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The large-scale PRACLAY heater test has been running since November 2014 in the HADES URF (Belgium) to examine the combined impact on the host Boom Clay of the thermo-hydro-mechanical (THM) perturbations due to gallery construction and a large-scale thermal load which simulates the heat dissipation of radioactive waste. To get an insight into the expected evolution of the test, which will be helpful to guide the test, numerical prediction of the PRACLAY heater test had been performed before the heater was switched on. The numerical prediction, based on the existing related knowledge before the switch-on of the heater test, took into account the complex coupled THM processes and interactions in all the involved materials of the experimental set-up (Boom Clay, bentonite, concrete lining, backfill sand, etc.) and the test phases from the gallery excavation till the end of heating which is planned in 2025. A large number of constitutive models and associated parameters required to define the numerical problem had been identified and calibrated on the basis of literature review, results of extensive laboratory tests and back analysis of many relevant in situ measurements from the HADES URF. The results of the numerical prediction are constantly and extensively compared with the in situ measured data obtained since the switch-on of the heater till 4-year stationary heating. Overall, it can be stated that the general Boom Clay THM responses in the PRACLAY heater test are in good agreement with the numerical prediction, which gives good indications on the model capacity and confirms the existing knowledge of the Boom Clay THM behaviour.

This paper describes the results of a back analysis of pillar failures at Troy Mine, Montana, and the use of this experience to make forward predictions on pillar stability in the nearby Montanore deposit which lies in a similar geomechanical setting. At Troy Mine, a progression of pillar failures in areas within the Middle Quartzite of the Revett formation led to the observed surface subsidence. The Troy Mine experience was used to understand the level of stresses and failure mechanism leading to the collapse of some pillars in the North Orebody to estimate pillar strength in quartzite beds within Troy’s mountainous terrain. The model elucidated that the dipping orebody geometry in relation to topography led to shear stresses in pillars at Troy Mine. Shear stresses resulted in significant loss of confinement in pillar cores (many theoretically in tension), even at width-to-height ratios that would be deemed stable under zero shear stress (flat seam under flat topography). A calibrated model was achieved, which allowed us to evaluate the impact that different pillar geometric characteristics (such as width, length, height, and shape) have on pillar performance under shear conditions for different depths and extraction ratios. Design charts were then generated to provide guidance on pillar geometry based on expected demand. Mine-wide models were developed to predict the level of vertical stress and horizontal shear stress for pillars in the different ore-bearing beds at Montanore. A sensitivity study was performed for various conditions, including extraction ratio, spatial location under the mountainous terrain, and local orebody geometry with the aim of performing a mine-wide evaluation of the factor of safety against shear. The results of the analyses performed in the present work show that the use of design methods that do not take the effect of shear stresses into account may result in under-designed pillars, while a false impression of rock mass strength could be derived from back analysis.

A major mining slope failure occurred in July 2012 on the East wall of the LAB Chrysotile mine in Canada. The major consequence of this failure was the loss of the local highway (Road 112), the main commercial link between the region and the Northeast USA. LiDAR scanning and subsequent analyses were performed and enabled quantifying the geometry and kinematics of the failure area. Using this information, this paper presents the back analysis of the July 2012 failure. The analyses are performed using deterministic and probabilistic limit equilibrium analysis and finite-element shear strength reduction analysis modelling. The impact of pit water infilling on the slope stability is investigated. The impact of the mining activity in 2011 in the lower part of the slope is also investigated through a parametric analysis.

An instrumented section of a 10-m-diameter circular shaft located at a depth of 1.2 km in an average quality rock mass was back analyzed to establish the rock mass behavior. Extensometers were installed radially at four locations and provided the primary data for the back analyses. Three- and two-dimensional continuum models were analyzed using the numerical codes FLAC3Dand Phase2 to assess the rock mass behavior. An initial set of mechanical parameters obtained from empirical relationships were found to give a reasonable match to the measured response of extensometers 2 and 4, when using a Mohr–Coulomb strain softening model. A different set of parameters were needed for FLAC3D when trying to match the significantly higher displacements recorded by only one of the extensometer. It was noted that regardless of the material model and corresponding parameters, the three-dimensional models were not able to give reasonable match to the results of all extensometers. It was shown that for the given problem, there is a theoretical limit for ratio of displacements predicted for different extensometers using a continuum isotropic material model. The two-dimensional models in Phase2, however, gave an apparently better overall match to all the extensometers. Comparison of the results of three-dimensional models with the assumed longitudinal displacement profile for the two-dimensional model indicated that the three-dimensional effects were not adequately captured in the two-dimensional model.

