Influences of soil moisture and salt content on loess shear strength in the Xining Basin,northeastern Qinghai-Tibet Plateau

Moisture and salt content of soil are the two predominant factors influencing its shear strength. This study aims to investigate the effects of these two factors on shear strength behavior of loess in the Xining Basin of Northeast Qinghai-Tibet Plateau, where such geological hazards as soil erosion, landslides collapse and debris flows are widespread due to the highly erodible loess. Salinized loess soil collected from the test site was desalinized through salt-leaching in the laboratory. The desalinized and oven-dried loess samples were also artificially moisturized and salinized in order to examine how soil salinity affects its shear strength at different moisture levels. Soil samples prepared in different ways(moisturizing, salt-leaching, and salinized) were measured to determine soil cohesion and internal friction angle. The results show that salt-leaching up to 18 rounds almost completely removed the salt content and considerably changed the physical components of loess, but the soil type remained unchanged. As salt content increases from 0.00% to 12.00%, both the cohesion and internal friction angle exhibit an initial decrease and then increase with salt content. As moisture content is 12.00%, the salt content threshold value for both cohesion and internal friction angle is identified as 3.00%. As the moisture content rises to 16.0% and 20.00%, the salt content threshold value for cohesion is still 6.00%, but 3.00% for internal friction angle. At these thresholds soil shear strength is the lowest, below which it is inversely related to soil salinity. Beyond the thresholds, however, the relationship is positive. Dissimilar to salinity, soil moisture content exerts an adverse effect on shear strength of loess. The findings of this study can provide a valuable guidance on stabilizing the engineering properties of salinized loess to prevent slope failures during heavy rainfall events.     

Inversion analysis of the shear strength parameters for a high loess slope in the limit state

It is difficult to obtain reliable shear strength parameters for the stability analysis and evaluation of high loess slopes.Hence,this paper determines slope elements and physical parameter of 79 slopes with heights of[40,120]m based on the measured loess slopes in the Ganquan and Tonghuang subregions of northern Shaanxi Province,China.In the limit state of the loess slope(stability factor Fs is 1.0),a fitting equation for the slope height and width is established.Then,the model is developed by stability analysis software-SLOPE/W,and the comprehensive shear strength parameters corresponding to different slope heights of the high slope in the study area are obtained by inversion using the Morgenstern-Price method.The results show that when the height of the slope increases,the cohesion c increases in the soil,and the internal friction angle j decreases.This change is consistent with the characteristics reflected in the composition and physical properties of the slopes,and the comprehensive strength parametric curves are very similar between the Ganquan subregion and the Tonghuang subregion.A landslide that occurred in Miaodian-zaitou of Jingyang County,Shaanxi Province,is selected to verify this inversion method,and the results show that the calculated shear strength parameters of the inversion are consistent with the measured value of the actual slope.     

Influence of root suction on tensile strength of Chrysopogon zizanioides roots and its implication on bioslope stabilization

Root tensile strength is an important factor controlling the performance of bio-slope stabilization works. Due to evapotranspiration and climate factors, the root moisture content and its suction can vary seasonally in practice and may not equal soil suction. The influences of suction and root moisture contents were investigated on Chrysopogon zizanioides(vetiver grass) root tensile strength. The root specimens were equilibrated with moist air in different suction conditions(0, 10, 20, and 50 kPa), prior to root tension tests. The root-water characteristic curve or relationship between root moisture and suction, was determined. The increase in suction resulted in decreased tensile strengths of the grass roots, particularly those with diameter of about 0.2 mm, which constituted 50.7% of all roots. For 1 mm roots, the tensile strength appeared to be unaffected by suction increase. The average root tensile strengths were used to estimate the root cohesion in slope stability analysis to find variation of safety factors of a bioengineered slope in different suction conditions. The analysis showed that the critical condition of slope with the lowest factor of safety would happen when the soil suction was zero and the root suction was high. Such condition may occur during a heavy rain period after a prolonged drought.     

