Cataclastic fault rocks in underground excavations

The present study makes part of a research project on the geological and geomechanical characterisation of weak cataclastic fault rocks encountered in underground excavation sites in the Alps. The project is composed of a geological and a geomechanical part, split up in two PhD theses and realised in parallel at the Laboratory of Geology (GEOLEP) and the Laboratory of Rock Mechanics (LMR) at the Swiss Federal Institute of Technology in Lausanne. By their very contrasting and unfavourable mechanical behaviour in underground excavations, weak cataclastic fault zones influence directly the excavation method, the safety of the working site, the choice of tunnel support, the long-term behaviour and the costs of the infrastructure. The construction of galleries across cataclastic fault zones bears furthermore the potential risk to modify the local and regional hydrogeological regime. Weak cataclastic fault rocks show a large scatter of geological and geomechanical properties. Due to their low cohesion, they alter rapidly when exposed to weathering. They can hence be studied and sampled in natural outcrops and exploratory boreholes only with great difficulty. Analyses and laboratory tests are laborious and require special sample preparation techniques and equipments. For these reasons still little is known about their geological, hydrogeological and geomechanical behaviour. The purpose of this study is to develop a detailed and objective differentiation and characterisation method for weak cataclastic fault rocks. Classifications and terminologies of (weak) cataclastic fault rocks are in fact still controversial. After a review of fault rock definitions and rock deformation mechanisms, the term kakirite has been retained in agreement with Heitzmann (1985) and Wyder (1997) as a general designation for weak, cohesionless cataclastic fault rocks. This term has been approved recently by the Swiss Society of Engineers and Architects (SIA) to designate fault zones made up of "loose, structureless material" (SIA Norm 199, 1998). Cataclastic fault zones have been studied and sampled in galleries currently under construction in different geological and tectonic contexts in the Alps. Research has been concentrated mainly on fault zones occurring in quartzo-phyllitic host rocks. For comparison, fault rocks from greenstones and serpentinite have been analysed too. To improve the understanding of the geomechanical and hydrogeological behaviour of weak cataclastic fault rocks, a new quantitative characterisation method has been developed. In a simplified approach, the geomechanical behaviour of (fault) rocks is considered to be controlled by two factors, namely the: mineralogical composition (mwVh), and rock fabric (TC, MC) The proposed mineralogical and structural characterisation of cataclastic fault rocks is referring to the meso- to microscopic scale, respectively to a scale comparable with common geomechanical laboratory tests. Mineralogy is expressed quantitatively by means of the mean weighted Vickers hardness, mwVh (Calembert et al., 1980a). The relative proportions of the principal rock forming minerals have been determined by semi-quantitative XRD-analyses. The mineralogical composition of kakirites, adjacent host rock samples and sieved kakirite grainsize fractions has been determined. Comparing the composition of the different grainsize fractions a clear dependency of grainsize on mineralogical composition has been found in kakirites. Quartz content is in general considerably higher in the clast than in the matrix fraction. The later is in fact composed mainly of phyllosilicates. This mineralogical grainsize fractionation is interpreted to be controlled by the original host rock fabric, the strength contrast between the involved mineral phases and alteration processes. Except kaolinite (Varzo), no other alteration mineral phases have been identified. The presence of (swelling) clay-minerals in very low proportions which cannot be detected by the performed whole rock XRD-analyses cannot be excluded however. Kakirite samples sieved for XRD-analyses show an extended grainsize distributions with in general high proportions of silt/clay grainsize fraction (mean 45%). This may explain the often observed very low permeability of kakirite zones. Their hydrogeological role is illustrated in the last part of this work by the means of a case history of a fault zone crossed by the gallery of Cleuson-Dixence. It demonstrates the often observed groundwater flow barrier-effect of cataclastic fault zones and its impact on the excavation work and the local hydrogeological regime. Rock fabric of kakirites has been characterised by thin section analyses with subsequent semiautomatic image analysis. It is based on a two-phase "clast-matrix" model, considering kakirites to be composed of varying proportions of "hard" clasts and "weak", fine-grained matrix. Clast properties (density, shape and relative orientation) are quantified by the texture coefficient (TC) of Howarth & Rowlands (1987). Pre-existing discontinuities in the matrix are characterised by a newly developed matrix coefficient (MC), involving the discontinuity density, roughness, mean length and relative orientation. The combined structural approach involving the MC and TC has shown to be appropriate to characterise and differentiate a wide range of weak cataclastic fault rocks by their microscopic rock fabric. Two opposite kakirite fabric types can be distinguished: first, a granular fabric, characterised by clasts taken in a fine-grained, isotropic matrix. Second, a discontinuity dominant, often schistose fabric wherein no clasts can be identified. A gradual transition is observed in-between these two end-member types. In order to assess the validity of the developed characterisation method, a first attempt of correlation has been made between the mineralogical-structural parameters and corresponding rock strength of kakirites tested by Habimana (1999). The found correlation is a strong motivation to continue the started research on cataclastic fault rocks with the presented mineralogical and structural approach.

