normally range between 0.06 and 2.00mm in size. Well-graded sand: A sand containing a proportion of all sizes of sand particles with a predominance of the coarser grades. Compact gravel and sand: Deposits require a pick for removal and offer high resistance to penetration be excavating tools. Loose gravel and sand: Deposits readily removable by hand-shovelling only. Uniform or poorly graded sand: The majority of particles lie within a fairly restricted size range.  Cohesive soils Clay: a natural deposit consisting mainly of the finest siliceous and aluminous products of rock weathering. It has smooth, greasy touch, sticks to there fingers and dries slowly. It shrinks appreciably on drying and has considerable strength when dry Stiff clay: A clay which requires a pick or pneumatic spade for its removal and cannot be moulded wit the fingers at its natural moisture content. Firm clay: A clay which can be excavated with a spade and can be moulded by substantial pressures with the fingers at its natural moisture content. Very soft clay: Extruded between fingers when squeezed in fist at its natural moisture content. Soft clay: A clay which can be readily excavated and can be easily moulded with the fingers at its natural moisture content. Boulder clay: a deposit of unstratified clay or sandy clay containing sub angular stones of various sizes. Silt: A natural sediment of material of finer grades than sand. Most of the grains will pass a 75 micrometer test sieve. It shows some plasticity, is not very gritty and has appreciable cohesion when dry.  Uniqueness of clay Apart from particle size, a major factor tat distinguishes clay from other soil constituents is article shape. Gravel, sand, and silt particles are approximately equidimensional (spherical and ellipsoidal in shape. This is because gravels sands and silts are the result of mechanical weathering. Clay particles results from chemical weathering. They are, therefore non-equidimensional (flat, platelike shapes) Because of their flat particle shape he surface-area-to-volume ratio of clays is several hundred to thousand times grater than the corresponding ratio for gravels, sands or silts. The behaviour of clayey soils is greatly influenced by the electrostatic forces that develop between the platelike surfaces. In presence of water these forces are repulsive, which increases the space between plates. Therefore, in the presence of water, clayey soils swell, and as water decreases, they shrink.  Organic soils Soil consisting of mainly of organic matter is called organic soil. The constituents of organic soil are fully or partially decayed plant matter, called peat, with little or no mineral constituents. Organic soils are highly compressible and unsuitable for building foundations. They are typically dark in colour and have a characteristic odour of decay Additionally, they may have a fibrous texture due to bark , leaves, grass, branches, or other fibrous vegetable matter. Organic soils form the top layer of soil, and are typically removed before construction begins. If the soil characteristics are desirable, they are often stockpiled and redistributed after construction is complete.  Soil Identification and classification tests Soils maybe subjected to a number of tests to establish their identity and classify them. Particle size distribution: This test determines the proportion of gravel, sand and clay in a particular soil. The soil is dried and sieved through a nest of sieves, and the weight retained of each soil is recorded. The results of the grading test are plotted on a graph. Sand has particles between 0.60 and 2mm, silt between 0.60 0.002mm and clay is less than 0.002mm. Liquid limit test: The object is to determine the moisture content at which the soil passes from plastic to liquid state. The moisture content is expressed as a percentage of the dry weight of the soil. Plastic limit test: This determines the moisture content at which the soil ceases to be plastic (soil sample can be rolled into a thread 3mm diameter without breaking) Plasticity index refers to the difference between liquid and plastic limits.  Casagrande classification chart This soil classification chart groups symbols of 2 letters for each soil, the first letter representing the size of the soil particles, and the second letter its main characteristic or property. First letter Second letter Gravel G Fines F Sand S High compressibility H Silt M Interme’ Compress.. I Clay C Low compressibility L Organic silts, clay O Poorly graded P Well graded W Clay C A poorly graded sand would have a group symbol of SP, if it were well graded it would be SW, and if it were a sandy clay it would be SC. Other tests include: Dry density tests: to determine the density of the dry soil in its natural position Standard penetration and consolidation test: to determine the compressibility and the shear vane, unconfined compression. Triaxial compression test: to determine the shear strength parameters. The compressibility and shear strength parameters are needed to ascertain the allowable bearing pressure.  Test Pit method Consists of digging pits of tenches that are large enough for visual inspection of the soil and for procuring samples for laboratory testing by getting inside the pit or trench. Considered to be the more reliable method because it allows direct inspection and assessment of several soil properties of the undisturbed soil through filed tests with simple instruments. Limitations include: expense. If water table is high, the death of inspection is limited.  Test boring method This method is more commonly used, involves obtaining soil samples by boring into the ground using a truck-mounted power- driven hollow stem, fitted at the end either with a cutting bit or a fluted drilling auger. When boring reaches the depth at which a sample is required, the cutting bit is withdrawn and a standard steel sampling tube is pushed into the soil through its full height and the core sample is removed. A tube that opens up into 2 semicircular halves, called a split-spoon, it is also used instead of the shelby tube.