Secondary Consolidation Coefficient Research Articles

A Case Study of Assessing the Accuracy of Secondary Consolidation Prediction Using Qualitative Approach

Infrastructure constructed, especially over soft organic soil layers, can be subjected to excessive settlement within its life cycle due to the complex behaviour of the soft soil with time. However, with necessary actions prior to construction, the impacts from such soil layers can be mitigated to some extent. The case under consideration is a leisure resort in Matara, a 15-storey hotel resting on a raft foundation near the southern coastal line of Sri Lanka. With time, some cracks have formed within the building, and an investigation was done to identify the cause for the crack formation. It has been determined that cracks have appeared due to the excessive settlement of the subsurface. Furthermore, ground investigation results suggest that a peat layer is beneath the building within a depth of 15-24m. As per the survey report on the settlement of the building, secondary consolidation of the soft soil significantly impacts the excessive settlement. Therefore, three methods were used to predict the secondary consolidation settlement of the peat layer beneath the building. The methods are prediction using empirical correlations, laboratory experiment results that will follow the constant coefficient of secondary consolidation throughout time and the qualitative method, which assumes that the secondary consolidation coefficient varies with time. Based on the above techniques, predictions were made, and results suggest that the qualitative method has a significant accuracy compared to the actual settlements of the building. These observations provide some proof that the coefficient of secondary consolidation varies with time according to the qualitative approach and does not remain constant throughout the lifespan of the building as suggested by conventional methods. KEYWORDS: Coefficient of secondary consolidation, Crack formation, Settlement, Soft soil, Time-dependent settlement

Elastic-viscoplastic model for coarse-grained soil considering particle breakage

This paper presents a novel elastic-viscoplastic constitutive model for reproducing the time-dependent behaviour of coarse-grained soil considering particle breakage, by integrating the Unified Hardening (UH) model, the elastic-viscoplastic (EVP) model and the overstress theory. The relationship between the degree of particle breakage and the loading rate is established, and the state variables associated with the critical state of coarse-grained soil are derived to jointly consider both time and particle breakage. A new three-dimensional elastic-viscoplastic constitutive model is then constructed through combining the one-dimensional viscoplastic hardening parameter with a secondary consolidation coefficient considering particle breakage. The proposed model requires 19 parameters and it can well describe the influence of time-dependency and particle breakage to the shear, dilatancy, and compression behaviours of coarse-grained soil with various confining pressures or initial void ratios. Comparisons between the model predictions and experimental data are employed to validate the capability of the proposed model to replicate the time-dependent behaviour of coarse-grained soil.

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.

Consolidation characteristics of high-water-content slurries improved by flocculation-enhanced surcharge (vacuum) preloading method

The high volume and unfavorable properties of high-water-content slurries make their treatment difficult and costly. Improving the slurries through prefabricated drains combined with surcharge or vacuum preloading represents an environmental and economic means of reusing these slurries as subsoil. As conventional methods are inefficient in dealing with low-permeability slurries, flocculation is often applied. In this study, the influence of the flocculant dosage on the drainage behavior is observed, and the secondary consolidation of the slurries is examined through laboratory tests. Flocculation is found to accelerate the consolidation process, although the flocculant dosage has an optimal value when combined with the surcharge preloading method. The coefficient of secondary consolidation is more significant in slurries improved by the flocculation-enhanced surcharge (vacuum) preloading method than in the case without flocculation. The use of flocculation must be carefully controlled.

