竖向开挖卸载应力引起的基坑坑底土体变形响应

针对基坑开挖引起的坑底土体和桩基础的回弹变形问题,基于Mindlin解和自重应力抵消法,给出了大面积基坑坑底中心区域的开挖回弹影响深度和回弹变形的计算方法。通过算例计算,对比了基于Mindlin解和基于Boussinesq解的竖向开挖卸载应力、回弹应力分布的区别,指出基于Mindlin解的解答更适用于基坑开挖的情况。研究结果表明:基坑坑底的竖向开挖卸载应力、回弹应力及回弹影响深度与基坑开挖宽深比、长深比和泊松比正相关;可采用基于Mindlin解的回弹应力的线性分布拟合公式直接计算坑底回弹变形。经上海地区的3个大规模基坑工程实例,验证了本文方法的适用范围和计算精度。

川藏铁路巴玉隧道开挖二次应力分布特征与岩爆风险分析

川藏铁路巴玉隧道施工中岩爆灾害突出,有必要分析该隧道施工过程的岩爆风险。采用数值模拟研究巴玉隧道开挖二次应力分布和损伤特征,基于Hoek判据评估开挖过程中的岩爆风险,分析隧道平行错开掘进对岩爆风险的影响。结果表明:数值模拟的塑性区与声波测试的损伤区结果较为一致;隧道开挖后最大主应力分布于拱顶与拱肩,具有强烈岩爆风险。边墙发生应力卸载,多表现为拉剪破坏的剥落与片帮;相比独头掘进,超前开挖平导降低了正洞即时型岩爆风险。但正洞与平导间出现的“马鞍形”应力升高区,引起平导边墙的破坏,甚至诱发平导时滞型岩爆。巴玉隧道大部分岩爆发生于拱顶和拱肩,与评估结果较为一致,边墙发生了岩板弯曲鼓折和岩片剥落。研究结论为深埋硬岩隧道的岩爆灾害分析预测和防控提供参考。

素土开挖边坡变形计算方法研究

土坡的变形是判断其是否安全的重要监控指标,文章提出了虚拟开挖应力的概念模拟土体开挖状态,通过等于2倍静止土压力的虚拟释放应力还原非弹性半无限体的开挖状态至弹性半无限体状态,建立了基于Mindlin解的素土边坡变形计算方法;利用编制的程序讨论了不同因素对素土边坡变形的影响。结果表明:素土边坡土体的最大侧向变形点约在边坡0.75倍开挖深度位置,其最大竖向变形点为靠近侧壁约为0.3H的位置,随着土体内摩擦角的增加边坡变形呈近似线性减小,随着边坡坡角的增大其侧壁最大侧向变形也逐渐增大。

卸荷软土的应力路径及对强度影响的研究

分析了基坑开挖作用分区的特征以及坑底土体强度变化规律,比较了软土卸荷强度与常规取土试验强度间的差异,对于基坑稳定性评估与加固设计具有重要意义。

基于MINDLIN解的土钉墙变形计算方法

土钉墙变形是影响其安全的关键指标,已有的研究没有给出其变形的合理理论计算方法。提出基于MINDLIN解的土钉墙变形理论计算方法,该方法通过虚拟开挖应力模拟土钉墙开挖状态,基于实测资料假定钉土剪力在潜在滑动面两侧沿土钉方向呈双三角形分布,利用MIDLIN应变解计算虚拟应力和钉土剪力在钉尾和潜在滑动面位置产生的侧向变形,根据钉土剪力在潜在滑动面位置等于0的特点得到钉土相对位移在潜在滑动面位置为0的结论,并利用该结论及土钉力学平衡条件得到钉土剪力计算模型的2个相关参数,最后通过钉尾变形和土钉自身拉伸变形的叠加得到土钉墙任意位置的侧向变形。通过与法国CEBTP大型试验1#墙实测数据的对比分析,初步验证该计算方法的合理性和可行性。

棚洞施作时机对高边坡稳定性及棚洞结构的影响

目前棚洞结构的设计通常是在稳定边坡的基础上进行设置的,一般不考虑棚洞对不稳定边坡的作用;而实际上棚洞结构也是一个大型的支挡结构体系,对边坡稳定性是有帮助的。为了分析棚洞对边坡稳定性的影响,提出"边坡开挖应力释放系数"的概念,通过对棚洞不同施作时机的数值模拟,分析不同施作时机下棚洞对高边坡稳定性以及边坡对棚洞结构内力的影响,从而为选取棚洞结构的合理施作时机提供依据。

