Unconsolidated Research Articles

Study of the Nonuniform Consolidation Characteristics of Soft Soils Using a Novel Model

The degree of soil nonuniformity consolidation varies continuously with the passage of consolidation time and is accompanied by nonlinear alterations in soil parameters. Existing theoretical models often assume a constant relationship between the parameters of the two soil layers, failing to consider the effect of nonuniform consolidation. This assumption does not align with real-world conditions and can lead to significant errors in calculation results. Hence, this study aims to investigate the dynamic changes in soil undergoing nonuniform consolidation and develop a mathematical model that accounts for this phenomenon. Based on the large-strain and double-layer models, an improved consolidation model was proposed, which considers nonuniform variations in consolidation with a vertical drain and corrections to calculations under the influence of the nonlinear relationships of soil parameters. The proposed improved model was validated by comparison with field test data, and the results were compared with those of the classical model. Finally, the effects of different consolidation parameters on consolidation behavior were investigated. The research is a reliable calculation method that incorporates the dynamic nonuniform changes in consolidated soil, enabling more accurate predictions of consolidation of foundations treated by vertical drains.

LAND SUBSIDENCE IN WUHAN REVEALED USING A MULTI-SENSOR INSAR TIME SERIES FUSION APPROACH

Abstract. Satellite Interferometric Synthetic Aperture Radar (InSAR) is widely utilized for topographic, geological, and natural resource investigations. However, many existing InSAR studies on ground deformation are limited to relatively short observation periods and single sensors. This paper introduces a novel method for fusing multi-sensor InSAR time series data, specifically designed to address scenarios involving partial overlaps and temporal gaps. The method employs a new Power Exponential Knothe Model (PEKM) to fit and fuse overlaps in the deformation curves, while a Long Short-Term Memory (LSTM) neural network predicts and fuses data during temporal gaps in the series. In this study, the city of Wuhan in China was selected as the experimental area. SAR datasets from COSMO-SkyMed (2011–2015), TerraSAR-X (2015–2019), and Sentinel-1 (2019–2021) were fused to map long-term surface deformation over the past decade. An independent InSAR time series analysis from 2011 to 2020, based on 230 COSMO-SkyMed scenes, was used as a reference for comparison. The correlation coefficient between the fusion algorithm’s results and the reference data is 0.87 in the time overlapping region and 0.97 in the time-interval dataset. The overall correlation coefficient of 0.78 demonstrates that the proposed algorithm achieves a similar trend as the reference deformation curve. Based on the long time series settlement results obtained through fusion, a detailed analysis of the causes of settlement was conducted for several subsidence zones. The subsidence in the Houhu area is primarily attributed to the consolidation and compression of soft soil. Soil mechanics were employed to estimate the expected completion time of subsidence and calculate the degree of consolidation for each year. The COSMO-SkyMed PSInSAR results indicate that the area has entered the late stage of consolidation and compression, gradually stabilizing over time. Furthermore, the subsidence curve observed in the area around Xinrong reveals that the construction of an underground section of subway Line 21 has caused significant settlement in that particular region. The high temporal granularity of the PSInSAR time series also enables precise detection of a rebound phase following a major flooding event in 2016. The experimental results demonstrate the accuracy of the proposed fusion method in providing robust time series for analyzing long-term land subsidence mechanisms. Additionally, these findings unveil previously unknown characteristics of land subsidence in Wuhan, thus clarifying the relationship with urban causative factors.

Ground settlement prediction for open caisson shafts in sand using a neural network constrained by empiricism

Accurate prediction of soil settlements induced by open caisson construction in sand is essential for safe and reliable delivery of critical underground urban infrastructure. This paper presents a novel prescriptive design approach using a neural network (NN) constrained by empirical relationships, referred to as an ’empiricism-constrained neural network’. The proposed approach is benchmarked using a traditional closed-form empirical expression. Both methods are calibrated using experimental data from reduced-scale laboratory testing for the prediction of surface and subsurface settlement trough shape and magnitude. The outcomes demonstrate that while both methods accurately capture the primary effect of caisson depth on surface and subsurface soil settlements, the NN approach exhibits superior prediction accuracy. These methods are developed in a form amenable for routine design use in industry and have the potential for broader applicability in other design scenarios, such as building damage assessment and risk-assessment exercises.

