Shanghai Clay Research Articles

Microstructure investigation of soft clay subjected to triaxial loading

The microstructure properties of soft clay in Shanghai were discussed in light of experimental observations from a series of triaxial stress path tests and field emission scanning electron microscope (FESEM) tests performed on undisturbed soft clay specimens. The microstructure changes of soft clay were observed during different stress paths tests. The microstructure parameters were analysed by fractal geometry theory. The results show that the mechanical properties of soft clay are subject to the effect of stress paths, variations in initial confining pressure, which are controlled by its microstructure. Shanghai soft clay is manifested by a typical skeleton microstructure with large pores and flat laminated clay particles. The total volume and number of pores increase, and the distributions of soil aggregated particles and pores become looser due to loading; thus, the void ratio and fractal dimensions of the pores and soil particles are positively correlated with the axial strain. They reveal that cementation between the soil particles decreases and that the microstructure of soft clay is damaged by not only pore enlargement, but also degradation of the soil particles. Damage onset, damage development and failure occur during the entire process of soft clay subjected to triaxial loading. The microstructure parameters affected by reduced triaxial compression (RTC) tests change to a larger degree than those affected by conventional triaxial compression (CTC) tests, indicating that there is weaker cementation between the soil particles in the RTC tests due to the stress path and negative pore pressure generation. The microstructure evolution of natural soft clay during loading can be used to analyse the macro-mechanical behaviours.

Non-Associated Flow Rule-Based Elasto-Viscoplastic Model for Clay

We develop a non-associated flow rule (NAFR)-based elasto-viscoplastic (EVP) model for isotropic clays. For the model formulation, we introduce the critical state soil mechanics theory (CSSMT), the bounding surface theory and Perzyna’s overstress theory. The NAFR based EVP model comprises three surfaces: the potential surface, the reference surface and the loading surface. Additionally, in the model formulation, assuming the potential surface and the reference surface are identical, we obtain the associated flow rule-based EVP model. Both EVP models require seven parameters and five of them are identical to the Modified Cam Clay model. The other two parameters are the surface shape parameter and the secondary compression index. Moreover, we introduce the shape parameter in the model formulation to control the surface shape and to account for the overconsolidation state of clay. Additionally, we incorporate the secondary compression index to introduce the viscosity of clay. Also, we validate the EVP model performances for the Shanghai clay, the San Francisco Bay Mud (SFBM) clay and the Kaolin clay. Furthermore, we use the EVP models to predict the long-term field monitoring measurement of the Nerang Broadbeach roadway embankment in Australia. From the comparison of model predictions, we find that the non-associated flow rule EVP model captures well a wide range of experimental results and field monitoring embankment data. Furthermore, we also observe that the natural clay exhibits the flow rule effect more compared to the reconstituted clay.

Multivariate probability distribution of Shanghai clay properties

In this study, a database is compiled for 11 clay parameters covering 50 sites in Shanghai with 4051 data points (labeled as SH-CLAY/11/4051). These sites are distributed in the eight districts of Shanghai covering an area of 145 km2. This is the first sizeable multivariate soil database compiled at the municipal level. General information, including the geology of Shanghai, the borehole locations and depths, the types of parameters, and the basic statistics associated with these parameters are presented first. The quality of the compiled data is then assessed by comparing them with the data in the published global database CLAY/10/7490 with respect to marginal statistics. Results show that data points in SH-CLAY/11/4051 fall within the range of CLAY/10/7490. It is not surprising that local Shanghai data are more clustered than those in CLAY/10/7490. The range of values spanned by each clay parameter is smaller at a regional/municipal scale than at the global scale. A preliminary study to understand what is “unique” in Shanghai clay data is conducted by comparing global correlations between any two parameters with the corresponding Shanghai versions. Some correlations found in SH-CLAY/11/4051 (e.g., normalized effective vertical stress and normalized undrained shear strength) are observed to be similar to those found in CLAY/10/7490. However, there are other distinct correlations produced by SH-CLAY/11/4051, indicating a regional/municipal effect. This study constructs a multivariate probability distribution based on SH-CLAY/11/4051 to capture these distinct correlations between Shanghai clay parameters. One useful application of this multivariate distribution is act as prior for Bayesian updating.

