Rock blasting vs. rock excavation

[civilman72]

I have a road project that is designed in an old creek bed, with large partially exposed rock (18" to 8' in diameter). Based on the geotechnical report, some of this rock is fractured and is "rippable" by a large excavator, other rock is expected to have to be blasted. Since the majority of the rock is not exposed, a lot of the actual size and type of rock to be excavated will have to be determined in the field. In order to try to accommodate this, we will be going with a unit cost bid schedule for the contractors, so that as actual quantities vary from the bid quantities, we can hold the contractor to a set fee per unit.

We have kept most of the road in fill, so I do not anticipate much more than 10% of the overall earthwork volumes to include rock removal/excavation. So, my question is, in a bid schedule is it typical to separate "Rock Blasting" from "Rock Excavation," and if so, do you differentiate the two by size of rock, or some other means?

TIA

 

[msucog]

i would leave it either rock (trench or mass) or not rock (and yes, a minimum size to satisfy the conditions--say cubic yard in size). let the contractor worry about how they will get it out. just be sure to state how the rock must be qualified and quantified. rock is probably the biggest headache i have since it's usually the biggest change order pay item for contractors. have all the terms and conditions set up on the front end of the project and never move forward unless the procedure and processes have been set in stone. by that i mean that someone should specifically explain how the rock will be qualified and how the rock will be quantified. it seems simple enough but has yet to be "simple". the contractor always has a high number than the testing lab or engineer. so document document document so that when the numbers don't match, someone can compare the calcs to see where the difference is. i've seen an owner almost pay for an extra 20k cy of mass rock on the first shot! luckily, i had my numbers calculated for everyone to see...the other guy had a computer program that told him the quantities. as it turns out, they didn't account for the 20' of cut that already taken place before the rock qualification/quantification took place. so they had 20' too much rock in their numbers. we fought for over 2 weeks over the issue until a meeting where they showed the graphics from the computer and i noticed that the original hill was still there in their graphics and is what their numbers was based on.

 

[BigH]

As msucog laments (?), the definition of rock can be one of the biggest headaches that one can develop on a road project. Normally it is "rippable" rock or "non-rippable" rock. Sometimes, there are clauses that say - rippable rock is rock that can be ripped by a D-8 dozer. On a Laotian project of mine, - how can I apply that to an "outcrop" that is hanging on a 1H:2V cut slope some 3 m up in the air? Ludicrous. Other specs give wave velocity ranges as to what is rippable and what isn't. Boulders are a different story - they are usually "treated differently" when more than 1 m3 in size (sometimes 1 yd3 in the USA).
Attending Underground Singapore 2007 conference back a year ago, there was an afternoon devoted to geotechnical baseline reporting. This might be the way to go. The geotechnical engineer will give the "%" of rock that he estimates will require blasting and the "%" of rock that can be ripped - rock being classified as particles greater than 1 m3 or some other volume. The contractor is given this information along with probably locations of each. This forms the baseline of his bid. If the blasting of rock is more, then he would be entitled to more money. If less, it would be a wash. This results in shared risk. Again, though, one is back what is rippable if it can't be 'proven' by a trial. In this case have an engineering geologist or geotech experienced in rock that would be agreed as independent by both contractor and owner/engineer. Then site decisions could be made by the independent expert when the two contracted parties cannot agree - sort of "on site" arbitration.
Just some thoughts.

 

[cvg]

you might consider including a set allowance for an estimated 10% quantity of rock removal. By doing this, your bids might come in more reasonable and you might avoid the change order. Maybe payment for the rock excavation should be based on the dimensions of the rock stockpile, not on the perceived in-place quantities

 

[miningman]

Why do you want to go this route??? Leave it up to the contractor to determine whether he wants to rip or drill and blast. I have seen D8's destroy their undercarriage when ripping areas that a competant driller / blaster would have out in a few shifts.

Similarily, a contractor who has a D10 / D11 sitting idle may be willing to take on the "ripping" risk more confidently than another contractor who feels he might have to sub contract the rock work.

I agree that the more geotechnical information that is provided to the bidders should reduce the contractor's risk...... and therefore his temptation to pad his bid..... but by giving the contractors the choice as to their proposed excavation techniques, you are allowing them to present their owm areas of expertise to both your and their financial benefit.

On a personal note, Im a great believer that explosives are a VERY cheap form of energy, and if in doubt, I will always drill and blast rather than risk excess wear on my heavy equipment.

 

[civilman72]

Appreciate all the input...

Hearing that "rock is probably my biggest headache," is actually reassuring, because this was my concern, and I guess I was not overthinking things.

Based on all your advice, I'm going with one line item - "rock excavation and stockpile." Trying to potentially define the means and methods to the contractor also is something I want to avoid.

Since I know my original quantity will vary from actual quantities, I will recommend to the owner that he hire the geotechnical engineer to be on-site during construction to verify the overall quantities of rock excavation vs. standard excavation, and also to define the difference between the two prior to the bidding process.