At-rest lateral earth pressure in compacted silica sand

ABSTRACTAt-rest lateral earth pressure is one of the major sources of disturbing force on retaining structures and provides resisting force for anchors and piles. A theoretical model is proposed, and experimentally validated, for the normally and overconsolidated at-rest lateral earth pressure coefficients in silica sand. In addition, the experiment illustrates a procedure to determine the maximum historic vertical stress equivalent to the standard and modified Proctor energies of compaction. The previous equivalent maximum historic vertical stress, which is found to be constant for the relative density reached by each energy of compaction, is then proposed as a parameter to determine the overconsolidation ratio within the backfill and its resultant at-rest lateral earth pressure distribution behind a vertical wall. The at-rest lateral earth pressure distribution incorporated with the overconsolidation ratio naturally results in a greater magnitude and is curved in contrast to the linear distribution characteristic of a normally consolidated state. A limit equilibrium analysis and a plane-strain particle-scale model of a granular media under critical state were used to derive the equations herein.

Anisotropy Study on the Process of Soil Permeability and Consolidation in Reclamation Areas: A Case Study of Chongming East Shoal in Shanghai

Anisotropic permeability is of great significance for assessing the consolidation and drainage mode of soil layers in reclamation areas, as well as for preventing and controlling ground settlement after project construction. This paper analyzes the anisotropic permeability of the inland and nearshore soil layers in Chongming East Shoal, Shanghai, and the formation mechanism of anisotropic permeability through permeability and scanning electron microscope (SEM) tests. The results highlight that compared with dredger fill and sandy silt, the horizontal permeability coefficient of underlying soft clay (USC) is significantly higher than its vertical permeability coefficient, which is more significant in nearshore USC. Interestingly, the upper clay (21.5 m) in the thickest clay layer shows greater anisotropic permeability than the lower clay (41.5 m). Due to the instability of seepage channels, the USC anisotropic permeability increases in a fluctuating manner as the hydraulic gradient increases. Microstructural parameters are used to reveal the mechanism of anisotropic permeability, which shows that a simple soil skeleton and structure, strong particle orientation, decreased particle abundance, increased particle roundness, decreased particle contact area, and increased pore area all contribute to the enhancement of permeability. Moreover, micro-parameters have been proposed to evaluate anisotropic permeability in terms of the effective seepage-pore area. This approach addresses the constraint of water films on the permeability efficiency of USC particles.

Consolidation analysis for soft soil with non‐Darcian flow considering variable discharge capacity of prefabricated vertical drain

AbstractGround improvement is a complex issue, and accurately predicting the consolidation and settlement of soft soil with prefabricated vertical drain (PVD) presents a significant challenge. Recent laboratory and field tests have highlighted the influence of the variable discharge capacity of PVD and the non‐Darcian flow behavior of soft soil on consolidation. However, existing theories have not yet considered these two factors simultaneously. To address this gap, a numerical solution for consolidation analysis incorporating non‐Darcian flow and variable discharge capacity of PVD was developed and applied in a test area. The results of this study demonstrate the significant impact of both non‐Darcian flow and variable discharge capacity on the consolidation rate. A comprehensive comparison between the findings of this numerical solution, the degenerate solution, and monitored data reveals clear differences. Notably, the non‐Darcian exponent (s) and discharge capacity parameters (A3) were found to exert a greater influence on consolidation behavior compared to other factors. These findings provide valuable guidance for the design and implementation of following airport ground improvement strategies.

