This study explores interpretation methods for determining the preconsolidation stress from one-dimensional consolidation test results. Twelve reconstituted clay specimens with targeted preconsolidation stresses of 60 and 120 kPa were prepared using commercial kaolinite and marine clays collected from coastal regions of South Korea. Five representative interpretation methods were applied, and the influence of maximum applied stress levels of 320, 640, and 1280 kPa was evaluated. The results indicate that the estimated preconsolidation stress varies considerably depending on both the interpretation method and the maximum applied stress, particularly for rounded compression curves with gradually changing virgin compression slopes. To address these limitations, a new interpretation approach is proposed. The method utilizes the rebound slope obtained from an unload–reload cycle and defines the virgin compression line through linear regression without identifying the recompression–virgin compression boundary. The proposed method demonstrated reduced sensitivity to the maximum applied stress and provided more reproducible estimates for rounded compression curves, although further validation using natural clays considering sample disturbance effects is required.

Railway idle land offers a large and underused space for solar power, yet its economic value has not been evaluated. Revenue depends on the volatile System Marginal Price (SMP) and Renewable Energy Certificate (REC) markets, whose uncertainty cannot be fully captured by static discounted cash flow (DCF) analysis. This study asks whether solar development on Korea’s railway idle land is worthwhile over the long term, and at what scale it should proceed. It applies an integrated DCF and real options analysis (ROA) framework to a proposed 438 MW project on the Honam Line in southern Korea. Price volatility is estimated from monthly SMP and REC data with a geometric Brownian motion model, and the options to expand and to contract are valued on a binomial lattice. The DCF yields a marginal net present value of USD 3.2 million. The expansion option adds USD 172.5 million and is exercised in 67% of states, raising the total project value to USD 175.7 million. Rising panel efficiency and falling capital costs move the project firmly into feasibility. Therefore, scale flexibility turns a marginal project into a strongly positive one, supporting the large-scale deployment of solar on railway idle land.