Construction over Frozen Ground on the Port MacKenzie Rail Extension Project

The Port MacKenzie Rail Extension project is an approximately 51.5-km-long rail spur extending south from the existing Alaska Railroad mainline in Houston, Alaska, to Port MacKenzie on the north end of the Cook Inlet. The rail spur alignment crosses largely undeveloped land and approximately 8 km of poorly drained, boggy wetland areas. Many of the wetland areas are incapable of supporting construction vehicle traffic during the summer season. To facilitate embankment construction in these wetlands during the summer, an approach to constructing the base of embankments over firm, frozen ground during the winter (winter embankment base stabilization (WEBS)) was developed. Several configurations to the WEBS construction were considered using soil fill and geosynthetics. A test section was constructed and instrumented using horizontal slope indicators and vertical thermistor strings during the first winter season of construction for four variations of fill thickness and reinforcement. The results of the test section and observations during construction of the WEBS during the first summer construction season were used to refine the approach for subsequent construction seasons. After three construction seasons, a significant amount of real-world performance and construction observations were available to evaluate the effectiveness of the WEBS over seasonally frozen ground. Additional comparisons of the WEBS construction approach to several embankments constructed in wetlands under summer conditions were also available. This paper provides settlement and ground temperature data collected at the WEBS test sections, observations of WEBS performance, recommended construction approaches over seasonally frozen ground, and a review of the efficacy of the recommended approaches. Conditions where WEBS should be utilized and lessons learned are discussed.