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.

Flexible roof support systems

(Note this is part of the series that usually appears on Sundays).

In the last Tech Talk, I discussed the use of rock bolts as a way of building, from relatively boken bits of rock, larger packages that could brace themselves against one another. In this way as they moved into the underlying opening they would act in the same way the blocks of a Roman Arch, and would become self-supporting. I have crudely tried to show the blocks with this figure:

Block building using rock bolts (after Jack Parker).

The intent is to create a compression zone just behind the face, and this can be shown with a photoelastic model, where a set of bolts is used to generate a central compressed zone. The black lines show zones of different compression, and while the bolts are tensioned, they compress the material between the ends.

Photoelastic model of bolt strengthening (after Lou Panek).

Using that model also allows me to point out the snag in just using bolts. You can see the small areas of tension between the bolts, where the compression cones from the plates has yet to intersect. Well if there are small loose pieces of rock there, they will fall out. And over time this “raveling” will get worse, so that any rock at the edges of the compressed pieces that crushes can also fall out. Over time this may mean that a critical block is no longer held properly in place, and it may collapse, bringing down the entire roof.

But when that raveling started, it was very small, and only the weight of the individual small pieces caused them to fall. This can be stopped, therefore, from happening by putting a layer of chicken wire (so-called because we used to use it to build chicken coops) between the bolts. This has no great strength in itself, but it holds the small pieces in place, so that they can’t fall out, and if they are still there, then the small pieces above them can’t fall out either, and so with a relatively weak addition, we gain longer term strength.

The snag, however, in today’s society, is that putting up chicken wire between bolts – since it has to be inserted between the bolt plate and the wall, is labor intensive, and slow. So, perhaps we can replace this thin metal support with something else – how about spraying on a thin layer of concrete?

The concrete can be kept relatively dry, and accelerators can be added to it, so that it can set very quickly, and in this way it is possible, using compressed air, to apply a layer of concrete up to about 6-inches in thickness to the wall, almost as a single layer. (There are application skills that are needed to do that over one’s head). Now we have build a relatively solid wall that the small stuff can’t get through, and the natural arch can build in the surrounding rock.

Applying shotcrete with the operator well back from the face (Evert Hoek)

(As a passing comment for those of a technical bent, concrete sets up as a relatively rigid liner and to get the rock walls to move and support themselves you shouldn’t therefore apply it too soon after making the opening, but if you have to, because the rock is really bad, you might put wooden strips in the shotcrete to give compression members so that the support can yield).

It is not, obviously, quite as easy as it looks, and it has the snag that after you have applied a layer, it is hard to tell what you have covered up. (Though I have only heard of one job where it was stripped off and re-applied). Questions of chemistry, and operator skill all play into getting it put on correctly, and if you are covering over a wet rock, you should allow a path for the water to get out, otherwise the shotcrete might peel off after a while.

Drains in a shotcrete wall. (Evert Hoek)

There is one further thing that can be done, and that is to add small fibers of steel or fiberglass. The problem with concrete is that it is not that strong in tension, and when the tunnel moves there can be some pull or bending of the liner, that could allow it to crack. But by putting less than 5% fiber into the mix, a much stronger layer can be created, especially if a small amount of silica fume is added to the mix, to both help lubricate the mix, and also to improve bonding.

Wires used to reinforce sprayed on concrete (shotcrete).

When this idea was first introduced it met with a lot of cynicism. And obviously it is not the answer in all cases (swelling and plastically deforming rock in particular can cause problems). But the story that I used to tell was of the operation on the West Coast. They were tunneling under a relatively poor sandy rock, that would not stay up. They had strong steel girders for support, and wooden lining between the arches, and still the walls were deforming into the opening.

Then, almost in desperation, they tried spraying the walls with shotcrete, before putting in the arches, and then spraying over those. After a while they increased the spacing between the arches. Then they replaced them with lighter arches, then they continued without arches.

The major advantage that shotcrete provides, particularly when it is combined with rock bolts and the judicious use of steel arches, is that of flexibility. As I mentioned in an earlier post, when you used to drive a tunnel under a harbor, for a certain metro system, you designed the tunnel for a certain estimated condition, and ordered the steel girders ahead of time. If conditions got worse, then you were out of luck, unless you were a lawyer.

However, with the shotcrete, bolt and arch system, the contractor, the owner and other interested parties can have their engineers meet about every 100 ft of tunnel advance and decide if the conditions are getting worse. If so then the shotcrete can be sprayed on a little thicker, the bolts moved a little closer, or light arches inserted. And the converse can also occur, with less support if it is not needed. This flexible approach became known as the New Austrian Tunneling Method, and while it was for a while very controversial, is now much more widely accepted.

