Permafrost

36
Thermal and Seepage Modeling Analysis of a Lined, Rockfill Dam
Founded on Marginal Permafrost
Keith F. Mobley, P.E 1 ., Andy R. Smith 2 , Edward Yarmak, Jr., P.E 3 .
(1.Northern Geotechnical Engineering, Inc; 2.Northern Geotechnical Engineering, Inc;
3.Arctic Foundations, Inc)
Abstract: As a part of the engineering required for development of a new open-pit gold mine
near Nome, Alaska, a thermal and seepage analysis was performed on the lined, tailing
retention dam to assess the impact on the foundation permafrost and to determine the seepage
water volume under the dam. The analysis included modeling the influence of thermosyphons,
with the intention that the artificial passive freezing would limit seepage through and under the
dam.
Modeling was completed using a coupled modeling package produced by Geo-Slope
International, Ltd. Their Seep-W package was coupled with the Temp-W package to allow for
analysis of the interaction between thermal changes and convective heat flow due to seepage.
Physical parameters for the foundation and construction materials were determined.
Environmental parameters were obtained from published data at nearby sites and adjusted such
that the model reached thermal equilibrium matching the site subsurface conditions. Several
scenarios were modeled to assess the influence of the physical and environmental parameters,
the effect of thermosyphons, and global warming. To model the anticipated three phase
development of the tailings dam, the model was split into five sets, with changes made to the
modeling mesh at the end of each model set. A long-term model run was made at the end to
assess the seepage and thermal performance of the development following closure of the mine
site.
With the dam liner, the seepage flow was forced through a small zone. The flow rates were
seasonally influenced by the thermosyphons, however the flow zone never completely froze. As
such, the seepage flows were significantly reduced in the winter while the thermosyphons were
active but summer flows were increased, effectively eroding the ice blockage created by the
thermosyphons. On an annual basis, the seepage volumes were reduced slightly.
This paper presents a summary of the results of the modeling completed for the project and
discusses the modeling techniques used to minimize the impact of the model constraints on the
site conditions. The limitations of numerical modeling on this project are discussed and an
assessment of the potential range of obtained results made.
Key Words: Coupled Numerical Analysis, Seepage, Convective Heat Flow, Thermosyphons,
Permafrost Foundation