Early Stages Of Consolidation Research Articles

Study on the consolidation behavior of horizontal drainage foundation under complex aquifer formation conditions in karst regions

Introduction: The consolidation behavior of horizontal drainage foundation under complex aquifer conditions in karst areas is a hot topic in the field of geotechnical engineering.Methods: This paper presents a modified piecewise-linear model for plane-strain consolidation. In this model, the distributed drainage boundary was used to describe the drainage performance of soil layer boundaries, and the UH model considering the time effect was selected to reflect soil’s rheological property. Through comparison with existing research, the validity of the calculation model in this paper was verified. Then several examples were used to analyze the consolidation behavior of the foundation under the combined action of rheological effect and distributed drainage boundaries.Results and discussion: Numerical studies show that the phenomenon of the increase of excess pore pressure exists in the foundation of the distributed drainage boundary after considering the rheology in the early stage of consolidation. Moreover, the larger the secondary consolidation coefficient and the initial over-consolidation parameter, or the smaller the pave rate and the thickness-width ratio, the above phenomenon is more obvious. In terms of the dissipation of the pore water pressure, the larger the secondary consolidation coefficient and the initial over-consolidation parameter, the slower the pore pressure dissipation, and the smaller the pave rate or the thickness-width ratio can achieve the above effects. In terms of the impact on settlement, the above-mentioned parameters are consistent, that is, the larger the corresponding parameter, the larger the corresponding settlement value.

Numerical analysis of time-dependent negative skin friction on pile in soft soils

The generation of negative skin friction (NSF) induces the additional axial force on the pile, which exerts a detrimental load rather than a beneficial one. However, the effect of soil creep on the long-term development of NSF has remained poorly understood. The study uses numerical analysis to investigate this effect. First, an elasto-viscoplastic model with an enhanced time integration algorithm is successfully implemented into a finite element code. Next, the two-dimensional axisymmetric pile-soil interaction model is established and properly calibrated with a known field case. Finally, parametric studies are conducted to examine different degrees of creep effect on the variation in NSF and neutral plane (NP) within the primary and secondary consolidation periods. According to the findings, a high creep coefficient of the soil results in an increase in NSF and descending trend of the NP. The creep induced delay of NSF is observed attributed to the increase in excess pore pressure during the early stage of consolidation. The NP position varies drastically at the commencement of consolidation when taking creep into consideration. Lastly, the study proposes an exponential prediction model to reflect the time dependence of the location of the NP.

A semi-analytical solution for consolidation of ground with local permeable pipe pile

Aiming at optimization of drainage holes for permeable pile, a scheme of local pervious segment is introduced in the body of pipe pile to accelerate the dissipation of excess pore water pressure caused by pile driving. An axisymmetric consolidation model is established for a single local permeable pile in saturated clay. The semi-analytical solution is obtained using the boundary transform and integral transform methods. The effectiveness of the proposed solution is assessed by comparing with the results of degradation analysis, finite difference analysis and model test. Upon the successful calibration of the solution, the impacts of pervious segment parameters, geometric dimensions of pipe pile and soil physical parameters on the consolidation of soils around the pile are analyzed. Results show that local permeable pile can accelerate the soil consolidation based on the premise of structural strength of the pile body, and the acceleration effect is mainly in the early stage of consolidation. With the increase of pervious segment length and position, aspect ratio of pile, and soil permeability coefficient ratio, the benefit of local permeable pile is more apparent, which can provide reference for design.

Green’s function analytical solution for free strain consolidation of soft soil improved by stone columns subjected to time-dependent loading

This paper proposes an analytical solution in terms of Green’s function formulations for axisymmetric consolidation of a stone column improved soft soil deposit subjected to time-dependent loading under free strain condition. The mathematical derivations incorporate the pore water flows in radial and vertical directions in stone column and soft soil synchronously. The capabilities of the proposed analytical solution are evaluated via worked examples investigating the influences of three common time-dependent external surcharges (namely step, ramp and sinusoidal loadings) on consolidation response of the composite ground. The examples show that a faster increase of load from an initial surcharge to an expected loading might generate more significant excess pore water pressure to be dissipated during the early stages of consolidation, but the dissipation rate in soft soil would speed up significantly afterwards. The column and soil settlements along with the differential settlement between them also proceed faster corresponding to the acceleration of loading – unloading processes. Finally, the proposed analytical solution is employed to evaluate the excess pore water pressure dissipation rate at an investigation point in soft clay of a case history foundation. The calculation results exhibit a reasonable agreement with field measurement data when various constant values of stress concentration ratio are substituted into the solution to reflect the increase of stress concentration ratio with consolidation time in real practice.

