The Geotechnical Group’s Sixth Annual William Barclay Parsons Lecture was held at the CUNY Graduate Center Recital Hall in Manhattan on April 17, 2008 and was presented by Dr. Gerhard Sauer, a noted tunnel engineer. Dr. Sauer was introduced by Dr. George Munfakh of Parsons Brinckerhoff. The sponsor for this lecture was Allentown Shotcrete Equipment.
Presented by: Gerhard Sauer, Dipl. Ing., Dr. tech., P.E., President, Dr. G. Sauer Group, Salzburg, Austria
Dr. Sauer began the lecture by introducing the topic areas to be covered: tunneling methods, contractual issues, toolbox items for construction, and special methods and topics. The tunneling methods discussed were mechanical methods (tunnel boring machines, raise borings, and shields) and hand mining (ribs and lagging, shotcrete, and NATM). The benefits of mechanized tunneling were discussed in terms of uniform tunnel geometries and rapid, efficient construction for longer tunnel lengths.
The specific hand mining method that Dr. Sauer spent the most time describing was the New Austrian Tunneling Method (NATM). This method is appropriate for underground structures with flexible geometry and variable geology, has a much lower mobilization time, and more controllable construction safety. The basic tunnel shape that works best for NATM is ovoid and the application of temporary support measures is immediate. The successful execution of this tunnel type requires experience, skillful execution, and careful instrumentation. The instrumentation results are used to modify the excavation support measures and provide stability. The toolbox items for NATM construction of tunnels include stabilization measures for face support, pre-support, and annular support. Implicit in the success of all these tools is the use of proper excavation, support sequencing, and shotcrete. Face support is provided by using face wedges and pocket excavations to limit inward movements prior to shotcreting. The simplest excavation sequence involves three stages in smaller tunnels with very stable conditions, but for larger tunnels, up to six stages may be involved. Pre-support may be necessary in some ground conditions and can take the form of rebar or grouted pipe spiling to give roof stability, jet grouting to provide stability for the crown, springline, or base. Overall annular stability is provided by the soil anchors or rock bolts installed in the roof and sidewalls to increase the arching effect. Dewatering is also used to increase stability.
Dr. Sauer reported that the special methods to allow successful tunneling are compressed air, ground freezing, the doorframe slab method, caisson method, and barrel vault method. Ground freezing can be conducted vertically and horizontally and has been successfully used on projects such as the Central Artery and Russia Wharf in Boston, Massachusetts. The doorframe slab method is suitable for shallow cover tunnels and requires first casting a roof slab beneath which a tunnel will be mined using NATM methods. The caisson method is used for shallow cover tunnels beneath water and requires that a below-water roof slab is constructed followed by tunnel excavation in between cutoff walls or under compressed air. The barrel vault method (BVM) involves using grouted pipes to create an umbrella or canopy in the crown prior to excavation, followed by adequate ground support like shotcrete and lattice girders. The BVM has been used for the Beacon Hill Station in Seattle, Washington and Seven Stars Tunnel in Tel Aviv, Israel. Dr. Sauer also discussed waterproofing as a major consideration for all NATM tunnels to reduce maintenance costs and extend the tunnel’s lifespan. Finally, Dr. Sauer discussed many case histories of NATM tunneling projects in the US and abroad. Of note was the Red Line project in Tel Aviv, Israel which involves multiple underground station complexes and tunnel intersections and crossovers. Local projects in New York City include the LIRR East Side Access (ESA) and PATH Exchange Place Station. The Northern Boulevard Crossing portion of the ESA project may involve either the BVM process or horizontal ground freezing. The infamous Devil’s Slide area of the Pacific Coast Highway in California is being bypassed with a new NATM tunnel.
Following a number of questions from the audience, Dr. Sauer was presented a Golden Apple from Geotechnical Group Chairman Terry Holman in appreciation for the lecture.