An elastic-viscoplastic model for time-dependent behavior of unsaturated soils

Modeling the time-dependent behavior of unsaturated soil is a topic that is poorly addressed by the current literature. This paper presents a novel elastic-viscoplastic constitutive model for reproducing the time-dependent behavior of unsaturated soil. The model is established using the theoretical framework of the Unified Hardening (UH) model and the Barcelona Basic Model (BBM) combined with Perzyna's overstress theory. A unified hardening parameter is introduced to modify the elastic-viscoplastic constitutive model to consider the effect of the loading history. A new expression for the secondary consolidation coefficient considering the effect of suction is presented, and the state variables related to the void ratio of unsaturated soil considering the time effect are derived. Combined with the one-dimensional viscoplastic hardening parameter, the viscoplastic scalar multiplier in overstress theory is presented, and a three-dimensional elastic-viscoplastic constitutive model is then constructed. With 17 parameters, the proposed model can describe the effects of time and suction on the shear, dilatancy, and compression behaviors of unsaturated soil with various initial void ratios and stresses. Comparisons between the model predictions and experimental data are employed to validate the capability of the proposed model in terms of reproducing the time-dependent behavior of unsaturated soil or soft rock.

On the hydro-mechanical behaviour of unsaturated damaged Callovo-Oxfordian claystone

During the excavation of drifts within the French Underground Research Laboratory (URL) in the Callovo-Oxfordian claystone (COx) formation, an excavation damaged zone (EDZ) was formed, which is expected to have adverse effects on the long-term safety of the radioactive waste repository. In this study, high pressure oedometer tests with controlled suction were performed to investigate the hydro-mechanical behaviour of damaged COx claystone. The hydraulic conductivity was measured under different vertical stresses. The suction and damage effects were analysed in terms of swelling, compression, hydraulic conductivity and creep. Besides, the evolution of micro-cracks inside the damaged claystone sample was also analysed through X-ray computed tomography (CT) and Mercury intrusion porosimetry (MIP). The oedometer results indicate that the damaged claystone has a lower swelling strain than the intact one, due to the effect of macro-pores created by damage. The damaged claystone with a higher suction exhibits a lower compressibility and larger yield stress. There is a linear correlation between hydraulic conductivity and vertical stress at zero suction. Compared with the intact claystone, the damaged one presents a lower compression index Cc. Besides, step compression index Cc* and secondary consolidation coefficient Cα both increase with the increase of vertical stress and with the decrease of suction, indicating the dependence of the compression and creep behaviours on stress and suction. A satisfactory linear relationship is observed between Cα and Cc* at each suction and the corresponding slope depends on suction. Microstructure observations from CT and MIP tests show that the micro-cracks inside the damaged sample after loading are more significant with the increase of suction, suggesting that the micro-cracks were closed more easily by loading at a lower suction. A damaged coefficient was identified to quantitatively analyse the evolution of micro-cracks.

Effect of clay with high water adsorption capacity on process of secondary consolidation of peat

Introduction. Primary consolidation of peat, underlying an embankment, is completed 3 to 5 years after its filling. It is followed by secondary consolidation, caused by removal of bound moisture from micropores. At this stage, the settlement rate depends on the degree of peat decomposition, temperature, content of various chemicals in the pore water, vibration effects, and other factors. To reduce the embankment deformation, caused by the secondary consolidation, peat is either modified by binding compounds, having filling agents, or other actions are taken to accelerate the settlement process for a short term and to decelerate it thereafter using temporary surcharge, electroosmosis and other actions.
 
 Materials and methods. The authors studied the effect of peat dehydration using clayey materials with high water adsorption capacity, such as bentonite and saponite waste generated by the diamond mining industry. 100 days oedometer tests were carried out. In the course of testing, vertical cylindrical elements, made of the above materials, were installed in the specimens. These cylindrical elements occupied about 10 % of the specimen volume.
 
 Results. Bentonite inclusions, formed without any peat extraction, had a powerful effect on development of deformations. Intensive dehydration of peat led to a 2–4 times increase in the coefficient of secondary consolidation сαε. Following 25 to 30 days of intensive settlement, the value of the cαε coefficient went down by an average of 1.5 times relative to its initial value. The effect of elements, made of saponite-containing waste, turned out to be different. The value of the cαε coefficient increased by 11 to 15 %, but 20 to 30 days later it went down by an average of 2 times relative to the initial value. Higher resistance of elements to vertical loading served as the factor preventing the development of settlement.
 