One-Dimensional Consolidation Analysis Considering Non-Darcian Flow and Self-Weight Based on Continuous Drainage Boundary

The one-dimensional consolidation problem of soft soil with self-weight and non-Darcian flow is studied by introducing continuous drainage boundary. A numerical solution is derived by using finite difference method and its correctness is proved by comparing with existing analytical and numerical solutions. Based on the present solution, the consolidation behavior of soil is detailedly analyzed by adjusting the different parameters. The results show that, the soil consolidation rate increases with the increase of interface parameters or external loading rate. The difference between the average consolidation degree obtained by the solution with continuous drainage boundary and that with traditional boundary is mainly in the early stage of consolidation, but in the late stage of consolidation, the difference between these two solutions becomes smaller. Compared with the soil consolidation solution considers non-Darcian flow, the soil consolidation rate with Darcy flow is faster.

Characterization of the Localized Agri-Food System (SYAL) for the Andean blueberry (Vaccinium meridionale Swartz) in the Boyaca Department, Colombia

The Andean blueberry (Vaccinium meridionale Swartz), known locally as agraz, possess a high level of anthocyanins and antioxidants and represents a socio-economical, environmental, and cultural alternative in Colombia. However, a characterization of the agri-food system has been done, limiting strategies for its valorization. The objective of this study was to characterize the Localized Agri-food System (SYAL) of the Andean blueberry in the Occidente and Ricaurte provinces of the Boyaca Department of Colombia. Three analytical axes were used within the SYAL approach: territorial anchorage, activation processes, and links between product quality and territory. The farmer product circuit methodology was used to identify agents linked to the product commercialization chain in the territory, and their participation was estimated. Data treatment was performed with descriptive statistics and multivariate analysis, specifically principal component analysis for numerical variables and multiple correspondence analysis for categorical variables. A SYAL was evident in the early stages of consolidation, with exceptional adaptations to the biophysical conditions in the territory. Finally, four groups of actors were identified, where the main discriminating variable was membership in an organization of farmers, a determining condition for business competitiveness in the territory.

Consolidation behavior of dredged ultra-soft soil improved with prefabricated vertical drain at the Mae Moh mine, Thailand

The effectiveness of the prefabricated vertical drains (PVDs) in the consolidation of ultra-soft dredged soil with various soil water contents (W) in Mae Moh mine, Lampang, Thailand was researched via a series of large-scale model tests and numerical analysis. Large settlements with the delay of excess pore pressures is a distinct behavior of ultra-soft soil. The PVD dimensions were found to have a significant effect on the rate of consolidation and the delay of excess pore pressure at low total vertical stress (σv). The smaller PVD dimension resulted in the smaller rate of consolidation and longer delay of excess pore pressure. The undrained shear strength (Su) of ultra-soft clay at various degrees of consolidation could be approximated by the vertical effective stress (σ′v) based on the SHANSEP where the σ′v was determined from the Asaoka's observational method. The finite element analysis with axisymmetric and plane strain models showed that the axisymmetric model produced an excellent settlement prediction. However, the excess pore pressures were not well predicted by the axisymmetric model, due to the delay of excess pore pressures at the early stages of consolidation. In practice, the plane strain models proposed by Chai et al. and Indraratna and Redana's methods are suggested to predict the consolidation settlement of the Mae Moh dredged soil improved with PVD. The outcome of this research will facilitate the geotechnical design of reclamation of ultra-soft dredged soil in Mae Moh mine and other similar soils.

Numerical Simulation of Vacuum Preloading for Reinforcing Soil inside Composite Bucket Foundation for Offshore Wind Turbines

The composite bucket foundation (CBF) with seven honeycomb subdivisions is a new foundation for offshore wind turbine structures. The bearing capacity of CBF can be improved by consolidation of soil inside the CBF, which is caused by the vacuum preloading method after installation. A three-dimensional numerical model is established to simulate the consolidation process of soil for CBF with and without subdivisions in terms of vertical settlement, pore water pressure and void ratio of the soil. This analysis investigates the reinforcement effect of the two foundation types to assess the influence of the bulkheads. The results obtained show that there are obvious reinforcement effects for both foundation types. In the early stage of consolidation, vertical settlement is rapid, and this becomes stable with time. The depth at which the pore water pressure becomes negative is the depth showing the main reinforcement. Vacuum pressure decreases continuously with increase in soil depth and time. In addition, the excess pore water pressure in the soil dissipates, which turns into the soil effective stress. Bulkheads provide vertical drainage channels in the soil and shorten the seepage path, allowing the extraction of more pore water. This is conducive to the improvement of shallow soil, while also decreasing the extraction of pore water in deep soil and the region of the soil that can be reinforced.

