Shanghai Clay Research Articles

Experimental investigation on mechanical behavior of natural soft clay by true triaxial tests

Laboratory tests on undisturbed and remolded Shanghai clay were conducted with a novel mixed boundary true triaxial apparatus. The strength and deformation behaviors of the structured soft clay at different Lode angles were carefully investigated. The relationship between the shear stress ratio and the dilatancy ratio of the clay in three different stress spaces, that is p–q space, TS stress space and tij modified stress space, are discussed in detail. In particular, the influences of the stress path and the structure of non–disturbed clay on the strength, deformation and failure criterion of Shanghai clay were quantitatively evaluated. The shear strength and dilatancy of undisturbed /remolded clay decreases with the increase of the Lode angle. It is also found that the shear strength and dilatancy of the undisturbed clay are larger than those of the remolded clay; while the influence of the structure on the dilatancy decreases with an increase in the Lode angle. In the π–plane, the maximum strength of Shanghai clay under different stress paths generally obeys the SMP failure criterion. The coincidence of the stress–dilatancy relationship given by the TS and tij stress spaces with the test results is much better than that given by p–q stress space. Furthermore, the linearity of the plastic potential curves of the undisturbed/remolded clay is more evident in tij stress space.

Effects of Overconsolidation and Structural Behaviors of Shanghai Clay on Deformation Caused by Deep Excavation

Effects of Overconsolidation and Structural Behaviors of Shanghai Clay on Deformation Caused by Deep Excavation

Enzyme-induced Carbonate Precipitation (EICP) Combined with Lignin to Improve the Unconfined Compression Strength (USC) of Shanghai Clay

In order to improve the mechanical properties of high moisture content and low strength of clay particles in Shanghai, the effects of different lignin content on the unconfined compressive strength of EICP-lignin-cured clay were studied by using the method of lignin combined with EICP technology to strengthen the clay by lignin combined with EICP technology. The experimental results showed that EICP-lignin could enhance the unconfined compressive strength of Shanghai clay. With the increase of lignin content, the unconfined compressive strength of clay first increased and then decreased, and there was an optimal content value. The optimal lignin content is 8%.

Experimental study of the dynamic characteristics of deep silty clay in Shanghai

To investigate the dynamic properties of deep silty clay, a comprehensive set of cyclic triaxial and resonant column tests were performed on undisturbed samples of silty clay containing sand from depths reaching 54.5 m in Shanghai. This study aimed to characterize the influence of confining pressure on the stress–strain behavior, pore water pressure, and dynamic modulus of the soil. The present findings revealed that the confining pressure, which is correlated with the depth of burial, significantly influenced the soil deformation. Moreover, elevated confining pressures resulted in increased stress within the hysteresis loops, a higher initial shear modulus, and a diminished ratio of excess pore water pressure. Based on the empirical data, a refined model was formulated for the shear modulus and damping ratio as functions of the shear strain for deep silty clay in Shanghai. These insights are intended to offer a robust reference point, facilitating calibration and guiding subsequent investigations into structures situated in the deep subterranean environments of Shanghai.

Investigation of creep characteristics and microscopic mechanism of cemented-soil subjected to freeze-thaw

With the development of underground construction, higher safety demands have been placed on construction methods. The improved freezing method, which combines artificial ground freezing and cemented soil stabilization techniques, is more suitable for studying the typical creep characteristics of muddy clay in Shanghai. However, research on the creep behavior of freeze-thaw cemented-soil is limited. In this study, the creep characteristics of freeze-thawed cemented soil were investigated through triaxial creep tests, and the micromechanism was explored using scanning electron microscopy (SEM). The results indicate that the creep deformation of freeze-thawed cemented soil is higher than that of non-freeze-thawed cemented soil. The freezing temperature decreased with an increase in deformation. By analyzing SEM images, the porosities of non-freeze-thawed cemented-soil and soil freeze-thawed at -5°C, -15°C, and -25°C, were extracted, which were 28.5%, 28.7%, 30.1%, and 39.1% respectively. During the freeze-thaw cycle, both the soil and cemented soil particles undergo movement and reordering. Freeze-thaw exacerbates creep degradation because frost heave weakens particle adhesion and affects structural stability. These results provide a scientific reference for the development of underground spaces in soft areas.

