Development of a soil abrasion test and analysis of impact of soil properties on tool wear for soft-ground mechanized tunneling

The primary objective of this thesis was to develop a new test device specifically designed to simulate the working conditions of soft-ground mechanized tunneling as much as possible and quantify different soil types in terms of abrasivity. The testing system which directly measures the wear on wear plates mounted on a propeller that is submerged in a chamber filled with soil is capable of simulating the high contact stresses between the tool and the soil, maintaining the original soil size distribution, field moisture conditions, and the possibility of applying high ambient pressures as well as soil conditioners.;Several key testing parameters such as soil overburden, pitch angle of the propeller, tool material hardness, moisture content, ambient pressure, particle angularity, grain size distribution, propeller speed, and applied torque to the soil were studied in order to see the impact of these parameters on soil abrasion. The parametric study was performed on various soil samples with known properties including grain size distribution, mineral content, and grain sphericity and roundness. This included high quartz-content Silica sand, Limestone sand, ASTM Graded sand, ASTM 20/30 sand and a sample of Silty sand. Based on the results of the parametric study, which showed the importance of soil particle size and angularity, moisture content, ambient pressure, etc. on soil abrasion, a standard setting for the testing system was selected. The standard setting includes a rotational speed of 60 rpm, a propeller pitch angle of 10°, and cover hardness of 17 HRC. In the standard setting, tests were performed at various moisture contents including dry sample, soil with water content dry of optimum compaction, and saturated soil.;By using the standard setting, the Penn State Soil Abrasion Index (PSAI) is developed in order to allow comparisons of the weight loss of covers in the same testing conditions for different soil types. The variation of wear on the cover as a function of time is expressed by using a power function in which W=ATb, where W is the wear in grams, T is time in minutes, and A and b are constants defining the shape of the curve.;In addition, a study on the effect of relative hardness on soil abrasion was performed in both dry and moist conditions. In this study, different cover hardness (17-60 HRC) and mixtures of Silica sand and Limestone sand at controlled proportions were used to create variable ratios of tool/mineral hardness. This study confirmed that the anticipated inverted S-curve that is well-known in tribology studies can be generated when the tool hardness is kept constant and material hardness is changed by changing the mixing ratio of abrasive and non-abrasive soils. The change in tool hardness will shift the inverted S-curve, meaning that the absolute value of relative hardness is not the determining factor for wear. Furthermore, the behavior of the tribological system and wear of tools in various soil mixtures will change with the presence of water.;Moreover, the effect of reducing the wear and torque by using soil conditioners is studied by using the testing device. The testing of various soil types by the soil abrasion testing device shows that the application of proper soil conditioning can reduce the abrasion, and hence the wear of the tools and inner parts of the tunneling machine as well as give a significant reduction of the required torque. The magnitude of the reduced wear and torque could be measured in the testing device.;Finally, a comparison was made between the Soil Abrasion Test (SAT) testing system developed by SINTEF/NTNU and the Penn State Soil Abrasion Testing system. This study showed several advantages of the Penn State Soil Abrasion system as compared to the SAT testing system such as quantifying the abrasivity of the fine-grained soil, non-abrasive material, and impacts of sphericity and roundness of the particles. The PSAI testing offers less operator sensitivity and higher repeatability and consistency of the test results, combination of two- and three-body abrasion as a main wear method, ability to perform tests with different moisture contents and by applying different ambient pressure, and there is no need for a change of grain size and shape of the soil samples during the sample preparation. (Abstract shortened by UMI.).