Collapsible Loess Research Articles

Fragipan Horizons: Definition, Properties, Genesis, and Influence on Soil Behavior

Many Missouri forest soils exhibit fragipans, which influence soil productivity, ecosystem services and land management. Fragipan bearing soils tend to occur where loess thickness is moderate (1 to 2 meters) or where the soil profile exhibits evidence of mass wasting of weathered limestone residuum. Consensus is consolidating around the self-weight collapse of loess and residuum after repeated wetting and desiccation. The use of gravel as an indicator of parent material differences and its correlation with fragipan development is not perfectly aligned, thus although most fragipans do exhibit a bisequal soil profile, the placement of the lithologic discontinuity is difficult given mass wasting, eluviation-illuviation, side slopes, and other soil processes that contribute to increasing the bulk density and conferring strength. Fragipan genesis is evolving; however, research involving Ecosystem Site Descriptions are a fusion of a land parcel’s soil properties, vegetational community, hydrology, and climate to guide land management. Ecological Site Descriptions associated with fragipan bearing soils are necessary, especially when making land management decisions.

Study on the collapsibility sensitivity of loess in different acid environment by consolidometer

Cement connection of loess is an important internal factor of loess collapse, which has a direct impact on the sensitivity of the collapse. In this study, different concentrations of nitric acid were added into the immersion water solution of loess collapsibility test. The collapsibility deformation and stability time were measured under different pressure and acid environment. The test results show that: in acid solution, the collapsibility coefficient of collapsible loess increases more under low pressure, and the collapsibility stability time is prolonged under all levels of pressure; Loess has different collapsibility sensitive pressure range in aqueous solution and acidic solution. The structural strength of collapsible loess has an important influence on the depth of collapsible loesr. It is very important to the constructions on the loess area

浅谈湿陷性黄土地基的评价及处理

浅谈湿陷性黄土地基的评价及处理

Study on the Hydro‐Thermal‐Mechanical Characteristics of the Lime‐Soil Compaction Pile in the Forming Process

To study the effect of the lime‐soil compaction pile on the collapsible loess foundation and the influence of water‐heat‐force characteristics on the soil during compaction, the field test of lime‐soil compaction pile treatment of collapsible loess foundation is carried out. This paper monitors the process of soil temperature, water, and soil pressure change in the process of pile forming. According to the macroprocess and micromechanism of lime‐soil compaction pile treatment of collapsible loess foundation, the qualitative law of water‐heat‐force change in the process of pile forming is obtained. Meanwhile, the influence of soil damage is introduced in the process of compaction expansion. The model of pore expansion under linear damage conditions in the plastic zone is established. The formula of radial stress distribution under damage conditions is given. The influence of different damage factors on the expansion process is analyzed.

Comparative study on compressive and uplift load tests of PHC pipe pile

There are 5∼8 m thick non self weight collapsible loess in the project site of a power plant. In order to study the bearing relationship between compressive and uplift piles in loess site, static load tests were carried out on PHC pipe piles with diameter of 400 mm and 500 mm and pile length of 12 m. Under the same pile top load, the displacement of uplift pile top is greater than that of compression pile top. Based on the measured load displacement curve, the uplift bearing capacity of PHC pipe pile with pile diameter of 500 mm is 32.5% higher than that of pile diameter of 400 mm. When the pile diameter is 400 mm and 500 mm respectively, the ultimate bearing capacity of PHC uplift pile is 26.24% and 27.70% of compression pile respectively. GM (1,1) model of grey system can fit and predict the load displacement curve of PHC pipe pile under compression and pull-out. Compared with the measured results,the relative error of GM (1,1) model of grey system is larger in the previous stages of load, and decreases gradually with the increase of load.

Construction Specifics of Oil Tanks with a Capacity of 20,000m3 on Large Strata of Collapsible Loess Soils

The article presents experience in compaction of a large stratum of collapsible loess soils (type II collapsibility) during the construction of 4 metal tanks with a capacity of 20,000 m3.

