Abstract:
Malcolm constructed a microtunnel jacking and a receiving shaft using the Cutter Soil Mixing method, whereby cement grout is mixed with the soil in-situ to form high quality soil-cement. The CSM method uses two vertically-mounted cutting wheels rotating about a horizontal axis to construct rectangular panels of treated soils. The walls of the two shafts were constructed by overlapping the CSM panels from ground surface to a depth of 47 feet, creating two compression rings of soil cement to act as jacking and receiving shafts. Internal diameters of the shafts were 25 feet for the receiving shaft and 32 ft for the jacking shaft. Width of CSM panels was 4 ft, and design unconfined compressive strength of the soil cement was 300 psi for the receiving shaft and 350 psi for the jacking shaft.
Malcolm created a bottom seal for each shaft by constructing overlapping 8-ft diameter jet grout columns from ground surface. The end result were micro-tunnel shafts consisting of soil cement material, tipping in a jet grout bottom seal with 200 psi unconfined compressive strength and low permeability. The jet grout plug thickness was 15 ft at the receiving shaft and 18.5 ft at the jacking shaft.
Site soils consisted primarily of soft to medium stiff clays, with interlayered loose sands. The water table was found at 25 feet below ground surface.
Continuity and strength of the soil cement and jet grout materials was corroborated through unconfined compressive strength of grab samples taken daily, complemented by cored samples. The use of CSM to construct the compression rings and jet grouting to form the bottom seal allowed for excavation of both shafts in the dry, without the need for dewatering, in spite of the high water table. The circular geometry of the shafts allow the soil cement to remain in compression, and hence no steel reinforcement was needed for the shafts.