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Bridge Foundations Over Granite Railway Cutting

Bridge Foundations Over Granite Railway Cutting

Bridge Foundations Over Granite Railway Cutting


As part of my final year at University I am performing a slope stability assessment of a 1.5km railway cutting, varying in height from 6-10m. The cutting runs through slightly weathered, strong, homogeneous granite and is capped with a 1-6m layer of boulder clay. The occasional faulted zone appears in the cutting but no significant stability problems have been identified.

The (currently disused) line is soon to be upgraded to a dual track system and the cutting therefore requires widening by approximately 2m. Due to the proximity of private propety either side of the cutting it has been decided to cut each slope face back by 1m, rather than taking the full 2m from just one face.

I am still in the early stages of analysing the data which I have collected - triaxial and shear disc tests have been arranged and scanline data is currently being input into a rockslope stability analysis program, so until I get these results back I won't know if any significant stability problems exist.

My report will be focussing on the excavatability of the granite within the cutting and the any subsequent slope stability problems which would be created by the widening process. I will also be looking at the bearing capacity of the granite either side of the cutting to asses the possibility of constructing a large footbridge across the railway.

I have set off in the usual direction - have reported on the geology, the structure etc but am now at the stage of assessing the rock mass. All we have been taught since I have been at University for the general classification of a rock mass is the use of various rock mass rating systems such as Bienawski etc.

However, I am aware that various new systems, such as GSI etc are beginning to appear which are supposed to offer a more accurate assessment of the rock mass for construction purposes and which avoid over-engineering, which apparently commonly occurs with the older systems.

Does anyone have any opinion on this matter? Is there a particular rock mass classification scheme which gives particularly good results with rock slope faces? How much better are these newer systems?

Also, as of yet I have found very little info regarding the construction of footbridge foundations directly onto the granite (at the site of the bridge <1m of overburden exists above the granite). If anyone can point me in the right direction I would be very grateful!

Many thanks!


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