Grouting Techniques for Rock Anchors Losing Grout

[jwatso33]

I was curious what ideas you guys may have for rock anchors in a Karst environment. Borings indicated a variable limestone but once through the upper few feet was fairly consistent. I have some anchors that are consistently not getting grout return after +3X theoretical grout volume with thickeners added. I am thinking adding post grout tubes but I thought you guys may have some other ideas.

 

[PEinc]

You may need to leave a casing pipe in place from the wall to the top of the rock socket. The casing will contain the grout above the socket. The socket will retain its own grout. If not, you haven't drilled your rock socket deep enough.
As long as the rock socket is filled with enough grout, it should not matter if you lose grout above the rock socket unless these are permanent tieback anchors. The driller needs to pay close attention to what is being drilled (rock or void).

http://www.PeirceEngineering.com

 

[oldestguy]

A very difficult thing to work with, especially where the voids can be very deep like 100+ft.

I'd sure like to see how someone overcomes that situation. I gave up on a few jobs and they went to a lighter project with many spans over voids.

 

[Ingenuity]

Back in the late 90's I did a design-build project in Guam with vertical rock anchors to anchor a 10' thick concrete seawall - designed for typhoon waves etc - founded on coral limestone.

1-[sup]3[/sup]/[sub]4[/sub]" dia 150 ksi Williams bar, double-corrosion protected anchors. 240 kip design load.

Anyway, it was very limited space drilling, 10' above sea level, and right on the coastline. It was an old US Navy dump site back in the 60's and therefore part of a remediation project. 100 each rock anchors total.

We started drilling and hit caverns, voids, etc in the karst limestone. We had initial grout takes of 400 + bags per hole with calculated takes of 10 bags! We ended up shortening up the anchors (from 50' total to 36' total, 15' free-length), cased the hole, installed post-grouting tubes. We purchased a U/W borehole camera and videoed each bore and found steel chain and old equipment in the hole, along with fish etc.

After shortening up the anchor length, we ended up pre-grouting each hole, removed the casing with a hydraulic casing puller(we were using using Stratex DTH), and re-drilled the hole the next morning. Very, very slow. We had to re-pregrout some holes after video review of the drilled pre-grouted hole.

We ended up mixing the large grout volumes with a volumetric mixer - truck-mounted - and pumped from a staging area many feet away from the seawall.

We ended up not having to use the post-grout tubes as out grout takes after pre-grouting were okay.

All anchors proof (or performance) tested to PTI requirements.

We lost a down-the-hole hammer in one of the voids! Not a fun project, but learnt a lot. Then the general contractor went bankrupt...a story for another day!