Natural vegetation influence over pipe jacking force : a case study in Metro Colombo Torrington Storm Water Project

Abstract

Pipe jacking is a popular trenchless tunneling technique in urban areas with advantageous in its different aspects. Torrington storm water tunnel is a project by the Sri Lankan government to tackle the storm water flooding issues related with metro Colombo. Torrington storm water tunnel is constructed by Slurry Pressure Balance TBM method (pipe jacking). The entire tunnel system is constructed through an uniformly graded granular sand medium. Soil Suction (sum of Matric and Osmosis) improve the shear strength of soil medium. Evapotranspiration and water uptake is seen to highly influence the development of capillary action throughout the soil and subsequently the matric suction in the soil. With increase of matric suction, the shear strength is improved. In pipe jacking, the imposed hydraulic force should be in equilibrium / stabilized with the sum of soil-concrete interface resistance force and face resistance force. In this case study, the matric suction induced shear strength improvement in soil in Torrington- Colombo area is considered. This improvement impacted the pipe jacking process by increasing the required hydraulic force. This force increment is proportional to the improvement of the shear strength in the particular soil medium. Three tunnel segments in the Torrington tunnel project are considered for this study. Soil shear strength parameters and GWT were estimated using borehole log data. At the Vadose zone of the respective sections, the imposed hydraulic pipe jacking force increased comparatively to the rest. Accordingly, the Vadose zone of the vegetation effect could be determined. Prior to pipe jacking, the pipe jacking models are considered in order to predict the maximum pipe jacking force and other fluctuations. There are several standard pipe jacking models proposed for different soil and geotechnical conditions. After analyzing by using three available models for sand, Alan Atalah method is found to be the most suitable and complying method for pipe jacking process in the Torrington tunnel project.