Soils properties and foundations
- 1. SOIL TYPES AND ITS BEHAVIOUR BUILDING TECHNOLOGY AND MATERIALS III TERM I
- 2. 2 SOIL Soil is a mixture of minerals, organic matter, gases, liquids and countless organisms that together support life on Earth. • Climate • Relief (Elevation, Orientation, Slope Of Terrain) • Organisms • Original Minerals All influence the type of Soil
- 3. 3 Six Different Types of Soils Found in India • Alluvial Soils: formed by the deposition of sediments by rivers • Black Soils: are made up of volcanic rocks and lava-flow • Red Soils: derived from weathering of ancient metamorphic rocks of Deccan Plateau • Laterite Soils: formed due to intense leaching • Mountain Soils: formed as a result of the accumulation of organic matter • Dessert Soils: generally sandy and deficient in organic matter
- 4. 4 Characteristics of soil Characteristics of soil that affects its behavior and which are considered while designing foundations are • Cohesion of particles • Internal friction • Permeability These characteristics are further classified depending on the quantity of following components: Rock Gravel and sand Chalk Clay Peat
- 5. 5 Rock • Rocks such as limestone, granite, sandstone, shale and hard solid chalk have a high bearing capacity. • Rock can be impervious, so topsoil is likely to require drainage as it is not possible to build soak pits to dispose of rainwater or surface water. Sand • Under load sand is slightly compressed due to discharge of water. • Because of its high permeability rate, compression is rapid. • Sand has negligible cohesion between particles thus doesn't have plasticity. • Thus weight of building on sand causes rapid compression with little disturbance to the surrounding soil. Gravel and sand • Dry compact gravel, or gravel and sand sub soils are most suitable for foundations when well drained and well confined. • They have good bearing value and are adequate for strip foundations. • Depth of 700mm is acceptable, as long as the ground has adequate bearing capacity. • If the water table is high, the bearing capacity is halved • In such cases a shallow, reinforced, wide strip foundation may be suitable.
- 6. 6 Chalk • It is a soft, white, porous sedimentary carbonate rock • Strip foundations are commonly used in chalk. • The depth of the foundation must be below 700mm • If the chalk is soft it will need to be excavated until firm chalk is reached. • Chalk soils can be prone to erosion Peat • Peat and loose waterlogged sand are poor subsoils. • If the peat can be stripped back to find suitable load-bearing ground of at least 1.5m depth, strip foundations may be suitable. • A reinforced raft foundation are preferred.
- 7. 7 Clay • 900-1,200mm layer of clay is subjected to movement due to expansion and shrinkage • Clay is very compressible but due to high cohesiveness the compression takes place slowly and it also affects the surrounding surface. • Excavation is done till a depth where the moisture content of the clay remains stable. • Strip foundation is sometimes acceptable but over digging shouldn’t be done as it leads to more stress on softer clay blow • Strip foundations with steel reinforcement are used at times • Minimum depth of 1m is required for foundation
- 8. 8 The choice of the appropriate type of foundation is also governed by following factors such as: • The nature of the structure • The loads exerted by the structure • The subsoil characteristics • The allotted cost of foundations • The first factor considered is loads from building on the foundation. • This load is a combination of dead load and imposed loads on the buildings. • Other loads such as wind loads, earthquake loads, snow loads etc. are also considered based on location.
- 9. FOUNDATION ON WEAK STRATA: RAFT AND PILE FOUNDATION BUILDING TECHNOLOGY AND MATERIALS III TERM I
- 10. 10 PILE FOUNDATION: FUNCTION: As with other types of foundations, the purpose of a pile foundations is: • To transmit a foundation load to a solid ground • To resist vertical, lateral and uplift load PILE FOUNDATION ARE ADOPTED FOR • Soils having Low Bearing capacity • Non availability of proper bearing stratum at shallow depths • Heavy load from the super structure for which shallow foundation may not be economical or feasible
- 11. 11 PILE FOUNDATION BASED ON FUNCTION 1. Bearing Piles 2. Friction Piles 3. Screw Plies 4. Compaction Piles 5. Sheet Piles PILE FOUNDATION BASED ON MATERIAL 1. Cement Piles 2. Timber Piles 3. Steel Plies 4. Composite Pile
- 12. 12 PILE FOUNDATION BASED ON FUNCTION
- 13. 13 1. BEARING PILES • Bearing pile are driven until it reaches to hard stratum • Pile acts as pillars supporting the super structure and transmitting the load down to hard stratum from the ground • Bearing Pile do not bear any load, they act as a medium to transmit the load 2. FRICTION PILES OR FLOATING PILES FOUNDATION • Friction pile are used when hard stratum is very deep or not available or soil is weak to carry the load of super structure • The load is transferred by the friction developed between piles and surrounding ground
- 14. 14 3. SCREW PILES • Construction of helical plates welded to hollow steel pile • Rapid installation can be done • Little installation noise or vibration • Sustain load immediately after installed • They do not work on rocky terrain • Shaft can be as small as 1.5x1.5 inches solid bar. 4. COMPACTION PILES When granular piles are driven in granular soil with the aim of increasing the bearing capacity of the soil the pile is termed as compaction pile
- 15. 15 5. SHEET PILES • Sheet pile differs from bearing or friction piles • Used rarely to furnish vertical support and are used to fulfill the function of a retaining wall • The ground anchor is provided to hold sheet in place
- 16. 16 PILE FOUNDATION BASED ON MATERIALS
- 17. 17 1. TIMBER PILE / WOODEN PILE ADVANTAGES: • The piles are easy to handle • Relatively inexpensive where timber is plentiful. • Timber is most suitable for long cohesion piling and piling beneath embankments. • For timber piles of length less than 14 meters, the diameter should be greater than 150 mm. • If the length is greater than 18 meters, the diameter of 125 mm is acceptable. • It is essential that the timber is driven in the right direction and should not be driven into firm ground
- 18. 18 1. TIMBER PILE / WOODEN PILE DISADVANTAGES: • Not suitable in all conditions. • The piles will rot above the ground water level if they are not treated properly. • Have a limited bearing capacity. • The timber should be in a good condition and should not have been attacked by insects.
