Investigations of the sequential excavation and reinforcement of an underground cavern complex using the discontinuous deformation analysis method

Two modifications are made to enable the use of the discontinuous deformation analysis (DDA) method to investigate the sequential construction of an underground cavern complex. One modification is the “block birth and death” approach. With the aid of a boundary block searching algorithm, this approach excavates the blocks in any closed domain step by step and is verified by a DDA example. The other modification is an updating algorithm of the rock bolt called the dull-reactivating method. In this new algorithm, the coordinates of the ending endpoints of the dull rock bolts are regularly updated and the corresponding rock bolts in the sequential reinforcement region are reactivated. The initial geostress field and the sequential construction of the underground cavern complex of the Dagangshan hydropower station in Southwest China are simulated with the modified DDA method. Valuable results are obtained by analysing the displacement, stress and rock bolt force. In the construction processes, the blocks around each cavern form a bearing capacity system to prevent the block deformation, and the rock bolts jointly support the stress release as a system. At the sidewalls of the main machine building and the downstream side wall of the tail surge chamber, the blocks cut by joints with a high dip angle suffer a strong stress release, and the tension forces of the several rock bolts are so large that they should be noticed, which provides a basis for the construction guidance of the project.     

Back analysis of geomechanical parameters by optimisation of a 3D model of an underground structure

One of the major difficulties for geotechnical engineers during project phase is to estimate the geomechanical parameters values of the adopted constitutive model in a reliable way. In project phase, they are normally evaluated by laboratory and in situ tests and, in the specific case of rock masses, by the application of empirical classification systems. However, all methodologies lead to uncertainties due to factors like local heterogeneities, representativeness of the tests, etc. In order to reduce these uncertainties, geotechnical engineers can use inverse analysis during construction, using monitoring data to identify the parameters of the involved formations. This paper shows the back analysis of geomechanical parameters by the optimisation of a 3D numerical model of the hydroelectric powerhouse cavern of Venda Nova II built in Portugal. For this purpose, two optimisation techniques were considered: one classical optimisation algorithm and an evolutionary optimisation algorithm. In the optimisation process, displacements measured by extensometers during excavation were used to identify rock mass parameters, namely the deformability modulus (E) and the stress ratio (K₀). Efficiency of both algorithms is evaluated and compared. Both approaches allowed obtaining the optimal set of parameters and provided a better insight about the involved rock formation properties.     

Estimation of the three-dimensional in situ stress field around a large deep underground cavern group near a valley

Understanding three-dimensional(3D) in situ stress field is of key importance for estimating the stability of large deep underground cavern groups near valleys.However,the complete 3D in situ stress fields around large deep underground cavern groups are difficult to determine based on in situ stress data from a limited number of measuring points due to the insufficient representativeness and unreliability of such measurements.In this study,an integrated approach for estimating the 3D in situ stress field around a large deep underground cavern group near a valley is developed based on incomplete in situ stress measurements and the stress-induced failures of tunnels excavated prior to the step excavation of the cavern group.This integrated approach is implemented via four interrelated and progressive basic steps,i.e.inference of the regional tectonic stress field direction,analyses of in situ stress characteristics and measurement reliability,regression-based in situ stress field analysis and reliability assessment,and modified in situ stress field analysis and reliability verification.The orientations and magnitudes of the3D in situ stress field can be analyzed and obtained at a strategic level following these four basic steps.First,the tectonic stress field direction around the cavern group is deduced in accordance with the regional tectonic framework and verified using a regional crustal deformation velocity map.Second,the reliability of the in situ stress measurements is verified based on the locations and depths of stressinduced brittle failures in small tunnels(such as exploratory tunnels and pilot tunnels) within the excavation range of the cavern group.Third,considering the influences of the valley topography and major geological structures,the 3D in situ stress field is regressed using numerical simulation and multiple linear regression techniques based on the in situ stress measurements.Finally,the regressed in situ stress field is further modified and reverified based on the stress-induced brittle failures of small tunnels and the initial excavation of the cavern group.A case study of the Shuangjiangkou underground cavern group demonstrates that the proposed approach is reliable for estimating the 3D in situ stress fields of large deep underground cavern groups near valleys,thus contributing to the optimization of practical excavation and design of mitigating the instability of the surrounding rock masses during step excavations.     

Coupled hydro-mechanical model for fractured rock masses using the discontinuous deformation analysis

A coupling analysis model is proposed to study the hydro-mechanical response of the fluid flow in fractured rock mass with the method of discontinuous deformation analysis (DDA). The DDA coupled hydro-mechanical model is interpreted in details by expressing the fracture fluid flow equations, the coupling process and the global coupled equations. For the mechanical response, the hydraulic pressure is determined first, followed by the coupled motion equations expressed under the DDA framework, to study the interaction between the fluid flow along the fractures and the movement of the rock blocks. In the fluid flow analysis, the cubic law is applied to study the steady flow along the fractures using the finite difference method (FDM). A real case of cavern excavation is analyzed by the proposed DDA coupled 2D hydro-mechanical model, to study the influence of fluid flow on the rock cavern stability during the excavation phase. The results show that the DDA coupled hydro-mechanical model is suitable for the stability and seepage analysis of practical engineering problems.     