湿陷性黄土地区挖填方场地稳定性关键技术研究

湿陷性黄土地区挖填方场地稳定性关键技术研究

Study on the acid environment to the sensitivity of soil

In this research, the tests of collapsible sensitivity of loess in acid environment were carried out. On the basis of this experiment, this author draw on the experience of predecessors, and put forward two indexes of collapsibility sensitivity evaluation of loess and its site, which are the time to complete 90% of collapsibility and collapsibility coefficient. The method was verified by taking the site with excessive collapsibility as an example. The results show that the method can accurately reflect the collapsibility sensitivity of collapsible site, and has certain guiding significance for engineering construction on collapsible site. In addition, the method is simple, scientific, and can be combined with the current specifications. It is easy to be accepted and promoted by the industry, and is a useful supplement to the existing few researches on loess collapsibility sensitivity.

Early Warning Mechanism of Loess Collapse Based on Stochastic Petri Net: A Case Study in China

Early Warning Mechanism of Loess Collapse Based on Stochastic Petri Net: A Case Study in China

Multiscale Analysis of the Strength Deterioration of Loess under the Action of Drying and Wetting Cycles

To study the strength degradation mechanism of compacted loess during dry‐wet cycles, 0–5 dry‐wet cycles tests and many triaxial compression tests were carried out on loess with an optimal moisture content. During the dry‐wet cycles, the loess samples were analyzed by nuclear magnetic resonance and scanning electron microscopy. Studies have shown that at the macro level, with increasing numbers of wet and dry cycles and increasing cycle amplitude, the cohesive force and internal friction angle of the loess decrease, and the shear strength of the loess deteriorates significantly. At the micro level, with the number of wet and dry cycles increasing, the connection between particles changes from surface‐to‐surface contacts to point‐to‐point or point‐to‐surface contacts. The edges and corners of the particles decrease, the roundness increases, the large pores gradually decrease, the small pores gradually increase, and the fractal dimension gradually increases. In terms of microscopic view, the NMR test shows that with increasing numbers of dry‐wet cycles, the T2 peak curve increases, the curve width increases slightly, the peak area gradually increases, and the porosity increases. From the macroscopic, mesoscopic, and microscopic multiscale analysis, the structure of loess is degraded under the action of dry and wet cycles; the strength of the loess is degraded significantly after 0 to 3 cycles and then gradually stabilizes. These research results can provide a certain reference value for the management of loess collapse geological disasters in semiarid climates.

The microscopic mechanisms of treating collapsible loess with vibratory probe compaction method

Although satisfactory treatment effect was achieved from a recent foundation reinforcement practice, where a foundation composed of collapsible loess was treated with the vibratory probe compaction method in northwest China, the mechanisms of the reinforcement was unclear. To reveal the reinforcement mechanism, a series of experimental tests, including scanning electron microscopy, mercury intrusion porosimetry, and X-ray diffraction, were carried out on loess samples before and after the treatment. A total of 600 particles were identified, and the particle diameter, perimeter, sphericity, directionality as well as the pore distribution were analyzed from a quantitative perspective. Based on the microscale analysis, it is indicated that the propagation of vibration caused the destruction of cementation of soil particles in the initial state and, at the same time, the size of the particles decreased while the sphericity increased. The particles of the intact samples were found to be directional, whereas the directionality was minimized by the vibratory probe compaction process. The broken small particles gradually accumulated and filled the pores of the soil skeleton under gravity, which lowered the porosity and altered the pore size distribution.

Experimental study of the failure mode and mechanism of loess fill slopes induced by rainfall

Loess fill slopes are vulnerable to heavy rainfall because of water sensitivity and collapsibility of loess. Studies on the failure mode and mechanism of loess fill slopes are limited and incomplete. In this study, a laboratory flume test is carried out to simulate the failure mode of loess fill slope by monitoring and analyzing its soil hydrological and mechanical parameters including volumetric water content, pore water pressure and horizontal earth pressure. The results show that under continuous rainfall, loess fill slope fails in a backward retrogressive failure mode including gully erosion, partial failure, slope toe failure, central slope failure and top slope failure stages. The failure mechanism of each failure stage has been explained based on the variation of slope hydrological and mechanical conditions with the rainwater infiltration. The evolution processes of the fissures and its effects on the slope failure have been investigated. It is revealed that fissures play an important role in failing the slope by generating preferential flow. Finally, some engineering measures are recommended for the prevention of loess fill slope failure.

