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Bearing capacity of Soil
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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

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

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  • 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. 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. * 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. * 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. 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. * 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.    
  • 8. ZB I2S i = C1C2 ( q − q ) ∫ ∆∈ E Z =0 S
  • 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. 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)