Faquan Wu

Geophysical exploration for a long deep tunnel to divert water from the Yangtze to the Yellow River, China

To identify problematic geological structures which would be encountered when driving tunnels in the high mountainous area of South West China, a joint geophysical and engineering geological study was made. The tunnels will form part of a project to divert water from the Yangtze to the Yellow Rivers. A controlled source audio-frequency magnetotelluric method (CSAMT) was carried out in 2004 between the Ma-ke and Jia-qu rivers in high, steep terrain. The paper discusses the method of data collection, processing, and analysis for the West route. The faults/fractures identified will need to be taken into account in the design and construction of the tunnel.RésuméAfin d’identifier des structures géologiques contraignantes qui pourraient être rencontrées lors du creusement de tunnels au travers des régions montagneuses de la Chine du sud-ouest, une étude couplée de géophysique et de géologie de l’ingénieur a été réalisée. Les tunnels feront partie du projet de détournement des eaux du bassin du Yangtze vers celui du Fleuve Jaune. Une méthode magnéto-tellurique (CSAMT) a été mise en œuvre en 2004 entre les rivières de Ma-ke et Jia-qu dans une région d’altitude présentant des pentes raides. L’article discute des méthodes de récupération, traitement et analyse des données pour l’itinéraire de l’Ouest. Les failles et fractures identifiées devront être prises en compte dans la conception et la construction du tunnel.

Influence of dip directions on the main deformation region of layered rock around tunnels

The influence of dip directions on the main deformation region of layered rock around tunnel is studied using a 3D distinct element code called 3DEC. We learn that the main deformation regions around tunnels are regularly changeful along with different intersection angles between dip direction and advancing direction of the tunnel. The specific deformation regions can be anticipated based on detailed engineering geological survey before tunnel excavation. Moreover, the common deformation pattern around tunnels is found to be structural deformation including bending and slipping of layered rock. The main deformation region is prone to occur at the region where the normal direction of rock layer points to the inside of tunnel because the minimum tensile strength of bedding plane appears on this region and rock bending is generally easier than rock slipping. An understanding of the main deformation regions around tunnels provides critical information for selecting the route of a tunnel, forecasting the main deformation region and designing an efficient support system.

Characteristics and mechanism of deep weathering of argillaceous limestones at Fengjie County, Three Gorges Region, Central China

The paper reports the ground conditions in the Sanmashan area where people were relocated as a consequence of the impoundment of the Three Gorges Reservoir. Examination of the geology indicates the strong calcareous rocks were not only highly fractured to considerable depths but were also decalcified in the near surface layers. A combination of the highly fractured state and decalcified nature of the material resulted in significant changes in the bedrock parameters between the northern and southern parts of the study area; the southern area having been affected by the deep down-cutting of the Yangtze River, release of stress and the development of three large landslides.RésuméL’article présente le cadre géologique de la région de Sanmashan où des personnes ont été installées en conséquence de la mise en eau du barrage des Trois-Gorges. Les études ont montré que les roches calcaires résistantes étaient non seulement fracturées à des profondeurs importantes mais aussi altérées dans les couches superficielles. La combinaison de la fracturation et de l’altération a pour résultat des changements importants des paramètres géotechniques du substratum entre le nord et le sud de la zone d’étude. Le sud de la zone a été affecté par le creusement profond du Yangtze, le relâchement des contraintes et la formation de trois grands glissements de terrain.

A Recognition and Geological Model of a Deep-Seated Ancient Landslide at a Reservoir under Construction

Forty-six ancient Tibetan star-shaped towers and a village are located on a giant slope, which would be partially flooded by a nearby reservoir currently under construction. Ground survey, boreholes, and geophysical investigations have been carried out, with results indicating that the slope consists of loose deposit with a mean thickness of approximately 80 m in addition to an overlying bedrock of micaceous schist and phyllite. Ground survey and Interferometric Synthetic Aperture Radar (InSAR) indicated that the slope is experiencing some local deformations, with the appearance of cracks and occurrence of two small landslides. Through using borehole logs with the knowledge of the regional geological background, it can be inferred that the loose deposit is a result of an ancient deep-seated translational landslide. This landslide was initiated along the weak layer of the bedding plane during the last glaciation in the late Pleistocene (Q3) period, which was due to deep incision of the Dadu River at that time. Although it has not shown a major reaction since the ancient Tibetan star-shaped towers have been built (between 200 and 1600 AD), and preliminary studies based on geological and geomorphological analyses incorporated with InSAR technology indicated that the landslide is deformable. Furthermore, these studies highlighted that the rate of deformation is gradually reducing from the head to the toe area of the landslide, with the deformation also exhibiting relationships with seasonal rainstorms. The state of the toe area is very important for stabilizing a landslide and minimizing damage. It can be expected that the coming impoundment of the reservoir will increase pore pressure of the rupture zone at the toe area, which will then reduce resistance and accelerate the deformation. Future measures for protection of the slope should be focused on toe erosion and some bank protection measures (i.e., rock armor) should be adopted in this area. Meanwhile, some long-term monitoring measures should be installed to gain a deep understanding on the stability of this important slope.