Loess Structure Research Articles

Influential Factors on the Stability of Building Slope

Based on the loess structure and topography of northern Shaanxi area, the construction slope model is built. The numerical simulation method is used to analyze the slope stability coefficient under conditions that the self-weight load, slope ratio and the distance from building to slope. The results are analyzed using soil mechanics. The results show that the construction load, slope ratio and d value have influences on the slope stability. The development trend of the slope stability coefficient with the change of the load depends mainly on the value of d. When the d value is less than 15m, the trend is rapid and continues to decrease; when the d value is 20m, the trend of the stability coefficient is gentle, and the stability coefficient increases with the increase of the load. The influential factors of the slope stability coefficient are different in different stages. The larger influential factors in the stage A is the building load, and the larger influential factor in the stage B is the slope ratio.

Analysis of retrogressive loess flowslides in Heifangtai, China

The Heifangtai terrace is located in the arid loess area in northwestern China, where a large number of landslides was induced by agricultural irrigation on the terrace, resulted in slope retreat at the terrace edge at an estimated rate of 0.024km2yr−1. On the basis of field investigation and monitoring, remote sensing and aerial images, and laboratory tests, we analyzed the characteristics of slope retreat and the failure mechanism of the loess flowslides. The retreat of slope is caused by the retrogressive behavior of the loess flowslide, of which new failure develops at the backwall of the preceding one. The groundwater level variation governs the failure process and mechanism of the flowslides. The observed loess flowslides were accompanied by rapid slope deformation, high velocity, and long runout distance exhibiting liquefaction and fluidization during the movement. In the event of a loess flowslide, a small-scale drained slope failure produces a concave recession first intensifies localized groundwater flow on the slope surface while the flow is being impeded by the deposit, which produces an undrained condition with increasing localized pore-water pressure, and in turn facilitates the subsequent flowslide. Results of the isotropically consolidated undrained triaxial tests (ICU) and constant shear stress drained triaxial tests (CSD) revealed the strain-softening behavior in the saturated reconstituted loess, and they showed that the excess pore-water pressure is associated with a small axial displacement and possibly facilitated by some alternations in the loess structure at the micro-scale.

Spatial variation of saturated hydraulic conductivity of a loess slope in the South Jingyang Plateau, China

Water infiltration in loess soils is an important trigger causing landslides, in which the hydraulic conductivity of the loess plays an important role. By analyzing a soil trench and soil samples from the South Jingyang Plateau, northwest China, a loess slope was investigated. A horizontal adit in a Lishi loess layer was also analyzed. Using variable head permeability tests the saturated horizontal hydraulic conductivity (Kh) and saturated vertical hydraulic conductivity (Kv) of the loess samples were measured. Results showed that the majority of the Kh values are larger than the Kv values, indicating that the loess exhibits permeability anisotropy. Lilliefors test was used to determine the normality of the Kh and Kv results. Test results indicate that Kh and Kv measurements of the loess samples collected in the trench and adit all show log-normal distribution, while a number of the Kh and Kv values also reflect a normal distribution. Geo-statistical analysis shows that the spherical model and the Gaussian model provide the best fit variograms of Kh and Kv values along the trench; analysis along the adit indicated the opposite results. The variogram range values suggest that the spatial correlation of loess Kv is greater in the vertical direction in a horizontal plane, while the spatial correlation of Kh in the adit is greater than that in the vertical direction. Loess structure rather than randomness decides the spatial variation of Kh and Kv measurements (except for Kh variation along the adit which depends on both structural factor and randomness). Kh and Kv values of loess tend to decrease with depth; results which can be described by an exponential decay model.

RETRACTED: Ground Settlement in Urban Structures Exposed to Geo-Environmental and Anthropic Hazards: A Case Study for Galati City

A case study including a detailed ground settlement analysis is made for a particular loess structure located in the city of Galati, Romania, in order to assess the interdependence of geo-environmental and anthropic settlement triggering factors in the urban system context. In the first part of the study, loess behavior and its effects on built environment are analyzed and discussed based on worldwide historic expertise and current issues. An integrative analysis of loess deposits in natural state, with reference to induced-hydraulic and stress conditions, which was performed by laboratory and in-situ investigations, is then reported. An assessment of basic geotechnical parameters, as well as small-scale modeling of loess settlements at wetting under self-weight were conducted in order to select the most representative experimental area, from the point of view of specific soil structure behavior. Field monitoring surveys were performed to obtain information for quantitative evaluation of settlement time-dependent evolution, by the simulation of excessive moisture. The processing and analysis of experimental data gathered for natural and improved ground conditions revealed that the settlement amplitude of loess deposits with high sensitivity to wetting under self-load can be significantly reduced by the selection of adequate consolidation solutions, based on realistic and undisturbed environmental conditions. The study of soil structure behavior under simultaneous settlement-triggering factors can be integrated in multi-hazard analysis as a support for efficient strategies and mitigation measures, with applicability in the urbanization process, building and foundation design and environmental protection.