Geological characterization of cataclastic rock samples using medical X-ray computerized tomography

Pierre Guillaume Christe

Technical difficulties associated with tunnelling operations in tectonized geological settings are frequently encountered. They may include instantaneous and delayed cavity convergence, sudden collapse of walls or roof of a gallery, outpouring of fault-filling materials and water inflows. These phenomena may negatively affect economical and safety aspects of construction sites. The present study refers to two previous research projects conducted at the EPFL addressing the problem of an improved geological and geomechanical characterization of weak cataclastic rocks in underground excavation works (Cataclastic fault rocks in underground excavations, a geological perspective, Bürgi, 1999; Caractérisation géomécanique de roches cataclastiques rencontrées dans des ouvrages souterrains alpins, Habimana, 1999). Cataclasis is an evolutionary rock degradation process in time and magnitude related to fault zone activity in the Earth's crust. A conceptual model describing the occurrence and variability of cataclastic rocks at shallow crustal conditions is proposed in this research. It describes environment of fault zones based on both theoretical considerations and field experiences collected during self-realized short-drilling operations performed in contrasted petrological settings. The model considers cataclasis from a regional perspective (macroscopic scale) down to its microscopic manifestations on rock materials. Cataclastic rock characteristics are strongly influenced by the degree of tectonic solicitation and the initial rock composition. Guidelines about specific damage microstructures diagnostic of the cataclastic intensity and function of the initial rock protolith can meaningfully assist the interpretation of rock specimens recovered from reconnaissance drilling operations. Despite the poor mechanical quality often observed for such samples and their readiness to collapse, this study demonstrates that the in situ extraction of cataclastic specimens can be largely improved with the use of a high-quality drilling equipment. Accordingly, with the availability of representative materials for subsequent studies, the uncertainties affecting the geological prognosis can be strongly limited at a preliminary stage of geotechnical studies. It is shown that the geotechnical characterisation of cataclastic rock cores by means of laboratory investigations requires combining information about rock microstructure and mineralogy with strain-stress relationship derived from triaxial compression tests. Such a framework makes it possible to interpret and explain the specific mechanical behavior of heterogeneous-anisotropic rock materials in a much more coherent manner. With this regard, the approach followed by Bürgi (1999) with a mineralo-structural index (MSI) determined on 2D thin sections and proposed to predict cataclastic rock strength based on geological evidences, is found to have potential for geotechnical studies. However, Bürgi's approach still faces heavy criticism in the light of the practical application. Its limitations are detailed and discussed in this study. Accounting for both aspects of the rock characterization procedure, an operational methodology is therefore proposed aiming at its application in geotechnical studies. The medical X-ray computerized tomography (XRCT) has been specifically implemented in this context for the analysis of geotechnical rock cores, as a non-destructive technique which allows a rapid indirect imaging of the rock core structure before any further analytical manipulations are conducted. The technique is found facilitating to a great extent the study of cataclastic materials. An irradiation protocol has been defined and an acquisition strategy minimalizing inherent artifacts of the XRCT technique has been tested. Obtained results are highly encouraging. Moreover, an image analysis tool has been developed in order to improve the segmentation of geological damage features from indirectly recorded X-ray models. As discussed in details within this study, a critical analysis of 3D segmented models corresponding to sample structures before and after mechanical tests is thought to be capable to improve the triaxial test interpretation. The XRCT is therefore envisaged as a bridge technique permitting an improved correlation between specific properties of cataclastic rocks based on indirect evidences. With it, the geological and mechanical characterizations can be performed on the exactly same samples, reducing uncertainty in the characterization procedure. This step goes towards the development of a precise geotechnical classification for cataclastic rocks. Based on the XRCT technique, it is also discussed how Bürgi's approach could be translated in a three-dimensional form to reach a wider agreement. Based on a database of carbonatic cataclasites, it is evidenced that cataclastic strength and deformation properties are strongly influenced by textural and structural characteristics of pre-existing damage microstructures. Strength data obtained in this research show that in the same fault zone environment, different mode of deformations affect cataclastic rocks and that peak triaxial strength can vary significantly. Heterogeneity and material anisotropy are prime factors affecting specific behaviors of cataclastic rocks. Therefore, it is suggested to incorporate geological features into constitutive frameworks which describe mechanical behavior of such materials. This last aspect constitutes a very stimulating research field where further work is needed today.

