Mark J. Farrow, P.E. & Terry W. Oswald, P.E.

A. detention vault to retain storm water runoff from North Central Expressway was designed as a series of deep elliptically shaped tunnels. Vault tunnels will generally have dimensions of 24 feet by 40 feet and be excavated in the Austin Chalk Limestone formation at depths up to 100 feet below grade. A geotechnical instrumentation monitoring system and preconstruction survey was designed for detection of ground movement during the excavation of the North Central Expressway (NCE) Cole Park Detention Vault in Dallas, Texas.

Two types of instrumentation were installed from the ground surface. Inclinometers and Multiple Position Borehole Extensometers (MPBX's) were installed in vertically drilled boreholes to monitor horizontal ground movements and vertical displacements in the limestone, respectively. Two additional types of instrumentation will be installed within the mined opening. Convergence Sections consisting of reference points anchored in the rock will be utilized to measure horizontal and diagonal convergence of the opening. Double position Borehole Extensometers (DPBX's) will be installed at locations specified by the Engineer during the tunneling operations to monitor displacements along the axis of the extensometer. This paper presents an overview of the tunneling instrumentation and preconstruction surveys implemented for this project.