Tunnel Construction Methods and their comparison
1. Introduction – This paper gives a general description of the tunnelling techniques such as cut and cover, drill and blast, bored tunnelling and sequential mining construction, reviewed for possible use in various projects.
A summary of environmental merits and demerits associated with these methods are also given.
Above tunnelling techniques are mostly used to construct small tunnels and find their applications in utility projects to a great extent.
2. Construction Methods:
a. Cut and Cover Tunnelling – Cut and cover tunnelling is a common and well-proven technique for constructing shallow tunnels. The method can accommodate changes in tunnel width and non-uniform shapes and is often adopted in construction of underground stations. Several overlapping works are required to be carried out in using this tunnelling method. Trench excavation, tunnel construction and soil covering of excavated tunnels are three major integral parts of the tunnelling method.
Most of these works are similar to other road construction except that the excavation levels involved are deeper. Bulk excavation is often undertaken under a road deck to minimise traffic disruption as well as environmental impacts in terms of dust and noise emissions and visual impact.
b. Drill and Blast – This tunnelling method involves the use of explosives. Drilling rigs are used to drill blast holes on the proposed tunnel surface to a designated depth for blasting. Explosives and timed detonators (Delay detonators)are then placed in the blast holes. Once blasting is carried out, waste rocks and soils are transported out of the tunnel before further blasting. Most tunnelling construction in rock involves ground that is somewhere between two extreme conditions of hard rock and soft ground. Hence adequate structural support measures are required when adopting this method for tunnelling.
Compared with bored tunnelling by Tunnel Boring Machine (see below), blasting generally results in higher but lesser duration of vibration levels. A temporary magazine site is often needed for overnight storage of explosives.
c. Bored Tunnelling by Tunnel Boring Machines (TBM) – Bored tunnelling by using a Tunnel Boring Machine (TBM) is often used for excavating long tunnels. An effective TMB method requires the selection of appropriate equipment for different rock mass and geological conditions. The TBM may be suitable for excavating tunnels which contain competent rocks that can provide adequate geological stability for boring a long section tunnel without structural support. However, extremely hard rock can cause significant wear of the TBM rock cutter and may slow down the progress of the tunnelling works to the point where TBM becomes inefficient and uneconomical and may take longer time than the drill-and-blast tunnelling method.
d. Sequential Excavation Method – This method is also known as the New Austrian Tunnelling Method (NATM). The excavation location of a proposed tunnel is divided into segments first. The segments are then mined sequentially with supports. Some mining equipments such as road-headers and backhoes are commonly used for the tunnel excavation. The ground for excavation must be fully dry for applying the NATM and ground dewatering is also an essential process before the excavation. Another process relates to the ground modifications such as grouting and ground freezing is also common with this method in order to stabilize the soil for tunnelling. This method is relatively slow but is found useful in areas where existing structures such as sewer or subway could not be relocated.
3. Environmental Merits and Demerits – Selection of the techniques to be adopted for construction of a tunnel section shall take into account the nature of the substrata and the levels of the tunnel involved. A summary of the environmental advantages and disadvantages associated with the construction methods is tabulated below:
|Tunnel Construction Methods||Environmental advantages and disadvantages
(on relative terms)
|Cut and cover tunnelling||Disadvantages:
|Drill and blast||Advantages:
|Bored tunnelling by TBM||Advantages:
|Sequential Excavation Method||Advantages:
* Partha Das Sharma; ‘Tunnel Excavation’: https://miningandblasting.wordpress.com/2009/12/16/tunnel-excavation/
*Partha Das Sharma; ‘Techniques of Controlled Blasting’: https://miningandblasting.wordpress.com/2009/09/02/techniques-of-controlled-blasting/
* Partha Das Sharma Blast design for Drifting and Tunnelling with Wedge and Burn Cut
Factors in Controlling Drilling and Blasting Costs in Mines – Discussion
The recent increase in ammonium nitrate and diesel oil prices have invited many operations in mine to re-examine their drilling and blasting methods to find ways of reducing costs. Under the changed scenario, drilling and blasting companies require to optimize their blasting and costs through a blasting optimization review. Some of the questions faced during blast optimization are discussed here.
Many operations only look for less costly explosives, which company feel would do the job. What can be done to reduce costs if we are only looking for less expensive methods for firing the same blasting pattern? Let’s examine the blasting pattern and supplies used and consider the following questions and how they relate to the specification prevailing in the field.
Do you really know what is your exact drilling and blasting cost actually should be? Countless times when we work with operations we find that their actual costs are much higher than their calculated ideal cost. Some times the reason for this is as simple as, for example, they are assuming a 4 by 4 m drill pattern while in actual fact the drill pattern on average is 3 by 3.5 m. This small difference in average pattern dimensions would already increase cost per ton by over 11%. There is Blasting Cost software available, which will accurately determine calculated ideal costs.