Driving a tunnel and holding the walls with bolts

Last week I wrote about the development of steel girders, and how they were designed to hold the immediate roof above the tunnel, but not the weight of rock all the way to the surface. By allowing a natural bridge of rock to develop around the opening it is possible to lower the amount of support that the rock actually needs. This takes a bit of time, but if you wait a short while before putting in a support, the relaxation of the rock around the tunnel into the open space will generate a zone of compression in the rock around the tunnel in much the same way as building a Roman bridge relied on the compression of the blocks around the opening to mutually support one another, and the load applied to the bridge. Because of this, the amount of support that is required can be reduced considerably from that needed if the tunneling team were to try and hold the rock wall in its original space. (For those of a scientific bent that want to look at this in more detail it is known as the “Method of Characteristic Lines” or the Convergence-Confinement Method and Charles Fairhurst, was an early proponent).


The problem that arises, with putting large rigid steel girders into the tunnel to hold it up is that these tend to be rather stiff and unyielding. One way of getting around this was to put the bottom ends of the arch girder into a frame where the “H” section of the girder had a wooden plank in the lower end of each side of the H. They were held tightly by metal straps, but as the weight of the overlying rock continued to increase as the walls moved into the hole, the girder would slide between the two pieces of wood, or stilts, yielding at a set load, and allowing the walls to deform and build up a natural arch.

Arch with stilts (Deputy’s Handbook).

But the design still had to estimate the size of the girders needed so that they could be on hand as the tunnel was driven, and if the design proved wrong, then usually the discussion ended in court and a lot of productive time was wasted. A more flexible method of holding up the walls was needed, and it came from the combination of two different rock support systems.

The first of these is known as a rock bolt. It has been suggested that the first rock bolts were just rods of wood that were stuck into holes in the roof, and wetted, so that they swelled and gripped the rock. While these worked passively (in other words they did not apply support to the rock until the rock moved, and their presence and acceptance of load then began to resist the movement) they were not, initially, seen as a great help in helping develop the natural tendency of the rock to hold itself. Now there are whole books that have been written on the use of rock bolts, cable bolts and cable trusses (a combination of the first two). I may offend some purists here, but I am going to chop through a lot of that development, and simplify a whole lot in what follows.

If a rock bolt could be inserted in a hole in such a way that it squeezed the rock along the borehole, then perhaps it could be used to build larger blocks of rock, from the rubble around a tunnel, and thus build the bricks, that would lock together as the rock moved into the tunnel, building that natural arch. To generate the compression in the rock, the rock bolt was designed in two main parts, a threaded rod that ran from the mouth of the hole, to the back, and an anchor system which, as the rod rotated through it, pulled a small wedge down into the anchor section, which was spread to grip the rock, and then resisted further pull down the hole, so that the rod went into tension as the wedge tried to advance further, and the rock between the anchor and the bearing plate on the end of the hole, was compressed.

Basic components of a mechanical rock bolt (after DSI )

There are a lot of things that can go wrong with this concept, and in the 1970’s when bolts in a mine were tested the vast majority were not working as they were supposed to, and as a result a regulation was passed that a mine should regularly test these bolts, by putting a torque wrench on the installation bolt head of a representative sample every day, correcting any with a problem. The torque would be a measure of the load the bolt was carrying, and how effectively it was holding the rock together.

This was an expensive requirement and alternate bolt designs were looked for, to find a way of installing rock support that would not need this testing. And the inventors went back to the idea, which I mentioned, of holding the rock along the full length, rather than just at two points. (I have at times suggested that this was looking for a way of using the “nail” idea, rather than the screw – nails are generally cheaper). There were two ideas that came along, The most popular was known as the “Split-Set” and was invented by a colleague of mine, Jim Scott. The idea was very simple, in essence take a steel pipe, just larger than the size of the hole being drilled, cut a slot along the length. Then compress the pipe and stick it into the hole, with a bearing plate on the bottom. The released pipe will grip the rock along its length, and provide immediate resistance to motion. There are two things that made it as successful as it became, internationally. The first was that it could not be tested conventionally, and thus the testing crews were not needed. But while that got the bolt an entry into mines it was its success in holding the roof in a simpler, yet cheaper way, that led to its widespread adoption.

How a split-set works

The alternate method was to fill the gap between the steel rod and the rock wall with a resin. This became known as resin bolts, or full-column rock anchors, and there have been a wide number of different designs. Very simplistically a hole is drilled, and cartridges containing a resin and a catalyst are inserted into the hole. Then a reinforcing rod is inserted into the hole, and spun as it is pushed to the back of the hole. This mixes the resin and catalyst so that the resin sets very quickly, and grips both the steel rod and the rock. It works well in rocks where, for example, the chemistry of the ground water might corrode an unprotected bolt.

How a resin bolt works

One can thus go into broken rock, and by using bolts to build larger blocks of rock, have these intersect one another and “build a bridge” to hold up the overlying rock. The problem comes with the raveling or crushing at the edges of these blocks, and the slow failure of the tunnel surface. This is now solved with shotcrete – but I’ll talk about that, and putting girders, bolts and shotcrete together in a later talk.

