Construction Techniques and Materials in the A3 Hindhead Tunnel Project - Research Paper Example

Construction Techniques and Materials in the A3 Hindhead Tunnel Project Submitted by: Submitted In Partial Fulfillment Of the Requirements in [Subject] Word Count: 1,972 [Date] Table of Contents Sections Page 1.0 Introduction 3 2.0 Tunnel Design 2.1 Geological Considerations 4 2.2 Cross Section Details 5 2.3 Safety Systems 6 3.0 Tunnel Excavation and Support 3.1 Construction System 6 3.2 Shotcrete Lining System 8 3.3 Waterproofing System 11 4.0 Conclusion 12 5.0 References 13 1. Introduction The A3 Hindhead project located in Surrey in the United Kingdom, is a dual carriageway linking Portsmouth and London. It is designed to lessen the gridlock around the A3/A287 traffic signal controlled crossroads. The new road is 6.9 km long and includes 1.8km of twin bored tunnels through the Devil’s Punch Bowl, an area of designated as an ‘Area of Outstanding Natural Beauty’ and ‘Site of Special Scientific Interest’. Figure 1 shows the location of the project and the existing A3 carriageway. Figure 1: Location of A3 Hindhead Tunnel (Ireland et al, 2007) Balfour Beatty was awarded with the construction of the tunnel which began in January 2007 with tunnelling works proceeding in February 2008. Tunnel bores broke through in 26 February 2009 and tunnel services are expected to be available in 2011. This paper investigates the design and construction used on the A3 Hindhead tunnel with focus on the functionalities and economic considerations of the techniques and materials used for the tunnelling such as ground supports, waterproofing system and fire-resistance used in the bored substructure 2.0 Tunnel Design 2.1 Geological Considerations Geological mapping indicate that the Hindhead area comprises of sequences of sedimentary deposits with the tunnel to go through the 90 meter thick Hythe Beds described as highly glauconitic, cross-bedded and variably sorted sand and sandstone layers. The Hythe beds are divided into 6 lithostratigraphic layers but the tunnel will pass only through 4 of them. The tunnel layout is also designed to be above the observed historical water table with only one location where the maximum water table conditions exceed the invert level. An illustration of the geology and tunnel path is shown in Figure 2: Figure 2: The Tunnel and the Geologic Layers (Highways Agency,2008) Geotechnical investigations using the shear box test and pressure meter was preferred over sonic testing and triaxial testing as the latter two either overestimate or underestimate the Elastic Modulus of the rock mass. The layers of the earth on which the tunnel will pass through have UCS classification of 2 to 5 MPa. Other properties were determined and are presented in the following: Upper Hythe A and B – Found to be comprised of medium dense, thinly bedded and thinly laminated, clean to silty and clayey fine and medium sand with weak to strong sandstone, cherty sandstone and chert. Upper Hythe C – Found to be comprised of locally weak to moderately strong, slightly clayey fine to medium sandstone occasional thin beds of clayey/silty fine sand’. Lower Hythe A – This layer has almost the same properties as Upper Hythe A but is heavily fractured with values ranging from 190 to 815 mm. Due to the weakness to resist horizontal stresses of the Hythe B strata on the northern end, the decision was made to adapt a low-point in the northern entrance in spite of the additional cost. 2.2 Cross Section Details 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. 1.2 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. Figure 3: A3 Hindhead tunnel cross section (Ireland et al, 2007) 2.3 Safety Systems 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. 3. Tunnel Excavation and Support 3.1 Construction System 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. Figure 4: TBM tunnel cross section (Ireland et al, 2007) 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 excavation which means that construction in the 4 ends could take place at the same time. With the TBM, however, needed to excavate 1 bore a time. A value engineering study also found that the TBM could only be financially viable for a 5 km long twin tunnel while the A3 Hindhead had only 3.6 km total length. It was also very difficult to procure a 12.0m diameter machine. Eventually, the government relaxed the exposure levels introduced by the HSE and the SCL was then subsequently applied to the construction of the twin tunnels. 3.2 Shotcrete Lining System As previously mentioned, the SCL technique involves a special form of concrete, known as shotcrete, sprayed at the bored perimeter after each excavation advance. Two advances in construction technology enabled innovations in the support measures and enabled designers to treat primary lining as permanent. Previously, the primary lining had to be considered as temporary because the corrosion potential of the steel lattice girder used as supports and in contact with the lining. However, the introduction of non-alkali accelerators for shotcrete enabled rapid hardening thereby eliminating the need for steel girders. Advances in 3-D surveying equipment also enabled accurate shape construction and spraying. A file photo in Figure 5 shows a robot theodolite taking GPS back sightings from other survey stations monitors the excavation activity. Figure 5: Robot Theodolite in Action in the Hindhead Project (Highways Agency,2008) Excavation in the north end of the tunnels was thru rock and involved a full face heading followed at a distance by the bench excavation. The typical pattern bolting used in rock tunnel support was deemed inappropriate due to the