Essai au pénétromètre dynamique

L'essai au pénétromètre dynamique (SPT Standard Penetration Test) est un sondage géotechnique fournissant les caractéristiques d'un sol. Ce test consiste à faire pénétrer dans le sol un échantillonneur standard (échantillonneur Raymond) sous les coups d'un marteau-pilon d’un poids de 63,5 kg et d’une hauteur de 76 cm [1]. Soit ce marteau, tombant en chute libre sur l'outil échantillonneur, trois mesures sont effectuées en comptant le nombre de coups nécessaire à chaque fois pour l'obtention d'une pénétration de l'outil de 15 cm. La résistance à la pénétration du sol est définie comme la somme des coups de la deuxième et de la troisième pénétration. Le test est effectué au fond d'un puits (éventuellement en modifiant le sol aussi peu que possible), creusé à la profondeur désirée. Avantages Les avantages sont divers : *s'étend sur un test standard, sans dépenses excessives ; *s'effectue sur tout terrain, tout au plus, en changeant l'échantillonneur cylindrique creux avec u

Rock fragmentation mechanism by TBM cutter and TBM tunnelling under stressed grounds

Lijun Yin

With the advance of technology, TBMs are becoming more versatile and TBM tunnelling has become a common tunnelling method. As TBM tunnelling is a special excavation method depending on the interaction between machine and rock mass under a certain geological circumstance, the rock mass properties and geological circumstance are directly related to the rock breakage process by TBM cutters and TBM excavation efficiency. Recently, more and more long tunnels at great depth are constructed by TBM method. In situ stress, as one of the important circumstance factors, has been paid more attention for TBM tunnelling at great depth. However, the influence of in situ stress on rock fragmentation by TBM cutter and TBM performance has not been explored thoroughly. Moreover, there is a gap for TBM performance prediction under stressed grounds. The objectives of this research are to study the rock fragmentation mechanism by TBM cutters with different confining stress, and to explore the influence of in situ stress on TBM performance, finally to develop a TBM penetration rate prediction model involving in situ stress. By indentation test with biaxial confining stress, the rock fragmentation processes with the different confining stress levels were studied for granite and marble based on the characteristics of force-penetration depth curve and the acoustic emission (AE) parameters (i.e. AE energy rate, hit counts, amplitude and location of AE events). The test results showed that the thrust force for crack initiation and the size of crushed zone increase with increasing confining stress. Thus, the confining stress restrains rock fragmentation by TBM cutter. The cracks gradually propagate parallel to the plane of the biaxial confining stress with increasing confining stress. While a critical confining stress level is observed from the tests of marble sample. As the confining stress is higher than the critical level, the stress-induced failure occurs in marble under the TBM cutter, and the thrust force for crack initiation and size of crushed zone decrease dramatically with the increase of confining stress. The rock fragmentation by TBM cutters is facilitated by the confining stress. To determine the stress level for stress-induced failure occurring on TBM tunnel face, the true triaxial rock burst test in laboratory is performed with the typical marble in the Jinping project. By simulating the stress state ahead of tunnel face after tunnel excavation, the stress-induced failure processes at the tunnel face are explored by the rock burst test. Combining the field investigation and the previous researches on the stress-induced failure, the different stress levels for stress-induced spalling and rock burst are obtained for TBM tunnels of the Jinping project. And then, a classification for the stress condition is proposed based on the influence of in situ stress on rock fragmentation and TBM performance. Subsequently, the field TBM penetration tests are performed to analyze the rock mass boreability under different stress conditions in three TBM tunnels. The TBM penetration curve, the results of muck sieve tests and comparison with the previous penetration tests showed that in situ stress has significant influence on rock breakage process by TBM and rock mass boreability. The validity of the classification for the stress condition is also verified by the penetration tests. Based on the data obtained from the penetration tests, the popular TBM penetration rate prediction models are performed. By comparing the prediction results, it is found that the rock mass characteristics (RMC) model is reliable and suitable to be modified to a new prediction model involving the parameter of in situ stress. Subsequently, extensive site data collection is conducted for the Jinping project to obtain rock mass properties, in situ stress conditions, TBM performance data and machine parameters. A database is then established for modifying the RMC model. Based on the previous conclusions obtained from this study, the modification for the RMC model is proposed dependent on the classification for the stress condition. The modified RMC model is obtained through nonlinear statistical analysis. It has potential application in the TBM tunnelling under stressed grounds.

