Borehole imagery is a very useful method to obtain discontinuity properties but its limits should be taken into account. We have worked on the investigations realized on a tunnel project in the French Pyrenees. Two horizontal core drillings were made and optical camera loggings were performed. We worked on portions of the boreholes to compare the discontinuities obtained on loggings imaging with manual measurements realized on core samples. Our study shows that even when borehole imagery is very well performed, biases are encountered: on one of the core drilling under study 32% of fractures measured on borehole samples were not identified on borehole images, and nearly 60% of the discontinuities seen in images were stratifications that have no mechanical role. Handmade characterization is quite long and needs high quality samples that are not easy to obtain. We focused on differences revealed by these methods and their consequences on tunnelling conditions.
During a dissolution process, bedding planes and cracks in the sedimentary rock itself enlarge and/or propagate over time. They move the water and expend the conduits through a combination of dissolution and abrasion of the surrounding rock. The ground stability is thus linked to the cracking state of the rock. The measurement of the resonant frequency enables the assessing of the cracking state of a rock. It is a non-destructive method based on the processing of a unit pulse excitation. Since karsts which are near the ground surface undergo climate variations, measurements have been carried out on sedimentary rocks subjected to frost cracking and also on marble samples subjected to heat cracking. The results highlighted the decrease of frequencies with cracking. This measurement thus appears to be an appropriate method to evaluate the cracking state of a rock and also to follow the evolution of the cracks inside the rock.
Air-dry and water-saturated samples of a sandstone and a mudstone from the Upper Silesian Coal Basin, Poland, were tested under uniaxial compression conditions with the aim of revealing the effect of moisture on the deformability of these sedimentary coal measure rocks. The volumetric deformation mode in the pre-failure domain was a matter of particular interest. Based on the test results it has been shown i.a. how the presence of water in rock affects the length of the range of linearity of stress-strain characteristics, the linear and secant moduli of axial of stiffness, the linear and secant Poisson’s ratios, the threshold of absolute dilatancy, the axial strain at failure and the dilatant volumetric strain at failure.
Sonmez, H. (Hacettepe University) | Kasapoglu, K.E. (Hacettepe University) | Coskun, A. (Hacettepe University) | Tunusluoglu, C. (Canakkale Ondokuzmart University) | Medley, E.W. (Geosyntec Consultants) | Zimmerman, R.W. (Imperial College)
Design and engineering facilities are often constructed in complex geological mixtures or fragmented rocks such as mélanges, fault rocks, coarse pyroclastic rocks, breccias and sheared serpentines. These types of geological materials are generally chaotic and mechanically and/or spatially heterogeneous rock masses, which are composed of relatively strong rock inclusions, surrounded by weaker matrix, and may be considered as bimrocks (block-in-matrix-rocks; Medley, 1994). The preparation of standard and representative cores from these types of rock masses for conventional laboratory experiments is almost impossible. In the literature, there are a few attempts to overcome this difficulty by developing empirical approaches based on case histories and laboratory studies on bimrocks. However, despite these attempts, there is no widely accepted empirical approach in the rock mechanics community. In this study, some conceptual equations, which are open to improvement, were generated by considering literature findings to predict strength of unwelded bimrocks.
Physical and mechanical rock properties which can be directly determined or estimated empirically are always considered for rock excavability, rippability, boreability and cuttability. Brittleness (B) and Cerchar Abrasiveness Index (CAI) of rocks have often been used by many researchers. Several brittleness concepts (BC) have been proposed and used for different areas such as rock, civil and ceramic engineering. In this work, four of the BCs obtained from uniaxial compressive (UCS) and tensile strength (TS) of rocks were used, considering the experimental data given in previous studies. First, the relationships between CAI and mechanical properties of rocks found in literature were summarised. Then, the BCs were determined from UCS and TS. The rocks investigated were classified based on their geological origins. The experimental raw data were assessed by regression analysis. The empirical equations in different significance levels were derived for all data and for each geological origin. The significance levels of obtained empirical equations were evaluated to statistical student t-tests. The equations obtained were finally generalised to geological origins and compared to the given equations in the literature.