Numerical analysis for the role of soil properties to the load transfer in clay foundation due to the traffic load of the metro tunnel

Abstract Soil properties play important role to the metro tunnel settlement induced by traffic load of metro operation. Numerical studies on this topic are still rare due to the lack of accurate dynamic constitutive models of clay. In this study, a new critical state based bounding surface plasticity model for natural structured clays is first developed. It is able to consider the initial and induced anisotropy of natural clay and the structure disturbance to the initial size of yield surface and to the initial adhesive stress by cyclic loading at low-stress levels. The applicability of the present model is evaluated through comparisons between the predicted and the measured results of numerous stress-path tests on Shanghai clay and Vallericca stiff clay. The model is then implemented as a user-defined model in a finite difference code. A typical tunnel in clay is simulated using the implemented model coupled with Biot’s consolidation, for which the stress distribution pattern in subsoils around tunnels due to train movement is investigated. The role of the dynamic constitutive model has been highlighted by comparing the simulation results of a static constitutive model and a dynamic constitutive model. The influences of the soil properties such as the anisotropy, structure ratio, and adhesive mean stress have been found significant to the load tranfer.

Twin EPB tunneling-induced deformation and assessment of a historical masonry building on Shanghai soft clay

This paper presents a comprehensive geotechnical and structural investigation of a historical multistory masonry building undercrossed by twin mechanized metro tunnels. The prototype is the case of the Shanghai Metro Line 11, which was recently constructed in Shanghai clay using two EPB machines. Field instrumentation is conducted before, during and after tunneling. The settlements along the greenfield surface and building sections are measured. A detailed three-dimensional finite element (FE) model is developed to reproduce the twin tunneling process under the masonry building characterized by the presence of uneven settlements to capture the interaction phenomenon occuring between the twin tunnels during construction. An advanced constitutive model, capable of simulating the behavior of Shanghai clay under the excavation, is employed for the numerical analyses. The FE model is first validated against a set of field instrumentation data, including both greenfield and building surface settlements in cases of single-tunnel passage and double-tunnel passage. Further analyses of the effect of the building presence on the ground movements are subsequently conducted. The results highlight that the presence of the building affects the settlement profiles and that significant differences are observed compared to greenfield conditions. The volume loss of the surface settlement trough after the passage of first tunel and twin tunnels is observed to be approximate 0.58% and 1.02%, respectively. A quantified assessment of the additional deformation of the historical building considering the existing deformation is conducted. No obvious damage to the building is observed during and after the tunnel drives, which confirms the successful tunnel passage beneath the historical building.

酒精作用下上海黏性土渗透特性试验研究

酒精作用下上海黏性土渗透特性试验研究

上海深厚软土层地铁车站基坑的侧向变形特征

上海深厚软土层地铁车站基坑的侧向变形特征

Stress path dependent small strain stiffness of Shanghai clay

The soil small strain stiffness plays an important role in many geotechnical applications such as machine foundations, deep excavations and earthquake ground response analysis. In this study, the small strain stiffness of saturated intact Shanghai clay specimen is measured in a hollow cylinder apparatus. The lateral earth pressure coefficient at rest is measured first during the one-dimensional consolidation. After consolidation, the very small strain stiffness is obtained by shear wave measurement using bender elements, while the degradation of small strain stiffness with strain is investigated by quasi-static loading with high resolution linear variable displacement transducers (LVDT). The effect of stress path on the small strain stiffness is also investigated. The results show that the small strain stiffness is nonlinear and significantly depends on the stress path.

