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# Bearing capacity of Soil

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Bearing capacity of soil, factors affecting and methods of calculating Bearing capacity of soil

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### Bearing capacity of Soil

1. 1. Strip footing resting on surface Load –settlement curve of dense sand or stiff clay* The load - Settlement curve in case of footing resting on surface of dense sandor stiff clays shows pronounced peak & failure occurs at very small stain.* A loaded base on such soils sinks or tilts suddenly in to the ground showing asurface heave of adjoining soil* The shearing strength is fully mobilized all along the slip surface & hencefailure planes are well defined.* The failure occurs at very small vertical strains accompanied by large lateralstrains.* ID > 65 ,N>35, Φ > 360, e < 0.55
2. 2. Strip footing resting on surface Load –settlement curve Of Medium sand or Medium clay* When load is equal to a certain value qu(1),* The foundation movement is accompanied by sudden jerks.* The failure surface gradually extend out wards from the foundation.* The failure starts at localized spot beneath the foundation & migrates out ward part by part gradually leading to ultimate failure.* The shear strength of soil is not fully mobilized along planes & hence failure planes are not defined clearly.* The failure occurs at large vertical strain & very small lateral strains.* ID = 15 to 65 , N=10 to 30 , Φ <30, e>0.75
3. 3. * The loaded base sinks into soil like a punch.* The failure surface do not extend up to the ground surface.* No heave is observed.* Large vertical strains are involved with practically no lateral deformation.* Failure planes are difficult to locate 222
4. 4. * The failure zones do not extend above thehorizontal plane passing through base of footing* The failure occurs when the down ward pressureexerted by loads on the soil adjoining the inclinedsurfaces on soil wedge is equal to upwardpressure.* Downward forces are due to the load (=qu× B) &the weight of soil wedge (1/4 γB2 tanØ)* Upward forces are the vertical components ofresultant passive pressure (Pp) & the cohesion (c’)acting along the inclined surfaces.
5. 5. ø General Shear Failure Local Shear Failure Nc Nq Nr Nc’ Nq’ Nr’0 5.7 1.0 0.0 5.7 1.0 0.015 12.9 4.4 2.5 9.7 2.7 0.945 172.3 173.3 297.5 51.2 35.1 37.7
6. 6. * Surcharge term is not affected.* Unit weight in term is γ = γsub + y ( γ – γsub) B Thus, qu = c’Nc + γ Df Nq + 0.5B γ NrWhen y = B ; W.T. at B below base of footing. qu = c’Nc + γ Df Nq + 0.5 B γ NrHence when ground water table is at b ≥ B, the equation is not
7. 7.
8. 8. ZB I2S i = C1C2 ( q − q ) ∫ ∆∈ E Z =0 S
9. 9. Sand & hard Plastic clay Clay Max.Settle. Diff.Settl Angular Max.Settle Diff. Angular distortion Settle. distortionIsolated foundationi) steel struct 50mm 0.0033L 1/300 50mm 0.0033L 1/300ii) RCC struct 50mm 0.0015L 1/666 75mm 0.0015L 1/666Raft foundationi) steel struct 75mm 0.0033L 1/300 100mm 0.0033L 1/300ii) Rcc struct. 75mm 0.002L 1/500 100mm 0.002L 1/500Theoretically, no damage is done to the superstructureif the soil settles uniformly.However, settlements exceeding 150mm may causetrouble to utilities such as water pipe lines, sewers,telephone lines & also is access from streets.
10. 10. n - −Sf sf ∑ (∫ mv ) iSF =)∫iH∆z ) i ........(2)z = = 0 Ho ∆ ( ∆ ∂ ( Ho mv ∆ ∂ ∆ i =1 − if both mv & ∆ ∂ are cons tan t , − SF = mv ∆ ∂ Ho ........ ( 1)