Shear rate effect on clay-structure interface strength properties in various interface boundary conditions

Based on the modified simple direct shear device which can directly measure the interface pore pressure and interface shear displacement, a series of interface shear tests and corresponding pure clay shear tests were conducted at an undrained state in constant normal load (CNL) boundary conditions or equivalent undrained state in constant volume (CV) boundary conditions. The clay-structure interfaces, consisting of seabed clay and Speswhite kaolin clay with overconsolidation ratios (OCR) of 1 and 3, were tested at three shear rates, respectively V1 = 0.0002 mm/s, V2 = 0.001 mm/s, and V3 = 0.01 mm/s. The results demonstrated that the shear strength of the clay-structure interface is lower than that of pure clay, and this difference is more pronounced under CV boundary conditions. In CNL condition, though the pure clay strength decreases with increasing shear rate at OCR = 1 and increases with increasing shear rate at OCR = 3, the shear rate effect on clay-structure interface strength is not obvious. In CV condition, the strength of the interface with the normally consolidated (NC) and over consolidated (OC) clay increases approximately linearly with the shear rate on the semi-logarithmic scale. the shear rate parameter ρ is used to describe the growth rate of pure clay or clay-structure interface shear strength with a tenfold increase in shear rate. As for normally consolidated clay, in CV condition, the corresponding shear rate parameter satisfies that ρ (with R1 roughness)> ρ (pure clay)> ρ (with R2 roughness). The rate parameter corresponding to NC seabed clay is significantly higher than the rate parameter corresponding to NC Speswhite kaolin clay. For OC clay, the shear rate parameter for interface strength is higher than that for pure clay, meeting with the relationship that ρ (with R1 roughness)> ρ (with R2 roughness)> ρ (pure clay).

Functional capabilities of the calculation methodology of railway approach embankment to engineering structures taking into account temperature effect and interaction of reinforcing piles with soil

For the smooth operation of the railway infrastructure, it is necessary to ensure the proper functioning of the roadbed embankment and engineering structures (pipe-culverts, bridges and overpasses), which corresponds to the long-term development programme of the open joint-stock company «Russian Railways» approved by the Government of the Russian Federation dated 19 March 2019 N 466-R. In the interface zones of nonhomogenous structures, namely roadbed embankments and engineering structures, ground distortion (soil settlement) occur, which leads to a change in the dynamic impact on engineering structures (pipe-culverts, bridges and overpasses) and, as a consequence, to their accelerated damage. The timeliness of the topic is determined by the target programmes for railway transport infrastructure optimisation, ensuring uninterrupted operation, reliability of engineering structures and roadbed. When designing engineering structures, a number of solutions are used to ensure reliable functioning of the roadbed — engineering structure system (pipe-culvert, bridge substructure, tunnel, laying of communications). The problem is that their application is ineffective without scientific and regulatory support and is limited by the lack of effective solutions to ensure the rigidity of the roadbed structure on the approaches to the engineering structure. In this connection, new constructive and technical solutions development for the interface of the roadbed — engineering structure system is an urgent task. The article considers railway transportations as a production process, the smooth functioning of which is ensured by smooth conjugation of approach embankments with engineering structures (bridges, pipe-culverts, underground communications). In the interface zone there are soil settlements, damage to the roadbed integrity, deformations leading to the appearance of additional dynamic forces and the emergence of irregularities and even destruction of the road pavement (on motorways), as well as damage to the engineering structures themselves.

Permeability of polymer-modified kaolinite and fly ash–kaolinite mixtures

With the goal of promoting the treatment and reuse of fly ash (FA), a series of permeability tests were conducted to investigate the hydraulic conductivity (K) of polymer (chitosan, polyethylene oxide (PEO) and xanthan gum)–FA—kaolinite (KA) systems under different confining pressures and overconsolidation ratios (OCRs). The addition of FA increased the K of KA due to changes in particle size and gradation, and the effect is not significant when the FA content is below 30%. PEO and xanthan gum increased the K of KA, with xanthan gum having a more significant effect, while chitosan reduces it. For FA–KA mixtures, chitosan and xanthan gum had negligible impact on the K, whereas PEO increased it by an order of magnitude. In addition, confining pressure restricts the flow channel while increasing the effective contact area between polymers and soil particles. The Koc/Knc of polymer-modified KA rapidly converges at high OCRs, while that of polymer-modified FA–KA converges at low OCRs. This study provides insight into the effects of polymer modification on the hydraulic conductivity of FA–clay mixtures and provides guidance for the application of polymer modification technology in soil improvement.

