Ground movements in EPB shield tunneling of Bangkok subway project and impacts on adjacent buildings

Abstract

Monitoring data of the first Bangkok subway project excavated by 8 EPB shields were analyzed to evaluate the ground movement characteristics and responses of adjacent buildings. Mostly the maximum ground surface settlements were in the range of 20–40mm, corresponding to 0.5–2.0% of tunnel volume loss, although few sections experienced larger settlements reaching 100mm. Larger ground losses tended to occur while excavating in sand or mixed soil layers and at locations of long duration stoppage. There was only a slight trend of relationship between the settlement and shield pressure. The predominant factor controlling the movement was the human related factor in shield operating which was not easy to predict. Obstructions from piled foundations and other known and unknown buried utilities and objects, resulted in non-conformance of ground surface settlement troughs to the Gaussian function, thus they could not be predicted by any empirical and analytical methods. Shield face pressures often caused outward ground displacements that complicated the final ground movements, particularly the lateral displacement which could not be predicted by available analytical methods. Long term consolidation settlements mainly occurred in the soft clay layer, but the amount and rate did not seem significant. On impacts to buildings, buildings on deep piles showed the least induced settlements. But for buildings on short piles, the settlements may be larger or smaller than ground surface settlements, depending on the distance from tunnel center line and pile tip depth.