Geotechnical Assessments for Trenchless Water Crossings in Alberta

The use of trenchless methods for pipeline water crossings in the Province of Alberta has grown significantly during the past decade, primarily as a result of increasingly strict water protection regulations. Because the feasibility of using a trenchless method is dependent on subsurface conditions, a geotechnical assessment should be performed during the planning stages. The results of this assessment are used in the selection of the most appropriate crossing method, and to assess the risk of encountering difficulties during construction. A discussion of the current state of practice for geotechnical investigations of trenchless projects in Alberta is initially presented. The importance of a good understanding of the engineering geological framework and associated geological-geotechnical complexities is highlighted. Recommended stages of a geotechnical assessment are described, namely the desk study, the site reconnaissance, and the field exploration, which involves drilling and sampling and geophysical surveys where appropriate. It is described how each stage interacts with, and is dependent on the others. It is noted that because the cost of a field drilling and sampling can be expensive when compared to the construction costs, obtaining as much information as possible during the desk study and field reconnaissance stages is considered to be a cost effective strategy. Drilling and sampling techniques currently used in Alberta are briefly reviewed, including continuous flight and hollow stem augering, wet rotary, and cased down-hole air percussion methods. Emphasis is placed on describing the advantages and limitations of each method. Some of the concepts presented are illustrated through evaluation of a recent case study of a recent major river crossing in Central Alberta.