presence of up to 2 meter sand layers. These layers have yield in low bond stress. There was no need for a close invert because of the stability of the rock structure and location above water table. The primary lining was supported on elephant’s feet. A typical cross section is shown in Figure 6. To protect against the failure of the overhead rock/crown, self-drilling GRP tubular spiles were installed when necessary. Figure 6: Typical primary lining in rock sections (Ireland et al, 2007) Excavation on the south end was thru sandstone and sand layers. The process was more complex and called for the use of ALWAG pipe umbrella pre-support system was installed. In this system, 25 pipes 12 m long and 139.7 mm in diameter were installed using a Sandvik Axera 8 two-boom jumbo. Figure 7 shows the installation. Figure 7: Installation of the ALWAG pipe umbrella supports (Wallis, 2009) The excavation in the sand-laden south section was more complex and involved three types of support: 1. Support Type 1: Sandy material is encountered by the heading only. 12m self drilling Glass Reinforced Plastic (GRP) face dowels were to be installed in the heading only. As with the rock excavation, the heading can proceed ahead of the bench. 2. Support Type 2: This is applied when the elephants feet of the bench is supported on unsound sandstone material. Face dowels are installed in the heading and bench. Both are to advance together with only a 2 m separation between them. 3. Support Type 3: The sandy material extends well below the tunnel thus requiring a closed invert. The bench and heading must move together with the invert close behind ( a maximum distance of 6 m). A typical sand excavation section is shown in Figure 8: Figure 8. Typical primary lining in sand sections (Ireland et al, 2007) The concrete to be sprayed were to be reinforced with steel fibre to provide a stable matrix and additional flexural strength. Design, however, considered the primary lining to be composed of concrete alone thereby giving additional safety factor. 3.3 Waterproofing System Water ingress is a common problem in tunnels due to percolation of water from the soil above. The challenge in previous waterproofing systems is that they are mostly sheet membranes which are prone to sagging. It was very hard for the secondary lining to support the membrane in the crown. Luckily, the BASF company, a construction material and chemical manufacturer, had developed a sprayed waterproofing product called the MASTERSEAL 345. The product is a dry, polymer-rich powder made of materials used in PVC membrane production. It has only one component and requires only water as a mixing agent. Application is done with a spraying boom equipped to introduce water to the dry-mix at the nozzle. The material can take a 100% tensile stress of its specified tensile strength and is able to bridge 2 mm cracks in the concrete surface. Figure 9 shows the application of the product: Figure 9. Application of Waterproofing Compound in Hindhead (Wallis, 2009) 3.4 Secondary Lining System Previously, the design called for concrete containing 35% Pulverized Fly Ash to minimize shrinkage and 2 kg/m3 of polypropylene fibres to prevent explosive spalling. However, the replacement of the sheet waterproofing membrane with a sprayed compound meant that shotcrete can be used as good bonding between the waterproofing compound and shotcrete was possible. It was agreed that 4 m cast concrete would be used for the sides and shotcrete for the crown (Wallis, 2009). This is shown in Figure 10: Figure 10. Secondary Lining Cast Sections (Wallis, 2009) 4.0 Conclusion The construction of A3 Hindhead Tunnel employed construction techniques and materials made possible thru the advances in construction technology. A3 Hindhead Tunnel provides a good example of how advances in the civil engineering profession is making previously tedious construction possible. Tunneling is a meticulous process that involves a careful analysis of the soil strata. Given the complexity and uncertainty in soil profiling, the way in which the engineering construction adapted and proceeded made it possible to determine best practices in construction. There are two significant innovations in construction technology used in the A3 construction. First is the shotcrete technology where concrete is sprayed and the second was the lining system which was also sprayed. This technology enabled the main contractor to proceed in a manner unheard of before. All this goes to show that the former practice of just likening it to other previous construction before may prove to be unnecessary as new technology can give significant gains. After all, providing new ideas and ways is what engineering is all about. References: British Tunnelling Society (2006). Occupational Exposure to Nitrogen Monoxide in a Tunnel Environment. Retrieved April 14, 2010 from London. http://www.britishtunnelling.org. Highways Agency (2008). A3 Hindhead Improvement Major Improvement Works Newsletter 10. UK: Highways Agency Publications Group. Ireland, TJ, Rock, TA and Hoyland P. (2007). Planning and design of the A3 Hindhead tunnel, Surrey, UK. Underground Space. in the 4th Dimension of Metropolises – Barták, Hrdina, Romancov & Zlámal (eds). Taylor & Francis Group: London. Wallis, Shani (2009). UK applies spray-on waterproofing. Retrieved April 14, 2010 from http://tunneltalk.com/Spray-on-waterproofing-Mar10-Hindhead-application-UK.php Williams, I., Neumann, C., Jäger, J.&Falkner, L. (2004). Innovativer Spritzbeton-Tunelbau für den neuen Flughafenterminal T5 in London. In Proc. Österreichisher Tunneltag 2004, Austrian Committee of the ITA, Salzburg, pp. 41–62. Salzburg: Die SIGN Factory. Read More