Sequential Excavation Method Tunneling (SEM)

Sequential Excavation Method (SEM) is a type of tunnel design and construction that started to gain popularity in the 1960s and 1970s. The essential component of the SEM approach is to take advantage of the natural capacity and strength of surrounding geology to support the tunnel with minimum cost and time required.

SEM and NATM

Unofficially and somewhat inaccurately, SEM is also known as the New Austrian Tunneling Method (NATM). Many tunnelling professionals believe each method is unique enough to warrant separate definitions. It can be said with some certainty that while there are similarities to SEM, the name New Austrian Tunneling Method was intended to distinguish it from the old Austrian tunnelling approach in the 1960s. Today many tunnel engineers assume a broader and more complex scope for sequential methods of excavation and prefer to not use the term “new” for a 50-year-old technology.

Sequential excavation was well known when the term NATM was coined in 1964, and many believe that SEM was developed around 200 years ago by miners that had to adapt their techniques to the needs of civil engineering works. In his 1963 book entitled ‘‘The History of Tunneling”, Sandstr?m talks about the tunnelling methods devised in the first half of the 1800s: “…the conventional practice used to be to advance a small pilot heading first in the forepoling manner described and subsequently expand it to full size in some other way.”

SEM Basics

SEM tunnelling is characterized by the sequential removal of ground material followed by the installation of support. The SEM process includes a thorough investigation of the ground and adjacent structures to create functional classifications for support and advance lengths (maximum unsupported excavation length).

Tunnel and geotechnical engineers use these classifications (or pre-planned scenarios) in combination with direct ground observations on-site to assess the result of the latest tunnel advance and recommend new round length and class of support system for the excavation operation ahead.

In SEM, the strength of the ground around the excavation is purposely mobilized to the maximum extent possible. This is achieved by allowing controlled deformation using an initial primary support with load-deformation characteristics appropriate to the ground conditions.

SEM Support

The support installation needs to be timed with respect to ground deformations and monitored by geotechnical instruments to assess developing deformations and make improvements to support selection for the work ahead.

Support measures usually include Shotcrete and additional reinforcement as needed, such as rebar mesh, lattice girders, bolts, or dowels in rock. Different arrangements might be used for different subdivisions of the tunnel cross-section like heading, bench, invert and side wall drifts.

In many projects, pre-support measures need to be used to protect the next advance before excavation and installation of support measures take place. Methods like spiling, forepoling, and pipe arch/canopy are among favourite techniques to improve the ground ahead.

In SEM, rather than using stiff support members that attract high loads to fight the ground deformation, flexible but strong support measures (like shotcrete lining) are used to redistribute loads into the ground by deflection and allow the ground itself to become an integrated part of the tunnel support system.

Ground Considerations

Another vital aspect of SEM is ground classification systems, which should be based on wide-ranging investigations and field observations. The ground response to tunnelling needs to be evaluated based on the data from the geological models in combination with the results from the investigation program and laboratory testing. These evaluations should also consider tunnel size, shape, overburden height, groundwater conditions, and environmental concerns.

The next step is to calculate the ground support needs and plan for “excavation and support” sequences. These results will be shown in the design documents as “Excavation and Support Classes” and form the foundation for the contractor to develop a bid to execute the tunnel work using SEM.

Finite Element Analysis

In most design efforts for SEM, a numerical model of the tunnel and support systems is constructed using a 2D or 3D finite element (FE) program for soil and rock applications. FE software can be used for a wide range of tunneling and underground projects to simulate the complex interaction between ground and structural elements and generally facilitates the excavation design and evaluation of support systems, groundwater seepage, consolidation, dynamic analysis, and much more.

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