Rock mass classification systems are used for various engineering design and stability analysis. These are based on empirical relations between rock mass parameters and engineering applications, such as tunnels, slopes, foundations, and excavatability. The first rock mass classification system in geotechnical engineering was proposed in 1946 for tunnels with steel set support. Design methods In engineering in rock, three design strategies can be distinguished: analytical, empirical, and numerical. Empirical, i.e. rock mass classification, methods are extensively used for feasibility and pre-design studies, and often also for the final design. Objectives The objectives of rock mass classifications are (after Bieniawski 1989):

Identify the most significant parameters influencing the behaviour of a rock mass.

Divide a particular rock mass formulation into groups of similar behaviour – rock mass classes of varying quality.

Provide a basis of understanding the cha

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

Analysis of TBM tunnelling performance in faulted and highly fractured rocks

Erika Paltrinieri

The thesis focuses on the study of the performance of TBMs in highly jointed and faulted rocks. Despite the great production rates recorded in favourable ground conditions, the TBM advance can be significantly slowed down in limiting geological situations such as extremely fractured and fault zones. After an introduction about the major issues resulting from tunnelling in bad ground conditions, a brief review of some TBM performance prediction models is reported and the most common parameters adopted for evaluating the TBM performance are identified. The geotechnical characterisation of fault zones is very difficult due to their heterogeneous nature (i.e. weak and strong rock components). This particular aspect is highlighted through a literature review about the geological/geotechnical description of fault rocks. In order to investigate possible relationships between difficult rock mass conditions and TBM performance, data of several tunnel projects are collected in a database by including information from the field, laboratory tests and literature. Preliminary analyses are carried out in order to identify correlations between TBM performance (i.e. penetration and advance rate) and rock mass parameters (e.g. rock strength, fracturing degree, etc.). Although some trends are identified, the scattered results confirm the difficulties in predicting the machine performance in complex geological environments. In order to obtain a more complete geotechnical description of disturbed zones, a classification system for highly fractured rock masses and fault zones is developed. Four âfault zoneâ classes are identified by considering the fracturing and the weathering degree of the rock mass. Furthermore, in order to analyse the response to mechanical excavation of the identified classes, a set of numerical simulations are run with the aim to investigate the TBM performance at the cutter-scale. The data recorded in the TBM-performance database are then analysed according to the new classification system. For each âfault zoneâ class, a reduction rate for selected TBM parameters is defined with respect to the tunnelling performance observed in good ground conditions. The results of these analyses are of great relevance as they allow to successfully quantifying the effect of altered rock mass conditions on the TBM behaviour. The results obtained in the previous steps are used to carry out probabilistic analyses of the tunnel construction time and costs by means of the Decision Aids for Tunnelling (DAT). The DAT are a software package that evaluates the influence of uncertainties related to both geotechnical conditions and excavation process on the final tunnel construction time and costs. In this framework, the TBM advance rate reductions previously defined for each âfault zoneâ class are input in the simulations. A reliable estimation of the effect of degrading geological conditions (i.e. faulted and highly fractured/crushed rocks) on the tunnel construction process is obtained. The results of this research provide useful insights regarding the TBM performance reduction in bad grounds, with respect to the âordinaryâ tunnelling conditions. The study highlights the need to better characterise, from a geomechanical point of view, the highly fractured and faulted rocks. This can be done by considering the parameters representative of the degraded state of the rock mass, instead of those commonly used for estimating the TBM performance in good rocks.

EPFL2015

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.