Åsa Fransson

Steering Parameters for Rock Grouting

In Swedish tunnel grouting practice normally a fan of boreholes is drilled ahead of the tunnel front where cement grout is injected in order to create a low permeability zone around the tunnel. Demands on tunnel tightness have increased substantially in Sweden and this has led to a drastic increase of grouting costs. Based on the flow equations for a Bingham fluid the penetration of grout as a function of grouting time is calculated. This shows that the time-scale of grouting in a borehole is only determined by grouting over-pressure and the rheological properties of the grout, thus parameters that the grouter can choose. Pressure, grout properties and the fracture aperture determine the maximum penetration of the grout. The smallest fracture aperture that requires to be sealed thus also governs the effective borehole distance. Based on the identified parameters that define the grouting time-scale and grout penetration an effective design of grouting operations can be set up.

Characterisation of a fractured rock mass for a grouting field test

An investigation was performed to find and describe a rock volume suitable for a grouting field test at the Aspo Hard Rock Laboratory, Sweden. Fixed interval length transmissivities and the corresponding number of fractures from geological mapping of a probe hole were used to calculate a probability of conductive fractures for analyses of data from individual boreholes. Furthermore, the transmissivity and specific capacity of the boreholes were compared to examine the robustness of the specific capacity. From the findings of the study, the probability of conductive fractures from probe hole data, the specific capacity and fracture frequency of individual boreholes were sufficient to construct a simplified model of the fracture and the rock volume. Moreover, the median specific capacity of the boreholes was a good description of the effective cross-fracture transmissivity. The field test was also carried out to demonstrate the usefulness of the methodology for improving the analyses of data from the hydraulic tests and geological mapping for a grouting fan.

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.

A swedish grouting design concept: Hydraulic testing and selection of grout

Some grouting boreholes take no grout and some boreholes take too much, two extremes related to grouting technique, grout properties and the properties of fractures intersecting the boreholes. Successful sealing of rock and soil demands an adequate description of the system to be grouted as a basis for grouting design and selection of grouting material. The basis for this Swedish concept of grouting design is the individual fractures and the hydraulic apertures, b, of these fractures. The hydraulic aperture is an important parameter to describe the grouting behavior and is used to determine if the grout can enter the fractures, the penetrability. The hydraulic aperture also determines the penetration length in addition to grout parameters e.g. yield stress, τ0, and viscosity, μg as well as grouting pressure and time. Knowing these parameters, a proper grouting technique can be adapted. Important input for both design and performance are simple and practical tests of rock and grout and the intention of this paper is to present a testing procedure and give examples from laboratory and field experiences that the approach actually works.

A Swedish Grouting Design Concept: Decision Method for Hard Rock Tunneling

Efficient grouting of hard rock requires adequate knowledge of the water-bearing fracture system in the rock mass. The observational method approach involves identifying different possible scenarios and relating them to predefined strategies for grouting design. Parameters useful in preparing a relevant description of the rock mass are presented, as well as a method for choosing a conceptual model. The implications of different fracture systems for the grouting design are discussed. A method is presented for deciding whether grouting is needed in order to ensure a high degree of probability that tunnel leakage will remain below the inflow requirements. The methods presented are applied to data from a real tunnel to illustrate the procedures.

Modeling early in situ wetting of a compacted bentonite buffer installed in low permeable crystalline bedrock

The repository concept for geological disposal of spent nuclear fuel in Sweden and Finland is planned to be constructed in sparsely fractured crystalline bedrock and with an engineered bentonite buffer to embed the waste canisters. An important stage in such a deep repository is the postclosure phase following the deposition and the backfilling operations when the initially unsaturated buffer material gets hydrated by the groundwater delivered by the natural bedrock. We use numerical simulations to interpret observations on buffer wetting gathered during an in situ campaign, the Bentonite Rock Interaction Experiment, in which unsaturated bentonite columns were introduced into deposition holes in the floor of a 417 m deep tunnel at the Aspo Hard Rock Laboratory in Sweden. Our objectives are to assess the performance of state-of-the-art flow models in reproducing the buffer wetting process and to investigate to which extent dependable predictions of buffer wetting times and saturation patterns can be made based on information collected prior to buffer insertion. This would be important for preventing insertion into unsuitable bedrock environments. Field data and modeling results indicate the development of a de-saturated zone in the rock and show that in most cases, the presence or absence of fractures and flow heterogeneity are more important factors for correct wetting predictions than the total inflow. For instance, for an equal open-hole inflow value, homogeneous inflow yields much more rapid buffer wetting than cases where fractures are represented explicitly thus creating heterogeneous inflow distributions.

Fracture Deformation Measurements during Grouting in Hard Rock

When a fracture system in crystalline rock is grouted the rock mass may deform. Such deformations may reduce the grouting efficiency since new flow paths are opened. The work presented here show that deformations occur at hydraulic tests and grouting and that deformation can be measured and evaluated as stiffness from in situ tests. Deformation measurements, hydraulic testing, and grouting was conducted in spring 2010 in the Hallandsas tunnel and hydraulic testing in a service tunnel in Gothenburg (Runslatt and Thorn, 2010). For measuring physical deformation recently developed equipment from Chalmers University of Technology was used. Deformations were measured seven times in the same borehole. Three measurements were during grouting, and the remaining four from water pressure tests. Most deformations occurred at pump pressures of 1-1.4 MPa, which is lower than the calculated normal rock stress. Stiffness has been evaluated in several ways, including a new method, (Fransson, et al., 2010). Generally the evaluated stiffness is lower in the Hallandsas tunnel than in the Gothenburg tunnel. The results show agreement with other in situ experiments.

Statistical Evaluation of Groutability Using Data from Hydraulic Tests and Fracture Mapping Case Studies from Sweden

Sweden has a long history of research within the field of rock fissure grouting in hard crystalline rock mass due to strict environmental requirements regarding allowable ground water draw down. These requirements normally implies that fractures down to aperture size between 50 to 100 μm needs to be sealed and within these ranges the size of the particles for cementitious grouting agents becomes a limiting factor. For a grouting design it is therefore of importance to consider the aperture size distribution of the rock mass in order to predict the groutability for both cementitious and non-cementitious grouting agents. Transmissivity data from hydraulic tests (water pressure tests) and number of fractures along a borehole can be assessed from core logging for further use as input for a statistical interpretation of fracture data to simulate an aperture size distribution. A methodology developed at Chalmers University of Technology in Gothenburg, Sweden, is proposed. The method is a statistical evaluation of groutability (SEG) and is based on the Pareto distribution. A computational design tool has been developed to simplify the use of the statistical evaluation and to make the research more accessible to end users, designers, in the grouting industry. The aim of this article is to present two case studies where the statistical interpretation of fracture data is performed by using the computational design tool and how the outcome can be of great use in finding a more accurate grouting design. The case studies include fracture data sets from two large infrastructure rock tunnel projects in Sweden; a road tunnel in Stockholm and a railroad tunnel in Gothenburg.