Passive Pressure Assumptions 4

[waytsh]

In the absence of a geotech report is there an assumption I can make for passive pressure and still sleep good at night? (other than 0 pcf)

What are you all doing in the situation of not having geotech reports available? I am curious to hear how you are handling this?

 

[DRC1]

I would say that more ofte tan not, I do no have any geotech report available. Good estimates can be made from estimate of soil and groundwater conditions, For granular soils, a coefficent of 3 is oftn used, again depending on soil conditions. For cohesive soils passive stress = unit weight times depth + cohesion. Cohesive soils are harder to accurately predict.
Even if a geotech report is not available borings might be. Try to get those. Then go see a local geotech and he can probly help you.

 

[fattdad]

For granular soils above the water table and in fairly good compaction, 300 to 350 pcf is reasonable (i.e., Kp=3 or so). We normally exclude the upper 2 ft of burrial. For clays, Kp can often be 1.0 (i.e., the "phi=0" condition). If there is a slope on the passive soil mass, you need to actually to a better calculating this stuff and be mindful of "global" stability. If there are water table conditions or multiple layers of soils, more information would be needed.

Good luck.

f-d

 

[jdonville]

waytsh,

We will generally recommend that passive pressure be neglected over the entire frost depth - which will vary by location. Unless your soils are unusually soft or loose, assuming a soil internal friction angle of 28 degrees should be conservative. Remember that FS(sliding) should be at least 1.5. However, I will also recommend that you have a geotechnical engineer review the subsurface data and your calculations prior to issuing your drawings.

 

[waytsh]

Thank you all for the responses. This is good information. I have been working with an old-timer who has been using a soil density of 65 pcf and Kp = 1.0 in his assumptions. Pretty conservative compared to what you all have mentioned. Although as far as I know he is not excluding any of the frost depth or upper portion of the soil.

On a related note, I have heard a lot of excuses as to why there is no geotech report and it seems that the most common is that it takes too long. This seems like a poor excuse to me in all but a few extreme cases. In any event, if this is the case would it be possible for a Geotech make a quick assesment of site conditions and give some conservative estimates from a single estimate? Something along the line of what DRC1 has mentioned. Or is this something I should not find many Geo's willing to do? It seems to me that it would be to the advantage of whoever is footing the bill to make this slight investment to help control foundation costs.

 

[fattdad]

Uh oh, I'm not sure we are all talking the same language. How will you use the original request for "passive pressure"? The reason that I ask is some "ol' timer" using a value of 65 pcf (forget Kp=1 for the time being) may be thinking of the earth pressures acting on a basement wall (i.e., at-rest pressure). Kp=1.0 is uniquely for a soil with a friction angle of 0. If you were using a Kp of 1.0, there is no soil that I'm aware of that has a density of 65 pcf (unless it's submerged) in which case you'd have to consider the water pressure, which would add another 62.4 pcf. So, there may be some way that a geotech could help you, but first a better explanation is needed.

f-d

 

[chichuck]

My experience with geotechs has been quite good in my area. They almost always try to be as accommodating as they can as regards to schedule. Maybe you have a very tight schedule, maybe those that are giving you the excuses are being a bit unreasonable.


That said, may I weigh in with an opinion? Maybe slightly off topic but I'd like to throw it in anyway.

I'm a structural type, my firm does mostly industrial (usually small power generation projects). We normally make an effort to get geotechnical info for each project.

My experience with geotechnical firms in our area has been very good. When we call them looking for help, they are very accommodating and professional. Of course we usually make a request for proposal, this is often done with a phone call and a follow up by email or snail mail. A phone call of two to discuss and clarify our requirements, get their requirements, terms and conditions, etc is next. They often can mobilize within one or two weeks, they are timely with reports and lab results. I've really not had problems with them at all. We all treat other professionally, and things go well. We recognnize going in that there is always SOME tome required for them to do their work, and we allow for that. Maybe we've just not had a difficult client or schedule recently, but we've not had problems with the geotechs.

