Unsaturated Ground Research Articles

Analytical approach for vertical dynamic responses of stiffened deep cement mixing pile in unsaturated ground

Stiffened deep cement mixing (SDCM) piles present excellent engineering performance compared with conventional pipe piles and thus are often applied to the structure foundation in coastal soft-soil area. This study intends to explore the vertical dynamic behavior of a SDCM pile in unsaturated soil by developing an analytical approach. In the proposed approach, the SDCM pile divides into two parts. The first part is considered as a composite pile formed by a prestressed high-strength concrete (PHC) pipe pile and a cement mixing pile through high-strength bonding, and the second part formed by the PHC pipe pile and an unsaturated soil column. The closed-form solution for the dynamic impedance of the SDCM pile has been determined and then verified by contrasting with the existing solutions, where the dynamic impedance is mainly characterized by dynamic stiffness and dynamic damping at SDCM pile head in the vertical direction. Numerical discussions are finally implemented to analysis the dependence of physical parameters of cement mixing pile, PHC pipe pile, and unsaturated soil on the vertical dynamic impedance of SDCM pile. It can be concluded that increasing the depth, radius and elastic modulus of the cement mixing pile at relatively low load excitation frequencies is beneficial for enhancing the vertical vibration resistance of the SDCM pile, while the reverse is true at higher load excitation frequencies. Quantitatively, under relatively low load excitation frequencies, the reinforcement depth of the cement mixing pile should not exceed 2/3 of the length of the PHC pipe pile, while its elastic modulus should be 5 ‰ to 2 % of the elastic modulus of the PHC pipe pile.

Thermo-hydro-mechanical behaviour of energy tunnel in unsaturated soils

Soils surrounding energy tunnels are often unsaturated, particularly in areas with a low water table. All studies of the mechanical behaviour of energy tunnels focused on saturated or dry soils. Through numerical simulations using a coupled thermo-hydro-mechanical model for unsaturated soils, this study aims to improve the understanding of the interactions between energy tunnels and unsaturated ground. The influences of water table position and soil type on the mechanical behaviour of energy tunnels were investigated. Results show that the thermally-induced lining convergence of energy tunnels does not change much with the water table position. Nevertheless, the other mechanical responses (e.g., settlements of ground and tunnel and lining stress changes) highly depend on the water table position. These mechanical responses are more significant when the water table is higher than the tunnel center, except for the larger settlements at a lower water table in clay. Furthermore, the above responses are underestimated if soils above the water table are assumed to be completely dry. The degree of underestimation is most significant for the clay, followed by silt and sand, e.g., a maximum of 54%, 23% and 10% in the change of circumferential stress, respectively. These results highlight that the unsaturated soil model should be used to evaluate the thermo-hydro-mechanical responses of energy tunnels in unsaturated soils, particularly for the clay ground.

Analytical approach for lateral dynamic behaviours of pipe piles enhanced by cement-improved soil in unsaturated ground

Analytical approach for lateral dynamic behaviours of pipe piles enhanced by cement-improved soil in unsaturated ground

Characterizing High Rate GNSS Velocity Noise for Synthesizing a GNSS Strong Motion Learning Catalog

Data-driven approaches to identify geophysical signals have proven beneficial in high dimensional environments where model-driven methods fall short. GNSS offers a source of unsaturated ground motion observations that are the data currency of ground motion forecasting and rapid seismic hazard assessment and alerting. However, these GNSS-sourced signals are superposed onto hardware-, location- and time-dependent noise signatures influenced by the Earth’s atmosphere, low-cost or spaceborne oscillators, and complex radio frequency environments. Eschewing heuristic or physics based models for a data-driven approach in this context is a step forward in autonomous signal discrimination. However, the performance of a data-driven approach depends upon substantial representative samples with accurate classifications, and more complex algorithm architectures for deeper scientific insights compound this need. The existing catalogs of high-rate (≥1Hz) GNSS ground motions are relatively limited. In this work, we model and evaluate the probabilistic noise of GNSS velocity measurements over a hemispheric network. We generate stochastic noise time series to augment transferred low-noise strong motion signals from within 70 kilometers of strong events (≥ MW 5.0) from an existing inertial catalog. We leverage known signal and noise information to assess feature extraction strategies and quantify augmentation benefits. We find a classifier model trained on this expanded pseudo-synthetic catalog improves generalization compared to a model trained solely on a real-GNSS velocity catalog, and offers a framework for future enhanced data driven approaches.