 Conclusions. Clay has a high water adsorption capacity, and its intrusion into peat results in accelerated water migration from micropores and boosts the rate of secondary consolidation. In addition to peat dehydration, secondary consolidation was accelerated by mechanical compaction in the process of adding clay into specimens followed by clay swelling.

One-dimensional non-linear rheological consolidation of clayey soils with Swartzendruber's flow law

Considering the non-linear creep characteristics of clays, the modified unified hardening (UH) relation is employed to reflect the visco-elastic-plastic deformation of soft soils. Simultaneously, Swartzendruber's flow in the form of a continuous function is captured to describe pore water flow through pores during consolidation. Thus, an upgraded one-dimensional (1-D) non-linear rheological consolidation (NRC) system of clays has been established, and its solutions are conducted using the finite volume method (FVM). Through comparisons against the theoretical solutions and laboratory test results, the effectiveness of the numerical algorithm and applicability of the modified UH relation is verified. Then, the effects of model parameters on non-linear rheological consolidation behavior were studied. The results indicate that ignoring the time effect of secondary consolidation coefficient overestimates the overall consolidation and settlement proceeding, and considering the viscous effect of the soils led to an unanticipated increase in excess pore-water pressure (EPP) at the initial loading period. Meanwhile, the rising EPP becomes more pronounced with the increase of the initial over-consolidated parameter, flow parameter, and soil thickness or the decrease of overlying load. Additionally, FVM and finite difference method (FDM) solutions, as well as the application scope of Darcy's law in the NRC system, are discussed.

Secondary consolidation characteristics of organic soil modified by bio-enzyme based on the Gibson model

In order to study the secondary consolidation characteristics of organic soil improved by bio-enzyme, the secondary consolidation test was carried out. First, the Gibson rheological model parameters were fitted according to the experimental results. Then, the relationship between rheological model parameters, Ca/Cc value, secondary consolidation coefficient Ca, and load was analyzed for different ratios of bio-enzyme. The results showed that: 1) the rheological model parameters were negatively correlated with the ratios of bio-enzyme and positively correlated with the load value. The rheological model parameters E1 (Kelvin elastic modulus) and η (Kelvin viscosity coefficient) reached the maximum value when the ratio of bio-enzyme was 0.01%. 2) The secondary consolidation coefficient was related to the load and showed a certain law. At 100 kPa, the secondary consolidation coefficient of samples reached the peak. When it was less than 200 kPa, the secondary consolidation coefficient changed obviously with the increase in load. When the load was greater than 200 kPa, the variation trend of the secondary consolidation coefficient tended to be gentle with the increase in load and finally tended to be constant. With the increase in load, the secondary consolidation coefficient Ca finally approached to a stable value. 3) The value of Ca/Cc of improved organic soil varied from 0.042 to 0.1 under various loads. In this article, the secondary consolidation characteristics of organic soil modified by bio-enzyme were studied. The secondary consolidation strain can be predicted by the established rheological model parameter equations.

Large-strain rheological consolidation analysis of multi-layered marine soft clays exhibiting natural structural characteristics

The destruction of marine soft clays’ macropore structure results in large-strain deformation during consolidation, as well as the potential to alter rheological deformation characteristics. A sectionalised regimes rheological model was adopted to describe the structural characteristics under the isotache framework, which led to a nonlinear variation of geometrical boundaries and material properties during consolidation. A semi-analytical theory combining numerical and analytical methods was developed, and the solutions for excess pore water pressure, degree of consolidation, and settlement were derived and computationally programmed. Using parametric analysis, the effects of rheological and structural characteristics on the consolidation behaviour of soft clay were discussed compared to the solutions of other theories. The coefficient of secondary consolidation in the transitional regime has an impact beginning in the middle of primary consolidation. The proposed solutions for degree of consolidation are smaller than the small-strain solutions during the early period of primary consolidation but change larger as consolidation progresses.