Experimental Study on the Seismic Response of Subway Station in Soft Ground

Shaking table tests were conducted on typical models of subway structures subjected to several seismic shaking time histories to study seismic response of subway structures in soft ground as well as to provide data for validation of seismic design methods for underground structure. Three types of tests were presented herein, namely green field test, subway station test, and test for joint structure between subway station and tunnel. The similitude and modeling aspects of the 1g shaking table test are discussed. The seismic response of Shanghai clay in different depths was examined under different input waves to understand the acceleration amplification feature in both green field and in the presence of underground structure. Damage situation was checked on internal sections of both subway station and tunnels by halving the model structure. Structure deformation was investigated in terms of element strain under different earthquake loadings. The findings from this study provides useful pointers for future shaking table tests on underground structures/facilities, and the seismic response characteristic of underground structure derived from the shaking table test could be helpful for validating seismic design method for subway station.

Strength of cement-stabilised clay by hardness testing

This paper presents an investigation of the evaluation of the unconfined compressive strength (UCS) of cement-stabilised Shanghai clay by hardness testing. Laboratory tests including the hardness test and the unconfined compression test were carried out. An innovative decision was taken to use the Shore-A durometer to measure the hardness of cement-stabilised clay. Based on the regression analysis of laboratory test results, an exponential equation is proposed to express the unique relationship between the UCS and the degree of hardness. By measuring the hardness of jet grout columns installed in Shanghai clay, the proposed exponential equation was applied as a trial for predicting their UCS. Comparison results suggested that the exponential equation proposed in this study may be used to produce a quick prediction of the UCS by measuring the hardness of samples in the field.

The Quasi-static Properties of Natural Marine Clay under Tidal Low Frequency Cyclic Loading in Yangtze Estuary, China

The enclosure engineering of tidal flats behind Changxing Island is taken as the research background. In order to explore the undrained cyclic response of natural marine clay on tidal level periodic changing, the stress-controlled low frequency cyclic tri-axial tests and static creep tests are conducted on natural K0-consolidated Shanghai clay. In the series I tests, samples are cyclically sheared by low-frequency cyclic loading until stability or failure, the accumulative behavior and reversible strain are studied. The effects of confining pressure and stress ratio are also investigated. Significantly, loading frequency is a key factor in controlling the cyclic behavior. In the series II tests, special attention is given to the low frequency cyclic vibration effect on the undrained cyclic tests by comparison with the static creep tests. The critical stress ratio of low frequency cyclic behavior is less than static creep strength.

Experimental Study on the Combined Effect of Cyclic and Static Loads on the Mechanical Properties of the Saturated Soft Clay Material

The combined effect of cyclic and static loads on the mechanical properties of the soft clay was experimentally investigated by conducting undrained cyclic triaxial tests on Shanghai clay. The results show that an increment in either static or cyclic load increases excess pore pressures and axial strains. For a given value of combined cyclic and static loads, the mechanical properties of the soft clay are more sensitive to the cyclic load. Furthermore, the accumulated excess pore pressure and axial strain for a larger cyclic stress ratio and a lower combined stress ratio might overcome that for a lower cyclic stress ratio and a higher combined stress ratio. The mechanical properties of the soft clay after the cyclic load was unloaded were also discussed. It was observed that the excess pore pressure and axial strain under the static load alone decrease gradually with time. The trend of them largely depends on the ratio of cyclic load to static load.

Experimental identification of plastic shakedown behavior of saturated clay subjected to traffic loading with principal stress rotation

Traffic loading causes the rotation of principal stress axis and generates a heart-shaped stress path in the deviatoric stress space. Using dynamic hollow cylinder apparatus (DHCA), a series of cyclic heart-shaped and cyclic triaxial undrained tests were performed on Shanghai clay through simultaneously varying the torsional shear stress and the normal stresses. Experimental results show that the accumulated undrained responses of clay at different stress levels can be described by the shakedown approach. The permanent strain and energy dissipation were used to classify the accumulated deformation patterns, including plastic shakedown, cyclic plastic creep and ratcheting (incremental instability). For the plastic shakedown, the IPC (increment per cycle) of permanent axial strain becomes negligible and the IPC (area of hysteretic loop per cycle) of energy dissipation tends to reach a low constant level after cycling. For the cyclic plastic creep, the IPC of permanent axial strain (plastic strain increment per cycle) and the IPC of energy dissipation become nearly constant after a large number of cycles. For the ratcheting, the IPCs of strain and energy dissipation increase after a limited number of cycles. Based on these results, a new method is proposed to define the boundaries of shakedown behaviors using the effective cyclic stress ratio.

