Biaxial creep test study on the influence of structural anisotropy on rheological behaviour of hard rock

Rheological characteristics are one of most important properties needed to be considered for the designing and construction for the long term stability and serviceability of underground structures in the rock mass. Up to date, although extensive studies on the rheological properties of rocks are available in the literature, most of existing studies reported the strain-time data for the axial deformation through compression rheological method and did not mention the lateral deformation, and mainly focused on the soft rocks at shallow depth. Thus, very limited attention has been paid to the rheological properties of deep and hard rock, neglecting the effects of structural anisotropy on the rheological properties. This paper presents a comprehensive in-depth study on the rheological behaviours of super-deep hard rock considering the effects of structural anisotropy by using the uniaxial and biaxial creep tests. The results revealed that significant creep behaviour can be observed in the hard rock specimens under high stress in the in-situ conditions, and the strain-time behaviour of hard rock exhibited brittle failure. The strain-time curves of hard rock exhibited two obvious phases of instantaneous creep and steady state creep without the phase of accelerated creep. Moreover, it was observed that the rheological behaviours, including the instantaneous modulus, transient creep duration, axial and lateral creep deformations, steady state creep rate, volumetric strain and contraction ratio are strongly affected by the structural anisotropy. Based on the experimental data, empirical models of the parameters governing creep behaviour have been established

Circulating tumor DNA clearance predicts prognosis across treatment regimen in a large real-world longitudinally monitored advanced non-small cell lung cancer cohort

Background: Although growth advantage of certain clones would ultimately translate into a clinically visible disease progression, radiological imaging does not reflect clonal evolution at molecular level. Circulating tumor DNA (ctDNA), validated as a tool for mutation detection in lung cancer, could reflect dynamic molecular changes. We evaluated the utility of ctDNA as a predictive and a prognostic marker in disease monitoring of advanced non-small cell lung cancer (NSCLC) patients.Methods: This is a multicenter prospective cohort study. We performed capture-based ultra-deep sequencing on longitudinal plasma samples utilizing a panel consisting of 168 NSCLC-related genes on 949 advanced NSCLC patients with driver mutations to monitor treatment responses and disease progression. The correlations between ctDNA and progression-free survival (PFS)/overall survival (OS) were performed on 248 patients undergoing various treatments with the minimum of 2 ctDNA tests.Results: The results of this study revealed that higher ctDNA abundance (P=0.012) and mutation count (P=8.5x10(-4)) at baseline are associated with shorter OS. We also found that patients with ctDNA clearance, not just driver mutation clearance, at any point during the course of treatment were associated with longer PFS (P=2.2x10(-1)6, HR 0.28) and OS (P=4.5x10(-6), HR 0.19) regardless of type of treatment and evaluation schedule.Conclusions: This prospective real-world study shows that ctDNA clearance during treatment may serve as predictive and prognostic marker across a wide spectrum of treatment regimens

3D characterization of microbially induced carbonate precipitation in rock fracture and the resulted permeability reduction

A new seepage control method for fractured rock is biogrouting through a microbially induced calcite precipitation (MICP) process. A study on the spatial distribution of biogrout in a rock fracture and its effect on permeability reduction is presented in this paper. A series of experiments together with 3D scanning and 3D flow simulation were performed on rock fractures with various initial apertures treated by bio-grouting. A lognormal distribution of MICP precipitates along the flow direction in a fracture was observed. The 3D flow simulation of biogrouted fracture has revealed that the routinely adopted parallel plate model (cubic law) for estimating permeability of channel flow is no longer applicable when the fracture aperture is less than the critical value of 0.7 mm based on this study. This is because partially clogging will occur when the fracture aperture is less than the critical value, resulting in a transition of the flow type from surface flow to channel flow. A semi-empirical equation which can account for the effect of flow type has been proposed for estimating the permeability reduction due to bio-grouting for rock fractures.Ministry of Education (MOE)Ministry of National Development (MND)The financial supports from the Ministry of National Development, Singapore (No. SUL2013-1) and the Ministry of Education, Singapore (MOE2015-T2-2-142) are greatly acknowledged

A modified optimization algorithm for back analysis of properties for coupled stress-seepage field problems