Seasonal variations in the influence of vegetation cover on soil water on the loess hillslope

Soil water is the key factor that restricts the restoration of the local ecological systems in the Loess Plateau of China. Studying the effects of vegetation types on soil water and its seasonal variation helps to understand hydrological characteristics and provides insights into the sustainable restoration of vegetation. Therefore, the Caijiachuan watershed was chosen as the research object to investigate the water status of a 0-10 m soil layer under different vegetation types including Pinus tabulaeformis, Robinia pseudoacacia, Platycladus orientalis, apple orchard, natural forestland,farmland and grassland. By comparing the difference between soil water of different land use types and that of grassland during the same period, the seasonal changes of soil water status of different types were judged. The results show that(1) in the 0-10 m soil layer, the largest value of soil water content was in the0.3-0.4 m layer, and the lowest was in the 5.6-5.8 m layer. The depths at which the vegetation cover influenced the soil water were up to 10 m;(2) among summer, fall and spring, the soil water storage wasthe highest in the fall. In addition, the lowest value of relative accumulation was in the fall, which was the period in which the soil water recovered;(3) the soil water in the 0-10 m layer was in a relatively deficient state in the artificial forestlands, apple orchards and native forestlands, while the relative accumulation was in the farmland. In addition, the relative deep soil layers(8-10 m) had more serious deficits in the areas in which P. tabulaeformis, R. pseudoacacia and the apple orchard grew;(4) during the study period, the farmland in the summer had the largest relative accumulation(182.71 mm), and the land under R.pseudoacacia in the fall had the lowest relative deficit(512.20 mm). In the Loess Plateau, vegetation cover will affect the change of deep soil moisture and artificial forest will cause soil water loss in different degrees.     

Stress-strain analysis of loess triaxial shear test by PFC3D

In order to compensate for limitations of microscopic study on loess triaxial tests, taking the loess in Longxi area as an example, the authors established the loess triaxial test model by using PFC3D software and simulating tfiaxial shear test under the different confining pressures in 0 kPa, 50 kPa and 300 kPa. Compared with laboratory triaxial shear test, the numerical simulation test has a guiding role in loess mechanical strength analysis.     

Experimental analysis of shear strength of undisturbed soil in leucaena forest in Jiangjia Ravine,Yunnan, China

Five leucaena trees of similar age were chosen in Jiangjia Ravine of Dongchuan, Yunnan Province, China, near which the soil samples were collected by digging profiles 2m in depth and 1m in width. In each section, soil samples at different depths were taken for direct shear experiments to determine the root amount and mechanical composition. It is found that the cohesion and internal friction angle of the undisturbed soil are related to the root amount, depth, clay content and breccias content. Cohesion correlates negatively with root content, a finding that differs from that of other researchers. In addition, internal friction angle correlates positively with all these factors.     

Field experiments on quantifying the contributions of Coreopsis canopies and roots to controlling runoff and erosion on steep loess slopes

Grass recovery is often implemented in the loess area of China to control erosion. However, the effect mechanisms of grass cover on runoff erosion dynamics on steep loess hillslopes is still not clear.Taking the typical forage species(Coreopsis) in semiarid areas as subject, this study quantified the effects of canopies and roots on controlling slope runoff and erosion. A series of field experiments were conducted in a loess hilly region of China. Field plots(5 m length, 2 m width, 25° slope gra...     

Estimation of soil reinforcement by the roots of four post-dam prevailing grass species in the riparian zone of Three Gorges Reservoir,China

