Chapter 2 Bearing capacity of foundations
Chapter 2 Bearing capacity of foundations
2.1 Introduction The soil must be capable of carrying the loads from any engineered structure placed upon it without a shear failure and with the resulting settlements being tolerable for that structure. This chapter will be concerned with evaluation of the limiting shear resistance, 周 or ultimate bearing capacity quit,of the soil under a foundation load. 阳喝
2.1 Introduction ▪ The soil must be capable of carrying the loads from any engineered structure placed upon it without a shear failure and with the resulting settlements being tolerable for that structure. ▪ This chapter will be concerned with evaluation of the limiting shear resistance, or ultimate bearing capacity qult, of the soil under a foundation load
2.1 Introduction A soil shear failure can result in excessive building distortion and even collapse. Excessive settlements can result in structural damage to a building frame,cracks and equipment failure. In many cases settlement criteria will control the allowable bearing capacity,however,there are also a number of cases where base shear dictates the recommended beating capacity
2.1 Introduction ▪ A soil shear failure can result in excessive building distortion and even collapse. ▪ Excessive settlements can result in structural damage to a building frame, cracks and equipment failure. ▪ In many cases settlement criteria will control the allowable bearing capacity, however, there are also a number of cases where base shear dictates the recommended beating capacity
2.2 Bearing capacity for footings on layered soils It may be necessary to place footings on stratified deposits where the thickness of the top stratum from the base of the footing d is less than the H distance computed as H=B tana/2. In this case the rupture zone will extend into the lower layer(s)depending on their thickness and require some modification of quit There are three general cases of the footing on a layered soil as follows: Case 1.Footing on layered clays (all =0) a.Top layer weaker than lower layer(c1<c2) b.Top layer stronger than lower layer(c1>C2)
2.2 Bearing capacity for footings on layered soils ▪ It may be necessary to place footings on stratified deposits where the thickness of the top stratum from the base of the footing d1 is less than the H distance computed as H=B tanα/2. ▪ In this case the rupture zone will extend into the lower layer (s) depending on their thickness and require some modification of qult ▪ There are three general cases of the footing on a layered soil as follows: ▪ Case 1. Footing on layered clays (all φ=0) ▪ a. Top layer weaker than lower layer (c1<c2 ) ▪ b. Top layer stronger than lower layer(c1>c2 )
2.2 Bearing capacity for footings on layered soils Case 2.Footing on layered -c soils with a,b same with case 1. Case 3.Footing on layered sand and clay soils a.Sand overlying clay b.Clay overlying sand Experimental work to establish methods to obtain qult for these three cases seems to be based mostly on models- often with B<75mm. Several analytical methods exist as well. Button(1953)used a circular arc to search for the approximate minimum(for the trial circle all in the top layer),give N.=5.5<2T
2.2 Bearing capacity for footings on layered soils ▪ Case 2. Footing on layered φ-c soils with a, b same with case 1. ▪ Case 3. Footing on layered sand and clay soils ▪ a. Sand overlying clay ▪ b. Clay overlying sand ▪ Experimental work to establish methods to obtain qult for these three cases seems to be based mostly on modelsoften with B<75mm. ▪ Several analytical methods exist as well. ▪ Button (1953) used a circular arc to search for the approximate minimum (for the trial circle all in the top layer), give Nc=5.5<2π