The Impact of Fracture Geometry on the Hydromechanical Behaviour of Crystalline Rock

Effective construction of tunnels in fractured crystalline rock requires a unified approach for handling rock mechanics and hydrogeological issues. Traditionally, rock mechanics and hydrogeology not only use different nomenclature, they also measure parameters such as e.g. aperture differently. A description of fractures that includes both fracture surface- and void geometry could be used as a basis for a conceptual model that allows complexity to be added to the descriptions of hydraulic and mechanical behaviour without contradictions. In this work, hydromechanically coupled experimental setups and methods were developed and used to improve a conceptual model of hydromechanical (HM) fracture behaviour at low compressive stress. Key aspects of the model are hydraulic aperture, fracture normal stiffness, the number of contacts between the surfaces, and the aspect ratio, i.e. the relationship between contact point distance and aperture, thus describing the voids between the surfaces. The experimental setups that were developed comprised equipment for in situ measurements of mechanical deformation due to stepwise hydraulic injection of fractures close to a tunnel, and a laboratory HM permeameter used in conjunction with fracture topography and aperture scanning. The latter produced high-resolution aperture maps of samples at 1.0 MPa, which were related to the flow rates, estimated hydraulic aperture and stiffness from the HM permeameter tests of the samples. Aiming at a common aperture-stiffness relationship for laboratory and in situ tests at different scales, the results were compared to a previously suggested relationship linking hydraulic aperture and normal stiffness. A relationship that has been devised from in situ hydraulic interference tests and is assumed to be valid for low comp-ressional stress across fractures with limited prior deformation. The few laboratory samples tested and the in situ tests performed show agreement with the aperture-stiffness relationship. A relationship and a conceptual model that have potential to provide support to future studies on hydromechanical behaviour of crystalline rock

Hydromechanical Behaviour of Fractures Close to Tunnels in Crystalline Rock

The deformation and stiffness properties of rock fractures are important measurable parameters when describing their hydromechanical behaviour. Deformation refers to aperture change. Stiffness refers to the amount of deformation per stress change to which a fracture is subjected. This thesis aims to investigate the stiffness and deformation behaviour of fractures in crystalline rock through in situ and laboratory experiments. The focus in this work is on fracture geometry due to geological stress history. This will result in increased conceptual understanding and accordance between hydromechanical and geomechanical fracture description and behaviour. The in situ measurements consisted of deformation measurements in boreholes and were conducted at the \uc4sp\uf6 Hard Rock Laboratory (HRL) and in the Hallands\ue5s Tunnel. The total deformation across the instrumented borehole sections was measured as an effect of hydraulic pressurisation of the fractures in the nearby rock volume. The results were assessed in terms of deformation and fracture stiffness.The laboratory experiments were conducted as cyclically loaded permeameter measurements of fractured rock core samples from \uc4sp\uf6 HRL with simultaneous deformation measurements across the fracture. The tested samples had various geological properties and revealed differences in hydraulic aperture and mechanical deformation behaviour across the experimental cycles.The stiffness to hydraulic aperture relationship followed a trend identified in the literature and deviations were given plausible explanations related to the geology and geometry of the samples. The results were discussed in the light of the sampled geology and the measurement methods. The measured deformations and corresponding stiffness were found to be reasonable in the light of available knowledge of the local geology and stress situation at the sites

Radial penetration of cementitious grout - Laboratory verification of grout spread in a fracture model

During the past two decades of research and development in the field of grouting in hard jointed rock, the design process has taken a number of significant leaps forward. A grouting design in hard rock can now be based on the penetration length of grout in individual rock fractures. For cementitious grouts, the most common rheological model used is the one for a Bingham fluid. The model is a conceptualisation of grout spread where two rheological properties of the grout viscosity and yield stress govern the penetration length along with the fracture aperture and applied grouting overpressure. This paper focuses on verification of radial Bingham flow of cementitious grout using a fracture model constructed from acrylic glass. Each test conducted using the fracture model was filmed, allowing the grout spread to be analysed as penetration length over time. The measured penetration lengths were then compared with analytical solutions derived for Bingham grout in a plane parallel fracture. The results indicate that the penetration of cementitious grout in fracture apertures of 125 gm and 200 gm is verified for up to 40% of the maximum possible penetration length. This can be compared to normal grouting, where the penetration lengths achieved are around 20% of the maximum penetration length

