Modeling the swelling extent of argillaceous soft rock using the PLAXIS swelling rock model

This paper addresses the challenges of numerically calculating the effective swelling extent in a test gallery situated in an argillaceous soft rock lithology using PLAXIS 2D and employing two different approaches, namely applying volumetric strain and the “PLAXIS Swelling Rock Model”. The focus is on a localized section of a tunnel excavation where swelling occurs in the invert due to artificial irrigation in an otherwise dry but swellable rock mass. Accurate long-term modeling of the swelling behavior of such lithologies is crucial for understanding the acting stresses on underground structures and improving the overall rock mass characterization. However, maintaining in-situ test equipment for extended periods on active tunnel construction sites for validation reasons often proves challenging.
The study utilizes long-term in-situ monitoring data from extensometers, stress strain meters, geodetic targets and a chain inclinometer installed in a test gallery of the Angath adit tunnel in Tyrol, Austria, combined with laboratory swelling test results. These datasets and associated back calculations are utilized as the basis of this study. Two different approaches for a prognosis of the final swelling extent in the invert of the test gallery on the basis of laboratory test results are used and compared with one another.
Results show that while numerous adjustable parameters in the modeling process of the PLAXIS Swelling Rock Model allow for flexibility, laboratory test results are not sufficient as sole input data to account for the model complexity and potential inaccuracies in achieving a realistic prediction. The problem of upscaling from rock to rock mass parameters appear to have a substantial influence on the results.