End Bearing piles in Lacustrine Clay? 1

[claytill]

I am working on a getechnical investigation in which there is a 10 m thick layer of lacustrine clay overyling clay till. My querie is as follows - is it common practice to use end bearing (under-reamed belled)CIP concrete piles in a lacustrine clay? If so, what bearing capacities would be suitable for the clay that has an approx cu of 50 kPa and what would be the best method to use to estimate settlement of the individual piles? Any feedback would be great.

 

[BigH]

Practically speaking, I don't see why you would want to consider "end bearing" piles in a borderline firm/stiff clay when you have a clay till at only 30 ft. I don't know anyone, in practice, who wouldn't just go down to the clay till. Better have a good positive support rather than a situation which is fraught with potential problems of settlement, low load carrying capacity, etc. As a quick back-of-the-envelope, qu can be taken as 9*Su assuming that L/d > 5 or so. Thus, for Su = 50 kPa; qu = 450 kPa; qall = qu/2.5 (to 3) = 150 to 180 kPa; settlement considerations (and don't forget group effect), though, may govern.
For good discussions on your questions, see Tomlinson's book on Pile Design and Construction. I find it the best on the market.

 

[jdonville]

BigH,

Excellent use of "fraught".

Claytill,

Current practice in allowable or working stress design for drilled shafts (what you're calling CIP underreamed piles) incorporates the allowable settlement into the determination of allowable design load for a particular shaft and soil geometry. Basically, you plug in the allowable settlement, the geometry of the pile, the strength of your soil column and back out an allowable design load.

The method is discussed in Das' Principles of Foundation Engineering (I have the second edition, but more recent editions are more up to date on the factors) and FHWA IF-99-025 Drilled Shafts - Construction Procedures and Design Methods (avaliable online from FHWA) which includes the nomographs for strain-compatibility between shaft friction and end bearing strength development.

In your case, you definitely want to maximize the strength of material at the tip of the drilled shaft in order to minimize the cost per unit of load capacity. Moreover, going deeper to improve the end bearing will give you a little more skin friction to help you out, too.

In this instance, save yourself and your client a boatload of headaches. Go deeper to the better soil if you're going to use deep foundations at all.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com