Optimization of construction scheme and supporting technology for HJS soft rock tunnel

For a soft rock tunnel under high stress in jointed and swell soft rock （HJS）, two construction schemes pilot-tunneling enlarging excavation and step-by-step excavation were optimized using FLAC20, and the deformation effects of the two construction schemes were verified by field tests. Based on engineer- ing geological investigation and mechanical analysis of large deformations, the complex deformation mechanisms of stress expansion and structural deformation of the soft rock tunnel were confirmed, and support countermeasures from the complex deformation mechanism converted to a single type were proposed, and the support parameters were optimized by field tests. These technologies were proved by engineering practice, which produced significant technical and economic benefits.

An experimental study of a yielding support for roadways constructed in deep broken soft rock under high stress

A rationally designed support for deep roadways excavated in broken soft rock under high stress was investigated. The deformation and failure characteristics and the mechanism of ''yielding support'' was studied for anchor bolts and cables. The rail roadway of the 2-501 working face in the Liyazhuang Mine of the Huozhou coal area located in Shanxi province was used for field trials. The geological conditions used there were used during the design phase. The new ''highly resistant, yielding'' support system has a core of high strength, yielding bolts and anchor cables. The field tests show that this support system adapts well to the deformation and pressure in the deep broken soft rock. The support system effectively controls damage to the roadway and ensures the long term stability of the wall rock and safe production in the coal mine. This provides a remarkable economic and social benefit and has broad prospects for fur- ther application.

Case study of performance assessment of overlapping shield tunnels with a small curve radius

Due to the network planning of subways and their surrounding structures,increasingly more overlapping shields with a small curve radius have been constructed.A newly constructed upper tunnel partly overlaps a lower one,leading to the extremely complex uplift of the lower tunnel caused by the construction of a new tunnel.Based on the shield-driven project that runs from the Qinghe Xiaoyingqiao Station to the Qinghe Station in Beijing,which adopts the reinforcement measures of interlayer soil grouting and steel supports on site,in this study,the uplift pattern of the lower tunnel and the stress characteristics of steel supports were investigated through numerical simulations and on-site monitoring.The study results show that among all tunnel segments,thefirst segment of the shield witnesses a maximum uplift displacement that increases with the horizontal space between tunnels.On using either interlayer soil grouting or steel-ring bracing reinforcement,the uplift of the tunnel lining exceeds the control value;by contrast,when these two measures are jointly applied,the uplift of the tunnel lining does not exceed a maximum value of 4.87 mm,which can satisfy the requirements of deformation control.Under these two joint measures,the soil strength between two stacked shield tunnels can be enhanced and the uplift deformation can be restricted with the interlayer soil grouting.Also,the segmental deformation and overall stability of the existing tunnel can be controlled with the temporary steel supports.The deformation of circumferential supports and segments is closely related to each other,and the segmental uplift is controlled by H-shaped steel supports.With the increase in the horizontal space between twin shields,the effect of the construction would gradually weaken,accompanied by a gradual reduction of the stresses of steel supports.Thesefindings provide a valuable reference for the engineering design and safe construction of overlapping shield tunnels with a small curve radius.K E Y W O R D Sfield monitoring,numerical simulation,overlapping shields,small curve radius,steel support stress,uplift of tunnel lining Highlights•A project of twin overlapping shield tunnels with a small curve radius is introduced.•Protective measures of soil grouting and H-shaped steel supports are proposed.•A 3D model in view of overcutting soils,grouting,and steel supports is established.•The performance of the overlapping shield tunnel with a small curve radius is evaluated.