In situ water immersion research on the formation mechanism of collapsible earth fissures

Loess collapse is an important factor in earth fissure formation, and earth fissures resulting from loess collapse are widely distributed in loess areas in China. To explore the basic features and formation mechanism of collapsible earth fissures, we conducted an in situ water immersion test in a loess area. This paper describes the genesis of collapsible earth fissures by monitoring the development processes of collapsible settlement and earth fissure formation. This paper demonstrates that the infiltration process can be divided into three stages, and the subsidence curve of the whole test process can be divided into six stages. During the test, the collapsible deformation gradually developed outward, the collapsible settlement decreased with increasing distance from the test pit, and the horizontal influence distance was approximately 18 m. The collapsible earth fissures were parallel to the test pit, and characterised by horizontal tension and vertical dislocation. With the collapsible deformation expanding outwards, the location of the earth fissure developed outwards. The tensile stress and tensile strain caused by the differential collapsible deformation caused the formation of collapsible earth fissures. When the tensile strain or tensile stress was greater than the ultimate tensile strain or tensile strength, the loess experienced tensile failure and formed an earth fissure.

Analysis of Land Subsidence Distribution and Influencing Factors in Yan'an New District Based on Time Series InSAR

Objectives The excavation and city in Yan'an New District is the largest geotechnical project in the collapsible loess gully region. Large-scale land creation, engineering construction and complex engineering geological conditions has caused a large number of uneven land subsidence, which is posing an increasing threat to the stability of urban infrastructure and public safety. Methods The ascending Sentinel-1A data stacks obtained during December 2017 to December 2018 were analyzed using the time series interferometric synthetic aperture radar (InSAR) technique, and the distribution characteristics of the post-construction land subsidence of the mountain excavation and city construction in Yan'an New District were obtained. The reliability of the detection results of InSAR technology was demonstrated by field investigation. Finally, based on the deformation monitoring results, the main influencing factors of land subsidence distribution were analyzed. Results (1) The results show that significant uneven land subsidence formed after the mountain excavation and city construction in Yan'an New District is mainly distributed in the filling area, and the maximum deformation rate reaches up to 120.1 mm/a. Field investigation validates the reliability and accuracy of the deformation results. (2)The remodeling of original loess and the change of its physical properties during the filling process is the main intrinsic factors of land subsidence in the filling area. Large-scale land creation determines the distribution of land subsidence, and the thickness of the filling are the main controlling factor of the distribution and size of land subsidence. In addition, human activities and changes in the geo-environment will also accelerate the development of land subsidence. Conclusions In order to achieve the early detection of potential geo-hazards and active risk prevention in the site selection and feasibility study stage of mega engineering projects in the Loess Plateau, the time-series InSAR technology can provide an effective technical that allows for the early detection of potential geo-hazards such as land subsidence and landslides, and provide a scientific basis for further monitoring and warning, planning and construction, geologic disaster prevention.

Interactions of landslide deposit with terrace sediments: Perspectives from velocity of deposit movement and apparent friction angle

The Loess Plateau has been deemed as a landslide-prone area in northwest China because of the unique platform geomorphology and the wetting-induced loess collapse. The interactions of landslide deposit and terrace sediment have been under-explored in the literature. This lack of research has inhibited the prevention and mitigation of loess landslide. This study summarises a total of 40 loess landslides in the South Jingyang Platform, Shaanxi Province; 4 out of the 40 loess landslides are investigated in detail, with an emphasis on the geomorphology feature and the internal geometry of geology. While the sandbox experiments and the discrete element modelling primarily aimed to reproduce the kinetic process of landslide deposit falling from the platform edge and colliding with terrace sediments. The field observation distinguished three domains that represent varying degrees of interaction between the landslide deposit and the terrace sediments, namely push forward domain, shear up/out domain and original terrace sediment domain. The push forward domain is defined as an area completely contained the loess deposit, with most distinct surface upheaval, while the shear up/out domain is defined as an area that significantly interacts with the push forward domain and possesses remarkable evidence of interactions. The original terrace sediment is defined as an area that is thoroughly not disturbed by the interactions. The internal geometry change and the geomorphology features, induced by the interactions of the deposit with the sediments, are reproduced using the sandbox experiments and the discrete element modelling. The sediments shearing upwards and the occurrence of shear liquefaction are interpreted from perspectives of the velocity of deposit movement and the apparent friction angle. The results are deemed to be useful in enhancing our understanding about the interactions of the landslide deposit with the terrace sediments and countermeasures against loess flowslides in the study area in future.

