Grouting in Karst Terrain

[nbr1]

When grouting in karst terrain (relatively deep depth to bedrock, say 50-ft plus) for the purposes of mititgation
of sinkhole development, what is the most rational methodology for determining maximum grouting pressures and
preventing hydrofracture in clayey soils....assuming no groundwater?

Also, is cap grouting at the bedrock interface the best approach, or should pressure grouting start at rock
and continue to near surface.

 

[oldestguy]

I'd be willing to bet that if you pose that question to a grouting contractor, that he will reply with a proposal based only on the cost per cubic yard, with the stipulation that you pay for what ever he pumps and there is no high limit for quantities pumped and no time limit. Oh yes, no guarantee and no claim for damages unintended. Let's see if you get meaningful answers from such sources. I think you are asking for a lot here.

 

[nbr1]

It just seems to me that there is a rational approach to developing grouting pressure refusal criteria,
for the purposes of say filling voids; perhaps based on vertical or horizontal earth pressures with
consideration to hydrofracture??

Only thing I can find is broad ranges referenced from past projects.

 

[BigHarvey]

Two approaches are possible :
1) you let the sikhole develop but you lay your slab on an array of rigid inclusions. The grid will be chosen accordingly with the most probable sinkhole diameter. This approach has already been used in France.
2) you grout. In this case, just like for any void, you start filling the voids with a mortar more than a grout in order to limit quantities. Once this phase is completed you do a pressure grouting phase with Bentonite cement. Don't worry about fracturing the soil, there is no such thing as permeation grouting ! ( see late articles by Cambefort ). Refusal criteria are established just as for any other pressure grouting jobs.

 

[BigHarvey]

attached is an article ( sorry, it's in french ) of the treatment, by grouting, of an accidental sinkhole created by a collapse of a tunnel in construction ( no water table )

 

[Prepakt1]

Grouting large voids underground. You have two major problems.
How big is the void and where does the void stop .It may be connected to other voids, usually impossible to tell.The volume of grout needed can be unknown.
Is water flowing and how fast. If it is flowing at any more than a crawl, it will wash out the grout.

You can pour anything you like into the hole from regular concrete to fine grout depends on what you want to achieve.
If the ground is full of interconnected voids and you just want to fill a specific hole, you can if necessary fill the void with a fabriform grout bag which will contain the grout flow and form a plug.

In previous projects where you have a lot of water flow our methods are usually to drill 6" or 8 "lined dia holes place a 1" grout tube to the bottom of the void and then fill l with 6" minus rock, then 2 " rock, vibrate it as much possible, then add pea gravel. This mixture will slow the water flow and then you grout from the bottom.

You then fine grout it from the bottom the mixture of rocks will help the grout bond. You can also add some expanders to you grout that will help block smaller voids

Intrusion Prepakt /marineconcrete.com

 

[nbr1]

My question is in regards to minimum pressure requirements, say once one is convinced all voids are filled.
Let's say you are at 80 feet below grade; is 10 psi okay, 50 psi, or 500 psi?

 

[Prepakt1]

As you grout you will get pressure spikes. I would say about 50 PSI should be more than reasonable but it will often spike above .Often when you are pumping the pressure will be minimal, depends on the depth and head required. Generally you pump until the grout reaches the surface again.It may appear anywhere around the area you are pumping. It also might be flowing into endless voids or river channels and never surface.You then have to decide how to fix the problem. A lot of pumps will do 250 Psi or more. This should not be required. It only takes 5 PSI or so to lift a building. When you pressure grout, the grout will eventually come to the top of your hole or the ground might start to rise. The head pressure if deep hole ,will cause it to percolate slowly into the ground, so it will settle and need to be topped up over a few hours. It should be solid by the following day.

Intrusion Prepakt /marineconcrete.com