Rock Failure of Quarry Highwall Below Existing Development 2

[Mad Mike]

I've recently been commissioned to carry out the geotechnical investigations and remedial design of engineering solutions for a rock-slide which now places a structure at risk.

The situation is that a major office block development was built approximately 10 years ago to encroach within 7' of an abandoned quarry highwall (near-vertical cliff face, estimated height 100'). The face of this quarry highwall has recently spalled off, detaching along a joint set parallel to its crest, which has led to tension cracks opening up in the asphalt immediately behind the highwall. The nearest structural columns are just 7' behind the highwall crest and about 4' behind the nearest tension cracks. Columns are undoubtedly supported on bases bearing onto rock.

I attach two photographs showing the situation- an elevation of the failed highwall and a profile immediately behind its face showing the existing columns. The tension cracks are located intermittently between the existing columns and the top of the highwall (which coincides with the boundary fence-line). The cracks are not particularly wide at this stage, but run perfectly parallel to the crest of the failed highwall, suggesting to me that a further joint set has opened up behind the face.

The bedrock geology is Tillite- it is well jointed but has no bedding to speak of.

I'm contemplating the best investigation methodology to use in this instance, and also the kind of engineering solution that might be required to stabilize the rock face, assuming the current failure has opened up a further joint set immediately behind the face. I was leaning toward a drone inspection of the highwall face, perhaps supplemented by orientated triple-tube core boreholes through the existing asphalt at the crest of the highwall.

Practically, I'm wondering how one might go about anchoring such a rock face beneath the structure. I have yet to inspect the site myself, but I foresee all sorts of challenges to the investigation, design and construction.

Any guidance, opinions or similar experiences would be most welcome.

Cheers,
Mike

 

[oldestguy]

This class of "rock" is mapped in large areas of South Africa as well as elsewhere. I notice many photos of such areas show vertical sides to valleys at streams, possibly indicating a natural "slope " there. One photo at subject site shows a natural slope of steeper than 45 degrees. Does that perhaps indicate a stable "rock " slope at 90 degrees is unlikely? Can "rock anchors" hold back a bunch of uncemented together "rock" pieces? Needs to be answered before trying it. Are there examples of rock anchors doing the job on similar "rock" faces in the area?

Edit: One photo shows automobiles inside the lower level. I'd try outside to do this first. If a drill rig is set up inside the garage as far to the front side as possible, installs inclined tie rods with plate and other wide "anchors" snugged against the "rock" face,that would secure the highest, most likely to fall away material. Combine those with lower tie rods installed from the street side. No work is needed from the high rock face.

 

[Mad Mike]

Hi all.

My apologies that all 3 photos were not attached- I'm having difficulty attaching multiple files- for every additional file I attach, it erases the previous attachment.

The current attachment shows clearly the tension cracks immediately behind the failed quarry high wall- they are 7 to 10 feet away from the nearest structural columns.

OG- I intend to put down 2No geotechnical boreholes (inclined, triple tube) to inspect the rock mass. The drill rig which will be used for rock bolt anchor installation is yet to be determined but the more I look at this situation, the more I believe that some form of grouting around the current column foundations would be required prior to drilling for rock bolts.

Miningman- I appreciate your uncensored statement "That rock looks rotten...and it WILL fail". I agree, and am moving forward on this basis. I will dig for historical data, but don't expect anything to come to light. Record-keeping is not South Africa's strong point. I think in the old days, the benches were simply removed towards the end of production, so no reason to believe there has been instability before.

OG- regarding your last post- we rock bolt this type of rock all the time, but never anything on this enormous scale. The unweathered (blue) tillite is usually very stable, with the occasional exception of major wedges/toppling. The brown tillite is as you note, well jointed and generally "blocky". We have a few old quarries around Durban with vertical high walls around 200 to 300' high, no benches, which have stood incredibly stable for decades- if you're familiar with Google Earth, go look at the coordinates 29deg 48' 50.0"S and 31deg 00' 45.0"E...one of our very public quarries that has been developed top and bottom despite the vertical high wall being 300'!

Mike

 

[theCorkster]

The first photo showing the vegetation is to me the tell tale, as miningman noted. Influence of water seeping through fractures/joints in the rock and the preponderance of vegetation with roots that will open joints/fractures is critical. Controlling all surface/near surface drainage will be key in the long term. We have bluff developments on 80-foot high near vertical weak sedimentary rock with overburden soil that perform well until the water arrives on the scene - landscape irrigation, landscape irrigation piping, broken storm drain pipes and even cracked pools.

 

[oldestguy]

Hey Corkster has hit it. Notice the quarry wall vegetation is most plentiful adjacent to the housing. Also note the photos that show inlets for surface water and I think roof water pipes also. Then note the buildings as collectors for rain water and likely they have drainage back towards the quarry. So the watershed , if you will, for the rock face at the buildings is much larger than that at the parking lot, etc. Chances are all that roof water and rear paved are is dumped to the most convenient place, likely the rock surface at the rear of the building. Explains pavement cracking there also.

