Soil Bearing Capacity decreases as footing size suggestion increases 2

[SatoshiNakamoto09]

Footing size 1.0m 1.50m 2.0m 2.5m 3.0m
Depth (m) (psf) (psf) (psf) (psf) (psf)
1 2778 2736 2694 2652 2611
2 3634 3362 3216 3133 3070
3 3592 3425 3258 3154 3091
4.5 4219 4135 4114 3947 3843
6 4532 4449 4365 4281 4198

Hello,

Im doing structural design, and for the design of foundation i got these values from the soil report(shown above).

Based on the bearing capacity evaluation indicated in the report, the geotechnical engineer used Hough Method(which is based on 25mm settlement) and using Skempton's ultimate net bearing capacit for fine grained soils.

I would like to ask any opinion from you if these results are quite acceptable? or is it possible to get lower bearing capacity as the suggested footing size to be used becomes larger?

I got 2 soil reports already from this geotechnical engineer but 2 different projects and it has similar results which is SBC decreases as footing size suggestion increases.

 

[fattdad]

Depends on the layering.

A bigger footing will allow for changing stresses to greater depths. If more compressible soils are at depth, then that will influence the allowable bearing to limit the settlement to 25mm.

I don't like the table; however. It seems more precise than our practice.

I also don't like the fact that the engineer has no idea of the structural approach. It's like there's no dialog between the geotech and structural engineers.

f-d

ípapß gordo ainÆt no madre flaca!

 

[BridgeSmith]

Seems perfectly normal and reasonable to me. The load spreads out as it goes down through the soil. Just for the sake of an extreme example, if we assume the angle of the spread is 45 degrees (1:1), at 1m below a 1m wide footing, the load is spread over 3m; the pressure at the 1m depth is 1/3 of what's applied to the footing. With a 10m wide footing, the load is spread over 12m at the 1m depth, producing a pressure at that depth of 5/6 of the pressure applied to the footing.

 

[BridgeSmith]

"It's like there's no dialog between the geotech and structural engineers."

Sounds like my world, except we don't have geotechs in the department across the parking lot. They're mostly geologists, so us structurals end up trying to muddle through doing half of a geotech's job, while the guys in Geology try to muddle through the other half.

 

[oldestguy]

I liken those tables to a "can of worms". It isn't all that precise, considering loads not usually precise and ground not that precise. I worked for Hough early on and his tables are not precise.

 

[Okiryu]

That table is strange...Footing sizes and Df are in meters and the allowable bearing pressures are in psf and as f-d mentioned the numbers are too “precise”...

 

[maxim22]

Some absence of precision indicates an understanding of the variability of ground, how well it is tested. Inappropriate accuracy sometimes due to engineers using calculations in ground structures more used to concrete and steel.

 

[Okiryu]

I heard also that it is better to be approximately correct rather than precisely incorrect...

 

[Settingsun]

Looks reasonably traditional. See image below.

I wouldn't get too hung up on the number of significant digits. The geotechnical engineer has done a calculation and reported the results. The variance in any row is less than 20% so a rounded number would give practically the same result. This way you get a little more visibility and insight into the expected soil behaviour rather than be left wondering how much the geotech rounded off and wrote off to 'factor of safety'. There may be good reason to keep the digits anyway as the units are mixed - maybe the project is wantonly converting from metric to imperial and losing accuracy if rounded at each step. A rounded number is only rounded in one set of units: 3000psf is 143.6kPa after all (or 150kPa is 3133psf).

 

[GeoEnvGuy]

Bearing capacity is determined in two ways, the ultimate bearing capacity (failure) and the serviceable bearing capacity (settlement). Starting from a point load your bearing capacity is determined by failure. As you increase footing size the bearing capacity peaks at the transition between failure and settlement control. Continuing to increase footing size your bearing capacity is now settlement controlled which decreases with size.

This is illustrated in Ralph Pecks Foundation engineering book.

The Canadian foundation engineering manual working stress design developed charts based on SPT N values using this concept. With the exception of flattening the failure side and straight lining the settlement side to a maximum footing size.

Looking at the table I wonder what is the frost depth? Why is he making a table showing so many capacities, why is it not providing the size required for the project?

 

[EireChch]

GeoEnvguy - apparently it is common in some countries to give a range of foundation sizes and depths. Its not common in the UK / Europe from what i know.

In the Middle East, we typically give 1-5m wide, by 1-5m deep. For various L/B ratios 1,2,3,5 and 10. Then we give rafts of 6x6, 10x10, 15x15 and 20x20m. And for all that we give modulus of subgrade reaction too!

Its a pain in the ass.

 

[Okiryu]

I think that Geotech investigations are commonly done before any structural analysis is completed. So, we do not get real loads from the SE (at least in my case, I typically do not get that information at the start of the Geotech analysis). But I think that we can have an idea of the loads based on the type of structure and previous experiences. Anyway, that's why I think that geotechs need to have some knowledge of structural engineering. We need to understand what the SE is planning to do (actually, we need to anticipate what the SE is planning to do). My typical recommendations are like: "...this is the recommended foundation system with this allowable bearing pressures (or capacities in case of piles) based on this range of X~Y assumed load and 25 mm settlement. If your structural analysis give you values outside that range, please contact us so we can refine our recommendations...". Anyway, we need to give "something" so people can start planning on costs (foundation costs are critical in most of my projects) or doing some preliminary design.

I would say that geotechs are good on guess estimates/assumptions...I think that we always need to make good assumptions because we are dealing with soils (and structural engineers !!) which are quite variable (and geotech investigations cannot cover each square foot of the site). That's remind me that I always tell my younger engineers that we need to have a sense and appreciation of the numbers... that is the base of good assumptions...

 

[BigH]

GeoEnvGuy - your explanation of Shear vs Settlement is okay - however,I, and it seems like many others use the following -
Allowable Bearing Capacity - this is based on shear strength (SF included)
Allowable Bearing Pressure - this is based on settlement (serviceability).
In almost all cases it is the allowable bearing pressure that governs. Say, for instance you have a settlement requirement of 10 mm - bearing capacity will basically be meaningless - the pressure to be applied would be the allowable bearing pressure.

Agree with those pointing out that determining bearing pressure (or capacity) to a single psf is a farce.