Bearing Capacity of Soil

[JN0589]

I have 2 questions related to the bearing capacity of soils during excavation.

The first relates to have a sloped cut next to an adjacent building to essentially eliminate underpinning. I've found a few samples that reference Meyerhof (ex: Naval DM 7.2-135). Based on this chart however (see attached), assuming an internal angle of friction of 30, a 30 degree slope and the slope starting directly below the building footing (b = 0 and D + 0), this results in a bearing capacity factor = 4 and thus q_ult = (120 pcf) * (2ft / 2) * (4) = 480 psf. This is a really low number and would definitely not support any building. How realistic is this number and is their any error in my calculations/assumptions?

The second question is in regards to the bearing capacity formula, Qu = cN_c + γDN_q + 0.5γBN_γ. The second term in this equation relates to the overburden pressure. Assuming your footing is 4 ft below soil (D = 4 ft), but approximately 10 ft away, there is a vertical cut supported by a soldier pile and lagging system, how is the bearing capacity affected?

 

[oldestguy]

Maybe it's just bum luck, but out of the many (maybe in the thousands)of reports I have written and given this recommendation, I have never heard of a structural problem.

"Don't exceed a cut slope of 1 to 1 down from edge of old footing"

In a couple of cases then I've witnessed they exceeded that slope and gotten away with it. For one of those cases, as an employee, I gave a written statement to the contractor that we (our company) no longer will be associated with his work. Our company president then said he understood my immediate job action and stood by me, even though that contractor had been a good client. The footing supported a 2 story brick structure housing day care children.

 

[PEinc]

If you have a building foundation sitting at the top of a 30 degree slope with phi = 30 degree soil, you should also check the slope stability.

http://www.PeirceEngineering.com

 

[killswitchengage]

i think in both cases you should conduct a numerical modeling . This is were they are the most needed since complex problems requires complex softs . try using FEM for this .
For the second question , you know that the load is transferred by a strip footing in a trapezoidal manner through the many layers . the existence of the supported cut slope will not undermine the bearing capacity of the footing , on the contrary it will improve it. Unless of course the slope is not well supported, your primary concern will be the added pressure on it because of the the footing. I also think of you have a compressible layer of clay and the cut slope is well supported ,the footing may settle deferentially . Well in the end , these are my humble opinion .

 

[BigH]

Wow - were we ever able to look at a problem before FEM?

 

[oldestguy]

Big H Gosh, for all those past years since 1956 I, musta had a great guardian angel. I just mentored with an eng tech yesterday on this very subject.

 

[Ron]

BigH...pull out your Pickett and a napkin and solve this one.

 

[BigH]

For the first part - try to check and wade through Bowles Section 4.9 on his 5th edition. One part you have is that the friction angle and the slope angle are the same; pretty well means you have a dry slope - any water seepage or so will cause you problems (see infinite slope). Having your footing on the edge of the the slope means that the Nq value will be reduced by half. So obviously, the bearing capacity of the footing will be greatly reduced. I think the best way to look at this is to do a slope stability assessment. You can include your footing (hard layer) in the model and then see what you will get - in reality - the bearing capacity of this, in my view, is not the governing factor - it is as PEinc says - the slope stability issue. Without the footing your slope stability FOS = 1 for a dry slope (Beta = 30deg; Phi = 30 deg).

As for the second - I would draw up the situation in section and then see if a 2V:1H slope of the footing near the shored soil would intersect or nearly intersect (being a bit below) the wall. If this is the case, then the footing will have an effect on the wall. However, you have asked about bearing capacity - and again, you could look up the Bowles reference and get a handle on it. Unless your footing is wide (say > 5 ft) the excavation being that far away should not affect the bearing capacity to any notable extent. Can you draw the "potential failure line" without it intersecting the shored excavation?

As is almost always the case, far too much attention is given to "bearing capacity" which is a shear phenomenon rather than on the allowable bearing pressure when is a servicabity (read that settlement or differential settlement) issue.

 

[fattdad]

the first case is a slope stability problem. the second case is a retaining wall problem. Devining the actual loads acting on the foundation may take some noodling, but adding a foundation load to a slope stability analysis (Culman's construction via calc pad - or using some software) is pretty well established. In other words, who cares whether you are within some, "safe" bearing capacity if the slope fails. Obviously, they are both one and the same.

Telegraphing the foundation load from the second case onto the soldier pile wall is pretty straightforward too. Again, it'll be hardest to figure out the magnitude of the foundation load. Bousinesq has equations for delta sigma H from point loads, line loads and aerial loads. Whatever you calculate, double the answer for retaining wall.

f-d

ípapß gordo ainÆt no madre flaca!

