Search

Estimating and preventing loess collapsibility are important for the design and construction of the Xi’an North–Airport Intercity Railway. Laboratory tests and large-scale in situ immersion experiments were carried out to assess the collapsibility characteristics, seepage field, and deformation properties of loess at sites located on the third terrace of the Wei River (Bai Qi Zhai village) and the Xianyang loess tableland (Bu Li village). The calculated and measured in situ self-weight collapsibility are 363.0 mm and 3.0 mm for the Bai Qi Zhai village experimental site (the third terrace with loess covering permeable alluvium) and 306.0 mm and 318.5 mm for the Bu Li village experimental site (loess tableland), respectively. The presence of underlying highly permeable strata inhibits the in situ collapsibility characteristics of the loess at the third terrace site. Along the project route, the third terrace of the Wei River and the loess tableland can be assessed as non-self-weight and self-weight collapsible sites, respectively, for which relevant foundation treatment methods can be suggested to alleviate the collapse settlement of the foundation of the project.

The Ma Lan loess is a typical structural soil due to its water sensitivity, collapsibility, and seismic vulnerability. In this study, Ma Lan loess samples were extracted from 17 sites dist...

The Qiyangou landslide occurred on July 182,010 in the Qinba Mountains of China and resulted in 29 deaths. The landslide or slope failure took place in carbonaceous slate of the upper Ordovician and lower Silurian Banjiuguan Group. A fault coincided with the location of the eastern scarp of the Qiyangou landslide. To study the geological and tectonic characteristics of the landslide, detailed field investigation, terrestrial laser scanner (TLS) survey and the quartz C-axis fabric analysis were conducted. Three-dimensional distinct element modeling was used to determine the potential slope failure mechanisms. The results demonstrated that the mechanism and stability of the slope changed as the internal discontinuity sets changed. The failure initiated on the eastern side along the fault plane and propagated along other pre-existing discontinuities to discontinuity set 1 (DS1). The shear strength of the overall block stability was contributed by the DS1. Based on the results of this study, we postulate that the wedge-planar failure was aided by the pre-existing fault.

The parameter b of the unified strength theory was set to depict the contribution of the intermediate principal stress on the failure of materials. A large number of true triaxial tests were carried out to determine the values and variation in the parameter b for loess, with which the effect of b on the estimation of the stability for a typical loess slope was then studied. The unified strength theory could be applied to fit the experimental failure loci of loess better than the traditional strength theories. The value of the parameter b greatly depends on the stratum age, remolding state, dry unit mass (dr) and water content (w) of the loess, and the appropriate value decreases with increasing dr and w. The value of b for the undisturbed Malan loess (L1) is much greater than that for the Lishi loess (L2). Comprehensively, the range of b = 0.2–0.5 is recommended to obtain the empirical value of the loess, with which the factor of stability (FOS) for the loess slope can be estimated with a 5–15% increase compared to that when applying the Mohr–Coulomb failure law.

Tremendous losses and threaten traffic safety were caused by ground fissures as it covers over half of the area of Xi’an, in China. This study summarizes the formation of ground fissures and found that variations in the dip angles and cross angles influence road deformation and cracking. Based on survey data, numerical simulation using MSC.MARC finite element software was conducted to analyze the impact of ground fissures on roads. The results show that the cross angle has a negative correlation with the deformation range and a positive correlation with the tensile and compressive stresses, plastic strain and shear stress. The dip angle has a negative correlation with the maximum tensile stress and plastic strain and a positive correlation with the vertical displacement of pavement, but it has little effect on the maximum compressive stress. It is recommended to take some engineering measures for prevention and treatment when road projects cross a ground fissure, such as avoiding ground fissures, changing the road structure, and establishing a monitoring system. These conclusions can serve as a reference for structural design and preventative measures for roads passing through active ground fissure zones.

The unified strength theory can account for the influence of intermediate principal stress using a specified parameter b, with which the determination of the values and influences of b is significant. In this research, a physical model test was carried out in combination with the numerical simulations to explore the effect of b on a sand foundation loaded by a shallow strip footing. Variation of parameter b would produce significant effect on the deformation characteristics, stress response, failure model, as well as bearing capacity of the sand foundation. In general, a larger b could be adopted to yield smaller magnitudes of stress. The calculated ultimate bearing capacity Pult increases linearly with increase in b, and an increment of 470% can be obtained using b = 1.0 relative to b = 0.0. The comparison of the bearing capacities of physical model test and numerical simulation suggests that b = 0.89–0.99 is the appropriate value for the experimental sand foundation, that ultimately results in a capacity increase in 420%–465% relative to the result of Mohr–Coulomb failure law.

