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    Abstract

    Underestimating the influence of principal stress rotation on the foundation soil during the construction is the cause of some engineering accidents. Series of experiments were done to study the strength characteristics of intact soft clay after undrained monotonic principal stress rotation. It was found that compared with the progress of undrained monotonic principal stress rotation, the principal stress direction when samples failed influenced the soils’ strength much more obviously. But the pore water pressure generated in the tests including principal stress rotation was much higher than that in the test of fixed principal stress direction shear. It was due to the shearing contraction of intact soft clay caused by principal stress rotation. A modified Lade-Duncan failure criterion was used to normalize these testing results and unify the soil’s failure criterion including principal stress rotation. It showed that initial anisotropy was one of the most important determinate factors of intact clay’s strength when principal stress rotation occurred during the construction.


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    Abstract

    The three-dimensional effects of pile head and the applicability of plane-section assumption are main problems in low-strain dynamic tests on cast-in-situ concrete thin-wall pipe piles. The velocity and displacement responses were calculated by a theoretical formula deduced by the authors. The frequency and influencing factor of high-frequency interference were analyzed. A numerical method was established to calculate the peak value and arrival time of incoming waves on top of the piles. The regularity along circumferential and the influence of radius or impulse width were studied. The applicability of plane-section assumption was investigated by comparison of velocity responses at different points in the sections at different depths. The waveform of velocity response at different points forked after the first peak, indicating that the propagation of stress waves did not well meet the plane- section assumption.


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    Abstract

    An adaptive criterion for shear yielding as well as shear failure of soils is proposed in this paper to address the fact that most criteria, including the Mohr-Coulomb criterion, the Lade criterion and the Matsuoka-Nakai criterion, cannot agree well with the experimental results when the value of the intermediate principal stress parameter is too big. The new criterion can adjust an adaptive parameter based on the experimental results in order to make the theoretical calculations fit the test results more accurately. The original elliptic-parabolic yield surface model can capture both soil contraction and dilation behaviors. However, it normally over-predicts the soil strength due to its application of the Extended Mises criterion. A new elliptic-parabolic yield surface mode is presented in this paper, which introduces the adaptive criterion in three-dimensional principal stress space. The new model can well model the stress-strain behavior of soils under general stress conditions. Compared to the original model which can only simulate soil behavior under triaxial compression conditions, the new model can simulate soil behaviors under both triaxial compression conditions and general stress conditions.


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    Abstract

    The high-frequency interference exists obviously in low strain integrity testing of large-diameter pipe pile when a transient point load is applied. An analytical solution of vertical vibratory response of large-diameter pipe piles in low strain testing is deduced in this paper. The analytical solution is verified by both numerical simulation and model test results. The time-domain velocity responses on pile top are analyzed. The calculation results indicate that the time-domain responses at various points suffer different high-frequency interferences, thus the peak values and phases of different points are different. The influence of vibratory modes on high-frequency interference is analyzed. It is found that the high-frequency interference at 90° point mainly derives from the second flexural mode, but for other points it mainly originates from the first flexural mode. The factors affecting the frequency and peak value of interference waves have been investigated in this study. The results indicate that the larger radius angle between the receiving and 90° points leads to greater peak value of high frequency wave crest. The least high-frequency interference is detected at the angle of 90°. The frequency of interference waves is decreased with the increase of pile radius, while the peak value is almost constant. The frequency is also related to pile modulus, i.e. the larger pile modulus results in greater frequency. The peak value varies with impulse width and soil resistance, i.e., the wider impulse width and larger soil resistance cause smaller peak value. In conclusion, the frequency of interference waves is dependent on the geometrical and mechanics characteristics of the piles such as pile radius and modulus, but independent of the external conditions such as impulse width and soil resistance. On the other hand, the peak value of interference waves is mainly dependent on the external conditions but independent of the geometrical and mechanics characteristics of the piles. In practice, some external measures should be adopted to weaken high-frequency interference such as using soft hammer, hammer cushion and adopting suitable receiving point.