Experimental analysis of liquefaction potential with variations of soil relative density and earthquake frequency

The resistance of sandy soil against liquefaction is influenced by its grain size and relative density. This study investigates the potential for liquefaction and settlement resulting from liquefaction. A single axis shaking table was employed, with a displacement of 2.5 cm and a box measuring 400 mm long, 400 mm wide and 600 mm height. Various relative soil densities were evaluated, namely 40%, 50%, 60%, and 70%, along with 1 Hz and 1.2 Hz seismic frequency variations. The experiment used three pore pressure transmitter sensors to measure the increase in pore water pressure (top, middle, and bottom), accelerometer sensors to measure acceleration, and settlement measurements at 12 points. The experimental findings prove a correlation between the relative density of the soil and its liquefaction potential. Increasing the relative density of the soil results in a decrease in settlement, ranging from a maximum settlement of 2.93 cm to a minimum settlement of 1.77 cm. Concerning the variation in earthquake frequencies, higher frequencies correspond to increased soil settlement, showing a difference of 15% to 33%. This study highlights the significance of earthquake frequencies and relative soil density in assessing the potential for soil liquefaction.

Analysis of Crustal Movements in the Angren-Almalyk Mining Industrial Area Using GNSS Data

This study aims to investigate the potential of GNSS data in earthquake zoning and risk assessment in a seismically active region. The study area is located in Uzbekistan, where the interaction of several tectonic plates results in frequent earthquakes. The study focused on two periods from October to October of the next year, respectively, for 2018 and 2019, 2019 and 2020 to analyze GNSS velocities and earthquake zoning maps to identify areas at high risk of seismic activity. The study also utilized GIS and spline interpolation to create continuous surface maps of risk zones based on earthquake hazard data. The results showed that the entire study area is at high seismic risk, with a tendency for rotational movement in the zone of intersection of tectonic plates. Velocities range from 1.56-20.3 mm/yr and 1.64-14 mm/yr for the two periods. Moreover, the analysis of vertical earth surface displacements indicates a tendency towards soil subsidence in the western part and uplift in the northeastern part of the study area. The study concludes that GNSS analysis can provide valuable insights into seismic hazard assessment and improve our understanding of the tectonic processes leading to earthquakes.

Thermo-mechanical behaviour of a kaolin-based clay soil

The thermal behaviour of KSS, a low-plasticity artificial clay made with kaolin clay, silt and sand, was investigated in a series of temperature-controlled oedometer tests, at temperatures between 5°C and 70°C, and at vertical pressures up to 2·4 MPa. The experiments investigated the effect of overconsolidation ratio (OCR), pressure level and repeated thermal cycling on thermally induced volume change. Thermal volumetric strains were found to be dependent not only on OCR but also on pressure level, contradicting previous experimental findings and highlighting the importance of even a small dependency of compression index Cc on temperature. Furthermore, thermal volumetric strains were irreversible on heating and cooling even for highly overconsolidated samples. Although irreversibility at high OCR values has been attributed to particle rearrangement and plastic accommodation in the past, an alternative explanation is put forward here, as yielding on the Hvorslev surface is expected to occur on unloading under one-dimensional (1D) conditions. The tests also revealed evidence of thermal creep for the initially normally consolidated samples. The influence of both current temperature and temperature history on the reloading response of mechanically overconsolidated KSS was tested and quantified in terms of their effect on the measured pre-consolidation pressure. The results from these tests were compared to results from the available literature referring to clays of similar and higher plasticity and the comparison highlighted that although soil plasticity can explain the observed quantitative differences between high-plasticity soils to a large extent, mineralogy, in addition to structure, may also play an important role for low-plasticity soils.

Research on the mechanism of plant root protection for soil slope stability.

