Tunnelling Methods

The choice of tunnelling method may be dictated by: geological and hydrological conditions, cross-section and length of continuous tunnel, local experience and time/cost considerations (what is the value of time in the project), limits of surface disturbance, and many others factors.

tunnel construction methods: Classical methods Mechanical drilling/cutting Cut-and-cover Drill and blast Shields and tunnel boring machines (TBMs) New Austrian Tunnelling Method (NATM) Immersed tunnels Special methods (Tunnel jacking, etc.)

The process for bored tunnelling involves all or some of the following operations: Probe drilling (when needed) Grouting (when needed) Excavation (or blasting) Supporting Transportation of muck Lining or coating/sealing Draining Ventilation

Classical Methods Among the classical methods are the Belgian, English, German, Austrian, Italian and American systems. These methods had much in common with early mining methods and were used until last half of the 19th century. Excavation was done by hand or simple drilling equipment. Supports were predominantly timber, and transportation of muck was done on cars on narrow gauge tracks and powered by steam. Progress was typically in multiple stages i.e. progress in one drift, then support, then drift in another drift, and so on. The lining would be of brickwork. These craft-based methods are no longer applicable, although some of their principles have been used in combination up to present day. Nevertheless some of the world’s great tunnels were built with these methods.

The English method (crown-bar method, figure left) started from a central top heading which allowed two timber crown bars to be hoisted into place, the rear ends supported on a completed length of lining, the forward ends propped within the central heading. Development of the heading then allowed additional bars to be erected around the perimeter of the face with boards between each pair to exclude the ground. The system is economical in timber, permits construction of the arch of the tunnel in full-face excavation, and is tolerant of a wide variety of ground conditions, but depends on relatively low ground pressures.

The Austrian (cross-bar) method required a strongly constructed central bottom heading upon which a crown heading was constructed. The timbering for full-face excavation was then heavily braced against the central headings, with longitudinal poling boards built on timber bars carried on each frame of timbering. As the lining advanced, so was the timbering propped against each length to maintain stability. The method was capable of withstanding high ground pressures but had high demand for timber.

The German method (core-leaving method) provided a series of box headings within which the successive sections of the side walls of the tunnel were built from the footing upwards, thus a forerunner of the system of multiple drifts. The method depends on the central dumpling being able to resists without excessive movement pressure transmitted from the side walls, in providing support to the top 'key' heading prior to completion of the arch and to ensuring stability while the invert arch is extended in sections. The Belgian system (underpinning or flying arch method) started from the construction of a top heading, propped approximately to the level of the springing of the arch for a horseshoe tunnel. This heading was then extended to each side to permit construction of the upper part of the arch, which was extended by under- pinning, working from side headings. The system was only practicable where rock loads were not heavy. The first sizeable tunnel in soft ground was the Tronquoy tunnel on the St Quentin canal in France in 1803, where the method of construction, based on the use of successive headings to construct sections of the arch starting from the footing, was a forerunner to the German system described above.

The Rove Tunnel near Marseille measured 22 x 15,40 m, and was excavated with multiple drifts.

Classical multiple face excavation

Mechanical Drilling and Cutting-Crushing Strength of rock

Roadheaders

Cut and Cover Method

The principal problem to be solved in connection with this construction method is to how to maintain surface traffic, with the least disturbance during the construction period. One method is to restrict traffic to a reduced street width, another to direct traffic to a bypassing street. Another way of supporting the sidewalls of open trenches is to substitute sheet-pile walls by concrete curtain walls cast under bentonite slurry (ICOS method), and using steel struts. This is especially a requisite in narrower streets trimmed with old sensitive buildings with their foundation plane well above the bottom level of the pit. This type of trench wall becomes a requirement for maintenance of surface traffic due to the anticipation of vibration effects potentially harmful to the stability of buildings with foundations lying on cohesionless soils.