TUNNELLING

1. TUNNELLING CE – 301 - Lecture 16 Lecturer ABDUL JABBAR Semester 2 15 Feb 10 – 18 Jun 12

2. Tunneling Definition • Tunnels are artificial underground passages opened at both ends constructed for different purposes. • Required for highways, railways, sewerage, water supply, public utilities and canals. • Necessitated when open excavation of strata becomes uneconomical, not only construction cost but for maintenance as well. Continued

3. History • The oldest tunnel was constructed 4000 years ago in ancient Babylon to underpass the bed of river Euphrates and to establish an underground connection between the royal palace and Temple of Jove. • The length of the tunnel was 1 km and it was built with the considerable cross-section dimensions of 3.6 m by 4.5 m. • In the second half of the 20th century, with the progress in development of both explosives and equipment, underground construction became feasible. Continued

4. Geological Considerations or ground properties before constructing any tunnel project: • The engineering properties of a rock generally depend not only on the matrix (structure formed by the minerals) but also imperfections in the structure such as voids (pore space ),cracks ,inclusions, grain boundaries and weak particles. • Pore spaces are largely made up of continuous irregular capillary cracks separating the mineral grains.

5. Physical discontinuities are present in all rock masses as a result of geological activities. • Introduction of defects into the rock mass due to human activities that alter the properties of the rock material. • The mechanical breaks in the rock have zero or low tensile strengths, increase rock deformability and provide pathways for water to flow.

6. Springline : The line at which the tunnel wall breaks from sloping outward to sloping inward toward the crown Terminology: Crown: The uppermost part of the tunnel Invert: The bottom (floor) of the tunnel Heading : The excavated face of the tunnel Station : The distance measured from the portal (chainage) Drift : A horizontal excavation Portal : The tunnel entrance Wall : The side of the tunnel

7. The choice of tunnelling method may be dictated by: • geological and hydrological conditions • Weak rock under high stresses leads to squeezing ground conditions. • Brittle strong rock high stress conditions may lead to rockbursting. • 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 Choice of method depends upon nature of strata and geometry of tunnel section Continued

8. Classification of Tunnelling Methods based on type of strata: • Methods divided into three categories • Firm ground- reasonable time available for installing conventional support. • Soft ground- Conventional support can not be installed. • Running ground- Special treatment required before starting excavation Continued

9. Tunnelling Methods Rough classification of methods • Excavation: An excavation process without removing the overlying rock or soil • Steps: excavating-remove muck-supporting-lining-ventilation-draining • Drill & Blast, hammer, roadheader, .... • Cut and cover: trenching to excavate and construct a tunnel, and then backfilling earth over it. • Immersedtunnel: lowering prefabricatedtunnel elements into a dredged channel and joining them up under water • Concrete • Steel Continued

10. Bottom-up method: The main site is excavated, with ground support as necessary, and the tunnel is constructed within. The tunnel may be of in situ concrete, precast concrete, precast arches, or corrugated steel arches. The trench is then carefully back-filled above the tunnel roof and the surface is restored.

14. Coffer Dam and hydroelectric Tunnel

17. Top-down method: Here side support walls and cap beams are constructed from ground level typically with slurry walls, or secant piling. Then a shallow excavation allows making the tunnel roof of precast beams or in situ concrete. The surface is then restored except for access openings. This allows early reinstatement of roadways, services and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is constructed.

22. Tunnelling in Firm ground • Traditional methods. Involving drilling and blasting • Full Face Method. In firm soils where full face excavation can hold it self for sufficient time to allow mucking and supporting operations. • In the full-face method, workers excavate the entire diameter of the tunnel at the same time. • This is most suitable for tunnels passing through strong ground or for building smaller tunnels. Continued

25. Top Heading and benching. When full face excavation is not possible. Heading should be sufficient distance ahead of benching. • In this technique, workers dig a smaller tunnel known as a heading. Once the top heading has advanced some distance into the rock, workers begin excavating immediately below the floor of the top heading; this is a bench. • One advantage of the top-heading-and-bench method is that engineers can use the heading tunnel to gauge the stability of the rock before moving forward with the project.

27. Excavation by tunnelling Methods • Drift Method. In case of large or pilot tunnel , Drift is made and then expanded. There can be more than one drifts. • Different ‘drift methods’ depending on location of drift can be used.

28. Tunnelling in Soft Strata • Tunnels constructed in soft materials require temporary support immediately or shortly after excavation. Way to provide support are: • Suitably spaced bents of wood • Suitably spaced bents of steel for lagging (covering) • Liner plates • Fore poling placed to retain material between adjacent bents • Temporary supports must be designed for higher working stress compared to those in permanent designs.

29. Tunnelling in Soft Ground • Instantaneous support is required • No drilling and blasting. • Forepoling is done. • Boards are driven ahead of the last ‘rib’, around periphery • Forepoles act as cantilevers beyond breasting • Soil is excavated after removing the breast board and new rib is erected Continued

30. Because stand-up time is generally short when tunneling through soft ground, cave-ins are a constant threat. To prevent this from happening, engineers use a special piece of equipment called a shield. • A shield is an iron or steel cylinder literally pushed into the soft soil. It carves a perfectly round hole and supports the surrounding earth while workers remove debris and install a permanent lining made of cast iron or precast concrete. • When the workers complete a section, jacks push the shield forward and they repeat the process.

