Excavation Unloading Research Articles

Application of D-CRDM Method in Columnar Jointed Basalts Failure Analysis

Columnar jointed basalt is a type of joint rock mass formed by the combined cutting effect of original joints and aphanitic microcracks. After excavation unloading, such rock mass manifested distinct mechanical properties including discontinuity, anisotropy, and proneness of cracking. On the basis of former research findings, this paper establishes a D-CRDM method applicable to the analysis of columnar jointed basalt, which not only integrates discrete element and equivalent finite-element methods, but also takes into account the coupling effect of original joints and aphanitic microcracks. From the comparative study of field monitoring data and strain softening constitutive model calculated results, it can be found that this method may well be used for the simulation of mechanical properties of columnar jointed basalts and the determination of rock failure mechanism and failure modes, thus providing references for the selection of supporting measures for this type of rock mass.

Analyze of the Affection of Excavation Unloading of the Rock Mass of the Spillway Tunnel of a Hydropower Station on the Slope’s Stability

After the excavation of the rock , the rock mass stress state changes and the quality of rock mass damage deterioration. Rock excavation of spillway tunnel of the hydropower station may have some influence on the stability of the tunnel-face side slope. In this paper, combined with the engineering practice, based on the unloading rock mass theory and through two dimensional finite-difference method. Selected seven feature points near the excavation surface as monitoring points for calculation ,then calculated the effect on the stress and strain of the slope caused by the excavation of the spillway tunnel and analysed the changes in the distribution of the slope rock mass plastic zone and the point factor of safety. The results show that spillway tunnel excavation in excavated slope have some impact on the excavated slope stability, and it is more obvious for the effect on stability of excavated slope in considering rock mass unloading effect than do not consider the rock mass unloading effect.

Research on the Deformation Mechanism of Outlet High Slope of Some Hydropower Station Diversion Tunnel

During the operation of outlet high slop of the hydropower station diversion tunnel, the monitoring data shown that the upper slope had an abnormal deformation tendency toward the upstream, which was different from the general slope deformation law. The paper analyzed the deformation mechanism of the high slop outside the hydropower station diversion tunnel in detail based on detailed analysis of the geological data. The analysis shown that the basic reasons of slope deformation were the upper hard and lower soft structure characteristics and the fully developed interlayer shear zone; the excavation unloading, reservoir impounding, reservoir water level change, rainfall, flood discharge atomization, rock weathering, water-rock long-term effects, and some other factors induce and aggravate the rock mass deformation.

Study on Stress Difference-Strength Ratio Rockburst Classification by Rock Unloading Test

Rockburst happened during underground excavation caused by stress re-distribution of surrounding rocks. Stress difference was critical factor to determine whether rockburst happened or not. The stress difference-strength ratio method of rockburst classification was put forward. Rockburst intensities of the same size and different cave shapes were calculated without considering the influence of excavation unloading and considering the influence of excavation unloading by finite element method. Results showed that the grade of rockburst predicted by the stress difference-strength ratio method was lower than by stress-strength ratio method. Rockburst grade was lower one grade for not considering the excavation unloading.

Ringlike failure experiment of thick-walled limestone cylinder specimens in triaxial unloading tests

In order to study the failure of surrounding rock under high in situ stress in deep underground engineering projects, disturbed by excavation unloading, we carried out triaxial unloading experiments using thick-walled cylinder specimens on a TATW-2000 rock servo-controlled triaxial testing machine in a laboratory. The specimens were made of limestone material, taken from Tongshan county, Xuzhou city, Jiangsu province, China. In our experiments, rock deformation and failure behavior was studied through loading and unloading of inner hole pressure of thick-walled cylinder specimens. At first, the axial stress, confining pressure and inner pressure were increased simultaneously to a specified designed state of stress. Then, keeping the axial stress and confining pressure stable, the pressure on the inner hole was decreased until the specimen was fractured. When the inner pressure was released completely but the specimen did not fracture, the confining pressure was decreased subsequently until complete failure occurred. Our experimental results suggest that traces of major circular ringlike fractures with a number of radial cracks often appear in thick cylinder walls. This type of ringlike failure phenomenon, similar to intermittent zonal fracturing characteristics of deep exploitation, has, so far, not been published. Our experimental results show that rock deformation and failure behavior of thick-walled limestone cylinders vary under different stress paths between loading and unloading. Tensile failure and orderly failure surfaces occur under unloading conditions while irregular damaged rock blocks are produced during loading failure. This type of triaxial unloading experiment provides for new research methodology and approach for thorough investigations on intermittent zonal fracturing in deep underground excavations.