Constitutive modeling structural behaviors of Shanghai deep clay

The rapid development of underground engineering requires a comprehensive understanding of deep soils. Existing study about the deformation and strength behaviors of Shanghai clay is mostly focused on shallow soft clay (generally within 30 m depth), while counterpart for deep clay is needed. This work investigates the physical properties and mechanical behaviors of Shanghai deep clay (block samples of layer (8) obtained at a depth of around 50 m), via conducting a series laboratory tests including physical property tests, onedimensional consolidation tests and undrained triaxial compression tests, performing constitutive modelling. The results show: (1) Shanghai layer (8) clay can be classified as a low-plasticity silty clay, and the void ratio measured in the laboratory of the block samples is larger than remoulded samples; (2) The clay is a normally consolidated clay, and the block samples show greater compressibility and stronger initial structure; (3) The block samples show contraction behaviors in the undrained triaxial compression tests; (4) The structural behaviors is well captured by structural bounding surface model.

Small-strain characterization of Shanghai clays by triaxial compression tests

The mechanical properties of the upper Shanghai Layers 2-6 clays at small strain levels have been extensively investigated. Nonetheless, limited data exist on their small-strain characteristics through K 0-consolidated undrained triaxial compression tests, with even less information available for the deeper layers 8 and 10. This study employed a series of K 0-consolidated undrained TC tests on intact samples from both the upper layers 2-6 and the deeper layers 8 and 10 of Shanghai clay. We evaluated the undrained maximum secant stiffness (Eu max) and reference shear strain (γ0.7 ). An increase in over-consolidation ratio (OCR) results in a higher E u max/p’ and a more rapid decay of the G/G max with the increasing shear strain (γ). As the value of γ 0.7 increases with I p and decreases with OCR, a correlation between γ 0.7 and Ip can be established. This study observed significant correlations between the E u max, effective stress (p’), and the maximum deviatoric stress, as well as between E u max and the cone resistance obtained from field cone penetration tests. These correlations offer methods to estimate E u max and γ0.7 values based on parameters that are more readily measurable, which provide critical references for the calculations and design parameters in foundation engineering within the Shanghai region.

Constitutive modeling of anisotropic small-strain stiffness behavior of Shanghai soft clay

Small strain stiffness plays an important role in determining the deformation of geotechnical engineering in soft soils. To model the anisotropic small strain stiffness behavior of soft clay, an enhancement to the bounding surface plasticity model is proposed. This enhancement involves incorporating the anisotropy in soil stiffness at very small strains and its non-linear degradation. Small strains are considered via cross-anisotropy elasticity, while the concept of intergranular strain is used to reflect the non-linear degradation of stiffness with strains and the effect of recent stress history. Using the proposed model, the triaxial test results on K0 consolidated Shanghai clay specimens are simulated to investigate the model’s performance. In the simulation, three typical shear stress paths are adopted. The comparison of the model simulations against experimental evidence is discussed, where the effects of anisotropy in initial soil stiffness and stress-induced anisotropy on the variation of small-strain stiffness, pore pressure, and shear-volume coupling response are highlighted.

Roles of corn starch and gellan gum in changing of unconfined compressive strength of Shanghai alluvial clay

Abstract This article aims to study the effects of corn starch and gellan gum (gellan glue) on the unconfined compressive strength of alluvial soft clay in Shanghai. Firstly, starch and gellan gum were evenly mixed into Shanghai cohesive soil in a certain proportion, and soil samples were prepared according to the optimal water content. Secondly, the unconfined compressive strength test was carried out according to the content of corn starch, gellan glue, and curing time. The strength characteristics of Shanghai soft clay under various test conditions were studied. Finally, the software for statistics and drawing was used to perform regression analysis and drawing on the experimental data. The results showed that in the unconfined compressive strength test, when the curing time was 28 days, the starch content was 3% mixed with 0.5% gellan gum, the unconfined compressive strength of the sample was the highest, and generally, it seems that the strength keep increasing with curing time.