Numerical Study: Influences of Local Radial Consolidation for Soft Clay

The permeability reduction behavior induced by local consolidation around a prefabricated vertical drain under equal vertical strain condition was investigated by finite element analysis (FEA) using a unit cell model. Taylor's equation was employed to consider the variation of permeability with the void ratio. The results of FEA indicate that at the early stage of consolidation, the ratio of permeability at the cell periphery and adjacent to the drain is about 8 for soft clay. Further, we compared the average degree of consolidation between the results of FEA and the analytical results from Hansbo's solution for radial consolidation. This showed that, without considering the influences of local consolidation, the analytical solution will overestimate the rate of consolidation.

Numerical analysis of surcharge preloading consolidation of layered soils via distributed sand blankets

Surcharge preloading consolidation of soft soils often implements a layer of fully arranged aggregate materials. The volume of drained water is abundant at the early stage of consolidation, but it reduces at middle and later stages, during which the fully arranged sand blanket will be a waste. In this investigation, a concept of distributed sand blankets is proposed to save aggregate materials. A series of finite element analyses have been performed on layered soils with distributed sand blankets. A mixed type of drainage boundary is assigned to a representative model, where a half sand blanket is perfectly pervious and a half width of soil among sand blankets is impervious. From parametric study, it has been found that a pave ratio between sand blankets and the total soil width can be selected in a range of 40%–60%, which will save aggregates by approximately 50% but cause an increase of consolidation time by less than 10%. For a fixed pave ratio, more evenly spaced sand strips with smaller width should be employed to optimize the design. The effectiveness of distributed sand blankets is not influenced by the anisotropy of hydraulic conductivity, elastic modulus, Poisson’s ratio, and thickness in multiple soil layers.

Surcharge preloading consolidation of reclaimed land with distributed sand caps

For land reclamation using the conventional surcharge preloading method, a sand cap layer is often fully placed at the ground surface of ultra-soft compressible estuary or marine clays. A novel strategy of distributed sand caps is proposed to save cost associated with sand materials. At an early stage of consolidation, the drainage capacity of distributed sand caps is less than the drainage demand, whereas at a later stage, the capacity exceeds the demand. Hence, the overall drainage capacity of distributed sand caps is equivalent to the drainage demand. A numerical model is developed to study the effectiveness of the proposed technique, and calculations are compared against theoretical solutions. A parametric study is conducted to optimize design parameters. It has been found that distributed sand caps should be uniformly placed with a size of 3% of the total width and a spacing of 1 times the size, such that sand materials can be saved by up to 50% without compromising the consolidation efficiency (an increase of consolidation time by less than 5%). The use of distributed sand caps will be effective regardless of soil properties, including hydraulic conductivity, elastic modulus and Poisson’s ratio.

One-dimensional nonlinear consolidation theory for soft ground considering secondary consolidation and the thermal effect

Based on the assumption that the change of the void ratio is caused by effective stress, creep effect and temperature, a one-dimensional nonlinear consolidation theory considering the thermal effect is established that contains the heat balance equation and the continuity equation. Verification and calculation examples are performed. The results show that the dissipation rate obtained using the present method is slower than that obtained using the traditional method. The pore water dissipation rate or consolidation rate considering the secondary consolidation and temperature is higher than that when only considering secondary consolidation. Heating accelerates the dissipation rate of pore water, especially in the early stage of consolidation.

