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Bored Pile (1m dia) in Rocks

Bored Pile (1m dia) in Rocks

Bored Pile (1m dia) in Rocks

(OP)
Is it possible to do a bored pile (of 10m deep) in hard rock (e.g  Granite of allowable compressive strength = 1500 kPa)?

RE: Bored Pile (1m dia) in Rocks

Yes. That is very weak granite,(220 psi vs 12000psi- 15000psi).

RE: Bored Pile (1m dia) in Rocks

you state allowable compressive strength of the granite.  Is that the actual strength, or an engineering value with factor of safety already worked in?

RE: Bored Pile (1m dia) in Rocks

Samples of granite with unconfined strengths below 1724 KPa (250 psi) are not considered to behave as rock.  Allowable contact pressure,(usable bearing pressure), ranges from 11491 Kpa(120tsf) to 958 Kpa(10tsf).  The RQD,(rock quality designation), is the determining factor for what part of the range to apply.

RE: Bored Pile (1m dia) in Rocks

(OP)
Sorry for the sutipd mistake, the presumed allowable
bearing pressure (kPa), is least 5000 kPa which has been factored down by a FOS of 2.5 - 3.

I am going to design a group of piles to support a bridge deck which will impose a huge horiz loads on the piles. I haven't done any SI but am pretty sure there will be a thick layer of Grade III Granite which may bear on Grade II granite. Since it is still at preliminary conceptual design stage, I wonder how depth of a rock socket is required to resist the horiz load..........

RE: Bored Pile (1m dia) in Rocks

Select a drilling method which can accomodate both soft and hard rocks.
Best solutions for this kind of situation are DTH hammers based solutions : Symmetrix with a single hammer ( max diameter 1 m ) or cluster drills with several hammers ( see Center Roch Inc , Berlin, PA ), max diameter 2m.

RE: Bored Pile (1m dia) in Rocks

Catjacob,

The depth of rock socket requied to resist the horizontal load can be figured a couple of different ways. You should contact a geotech who can provide specific recommendations for in-situ tests of the rock mass and perform the evaluation of the needed socket depth. You should plan on performing at least 15 feet of rock coring/boring at the pier locations (more if advised by the geotech).

Jeff

RE: Bored Pile (1m dia) in Rocks

Catjacob,

My suggestion for coring depth was for the subsurface investigation.

Jeff

RE: Bored Pile (1m dia) in Rocks

There are many method for drilling in hard rock however I agree with BigHarvey that the DTH Hammer is the most efficient.
You mention the strength of the rock however is this consistent all the way to proposed tip elevation?
Is there overburden and if so how much?
If drilling in consolidated rock then the cluster hammer will be most likely the most efficient way to go - less air and typically faster than a mono-hammer and sizes as large as you need (which will be more a function of the rig and drill steel as Center Rock can make them as large as you need; see www.centerrock.com)
If drilling in unconsolidated rock and/or overburden then a casing advancing system such as Symmetrix or GeoRocFor would be most likely the best. Symmetrix is limited to 1m and GeoRocFor can go to 1.5m if needed.

RE: Bored Pile (1m dia) in Rocks

(OP)
Thanks you very much!!!

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