Estimation of tensile strength of sandy soil from soil–water characteristic curve

Acta Geotechnica - During heavy rainfalls, the surface soil on a slope may be eroded and the erosion is much dependent on the tensile strength of soil. In addition, the tensile strength of soil is...     

Bonding degradation and stress–dilatancy response of weakly cemented sands

The progressive bond breakage of artificially cemented sands induced by shear straining was investigated through conventional isotropically consolidated drained triaxial compression tests. Sand specimens were prepared with a low degree of cementation by adopting a chemical grout. Test results were interpreted in terms of two stress–dilatancy theories for cohesive-frictional materials proposed in literature. The influence of debonding on the stress–dilatancy behaviour of cemented sands was analysed with particular emphasis on the ‘delayed dilatancy’ phenomenon. A bonding degradation curve was determined for each test relating the interparticle cohesion (c) to the magnitude of the total plastic strain vector (ε_d) and a bond degradation rate factor (D_c) was assessed from each curve. The maximum value of interparticle cohesion (c₀) before the onset of bond degradation under shearing was found to correspond with a sharp decrease in the soil stiffness of the specimens. The influence of the effective confining stress (p^′_c) on both c₀ and D_c parameters gathered from each test was also ascertained.     

Modelling of fibre–cohesive soil mixtures

A new constitutive model for fibre-reinforced cohesive soil is proposed. The model combines a Cam-Clay like bounding surface model with an elastic–plastic one-dimensional fibrous element model. A “smearing procedure”, which can consider any spatial distribution of fibre orientation, is employed to transform discrete tensile forces developed in the fibres into stresses for the composite material. The fibre stress contribution is bounded by both degradation of soil–fibre bonding due to pull-out mechanism and tensile strength of the fibres. Eventual occurrence of fibre breakage is also considered. The model performances are analysed for both consolidation and shearing loading modes, and qualitative comparison is performed with experimental data available in the literature. For consolidation loading, tensile stresses are not developed in the fibres and thus the fibre effect is rather limited. For drained shear loading, addition of fibres can result in a consistent shear strength increase. The beneficial effect of fibres seems to be controlled by two parameters: the fibre tensile stiffness and the fibre/soil strain ratio that accounts for any possible slippage or shear deformation at the fibre/soil matrix interface. For undrained shear loading, the strengthening effect of the fibres appears to be counteracted by the increase in pore water pressure, induced by the additional confining contribution of the fibres. In agreement with published experimental data, the model suggests also that the moisture content is a key factor governing fibre effectiveness for undrained shearing. Finally, analysis of the model predicted critical states for fibre-reinforced cohesive soil is provided.     

Three dimensional Finite Element modeling of seismic soil–structure interaction in soft soil

Earthquakes in regions underlain by soft clay have amply demonstrated the detrimental effects of soil–structure interaction (SSI) in such settings. This paper describes a new three dimensional Finite Element model utilizing linear elastic single degree of freedom (SDOF) structure and a nonlinear elasto-plastic constitutive model for soil behavior in order to capture the nonlinear foundation–soil coupled response under seismic loadings. Results from an experimental SSI centrifuge test were used to verify the reliability of the numerical model followed by parametric studies to evaluate performance of linear elastic structures underlain by soft saturated clay. The results of parametric study demonstrate that rigid slender (tall) structures are highly susceptible to the SSI effects including alteration of natural frequency, foundation rocking and excessive base shear demand. Structure–foundation stiffness and aspect ratios were found to be crucial parameters controlling coupled foundation–structure performance in flexible-base structures. Furthermore, frequency content of input motion, site response and structure must be taken into account to avoid occurrence of resonance problem.     

Discrete element modeling of shear wave propagation in carbonate precipitate–cemented particles

Bioremediation is widely used to improve ground soil by introducing calcium carbonate (CaCO₃). Shear wave velocity (Vs) is usually adopted to evaluate effect but the microscopic mechanism is unclear. The discrete element method (DEM), a promising tool for simulating the behaviors of cohesive and noncohesive materials, was used in this study to simulate Vs evolution and wave propagation path of sand reinforced by calcite precipitates. Two basic calcite precipitate forms are proposed for representing individual calcite precipitation (CaCO₃-P) and calcite aggregation (CaCO₃-C). Contact cementation between adjacent sand grain pairs was the primary association pattern for calcite precipitates at a low calcite content. At a higher calcite content, the preferential shear wave propagation pathway is the clusters cemented by CaCO₃-C. With calcite content increasing from 0 to 9%, the coordination number and average contact force increased. Vs increased from 169.73 to 2132.64 m/s but had high variability due to the spatial distribution. The results suggest that the calibrated DEM model can elucidate the microscopic mechanisms and evaluate the enhancement effect of microorganism-reinforced soil.

