by Olle Karlsson, Manager Ground Support, Atlas Copco Rock Drills AB

The primary aim of a tunnelling rig is to drill high quality blast holes, quickly, safely and economically. But now drillers are discovering that certain production rigs can also be the perfect partners in rock reinforcement.

More and more tunnelling is now being conducted in soft rock and soil - geological conditions that require considerable ground support and different reinforcement methods from those used in hard and competent rock.

During the planning stage, today's tunnel designers attempt to predict the various types of ground support that will be needed by comparing geological data with experiences derived from similar projects or by applying standard classification systems.

The information is used to compile a geo-mechanical classification which forms the basis for the various types of ground support methods to be applied throughout the project. (Fig 1.)

Ground support is time-consuming and often heavy and hazardous. It also requires different types of bolts and bolting methods. But certain types of tunnelling rigs designed primarily to drill blast holes are becoming key players in the process.

A good example

Boomer rigs from Atlas Copco, for example, are versatile and easy-to-operate units which are not only excellent tools for blast hole drilling, but also offer solutions for different types of safe, quick and simple rock reinforcement. This includes the installation of various reinforcement tools such as different types of radial bolts, pipes and bolts for spiling and for forepoling of the tunnel face.

Equipped with a few standard options, the Boomer rig is easily converted into a very productive bolting unit. The service platform is a must and together with a Swellex pump and an installation arm, including the chuck, the rig is able to install Swellex bolts of any type.

Fig 2. The Boomer rig is capable of installing self drilling rock bolts in most applications such as radial bolting, spiling and forepoling and face stabilization.

SDR most favoured

In situations where long bolts are required, connectable Super Swellex or Self-Drilling Rock bolts can be used with the RAS (Rod Adding System) and the BSH 110 (drill steel support). Conventional bolts, including Swellex, cannot be used in cases where the drill hole collapses as the drillstring is withdrawn. In this situation, the use of SDR (Self-Drilling Rock bolts) has gradually become a favoured alternative. (Fig 2.)

The most common method is still radial bolting in which most types of bolts are used. And here, the Swellex bolt is undoubtedly the fastest and most efficient way of achieving immediate rock support.

In this method, the hole is drilled and the bolt is inserted with the installation arm and the chuck. It is then inflated until the pump stops automatically, signalling that the bolt has been fully set and is sufficiently strong and safe.

The SDR type is preferable in poor ground conditions. It is an externally threaded pipe or bolt, equipped with a sacrificial drill bit which can be matched to the prevailing rock and soil conditions. (Fig 3.)

Easily extended

The bolts range from 2 -12 metres long and by selecting a common thread type, such as R32 or R38, they are readily matched to standard shank adapters. When needed, these can be extended to practically any length by just connecting another bolt section to a coupling with matching load-bearing capacities.

The SDR system is designed to be used in combination with grout and this can be done in several different ways. The simplest way is to install a full round of bolts, kept in place by retainers if needed, and then to inject grout through the hollow bolt, via a special adapter, and into the surrounding rock.

Fig 3. Principle of the SDR, Self-Drilling Rockbolt.

Alternatively, drilling and grouting can be performed simultaneously by placing an adapter and swivel between the shank and the bolt. The grout adapter is fed with grout which has a cement-water ratio of 0.35 to 0.45. As it is impossible to avoid grout spillage, the Boomer rig should not be used for this part of the work cycle.

When tunnelling in poor rock conditions there can be a need to stabilise the face to prevent collapse. If non-collapsible drill holes can be achieved, Swellex bolts are again the obvious choice. Depending on the rock conditions, new bolts are installed every third or fourth round. Furthermore, as the Swellex bolts are easy to cut, they are less harmful to digging machinery.

Quite often in soft rock, especially in soil, there is a need for installing an umbrella-shaped hole pattern to protect workings at the front - so-called spiling or forepoling. The most common method is to install a number of SDR bolts in a fan pattern just outside and around the tunnel periphery.

When grouted, these form an arch, reducing a substantial part of the rock stresses in and around the face. This fan drilling procedure is repeated as often as required with a certain overlap between the fans.

A case for Odex

When heavy duty spiling is required, Atlas Copco can offer the unique ODEX method adapted to the Boomer, known as the Boodex method. This consists of the Boomer with its feeds adapted to the top hammer ODEX drilling system with an ODEX head and hitting shoe welded to a pipe 114 mm in diameter. The pipe has longitudinal slots or holes along its entire length. The ODEX head, the pipe and the drillstring are placed in position on the feed and connected to the COP hammer by standard R or T threads.

During drilling, the pipe follows the ODEX head into the drill hole to the desired depth. Once in place, the annular space between the pipe and the ground is filled under pressure with grout, which seeps into the surrounding rock/soil through the slots and holes in the pipe. A rigid core builds up inside the pipe and when all the holes have been prepared and the grout is cured, it creates a strong arch-shaped element.

Major step forward

By being adapted to the use of Self Drilling Rock bolts, the new generation of Boomer rigs represents a major step forward in terms of versatility. Therefore, by choosing Atlas Copco as a supplier and partner, tunnelling contractors gain access to equipment, accessories and competence in tunnelling as well as ground support, and not only in hard rock but also in very poor and difficult geological conditions.

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