Dynamic leakage mechanism of gas drainage borehole and engineering application

Borehole leakage not only affects the gas drainage effect but also presents considerable risk to human security. For the research on the leakage mechanism of gas drainage borehole, the rheological and visco-elastic-plastic characteristics were considered to establish the mechanical model of coal mass around borehole, which is used to analyze the leakage mechanism and deduce the dynamic leakage model. On the basis of the real coal seam conditions, the variation rules of the stress, leakage ring, and air leakage amount were analyzed through numerical simulation, and the influence factors of air leakage amount were also investigated to provide the theoretical basis for the sealing technology. Results show that the air leakage amount of borehole is inversely proportional to the increase in supporting stress and sealing length, and directly correlated with the increase in borehole radius and softening modulus. Using theoretical analysis, we design a novel active supporting sealing technology that can use grouting material to seal the fractures to reduce the leakage channels and also provide supporting stress to prevent borehole deformation. The engineering test results indicate that the average gas concentration with the novel active supporting sealing technology is increased by 162.12% than that of traditional polyurethane sealing method. Therefore, this technology not only effectively resolves borehole leakage but also significantly improves the gas drainage effect.

Bolt support mechanism based on elastic theory

Mechanical model of anchorage surrounding rock considering tray effect was established based on elastic theory,in order to study the mechanism of bolt supporting.Elastic solutions of normal force at point in the interior of a semi-infnite solid were obtained by means of classical displacement function method in elasticity.The factors which influence stress of bolted surrounding rock,such as the length of bolt and tray effect,were analyzed.The absolute value of stress along bolt axes decreased rapidly with an increase in radical distance and the maximum appeared near ends of bolt.With increasing radical distance,the value of radical stress changed from positive to negative roughly and then increased to zero,with maximum at the middle of bolt.The evolution of hoop stress as radical distance increasing was similar with stress along bolt axes.With an increase in depth,the radical effect ranges of all normal stress components were reduced.These suggest that the effect from tray on stress along bolt axes of bolted surrounding rock could be neglected,except near surface of surrounding rock.

Stability control of surrounding rocks for a coal roadway in a deep tectonic region

In order to effectively control the deformation and failure of surrounding rocks in a coal roadway in a deep tectonic region, the deformation and failure mechanism and stability control mechanism were studied. With such methods as numerical simulation and field testing, the distribution law of the displacement, stress and plastic zone in the surrounding rocks was analyzed. The deformation and failure mechanisms of coal roadways in deep tectonic areas were revealed: under high tectonic stress, two sides will slide along the roof or floor; while the plastic zone of the two sides will extend along the roof or floor,leading to more serious deformation and failure in the corner of two sides and the bolt supporting the corners is readily cut off by the shear force or tension force. Aimed at controlling the large slippage deformation of the two sides, serious deformation and failure in the corners of the two sides and massive bolt breakage, a ‘‘controlling and yielding coupling support'' control technology is proposed. Firstly, bolts which do not pass through the bedding plane should be used in the corners of the roadway, allowing the two sides to have some degree of sliding to achieve the purpose of ‘‘yielding'' support, and which avoid breakage of the bolts in the corner. After yielding support, bolts in the corner of the roadway and which pass through the bedding plane should be used to control the deformation and failure of the coal in the corner. ‘‘Controlling and yielding coupling support'' technology has been successfully applied in engineering practice, and the stability of deep coal roadway has been greatly improved.

Relection of elastic waves at the elastically supported boundary of a couple stress elastic half-space

The relection elastic waves at the elastically supported boundary of a couple stress elastic half-space are studied in this paper. Different from the classical elastic solid, there are three kinds of elastic waves in the couple stress elastic solid, and two of them are dispersive. The boundary conditions of a couple stress elastic half-space include the couple stress vector and the rotation vector which disappear in the classical elastic solids. These boundary conditions are used to obtain a linear algebraic equation set, from which the amplitude ratios of relection waves to the incident wave can be determined. Then, the relection coeficients in terms of energy lux ratios are calculated numerically, and the normal energy lux conservation is used to validate the numerical results. Based on these numerical results,the inluences of the boundary parameters, which relect the mechanical behavior of elastic support, on the relection energy partition are discussed. Both the incident longitudinal wave（the P wave） and incident transverse wave（the SV wave） are considered.