     

Analysis of soil resistance on laterally loaded piles based on 3D soil–pile interaction

The load distribution and deflection of large diameter piles are investigated by lateral load transfer method (p–y curve). Special attention is given to the soil continuity and soil resistance using three-dimensional finite element analysis. A framework for determining a p–y curve is calculated based on the surrounding soil stress. The appropriate parametric studies needed for verifying the p–y characteristic are presented in this paper. Through comparisons with results of field load tests, the three-dimensional numerical methodology in the present study is in good agreement with the general trend observed by in situ measurements and thus, represents a realistic soil–pile interaction for laterally loaded piles in clay than that of existing p–y method. It can be said that a rigorous numerical analysis can overcome the limitations of existing p–y methods to some extent by considering the effect of realistic three-dimensional combination of pile–soil forces.     

Numerical modeling of soil–structure interface of a concrete-faced rockfill dam

Concrete-faced rockfill dams (CFRD) are widely used in large-scale hydraulic projects. The face slab, the key seepage-proof structure of great concern, has a strong interaction with the neighboring gravel cushion layer due to a significant difference in their stiffness. An elasto-plasticity damage interface element, a numerical format of the EPDI model, is described for numerical analysis of a CFRD that can trace the separation and re-contact between the face slab and the cushion layer at the interface. As verified by simulating slide block and direct shear interface tests, this element was confirmed to capture effectively the primary monotonic and cyclic behaviors of the interface. This element can easily be extended to the finite element method (FEM) programs that involve the Goodman interface element. The analysis of a typical CFRD showed that the interface model describes a significant effect on the stress response of the face slab under different conditions, including dam construction, water storage, and earthquake. Treatments of the cushion layer, such as an asphalt layer, changed the behavior of the interface between it and the face slab, which resulted in a significant effect on the stress response of the face slab. The top of the face slab exhibited a significant separation from the cushion layer during construction, induced mainly by construction of the neighboring dam body.     

Predictions of the effects of Qinghai-Tibet highway rebuilding project on soil erosion

In order to enhance the road capacity and guarantee the commodity transportation to Tibet, the Ministry of Communications conducted renovation and rebuilding of the Qinghai-Tibet Highway. It is of great importance to conserve the water and soil well durin…     

Analysis of soil biogeochemical characteristics in the process of karst rocky desertification

The sampling method of karst soil is always the one used for non-karst area. However, if the samples can represent the plot, it still needs further investigation due to high degree of spatial variability of karst soil. The study results of soil from the Maolan karst virgin forest in Guizhou Province elucidated that only the surface area-weighted samples could represent plot soils in the karst ecosystem. The remediation of karst rocky desertification has currently become a national issue, but soil degradation research on rocky desertification genesis types and processes is still insufficient. Soil physical, chemical, biological characteristics and enzymes activities from plots of different rocky desertification genesis types and grades in the Cha＇eryan catchment in the Huajiang Gorge area, Guizhou, were studied. The results showed that woodcutting and land reclamation influenced soil biogeochemical characteristics, and the influence of different disturbing ways was huge. The author suggested a soil degradation index system which could illustrate the rocky desertification genesis type and extent. The degraded modes of woodcutting and reclamation sequence were different, both had no synchronism with rocky desertification grades divided by current landscape index. Only soil of serious grade degraded evidently in wood cutting sequence. On the contrary, soils of slight, medium and serious grades degraded evidently in the reclamation sequence.     

Strength characteristics and mechanisms of salt-rich soil–cement

Salt-rich soft soils have not only general characteristics of common soft soils, but also contain high contents of Mg²⁺, Cl^?, and SO₄^2?, which have negative effects on deep mixing method using cement to treat soft soils. Laboratory and field tests were conducted to investigate the effects of changing cement incorporating ratio, water content, cement mixing ratio, and contents of Mg²⁺, Cl^?, and SO₄^2? on the unconfined compressive strength of the salt-rich soil–cement. The microstructure of soil–cement and the mechanism for the strength change of salt-rich soil–cement were investigated using X-ray diffraction, scanning electronic microscopy (SEM), and backscattered diffraction technology. It was found that an increase of cement incorporating ratio enhanced the strength of soil–cement but reduced its strength when water is added. Different amounts of Mg²⁺, Cl^?, and SO₄^2? not only caused the difference in the microstructures of salt-rich soil–cement but also influenced the soil–cement strength.     

