عنوان مقاله [English]
To extend the application of earth pressure balance (EPB) machines to different geological conditions, the soil to be excavated must be treated with soil conditioning additives. The additives are injected at several points on the EPB machine; ahead of the cutting head, in the pressure chamber and along the screw conveyor. The behavior of the excavated soil, from mixing to extraction, depends mainly on the workability of the conditioned soil. Workability is the ease with which the excavated soil can be mixed with additives, compressed in a pressure chamber, flow freely from the face and be extracted by a screw conveyor. Workability is an indicator of the plasticity of the support media and the cohesiveness of the soil and is, thus, a criterion for applicability of EPB tunneling. The factors that affect the workability of the conditioned soil are water content, soil grade and size, additives and conditioning parameters. In this study, the effect of soil grade and size on the workability of conditioned soil for EPB-TBM tunneling is assessed using a laboratory slump test. The influence of fine grains on the workability of conditioned soil is also investigated by testing mixtures with different soil-foam contents. The test results show a quantitative relationship between fine-grains and workability for the application of EPB shields. The results clearly indicate that decreasing the fine grain content becomes stronger with an increase in the slump of the conditioned soil. It is concluded that a fine grain content of 25% to 60% produces the most normal behavior of the percentages tested in this study.
EPB machines in closed mode make the use of the excavated soil to support the face. The support medium must fulfill a range of characteristics that include workability, water permeability, stickiness and abrasivity (Thewes et al., 2010). Not all types of soil are capable of being processed into support media having the required characteristics, which means that conditioning of the excavated soil may be necessary (Thewes and Budach, 2010). EPB-TBMs have increased in popularity since their first applications in Japan in the 1970s. This is in response to the mechanical improvements in EPB machines and the more effective use of additives (mainly foams and polymers) injected into the screw conveyor, bulk chamber and ahead of the cutting head to condition the soil (Peila et al., 2007). Conditioning is done by modifying the soil into a plastic, pulpy, impermeable paste that can correctly control tunnel face ground movement. It applies stabilizing pressure to the face, controls water flow, decreases wear on tools, the cutterhead and the screw conveyor and permits easy handling of the muck during transport (Peila et al., 2009). Excavated soil-foam mixtures must comply with requirements for use as a support medium. Testing can determine properties such as density, shear strength, water permeability, stability and workability (Thewes et al., 2010). The behavior of the conditioned soil relies mainly on its workability. Factors affecting workability include the size and grade of the soil, water content, use of admixtures and conditioning parameters. The size and grade of the soil controls ground behavior and workability of the support medium during EPB excavation (Thewes et al., 2010; Peila et al., 2008). Workability can be measured using a slump test. In this study, the influence of fine-grained soil on the workability of conditioned soil is examined using a laboratory slump test. Soil-foam-mixtures of different soil sizes are also analyzed using statistical methods at different foam injection ratios (FIR) under atmospheric conditions.
Methodology and Approaches
Slump testing has been performed according to ASTM C143 standard to measure the workability of the generated support medium (foam + excavated soil) on fresh concrete and self-compacting concrete. Previous research indicates that the slump cone test offers a good indication of the workability of the conditioned material. A laboratory foam generator has been used to generate foam for conditioning. A commercial foaming agent with a surfactant concentration of 2.5% has been applied in this investigation. Four frothy soil samples have been tested for workability and suitability of the conditioned materials for an EPB shield. The soils have artificially been prepared in the laboratory by mixing different percentages of fine grains, sand and gravel. Testing has been carried out with different FIRon different types of soils.
Results and Conclusions
The tests performed show that it is possible to determine the influence of different properties on the conditioned soils with foam that are essential for tunneling. The fine grain content has been used as an independent variable in the selected regression model to predict slump. It is concluded that the fine grain content and FIR are useful properties for prediction of slump in EPB tunneling.