The study is aimed at developing a new algorithm for back analysis of rock mass parameters based on the observed displacements of tunnel excavations under coupled stress-seepage field problems which are generally encountered in hydraulic tunnel projects. The back-analysis algorithm is developed by incorporating the Levenberg-Marquardt optimization technique with complex-variable differentiation method. Additional auxiliary technique is also incorporated to enhance the convergence and stability of the proposed algorithm during the back-estimation of multiple rock mass parameters. A hypothetical hydraulic tunnel case was used for testing and validating the proposed algorithm by incorporating the method in a finite element code in which multiple rock mass parameters (such as modulus, permeability, and in situ stress) were treated as target parameters. Results show that the multiple rock mass parameters can be accurately and efficiently estimated by back analysis using a newly developed algorithm for coupled stress and seepage fields encountered in tunneling. The proposed algorithm can be used for predicting excavation behavior, particularly, the stress-induced deformations at subsequent stages of tunnel excavation under coupled multiple fields (e.g. stress and seepage fields)

A new shear strength criterion of three-dimensional rock joints

The presence of rock joints substantially reduces the rock mass strength. The deformability and stability of natural and engineered rock structures, such as rock slopes and underground caverns, are highly afected by rock joints along which sliding can easily occur. Natural rock joints are inherently rough with randomly distributed asperities that control the shear behaviour. Predicting the shear strength of rough rock joints is crucial for the stability analysis and support design of rock-engineering structures.Yingchun Li thanks the financial supports from the National Natural Science Foundation (51809033 and U1765107), the China Postdoctoral Science Foundation (2019T120208), the National Key Research and Development Plan (2018YFC1505301), and the Fundamental Research Funds for the Central Universities (Grant No. DUT19RC(4)022)

Reinforcement Mechanism and Erosion Resistance of Loess Slope Using Enzyme Induced Calcite Precipitation Technique

The disaster of loess slope seriously threatened the safety of people and property. Enzyme Induced Calcite Precipitation (EICP) was demonstrated as an environmentally friendly soil improvement method. However, few studies have focused on the improvement effect of EICP on loess slopes. In this study, a series of tests were conducted to investigate the effect of EICP and added either basalt fiber (BF) to the loess or polyvinyl acetate emulsion (PVAC) to the solution on the erosion resistance of loess slopes. The results showed that all of the EICP, EICP-BF, and EICP-PVAC treatments could improve surface strength (SS). The addition of 50 g/L PVAC achieved high SS because the network structure formed by PVAC promoted the affixation of CaCO3. The thickness of the crust layer decreased with the increasing BF content or PVAC concentration. With the increasing number of EICP treatment cycles, the CaCO3 content increased progressively, but the increase rate decreased. For rainfall erosion, the time until erosion occurred was delayed and the stability was improved for loess slopes treated with EICP, EICP-BF, and EICP-PVAC. The high erosion resistance of loess slopes treated with EICP-0.5% BF, EICP-30 g/L PVAC, and EICP-50 g/L PVAC was attributed to the stable spatial structure formed by CaCO3 precipitation and the additional cementation provided by high BF content and PVAC concentration. The addition of 0.5% BF effectively inhibited the development of surface cracks in loess slope after dry–wet cycles. With the increasing number of dry–wet cycles, the accumulative loess loss weight of slopes treated with various methods increased gradually. Among all treatment methods, the number of dry–wet cycles had less effect on EICP-30 g/L PVAC treated loess slopes. This study provided guidance for loess slopes prevention

Laboratory investigation on rheological properties of greenschist considering anisotropy under multi-stage compressive creep condition

The influence of structural anisotropy on time-dependent behaviour of foliated metamorphic greenschist collected from deep underground tunnel (−1300 m) in China was investigated to acquire the fundamental knowledge of the long-term mobility of greenschist in nature. Considering the structural anisotropy, greenschist samples with different inclination of foliation/bedding planes (i.e., θ = 0°, 35°, 45°, 60° and 90°) was prepared for creep test. Multi-stage constant loading method is employed in this test with each stage equals to 10 MPa, 20 MPa, 30 MPa and 40 MPa respectively for presenting analogous situation of variable stressing of greenschist caused by over laying rocks and tectonic stresses. The test results revealed the significant effect of structural anisotropy on the creep development (i.e. instantaneous strain, transient creep, steady-state creep) and failure mode. The control of structural anisotropy on steady-state creep rate is strengthened at high stress level, compared to that is weaken on other properties (i.e. instantaneous strain, transient strain), which is mainly due to the evolution of tensile and shear crack along and cross the bedding plane at high stress level. The three types of failure models of greenschist (i.e. shear along the bedding plane, cross the bedding plane or compound) were observed which hints that the long-term mobility of greenschist rockmass is mainly dependent on characteristic of structural anisotropy and stress conditions. In addition, based on the experimental results, creep models were proposed for greenschist considering the bedding plane and their parameters were obtained using test result. We believe that these test results and the models would throw some insights and benefit practicing design engineers and numerical modellers

