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.
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..........
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.
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).
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
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.
