Grain Size Distribution in HDD
For HDD construction, soils with 50% or more gravel-sized material pose significant challenges. Specifically, coarse gravel (3/4 inch or larger) cannot be efficiently suspended in drilling fluid, making it difficult to maintain borehole stability. To mitigate this, it's crucial to avoid significant layers of coarse material, either by adjusting the drilling path or using conductor pipes. For HDD to be feasible, most of the coarse material should be smaller than 3/4 inch, and the soil should contain 25-30% fines to help maintain borehole stability. Additionally, keeping gravel content below 50% by weight is essential to reduce risks during drilling.