4 Foundation is the part of the structure which is in direct contact with the ground to which loads are transmitted.
5 A weak foundation destroys the work which is built upon it.
6 Functions of foundation Even distribution of loadReduction of load intensityReduction of differential settlementSafety against sliding and overturningSafety against underminingProvide firm and level surfaceProtection against soil movement
7 Requirements of a good foundation Constructed to sustain dead load and imposed load and transmit them to underlying soilRigid-to avoid differential settlementTaken to sufficient depthPerformance should not be affected due to any unexpected future influence
8 Factors for the design Ultimate bearing capacity Safe bearing capacityAllowable bearing capacityUniform settlementDifferential settlementBearing capacity of the soil2. Settlement of foundations
9 BEARING CAPACITY Supporting power of soil without any failure Depends on :Properties of soilPosition of water tablePhysical features of foundation like type, size & shapeUltimate Bearing Capacity : minimum gross pressure intensity at the base of foundation that the soil fails in shearNet Ultimate Bearing Capacity : minimum net pressure intensity at which the soil fails in shearNet safe bearing capacity : obtained by dividing the net ultimate bearing capacity of the soil with a suitable factor of safetySafe bearing capacity : Maximum pressure the soil can carry safely without the risk of shear failure
10 Settlement of foundations No settlementTotal settlementDifferential settlementUniform settlement is usually of little consequence in a building, but differential settlement can cause severe structural damage.
11 Settlement of foundation means the sinking of foundation as a consequence of compression or deformation of the soil under the foundationEqual Settlement :The structures settles by uniform amount at each and every portion of the structureUnequal Settlement:The amount of settlement is different at different parts of the building
12 Causes of differential settlement: Causes of settlement:Due to weight of the structure transmitted to the soilDue to increased load on the surrounding soilDue to excavation near the foundationLowering of water tableVibrations from moving machineriesDeterioration of concrete by the chemical action of soil, seawater etc…Due to mining and tunnelling operationsCauses of differential settlement:Non uniform load distribution on foundationsNon uniformity of soil typesPercolation of waterOverlap and concentration of stresses due to presence of adjacent foun dations
13 Causes of foundation failure Unequal settlement of subsoil under the foundationUnequal settlement of the masonryLateral movement of earthShrinkage of soil bed due to seasonal variation of moisture contentThe penetration of the roots of treesAtmospheric actionLateral escape of the soil beneath the foundationHorizontal movement of the soil adjacent to the structure
14 Types of Foundations Shallow Foundations D<=B Deep Foundations D>BD - Depth of foundationB – Width of foundation
15 Shallow foundations Types of shallow foundations Isolated or column footingWall or strip footingCombined footingContinuous footingCantilever footingRaft or mat foundation
16 Isolated or column footing When the load on the column is less, a spread is given under the column
20 Combined footingWhen a foundation or footing is constructed for two or more columns is called as combined footingTwo individual footings overlapWhen bearing capacity is lesswhen footings are constructednear boundaries of the plotTrapezoidal footing – when columnloads vary considerably
22 Continuous footingA single continuous reinforced concrete slab is provided as foundation for three or more columns in a row. Continuous footing is more suitable to prevent the differential settlement in the structure and for the safety against earthquake.
33 Raft foundations are suitable when The building loads are heavyThe allowable soil pressure is smallIndividual footings would require more than half the building areaIn highly compressible soilWeak spots and loose pockets in soil mass are suspected
34 Deep FoundationThe foundations having very large depth compared to width are called deep foundationsE.g. Pile foundationsWell foundations
36 Pile FoundationPiles are long slender members driven into ground or cast at the site. Pile foundations are common where the soil conditions are unfavorable for the use of shallow foundations
37 Suitability of pile foundations Loose foundation soil but hard strata is available at a depth of mHeavy dead and live loadsNear seashore or riverbed where scouring action of water occursPosition of water table is likely to fluctuate appreciablyCanals or deep drainage lines near by
38 Classification of Piles Method of load transferFunction or actionComposition and materialInstallation
39 Classification based up on method of load transfer End bearing PilesFriction Piles
40 End bearing pilesEnd bearing piles-Used to transfer load to a suitable bearing stratum
41 Friction pilesFriction Piles- used to transfer the loads to a depth by friction along the surface area of the piles.
42 Classification based up on function or use Compaction PilesTension or uplift pilesAnchor PilesFender PilesSheet Piles
54 CAST IN SITU PILESCASED CAST IN SITU PILESUNCASED CAST INSITU PILES
55 Well FoundationsA well foundation is a well type structure, which built at the ground level and sunk into the soil at the required level.The bridge pier will be resting on the top of the well foundation.The bottom of the well is plugged with concrete.The top is covered with a well cap which is a thick concrete slab
56 The bottom edge of the well foundation consists of a cutting edge The bottom edge of the well foundation consists of a cutting edge. The different c/s adopted for well foundations areCircularTwin circularDouble DDumb bellTwin hexagonalRectangular
57 Components of well foundation Well curbCutting edgeSteiningBottom plugTop plugWell cap
58 WELL CAPTOP PLUGSTEININGSAND OR SOIL FILLINGWELL CURBBOTTOM PLUGCUTTING EDGE
59 Well curb Cutting edge Steining Bottom plug Top plug Well cap Support the wt of the wellCutting edgeSharp angle for cutting the soil without making it too weakSteiningWalls of the wellBottom plugConcrete plug provided to balance the soil pressureTop plugConcrete plug provided above the sand/soil fillingWell capServes as platform for the supporting members of the superstructure
65 Machine foundationTo support the dynamic forces produced by the operation of the machineTo avoid large settlements at resonance, natural frequency of foundation should be different operating frequency of the machine
66 Types of machines Impact type Reciprocating type Centrifugal type Presses, forge hammersReciprocating typeCompressors, enginesCentrifugal typeMotors, turbinesMiscellaneous typesMachine foundation
72 PLATE LOAD TESTIt is a field test used to determine the ultimate bearing capacity of soilA pit is dug up to the foundation levelA square plate of 300mm x 300mm & 25 mm is placed at the centre of the pitA dial gauge is connected to the test plateNow weights in the form of sand bags are placed on the platforms in equal increments.The test is continued till the failure occurs or the plates settled by 25 mm whichever occurs earlierThe load settlement curve is then recorded.
75 STANDARD PENETRATION TEST Test is conducted in a bore hole 50 to 150 mm in diameterSplit spoon sampler (pipe-like weight) is driven into the ground by a weight of 65 kgweight is repeatedly raised and dropped a distance of 750 mm to drive in samplernumber of blows required to drive the sampler for a penetration of 300mm is recordedThe number of blows is known as penetration number (N Value)There are empirical charts from which the bearing capacity can be calculated based up on the N Value
76 Methods for improving Bearing Capacity CompactionDrainageVibratory methodsChemical stabilisationGroutingGeotextiles