32. Tunnelling in Running Ground • Many methods • Principle is, Linear plates are pushed in starting from crown • Cavity is excavated • Additional linear plates are pushed one by one and bolted • Arch section gradually widened down to springing line. Continued

33. Tunnelling in Rocks • Tunnels are driven in rocks by drilling holes in the rock face, loading the holes with explosives, blasting and removing the broken rock • Each sequence full cross section of the tunnel may be excavated, or one or more drifts may be there. • Drilling pattern for holes of explosives may differ depending upon rock type, cross section, experience expertise and type of explosive suggested. • Methods are similar in principle like • Full Face Method • Top Heading and benching • Drift Method Continued

34. Tunneling through hard rock almost always involves blasting. Workers use a scaffold, called a jumbo, to place explosives quickly and safely. • The jumbo moves to the face of the tunnel, and drills mounted to the jumbo make several holes in the rock. The depth of the holes can vary depending on the type of rock, but a typical hole is about 10 feet deep and only a few inches in diameter. • Next, workers pack explosives into the holes, evacuate the tunnel and detonate the charges. After vacuuming out the noxious fumes created during the explosion, workers can enter and begin carrying out the debris, known as muck, using carts. • Then they repeat the process, which advances the tunnel slowly through the rock.

37. Fire-setting is an alternative to blasting. In this technique, the tunnel wall is heated with fire, and then cooled with water. The rapid expansion and contraction caused by the sudden temperature change causes large chunks of rock to break off. The Cloaca Maxima, one of Rome's oldest sewer tunnels, was built using this technique. • The stand-up time for solid, very hard rock may measure in centuries. In this environment, extra support for the tunnel roof and walls may not be required. However, most tunnels pass through rock that contains breaks or pockets of fractured rock, so engineers must add additional support in the form of bolts, sprayed concrete or rings of steel beams. In most cases, they add a permanent concrete lining

38. Sequence of operation in Rocky Strata (Drill and Blast) • Marking tunnel profile. • Setting up and drilling • Loading explosive and blasting • Removing the foul gases • Checking • Scaling • Mucking • Rock bolting and lining Continued

39. Modern Tunnel Construction Methods: • Drill and blast • Mechanical drilling/cutting • Cut-and-cover • Immersed tunnels • Tunnel boring machines (TBMs) • New Austrian Tunnelling Method (NATM) Continued

40. Road headers

43. Types of support Steel arches • Steel ribs are used for reinforcement of weaker tunnel sections, and give rigid to semi-rigid support. The ribs are made from I-beam or H-beam structural steel bent to conform to the requirements of a particular tunnel cross-section. • Timber may be used for packing between the beams and the rock. However, providing continuous bedding against the rock may considerably increase the load-bearing capacity of the arches. Continued

45. Rock bolts • Steel bolts are frequently set in holes drilled into the rock to assist in supporting the entire roof or individual rock slabs that tend to fall into a tunnel. Rock bolts maintain the stability of an opening by suspending the dead weight of a slab from the rock above Continued

46. Shotcrete • Pneumatically applied mortar and concrete are increasingly being used for the support of underground excavations. • A combination of rock bolts and shotcrete has proved an excellent temporary support for all qualities of rock. • Shotcrete is best known in tunnelling as an integral component of the NATM (New Austrian Tunnelling Method). • Quick-setting concrete is sprayed onto the bare rock surface immediately after excavation, and rapidly hardens to form a preliminary support until the final lining of conventional poured concrete can be installed. Continued

47. Wire mesh • Wire mesh is used to support small pieces of loose rock or as reinforcement for shotcrete. • Two types of wire mesh • chain link mesh commonly used for fencing and it consists of a woven fabric of wire, tends to be flexible and strong • weld mesh. commonly used for reinforcing shotcrete and it consists of a square grid of steel wires, welded at their intersection points.

49. Ventilation during construction • Why required? • To furnish fresh air for the workers • To remove the dust caused by drilling, blasting, mucking, diesel engines, and other operations • To remove obnoxious gases and fumes of explosives • How done? • Mechanical ventilation is usually supplied by electric fans, as for example axial flow pressure fans. • If air is blown into a tunnel, it may be forced through a lightweight pipe or fabric duct. • If air is exhausted, it is necessary to use a rigid duct that will not collapse. • The exhaust method has the advantage of more quickly removing objectionable air from spaces occupied by the workers. Continued

50. Ventilation of tunnels • Mechanical ventilation systems provide the temperature, humidity and air velocity conditions necessary to give tunnel users a reasonable degree of comfort during normal operation. • When a fire occurs in a tunnel, the system must also provide a safe evacuation route for tunnel users and access for fire fighting services. • The choice and design of a ventilation system depends on these main factors: • tunnel length and volume • admissible air pollution around tunnel portals • fire safety considerations. • Key pollutants include carbon dioxide, nitrogen oxides, nitrogen dioxide, hydrocarbons PM10 and lead. • Better understanding of ventilation techniques • Awareness of new safety and environmental legislation.