ALBERT

Description: Slope movements of the deep-seated Campo Vallemaggia landslide in the southern Swiss Alps have been reported for over 200 years. Surface and borehole investigations of the unstable mass reveal an up to 300 m deep complex structure incorporating 800 million cubic metres of disturbed metamorphic rocks divided into blocks along primary fault zones. An average slide velocity of approximately 5 cm/year can be calculated from various monitoring data recorded between 1892 and 1995. Block movements primarily involve mechanisms relating to multiple shear surfaces, but in cases where slide blocks are constrained by other blocks, creep deformations are observed. Borehole investigations revealed the presence of artesian water pressures, which when integrated with inclinometer and surface geodetic data, helped to provide key insights into the underlying instability mechanisms. This paper reports the findings of an extensive mapping, geophysical, and monitoring investigation carried out over a 20 year period. Results from the analysis are presented with respect to the hydromechanical factors controlling the unstable mass, the significance of which were instrumental in resolving conflicts with regards to the slope mitigation measures required to stabilize the slope. In Part II (see companion paper, this issue), the stabilization works performed at Campo Vallemaggia and their effectiveness are presented.

Description: Recent work at the Underground Research Laboratory of Atomic Energy of Canada Limited in Pinawa, Manitoba, has shown that high compressive stresses near the tunnel face significantly contribute to the loss of strength, and eventual failure of the rock, through stress-induced brittle fracturing. A program of laboratory testing has been undertaken to investigate the effects of brittle fracture on the progressive degradation of rock mass strength. The work carried out in this study involves a detailed analysis of the crack initiation and propagation thresholds, two key components in the brittle-fracture process. This paper describes new techniques developed to enhance existing strain gauge and acoustic emission methodologies with respect to the detection of these thresholds and their effects on the degradation of material strength.Key words: tunnel, rock failure, brittle fracture, crack initiation, crack propagation.

Description: For more than 200 years, the villages of Campo Vallemaggia and Cimalmotto have been slowly moving on top of a deep-seated landslide in the southern Swiss Alps. Numerous mitigation measures have been carried out during this time to stabilize the landslide but with limited to no success. Those attempts largely focussed on minimizing erosion at the toe of the landslide. More recently, the need to stabilize the slope began to intensify, as with each passing year the two villages were being pushed closer to the edge of a 100 m high erosion front at the foot of the landslide. This led to an extensive investigation and monitoring campaign to better understand the factors controlling the landslide movements, which as reported in Part I (see companion paper, this issue), pointed to high artesian pore pressures as being the primary destabilizing mechanism. Here in Part II, the arguments supporting the need for a deep drainage solution are reported, as is the history, implementation, and measured response of the Campo Vallemaggia landslide to the various mitigative measures taken. Numerical modelling results are also presented, based on hydromechanically coupled distinct-element models, to help demonstrate why deep drainage succeeded where other mitigation measures failed.

Description: The effects of sampling disturbance on the laboratory-derived mechanical properties of brittle rock were measured on cored samples of Lac du Bonnet granite taken from three different in situ stress domains at the Underground Research Laboratory of Atomic Energy of Canada Limited. A variety of independent measurements and scanning electron microscope observations demonstrate that stress-induced sampling disturbance increased with increasing in situ stresses. The degree of damage was reflected in laboratory measurements of acoustic velocity and elastic stiffness. Examination of the stress-induced microfracturing characteristics during uniaxial compression of the samples revealed that the degree of sampling disturbance had only minor effects on the stress levels at which new cracks were generated (i.e., the crack initiation stress threshold). Crack-coalescence and crack-damage thresholds, on the other hand, significantly decreased with increased sampling disturbance. The presence of numerous stress-relief cracks in the samples retrieved from the highest in situ stress domains was seen to weaken the rock by providing an increased number of planes of weakness for active cracks to propagate along. A 36% strength decrease was seen in samples retrieved from the highest in situ stress domain (sigma1 - sigma3 approximate 40 MPa) as compared with those taken from the lowest in situ stress domain (sigma1 - sigma3 approximate 10 MPa).Key words: sample disturbance, brittle fracture, crack initiation, crack propagation, material properties, rock failure.

Description: Hydraulic fracturing is the primary means for enhancing rock mass permeability and improving well productivity in tight reservoir rocks. Significant advances have been made in hydraulic fracturing theory and the development of design simulators; however, these generally rely on continuum treatments of the rock mass. In situ, the geological conditions are much more complex, complicated by the presence of natural fractures and planes of weakness such as bedding planes, joints, and faults. Further complexity arises from the influence of the in situ stress field, which has its own heterogeneity. Together, these factors may either enhance or diminish the effectiveness of the hydraulic fracturing treatment and subsequent hydrocarbon production. Results are presented here from a series of two-dimensional (2-D) numerical experiments investigating the influence of natural fractures on the modeling of hydraulic fracture propagation. Distinct-element techniques applying a transient, coupled hydromechanical solution are evaluated with respect to their ability to account for both tensile rupture of intact rock in response to fluid injection and shear and dilation along existing joints. A Voronoi tessellation scheme is used to add the necessary degrees of freedom to model the propagation path of a hydraulically driven fracture. The analysis is carried out for several geometrical variants related to hypothetical geological scenarios simulating a naturally fractured shale gas reservoir. The results show that key interactions develop with the natural fractures that influence the size, orientation, and path of the hydraulic fracture as well as the stimulated volume. These interactions may also decrease the size and effectiveness of the stimulation by diverting the injected fluid and proppant and by limiting the extent of the hydraulic fracture.