Also, whenever I've had questions about how to put together a rfp, such as how do I deal with (fill in the blank here: settlement in a river saturated soil, resistivity, corrosion protection), I've most always been able to get some help from the geotech.

So, I'd like to send out kudos, thanks, tip o the hat, red star for the helpful geotechs I've dealt with in the past. Thanks, guys!


regards,


chichuck

 

[waytsh]

No, this is not an at-rest condition. It is being used in part to resist the outward thrust of a footing supporting a moment frame. I had assumed he was conservatively using the dry density of the least dense soil(my Lindeburg shows OH: 65-100 pcf), and the lowest range of Kp for cohesive soil (Lindeburg shows Kp = 1-2 for cohesive soil). I realize this is probably extreme but it seems like the only choice when no information is available. Welcome your input on this.

p.s. - I have other soil references but Lindenburg is the only one not in the office right now.

 

[waytsh]

chichuck,

I second that. Although my experience has been very limited I have always found them professional and very helpful.

 

[DRC1]

the concept of passive pressure is different for cohesive siols than it is for granular soils. Normally consolidted clays (which are the majority of clays)are generally weakest in the undrained or phi = 0 state. Given time, the soil will drain and gain strength due to modest friction. Overly consolidated clays generally do not change much with time, but can actually lose strength over time as cohesion drops more than friction increases.
Assuming a phi=0 state, the active and passive pressures are rectangular distribution (hence Kp=Ka=1) The passive pressure is equal to the overburden pressure at the bottom of the cut plus 4 times the cohesion. Generally the cohesion is divided by a factor of safety of 1.5 to 2.
Behaviour of clays is also efectedby wetting & drying, causing shrinking & swelling. drying can cause tension cracks, which can refill with water and cause unaticipated surcharge. Some clays expand considerably upon wetting, some with fine layers of silt apear stable until slightly vibrated when they can suddenly flow. Often saturated silts can appear clayey to inexerpienced drilling inspectors and get reported on the logs as a clay with misleading strength. Clays can be fussy and generally do not have enough data to fully characterized. I would recomend you work with your geotechnical engineer.

 

[fattdad]

Getting back to the original question, in the absense of a geotechnical report, I'd recommend you go to the site with a post-hole digger and see for yourself whether the soils truly classify as OH (i.e., organic fat clay). This is a hard one to guess at visually, as you can't just base it on the presence of organic matter (i.e., if you see flecks of root hairs, etc.). Rather OH is classified on the basis of variation in two trials of the liquid limit test. Trial one is using prepared using the "wet method" and trial two is prepared using the oven dried method. If these two tests yield a greater than 25 percent change in the liquid limit value then it's an organic soil.

If you are an engineer (i.e., trained in using methods to derive a solution), you should be able to dig an auger hole and classify the soil. You know what's there: mostly sand, mostly silt/clay, any gravel, etc.? If it's truely organic fat clay, the next question is how will you actually get the foundation constructed? You'll dig a hole pour the concrete and backfill with something. What will be in intimate contact with the foundation and how far laterially will it extend? There may be some structural benefit to the backfill material.

Thanks for the kind words.

f-d, geotechnical engineer
richmond, virginia

 

[eric1037]

waytsh:

Depending on where you are located, you should be able to get at least some preliminary info from your local geotech. Heck, if you would have called me the day that you first posted, I would probably be drilling right now. (We are a bit slow in Michigan right now.)

I can usually provide preliminary verbal or e-mail info within a day after drilling.

 

[BigH]

eric1037 - maybe you should be looking to move some of your ops to Alberta! They are smoking right now!

 

[COEngineeer]

I hardly ever use the passive pressure because I dont have that much soil at the toe. Plus you are suppose to neglect a foot or two anyway. I usually just use the friction of the base from dead load and make sure thats bigger than the lateral force. I would probably start using the passive pressure if I use a key.