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    Abstract

    A unified nonlinear strength criterion (i.e UNS criterion) is proposed, for the sake of versatility, to capture the complex strength behaviors of frictional materials in geotechnical field. It covers wide ranges on the meridian and octahedral planes to describe nonlinear strength behaviors of soils. The Modified Cam-Clay model, incorporating the unified nonlinear strength criterion, is employed as an example to derive working mathematical equations and to illustrate yielding surfaces in three-dimensional stress space for improving the model’s predictive capability. The unified nonlinear strength criterion, demonstrated here, is capable of capturing the experimental results of different types of soils on the meridian and octahedral planes. In addition, the revised model, based on this unified nonlinear strength criterion, though very simple, is versatile to predict the true triaxial test results from literature when considering the influences of the intermediate principal stress on strength and deformation under complex stress conditions.


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    Abstract

    Laboratory tests on the large post-liquefaction deformation of saturated Nanjing fine sand were performed by using a hollow cylinder apparatus. The stress-strain responses and the characteristics of excess pore water pressure after liquefaction were studied. It was found that the relationship between deviatoric stress and axial strain presented a sigmoid curve, and there was a good linearity relationship between normalized pore water pressure and deviatoric stress. On this basis, a constitutive model of stress-strain responses and a dissipation model of excess pore water pressure were established. It was found that the results predicted by the two models were in good agreement with the experimental data. The influence of relative densities and confining pressure on the characteristics of liquefied soil were studied. The results showed the relative densities and initial effective confining pressure all had an important influence on the stress-strain responses of liquefied saturated Nanjing fine sand. However, the dissipation model of excess pore water pressure after liquefaction was only affected by the confining pressure.


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    Abstract

    A 3D bounding surface model is established for rockfill materials, which can be applied to appropriately predict the deformation and the stabilization of rockfill dams. Firstly, an associated plastic flow rule for rockfill materials is investigated based on the elaborate data from the large-style triaxial compression tests and the true triaxial tests. Secondly, the constitutive equations of the 3D bounding surface model are established by several steps. These steps include the bounding surface incorporating the general nonlinear strength criterion, stress-dilatancy equations, the evolution of the bounding surface and the bounding surface plasticity. Finally, the 3D bounding surface model is used to predict the mechanical behaviors of rockfill materials from the large-style triaxial compression tests and the true triaxial tests. Consequently, the proposed 3D bounding surface model can well capture such behaviors of rockfill materials as the strain hardening, the post-peak strain softening, and the volumetric strain contraction and expansion in both two- and three-dimensional stress spaces.


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    Abstract

    Rockfill material is widely used for construction of high rockfill dam due to its facility, economical cost, high strength and effective aseismatic property. It is provoked profoundly to propose a suitable constitutive model for a better application of this material. The dilatancy equation of rockfill material plays a significant role in the constitutive model. For the sake of simplicity, a dilatancy equation is established by the linear least square method on the basis of the rearranged data of rockfill material in the true triaxial tests. Based on the fact that the rearranged data at different initial confining pressures are aligned in a narrow band, the dilatancy behavior of rockfill material is independent of the initial confining pressure. However, different from the initial confining pressure, both the intermediate principal stress ratio and the specimen density exhibit a remarkable influence on the dilatancy behaviors of rockfill material. Furthermore, the predictions of the proposed dilatancy equation are in a good agreement with the rearranged test data of rockfill material at different specimen densities and stress paths.


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    Abstract

    This paper presents finite element analyses of negative skin friction on a single pile under various conditions. Negative skin friction is a common problem if a pile is designed in a highly compressible soil. There are two most important parameters in estimating the load caused by negative skin friction: (1) the distribution and magnitude of skin friction and (2) the location of the neutral plane. The neutral plane is the location where the pile and soil settle the same amount or have no relative displacement. Negative skin friction is a very complex phenomenon influenced by many factors. In this paper, a two-dimensional axisymmetric model is built in the finite element program, ABAQUS. The model is first verified with a known case history. A systematic parametric analysis is performed to investigate the influence on both the neutral plane and the magnitude and distribution of negative skin friction along the pile length of various influencing factors, including the consolidation time, the properties of pile/soil interface, the lateral earth pressure coefficient, pile-soil limiting displacement, the intensity of surcharge, and soil stiffness. Based on the analyses, it is found that the location of the neutral plane is significantly influenced by the consolidation time and the stiffness of bearing layer. The distribution and magnitude of negative skin friction is influenced mainly by the pile/soil interface, soil compressibility, and the surcharge intensity. Based on the field measurements from literature and this investigation, a simple design procedure is proposed for estimating the pile load caused by negative skin friction.


