deep foundation 1

[deereman]

I recently designed a timber pile foundation system based on reccomendations from the soils report. The contractor now wants me to look at doing caissons instead. The soils report doesn't give a net allowable end bearing capacity until a depth of 50 feet. I know this may be site specific but how deep is practical for a drilled pier. Also are there typical pier diameters (any one size more standard than others?)

Is it a concensus that I should not design based on skin friction (based on skin friction I could use a 30' caisson versus a 50' one based on end bearing?)

 

[Zambo]

In the UK we refer to "drilled piers" as bored piles. Typical diameters I have worked with are 1.2m and 1.5m depths of 50 or 100m are not unusual.

If your timber pile is no longer acceptable for the contractor why doesn't he go for driven concrete piles?

 

[GeoPaveTraffic]

There are typical pier sizes, check with some local contractors or the soils engineer to see what is generally used in your area. Here the sizes would be 30", 42", 48", 54", 60", and 72". As for using skin friction, that depends on the type of soils and the loads, talk to the engineer that prepared the report.

 

[BigH]

Both GeoPaveTraffic and Zambo give good information. In Bangkok, 70m plus is common.
Why does he want to change from timber piles? I take it your timber piles are founded higher than 50ft (which is pretty long these days although up to 60ft used to be common in Canada) which makes them frictional piles. If he wants to go deeper, you might consider monotube piles which are tapered. The choice depends on a number of factors; the geotechnical engineer must know the deep foundation types that are common to the area which is why he apparently recommended timber piles. Perhaps there is a scarcity of timber piles in your area and he wishes to go for a higher load with less installations - or, perhaps, he is tight on equipment and can provide one for the drilled pier but doesn't have a pile driver??
As far as the name goes, you will find various names such as caissons, drilled caissons, drilled-in shafts, drilled shafts, drilled piers, drilled piles, bored piles and there are probably other "local" names as well.

 

[Zambo]

BigH,

I am now used to using the terminology drilled pier or bored pile - quite straightforward. What I don't get is north Americans refering to them as caissons. Surely these are entirely different structures wherever you come from and can only refer to a shaft or box with a hollow core. BTW in Bangkok the same clay which requires 70m long bored piles is also excellent for sinking caissons.

Zambo

 

[BigH]

Zambo - can't tell you the origin of the word caisson - but they are called that locally (perhaps started in Chicago?). I remember starting with a company in Toronto and the form for the bored pile (et al) was called "Caisson" - then a few years later, we had caissons for wharfs that were 40m long by 10m wide. Go figure - . . . Here in India, they use the terminology well foundations rather than caissons for large diameter (6m or more) deep foundations sank using clam-shell for digging out inside muck - and lots of kentledge to keep them sinking straight!

 

[deereman]

THE DESIGN OF THE PIER IS BASICALLY THE SAME AS A COLUMN ISN'T IT, WITH AN UNBRACED LENGTH OF 5 TO 10 FEET. CAN THE PIERS BE UNREINFORCED?

HOW DOES THE CONSTRUCTION WORK WHEN YOU ARE BELOW THE WATER TABLE?

 

[UcfSE]

Drilled piers can be constructed without rebar all the way to the bottom unless it is required by analysis and imposed forces. Engineers may or may not wish to reinforce the top part of the pier. Some will also impose the same minimum reinforcing as columns, i.e. 1%. If you have uplift or moment in the pier for instance you will need some reinforcing.

The soil typically will provide bracing. You will need to consider whether you may have a situation in the future where you loose some of that soil near the top, perhaps due to flooding, scour or something else. That will obviously affect your unbraced length.

In your original post you asked about using skin friction. Driller pier capacity comes from end bearing and skin friction. The ultimate capacity due to skin friction occurs at a different displacement than does that for end bearing, so you cannot simply add the skin friction capacity to the end bearing capacity and come up with a capacity for the pier. It is my understanding that engineers in the US will use one or the other, either skin friction or end bearing but not both. Which you choose is up to you but I would definitely consult your geotech for his opinion. Engineers in Europe have a more complex analysis that involves adding the end bearing and some portion of the skin friction. Please keep in mind that I don't have much design experience with drilled piers, just academic .

 

[jheidt2543]

A couple of good references:

"Drilled Pier Foundations" by Woodward, Gardner and Greer, McGraw-Hill, 1972

"Analysis and Design of Laterally Loaded Piles and Caissons in a Layered Soil System" by T.R. Naik and A.H. Peyrot, ASCE Specialty Conference at the University of Wisconsin, in Methods of Structural Analysis, Vol II, published by ASCE, 1976

 

[jdonville]

deereman,

Drilled shafts/piers/whatever are constructed by boring a hole and then filling with concrete (under gravity pressure only) after removing the auger or other excavation tool.

Clayey soils will generally have enough short-term strength to maintain the hole without using casing. Soft clays and granular soils will generally tend to cave or slough and need to be cased to maintain the design cross-section. Shafts constructed in marine environments or places where soil casing will be difficult to work with can be constructed using a slurry to maintain the stability of the hole and displace water. Casing will still be required through the water in order to contain the slurry.

Further complicating matters, the tip may have a diameter larger than the shaft - sometimes referred to as a bell (due to the shape) or an underream. Drilled shafts should be spaced at least 3 tip diameters apart, center-to-center.

Research has shown that the skin friction will mobilize at a different rate than the end bearing. Therefore, depending on the size of the shaft, tip, length of the shaft and allowable settlements, a strain-compatibility analysis will need to be performed in order to best estimate the safe working loads.

The publication "Drilled Shafts: Construction Procedures and Design Methods" is available for free download from FHWA (

http://www.fhwa.dot.gov/bridge/geopub.htm)

and discusses the design and construction practices commonly used in the US in far more detail than can be done in this forum.

Hope this helps.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[jakin]

Another useful reference: NAVFAC Design Manual 7.2 - Foundations and Earth Structures