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

 

[GeoEnvGuy]

Given the proximity of the building to the edge of the drop off I don't think the solution will allow for slope flattening. So your left with reinforcement probably with rock bolt anchors. You can either pepper the existing slope based on rock quality or you can implement a monitoring system to find where the slope is moving to target reinforcement.

For newer open pits they are using a ground based lidar system to generate a heat map of where the movement is happening. They measure the rate and acceleration to predict rockfall. The older way was a prism based system with manual checks.

The lidar systems are very pricey and take time to detect movement which may not be occurring yet on some parts of the quarry wall, so you may be better off to do an assessment of the rock and just pepper the entire slope with anchors.

 

[Mad Mike]

GeoEnvGuy- thanks for your feedback.

I attach a regional perspective of the affected structures, which you can now see are situated precariously above the highwall, absolutely no space for slope flattening.

I was thinking exactly what you've said- rock bolt anchors through the potentially unstable rock mass below the structure, possibly with additional anchors through the column base foundations as a precautionary measure.

I suspect the tension cracks were an immediate response to the original failure, so I'm not expecting them to move but will have them monitored nevertheless.

Have you any thoughts on the best way of investigating the rock-mass in this case? I'm thinking of omitting boreholes and instead just carrying out a detailed mapping of the exposed quarry face using drone imagery, which I see as a cheap and practical alternative.

Mike

 

[PEinc]

Get a big crane. Hang a platform with a drill sitting on the platform. Drill and grout hollow core, threaded bar, rock bolts with bearing plates and nuts. Add wire mesh fabric as needed. See Geobrugg.

https://www.geobrugg.com/en/Rockfall-Protection-77472,7858.html?branche_id=59494

http://www.PeirceEngineering.com

 

[miningman]

Some sort of rockbolting in the very near future is a no brainer. IMO, the owner should be informed that immediate remedial action is necessary. Any delay due to procrastination or desire to monitor , in order to evaluate how bad the problem is , can only lead to further problems.

Too many people think of rock bolting as a procedure that can be implemented once movemement is observed. In fact its best usage is to reinforce the rock mass before movement takes place.

A cost comparison between rockbolting and the financial loss to be incurred if the buildings are lost might help focus the owners attention

 

[GeoEnvGuy]

Who approved that site plan, and where is the pre-development stability report? The immediate requirement is rock-bolting the long term requirement is to determine if there is a risk of a larger wedge or planar failure. This will require drilling to determine which other reinforcement measure would be required in the long term.

EDIT In terms of investigation techniques for mapping the discontinuities you will probably be best doing photography from the base access road and have a long spool measuring tape sent down over the slope. For investigating larger failure planes standard core logging using the RMR system would be needed. The best book I have seen on this subject is Hoek and Bray's Rock Slope Engineering and subsequent editions.

 

[oldestguy]

In this case even as the engineer, I'd have my attorney made aware of this and the agreement should be ok'd by him if not so already. You sure don't want to be hauled in to the court battle should this thing fail at some time. Even installing tie backs some vibration can trigger movements.

 

[BigH]

Besides the above good comments, you might wish to contact a very experienced engineering geologist - likely much better about mapping and developing the geological model - from which you can then recommend suitable design options. OG's point about getting your lawyer involved is quite good and likely very prescient.

 

[Mad Mike]

I really appreciate the feedback from everyone, thank you.

The original developer had retained the services of an incredibly experienced geotech (retired,~70yrs old), who I believe looked at the quarry face and casually deemed it to be stable. No detailed analyses or reports were ever published to support this. I'm not sure if corners were cut in placing the structures so close to the highwall, but its quite possible that some blame lies with the developer.

Everyone involved knows this is an emergency situation and I will be getting onto the site as soon as possible.

GeoEnvGuy- appreciate the comments on investigation methodology and yes, I would view rock-bolting beneath the structure as an immediate measure to stabilize the structure. If there are larger wedges, the alternative would then be to abandon the buildings. My feeling from the photographs, this is not the case, but I will certainly be analysing all options.

BigH- I'm an engineering geologist myself- I've mapped plenty of faces and done the kinematic models; just never on a face as inaccessible as this one!!!

I can see very well the absurdity of the structures being positioned where they are, in case there is any doubt, but sometimes we inherit the problems of others...

Cheers,
Mike

 

[oldestguy]

In addition to yourself now, considering your experience,I'd get a nationally known geotech involved to make sure you are doing things right, etc. I'd set a survey line along that fenced walk to monitor position horizontally and vertically. If the minor disturbance of drilling tie-backs triggers movement, you certainly need to know that early on. It would not hurt also to monitor floor elevations inside the building especially near the side at the drop off. You can do that with a hose filled with (no bubbles) water and fitted with clear hose at each end, making a portable level, will you. Takes only two persons to do that.

Finally, make a cost estimate of this reinforcement work as well as a cost estimate for moving the buildings to another site.

 

[Mad Mike]

Thanks OG- agree with the monitoring requirements and the like.

I'm excited to see how this unfolds.

