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Underground excavation dates back to the days of cavemen, who used pagan methods to expand the caves for creating shelters for dwelling. However, in the modern context, the application range is far more varied. Complex excavation techniques and ultra modern equipment now execute such works.

During the past decade, the industry has witnessed that most leading manufacturers are offering a series of new generation equipment with inbuilt systems and features to provide high productivity, high quality and accuracy of work; operational ease, ergonomically designed operator cabins, safety and lowered OPEX costs. The latest trend in this equipment category also provides viable alternatives to the traditional drill and blast method in practice since inception, wherever favourable geological conditions exist.

India has also had its own share of underground excavation in infrastructure and mining spaces. Going forward, new initiatives by the government to accelerate infrastructure development in the country substantially increase mining outputs and announcing metro rail projects in Tier-II and Tier-III cities are positive indicators for underground excavation equipment suppliers.

Changed perceptions
A tunnelling project is always a race against time and cost. You have to get the project right and get it right from the start. This requires not only deep knowledge and years of experience in underground construction, but also a proper range of equipment that works, whenever and wherever needed.

?Changes in rock deformations, stresses, strains and mechanically induced pore pressure in an undrained condition, during excavation can be foreseen through instrumentation,? says BV Raisinghani, President (Purchase), Jaiprakash Associates Ltd.

Unpredictable Himalayan geology is a major constituent of underground excavation projects. This type of uncertainty prevails from Sikkim in the East, all the way across to Jammu and Kashmir in North India. As a result, in many projects, use of New Austrian Tunneling Method (NATM) has to be adopted to complete the job. NATM is a slow and cumbersome process involving one support at almost every metre of tunnel.

Viable challengers
Geotechnical surveys and geological profiling is a standard prelude to conceptualising underground excavation projects. The collected data provides an idea and locations of the class of rock, soil, sub terrain water bodies, cavities, geo-fault zones and other relevant conditions, one is likely to encounter during excavation. This forms the basis of selecting the most appropriate excavation equipment for a given location. Traditionally, Drilling Jumbo is universally applicable excavation equipment. This versatile drilling equipment can be used irrespective of the geological conditions. However, the application of this equipment is synchronous with the Drill and Blast Method (DBM) of underground excavation (refer box for the steps involved). Progress is slow while using DBM and various equipment categories in large numbers are required to expedite this process. The average progress ranges from 90 to 120 m length or a record 100,000 cu m of excavation per month, under favourable geological conditions. On the other hand, in case of Rock Class 3 to 5 to loose soil (bad to worse to worst), the progress is as low as 15 m or 10,000 cu m per month. Depending upon the dimensions of the excavation, single, double or triple boom jumbo of small to large working envelopes are deployed. Modern jumbos are equipped with numerous productivity enhancing, tunnel profiling laser guidance, safety and other features to meet with the diverse range of work requirements.

Road Header is the best breed excavation solution for tunnelling, mine development, and mine production for soft to medium strength rock formations. Performance prediction is an important factor for successful road header application and generally deals with machine selection, production rate and bit consumption. Among many different parameters, brittleness is also one of the material properties related to breakage characteristics and can be used as a cuttability parameter from a mechanical excavation point of view.

Global trends, environmental restrictions, and other market conditions in the last decade forced mining companies all over the world to be more profitable and competitive. One of the ways to be more profitable operations is to use mechanical miners such as road headers, continuous miners, impact hammers and tunnel boring machines, for ore extraction and excavation of development drivages. Since these miners allow for continuous operation, it is expected that mechanisation of mines with mechanical miners would increase productivity, decrease production cost and improve competitiveness, which can lead to move away from the conventional drill and blast method. Road headers are a unique class of mechanical excavation machines that break rock by utilising tungsten carbide tipped cutting tools laced in a specific geometry on a rotating cutting head. Road headers, first introduced and developed for mechanical excavation of coal in Hungary in the early 1950s, are the extensively used partial-face excavators in the mining industry, particularly in coal mining and industrial minerals. Road headers are versatile excavation machines used in tunnelling, mine development, and mine production for soft to medium strength rock formations. They are favoured in mining operations due to a high degree of mobility, flexible cutting profile (horseshoe), and selective mining, providing immediate access to the face and the capability to cut medium rocks with a compressive strength of up to about 100 MPa.

