Conventional Tunneling Turned Unconventional – Author’s Reflection
Here in the United States we used to refer to tunneling by drill-and-blast as “conventional” tunneling, which I guess leaves tunneling by TBM or other mechanized means to be referred to as “Unconventional.” However, with the evolution of TBM technology it becomes more and more rare to do tunneling by drill-and-blast and as such we may want to think about turning the expression around and start referring to tunneling by drill-and-blast as “unconventional” tunneling.
Tunneling by drill-and-blast is still the most common method in the Underground Mining Industry while Tunneling for infrastructure projects is more and more becoming mechanized tunneling by TBM or other methods. However, in short tunnels, for large cross sections, cavern construction, cross-overs, cross passages, shafts, penstocks, etc., Drill and Blast is often the only possible method. By Drill and Blast we also have the possibility to be more flexible to adopt to varying profiles compared to a TBM tunnel that always gives a circular cross section especially for highway tunnels resulting with a lot of over excavation in relation to the actual cross section needed.
In the Nordic Countries where the geological formation of underground construction is often in solid hard Granite and Gneiss which lends itself to Drill and Blast mining very efficiently and economically. For example, the Stockholm Subway System typically consists of exposed Rock surface constructed using Drill and Blast and sprayed with shotcrete as the final liner without any Cast-in-Place Lining.
Currently AECOM’s project, the Stockholm Bypass which consisting of 21 km (13 miles) highway out of which 18 km (11 miles) is underground under the western archipelago of Stockholm is under construction, see Fig. 1. These tunnels having variable cross sections, to accommodate three lanes in each direction and on and off ramps connecting to the surface are being constructed using Drill and Blast technique. This type of projects is still being competitive as Drill and Blast due to the good geology and the need for variable cross section to accommodate the space requirements. For this project several access ramps have been developed to split the long main tunnels into multiple headings which will shorten the overall time to excavate the tunnel. The tunnel initial support consists of rock bolts and 4” shotcrete and the final liner consists of waterproofing membrane and 4 inch shotcrete suspended by bolts spaced around 4 by 4 feet, installed 1 foot from the shotcrete lined rock surface, acts as a water and frost insulation.
Norway is even more extreme when it comes to tunneling by Drill and Blast and have over the years refined the methods for Drill and Blast to perfection. With the very mountainous topography in Norway and the very long fjords cutting into the land, the need of tunnels under the fjords for both Highway and Rail is of great importance and can considerably reduce the travel time. Norway has more than 1000 road tunnels, which is the most in the world. In addition, Norway is also the home to countless hydropower plants with penstock tunnels and shafts which are constructed by Drill and Blast. During the period of 2015 to 2018, in Norway alone, there was about 5.5 Million CY of underground rock excavation by Drill and Blast. The Nordic countries perfected the technique of Drill and Blast and explored its technologies and state-of-the art throughout the world. Also, In Central Europe especially in the alpine countries Drill and Blast is still a competitive method in tunneling in spite the long length of tunnels. The main difference to the Nordics tunnels is that most of the Alpine tunnels have a Cast-In-Place final concrete lining.
In the North East of USA, and in the Rocky Mountains regions there are similar conditions as in the Nordics with hard competent rock allowing economical use of Drill and Blast. Some examples include the New York City Subway, the Eisenhower Tunnel in Colorado and Mt McDonald Tunnel in the Canadian Rockies
Recent transportation projects in New York such as the recently completed Second Avenue Subway or the East Side Access project has had a combination of TBM mined running tunnels with Station Caverns and other auxiliary space done by Drill and Blast.
