Geostatistical estimation of Ground Class prior to and during excavation for the Caldecott Tunnel Fourth Bore project

摘要

One of the most significant challenges when tunneling in rock is predicting the true tunneling conditions along the alignment. Prior to excavation, engineers rely on sparse borehole information and a knowledge of the regional geology and tectonics to estimate anticipated tunneling conditions and associated spatial uncertainty. As more data become available during excavation, in some projects, geologists and engineers construct maps of the tunnel face that record the tunneling conditions at closely-spaced intervals along the tunnel, which can be used to update predictions of tunneling conditions just ahead of the face. This work used a variogram-based geostatistical algorithm (indicator kriging) with both borehole data and face maps from the completed Caldecott Tunnel Fourth Bore in California, USA to predict anticipated tunneling conditions and associated uncertainty both prior to and during excavation to integrate information available at different times during the life of the project. First, borehole data available prior to excavation were used to estimate anticipated tunneling conditions using two methods which attempt to represent a relationship between measured borehole data and anticipated tunneling conditions; these methods were compared using the face maps collected during excavation as verification. When comparing the two methods in the context of two geologic units from the Caldecott Tunnel Fourth Bore, one of the methods produced reasonable uncertainty predictions for both geologic units, while the second produced excellent uncertainty predictions in one geologic unit and poor uncertainty predictions in the other geologic unit due to the geometrical configuration of the boreholes. The former method was used to assess the impact of face maps on predicted tunneling conditions in both geologic units, and it was found that a large reduction in uncertainty can be achieved 1-5 m ahead of the face when incorporating face maps into updated geostatistical calculations.