EPFL2009

Les circulations d'eau dans les massifs cristallins alpins et leurs relations avec les ouvrages souterrains

The present study has been achieved within the framework of the scientific program AQUALP, supported by the EVIAN Foundation. Its theme is the study of the hydrogeology of the Alpine crystalline. The AQUALP programme consists of two co-coordinated parts : while the "source" part, lead at the Savoy University, studies natural circulation towards springs, the "work" part, which concerns this study, has focused on the interaction between civil engineering works and underground waters. The approach adopted is that of a field study during which water inflows encountered in various Alpine underground works and replaced in their geological contexts were followed up as from the discharge, the physico-chemical and the isotopic chemistry point of view. A tracer test and numerical simulations of underground flows were also undertaken. Main results The summary of the hydrogeological data in the numerous existing underground works allowed to calculate, on the basis of simple analytical solutions, the distribution of the hydraulically conductivity of the massifs crossed. These results show that the deeper one goes the more the hydraulical conductivity decreases. They bring us to specify the thickness of the decompressed zone, i.e. about 500 m. This zone strongly influences the underground flow in these massifs. Presence of mica inhibits the hydraulical conductivity in conferring a more plastic comportment to the rocks during past tectonic phases. The temperature of the water inflows encountered is a complex function of the thickness of the overlying rock and the quantity of waters circulating in the massif. Therefore it constitutes a precious indicator, during the excavating of underground works, of the presence of possible more permeable sections in formations to come. A numerical simulation of such a case has been undertaken and shows the possibility of using this tool to improve the expectation of water inflow in the work. The temperature study as well as the application of the chalcedony geothermometer to the entire of the inflow studied has shown the existence of ascending flow systems, fed by precipitation. Waters circulating in the Alpine massifs have a chemical composition strongly influenced by the reservoir rock. Chemical analyses of major and trace elements have allowed to distinguish waters issued from the various petrographies encountered. One must first of all underline the complementarity of major and trace elements analyses. The discriminating power of these two types of analyses does not apply to the same type of rocks. The major structures hydraulically conductive in crystalline Alpine massifs have been defined. One can mainly quote isolated fractures, strongly fractured and tectonised zones, kakirite zones, the decompressed zone, the down-bending zone, contacts between distinctive hydrogeological units and large metasedimentary structures. Hydraulical and physico-chemical characteristics of waters associated to each type of defined structures have been determined. These structures are straightened and induce water circulation close to the vertical, from the surface towards the work. The infiltration zones of waters determined on the basis of the orientation of major structures generally coincides with the altitudes calculated on the basis of isotopic analyses of waters (0-18). Some anomalies have nevertheless been put into evidence. They are mainly relative to circulation in the decompressed zone or in overlying Quatemary beds. Relationship between surface waters (barrier lakes or rivers) and underground works have also been put in evidence on the basis of the interpretation of isotopic results. Numerous water inflows are poor in tritium and attest of the time of transit through to the work in the order of a number of decades, in relation with the weak permeability observed in depth. These waters are often rich in sodium, because of the alteration of silicates contained in rocks. More recent waters have been encountered in shallow depths, in the decompressed zone or in association with a great geological fault. The examples of the interaction between underground works and springs are systematically linked to important water inflows in the works either through a decompressed zone of the massif, or through a particularly transmissive zone connected with the surface. These examples show that in this fissured environment, extremely divided into compartments and anisotropic, the drying up of springs is not linked with the progressive lowering of a water table along side the work but at punctual and localized inflows. A conceptual model of the hydrogeology of the Alpine crystalline is proposed in this study. It is accompanied with an interaction model between a work and the natural middle. It must help the project manager to foresee the water inflows in underground works and to adopt the optimal solution to minimize the impact of the work on water resources.

EPFL1998

Analysis and estimation of gripper TBM performances in highly fractured and faulted rocks

Erika Paltrinieri, Federica Sandrone, Jian Zhao

This paper focuses on performance analysis of Tunnel Boring Machine (in particular gripper TBMs) in highly jointed rock masses and fault zones. In order to investigate possible relationships between these difficult excavation conditions and TBM performance, the data of several tunnel projects have been collected in a specific database (TBM-performance database). Despite the difficulties in gaining complete TBM data sets and detailed geological information, this database compiles data from the field, laboratory tests and literature. Preliminary analyses have been carried out in order to find possible correlations between TBM performance parameters (penetration and advance rates) and rock mass characteristics, such as rock strength and fracturing degree, generally used in common TBM performance prediction models. Although some trends could be identified, the scattered results confirm the difficulties in predicting the machine performance in complex geological environments starting from characterising indexes proper to good rocks. Then, a classification system for highly fractured rock masses and fault zones has been developed, based on representative parameters, providing a more complete geomechanical description of disturbed zones. Four “fault zone” classes have been identified. For each class a reduction rate of the selected TBM-performance parameters (i.e. cutterhead rotation speed, penetration rate and daily advance rate) has been evaluated with respect to the tunnelling performances recorded in good ground conditions. The results of these analyses allow effectively quantifying the effects of degrading rock masses on the TBM performances.

Elsevier2016

Geological characterization of cataclastic rock samples using medical X-ray computerized tomography

Pierre Guillaume Christe

EPFL2009

Les circulations d'eau dans les massifs cristallins alpins et leurs relations avec les ouvrages souterrains

EPFL1998