Soil erosion and bank degradation is a major post-dam concern regarding the riparian zone of the Three Gorges Reservoir. The development and succession of vegetation is a main countermeasure,especially to enhance bank stability and mitigate soil erosion by the root system. In this study, the roots of four prevailing grass species, namely, Cynodon dactylon, Hemarthria altissima, Hemarthria compressa, and Paspalum paspaloides, in the riparian zone were investigated in relation to additional soil cohesion. Roots were sampled using a single root auger. Root length density(RLD) and root area ratio(RAR) were measured by using the Win RHIZO image analysis system. Root tensile strength(TR) was performed using a manualdynamometer, and the soil reinforcement caused by the roots was estimated using the simple Wu's perpendicular model. Results showed that RLD values of the studied species ranged from 0.24 cm/cm3 to20.89 cm/cm3 at different soil layers, and RLD were significantly greater at 0–10 cm depth in comparison to the deeper soil layers(10 cm). RAR measurements revealed that on average 0.21% of the reference soil area was occupied by grass roots for all the investigated species. The measured root tensile strength was the highest for P. paspaloides(62.26MPa) followed by C. dactylon(51.49 MPa), H.compressa(50.66 MPa), and H. altissima(48.81MPa). Nevertheless, the estimated maximum root reinforcement in this investigation was 22.5 k Pa for H.altissima followed by H. compressa(21.1 k Pa), P.paspaloides(19.5 k Pa), and C. dactylon(15.4 k Pa) at0–5 cm depth soil layer. The root cohesion values estimated for all species were generally distributed at the 0–10 cm depth and decreased with the increment of soil depth. The higher root cohesion associated with H. altissima and H. compressa implies their suitability for revegetation purposes to strengthen the shallow soil in the riparian zone of the Three Gorges Reservoir. Although the soil reinforcement induced by roots is only assessed from indirect indicators, the present results still useful for species selection in the framework of implementing and future vegetation recovery actions in the riparian zone of the Three Gorges Reservoir and similar areas in the Yangtze River Basin.     

Responses of soil moisture to vegetation restoration type and slope length on the loess hillslope

Soil moisture, a critical variable in the hydrologic cycle, is highly influenced by vegetation restoration type. However, the relationship between spatial variation of soil moisture, vegetation restoration type and slope length is controversial. Therefore, soil moisture across soil layers (0-400 cm depth) was measured before and after the rainy season in severe drought (2015) and normal hydrological year (2016) in three vegetation restoration areas (artificial forestland, natural forestland and grassland), on the hillslopes of the Caijiachuan Catchment in the Loess area, China. The results showed that artificial forestland had the lowest soil moisture and most severe water deficit in 100-200 cm soil layers. Water depletion was higher in artificial and natural forestlands than in natural grassland. Moreover, soil moisture in the shallow soil layers (0-100 cm) under the three vegetation restoration types did not significantly vary with slope length, but a significant increase with slope length was observed in deep soil layers (below 100 cm). In 2015, a severe drought hydrological year, higher water depletion was observed at lower slope positions under three vegetation restoration types due to higher transpiration and evapotranspiration and unlikely recharge from upslope runoff. However, in 2016, a normal hydrological year, there was lower water depletion, even infiltration recharge at lower slope positions, indicating receiving a large amount of water from upslope. Vegetation restoration type, precipitation, slope length and soil depth during a rainy season, in descending order of influence, had significant effects on soil moisture. Generally, natural grassland is more beneficial for vegetation restoration than natural and artificial forestlands, and the results can provide useful information for understanding hydrological processes and improving vegetation restoration practices on the Loess Plateau     

Study on influence factors of cement-stabilized soil compressive strength

The cement dry jet mixing method has been used to reinforce soft cohesive ground to increase the strength of soft cohesive ground and to decrease its deformation. The study briefly introduces the curing mechanism of cement-soil,presents the factors of influencing on compressive strength,mainly analyses the factors including cement mixing ratio,cement strength grade,curing age,moisture content and soil texture and puts forward some rational proposals at last.     

Thermal-water-salt coupling process of unsaturated saline soil under unidirectional freezing

Salinization and desertification are closely related to water-salt migration caused by a temperature gradient. Based on the Darcy Law of unsaturated soils, the law of energy conservation and the law of mass conservation, the thermal-water-salt coupling mathematical model of unsaturated frozen saline soil was established. The model considered the latent heat of phase change, crystallization impedance, crystallization consumption and complete precipitation of solute crystallization in ice. In orde...     