Advancements in Drilling Techniques
The use of drill jumbos has over the years evolved from the primitive hand held drills or one boom jumbos to the computerized self-drilling Multiple-Boom Jumbos where drill patterns are fed into the on-board computer allowing rapid and high accuracy drilling to a pre-set accurately calculated drill pattern. (see Fig. 2)
The advanced drilling jumbos come as fully automated or semi-automated; in the former, after completion of the hole the drill retracks and moves automatically to the next hole position and starts drilling without the need of positioning by the operator; for the semi-automatic booms the operator move the drill from hole to hole. This allows one operator to effectively handle drill jumbos with up to three booms with the use of the on-board computer. (see Fig. 3)
With the development of Rock Drills from 18, 22, 30 and up to 40 kW of impact power and high frequency drills with feeders holding up to 20’ drifter rods and the use of the automated Rod Adding System (RAS), the advance and speed of drilling has improved greatly with actual advance rates of up to 18’ per round and hole sinking between 8 – 12 ft/min depending on the type of rock and the used drill. An automated 3-boom drill jumbo can drill 800 – 1200 ft/hr with 20 ft Drifter Rods. The use of 20 FT drifter rods needs a certain minimum size of tunnel (about 25 FT) to allow rock bolts to be drilled perpendicular to the tunnel axis using the same equipment.
A recent development is the use of multi-function jumbos suspended from the tunnel crown allowing multiple functions to proceed simultaneously such as drilling and mucking. The jumbo can also be used to install lattice girders and shotcrete. This approach overlaps sequential operations in tunneling resulting in time saving on the schedule. See Fig 4.
Advancement in Explosive Charging Methods
The use of bulk emulsion to charge the holes from a separate charging truck, when the drill jumbo is being used for multiple headings, or as a built-in feature to the drill jumbo when a single heading is being excavated, is becoming more common unless there are local restrictions for this application. This method is commonly used in various areas around the world, with two or three holes can be charged at the same time; the concentration of the emulsion can be adjusted depending on which holes are being charged. The cut holes and bottom holes are normally charged with 100% concentration while contour holes are charged with a much lighter concentration of about 25% concentration. (see Fig 5)
The use of bulk emulsion needs a booster in the form of a stick of packaged explosives (primer) which together with the detonator is inserted to the bottom of the holes and is needed to ignite the bulk emulsion that is pumped into the hole. The use of bulk emulsion reduces the overall charging time than the traditional cartridges, where 80 – 100 holes/hr can be charged from a charging truck equipped with two charging pumps and one- or two-person baskets to reach the full cross section. See Fig.6
The use of wheel loader and trucks is still the most common way to do the mucking in combination with Drill and Blast for tunnels having adit access to the surface. In the case of access via shafts the muck will be carried mostly by wheel loader to the shaft where it will be hoisted to the surface for further transport to the final disposal area.
However, the use of a crusher at the tunnel face to breakdown the larger rock pieces to allow their transfer with a conveyor belt to bring the muck to the surface is another innovation which was developed in Central Europe often for long tunnels through the Alps. This method greatly reduces the time for mucking, especially for long tunnels and eliminates the trucks in the tunnel which in turn improves the working environment and reduces the needed ventilation capacity. It also frees-up the tunnel invert for concrete works. It has an additional advantage if the rock is of such quality that it can be used for aggregate production. In this case the crushed rock can be minimally processed for other beneficial uses such as concrete aggregates, rail ballast, or pavement. To reduce the time from blasting to application of the Shotcrete, in cases where stand-up time can be an issue, the initial shotcrete layer can be applied in the roof before the mucking is done.
When excavating big cross sections in combination with poor rock conditions the Drill and Blast method gives us the possibility to divide the face to multiple headings and applying the Sequential Excavation Method (SEM) method for the excavation. A center pilot heading followed by staggered side drifts are often used in SEM in tunneling as can be seen in Fig 7 for the top heading excavation of the 86th Street Station on the Second Avenue Subway project in New York. The top heading was excavated in three drifts, and then was followed by two bench excavations to complete the 60’ wide by 50’ high cavern cross section.
