1. Distribution of Seasonal Frost-heaving Soil of Subgrade and Harms Caused by Frost-heaving  

1.1 Survey of Seasonal Frozen Soil

Climates along Harbin-Dalian passenger special line change greatly. Dalian City belongs to warm region; Dashiqiao, Anshan and Shenyang City belong to cold region; and north of Shenyang belongs to severe cold region according to climatic zones divided as their influences to railway engineering.  Maximum frozen depth of soil along Shenyang-Dalian section and Shenyang-Harbin section are 0.93～1.48m and 1.48～2.05m, separately.

Part of the surfaces on K0+000～DK144+000 section along the entire line are bedrock. Most parts within 2m depth below the surfaces of other sections are viscous soil layers which natural water content before freezing is high. Groundwater at some sections provides recharge conditions for frost-heaving of soil. Classified according to seasonal frozen soil frost-heaving nature, it belongs to frost-heaving soil and strong frost-heaving soil.

1.2 Harms Caused by Seasonal Frozen Soil

Harbin-Dalian passenger special line with ballastless track is located at severe cold region. As bearing granular structure under the track, great inhomogeneous deformation of subgrade will be generated through long-term and repeated actions of train load, temperature variation, rain and snow, especially subgrade frost-heaving and thaw settlement during cold winter. Irregularity of railway track caused by the above reasons will seriously endanger the safety operation of passenger special line, and cause great trouble for railway maintenance.  Therefore, effective frost-heaving prevention measures should be taken to guarantee the stability of subgrade structure.

2. Domestic and Foreign Research Status

2.1 Domestic Research Status

It is quite common for railway frost damage in seasonal frozen soil region of our country.  According to statistics of data in 2000, more than 100 places from Xining to Germu section in Qinghai-Tibet railway suffered from severe damages with frozen depth more than 50mm; in northeast region, from 1984 to 2000, there are 803 places of damages caused by seasonal frozen soil on the railway within the jurisdiction of the former Tonglao Substation of Shenyang Railway Bureau. Their total length reaches to 77.6km, and the affecting time can be more than 2 months each year.  Subgrade damage caused by frost-heaving adds lots of working loads to the maintenance of the railway line. It will also seriously endanger safety running.  In capacity expansion and revamping project in 2000, China Railway First Survey and Design Institute adopted subgrade bed replacement, partition layer setting, dewatering facilities, and other measures according to characteristics of the damage, and effectively solved the frost damage of existed subgrade.

2.2 Foreign Research Status

For the construction of passenger special line in severe cold regions in German, influences of frost-heaving have been taken into account and corresponding engineering countermeasures have been taken:

1. A disconnected protective layer which is composed by antifreezing and non-cohesive gravel is laid between ballast bed and frozen-changed soil subgrade. This guarantees the frozen-changed subgrade to a depth which can not or hardly reach by freezing. This protective layer is named as freezing protective layer (FSS). This layer must have the following features: blocking capillary water action, good permeability, rolled and compacted, and stable transition function correspond to existed subgrade soil.

2. To prevent freezing from approaching the soil subgrade, lay a layer of heat insulation course (thermal insulation board) between ballast bed and frozen-changed soil subgrade.

For the sake of economy, complete protection of frost and partial frost-heaving should be distinguished.  Germany implementation guideline DS836 only requires partial protection of frost for passenger special line, which is partially adopts FSS (frost-heaving prevention) layer within frozen depth.      

In Japan, frost-heaving vulnerable soil within the frozen depth in cutting and low-filled embankment of severe cold region is completely replaced with minor frost-heaving soil, or thermal insulation board and other heat insulation measures are taken according to rigid and deformation requirements of the subgrade bed.

Seasonal frozen soil is widely distributed in America, Canada, Russia, China, and some other countries. Although abundant accomplishments have been achieved in soil frost-heaving and thaw settlement treatment, there are almost no passenger special lines in these countries. Therefore, the influence of season frozen soil to the stability of subgrade of passenger special line has not been studied yet.  A passenger special line is under construction in seasonal frozen soil region of Hokkaido in Japan.  Therefore, few accomplishments can be referenced.