Foundation evaluation of underground metro rail station using geophysical and geotechnical investigations

摘要

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 construction activities often leads to undesired stability issues with resultant appearance of weak zones. The reinforcement 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 Vs values to compare the subsurface conditions identified under pre- and post-excavations. The results confirmed the presence 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.