Stability of a bridge abutment?
Stability of a bridge abutment?
(OP)
I have been working on a project that involves stabilizing a natural slope on a riverbank in order to construct a bridge abutment. There are two slip surfaces, the upper is an active (according to SI data); however, there is a potentially deeper slip surface that may become active if the upper surface is stabilized.
I have back analyzed the upper and lower failure by matching total head contours to field data. Lab testing was conducted for a majority of the materials (mostly over consolidated fine grained soils) to determine strength properties. The two potential shear planes are fairly thin (maybe 0.5 to 2 m). I have then implemented methods to stabilize the main landslide surfaces. The issue is that although I can (and have) applied measures to stabilize the main upper and lower slip surfaces I have noticed failures occurring just behind the bridge abutment on the potential lower slip surface. Are these failures realistic to consider? 2-D slope analysis considers the slope is continuous into the page and the 3-D shape of the finished embankment will have side slopes, etc? Is a 2-d analysis over conservative?
Thanks for any input.
I have back analyzed the upper and lower failure by matching total head contours to field data. Lab testing was conducted for a majority of the materials (mostly over consolidated fine grained soils) to determine strength properties. The two potential shear planes are fairly thin (maybe 0.5 to 2 m). I have then implemented methods to stabilize the main landslide surfaces. The issue is that although I can (and have) applied measures to stabilize the main upper and lower slip surfaces I have noticed failures occurring just behind the bridge abutment on the potential lower slip surface. Are these failures realistic to consider? 2-D slope analysis considers the slope is continuous into the page and the 3-D shape of the finished embankment will have side slopes, etc? Is a 2-d analysis over conservative?
Thanks for any input.





RE: Stability of a bridge abutment?
You didn't indicate what your computed FS was post construction of the landside remediation but if these factors of safety are high enough your problem may simply be short term until a resistant strength from your repair is mobilized. Problem is that additional movement may occur under greater loading, such as can happen under higher GW conditions, where additional movement is needed to mobilize additional strength.
RE: Stability of a bridge abutment?
The FS of the upper surface is greater than 1.5 and has no further issue once the upper stabilization is implemented (shear key). The lower slip surface is too deep for a shear key and is under artesian conditions so a large excavation is not recommended.
The FOS of the lower slip is 1.2 before stabilization, but can be increased to > 1.5 with the incorporation of stone columns to intercept the shear zone and provide drainage of the artesian pore water pressure conditions.
The issue is rock columns are expensive and a lot are required to deter a failure from just behind the abutment. So if a failure right behind the abutment is simply an effect of 2-D modeling then the stabilization of the lower slip changes greatly in effort/cost.
I will check the uniform geometry vs. depth to failure.
RE: Stability of a bridge abutment?
RE: Stability of a bridge abutment?
RE: Stability of a bridge abutment?
The current active slip surface(upper) is the easiest path of failure (toe exits at the river). If we stabilize this slip, there is concern that a deeper potential slip surface may become active. High pore water pressure in this zone indicate the potential for a 'tight' zone that could be prone to shear failure. The artesian conditions are caused by a major regional aquifer.
Based on recent conversations with colleagues, I have been led to believe my analysis is valid and the potential failures through the abutment are realistic and must be addressed.