Failure mode analysis of jointed rock masses around underground opening under excavation unloading

The failure modes and loosened mechanism for jointed rock masses around an underground opening under excavation unloading are studied using the discontinuous deformation analysis (DDA) method. A sequential excavation algorithm for jointed rock masses is proposed and implemented into the original DDA program. Different combinations of joint dip angle are used to study their effects on the failure modes and deformation behaviour of jointed rock masses. The results of both DDA and physical model show that the failure modes of jointed rock masses around the underground opening are mainly dependent on disadvantage combination of free excavating surface and discontinuities surfaces. The different joint orientations result in different failure modes and collapse areas of surrounding rocks. Opening and sliding failure in the left and right sidewalls of the underground opening occur for the orthogonal joint set with dip angles of 30×120°, while toppling failure was observed in the right sidewall for the joint set with dip angles of 45×135°.

The settlement characteristic of underground urban complex in Shanghai

The effects of excavation unloading, construction reloading and underground water on basal heave of excavation projects were presented and analyzed based on the measurement results of an underground urban complex which was located in Shanghai. The effects on water pressure and building settlements were analyzed as well. The numerical analyses by finite element method (FEM) were conducted. It showed that the soil under the excavation base continued to heave during the following certain construction stage. It also found that the bearing capacity of uplift piles which supported the buildings affected the structure quality significantly. The conclusions can be applied in future projects.

Excavation unloading destruction phenomena in rock dam foundations

Excavation unloading destruction (EUD) is a common phenomenon in the large-scale construction sites in Southwest China, where intense horizontal tectonic stresses are present in the rock mass. The paper discusses the Xiaowan hydropower dam where stresses of 23–35 MPa were measured in the strata in the valley sides and 44–57 MPa in the floor of the valley. It describes the characteristics of EUD, discussing the laminar, arched and imbricate fractures which occurred when the near surface strata were excavated for the dam construction. The EUD fractures were largely restricted to the upper 6 m below the excavation and were particularly pronounced in the top 2 m. It is concluded that the typical Mohr Coulomb formula is not applicable where a rock mass near its critical stress is rapidly unloaded. A better understanding of EUD is important for the prediction of rock bursts etc. and hence safety in large engineering constructions in highly stressed strata.

Creep Modeling in Excavation Analysis of a High Rock Slope

Based on the distinct element method, a numerical procedure is presented for simulation of creep behavior of jointed rock slopes due to excavation unloading. The Kelvin model is used to simulate viscous deformation of joints. A numerical scheme is introduced to create incremental contact forces, which are equivalent to producing creep deformation of a rock-joint system. The corresponding displacement of discrete blocks due to creep deformation of contact joints can be calculated by equilibrium iteration. Comparisons of results between the numerical model and theoretical solutions of a benchmark example show that the presented model has excellent accuracy for analysis of creep deformation of rock-joint structures. As an application of the model, residual deformations of the high rock slopes of the Three Gorges shiplock due to excavation unloading and creep behavior are investigated. By simulating the actual excavation process, the deformation history of a shiplock slope is studied. Good agreement has been achieved between numerical prediction and field measurements. It demonstrates the effectiveness of the presented model in analysis of the creep deformation due to excavation unloading of high rock slopes.

