Passive Pressure Assumptions
Passive Pressure Assumptions
(OP)
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?
What are you all doing in the situation of not having geotech reports available? I am curious to hear how you are handling this?





RE: Passive Pressure Assumptions
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.
RE: Passive Pressure Assumptions
Good luck.
f-d
RE: Passive Pressure Assumptions
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.
RE: Passive Pressure Assumptions
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.
RE: Passive Pressure Assumptions
f-d
RE: Passive Pressure Assumptions
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
RE: Passive Pressure Assumptions
p.s. - I have other soil references but Lindenburg is the only one not in the office right now. ;)
RE: Passive Pressure Assumptions
I second that. Although my experience has been very limited I have always found them professional and very helpful.
RE: Passive Pressure Assumptions
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.
RE: Passive Pressure Assumptions
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
RE: Passive Pressure Assumptions
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.
RE: Passive Pressure Assumptions
RE: Passive Pressure Assumptions