Increase in allowable pressure for depth of footing 1

[JimParks]

Hi folks,

I have a question regarding the increase in allowable soil bearing pressure per foot of depth of a footing.

The allowable soil bearing pressure (from the code or typical soil reports) begins at a minimum 12" below grade and can be increased, in combination with increase for width, 20% per foot of depth (or width) up to a maximum 300% (combined).

My question: Is the depth measured from the natural (original) grade, minus 12", of an unimproved site or from the finished grade?

The debate here arose when discussing the allowable bearing pressure at a footing for a retaining wall in a cut situation. One school of thought was the depth should be measured from the finished grade at the toe, which was to be the new design grade, and used throughout the width of the footing. The other was that the allowable should be determined relative to the depth of the footing from the original grade, as the soil being removed had been providing an overburden at that location for some time.

A lunch is riding on this.

Thanks in advance.

Jim

 

[jheidt2543]

I don't think I've ever heard of this. What is the rational behind the increase? Is it because of the removal of the overburden? Most soils reports I have seen give an allowable soil bearing pressure at various depths depending on changes in soil type or depth of proposed footings.

Now I'm interested in the answer too!

 

[GeoffD]

Condider a circular failure centred on the outside edge of the foundation. In order to fail the disturbing forces have to overcome frictional forces along the line of failure, which are influenced by normal pressures across the failure surface, and would also have to lift up the wedge of soil resisting failure, which is a function of the weight of soil forming the wedge.

With a foundation at ground level the failure path is relatively small, the normal forces small and the wedge of soil resisting movement is small therefore factor of safety is lower than a deeper foundation with longer failure surface, greater normal forces across that surface (due to weight of overlying soil) and a greater weight of soil forming the wedge of soil that would need to be uplifted for failure to occur. Therefore it is depth below formation level that's important.

With regards reducing levels and removal of overburden there is in theory a net reduction in vertical stress in e.g. a foundation trench but it is a relatively small addition to the allowable bearing capacity.

I would not assume if levels were reduced by say 10 metres with say a reduction in stress of approx 200kPa that it would be safe to add this to an allowable bearing capacity for a foundation at the new ground level, as the failure conditions would be exactly the same in terms of length of failure surface, normal stresses across that surface and weight of soil resisting movement.

 

[JimParks]

Thanks Geoff,

The circular failure is what settled the argum.... discussion here.

Thanks again,

Jim

 

[BigH]

While I realize that this is from a few months back, sometimes, for later reference, additional comments might be welcome.

Jim Parks indicated allowable bearing pressures in his opening query. The base pressure allowed in design will depend on two conditions being met - (1) no distress due to shear movements (and GeoffD covered that) and (2) settlements within tolerable range so that the structure remains integrally intact to perfom its function.


Allowable bearing capacity is based on the shear conditions. Shearing is based on gross pressures. Use the post constrution conditions for shear stability will be dependent on the strength values and post-construction slip planes.

Limiting settlements must also be determined - these dictate the allowable bearing pressures that can be taken to prevent structure distress. For this condition (i.e., settlement) you can use the OGL - as settlement is based on the additional or net new loadings.

Having determined this, use that which is least.

Personal Comment: I originally took up geotechnical engineering as it appeared to be the least governed by codes - it permitted a bit of freedom in arriving at sound engineering solutions. Do you think that the original thinkers used such codes? We would never have developed. Geotechnical engineering is now becoming too code oriented, in my view. Codes should be used as guidelines (and they should say that) - sound practical knowledge and suitable analyses in the form of good engineering principals that have developed one's experienced judgment is the key to our field - not bloody codes. I see how codes (India's for example) stiffle sound engineering judgment because no one want to go against the written "rule". How does a code know all the idiosyncracies of a particular site? - it is generic.

Best regards.

 

[Focht3]

BigH is right. I avoided going into environmental engineering because it was obvious to me (in the early 1980's) that government was going to impose codes and rules to "assure" that contaminated sites were cleaned up "properly." In my view, most of the real engineering has been taken out of environmental engineering projects. I was able to work on one of the original SuperFund projects before the rules and regulations were established. (The waste generator negotiated an agreement with the EPA, and managed the clean-up themselves. It was a very interesting and challenging job that was completed in far less time and at far less cost than similar EPA-run projects.)

Unfortunately, the proliferation of codes reflects (in part) our profession's inability - or unwillingness - to communicate the uniqueness of all of our assignments. And I fear that the quality of our profession is deteriorating, which also may be contributing to this problem.

I believe that there is a reason that so many geotechnical engineers have historically been officers in ASCE at rates far above our numbers within ASCE or the civil engineering profession: many of us are innovative and independent thinkers. The proliferation of "codes" and "regulations" in our profession - if it occurs - will spell a drastic decrease in innovation in our profession. A sad day, indeed.

I, for one, will go that way kicking and screaming.

 

[BigH]

Focht3: Hear!!!! Hear!!!!! But, who will listen and lead us away from impending disaster??? from mediocrity? Don't think any of our bureaucratic lawmakers, lawyers and, unfortunately, our associations will.

"Rules freeze companies inside a glacier; innovation lets them ride sleighs over it." (Ricardo Semler)