Suppliers of equipment for drill and blast tunnelling aim to provide the marketplace with continuously improving products. Drill rigs are becoming more cost effective and the quality of the drilled blast hole improving with respect to accuracy. This means in simple terms that blast holes are now produced at lower prices, in shorter times and with improved tolerances. This is not something the manufacturer aspires to on idealistic grounds. It is simply a result of the intense competition within the drilling market, as experienced in most segments of the manufacturing industry.

There are however flaws surrounding the technique that pose a real threat to these continued improvements. One of the most serious is the misuse of the logging function by drill and blast operatives. Such misuses of the improvements will certainly hamper development of new and better products, and consequently threaten the realisation of cheaper and better-excavated tunnels. Being a manufacturer it is felt that it is our obligation to highlight this problem.

The misuse problem in drilling and logging

The use of guidance tools in blast hole drilling has become increasingly popular over the past decade. Its use has expanded as the capability of the technology has improved. Today, most tunnelling projects in the construction industry apply one or another type of guiding tool to improve the accuracy of the location of the blast holes. In parallel to this, the means for monitoring excavated tunnel profiles have become increasingly more sophisticated. Today, there are a number of suppliers that offer electronic instruments that can accurately and with high capacity monitor the tunnel profile, even with dense grids. This activity is often named scanning, whereby over- and under-break can easily be registered at a moderate expense, and is frequently used on tunnelling sites.

These guiding systems for the drill rigs will, more often than not, have logging capacity. This enables the user of the rigs to check how the drilling has been carried out with respect to the collaring position of the blast hole as well of the alignment of the feed and drill string. He can consequently evaluate how well the location of the perimeter holes match the scanning, and he is given an opportunity to adjust his drill plans to compensate for errors and to optimise the drilling with respect to over-and under-break. The principles of using two tools: one for the primary settings and a second for check up as described above, are commonly applied in many other industrial activities where defined tolerances are due to be maintained.

Quality control in steel plate rolling

To illustrate and hopefully simplify the issue of misuse of logging technology in drill and blast tunnelling, an example has been chosen that compares the drill and blast technique and the industrial mass production of steel plates. In steel mills big lumps of glowing steel enter one end of the production hall, and are then squeezed between a series of roller sets where each set of rollers reduces the thickness of the steel plate closer and closer to the final required thickness.

Certainly there should be a notable correlation between the gap of the final roller set and the thickness of the final steel plate leaving the production hall. The thickness of the plate is continuously monitored as it is being produced. Before leaving the plant it’s dimensions have to conform within defined limits. The steel plate manufacturer needs to come as close as possible to the nominal and requested limits. Any excess thickness will result in higher steel consumption, that he has to pay for out of his own pocket. However, if the thickness falls below the set tolerance, the plate is likely to be rejected.

Let us now assume that a steel plate has a thickness that is less than acceptable according to the set tolerance. For one reason or another the plate slips through the quality control at the production facility and is delivered to a customer, who notices the faulty delivery. The customer reaction would be either to reject the delivered plate, or to ask for compensation in some way. He would claim that the steel plate is not the product he had ordered. It is extremely unlikely that he would start a discussion on how the steel mill owner had been running his rollers with respect to the set distance between them and his continuous monitoring of the thickness in the production line.

Discussion

What, you may ask, has this example to do with over- and under-break of the tunnel profile? However, there are some clear parallels. How the tunnel builder utilises the capacities of the guiding system and the logged results should be entirely his or her own responsibility. A log showing that the actual collaring of a blast hole has been placed inside the theoretical tunnel profile, and therefore does not correspond to the computers planned profile, does not necessarily mean that it is faultily placed.

There might be several reasons for the selection of an adjusted collaring position. It may have been impossible to collar at the preset position due to slippage of the drill bit. Alternatively, the driller may have been aware that the true collaring point fell further out than the position shown on the screen. In the latter case the scanning has indicated that the guiding system of the drill rig places the collaring point of a blast hole too far out. Another reason might be that it has proven to be more profitable for the tunnel builder to accept a higher degree of under-break as a result of the blasting activity and later eliminate it by use of other excavation means like hydraulic breakers.

It can be very costly to ensure that no under-break occurs during excavation. It is always profitable to accept a limited percentage of under-break in the first excavation step and then return to remove it by drill and blast or other means like hydraulic breaker or roadheader. The calculation of the under-break percentage (meaning the amount of total tunnel surface inside the theoretical profile) depends on the cost for removal of the under-break against the cost for ensuring that no under-break occurs in the primary drill and blast excavation step. This means that the only party capable of evaluating the amount of under-break that can be tolerated is the contractor himself, who is responsible for the excavation as he has the full knowledge of all costs involved.

Summary

The situation is the same for the manufacturer of the rolled steel plates. He has to accept a small percentage of too thin plates that will be rejected and scrapped. To design a steel plate production line that eliminates the production of any faulty plates is just not profitable. It is up to the steel manufacturer to find a compromise, i.e. reach a tolerable percentage of steel plates that have to be scrapped.

By giving this parallel example from the steel rolling industry it is hoped that a better understanding of how benefits can be derived from guiding monitoring and logging of drill holes.

Logged drilling results should never be held against a tunnel contractor for carrying out faulty drilling, unless special clauses in the contract clearly say so. Normally, it is mandatory for the tunnel contractor to remove under-break at his own expense, unless the contract documents say differently. That expense includes replacement of all support measures like shotcrete that might have to be taken away.

The logging function of the drill rig guiding system is meant to assist optimisation of the drilling process and should not be used for any other purposes.

Related Files
Screen dump of planned position (red) and actual position (blue) of blast holes in the upper left hand corner of a round