In order to minimize water intrusion into the tunnel during excavation, pre-excavation grouting is often used. Pre-excavation grouting of the rock is mandatory in Scandinavia in order to address the environmental requirements regarding water leakage into the tunnel in order to minimize the construction impact on the water regime at or near the surface. Pre-excavation grouting can be done for the entire tunnel or for certain areas where the rock condition and the ground water regime require grouting to reduce water intrusion to a manageable quantity such as in fault or shear zones. In selective pre-excavation grouting, 4-6 probe holes are drilled and depending on the measured water from the probe holes in relation to the established grouting trigger, grouting will be implemented using either cement or chemical grouts.
Normally a pre-excavation grouting fan consists of 15 to 40 holes (70-80 ft long) drilled ahead of the face and grouted prior to excavation. The number of holes depends on size of tunnel and the anticipated quantity of water. The excavation is then done leaving a safety zone of 15-20 ft beyond the last round when next probing and pre-excavation grouting is done. Using the automated Rod Adding System (RAS), mentioned above, makes it simple and fast to drill the probe and grout holes with a capacity of 300 to 400 ft/hr. The pre-excavation grouting requirement is more feasible and reliable when using the Drill and Blast method compared to using a TBM
Advancement in Safety of Drill and Blast Excavation
Safety in Drill and Blast tunneling has always been of major concern requiring special provisions of safety measures. In addition to the traditional safety issues in tunneling, construction by Drill and Blast the risks at the face including drilling, charging, scaling, mucking, etc. add additional safety risks that must be addressed and planned for. With the advancement of technologies in Drill and Blast techniques and the application of risk mitigation approach to safety aspects, the safety in tunneling has significantly improved in recent years. For example, with the use of automated jumbo drilling with the drill pattern uploaded onto the on-board computer, there is no need for anybody to be in front of the drill jumbo cabin thus reducing the potential exposure of workers to potential hazards and thus increasing their safety.
The best Safety related feature is probably the automated Rod Adding System (RAS). With this system, mainly used for long hole drilling in connection with pre-excavation grouting and probe hole drilling; the extension drilling can be done fully automated from the operators cabin and as such eliminates the risk to injuries (especially hand injuries); otherwise the rod adding was done manually with workers being exposed to injuries when adding rods by hand. It is worth noting that The Norwegian Tunnelling Society (NNF) issued in 2018 its publication No. 27 entitled “Safety in Norwegian Drill and Blast Tunnelling”. The publication addresses in a systematic manner measures related to the health, safety and environmental management during tunneling using Drill and Blast methods and it provides best practice for the employers, the foremen and the tunnel construction workers. The publication reflects the state of the art in safety of Drill and Blast construction, and it is downloadable free from the Norwegian Tunnelling Society website: http://tunnel.no/publikasjoner/engelske-publikasjoner/
Drill and Blast used in the right concept, even for long tunnels, with the possibility to split the length into numerous headings, can still be a viable alternative. Significant advancements have been made recently in equipment and materials resulting in enhanced safety and increased efficiency. Although mechanized excavation using TBM is often more favorable for long tunnels with a constant cross section, however in case there is a breakdown in the TBM resulting in long stoppage, the whole tunnel come to a standstill whereas in Drill and Blast operation with multiple headings the construction can still be advancing even if one heading run into technical problems.
About the Author:
Lars Jennemyr is an expert Tunnel Construction Engineer in AECOM New York’s office. He has a life time of experience in underground and tunneling projects from around the world including South East Asia, South America, Africa, Canada and the USA in transit, water and hydropower projects. He has extensive experience in conventional and mechanized tunneling. His special expertise includes rock tunnel construction, constructability, and construction planning. Among his projects are: the Second Avenue Subway, 86th St. Station in New York; the No. 7 Subway Line Extension in New York; the Regional Connector and the Purple Line Extension in Los Angeles; Citytunnel in Malmo, Sweden; the Kukule Ganga Hydro Power Project, Sri Lanka; Uri Hydro Power Project in India; and the Hong Kong Strategic Sewage Scheme.