２次元弾性解と個別要素解析によるトンネル支保特性曲線の考察

Active mobilization of bearing capacity of rock mass surrounding an underground opening characterizes the NATM (New Austrian Tunneling Method) concept of supporting. There have been many attempts to give quantitative expressions for this capacity but no one has ever completely solved the question. In the first part of this paper the theoretical relations between support pressure and tunnel wall displacements are briefly discussed. We derive a simplified solution of a circular lined-tunnel in an elastic ground under the plain strain for full-slip and no-slip conditions. These two conditions at the ground-lining interface make it possible to explain the effects of shear stress transmission and relative shear displacement. The lateral free-field ground stresses for this solution are restricted to a fixed value of the vertical stresses (σV*=-p, σH*=-kp). An actual tunnel opening is excavated and supported after the load corresponding to the free-field stresses. We introduce an assumption of the actual “excavation unloading” conditions that occur during tunneling. By advancing this derivation, the strict solution is utilized successfully to construct a theoretical characteristic curve of tunneling. The characteristic curve shows clearly the bearing capacities of a tunnel lining, the surrounding ground and the total by using an index, “relative-support-stiffness”.The elastic theory of the stress distribution around tunnels demonstrates that the deviatoric stresses are maximum at the periphery of the excavation. Therefore, the rock mass may yield in the overstressed zone surrounding the excavation. Discussion here should include the dependence of the characteristic curve on the zone of yielded ground and the discontinuity in the ground. We have carried out numerical simulations for a discontinuous ground with Distinct Element Modeling. The realistic ground reaction curves by the analyses are examined with the theoretical solution derived above. We will learn that while the ground may work, more or less, as a load bearing structural component to the load corresponding to the free-field stresses, overestimation must be avoided.

New Method for 3D and Asymmetrical Slope Stability Analysis

A new three-dimensional (3D) slope stability analysis method is developed based on two-directional moment equilibrium. This method calculates not only the safety factor but also the possible direction of sliding for semispherical and composite failure surfaces. As a result, the possible errors associated with assuming a plane of symmetry in 3D stability analyses are eliminated. Another advantage of the new method is to eliminate the tedious work on the coordinate transformation prior to the analysis. Two examples of symmetrical failure surfaces are used to verify the basic formulation in the present study. Three additional examples further demonstrate the applicability of the proposed method in analyzing 3D asymmetrical failure surfaces. An analysis on a slope, subject to asymmetrical excavation unloading and geological conditions, shows that using the method of one-directional moment equilibrium may give an overestimated safety factor of the slope.

Two-dimensional elastoplastic analysis of a long, cylindrical cavity under non-hydrostatic loading

This paper describes a two-dimensional elastoplastic model of a long, cylindrical cavity in an infinite rock mass subject to non-hydrostatic far-field stress loading. The rock is assumed to behave as an elastoplastic dilatant material that is characterized by a cohesive frictional yield strength. The analysis is based on a solution of the elastoplastic interface that is applicable when the underground cavity becomes completely surrounded by a zone of failed rock, and for stress conditions for which the problem is statically determined. This latter restriction limits the range of deviation from a hydrostatic stress condition that can be analyzed with the model. The stress and displacement field induced by excavation unloading of the cavity is determined on the basis of a proportional load trajectory that involves a monotonic decrease of internal pressure in the cavity. For this particular load history, the elastoplastic boundary grows in a self-similar manner. This allows both displacement and stress field to be calculated in a unit plane, defined by dividing all physical lengths by the characteristic size of the plastic zone. This feature of the solution eliminates the need to solve continuously for the stress and displacement during the evolution of the plastic region. Explicit formulation of the stress field in the plane and of the displacement field in the elastic region is provided; determination of the displacement field in the elastic region is obtained numerically using the method of characteristics. Some important specific results are discussed; a single parameter, the “obliquity” ( m), defined as the ratio between the stress deviator at infinity and the yield limit, is found to control the shape of the failed rock region and the uneven closure of the cavity. It is found also that the direction of maximum closure may become perpendicular to the maximum far-field compressive stress under some loading conditions.