Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo Foundation evaluation of underground metro rail station using geophysical and geotechnical investigations B. Butchibabu a , Prosanta Kumar Khan b, ⁎ , P.C. Jha a a National Institute of Rock Mechanics, Banashankari 2nd Stage, Bangalore 560 070, India b Department of Applied Geophysics, Indian Institute of Technology (Indian School of Mines), Dhanbad, India ARTICLE INFO Keywords: Cross-hole seismic survey P-wave and S-wave velocities Electrical resistivity tomography Standard penetration test Weak zones Foundation evaluation ABSTRACT Excavations for underground space developments in a thickly populated urban area are sometimes hindered by unexpected formidable consequences like formation of sinkholes or uneven settlement of foundations. Such troublesome incidences occasionally damage structures in the vicinity of excavations. Although adequate design guidelines and codes of practices are adopted, the disturbance of subsurface regime due to excavation or con- struction activities often leads to undesired stability issues with resultant appearance of weak zones. The re- inforcement of weak zones by grouting is one of the common remedies, without demarcating the weak or the affected zones becomes counter-productive. Excavation at a location in Chennai mega-city for metro rail was severely affected by soil piping with development of sinkholes and weakening of the surrounding strata. Subsequently development of cracks in the adjacent buildings, and caused tilting of the nearby taller structures. The mandated the geophysical and geotechnical investigations were carried out to demarcate the affected area to avoid further deterioration of the site. Cross-hole seismic P- and S-wave velocities were estimated between10- pairs of boreholes up to 20 m depth on both the sides of the excavation. Two electrical resistivity tomography (ERT) profiles (each of 141 m in length) were carried out in the same location. Cross-hole S-wave velocity profiles and resistivity tomograms clearly indicate the presence of weak zones, and possible sinkhole locations. Site specific empirical relationship was established between S-wave velocity and standard penetration test (SPT) N-value. This relationship was further used for determination of SPT N-values from the measured V S values to compare the subsurface conditions identified under pre- and post-excavations. The results confirmed the pre- sence of weak zones, which are responsible for the weakening of the surrounding strata in the foundation. Geophysical and geotechnical investigations, similar to present investigation, can be useful for designing metro rail projects in a subsurface environment comprising alluvium, sand, clay mixed with silt, pebbles and boulders, lying near a region of ground water pocket in a thickly populated urban area. 1. Introduction With rapid urbanization and industrialization, infrastructure de- velopments like rail, road, public transport, etc. are also concurrently done to keep pace with the requirements. A number of major cities have been witnessing construction of urban metro rail network for mass transportation in India. However, the optimal alignment of a metro rail project, planned for mega-cities, becomes difficult in a subsurface en- vironment that comprises both granites, and gneisses along with sedi- mentary formations (viz. shale and sandstones); occasionally, younger alluvium of variable thickness, usually 3.0 to 30 m, is also encountered. Part of an urban metro network is sometimes designed to pass through underground tunnels (mostly below thickly populated areas). Incidentally, the underground stretches in a metro rail project for Chennai mega-city in India were designed for excavation using Tunnel Boring Machine (TBM). A section of the ground was opened with box- type excavation (shaft) measuring in the dimension of 200 m × 50 m × 20 m for deployment of TBM. The side-walls of the excavation was supported by two parallel diaphragms up to a depth of 24 m over 200 m lateral length. While the excavation was underway, water mixed with fine sand and silt started oozing out from joints be- tween different segments of both the diaphragm walls into the box-cut area. Prolonged inflow of the slurry into the excavation area resulted in the development of sinkholes (Fig. 1) on the unexcavated side of the diaphragm walls. Along with the weakening of the surrounding strata, cracks started appearing in the adjacent buildings and even some tilting was also reported in the nearby taller structures. During reinforcement work using grouting with bentonite-cement mix grouting, the grouting https://doi.org/10.1016/j.enggeo.2018.12.001 Received 25 November 2017; Received in revised form 11 November 2018; Accepted 2 December 2018 ⁎ Corresponding author. E-mail address: khanprosanta1966@gmail.com (P.K. Khan). Engineering Geology 248 (2019) 140–154 Available online 04 December 2018 0013-7952/ © 2018 Elsevier B.V. All rights reserved. T