A review of shear and tensile strengths of the Malan Loess in China

Loess deposits cover about 640,000km2 of China, equivalent to about 6.7% of the total area of the country. The Loess Plateau alone covers approximately 317,000km2, making it the largest bulk accumulation of loess on the Earth. Land instability hazards in the Loess Plateau are frequent and varied, among which the cracking-sliding (Beng-hua, in Chinese) failure is a typical mode. Cracking-sliding failures commonly occur in the Malan Loess at volumes of the order of 100m3 and cause more than 100 deaths per year. As the tension in the upper part of the failure plane and shearing/sliding along the lower part mobilize cracking-sliding failures, this failure mode demands an investigation into the tensile and shear strengths. This paper presents a review of these properties based on a large collection of literature. Both tensile and shear strengths of the Malan loess decrease with water content and increase with dry density, resulting in close multi-regression correlations. Large differences between undisturbed and disturbed tensile strengths of the loess indicate the significance of its structure. Shear strength of the loess is direction dependent, indicating strong anisotropy of the loess structure. Test method, sample size and loading rate all influence the measured tensile strength.

Small-strain stiffness and damping of Lanzhou loess

This study examines the small-strain stress-strain properties during cyclic loading of Lanzhou loess deposits, characterized by a very high void ratio (>0.9). Torsional resonant column (RC) tests are performed in order to investigate the effect of water content, confining pressure, consolidation time and soil structure strength on the small-strain stiffness (G0) and damping (Dmin) of loess. It is proved that shear modulus G0 at low strain levels is very sensitive to water content while the effect of water content on small-strain damping Dmin is relatively small. As water content W approaches the plastic limit PL, there is a dramatic decrease of G0 compared to drier loess specimen; G0 has the tendency to reach a stable or constant value with further increase of water content which is more than the PL of loess. The small strain damping Dmin decreases continuously with water content for unsaturated loess and then tends to increase slightly for almost fully saturated loess. A linear relationship between G0 and confining pressure σ′m in log-log coordinates for different water contents is observed for Lanzhou loess. Similarly, Dmin decreases linearly with σ′m. A power function is proposed to describe the relationship between G0 and confining pressure σ′m where the exponent m varies with the water content of loess. G0 is also affected by the consolidation duration and a power function is proposed between normalized G0 and normalized consolidation time. G0 is finally strongly affected by the structure of loess and the type of specimen. For undisturbed loess samples, G0 is always higher than reconstituted ones and the difference decreases with the degree of saturation; for fully saturated samples the influence of the structure strength between undisturbed and reconstituted samples becomes insignificant. Likewise, Dmin of undisturbed loess is higher than that for remolded loess under the same confining pressure and the difference between them reduces with the degree of saturation as well.

CRYOGENIC PROCESSES IN LOESS

This paper presents a new approach to the analysis of the genetic nature of the mineral substance of loessial rocks. At the present time, the prevailing view on this issue is the eolian accumulation of loess, while the influence of other factors of formation has not been practically taken into account. However, loess accumulation can be explained by other mechanisms, e.g., active processes of cryogenic weathering under a very harsh climate. The latter concept is based on the results of analysis of wedge-shaped structures in loess thickness, as well as numerous data of spore-pollen, microfaunistic, and other types of analysis. Further developing concepts of loess formation, the authors made an attempt to assess the degree of influence of cryogenic processes on the composition and structure of loess. The proposed method is based on a differentiated analysis of the distribution of the main rock-forming minerals (quartz and feldspars) along the granulometric spectrum. Two criteria are proposed − the coefficient of cryogenic contrast and the heavy fraction coefficient (i.e., the coefficient of distribution of heavy minerals) − which allow determining the degree of participation of cryogenic processes, as well as aeolian and aqueous sedimentation, in the formation of loessial rocks. This method was used to study two sections of loessial thickness − in the south of the Russian Plain and within the Loess Plateau of China. The results of the study revealed the role of cryogenic factors in the formation of the composition of the loess horizons of soil-loess sequences of different territories. Particularly clearly the effect of cryogenesis was manifested in the loess section in the south of the Russian Plain. In the section of the Loess Plateau, only the youngest deposits of the last formation stage are affected by cryogenesis. It follows that not only within the long-term periglacial permafrost zone, but also under the conditions of seasonal freezing in the Pleistocene, the processes of cryogenic transformation of deposits could have developed, which contributed to the formation of the composition and properties of loess of sufficiently high thickness.

