Journal of Cold Regions Engineering

Wastewater precipitation ponds (fellingsdams) are conventional stabilization ponds adapted to cold climate by the use of chemical precipitation to attain sufficient removal efficiency of impurities, primarily phosphorus. The objective with this investigation was to study the influence of an interruption of the dosage of coagulant during summer periods at two fellingsdam systems (Orrviken and Lockne) in the middle of Sweden. The investigation took place over two years characterized by unusual precipitation conditions; 2001 was intense in precipitation whereas summer 2002 represented a dry season. The results showed that there is a potential to utilize the summer biological activity in fellingsdams. At Orrviken the effluent quality measured as organic matter and phosphorus in the effluent was just slightly above the values that were reached by chemical precipitation. At Lockne the performance was lower. The organic matter reduction at Orrviken in the summers of 2001 and 2002 were 71 and 67%, respectively, compared to previous years using precipitant when the average was 78%. At Lockne, however, the values in the summers of 2001 and 2002 were 36 and 18%, respectively, compared to previous years using precipitant when the average was 55%. The phosphorous reduction at Orrviken in the summers of 2001 and 2002 were 85 and 89%, respectively, compared to previous years using precipitant when the average was 95%; at the Lockne plant, the phosphorous reduction during the summers of 2001 and 2002 were 60 and 66%, respectively, compared to the previous years' average of 86%. The nitrogen reduction varied considerably over the two summer periods. The reduction at Orrviken was 13% in 2001 and 58% in 2002; the reduction at Lockne was 13% in 2001 and 33% in 2002. Reference values of nitrogen reduction during normal operations were not available.

Measures were taken to protect the thermal regime of the roadbed embankment after construction and to lower permafrost temperatures in the rich-ice and warm permafrost regions along the Qinghai-Xizang Railway. However, these measures were taken only for some sections of the railway, leaving many sections unprotected. This article addresses those areas where no measures were taken and presents analysis of the variation of soil temperatures under the embankment in seasonal frozen soil areas, degrading permafrost areas, and warm and cold permafrost areas. The results show that soil temperatures, maximum seasonal freezing depth, and the permafrost table under the embankment differ according to the different frozen soil areas after embankment construction. In seasonal frozen soil areas and degraded permafrost areas, the seasonal frost layer remained frozen the next year under the shaded shoulder of the embankment. In degrading permafrost areas, a thaw layer between the permafrost table and the bottom of the seasonal frost formed under the embankment. In warm permafrost areas, the permafrost table under the embankment was unstable and soil temperatures near the permafrost table showed an obvious increasing trend. In cold permafrost areas, the permafrost table under the embankment was clearly raised and temperature lowered in the soil near the permafrost table, which is advantageous to permafrost thermal stability under the embankment. In particular, the differ-ence in solar radiation from the slope exposed to the sun to the shady slope of the embankment is responsible for the difference in the soil thermal regime and the permafrost table, which potentially can affect roadbed stability.

The crushed-rock embankment and duct-ventilated embankment have been used as effective cooling measures to protect permafrost underlying the Qinghai-Tibet Railway from thawing in China. These two cooling techniques are not directly applied to the Qinghai-Tibet Expressway, however, due to the large width and higher temperature of pavement surface. Therefore, considering the heat transfer characteristics of crushed-rock interlayer embankments and duct-ventilated embankments, we designed the crushed-rock interlayer embankment with ventilated duct. For cold regions engineering projects, the thermal regime is the most important factor that determines the stability of construction. To investigate the thermal stability of this new type of embankment, a three-dimensional numerical model was developed based on heat and mass transfer theory. The model includes coupled heat transfer between the airflow and the duct wall, air convective heat transfer within the crushed-rock interlayer, and heat conduction with phase change in the soil layer. The computational results indicated that the numerical model can reasonably solve the coupled heat and mass transfer for the crushed-rock interlayer embankment with ventilated duct. Based on an assumption that the mean annual air temperature will increase by $2.6°\text{C}$ in the next 50years, it was determined that in areas where the mean annual air temperature is currently $-4.0°\text{C}$ , the crushed-rock interlayer embankment with ventilated duct can be an effective measure to decrease the underlying ground temperature and ensure the stability of the Qinghai-Tibet Expressway in permafrost regions.