So suppose tie backs are installed. Will they be permanent as long as that water source is still there?

Gosh, if it were not for rainwater and the like we geotech engineers wouldn't have much to do.

 

[PEinc]

Tieback anchors or rock bolts could be encapsulated in corrugated, PVC sleeves (double corrosion protected) before they are grouted into the bedrock. Ground anchors commonly extend below the ground water elevation.

http://www.PeirceEngineering.com

 

[Mad Mike]

I appreciate the comments regarding the vegetation, storm water drainage and the likes. No doubt they are all contributing factors.

My real purpose in this investigation, other than to design the stabilizing measures, is to point a finger of liability at the original development team.

In this regard, we can take away the trees at the crest and potential water ingress, but I still feel confident that gravity would ultimately have its way with that rock face. If we look at the base of that failure, the release plane is not some lubricated joint surface- it is a jagged shear through the weathered rock. Hydrostatic pressure on the main joint parallel to the face may very well have been a driving force, but I can't see the factors of safety on that rock face being fit for development even in the dry state.

Cheers,
Mike

 

[oldestguy]

Assuming gravity will do its thing some day, perhaps "face the music" and install a position monitoring system that will trigger when there is movement and the building can be evacuated. Under most likely situation the parts of the structure next to the quarry face are well connected to the reminder, there is little risk to the occupants in case of the face falling away. Cheapest way to resolve the danger IF it is positive the support cannot be saved permanently.

 

[miningman]

Mike, no doubt local laws and practices are different to those in North America, but based on the facts identified todate, I would enjoy being an expert witness for the developers team. The building is 10 years old. How long is the developer on the hook in the event the owner fails to take care of normal maintenance?? When , and how often did the owner trim / cut back the vegetation in the affected area??

Who did the crack sealing, under whose supervision, and basically just papered over a serious problem without red flagging the issue???

Of course at this stage it doesnt matter on our disagreements...... the lawyers will ultimately be the only winners

 

[oldestguy]

On the above comment "Who did the crack sealing", not critical. More important is what is the condition of the buried pipes draining the inlets and the roof drains? My guess is they extend only the minimum towards the face and, with the visible crack, how many more down there.

Crack merely is the signal of "bad stuff" down below. Sealing it is like correcting a broken lock on a safe that was burglarized.

 

[Mad Mike]

Miningman, it will become an interesting legal predicament no doubt. Regarding the vegetation- those trees you're looking at fall outside of the commercial property and are inaccessible without taking down the boundary fence, so maintenance there was never the owner's responsibility. For what it's worth, none of those trees have particularly impressive root systems and so I'm not placing too much emphasis on them as a driving force.

OG- I still need to obtain the storm water management details. No doubt the bulk of storm water is taken back toward the main street into Municipal pipes, but there may be small discharges onto the quarry face side that I would need to check up on.

Best,
Mike

 

[miningman]

Madmike, anything new on this???

 

[Mad Mike]

Miningman, I've currently got a drilling contractor our there coring the inclined boreholes.

Client produced a litany of old Architectural plans, all of which showed storm water being taken back to the road front; but then, an approved deviation to a plan, showed storm water being released directly onto the rock face, as Oldest Guy suspected might be the case.

I will be flying a drone up there to try and determine the extent of storm water release onto the face- I think it is just auxiliary small volumes, but probably discharging directly into the joint along which the failure occurred.

Nothing has been forthcoming from the original Geotech guys- they're simply unavailable or noncommittal, presumably shelled up in the face of a major claim. I've not managed to get any formal papers out of them.

In about a week I will have logged the core boreholes and obtained the drone photographic data- I will post again when I have something meaningful to report.

Best,
Mike

 

[ATSE]

Hoek quotes Terzaghi as saying "...we were overstepping the limits of our ability to predict the consequences of our actions" regarding rock engineering.
Engineers and geologists that push the envelope with rock should not be surprised at adverse consequences. Even a non-engineer would look at that development and say, "Are you sure this is going to work in the long term?"

 

[MiningLad]

This is an interesting problem and will be keeping an eye out for future developments. With the drone, are you thinking of doing some photogrammetry to map the face?

 

[Mad Mike]

MiningLad,

It's my first time dabbling in drones for field data and I'm not sure exactly what they are capable of, so I've just asked for high-resolution photographs.

The majority of my field data (drilling / trenching / drone photography) is in and I will try to post some more photos soonest. Using the drone, I've ascertained that there are several pipes discharging storm water onto the rock face, including a couple of points immediately above where the failure occurred.

I've managed to discuss the case with the original geotech- as I'd mentioned a very experienced professional now close to retirement- he admits that no stability assessment was ever done by him. Seems he was suckered into a brief assessment by a developer "friend", who he has since regretted ever dealing with. He's very stressed by the situation and I would love to be able to get this through without incriminating him any more than necessary.

Best,
Mike

 

[oldestguy]

I'd guess the presence of so much vegetation at the building portion of the rock face that your photos would then show that Next what to do about it..