Long-Term Benefits of Stabilizing Soil Subgrades

Chemical admixtures have been used extensively since the mid-eighties in Kentucky to improve bearing strengths of soil subgrades. Most pavements in Kentucky are constructed on clayey soils. Although short-term observations at a small number of sites showed that chemical stabilization worked very well, a need existed to perform a more comprehensive review and to assess the long-term benefits of this subgrade stabilization method. The main intent of this study was an attempt to address questions concerning bearing strengths, longevity, durability, structural credit, economics, and performance of pavements resting on soil subgrades mixed with chemical admixtures. In-depth field and laboratory studies were performed at fourteen roadway sites containing twenty different treated subgrade sections. Ages of the sites range from about 8 to 15 years. About 455 borings were made at the various sites. Air, instead of water, was used as the drilling media. In-situ CBR tests were performed on the treated subgrades and the untreated subgrades lying directly below the treated layers. Index tests and resilient modulus tests were performed on samples collected from the treated and untreated subgrades. Falling weight deflectometer (FWD) tests were performed. At the 85th percentile test value, the in situ CBR values of subgrades mixed with hydrated lime, Portland cement, a combination of hydrated lime and Portland cement, and a byproduct (MKD) obtained in the production of hydrated lime were 12 to 30 times greater than in CBR values of the untreated subgrades. In-situ CBR values of the treated layer ranged from 24 to 59 while the in-situ CBR of the untreated layer at the 85th percentile test value was only 2. Based on rating criteria of the Kentucky Transportation Cabinet, the conditions of the pavements at twelve sites could be rated “good” at the time of the study– pavement ages were 8 to 15 years– and “good” at the end of the twenty-year design period, based on projected data. At two sites, thin asphalt overlays had been constructed after 15 years. However, accumulated values of ESAL at those sites had exceeded or were near the values of ESAL assumed in the pavement designs. At the 20th percentile test value, rutting depths of the pavements resting on the treated subgrades were less than about 0.27 inches. Structural layer coefficients, a3, for use in pavement design of the different chemically stabilized subgrades were developed. The proposed values were verified at sites where reduced pavement thickness was used and “in service” structural coefficients could be observed. Back-calculated values of FWD modulus of the treated layers were about two times the values of modulus of the untreated subgrade. Resilient modulus of the treated subgrades was larger than the resilient modulus of the untreated subgrades. Moisture contents at the top of the untreated subgrade layers showed that a “soft” layer of material exists at the very top of the untreated subgrade. This soft zone did not exist at the top of the treated layer. This discovery has significant engineering implications. Future research will focus attention on an in-depth examination of this weak layer of soil. Chemical admixture stabilization is a good, durable and economical technique for improving subgrade strengths