ANALYSIS OF BRIDGE PIER FOUNDATION ON JOINTED ROCKS

Abstract

A 310 m long bridge proposed at Bagchhal, on Govind Sagar reservoir, Bilaspur, Himachal Pradesh. The bridge will be supported by two piers namely P1 and P2 situated on left and right bank of the river respectively. Centre to centre distance between piers will be 185 m. During construction of Pier P-2, actual condition of site was found to be differing from condition mentioned in design. Only a rock cover of about 5 m horizontal extent is available, instead of more than 30m (as reported earlier in design), to support the lateral pressure coming from the socketed length of the pier. The rocks present at site indicate presence of lower Shiwalik rocks with alteration of sandstone/ clay rocks. It was decided to check the stability analysis of existing design and apply some corrective measures, if needed. In the present study the probabilistic analysis of input parameters needed for UDEC analysis has been done to find out their representative values. Probabilistic analysis has been preferred because in deterministic analysis usually single fixed values (typically, mean values) of representative orientation, rock mass deformation modulus, joint frequency, trace length of discontinuities, strength parameters etc. are used in the analysis. The stability analysis is normally carried out with one set of geotechnical parameters. However, most input values measured in the field or obtained by laboratory tests which are used subsequently to analyze the problem show a wide scatter across a significant range rather than being a fixed single value. Numerical analysis has been done by using UDEC. Input parameters of modelling are based on the statistical analysis. Deformations and settlements have been checked under different loading conditions. Response of single stage and multistage modelling are analysed. To check overall stability of the pier and foundation system wedge analysis has been also done with the help of standard software.