The technology of grouting

For the last two technical talks I have been explaining how, by bolting rock layers around an opening, spraying concrete on the walls of the opening or a combination of both, one can stabilize openings underground. Sometimes, however, the rock itself is relatively weak. (This is a common problem when driving subway systems under major cities, as an example). In these cases the rock won’t have enough strength to hold the walls, until a stronger tunnel lining system is installed. Often that is pre-cast concrete liner segments, but the walls have to be kept open until they can be installed.

Another problem, which is common in almost every underground excavation, is that there is water moving through some of the rocks and, if not dealt with it can fill your excavation very quickly, even if the flow rates don’t initially sound that high. There are two fundamental ways of both increasing rock strength and stopping fluid flow through the rock. The first is to inject some form of grout into the rock, under enough pressure that it will fill all the cracks and then set. The alternative, and becoming more popular method, is to lower the temperature of the rock, so that the water freezes. This increases the rock strength, stops the water flow, and is temporary, rather than a permanent change that can be reversed, after the tunnel is finished. I’ll talk about Ground Freezing in a later post, and in this one I will run through some of the simpler descriptions of Grouting.


When the drillers at the Deepwater Horizon well in the Gulf wanted to stop the flow of oil out of the reservoir into the well, or drilled hole, they pushed a cement down into the well, to fill the gap between the internal casing they were leaving in the well, and the surrounding rock. That initial injection of cement into the well did not work well, and the oil was able to penetrate through cracks, weakness layers or perhaps zones where the cement had not properly set. When the well was finally plugged towards the end of the operation, cement was pumped down the well at higher pressure so that it not only filled any gaps that the oil and gas had been flowing though in the original cement plug, but also moved back into the rock, filling the flow channels that had developed to carry oil and gas from further back in the rock, into the well.

Drilling pattern to inject grout around a tunnel line.

In a grouting operation, that is the goal. Normally a ring of holes are drilled into the wall of the tunnel so that they fan out around the planned tunnel path, and they are about 40 ft long. At this point the cement is brought to the site, ready to be injected. However, it is not just a case of bringing in say 8 bags of cement from the local hardware store. When dealing with the choice of cement, its physical and chemical contents and the pressure at which it will be pushed into the rock there are a number of factors that have to be established first.

The temperature and water chemistry of the surrounding rock are some of the initial critical factors. Changing either will change how fast the cement sets, or if it will. The object in this case is to get the cement to flow into the cracks around the drilled holes, so that the cement will flow to fill those spaces completely, before it sets. But it has to set in a reasonable time for work to continue on schedule. And since water chemistry in a tunnel, and temperature, change – so the mix has to be altered to accommodate those changes.



The next thing that has to be checked depends on the size of the cracks that the cement is being injected into. If the cracks are very thin, and the cement contains particles that are bigger than the crack, then the wall of the opening will act as a filter paper, stopping the cement particles from getting back into the crack and filling it. On the other hand if the particles are too small, then they will not bridge together to block the crack, and stop the fluid flow long enough for the cement to set up. If there are too many large particles then, when they lock together, they leave too large a gap between them, and fluid can still flow, and the rock will remain weak.

Over the years a rough correlation has been developed between how much fluid is flowing though the rock, and the type of rock, and the size of initial particles needed to provide an initial seal of it. But, as with cementing in an oil well, when the first cement injection has been finished, and allowed to set, then the rock is tested to see if the flows have stopped. Very often they have not, though hopefully they have diminished. This is because the first shot into the rock is more aimed at narrowing the flow passages and slowing the flow of fluid through the rock so that when finer particles are used, in secondary grouting, they won’t be carried away into the rock, before they can set up and block the remaining passages.

And so, typically, after the first grouting operation, there will be a second, to further fill the narrower passages in the rock, and those bits not properly sealed by the first injection. The cement grouts also act to give some strength to the rock, since they are filling the spaces within the rock structure with the set-up cement, that has some strength to it.

Rock after grouting (white lines)

However to fill the finer cracks, and to stop the flow cement may be too coarse a material in some of the rocks found. In such a case, then a chemical gel might be injected into the ground to fill those finer cracks. These tend to set up rather more like Jello, and while strong enough to resist water flow, do not usually give any additional strength to the rock.

There is one caution in injecting grouts into the rock that has to be borne in mind (and I know of cases where it wasn’t). Grouting operations force liquid into existing cracks within the rock surface. The liquid hopefully fills those cracks, before it sets, but if it is injected at too high a pressure, then the force on the walls of the crack can cause it/them to grow. At that point the rock will become weaker, instead of stronger, and the section in the tunnel roof/walls that is already open can fall in. Which is not good!