What types of initiators are you using in the blast? Are you using a redundant path system for a shock tube initiation system? Is the redundant path really needed or are you paying double for the cost of the initiators to protect the manufacturer for product defects? Are you paying double for initiators to cover mistakes made by the blasting crew? A single path shock tube system is used by many operations worldwide. Why would it not work in your operation?
One of the lowest cost methods to produce better fragmentation is to use the proper delay times in the blastholes. How do you know if the delay times used produce the best fragmentation, back wall control, and lowest vibration in your particular operation. Selection of proper delay times are site specific and depend on the local geology. Do your blasters know how to select the proper delay times? Using the wrong delay times can greatly increase your production costs.
How many cast primers are used routinely in each blasthole? Is more then one really needed in all holes? What size cast primer are you using: 500 gm, 400 gm, 250 gm, 100 gm? What size and how many primers are really needed from a technical standpoint?
What explosive are you using as the main explosive charge? Many operations are using more expensive, more energetic explosives than needed with the same drill pattern they would use for ANFO. All that results is additional throw of the broken for the additional cost.
Are you using cartridged explosives rather than bulk explosives? The additional explosive that you can place into the blasthole because you are filling the annular space that you would not fill with cartridged explosives will allow you to reduce the drilling cost by expanding the drill pattern.
What do you do with the used motor oil that you generate from your equipment? Some companies use the old motor oil as part of the fuel for their ANFO or in the manufacture of their emulsions rather than paying to have the used oil taken away. Research has shown that used oil diluted with diesel oil produces as much or more energy than pure diesel oil in ANFO.
Deck loading is used in many operations to reduce the quantity of explosives per delay in order to reduce vibration. Deck loading increases the time it takes to load a blast, increases the number of initiators and primers needed, and often produces less efficient fragmentation than when using a full column of explosives. In my experience most blasts are not efficient when it comes to minimizing vibration. If the blast efficiency is improved then deck loading may not be necessary. By considering these factors and others, many operations have saved as much as 40% of their explosive costs without effecting their blast performance.
A bigger picture must be considered if we are truly concerned about reducing production costs. Blasting is only the first step in the production process for mines and quarries and the costs of this first step is normally only 8% to 12% of the total costs. The total product costs are composed of: drilling, blasting, secondary breakage of oversize, digging, haulage, crushing. Blasting affects every step in the production cycle. What is important is to reduce total costs. If you try to reduce explosive costs alone you may raise drilling cost per ton, secondary breakage costs, digging costs, hauling costs, and crushing costs. If explosives cost would increase but produce better breakage and cost reductions could made in the other production costs then total production cost may radically decrease.
If savings on drilling and blasting costs are desired it is important to determine the actual costs of the production process. What are the drilling and blasting costs per ton? What percent of the blast is oversized and what does it cost to break up the oversized boulders? What is the average cost per ton to dig and load out the blast? What does it cost to haul the material to the crusher? How many trucks dump at the crusher per hour? With good blasting you may be able to greatly increase the truck count per hour. What are the average crushing costs. How many tons per hour go through the primary crusher? Most operations either have the raw data needed or can easily get this data to be able to determine a total production cost.
The major problem we find is that many operations do not have a good understanding and easily obtainable data on of the costs associated with secondary breakage, digging and haulage costs and how this would be affected by better drilling and blasting procedures. It is much easier to look at explosive costs. To totally optimize costs and to make informed decisions, all production costs must be considered when selecting drilling and blasting methods.
Drilling and blasting is only the first phase of the production cycle but influences all costs for all the other activities.
Characteristics of Rock and Geology influence Surface, UG and Tunnel Rock Blasting Results
1. GENERAL CHARACTERISTICS OF ROCK – The result of any blast is more dependent on the characteristics of the rock than on the explosives being used to break it. The more important characteristics of the rock influencing the blasting result include:
- Tensile and compressive strength,
- Density and
- Seismic velocity (acoustic velocity).
a. Tensile and Compressive Strength – Most types of rock have a compressive strength which is 8 to 10 times greater than the tensile strength. These properties are important factors in rock blasting.
|Type of rock||Compressive strength (kg/cm2)||Tensile strength (kg/cm2)|
b. Density – Rock of high density is normally harder to blast than rock of lower density. One reason for this is that high density rock is heavier to move during detonation.
c. Seismic Velocity – The seismic velocity (acoustic velocity) of the various types of rock varies from 1500-6000 m/s. Hard rock of high seismic velocity will shoot more easily, specially when explosives with high velocity of detonation are being used.
|Type of rock||Density (kg/dm3)||Seismic velocity (m/s)|
2. ROCK STRUCTURE – The planning process should include a survey of the rock structure and other rock characteristics so that the drilling and loading pattern and direction of advance can be optimized as far as possible.
The rock structure included cracks and fissures and other zones of weakness.
Two expressions commonly used to describe the rock structure are ‘strike’ and ‘dip’. Strike is the horizontal direction of the structure on the rock surface. Dip is the angle of the structure relative to the horizontal rock surface.
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