Shear behavior of greenschist along foliation plane considering anisotropy

The shear behavior of rock mass along its structural plane, i.e., joint, bedding plane, and foliation plane, plays a key role in determining rock strength which is highly anisotropic. The shear behavior along the foliation plane is rarely addressed from engineering geology point of view in the effect of anisotropy of on the shear behavior along foliation plane, not to mention the effect of anisotropy. In this study, the direct shear behavior of greenschist along the foliation plane is investigated using both intact and sheared greenschist samples. Identical concrete replicas for the sheared greenschist samples are prepared using 3D scanning and printing, in this study, and sheared in four directions, i.e., 0°, 90°, 180°, and 270°, under four normal stresses (σ n / σ c = 0.1 , 0.2 , 0.3 , 0.4). The intact greenschist samples show significantly higher shear strength compared to the sheared samples due to the bonding strength along foliation plane. The shear behavior of greenschist along foliation plane is anisotropic due to the 3D roughness of foliation plane, and the anisotropic behavior diminishes as the normal stress increases. Moreover, the replicas of sheared greenschist samples shows same shear strength as the sheared greenschist samples, thus suggesting that the concrete is suitable for simulating greenschist shear behavior, including the peak shear strength, residual shear strength and stiffness, which also hints that the shear behavior of the sheared greenschist is dramatically controlled by surface roughness of the joint rather than rack by comparing the results of the shear greenschist with concrete replicas.This work was financially supported by National Natural Science Foundation of China (Grant Nos: 41602287, 51578408, 51509219). Soumyajit Mukherjee handled this article and reviewed twice. Anonymous reviewers are thanked for detail comments in two rounds

Scale Effects on Shear Strength of Rough Rock Joints Caused by Normal Stress Conditions

Scale effects on the mechanical behavior of rock joints have been extensively studied in rocks and rock-like materials. However, limited attention has been paid to understanding scale effects on the shear strength of rock joints in relation to normal stress σn applied to rock samples under direct shear tests. In this research, a two-dimensional particle flow code (PFC2D) is adopted to build a synthetic sandstone rock model with a standard joint roughness coefficient (JRC) profile. The manufactured rock model, which is adjusted by the experiment data and tested by the empirical Barton’s shear strength criterion, is then used to research scale effects on the shear strength of rock joints caused by normal stresses. It is found that the failure type can be affected by JRC and σn. Therefore, a scale effect index (SEI) that is equal to JRC plus two times σn (MPa) is proposed to identify the types of shear failure. Overall, shearing off asperities is the main failure mechanism for rock samples with SEI > 14, which leads to negative scale effects. It is also found that the degree of scale effects on the shear strength of rock joints is more obvious at low normal stress conditions, where σn < 2 MPa

Assessment of New Bio-Cement Method for Sand Foundation Reinforcement

Microbially induced carbonate precipitation (MICP) is a new method used in recent years to improve the soil. However, this method still faces challenges related to low grouting reinforcement strength and efficiency. In this study, both the bio-cement infiltration method and bio-cement mixed method for sand foundation were proposed, and physical model tests were conducted to investigate the mechanical properties of sand treated with the bio-cement method. The results showed that the bio-cement maximized the utilization rate of bacterial liquid and reduced the waste caused by the loss of bacteria compared with traditional methods. Both the size of the reinforced area and bearing capacity of the sand reinforced by bio-cement infiltration method were controlled by the volume ratio of the bio-cement, calcareous sand powder, and the inflow rate. The maximum bearing capacity was 125 N when using a mixture of bio-cement and calcareous sand powder with a ratio of 400/80, with an inflow rate of 20 mL/min. The UCS of the sand reinforced by the bio-cement mixed method gradually decreased from 3.44 MPa to 0.88 MPa with depth, but increased with increasing CaCO3 content. The CaCO3 crystals were primarily concentrated at the contact point between the particles, and the formed crystals were mainly polyhedral. Reduction in the CaCO3 content mainly occurred in the central deep part of the reinforcement area. The result provides an experimental basis for the use of bio-cement in the reinforcement of sand soil foundations