During the planning of the Mae Ngad-Mae Kuang project a comprehensive geological/geotechnical investigation program led to an optimization of the alignment with respect to technical and environmental risks. Different options for the construction of the project were evaluated, including NATM and open, as well as shielded TBM excavations, with a maximum of 5 headings from two access tunnels and one portal. After establishing the geological model and characterization of the rock mass, the rock mass reaction to the excavation was analysed along the entire alignment, which for that purpose was divided into sections of 50m each. Depending on the ground behaviour excavation and support methods, as well as auxiliary measures were selected. Time and costs were assigned to each activity.
In addition to the variation in construction methods, the geological risk was assessed by using expected, optimistic and pessimistic rock mass parameters. This evaluation showed a variation in excavation time of up to 8 months depending on the construction methods chosen. A comparison of construction costs revealed that due to the relatively low labour costs in Thailand the option with the greatest share in drill and blast excavation is the most economical one.