In civil construction, they find wide use for excavation of tunnels (railway, roadway, sewer, diversion tunnels, etc) in soft ground conditions, as well as for enlargement and rehabilitation of various underground structures. Their ability to excavate almost any profile opening also makes them very attractive to those mining and civil construction projects where various opening sizes and profiles need to be constructed. Other advantages such as low life cycle costs and combined excavation, muck collection and loading capabilities, also make use of these machines more desirable for contractors. One major fall out is the huge amount of dust generated in this application, for which adequate de-dusting units are necessary to extract dust from the work location. Another viable alternative to DBM is Crawler Type Tunnel Excavator. These versatile machines can be equipped with a variety of application-specific digging/cutting attachments and articulated boom options. Such machines can also use hydraulic rotating cutters, which are highly productive.

It may be recalled that traditionally the tunnel heading and loading the collected muck into the dump trucks or rail wagons has to be done by two separate equipment. This is a time consuming activity and a major part of the excavation cycle. To challenge this difficult task and substantially reduce time, use of a combined Tunnel Heading and Loading Machine is one of the best options. Secondly, this type of machine is suitable for application in smaller diameter tunnels wherein separate equipment like excavator and loader cannot be accommodated.

In large size underground excavations, example: underground caverns for strategic storage of petroleum crude and gas, where the size is 30 m wide, 200 m length and 30 m height or more, benching becomes essential. The excavation commences at the crown of the cavern and is progressively benched downwards to achieve the required height. Instead of utilising the standard drilling jumbos, using multi-axis, crawler-mounted benching drills is faster and viable option. This equipment provides the flexibility to drill vertically or horizontally or angular and even at the lower end near the floor which is inaccessible to drilling jumbo. Using such equipment, productivity as high as 100,000 tonne per month is achievable.

Loading and haulage of excavated material is another challenge to be addressed. High capacity (bucket size 6 cu m) wheeled loaders with advanced productivity and safety features for large tunnels, Low height Loaders or Scoop trams, Low height Dumpers (LHD) and Articulated Dump Trucks (ADT) come to the rescue. These are all specialised, high tech but expensive equipment. However, their contribution to productivity more than offsets their acquisition costs.

?The world of underground mining is becoming increasingly challenging. Issues associated with deeper ore bodies, increasing costs and the drive for greater social responsibility continue to pressure the viability of many operations. In response to this, manufacturers are developing tailor-made equipment to provide a comprehensive range of automation solutions that deliver project-specific safe and productive mining. Underground drilling rigs, tunnel boring machines, road headers and multi-utility vehicles have gone through major design changes,? informs Raisinghani.

Conclusion
Industry veterans opine that underground excavation or tunnelling is An Art! Knowledge-based decisions and prudent selection of alternative equipment to be deployed under various conditions, enables the successful execution of the projects, even under the most difficult and unique challenging conditions.

UG excavation applications

• Hydro Electric Power

-Head Race Tunnel
-Tail Race Tunnel
-Power House
-Vertical shafts

• Transportation

-Mountain Roads
-Railways

• Urban Infrastructure

-Metro Rail
-Utilities
-Water management
-Sewerage
-Storm water drains
-Others

• Irrigation and Water Supply

-Inter connecting reservoirs
-Long distance water supply
-Irrigation channels

• Petroleum Products Storage

-Caverns for crude storage
-Caverns for gas storage

• Sub-aqueous crossings 

-Under sea
-Under rivers
-Dredging

• Mining 

-Coal
-Oil shale
-Uranium
-Other geological materials

Occupational Hazards

• Unpredictable geology
• Sub terrain water bodies
• Hot or cold water ingress
• Combustible gases
• Loose earth or rock collapse
• Swell or Shrinkage
• Environmental conditions
• Safety

UG excavation basic work sequence

• Drilling
• Evacuation:

Personnel & Equipment

• Blasting
• Defuming
• De mucking:

Loading & Haulage

• Scaling
• Rock Bolting
• Shotcreting
• Supporting
• Auxiliary job-specific steps