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Excavation in Rock

Excavation in Rock

Excavation in Rock

"metavolcanic rock with occasional fracured zones"

Spread footings are to be embedded in the rock.  I haven't seen this before, how are excavations in rock done?

Is it practicable to bore holes in rock, for end bearing caissons?


RE: Excavation in Rock

Rock excavation is accomplished by a variety of techniques depending on weathering, strength, fracturing, etc.  Some rock can be excavated with normal excavating equipment.  Hoe rams are like giant jackhammers and are sometimes used.  Blasting is another technique.

Rock-socketed caissons are relatively common.  Many times the side friction develops before end bearing.

RE: Excavation in Rock

I think in this case, although not stated in the reports, damage to the rock stucture should be avoided.  Definitely no blasting, would the Hoe rams have negative effects on suf-footing structure?

The caisson sounds interesting, that was more or less a passing thought--I was thinking end bearing, and then extending it all the way up as a column.  Good stuff.

RE: Excavation in Rock

I am not sure of what you are trying to build, but I pressume that the rock is surficial or very shallow since you require the footings to be embedded in the hard stratum.  If this is the case, the project will require excavating through the rock for the installation of underground utility lines, drainage, etc.  The same techniques used for this type of excavation is usually applicable to the excavation of foundations.

RE: Excavation in Rock

The footing needs to be embedded to prevent scour and would like to maintain the surficial rock structure in tact as much as practicable.  The deeper the better, but it looks like spread footings will be selected.

I was more or less writing in this forum, because I thought someone might have some experiences (things that went wrong?) or web links to the actual construction equipment and projects, pictures are worth a thousand words.  There isn't much information on rock excavations in my engineering references DOTs, USACE, Bowles, Das, etc...  Almost seems like I should be looking at mining or oil drilling material references?     

RE: Excavation in Rock

The metavolcanic rock may be quite hard and offer considerable bearing capacity for your footing load.  However, the fractured nature may detract from its suitability.  At a minimum, I would investigate the fracture density for sure, and determine how favorable the rock structure is with respect to the footing design.  "...to prevent scour..." suggests shallow depth of footing.

You may wish to consult this publication:

US Army Corps of Engineers, ROCK FOUNDATIONS, EM 1110-1-2908

link: http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-1-2908/

Also, http://www.rocscience.com/hoek/Hoek.asp

RE: Excavation in Rock


Another thing to consider is transfer of lateral loads and moments to the rock. Based on these loads, a shallow foundation may not fit the bill.

In any case, your geotechnical engineer/consultant should have recommended a foundation type based on the loads and given construction or design recommendations related to the geometry and method of excavation for the rock.

Scour concerns indicate that this is a bridge or raised structure of some sort. However, the questions that you are asking lead me to suspect that you don't have a geotechnical engineer/consultant on the project. If not, this forum is ill-equipped to give you the kind of specific advice you need, and you had better engage a geotech sooner rather than later.


Jeffrey T. Donville, PE
TTL Associates, Inc.

RE: Excavation in Rock

Lateral loads and moments can be resisted through the use of rock anchors.  These may be required.

RE: Excavation in Rock

Blasting is the most effective means to remove rock, esp hard rock. The bearing capacity of poor rock or fractured rock is still signicantly better than the bearing capacity of soil. You may want to reconsider blasting unless external constraints like close proximity to sensitve structures prohibit it.
Generaly the rock must be over broken, that is the blast zone extends 2-3 feet below the bottom of the footing level. This rock can be cleaned out and filled with concrete or left and the footing built o top of i. This would depend in your structures needs. Generally the rock is blasted prior to removing the overburden as the over burden provides confining pressure, sound dampining and prevention of fly rock. If the surface is fractured, and that is a concern, it generally remidied by slush grouting.
Proerly designed and executed, blasting is generally the most effective way to remove rock.

RE: Excavation in Rock

Blasting can be accomplished in any environment and is the most cost effective and efficient way to remove rock.  Even if significant cracking occurs, grout or concrete will remedy the problem.

Most footings and foundations in New York City have to be blasted.  Other structures and buildings are inches away at times.

The key is to find a good Geotech firm familiar with blasting.  We use GZA, they are excellent.  Also, find a blasting contractor that specilizes in this type of work.

All of our research shows that rock hammers open up all of the seams in the rock.  If you hammer first, and then have to blast, it is much harder to break the rock and is not as safe due to loss of energy confinement.

I find that people who say "no blasting" do not have much experiance in this area or have had a few bad experiances.  This normally happens when going with the lowest bidder.

Remember that vibration damage criteria are based on wave frequency.  When blasting real close, the frequencies are in the thousands, so normal limits do not apply.

Frank Lucca M.I.Exp.E.

RE: Excavation in Rock

Before I was an engineer, I remember some blasting for a building.  I don't intend on switching geotech firms.  And it would seem to me the contractor would be the one to determine the final technique, as long as it meets all the requirements.  It sounds like little drilled charges would work fine.  But, it seems like with today's technology we would have some hand tools, or something less brute force, that would be capable.  Anyways, it just got my curiosity going, and I wondered what the latest and greatest is.  Thanks exploengineer and DRC1.  

I think you all would have liked my response to djonville, but it was deleted from the thread awful fast.  I think this is about my 10th handle.   

RE: Excavation in Rock

Just found this website.  I doesn't have exactly what I wasn't looking for.  Nonetheless, it looks like a kick butt website with photos and links take a look.


RE: Excavation in Rock

Since scour is a concern, you are most likely working along a river or large body of water, which means there are slopes.  It is very important to make sure there are no daylighting joints that could slide (too steep for friction to keep stable; flat enough that the down-dip edge is exposed in a cut or river bank).  Vaguely recall that PennDOT had a similar problem with a bridge footing a couple years ago, which, fortunately, was recognized before they put the bridge on it.


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