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This chapter provides guidance for the determination ofthe ultimate and allowable bearing stress values for foundationson rock. The chapter is subdivided into foursections with the following general topic areas: modesand examples of bearing capacity failures; methods forcomputing bearing capacity; allowable bearing capacity;and treatment methods for improving bearing capacity
Modes of failure, methods for estimating the ultimateand allowable bearing capacity, and treatments forimproving bearing capacity are applicable to structuresfounded directly on rock or shallow foundations on rockwith depths of embedments less than four times the foundationwidth. Deep foundations such as piles, piers, andcaissons are not addressed.b. As a rule, the final foundation design is controlledby considerations such as deformation/settlement, slidingstability or overturning rather than by bearing capacity.Nevertheless, the exceptions to the rule, as well as prudentdesign, require that the bearing capacity beevaluated.Section IFailure Mode
Bearing capacity failures of structures founded on rockmasses are dependent upon joint spacing with respect tofoundation width, joint orientation, joint condition (openor closed), and rock type. Figure 6-1 illustrates typicalfailure modes according to rock mass conditions as modifiedfrom suggested modes by Sowers (1979) andKulhawy and Goodman (1980). Prototype failure modesmay actually consist of a combination of modes. Forconvenience of discussion, failure modes will be describedaccording to four general rock mass conditions: intact,jointed, layered, and fractured.
6-4. Intact Rock Mass
For the purpose of bearing capacity failures, intact rockrefers to a rock mass with typical discontinuity spacing(S term in Figure 6-1) greater than four to five times thewidth (B term in Figure 6-1) of the foundation. As arule, joints are so widely spaced that joint orientation andcondition are of little importance. Two types of failuremodes are possible depending on rock type. The twomodes are local shear failure and general wedge failureassociated with brittle and ductile rock, respectively.a. Brittle rock. A typical local shear failure is initiatedat the edge of the foundation as localized crushing(particularly at edges of rigid foundations) and developsinto patterns of wedges and slip surfaces. The slip surfacesdo not reach the ground surface, however, endingsomewhere in the rock mass. Localized shear failures aregenerally associated with brittle rock that exhibit significantpost-peak strength loss (Figure 6-1a).b. Ductile rock. General shear failures are also initiatedat the foundation edge, but the slip surfaces developinto well defined wedges which extend to the groundsurface. General shear failures are typically associatedwith ductile rocks which demonstrate post-peak strengthyield (Figure 6-1b)
.6-5. Jointed Rock Mass
Bearing capacity failures in jointed rock masses aredependent on discontinuity spacing, orientation, andcondition.a. Steeply dipping and closely spaced joints. Twotypes of bearing capacity failure modes are possible forstructures founded on rock masses in which the predominantdiscontinuities are steeply dipping and closelyspaced as illustrated in Figure 6-1c and 6-1d. Discontinuitiesthat are open (Figure 6-1c) offer little lateralrestraint. Hence, failure is initiated by the compressivefailure of individual rock columns. Tightly closed discontinuities(Figure 6-1d) on the other hand, providelateral restraint. In such cases, general shear is the likelymode of failure.b. Steeply dipping and widely spaced joints. Bearingcapacity failures for rock masses with steeply dippingjoints and with joint spacing greater than the width of thefoundation (Figure 6-1e) are likely to be initiated by splittingthat eventually progresses to the general shear mode.c. Dipping joints. The failure mode for a rock masswith joints dipping between 20 to 70 degrees with respectto the foundation plane is likely to be general shear(Figure 6-1f). Furthermore, since the discontinuityrepresents major planes of weakness, a favorably orienteddiscontinuity is likely to define at least one surface of thepotential shear wedge
6-6. Layered Rock Mass
Failure modes of multilayered rock masses, with eachlayer characterized by different material properties, arecomplicated. Failure modes for two special cases, however,have been identified (Sowers 1979). In both casesthe founding layer consists of a rigid rock underlain by asoft highly deformable layer, with bedding planes dippingat less than 20 degrees with respect to the foundationplane. In the first case (Figure 6-1g), a thick rigid layeroverlies the soft layer, while in the second case (Figure6-1h) the rigid layer is thin. In both cases, failure isinitiated by tensile failure. However, in the first case,tensile failure is caused by flexure of the rigid thick layer,while in the second case, tensile failure is caused bypunching through the thin rigid upper layer. The limitingthickness of the rigid layer in both cases is controlled bythe material properties of each layer
.6-7. Highly Fractured Rock Masses
A highly fractured rock mass is one that contains two ormore discontinuity sets with typical joint spacings that aresmall with respect to the foundation width (Figure 6-1i).Highly fractured rock behaves in a manner similar todense cohesionless sands and gravels. As such, the modeof failure is likely to be general shear.
6-8. Secondary Causes of Failure
In addition to the failure of the foundation rock, aggressivereactions within the rock mineralogy or with groundwater or surface water chemistry can lead to bearingcapacity failure. Examples include: loss of strength withtime typical of some clay shales; reduction of load bearingcross-section caused by chemical reaction between thefoundation element and the ground water or surface water;solution-susceptible rock materials; and additional stressesimposed by swelling minerals. Potential secondary causesshould be identified during the site investigation phase ofthe project and implimentation. Once the potential causes have been identifiedand addressed, their effects can be minimized.
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