Deep Soil Mixing for Retention of Excavations
The Deep Mixing Method (DMM) is an in-situ modification technique which is performed to improve strength, reduce liquefaction potential, lower permeability, reduce deformations and allow construction in difficult areas. In particular, DMM has been used to create wall type geometries which function as excavation support or hydraulic cutoff walls. These techniques are classified based on geometry, construction techniques, type of stabilizers, state of stabilizers, and the method of mixing (Bruce, 2000). The primary goal is to provide retention and reduce seepage in a cost effective manner. This is generally achieved by mixing slurry containing combinations of ordinary Portland cement, bentonite, fly ash, set retarders, or super plasticizer with the in-situ soils by using a series of overlapping auger flights (figure 1). In the U.S., this method for construction is known as the Soil Mixing Wall technique (SMW) as a trade name.
Figure 1. Auger mix paddles used in the SMW method (Bauer, 2012)
The main aspects of construction of SMW walls are summarized in Figure 2. As shown, construction is separated into two distinct categories by operation. Operation II is accomplished by using a multiple axis auger mixing tool and shaft like the one shown in Figure 1 which is drilled into the ground at a specified rate by a drill rig (Proboha, 2001). Drilling power can be focused on one single shaft to penetrate problematic layers and different auger shafts can be used to accommodate cohesive soils, sands, and gravels (Taki and Yang, 1991). Before mixing and injection, a guide trench is excavated to handle the spoils produced during the process.
Figure 2. Construction aspects of DMM (Proboha, 1998)
Operation I consists of a batch mixing plant which mixes the cement slurry with bentonite or other binders and provides the mixture to the drill rig by means of a pressurized delivery pump (Figure 3). To ensure that the mixture is homogenous, an agitator re-mixes the final product before delivery to the rig. The plant also typically contains a computer to monitor flow and batching scales for quality control (Taki and Yang, 1991). Quality control during construction is monitored at the batch plant. According to FHWA, there are 3 levels of increasing complexity which categorize quality control. Level 1, the most basic level, is often specified in projects with very simple construction and predictable site conditions where only a single user is needed to monitor results (Bruce, 2000). In larger more sophisticated projects, higher levels are specified to fully automate the process.
Figure 3. Construction operations of DMM (Bauer, 2012)
Sites that have low tolerance to vibration and noise disturbance, high groundwater tables, and in-situ soils that are ideally suited to form either low permeability or high strength and durability mixed columns are well-suited for this technique (Proboha, 1998). One of the main advantages of DMM over many of its competitors is that it generates moderately low noise pollution and very low vibration (Figure 4).
Figure 4. A Comparison of the environmental impacts of various techniques (Proboha, 1998)
The SMW method is often in competition with various other ground improvement techniques including permeation grouting, soil nailing, and jet grouting (Rutherford, 2004). Alternative wall type construction techniques include sheet pile walls, lagging walls, secant/tangent walls, and slurry walls (diaphragm walls). The main advantages of DMM that make it a competitive and viable ground improvement technique are that it produces fewer spoils than slurry wall applications, creates low noise and vibration disturbances, can be adapted to handle complex fills and obstructions, can be used for near-surface groundwater tables, and can be used with tieback anchors and struts to provide structural support (FHWA, 2000). The disadvantages of the technique are:
- anchorage of tiebacks can cause localized areas that fail to seal off the wall
- disposal of excavated spoils can be costly
- specialty contractors and equipment are required
- freeze-thaw cycles can cause durability problems by flaking away the soil-cement surface through a process called surficial crumbling (Rutherford, 2004).
This paper focuses on the application, construction aspects, design considerations, and lessons learned from case studies involving DMM for excavation support and hydraulic cutoff walls. The goal will be to present some of the fundamental concepts of deep soil mixing and analyze the trends among the projects that have used it successfully.
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