Grouting is a common method of sealing rock and repairing concrete structures (Turkmen 2003; Butron et al. 2010; Hoien and Nilson 2013; Lim et al. 2013; Bras et al. 2013), and there are many examples of its application to the engineering of dam foundation improvement (Rafi 2010; Saeidi et al. 2013; Rostami Barani et al. 2014). Determining a need for grouting is a main concern in dam foundation improvement (Deere 1982; Ewert 1985). A useful design tool for ground treatment must be able to predict both zones of low water take and low grout take. Our approach is to use the degree of water-bearing structures of the rock fracture to produce reliable zoning map of the dam foundations. For this purpose, the database of the grouting works at Sumbar dam was analyzed. The main objective is to produce a criterion to be used during grouting and capable of a reasonable estimation of the expected sealing through various ground conditions. A systemic statistical analysis to assess the correlation between the joint hydraulic factor (JH) and cement take (CT) is considered; the joint hydraulic fracture (JH) is the mean conductivity per conducting fractures. It is found that the proposed factor can be successfully applied to estimate the engineering treatment for a wide variety of rock foundations. 2 Concept of Joint Hydraulic Factor (JH)

This paper deals with the results of the back analysis performed on an auxiliary adit of a dam site excavated in a sedimentary rock succession whose lithological and mechanical characteristics may give rise to squeezing phenomena in unfavourable conditions.

This paper deals with a study on the geomechanical behavior of the rock mass around the tunnel. The Convergence Confinement Analysis is one of the most appropriate way to estimate this behavior. Elasto-plastic models proposed by Hoek and Brown, Egger and Kastner were adopted in this study. Measurements of the preceding displacement were performed in a road tunnel in Japan, the geology of which is Neogene squeezing mudstone. The measured data was compared with the results of Convergence Confinement Analysis. The preceding displacement was about 40% against the upper bench face converged displacement and 17% against the final converged displacement. Meanwhile, it increased drastically in lower bench excavation step. Back analysis was also executed to calculate the extension of the plastic zone of the rock mass, which was half of that estimated by Convergence Confinement Analysis in upper bench excavation step. But, in lower bench excavation step, the results of the two analyses were very similar. The support pressure and support stiffness were also calculated.

An Underground Research Laboratory (URL) has been constructed by Atomic Energy of Canada Limited (AECL) to study various aspects of disposing of nuclear fuel waste in the Canadian Shield. In order to ensure good quality and long life of the construction, one important factor to be considered is the in situ stress field. As part of the research program, stress changes have been measured during the advance of a tunnel to monitor the stress response to excavation. It is desirable to estimate the state of in situ stress from these stress change measurements and to compare the results with overcoring data. This paper describes the application of a recently developed stress change fitting technique for stress determination. The basic principle and the fitting procedure are briefly summarized and the back-analysis process for the data collected at the URL is presented in detail. A boundary element model was used to predict the stress changes as required by the fitting approach. This new method of in situ stress determination was successfully applied in this case. The back-analyzed stresses agree very well with those determined from overcoring tests. The quality of individual measurements was assessed and the required minimum number of measurements for a conclusive data interpretation is evaluated.

The Timpagrande underground powerhouse, built between 1977 and 1978, was excavated in a highly fractured granite formation having poor mechanical properties. In this paper a description is given of the in situ and laboratory tests performed to characterize the rock mass, of the criteria adopted during the design stage to define the relevant geotechnical parameters, of the methods used for the construction and of the system adopted to monitor the real behaviour of the structure. Finally, the results of a back analysis are presented, which attempt to identify the real values of the geotechnical parameters of the rock mass on the basis of the control readings.