Characterisation of Fractured Rock for Grouting Using Hydrogeological Methods

Doctoral thesis, 2001

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.