Loess is a special geotechnical material with strong structural properties, and the microstructural characteristics of loess significantly influence its macroscopic physical features, mechanical properties, and catastrophic behavior. In this paper, serial samples were extracted from the continuous loess and paleosol strata of the Bai Lu tableland; with these samples, the optical microscopy-based serial sectioning method was adopted to study the quantitative characterization and variation in the loess microstructure. Three-dimensional characteristics and quantitative parameters of the particles, pores, and throats of the loess were obtained. The results indicate that the volume, Eq-Radius, and major-minor axis ratio of the loess particles satisfy third-order, third-order, and second-order Gaussian distributions, respectively. The Eq-Radius of the loess pores and throats satisfies a first-order Gaussian distribution, and the throat channel length satisfies a gamma distribution. With increasing stratum depth, the particles become more flattened, the throat radii become larger and the pore channels become slenderer. The variation in fitting parameters and the correlations between the macrophysical and mechanical properties of loess were then explored. The study of the microstructure of loess contributes to a better understanding of the catastrophic behavior of loess and the physical mechanism of geologic hazards in this area.

Triaxial experiments were performed to study the mechanical behavior of fissured loess. The experimental results showed that confining pressure and fissure dip each yield influence on the mechanical characteristics of loess. As the confining pressure increased, the peak strength of fissured loess was found to consistently increased; conversely, the increasing trend differed for samples with various fissure dips. Additionally, the stress–strain relationship of fissured loess was analyzed via a double parameter binary-medium model. This model was designed based on the homogenization theory and regarded fissured loess as a composite material comprising loess matrix elements and fissure elements. The Duncan–Chang hyperbolic model and modified Cam-Clay model were employed to describe the stress–strain relationship of loess matrix elements and fissure elements, respectively. The fitting results demonstrate that this model is suitable for modeling fissure development in loess.

The unified strength theory takes into consideration the effects of intermediate principal stress, and a series of yield surfaces can be determined using various values of parameter b which reflects the effects of the intermediate principal stress. The lower bound (obtained using b = 0.0) is the widely used Mohr–Coulomb failure law, and the upper bound (obtained using b = 1.0) is generalized as the twin-shear strength theory. In this study, a combination of a physical model experiment and numerical simulations using unified strength theory is used to analyze the influence of b on the failure and stability of a loess slope. An analysis shows that the size of the computed failure zone decreases noticeably with an increase in b. In contrast, the factor of safety (FOS) of the slope increases linearly with an increase in b, and an increase of 23–25% in the FOS can be obtained using b = 1.0 as compared to that for b = 0.0. A comparison of the physical model experiment and simulation shows that the range of b = 0.25–0.50 is valid for determining the failure characteristics and stability of the experimental loess slope.

The unified strength theory is one of the most suitable theories to study the influence of the intermediate principle stress, of which the influence could be considered using a parameter b. A physical model experiment and numerical simulation were conducted to comparatively study such influences for a loess foundation supporting a strip footing, and the proper value of b for the foundation was determined. The value of b has a significant influence on the calculation of the stress distribution, bearing capacity, and failure characteristics of the foundation. The calculated bearing capacity increased linearly with increasing b, and an improvement factor of 60–100% could be obtained by using b = 1.0 over using b = 0.0. Based on the comparison, the authors found that b = 0.17–0.20 resulted in a valid fit for the studied loess foundation, which provided an improvement in the bearing capacity of 15–18% over what was calculated using the Mohr–Coulomb failure law.

Groundwater is the essential and predominant source of drinking and irrigation water supplies in most arid and semiarid regions, particularly in Loess Plateau of China. The threat to groundwater quality by nitrate pollution has always been of considerable concern in loess regions. In this paper, correspondence analysis, stoichiometric analysis, ratios of major ions and some qualitative analysis are employed to discuss the spatial variability of nitrate (NO3 −), identification of NO3 − pollution sources and its potential pollution paths based on 112 water samples in Tongchuan region on the middle edge of Loess Plateau of China. Results indicate that the NO3 − concentrations ranged from 0.25 to 173 mg/L, 20.54% of water samples exceeded the Standards for Drinking Water Quality from China (≤44 mg/L), and 15.18% of samples exceeded the Guidelines for Drinking Water Quality from World Health Organization (≤50 mg/L); the water samples with high NO3 − concentrations primarily concentrated in river valleys, and the types were mainly pore waters and individual surface waters. Sewage or manure from livestock effluent was the primary sources of NO3 − in some surface waters, pore water samples with high NO3 − were closely related to agricultural input or a mixing process. Groundwater recharges from surface water and vertical infiltration in river valleys with shallow groundwater depth were considered as the potential pollution paths for NO3 − ions in the study area based on nitrate nitrogen distribution in soil profile. Driven by concentration difference and local seepage system, these NO3 − ions diffused around and groundwater buried in great depth was consequently polluted. Therefore, river valleys should be listed as the prior protection areas in loess regions; in addition, the time lag between land use changes and groundwater quality response should also be paid attention to in nitrate pollution prevention, particularly in the thick unsaturated zones.