Deformation Characteristics of Soft Soil Induced by Deep Excavation and Its Impact on Adjacent Tunnels: A Case Study in Shanghai

This research aims to comprehensively study the deformation characteristics of an excavation pit of 7,600 m2 (117 m long, 65 m wide and 20.8 m deep) in soft clay in Shanghai, China. The deformation characteristics of the mixing pile in both the vertical and lateral directions, ground surface movement, and surface displacement in the adjacent tunnels were comprehensively investigated. The results indicated that the maximum lateral deflection was approximately 85 mm at the end of the excavation and that the maximum lateral displacement, δhm, was in the range between 0.12%He and 0.41%He. The factor of safety of the excavation was approximately 1.6, which was larger than that of most reported sites in Shanghai. The relationship between heaving and excavation depth was strong between 0.0047%He and 0.056%He, and a relatively large settlement was observed at a distance of approximately 0.2He–0.3He. The tunnel surface of the eastbound line deformed upward and inward with a maximum horizontal convergence of 4 mm. A multilayer regression model was also validated, showing a maximum discrepancy of 26.5 mm. This research provides a valuable historical case for the documentation of the deformation characteristics of excavation pits close to existing foundation pits and tunnels.

Adsorption performance of bentonite and clay for Zn(II) in landfill leachate

BackgroundThe increasing prevalence of the Galvanized and dry battery industries has led to a rise in zinc proportions in landfills, posing environmental risks. This study explores the potential of bentonite, renowned for its metal adsorption capability, as a landfill barrier material.MethodsAdsorption characteristics of Zn(II) in natural bentonite and Shanghai clay were investigated. Various factors affecting Zn(II) adsorption, including pH, Na ion strength, contact time, initial Zn(II) concentration, and adsorption temperature, were analyzed through batch-type adsorption tests.ResultsThe change in pH and Na strength demonstrates no significant impact on the adsorption of Zn(II) onto bentonite, highlighting the strong selectivity of bentonite for Zn(II). Conversely, the equilibrium adsorption capacity of Zn(II) onto clay increases with rising pH or decreasing ion strength. The Zn(II) adsorption onto bentonite is well-described by the pseudo-second-order, intra-particle diffusion, and Elovih models, each achieving an R2 value exceeding 0.9. While both physical and chemical adsorption coexist in Zn(II) adsorption onto bentonite or clay, the primary determinant of the adsorption rate is chemical adsorption. The adsorption onto bentonite is spontaneous, whereas onto clay it is not. The mechanism involves van der Waals forces, ion exchange, and chemical actions such as inner-sphere complexation for Zn(II) adsorption onto both bentonite and clay.

Fractal thermal conductivity of unsaturated soils considering pore heterogeneity

Accurately predicting the thermal conductivity of unsaturated soils is crucial for assessing and simulating the long-term environmental impact of subterranean engineering structures. This study presents a fractal thermal conductivity model for unsaturated soils that considers the influence of heterogeneous pore topology, water content and volumetric deformation. Based on the results from the scanning electron microscopy (SEM) and mercury intrusion-extrusion cyclic porosimetry (MIECP) tests on Shanghai clay and Kaolin, we introduce the pore-throat structure to describe the soil heterogeneity and deal with the coupling of solid-pore thermal properties through the mixing law at the pore scale. Together with the self-similar fractal law, the thermal conductivity model is upscaled from the pore scale to the representative element volume (REV) scale. During the process, heterogeneity factors are introduced to describe differences between local and bulk properties, such as porosity and saturation. Thermal property tests of Shanghai clay and Kaolin with varying porosities are then conducted to validate the proposed thermal conductivity model. Additionally, nineteen published datasets on thermal conductivity are also utilized to confirm the adaptability and robustness of the model. The results show that the proposed model predicts well the thermal conductivity of unsaturated soils. Moreover, when compared to existing models, the proposed model offers superior performance in describing the thermal properties of unsaturated soils.

Multivariate probability distributions for index and mechanical clay parameters in Shenzhen

Probabilistic site characterization is the cornerstone of data-centric geotechnics, which underpins the digitalization in geotechnical engineering. The essential aspect of probabilistic site characterization is to reasonably infer the probability density distributions of geotechnical parameters, based on field and laboratory data. In this paper, a clay property database, SZ-CLAY/11/5130, was established by collecting geotechnical investigation reports from 52 engineering projects in Shenzhen city. This SZ-CLAY/11/5130 database was first compared with the global clay database CLAY/10/7490 and the Shanghai clay database SH-CLAY/11/4051, in terms of their sample ranges, correlations, and scatter plots. Then, the multivariate probability distribution of eight clay parameters was established using the training records selected from SZ-CLAY/11/5130, to characterize the generic correlations between multiple clay parameters. Based on Bayesian theory, the posterior distributions of clay mechanical parameters from a construction site in Shenzhen city were updated using both the site-specific correlations and the generic correlations given by SZ-CLAY/11/5130. The constructed multivariate probability density distribution is necessary for this updating step.

