AboutRobert Cummings, P.E. Expertise I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.
Experience 30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.
Organizations Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.
Publications Mining Engineering, AEG Bulletin.
Education/Credentials BS and MS Geological Engineering
Question We may need to remove up to 250 CY of Fordham Gneiss in Westchester
county, NY. What kind of production can we expect and what size
machine do you recommend? Hammering is also a sensitive issue in this neighborhood, but we can do some.
Thanks a million!
Answer I can't give you many specifics without knowing how the rock is situated. If it is in a confined area, such as a trench or basement, it will be slower than if it is (for example) a side hill outcrop. It also depends greatly on the state of weathering and fracturing in the rock mass. It also depends on the geometry of the removal: is it deep but narrow and long, or shallow and wide, etc.
Since you speak of hammering I assume the rock is relatively hard and fresh. Removal of 250 cy of such rock by free-digging may be very slow if it is possible at all. By hammering, you will be able to increase the rate of production. Assuming it is around 5 ft thick, strong (25,000 psi), and massive with only a few tight fractures, the Caterpillar Performance Handbook indicates that a 10,000 ft-lb hoe-ram would be expected to produce around 200 cy max in an 8-hr shift. The production of a 5,000 ft-lb hoe-ram would be a little less than half that. If there are sub-horizontal fractures and a couple of sets of steep fractures, the production could more than double.
All this assumes there is another machine available, or the hammer and bucket are the quick-connect type to allow rapid changeover on the same machine, to remove the excavated material so the hoe-ram does not get muck-bound. The estimates do not take into account down times for machine moves, equipment changeovers, and relocations for mucking. I would discount the production estimates 25-40% for that.
My experience with massive gneisses has been that if the excavation is confined at all, or is vertically extensive (more than 4-5 ft) the production rates tabulated by Caterpillar are optimistic. If there are few fractures it will be advisable to pre-drill the rock (think 3 in. holes on 1.5-2 ft centers) to provide surfaces to break to. Otherwise there is the real possibility that you will hammer away on the rock for days and produce only little piles of gray dust. This is because, without discontinuities to break to, the rock will spall to curved surfaces and then the hoe-ram has nothing to bite into.
You should consider blasting. Using modern and knowledgeable techniques there are few if any realistic reasons why 250 cy cannot be blasted safely and without damage to the surroundings. Fly rock is controllable with post-covering (mats or inert earth) which also suppresses noise. All you hear is a soft crump. Vibrations are minimized by means known to most good blasters and consultants.
I personally worked on one case -- a basement excavation in hard granite with very few, tight fractures -- where a hoe-ram worked for weeks and finally the Owner resorted to blasting with 3-7 ft of rock to go. The reasoning was that if you are going to pre-drill the rock for a hoe-ram you might as well place explosives in the holes and forget the hoe-ram. There were houses within 50 ft on either side and the adjoining property owners were as wealthy, fearful, uninformed, and generally blasting-adverse as you are likely to encounter, even in Westchester County. However they were also fed up with the endless dust, noise, and vibration from the hoe-ram. We were able to blast a 9-hole test pattern (covered the shot with dirt) with no noise and no detectable vibration, and the homeowners were convinced. Thereafter the project was completed, to everyone's enthusiastic relief, in about a week by blasting little shots and covering each of them to avoid fly rock.
