Foundation Design 3

[LeonhardEuler]

Hello,

I am analyzing what I believe to be a mat foundation, or large spread footing? It is approximately 40'x20' and is loaded via two concrete "walls", or "pedestals", that sit 25' apart and each run the full width of the slab. The structure the pedestals support is subjected to wind load as well as 500 kips of gravity load. The foundation itself is over 6 foot in height, but no part of it is below grade...

From what I am seeing in Das foundation design book I can find bearing pressure on soil by simply dividing gravity load by area i.e. P/A. Easy enough. However, I am having difficulty defining the applied soil pressure resulting from the moment caused by wind loads. I have seen on here assume infinitely rigid slab and use M/S is this accurate?

What I am looking for is a good design procedure and/or reference for designing this type of foundation.

 

[LeonhardEuler]

steveh49 Yes, the project is in Florida and all other borings show medium-dense, coarse grained sand. I actually did convert the smallest SPT N-value seen into a phi angle (30 blows per foot) and used Meyerhoff and ended with a 10,000 psf load bearing value, before factor of safety. Perhaps it would be better to slap a F.S. of 5 on that, explain that the boring is close enough given such a high F.S., and use the Meyerhoff equivalent area method for bearing capacity equations.

 

[oldestguy]

OG here again. For a slab this large the various methods for bearing capacity mentioned are a failure by soil rupture. That is not the first thing that happens which is a "failure". Failure of a slab this large is excessive settlement. For the high blow count, and for the pressures being mentioned, there is likely no excessive settlement. A few methods for computing settlement of granular soils are out there, but consolidation based theories don't apply to sand. If you wish, research the method addressed by Hough back in the 50's. ASCE August 1959 SM4, paper #2135 "Compressibility as the Basis for Soil Bearing Value". He ties it to blow count.

 

[Ron]

LeonhardEuler....where in Florida is the project located. That can make a huge difference in what we opine here.

 

[LeonhardEuler]

RonEast coast about a mile from the ocean. Most of what we have in this area is A3 sand and in my particular area hydraulic fill starting at about 50 feet.

oldestguy I did see those and from what I have gathered there are SPT conversions from Hough that give an allowable soil bearing pressure that results in 1 inch of immediate settlement. I would imagine the Florida Building Code's number is derived from settlement as well, but it is impossible to tell, because they don't offer their calculation procedure. If I am remembering right from my foundation design classes I believe terzaghi offers some settlement procedures for cohesionless soil.

 

[LeonhardEuler]

Digging through some of my university notes and it seems we used Schmertmans method for settlement of shallow foundations.

 

[Settingsun]

The good news in this case is that it's a moderate load increase in a foundation that was 'load tested' simply by holding up the previous tank. Has the old tank been in place for several years and experienced some decent storms?

If so and there are no signs of excessive or differential settlement, you're unlikely to have any significant problems with the 17% load increase which is still essentially centred on the footing. Just another level of confidence on top of the building code allowable pressure and your Meyerhof calc.

 

[LeonhardEuler]

Steveh49 Yes, it has been in operation for years and shows no sign of excessive settlement

 

[Ron]

History of settlement under gravity loads only is just one piece of the information. It's great if it has performed well in that respect. Even though the site is on the east coast of Florida, it has likely never seen the design wind loads to consider for overturning or sliding. The foundation has no embedment in the soil so it is certainly susceptible to sliding, overturning and scour under the various potential load considerations.

 

[LeonhardEuler]

Roninteresting that you mention Scour. I was worried about this, but found no method of analysis to show that it wont be a problem. I assume its a continued monitoring operation.

 

[geov86]

Hi leonhardeuler. Did you include the depth of the water table in regards to the foundation. If you didn't you should, because we are talking about a coarse grained sand and you don't want to mesh it up with water when you have this kind of soil as friction is an essential factor regarding the bearing capacity. Keep in mind that the settlement of a footing in dry sand may go approximately to double if the water table rises and floods the formation. Now as for the settlement, generally settlement in a granular soil such as coarse grained sand is not that much of a trouble (usually they have large Es) and any occurred deformation is expected to be at the foundation edges.

In general, here are some things to have in mind from a geotechnical point of view. First, the bearing capacity usually increases when the lateral pressure from either the upper formations or the applied loading is increased. Especially the upper formations are really important in regards to bearing capacity in coarse grained sand. in addition, The bearing capacity of the foundation goes to half if the ground is flooded with water.Last but not least, keep in mind that this soil can become compacted rather easily if a vibration (i.e earthquake) takes place.

 

[LeonhardEuler]

geov86 Yes, I did account for the water table which is at about 3' here. It's flooded always