Laboratory model tests on water inrush in foundation pit bottom

Numerous deep foundation pits were constructed in China in the course of urbanization. Water inrush is one of the most important causes of foundation pit accidents. At present, few images and detail courses of the water inrush have been recorded and reported. The evidences for the hypothesis of water inrush calculation are not enough. In this study, model tests were performed to verify the water inrush course in foundation pit bottom. The water inrush modes of foundation pits were analyzed for the aquitards that included clay and silty clay in Shanghai, China. The deformation and failure characteristics of the layers under different water pressures were obtained. The deformation course without water inrush was divided into three stages: continuous deformation, progressive deformation, and equilibrium stages. The deformation course with water inrush was divided into four stages: continuous deformation, progressive deformation, shear failure, and water inrush and sinking stages. The course included creep deformation and micro-cracks development before failure. It was not an instantaneous phenomenon but a time-depending one. The pore water pressure was the response of aquitard to boundary water pressure, which indicated the seepage in low-permeable aquitard and the development of micro-cracks. The water inrush formula was verified and tested using the model test results. The limit equilibrium method had larger safe reserves, and the prestressed homogeneous continuous-beam method was relatively accurately.

An efficient parameter identification procedure for soft sensitive clays

The creep and destructuration characteristics of soft clay are always coupled under loading, making it difficult for engineers to determine these related parameters. This paper proposes a simple and efficient optimization procedure to identify both creep and destructuration parameters based on low cost experiments. For this purpose, a simplex algorithm (SA) with random samplings is adopted in the optimization. Conventional undrained triaxial tests are performed on Wenzhou clay. The newly developed creep model accounting for the destructuration is enhanced by anisotropy of elasticity and adopted to simulate tests. The optimal parameters are validated first by experimental measurements, and then by simulating other tests on the same clay. Finally, the proposed procedure is successfully applied to soft Shanghai clay. The results demonstrate that the proposed optimization procedure is efficient and reliable in identifying creep and destructuration related parameters.

Influence of sodium silicate and promoters on unconfined compressive strength of Portland cement-stabilized clay

Eco-friendly sodium silicate and promoters, which are compatible with cement and are used to obtain super properties, have been developed into a variety of soil stabilizers. This paper investigates the possibility of using cement and sodium silicate admixed with composite promoters to improve the strength of soft clay in Shanghai, China. The influential factors involved in this study are the type of promoters, the proportion of each binding agent, the binder content, and the curing time. The unconfined compressive strength of stabilized clay at different ages is tested. Based on an orthogonal experiment, the selected clay stabilizer (CSCN) is determined. More importantly, it is found that much less CSCN is needed to achieve the equivalent improvement in strength compared with cement, which illustrates that CSCN can be a more effective and eco-friendly clay stabilizer. Mineralogical and microstructural tests are performed to reveal the possible mechanisms controlling the strength development. The effect of CSCN on cement hydration and pozzolanic reactions is discussed. Microstructural analyses confirm the formation of hydration and pozzolanic products, and show that the clay tends to form more compact microstructures after being stabilized with CSCN.

Laboratory evaluation on the effectiveness of polypropylene fibers on the strength of fiber-reinforced and cement-stabilized Shanghai soft clay

