Excessive grout take-up in coarse gravel layer - Down Hole Drilling - Rock Anchors

[sstanton01]

Please review the attached sketch. We're looking for suggestions on how to stabalize the course gravel cobble layer. We are duplex drilling with 7 5/8" dia. 0.5" wall casing. Using a 6 1/4" button bit on a 6" down the hole hammer with 4 1/2" inner drill steel. We stopped advancing the casing when we hit a boulder. Thought it was bedrock. Advanced the hammer out the end of the casing using 4 1/2" inner drill steel. After about 3' of rock drilling the hammer broke through and went into a softer gravel-cobble area fairly quickly. We decided to grout the area and try to redrill it. We have tried 6 times and each time we find the gravel continues to collapse on us. We are afraid we're not going to get the anchor past the area. Unfortunately the first grout attempted locked in the casing and we can no longer turn it to try and advance furthur. It seems as though the grouting attempts run horizontally and don't fill the void we are creating. We do get ground water to displace out the cassing up to elevation 8'. We see that water slowly retracting down the casing indicating that we're getting a slight pressure grout. We're Looking for suggestions on how to stabalize this area. Your thoughts will be greatly appreciated.

 

[DanielSgeo]

Wow that’s a tough spot. Can you get a tremie and a packer down there and then use a low slump grout like compaction grout piles?

 

[sstanton01]

We can get down there with a tri-cone bit on the 4 1/2" inner drill steel. We can tremmie pump grout through the bit. I'm not sure what a packer is. Low slump grout is a thought we had too. Can you send a link to a packer?

 

[DanielSgeo]

Packers are inflatable rubber devices that you can use to seal off portions of a borehole. Usually, they are used for fracking or pump tests of isolated layers. The following link is for a wireline rig but it gives you the idea. I don’t know if they make a setup for a solid stem setup.

https://cmeco.com/newsletters/Wireline_Packer_Assembly_CME_Update_August_2018.pdf

 

[sstanton01]

Thanks for the tip. We will research this option.

 

[MTNClimber]

Can you sub a 6.25" rock socket for a longer 5.5"? Slip in the 5.5" inside the 6.25" and duplex through the unstable layer?

You may have to get in contact with a grouting specialist who can pressure grout a quick setting grout to stabilize the hole. I see you're on the coast in southern New England. Reach out to Bob Tonning at Phoenix Foundation.

 

[sstanton01]

Yes MTN Climber, we have 5.5" casing with 3 1/2" inner drill steel. I was thinking of working with just that. Problem is I have to extract the casing as we grout and the grout doesn't seem to want to fill the void. Like your grout suggestion. Thanks for the tip. We'll get in touch with Phoenix Foundation.