Migration Behavior of NH4+ and Na+ in a Bentonite-Clay Mixed Soil Column and Numerical Simulation

The landfill barriers effectively prevented the migration of high-concentration pollutants, such as NH4+ and Na+, from the landfills to the surrounding environment. However, due to the high hydraulic head inside the landfill compared to the surrounding environment, NH4+ and Na+ can migrate towards the outside of the landfill barrier with the infiltrating solution, potentially causing harm to the surrounding environment. To address this, saturated mixed soil column samples made of bentonite and Shanghai clay, with bentonite contents of 3% and 10%, were used in this study. Permeability coefficients of the column samples in solutions are obtained by using permeation tests, and using NaCl and NH4Cl solutions with concentrations of 37.4 mmol/L and 74.8 mmol/L, respectively. The concentration-depth result of the column samples after permeation tests was determined using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Ion Chromatography (ICS-1100). Numerical simulations are used to investigate the effect of downstream solute concentration of the barriers on upstream solute concentration, dry density, and bentonite content of the barriers. The results indicate that the permeability coefficient of the soil column samples exposed to NH4Cl solution is greater than that of samples exposed to NaCl solution. This can be attributed to the stronger cation exchange of montmorillonite for NH4+, resulting in less swelling of the bentonite and more micro-pores, leading to an increase in the permeability coefficient. The concentration of Na+ is higher than that of NH4+ at the same depth of the column samples, indicating that Na+ has a higher migration rate in the column sample. This could be attributed to the relatively fast diffusion of Na+ on the surface of the bentonite and larger hydration radius of Na+. According to the simulation results, the recommended values for the bentonite clay mixed-soil barrier wall are as follows in this study: a thickness of 43 cm, a dry density of 1.5 g/cm3, and a bentonite content of 5%.

A state dependent constitutive model for unsaturated soil-steel interfaces considering bonding effect

The frictional behaviours between unsaturated soils and structure are controlled mainly by the complex phenomena that develop within a thin soil layer close to the contact area. The constitutive model for soil-structure interfaces (SSI) must be established to predict the performance of related soil-structuresystems accurately. Multi-stage suction-controlled ring shear (RS) tests with three different upper annular platens were carried out on an equal number of statically compacted samples of Shanghai clay under controlled suction states for a given net normal stress. A state dependent constitutive model for unsaturated soil-structure interfaces based on critical state concept considering bonding effect was proposed. In all stress conditions, the tested and predicted results for smooth interface and rough interface are well matched.

Constitutive modeling for cyclic responses of saturated soft clay under principal stress rotation induced by wave loads

A bounding surface model is proposed to reproduce the cyclic responses of saturated soft clay under principal stress rotation induced by wave loads. While extensively reported in experimental studies, this particular type of clay response has not been adequately addressed by constitutive modeling. By incorporating the mapping rules of relocatable projection center into the bounding surface, the plastic effects under pure principal stress rotation can be established. To represent the non-coaxiality during pure principal stress rotation, the presented model employs a non-coaxial flow rule sharing the same directions with the non-coaxial stress rate. However, it has been modified to related to the coaxial strain rate and the current stress ratio. Besides, the anisotropic elasticity is introduced for simulating the effects on plastic accumulation behavior during principal stress rotation. The developed model is validated through pure principal stress rotation tests of Shanghai clay and Wenzhou clay. The comparisons show that the proposed model can reasonably reproduce cyclic responses of saturated soft clay under principal stress rotation induced by wave loads.

Research on water migration and deformation characteristics of coastal composite strata under artificial ground freezing affected by seepage

When the artificial ground freezing (AGF) method is used to construct underground structures in the coastal composite strata, the differences between characteristics of soft clay and silty sand under seepage often increase the risk of freezing construction and the difficulty of deformation control. In order to find out water migration and deformation characteristics of costal composite strata under seepage condition in freezing process, a series of unidirectional freezing tests of silty sand and soft clay in Shanghai and AGF model experiments on composite strata of soft clay and underlying silty sand were carried out under seepage condition. The results of unidirectional freezing tests of single soil type showed that the layered deformation curve of silty sand and soft clay can be divided into two types that steep mode of silty sand and gradual mode of soft clay. It also indicates that the water migration is fully developed within soft clay during freezing; but most of water inside silty sand froze quickly, thus water migration was not developed as that much in soft clay. In AGF model experiments, the seepage contributed to the development of water migration inside composite strata. However, the deformation characteristics of composite strata under seepage condition were evidently different from that of unidirectional freezing tests. The frost heave force development of the lower silt sand in the area directly affected by seepage was gradual mode, while that of the upper soft clay in the area indirectly affected by seepage was steep mode. The above results demonstrated that influence on freezing process of seepage is greater than that of soil properties in freezing process. Through research results, some optimization strategies can be made for the relevant measures to deal with the adverse effects of seepage and frost heave in the practice of the freezing method for coastal composite strata.