X-ray micro-computed tomography (μ-CT) for 3D characterization of particle kinematics representing water-induced loess micro-fabric collapse

The 3D characterization of particle kinematics representing the changes in fabric observed during water-driven collapse of loess is, to the best of our knowledge, performed for the first time at the micron scale. An apparatus is specially designed to perform collapse tests on loess specimens of several millimeters in size and to capture, using X-ray tomography, particle-scale microfabric features of the sample in initial, loaded and flooded (i.e. collapsed) states. Although similar approaches have been used to assess problems such as microstructural evolution in sand, the application of these methods to assess loess collapse behaviour at the micron scale constitutes the novelty of this work. Individual particles within the specimen in the initial and deformed configurations are identified and tracked through an iterative segmentation technique and the particle tracking method ‘ID-track’. This allowed determination of particle displacement and rotation. The displacement field within the collapsed sample is found to be less uniform compared to that within the unwetted sample under loading. Coefficient of heterogeneity is defined to quantify the level of non-uniformity of the particle displacements within the deformed sample, revealing higher heterogeneity in the deformation of the collapsed sample compared to that of the unwetted sample under loading. It is also found that particles tend to rotate around axes perpendicular to, rather than parallel to, the direction of major principal stress, and the rotation magnitudes appears to be controlled largely by the size of particles while their morphology plays a minor role. The influence of porosity on the collapse process is quantified significant heterogeneous volumetric strains are observed at the single particle scale. It is also shown that the evolution of particle-to-particle contacts is much more complex than previously stated. The micron-scale investigations of individual particle kinematics following loading and wetting offer exciting new avenues for visualization and an enhanced capability for quantification of loess collapse processes.

Replacement Construction Technology of Collapsible Loess Subgrade in Typical Subtropical Humid Climate Area

Aiming at the problem of insufficient bearing capacity of collapsible loess in highway subgrade engineering, taking the second phase of KKH project in Pakistan as an example, Based on the analysis of the physical and mechanical properties of collapsible loess in this area and the actual situation of the project, the most effective construction technology is explored. The construction technology of replacing natural gravel is analyzed, and summarizes the actual operation of the construction methods, to provide some theoretical basis and construction reference for related projects. The results show that the design and construction requirements can only be met under the condition of strict implementation of the norms and standards, combined with the actual situation of the project, and the most effective construction technology explored can be applied to the construction of highway subgrade.

The application of bio-cementation for improvement in collapsibility of loess

Bio-cementation is currently appraised to solidify sandy soils, but only few studies use it to cement loess soil particles. The effects of the addition of loess soils on bacterial growth, urease activity, and productive rates for calcium carbonate were studied. Moreover, bio-cementation tests were conducted to improve the collapsibility of loess soils. The results showed that adding increased loess soils increased the pH of supernatant, while the addition of loess soils slightly affected the growth and urease activity of Sporosarcina pasteurii. The productive rates for calcium carbonate in samples treated with calcium chloride and calcium acetate are larger than those treated with calcium nitrate. The optimum concentrations of calcium ions and urea in the cementation solution for bio-cementation tests were 0.75 M and 1.0 M, respectively. The coefficients of collapsibility of samples with various initial densities all decreased via bio-cementation, but the decreasing ratios were different. The productive rates for calcium carbonate were larger for the samples with larger densities. The coefficients of collapsibility decreased with the increase in treatment cycles. Moreover, there were larger increasing ranges of productive rates for precipitation from 4 to 6 cycles. Adding bacterial suspension and cementation solution together enabled better treatment effects for the samples with smaller densities, however, while adding them separately was more suitable for the samples with larger densities.