Semi-analytical solution of pore-water pressure in unsaturated ground and infinite slope considering highly nonlinear soil hydraulic properties

A multi-exponential function (ME) is proposed to depict the highly nonlinear soil water retention curve and hydraulic conductivity function. Then, semi-analytical solution of pore-water pressure in an unsaturated infinite slope at steady state is derived using ME to depict soil hydraulic properties. The solution is verified with experimental and numerical results. It is found that ME can better describe the highly nonlinear hydraulic properties of soil than the single exponential function (SE), which is commonly-used previously. At steady state, the adoption of SE results in overestimating negative pore-water pressure (PWP; i.e., matric suction) by about 25 kPa and slope’s factor of safety (FOS) by 100% under rainfall, but underestimating negative PWP by about 50 kPa under evaporation. The difference between PWP and FOS calculated by using SE and ME becomes more significant for soil with bimodal hydraulic conductivity function than that with unimodal one. This difference reduces at a larger slope angle and deeper depth. Moreover, this difference generally increases at a lower SME/SSE, where SME and SSE represent the suction at which the unsaturated hydraulic conductivity expressed by ME and SE equals the applied rainfall intensity or evaporation rate, respectively. When SME/SSE is greater than 0.1, the difference basically diminishes.

Analytical solution for the horizontal dynamic response of strength composite piles in fractional viscoelastic unsaturated ground

The analytical representation of the dynamic interaction between a strength composite (SC) pile and unsaturated soil under lateral excitations at the pile head is studied using three-dimensional continuum modelling. The fractional standard line solid (FSLS) model is utilized in the governing equation of unsaturated soil to characterize the flow-independent viscosity of the soil skeleton. Timoshenko beam theory is employed to describe the horizontal dynamic behaviour of SC piles composed of cement-soil mixing (CM) piles and precast high-strength concrete (PHC) pipe piles. A closed series form solution of the horizontal dynamic behaviour for an SC pile embedded in unsaturated soil under lateral excitation at the pile head is deduced theoretically. The solutions of the proposed model are compared with those captured from existing solutions. Finally, the effects of physical parameters in the SC pile-unsaturated soil system on the horizontal dynamic behaviour of the SC pile are evaluated with analytical examples and parametric study. The results indicate that the radius and elastic modulus of the SC pile have a critical influence on the horizontal dynamic behaviour of the SC pile, the FSLS model parameters are the key factors influencing the horizontal dynamic behaviour of the SC pile, and the horizontal dynamic behaviour of the SC pile varies with the change in liquid saturation of unsaturated soil. The research results provide theoretical references for the anti-vibration design and safe operation of SC piles.

Hydrochemical Characterization and Assessment of Groundwater Quality in the Mining Environment of Afema Township (South-East of Côte d’Ivoire)

Afema Township, located in the department of Aboisso, was mined from 1992 to 1998. At the end of the exploitation, the sites did not undergo any real rehabilitation work. In order to determine the impact of these mining activities on the physicochemical characteristics and quality of groundwater, this study was undertaken. The results of the water analyses collected in the mining area were processed using Piper's triangular diagram and Principal Component Analysis (PCA). The methodological approach consisted, first of all, in determining the hydro-facies of the waters studied, then in dividing them into different groups on the basis of their hydro-chemical similarity and in identifying the factors likely to explain both the origin of the parameters studied and their correlation. Finally, the overall quality of these waters was estimated from the calculation of the quality index. The results showed that the waters studied were divided into two main families: chloride-calcium-magnesium waters and bicarbonate-calcium-magnesium waters. They were on the whole weakly mineralized, with an average electrical conductivity of 195.76 μS.cm-1. This mineralization was controlled by two essential phenomena, the residence time of water in the aquifers and surface infiltration and leaching from unsaturated ground. The physico-chemical quality of the groundwater studied complied with the standards of potability recommended by the WHO. However, the results also showed high levels of cadmium (0.052 mg.L-1) and lead (0.058 mg.L-1); this explained the overall poor quality of these waters according to the water quality index. Thus, the studied waters presented a real risk for human consumption. The consumption of these waters by local communities exposes them to health risks.