 

[rconner]

Interesting inquiry and subsequent comments. While I may not be understanding the situation correctly OSHA in standard at

https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10775

now requires (and see the specific language in Sect. 1926.651(i)(2)(iii) ) :

"1926.651(i)
Stability of adjacent structures.
1926.651(i)(1)
Where the stability of adjoining buildings, walls, or other structures is endangered by excavation operations, support systems such as shoring, bracing, or underpinning shall be provided to ensure the stability of such structures for the protection of employees.
1926.651(i)(2)
Excavation below the level of the base or footing of any foundation or retaining wall that could be reasonably expected to pose a hazard to employees shall not be permitted except when:
1926.651(i)(2)(i)
A support system, such as underpinning, is provided to ensure the safety of employees and the stability of the structure; or
1926.651(i)(2)(ii)
The excavation is in stable rock; or
1926.651(i)(2)(iii)
A registered professional engineer has approved the determination that the structure is sufficiently removed from the excavation so as to be unaffected by the excavation activity; or
1926.651(i)(2)(iv)
A registered professional engineer has approved the determination that such excavation work will not pose a hazard to employees."

[Of course even the edge of spoil piles from the excavation are permitted no closeer that 2 feet to the "edge" of the excavation.] All have a good weekend.

 

[Mccoy]

I've always been wondering, is there around a registered professional engineer, sane of mind, who is willing to guarantee in writing the safety of an unsupported trench or excavation (unless it's unfractured and unaltered rock)?

A registered professional engineer has approved the determination that the structure is sufficiently removed from the excavation so as to be unaffected by the excavation activity; or
1926.651(i)(2)(iv)
A registered professional engineer has approved the determination that such excavation work will not pose a hazard to employees."

http://www.mccoy.it

 

[BigH]

Mccoy - Excavations from the "safety" (OSHA) point of view is confined to ones with a width of 15 ft (5 m) or less - wider excavations as for a building (or highway) are much wider and would not be covered by OSHA. However, from a liability point of view, the geotechnical engineer of record would have to address the issue of overall stability (footing and/or slope). This is my understanding - although in Asia things are different . . . .

 

[fattdad]

please note that anything OSHA related is independent of the completed works. They are focused on worker safety - temporary stuff that is not a part of the betterment of the completed project.

f-d

ípapß gordo ainÆt no madre flaca!

 

[rconner]

While it is true there is a defined subset of excavations 15 feet or less in width that are referred to by OSHA specifically as "trenches" with some specific coverage and requirements, note however,

"1.4 Requirements of the Excavations Standard

OSHA has published a number of documents that describe the provisions of the Excavations Standard for employers and employees in construction10. The standard applies to all open excavations made in the earth's surface, which includes trenches."

 

[Mccoy]

Having seen my share of suddenly collapsed unsupported trenches and related casualties and fatalities, I would never reccomend the excavation of an unsupported trench as far as workers' safety goes.
Even in rock, it may happen that some loose debris detaches from the walls with serious consequences.

That's a very risky issue, how would I determine that an excavation work will not pose risk to the employees without extensive soil investigation which is very rarely done? Even then there would be a non-zero probability of failure. The OSHA provisions do not speak about 'reasonable risk' or allowed probability of failure, so it sounds like any casualties would be full responsibility of the engineer who approved the unsupported excavation.

http://www.mccoy.it

 

[rconner]

While maybe digressing a little from the OP, I think OSHA for the last few years has had a sort of catchy (so to speak) phrase in their excavation area,

"An unprotected trench is an early grave."

[Maybe the same could be said of any inadequately protected or unprotected shoulder to any deep excavation, only with a trench it sort of doubles ones chances.]

 

[aeoliantexan]

Referring back to the original question, there is a simple way I use to think about excavations next to foundations. If the soil is truly cohesionless, the angle of repose and the friction angle are virtually equal. The factor of safety of cohesionless soil at the angle of repose is 1.0, before any foundation load is added. If you have ever tried to walk up the side of a pile of sand, or grain, dumped from a truck or a conveyor, you experienced bearing capacity failures with each step.

A little cohesion, even apparent cohesion, can enable sand to stand steeper than the friction angle if the slope is not too high. Apparent cohesion can disappear with drying or seepage.

Cohesive soil can stand vertical to some height. I have seen excavations cut vertically at the edge of spread footings. This looks like an unconfined compression test viewed from one side, and indeed, the ultimate bearing capacity of the footing is about equal to the unconfined compressive strength, if there are no joints, fissures, or other discontinuities present. Footings on cohesive soil are commonly designed for an allowable bearing capacity equal to the unconfined strength of the soil, with a theoretical safety factor of 3. The excavation changes the safety factor from 3 to 1, assuming the designer accurately evaluated the unconfined strength

All of the above conditions are very dangerous. Knowledge of the footing load, the soil properties, and the geometry are needed to evaluate the ultimate bearing pressure, Yes, if the height is sufficient to bring the weight of the soil into play, a slope stability analysis is needed also.

Failures of shallow foundations into excavations are sudden and catastrophic. A proper investigation and analysis are needed.