Jie Cui

Application of computerized tomographic scanning to the study of water-induced weakening of mudstone

Water generally plays a harmful role in the stability of mudstone and has induced many geological disasters in mudrock strata. To visually examine water-induced deterioration of mudstone, X-ray computerized tomographic scanning was used to detect the real-time interaction between mudstone and water. This facilitated real-time visual observations of the evolving characteristics of water-induced cracks within the samples. According to these experimental findings, assisted with scanning electron microscopy and X-ray diffraction, the water-weakening mechanisms of mudstone can be divided into the following steps: (1) micro-sized discontinuities provide an initial path for water to invade the rock; (2) the incursive water induces volumetric swelling of clay minerals and dissolution of carbonate; and (3) these effects drive the propagation and connection of cracks. This understanding of the processes of degradation provides a visual evidence for the assessment of mudrock performance with respect to the stability of landslides, debris flows, and susceptibility of subsidence in mudstone strata.

Failure Mechanism of Unbonded Prestressed Thru-Anchor Cables: In Situ Investigation in Large Underground Caverns

Prestressed rock anchor cables are commonly utilized in geotechnical and mining engineering (Peliua et al. 2000; Tezuka and Seoka 2003; Koca et al. 2011) because their installation increases the effective strength and stability of the reinforced rock (Maejima et al. 2003; Li et al. 2012). Nevertheless, in some cases, prestressed cables have failed in large underground caverns or tunnels because of excessive deformation of the anchored rock mass or inadequate assumptions used when designing the cables (Li 2004; Lu et al. 2011; Gong et al. 2011). During the failure process of anchor cable, the stress redistribution and progressive deterioration of the reinforced mass rock can adversely affect the overall stability of the free surface, manifested as large deformation or indeed collapse of rock mass (Galvez et al. 2006; Zhu et al. 2010). Therefore, a deeper understanding of the mode and mechanism of prestressed anchor cable failure will better inform the design process of anchor cables to mitigate future failures. Prestressed anchor cables can be categorized into three general groups based on the anchoring method (Jarred and Haberfield 1997; Chen and Yang 2004): (1) tip-grouted anchor cables that have grout-bonding segments and free segments; (2) fully grouted anchor cables with anchor wires that are fully bonded with the rock; (3) prestressed thru-anchor cables that have two anchor bases without grout-bonding segments. The tip-grouted and fully grouted prestressed cables have been the subject of research more than the thru-anchor cables because they have more extensive applications (Spang and Egger 1990; Hyett et al. 1995; Serrano and Olalla 1999; Huang et al. 2002; Cai et al. 2004; Ugur et al. 2011). Even less attention has been paid to the unbonded prestressed thru-anchor cables (UPTACs) based on the limited information available in the academic and professional literature. The mechanical interactions of UPTACs are different from those of groutbonding cables because the UPTAC does not have a groutbonding section, but has two anchor bases. Under loading, the prestressed cable can restrain the deformations of the anchored rock, and the anchored rock can also transfer the rock stress to the cable via the bridge of anchor bases. This paper focuses on evaluating the failure mechanism of UPTACs based on a case study of underground caverns in Sichuan Province, China. Several external failure modes of the UPTACs observed in this project are first presented and special design techniques for the UPTACs in the large underground caverns are summarized based on in situ investigation: failure depth of disabled UPTACs, break face, measured working load and installation method.

Time-Dependent Damage Investigation of Rock Mass in an In Situ Experimental Tunnel

In underground tunnels or caverns, time-dependent deformation or failure of rock mass, such as extending cracks, gradual rock falls, etc., are a costly irritant and a major safety concern if the time-dependent damage of surrounding rock is serious. To understand the damage evolution of rock mass in underground engineering, an in situ experimental testing was carried out in a large belowground tunnel with a scale of 28.5 m in width, 21 m in height and 352 m in length. The time-dependent damage of rock mass was detected in succession by an ultrasonic wave test after excavation. The testing results showed that the time-dependent damage of rock mass could last a long time, i.e., nearly 30 days. Regression analysis of damage factors defined by wave velocity, resulted in the time-dependent evolutional damage equation of rock mass, which corresponded with logarithmic format. A damage viscoelastic-plastic model was developed to describe the exposed time-dependent deterioration of rock mass by field test, such as convergence of time-dependent damage, deterioration of elastic modules and logarithmic format of damage factor. Furthermore, the remedial measures for damaged surrounding rock were discussed based on the measured results and the conception of damage compensation, which provides new clues for underground engineering design.

Insights into statistical structural characteristics and deformation properties of columnar jointed basalts: field investigation in the Baihetan Dam base, China

The columnar jointed basalt (CJB) in the left-bank dam foundation of the Baihetan hydropower station, China, is a special jointed rock mass with a columnar cylinder structure. A field investigation revealed three types of joints developing in the CJB: (1) columnar joints, (2) internal steeply dipping joints (ISDJ), and (3) internal gently dipping joints (IGDJ). A uniform distribution survey lines method (UDSLM) is proposed to describe quantitatively the spatial structural features of CJB exposed in limited outcrops. The visible spacing of internal joints in CJB is found to evolve; joint spacing decreases with increased degrees of weathering, disturbance, and time after unloading excavation. Combined with the actual structure, the formation mechanism of CJB is discussed from the structure of a single column to the whole rock mass, based on the contraction hypothesis. A columnar joint tensor, which is closely related to the CJB deformation characteristics, is established for describing the complex columnar joint network in the three-dimensional space based on the measurement data with the UDSLM. In addition, the structural characteristics of intact columns were exposed through dismantling cuboid specimens of 0.5xa0m edge lengths and 1.0xa0m height, verifying the rationality of the UDSLM and the evaluation of structural characteristics of CJB in the Baihetan dam foundation. This in situ study presents an in-depth understanding of CJB structure and provides meaningful guidance for excavation and supporting reinforcement for the CJB exposed on the Chinese Baihetan dam foundation.

Statistical Characterization of the Mechanical Parameters of Intact Rock Under Triaxial Compression: An Experimental Proof of the Jinping Marble

We investigated the statistical characteristics and probability distribution of the mechanical parameters of natural rock using triaxial compression tests. Twenty cores of Jinping marble were tested under each different levels of confining stress (i.e., 5, 10, 20, 30, and 40xa0MPa). From these full stress–strain data, we summarized the numerical characteristics and determined the probability distribution form of several important mechanical parameters, including deformational parameters, characteristic strength, characteristic strains, and failure angle. The statistical proofs relating to the mechanical parameters of rock presented new information about the marble’s probabilistic distribution characteristics. The normal and log-normal distributions were appropriate for describing random strengths of rock; the coefficients of variation of the peak strengths had no relationship to the confining stress; the only acceptable random distribution for both Young’s elastic modulus and Poisson’s ratio was the log-normal function; and the cohesive strength had a different probability distribution pattern than the frictional angle. The triaxial tests and statistical analysis also provided experimental evidence for deciding the minimum reliable number of experimental sample and for picking appropriate parameter distributions to use in reliability calculations for rock engineering.