geotech 3

[5125er]

we excavated to 5 feet and remove the loose fill and the strata at this elevation is a virgin ground with single digit blow counts. the game plan is to fill it in 12 inch lifts with RCA and compact it to 95% of maximum dry density. how you determine the bearing capacity of the compacted recycled material and how far below the surface the load will impact the fill.

 

[msucog]

depends on the loads, area loaded, and characteristics of soil, plus a few other things. undercutting 5 feet and replacing means the footing is something like 2-4 feet in the ground...or essentially bearing on the residuum. i suggest that if you really want to know, you should perform some borings, obtain soil samples, run a few tests, perform some analysis and use engineering judgement to evaluate what a "reasonable" bearing pressure/capacity might be for your site. evaluating settlement potential would probably be a good place to start.

 

[msquared48]

You state "at this elevation". That concetrns me. What about below that? I suggest that at least a couple of borings to 20 feet or so would verify your design. I agree with msucog.

Mike McCann
McCann Engineering

 

[fattdad]

The determination of bearing capacity at any single level is dependent on the supported load and the depth of stress increase. For starters, consider the soil profile beneath your footing depth. Most likely the soft clay will limit your bearing capacity. You would then consider the undrained shear strength and multiply that by 5.6 (IIRC) for the ultimate bearing pressure. Divide that by three for the allowable bearing pressure. Let's say that's 1,000 psf. If you then have three ft of "headroom" between the concrete bearing grade and the top of the soft clay (i.e., owing to the thickness of the newly-placed aggregate) you would then reverse figure what bearing pressure would provide a load spread to limit the pressure on the clay to 1,000 psf. This requires an interative step where you'd anticipate a bearing pressure, consider the column load and then work either Bousinnesq or Westergaard equation.

After you do all this you would then consider the settlement response of the "safe" bearing pressure. Unlikely that the settlement in the newly-placed fill will amount to much, but the soft clay may be a different matter. If settlement is too great, then you'd have to further limit the bearing pressure. Bear in mind that you would want to consider settlement via soil modulus and ALSO one-dimensional consolidation coefficients.

Hope these ramblings help.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Following fattdad - he is likely quite correct in using 5.6/3 for allowable bearing "capacity" - but if you factor in the foundation geometry, etc. it can be more. He is using, really 1.9x undrained shear strength. I typically use 2x - it is easier to do mentally in a second or so. It will make so little difference. As fattdad implies, it is seldom the allowable bearing capacity that governs but the settlement which will give the allowable bearing pressure that can be used.

 

[msucog]

my experience as well--"it is seldom the allowable bearing capacity that governs but the settlement which will give the allowable bearing pressure that can be used"