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5.3. Bearing Capacity of MAT Foundations

The gross ultimate bearing capacityof a mat foundation can bedetermined by the same equationused for shallow foundations seesection 3.!" or


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chapter 3 gi#es theproper #alues of the

bearing capacityfactors$ and theshape$ depth andload inclinationfactors". The term Bin %q. 3.&5" is thesmallest dimensionof the mat. The netultimate capacity is '

. 3.()"

Figure 5.2 (Continued) (b) FlatPlate thickened under column.

b"


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A suitable factor of safety should be used tocalculate the net allowable bearing capacity.

For rafts on clay$ the factor of safety shouldnot be less than 3 under dead load andma*imum li#e load. +owe#er$ under themost e*treme conditions$ the factor of

safety should be at least (.!5 to & for raftsconstructed o#er sand$ a factor of safety of3 should normally be used. ,nder mostwor-ing conditions$ the factor of safetyagainst bearing capacity failure of rafts onsand is #ery large.for saturated clays with and #erticalloading condition$ %q. 3.&5" gi#es

.................... 5.


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/here

Figure 5.2 (Continued)(c)Beams and slab


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From Table 3.5$ for


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Figure 5.2 (Continued) (d) Slabwith basement wall

d"


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+ence the net ultimate bearing capacity is

For F0 1 3$ the net allowable soil bearing capacitybecomes


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The net allowable bearing capacity for matsconstructed o#er granular soil deposits can beadequately determined from the standardpenetration resistance numbers. From %q. 2.53"$ forshallow foundations

/hen the width$ B is large$ the precedingequation can be appro*imated assuming3$& B ( 3$& B" as


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4n %nglish units$ %q 5.(&" may bee*pressed as


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ote that %q. 5.(3" could ha#e been deri#ed from %qs.2.52" and 2.56"

ote that the original %qs. 2.53" and 2.56" were for asettlement of ( in. &5$2 mm" with a di7erential settlementof about 8$!5 in () mm". +owe#er$ the widths of the raftfoundations are larger than the isolated spread footings. Astable 2.3 shows$ the depth of signi9cant stress increase inthe soil below a foundation depends on the foundation

width. +ence$ for a raft foundation$ the depth of the :one ofin;uence is li-ely to be much larger than that of a spreadfooting. Thus the loose soil poc-ets under a raft may bemore e#enly distributed$ resulting in a smaller di7erentialsettlement. +ence the customary assumption is that$ for a

ma*imum raft settlement of & in. 58$ mm"$ thedi7erential settlement would be 8$!5 in () mm". ,sing thislogic and conser#ati#ely assuming that Fd wquals ($ wecan appro*imate %qs 5.(&" and 5.(3" as


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4n all cases$ q should be less than or equal to

Figure 5.4 De nition o! net "ressureon soil caused b# a mat !oundation


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%*ample 5.(


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%*ample 5.&


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5.2


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The term %>4b can be e*pressed as

.............................. 5.( "

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