Timber piles in organic soils 5

[JackTrades]

I'm in the initial stages of the design of a pre-engineered metal building foundation and have just received the soils report. The report indicates 2 to 4 feet of sand (SP) and then organics laden sand (SM/OL) to about 18 feet. The report suggests a c.i.p. structural slab and beams on timber piles driven to refusal. My question is: Is bearing reinforced concrete on timber piles something that is done these days? I know this was common in the past but I've never taken that route so far in my 20+ year career. Will the organics have an effect on the timber? What about connections for uplift of the concrete at the frame columns, if present? Any advise would be appreciated.

-Jack

 

[BigH]

Timber piles are a solid and well-established type of deep foundation. You are correct in that in many areas, engineers have shied away from them - for, in my opinion, no apparent good reason. As with any pile type, there are pros and cons - that is why there are so many types from which to choose; no single one is god's answer to all situations. I see no reason to worry about the use of timber piles - you will treat them and they will last. One problem that does occur from time to time is that timber piles can rot if they go through groundwater fluctuations (dry/wet cycles). If always beneath the water table - no problem. If treated and not in a offshore (including marina type) environment, I'd see no problem. The Swedish Parliament Building has been on timber piles for over a 1000 years. The geotech is obviously comfortable with timber piles in your area. You are paying him good money - so take his advice. I once specked timber piles while the structural engineer wanted steel - he was in a steel town of northern Ontario. But, northern Ontario is also timber land. In any event, I saved the school district some $85k on his school addition foundation design - something like 30% of the costs. See SlideRuleEra's web site for many papers (mainly about New Orleans) on timber piles. Also do a search for the Timber Pile Manual.

 

[JackTrades]

Thanks BigH.

I'm assumimg that groundwater flucuations would not effect the pile durability due to the lack of oxygen. Are there concerns with the groundwater flucuation within the organic strata that the piles will be in?

-Jack

 

[cbosy]

No to your question JackTrades. Be careful about driving them to "refusal." Overdriving can damage them via "brooming." With a 15000 ft-lb hammmer, generally 3 blows on the last inch will get you a 40-50 ultimate capacity pile and there is no need to drive it harder.

 

[BigH]

cbosy has a good point on overdriving - you can always put a toe pad on them, or a ring about the tip to help prevent brooming - but don't drive too hard. Remember, with timber piles, they are tapered and you will also gain from that in cohesionless soiils (see Nordland's formulations). I'd also suggest you check, in construction via Gates formula as modified by Flaate.

 

[JackTrades]

BigH and cbosy - thank you both for your assistance. -jack

 

[SlideRuleEra]

For piling / concrete uplift resistance see page 51 of the TECO catalog at this link

http://www.clevelandsteel.com/divisions/construction/PDF/Entire_Catalog_print.pdf

Concur with BigH - Treated timber pile have excellent longevity, even in the conditions that you describe. If really want them to last "forever", go with 2.5 Lb/Cu Ft, CCA treatment - this is four times typical "permanent" CCA protection for timber piles. (It is normally specified for use in sea water).

http://www.SlideRuleEra.net

 

[JackTrades]

Thanks SlideRuleEra for the advice and the Cleveland connections link.

 

[eric1037]

A few other comments on timber piles:

- In my experience if the piles are designed to be longer than about 30 ft., they become relatively expensive due to shipping costs. I have used timber piles up to 40 ft. long, but I wouldn't recommend much longer than that.

- Timber piles are relatively difficult to splice. Therefore, you should be relatively comfortable with the length. It's good practice to specify them a bit longer than necessary in case they do not meet the driving criteria at the recommended length. They are easy to cut, but remember the average diameter will decrease if you cut off the pile.

 

[boo1]

nice post thanks guys