Prediction of hydraulic conductivity for soil–bentonite mixture

Owing to its low hydraulic conductivity, soil and bentonite mixture is applied as a liner material. However, the experimental determination of hydraulic conductivity, which is controlled by various physical, chemical and mineralogical factors, requires an expensive and time-consuming setup. In the present work, multigene symbolic, genetic programming was used to model functional relationships for hydraulic conductivity. The developed model was able to generalize highly nonlinear variations in data as well as predict system behavior from experimental observations. It was found that the predictions obtained from developed model agree well with experimental observations.     

Influence of plant on the extractability of estrogen in the rhizospheric soil

Livestock wastes applied to agricultural land are potential sources of steroidal hormones （estradiol （E2）, estrone （E1）, and estriol （E3）） that can adversely affect the aquatic ecosystem as endocrine disruptors. But the effects of plants on the fate of estrogen in the environment are not clearly known. In the present report, the behavior of E1, E2 and E3 added to the rhizospheric soil in various concentrations and its effect on plant growth were examined by estrogen batch sorption test, plant pot test and estrogen extraction test. Trifolium repens was grown on the soil spiked with E2 or a mixture of E1, E2 and E3. Pots without plant were prepared as controls. All pots were watered in 50 ml dose almost every day, and the rhizosheric soil and plant body were collected 7, 19, 26, 33 days after planting. The soil was sequentially extracted with Milli-Q water, and methanol / 1M acetic acid solvent followed by methanol wash at the solid to liquid ratio of 1 ： 4 （g/mL）. Extracts were analyzed for El, E2 and E3 using a Shimadzu GCMS-QP5050. Plant dry weight （4 hours, 105 ℃） was measured as an indicator of plant growth. In batch sorption tests, E1 and E2 concentrations in the liquid phase significantly decreased within 24 hours after the addition of estrogen. The sum of methanol / 1M acetic acid extractable E 1 and E2 in the soil decreased to half of the initial added amount within 7 days. Reduction in total estrogenic activity of the soil extracts （estimated as 0.01CE1＋CE2＋0.1CE3 where CE1, CE2 and CE3 are respective El, E2 and E3 concentrations per unit soil weight） continued for 1 month due to slow oxidation of E2 to E1. Desorption of E2 from soil during the pot experiment was higher than expected from the sorption experiment.     

Consolidation behavior of soil–cement column improved ground

Columnar inclusion is one of the effective and widely used methods for improving the engineering properties of soft clay ground. This article investigates the consolidation behavior of composite soft clay ground using both physical model tests under an axial-symmetry condition and finite element simulations using the PLAXIS 2D program. It was determined that the final settlement and the rate of consolidation of the composite ground depended on the stress state. For an applied stress that is much lower than the failure stress, the final settlement of the composite ground was lower, and the consolidation was rapid. When the soil–cement column failed, the stress on the column suddenly decreased (due to strain-softening); meanwhile, the stress on the soil increased to maintain the force equilibrium. Consequently, the excess pore pressure in the surrounding clay increased immediately. The cracked soil–cement column acted as a drain, which accelerated the dissipation of the excess pore pressure. The consolidation of the composite ground was mainly observed in the vertical direction and was controlled by the area ratio, which is the ratio of the diameter of the soil–cement column to the diameter of the composite ground, a. The stress on the column was shown to be low for a composite ground with a high value of a, which resulted in less settlement and fast consolidation. For a long soil–cement column, the excess pore pressures in the surrounding clay and the column were essentially the same at a given consolidation time throughout the improvement depth. It is proposed that the soil–cement column and surrounding clay form a compressible ground, and the consolidation occurs in the vertical direction. The composite coefficient of consolidation (c_v(com₎) that was obtained from the physical model test on the composite ground can be used to approximate the rate of consolidation. This approximation was validated via a finite element simulation. The proposed method is highly useful to geotechnical engineers because of its simplicity and reliable prediction.     

Advances on bioremediation of off-contaminated soil in cold region

Petroleum pollution in the soil is a common problem in the world. The pollution may not only cause resource waste, but also may result in environment destruction, biology subsistence crisis and human health damage gradually. Biological techniques can be used to remove and transfer petroleum contaminants in the soil. Bioremediation of petroleum-contaminated soil, which is cost-effective,safe and friendly to environment, is promising. Low temperatures and lack of available nutrients often limit the rate of microbial degradation of petroleum hydrocarbons in contaminated soils in cold region. Some scholars carried out bioremediation technology research on oily soil in cold area. Scientists attempted many measures to increase the temperature of the field. A multidisciplinary team of engineers, microbiologists and electricians has designed and installed a thermally （TIS） enhanced biopile in oil-contaminated soil in Prudhoe Bay, AK. Covered with a black plastic sheet, the pile can also improve temperature condition. Nutrient is another important factor affecting bioremediation. Because of the different constituents in the soil, the proportion of elements is different. To optimize nutrient amendments for the remediation of a long-term hydrocarbon-contaminated site at the Old Casey Station in Antarctica, results showed that the effects of nitrogen （and phosphorus） on microbial are evident. If the method of fertilizing inorganic nutrients is improper, salinity of the soil may be increased and the osmotic potential may be impacted. J.L.Walworth et al.     

