Technologies: Geotechnical Modeling

 

Static and Seismic Modeling in Geotechnical Engineering

Hart Crowser's geotechnical engineering division uses a combination of traditional slope stability and state-of-the-art displacement-based modeling to analyze the static and seismic performance of existing or new walls and embankments with or without structures.

Slope stability modeling

We use the SLOPE/W software package to conduct slope stability analyses. SLOPE/W uses the limit equilibrium method and search techniques to find the minimum factor of safety for a given input geometry and soil profile. A horizontal acceleration is applied to the zone of soil within the potential failure surface to account for seismic loading in pseudo-static analyses. The result of a limit equilibrium analysis is a factor of safety. A factor of safety less than one indicates the slope is unstable, while a factor of safety greater than one indicates a stable slope.

Displacement-based modeling

We use the FLAC software package to perform two-dimensional finite difference analyses to estimate slope and/or structural movements due to construction activities or seismic shaking. This software simulates the behavior of soil, rock, and structural members to evaluate the static or seismic performance of the model. Displacement-based modeling complements slope stability modeling because it calculates displacements rather than a factor of safety. While a factor of safety provides a general index of stability, it doesn't correlate well to how much a slope may move. FLAC allows the engineer and owner to understand the consequence of construction activity and/or seismic shaking by predicting the displacements throughout the model to determine if the facility will remain serviceable. We have recently used FLAC for projects involving deep excavations, retaining walls, buildings on slopes, and wharf deepening.

Recent Project Experience

75-foot-deep Excavation for the 20-story Millennium Tower in Seattle, WA

The excavation for this 20-story building was retained by soldier piles and tiebacks adjacent to a deformation sensitive, 17-story building constructed in 1918. Hart Crowser used FLAC to model the construction sequence of incrementally excavating lifts, installing tiebacks, and stressing tiebacks. The deformations calculated by the FLAC model compared well with the measured deformations.

Puget Sound Naval Shipyard: Building 850 in Bremerton, WA

Hart Crowser performed slope stability and displacement-based analyses for seismic rehabilitation of this existing 6-story structure that was built into a hillside retained by soldier piles and tiebacks. Hart Crowser used FLAC to model the seismic stability of the soil and building with special emphasis on calculating deformations and the dynamic forces in the tiebacks that connect the building to the soil.

Terminal 5 Channel Deepening in Seattle, WA

Hart Crowser worked closely with the structural engineer on this project to accomplish 5 to 10 feet of wharf deepening along the 2,740-foot pier without diminishing the existing static or seismic stability of the wharf. The added depth was required to allow post-Panamax container cargo vessels to berth at the terminal. Hart Crowser used slope stability and FLAC modeling to show that the structurally improved post-dredging conditions were more stable than conditions prior to dredging. Seismic improvements (installation of compaction/pinch piles) to the under pier slope were modeled for use in a cost/benefit analysis for the project. The dredging and sheet pile installation has been performed and was subjected to the 2001 Nisqually Earthquake (M=6.8). The sheet pile wall performed well with no noticeable earthquake movement.

Third Runway Embankment in SeaTac, WA

This project requires the placement of 17 million cubic yards of fill and construction of three mechanically stabilized earth (MSE) walls. Hart Crowser used FLAC to model the static construction of MSE walls up to 135-feet-tall that were steel-reinforced and had concrete facing panels. The actual construction sequencing was used in the modeling while the reinforcement and deformations throughout the model were monitored. After construction, the model was subjected to seismic shaking. Peak forces in the reinforcement were recorded during shaking and permanent displacements were provided to the design team. The design team used FLAC as a check of the slope stability-based design.