EPFL2013

Micromechanical Study of Rock Fracture and Fragmentation under Dynamic Loads using Discrete Element Method

Tohid Kazerani

The study presented in this thesis aims to numerically explore the micro-mechanisms underlying rock fracture and fragmentation under dynamic loading. The approach adopted is based on the Discrete Element Method (DEM) coupled to the Cohesive Process Zone (CPZ) theory. It assumes rock material as assemblage of irregular-sized deformable fragments joining together at their cohesive boundaries. The simulation, which is referred to as Cohesive Fragment Model (CFM), takes advantage of DEM particle/contact logic to handle the fragments and boundaries in between. In this idealization, mechanical properties of particle and more dominantly those of contact control macroscopic response of the particle assemblage. A rate-dependent orthotropic cohesive law is developed for DEM contacts to capture rock material specific features, e.g. brittleness, anisotropy and rate-dependency. Rock experimental behavior is then modeled in order to assess individually the sensitivity of results to grain size, confining pressure, micromechanical parameters, stored strain energy, loading rate etc. The thesis is organized to approach the problem systematically. First, CFM application for static analysis is examined. It is shown that CFM quantitatively and qualitatively predicts compressive and tensile failure of hard and soft rocks as well as shear strength, dilatancy and degradation of rough rock joints. CFM micro-parameters, i.e., stiffness of particle and strength, stiffness, and friction of contact are calibrated using a combination of statistical disciplines and original closed-form expressions. The calibration process provides useful physical interpretation for each micro-parameter in terms of standard rock mechanical properties. These interpretations enable to understand how macroscopic behavior of rock material originates from its mineral microstructure. Energy needed to fully open a contact, the contact energy numerically represents material fracture energy in CFM. Experimental investigations suggest that fracture energy is independent of loading rate in quasi-static circumstances. Thus, contact energy is simply assumed as constant in static analysis. However, simulation on fast fracturing by CFM warns that this assumption causes serious deviations in fracture dynamic analysis. Laboratory observations reveal that fast-moving fracture consumes more energy than slow-moving one does. This inspires to consider contact energy as variable and rate-dependent to provide the model with the appropriate prediction of the fracture energy release process. Applying this new approach, fracture behavior of PMMA plates is investigated under different levels of stored strain energy. As the final stage, dynamic fracture toughness of rock samples, measured by the split-Hopkinson pressure bar test, is simulated and promising results are obtained. They demonstrate how numerical modeling can practically aid experimental methods in terms of measurement verification, error estimation, and performing appropriate corrections. The studies suggest that DEM is an effective and convenient tool to investigate fracture and fragmentation problems. While predictions by continuum models are restricted only to crack initiation, simulation by DEM made it possible to track both the initiation and progression of fracture over time by following consecutive damage of contacts. Moreover, the research specifically demonstrates that the proposed contact model properly predicts the experimental behavior of rock fracture under static and dynamic loading. This result verifies the model validity and adequacy for rock fracture analysis.

EPFL2011

Study on rock mass boreability by TBM penetration test under different in situ stress conditions

Lijun Yin, Jian Zhao

Rock mass boreability is a comprehensive parameter reflecting the interaction between rock mass and a tunnel boring machine (TBM). Many factors including rock mass conditions, TBM specifications and operation parameters influence rock mass boreability. In situ stress, as one of the important properties of rock mass conditions, has not been studied specifically for rock mass boreability in TBM tunneling. In this study, three sets of TBM penetration tests are conducted with different in situ stress conditions in three TBM tunnels of the Jinping II Hydropower Station. The correlation between TBM operation parameters collected during the tests and the rock mass boreability index is analyzed to reveal the influence of in situ stress on rock mass boreability and TBM excavation process. The muck produced by each test step is collected and analyzed by the muck sieve test. The results show that in situ stress not only influences the rock mass boreability but also the rock fragmentation process under TBM cutters. If the in situ stress is high enough to cause the stress-induced failure at the tunnel face, it facilitates rock fragmentation by TBM cutters and the corresponding rock boreability index decreases. Otherwise, the in situ stress restrains rock fragmentation by TBM cutters and the rock mass boreablity index increases. Through comparison of the boreability index predicted by the Rock Mass Characteristics (RMC) prediction model with the boreability index calculated from the penetration test results, the influence degree of different in situ stresses for rock mass boreability is obtained. (C) 2014 Elsevier Ltd. All rights reserved.