BOBWALTON (Geotechnical) (OP)
14 Jan 06 11:10
Howdy: After doing a search on helical foundations, I discovered the following post: thread256-115353
by renRosen which has been placed in the archive. I would like to revive the question:
renRosen: "I'm starting the process of having built for me a home (2700 sf) on a hillside outside of San Francisco. The slope is pretty severe (1(rise)/2(run)). Bedrock is around 10 feet down. Foundations here are very expensive ($100K-$200K) and almost always use concrete piers and gradebeams. I've heard that helical piers are ofen less expensive and quite effective, but they don't seem to be used very often here. I'm trying to figure out whether helical piers can be used on a job like this and whether they tend to be more cost effective. Any insights (including thoughts on other foundation systems which could be explored) would be greatly appreciated."
Then came one reply.
GeoPaveTraffic: "I would stick with concrete piers and grade beams. They have the advantage of being a stiff system, whereas most of the helical piers are not very stiff. Another consideration is that concrete piers can be ground partway into the surface of the bedrock, the helical piers cannot be.
Given the potential for earthquake loads and slope/mud slides in the area, I would go with as stiff a system as possible well tied together. That way if one pier fails, the others can take the load and keep the house from moving."
As a former pole house builder and helical specialist, I would like to learn if anyone has successfully designed and constructed a hillside foundation of high capacity helical piles- 6" to 24" diameter tubular steel columns which are screwed into the soil using high torque- 10 to 20 foot-kip (or greater)? The arguments of GeoPaveTraffic may be valid as to slender square-shaft helical piers which have very little lateral capacity and may not penetrate formational soils. However, using a boom truck or hydraulic excavator with a high-torque hydraulic drive, I suspect that high-capacity galvanized steel helical screw piles would make a dandy hillside foundation- much more cost effective than caisson grade beam and just as stiff!