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.