Fracture effect on the hydro-mechanical behaviour of Callovo-Oxfordian claystone

Excavation of the galleries for the geological disposal of radioactive waste can induce an excavation damaged zone (EDZ), which can have consequences on the long-term safety of the repository. In this study, intact Callovo-Oxfordian (COx) claystone taken from the Meuse/Haute-Marne Underground Research Laboratory in France was artificially fractured in different directions, and the hydro-mechanical behaviour was investigated by performing high pressure oedometer test and hydraulic conductivity test. Comparison between the fractured and intact COx samples allows identification of the effects of fracture in terms of swelling, compression, hydraulic conductivity and creep. It appears that the presence of fracture decreases the swelling strain and this phenomenon depends on the infiltration path and the orientation of fracture. In particular, the infiltration path affects the global swelling rate of fractured claystone. The self-sealing of fracture is mainly attributed to the swelling of COx claystone grains in the fracture zone. Lower compression coefficient a, higher compression index Cc* and higher secondary consolidation coefficient Cα are observed for the fractured claystone, indicating a higher compressibility and creep than the intact one. The compression and creep behaviours depend on both the orientation of fracture and the stress level. Comparison of hydraulic conductivities between fractured and intact claystone indicates that the hydraulic conductivity can be partially recovered due to the collapse of macro-pores in the filled fracture zone. This is also confirmed by analysing the evolution of consolidation coefficients cv with stress. Beyond a stress level about 16 MPa, the fracture effect can be neglected, but the compressibility increases further due to grain breakage. Interestingly, this stress is close to the in-situ stress before excavation.

CONSOLIDATION CHARACTERISTICS OF A SILTY CLAY: CONCERNING THE EFFECT OF SOIL DISTURBANCE

In this paper, undisturbed specimen of a silty clay constituting of Phu Bai formation (ambQ21-2 pb) was collected from boreholes in Hue city and surrounding areas. The soil, under both undisturbed and disturbed conditions, was then subjected to standard one-dimensional consolidation tests with 7 loading increments. It is shown from the experimental results that the time to the end of primary consolidation (EOP), determined by Log Time method (tLT) and 3-t method (t3T), decreases with the load increment and under the same vertical stress, the primary consolidation of disturbed silty clay finish at a shorter time than those of the undisturbed one. The coefficient of secondary consolidation (Cα) increases with the vertical stress and reaches the maximum values before decreasing. The obtained values of Cα = 0.005 - 0.020 suggest a relatively low secondary compressibility of the silty clay constituting of Phu Bai formation.

One-dimensional rheological consolidation analysis considering self-weight stress and non-Darcian flow under time-dependent loading

To evaluate the settlement of saturated clay ground under the time-dependent loading, the unified hardening (UH) constitutive model considering the time effect was used to describe the elastic visco-plasticity of the soil and Hansbo's flow equation was applied to reflect non-Darcian flow. Thus, the classical Terzaghi consolidation equation was modified considering the influence of gravitational stress of the ground. The finite volume method was introduced to solve the numerical solution of the modified equation, and the corresponding program was compiled based on the above algorithm. The applicability of the present program was validated by comparing with the existing results in the literature. Then the effect of some parameters on the consolidation behaviour was investigated. The calculated results showed that the excess pore water pressure in the ground is accumulated at the initial stage of loading. Furthermore, when the viscosity of the soil is considered, the maximum excess pore water pressure in the ground away from the permeable boundary can even exceed the load imposed on the upper surface of the ground. In addition, prolonging the construction period, thickening the soil layer, increasing the secondary consolidation coefficient and the parameters of Hansbo's flow will delay the consolidation process of ground.

