2.2 Bearing capacity for footings on layered soils The use of trial circular arcs can be readily programmed for a computer for two or three layers using su for the layers. It is suggested that the circular arcs be limited to cases where the strength ratio CR=c2/c1 of the top two layers is on the order of 0.6<CR<1.3. where CR is much out of this range there is a large difference in the shear strength of the two layers,Nc is obtained using a method given by Brown and Meyerhof (1969)based on model tests as follows:equation (2.2)~(2.6)
2.2 Bearing capacity for footings on layered soils ▪ The use of trial circular arcs can be readily programmed for a computer for two or three layers using su for the layers. ▪ It is suggested that the circular arcs be limited to cases where the strength ratio CR=c2 /c1 of the top two layers is on the order of 0.6<CR≤1.3. ▪ where CR is much out of this range there is a large difference in the shear strength of the two layers, Nc is obtained using a method given by Brown and Meyerhof (1969) based on model tests as follows: equation (2.2)～(2.6)
2.2 Bearing capacity for footings on layered soils When the top layer is very soft with a small d /B ratio, consideration should be given to placing the footing deeper onto the stiff clay or to using some kind of soil improvement method. Model tests indicate that when the top layer is very soft it tends to squeeze out from beneath the base and when it is stiff it tends to punch into the lower softer layer [Meyerhf and Brown(1967)]. If quit >4c+q the soil may squeeze from beneath the footing (the "lower-bound"solution). It is suggested for -c soils to obtain modified pand c value as follows:①④ If the top layer is soft you should check for any squeezing using Eq.(a)
2.2 Bearing capacity for footings on layered soils ▪ When the top layer is very soft with a small d1 /B ratio, consideration should be given to placing the footing deeper onto the stiff clay or to using some kind of soil improvement method. ▪ Model tests indicate that when the top layer is very soft it tends to squeeze out from beneath the base and when it is stiff it tends to punch into the lower softer layer [Meyerhf and Brown (1967) ]. ▪ If qult >4c1+q the soil may squeeze from beneath the footing (the “lower-bound” solution). ▪ It is suggested for φ-c soils to obtain modified φand c value as follows: ①～④ ▪ If the top layer is soft you should check for any squeezing using Eq. (a)
2.2 Bearing capacity for footings on layered soils For bases on sand overlying clay or clay overlying sand,first check if the distance H will penetrate into the lower stratum.If H>d1 you might estimate qult as follows:①③ ■ A possible alternative for o-c soils with a number of thin layers is to use average values of c and o in the bearing capacity equations of Table 2.1 obtained as Eq.(c)and (d). A slope-stability program written by Bowles can be used to obtain the bearing capacity for layered soils. 超
2.2 Bearing capacity for footings on layered soils ▪ For bases on sand overlying clay or clay overlying sand, first check if the distance H will penetrate into the lower stratum. If H>d1 you might estimate qult as follows: ①～③ ▪ A possible alternative for φ-c soils with a number of thin layers is to use average values of c and φ in the bearing capacity equations of Table 2.1 obtained as Eq. (c) and (d). ▪ A slope-stability program written by Bowles can be used to obtain the bearing capacity for layered soils