The uniaxial compressive strength is the vital test used for designing rock engineering projects. Owing to the difficulties associated with preparing rock samples from weak, highly fractured and thinly bedded rocks, the predictive models were developed by many researchers. Recently researchers have begun evaluating the potential of neuro-fuzzy hybrid approach in several fields of studies. This paper presents the application of an adaptive neuro fuzzy inference system to predict uniaxial compressive strength of rocks using 126 data sets. For this purpose uniaxial compressive strength, Schmidt hammer, point load index, sound velocity, Physical properties, and Tensile Strength tests were applied. To control prediction performance of the obtained models, the root mean square error index was calculated as 13.65 from the neuro-fuzzy and 15.60 from the multiple regression model for test data set. The results are vastly promising. A comparative analysis proves that the adaptive neuro fuzzy inference system outperforms conventional methods.
Castellanza, R. (Politecnico di Milano) | Nova, R. (Politecnico di Milano) | Gerolymatou, E. (National Technical University of Athens) | Fusi, N. (Universitàdegli Studi di Milano-Bicocca) | Barberini, V. (Universitàdegli Studi di Milano-Bicocca) | Crosta, G. (Universitàdegli Studi di Milano-Bicocca)
High porosity soft rocks are characterized by a microstructure that collapse under loading exerting relevant volumetric strains in localized tabular zones perpendicular to the loading direction, called compaction bands. A detailed experimental study of the formation and the evolution of compaction bands will be presented. A strain-controlled oedometric apparatus able to detect radial stress has been used for testing artificial and natural soft rocks. Initially, the mechanical response is essentially elastic until bonds are progressively broken. The axial strain-stress curve exhibits an unloading branch and a sort of curl appears in the stress path. Then, the behaviour becomes similar to an unbonded soil. A series of Micro-CT X-rays scansions are performed at different levels of total axial strain. Two compaction bands are formed one at the bottom and one at the top, due to the friction between the soft rock and the loading cups. By increasing the strain, the thickness of the two bands increase towards the center of the specimen, so that the whole specimen is completely destructured. Finally the experimental behaviour is reproduced with an elastoplastic constitutive model and the occurrence of compaction bands is theoretically predicted.
The excavation of the Power House Cavern (PHC) for the Siah Bishe Pump Storage Project was accompanied by an extensive monitoring program. The gathered monitoring data was evaluated and used as a basis for the back analysis—special emphasis was put on the extensometer measurements. Firstly, an FE-model with a simplified geological setup of the PHC, which was initially used before the cavern was excavated (Design Model), was adapted to the as-built conditions, i.e. the excavation sequence and as-built support measures were implemented. At this stage of the back analysis it was already noticed that the initially (before excavation) presumed rock mass parameters and shear zone parameters had to be reduced to achieve a match between measured and calculated deformations. Furthermore, the analysis revealed that a comparatively small increase of the horizontal in-situ stress leads to similar results as a reduction of the rock mass parameters.
Bandic, M. (Investinženjering d.o.o.) | Galjan, B. (Investinženjering d.o.o.) | Barbalic, I. (Institut IGH d.d) | Štambuk Cvitanovic, N. (Institut IGH d.d) | Vrkljan, I. (Geotechnical Laboratory, Institut IGH -University of Rijeka)
After noticing poor rock penetration performance during pile excavation works, contractor induced additional site investigation to support his claim on higher rock strength than previously inspected. Results of 115 point load tests and 97 uniaxial tests were fitted to Weibull and Gamma probability density functions. One-sided confidence interval with confidence level of 95% (CI95) for strength value which is higher than 95% of population (i.e. 95% percentile) was obtained with “Likelihood Ratio Confidence Bounds” method. Suggestion on number of specimens is given for percentile interval estimation in future geotechnical investigation works.
In order to monitor mechanical behaviors of weak rock and coal surrounding gates of fully mechanized top-coal caving (FMTC) face and provide guidance for gate layout, supporting design and safe mining, experimental analysis including lab and spot testing are put forward by analyzing deep displacement, surface displacement, stress redistribution and supports loading. The results show that rock surrounding gateways lies in abutment pressure decrease zone near working face, support loads decreasing, but both supports and surrounding rock deformation are the most acute. The deformation of surrounding rock mainly appears in abutment pressure influence zone. Reasonable roadway supporting should control the deformation of surrounding rock in intense stage of mining influence. Consequently rock pressure and behaviors of weak rock in FMTC are lower occurrence in thick seam mining and supporting design ideas of tailgate and maingate should be changed from loading control to deformation control.