变形协调情况下土钉墙内力和位移的计算方法

土钉墙的内力主要包括土钉轴力、钉土剪力和面层受力,而土钉墙变形的过程是土钉、面层和土体之间相互协调共同作用的过程,传统的计算模型都是从力矩平衡的力学角度出发,不能合理解释土钉内力和变形的相互关系。通过对土钉墙受力和变形机制的分析,利用Mindlin解计算在虚拟开挖应力和钉土剪力共同作用下土体侧向位移,结合钉土剪力和钉土相对位移的线性关系以及钉土剪力在最危险滑动面位置等于0的特点,计算土钉轴力、钉土剪力和土钉墙在开挖面位置的变形。土钉被动区段钉土剪力超出其极限值的部分转移至面层,假定面层受力增量呈三角形分布,根据静力平衡得到面层受力分布。通过与工程实测资料的对比分析,初步验证土钉墙内力和变形计算方法的合理性和可行性。

Numerical simulation of optimum mining design for high stress hard-rock deposit based on inducing fracturing mechanism

The 3D numerical simulation model of deep hard-rock deposit in Kaiyang Phosphate Mine of Guiyang was established based on the practical engineering using 3DEC numerical simulation software. The distribution characteristics of displacement fields and plastic zones of the orebody were simulated in three different excavation cases, including the case of excavation artificial inducted roadway in the orebody, the case of horizontal or vertical excavation direction and the case of the upward or downward excavation order. The simulation results indicate that the plastic zone and displacement field of surrounding rock around the inducted roadway are continuously increasing with the increase of the exposure time after digging an artificial inducted roadway in the orebody. Thus the raw rock ore becomes easier to be fragmented, which provides advantageous conditions for roadheader to cut high stress hard-rock. It is worthy noting that there is a large difference in effective utilization of deep ground pressure between horizontal and vertical excavation directions. The later can produce larger deformation and fracture zone than the former on the rock mass around the deduced roadway, which means that the later may utilize the high ground pressure more effectively to break hard-rock. And the obtained results also show that upward excavation order is more helpful for ground pressure to break rock than downward excavation order.

Unloading responses of pre-flawed rock specimens under different unloading rates

Based on the stress redistribution analysis of rock mass during the deep underground excavation, the unloading process of pre-flawed rock material was simulated by distinct element method (DEM). The effects of unloading rate and flaw inclination angle on unloading strengths and cracking properties of pre-flawed rock specimens are numerically revealed. The results indicate that the unloading failure strength of pre-flawed specimen exhibits a power-function increase trend with the increase of unloading period. Moreover, combined with the stress state analysis on the flaws, it is found that the unloading failure strength increases with the increase of flaw inclination angle. The cracking distribution of pre-flawed specimens under the unloading condition closely depends on the flaw inclination angle, and three typical types of flaw coalescence are observed. Furthermore, at a faster unloading rate, the pre-flawed specimen experiences a sharper and quicker unloading failure process, resulting in more splitting cracks in the specimens.

Dynamic response and failure of rock in initial gradientstress field under stress wave loading

Once an opening is created in deep underground,the stresses surrounding the opening will be redistributed,inducing a gradient stress field.To understand how the ground rock in such a gradient stress field responses to dynamic stress loading,the gradient stress distribution at a circular opening was first analyzed and the propagation of 1D stress wave in rock mass under gradient stress field was theoretically derived.By using an implicit to explicit solution method in LS-DYNA code,the dynamic mechanical behaviors of rock in gradient stress field were numerically investigated.The results indicate that the damage is mainly produced at or near the free face,partly due to the straight action of compressive stress wave and the tensile stress wave generated at the free face.The range of the induced damage zone is narrowed under the conditions of higher gradient stress rate and lower dynamic stress amplitude.However,under lower gradient stress field and higher dynamic stress,the damage becomes severer and wider with discontinuous failure regions.

Permeability and pressure distribution characteristics of the roadway surrounding rock in the damaged zone of an excavation

Research on the permeability and pressure distribution characteristics of the roadway surrounding rock in the excavation damaged zone(EDZ) is beneficial for the development of gas control technology. In this study, analytical solutions of stress and strain of the roadway surrounding rock were obtained, in which the creep deformation and strain softening were considered. Using the MTS815 rock mechanics testing system and a gas permeability testing system, permeability tests were conducted in the complete stress-strain process, and the evolution characteristics of permeability and strain were studied over the whole loading process. Based on the analytical solutions of stress and strain and the governing equation of gas seepage flow, this paper proposes a hydro-mechanical(HM) model, which considers three different zones around the roadway. Then the gas flow process in the roadway surrounding rock in three different zones was simulated according to the engineering geological conditions, thus obtaining the permeability and pressure distribution characteristics of the roadway surrounding rock in three different zones. These results show that the surrounding rock around the roadway can be divided into four regions-the full flow zone(FFZ), flow-shielding zone(FSZ), transitive flow zone(TFZ), and in-situ rock flow zone(IRFZ). These results could provide theoretical guidance for the improvement of gas extraction and gas control technology.

