Question

For the 10 soil or rock conditions indicated below to select the most likely tunneling method. More one answer may apply but select what you consider is the best answer. Explain your answer in one or two sentences. Do not consider the need for construction shafts in general and launch or retrieval shafts. Unless otherwise indicated soil or rock described is for the entire tunnel alignment and profile.

Your answer will be one or more of the following:

1. Cut-and Cover Tunneling

2. Drill and Blast Tunneling

3. Soft Ground Tunnel w/ Open Face Shield

4. Main Beam Rock TBM

5. Soft Ground Tunnel w/Open Face Shield Compressed Air

6. Single Shield Rock TBM

7. Earth Pressure Balance (EPB) TBM

8. Double Shield Rock TBM

9. Slurry Shield (SFM) TBM

Soil or Rock Conditions:

1. Tunnel mined through a thick stratum of impermeable hard clay below groundwater.

2. Tunnel mined through the massive unjointed rock below groundwater.

3. Tunnel mined through the fractured rock below groundwater.

4. A tunnel is being mined through silty sand above groundwater.

5. A tunnel is being mined through silty sand below groundwater.

6. A tunnel is being mined through medium to coarse sand below that groundwater without dewatering.

7. A tunnel is being mined mixed face below groundwater.

8. A shallow large (60 ft. wide by 1000 ft. long) island platform station constructed in soil and rock.

9. A tunnel mined through medium to coarse sand below groundwater with dewatering permitted.

10. A station is mined in rock. Except for entrances and exits, no penetrations of the ground surface are permitted.

11. Groundwater is not a problem in rock tunneling. True or False

12. Compressed air is not required for EPM or Slurry Shield TBMs. True or False

13. To assist in the design of a large diameter highway tunnel a small diameter “pilot tunnel” will be mined first to determine rock mass characteristics. The Q-value used to assess ground support for the pilot tunnel may be the same as that of the large-diameter tunnel. True or False

14. If the Q value is the same, the support requirements for the pilot tunnel will be the same as the large diameter tunnel. True or False

15. Main beam rock TBMs mine rock continuously except when they stop to replace disc cutters. True or False

Answer 1

Cut and cover is the oldest method of tunnelling: the basic concept involves the digging of a trench, the construction of a tunnel, and then returning the surface to its original state.
As such, it is a disruptive technique, but it is also usually the most economical construction method. Where the tunnel alignment is beneath a city street, the construction may cause interference with traffic, utilities, businesses and other urban activities. The disruption, however, can be lessened through the use of proper staging, decking over the excavation to restore traffic, or by implementing a top- down-construction technique. While cut and cover is a technique usually reserved for relatively shallow tunnels, it is not uncommon to see it used at depths of around 20m (60 feet), but rarely does it exceed 30m (100 feet).