In order to investigate the impact of herbaceous root development on soil slope stability in expansive soil areas, the research was conducted in the soil slope experimental area of Yaoshi Town, Shangzhou District, Shangluo City. Three types of herbaceous plants, namely Lolium perenne, Medicago, and Cynodon dactylon, were planted to examine their influence on slope stability. The results indicated that Lolium perenne had significantly higher root length density and root surface area density compared to Cynodon dactylon and Medicago. However, the root weight density of Cynodon dactylon was found to be highest. The roots of Lolium perenne, Cynodon dactylon, and Medicago were predominantly observed in diameter ranges of 0 < L ≤ 1.0 mm, 0 < L ≤ 2.5 mm, and 2.5 < L ≤ 3.0 mm, respectively. The roots of herbaceous plants have the ability to enhance water retention in soil, resist hydraulic erosion of slope soil, and reduce soil shrinkage and swelling. During the initial phase of herbaceous planting, there is an accelerated process of organic carbon mineralization in the soil. The roots of herbaceous plants play a crucial role in soil consolidation and slope protection. They achieve this by dispersing large clastic particles, binding small particles together, altering soil porosity, enhancing soil water retention, and reducing soil water infiltration. It was found that Lolium perenne and Medicago, which have well-developed roots, exhibited superior slope protection effects. These findings contribute to the theoretical understanding for the implementation of green ecological protection technology on soil slopes.

Effects of pre-existing damage on vertical load-bearing capacity of masonry arch bridges

In-service masonry arch road bridges, mainly realised before the first half of the last century, represent a wide portion of the entire worldwide infrastructural asset. Given their age, during their service life these structures could have experienced damage due to anthropic (i.e. traffic) and natural (i.e. earthquakes, soil settlements, degradation, etc.) actions which may have inevitably affected their load-bearing capacity. The present study addresses the problem of the residual capacity estimation of damaged bridges by investigating the impact of previous loading on the actual strength of the structure. In particular, reference to a past experimental activity retrieved from the literature on reduced-scale bridges subjected to concentrated vertical loads has been made to calibrate a reliable detailed finite element model in Abaqus software. Then, damage of different extent has been introduced by simulating the transit of vehicles of various weights on the structure and the residual capacities of the bridge have been assessed and compared against the undamaged configuration. The results confirm that pre-existing damage due to traffic loading may significantly influence the capacity of such structures, with peak load reductions up to 60% estimated through the proposed methodology.

Coupled creep and nonlinear consolidation of soft soils in disturbed state concept framework

A simplified solution for coupled creep and nonlinear consolidation of soils has been presented based on the disturbed state concept (DSC). The nonlinearity of the problem due to the change of the material properties and thicknesses of the soil layer, and creep was considered in the proposed solution. The state of the soil before primary consolidation was considered as initially relatively intact, and after primary consolidation during the pure creep phase was considered as finally fully adjusted state. The state of the soil during the coupled creep and consolidation process was related to its relatively intact and fully adjusted states using two consolidation and creep state functions that have been derived based on the numerical solution of the problem. Using the consolidation state function, the solutions of Terzaghi’s theory of consolidation in two initial and end of the primary consolidation states were interpolated to achieve the solution of the nonlinear primary consolidation phase. The creep state function was used to implement the effect of the creep deformation during and after the primary consolidation in the settlement calculations. The result of the proposed method was verified by the results of the numerical and approximate methods along with laboratory data in the literature.

Analytical solution for consolidation of saturated viscoelastic soils around tunnels with general Voigt model

This paper presents a novel analytical solution for the consolidation behavior of viscoelastic saturated soft soil subjected to large-scale ground loading. The rheological properties of clay are described using the general Voigt model. Based on the Terzaghi-Rendulic theory, the governing equations for the dissipation of excess pore water pressure in the surrounding soil mass of a tunnel are established under the first and second boundary conditions. The governing equations are solved using the complex variable method. The obtained solutions are verified by reducing them to the forms of three traditional rheological models, demonstrating the reliability of the proposed approach. Finally, based on the established solutions, the dissipation characteristics of excess pore water pressure around the tunnel are analyzed. The case results indicate that the soil permeability coefficient (k), the independent Newtonian viscosity coefficient (K0) and Hooke’s spring modulus (E0) in the general Voigt model have significant influences on the dissipation of excess pore water pressure and the degree of consolidation. A larger k, K0, and E0 lead to faster dissipation and consolidation development, while a greater tunnel buried depth (b) results in slower consolidation process. The influence of k on excess pore water pressure dissipation is more significant than that of K0 and E0. For the first boundary type, consolidation is instantaneous when k > 0.1 m/d. For the second type, when k < 0.0001 m/d, excess pore pressure remains unchanged within 100 d. The permeability condition of the tunnel has a considerable impact on the distribution of excess pore water pressure in the soil layer directly above the crown. When the tunnel is fully permeable, the effects of k, K0, and E0 on the dissipation of excess pore water pressure are more pronounced in the early stage, with almost complete dissipation of excess pore pressure above the tunnel before 100 d. However, when the tunnel is completely impermeable, their effects are more prominent in the later stage, and by 100 d, the maximum excess pore pressure within the depth range is 25 % of the initial.