In the main it is not economic to keep injecting more and finer grouts into a rock until the flow is totally stopped. As the flows diminish the costs to stop them rise, and so it is usually the case that the operator accepts a certain small flow rate as the most economic alternative, and makes arrangements to deal with that water as it enters the tunnel. (If not I have seen tunnel floors lifted by the pressure that develops in the water trapped behind them).

Of course, if all else fails, then you can cut a slot into the wall and fill it with cement to completely seal off the excavation – though this is often done with a series of drilled holes, it can also be done with a variety of rock saw.

Grout wall exposed to show the 12-inch thickness and integrity.

Sometimes my predictions turn out to be wrong

Last Thursday I posted the first half of a letter that I wrote to OMNI magazine back in 1979, and in it I made some cost estimates for the potential for a satellite in space beaming energy back to the Earth. I then made some additional cost estimates for solar energy.

On a day when there is growing concern that energy prices are again heading upwards, and with OPEC maintaining control by discussing an increase in production I am going to perhaps weaken your faith in me as a prophet (I consider that OPEC can only continue to maintain this control for another year or so before they too run out of enough oil to satisfy demand – providing the price remains reasonable). I am going to do this by adding the second half of the letter that I sent to OMNI. This is where I became less of a prophet than I had expected, and I will discuss some of those inaccuracies in a follow-on post. But first back to that 30-year old letter:

The next alternative is the biomass option. Firstly have seen figures that it takes 80 gal of gasoline equivalent to raise an acre of corn (including fertilizer) and that 4 gal of gas input to biomass gives 2.5 gal of alcohol out not an equitable exchange. Mr. Pohl's argument that "burning biomass does not (add to atmospheric CO2) ." is specious. Not only does accelerating the decay time increase the rate of change and the volume involved, but it also disregards that portion of the material which turns into humus. I'm sorry but biomass is not a significant option either. Hydro electric and wind power are very site selective and, as a practical but mundane point, can a TV oriented society realistically be expected to tolerate that much TV interference (from windmills). No, while every little counts I'm afraid that this is all this will amount to.

And thus we come to the big four; oil, gas, nuclear, and coal (I'd like to leave geothermal until later).

There is no question that our petroleum based oil and gas is very rapidly diminishing. Price decontrol will have very little long term effect on this situation and recent studies have shown that reserves are often even lower that predicted . It hardly bears repeating that the reason this is not too evident at the pumps is that we now import almost half of what is used. The fuel component is significant in almost all manufacturing and if our suppliers abroad put up their prices (as they will continue to do), then the result of course, is that inflation is virtually guaranteed for as long as this continues.

The Iranian oil stoppage, and the declining levels of the international oil pool all urge that something must be done, in the short term as well as the long.

In this regard we need all the domestic fuel supplies we can develop, for at least the remainder of this century until the time in the next that the SPS or fusion becomes practical. I stress domestic, because the need for supplies is critical and must be guaranteed, and from abroad this is not a sure thing.

This, in turn, casts grave doubts on the economic installation of more nuclear reactors. Figures for economically recoverable domestic uranium seem to center about 380,000 tons, while demand projections for the reactors in and currently planned appear to vary up to 1,500,000 tons, depending on whose figures you believe.

In 1974, when the price of uranium was $7.90, spokesmen for the power companies were quoted as saying that nuclear power stations would become uneconomic if the price doubled . The price is currently $15 and this supply will run out in the near future, putting the price up further . To those who start waving the breeder flag, I would rejoin that it takes 20 years to double the fuel supply, that it will take 10 to 15 years to get one built and that already moves us into the next century.

To be honest I don't know who to believe on gas. In 1977 we had a shortage of gas and thousands of businesses closed. I have heard that this caused a lot of companies to switch out of gas and this resulted in the current surplus, not the fact that we found that much more. In either case it is currently a popular fuel again, yet available data would indicate our supply is even worse than that of oil, a point I will return to later.

So far I have been very negative, not through malice but because energy costs are going up and we need to understand the realistic options that face us in the remainder of this century.

It is common these days to hear cries from Washington at something must be done and conservation is the cry. But after adding insulation to my house, turning the thermostat down to 60 and cutting out pleasure driving I don't see that I can do much more in that regard. The population continues to grow and energy demand per capita will grow with it. To give just one reason, the ore required to produce metals gets thinner every year and more must be mined to give us the same volume of material.

There is also a direct correlation between energy levels and jobs, to prove which, I attach a graph from a paper by Congressman McCormick.

To make this point another way, after that well known actor made his walk along the canyon for the TV cameras in the campaign which stopped Kaiparowits we did not see him in Watts explaining to the young unemployed that his actions helped ensure there would still be no jobs in the 1980's.

