GEOTECHNICAL INVESTIGATIONS FOR TUNNELING IN GLACIAL TERRAIN

Due to the transport mechanics of the depositional environment, glacial soils display a very wide range of grain sizes and behavior. Several nonstandard drilling, sampling, and analytical techniques are therefore required to characterize the subsurface. This presentation summarizes the geotechnical investigation for the design of a large tunneling project in glacial terrain. The presentation focuses on the challenges presented by the glacial terrain and the techniques required to identify and characterize it for design and contract document preparation, and the improvements in understanding of the sub-surface which can be gained through the use of specialized investigative techniques. Glacial terrain can present significant challenges to the tunnel construction process, as well as to the geotechnical investigator tasked with characterizing and describing the geologic materials for tunnel design and bidding purposes. Glacial soils often consist of a wide range of particle sizes, frequently present in stratified layers that respond differently to the tunneling process. Typical sampling methods recover small volumes of material, which are appropriate for relatively fine-grained soils. This project also required non-traditional sampling methods to recover the large sample volumes necessary to characterize the coarse-grained materials. Characterization of these soils for tunnel design and construction required the application of several specialized investigation techniques. These methods include 4.25 inch (ID) hollow stem auger borings with samples obtained by driving 2 and 3 inch diameter split spoon samplers, 6 inch diameter sonic borings which obtained a continuous 4 inch diameter core, and large diameter (36-42 inch) caisson borings. Additional measures were used to try to estimate the number and hardness of boulders expected in the tunnel envelope utilizing observations of production at a dragline based aggregate quarry that is close to the alignment, and characterizing previously mined boulders. An attempt was made to characterize all of the materials that could have an effect on the construction of the project from the clay sized rock flour to the potentially house sized glacial boulders.