Bearing Capacity of Rocks - IITK - Indian Institute of ...

1 Bearing Capacity of RocksIntact Rock MassIntact Rock MassA rock mass with joint spacing greater than 4 to 5 times the width of the footing. Local and General shear failure is associated to brittleGeneral shear failure is associated to brittle and ductile rock Rock MassJointed Rock MassGeneral Shear FailureLocal Shear FailureLocal Shear Failure Compressive Failure:AhtidbltidA case characterized by poorly constrained columns of poor FailureFor widely spaced and vertically orientedFor widely spaced and vertically oriented discontinuities, failure generally initiates by splitting beneath the foundation. The ultimate Bearing Capacity is given by:Guideline properties of Rock Mass ClClassesAllowable Bearing Stresses on Rock MassesFoundations on Fr actur ed Rock For mation30 MPa)Note: Use maximum qa < quwhere qu=compr essive str ength2025ress qa (Mwhere qu compr essive str engthof intact r ock specimens15earing Str)130/(1)16/(1)(RQDRQDMPaqALLOWABLE + NOTE: 1 MPa = 10 tsf510owable BePeck, et al.

2 (1974)NOTE: 1 MPa = 10 tsf00 102030405060708090100 Allo()ApproximationRock Quality Designation, RQDIn case of rock mass with favorable discontinuities, the net allowable Bearing pressure may be estimated from:qa = qc* NjWhere qc=average uniaxial compressive strength of rock coresWhere qc average uniaxial compressive strength of rock coresNj= empirical coefficient depending on the spacing of the discontinuities/3BS+[])/(300110S += = thickness of discontinuity = thickness of discontinuityS = spacing of discontinuitiesB = width of footingThe above relationship is valid for a rock mass with spacing of continuities > m, < 10 mm (15 mm if filled with soil) and B>03mB > Foundations:The allowable Bearing Capacity of socketed piles is given by:qa= qc* Nj* NdWhereWhereNd= + h/dh =depth of socket in rockd = diameter of socketDetermination of Net Allowable Bearing Pressure from PtTtPressuremeter Test:[])(31fldfaDPKDq +=Where qa= allowable Bearing pressure in t/m2Pl= limit pressure determined by the Pressuremeter in t/m2 Df= overburden pressure in t/m2 Values of KdValues of KdDepth of footingKdDepth of footingKdLoad at rock at radius/width of foundation at 4 times radius/width of unit Load at 10 times radius/width of fd unitPlate Load TestPlate Load TestCorrection Factors : (Not required for RMR)Co ect oacto s ( ot equ ed o) Ground Water Table: (a) Rock with discontinuous joints with opening less(a) Rock with discontinuous joints with opening less than 1mm wide (b) Rock with continuous joints with opening 1 to 5 id fill d ith07t 05mm wide filled with gouge to (c) Limestone/ Dolomite deposit with major cavities filled with soil to Cavities: Major cavities inside limestone - Slope and orientation of joints.

3 ()Fi i ttiftijit i th l(a) Fair orientation of continuous joints in the slope 1 to (b) Unfavorable orientation of continuous joints in(b) Unfavorable orientation of continuous joints in slope to