I say this not meaning to be facetious, but to point out that those whose major concern is with the environment must accept the social burden which is implied. Those who delay the construction of a power station must accept some of the blame for the resultant rate increase when the plant is built, or the unemployment which will result from the ultimate lack of power if it is not.

What I also am seeking to establish is that we are in a mess and while we need the long term solutions which will perhaps be brought about by fusion or the SPS system we are also in desperate need for some short term solutions as well.

In this regard I would like to take exception to the remarks by Mr. Pohl who writes off coal mining in a short paragraph. I regret this because it is a very common occurrence when one reads reports on the current energy situation by a wide variety of people and unfortunately the attitude it conveys is pervasive. If one might first of all point out the fact that soil is dirty does not stop farmers from growing crops, and the thousands of fatalities a year do not stop Americans from driving cars. The dirty characteristic which is attached to the industry is, regrettable and based on history; more than current fact. It is not true, for example, that strip mining ruins everything it touches and there are areas in Texas and Wyoming, among others which would show that the 1,000 plus dollars put into each acre of reclaimed land have left the land in much better condition than it was before strip mining occurred. This does not make strip mine coal ruinously expensive and once a recognized set of regulations can be established and operated under I would expect that coal mining prices will stabilize. I would point out in this regard the experience of the National Coal Board in Britain would indicate that land can be restored to at least as good a condition after strip mining as it was beforehand.

One must accept that people are killed in coal mines and that much of the coal contains sulfur but surely these should pose challenges to science (as does developing "cheap" solar cells) rather than be shrugged off as absolute disqualifiers. Surely if we can develop robots to operate on Mars we can develop robots to operate within a thousand feet of the ground surface in coal seams and thereby make mining operations safe so that miners aren't killed.

This is perhaps a challenge for the future, however, in the short term while coal mining will produce as much energy as is required of it surveys indicate that, since its major use will be in power generation that to the turn of the century the supply will be demand limited rather than supply limited. Coal mines also take somewhere between 5 and 10 years to develop as do the power plants which must supply them.

Where then can we turn for more answers to the energy problem in the short term and what other techniques can possibly be used in the medium term. I would like to put forward two suggestions. Firstly, there is within the United States somewhere in the region of 4 trillion tons of coal of which proven reserves down to 3,000 ft run at levels of approximately 1,700 billion tons. This coal contains anything from 140 to 700 cu ft of methane per ton, and therefore gives a readily available additional volume of perhaps 500 trillion cu ft of gas. We currently use approximately 20 trillion cu ft of gas a year with conventional resources estimated at 220 trillion cu ft. (Hence my earlier comment about gas supplies.) One therefore would triple the amount of natural gas available if this resource were adequately developed. Since this supply occurs in deposits less than 3,000 ft from the surface it can very easily be accessed for utilization. It is, however, a reserve which is currently not being exploited mainly because of legal entanglements as to who exactly owns it.

It is frequently said that Congress cannot legislate technology however, in the situation which faces us, as we move towards a solution to an energy crisis, this is one instance where a move by Congress in regard to deciding on the exact ownership of this gas would free up a major resource rapidly as a means of supply. It would also, serendipitously, make later mining of the coal a much safer operation. In regard to the CO2 build up, one must accept that there is a problem which must be addressed. But, while we are responsible to future generations we are equally responsible to the current and past generation. Those people now returned have as much if not more right to power in their lifetimes and must also be factored into any solution.

The second option I would propose in relation to more proper use of underground space. This last winter a house built relatively close to mine with approximately the same sq footage was left unoccupied and unheated. The temperature inside never fell below 56 degrees.

As I have mentioned earlier it cost me up to 5,500 kilowatts/month to maintain the temperature inside my house at only 6 degrees higher. There would thus be great savings if a move were made to put at least part of future construction underground. To those who say it is expensive, traumatic, and unsafe, I would add three further facts, firstly, that the underground house cost $32/sq ft to build and this $32 is the same price as current super surface house construction in the Rolla area. Secondly, studies in Texas have shown that school children are if anything less anxious when taught in an underground school than they were on the surface. Thirdly, caves in Missouri survived, with no evident damage the worst earthquake in U.S. history. These structures are of course safer and much better able to withstand tornado, wind storms, ice storms, and other hazards of the weather which are prevalent in these times. The development of this technology is really already with us. It of course can only be applied to novel construction but nevertheless the savings which it would lead to in the long term would be not only in energy but quite frequently also in aesthetics and also in other potential areas since one can for example grow vegetables on one's roof. (Something one could not do underneath the solar collector which would cover my backyard.)

Well that is my response to the energy articles in your last issue. You must forgive me for being a little long winded but the matter is a little complex.

Thank you for your kind attention.

Respectfully I remain,

Yours sincerely,

Well that was the letter - we'll talk about how it really turned out next time.