Drilling and Grouting Challenges in Broadly Graded Soils

Broadly graded soils are characterized by having high coefficient of uniformity and typically are deposited in fluvial and alluvial environments. Galciofluvial soils are common broadly graded soils found in glaciated areas in the northern Continental United States and other locations around the world. Particle sizes in glaciofluvial soils range from fine sand to boulder sizes and typically have very little fines. The coarse fraction poses some problems for drilling and some methods of grouting, but the greater problems occur due to internal instability. Internal instability results from the inability of the coarse fraction of the soil to act as a filter to prevent movement of the finer fraction. This causes water based drilling methods and grouting to wash the finer fraction out leaving the open graded coarse material. This can induce settlement of the ground surface during drilling and grouting. Drilling and grouting in these soils can also increase the permeability of the ground, require much larger volumes of grout to complete the project than initially anticipated, and limit the effectiveness of some grouting methods. While some practitioners are knowledgeable about these conditions and their impacts, the impacts to drilling and grouting have not been well documented in the literature. This paper presents a summary of the nature of these soils including appropriate references, a discussion of the specific impacts to drilling and grouting, and summaries of example cases where these soils have impacted construction cost schedule, and performance.