Driven by the unprecedented process of urbanization across the globe and rise of need in infrastructure such as transportation, water, sewer, utilities, storage space, etc., the world has witnessed rapid expansion of the tunneling and underground construction industry in the past few decades. Among all tunnel construction methods, the growth of soft-ground tunneling using Earth Pressure Balance (EPB) Tunnel Boring Machines (TBMs) has been the most significant. EPB TBMs use the excavated muck in the machine chamber as a supporting medium before moving it to the muck haulage system. This is conducted to achieve the desired functionality of the conditioned muck, such as maintaining face stability while ensuring the designed advancing rate. To modify the behaviors of the muck, soil conditioners are often injected to the tunnel face and the machine chamber so the conditioned muck will exhibit certain properties such as good balance between flowability and viscosity, low abrasiveness, and low stickiness. There is no universally accepted approach for evaluation of these properties, leading to the current industry trend of using trial-and-error evaluation approaches.
The primary purpose of this thesis was to develop a new system for large-scale measurement of the rheology of conditioned soil for application in EPB TBM tunneling. The development is presented in three phases:
1. The first phase was to evaluate the feasibility of measuring the rheology of conditioned soil using existing conventional small-scale rheometers designed for liquid testing in industries such as biological and chemical engineering. The results show that the small-scale rheometers are capable of testing the yield stress and viscosity of liquid using oscillation sweep and strain ramp methods. The gaps between the vanes and the cells of the rheometers, however, are not large enough to accommodate the free flow of soils containing coarse sand or even larger particles. The torque and axial force capacities of the rheometers are also insufficient to test firm soils. It was therefore necessary to develop a large-scale rheology measurement system for evaluating rheology of conditioned soil in the context of EPB tunneling applications.
2. The second phase was to develop a large-scale rheology measurement system and conduct preliminary testing to verify the viability of the proposed system for assessment and measurement of soil rheology. Built upon the prototype of the existing Soil Abrasion Index (SAI) testing machine, a Variable Frequency Drive (VFD) was incorporated to allow for control of the rotational speed of the propeller. The pitched propeller was used as a preliminary configuration for assessment of the shear resistance vs. shear rate relationship for various mediums. Several steps were taken to establish the feasibility of the system, including device calibration in air and water, testing on a poorly graded sand with different water content conditions, and back analyses of rheological parameters using Computational Fluid Dynamics (CFD) modeling. The results demonstrate that the system of combining lab experiment and CFD modeling is a feasible method to determine rheology of conditioned soil. The Bingham plastic model is shown to be a suitable model to represent the rheology of conditioned soil. The operational range of the system’s rotational speed for this purpose is between 3 rpm and 60 rpm. Furthermore, a parametric study using CFD modeling was conducted to evaluate the optimal configuration of the propeller. The results show that the auger geometry with a similar diameter to that of the existing pitched propeller is the optimized configuration for measurement of soil rheology.
3. The third phase was to verify the system with an auger propeller by testing various scenarios including different soil types with various water content conditions and conditioning parameters (i.e., Foaming Agent Concentration, Foam Injection Ratio, Foam Expansion Ratio), test durations, ambient pressures, and compressibility settings in CFD models. A general measurement protocol was set up for future assessment. The results proved the ability of the system in measuring the variation in the rheology of the soil mix, using different conditioning parameters.
A preliminary study was conducted to assess the preparation and rheology evaluation of conditioned clayey soils. For testing rheology of foam conditioned clay, the major challenge was to mix clay and foam homogeneously in a timely manner. Six different mixing methods were tested, and none of them proved to provide a quality clay-foam mixture for subsequent rheology testing. For clay clogging evaluation, mixtures of clayey soil at various water content levels were tested in the proposed rheometer with the goal of finding the possible impact of surcharge loading on clay clogging potential using propeller torque as an indicator. The results show high variability in torque with respect to different surcharge loading. Due to the torque limit of the rheometer, limited testing of the clay at water content near plastic limit was conducted, which showed high tendency of clogging.
With the newly developed rheology measurement system, soils with different natural compositions may be conditioned with various soil conditioning agents such as water and foam, and the rheology of the conditioned soils can be characterized. This will lead to establishing a database of soil rheology based on soil conditioning parameters, which can be combined with CFD modeling of EPBMs to predict the machine response and optimize soil conditioning practices.
|Commitee:||Sampaio, Jorge, Walton, Gabe, Hedayat, Reza|
|School:||Colorado School of Mines|
|School Location:||United States -- Colorado|
|Source:||DAI-B 81/12(E), Dissertation Abstracts International|
|Subjects:||Civil engineering, Geophysical engineering|
|Keywords:||Computational fluid dynamics, Earth pressure balance TBM, Rheology, Soil conditioning, Viscosity, Yield stress|
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