Northeast Boundary Tunnel – Mount Olivet Road Drop Shaft

The Mount Olivet Road Drop Shaft (MOR-DS) support of excavation (SOE) is a circular unreinforced concrete shaft with an inside diameter of a 30.5 ft (9.3 m) and depth of 120.5 ft (36.7 m), constructed using slurry wall methods. The SOE for the adjacent Mount Olivet Road Approach Channel (MOR-AC) and Mount Olivet Road Ventilation Vault (MOR-VV) near surface structures (NSS) consists of lean mix concrete walls and cement-bentonite walls, respectively, both constructed using slurry wall techniques and reinforced with embedded soldier piles. NSS SOE required multiple levels of preloaded internal bracing, with excavation depths of up to 75 ft (22.9 m). The original design construction sequence for the shaft and NSS SOE required that the MOR-DS be constructed in its entirety and backfilled to ground surface prior to excavation of the adjacent NSS SOE. After installation of the drop shaft and NSS SOE an accelerated construction sequence was implemented requiring the analysis of the excavation support systems to determine if the excavations could be open concurrently without adversely impacting the partially constructed permanent MOR-DS structure. The MOR-DS was analyzed using the finite element analysis (FEA) software, MIDAS GTS, to demonstrate that the unbalanced loads applied to the MOR-DS composite structure due to the revised sequence would not exceed the strength and serviceability requirements per ACI-350 or superseded by the more stringent project requirements. The results of the FEA were used to set limits for geotechnical instrumentation consisting of strain gauges located on the brace levels of the NSS SOE. A comparison of the SOE design hand calculations, FEA model results and instrumentation data is presented.