huangyhg

helical piles supporting foundation wall
i am considering preventing further settlement on a plain (unreinforced) concrete foundation wall by using helical steel piles attached to the foundation wall along their sides (pile manufacturers have special fittings for this).
walls are the usual 10 to 12 inches thick by 9 to 10 ft high, supporting 2 story frame residential construction.
i don't want to overstress the concrete when it becomes a grade beam. what spacing of piles makes sense, relative to concrete strength?
i am looking at aci 318 which addresses plain concrete design, but specifically prohibits plain concrete beams supported by piles (in new construction).
however, using helical piles is commonly done to deal with settlement, so how should i design spacing where concrete failure might be the controlling factor.
of course piles have their capacities also, but i'll deal with that after i decide on concrete span.
i also need a reference for analyzing pile depth based upon soil conditions. anybody know where to look.

usually the helical pile installer has guidelines that relate installation torque to pile capacity. in most cases, the pile installer will want to know what capacity you need at each pile and then the installation depth will depend on the torque developed as they install it. as the pile goes deeper, they use extension couplers to keep going until the required torque is met.
the span of your wall will behave in a shear/deep beam fashion and not as a flexural grade "beam". i would think that spacing the piling a max. of 6 to 8 feet would keep the wall in a more pure case of shear and work fine.
to support a 2 story with basement which weighs about 3000 #/lf we would put piles about 6' apart and excavate the outside as the weight of our cohesive soil clinging to the wall is a lot. piles follow the standard bearing equation formula, so are dependent on helix area. as an example, we use a helix about 14" across with an area of 1.12 sf. q=a(nc)c c=125n and nc=9. so we would need to found in soils with a blow count about 35 to get a 2:1 sf. no worries with a wall that deep of overstressing. however if it has had cracks patched it may not move much until these are cleaned out. my concern with a side mounted flange would be the shear of the bolts/concrete, a new section in aci 318.
pslem - good point about the soil weight - i forgot to mention that - the soil sitting on the sides of the walls will produce an overburden on the piling over long term.
thanks
my blow counts are 14 to 16. so looks i need triple helices with the 14 inch diameter helices
how many helices per shaft is the practical limit (or industry standard), or can you just add helix sections as you need them, for example can you have 5 helices.
and how is the area of each helix figured? say i wanted to use 10 or 12 inch diameters
is 14 inches the biggest diameter available?
just figure the area as a circle. don't worry about deducting the shaft. i differ from the industry some. you would not want to bear in soils with a blow count less than n=7 as creep might be a problem. i think the industry says 4 or 5. also we do a lot of new construction so use only single helix as additional helices will show greater settlement for a given load. this is not a problem in underpinning as they will be preloaded. typically 14" is the largest for the smaller shaft sizes as you start to get coning when they are heavily loaded.
the optimist sees the glass as half full. the pessimist sees the glass as half empty. the engineer see the glass as too big.
thanks
i also want to work out pile capacity q for granular soils
using
q = a x q x nq
a = helix area
q = effective overburden pressure
nq =bearing capaicty based upon soil friction angle
what is effective overburden pressure? how is it figured?
another one of my texts calls it "effective vertical stress at the level of the pile tip". still don't understand.
friction angle - does this come from description of soil from borings. if so, where can i find the correlation
if not, what is conservative to use.
i have some charts for friction angle vs nq.
pt999,
you just stepped past your line of geotechnical competency. you don't sound like you should be designing these helical piers. send the borings to chance and ask for help.

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