Improvement of Physical-Chemical and Rheological Properties of Ghardaïa Loess (Southern Algeria) Using Bentonite Clay and Lime

AbstractLoess is a collapsible soil; when it collapses, it can cause significant damage to structures built on it. Improvement in the stability and strength performance of loess is necessary to meet engineering needs. In the present study, the effects on the physical-chemical and rheological characteristics of Ghardaïa loess of adding bentonite and lime (southern Algeria) were examined. Rheological characterization of suspensions was implemented to assess the mechanical sensitivity of the bonds and the structural inter-particle resistance to both the chemical effect and mechanical impact. By analyzing the viscosity results and the evolution of the rheological parameters, the improvements needed in terms of the resistance characteristics of the loess-bentonite and loess-lime mixtures were evaluated and confirmed. The loess physical sensitivity was examined through grain-size distribution and plasticity properties. The pH and electrical conductivity of the mixtures were also used to explore structural modifications. Physical test results showed that introduction of the additives changed the loess texture and improved the plasticity of mixtures. Chemical examination (via change in pH and electrical conductivity) revealed the structural changes in the mixtures studied. Rheological test results showed that increasing concentrations of bentonite and lime improves the mechanical strength and increased the yield stress, consistency, and viscosity of the suspensions. The creation of cement interactions between mixture particles explained the increase in those parameters. Hydration, agglomeration, and inter-particle flocculation induced by the additives promoted these interactions. The experimental results led to the conclusion that bentonite and lime may represent an effective means to improve the performance in terms of preventing loess collapse and to increase its resistance to mechanical impact. The results presented in the present study may provide a geotechnical and rheological working database for the control and treatment of loess collapse and landslides in the region under study. Technical data related to loess may, therefore, be beneficial in terms of civil engineering, public works, hydraulics, and the manufacture of construction materials.

Removal of nitrogen from rainwater runoff by bioretention cells filled with modified collapsible loess

The construction of bioretention facilities in collapsible loess areas faces some problems such as low infiltration rate of rainwater runoff and system blockage, which seriously affect the treatment effect of bioretention cell on rainwater runoff. According to the different filler matrix, three bioretention cells with improved filler were constructed, the 1# bioretention system was filled with 50% of the construction waste mixed with 50% loess, and the 2# bioretention system was filled with 50% concrete sand mixed with 50% loess, the 3# bioretention system was filled with 45% concrete sand +10% wood chips +45% loess mixed in the bioretention cell. The results show that the removal rate of NH4+-N is the highest when the height of the inundation zone of the bioretention cell is 600 mm (more than 98%). The change of hydraulic load and submerged zone height had a significant effect on the removal rate of NO3−-N in bioretention cell (P < 0.05). The removal rate of NO3−-N in the bioretention cell decreased with the increase of hydraulic load, and the bioretention cell had the highest removal rate of NO3−-N (48.85%) under low hydraulic load conditions. The removal rate of NO3−-N in the bioretention cell increased with the increase of the height of the inundation area, and the removal rate of NO3−-N was the highest when the inundation area height of the bioretention cell was set at 600 mm (98.33%). The improved bioretention cell has a better effect on NH4+-N treatment during the wet period, and the NH4+-N concentration of the effluent is below 0.4 mg/L. The NO3−-N concentration in the effluent of the three improved bioretention cells during the wet period differed greatly. Among them, the bioretention cells using sawdust modified fillers had the highest removal rate of NO3−-N. The collapse amount of the three improved bioretention cells was below 0.5 cm, and the permeability coefficients were above 2 cm/h. The experiment proves that the three methods to improve the collapsible loess filler can be applied to the construction of the bioretention cell in the collapsible loess area so that the bioretention technology can be effectively promoted in the loess area. This experiment has great significance to the construction of bioretention cells in the loess area.