RESEARCH ARTICLES CURRENT SCIENCE, VOL. 118, NO. 5, 10 MARCH 2020 759 *For correspondence. (e-mail: rajib@iitism.ac.in) Seismic behaviour of RC building with raft foundation in the Ganges basin, India J. S. Rajeswari 1 , Rajib Sarkar 1, *, Sekhar Chandra Dutta 1 , Jai Prakash Singh 2 and Ranjeet Saw 3 1 Department of Civil Engineering, Indian Institute of Technology (ISM), Dhanbad 826 004, India 2 Jharkhand Urja Sancharan Nigam Limited, Ranchi 834 004, India 3 Water Resources Department Madhya Pradesh, Bhopal 462 003, India Many highly populated and important cities of India are situated in the Ganges basin. Deep alluvium depo- sit of this basin enhances the earthquake vulnerability of these cities due to amplification of seismic energies in the case of an earthquake. Raft foundations are generally provided for critical facility buildings due to their perceived effectiveness against differential set- tlement during earthquakes. However, the literature available on seismic behaviour of buildings with raft foundation considering soil deformability is relatively limited. In this context, a full three-dimensional finite element model of a four-storeyed building with raft foundation considering the typical layered soil profile of the Ganges basin has been developed in this study. The effects of different seismic parameters on the structural responses and moments induced in the raft have been studied with ground motions from 10 dif- ferent earthquakes. Since the alluvium deposit of the Ganges basin is prone to get liquefied, effects of lique- faction of soil on the building with raft foundation have been considered simplistically. The results show that the raft foundation reduces the lateral displace- ment of the structure considerably. However, an increase in the vertical settlement of the raft in case of liquefiable soil is a matter of concern. Keywords: Layered soil profile, raft footing, river basin, soil–structure interaction. Due to the presence of soft alluvium deposits in the surfi- cial layers, most parts of the Ganges basin are highly vulnerable to the amplification of seismic waves during earthquakes 1–3 . This necessitates proper analysis and design of foundations under seismic loading conditions in such areas. Traditionally, foundations are designed with- out considering the interactive nature of footing and soil. Since the foundations are subjected to large inertial and kinematic loads in addition to vertical gravity loads dur- ing earthquakes 4 this negligence may result in inaccurate estimation of forces and moments leading to unsafe design of foundations. To capture the real behaviour of the system, analyses must be carried out considering the soil, foundation and superstructure as a single unit 5–10 . The use of raft foundation is generally preferred for low-rise structures founded on soft ground, considering its beneficial role in reducing the differential settlement during earthquakes. The behaviour of the raft foundation during earthquakes is highly influenced by the subsoil profile, since the seismic forces get significantly mod- ified by variation of the subsoil layers 11 . In the case of the Ganges basin, existence of soft alluvial deposits in the subsoil tends to amplify the seismic ground motion 12 , causing higher forces in the raft foundation. Hence a de- tailed study on the seismic behaviour of raft foundation in alluvial deposit is necessary. The finite element method (FEM) of analysis is a well- accepted numerical tool for solving various engineering problems. Wardle and Fraser 13 utilized FEM to study the behaviour of uniformly loaded rectangular rafts resting on a homogeneous elastic layer. Surface elements were used for modelling interaction between the raft and the soil. The results indicated that the displacement and bending moment of the raft foundation are highly influ- enced by variation in rigidity of the raft, length-to- breadth ratio, depth of the soil layer and Poisson’s ratio. King and Chandrasekaran 14 presented a finite element procedure for analysing a plane frame supported on a combined footing. The frame and combined footing were modelled using beam-bending elements and the soil using plane rectangular elements. The raft–soil interface was simulated using zero-thickness friction elements. Later, King and Chandrasekaran 15 developed a full three- dimensional finite element formulation for studying the immediate and long-term behaviour of the space frame– raft–soil system. Brown and Yu 16 examined the effect of sequence of construction on the interaction behaviour and found that the effective stiffness of a building during construction is about half the stiffness of the completed structure. Tahghighi and Rabiee 17 demonstrated the importance of considering foundation flexibility for the safe and economical design of structures under earth- quake loading. Liquefaction of soil is one of the leading causes of dam- age to structures during earthquakes, in case structures