Cobbles are an important consideration when reviewing any geotechnical baseline report (GBR) for a soft ground earth pressure balance (EPB) tunnel boring machine (TBM) tunnel. How many times have you read a GBR that says, “For Reach Y, Cobbles are anticipated to be X percent of the total excavated volume of material?” Engineers use this language to better define the ground and set a baseline. Contractors review this language closely when they are bidding on the work or when they are wondering whether the ground presents a differing site condition.
