As an important part of farmland, the slope farmland is widely distributed in the central and western plateau mountain region in China. It is necessary to scientifically evaluate the slope farmland quality (SFQ) and analyze the spatial structure characteristics of SFQ to ensure reasonable utilization and partition protection of slope farmland resources. This paper takes the typical plateau mountain region—Yunnan Province in China—as an example and systematically identifies the leading factors of SFQ. The sloping integrated fertility index (SIFI) is adopted to reflect the SFQ. The evaluation system is built to quantitatively evaluate the SFQ and the spatial structure characteristics of SFQ were analyzed by a geostatistical model, autocorrelation analysis and spatial cold–hot spot analysis. The results show that the SFQ indexes in Yunnan Province are between 0.36 and 0.81, with a mean of 0.59. The SFQ grade is based on sixth-class, fifth-class, seventh-class and fourth-class land. The SFQ indexes present a normal spatial distribution, and the Gaussian model fits well with the semi-variance function of the spatial distribution of SFQ indexes. Furthermore, the spatial distribution of SFQ indexes is moderately autocorrelated. The structural factors play a major role in the spatial heterogeneity of SFQ indexes, but the influence of random factors should not be ignored. The spatial distribution of SFQ grades has a significant spatial aggregation characteristic, and the types of local indicators of spatial association (LISA) are based on high–high (HH) aggregation and low–low (LL) aggregation. The cold spot and hot spot distributions of SFQ grades display the significant spatial difference. The hot spot area is mainly distributed in Central Yunnan and the Southern Fringe, while the cold spot area mainly distributes in the Northeastern Yunnan, Northwestern Yunnan and Southwestern Yunnan. This study could provide a scientific basis for SFQ management and ecological environment protection in the plateau mountain region.
Ultra-high-performance concrete (UHPC) is a new type of high-performance cement-based composite. It is widely used in important buildings, bridges, national defense construction, etc. because of its excellent mechanical properties and durability. Freeze thaw and salt erosion damage are one of the main causes of concrete structure failure. The use of UHPC prepared with multi-walled carbon nanotubes (MWCNTs) is an effective method to enhance the durability of concrete structures in complex environments. In this work, the optimal mix proportion based on mechanical properties was obtained by changing the content of MWCNTs and water binder ratio to prepare MWCNTs UHPC. Then, based on the changes in the compressive strength, mass loss rate, and relative dynamic modulus of elasticity (RDME), the damage degree of concrete under different salt erosion during 1500 freeze-thaw (FT) cycles was analyzed. The changes in the micro pore structure were characterized by scanning electron microscope (SEM) and nuclear magnetic resonance (NMR). The test results showed that the optimum mix proportion at the water binder ratio was 0.19 and 0.1% MWCNTs. At this time, the compressive strength was 34.1% higher and the flexural strength was 13.6% higher than when the MWCNTs content was 0. After 1500 salt freezing cycles, the appearance and mass loss of MWCNTs-UHPC prepared according to the best ratio changed little, and the maximum mass loss was 3.18%. The higher the mass fraction of the erosion solution is, the lower the compressive strength and RDME of concrete after FT cycles. The SEM test showed that cracks appeared in the internal structure and gradually increased due to salt freezing damage. However, the microstructure of the concrete was still relatively dense after 1500 salt freezing cycles. The NMR test showed that the salt freezing cycle has a significant influence on the change in the small pores, and the larger the mass fraction of the erosion solution, the smaller the change in the proportion of pores. After 1500 salt freezing cycles, the samples did not fail, which shows that MWCNTs UHPC with a design service life of 150 years has good salt freezing resistance under the coupling effect of salt corrosion and the FT cycle.
Few studies have been published on the dynamic centrifuge model test of cohesive soil under earthquake action. The seismic response of cohesive soil foundation and tunnel was studied by the centrifuge experiment and numerical modelling. Through a comparison of the acceleration results of tunnel and cohesive soil foundation and the pore pressure and displacement of cohesive soil foundation, the influence of tunnel on cohesive soil foundation is discussed. The weak position of the tunnel under earthquake is predicted by effective numerical modelling. The results show that: (1) Under the Parkfield seismic wave, the natural frequency of the cohesive soil foundation with the tunnel is about 0.3 Hz, which is the most clear for the amplification of the low frequency component and the amplification of the seismic acceleration from bottom to top; (2) The acceleration response of the tunnel itself is small, and the effect of seismic wave on the surrounding soil is weakened due to the existence of tunnels; (3) The maximum bending moment and shear force appear at the corner of the rectangular tunnel, and the maximum axial force appears at the top of the rectangular tunnel; (4) The lateral displacement of the surface soil is the largest, and the pore pressure reduction in the middle soil is the largest compared with other soil layers. The existence of tunnels weakens the liquefaction potential of the surrounding soil.
Fire occurred in underground engineering such as tunnel will cause the change of the temperature field in the surrounding soil. The study on thermal conductivity of soils in fire environment is very important. This paper focuses on the experimental study on heat transfer characteristics of soft clay in Shanghai in high temperature environment over 100°C. The test results show that the change of internal temperature of initial saturated soils can be roughly divided into four stages, namely rapid heating stage I, constant temperature stage II, the second rapid heating stage III and the final constant temperature stage IV, when the drainage and exhaust are allowed in the high temperature environment. There is a peak value in the high temperature curve, the higher the temperature the bigger the peak value. According to comprehensive analysis of the heating curves under different temperature, high temperature has significant influence on thermal conductivity of soils and causes the increasing of thermal conductivity for wet soil and dry soil. The thermal conductivity of dry soil is relatively smaller than that of wet soil.
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