Fault (geology)

In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as the megathrust faults of subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes. Faults may also displace slowly, by aseismic creep. A fault plane is the plane that represents the fracture surface of a fault. A fault trace or fault line is a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted on geologic maps to represent a fault. A fault zone is a cluster of parallel faults. However, the term is also used for the zone of crushed rock along a single fault. Prolonged motion along closely spaced faults can blur the distinction, as the roc

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

Load-strengthening versus load-weakening faulting in gouge-bearing faults: insights from triaxial saw-cut experiments

Carolina Giorgetti, Iskander Louhichi, Marie Estelle Solange Violay

Brittle reactivation of pre-existing faults is theoretically constrained by their friction, the stress field orientation, and magnitude. Thus, following the Mohr-Coulomb failure criterion, the increase in tectonic shear stress leads to the reactivation of the fault whenever the shear stress achieves its frictional strength. Moreover, the increase in shear stress can occur following different fault loading paths: commonly, reverse faulting is accompanied by an increase in normal stress (load-strengthening path), while normal faulting is accompanied by a decrease in normal stress (load-weakening path). However, how the loading of the fault influences its frictional strength, i.e. its reactivation as well as its seismic behavior, is still poorly understood. To investigate this, we conducted triaxial saw-cut experiments under different loading conditions. We simulated pre-existing fault zones placing a layer of quartz gouge in a saw-cut through a cylinder of Rothbach sandstone. We investigated faults favorably and unfavorably oriented for reactivation, i.e. oriented at 30. and 50. to the maximum principal stress. These experimental faults were subjected to load-strengthening and load-weakening paths. Different confining pressures (i.e. minimum principal stress) were tested in such a way as to reactivate faults, that undergo different loading paths, at comparable shear and normal stresses. In addition, acoustic emissions were monitored during fault deformation. Consistently with previous studies, our results show that increasing the angle to the maximum principal stress from 30. to 50., the frictional strength (i.e. the apparent friction) of gouge-bearing faults decreases. Furthermore, the frictional strength of unfavorably oriented faults (50.) is affected by the loading path, showing slightly lower frictional strength for load-strengthening than for load-weakening path. These observations suggest that the potential for reactivation of thick, gouge-bearing faults depends on the loading conditions and it cannot be properly assessed assuming a zero-thickness fault plane.

2019

Load-strengthening versus load-weakening faulting in gouge-bearing faults: insights from triaxial saw-cut experiments

Carolina Giorgetti, Iskander Louhichi, Marie Estelle Solange Violay

2019

Analysis of TBM tunnelling performance in faulted and highly fractured rocks

Erika Paltrinieri

EPFL2015

Analysis of TBM tunnelling performance in faulted and highly fractured rocks

Load-strengthening versus load-weakening faulting in gouge-bearing faults: insights from triaxial saw-cut experiments

A study of the TBM performance in fault zones and highly fractured rocks

Load-strengthening versus load-weakening faulting in gouge-bearing faults: insights from triaxial saw-cut experiments

Analysis of TBM tunnelling performance in faulted and highly fractured rocks

A study of the TBM performance in fault zones and highly fractured rocks