Blasting to Arrest Sliding Plane Failures in Hard Rock

[Mad Mike]

Has anybody ever heard of this method being used to treat cut hard rock embankments that are failing along a discrete sliding plane?

I've got some high (40 - 50')shale rock cuttings which I've designed to a 1:2(V:H) cut slope, however the shale bedding dips directly out of the slope at ~15deg and, despite the shale being relatively hard rock, I have field evidence suggesting that saturated clay films may be present on the bedding planes at depth.

In principle, if I were to drill and blast the shale bedding within the proposed cut face, I could disrupt the sliding planes and then excavate the face to a stable 1:2(V:H) batter. This would be more economic than losing industrial platform space by flattening my slopes to 15deg.

Any thoughts or major concerns?

Mike

 

[emmgjld]

It sounds good, until you consider the use of explosives in quarries.
I am wondering how you could disrupt the natural fracturing or bedding planes without lifting the upper mass, creating a lower density mass.

 

[oldestguy]

I once did an "in-place" shear strength test, on a one foot square block. Needed a vertical reaction and a horizontal reaction. Vertical was a back-hoe bucket sitting on a post near the test. Used a calibrated jack for the load. Shearing load was by a calibrated hydraulic cylinder and pump. It's reaction was a nearby cut earth bank. Dial gauges nee3ded for strain measurement. This test could then verify the need or lack of need for fooling with the slope.

 

[miningman]

Without a drawing , I'm having difficulty imagining the problem here. However if its something like I think it is , sure absolutely no problem. The proviso is ensuring that the blasters use perimeter blasting techniques, and proper timing to ensure that the broken rock moves out rapidly to allow room for the rows behind to move outwards without excess gas volume creating undesired reactions

 

[Mad Mike]

I appreciate all the responses- attached is a sketch showing the situation as it relates to my proposed cut rock embankment.

In my case- since I'm not confident of the level of the sliding plane(s), I will have them cut the embankment first and then visually confirm the sliding planes by inspection of the cut face. I would then want to drill and blast the sliding planes to reduce them to rubble.

If disturbance of the intact rock above is a concern, we could blast the entire rock embankment and reduce it all to rubble with a high friction angle. I'm really grateful for the opinions of the mining guys- though in this specific case, I doubt disturbance of the rock mass is a problem with such a flat rock face.

For the geotech guys- the cost of stabilizing my rock embankment by any other means would be astronomical- keep in mind that by the time I've cut the bank, it's too late to trim the top back since we're on the site boundary. No matter how much blasting is required to reduce my rock to rubble, it is still going to be cheaper than the engineering alternatives (stability berm, ground anchors, anchored walls etc.)

Mike

 

[Mad Mike]

Sorry my previous attachment would not open for some reason. Reattached.

 

[fattdad]

I'm not too sure how to help. Not all shales are the same. Something about durability?

Here's my problem: Shale degrades in time. We don't know how that shale has performed in the pre-excavated condition. Is this an are where the ravages of time lead to softening of the otherwise intact shale (i.e., via freezing/thawing, heating/cooling, wetting/drying, etc.)?

Do you really think the interface friction along the 15-degree plane is less than 22-degrees (i.e., to allow a safety factor of 1.5)? Where's the water?

In general, I've never heard of this approach. That said, it may work?

f-d

ípapß gordo ainÆt no madre flaca!

 

[Mad Mike]

Thanks fattdad,

My shales are relatively indurated - slake testing on the material recorded a durability index of 90% - I am assuming them to classify as "hard and non-durable".

I'm in a very humid, wet coastal region where degradation of shales is a common problem. With that said, my shales show relatively little weathering and I believe the degradation in a free-draining, blasted rubble heap would not be sufficient to destabilise a 1:2 (V:H) slope.

This 15-degree plane is definitely a problem in my area- we've had a number of serious landslides in the same shale formations, along sliding planes as flat as 10 degrees, slightly less even. The clay films that lead to these sliding problems are usually only a tenth of an inch thick, incredibly persistent laterally, and often sandwiched between hard rock. The clay films are always wet- there is a structural control on water through steeply inclined joints which intersect the bedding planes. There is rarely any seepage through the planes, just wetness.

I recently had a colleague deal with a sliding failure on a similar plane of not more than 5-degrees, effectively horizontal by normal standards...you would have to see it to believe it- it completely destroyed 2 newly built houses at the top of the cutting and was eventually stabilized with rock-anchors and gabion walls.

As a last point, I'd like to consider this treatment hypothetically- let's just assume my shale was perfectly durable- a durable rock heap would have a friction angle that allows it to be steepened to at least 1:1,5(V:H) and the costs of blasting could easily be recovered from saleable platform area. I'd be far happier dealing with the (in)stability of a rubble embankment than that of a hillside sliding down onto my site!

I need to find a case history before I would ever recommend this- I'm not ready to make a martyr of myself just yet!

All the best,
Mike

 

[fattdad]

MM,

Now that you've addressed my questions, I'll have a bit of further pondering. . .

Now I'm imagining that you have such surgical precision on the 15-degree planer surface you could drill and blast it all to smitherines under confinement? With all the rubblized shale present where the slip plane used to be, the interface friction angle would want to increase, eh?

Can we actually, "Get" what we imagine; however?

Let me continue my pondering.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Mad Mike]

Fattdad,I trust your pondering more than my own.

I would only know the exact location of the sliding plane(s) once the first cutting (blasting) is carried out- 3D prediction of the sliding plane(s) is simple in my case since they're perfectly planar along the shale bedding. We would then have to re-drill and blast the cutting upon discovery of said sliding plane(s).

All I want from my slope is a 1:2(V:H) cut batter- I don't think that's too much to ask from a rubble heap- more important though would be ensuring that the rubble heap has enough weight to resist the sliding plane(s) shearing through it.

I suspect there is enough doubt surrounding this "method" that I won't be recommending it- my problem is that I don't much enjoy using structural members (rock anchors) for supporting these slides either...damn Developers and their constant quest to gain platform space- I should just tell them to cut it at 15deg and be done with it!

Anyhow, I greatly appreciate the discussion- thank you.

Mike