durability of wood piles

[digger95]

I have a project with miscellaneous fill (mostly fine silty sand) to 7-feet, peat to 10-feet, underlain by medium dense fine sand to about 18-feet where the sand becomes dense to very dense N=50 for 4 inches) . This sand was encountered to the maximum boring depth of 35-feet below grade.

Groundwater was encountered at 10' below grade.

The design is for a 30,000 sf warehouse w/ 28' precast concrete walls.

It seems logical that (timber) piles would be the way to go, however what is thier lifespan? I heard they are good for at least a hundred years, below groundwater, longer.

Can anyone confirm this?

My second question is regarding using a floating floor slab vs a structural slab. Does is seem logical to put the slab of grade beams, or just float it an tie it into the footing? My gut tells me to put it on grade beams to eliminate the risk of differential settlement.

 

[SlideRuleEra]

Untreated wood will last virtually forever (several centuries... and counting, anyway) if it is continuously submerged in fresh water. However, if I understand your situation, the problem will be with the 10 feet, or so, of timber that is above the water table. That portion of untreated wood could deteriorate relative quickly, depending on soil conditions. If you choose to use timber piling, suggest that treated timber be considered. A typical lifespan of properly treated timber (above the water table), would be many decades. Here are some links for reference:

American Wood Preservers Association

http://www.awpa.com/index.asp

Koppers Company

http://www.koppers.com/htm/PandS_Util_Pilin.html

USDA Forest Products Lab

http://www.fpl.fs.fed.us/documnts/fplrn/fplrn002.pdf

As for you other question, a monolithic floor would help tie the entire foundation system together. I would consider it preferable.

http://www.SlideRuleEra.net

 

[BigH]

SlideRuleEra is correct on the lifespan - I remember reading that the Swedish Parliament building is on wood piles and it was built before 1000 AD. There is no question that you must treat the timber pile. There is a timber pile manual that you can download from the internet. Not sure of the URL - when I downloaded before, I never saved them. One point - a project in British Columbia at a wood mill wanted to inspect the timber piles. They opened up the floor, excavated about the piling, pumped out the water and did some visual inspections. Pile was near perfect. Closed it up. Went back a few years later - opened it up again - pile just about had it. Even a single wet/dry cycle can have a disastrous effect if not properly treated. Otherwise, timber piles are a great alternative and one that many won't/don't use anymore for one reason or another.

 

[dmoler]

The problems in warehouses with floating slabs on peat and organics I have encountered is that the load on the slab are not really spread out. They will stack heavy stuff, say, against the wall on one edge of the slab, and then that edge of the slab will slowly sink over time. Adding light reinforcement tie-ins to grade beams tend to cause the sinking with cracking of the grade beams due to light tie-ins, so true structurally supported slabs would help.