Soil conditioning plays an important role in reducing soil abrasivity and wear of cutting tools in earth pressure balance shield tunneling, leading to service life extension of cutting tools. In this article, an experimental study to assess the abrasivity of conditioned soil with different conditioners, namely foam and slurry, was carried out using a soil abrasion tester. The abrasive characteristics of the tested soil samples were quantitatively assessed through the measurement of weight loss of the steel cubes fixed on the propeller. Preliminary test results indicate that both foam and slurry can modify the sandy gravel to a suitable state according to slump test and decrease soil abrasivity significantly. The torque, which reflects the interaction between the propeller and tested materials, is used to provide insights into the soil-tool contact. Potential relationships between tool wear and tool position are also investigated.

Soil conditioning is of great significance for tunneling in soft ground with earth pressure balance shield (EPBS) machines, which leads to safe, highly efficient, and high-quality tunneling. To study the effects of soil conditioning on properties of a clay-sand-gravel mixed soil encountered in an EPBS tunneling project in Beijing, a series of geotechnical test methods was carried out based on laboratory test in this paper. The decay behaviors of foam particles generated by the Waring-Blender method were studied first using the image particle analysis system, and then the feasibility of soil conditioning on the mixed soil was qualitatively and quantitatively assessed through the mixing test, slump test, and friction coefficient test. The preliminary test results indicate that drainage of water in liquid film plays an important role in the decay of foam microstructure. The viscosity and flowability of the conditioned soil were modified dramatically by using various amount of water and foam, and a suitable state meeting the requirement of EPBS machines was obtained. The net power and mixing time, which reflects the interaction between the blending rods and tested soils, as well as the slump value, providing the overall indication for liquidity of the conditioned soils, and friction coefficient, reflecting the friction between steel and tested soils, were used to provide insight into the variation in viscosity and flowability of the tested soils.

Cutting tool wear is a critical problem for tunneling in soft ground with the earth pressure balance shield. In this study, a cutting tool tester was developed in the China University of Geosciences in Beijing, and a series of tool wear tests were conducted to investigate the characteristics of tool wear in granular soils. The characteristics of cutting tool wear can be quantitatively assessed through the ratio (M0 − M)/M0 (i.e., tool wear rate, %; M0 and M are the mass of the tested blocks before and after the wear test, respectively). The effects of five important factors, including mineralogy, moisture, soil conditioner, test time, and sand content, on tool wear were considered in various test conditions. According to the primary test results, it is observed that (1) high soil hardness produced significantly higher tool wear than low soil hardness in dry condition; (2) with increasing water content, the tool wear gradually increased to a maximum value and then decreased; (3) the foam conditioner mixed in sandy gravel reduced the tool wear dramatically regardless of the cutting tool material, e.g., cemented carbide and Q235 steel; (4) the tool wear increased with the test time in the dry sand mixture, and their relationship showed a good agreement with a power function; and (5) the tool wear decreased as the sand content increased in dry sand-gravel soil mixtures. The reasons for these phenomena were also explored.

Estimation of ultimate bearing capacity (UBC) of pre-stressed high-strength concrete (PHC) pipe pile is critical for optimizing pile design and construction. In this study, a standard penetration test (SPT), static cone penetration test (CPT) and static load test (SLT) were carried out to assess, determine and compare the UBC of the PHC pipe pile embedded in saturated sandy layers at different depths. The UBC was calculated with three methods including the JGJ94-2008 method, Meyerhof method and Schmertmann method based on in-situ blow count (N) of SPT (SPT-N) which was higher than the values recommended in survey report regardless of pile length. The average UBC values calculated with cone-tip resistance and sleeve friction from CPTs was also higher than the value recommended in the survey report. Moreover, the actual UBC values directly obtained by load-displacement curves from SLTs were in line with the calculated values based on in-situ SPTs and CPTs, but approximately twice as high as the values recommended in the survey report regardless of pile length. For the SPT method, the application of bentonite mud in saturated sand layers is critical for the assessment of pile capacity in the survey phase, CPTs can provide reliable results regardless of soil characteristics and groundwater if the soil layer can be penetrated, and SLTs are necessary to accurately determine the UBC in complex stratum.