Enhancing early consolidation of human episodic memory by theta EEG neurofeedback

Consolidation of newly formed memories is readily disrupted, but can it be enhanced? Given the prominent role of hippocampal theta oscillations in memory formation and retrieval, we hypothesized that upregulating theta power during early stages of consolidation might benefit memory stability and persistence. We used EEG neurofeedback to enable participants to selectively increase theta power in their EEG spectra following episodic memory encoding, while other participants engaged in low beta-focused neurofeedback or passively viewed a neutral nature movie. Free recall assessments immediately following the interventions, 24h later and 7d later all indicated benefit to memory of theta neurofeedback, relative to low beta neurofeedback or passive movie-viewing control conditions. The degree of benefit to memory was correlated with the extent of theta power modulation, but not with other spectral changes. Theta enhancement may provide optimal conditions for stabilization of new hippocampus-dependent memories.

Enhancing consolidation of a rotational visuomotor adaptation task through acute exercise.

We assessed the effect of a single bout of intense exercise on the adaptation and consolidation of a rotational visuomotor task, together with the effect of the order of exercise presentation relative to the learning task. Healthy adult participants (n = 29) were randomly allocated to one of three experimental groups: (1) exercise before task practice, (2) exercise after task practice, and (3) task practice only. After familiarization with the learning task, participants undertook a baseline practice set. Then, four 60° clockwise rotational sets were performed, comprising an adaptation set and three retention sets at 1 h, 24 h, and 7 days after the adaptation set. Depending on the experimental group, exercise was presented before or after the adaptation sets. We found that error reduction during adaptation was similar regardless of when exercise was presented. During retention, significant error reduction was found in the retention set at 1 h for both exercise groups, but this enhancement was not present during subsequent retention sets, with no differences present between exercise groups. We conclude that an acute bout of intense exercise could positively affect retention, although the order in which exercise is presented does not appear to influence its benefits during the early stages of consolidation.

One-dimensional consolidation of saturated degradable porous media with degradation-dependent compressibility

For degradable porous media (DPM) experiencing solid particle or mass loss, the volume change process is complex. To investigate the pore water pressure response and associated deformation during the consolidation process of saturated DPM theoretically, the generalized governing equation for one-dimensional (1-D) consolidation of saturated DPM was proposed. Then, saturated municipal solid waste (MSW) was taken as an example. The generalized and simplified 1-D consolidation models were established for saturated MSW, respectively. Analytical solution to the simplified 1-D consolidation model considering degradation-dependent compressibility only was derived for saturated MSW. Based on this analytical solution, case studies were carried out. Excess pore water pressure (usw) being larger than the initial value was found in deeper waste during the early stage of consolidation under a constant surcharge load. This is because the loading effect resulted by the increase of compressibility is greater than the unloading effect resulted by the increase of void space. An increase of degradation rate coefficient c could result in greater loading effect, and hence causes increasing usw in waste. The numerical results are of wider interest, being applicable to other DPM.

Effect of water content and soil texture on consolidation in unsaturated soils

Soil consolidation, involving time-dependent coupling between deformation of a porous medium and interstitial fluid flows within it, is of relevance to many subsurface engineering problems. A comprehensive model of poroelasticity for consolidation in unsaturated soils has been recently developed by Lo et al. (2014), but it still remains elusive how variations in soil texture and water content affect consolidation behavior, and the underlying parameters deriving this behavior.In the current study, a boundary-value problem is first setup corresponding to two symmetric semi-permeable drainage conditions, and then solved analytically for describing the excess pore air and water pressures along with the total settlement in response to time-invariant external loading using the Laplace transform. These solutions are numerically calculated for unsaturated soils with eleven texture classes as a function of three initial water saturations as representative examples. Our results reveal that the excess pore water pressure and time-dependent total settlement are indeed significantly sensitive to both soil texture and initial water saturation. We demonstrate that the coefficient of consolidation for water and its loading efficiency are two important physical parameters controlling consolidation behavior. With respect to the same soil texture, the coefficient of consolidation for water increases with an increase in initial water saturation, taking a value approximately four to five orders of magnitude greater in saturated soils than that in unsaturated ones. For a given initial water saturation, the rate of dissipation of excess pore water pressure is smallest in clay, followed by silty clay, silty clay loam, sandy clay, clay loam, silt loam, loam, sandy clay loam, sandy loam, loamy sand, and sand. A comparative study shows that in the early stage of consolidation, unsaturated soils bear smaller excess pore water pressure, but its dissipation is completed faster in saturated soils. The loading efficiency for water exhibits a concave upward relationship with initial water saturation in silty clay and clay, whereas a positively-correlated relationship between the efficiency and initial water saturation is observed in other soil textures. Unlike saturated soils, a considerable amount of total settlement is shown to occur immediately after application of external loading.