Ground Settlement in Urban Structures Exposed to Geo-environmental and Anthropic Hazards: A Case Study for Galati

A case study including a detailed ground settlement analysis is made for a particular loess structure located in the city of Galati, Romania, in order to assess the interdependence of geo-environmental and anthropic settlement triggering factors in the urban system context. In the first part of the study, loess behavior and its effects on built environment are analyzed and discussed based on worldwide historic expertise and current issues. An integrative analysis of loess deposits in natural state, with reference to induced-hydraulic and stress conditions, which was performed by laboratory and in-situ investigations, is then reported. An assessment of basic geotechnical parameters, as well as small-scale modeling of loess settlements at wetting under self-weight were conducted in order to select the most representative experimental area, from the point of view of specific soil structure behavior. Field monitoring surveys were performed to obtain information for quantitative evaluation of settlement time-dependent evolution, by the simulation of excessive moisture. The processing and analysis of experimental data gathered for natural and improved ground conditions revealed that the settlement amplitude of loess deposits with high sensitivity to wetting under self-load can be significantly reduced by the selection of adequate consolidation solutions, based on realistic and undisturbed environmental conditions. The study of soil structure behavior under simultaneous settlement-triggering factors can be integrated in multi-hazard analysis as a support for efficient strategies and mitigation measures, with applicability in the urbanization process, building and foundation design and environmental protection.

Spatial characteristics and controlling factors of chemical weathering of loess in the dry season in the middle Loess Plateau, China

AbstractHydrochemistry methods were used to decipher the weathering and geochemical processes controlling solute acquisition of river waters in the dry season in the middle Loess Plateau (MLP), one of the most severely eroded areas and turbid riverine systems in the world. River waters were neutral to slightly alkaline with pH varying from 7.6 to 9.6. The total dissolved solids decreased from northwest to southeast with a mean value of 804 mg/l, much higher than the global average and other large rivers in China. Ternary diagram showed that river waters were dominated by Na+, HCO3−, and Cl− with the main water‐type of HCO3−–Cl−–Na+. Saturation index values, Mg2+, Ca2+, and HCO3− analyses indicated the preferential Ca2+ removal by calcite precipitation. Gibbs plots and stoichiometry plots indicated that the dissolved solutes were mainly derived from rock weathering with minor anthropogenic and atmospheric inputs. Samples in the northwestern basin are also influenced by evaporation. A forward model of mass budget calculation showed that, owing to high soluble characteristics, evaporite dissolution was a major feature of river waters and contributed 41% to the total dissolved cations on average, while carbonate and silicate weathering contributed 28%,and 25% on average, respectively. Besides evaporite dissolution, cation exchange is also responsible for the high concentrations of Na+ in river water. Spatial variations showed that evaporite dissolution and silicate weathering were higher in the northern basin, whereas carbonate weathering was higher in the southern basin. Different from most rivers in the world, the physical erosion rates (varying from 117.7 to 4116.6 t/km2y) are much higher than the chemical weathering rates (varying from 3.54 to 6.76 t/km2y) in the MLP because of the loose structure of loess and poor vegetation in the basin. In the future, studies on comparison of water geochemistry in different seasons and on influence of different types of land use and soil salinization on water geochemistry, denudation rates, and water quality should be strengthened in the MLP. These results shed some lights on processes responsible for modern loess weathering and also indicate the importance of time‐series sampling strategy for river water chemistry. Copyright © 2016 John Wiley & Sons, Ltd.

