increase in stress calculation for layed soil

[Marissa]

I have a question on calculating the increase in stress under square foundation, when soil are layered. In Das' Foundation of geotechnical engineering ', Sec 7.12, there is an equation, delta(sigma)average=(delta(sigma)top+4*delta(sigma)mid+delta(sigma)bottom)/6

In another similar problem, the solution use Westergaard chart DM7.1-176 for layed soils. Using the two methods to solve the same problem I got different answers. Can someone tell me what's the difference between these methods, and when to use what if the problem didnot specify. Thanks.

 

[Venpr]

I have a question on calculating the increase in stress under square foundation, when soil are layered. In Das' Foundation of geotechnical engineering ', Sec 7.12, there is an equation, delta(sigma)average=(delta(sigma)top+4*delta(sigma)mid+delta(sigma)bottom)/6
In another similar problem, the solution use Westergaard chart DM7.1-176 for layed soils. Using the two methods to solve the same problem I got different answers. Can someone tell me what's the difference between these methods, and when to use what if the problem didnot specify. Thanks.

Marissa

I look to all my references (Das, diferents editions, NHI Manual Vol 1, NFEC DM 7.01 Geotech Eng portable Handbook R.W. Day, etc) for "increase in stress under square foundation for layered soil" don't see this equation, I think for test porposes the using the chart based on Westergaart theory is you best shot, that is the way that i seen before on the geotech depth test.

 

[Marissa]

Marissa
I look to all my references (Das, diferents editions, NHI Manual Vol 1, NFEC DM 7.01 Geotech Eng portable Handbook R.W. Day, etc) for "increase in stress under square foundation for layered soil" don't see this equation, I think for test porposes the using the chart based on Westergaart theory is you best shot, that is the way that i seen before on the geotech depth test.

 

[Marissa]

Venpr,

Thanks. That's good. The equation is shown in his another book, too. Anyway the chart is much easier to use.