Low cost monitoring system to diagnose problematic rail bed : case study of Mud Pumping Site

by Eng Sew Aw

Institution: MIT
Department: Civil and Environmental Engineering
Degree: PhD
Year: 2007
Keywords: Civil and Environmental Engineering.
Record ID: 1811215
Full text PDF: http://hdl.handle.net/1721.1/42051


This thesis describes the development of low cost sensors and wireless sensor network (WSN) platform aimed at characterizing problematic rail beds (subgrade). The instrumentations are installed at a busy high-speed Northeast Corridor (NEC) railway site that is experiencing recurring mud pumping problems and requires frequent (bi-monthly) track maintenance. The field instrumentation program addresses some of the challenges faced with designing and installing the subgrade sensors: adoption of new low-cost sensor technologies, fabrication for harsh railway environment, ease of installation with minimal traffic disruption; adoption of advances in WSN for remote data gathering, two-way communications, back-end processing, and near real-time viewing of data. The subgrade sensors consist of piezometers, accelerometers, liquid-based settlement probes, and temperature sensors (thermistors). The piezometers measure the long-term variation in the ground water table and short-term load induced pore pressures. The accelerometers measure the dynamic wheel-induced accelerations at the tie and the top subgrade. The liquid-based settlement probes are designed for long-term settlement measurements although they are not important at this particular site that exhibits greater track vibrations than long-term settlement. The temperature sensors installed at three different levels are able to track the atmospheric, top subgrade, and deep subgrade temperatures. The field instrumentation installed at the problematic NEC site provides valuable insights into factors causing mud pumping. The field data suggest that water, widely acknowledged one of the causes of mud pumping, is not derived from the in-situ ground water table, perched water table at the top subgrade (that is under hydrostatic matrix suction), or rain (merely replenished the matrix suction in the ground). An energy-based method, modeled after the Arias Intensity, is used to characterize the response of the track to train loadings based on measured acceleration. This method is able to track the serviceability of the rail track (effect of maintenance and gradual build-up of track deterioration), investigate the damage factor of trains on the track (high-speed Acela is found to be most damaging), and establish that the track is extremely stable during winter due to frozen soil but dramatically deteriorates during ice thaw.