Consulting: Civil Engineering

Slope and Portal Stability

Although the vast majority of Itasca's work in slope stability has been for open-pit mines, Itasca does have significant experience in typical civil engineering slopes, including tunnel portals. One of the larger slope stability studies performed by Itasca involved the excavation of a slope for a cooling water channel at the Paiton thermo-electric generating plant in Indonesia. Here, excavation of the channel caused cracking of the main national highway located at mid-height in the slope. Itasca was retained to determine the cause of the movement and evaluate remediation options. One result of the study was the need to develop a special procedure for determining factors of safety using explicit finite-difference codes, such as FLAC.

Currently, determining factors of safety with Itasca codes is a routine matter. The determination involves adjusting shear strength parameters to find the values that put the slope at the limit of stability (i.e., a safety factor of one). The safety factor for the slope is computed by comparing the limit parameters with actual strength parameters. This technique, called the shear-strength reduction technique, has an important advantage over more traditional limit-equilibrium solutions. Limit equilibrium solutions are restricted to prescribed failure surface geometries (circular, log spiral, segmented, etc.). Use of the shear-strength reduction technique in combination with Itasca codes allows failure surfaces to develop naturally, resulting in a more accurate estimate of the safety factor. Itasca has also developed functions using FISH, the built-in programming facility in all Itasca codes, that allow automatic determination of probability of failure using Rosenbleuth's point-estimate method.

Itasca has recently developed a special program to study slope stability called FLAC/Slope. This code allows rapid generation of problem geometries and factor-of-safety calculation using the shear-strength reduction technique. One particular feature of this code is the ability to overlay DXF plots (e.g., from AutoCAD) to speed model generation. Users can also specify water tables and pseudostatic earthquake loading.