Reducing the secondary consolidation of peat ground using vacuum consolidation

This paper discusses secondary consolidation of peat ground improved by vacuum consolidation method, and quantitatively presents the degree of reduction of the secondary consolidation. When an embankment is constructed for roads, river dykes or other purposes on peat ground, settlement after the construction due to remarkable secondary consolidation of peat ground is a serious problem. Therefore, to clarify the reduction effect on secondary consolidation obtained by using the vacuum consolidation method for peat ground, a trial embankment and elasto-viscoplastic finite-element (FE) analysis were performed. It was proved, with the trial embankment, that the surcharge effect by unloading vacuum pressure reduces the secondary consolidation. The coefficient of secondary consolidation (Cα) of peat ground improved by vacuum consolidation was approximately 30−50% that of non-improved peat ground. The trial embankment and the FE analysis revealed that the reduction effect could be expressed quantitatively in relation to the over-consolidation ratio (OCR), and Cα decreases hyperbolically with an increase of the OCR with the maximum curvature near an OCR of 1·3.

Experimental Study on Consolidation Characteristics of Compacted Loess

The prediction of foundation settlement is an important topic in loess filling engineering. Based on a filled foundation in Yan’an, China, this study explores the consolidation characteristics of compacted loess with different compaction energy and consolidation pressure through consolidation tests, analyzes the strain-time curve and refines the curve within 2 h, separates the primary and secondary consolidations, and obtains the critical time point between the primary and secondary consolidations. Deformation rate S t ′ and cumulative deformation St were introduced to analyze the S t ′  − St curve at the secondary consolidation stage; the secondary consolidation coefficient was employed to describe the secondary consolidation characteristics of compacted loess. According to the secondary consolidation characteristics, a prediction model of loess settlement considering different compaction energy and fill thickness was proposed, and the applicability of the model was further analyzed. The model will facilitate in guiding the design and construction of loess filling engineering.

Probabilistic characteristics analysis for the time-dependent deformation of clay soils due to spatial variability

Consolidation and creep are two types of time-dependent deformation of clay soil. However, the soil deformation presents probabilistic behaviours when the spatial variability is considered. The Monte–Carlo method is used to sample anisotropic random fields that are mapped into two-dimensional random finite element models (RFEM) to analyse the influences of spatial variability on the time-dependent deformation of clay soil. The anisotropic random fields of soil parameters are generated by a spectral representation method (SRM) to describe the spatial variability of clay soil. The consolidation and the creep problems are modelled by Biot’s consolidation theory and the creep model proposed by Yin et al. (2010) respectively. The spatial variability of Young modulus and permeability coefficient in probabilistic consolidation analysis and the secondary consolidation coefficient in probabilistic creep analysis are emphasized by the RFEM simulation. The results show that the scale of fluctuation (SOF) in vertical direction and the coefficient of variation (COV) of Young modulus and permeability have significant influences on the probabilistic characteristics of consolidation behaviour. Besides, the bunched region of low permeability soil produces a blockage effect, which seriously affects the consolidation characteristics. It is found that the uncertainty of creep deformation will accumulate over time until it remains constant. The variability of creep deformation increases with the increase in the vertical SOF and COV of the secondary consolidation coefficient. The research also provides insightful clues for the time-varying reliability design related to the time-dependent deformation of clay soil.

Study on Creep Characteristics of Expansive Soil in High-Fill Channel of South-to-North Water Transfer Project

In the Nanyang section of the midroute of the South-to-North Water Transfer Project, the expansive soil is often used as a filler for high-fill channels. After the channel is stabilized, the expansive soil undergoes creep deformation over time. Studying the creep characteristics of expansive soils in different environments is particularly important for evaluating the safe operation of high-fill channels. In the current study, the creep test of expansive soil under different moisture content and dry density was carried out. It is proposed that the slope of the fitted straight line in the compression curve of the expansive soil can be used to represent the secondary consolidation coefficient of unsaturated expansive soil, and the variation law of the secondary consolidation coefficient under different environmental factors is obtained. The modified Bjerrum calculation method considering the influence of additional load and lateral deformation yields the postexpansion soil settlement curve model to determine the control index range of the project site. Moreover, it is also observed that the secondary consolidation coefficient of unsaturated expansive soil increases with the increase of moisture content and decreases with the increase of dry density. The coefficient of secondary compression of unsaturated expansive soil is linearly related to dry density and moisture content. After the preconsolidation treatment of the expansive soil, when the load level is less than the preload, the secondary consolidation coefficient is smaller, otherwise the secondary consolidation coefficient is larger.

