Shallow shear wave investigation and its relation with litho-logs and delineation of geological structure

Abstract

Shear wave velocity (Vs) plays an important role in estimation of various engineering parameters. This thesis utilizes various methods to estimate one- and two-dimensional shear wave velocity profile and its application in development of regression relation based on the type of formation obtained at different depths and estimation of two-dimensional shear wave velcoity structure for lineation of shallow structures. The state of Uttarakhand in India which is the site of numerous ongoing projects is the locale of a major railway project from Rishikesh to Karnprayag by Rail Vikas Nigam Limited (RVNL). This region is highly susceptible to seismic activity, which is why earthquake-resistant design criteria is rigorously applied in construction practices. The shear wave velocity profiles at twenty-six different sites were estimated using both active and passive seismic exploration methods. The joint fit gives a one-dimensional velocity structure. The obtained shear wave velocity shows a consistent correspondence with bore log data and notably, there is a strong correlation between variations in shear wave velocity and lithology that is dependent on depth. It has been seen that shear wave velocity tends to rise with depth within similar formations. Data from twenty-four shear wave profiles have also been used from Garhwal Himalayas provided by RVNL to prepare a linear regression relation of shear wave velocity in different lithological formations with respect to depth of formation. The validation of this relationship is done by comparing observed and calculated velocity of shear wave at those sites that are excluded in the computation of the regression relation. A strong indication that the findings are reasonably well within the range of acceptability is provided by the Root Mean Square Error (RMSE) that was derived from the established relationship for the different types of lithology. The developed relation has been further validated by calculating shear wave velocity profile for borelog data obtained at two sites that are not included in the data set used for developing regression relations. Comparison of two velocity sections clearly shows that calculated velocity profiles match closely with that from the seismic survey and thereby establish the efficacy of the developed regression relationship. A major railway project, CharDham Project is undergoing in the state of Himachal Pradesh which is located in seismic zones IV and V in the seismic zoning map of India. Given the region’s susceptibility to seismic activity, it is always necessary to design structures according to the safety standards available in the country. Averaged shear wave velocity up to a depth of 30 meters (Vs30) is a crucial factor in designing earthquake-resistant structures in seismically active areas. In i this part, a study was conducted to determine in-situ Vs using active and passive methods at various locations covering the ongoing Bhanupalli-Bilaspur-Beri railway project. This facilitated earthquake hazard assessment and Vs30 calculations. Shear wave profiles for various stations have been derived using litho logs and compared to existing shear wave profiles and a strong agreement is found between these two data sets. When comparing two profiles, one developed from relationships and one from a seismic survey, a significant match is observed, confirming the value of the developed relationship. The application of a two-dimensional shear wave velocity profile for identification of the extent of rupture is made in this work for the location of Parwan Dam in the state of Rajasthan. Parwan Gravity Dam is under construction stage in the Jhalawar district of Rajasthan, India. A thin sub-vertical surficial fracture trending N 75°W to S 75°E has been observed in the foundation area of the dam. Geophysical techniques are utilized extensively in the field of civil engineering, and exploration geophysics for the assessment and construction of large-scale infrastructures such as dams. The combination of Multichannel Analysis of Surface Waves (MASW) techniques together with Electrical Resistivity Tomography (ERT) and Seismic Refraction Tomography (SRT) have been used to image the extent of shallow subsurface geological structures. These methods provide critical information about the subsurface conditions without the need of extensive drilling and excavation. These surveys have been carried out along several profiles in the longitudinal direction and along the transverse direction to the fault axis. A total of nine MASW profiles were conducted of which eight are transverse profiles and one is a longitudinal profile. A total of thirteen refraction and resistivity profiles were conducted of which nine were transverse profiles and four were longitudinal profiles. In this part of the thesis, the subsurface distribution of seismic wave velocity and electrical resistivity have been studied to identify any possible anomalous zone in bedrock and to detect the downward extension of surface fracture of brittle fault using the afore mentioned methods. The analysis of these survey results has investigated the vertical and lateral extent of the surface fracture of the fault. The analysis of the results indicates that a very tight and narrow fracture is present in the shallow subsurface.