Influence of stress releasing ratio and boundary scope on 2D FEM simulate

Give constrains of costs and technology in analysis,actual practice of 2D FEM is widely popular and demanded.In order to take advantage of 2D FEM to simulate 3D stress state,the concept of stress releasing ratio was generally introduced to represent the 3D constraint effect.For example,the simulation analysis of tunnel excavation is based on the measured actual deformation to provide stress releasing ratio.In the engi- neering of open excavation,the construction is,most of the case,targeted on alluvial de- posit with relatively soft stratum.However,the 2D FEM simulation lacks a clear and ra- tional basis in how to represent the effects of 3D constraint.Thus,in order to investigate the problem above,the author analyzed same engineering using both 2D and 3D individu- ally,and compared the corresponding results.Based on the 3D analysis,factors including the relationship between the model's scope,stress releasing ratio,and construction condi- tion of 2D analysis were also examined.

Dynamic characteristics of high stressed red sandstone subjected to unloading and impact loads

In the process of deep projects excavation,deep rock often experiences a full stress process from high stress to unloading and then to impact disturbance failure.To study the dynamic characteristics of three-dimensional high stressed red sandstone subjected to unloading and impact loads,impact compression tests were conducted on red sandstone under confining pressure unloading conditions using a modified split Hopkinson pressure bar.Impact disturbance tests of uniaxial pre-stressed rock were also conducted(without considering confining pressure unloading effect).The results demonstrate that the impact compression strength of red sandstone shows an obvious strain rate effect.With an approximately equal strain rate,the dynamic strength of red sandstone under confining unloading conditions is less than that in the uniaxial pre-stressed impact compression test.Confining pressure unloading produces a strength-weakening effect,and the dynamic strength weakening factor(DSWF)is also defined.The results also indicate that the strain rate of the rock and the incident energy change in a logarithmic relation.With similar incident energies,unloading results in a higher strain rate in pre-stressed rock.According to the experimental analysis,unloading does not affect the failure mode,but reduces the dynamic strength of pre-stressed rock.The influence of confining pressure unloading on the shear strength parameters(cohesion and friction angle)is discussed.Under the same external energy impact compression,prestressed rock subjected to unloading is more likely to be destroyed.Thus,the effect of unloading on the rock mechanical characteristics should be considered in deep rock project excavation design.

The mitigation negative effect of tunnel-blasting-induced vibrations on constructed tunnel and buildings

Provided the results of a research conducted to investigate the relationships between the empirical vibration attenuation equation of Peak Particle Velocity （PPV） and the Scaled Charge （SC） through testing the blasting-induced vibrations on the spot of Wanshishan tunnel based on 96 vibration recordings. It is found that the maximum charge amount per delay in Wanshishan tunnel excavating is determined by the buildings on the surface and the constructed tunnel nearby. Considering that the repeated blast loading in tunnel blasting caused accumulative effects of damage on buildings, comfortable threshold damage limits of PPV to maintain buildings safety was given. Dynamic Stress Ratio （DSR） was adopted to study the stability of constructed tunnel on the action of blasting induced vibrations. The method to determine specific maximum charge amount per delay in Wanshishan tunnel excavation was given. It is proved that the findings in this study are very effective to control the negative effects of blasting-induced vibrations on buildings on the surface and constructed tunnel nearby.

Deformation effect of lateral roof roadway in close coal seams after repeated mining

This paper analyzed the deformation mechanism in lateral roof roadway of the Ding Wu-3 roadway which was disturbed by repeated mining of close coal seams Wu-8 and Wu-10 in Pingdingshan No. 1 Mine. To determine the strata disturbance scope, the strata displacement angle was used to calculate the protection pillar width. A numerical model was built considering the field geological conditions. In simulation, the mining stress borderline was defined as the contour where the induced stress is 1.5 times of the original stress. Simulation results show the mining stress borderline of the lateral roadway extended 91.7 m outward after repeated mining. Then the original stress increased, deforming the road- way of interest. This deformation agreed with the in situ observations. Moreover, the strata displacement angle changed due to repeated mining. Therefore, reselection of the displacement angle was required to design the protective pillar width. Since a constant strata displacement angle was used in traditional design, the orooosed method was beneficial in field cases.

Experimental study on the variation law of coal temperature during excavation in coal mines

By testing the temperature of the coal and the stress of the working surface, we got the variation law of coal temperature and coal stress during the excavation. The result shows that the activities of mining affect the coal temperature, the fluctuation of coal temperature and the coal stress is synchronous. During the smooth change of crustal stress, the coal temperature basically keeps unchanged, when the dynamic phenomenon appears, the coal temperature changes, as well the coal stress. Therefore, we can use the online coal temperature monitoring system to test the coal temperature of the working surface continuously, and it can provide basic information for forecasting coal mine power disaster before it happens.