Feasibility Study of Loess Stabilization with Fly Ash–Based Geopolymer

Loess, which primarily consists of wind-transported silt and clay particles, is widely distributed around the world, including the central Asia, central Europe, the northwestern and central United States, Alaska, and South America. The metastable structure of natural loess has resulted in construction delays and catastrophic failures. To explore the potential applications of soil improvement with green materials, this paper presents an experimental study of applying fly ash–based geopolymers for the loess stabilization. Two different precursors are employed to the investigation. It is found that potassium hydroxide renders a higher unconfined compressive strength than sodium hydroxide to the samples when the same fly ash/loess ratio is applied. This agrees with the results of the mechanical properties of the two different geopolymers. With an increasing fly ash/loess ratio, the compressive strength and Young’s modulus increase. The microstructural characterization unveils that a compact and stable microstructure has been developed in the stabilized loess. With the aid of energy dispersive X-ray spectroscopy and X-ray diffraction, it is observed that it is the binding effect of geopolymer gel that contributes to the improvements in the mechanical properties of stabilized loess samples.

Loess: Rock, sediment or soil – What is missing for its definition?

Loess is commonly defined as an accumulation of windblown silt. However, the complex mechanisms that are responsible for most of the structural characteristics of loess require a more precise explanation. The common definition of loess ignores a set of processes that start during and after the subaerial deposition of silt. The term loessification has been used by a number of authors to refer to the quasi-pedogenic/quasi-diagenetic processes that result in the typical aggregation of loess; however these mechanisms are rarely described in detail. Depending on the researcher's background, loess is classified as sediment, soil or rock.This review gives an overview on loess definitions through time and evaluates the main concepts related to loess formation. Several gaps of knowledge are identified that require a number of specific studies related to different aspects of loess formation in various environments. We propose to 1) differentiate primary loessification which initializes the formation of loess structure from secondary loessification which takes place subsequently and 2) define loess-like sediments as deposits that experienced loessification but were not transported aeolian. Mainly aeolian processes and loessification are of equal importance to define loess.

ENGINEERING-GEOLOGICAL CONDITIONS OF DNIEPER VALLEY ON AREA OF DNIEPER WATER-STORAGE BASIN

The features of engineer-geological conditions on a site of break of Dnieper through the Ukrainian shield are considered. This territory tested anthropogenic influence since olden times. Most changes happened after building of a dam of multifunction Dnieper water-storage basin. The problem of navigation was decided at flooding of the Dnieper rapids. The engineer-geological problems of adjoining to this area of valley territory are also related to distribution of grounds of loess structure (periglyacial area of the Dnieper glacier).

Hydrocollapse of Semi-arid Soils

Problematic soils are those that make the construction of structures extremely difficult. They include collapsible soils which are widely distributed in North America, Europe, Russia and Asia. They show low dry density and moisture content in all testing schedule due to presence of cementation bonds or loess structure especially at unsaturated state in arid or semi-arid regions. Therefore, conditions in arid and semi-arid climates favor the formation of the most problematic collapsible soils. Arid and semi-arid soils are mainly found in the Middle east (Arabian desert). The basic objectives of this study are to describe the hydrocollapse properties and characteristics of semi-arid soils of Najran (Saudi Arabia), it also investigates the collapsibility characteristics and behavior of the materials with particular reference to their response to wetting under different stress levels. Undisturbed soil samples from Najran area situated in the south of Saudi Arabia have been collected and studied by using both single and double Oedometer tests. The results show that the highest value of collapse potential (CP = 12.76 %) of Najran soil is given by double Oedometer tests for a depth of 1.5 m which represents a very severe trouble.

Reliable Design and Study on Highway Tunnel Lining Structure

The article proposed the reliability calculation method for tunnel lining design combining Rock-Mechanics model and Monte Carlo finite element method.The reliability calculation model of tunnel lining structure design was established considering tunnel character. The types of the statistical characteristics and distribution of axial force for the big cross section of the loess tunnel structure had been summarized. The study result stated that the method is feasible. It has important theoretical guiding significance to the practice structure reliability design of tunnel and underground engineering.

Humidification vibration deformation behavior of intact loess

A parameter, known as the parameter of humidification vibration deformation, was proposed, describing quantitatively the impact of water content on vibration settlement deformation, and its relationship with humidification water content, dynamic shear stress peak value, initial consolidation stress and vibration frequency was built. The result shows that 1) the parameter of humidification vibration deformation increases with the vibration shear stress peak value increasing. 2) The humidification water content has significant influence on the curve of the parameter of humidification vibration deformation and the peak vibration shear stress. When the humidification water content is low, the curve increases slowly. However, when the humidification water content is high, the curve increases rapidly. 3) Initial consolidation stress has significant influence on the humidification vibration deformation coefficient. When initial consolidation stress is not large enough to destroy the loess structure, with initial consolidation stress increasing, the humidification vibration deformation coefficient decreases. On the contrary, the humidification vibration deformation coefficient increases with initial consolidation stress increasing. 4) With the increase of vibration time, the parameter of humidification vibration settlement shows an increasing trend overall. The initial dynamic shear stress peak value and humidification water content all have significant effects on the curve of the parameter of humidification vibration settlement and vibration time. However, the humidification water content is even more significant.