Analytical solutions for nonlinear consolidation of soft soil around a shield tunnel with idealized sealing linings

This paper presents the analytical solutions for nonlinear consolidation of soft soil around a shield tunnel with idealized sealing linings. By introducing the empirical relation between permeability and compressibility, along with the conformal transformation, the governing equations of nonlinear consolidation are established, and the corresponding analytical solutions are derived. Then, the Terzaghi consolidation solutions are derived from the degenerate governing equation of nonlinear consolidation. Through the predictions of different consolidation theories in both completely permeable and impermeable lining conditions, the influences of a tunnel acting as a drain and impacting the dissipation of pore pressure, degree of consolidation, long-term ground settlements and ground settlement rates are investigated. During the early stages of consolidation, the case studies reveal that the predictions made by this study strongly agree with the field data when a completely permeable lining is applied. This study confirms that a tunnel acting as a drain can accelerate the consolidation of soil and enlarge soil deformation due to consolidation. During long term consolidation, a notable nonlinearity of the soil consolidation is exhibited by a small and gradually decreasing settlement rate, showing agreement with the tendency of field data from the impermeable conditions.

Improving final settlement predictions of the observational method

Preloading is a popular technique in ground improvement works on soft compressible soils. Observational methods such as the Asaoka or hyperbolic methods are widely used in practical applications to assess final settlement and the degree of consolidation during preloading. In this paper it is argued that the major cause of inaccuracy in the final settlement predicted by the Asaoka method, especially in the early stages of preloading (short assessment periods), is neglect of the effects of creep in Asaoka's solution. This may also be the cause of the lack of agreement between observational methods and the piezometer method at higher degrees of consolidation. In this paper a method is proposed that can improve ultimate settlement predicted by the Asaoka method, by considering the effects of creep. By applying this method to two sets of settlement–time data available in the literature, it was found that the final settlements can be estimated with greater accuracy compared with the Asaoka method, even in the early stages of consolidation. Additionally, the proposed method can predict future settlements accurately. The degrees of consolidation predicted by this method showed good agreement with pore pressures recorded in piezometers.

Perirhinal-Hippocampal Connectivity during Reactivation Is a Marker for Object-Based Memory Consolidation

The present study utilized event-related fMRI to address the role of the human perirhinal cortex (PRC), and its interactions with the hippocampus, in memory consolidation. Participants encoded object-based and scene-based associations and then restudied them either after a "long" or "short" delay during which consolidation could occur. We found that BOLD activation in left PRC and hippocampal-PRC functional connectivity were significantly enhanced during the restudy of the long versus short delay word-object pairs. Secondly, hippocampal-PRC connectivity during restudy of the long delay word-object pairs predicted a subsequent reduction in associative forgetting. By contrast, hippocampal-PRC connectivity did not predict subsequent resistance to forgetting for the short delay or novel associations. Together, these results provide evidence for perirhinal-hippocampal interactions in the selective consolidation of object-based associative memories and provide support for the notion that, during early stages of consolidation, memories become more distributed across brain regions.

Morphology and microstructure of chromite crystals in chromitites from the Merensky Reef (Bushveld Complex, South Africa)

The Merensky Reef of the Bushveld Complex consists of two chromitite layers separated by coarse-grained melanorite. Microstructural analysis of the chromitite layers using electron backscatter diffraction analysis (EBSD), high-resolution X-ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals: fine-grained idiomorphic and large silicate inclusion-bearing crystals. The lower chromitite layer contains both populations, whereas the upper contains only fine idiomorphic grains. Most of the inclusion-bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre-existing smaller idiomorphic crystals. Two possible mechanisms have been proposed for sintering of chromite crystals: (1) amalgamation of a cluster of grains with the same original crystallographic orientation; and (2) sintering of randomly orientated crystals followed by annealing into a single grain. The EBSD data show no evidence for clusters of similarly oriented grains among the idiomorphic population, nor for earlier presence of idiomorphic subgrains spatially related to inclusions, and therefore are evidence against both of the proposed sintering mechanisms. Electron backscatter diffraction analysis maps show deformation-related misorientations and curved subgrain boundaries within the large, amoeboidal crystals, and absence of such features in the fine-grained population. Microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers, and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation. An alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals, formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the Merensky Reef. The upper chromite layer formed from a subsequent magma influx, and hence lacked a mechanism to form dendritic chromite. This accounts for the difference between the two layers.