Cement grouting pressure

[karimovic]

hey everyone this is my first participation in here so hope u all be helpful

I'm owrking on a project where the soil is weathered rock.. so no problem with bearing capacity or settlement.. but the problem is the existence of cavities.. one of the boreholes already identified a 1.2m thick cavity.

we assigned a subcontractor to do cement grouting on a 5mx5m grid and to a depth of 6m (because its a raft foundation)

now the problem is: What grouting pressure to use?

the consultant suggested a 1-2 bar pressure.. whereas the contractor says its insufficient at all because it wont be enough to infill the grout in between the nodes of the grid.. the contractor also says that a 1 bar (atmospheric) pressure is only sufficient to fill the drilled holes and to infiltrate into the ground and improve the soil.

the consultant says: we don't need to grout all the site because it's expensive.. only local grouting of the boreholes is enough since the soil is rocky and strong..

we just can't find a consensus.. any suggestions?? I can't find any pressure limit for grouting in the codes ( only a rule of thumb that results in a 1.2 bar pressure), however i found a pressure limit for anchoring in the BS code that states a limit of 20bar ( not sure but close to 20bar )..

plz help!

 

[karimovic]

*Correction: the contractor also says that a 1 bar (atmospheric) pressure is only sufficient to fill the drilled holes and NOT* to infiltrate into the ground and improve the soil.

 

[miningman]

Grouting is as much of an art as it is a science. Heres a few additional perspectives that might help you make a decision. If the local geology was completely water saturated, and if the rock conditions were extremely permeable or pervious, at approx 0.55 bar you would have grout starting to appear at surface in large quantities. I derive this on the basis of a 6 meter deep hole ( 20 feet) and a hydrostatic pressure of 41 psi per 100 feet. Altho this might seem irrelevant bear with me.

Given that all fluids take the path of least resistance, unless you have some extremely detailed knowledge of the local geology, grout from an injection point 6 metres below surface is far more likely to flow laterally 3-4 metres than it is to flow vertically 6 metres. If you manage to get grout to come to the surface, you have probably achieved as much lateral grout flow as you can get.

Obviously if you managed to seal the leaks at surface, you could apply increasing ;pressure to PERHAPS improve lateral flow, but now the danger is hydrofracturing / hydrofraccing. You call your geology weathered soil. I call it rotten rock. All rock close to surface is weathered to some extent and has major planes of weakness. Ill guranteee that well befor 20 bar , at a depth of only 6 metres you'd be achieving break up of your rock mass and tghe surface elevation will be increasing adequately that your surveyors could measure it and the volume of grout being pumped would start to rise astronomically.

You need to focus as much on the type of cement you will be using. Search for Microsill products. Altho Ive used regular type 10 , Microsill has some major advantages.

 

[Ron]

Compute the lateral soil pressure...the grouting pressure need not be much more than that. Use a 1.5 safety factor and compute the lateral grouting pressure.

 

[karimovic]

miningman, thank you for the sharing your knowledge on this topic.
can you please refer me to an article/code/paper that mentions the required pressure to be used? or is it as you said an art that isn't yet well scientifically researched?

 

[karimovic]

Ron, rock's strength is well higher than the grouting pressure advised by the contractor (20bar), i don't think lateral strength of rock is a major issue in this case.

 

[racookpe1978]

So, he has at least one "hole" 1.5 meter - either in diameter or in wall-to-wall thickness, of an undetermined length and width - in the middle of his intended pad.

For this specific hole, doesn't he absolutely have to drill though it, fill fro below with the top of the both located and vented, then fill at least that hole until the grout is coming up for the vent hole at a 2x air pressure?

 

[karimovic]

Racookpe1978, during cavity probing campaign ( which is drilling boreholes into a 5x5m grid to locate probable cavities, and then grouting them ) we discovered cavities, the biggest one was composed of 2 cavities below each other, the first is 1.5meters in height, and then 20cm below it, we found another cavity of 1m height. volume dimensions can not determined until grouting.

 

[miningman]

Let me make a guess. The local geology is a limestone of some sort?? If I'm correct, this is known as "karst" and Ill predict that the eventual volume of grout will be a very unpleasant surprise to all involved. If I'm wrong, then you've got even bigger problems. Karst geology is known, and large voids are to be expected. If its something more like a sandstone or granite, then grout volumes will be manageable but engineering of the grout program is more difficult. I couldnt provide specific advice unless I saw every bore log. I'm curious , over what surface area are you trying to consolidate.

And no , sorry I cant give articles or codes. There are a number of relevant articles Ive seen over the years. Google the tailings dam failure in the Phillipines about 15 years ago..... thuis was caused by a lack of appreciation of the potential for hydrofraccing due to over pressurization, at relatively shallow depths.

 

[racookpe1978]

The Hoover Dam almost lost everything due to bad grout practices (too little grout, no venting of what holes were grouted, not deep enough holes, not completely filling holes that were found while drilling, not a close enough grid for the holes, etc.)

The dam concrete was fine. It was just found to be "not" as attached to the canyon walls quite as firmly as they expected.

 

[miningman]

Check out the technical papers section of this web page

http://www.haywardbaker.com/WhatWeDo/Applications/VoidFilling/default.aspx

 

[nbr1]

In my opinion, current state of the art is very lacking with respect to grouting pressure assignment. If you read ASCE/G-153-10 "Compaction Grouting Consensus Guide"; it barely touches this subject with respect to a rational grouting pressure specification.

Schmertmann (I forget what year, sometime in the 80's) published 'A Design Theory for Compaction Grouting' which accounts (I recall) for grouting pressure, pier size/spacing, and soil arching effects; however, I've not heard of this being used extensively in practice. I would like to hear if anyone has used this with success; especially in clayey soil sites.

I think with all of the variables, grouting pressure is just a very difficult thing to specify beforehand. In my opinion, it should be recognized that karst areas are generally 'good areas' intermixed with 'poor areas'...; and after an initial grouting pass, these areas can be more fully understood and the grouting pressures refined based on this knowledge. I also like to use the initial pass to identify the lower rock areas, which from my experience tend to be more problematic in terms of grout takes.

If anyone knows of a solid research report on grouting pressures in karst...please let me know.

 

[Prepakt1]

Grout pressure at the pump is not the same as the pressure at the end of the grout tube it can drop by 75% or more depending on length. Specifying grout pressures is pointless. Grout voids at whatever pressure you think is working its all a matter of the site.

Intrusion Prepakt /marineconcrete.com