Supporting underground tunnels

With thoughts of the miners in Chile, and suggestions that a rescue tunnel will also be started to try and drive down through the broken rock, I thought I would write a little today about holding rock together. For those of a slightly cynical nature, I recently used the example of the Forth Road Bridge, in comparison with one of the bridges that the Romans built in France. The latter is over 2,000 years old and still standing, the former was built in 1964 and is now being scheduled for replacement, since the cables in the old one are corroding and snapping. The point of the comparison being that if you know what you are doing with rock, you can build a structure that can carry load for a long time. (And if you think about it, the rock that the Romans built with is bits of broken rock, rather than the solid structure that a tunnel starts out drilling through).

The Roman Bridge at Chaves (James Martin )

When man first started digging into rock, whether to get flints, create shelter or extract some coal that would burn, it was a relatively slow process. The miner relatively slowly dug out the rock, giving the loads around the hole some time to redistribute, and small holes could be made that would be stable for a long time. That is only a generalized statement and not always true, there are several factors that limit its validity. The first is water. Of the underground hazards water is one of the worst. In its most visible form it can fill the hole, and drown all those in it. There have been many disasters where water has invaded a mine, either from the surface, or from nearby underground workings. (For example Quecreek).

But water has a more insidious role underground, travelling into the mine with the air that the miners must breathe. Underground the ground temperature stays quite constant, and for many mines nearer the surface, the temperature can be quite cool (one I was in recently was at 58 F). So in the days of high humidity of the summer, the moist air moves into the mine, meets the cooler rock, and the moisture condenses onto the rock. It soaks into the rock, and usually weakens it – some shales (the more common rock in coal mine roof) will lose 60% of their strength when they get wet. And it is often why there are more accidents from roof falls in the summer – while in the winter it is more the time for gas explosions – another story).

The second problem that affects the rock relates to the number of cracks in it. If you drive along a road and pass through a road cut where you can see the cracks that develop around individual lumps of rock. Some, the bedding planes, are formed when the rock was first laid down as a sediment, some were formed as the rock was twisted and distorted over geologic time, or during the time that it changed from the sediment into the rock structure encountered today. And some cracks are made as the opening in the tunnel is made. We want to break the rock in the tunnel face into small pieces that we can pick up and carry away, and so we place explosive in known patterns of drilled holes in the face that will break the rock into bits, when the explosive is set off. Generally the charged holes are set off in a sequence, so that after breaking out the middle of the face, successive rounds (they are set off with timed detonators) will blast successive layers of rock into the opening until the desired shape has been removed. In the process some cracks from the outer ring of blast holes will extend out beyond the intended wall of the tunnel and into the final wall. (Seems to happen more on Mondays and Fridays for different reasons).

So there are several concerns that face an engineer that is going to try and drill a tunnel through rock, whether it is solid or already broken into boulders and smaller pieces. The first is to get some idea of the general strength of the rock – designs that work in something like a granite won’t work in a very weak shale, for example. Once the rock strength is known, then the amount of cracking, either natural or man induced needs to be found. There is a very simple way of doing this that a group at Urbana/Champaign developed called the Rock Quality Designation (RQD), under Don Deere to simplify how it is measured, you drill a core through the rock layers that you want to drive the tunnel under. You recover the core and measure the core lengths that are more than 4-inches long. That total value, divided as a percentage over the length of core recovered gives you the RQD. Over time (it is now 40-years old) it has been shown to give a very good first estimate as to how badly broken the rock is, and it is used in many design programs to decide how best to hold the roof up.

When working out how to hold the roof of the tunnel up, the engineer knows that he is not trying to hold the weight of all the rock between the tunnel and the surface. The work that most of us used to refer to as the basis for the support of the tunnel was written by Karl Terzaghi . Again, to simplify a relatively complex subject, he came up with a simple method of classifying rocks so that, the designer of the support would know how much rock load from above the tunnel, the supports would have to carry. And very often it was only a small additional amount above the height equaling the width of the tunnel. (The presence and actions of water being the main factor that would make it a lot worse).


Karl was starting the knowledge base that now allows engineers to classify rock and thus design the tunnel supports before the tunnel gets started. It was only a start, however, because back in those days (beginning in 1925) most of the tunnels were supported with large steel arch girders. Because those had to be ordered and delivered before the tunnel was started, getting that size wrong could be very expensive, and there have been many lawsuits as to whose fault that was. (Very awkward, for example, if the tunnel is half-way under a harbor when you discover the steel beams aren’t big enough).

Possible heights of overbreak that have to be supported over the tunnel

Since then a new method of support, which works more on helping the rock to support itself, along the lines of the Roman arch bridges, has been shown to often be more effective. Although it has been generally more effective, and more flexible, it has become more popular, but I will write about it, and the change from steel arches to sprayed on concrete (shotcrete) and rock bolts, next time.