 

[Dozerman56]

From a contractor's perspective: The biggest hassles involved in rock excavation are boulders and blasting shallow rock cuts. Will you be providing a maximum particle size you're going to allow in your fills? You say 10% of excavation total may be rock, is the excavation total 25,000 CY or 2,500,000 CY? Yes, a D10 or D11 will rip some fairly serious rock, but you don't just drop one off at the site. To mobilize one of these beasts you're going to take it apart, put it on 6 or 8 tractor trailers and devote 6-8 mechanics for 3-4 weeks.

The biggest advantage to going unclassified is noone has to measure anything, which as mentioned above can be a real pain. However, if you don't have enough information to make the contractors comfortable, or if conditions vary wildly, any contractor worth his salt will want a substantial risk premium.

If you want to include an allowance quantity, make sure it's something the contractors can come reasonably close to verifying or you're begging for unbalanced bids. Finally, is there any reason you can't set the unit prices in your bid documents? Of course you need to make them something reasonable for the conditions and market.

 

[thegraduate]

good points here...

from a contractors point of view...

i would get a geotech to go out and grab some x-ray diffractions on the rock to determine the exact qty of non-rippable. At least this way you will be able to determine yourself how much rock is present and the exact locations - thus you might be able to include a provisional item in the schedule for this (and bulk the $$ up) or leave it out, and keeping in mind that you know how much rock there is, then you might be able to reduce your tender and make up for it (and more) with this variation.

knowing where the rock is, and determining when you will come across it in your works programme will also be important.


from your point of view... id also just get a geotech in to let you know exact qtys and where, and then let all the tenderers know.. you will expose yourself to less risk this way.

 

[civilman72]

Thanks for the continued thoughts...

This is a very remote site, with very thin roads as access, so mobilizing anything bigger than a D9 will be a major challenge. The entire road construction includes about 15k cu. yds of cut and fill, so paying for x-ray diffraction to determine sizes of rock is probably finanicially infeasible for such a small job. I'm thinking that blasting will be the most likely way the contractors decide to remove the rock.

I still think getting the geotech involved prior to going to bid and during the construction process will be my best route.

 

[dgillette]

X-ray diffraction? How would that distinguish between rippable and nonrippable rock? How far can x-rays penetrate rock, and then, how do you measure the diffracted rays?

 

[cvg]

one issue with blasting is that overloading the drill holes may remove more rock than is necessary to reach the subgrade. This will require hauling off more rock and then re-filling to get up to subgrade level. Make sure you cover this in your specs and allow for it in your estimate.

 

[Boise]

As cvg mentioned there will be "overdrill' or "subdrill" and often excess "overbreak". Also don't forget that 5,000 "bank cu/yd" will swell after blasting. The swell factor depends on the type of rock, but I have seem as much as 25-30% in some granite. Often contractor will try for "extras" because they excavate and haul more than the blast quantity.
You migh want to take miningmans advice and leave it up to the contractor to blast or rip...under most conditions I would definitely blast

 

[miningman]

Where geographically is this project located?? IMO, some of the posts here may be factual but might also be confusing for someone unfamiliar with the practicalities of blasting.

Yes subdrill and overbreak are facts of life, as is oversize, and issues related to mobilization etc etc.

If I understand previous posts correctly, this is a very small job in a remote area. 15,000 cu yards total, only 10% of which is rock. I would be a lot more concerned with the logistics of mobilizing the equipment, and living arrangements for the crews.

Remote is subjective. Helicopter support?? Dedicated camp??

 

[civilman72]

Good points about overbreak and also swell factor. I know exactly what you guys mean and this is another factor that I will consider and include/identify in the bid documents.

With regards to the site "remoteness." Using the term "remote" was a little misleading; I should have stated that the access to the site is "limited." Access to site is via a 7-8 mile gravel road, no more than 20' wide, with sharp curves in areas. Even bringing a low-boy to the site will be a challenge.

 

[moe333]

I think the posters must be talking about seismic refraction for rippability evaluation rather than X-ray diffraction.

 

[dgillette]

That makes more sense to me. There is at least one correlation that I've seen, but it's obviously +/- a whole lot. I've only tried it once, 16-18 years ago, for a site with highly fractured andesite with calcite filling most of the joints. As I recall, the contractor's D8 went through it like a bullet through butter (well, maybe not quite that easily), in spite of fairly high velocities, and we only had to shoot one small area. We speculated that the calcite made the velocity higher than it would have been with only granular material in the joints, or maybe the D8 was just having a good day.

 

[eea]

I completely agree with miningman, leave it up to the contractor's choice under a single pay item. I had a similar situation with on site versus off site borrow. 3 of the 4 bidders bid it as estimated, but the low bidder bid offsite at 1 cent per yard stating that they believed the project material would balance. Based on test borings, topsoil deposits were such that worst case we'd have 50-50 on vs off and at best 100-0 it would balance. We discussed with the owner and settled on 2/3 on and 1/3 off when we assembled the bid package. They finished right on the bubble. A gamble that could have really cost them.

I think you might be setting yourself up for an unbalanced bid offering two pay items for rock exc. I'd leave it their means and methodology. I suspect that this would in keeping with your DOT's standard practice, it is here.