Stability of a bridge abutment? 1

[cds922]

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.

 

[ntschwanz]

A sketch would be nice but based on your description a few things come to mind. After installing a stabilization scheme there can be additional displacement needed to mobilize resistance. This may become evident in the formation of tension cracks. You mention failures post construction and if you are getting excessive horizontal and vertical displacement that are affecting performance of the bridge abutment then the constructed measures may not be sufficient. I'm assuming that the lower slip surface continues to show no displacement in your SI so that this is simply a potential slip surface based on analysis with a low factor of safety. It sounds like you have conducted a detailed analysis in that you have calibrated a seepage model to field conditions, you have done drilling and testing to understand the stratigraphy and material properties and likely have coupled the seepage and stability analysis. I have only dabbled in 3D analysis but if the ratio of uniform geometry into the paper to depth of slip surface is say 3 or more, then 3D effects may be small.

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.

 

[cds922]

Thanks for the response.

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.

 

[ntschwanz]

For references on 3D slope stability suggest searching Prof. Tim Stark, U of Ill. The 3:1 ratio I mention is just an estimate from memory.

 

[VAD]

Interesting case. A few questions. What is the reason for failure of the riverbank. Has this occurred because of toe erosion caused by river action or has this failure resulted from groundwater seepage from higher fround to the river. Are there indications of similar failure outside the location of the location of the proposed bridge abutment/ alignment. What do you think is the reason for the artesian pressures. I am sure that you have examined these issues. The answer to your problem could perhaps be found in reflecting on some of these issues.It does not appear that you have implemented any measures in the field but you are basing your questions on your analysis. I am not sure if I am correct in saying this. How would stabilizing the upper slide influence failure of the lower slide.

 

[cds922]

The particular bank is on the outside bend of a river meander, so it is actively being eroded. Down cutting during the last deglaciation oversteepened the slopes and caused the slope to fail and/or created shear zones, or failed along glacially disturbed shear zones. Failures of this nature a common place throughout the river valley.

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.