Stability analysis of underground oil storage caverns by an integrated numerical and microseismic monitoring approach

Underground storage in unlined caverns is of great significance for storing energy resources. Construction of underground storage caverns is an extremely complex process, involving extensive multi-bench excavation and strong unloading. Excavation-induced damage of surrounding rock masses may lead to instability of underground storage caverns. The aim of this paper is to put forward a method by integrating numerical simulation and microseismic monitoring for evaluation of cavern stability. A novel numerical method called Continuous–Discontinuous Element Method (CDEM) is applied to simulate micro-cracks under excavation-induced unloading conditions. Meanwhile, a microseismic (MS) monitoring system is employed to monitor real-time MS events during construction of storage caverns. Numerical results are validated using the monitoring data from the MS monitoring system. The integrated method is proved to be successful in capturing micro-cracks in underground storage caverns. Local instability, potential unstable zones and micro-crack evolution are analyzed, and cracking mechanisms are also discussed.     

Application of numerical analysis principles and key technology for high fidelity simulation to 3-D physical model tests for underground caverns

Numerical simulation and model testing are the main research methods of solving geo-technical and underground problems. In the 1980s, physical simulation (model testing) was one of the most popular methods to solve these types of problems, but since then, with the rapid development of IT technology, numerical simulations have rapidly become dominant. However, the complexity and uncertainty of geological conditions and geo-technical engineering problems represent a great challenge to the numerical method, leading some researchers to question how far numerical simulation can go. In this case, the physical simulations have begun to reappear in research and the use of physical models has seen valuable progress. In the past, problems have been solved either with physical simulation or numerical simulation, or just by comparing the results of these two methods simply. But very little attention has been paid to the question of how to combine the advantages of both and improve them. In the 3-D geo-mechanical model of the underground caverns of the Xiluodu hydropower station, the advantages of the two methods were combined and some of the primary principles of the numerical method were combined with the physical model process. Considering that the boundary condition has a great effect on the stress distribution, a larger test model was set up. The numerical discrete principle was also been used to simulate an initial geo-stress field. The basic principle of cavern excavation according to requires that loading comes first, followed by excavation. According to this idea, we developed a new technique, in which a high fidelity simulation of the excavation process and a very difficult cavern excavation were successfully achieved for the first time. The new method produced test results more satisfactory, and this simulation represents an obvious improvement of existing techniques. We also conducted a numerical simulation with almost the same conditions, and compared the results of the model test and the numerical simulation.     

等截面抗拔桩承载变形特性离心模型试验及数值模拟

抗拔桩的承载变形问题正受到越来越多的重视。对抗拔单桩的承载变形特性进行了离心试验研究和数值模拟研究。通过做离心模型试验,分析抗拔桩在竖向上拔荷载作用下的Q–s曲线、桩身轴力曲线,然后通过建立桩土接触模型,进行数值模拟,将结果与离心试验结果进行对比。通过上述两方面工作,可以看到抗拔单桩承载变形特性呈现如下性状,抗拔力在起始阶段随上拔位移呈线性增长,随着上拔位移进一步增大,抗拔力出现非线性增长的特性,然后突然出现拐点,上拔位移迅速增大,抗拔力达到极限,抗拔桩破坏。     

可液化地基单桩基础离心机模型试验的三维数值分析

文章通过三维水土耦合动-静一体有限元程序DBLEAVES对饱和砂土地基（Dr=40%）单桩基础的离心机模型试验进行模型试验相对应的原型三维有限元数值模拟和分析。对比分析小震（峰值加速度0.08g）和大震（峰值加速度0.47g）情况下的土体加速度、超静孔隙水压、沉降位移以及桩身弯矩等变化规律。其中,地基土的性质采用应力诱导各向异性的交变移动弹塑性模型模拟,基桩采用弹性梁单元模型模拟。结果表明：①超静孔隙水压会“隔断”振动波的传递,当土体接近完全液化时,土表面峰值加速度会明显小于输入波峰值加速度,而当超静孔隙水压比较小时,土表面加速度相对于输入波则可能会放大;②地震时所达到的最大超静孔隙水压比是地基土沉降量的主要因素之一,且一大部分沉降发生在震后的孔压消散期;③数值模拟与模型试验结果的对比分析表明,交变移动模型可以较好地反映土体在交变荷载下的动力响应特性,验证了所采用的DBLEAVES程序和有限元方法的有效性。     

Numerical analysis on seismic behavior of soil around pile group by 3D effective stress finite element method

The behavior of soil around a pile group was investigated based on a series of dynamic effective stress analyses through the three-dimensional finite element method, simulating centrifuge shaking table tests on soil-pile-structure systems with closely spaced piles reported in a previous study (Suzuki et al., 2006). The computed results show that the mechanism of subgrade reaction in a pile group in dry ground is different from that in saturated ground. The horizontal compression normal stress of the soil at the front side of the pile mainly contributes to the subgrade reaction in dry sand, while pore water pressure reduction at the back side of the pile is the main cause, rather than the normal stress in saturated sand. A series of computed results were found to support the presumed mechanisms of closely spaced pile groups described in a previous experimental study.