Effects of shrub on runoff and soil loss at loess slopes under simulated rainfall

Improved understanding of the effect of shrub cover on soil erosion process will provide valuable information for soil and water conservation programs.Laboratory rainfall simulations were conducted to determine the effects of shrubs on runoff and soil erosion and to ascertain the relationship between the rate of soil loss and the runoff hydrodynamic characteristics.In these simulations a 20° slope was subjected to rainfall intensities of 45,87,and 127 mm/h.The average runoff rates ranged from 0.51 to 1.26 mm/min for bare soil plots and 0.15 to 0.96 mm/min for shrub plots.Average soil loss rates varied from 44.19 to 114.61 g/(min·m~2) for bare soil plots and from 5.61 to 84.58 g/(min·m~2) for shrub plots.There was a positive correlation between runoff and soil loss for the bare soil plots,and soil loss increased with increased runoff for shrub plots only when rainfall intensity is 127 mm/h.Runoff and soil erosion processes were strongly influenced by soil surface conditions because of the formation of erosion pits and rills.The unit stream power was the optimal hydrodynamic parameter to characterize the soil erosion mechanisms.The soil loss rate increased linearly with the unit stream power on both shrub and bare soil plots.Critical unit stream power values were 0.004 m/s for bare soil plots and 0.017 m/s for shrub plots.     

ECO-ENVIRONMENT CHANGE AND SOIL EROSION PROCESS IN THE RECLAIMED FORESTLAND OF THE LOESS PLATEAU

1INTRODUCTIONSerioussoilerosionhasmadetheeco-environmentfragileintheLoessPlateau.InthemodernsoilerosionoftheLoessPlateau,theman-madeerosionwasthedominanterosionpattern (TANGetal.,1993a,1993b).Manyhumanactivitiesacceleratesoilerosion,butthemostinfluentialactivitiesaretodestroyforestandgrass,eventodestroyallvegetation.Reclamationbydestroyingforestlandandgrasslandartificiallycouldrapidlyincreasesoilerosionintensityinshorttimeandenlargeerosionspacegradually,evenchangeerosiondirectionandwor…     

Characteristics of Channeling Flow in Cultivated Horizon of Saline Rice Soil

1 Introduction The preferential flow tends to result in the loss of irriga- tion water and nutrients. The formation of the preferen- tial flow was caused by various factors, such as soil structure, tillage, worm cavity and root channeling rup- ture (Pot et al., 2005). Besides, frozen rupture can not be ignored in the northeastern China where the lowest tem- perature reaches -40℃ (Wang et al., 1993). There are several types of preferential flow, such as bypass flow, channeling flow and macro…     

Biogeochemical conversion of sulfur species in saline lakes of Steppe Altai

The aim of the present research is to identify the main mechanisms of sulfur behavior in saline lakes in the course of time and followed transformations in their chemical composition. The influence of water on chemical composition of biochemical processes involved in decomposition of organic matter was determined by the study of behavior of reduced forms of sulfur in lakes. The determination of reduced forms of sulfur was carried out by successive transfer of each form of sulfur to hydrogen sulfide followed by photometric measurements. The other chemical components were determined by standard methods(atomic absorption, potentiometric method, titration method and others). The salt lakes of the Altai steppe were studied in summer season 2013–2015. Analysis of the chemical composition of the saline lakes of Altai Krai has shown that carbonate-, hydrocarbonate-and chloride ions dominate among anions; sodium is main cation; sulfates are found in subordinate amounts. Reduced forms of sulfur occur everywhere: hydrogen and hydrosulfide sulfur S~(2-) prevail in the bottom sediments; its derivative—elemental S~0—prevails in the lakes water. The second important species in water of soda lakes is hydrosulfide sulfur S~(2-), and in chloride lakes is thiosulfate sulfur S_2O_2~(3ˉ). The lag in the accumulation of sulfates in soda lakes in comparison to chloride lakes can be explained by their bacterial reduction, followed by the formation and deposition of iron sulfides in sediments. In chloride lakes gypsum is a predominantly barrier for sulfates.     

