青藏高原多年冻土区路基工程地质研究

RESEARCH ON ENGINEERING GEOLOGY OF THE ROADBED IN PERMAFROST REGIONS OF QINGHAI-XIZANG PLATEAU

回顾了青藏高原多年冻土区近年来所开展的路基工程地质研究工作 ,指出冻土工程地质勘探在方法上有很大进展 ,总结出了一整套冻土区工程地质勘察的暂行规定和细则。近年来的工作特别注重了对冻土地温和含冰量的研究 ,这是冻土工程地质中特有的十分重要的参数。在大量观测试验的基础上 ,总结出了有关路基稳定性的若干重要结论。据此提出 ,在气候转暖的背景下 ,高温冻土区筑路必须采用“冷却路基”的方法。为寻求气候转暖的对策 ,冻土工程地质预报的研究也取得了很大的进展 ,对冻土区路基工程地质的研究方向提出了新的建议。

In the permafrost regions of the Qinghai Xizang Plateau, many important engineering constructions,such as the gas pipeline from Golmud to Lhasa, fiber cable from Lanzhou to Xining and to Lhasa, Qinghai Xizang (Qing Zang) Highway (HWY 109), Xining Kangding (Qing Kang) Highway (HWY 214) and Xining Zhangye (Ning Zhang) Highway (HWY 227), have been completed. To summarize the engineering geological works of these constructions is of significance for guidance to the on going construction of Qinghai Xizang Railway, and to the future projects such as the construction of electric cable system and the diversion of river water from south to north in the western China. 1 Engineering geologic survey of permafrost The distribution of permafrost, especially the underground ice, is usually discontinuous in space. This phenomenon creates difficulties for geological investigation. Application of ground penetrating radar has greatly eased such difficulties and increased the accuracy of detection. A series of detailed regulations for engineering geological survey in permafrost regions of the Qinghai Xizang Plateau have been worked out based on practical investigations and theoretical researches over the past several decades. These regulations have been applied for guidance to the engineering geological works for Qinghai Xizang Railway construction in the permafrost region. Ground temperature and ice content are two main factors controlling the roadbed stability. They are also the unique indices for the engineering geological survey in permafrost regions. Many researches on these two factors have been conducted in the past several years. It has been found that 76% of the permafrost along the Qing Zang Highway is high temperature permafrost, and 59% belongs to high ice content permafrost. The thermo stability of roadbed in permafrost regions has been classified and evaluated based on the ground temperature and ice content. 2 Forecast of permafrost engineering and geological perspectives To forecast the changes of permafrost under natural conditions and mankind activities is important to guarantee the accuracy of engineering design. About 12 monitoring stations have been established and ground temperatures in more than 150 boreholes have been monitored along Qing Zang Highway since 1973. The longest record for continuous observation reaches 12 years. Six sections and 65 boreholes are currently still under regular monitoring along the highway. For the Qing Kang Highway, monitoring on experimental embankment has been conducted in Huashixia of Qinghai Province since 1996, and has covered an accumulated distance of about 2 200 km, including 9 sections over 4 different permafrost types. Based on the analysis of the large amount of data, the following conclusions can be drawn and are of importance to research on roadbed engineering geology in permafrost regions. (1) There is an obvious trend of permafrost degradation. The north limit of permafrost has retreated about 0.5～1 km southwards, and the south limit has retreated about 1～2km northwards under natural condition. Combined with the effects of road, the predominantly continuous permafrost has decreased about 28 km from 550 km in 1979 to 522 km in 1991, while the island permafrost decreased about 19 km from 210 to 191 km. The main damages to the road along the highway are caused by thawed settlement and frost heave. Taken the Qing Zang Highway as an example, about 85% of the roadbed damages can be attributed to thawed settlement, and the other 15% are resulted from frost heave and frost boiling. (2) The stability of roadbed in permafrost region is closely related with ground temperature. After the pavement of asphalt on the Qing Zang Highway, 60% of the road underlain by permafrost forms the thawed inter layer and most of such sections belong to high temperature permafrost. The monitoring data indicate that measures by increasing heat resistance (such as increasing the height of roadbed and application of heat insulation materials) play a role in low tem