Rock Blasting Volume Calculations

[Rjeffery]

Rock was encountered on my job site while excavating for a stormwater mamagement pond. The contractor has offered a unit price for the blasting of the rock. I have asked the contractor to explain how they will calculate this volume but it seems they are adding in the volume of the overburden which does not need to be blasted to be made excavatable. How is this volume normally calculated and how is it calculated when trenching for utilities is involved?

 

[dicksewerrat]

Rock excavation should be calculated by volume removed. 100 foot long trench, 6 feet wide, 7 foot deep should be 4200 cubic feet. Overburden is common excavation.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[GeoPaveTraffic]

You have two choices to measure the rock. First wait until the excavation is complete then look at the rock walls, before any backfill is placed, and measure as disksewerrat indicated. Second is to have the contractor remove all of the overburden, then have a surveyor shoot the top of the rock. It is then easy to have the survey or engineering company compare the top of rock to the proposed excavation and calculate the quantity.

 

[DRC1]

Have the contractor give you a break down on the price. If it is for unclassified excavation (ie overburden and rock combined)then the price should be okay. If possible, you you want to leave the overburden, drill and shot the rock and excavate both. The overburden acts to contain the blast which increases the effectiveness of the shot, and controls fly rock. Properly blasted the excavation is a little, but not a lot slower than earth. Depending on project requirements, the shot rock may or may not be suitable for reuse on the site which may also effect the price. As long as you know that you have to blast and the limits of the blasting, it really is not necessary to calculate the volume of rock and stripping the overburden to stop and measure the rock can add to the project expense for little benifit.

 

[exploengineer]

Rock Volume in Construction blasting is based on bank cubic yards. This is rock in place.

In blasting, you must drill deeper then needed to facilitate breaking the rock to grade. this extra drilling is called subdrill. Many contractors will try and charge for this.

So, accepted practice is (length x width x cut depth)/27.

In regards to overburden. It depends how much is present, and if removed, can the cut depth still be blasted safely. Its hard to blast for 2 feet of rock. We will not load less then a 6 foot hole for safety reasons. If you strip the overburden and then you must drill alot deeper, your costs go way up for blasting and fill.

Normally, if the cut is less then 10 feet, leave the overburden if possible (it helps for control and containment) and pay them for it. But, they might find that its too difficult to keep an open hole and have to strip it anyway.

I hope my rambling helps.



Frank Lucca M.I.Exp.E.

http://www.terradinamica.com

 

[PBroad]

Also note the broken volume of rock is about 50% larger than insitue volume, so if blaster is on a break only contract you will need more trucks to move the rock than the pre-blast volume.

 

[Dozerman56]

The rock can be measured either by first excavating the overburden off it or by observing the drilling and measuring the depth of overburden they're drilling through. Both you (or your inspector/soils tech/surveyor) and the contractor should be involved in gathering the measurements jointly. If the contractor does the volume calculation, you or your surveyor should check it.