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Underground excavation can be broadly classified into tunnelling method and cut and cover method. Construction machinery used for both type of excavation is completely different. Tunnelling machines are used for tunnelling/boring and find application in metro rail tunnel, urban water and sewage pipelines, electricity cables etc. The major equipment deployed are tunnel boring machine, micro tunnelling machine, pipe pushing, and directional drilling machine. Excavating machinery are generally used for cut and cover application like installation of shallow utilities, metro station, oil and gas pipelines, water and sewage pipelines etc, and machinery deployed are excavator, trencher etc.

Pipeline excavation in rocky condition can be done either by an excavator with rock breaking attachment, drilling and blasting, or using a chain trencher. Trenchers are deployed for installing large scale pipelines and give a linear cut having width anywhere between a couple of inches to a few feet and depth of up to 24' based on the machine model and configuration. A trencher can easily excavate in soil, hard rock condition or in permafrost and desert areas. It is important to evaluate the competing technologies based on total cost of the project from excavation to back filling and time taken to complete the same. Some of the advantages of trenchers are:

• Productivity advantage - Trencher can give higher speed of execution. A large trencher (90-120 tonne class) can substitute up to 15 excavators (20-25 tonne class)
• Minimise excavation volume - Less cubic metre of excavation per linear meter of trench. Reduction in excavation area by at least 30 per cent compared to an excavator
• Lower operating cost per linear metre - Digging cost per linear metre with trencher is about 30-35 per cent lower than that of a rock breaker
• Maximise re-use of trench spoil - Spoil generated is typically down ?? and can be easily used for backfilling.
• Fewer machines working at site - More productivity compared to excavator leading to lesser machine working at site.
• Environmental friendly - Less machine means less CO2; also minimises negative environmental effects of truck haulage and waste disposal
• Reduce or eliminate risks and costs arising from use of explosives
• Safer site -No swinging arms unlike excavator. Also moving machinery, less truck haulage and handling of waste material; overall a safer site.
• Less noise at the job site - Trencher produces less dust, vibration and noise than other methods.

Tesmec has about 30 years background in trenchers for rock and hard rock soil.
Tesmec trenchers are designed in order to allow the user to achieve high performances in any kind of soil, especially rock and hard rock.
High performances of a trencher means:

• High productivity: As general information, the productivity depends on the soil conditions and the material of the road surface. A high level of productivity can be achieved thanks to a powerful engine.
• High directional control: The trench must follow a linear path, avoiding any undesired bend even in presence of boulder in the soil. This result is obtained by the use of the tracks instead of the tyres.
• High stability: The trench must not have any vertical movement or jumping during the digging operations, even in the presence of hard rock. This result is allowed by the use of a well-dimensioned frame and a well-balanced weight of the structure.

Tesmec Trenching equipment was specifically developed as economical and time-bound solution for opening ditches for large diameter pipelines. This equipment can operate under extreme conditions: with its monolithic frames ensures reliability, rollover protection structure cabins provide operator safety, and in-house developed, TrenchTronic digital control improves ease of use and performance. The digital control can be stated as a revolution because it also brought together the Remote Monitoring (Re.M) and TrenchIntel 3DGPRS guidance, allowing automatic guidance and digging, remote troubleshooting and fleet management with relevant benefits in precision of the work, productivity and jobsite management. The new range of Tesmec premium selection teeth is specifically focused on increasing tool lifetime while maintaining a high level of penetration and abrasion resistance: Tesmec special carbide tools are more durable than standard products and allow reduced diameters that results in lower vibration levels with benefits on machine reliability.

We would like to present three jobsites across the geography where Tesmec trenchers have worked and made a difference.

Long distance pipeline in Chile
In 2014, two 120 km pipelines were constructed to deliver desalinated water to the copper mines in Chile. It is a complex project given the distance between the port and the mines combined with the remote location and high altitude; also the region is world's driest desert. Three Tesmec 1675 chainsaw trenchers with 10' depth and 60? width were deployed and backed with after sales service and spare parts from Tesmec factory. The machines have been digging satisfactorily ever since and the project is expected to commission in 2017.

Entrenching great performance in Mexico
In 2014, Mexico government liberalised the energy sector, thus launching many relevant energy projects. Among these, Los Ramones natural gas pipeline integrated project for Gasoductos del Noreste (GDN) is responsible for delivering up to 2.1 billion cubic feet a day of gas from shale gas locations in Texas, US, to Guanajuato, a city in the centre of Mexico, with a total length of about 800 km. It's the largest infrastructure of this kind in Mexico.

Two Tesmec trenchers were involved in the second phase of this project, delivered by Grupo Desarrollo Infraestructura S.A. de C.V. (GDI), one of the largest pipeline and mining contractors in Mexico, with the mission to ensure a daily production of 1,500 linear metres to install a 42? pipe in a 58? wide trench. Tesmec trenchers excavated the precisely shaped trench, minimised the excavated volume, and increased production.

The 1675 chainsaw was used to dig the rocky ground, a mix of sandstones and volcanic rocks porphyry type, while the 1475 bucket wheel excavated the loose ground and fractured rock. Excavated material, produced by the 1675 chainsaw, was fine grained and immediately reusable for bedding and backfilling by simple screening with padding machines, thus resulting in cost savings, fast execution, reduced erosion and lower risks of collapse.

Hard rock trenching in Croatia
The gas pipeline project of Lika-Dalmatia, built in 2010, consists of a 20? main line 290 km long, with 160 km of 12? connection lines (from Josipdol to Dugopoljie) in hard limestone.

Tesmec supplied one unit 1275 rocksaw, two units 1150 chainsaw and a 1475 chainsaw for the project. These different models were necessary to operate with the 'pilot trench' technique: a narrow trench that is cut in the first pass is later enlarged to reach the final requested size by dividing the hard rock volume to smaller parts and allowing the use of smaller size trenchers while maximising the effectiveness of the large size machines to achieve final dimensions.

The 1275 rocksaw trencher first dug a narrow trench in the strong limestone (from 70 to 100 MPa UCS, solid to fractured) and then the 1475 chainsaw trencher enlarged the ditch to its final width, clearing it. The 'pilot trench' effects of combining a small and a large cutting machine on this job multiplied the excavation efficiency of the enlarging machine by three times.

The obtained result was an increase of the productivity from average 24 m/h of the 1475 chainsaw in the stand-alone mode to average 75 m/h when 1275 rocksaw + 1475 chainsaw in sequence were used.

Innovative solutions for tomorrow
Tesmec is looking ahead and developing stronger equipment with bigger attachments and improving the assistance and service. The entire range of Tesmec trenchers is constantly innovated to gain new substantial advantages in terms of productivity and cost efficiency.

The author is Executive Director, Trenchless Toolbox Pvt Ltd.