Numerical modelling of in situ stress calculation using borehole slotter test

Accurate determination of in situ stress magnitude and orientation is an essential element of the design process of all underground openings. A stress calculation method was proposed for the new stress measurement technique using the borehole slotter device. Two major objectives were the focus of this research work. The first goal was to simulate the slotter test process numerically and delve into the mechanisms involved in this test. A precise 3D numerical model of a typical slotter test condition was constructed using the FLAC3D code. The effects of variations in rock mass deformation modulus on the strain/stress relieve, and thus borehole slotter test results, were investigated numerically. The second objective of the work was to employ 3D modelling in the interpretation of slotter field tests conducted at Bakhtiari dam site. These tests were aimed at determining the stress magnitude and orientation to be used in the design of underground chambers and tunnels associated with Bakhtiari dam. The stress regimes measured in field were applied as boundary condition on the constructed 3D model and a backward analysis was conducted. For each case the actual strain field measured was compared against strain field calculated numerically. Accordingly, the boundary condition (stress magnitude and orientation) associated with the model results that provided the best fit to the measured data was determined as the governing stress regime. A good agreement was achieved between numerical and field test results. The obtained numerical results provided valuable insights in selecting the governing in situ stress condition from a set of recorded field data.     

Influences of changes in mechanical properties of an overcored sample on the far-field stress calculation

The concentration factors for the determination of far-field stresses have been calculated based on three-dimensional FLAC^3D simulated results. FLAC^3D is an explicit finite difference computer program. Three-dimensional numerical simulations have been conducted using FLAC^3D to determine the influence of altered overcored sample parameters on the far-field stress calculation. Both Young's modulus and Poisson's ratio are known to change during overcoring in highly stressed rock as a result of stress-relief damage. Modification factors have been proposed to take into account the influence of the reduction in Young's modulus on the far-field stress calculation. The influence of change in Poisson's ratio has also been analyzed. A method of determining a Young's modulus value suitable for use in the far-field stress calculation is described and the influence of reduction in Young's modulus of an overcored sample on the determination of the stress ratio (SR) in the RPR method is analyzed. Using the proposed modification, the in situ stresses for the Atomic Energy of Canada Limited's Underground Research Laboratory are re-calculated based on 21 deep doorstopper measurements. The re-calculation shows an increase in the trend of stress magnitudes versus depth in the URL.     

Determination of the natural stress state in a Brazilian rock mass by back analysing excavation measurements: a case study

The Serra da Mesa Hydroelectric Power Plant, located in the Tocantins river, 210 km north of Brasilia, Brazil, has been completed and power (1200 MW) has been generated since 1998. This project includes one of the largest underground structures in Brazil, totalling 550,000 m³ of underground excavations in rock for the hydraulic circuit which was excavated in very high quality granite. Geotechnical investigations, laboratory tests and geological mapping showed that the rock mass could be considered as a continuous, homogeneous, isotropic and linearly elastic (CHILE) material.

In situ tests, for obtaining the natural stress tensor, namely hydraulic fracturing and small flat jack tests (SFJ), were executed. The hydraulic fracturing tests were performed in two boreholes, at the planned position of the future underground structures. SFJ were executed in a test gallery especially constructed for the purpose. These latter tests confirmed the in situ rock stress data obtained from the hydraulic fracturing tests.

This paper presents a new technique for interpretation of the SFJ results. This is achieved by inputting the SFJ measurements into a 3D program that compiles the influence matrix of the excavated rock mass domain and then, via the least square technique, the determination of the stress tensor. All the equations are fully developed and the methodology is presented in its entirety. The successful application of the methodology is also presented, with comparisons between the results obtained and the in situ stress tensor determined by other methods.     

Study of the in situ stress field in a deep valley and its influence on rock slope stability in Southwest China

Bulletin of Engineering Geology and the Environment - The in situ stress field in valley areas is an important factor to consider in the construction of hydropower projects in southwest China. Due...     

Experimental and numerical modelling of the three-dimensional incompressible flow behaviour in the near wake of circular cylinders

An experimental investigation was carried out to study the structure of the flow field around three-dimensional circular cylinders. The study of the flow field around an obstacle was performed in a wind tunnel using a particle image velocimetry (PIV) system. The flow of a fluid around an obstacle with a different velocity to the oncoming flow was examined. The results showed the dependence of the flow structure around the obstacle on its Reynolds number, and the spacing between a pair of obstacles. Detailed quantitative information of turbulence parameters in the vicinity of the obstacle was attained. Extensive wind tunnel experimental results are presented and compared with numerical simulation. A three-dimensional numerical model with Reynolds stress model (RSM) turbulence and a non-uniform grid system were used to examine the effects of a single cylinder and two cylinders in tandem on the flow. The principal objective was to analyse three-dimensional flow past a single cylinder and two circular cylinders placed in tandem by combining the application of a PIV experimental technique and an RSM turbulence model.For the case of two cylinders in tandem, the flow patterns are characterized in the gap region as a function of the distance between the cylinders. A good level of agreement was found between the experimental results of flow and numerical simulation.     

Pedestrian wind comfort around a large football stadium in an urban environment: CFD simulation, validation and application of the new Dutch wind nuisance standard

Three-dimensional steady Reynolds-averaged Navier–Stokes (RANS) Computational Fluid Dynamics (CFD) simulations are used in combination with the new Dutch wind nuisance standard to assess pedestrian wind comfort around a large football stadium in Amsterdam, before and after the addition of new high-rise buildings. The focus of the study is on the elevated circulation deck and the surrounding streets and squares. CFD validation is performed by comparison of the simulated mean wind speed at the deck with full-scale measurements. The important effect of local ground roughness specification on the simulated wind speed values is indicated. Application of the Dutch wind nuisance standard shows that wind comfort at the elevated circulation deck is only slightly influenced by the new buildings. Wind comfort at the surrounding streets and squares however significantly deteriorates. Finally, the results obtained by the Dutch wind nuisance standard are compared to those obtained by a more simplified procedure for the transformation of wind statistics to the building site, as used in earlier studies. The more sophisticated transformation procedure in the Dutch standard was successfully validated based on full-scale measurements in earlier research. Comparison of the Dutch standard results in this study with those of the simplified procedure shows that the latter provides overestimations by up to 25% for the highest discomfort probabilities. This type of large discrepancies can significantly change the outcome of wind comfort studies.