Qihao Yu

Continuous and simultaneous measurement of reflector depth and average soil-water content with multichannel ground-penetrating radar

Ground-penetrating radar is a fast noninvasive technique that can monitor subsurface structure and water-content distribution. To interpret traveltime information from single common-offset measurements, additional assumptions, such as constant permittivity, usually are required. We present a fast ground-penetrating-radar measurement technique using a multiple transmitter-and-receiver setup to measure simultaneously the reflector depth and average soil-water content. It can be considered a moving minicommon-midpoint measurement. For a simple analysis, we use a straightforward evaluation procedure that includes two traveltimes to the same reflector, obtained from different antenna separations. For a more accurate approach, an inverse evaluation procedure is added, using traveltimes obtained from all antenna separations at one position and its neighboring measurement locations. The evaluation of a synthetic data set with a lateral variability in reflector depth and an experimental example with a large variability in soil-water content are introduced to demonstrate the applicability for field-scale measurements. The crucial point for this application is the access to absolute traveltimes, which are difficult to determine accurately from common-offset measurements.

Thermal–moisture dynamics of embankments with asphalt pavement in permafrost regions of central Tibetan Plateau

Subsurface moisture content is one of the critical factors that control the thermal dynamics of embankments. However, information on the subsurface moisture movement and distribution in embankments is still limited. To better understand the coupled water and heat transport within embankments, subsurface temperature and moisture of an asphalt pavement highway were extensively measured from 2009 to 2011. Collected data indicate that pure heat conduction is the overall main mechanism of heat transport in the embankment and heat convection plays a relatively unimportant role in heat transport. The results also indicate that subsurface moisture and temperature dynamics in the asphalt layer is strongly related to the rainfall events, while the subsurface moisture content below the road base course maintains relatively constant. Rainfall in summer leads to rapid cooling of the subsurface soil. Our results suggest that frequent and small rainfall events favour the thermal stability of the embankment due to the loss of latent heat of water evaporation. Moisture migration during freezing still occurred in the gravel fill and the water infiltrated into the active layer during thawing period. Freezing-induced water migration may result in the increase in water content of the embankment and the decrease in compactness of gravel fill.

Laboratory testing on heat transfer of frozen soil blocks used as backfills of pile foundation in permafrost along Qinghai-Tibet electrical transmission line

Generally, construction for pile foundation in permafrost has to be carried out in winter to minimize the thermal distribution to the underlying or surrounding permafrost. Thus, there exists a problem that it is hard to meet the stipulated requirement to the compaction degree of the backfilled frozen soil blocks around the pile foundation excavated quickly. In order to study the effect of froze soil blocks on the heat transfer process between pile and permafrost during the construction of the Qinghai-Tibet electrical transmission line in winter, some laboratory tests were carried out for the highly porous frozen soil blocks and the naturally compacted thawed soil body, respectively. In addition, the thermal conductivities were calculated under different temperature gradient according to the measured thermal data. Results show that the convective heat transfer occurs in the highly porous frozen soil blocks at negative temperature corresponding to winter time, which is favorable for refreezing the pile foundation and lowering permafrost temperature. However, backfilling the highly porous frozen soil blocks hardly meet the requirement of compaction degree. It has dual effect on the stability of tower foundation depending on the specific site conditions such as permafrost temperature, ice content, soil type, permeability, hydraulic condition, and embedded depth of pile. Results also show that the equivalent thermal conductivity of the frozen soil blocks is over five times more than that of the thawed soil body on average. This is because the convective heat transfer occurs in frozen soil blocks in winter, which has stronger heat exchange effectiveness and can diminish refreezing time. Tests have revealed the process of heat transfer of frozen soil blocks used as fills around the pile foundation in permafrost, verified its thermal semiconductor effect, and accumulated and expanded data of the thermal conductivity.

The role of rainfall in the thermal-moisture dynamics of the active layer at Beiluhe of Qinghai-Tibetan plateau

The active layer in permafrost regions plays an important role in energy exchange between permafrost and atmosphere. Rainfall is one of the dominant factors affecting thermal-moisture dynamics of the active layer. To better understand the thermal-moisture dynamics and the interaction between rainfall and the active layer in-detail, in situ experiment was carried out and soil temperature, soil moisture and soil heat flux of the active layer were measured from 2007 to 2009. The observation data demonstrated that the volumetric soil water content of the active layer remained fairly constant during the winter and had a notable fluctuation resulted from evapotranspiration and rewetting from rainfall events in summer. The daily variation amplitude of soil temperature and soil heat flux in summer was bigger than that in winter. Soil moisture content increased and soil temperature decreased after rainfall. Rainfall in summer led to the change of surface energy balance and caused subsurface soil cooling. The convective heat transfer from water infiltration reduced the temperature gradient along depth and changed near-surface heat fluxes. The increase in rainfall may mitigate permafrost degradation on the Tibetan Plateau.

Thermal effects of lateral supra-permafrost water flow around a thermokarst lake on the Qinghai-Tibet Plateau

State Key Laboratory of Frozen Soils Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, China 2 Institute of Environment Physics, University of Heidelberg, Heidelberg, Germany Correspondence QihaoYu, State Key Laboratory of Frozen Soils Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, China. Email: yuqh@lzb.ac.cn

The risk evaluation of frost jacking for tower foundations along Qinghai- Tibetan transmission line and anti-heave measures

The Qinghai-Tibetan ±400 kV direct current grid interconnection project runs across 1038 km of permafrost and seasonally frozen-ground in the interior of the Qinghai-Tibetan Plateau. The mean annual air temperature of the Qinghai-Tibetan Plateau varies between -3 ℃ and -7 and the minimum air temperature ℃ is lower than -37 in short durations. The active ℃ layer is subjected to annual freeze-thaw cycle and its thickness varies between 2 m and 3 m. significant heave force is expected due to the existence of extensive frost susceptible soils and cold weather. The tower foundations tend to be jacked out of the ground and result in expensive maintenance costs and foundation failure, which significantly threatens the safety and normal operation of the transmission line. Therefore, it becomes the first concern to prevent the frost jacking failure for design and construction of the Qinghai-Tibetan transmission line. To protect the transmission system from damage, it is necessary to evaluate the engineering risk and to employ some effective countermeasures to mitigate the frost-related damages. To evaluate the risk and provide reasonable suggestions for design and construction, a safety coefficient calculation which involves with the frost penetration, frost heave force, freezing strength as well as loads is conducted. The results show that the spread-type footing has more excellent performance to resist uplift loads than drilled shaft. To improve the safety of foundations, anti- heave measures including non-frost susceptive soil backfill, bevel foundation design, surface treatment and two-phase closed thermosyphon, etc were proposed and applied in the construction of the Qinghai-Tibetan grid project. The advantages and applicability of anti-heave technical solutions are described.