What Does Sequential Excavation Method (SEM) Mean?
The sequential excavation method (SEM) is used for the construction of shallow mined tunnels that have ground conditions that are fully dry or have been effectively dewatered.
The method is also known as the new Austrian tunneling method (NATM), as the idea for this method originated in Austria. It gained popularity for its use of the available geological strength that is inherent in the surrounding rock mass for stabilizing the tunnel, making it economically advantageous. This method is preferred where existing structures such as sewers or subways exist and cannot be relocated.
Trenchlesspedia Explains Sequential Excavation Method (SEM)
The principle behind SEM is the integration of the behavior of rock mass under load and the continuous monitoring of underground construction, also referred to as the "design as you go" approach. This method does not depend on any specific technique for excavation and support, rather depending on observed site conditions and prevailing rock strength, optimized support is provided.
The proposed tunnel location is first divided into segments, which are then mined using an excavator and road header in a sequential manner by providing supports. As soil from each section is removed, pressurized concrete known as shotcrete is sprayed on the ceiling, walls and sides. Additional structural support is provided by installing lattice girders.