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Subsurface map of Delhi


The study reveals that the thickness of soil cover in Delhi area varies from nil to beyond 200m depths. This unusual variation is due to the appearance of NNE-SSW Delhi-Hardwar Ridge. Thickness of soil cover is generally much deeper towards eastern region than that of the west. In some places abrupt changes in the thickness of soil cover can be observed in this area, mainly due to the presence of faults. This narrow ridge develops in the form of a buried ridge passing from Chanakya Puri to Old Yamuna Bridge through Rail Bhawan, Ashok Road Connaught Place, Ramlil Ground and Chowri Bazar where bedrock is at very shallow depth varying from 5 to 20m below ground level. The thickness of soil cover around east of the ridge in North Delhi varies from almost nil to 30m, with a gradual easterly slope towards the river Yamuna. Towards west of the ridge, the thickness also varies from almost nil to 30m and beyond, with an abrupt deepening has been shown in north beyond 180m near Roshanara Garden. In the southern part of Delhi around Gurgaon and Palam, the thickness of soil cover generally starts varying from 75m and goes much beyond 120m, except in the vicinity of rock exposures. In the east-west direction the thickness of overburden varies from almost nil in the Link road - Pusa road to as much as 60m in the Patel road area. The thickness gradually increases in the axis of Jawahar Lal Nehru road, Connaught Place, Rajghat area starting from 8m to 50m and goes beyond 120m in the Yamuna riverbed. However, more realistic estimation of thickness of soil cover of the area can be done by collecting the grid wise subsurface features of the area.

Conclusions
This study has been under taken by CBRI as an in-house R & D project, and the authors could produce the above subsurface map with the help of limited sets of data and surfer software. As more and more detailed layer wise borehole data are being collected and processed, more realistic and reliable subsurface maps may be prepared in near future. GIS may also be used for producing better results. Seismic microzonation of big cities of the world have been taken up by groups of research workers, and such subsurface maps may serve as powerful tools for seismic microzonation of Delhi in near future, if need be.

Acknowledgments
This paper is being published with the kind permission of the Director, CBRI, Roorkee. Thanks are also due to Mr. Pradeep Kumar, Scientist, CBRI for his help extended during the data collection. The authors express their sincere thanks to Dr. K.S. Rao, Associate Professor, IITD and Dr. V.M. Sharma, AIMIL, New Delhi and organizations like GSI, RITES, CGWB, DDA and CSMRS who readily provided the data for this study. 

References
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