Laboratory Investigation of TDR System to Monitor MSW Landfill Settlements

Settlement in municipal solid waste (MSW) is an important factor affecting landfill management, whether the landfill is operated as a conventional or bioreactor landfill. Mechanical settlement results from compression and consolidation caused by the weight of the waste. Biological settlement occurs following deterioration and subsequent volume loss due to corrosion, oxidation, combustion and decay. Bioreactor landfill technology aims to optimize biological settlement through controlled leachate recirculation. Settlement measurements can allow managers to better estimate waste degradation rates, locate "hot" and "cold" spots, and increase bioreactor efficiency by directing leachate to low-settlement zones. This paper reviews past and current efforts to monitor MSW landfill settlements and presents a new method for monitoring settlement in landfills. Time domain reflectometry (TDR) is a technique utilized to locate and quantify physical changes along a transmission line, such as a coaxial cable, by tracking voltage reflections. While the TDR technique does not work well when cables are pulled in direct tension, it has been shown that prescribing loops along a cable significantly enhances the sensitivity of TDR measurements in tension. When cables are stretched such that the loop radius progressively decreases, voltage reflection amplitude increases as the loop kinks at its apex. The proposed method was evaluated experimentally in a settlement tank filled with a simulated daily cover composed of a moist paper-sand mixture. The tank is a steel frame structure with tempered glass sheets as sidewalls for visual observation of waste settlement along the boundaries. A galvanized steel plate centered at the base of the tank can be lowered to create a subsidence zone. A series of experiments were performed using single-loop and multi-looped cables embedded in the waste and monitored as the plate is lowered. The results show that loops located within the subsidence zone compress as the waste settles, and that the magnitude of voltage reflection increases as the waste continues to settle. Thus the method has the potential to detect and monitor local settlement in MSW landfills.