Time Domain Reflectometry (TDR) was recently introduced as the most effective non-destructive testing method for soilnailing inspection. Current practice of TDR inspection was conducted through a pre-installed single-core electric wire alongside the rebar within the soil nails. However, the aforementioned practice is shown to be hindered by its inability to decouple the effect of rebar length from possible grout defects, and by excessive overestimation of rebar length if the wire is coiled around the rebar. A new TDR waveguide construction and a corresponding decoupled data reduction method (for both soil-nail length and grout condition inspection) were proposed in this study. Feasibility and advantages of the new approach were experimentally verified with two types of TDR device. Based on the proposed methodology, a portable, low-cost, and low-speed TDR device was shown to suffice as a quick and economical tool for quality inspection of soil-nailing works.