Study on vibration isolation performance of composite multilayer wave impeding block based on wave impedance ratio under an underground dynamic load

Based on the theory of single-phase elastic medium and unsaturated porous medium, the vibration isolation effect of composite multilayer wave impeding block (WIB) in the unsaturated ground under an underground dynamic load is investigated. The results show that the best vibration isolation effect can be obtained by designing the wave impedance ratio between the composite multilayer WIB and unsaturated ground. The composite multilayer WIB improves the vibration-damping bandwidth compared with the homogeneous WIB. The vibration isolation effect is better the closer the embedded depth of the composite multilayer WIB is to the vibration source, and its vibration isolation effect increases significantly with the increase of thickness, but when the thickness of the WIB exceeds a certain critical thickness, its vibration isolation effect decreases with the increase of thickness. Soil saturation has a significant effect on the vibration isolation effect of composite multilayer WIB in the unsaturated ground, and the composite multilayer WIB can achieve a better vibration isolation effect at low saturation.

Effects of thermal cycles on soil behaviour: theoretical and experimental studies

Fundamental understanding and proper modelling of soil behaviour under thermal cycles are increasingly important and essential for the analysis and design of many emerging infrastructures, such as geothermal structures and embankment-atmosphere interactions under a changing climate. Previous studies mainly focus on monotonic thermal loading of thermo-mechanical behaviour of soils. Based on a unified, state-dependent theoretical framework in the form of compliance matrix, a new constitutive model is developed to simulate the cyclic thermo-mechanical behaviour of saturated and unsaturated soils. This new bounding surface model is formulated in terms of Bishop’s stress and suction. Apart from the loading and bounding surfaces, a memory surface is incorporated in the model to simulate cyclic thermal behaviour of soils. To verify the new model, computed results are compared with measured data from cyclic heating-cooling tests on saturated and unsaturated soils at various suctions. Based on this new model, two engineering applications are analysed including cyclic thermally loaded floating energy pile foundations and a deep excavation in the unsaturated ground. Consistent results are obtained between computed and measured data.

Analytical solution for axisymmetric consolidation of unsaturated composite foundation incorporating permeable columns and impermeable columns

AbstractComposite foundation is a widely used foundation treatment technology in geotechnical engineering. This study are derived governing equations to axisymmetric consolidation of unsaturated composite foundation (USCF) incorporating permeable columns and impermeable columns under instantaneous loading, and obtained the final solutions of excess pore pressures (EPPs) by using mathematical methods such as the variable separation, decoupling, finite Hankel transform and its inverse transform. The analytical solution obtained under free strain conditions is more general and representative, as it can be readily degraded to solutions for a variety of consolidation models and the EPPs can be conveniently calculated at any point in the calculation cell. Through degrading proposed solutions to the special case to verify their correctness, the comparative results were found well‐matched to the existing literature solutions. The effect of various parameters on the consolidation performance of USCF was analyzed, where the studied parameters include the compression modulus of columns, radius ratio for permeable columns, and the number of impermeable columns. The results show that increasing the compression modulus and area replacement rates of columns can accelerate consolidation and reduce settlement, and the latter is more effective. Compared to reinforced composite foundations with only a single form of the permeable or impermeable column, the combination of permeable and impermeable columns can significantly shorten the consolidation time and enhance the bearing capacity of the unsaturated ground.

Numerical analysis of wave propagation and vibration attenuation effects of periodic scattering piles in unsaturated poroviscoelastic ground

ABSTRACT The elastic wave propagation and vibration attenuation zones for periodically scattering piles in unsaturated ground were investigated using Finite-Element Method (FEM) and Bloch periodic theory. The unsaturated ground was modelled by unsaturated poroviscoelastic medium with the weak form governing equations derived from the mass and momentum conservative equations using Galerkin method and Fourier transform. The quadratic polynomial eigenvalue equations for the unsaturated medium were obtained using the Bloch periodic theory, and solved by FEM for the complex eigenvalues to evaluate the band gaps of the pile-soil system. The eigenvalue bands combined with the frequency response function were used to analyse the mitigation effects of piles in unsaturated ground with different soil properties and piles parameters. This research found that the pile-soil system has three propagational waves, in which the shear wave and the slow longitudinal wave carry low-frequency vibrations that undermine the attenuation effects of the piles barrier. The increasing soil saturation degree affects the attenuation effects by reducing the soil modulus and increasing the fluid content. And both of those two mechanisms reduce the frequency of the attenuation zones (AZ), which improves the low-frequency vibration attenuation, and reduces the width of AZ that could reduce the vibration mitigation effects.