An Investigation of Time-Dependent Deformation Characteristics of Soft Dredger Fill

The creep characteristics of soft clays have been studied for decades. However, the lateral deformation of soils is not allowed during the commonly used one-dimensional consolidation tests, which cannot describe the real deformation features of soils in practice. On the other hand, the influence of drainage distance on the mechanical properties of soil is still controversial, classified as hypothesisAand hypothesisB. For a better understanding of deformation characteristics of soft clay, especially which in long-terms, a series of conventional oedometer tests as well as novel geometric confined consolidation tests was conducted on soft dredger fill. The results show that the secondary consolidation coefficient of the soil sampleCαwould increase firstly, followed by a small decrease with the increase of consolidation pressure generally.Cαwould decrease with the consolidation time and also be reduced by preloading. The strain at the completion of primary consolidation would increase with the drainage distance, but theCαwould be affected little. Both compression indexCcandCαof soft clay would reduce after preconsolidation, in which two parameters show an approximate linear relationship. The creep coefficient of soft clay under the geometric confinementCαε kis larger than that under the oedometer testCαε. However, the trends of the relationship between the creep coefficient and loading are consistent regardless of the confinement conditions.

Test study on secondary consolidation behavior of soft soil under different loading paths

Based on a series of one-dimensional consolidation tests for undisturbed samples in Guangzhou Nansha, the relationship between secondary consolidation properties and the stress history, load ratios and overload pre-pressure had been researched. The results show that the division of primary and secondary consolidation of soft soil is not only associated with the consolidation pressure, but also with loading ration, soil compression process and soil permeability decreased, etc factors. The main factors to affect the secondary consolidation coefficient are the loading ratio, pre-consolidation pressure, load duration, pre-consolidation pressure pc etc. Cα presents different regularities. For example, the larger the ratio is, the greater the influence of the coefficient of secondary consolidation is; secondary consolidation coefficient has a peak value when consolidation pressure equal to the pre-consolidation pressure;the longer duration of the front load, the smaller secondary consolidation coefficient of the later load. After preloading, the secondary consolidation ratio Cα and compression index Cc is a constant: Cα/Cc≈0.03. The test results indicate that preloading processing is a good way to reduce post-construction settlement: the secondary consolidation coefficient is small when the soil is over-consolidated state, and it can greatly reduce the amount of secondary consolidation deformation.

Settlement of jacked piles in clay: Theoretical analysis considering soil aging

This paper develops an analytical approach to estimate the long-term load-displacement behaviors of jacked piles in clayey soils. Apart from considering the effects of pile installation, subsequent re-consolidation and loading, the proposed approach properly considers the effects of soil aging on the long-term behavior of the surrounding soil so as to determine the load-displacement response of the jacked pile in the long run. An elastic-viscoplastic soil model is incorporated into the load transfer method to predict long-term soil properties. The comparison between the predictions from the presented approach and the data measured from the field pile loading test shows the validity of the proposed method. Parametric studies are conducted to illustrate the effects of the in-situ overconsolidation ratio, internal friction angle and the secondary consolidation coefficient on the long-term load-displacement behaviors of old piles. Results show that the proposed approach not only is capable of predicting the short-term set-up of jacked piles, but also could yield reasonable long-term load carrying behavior. The three parameters pertaining to the strength and stiffness of soil have significant impacts on the long-term load-displacement curve of the pile.