Design and Construction of a Soil Cement Mixed TBM Retrieval Shaft in Porous Limestone

Deep soil mixing (DSM) was used to form an 8.5-m (28-ft) diameter retrieval shaft for a 3-m (10-ft) diameter tunnel boring machine constructed up to 26.5-m (87-ft) deep in porous limestone in Miami, Florida. Three-dimensional finite element modeling was used to simulate the construction sequencing and final break-in of the tunnel boring machine. The initial design parameters for the soil-cement material was based on reported test results from a nearby site in similar ground conditions. An independent check on the stresses around the tunnel opening was performed using analytical solutions. The final design incorporated seventy-five, 2.1-m (7-ft) diameter soil cement columns forming a shaft with an effective wall thickness of 2 m (6.5 ft) and a 4.6-m (15-ft) thick base plug. A full-scale trial program on site indicated that the in situ DSM strength parameters were lower than expected and the drilling rates were significantly reduced at the lower elevations. Based on the strength results of the trial, the shaft design was re-evaluated and the construction sequence was revised to accommodate the drilling and mixing challenges found in the trial. A comprehensive quality assurance program undertaken during construction indicated the compressive and tensile stresses of the DSM obtained from wet grab and cored samples were greater than the revised design values. Inclinometers and settlement points indicated that shaft wall deformation was negligible and ground settlements were less than 2 mm (1/16 in.).