Experimentally Derived Mechanical and Flow Properties of Fine-grained Soil Mixtures

As silt content in mudrocks increases, compressibility linearly decreases and permeability exponentially increases. We prepared mixtures of natural Boston Blue Clay (BBC) and synthetic silt in the ratios of 100:0, 86:14, 68:32, and 50:50, respectively. To recreate natural conditions yet remove variability and soil disturbance, we resedimented all mixtures to a total stress of 100 kPa. We then loaded them to approximately 2.3 MPa in a CRS (constant-rate-of-strain) uniaxial consolidation device. The analyses show that the higher the silt content in the mixture, the stiffer the material is. Compression index as well as liquid and plastic limits linearly decrease with increasing silt content. Vertical permeability increases exponentially with porosity as well as with silt content. Fabric alignment determined through High Resolution X-ray Texture Goniometry (HRXTG) expressed as maximum pole density (m.r.d.) decreases with silt content at a given stress. However, this relationship is not linear instead there are two clusters: the mixtures with higher clay contents (100:0, 84:16) have m.r.d. around 3.9 and mixtures with higher silt contents (68:32, 50:50) have m.r.d. around 2.5. Specific surface area (SSA) measurements show a positive correlation to the total clay content. The amount of silt added to the clay reduces specific surface area, grain orientation, and fabric alignment; thus, it affects compression and fluid flow behavior on a micro- and macroscale. Our results are comparable with previous studies such as kaolinite / silt mixtures (Konrad & Samson [2000], Wagg & Konrad [1990]). We are studying this behavior to understand how fine-grained rocks consolidate. This problem is important to practical and fundamental programs. For example, these sediments can potentially act as either a tight gas reservoir or a seal for hydrocarbons or geologic storage of CO2. This study also provides a systematic approach for developing models of permeability and compressibility behavior needed as inputs for basin modeling.