Characterization of Distresses in the Soft Foundation Soil Caused by Highway Embankment to Formulate Remedial Measures

Abstract

Soft foundation soils under highway embankments are susceptible to distresses due to varying loads and climatic conditions. Their behavior is unpredictable, they have greater affinity to high compressibility and low permeability. Embankment roads constructed over such soils are prone to structural deformations, reduced service life and permanent failure at times. This hinders traffic operations, adds misery to the life of commuters and falls heavy in terms of maintenance expenditure. This study aims at evaluating the behavior of soft soil foundation, estimate magnitude of settlement and evaluation of stone column efficacy vis-a-vis its suitability for ground improvement in future road construction projects. Pakistan is in process of expanding road infrastructure in terms of motorways and expressways to link China Pakistan Economic Corridor (CPEC) from across the country. Swat motorway is the one such mega project, presently under construction. It measures 81km in length and an expenditure of Rs. 43 billion is estimated for its completion. The road runs through irrigation land, criss crossed with irrigation channels and natural streams. Considerable portion of about 5-7km lies in area either waterlogged or with very high water table. Very high water table and soft soil up to 10m depth was detected during geotechnical investigation of Swat Motorway. Soil is weak, thus presents low bearing capacity. This renders the existing foundation soil inherit potential for settlement if left untreated before road construction. Settlement up to 1m was predicted during initial geotechnical investigation. However, the client was convinced to construct the road following typical design in vogue that does not have any room for improving soft foundation soils using any of the existing ground improvement techniques. Sequel to settlements observed in existing motorways M1, M2, Grand Trunk(GT) road, and predicted settlement in Swat Motorway, several ground improvement techniques were considered and stone columns were selected for mitigating the settlement potential of foundation soil. Stone-C software was used for designing stone columns. Numerical simulations using Fast Lagrangian Analysis of Continua (FLAC), a 2D finite difference program were performed for unimproved foundation soils to predict xix settlement potential. Efficacy of stone columns as a ground improvement technique was also validated through numerical simulations. The study also includes formulation of indigenous real-time monitoring framework for Pakistan. Major findings of the research display the predicted short term settlement (undrained conditions) of unimproved soil to be 0.16m while long term settlement (drained conditions) to be 0.21m. Stone columns-improved foundation soil displayed significant reduction in settlement. Short term settlement reduced from 0.16m to 0.007m while long term settlement reduced from 0.21m to 0.0095m. It is believed that the study will help formulate remediation measures for future road construction in areas with soft soils and high water table in Pakistan, thus reducing soft foundation soil hazards related to highways, minimizing maintenance cost and help improving roadways serviceability. Major stakeholders like NHA, FWO, CWO, Pakistan PWD, will benefit from this research specially once it comes to highways and motorways construction in areas inheriting soft foundation soils.