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Mechanical Excavation vs Blasting

Mechanical Excavation vs Blasting

Mechanical Excavation vs Blasting

I am involved in a project where the Owner would like to omit drilling and blasting as a method for rock excavation; however, rock excavation will
be required for foundations and utilities for numerous(>30)residential structures on shallow foundations (no basements). The site is relatively level with rock near existing grade in some areas (say 1/3 of the site). Rock excavation depths are not likely to be greater than 3 to 5 feet; where required for foundation support or utilities. The rock is limestone (Core Recoverly 10 to 80%; RQD 0 to 40%). SPT values in the limestone range from 50/6" to 50/0" and auger refusal. No coring was performed (although data is available from an adjacent area).

Here is my concern: If we catorgorize excavation as 'unclassified', should we anticipate a changed site condition claim by the Contractor? My worries are that the Contractor will claim that he cannot get adequate 'PRODUCTION' with hoe-ramming or other mechanical means and then claim that it was assumed that mechanical means would be 'productive' since blasting was not permitted. I guess this is not a question of 'rippability' as much as 'practicablity of rippability'; since most materials could probably be ripped if you are willing to pound on the rock for days or weeks. Also, my concern is that the Contractor may claim we did not adequately characterize the bedrock since no coring/lab testing was peformed at the site.

Any comments would be appreciated.


RE: Mechanical Excavation vs Blasting

Why not bring an experienced blaster to meet with the owner and see what he can say about careful blasting techniques? I have found that communicating with those that do the work, well before writing documents, can help greatly in solving questions like this and eliminating job arguments.

RE: Mechanical Excavation vs Blasting

If you blast and there are nearby homes make sure that you conduct seismic readings and perhaps be allowed to do home inspections.

RE: Mechanical Excavation vs Blasting

I don't think blasting would be an issue. It's a relatively remote area (more remote than I've seen blasted in the past).
I'm more concerned that it won't be allowed (for whatever reason the Owner provides) and change orders follow; due to contractor claims of difficult 'mechanical excavation'.

RE: Mechanical Excavation vs Blasting

Make sure that the owner recognizes that explosives are a very cheap form of energy and if he choses to dictate that explosives cant be used , then he WILL incur the additional costs. It might be difficult to estimate how much these extra costs might be..... that is where bringing in an experienced blaster will generate some semi realistic numbers...... but from the data provided , I'd say it can be ripped easily, but a D11 would be a lot more effective than a D6. THis is where any numbers generated become vague and subject to claims for extras. The contractor is entitled to a reasonable expectation of profit, and this is a classic case of "Pay me now or pay me later" If the owner or GC is unwilling to do core drilling etc , then any restrictions on the method of working will justifiably lead to additional claims.

RE: Mechanical Excavation vs Blasting

Are you developing the technical specs and contract documents? There are plenty of specifications that will better define the rock, by qualifying the rock hardness and/or defining rock rippability by machine type. If these are defined prior to construction, including unit costs for rock removal and unclassified excavation, you should be able to avoid any major change order issues.

RE: Mechanical Excavation vs Blasting

I can't understand why someone would not want to blast. The owner could even be the guy that gets to hit the switch.

Richard A. Cornelius, P.E.

RE: Mechanical Excavation vs Blasting

provide the boring logs and geotech report to the bidders. do not call it "unclassified excavation", clearly indicate that it is expected to be rock. since the conditions are fairly well known prior to bidding, I really question why a change order is expected.

in order to provide the necessary data for a good controlled blasting plan which will control overbreak and produce rock of a size that can be easily excavated, you might need more field data and a report from a geologist which addresses the parameters required for the blaster. In addition, you would need confidence in the blaster that they are able to develop a plan that will not result in excessive overbreak resulting in an increase in the quantity of excavation and backfill.

chemical rock splitting may be just as effective as blasting. it will certainly reduce any overbreak which if it happens is more of a concern for change orders than anything else in my opinion

RE: Mechanical Excavation vs Blasting


That limestone can probably be ripped instead of blasted.

Here is the semi-short version of my one experience with something similar:

In the mid 1980s, I was on the design team for a project that included a 2.5-million-gallon (120' dia x 32' high) ground-level welded steel tank for reclaimed water for a municipal water district in southern California. The tank site was on a ridge of unfractured basalt, which is much stronger than your limestone. In fact, the geotech had told me (tongue-in-check) that he hesitated to give us the actual allowable bearing strength of the rock because he didn't think we would believe him. I don't remember that number or the more conservative number he suggested we use for designing the tank's ringwall foundation, but both were much higher that I had ever seen for "dirt." Also, the tank site was only about 200 yards from the nearest homes and the city we were in didn't like the water district or this project and had recently enacted (due to our upcoming project) strict limitations on blasting and onerous requirements if blasting was used.

For hydraulic reasons and to minimize viewshed impacts, we set the tank bottom elevation about 25 below the top of the ridge. This design required the excavation of about 25,000 cubic yards of rock to create the pad for the tank. Even with the city's new requirements, we had assumed the rock would need to be blasted and had prepared the PS&Es to reflect that. However, the low bidder had other ideas. He proposed to rip the rock with a D10 and a single ripper tooth. The engineer's estimate for the earthwork contract was around $450,000, assuming blasting (the tank and pipeline were contracted separated). The low bid (which was based on ripping) was around $275,000. At the end of the project, the contractor told my project manager that he had actually made a little more money than he had originally budgeted for. He had also finished about two weeks early.

The ripper tooth was mounted on the bottom end of a large hydraulic ram that was attached to the back of the D10 via a 2-foot-square by 1.5-inch thick steel plate. When the ram was actuated, the ripper tooth wouldn't go immediately into the rock. Instead, it lifted up the back end of the D10 about 18 inches or so. A D10 weighs about 90 tons, so the force on the tooth was about 45 tons. IIRC, the cross section of the tooth (ignoring the rounded tip) was about 0.5" x 6" perpendicular to the direction of travel. This resulted in an average pressure of about 30,000 psi, although it was higher along the line of contact. The weight of the D10 would slowly (but not too slowly) force the tooth into the rock. Once the D10 was level again, the 700-hp diesel engine was throttled up and the tooth ripped an 18-inch-deep furrow into the rock. At the end of a furrow, the D10 was turned around and the process was repeated to create a new furrow that was offset about 3 feet from the previous one. Unlike with dirt, the furrows in the rock were much wider than the ripper tooth, probably due to propagation of cracks in a solid media. Back and forth it went. As each layer was furrowed, a D9 moved the rubble out of the way, then the D10 went back at it.

One day I went to the site to see how things were going (at the time, I was inspecting two other projects in the area and this one was halfway between the two). I found equipment but no activity. I asked our inspector what the story was. He took me to the back of the D10, where he showed me the 1.5-inch-thick steel plate. It was ripped down the middle like a phone book due to fatigue fracture. Our inspector told me that the contractor was sending up a new plate and a welding rig and in the meantime, the contractor's crew had taken an early lunch.

"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

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