GUWAHATI, May 1 � Guwahati has a thick stratum of loose alluvial deposit and it is likely to experience phenomena like soil site amplification, liquefaction, cyclic mobility, lateral spreading etc., in the event of a massive earthquake. Without proper geo-technical explorations, designs of structures in such a natural backdrop of the city, cannot be made safe.
Geo-technical exploration has a major role in making the structures safe in such conditions. The starting point of any engineered structure is the fool-proof geo-technical exploration, said Dr Diganta Goswami, a teacher at the Assam Engineering College here and a leading geo-technical engineer, who was instrumental in identifying landslide-prone areas of Guwahati in 2012, under a survey named the Rapid Visual Screening Survey (RVSS) of the Guwahati Hillocks.
However, he said till now geo-technical investigation in Guwahati is mainly confined to submission of a geo-technical report to the appropriate authorities, where the number, spacing and depth of the boreholes are decided mostly by the owners of the structures only. Geo-technical engineers are normally not consulted for planning the geo-technical investigation programmes for safe designing of the structures.
Pile load test: Again, he said most of the buildings in Guwahati are on pile foundation and pile load test, which is mandatory and is guided by IS Code 2911, is not properly followed in most of the cases.
Quality control: Moreover, a good structural design may prove to be ineffective if proper quality control and quality assurance measures are not adopted. For example, he said mere concrete cube test is not adequate for ensuring the quality of the structures. To ensure quality of the structures, one must go for slump testing, fineness modulus of sand testing and aggregate impact value testing to know the coarse aggregate, setting time of cement test etc.
Inclination, free space: In Guwahati, there are at least a dozen leaning buildings, which may be directly attributed to geo-technical failure.
In some Guwahati areas, free space between the buildings is so little that during an earthquake, possibility of thumping (knocking mutually) of the buildings cannot be ruled out. Such phenomena are more dangerous, especially when a taller building is very close to a relatively shorter building. Highly commercial areas like Fancy Bazar have been found facing such situations, said Dr Goswami.
Hill slopes: Further, he said lots of the buildings in the hilly areas of the city, have been constructed near the hill slopes and the behaviour of such structures may be quite different to that of those in the plain areas at the time of an earthquake. However, this point is not emphasised during construction of structures in Guwahati.
The stability of the slopes due to structure load and cutting of earth for construction of the structures is not studied even during the static loading period, not to speak of the situation in an earthquake condition.
While conducting the RVSS in the city hillocks in 2012 on behalf of the Kamrup (Metro) district administration, the AEC team identified 366 sites as vulnerable to landslide and could foresee the resultant damage of the structures in and around them, which pose threats to human lives of those areas, said Dr Goswami.
In the event of a major earthquake, landslide may lead to disruption of road connectivity in many hilly areas of the city, he warned and suggested scientific formulation and implementation of a special hill area building and infrastructure bye-law for the city hill areas to lessen earthquake-related casualty rates in such areas.