Grundl\ue4ggande egenskaper f\uf6r injektering och intr\ue4ngning av bruk

Injektering i h\ue5rt berg inkluderar vetenskaper som fl\uf6de av v\ue4tskor, reologi, hydrogeologi, material k\ue4nnedom, geologi och annat. Att faktiskt f\uf6rst\ue5 och ha en k\ue4nsla f\uf6r hur fl\uf6de fungerar i sprickor \ue4r viktigt. Detta kan l\ue4tt gl\uf6mmas av och energi l\ue4ggs allt som oftast p\ue5 andra delar exempelvis p\ue5 materialetkunskap. Denna artikel syftar till att beskriva hur fl\uf6de sker i sprickor och b\uf6r kunna bidra till en k\ue4nsla av hur fl\uf6det av bruk faktiskt sker. Artikeln utg\ue5r fr\ue5n framtagna samband f\uf6r spridning baserat p\ue5 Binghamfl\uf6den f\uf6r att beskriva hur yttre h\ue4ndelser som kan upptr\ue4da vid vanlig injektering kan p\ue5verka spridningen. En verifikation av intr\ue4ngningsl\ue4ngder i en nyligen tillverkad sprickmodell anv\ue4nds f\uf6r att visa hur ett Binghamfl\uf6de beter sig. B\ue5de spridning och hur ett sambandsh\ue5l p\ue5verkar spridningen visas samt hur detta kan p\ue5verka designkriterier. Filmvisning av injekteringsfl\uf6de i sprickrepliken kommer s\ue4kerligen bringa lite mer klarhet i hur teoretiska ber\ue4kningar kan anv\ue4ndas som modeller

Assigning Fracture Stiffness from In-Situ Deformation Measurements

Fracture stiffness is varying between fractures and is influenced by its proximity to a tunnel opening; if the behavior close to the opening is of interest for modelling efforts, then it may be better to use such data as input rather than high-stress laboratory measurements. A handy method for in situ testing of deformation (stiffness) and transmissivity would be beneficial to obtain data for numerical modeling of the near field of an excavation. We describe a measurement method under development that uses an anchor in a borehole and measures deformations between the anchor and the rock surface. Measurement of deformations is done during a stepwise constant head injection test providing information about both hydraulic and mechanical properties. Deformations and applied pressure is used through the effective stress concept to calculate fracture stiffness. Deformation measurements have been conducted in the TASO and TAS04 tunnels at 410 – 420 m depth at \uc4sp\uf6 Hard Rock Laboratory (HRL), and in the Hallands\ue5s tunnel, Sweden. Results show deformations in tested fractures 0.2 – 3 m below tunnel floor in the order of a few to tens of micrometers for injection overpressures in the order of 0.5 – 0.6 MPa. Stiffness is traditionally described as either normal stiffness or shear stiffness, the design of the experimental setup here does not allow for this distinction directly from the results; however knowledge of the orientation of tested fractures and coupling of the results to injection rates may help in discerning type of deformation

Fracture Aperture Measurement and Consequences for Grouting

The hydraulic and the mechanical apertures of fractures and the relation between them are of interest for hydromechanical (HM) coupling and design of grouting works and reinforcement. The fracture geometry will influence water inflow to underground constructions, penetrability and penetration length of grout and mechanical properties of the fracture. This paper aims at presenting fracture geometry measurements on one fracture sample and to use this as a basis for a discussion on consequences for grouting. To measure surface topography of the two sides of a rock core fracture sample from the TASS-tunnel, \uc4sp\uf6 HRL (Sample and sampling described in Ericsson et al. 2009) commercial equipment for stereo photogrammetry was used. Prior to scanning each surface, their relative positions were determined at 1.0 MPa confining pressure. The procedure enables a computer comparison between the surfaces, rendering an aperture map for the specific confining pressure. The measured surface geometry provided a data set that was put in the context of hydraulics, mechanics and hydromechanics. Comments on how the applications inflow, grout spread, fracture deformation and block stability can be related to the fracture geometry are given