Characterisation of Fractured Rock for Grouting Using Hydrogeological Methods

Sealing of tunnels by grouting demands knowledge concerning the fractured rock and the grout, followed by an appropriate choice of strategy and equipment. In this thesis, the characterisation of rock is the main issue and, since it addresses the engineering problem of producing a description for grouting, the properties of grout and the grouting procedure need to be considered. The conceptualisation is based on a grouting fan with a number of boreholes that cross fractures of different lengths and orientations and with varying ability to transmit water. These parameters are used to form a simplified model, in which discrete fractures and fractures along boreholes are considered central for further investigations. The ability to transmit water, or the transmissivity of the fractures, is a key parameter since it reflects the fracture aperture, an aperture that will influence both the penetration length and the volume of grout. Transmissivity, orientation and length of fractures are obtained by means of hydraulic tests and geological mapping. The question asked is whether these methods are useful and robust enough for grouting predictions. Investigations of geometry for a discrete fracture were performed using analytical and numerical approaches followed by laboratory and field experiments. For a larger scale and fractures along boreholes, a compilation of field data was made followed by the development of a non-parametric method, which gives a possible transmissivity distribution for fractures crossing a borehole. According to these investigations, the transmissivity or a closely related and more easily obtained parameter, referred to as the specific capacity, gives a good description of the fracture aperture. Furthermore, the median specific capacity of a small number of tests described the fracture in general, i.e. the effective or cross-fracture transmissivity. Probe holes have proved useful as a basis for choosing a grouting strategy and, here, the non-parametric method resulted in a low frequency of conductive fractures with log-normal distributed transmissivities. These parameters were used to analyse data from individual boreholes, which were subsequently connected to form a simplified model. The methodology was verified with a field experiment and should be useful in a grouting project

Geology, water inflow prognosis and grout selection for tunnel sealing: Case studies from two tunnels in hard rock, Sweden

Successful sealing of tunnels by injection of grout into the fractured rock demands knowledge concerning the geological and hydrogeological conditions. This paper aims at presenting how a description of the geology and hydrogeology can be used as a basis for a grouting design. Important issues that are considered here are: identification of the extent of the grouting based on inflow requirements and a prognosis of tunnel inflow; and selection of grout based on the performance of grouting materials. Two tunnel sections from two different tunnels in crystalline rock were used as case studies. The first tunnel section (70 m) is found in \uc4sp\uf6 Hard Rock Laboratory (south east Sweden) at 450 m depth. The rock is sparsely fractured and the main rock type is a diorite. The second tunnel section (36 m) is found in the Hallands\ue5s tunnel (south west Sweden) excavated through a horst and at approximately 100 m depth. The fracture network is well connected. In this section the rock consists of gneiss and amphibolite.A conclusion based on the two case studies is that the principal descriptions of the geology and hydrogeology for the two tunnel sections identify general differences between the two tunnels. This explains in part the deviation found between early inflow prognoses and the measured inflows. Further, the estimated hydraulic aperture from hydraulic tests can be used as a basis for selection of grout. Finally, the fracture frequency or the variation in fracture frequency (e.g. between neighboring borehole- or tunnel sections) in combination with hydraulic tests indicate what areas to focus on when grouting

The use of the Pareto distribution for fracture transmissivity assessment

Abstract For many applications data on the transmissivity distribution of individual fractures are necessary, i.e., discrete fracture network (DFN) modelling of groundwater flow and transport of solutes in fractured rock and design and performance of rock grouting. Using borehole data from the \uc4sp\uf6 Hard Rock Laboratory, Sweden, it is shown that evaluated fracture transmissivities from three boreholes at \uc4sp\uf6 can be well described by a Pareto or power-law distribution. Evaluated distribution parameters for the three boreholes are similar, which indicates that the Pareto distribution is a robust tool to assess three fracture transmissivity distributions. Using the evaluated distribution parameters random simulations of the original interval test data show that the approximate lognormal distributions of these are reproduced. This strengthens the credibility of the approach. It is shown how the distribution parameters can be assessed from incomplete data using the properties of the distribution. Finally, Pareto distribution transmissivities also imply Pareto distribution apertures of the fractures

Hydrogeological reference conditions for assessment of environmental impact and for grouting design

Dewatering of groundwater resources induced by leakage into underground constructions can cause land subsidence, damage to constructions and their foundations and disturbances of groundwater dependent ecosystems. To reduce the environmental impacts, safety measures, e.g. sealing fractures by grouting to reduce inflow of groundwater or artificial recharge to maintain groundwater levels, must be implemented. Site investigations of the total geological and hydrogeological conditions at a site before construction is, due to financial aspects, most often not possible. To handle these uncertainties in the design- and construction process, it is suitable to use the observational method, which include the idea of identification, confirmation or rejection, and revision of the most probable and unfavorable conditions, and predefined technical design solutions for conditions that can reasonable be anticipated or foreseen. To assess the geological and hydrogeological conditions at an early stage of a project we suggest that geological and related hydrogeological reference conditions are used. Fundamental to our approach using reference conditions is the grouping of materials with similar geological and hydrogeological conditions and engineering characteristics. In this paper, we present conceptualizations of five reference conditions common in western Sweden and two examples of reference conditions in Singapore. The conceptualization of reference conditions includes a description of: the geological material; the hydrogeological properties and behavior within the environment; and the engineering characteristics related to water control and grouting. Examples of technical design solutions used to adopt to project specific requirements for inflow and drawdown for underground constructions constructed in environments representing one of the suggested reference conditions in western Sweden are also presented to exemplify the application of reference conditions for technical design

Approach for early engineering geological prognosis adapted to rock grouting design

Planning of suitable water-mitigating measures, such as grouting, can be valuable for feasibility studies of underground construction projects. However, grouting design work becomes more difficult if hydraulic properties of the rock mass are insufficiently investigated and the geological information presented in the engineering geological prognoses mainly focus on stability-related parameters. The purpose of this study has been to demonstrate how geological information useful for grouting design can be compiled in engineering geological prognoses during early phases of tunnel projects. Attention is also given to relating geological settings to grouting design classes during the construction phase. The suggested approach was exemplified for the Hallands\ue5s project, Sweden, using low-cost geological information available during the original feasibility study of the project. The prognosis included a conceptualisation of the hydraulic behaviour of the rock mass at the project site, with hydraulic domains representing the flow regimes found in different geological units. The need for sealing measures along the tunnel alignment was expressed with inflow estimates based on data from the SGU Wells archive. This combination of qualitative and quantitative estimatesindicated grouting design prerequisites for both favourable and unfavourable scenarios, which could be useful for grouting design classification during construction. Thehydraulic domains facilitate the understanding of the relation between geology and grouting design, which is essential for tunnel construction work