allowable bearing capacity

huangyhg

allowable bearing capacity
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
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get a different geotech who is interested in the job. yours probably isn't interested enough.
also, verify with the geotech if the 1500 psf is net or gross allowable.  often, the number they give you has already accounted for the weight of the soil above it.
defrazie,
keep in mind that the depth of the soil is directly proportional to bearing capacity. q=d*gamma is one part of the ultimate bearing capacity of the soil. terzaghi's equation will give the true ultimate bearing capacity. the 1500psf is just a conservative guess. bearing capacity is a part of the design, but you will need to determine your bearing capacity to see if your soil heights, surcharge, and width of footing/wall exceed this. good luck.
talk to the geotech about the problem.  sounds like the net vs gross may be the big issue here- you and the geotech need to make sure you're talking about the same things.
i checked with the geotech and he said that i must include the weight of the soil in my bearing calculations.  he said that he would have to provide helical piers to handle the load.  i don't think this is logical.  if you are determining an effective bearing pressure you subtract any overburden from the design bearing pressure.  i.e.  1500psf-150psf(12" ftg)-115pcf*13'-10"(soil above ftg)= negative bearing capacity????  the soil was there before it was excavated out, we place a retaining wall footing and put the soil back.  the only additional pressure that has been added is the surcharge load and the moment from overturning.  
what kind of soil do you have, unit weight, c-phi?  1500 psf is pretty low.  
i would go over what the geotechnical engineer assumed in his calculation of the bearing capacity and settlement and check to see that his assumptions are reasonable.  he would need to assume loads and footing sizes and depths to arrive at a suitable bearing capacity.  perhaps this has not been taken into account properly.  
if you have a very weak or loose soil you may be dealing with a failure mode other the general bearing capacity failure, though i'd say it is doubtful.   
try reversing the toe and the heel, making the toe much larger than the heel.
check in the boring test logs and at consolidation results that the foundation soils are not compressible.  many times the computed bearing, specially at deep embedments are high, but settlement estimates force us to cut the design bearing presssures.  
say the worst case scenario-that indeed your footing soils are compressible remove and replace with crushed rock up to one footing diameters in depth and have the geotech recalculate if 2500-3000 psf is appropriate.  by replacing compressible soils with rock, you take the limiting settlement factor out & you also increase allowable pressure so the footing widths are smaller and consequently the "zone of influence" in the foundation soils are also smaller.
i think that as a first step, advise us of the foundation soil - stratigraphy, n values, undrained shear strength, etc.  secondly, the term allowable bearing capacity gets bandied about quite a bit.  is it actually a restriction on the bearing capacity or is the geotechnical engineer, as some have alluded to, limiting the value he gave you due to settlement? - which, then in my view, should be called the net allowable bearing pressure. we've had several threads on this issue in the past.
    if, in fact, it is the allowable bearing capacity, what is the factor of safety that the geotech is proposing.  if he is using sf=3 and presuming you have a clayey founding material, then your n values would likely be in the order of 5 to 7? and/or undrained shear strengths in the order of 35 to 45 kpa (700 to 900 psf)?  do you have any desiccated crust?  do you need to use a cantilevered retwall?  can you use an mse wall? - more forgiving to settlement and typically, it is permissible to use a fs of 2 in this kind of analysis.
  another question - how is the footing level of the retwall in relation to the original ground level?   
the 1500 psf probably applies to the material under the toe. allowable bearing pressure increases with depth, so before you excavated the material, you had an additional 14 feet of material on top of this soil, which would raise the allowable bearing. if you have reviewed your numbers with the geotech, and he says you need piles, then you probabaly do, although big h has some options worth dicussing with the geotech.