Deyu Qian

Experimental Investigation of Sandstone under Cyclic Loading: Damage Assessment Using Ultrasonic Wave Velocities and Changes in Elastic Modulus

This laboratory study investigated the damage evolution of sandstone specimens under two types of cyclic loading by monitoring and analyzing changes in the elastic moduli and the ultrasonic velocities during loading. During low-level cyclic loading, the stiffness degradation method was unable to describe the damage accumulations but the ultrasonic velocity measurements clearly reflected the damage development. A crack density parameter is introduced in order to interpret the changes in the tangential modulus and the ultrasonic velocities. The results show the following. (1) Low-level cyclic loading enhanced the anisotropy of the cracks. This results from the compression of intergranular clay minerals and fatigue failure. (2) Irreversible damage accumulations during cyclic loading with an increasing upper stress limit are the consequence of brittle failure in the sandstone’s microstructure.

Focus Energy Determination of Mining Microseisms Using Residual Seismic Wave Attenuation in Deep Coal Mining

Based on the energy attenuation characteristics of residual wave in deep rock, a method was developed to determine the microseismic focus energy. Differential energy loss in infinitesimal spreading distance is logically deduced, upon which energy attenuation equation was established. With a logarithmic transformation, a linear relation of the residual seismic energy with distance is formulated. Its intercept was used to determine the microseismic focus energy. The result is compared with that determined by the energy density method. The reliability of the determined focus energy and the impact of the built-in velocity threshold on the residual wave energy computation are discussed. Meanwhile, the energy absorption coefficient used for representing the absorption characteristics of the rock medium in the mining region under study is also clarified. Key findings show that the microseismic focus energy confirmed by the residual wave attenuation is reliable. The result’s accuracy is quite high, especially for the events in deep rock with great homogeneity. The developed focus energy computation method is closely dependent on the integrity of waveform, accuracy of repositioning, and reliability of effective components extraction. The new method has been shown to be effective and practical.

Long-Term Mechanical Behavior of Nano Silica Sol Grouting

The longevity of grouting has a significant effect on the safe and sustainable operation of many engineering projects. A 500-day experiment was carried out to study the long-term mechanical behavior of nano silica sol grouting. The nano silica sol was activated with different proportions of a NaCl catalyst and cured under fluctuating temperature and humidity conditions. The mechanical parameters of the grout samples were tested using an electrohydraulic uniaxial compression tester and an improved Vicat instrument. Scanning electron microscope, X-ray diffraction, and ultrasonic velocity tests were carried out to analyze the strength change micro-mechanism. Tests showed that as the catalyst dosage in the grout mix is decreased, the curves on the graphs showing changes in the weight and geometric parameters of the samples over time could be divided into three stages, a shrinkage stage, a stable stage, and a second shrinkage stage. The catalyst improved the stability of the samples and reduced moisture loss. Temperature rise was also a driving force for moisture loss. Uniaxial compressive stress-strain curves for all of the samples were elastoplastic. The curves for uniaxial compression strength and secant modulus plotted against time could be divided into three stages. Sample brittleness increased with time and the brittleness index increased with higher catalyst dosages in the latter part of the curing time. Plastic strength-time curves exhibit allometric scaling. Curing conditions mainly affect the compactness, and then affect the strength.

Frictional analysis of pipe-slurry-soil interaction and jacking force prediction of rectangular pipe jacking

Abstract Predictive model and formula of jacking force has been well-established for circular pipe jacking in the past decades. However, the frictional mechanism of pipe–soil–slurry and jacking force prediction of rectangular pipe jacking has not well been understood. Nowadays, the rectangular pipe jacking is widely used, because of its over 20% larger effective usable area than that of circular pipe as underground facilities, with the same cross-sectional area. Nevertheless, the corresponding prediction models of thrust of rectangular pipe jacking are lacking. In order to further understand the frictional mechanism of slurry rectangular pipe jacking, five classical analytical calculation models have been proposed, considering the pipe–soil–slurry interaction, and prediction formulas are obtained. Then, 3D finite difference method was introduced to estimate the jacking force by using FLAC3D. Finally, three case studies have been carried out to validate the proposed models. The results show that the numerical and theoretical results have good consistency, and what is more, the measured jacking force can be well included within the ranges of analytical models presented in this paper, which can improve the thrust prediction of rectangular pipe jacking.

Impacts of Cyclic Loading and Unloading Rates on Acoustic Emission Evolution and Felicity Effect of Instable Rock Mass

Uniaxial cyclic loading-unloading compression experiments with schemes of six loading rates and six unloading rates were carried out on the combined testing platform. Impact of loading and unloading rates on rock AE characteristics was revealed. Results show that increasing loading and unloading rates resulted in decreasing total AE rings in the entire rock deformation and failure process. Increasing loading rate decreased the AE rings at loading stages, and increasing unloading rate decreased the AE rings at unloading stages in the same cycle. Total AE counts had a negative linear relationship to the loading and unloading rates. The loading stage was the active period of the AE phenomena, and impacts of the loading rate on the AE characteristics were more apparent. Especially when the loading rate was greater than 2.0  kN/s, brittle failure of rock specimens became noticeable. After the cyclic load reached the uniaxial compressive strength of 1/3∼ 1/2 times, the rock Felicity effect became more obvious. With the increase of the loading rate, the Felicity ratio decreased in the elastic stage and increased a little in the plastic stage, whereas with the increase of the unloading rate, the Felicity ratio decreased gradually in the elastic stage and remained almost the same in the plastic stage.

The Influence of Heading Rate on Roof Stability in Coal Entry Excavation

Coal entry heading is one of the most hazardous activities in coal mine operations because a certain area of an unsupported roof inevitably forms and poses a significant threat to the safety of miners. In order to accelerate the coal entry heading, a simplified method including theoretical analysis and laboratory and in situ tests was developed to predict the influence of heading rate on the stability of the unsupported immediate roof. The results demonstrate that the deflection of the unsupported immediate roof at the heading face is on a scale of millimetre; hence, monitoring the deformation by conventional observation methods is difficult. The proposed model shows that, within the unsupported immediate roof, the peak values of normal stresses σx (perpendicular to the direction of excavation) and σy (parallel to the direction of excavation) and shear stresses τxz (perpendicular to the direction of excavation) and τyz (parallel to the direction of excavation) have different changing trends. The peak values of σx and σy both rise with the increasing advancing distance; however, σy reaches the tensile strength within a shorter range than σx. Moreover, the peak values of τxz and τyz initially increase with the increasing advancing distance and then stabilize or decline. The major threat to roof stability at the heading face is tensile failure parallel to the heading direction. According to the industry practices, it is proved that our method can make a good prediction of the mechanical state of the unsupported immediate roof, further deriving the heading rate with a considerable safety margin.