NATM / SEM Construction Method

HISTORY

The New Austrian Tunneling Method (NATM), also referred to as Sequential Excavation Method (SEM) or Shotcrete Method, was developed in the 1950s when shotcrete was first used systematically to stabilize squeezing ground in a water diversion tunnel at the Runserau Hydroelectric Power Project in Austria. In the following years, the method was advanced in theory and practice and adapted to be suitable for virtually all ground conditions. Since its first application in an urban environment in the Frankfurt Clay in 1968, means and methods have evolved further and a substantial number of NATM/SEM tunnels have been constructed, many of them in dense urban settings, with adverse ground conditions and low overburden.

In the US the method has been used since the early 1980s, with the first applications being in Pittsburgh and Washington D.C. From there on NATM/SEM was used on a variety of transit projects including Dallas, TX, Boston, MA, Dulles, VA and Seattle, WA.

DESCRIPTION

NATM/SEM enhances the self-supporting capacity of the rock or soil by mobilizing the strength of the surrounding ground. It is today by far the most resistant tunnel support system in earthquake endangered zones.

The tunnel excavation is carried out in increments (headings and rounds), which are supported by flashcrete immediately after exposure followed by the installation of the initial lining consisting of reinforced shotcrete. The lining has a defined stiffness to allow controlled stress relaxation around the opening, minimizing the section forces and hence allows for a cost effective structural design. In addition, various ground support, face support, pre-support and ground improvement measures are utilized to ensure the stability and safety of the tunneling operation and minimize settlements at the surface. For extremely unfavorable ground conditions special methods like ground freezing, tunneling under compressed air, etc. have been developed and utilized in the past.

During construction, the deformations in and above the tunnel structures are continuously recorded, monitored and interpreted to verify the design assumptions and assess the stability and the appropriateness of the applied excavation sequence and support elements. The interpretation of the monitoring data is fed back to the ongoing construction and adjustments can be made if necessary.

After completion of the excavation and initial support, the waterproofing system is installed sandwiched between initial and final lining, consisting of a protective geotextile, a flexible waterproofing membrane, enhanced by a sectioning system for remedial repair of possible leaks.

The last step is the installation of the final lining, which is designed to withstand ground loads, hydrostatic loads and seismic loads according to the design criteria. The final lining can be either reinforced cast-in-place concrete or shotcrete, depending on the length of the tunnel and the variability of the cross section.

NATM/SEM TUNNEL DESIGN

The NATM/SEM tunnel design follows the principles outlined above and relies heavily on engineering judgment and experience from previous projects, but also on advanced Finite Element Modeling Tools to determine the appropriate excavation sequences and support measures.

Some of the key factors to be considered are:

• Ovoid cross section with a domed excavation face to prevent stress concentrations; a rounded invert in soft ground.
• Timely installation of sealing shotcrete / flashcrete and the initial shotcrete lining to prevent deterioration and disintegration of the soils and to provide continuous, smooth support around the tunnel perimeter and the tunnel face.
• Ring Closure, in soft ground within one tunnel diameter to prevent loosening and movement of the surrounding ground and excess settlements.
• Subdivision of the faces into top heading / bench / invert or smaller drifts and adjustment of round lengths to be able to control and stabilize the excavation at any stage.
• Ability and necessity to make adjustments in the field to deal with actual ground conditions encountered.

Depending on the actual ground conditions encountered during construction, support measures ("NATM/SEM Toolbox Items") are used on an as needed basis to ensure stability of the tunnel face and the surrounding ground. They include:

• Pre Support Measures: e.g. Rebar Spiling, Grouted Pipe Spiling, Grouted Barrel Vault.
• Face Stabilization Measures: e.g. Face Stabilization Wedge, Face Bolts.
• Annular Support Measures: e.g. Soil Nails, Temporary Invert.
• Ground Improvement Measures: e.g. Dewatering, Grouting.

This approach provides a high degree of flexibility during construction and makes it possible to control virtually all kinds of ground conditions, thereby greatly reducing the risks of NATM/SEM construction. It does require a representation of the designer during the construction phase to ensure the adequate application of the various measures.

CRITICAL SUCCESS FACTORS

Experience from past projects has helped identify several critical success factors for safe and cost effective NATM/SEM tunneling.

• Prescriptive design with a certain degree of flexibility for adjustments according to actual site conditions encountered.
• Unit price contracts to support flexibility, adaptability and balance risk sharing - a proven way to a cost effective and state-of-the-art product.
• The designer’s representation and involvement in the construction phase.
• Skilled contractors familiar with the principles of NATM/SEM and the utilization of the ground support measures.
• Experienced and competent construction management, site supervision and quality control.