Coarse vs. Fine Grained Soils
In HDD, soils are categorized into two main groups: coarse-grained (granular) and fine-grained (cohesive). Coarse-grained soils include sands, gravels, cobbles, and boulders, with behavior largely dependent on particle size and distribution. Fine-grained soils, such as silts and clays, are classified based on their plasticity. The Unified Soil Classification System (USCS) is used to label soils, with symbols indicating type (e.g., G for gravel, S for sand) and characteristics (e.g., P for poorly graded, W for well-graded). For HDD, granular soils with a certain percentage of fines (e.g., 25-30%) are preferred as they help maintain the fluid condition during drilling. Fine-grained soils are further classified by their liquid limit and plasticity, influencing their behavior during the drilling process.