Intact Loess Structural Loss Parameter in Humidification Process

Based on soil structural view, the mechanism of loess structure is analyzed in humidification process. The concept of the loess structure loss is proposed.And quantitatively parameter described the loess structure loss is set up, that is structure loss parameter. And then the influence laws of humidification and loading on loess structure loss are discussed. The results show that structure loss parameters is increasing with the increase of the humidification water content. The moistening collapsibility has a good linear relationship with loess structure loss parameters.

Study on Dynamic Strength Characteristics of Malan Loess

Vibration effect is a major inducing factor of loess slope hazards and the basis of revealing loess slope mechanism, which often changes loess structure and decreases loess strength. Based on the GDS Lab System, the dynamic triaxial test was performed by using equivalent sine wave under strain controlling. The results are described as follows: ① there is an obvious phenomenon of stress relaxation during the cyclic loading test on Malan loess, and the degree of stress relaxation decreases with the increase of confining pressure and dynamic strain. Under the low confining pressure, Malan loess reaches certain strain only by small dynamic stress within 2% strain ranges. Under high confining pressure, Malan loess will also reach certain strain value by small dynamic stress within 1% strain ranges. At the same time, with the increase of confining pressure, the peak value of dynamic stress increases gradually. ②The dynamic failure stress of Malan loess increases linearly with the increase of confining pressure. The correlation coefficient is 0.99 and the linear regression equation is бd = 0.001б3 + 0.019. ③On the basis of Molar Coulomb failure criterion, the index of dynamic strength was c =3.36kPa, φ=16°. In comparison with the static strength, the internal friction angle decreases significantly while the cohesion slightly.

Duncan-Chang Nonlinear Elastic Model Considered Loess Structure

Duncan-Chang nonlinear model has been modified and applied to the structural loess calculation. Based on structural studies and conventional triaxial tests, this paper has analyzed the mechanical properties of intact loess and the relationship between the stress ratio structural parameters and the strain, then the expression of generalized shear strain and stress ratio structural parameters are given to facilitate the engineering applications. On this basis, the stress-strain curve of intact loess was corrected by the use of the stress ratio structural parameters. The form of the intact loess stress-strain curves which have been revised has changed hardening from the softening or weak softening. The results show that the modified stress-strain curves of intact loess can apply Duncan- Chang nonlinear model to calculate and the model parameters are reasonable and effective. This method provides Duncan-Chang nonlinear model which is widely used in engineering with a new ways and means in intact structural loess application.

Analysis on ROI Characteristics in CT Images of Q2 Loess under Freezing Temperature

Freeze-thaw action is one of causes to loess disaster. Loess structure and mechanical properties are quite different due to its structural model, unsaturation and salt content, which make loess freezing much complex. The samples from the site are set with different moisture content and lowered their temperature to 0°C, -10°C and -20°C respectively, and then the microscopic damage propagation and damage structural variation in the internal loess have been studied under the different freezing temperatures through CT scanning experimental technique. With zoning methods and zoning statistic frequency, the process of damage evolution has been analyzed, which provides specific guideline for researches on geotechnical engineering in loess region.

Stress-strain relationship with soil structural parameter of collapse loess

Through the tri-axial shearing tests of unsaturated intact loess and based on the concept of comprehensive soil structural potential, this paper reveals the changing laws of soil structural property under the tri-axial stress conditions and establishes a mathematical expression equation of structural parameters, whereby reflecting the effects of unsaturated loess water content, stress and strain states, which is introduced into the shearing stress and shearing strain relation to obtain the structural stress-strain relation. The tests reveal that the loess dilatancy is of shearing contraction and shearing expansion, whereby indicating that there is a good linear relation between the stress ratio and shearing expansion strain ratio. The larger consolidation confining pressure is, the larger the stress of shearing contraction and expansion critical point is; and the larger water content is, the smaller the strain ratio of shearing contraction and expansion critical point is. Finally, the constitutive model is established to reflect the variation in loess structure, stress-strain softening and hardening, and shearing contraction and shearing expansion features. Through the comparative analysis, the stress-strain curves described by the constitutive relationship are found to be in good conformity with test results, whereby testing the rationality of the model in this paper.