Evaluation of the Organic Soil Compressibility from In-Situ and Laboratory Tests for Road Application

Organic soil is characterised by high compressibility and should be improved so that it can be used for construction. The use of every method of soil improvement requires knowledge of the compressibility parameters. One of these parameters is the constrained modulus. The constrained modulus can be determined using laboratory or in-situ tests. In this study, the constrained modulus of organic soil was determined using oedometer and piezocone tests (CPTU). The author analysed subsoil under an approximately 250 m section of a designed road in north-eastern Poland. The constrained modulus of organic soil sampled from four different depths was determined in oedometer tests. Piezocone tests were conducted at 18 points located every 15 m along the length of the section concerned. To determine the constrained modulus based on the cone resistance from CPTU tests, the knowledge of the α and αM coefficients is needed. For the tested soil, the optimal range of the α coefficient from 0.4 to 0.7 was determined. The αM coefficient ranged from 0.4 to 0.8. The value of the constrained modulus of organic soil obtained from the oedometer tests, depending on the effective stress, ranged from approximately 100 kPa to 400 kPa. The constrained modulus of the tested soil decreased with depth, which both research methods proved.