Advances on bioremediation of oil-contaminated soil in cold region

Petroleum pollution in the soil is a common problem in the world. The pollution may not only cause resource waste, but also may result in environment destruction,biology subsistence crisis and human health damage gradually. Biological techniques can be us…     

Organic carbon and its oxidation stability characteristics of shallow soil in frozen soil area of Three River Source Region北大核心CSCD

As the most important part of the global carbon cycle,soil carbon pool is the largest carbon pool in terrestrial ecosystems. Soil carbon pool in permafrost regions is the most sensitive carbon pool to climate change. Weak climate change will have a huge impact on the organic carbon production in the shallow soil,and then affect the regional landscape and ecology. As an indicator reflecting the antioxidant capacity of soil organic carbon,oxidation stability affects the quantity and quality of soil organic carbon,and its variation has a certain regularity in the alpine permafrost region under the influence of climatic factors. In order to explore the distribution characteristics of soil organic carbon and its oxidation stability in frozen soil,based on the experimental data and the climatic data from 2011 to 2019,the random forest model was used to conduct multi-factor digital mapping on soil organic carbon content,soil organic carbon components with different oxidation difficulty degrees,and soil organic carbon oxidation stability coefficient and environmental variables（average annual precipitation,average annual sunshine hours,average annual air temperature,and altitude）and analyze the controlling factors. The results showed that the model had an interpretation degree of more than 54% for the shallow soil organic carbon in frozen soil area of Three River Source Region,and the digital mapping could reflect the distribution of soil organic carbon well. Soil organic carbon was mainly affected by precipitation and sunshine duration,and temperature took second place. The spatial distribution of components with different oxidation difficulty is different,but the oxidation stability has the distribution characteristics of high in the north and low in the south. Cold and dry are conducive to improving the oxidation stability of organic carbon in shallow soil of frozen soil area. © 2022 Science Press (China).     

A theoretical–experimental approach to elastic and strength properties of artificially cemented sand

Evaluating the behavior parameters of soils and soil-binder mixes by means of theoretical models that are supported by laboratory tests still remains a key challenge in foundation design. In this context, the paper investigates some aspects of the mechanical behavior of artificially cemented sands (ACS) by means of experimental characterization and micromechanics-based modeling. Particular emphasis is given to the increase in elastic stiffness and strength brought by cementation. Based on the concept of a fictitious continuum medium and the homogenization theory, the effective elastic properties of ACS are evaluated using the Mori–Tanaka and self-consistent schemes. The elastic micromechanical approach is supported by bender element tests. Finally, the effective strength properties of ACS are assessed by means of micromechanics-based failure criterion formulated within the context of non-associated plasticity. Validation and calibration of the theoretical model are achieved by comparison with data from unconfined compression tests.     

Research status and prospect of tensile strength of frozen soil北大核心CSCD

A comprehensive grasp of the research status of tensile strength of frozen soil is the basis for further research. Firstly,the typical methods that can be used to test the tensile strength of frozen soil are introduced,and the test conditions,sample forms and stress mechanism of different test methods are described in detail. The advantages and disadvantages of typical tensile strength test methods are compared and listed. Secondly,the research work and shortcomings based on different test methods are summarized. Then,the latest research progress of the influence of temperature,water content,loading（deformation）rate,soil quality and sample size on the change law of frozen soil tensile strength is comprehensively analyzed. Finally,it is proposed to develop and improve the research method and system of frozen soil tensile strength,and increase the testing research of warm frozen soil tensile strength,so as to obtain the prospect of more accurately simulating the tensile failure behavior of frozen soil. It is pointed out that the internal cause of the formation of the tensile strength and the tensile failure mechanism of frozen soil should be thoroughly revealed by combining the research methods of microstructure and digital image technology of frozen soil. Based on the multi-factor test,a more perfect prediction method of frozen soil tensile strength is explored. Meanwhile,expand the in-situ test research on the tensile strength of frozen soil,and strengthen the parallel research ideas of indoor and outdoor double tracks. Through the analysis of the research status and development trend at home and abroad,it provides reference and guidance for the experimental study of frozen soil tensile strength,the improvement of theoretical model of frost heave,geotechnical engineering design in cold regions and artificial freezing reinforcement engineering. © 2022 Science Press (China).