Allowable Bearing Capacity 1

[Steel2Steel]

I have a situation where I have a foundation/retaining wall. The wall is 16' tall and is retaining 13'10" of soil on the one side. On the other side, the grade is sloping and at a particular location is 9'-6" above the top of the footing. I have a 100PSF surcharge load on the side the soil is being retained. The geotechnical engineer has given me an allowable bearing capacity of 1500 PSF. I only need a heel of 6'0" or so for overturning but the problem is I can never get my bearing pressure below 1500. The reason being the (height of soil * 115PCF) + (150 PSF for the concrete ftg) + (100 PSF Surcharge) = 1840 PSF which is greater than 1500PSF. This is without the additional load due to the moment. The soil load alone is 1590 PSF. I don't beleive this to be reasonable. I would think, most of the time, the deeper excavation, the greater your bearing capacity would get. I also thought about looking at an effective pressure by doing the following: Since I have 9'6" of soil on the passive side and 13'-10" of soil on the other.9'-6" + thickness of ftg 12", multiply by 115PCF and add this to the 1500PSF given by the geotechnical engineer. This would give me an effective bearing capacity of 2700 PSF, only at this retaining wall condition. The rest of my footings have been designed for 1500 PSF. At this depth of 13'-10", basically the weight of the soil is crushing itself?? The bigger an deeper you make the footing the more you are taking away from the bearing capacity???? Thanks in advance, David

 

[BigH]

Dafrazie - I've seen this post in the Structural thread - no need to post twice (and it is frowned on). Most geotechs also read the structural threads.

 

[dgillette]

Just guessing, the geotech gave you 1500 for the bearing capacity for a very shallow foundation. The bearing capacity increases with embedment. Look at DM-7 or other handbook, and you will see the factors for increasing the bearing capacity with depth, and some explanation. Or, call up the geotech and ask him.