All the best,
Mike

 

[oldestguy]

A final question. Who is paying the bills? If trouble develops along the line, will you be paid? I'd want monthly payments based on time involved.

 

[oldestguy]

This old guy was wondering about site utilities and possible leaks. Notice the storm drain inlet next to the fence area. Where do these things and plumbing waste water go? Are there leakages under the building triggering movements? Experienced plumbers in the area may provide some useful info on this.

Edit: Should it be determined that leakages in the waste water lines is present and likely causing the rock mass below the building to be unstable, then there may be sufficient risk, in spite of rock bolting to abandon the buildings and of course the bolting..

 

[miningman]

Additional comments for consideration. From the photos, I would suggest that the vertical rock face in question is not a natural surface. Look at the faces at either extremity. To me this has all the characteristics of an old quarry. If correct, this might have contributed to the erroneous idea that this
is a stable face.

Look at the vegetation growing out of the rock face just below the affected building. Compare this with the complete lack of vegetation further along. The existing vegetation can only contribute to instability.

rock bolting the face in a conventional manner is certainly worthy of evaluation but there is another option. Using the face height of 100 feet as a reference, it may be feasible to cable bolt the entire rock mass with drills set up behind the upper road. This eliminates the need for the expensive crane and allows the drillers to work with both feet on the ground..... a major improvement in manpower efficiency.

If mapping can identify the most unstable areas, cable bolting could be focussed on those areas , rather than the expense of large scale area bolting

 

[GeoEnvGuy]

The rock face is a quarry as stated in the subject line, I would also want to know how deep the quarry went as it appears there is water pooled at the base of the rock face. Also what is the quality of the rock below the water line? Was the quarry backfilled or is it an open hole? if you need to drill at the toe access may be costly.

I could only speculate that during the life of the quarry there may have been a dewatering effort which has now been dismantled.

 

[Mad Mike]

Thanks for all the feedback- sorry that our time-zones aren't aligned, hence my slow response.

OG- appreciate the comment regarding leaking services- I will dig around in this regard. The main focus of this exercise, independent of the variables, is whether that site was ever fit for development, which is likely going to be the subject of a major court case, for which my firm would need to assume the "expert witness" position. The serious costs associated with rock-bolting that face will also be a subject of dispute, no doubt.

Miningman- correct this is a quarry we're dealing with. I really like the idea of your cable bolting from behind approach...I will certainly be looking at it in respect of costs and practicality.

GeoEnvGuy- fortunately in this area the quarry does not extend much deeper than the water. Perhaps 10' at most- I know this for a fact so I'm not looking to carry out any drilling at the base.

Update: My estimate of 100' for the highwall was way off- the quarry face is approximately 200'. I'm inspecting the tension cracks this morning and will then look at what form of monitoring to put in place as an interim measure while conducting the detailed investigations.

Cheers,
Mike

 

[Mad Mike]

So I've returned from my first site inspection and taken some measurements.

I attach a slightly better photograph of 1. the rock failure of the quarry face; 2. the profile behind the failure and 3. an elevation view of the intact quarry face immediately adjoining the failure, showing the rock weathering profile more clearly.

The currently sealed tension cracks are at their closest 2m from the nearest structural columns, and stepping back approximately 2m from the crest of the quarry highwall. There is no structural damage beyond the cracked asphalt at this stage.

I've advised for the time being that the area need only remained cordoned off, and I will be inspecting the asphalt seal on a weekly basis to see if the tension cracks open any further (we're just entering our very dry winter season so no major rains expected).

What is very evident now, and clearly visible from the quarry face elevation photo I've attached, is that the upper half of the quarry face exposes a brown, moderately weathered, well jointed tillite which at that height, would allow interconnected joints to communicate. Looking at the failure, the main release joint runs perfectly parallel to the rock face, with movement facilitated by the regular joint blocks in the weathered tillite. Below this weathered tillite occurs unweathered blue, massive tillite which is stable.

One of the main data sets I require for the design is the spacing and infill on the main release joint set, parallel to the quarry face. I'm thinking the best way to measure this would be a couple of inclined boreholes, drilled about 40' back from the face at a 45deg angle, advanced until the drill bit breaks through the quarry face. Supplemented with joint measurements from the quarry (+ drone photography).

Have a great Friday guys.

Best,
Mike

 

[PEinc]

Only one photo was attached.

http://www.PeirceEngineering.com

 

[oldestguy]

Is there any history of what the proposed drilling method might do to trigger movement or weakening the mass? Will water be used? Air? Can this "rock" be cored and any recovery examined? Any info on trenches dug for the original construction as possible paths for surface water entering?

 

[miningman]

I would also be looking for historical data in terms of when was the quarry last active?? Was blasting done using conventional benches?? If yes , when and over what time period was the failed benches removed from the toe of the rock face? How old is the building??.

As soon as the lawyers are involved , I suspect these will be very relevant questions. The answers may pay a part in the allocation of blame / responsibility. I suspect the building is effectively condemned. That rock looks rotten and at some point, the owner has to be made aware that regardless of stabilization efforts , the whole rock mass is moving and it WILL fail