How can I seal off underflow in a weir? 1

[soileater]

I am trying to come up with a practical method of sealing off the underflow under a proposed new weir in a very inaccessible gorge. I dont know the depth of the rock and the river bed is filled with large boulders. Do I drill & grout or what?

 

[dgillette]

What material are you talking about grouting? The alluvium? If the critical layers don't contain too much in the way of fines or fine sand, there is a possibility of permeation grouting. Probably can't drive sheet pile if many boulders are present. How about excavating a c/o trench and backfilling with lower permeability material?

If it's the bedrock in need of grouting, conventional cement grout may be all you need.

Might get SOME benefit from an upstream blanket, but it's not a very reliable solution - depends on where the seepage gets into the critical material.

Can't say much more without knowing specifics. If you don't know the specifics either, better get some exploration.

Regards,
DRG

 

[Prepakt1]

Rock dam grouting.
Not easy, need to drill then grout .On a similar project a 80 ft deep rockfall filling a gorge we used a 7" drill bit 3 lines of grout holes at 2- 3 ft centers. Downstream line was first filled with 3 " rock to fill voids in large rocks, middle line was filled with 1" rock, again to fill large voids and upstream line of holes then filled with pea gravel. The intention was that the gravel and pea gravel would migrate downstream and gradually fill the voids in the 3" rock that was first dropped into the downstream line of holes.This would reduce the void sizes to as small as possible and enable grouting ,if you have some water flow.
You will need to keep topping up the various holes
as they settle.
If if you cannot slow the flow enough to grout, you need to stop the flow as much as possible by building a higher temp dam downstream to reduce the pressure differential in the first then you can grout.
Environmental considerations if fish are present should also be considered.The ph downstream should also be monitored. Grouting if ph levels are critical can be very slow to reduce the volume/hr of grout leaking into the flow to minimise ph level changes.

Alternatively if you have one or a number of definite entry channels you can use fabriform concrete
bags or mat as a plug, or just one of varois types of liners, a definite answer depends on the actual situation.

Intrusion Prepakt /marineconcrete.com

 

[msquared48]

For a gravity-arch dam constructed in the late 50's early 60's on the Skagit River in Washington State, a freeze curtain was used. Worked very well during construction. Dam is still there too. That's a very good sign...

Mike McCann
McCann Engineering