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    Abstract

    A bounding surface model incorporating a unified nonlinear strength criterion is proposed. The proposed bounding surface model contains 9 model parameters, which can be determined from the conventional triaxial tests. The bounding surface model can reproduce such behaviours as the strain hardening, the post-peak strain softening, and the volumetric strain contraction and expansion. Based on the comparisons between the predictions and the test results, the proposed strength criterion and model can well reproduce the experimental results of the strength and stress-strain behaviours of rockfill material in three-dimensional stress space. The strength behaviour of rockfill material is summarized as: (a) the failure stress ratio decreases with the initial confining pressure on the meridian plane; (b) the failure deviatoric stress decreases with the Lode angle from 0° to 60° on the deviatoric plane. The stress ratio decreases with increasing one of such factors as the initial void ratio, the intermediate principal stress ratio and the minor principal stress at the same strain when the other factors are given.


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    Abstract

    EPS composite soil is one type of premixed lightweight fills studied by numerous researchers. However, one aspect that has not been fully understood is the creep behaviors which may have significant effect on the design and application of EPS composite soil. In this paper, the results of a series of oedometer creep tests and triaxial undrained creep tests on EPS composite soil were presented. Four main influencing factors were identified and their effects on the creep behaviors of EPS composite soil were studied. Three well established creep models, namely, Findley model, Singh & Mitchell model, and Mesri model, were used to simulate the creep behavior of EPS composite soil. This study shows that the Findley creep model fits the test results the best. A semi-empirical creep model was also proposed to model the creep behavior under axisymmetric conditions. In this model, the creep strain was divided into instant and viscous elastic strain as well as instant and viscous plastic strain which were simulated by element models and empirical equations, respectively. It was shown that the proposed creep model was able to precisely predict the creep strain of EPS composite soil.


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    Abstract

    The application of fiber reinforced plastic (FRP), including carbon FRP and glass FRP, for structural repair and strengthening has grown due to their numerous advantages over conventional materials such as externally bonded reinforcement (EBR) and near-surface mounted (NSM) strengthening techniques. This paper summarizes the results from 21 reinforced concrete beams strengthened with different methods, including externally-bonded and near-surface mounted FRP, to study the strain coordination of the FRP and steel rebar of the RC beam. Since there is relative slipping between the RC beam and the FRP, the strain of the FRP and steel rebar of the RC beam satisfy the quasi-plane-hypothesis; that is, the strain of the longitudinal fiber that parallels the neutral axis of the plated beam within the scope of the effective height (h0) of the cross section is in direct proportion to the distance from the fiber to the neutral axis. The strain of the FRP and steel rebar satisfies the equation: ɛFRP=βɛsteel, and the value of β is equal to 1.1–1.3 according to the test results.


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    Abstract

    X-section cast-in-place concrete pile (XCC pile) is a new type of pile foundation, which has an X-shaped cross section. Compared to the traditional circular pile of the same cross-sectional area, the bearing capacity of an XCC pile is higher due to increased cross-sectional perimeter. Since Geddes solution is based on St. Venant’s principle, leading to the results independent of the cross-sectional geometry and size, large differences are induced when estimating the soil stress distribution for XCC pile foundations. This paper derives a modified analytical solution, which is dependent on the cross-sectional geometry of XCC pile, from Geddes solution. Validation of this modified solution was conducted through three-dimensional numerical analysis and proven more suitable for XCC pile foundations. Parametric study on three geometrical parameters is conducted using this modified solution. The results indicate that the stress in founding soil due to skin friction decreases with increasing pile radius and central angle of concave, but increases with increasing length of flat side. The stress due to end-bearing decreases with increasing pile radius and length of flat side, but increases with increasing central angle of concave. From the parametric studies, the recommended dimensions of XCC pile radius, length of flat side, and central angle of concave are recommended ranges from 200 to 600 mm, 30 to 60 mm, and 90° to 150°, respectively.