- 19. 19 PRECAST CONCRETE PILES ADVANTAGES • The piles can be cast before excavation. • Can be inspected before casting can easily be cut or extended to the desired length. • Relatively inexpensive. • Pile lengths are readily adjustable. DISADVANTAGES • Concrete cannot be inspected after completion. • Light steel section or Precast concrete shells may be damaged or distorted by hard driving. • Cannot be driven where headroom is limited • Time consuming. Cannot be used immediately after the installation.
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- 21. 21 CAST IN-SITU ADVANTAGES • Length can be readily varied to suit varying ground conditions. • Soil removed in boring can be inspected • Can be installed in very large diameters and lengths • End enlargement up to two or three diameters are possible in clays. • Material of piles is not dependent on handling or driving conditions. • Can be installed with out appreciable noise or vibrations. • Can be installed in conditions of very low headroom.
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- 23. 23 CAST IN-SITU DISADVANTAGES • Supervision has to be done so that the concrete is placed under ideal conditions. But in situ piles cannot be subsequently inspected. • Water under artesian pressure may pipe up pile shaft washing out cement. • Cannot be readily extended above ground level especially in river and marine structures. • Boring methods may loosen sandy or gravely soils requiring base grouting to achieve economical base resistance.
- 24. 24 3. STEEL PILES • Suitable for handling and driving in long lengths. • Their relatively small cross-sectional area combined with their high strength makes penetration easier in firm soil. • They can be easily cut off or joined by welding. • Anti corrosion coating is applied on all surfaces to protect the metal The types of steel piles commonly used are a) H-piles b) Pipe piles c) Screw piles
- 25. 25 4. COMPOSITE PILE FOUNDATION • Combination of different materials in the same of pile.
- 26. 26 RAFT FOUNDATION: • The raft foundation is a continuous footing that covers the entire area beneath a structure and supports all the walls and columns. • The principal of any raft foundation is to spread the load of the entire area of the site. • It may be stiffened by ribs or beams incorporated into the foundation. • They are made up of concrete and heavily reinforced with steel, so entire foundation will act as a unit. • The term mat is also used for the foundation • A raft is an alternative to piles as it can be less expensive.
- 27. 27 WHERE RAFT FOUNDATIONS ARE USED ? • The raft foundations are used where the column load are heavy and thus requiring large bases. • Where the bearing capacity of soil is low, resulting in the need of large base. • If the subsoil is soft compressible such as soft clay or peat. • If the bottom of the structure is located below the ground table level. • It is also used where the area covered by spread footings is more than half the area covered by the structure.
- 28. 28 FLAT PLATE RAFT FOUNDATION • These are constructed of uniform thickness over the whole raft area..
- 29. 29 FLAT PLATE THICKENED UNDER FOUNDATION • These are constructed of uniform thickness over the whole raft area..
- 30. 30 BEAM AND SLAB RAFT FOUNDATION • Beam and slab rafts are alternatives to the solid slab raft and are used where poor soils are encountered. • The beams are used to distribute the column load over the area of the raft, that results in the reduction of the slab thickness. • The beams can be up stand or down stand depending upon the bearing capacity of soil near the surface.
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- 32. 32 CELLULAR RAFT FOUNDATION • These type of foundations are used on soft compressible subsoil such as soft clay or peat. • These rafts are used on sites subject to severe mining activity or in areas of poor ground where large bending moments are to be resisted.