The Role of Geological Uncertainty in a Geotechnical Design-A Retrospective View of Freeway No. 3 Landslide in Northern Taiwan

The importance of the geological model in a geotechnical engineering project has long been recognized. However, the uncertainty associated with the geological model has rarely been quantified and explicitly considered in the geotechnical design. This paper explores the role of the geological model uncertainty and the benefit of reducing such uncertainty in a geotechnical design. To this end, the landslide that occurred on April 25, 2010, at 3.3K of Freeway No. 3 in Northern Taiwan, referred to herein as NH-3 Slope, is reanalyzed with various assumed geological models. Traditionally, the design (i.e., a choice of design parameters such as slope height, slope angle, and supporting anchors) of a weathered rock slope seeks to satisfy a target safety factor (FS). In the case of NH-3 slope, the apparent dip angle (i.e., inclination) of the slip surface along the dip direction of slope constitutes the geological model of concern. Different survey techniques produce data with varying degrees of accuracy and precision (or uncertainty) in the measured apparent dip angle. For example, data from a Regional Geological Map (RGM) typically yield a low-precision estimate (with a high level of uncertainty) of the bedding plane attitudes; in contrast, LiDAR can significantly reduce the uncertainty of the derived bedding plane attitudes. By lowering this uncertainty, the variation (or uncertainty) in the resulting FS may be reduced, which tends to yield a lower failure probability (P-f), given other conditions being the same. From a design perspective (to achieve a target P-f), the use of LiDAR data (with lower uncertainty), as opposed to the use of RGM data (with higher uncertainty), may result in an acceptable design at a lower cost. The benefit of reducing the geological model uncertainty in a geotechnical design is demonstrated through a retrospective analysis of the NH-3 slope, under both the "as-design" condition and the "failure" state.