In situ digital testing method for quality assessment of soft soil improvement with polyurethane

This study purposes an in situ testing method on quality assessment of soil improvement. Factual drilling data includes the spatial distribution and in situ strength of untreated and treated soil along three different drillholes measured by on-site drilling monitoring method. These factual drilling data can characterize the degree of soil improvement by penetration injection with permeable polyurethane. Result from on-site drilling monitoring shows that the linear zones represent constant drilling speeds shown in the plot of drill bit advancement vs. net drilling time, which indicates the spatial distributions of soil profile. The soil profile at the study site is composed of four layers, which includes fill, untreated silty clay, treated silty clay, and mucky soil. The results of soil profile are verified by the parallel site loggings. The constant drilling speeds profile the coring-resistant strength of drilled soils. By comparing with the untreated silty clay, the constant drilling speeds of the treated silty clay have been decreased by 13.0–62.8%. Two drilling-speed-based indices of 61.2% and 65.6% are proposed to assess the decreased average drilling speed and the increased in situ strength of treated silty clay. Laboratory tests, i.e. uniaxial compressive strength (UCS) test, have been performed with core sample to investigate and characterize in situ strength by comparing that with drilling speeds. Results show that the average predicted strengths of treated silty clay are 2.4–6.9 times higher than the average measured strength of untreated silty clay. The UCS-based indices of 374.5% and 344.2% verified the quality assessment (QA) results by this new in situ method. This method provides a cost-effective tool for quality assessment of soil improvement by utilizing the digital drilling data.