Removable panels for concrete retaing walls

[Patgeotech]

We are currently undertaking rock slope remediation works to stabilise 30 year old rock slopes for a mountain pass. Concrete retaining walls were built at locations along the pass to collect/block rock falls. The natural bedding plane is unfavourable (dips down towards the road) and large rock masses (25 to 100 ton) are prone to sliding. Huge quantities of rock have been trapped behind the retaining walls and we are cleaning up behind them to improve the collection holding capacity (current walls are full to the top of the retaining wall). To avoid the need for specialised maintenance work (say, rock barring or rock splitting on the rock slope), I have been wondering about cutting into the existing concrete retaining walls and installing removable reinforced panels that could be lifted by an excavator to gain access behind the wall. Has anyone used such panels in retaining walls? I would appreciate your thoughts and design ideas. The current walls are approximately 4 to 5 metres in height and the rock slopes are approximately 30 to 60 metres high (from road level).

 

[itdepends]

I am not a geotechnical engineer (not even close) but the thought that springs to mind is that if the retaining wall already has material behind it- won't that material be pushing against any removable panels making them
a) very difficult to remove if they slide vertically
b) somewhat hazardous to remove if you undo the locking bolts/flanges that are holding them in position if they come out horizontally.

 

[kslee1000]

Why not design access man-door (1m x 2m)with metal gate, which is less prone to impact from the falling rocks.

 

[Patgeotech]

I should have added that the rock mass above part of the wall has been stabilised and the risk of rockfalls completely taken away at this point, however there is still a need to get in behind the wall to remove rock from other areas of the wall (90 metres long). The rock formation on the other side is unforgiving and prone to unpredictable rockfalls, hence the need for the retaining wall. The plan is to provide the access panel where we have stabilised the rock mass above. The removable panel would therefore provide security against perhaps small impacts (although unlikely) and prevent th public wondering behind the wall (so a safety issue as well).

 

[cvg]

how about just putting rock fall steel netting across the opening that could be easily removed and replaced during maintenance?

 

[msquared48]

Any way to safely access the rock fill from the ends of the wall rather than the middle?

Mike McCann
McCann Engineering

 

[msquared48]

Or abandon the opening idea and use small portable tower crane with a clamshell to remove the smaller debris. The larger rocks could be broken down to size with borings and chemicals for removal by the crane.

Mike McCann
McCann Engineering

 

[Patgeotech]

To illustrate what we are working with I have downloaded a photo of one of the cut rock slopes.

I have abandoned the removable panel idea and focussed my attention to the crest of the slopes as well as the unfavourable bedding planes. Access will always be a problem behind the wall, so I am going to concentrate on eliminating the risk area (i.e. the crest). We are going to chase the crests along the bedding planes and then rock bolting with restraining mesh draping.

The existing wall is about 4 metres high and you can see where one of the rock masses slid through and over the side of the road. Took out a 3 thick gabion structure as well - gabion structure was placed just off the centre line of the road to catch minor cobbles and boulders, however it met with a heavier 'boulder'. The rock mass/sheet only slid from about a 15 metre height - was probably about 15 metres by 10 metres and about 1.5 metres in thickness! The mass of the rock mass must have been something and probably nothing would have stopped it. I had a gut feeling that something was moving and we were due to rock bolt the rock sheets, however mother nature beat us to it. We experienced two days of 100mm rainfall in an area in South Africa where the monthly average is approximately 50mm. The rock sheets were 'lubricated' along the bedding planes and joints - we were then in a dangerous position for rock bolting (which I called off)

Proving to be a challenging project. Any ideas about other methods to stabilise crests would be welcome. I am looking into fibre-reinforced shotcreting as a possible alternative.We have captured a movie clip of an entire 20 metre section of crest toppling down (purely a natural rockfall as no work had been done in this area - I will try and attach it to the next thread).