Y. Pandey and R. Dharmaraju
Scientists, Central Building Research Institute
Roorkee – 247 667, India
Tele: 91-01332-82372/82214, Fax: 91-01332-72272/72543
Delhi as the seat of power has been the focus of attention for administrators and planners. Its fast growing population, natural and manmade disaster proneness, changing security perceptions and pressure for maximum utilization of space call for exploring newer avenues for development. Sky scrappers as well as the sophisticated underground living and working space are the features of inevitable future.
Subsurface maps of Delhi, emerged as an outcome of a research project undertaken by Central Building Research Institute, are the essential prerequisite of further urban growth of the mega city. Whether it be new development of any uninhabited area, delineation of earthquake prone localities, building up a failsafe corridors creating strategically sensitive space, subsurface maps are one of the best required tools.
In this paper, authors discuss about the methodology used for preparation of subsurface maps as well as their results.
Delhi being the capital, has always been the seat of power right from ancient times, except for some decades during the British period. Its strategic geographic location, as well as the typical geological geomorphological setting makes it so important. Its importance is to attract people, industrial and strategic establishments in great proportion, simultaneously converting the mega city into a time bomb being vulnerable to natural and manmade hazards. It has to expand with faster pace to cater the growing need, but preparedness to efficiently manage the said disasters should be seen as imminent urgency by the administrators and planners.
Delhi region falls in zone IV of seismic zoning map of India, which is located on the margin of Himalayan foredeep. The zone has fairly high Seismicity with general occurrence of earthquakes of 5-6, a few of magnitude 6-7 and occasional incidence of 7.5-8.0 magnitude shocks (RITES Report, 1996). The first recorded major earthquake in this region occurred on 15th July 1720 of intensity IX (Chandra, 1992; Srivastava & Somayajulu, 1996; Tandon, 1975). Subsequently other historical events occurred in 1803, 1819, 1905, 1934, 1937, 1945, 1949, 1958, 1960, 1966, 1975, 1980, 1994 during which intensities between VII and IX are believed to have been experienced as indicated by damage pattern. During recent times the area has been shocked by a number of earthquakes, out of these earthquake of 27th August 1960 with magnitude 6.0 and having epicentral tract between Delhi and Gurgaon was most significant. Recent Himalayan earthquake namely Uttarkashi and Chamoli were experienced in Delhi, and ultimate probability of major earthquakes in the region should make the authorities awake to the problem.
The earthquake history of Delhi region indicates fairly high Seismicity for the city of Delhi. The most active area of the region is considered to be the trijunction of the Delhi-Hardwar ridge, Lahore-Delhi ridge and axis of Delhi folding. Most of shocks are interpreted to be shallow focus and have concentrated around Sonepat, Rohtak and Gurgaon region in and around Delhi may be considered as seismically very active and the tectonic elements of the area are considered capable of generating an earthquake of magnitude 7 on Richter Scale (Chadha & Mathur, 1995).
Some areas of Delhi, due to their unique subsurface settings, are relatively more prone to damage in case of such eventualities. In such cases, apart from the density of population and type and quality of construction of buildings, thickness of sedimentary deposits play very critical role. This paper gives an account of CBRI’s effort to map Delhi on the basis of thickness of soil cover, which may be very useful to the common people, Government and private builders, administrators, planners, rescue operation managers and many more sections of the society. Importance of subsurface features
During an earthquake, actual ground shaking at a particular location mainly depends on three factors; Source mechanism, propagation path and local soil condition. The previous two parameters may be the same in a city, but different soil conditions may create different degree of damage in some locations. Though few meters of the top surface of the ground may be composed of different layers of soil compositions, but in the present study we have taken the sedimentary and filled up overburden above the bedrock as an individual factor.
It has been repeatedly observed that some localities behave differently in terms of damage to the engineered structures in the same city during an earthquake. In recent years, 1985 Michoacan, Mexico earthquake (Singh et.al., 1988), 1988 Armenian earthquake (Borcherdt et.al., 1989), 1989 Loma Prieta earthquake (Hough et.al., 1990, Borcherdt et.al., 1992) and some other devastating earthquakes have clearly proved this phenomena if one goes through their damage patterns. Iyengar (1999) refers some of the well known facts of 1934 Bihar earthquake, 1964 Nigata Japan earthquake, 1964 Alaska USA earthquake, 1971 San Fernando earthquake, 1991 Uttarkashi earthquake and 1993 Latur earthquake where structures located on loose granular or sandy soils has experienced major damages. Recent Chamoli earthquake (29th March, 1999), which was severely felt in Delhi, caused some damages in buildings standing on very thick layer of sedimentary deposit.
It had already been established about a century ago that ground motions are substantially amplified near the ground surface with thick sedimentary cover (Milne, 1898). Since then numerous studies have been undertaken by different research workers to substantiate the fact, in different parts of the globe. Studies on the subject are still being persued with the sole intention of enriching and authenticating the database. Review of the published literature on the subject has been presented by Aki (1993) and Bard (1995) in near past. Aki (1988) concluded in his review paper, among other things, that soil sites show higher amplification than rock sites by a factor of 2 – 3.
Delhi has some heavily populated colonies on deep deposits of alluvium in various locations, particularly in the trans Yamuna area. These areas are seismically vulnerable as large ground motion amplifications are expected due to their typical ground compositions. On the basis of past earthquake records Khatri (2000) makes observation that Delhi region is likely to experience a peak ground acceleration of 0.21g with the shaking duration of about 30 seconds, with a 10% of probability of excedence in a time window of 50 years. Recent Chamoli earthquake, originated about 400 kms away from Delhi could generate peak ground acceleration of the order of 0.01g in Delhi (CBRI Report 1998).
Given the greatly increased level of damage that can be produced, it is of practical importance to develop methods for assessing the nature of, and potential for, sediment amplification and design of critical and strategically essential infrastructurd facilities.
Subsurface map of Delhi
Generally the bedrock topography in Delhi region is undulating with several humps and depressions. The depth of bedrock varies from near surface to beyond 200 meters deep from place to place, which is caused mainly due to the complex tectonic features of the area and thick sediments deposited on the bedrock. Several agencies like GSI, RITES, CGWB and others have carried out geotechincal investigations in Delhi for various purposes. The investigations carried out by the different agencies mainly include geophysical surveys, geotechincal investigations and drilling of shallow as well as intermediate depth drill holes with a view to either determine the nature of material lying below the surface, locate the possible source of ground water and delineate the bedrock profile.
With the sole purpose of generating a three-dimensional subsurface map of Delhi, an attempt has been made to collect the subsurface data available with various agencies. About 100 subsurface details spread all over Delhi have been collected and analyzed. As the informations collected originated from different sources, there was no technical uniformity in them. The information collected by a drinking water locating body has to be different in terminology from the information gathered by a geotechincal/construction agency. Authors tried their best to take up the cases of uniform nature to the maximum possible extent. Out of these, actual thicknesses of sedimentary deposit at about 50 well spread out locations were evaluated for the preparation of subsurface map. A three-dimensional map showing the thickness of sedimentary cover from ground surface covering an area of 18/20km has been generated with the help of Surfer software package. Fig.1 and 2 give the details of the topography and sedimentary deposit thickness variations from place to place in the studied area 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.
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.
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.?
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