Induction of ice VII structure with secondary shock compression by backward Raman scattering in plasmas

Forward stimulated Raman scattering and backward stimulated Raman scattering (BSRS) are measured when an intense 532 nm nanosecond pulsed laser is focused into water and heavy water. The investigation reveals a significant observation: the formation of the ice VII structure exclusively occurs in the backward direction when optical breakdown takes place, provided that the input energy falls below 90 mJ for liquid water or 75 mJ for heavy water. This phase transition is attributed to secondary shock compression, which comes from energy transfer and compression between the BSRS in plasmas with the pump laser. The optical breakdown experiment under pre-pressure reveals that the shock compression in the back direction is approximately 2.3 times that of the forward direction. This research is useful for shock compression and dynamics in plasmas.

Undrained expansion analyses of a cylindrical cavity in anisotropic sands incorporating the SANISAND model

AbstractSoils are known to be inherently anisotropic, resulting in complex responses to loading. This paper aims to develop an elastoplastic solution for the undrained expansion of a cylindrical cavity in sands adopting a non‐associated and anisotropic model, SANISAND. The rigorous derivation of the stress‐strain state of the soil element is provided following a standardized solving procedure. The dilatancy and crushing of the soil are invoked in the three‐dimensional cavity expansion solution by adopting the critical state soil mechanics and limiting compression curve, respectively. By combining this with a governing equation that considers the undrained condition, the stress‐strain state of the surrounding soil around the cavity can be determined. A subroutine is then implemented into the ABAQUS FEM simulation to verify the solution. The solutions are also validated against those based on an isotropic model, and anisotropic sand is used to investigate the effects of the initial effective mean stress, at‐rest coefficient of earth pressure, and overconsolidation ratio on the stress distribution, stress path, and boundary surfaces.

Poroelastic response of unsaturated soils to cyclic loads with arbitrary waveforms

AbstractWe develop an analytical solution to the problem of one‐dimensional consolidation of unsaturated soil subjected to cyclic loads with arbitrary waveforms. The solution predicts the excess pore water and pore air pressures and the accompanying vertical compression in a poroelastic, unsaturated soil material. Cyclic loading occurs in a variety of engineering applications and often generates higher excess pore fluid pressures and larger vertical compression than does a time‐invariant load. In the present study, the loading function is allowed to take on any arbitrary waveform represented by a Fourier trigonometric series. Analytical solution to the boundary‐value problem in one dimension is given in closed form describing the frequency‐independent and frequency‐dependent components of the poroelastic response. We verify the analytical solution through representative examples involving cyclic loads with square and triangular patterns. Apart from the shape of the forcing function, we also investigate the effects of initial water saturation, soil texture, and excitation frequency on the system response.

Soil Consolidation Analysis in the Context of Intermediate Foundation as a New Material Perspective in the Calibration of Numerical–Material Models

This paper presents the authors’ research results from an analysis of intermediate foundations as well as slab and pile foundations in the context of soil consolidation. Looking at soil as a building material that changes its properties over time is very important from the point of view of the safety of construction, implementation, and operation of building structures. In addition, soil can be parameterized in such a way as to accurately describe its possible behavior under service loading. Of great interest is the phenomenon of consolidation, which is based on the reduction of soil volume over time under constant loading. This study explores existing piles and replicates soil conditions to understand individual and grouped pile behavior in combined pile–raft foundations (CPRF). To assess pile settlement from primary and secondary consolidation phases, 13 field measurements on concrete columns in gyttja clay were conducted. Analyzing data from these tests allowed engineers to accurately calibrate a numerical model. This calibrated model was instrumental in designing high-rise buildings, ensuring stability and safety. This study emphasizes the importance of understanding soil behavior, particularly consolidation phenomena, in optimizing foundation design and construction practices.