Soil, cement, then mix.

Cutter Soil Mixing (CSM) is a highly effective method for the construction of watertight shafts under adverse soil conditions. This in situ soil-cement mixing process has the distinct advantage of real-time monitoring of vertical alignment (x- and y-axes), which cannot be found with other, traditional soil-cement mixing processes. Real-time monitoring of the vertical alignment minimizes the risk of pile or wall panel drift. This common problem often occurs with the traditional soil-cement mixing processes when shafts start exceeding depths of about 60 or 70 feet.

The CSM method creates rectangular panels by shearing the soil with horizontally mounted cutter wheels attached to a stiff Kelly bar. This is very different from traditional double or triple auger-type mixing paddles on soil-cement drill rigs. During the CSM excavation (shearing) cycle, water is added in predetermined amounts to fluidize the material in place if it is predominantly cohesive (clayey). If the material is predominantly cohesionless (silt, sand and gravel), bentonite is added to the water. This keeps the granular material in suspension, which facilitates mixing with the grout, and keeps the granular material from collapsing onto the cutterhead. In essence, the CSM method can be considered a hybrid of the soil-cement and slurry wall methods. It takes advantage of both the economies inherent with in situ soil mixing and the excavation support benefits of slurry wall panel excavation.

The cutter wheels for the CSM method mount on a horizontal axis and turn on a vertical axis. The cutter wheels typically rotate upwards towards each other directing the soil cuttings towards shear blades. Shear blades provide additional breakdown of the cuttings for mixing with the water that is injected at the wheel confluence. Additionally, an inclinometer mounted in the head assembly provides real-time data for the x- and y-location of the cutterhead as it excavates through the ground. The operator can change the wheel rotation to assist in counteracting deviations of the head in the plane of the cutter wheel rotation.

After the prescribed panel depth is achieved, the retraction process begins. At this time, cement grout is mixed with the fluidized soil to form a soil cement mixture. To construct the shaft, the panels are interlocked to form a contiguous ring of panels, similar to the way secant pile walls are interlocked. After the shaft panels are cured, excavation of the soil within the shaft interior can commence.

The accuracy of individual panel installation is a distinct advantage of the CSM method. It can provide a high level of confidence that individual panels are sufficiently interlocked to provide continuous straight or circular walls. Primary alternating panels are typically constructed first and allowed to cure. Secondary overlapping or face-to-face panels are then cut into and between the primary panels to form continuously interlocking panels. The panels can be constructed straight in the case of a wall or in a circular pattern in the case of a shaft. Overlapping panels can be cut into hardened panels (cured-on-cured method) or wet/partially cured panels (fresh-on-fresh or wet-on-wet method). When the application is a wall or shaft, reinforcing piles can be slotted into the panel while the soil cement is still wet. In the case of a shaft, internal panel reinforcing is not necessarily required as the design can utilize a hoop stress or “compression only” approach. In lieu of panel reinforcement, the shaft interior can be lined with shotcrete for added strength. p>

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