Analysis of the effect of soil improvement in the frequency domain

In this study, the effects of calcium aluminate cement (CAC) stabilization on the strength and wave propagation behavior of four clay types with different engineering properties Pazaryeri clay, Şile clay, Ukrainian clay, and organic clay were investigated. For stabilization purposes, CAC was added to the clay samples at rates of 0%, 10%, and 20%, resulting in a total of 12 different variations. Each mixture was molded based on its optimum moisture content and subjected to experimental tests after 0, 7, and 28 days of curing. To determine the physical and index properties of the soils, Atterberg limit tests, specific gravity determination, sieve and hydrometer analyses were conducted. The compaction characteristics were determined using the standard Proctor test, and the maximum dry unit weight (MDD) and optimum water content (OWC) values were identified. Unconfined compressive strength (UCS) and ultrasonic pulse velocity (UPV) tests were performed to assess the effects of the cement content and curing time. The signals obtained from UPV tests were transformed into the frequency domain via Fast Fourier Transform (FFT), and amplitude values were analyzed. The results showed that both increasing CAC content and longer curing durations led to significant improvements in UCS and UPV values across all soil types. The highest strength and UPV values were observed in the samples containing 20% CAC and cured for 28 days. Additionally, amplitude values increased proportionally with cement content and curing time, with Şile clay displaying the highest acoustic performance. A strong regression was observed between UCS and UPV, suggesting that UPV data could be used for estimating soil strength. In this context, stabilization with CAC was found to effectively improve both mechanical and dynamic properties of the clayey soils, while UPV testing proved to be a non-destructive and efficient method for monitoring such improvements. The results of the frequency analysis revealed that the dominant frequencies shifted toward higher values with increasing cement content and curing duration.