Introduction to Chemical Stabilization of Unstable Trackbeds

Trackbeds are typically composed of all granular materials comprised of ballast and subballast over compacted subgrade. Most poor performances of railroads can be attributed to poor and unstable subgrade conditions. Below the surface, the instability of the subgrade material can propagate through the granular zone leading to excessive settlements and deformations of the railway. Conventional subgrade restoration in the trackbed system requires the removal of the granular materials and over-excavation of soft unstable subgrade materials, moisture adjustment, re-compaction, and sometimes chemical stabilization of the subgrade soils. Since these procedures are considered very expensive in terms of construction equipment, railway outage time, and labor force, alternative solutions for consideration and evaluation are essential. Injection of expansive foam (polymer based) materials is a relatively recent method that has been used in various applications of soil stabilization in the roadway industry. This technique relies on the injection of rigid-polyurethane foam, which is a high-density, expanding, thermoset, hydro-insensitive and environmentally neutral polyurethane-resin product, into the soft and unstable soil to improve their shear strength and stability index. In addition, the stabilized zone acts as a waterproof membrane protecting moisture sensitive subgrade, and acting as a separation layer to eliminate pumping and contamination of the granular subballast at saturated fine grained conditions. The objective of this paper is to evaluate the practicability of polyurethane stabilized soft and unstable subgrade under unbounded granular trackbeds to mitigate future deformation, restore railway foundation, and reduce trackbed repair cost and outage time.