| An experimental and theoretical investigation of the prospects for improvement of barrier material resistance to freeze-thaw and desiccation-induced stresses through polymer amendment was conducted. The polymers tested were nonionic polysaccharide (guar gum), cationic polyacrylamide (PAM) and cationic chitosan. Concentrations ranging from 0.25 to 8.00 g/L were used and mixed with four barrier materials, namely, Na-montmorillonite, 70:30 sand:clay mix, 90:10 sand:clay mix and Boston Blue Clay excavated as part of the Central Artery/Tunnel project in Boston, MA. Soils and solutions were characterized on the basis of liquid/plastic/shrinkage limit, moisture-density relationships, viscosity and rheology, specific surface area (SSA), X-ray diffraction (XRD), dielectric constant, batch testing, swell testing, freeze-thaw effects on hydraulic conductivity and cracking as well as desiccation rate and cracking. In response to polymer amendment, the crack area density (CAD) generally improved for all materials subjected to either desiccation or freeze-thaw action. Unamended Boston Blue Clay resulted in a CAD of nearly 70% at a depth of 1.7 cm from the top of the specimen in response to five freeze-thaw cycles at temperatures ranging from -20 to +20°C. The corresponding CAD when modified with PAM at a concentration of 0.50 g/L was less than 8%. Observed improvement is attributed to polymer-clay bonding. The SSA of Na-montmorillonite increased from 18.6 to 36.8 m2/g when amended with 0.25 g/L of PAM, because of interlayer bonding reactions. In addition, a theoretical approach to simulating field-length freezing durations over laboratory-tractable time periods was developed. It is postulated that an entire winter may be simulated in a few weeks or even days by maintaining damage similarity as defined by temperature and time. |