Influence of non-dimensional strength parameters on the seismic stability of cracked slopes

Cracks in rock or soil slopes influence the stability and durability of the slopes. Seismic forces can trigger slope disasters, particularly in the cracked slopes. Considering the nonlinear characteristics of materials, the more generalized nonlinear failure criterion proposed by Baker is adopted. The influence of non-dimensional strength parameters on the stability of cracked slopes under earthquakes is performed using the upper bound limit analysis. The seismic displacement is calculated by adopting the logarithmic spiral failure surface according to the sliding rigid block model. Based on the existing studies, two methods for the stability analysis of cracked slopes under earthquakes are introduced: the pseudo-static method(with the factor of safety(Fs) as an evaluation index), and the displacement-based method(with the seismic displacement as an evaluation index). The pseudo-static method can only determine the instantaneous stability state of the cracked slope, yet the displacement-based methodreflects the stability variation of cracked slopes during earthquakes. The results indicate that the nondimensional strength parameters affect the factor of safety and seismic displacement of slopes significantly. The non-dimensional strength parameter(n) controlling the curvature of strength function shapes on the slope stability is affected by other parameters. Owing to cracks, the effect of non-dimensional strength parameters on seismic displacement becomes more significant.     

A new numerical model of ring shear tester for shear band soil considering friction-induced thermal pressurization

In this study, a new numerical model of ring shear tester for shear band soil of landslide was established. The special feature of this model is that it considers the mechanism of friction-induced thermal pressurization, which is potentially an important cause of high-speed catastrophic landslides. The key to the construction of this numerical ring shear model is to realize the THM(thermo-hydro-mechanical)dynamic coupling of soil particles, which includes the processes of frictional heating, the...     

Effect of cement on the stabilization of loess

Considering the potential use of cementstabilized loess (CSL) as a construction material for structures that are subjected to frequent loess landslides, this paper explores the stabilization and improvement of geotechnical characteristics of loess achieved by the addition of 0%-9% cement by dry weight. Laboratory evaluations investigated the consistency limits, compaction, compressibility, California bearing ratio (CBR), direct shear strength, and unconfined compression strength (UCS) of CSL for different curing stages. A durability index was quantified to estimate the influence of wetting-drying (w-d) cycles on CSL strength, and an optimum cement dosage was also identified. The results reveal that the cohesion of CSL is substantially more sensitive to structure than its friction angle and that cohesion is responsible for shear strength increase after remoulding. The cement proportions have an effective role in the enhancement of compressibility. The development of UCS can be categorized into the early stage (<14 days) and the later stage (>14 days). The increase in strength primarily occurred in the first 14 days. The w-d cycles have a significant influence on the decrease in compression strength. The CBR value increases with increments of additional proportions and compaction times. The relationships of UCS versus the compressibility modulus and UCS versus CBR are established to facilitate the mix design for strength. A rational predictive exponential equation is proposed to predict the durability index for different w-d cycles.     

Characteristics of grain size distribution and the shear strength analysis of Chenjiaba long runout coseismic landslide

Study on the grain size distribution characteristics and the frictional strength behavior of the slide deposits are helpful to disclose the landslide runout process and understand the mechanism of a long runout landslide. We performed grain size distribution analysis on samples collected from Chenjiaba landslide induced by Wenchuan earthquake. The grain size distribution of samples from the landslide sections quantitatively depicts a gradual coarsening upward grading from shear zone to the top section. Then a multistage-multiphase ring shearing approach was used to determine a comparative shear strength behavior of samples from each landslide section. In this method, a sample was sheared continuously for large displacement and fast rate on different normal stress conditions. The multiphase shear mode with a maximum of 105 mm/min rate has allowed observing the qualitative change and patterns of the frictional resistance behaviors of soils under different normal stresses. The results of coefficient of friction values under multiphase shear mode have shown substantial post peak shear weakening behaviors after large shear displacement that can be narrated with long runout processes. The shear strength test results indicate that the shear zone samples have developed higher friction angle values compared to overlying section samples, on the last phase of shear process, which may be very important to understand the braking mechanism of a long runout landslide.