Boring Log, Discontinuities, question...

[Nick42]

So, I'm going over a bunch of data for a project, and some of it is 30 or so boring logs. 2 of the borings, go about 70 feet into granite. I'm not sure how to interpret a few comments, and some suggestions would be great.

The note says this:

"
GRANITE
Weathering: Generally fresh with some stained joints
Discontinuities: Shallow or low angle joints observed, 6.38', 7.05', 15.05', 16.78', 19.1', 26.57', 31.28', 39.54', 40.06', 42.81', 43.94', 54.52', 58.3', and 59.23'
Color: Pink, gray and black specked to about 20', then black and gray to about 25.6' then gray, pink and black specked to about 39.5' then black and gray to about 42.7' then gray and black specked to 50.5' then gray, pink and black specked to 60'.
Hardness: Very hard
"

What I don't really know how to interpret is the Discontinuities, there are 2 rock boring samples, and all the numbers are different, and there are about twice as many on the second boring.

 

[Nick42]

I was mistaken, there are 2 more borings into the rock, but by a different company. On theres they do not list discontinuities, but at various points on the boring they use descriptions like "(FRAC., low angle)" and "(FRAC., high angle)" I'm assuming this is the same discontinuities or similar ones to the other borings. But not sure what to take away from the data.

 

[MarineConstruction]

What exactly are you trying to gain from the logs?

 

[Nick42]

Gain knowledge about how the rock will behave for blasting and excavation. Also gain and understanding of what they mean, or what the implications of fractures are in the boring data.

 

[PEinc]

Why don't you talk to the company that did the borings? Their geotech or geologist could tell you exactly what you want to know.

http://www.PeirceEngineering.com

 

[Nick42]

The borings are from 2004 and 2005, the geotech or geologist was not able/willing to talk about borings that are so old.

 

[PEinc]

Not for free, that is!

http://www.PeirceEngineering.com

 

[fattdad]

give the logs to a blaster. There is no doubt you'll need to blast, that's for sure. It's hard rock and the fractures are not very close together to anticipate ripping. Do the logs convey "RQD" measurements? I'm thinking you have pretty good "rock quality designation" values.

The implication of fractures are that there is limited weathering along the joints. Low angle can be a good thing as it's less likely a fracture trend will daylight on a cut face and facilitate sliding. High-angle fractures can be a problem, but you don't have orientation to know the strike of the fractures anyhow.

Where is this site?

f-d

¡papá gordo ain’t no madre flaca!

 

[Nick42]

There are RQD %'s on the first boring the RQD %is all 100% and then one segment is 95%. On the second one they range from 72% to 100% with most being around the low 90's. The rock to be blasted and excavated is about a 500ft long, 100ft wide and 70ft deep hole. The owner wants cookie cutter smooth edges. If a competent blaster using standard means and methods went at the rock, should I expect nice clean edges? Or do the low and high angle fractures tell me that the edges will not be clean?

I just graduated from University and this is one of the first things I have been tasked with. I'm going all over rock mechanics books, and geo-technical books, to get the best understanding I can. But there is really no one who can give me a hand here. As PEinc said for free that is...

 

[fattdad]

Your hardest job is to manage the owner's expectations. That's the truth whether you've just graduated or whether you've been at it awhile.

f-d

¡papá gordo ain’t no madre flaca!

 

[MarineConstruction]

If you discuss it with the blasting firm, they should be knowledgeable in several different techniques that can be used to allow for smooth edge walls. One technique is to line drill the perimeter with appropriate spacing and light loadings to pre-split the rock. This will cause the rock to fracture from hole to hole and will result in a smooth face. You can also use this same technique when in full production blasting by using tighter spacing and lighter loading for your perimeter holes. I've used a product called Dynosplit for multiple large structures and underground chambers which resulted in minimal damage to the face.

Regards.

 

[MarineConstruction]

See the below for some general information which will guide you in the right direction.

http://www.docstoc.com/docs/10641987/Techniques-of-Controlled-Blasting

 

[Nick42]

Thanks for the suggestions. The company I work for is the blasting firm involved. We are a highway heavy contractor and blasting is one of the main aspects of the company. The plan was to drill line holes and presplit the face. What I'm trying to determine is if anything in the boring logs would show that this would fail. Fail in the sense that large portions of the face fall into the hole, and things like major fractures and cracks could be created, thus failing to provide a smooth face.

So far the only thing I have found that could possible lead to that is these fractures mentioned, but I don't know if they tell me that or not.

 

[MarineConstruction]

If that is your concern, then typically it is high angled joints that will cause wedges to form and thus cause failure of the face. Based on the description, this doesn't appear to be the case. If you load your holes properly, it should break from hole to hole and cause minimal damage/back break to the face.

 

[Dozerman56]

Nick

This is a great place to seek knowledge but don't overlook resources at your fingertips. It's entirely possible folks in your company have worked on projects in this area and may be familiar with the characteristics of the rock.

 

[DRC1]

A couple of things that may help
1.) An engineering geoligist should be able to provide the insite you are looking for
2.) Presplitting can give you nice edges, but if you are blasting a hole you will need to plan for a relief face.
3.) The joints ar 7 feet apart more or less and the rock is higly cometent based on RQD's. I am assuming the breaks are not drilling breaks. The logs do not mention anything such as soft material on the seams that would give the impression that theses seams are large and would relieve pressure.
4)My big question is what happens after the material is blasted and possibly excavated. Fractures are frequently orientated. A fracture at a critical angle with 70 ft of overburden could slide. The owners cookie cutter edges could be the least of your worries. If you have jointing with a 70 ft exposed face, someone needs to consider stability proir to begining work.

 

[Nick42]

Thanks for the info, and suggestions. What I am doing now, is research and data gathering. My goal is to stay out of reality and determine what we should have known.

I can answer you number 4, since the hole is 1/2 done, blasted out and excavated, with a ramp. The blasters decided to do it in 2 lifts, so its at about half depth. It is awful, major failures, there is only 1 location that the outcome looks nice. There are parts where instead of a nice clean 90 degree edge it slopes from 10 ft back all the way to the bottom.

One of the blast locations was really strange. They loaded a set of pre-splits, and do to all the fracturing and problems, they reduced the loads in the holes, hopping to have a softer hit and still get the breaking. One of the owners engineers was worried that they would not get any breaking with the reduced load. Instead they got way more. The 20 ft area that was to be pre-split had almost no breaking, instead the wall was failing on ether side as far as 30ft away.

The company has blasted in this area over the last 50 years, and worked in locations near to this one. Never have the blasters had so many troubles. Going into it from the borings they expected the rock to be like other rock in the area. Now that it turns out it was not, I was tasked with going back and seeing if we should have expected this.

All blasting has stopped for the time being, as new plan is being evaluated to go forward and address the stability. I'm not part of that, as my task is to go look back words. I do know we had 3 new borings done (at this 1/2 way point), to compare with the original ones, and they are much much worse. RQD averaging around 75%, where the original ones average at 90%.

My bass says this is the job from hell... Good start for an EIT eh...

 

[Dozerman56]

Yes, it's a heck of a start for an EIT, sounds like your boss is right. It also sounds like your company is headed for major legal issues with this project. At this point, while your bosses undoubtedly want you to learn everything you can and handle as much of the issue as you can, need to realize they need someone on board with enough gravitas to help them come out whole.