Case study of post uplift in deep excavation of a subway station in thick soft clay using long pile foundations

There is growing engineering concern about the base heave and post uplift phenomena in deep excavation in soft clay, which may pose a risk of instability of retaining systems. The purpose here is to conduct a detailed case study on the post uplift observed in a 17.6-meter-deep braced excavation of a subway station in thick soft clay (total thickness up to 42 m) in Shanghai. In this case, a large uplift up to 87 mm unexpectedly developed for the post founded on a 30-meter-long pile foundation. Efforts were first made to examine the complex relationships between the post uplift with the excavation depth (H), Terzaghi’s safety factor against base heave (Fs) and maximum deflection of retaining wall. A simplified approach for soil-post-strut interaction analysis was then proposed and used for quantitative research. The working characteristics of the long pile foundation under low safety factor against base heave (Fs < 1.5) are summarized as following: (a) the back-analyzed neutral plane, where soil uplift equals the post uplift, lies at approximately 0.68 times the pile length from the pile top; (b) deep soil movement below the neutral plane results in the observed post uplift; (c) strut reaction plays a minor role in the restriction of post uplift. The influence of base treatment and excavation/construction procedures on post uplift and the principles of pile foundation design are also discussed in this paper.

Research on the Effect of Urease on the Energy Absorption in Shanghai Clay by Unconfined Compressive Test

Urease was attempted to improve the quality of soil as a novel material rising in recent years. The unconfined compressive tests (UCT) have been carried out on soils mixed with urease at a series of concentration. The unconfined compressive strength (UCS), energy absorption (Ei), cumulative energy absorption (Ei total) of 4 types of samples was also analyzed to evaluate the effect of reinforcement. Results from tests show that a certain concentration of urease has a positive effect on UCS and Ei of the clay, and it appeared best with the concentration of 3g/L. Urease of 4g/L achieved the maximum value in energy absorption within certain strain range. Conclusion is differed for Ei total as the values of samples mixed with urease are all decreased compared to control sample. It is important for civil engineers to understand the stabilizing mechanism of urease for design and construction of civil engineering project.

Performance of soils enhanced with eco-friendly biopolymers in unconfined compression strength tests and fatigue loading tests

Recently, biopolymers have emerged in soil stabilisation. The efficiency of biopolymers in ground improvement is mainly dependent on biopolymer types, soil types, biopolymer contents, curing periods, thermal treatment and mixing methods. However, the effect of the initial moisture content during sample preparation stages, on the mechanical behaviours of biopolymer-treated soils, has not been fully understood. The first part of this study probed the role of initial moisture content, in treating Shanghai clay with the xanthan gum by performing standard proctor compaction tests, Atterberg limit tests, unconfined compression strength (UCS) tests and microstructural analysis, while the second part contributed to capture the fatigue behaviours of the samples treated with an ideal moisture content by performing constant-amplitude and stepping-amplitude fatigue loading tests. Our results showed that the improvement appeared to occur from an average optimum moisture content for the treated soils (treated optimum), which was 3% wet of the untreated optimum. As the initial moisture content increased, the UCS values were elevated. However, there existed an ideal initial moisture content leading to the maximum strengthening efficiency. For xanthan gum content (i.e., the mass of xanthan gum with respect to the mass of dry soil) ranging from 1.0% to 5.0%, this ideal value was between 1.1 and 1.2 times the treated optimum. Our results also indicated that xanthan gum, as a biopolymer soil strengthener, was efficient in increasing either fatigue life or bearing capacity, under repeated loading for xanthan gum-soil matrices, when compared to untreated soils. While the untreated soils failed at the stress level of only half the UCS, the xanthan gum-treated soils with a 3.0% xanthan gum content sustained at the end of the tests. These data imply the potential use of xanthan gum in soil stabilisation, under repeated loads.