Qu Jianjun

Effects of freeze-thaw cycles on soil mechanical and physical properties in the Qinghai-Tibet Plateau

Extreme freeze-thaw action occurs on the Qinghai-Tibet Plateau due to its unique climate resulting from high elevation and cold temperature. This action causes damage to the surface soil structure, as soil erosion in the Qinghai-Tibet Plateau is dominated by freeze-thaw erosion. In this research, freezing-thawing process of the soil samples collected from the Qinghai-Tibet Plateau was carried out by laboratory experiments to determinate the volume variation of soil as well as physical and mechanical properties, such as porosity, granularity and uniaxial compressive strength, after the soil experiences various freeze-thaw cycles. Results show that cohesion and uniaxial compressive strength decreased as the volume and porosity of the soil increased after experiencing various freeze-thaw cycles, especially in the first six freeze-thaw cycles. Consequently, the physical and mechanical properties of the soil were altered. However, granularity and internal friction angle did not vary significantly with an increase in the freeze-thaw cycle. The structural damage among soil particles due to frozen water expansion was the major cause of changes in soil mechanical behavior in the Qinghai-Tibet Plateau.

Sand drift encroachment in the Dunhuang Mogao Grottoes District and its control

Analysis on sand drift damages to the Dunhuang Mogao Grottoes showed that the actual sand drift hazards in the area are mainly attributed to the gobi wind-sand stream formed by sand coming from the Mingsha Mountain (megadune), while the migration of the megadune and its frontal edge dunes is actually not a major threat to the grottoes. The gobi belt on the cliff top of the Mogao Grottoes is not only a natural sand transport field but also a natural protective screen which prevents the megadune and its frontal edge dunes from advancing towards the grottoes or burying the grottoes. Therefore efforts to protect the Mogao Grottoes should be focussed on halting the gobi wind-sand stream rather than on checking the migration of sand dunes. For this reason, effective sand blockade and stabilization measures should be adopted at the frontal edge of the megadune but no sand retarding works are allowed to be set up in the gobi belt so as to avoid sand accumulation and entering the grottoes.

Pulsatory characteristics of wind velocity in sand flow over typical underlying surfaces

Pulsatory characteristics of wind velocity in sand flow over Gobi and mobile sand surface have been investigated experimentally in the wind tunnel. The primary goal of this paper is to reveal the relationship between pulsatory characteristics of instantaneous wind speed in sand flow and the motion state of sand grains. For a given underlying surface, pulsation of wind velocities in sand flow on different heights has a good correlation. As the space distance among different heights increases, fluctuation of instantaneous wind speed presents a decreasing trend and its amplitude is closely related to the motion state of sand grains and their transport. Pulsatory intensity increases with the indicated wind speed, but its relative value does not depend on it, only agrees with height.

Experimental study of surface texture and resonance mechanism of booming sand

The sound-producing mechanism of booming sand has long been a pending problem in the blown sand physics. Based on the earlier researches, the authors collected some silent sand samples from Tengger Desert, Australian Desert, Kuwait Desert, beaches of Hainan Island and Japanese coast as well as the soundless booming sand samples from the Mingsha Mountain in Dunhuang to make washing experiments. In the meantime the chemical corrosion experiment of glass micro-spheres, surface coating experiment and SEM examination were also conducted. The experimental results show that the sound production of booming sand seems to have nothing to do with the presence of SiO2 gel on the surface of sand grains and unrelated to the surface chemical composition of sand grains but is related to the resonance cavities formed by porous (pit-like) physical structure resulting from a number of factors such as wind erosion, water erosion, chemical corrosion and SiO2 gel deposition, etc. Its resonance mechanism is similar to that of Hemholz resonance cavity. Under the action of external forces, numerous spherical and sand grains with smooth surface and porous surface are set in motion and rub with each other to produce extremely weak vibration sound and then become audible sound by human ears through the magnification of surface cavity resonance. However the booming sands may lose their resonance mechanism and become silent sand due to the damping action caused by the invasion of finer particles such as dust and clay into surface holes of sand grains. Therefore, clearing away fine pollutants on the quartz grain surface is an effective way to make silent sand emit audible sound.