This research carries out the model test and numerical simulation, studying the time-varying response regularity of surrounding rock construction mechanics. The purpose was to ensure the construction safety of a double-arch tunnel without a middle pilot tunnel and to prevent rockburst or large deformations of surrounding rock in a stress concentration areas. To resist shear deformation of the steel arch of the first tunnel, during construction of the secondary lining of the lower step of the following tunnel near the side of the first tunnel, shear reinforcement is added to form a shear wall. To resist the stress of upward uplift in the inverted arch area, the spacing of steel reinforcements in the inverted arch is encrypted to reduce disturbance to the supporting structure. To reduce disturbance to the first tunnel caused by the blasting construction of the following tunnel, this article carried out the SHPB test on field rock samples and suggests that the reserve 2 m width of the surrounding rock (the lower heading of the following tunnel near the side of the first tunnel) is mechanically broken.

When the Holmquist-Johnson-Cook (HJC) constitutive model is used to simulate limestone under the impact load, problems of a compaction stage not being characterised and low dynamic peak stress prediction accuracy are observed. The numerical simulation and experimental results were inconsistent. In this study, we proposed a modified HJC constitutive model and parameter determination method. Based on the characteristics of the dynamic stress-strain curve of limestone and relationship between axial and volumetric strains, the linear elastic phase of the state equation of the original HJC constitutive model was modified, and a new state equation was proposed. The yield surface of the original HJC constitutive model was modified on the basis of the sensitivity analysis method and limit surface theory, and a method for determining the parameters of the modified HJC constitutive model of limestone was proposed. The modified model and parameter determination method were experimentally verified using the split Hopkinson pressure bar (SHPB) and a high-speed camera. The results showed that the state equation curve of limestone under the impact load was divided into compaction, linear elastic, and fully compacted stages. After the introduction of the pressure parameters M, P, and Q, the nonlinear change in the stress-strain curve during the compaction stage was highly consistent with the SHPB results. The strength parameters fc, A, B, and N exhibited the maximum impact on the dynamic strength of limestone. After the strength parameters A, B, and N were modified for the yield surface, the prediction accuracy of limestone dynamic peak stress was over 97%, the prediction error rate decreased by more than 10%, and the reliability of numerical simulation results improved. These results can provide a simple and feasible numerical simulation method for the dynamic analysis of rock materials.

High-sensitivity magnetometry is of critical importance to the fields of biomagnetism and geomagnetism. However, the magnetometry for the low-frequency signal detection meets the challenge of sensitivity improvement, due to multiple types of low-frequency noise sources. In particular, for the solid-state spin quantum magnetometry, the sensitivity of low frequency magnetic field has been limited by short T * 2 . Here, we demonstrate a T2-limited dc quantum magnetometry based on the nitrogen-vacancy centers in diamond. The magnetometry, combining the flux modulation and the spin-echo protocol, promotes the sensitivity from being limited by T * 2 to T2 of orders of magnitude longer. The sensitivity of the dc magnetometry of 32 pT/Hz 1/2 has been achieved, overwhelmingly improved by 100 folds over the Ramsey-type method result of 4.6 nT/Hz 1/2 . Further enhancement of the sensitivity have been systematically analyzed, although challenging but plenty of room is achievable. Our result sheds light on realization of room temperature dc quantum magnetomerty with femtotesla-sensitivity in the future.

After removing the mold from the secondary lining concrete of a tunnel, problems such as honeycomb and hemp surface easily occur. To obtain self-compacting fair-faced concrete (SCFFC) that can meet strength requirements and effectively solve the above problems, this research prepared SCFFC with different mix proportions and performed slump expansion, slump, J-ring expansion and mechanical tests. Additionally, this research comprehensively analyzed the SCFFC based on fuzzy mathematics to study its apparent quality and service performance. This research aimed to solve problems such as uneven bubbles and poor bubble diameter in C30 SCFFC, through a combination of defoaming and air entraining by adding defoamer and air-entraining agent according to different proportions for compound treatment. The defoamer dosage was 0.5‰ of that of water reducer, and the air-entraining agent dosage was 0.1‰ of that of cement. The workability and clearance passability of the concrete were optimal. At the same time, the apparent holes in the SCFFC were small, as were their area and quantity. The distribution and apparent color of the SCFFC were uniform. Considering the factors affecting the service performance of concrete and after a comprehensive analysis of the samples’ weights, subjection degree, variability, stability, and strength index, this research found that the ratio for obtaining a C30 SCFFC material with the highest apparent quality and service performance was as follows: cement:machine-made sand:crushed dtone:fly-ash:water = 4:8.6:9.3:1:2.2. The water reducer comprised 1.0% of the total mass of the cementitious materials. The defoamer dosage was 0.5‰ of that of water reducer, and the dosage of air-entraining agent was 0.1‰ of that of cement.

This research aims to study the surrounding rock loosening pressure variation law of tunnel in the fold area. Based on the calculation method of surrounding rock loosening pressure for shallow tunnel, a new calculation model of the surrounding rock pressure was proposed for tunnel in the fold area; through this calculation model, the effects of tectonic stress (F), the angle ( φ 1 ) between tectonic stress and horizontal plane, tunnel buried depth (h), friction angle ( θ ), the multiple (k) between tectonic stress and rock mass gravity in the upper part of the tunnel, lateral pressure coefficient ( λ ), and tunnel midline offset (t) on tunnel surrounding rock loosening pressure in fold area are studied, respectively. Results show that in the anticline area, when φ 1 increases, the vertical loosening pressure (q) decreases; when q > 0, the surrounding rock is in the elastic deformation stage, and q decreases monotonously as F increases; when q < 0, the rock mass is in the initial stage of failure, and as F continues to increase, the number of internal cracks increases, the rock mass reaches its ultimate bearing capacity and then fails completely, and q increases linearly in this process; q decreases with the increase of θ and k; the greater k is, the easier it is to reach its bearing limit; the horizontal loosening pressure (e) increased monotonously with the increase of h and λ . The research process of surrounding rock loosening pressure of tunnel in the syncline area is similar to that of tunnel in the anticline area; q decreases with the increase of θ and λ ; q monotonically increases with F increasing.