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    Abstract

    The particle breakage during specimen compaction had more significant influence on the position of the breakage critical-state line (BCSL) of Tacheng rockfill material (TRM) in the e-lnp′ plane than the particle breakage during shearing, based on the large-scale triaxial compression tests on TRM in a wide range of densities and pressures. The state-dependent dilatancy and the plastic modulus were correlated to the breakage index, based on the formulations of the BCSL of TRM in the e-lnp′ plane. The state-dependent model considering particle breakage was proposed for TRM within the framework of the generalized plasticity theory. The proposed model contained fourteen material constants. The test data of TRM from Group A were adopted to determine these material constants, while the test data from Group B were used independently to validate the model predictive capacity. The comparisons between model simulations and test data illustrated that the model with consideration of particle breakage could well represent the stress-strain behaviors of TRM, e.g., the strain hardening and volumetric contraction behaviors at a loose state and the strain softening and volumetric expansion behaviors at a dense state, and also the particle breakage behaviors of TRM.


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    Abstract

    The propagation of stress waves in a large-diameter pipe pile for low strain dynamic testing cannot be explained properly by traditional 1D wave theories. A new computational model is established to obtain a wave equation that can describe the dynamic response of a large-diameter thin-walled pipe pile to a transient point load during a low strain integrity test. An analytical solution in the time domain is deduced using the separation of variables and variation of constant methods. The validity of this new solution is verified by an existing analytical solution under free boundary conditions. The results of this time domain solution are also compared with the results of a frequency domain solution and field test data. The comparisons indicate that the new solution agrees well with the results of previous solutions. Parametric studies using the new solution with reference to a case study are also carried out. The results show that the mode number affects the accuracy of the dynamic response. A mode number greater than 10 is required to enable the calculated dynamic responses to be independent of the mode number. The dynamic response is also greatly affected by soil properties. The larger the side resistance, the smaller the displacement response and the smaller the reflected velocity wave crest. The displacement increases as the stress waves propagate along the pile when the pile shaft is free. The incident waves of displacement and velocity responses of the pile are not the same among different points in the circumferential direction on the pile top. However, the arrival time and peak value of the pile tip reflected waves are almost the same among different points on the pile top.


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    Abstract

    A unified plastic modulus parameter for the bounding surface plasticity model is introduced in order to maintain the identical responses of modeling for both the two-dimensional and three-dimensional stress space with the same model parameters. Also discussed are the influences of the plastic modulus parameter on the stress-strain relationship and the plastic modulus. The model is more sensitive in modeling the stress strain responses when the plastic modulus parameter is small. The plastic modulus parameter has a great influence on the magnitude of the plastic modulus, especially at the initial loading stage. The plastic modulus asymptotically tends to zero at the end of loading.


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    Abstract

    This paper presents an elastic solution to the pressure-controlled elliptical cavity expansion problem under the anisotropic stress conditions. The problem is formulated by the assumption that an initial elliptical cavity is expanded under a uniform pressure and subjected to an in-plane initial horizontal pressure 0 and vertical pressure σ0 at infinity. A conformal mapping technique is used to map the outer region of the initial elliptical cavity in the physical plane onto the inner region of a unit circle in the phase plane. Using the complex variable theory, the stress functions are derived; hence, the stress and displacement distributions around the elliptical cavity wall can be obtained. Furthermore, a closed-form solution to the pressure-expansion relationship is presented based on the elastic solution to the stress and displacement. Next, the proposed analytical solutions are validated by comparing with the Kirsch’s solution and the finite element method (FEM). The solution to the presented pressure- controlled elliptical cavity expansion can be applied to two cases in practice. One is to employ the solution to the interpretation of the shear modulus of the soil or rocks and the in-situ stress in the pre-bored pressuremeter test under the lateral anisotropic initial stress condition. The other is the interpretation of the membrane expansion of a flat dilatometer test using the pressure-controlled elliptical cavity expansion solution. The two cases in practice confirm the usefulness of the present analytical solution.


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    Abstract

    An elastoplastic constitutive model for overconsolidated clays is established in the framework of the critical state theory and bounding surface plasticity theory. The bounding surface is defined as the maximum yield surface in the loading history. A yielding ratio, i.e., an internal variant, is defined as the size ratio of the current yield surface to the corresponding bounding surface. The yielding ratio instead of the overconsolidation ratio (OCR) is used to evaluate the strength and stress-strain behaviors of overconsolidated clays in the shearing process. The bounding stress ratio incorporating the effect of the yielding ratio is used to characterize the potential failure strength of the overconsolidated clays. The dilation stress ratio taking into account the effect of the yielding ratio is applied to describe the dilatancy behaviors of the overconsolidated clays. Comparisons between model predictions and test data show that the proposed model could well capture the strength and stress-strain behaviors of normally consolidated and overconsolidated clays.


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