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Civil Engineering Construction

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The two tunnels were identical in design with cross passages at 100 meter nominal centres. There were two lanes per bore and were 3.65 meters wide each with full batter curbs.2 meter wide verges were also provided on each side of the tunnel to provide sight lines restricted by the horizontal curvature of the tunnel and to also serve as emergency points wide enough for wheelchair access. The vertical clearance was 5.03 meters with an additional 250 mm for monitoring and other equipments.For drainage, a continuous system was placed beneath the curb. Cable duct banks, fire main and pump main were also buried under the carriageway. Ventilation fans, lighting and communication systems were placed overhead.All of these requirements meant that an excavated diameter of 11.6 m was needed with a 10.6 meter internal diameter. The cross section adapted for the tunnel was the typical horse-shoe shape. This is shown in Figure 3.A total of 20 jet fans were installed per bore for smoke control. Provisions for sprinkler systems were not included as it presented another significant cost especially for operation and maintenance. Cross passages serving as emergency access were provided and equipped with fire hydrants, fire extinguishers and emergency telephones.Originally, the excavation was to be undertaken using diesel-based sequenced shotcrete lining system (SCL) construction where concrete is sprayed on the bored perimeter after each excavation cycle. However, new changes in limits adopted by the UK Health and Safety Executive regarding levels of exposure to Crystalline Silica (from 0.3 mg/m3), Nitrous Oxide (from 25 ppm to 1 ppm) and Nitrous Dioxide (from 3 ppm to 1 ppm) were not achievable thru the use of diesel-based methodology. As an alternative, an Earth Pressure Balance Tunnel Boring Machine (TBM) was considered with the preliminary design shown in Figure 4.TBM allowed for a straight construction from the north to south end of the twin tunnels hence eliminating the need to orient the tunnel thru the optimum material. The method also allowed for the application of a single-pass lining compared to the multiple-lining pass required in SCL. Nonetheless, construction of cross passages and emergency points in a TBM system were more costly. The benefit cost ratio (BCR) for the openings in SCL was greater than 1 while the BCR for the TBM were less than 1. SCL boring also allowed for multiple, concurrent
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preview essay on Civil Engineering Construction
  • Pages: 8 (2000 words)
  • Document Type: Essay
  • Subject: Unsorted
  • Level: Undergraduate
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