This paper presents a laboratory evaluation on the strength behavior of cement–clay admixture improved by polypropylene fiber. Waste polymer textile bags are harmful to the environment and sustainable means to reuse them are urgently being sought. A novel method to recycle these waste polymer textile bags is applying them in soft soil improvement projects, such as compacted-pavement base/subbases. In order to verify the effectiveness of fiber bundles used in soil mixing, a series of laboratory investigation are conducted on fiber-reinforced cement-improved soft Shanghai clay. In the tests, two types of polymer fibers were employed; the first one is monofilament polypropylene fiber and the other is fiber bundles split from polymer textile bags. The tests were conducted using unconfined compressive strength (UCS) test after the specimen of fiber–soil–cement admixture were cured for some period. The results show that fiber additive can significantly improve the strength and ductility of the cement treated Shanghai clay. The UCS of fiber–soil–cement admixture is related to both content and length of fiber. Both two kinds of fiber-reinforced cement clay reached their peak strength at fiber content of 0.5%. UCS will slowly reverse if the fiber content continues to increase. Even though the polypropylene fiber works better than the fiber bundles, the difference is less than 5%. These results indicate that the fiber bundles split from polymer bags can be used as the reinforcement in soil mixing, thus providing a novel approach to treat waste polymer textile bags.

Consolidation and creep behaviors of two typical marine clays in China

This paper presents an experimental investigation into the deformation characteristics of two typical marine clays obtained from Dalian and Shanghai, respectively, in China. Three kinds of laboratory tests, i.e. conventional oedometer tests, one-dimensional and triaxial creep tests were carried out. The results obtained from consolidation tests demonstrate linear e−logσ v relationships for Shanghai clay at normally consolidated state, while partly or even global non-linear relationships for Dalian clay. The compression index C c for both clays follows the correlation of C c = 0.009(w L −10) where w L is the liquid limit of soil. The relationship between logk v (k v is the hydraulic conductivity of soil) and void ratio e is generally linear and the hydraulic conductivity change index \(C_{k_v }\) can be described by their initial void ratio for both clays. The secondary compressibility of Dalian clay lies in medium to high range and is higher than that of Shanghai clay which lies in the range of low to medium. Furthermore, based on drained triaxial creep tests, the stress-strain-time relationships following Mesri’s creep equation have been developed for Dalian and Shanghai clays which can predict the long-term deformation of both clays reasonably well.

Depositional environment and geotechnical properties of Shanghai clay: a comparison with Ariake and Bangkok clays

Shanghai soft clay was deposited in the Yangtze River Delta in the Late Pleistocene and Holocene eras. The general engineering properties of this clay have been studied in the last few decades. However, limited research on its depositional history and impact on geotechnical properties is available. In this paper, the depositional history of Shanghai clay was reproduced based on sea level changes and other geological information. The unique depositional environment was compared with two other Asian clays, Ariake clay in Japan and Bangkok clay in Thailand. The physical, mineralogical and mechanical properties of different strata were investigated by a series of laboratory tests. It was found that silt-sized grain particles dominated the clay materials and illite is the major clay mineral, while clay-sized grains and smectite are predominant in Ariake and Bangkok clays. The liquid limit, plastic limit and activity are lower than the Ariake and Bangkok clays. Oedometer tests and undrained triaxial tests indicated a less sensitive structure. Stronger hydraulic conditions (faster stream velocity) and the origin of material in the middle and lower reaches of the Yangtze River account for the large amount of silt-sized particles and illite. A less saline depositional environment is the main reason for the weaker structure in Shanghai clay than the two other Asian clays.

Rapid field evaluation of the strength of cement-stabilized clayey soil

This paper presents an investigation into the hardness and unconfined compressive strength (UCS) of cement-stabilized Shanghai clay. Various specimens with different cement content and curing time were made in order to conduct hardness tests and unconfined compression tests. The Shore-A durometer was used in a pioneer manner to measure the hardness of stabilized clay. Hardness tests suggest that the measured values of degree of hardness ranged from 1 to 87 DH, and the unconfined compression tests showed that the measured UCS varied from 56 to 1,061 kPa. Test results indicate that the average degree of hardness decreases dramatically with an increase in the clay-water/cement ratio. It is shown that the relationship between the degree of hardness and the curing time is approximately linear in the semi-logarithmic plot. An exponential equation is proposed to express the unique relationship between the UCS (q u ) and the ratio of degree of hardness (S/S r) by regression analysis. The expected prediction range of UCS using the Shore-A durometer was calculated to be between 0.04 and 1.79 MPa. The exponential equation proposed in this study may be used to produce a quick prediction of UCS by measuring the hardness of samples in the field.