Lynx Case Study (Geotechnical)

3D Geotechnical Characterization for a Tunnel Project

The 3D geoscience computer modeling capabilities of Lynx Geosystems Inc. (Vancouver, Canada) have been applied to a geotechnical characterization for the Kungkuan Station and Tunnels contract in Taipei, Taiwan. The project involves face-borer construction of twin 5, diameter tunnels, representing a 1.6 km extension of the Taipei Metro between Kungkuan and Wanlung Stations. Tunneling contractor for the project is Morrison-Knudsen (Taiwan). The highly variable soil/rock stratigraphy and the variation of geotechnical conditions along the tunnel alignment dictate a need for enhanced appreciation of subsurface conditions prior to commencement of boring operations. Geotechnical characterization includes creation of a 3D database of site investigation records, 3D interpretation of soil and rock volumes, geostatistical prediction of CPT and RQD values from borehole samples, and spatial analysis of conditions along the tunnel alignment. In the words of Robert Page, MK Project Director - Tunnels, the results of the characterization will be used to:

"...make our field personnel and TBM operators aware of the potential geologic conditions they could face as our tunnels are mined as well as for a general comparison with the actual conditions encountered."

3D Geoscience Modeling Technology
The applied modeling technology was originally developed to support mineral resource and mining applications. It has since been rationalized to a generic form that extends its application to the environmental, waste management, geotechnical and tunneling fields. Primary data types include characteristics (e.g. lithology), variables (e.g. CPT, RQD) and 3D coordinates (UTM or local). Data structures include borehole logs, maps, surfaces, volumes and 3D gridded variations. Analytical tools include geostatistics, 2D/3D visualization and spatial (volumetrics) analysis. Modeling algorithms include interactive 3D interpretation of complex geology, volume generation for simple geology and excavations, and geostatistical prediction of variables.

Site Conditions and Investigation Records

The highly variable soil stratigraphy includes continuous and discontinuous layers and lenses of gravel, sand, silt and clay. The soils overlay eroded, inclined layers of sandstone, shale and tuff that approach the surface in some areas. Site topography is relatively flat, with a water table at approximately 8m depth. Tunnel alignment depth varies between 15-20m and intersects both soil and rock material.

The primary data source comprises 66 borehole logs containing records of soil and rock lithology, RQD values in rock material, and CPT values and USCS designations in the soil material, at approximately 1m intervals, to an average depth of 30m. The logs originate from three phases of site investigation with variable record formats. The first phase of the characterization involves correlation of the logs in an appropriate electronic format in a 3D spatial database. Other data sources include water table elevations, and tunnel alignment and geometry, stored in map data structures.

Interpretation of Soil/Rock Stratigraphy
Variability of stratigraphy in all three dimensions dictates a full 3D interpretation, to a depth of 30m and extending laterally either side of tunnel alignment. The initial interpretations are performed on three vertical viewplane sections oriented approximately along tunnel alignment, using interactive graphics techniques with the borehole lithology intersections as a background reference. These longitudinal interpretations are extended 40m laterally either side and are reinterpreted at the extremities to provide a full 3D representation of stratigraphic volumes. Because of inconsistencies in the recorded borehole intersections, the soil interpretations are simplified by combining sand and gravel observations, and silt and clay observations.

Prediction of CPT and RQD Values
Spatial variability (semi-variogram) analysis of 689 CPT soil samples indicates a range of influence of 45m...  this is the average distance at which sample values become independent of each other, i.e. have no spatial correlation. Analysis of 387 RQD rock samples indicates greater spatial correlation with a range of 110m. These range values are applied with geostatistical kriging algorithms to predict the spatial variation of CPT value in soil material and RQD value in rock material respectively at regular intervals of 20m longitudinally,10m laterally and 3m vertically.

Analysis of Tunneling Conditions
Tunnel alignment and geometry combined to generate a volume representation of the twin tunnels, cross passages and station outlines for spatial analysis purposes. A volumetric intersection analysis of this representation with the 3D stratigraphic model provides an indication of the different soil and rock volumes intersected by the tunnels. In similar fashion, the tunnel representation is intersected with the predicted spatial variations of SPT and RQD value to obtain an appreciation of the variation in conditions along the alignment. The ability to generate 2D sections and 3D views at any orientation through the site characterization models allows rapid appreciation of the complex conditions... and since all information is spatially referenced by real world project coordinates, it can be readily correlated with construction information.

Characterization Summary
3D geoscience modeling techniques have not been extensively applied to tunneling applications in the past. Nonetheless, this relatively simple example demonstrates their effectiveness for obtaining an enhanced appreciation of subsurface conditions prior to tunneling operations. This characterization was commissioned by Morrison-Knudsen (Taiwan) under a consulting contract with the Vancouver, Canada office of Lynx Geosystems Inc.