Semi-analytical solutions to the one-dimensional consolidation for double-layered unsaturated ground with a horizontal drainage layer

In order to accelerate the consolidation rate of double-layered unsaturated ground with poor hydraulic conductivity, the sand blanket is always set between two adjacent unsaturated soil layers to produce drainage paths along the horizontal direction in practical engineering. Based on the one-dimensional consolidation theory of unsaturated soil proposed by Fredlund and Hassan(1979), the semi-analytical solutions of excess pore-air and pore-water pressures, and settlement of the double-layered unsaturated ground with the horizontal drainage blanket are obtained by Laplace transform technique in this paper. Additionally, the current solutions can degenerate into those under the single-layered unsaturated model, which are then utilized to compare with existing solutions in literature, the significant agreement indicates that the present solutions are reliable and general. At last, a case study is adopted to investigate the influence of the vertical position of the drainage blanket, and permeability coefficients ratio of two adjacent soil layers on the consolidation of the double-layered unsaturated ground. Results show that the horizontal drainage layer will effectively accelerate the consolidation process, and the setting depth of 0.7H works best. Moreover, the smaller the ratio of the permeability coefficient of the upper layer to that of the lower layer (with the permeability coefficient of the upper layer remaining constant), the faster consolidation will develop.

Influence of transient flow during infiltration and isotropic/anisotropic matric suction on the passive/active lateral earth pressures of partially saturated soils

In this study, the influence of transient flow on the active and passive lateral earth pressures of variably saturated backfills is thoroughly evaluated employing the lower bound theorems of the finite element limit analysis with second-order cone programming. In order to account for the tempo-spatial variations of suction stress within the soil stratum during infiltration, a closed-form solution derived for one-dimensional transient flow through variably saturated porous media is adopted. The unsaturated ground condition is simulated by incorporating the well-established unified effective stress approach into the soil yield function. The soil medium is considered to be both isotropic and anisotropic, due primarily to the various distributions of suction stress along different directions within the partially saturated porous medium. In order to model the soil suction stress anisotropy, an iterative procedure is employed by differentiating between the mobilized suction stress within the horizontal and vertical planes. In general, the influence of the transient flow condition is magnified for cohesive soils compared with granular backfills. In either case, such an effect is more noticeable for the active lateral earth pressure as compared to the passive counterpart. In the anisotropic condition, while the suction stress anisotropy within the soil medium has fairly negligible influence on the contribution of transient flow to the lateral earth pressure coefficient in the active state, the influence of elapsed infiltration time on the passive earth pressure coefficient turns out to be more at play at lower anisotropy ratios.

The importance of topographic gradients in alpine permafrost modeling

Alpine permafrost environments are highly vulnerable and sensitive to changes in regional and global climate trends. Thawing and degradation of permafrost has numerous adverse environmental, economic, and societal impacts. Mathematical modeling and numerical simulations provide powerful tools for predicting the degree of degradation and evolution of subsurface permafrost as a result of global warming. A particularly significant characteristic of alpine environments is the high variability in their surface geometry which drives large lateral thermal and fluid fluxes along topographic gradients. The combination of these topography-driven fluxes and unsaturated ground makes alpine systems markedly different from Arctic permafrost environments and general geotechnical ground freezing applications, and therefore, alpine permafrost demands its own specialized modeling approaches. In this work, we present a multi-physics permafrost model tailored to subsurface processes of alpine regions. In particular, we resolve the ice–water phase transitions, unsaturated conditions, and capillary actions, and account for the impact of the evolving pore space through freezing and thawing processes. Moreover, the approach is multi-dimensional, and therefore, inherently resolves the topography-driven horizontal fluxes. Through numerical case studies based on the elevation profiles of the Zugspitze (DE) and the Matterhorn (CH), we show the strong influence of lateral fluxes in 2D on active layer dynamics and the distribution of permafrost.

Improving computational efficiency of numerical modelling of horizontal ground source heat pump systems for accommodating complex and realistic atmospheric processes

Modelling horizontal ground loops for a horizontal ground source heat pump (HGSHP) system is complex and computationally expensive. The computation precision is highly reliant on the prescription of an undisturbed ground temperature in the unsaturated ground as well as realistic and accurate atmospheric processes at the ground surface boundary. Conventionally, modelling of such a system would include direct application of the atmospheric processes at the soil-atmosphere boundary and solve it in a single-stage approach. However, low efficiency is found for large spatial domain and long-term transient problems as the boundary processes need to be solved and expressed in terms of primary model variables at each simulation time-step. This paper proposes an equivalent two-stage modelling approach, for the first time, based on an advanced coupled thermal-hydraulic (TH) model to improve computation efficiency while maintaining adequate accuracy. In this approach, firstly, the model is solved for an intact ground that is imposed by complex atmospheric processes, e.g., rainfall, solar radiation, humidity, evaporation, etc. at the soil-atmosphere boundary, and the spatial and temporal variations of the primary model variables are recorded. Afterwards, the recorded data are incorporated in the simulator, as model inputs, for the same ground including a HGSHP system. Predicted results from both 2D and 3D simulations show that the ground temperatures calculated by the proposed two-stage approach are in good agreement with that of the traditional single-stage approach. However, the two-stage approach is computationally robust. For the presented 2D and 3D simulations, it required only 32% and 37% of the time of the single-stage approach, respectively, while maintaining great accuracy. This demonstrates the utility of the proposed two-stage approach for modelling complex scenarios of realistic HGSHP systems installed in a large spatial domain and for long-term operation.

One‐dimensional consolidation for multilayered unsaturated ground under multistage depth‐dependent stress

AbstractDue to the phenomenon of stress decay in space, the total vertical stress varies with depth in the layered soil stratum. The physical properties of unsaturated soil stratum and the nonuniformity of external load in vertical direction pose significant effects upon the consolidation process, while related works have viewed modest development at present. Therefore, the main objective of this study is to explore the one‐dimensional (1D) consolidation patterns of multilayered unsaturated ground considering depth‐dependent stress by a semi‐analytical procedure. According to the 1D consolidation theory of unsaturated soils put forward by Fredlund and Hasan, the semi‐analytical solutions of excess pore pressures and settlement are derived with the combination of Laplace transform and matrix transfer method. Crump's method is then adopted to obtain the analytical solutions in the time domain, and the corresponding computing programs are compiled. Furthermore, the validity work of proposed solutions is conducted by comparing the current solutions in this paper with degenerated ones under constant load in existing literature. Concentrating on the double‐layered unsaturated ground, and by considering different physical parameters of separate layer, the 1D consolidation characteristics under depth‐varying stress resulting from two‐stage load are investigated. The major conclusion can be drawn as that the settlement with constantly distributed stress along depth is overestimated compared with that considering the depth‐varying stress.

Theoretical study of P1 wave passing through a double-layer wave impeding block in the unsaturated soil

Based on the propagation theory of elastic waves in unsaturated porous medium and single-phase elastic medium, the calculation model of a double-layer wave impeding block (WIB) as vibration isolation barrier in unsaturated soil foundation is established. The analytical solution of the transmission/reflection amplitude ratio of P1 wave passing through a double-layer WIB in unsaturated soil foundation are derived by Helmholtz vector decomposition theorem. The influences of physical and mechanical parameters such as saturation, wave impedance ratio, density, and shear modulus on the vibration isolation effect of the double-layer WIB when P1 wave passing through the double-layer WIB in the unsaturated soil foundation are analyzed by parametric studies. The results show that the saturation has a small effect on the transmission/reflection coefficients of P1 and S waves and a significant effect on the transmission/reflection coefficients of P2 and P3 waves. The wave impedance ratio and the shear modulus and density of the double-layer WIB have a significant effect on the transmission/reflection coefficients. The optimal shear modulus and wave impedance ratio of the double-layer WIB can be selected to obtain the best vibration isolation effect, which provides theoretical guidance for the application of the double-layer WIB in the field of unsaturated ground vibration control.

Dynamic response of unsaturated poroelastic ground underlying uneven pavement subjected to vehicle load

In this study, the dynamic response of unsaturated ground coupled with an uneven model due to vehicle load is investigated. The governing equation of the unsaturated poroelastic ground is established by extending Biot's theory. The uneven pavement is simplified as a Kirchhoff small-deflection thin plate. The vehicle is modeled as a spring–mass damping system, while considering the effect of the uneven pavement. The dynamic wheel-uneven pavement force is calculated by regarding the pavement roughness as a sinusoidal wave. The solutions in the wavelength domain are presented by applying a double Fourier transform. Subsequently, the corresponding numerical results in the space domain are obtained by employing the numerical method of the double inverse Fourier transform. Next, the stress paths of the unsaturated soil element caused by the vehicle load are calculated and analyzed. The numerical results show that the additional dynamic load caused by the uneven pavement is affected by pavement roughness and vehicle velocity. The increasing vehicle velocity slightly affects the vertical normal stress, shear stress, and stress path. However, the peaks of the horizontal normal stresses increase, and the vertical displacement decreases. The ground dynamic response is significantly and positively correlated with the axle load. As saturation increases, the vertical displacement and dynamic stresses of the ground soil decrease slightly, and the stress path is reduced.

A Ground-Motion Model for GNSS Peak Ground Displacement

ABSTRACT We present an updated ground-motion model (GMM) for Mw 6–9 earthquakes using Global Navigation Satellite Systems (GNSS) observations of the peak ground displacement (PGD). Earthquake GMMs inform a range of Earth science and engineering applications, including source characterization, seismic hazard evaluations, loss estimates, and seismic design standards. A typical GMM is characterized by simplified metrics describing the earthquake source (magnitude), observation distance, and site terms. Most often, GMMs are derived from broadband seismometer and accelerometer observations, yet during strong shaking, these traditional seismic instruments are affected by baseline offsets, leading to inaccurate recordings of low-frequency ground motions such as displacement. The incorporation of geodetic data sources, particularly for characterizing the unsaturated ground displacement of large-magnitude events, has proven valuable as a complement to traditional seismic approaches and led to the development of an initial point-source GMM based on PGD estimated from high-rate GNSS data. Here, we improve the existing GMM to more effectively account for fault finiteness, slip heterogeneity, and observation distance. We evaluate the limitations of the currently available GNSS earthquake data set to calibrate the GMM. In particular, the observed earthquake data set is lacking in observations within 100 km of large-magnitude events (Mw>8), inhibiting evaluation of fault dimensions for earthquakes too large to be represented as point sources in the near field. To that end, we separately consider previously validated synthetic GNSS waveforms within 10–1000 km of Mw 7.8–9.3 Cascadia subduction zone scenario ruptures. The synthetic data highlight the importance of fault distance rather than point-source metrics and improve our preparedness for large-magnitude earthquakes with spatiotemporal qualities unlike those in our existing data set.

Dynamic responses of unsaturated ground with or without embankment under moving loads using 2.5D FEM with perfectly match layer

ABSTRACT The dynamic responses of an unsaturated ground with or without an embankment are analysed using 2.5D finite element method (FEM) to investigate the effects of the soil saturation and the embankment. The Galerkin method is utilised to establish the weak form governing equations for an unsaturated poroviscoelastic medium in the framework of 2.5D FEM. To reduce the reflected waves from the truncated boundaries of the 2.5D FE model, the formulations of the perfectly match layer (PML) technique are coupled with the governing equations of the unsaturated medium. The unsaturated ground vibrations with an embankment under moving loads are analysed by the proposed 2.5D FE approach, to investigate the influences of an embankment on unsaturated ground vibrations. The results of this research found that the PML technique can effectively reduce the reflected waves in the 2.5D FE model of unsaturated grounds. The increased soil saturation would increase the ground vibration amplitude, as well as the pore water pressure. Constructing embankment on the unsaturated ground could reduce the vibrations amplitude under the moving load, however increase the ground vibrations at the foot of the embankment.