Consolidation Theory for a Stone Column Composite Foundation under Multistage Loading

The consolidation theories considering instant load cannot fully reveal the consolidation mechanism of a stone column composite foundation used in the expressway embankments due to the time effect of loading; that is, the expressway embankments are often constructed in several stages for a long time. Meanwhile, owing to the special property that the pile-soil stress ratio is larger than 1, the consolidation theory for sand drain well foundation cannot be used directly in the consolidation analysis of stone column composite foundation. Based on the principle that the vertical load applied on the composite foundation is shared by the stone column and the surrounding soil, the governing solutions for the stone column composite foundation under a multistage load are established. By virtue of the separation of variables, the corresponding solutions of degree of consolidation for loading stage and maintaining load stage are derived separately. According to the Carrillo theorem, the solution for the average total degree of consolidation of entire composite foundation is also obtained. Finally, the reasonableness of the present solution has been verified by comparing the consolidation curve calculated by the present solution with that measured by site test

Evaluating mechanism and inconsistencies in hydraulic conductivity of unsaturated soil using newly proposed biochar conductivity factor

In the past few decades, numerous studies have been conducted to promote the use of biochar as a soil amendment and most recently, for compacted geo-engineered soils. In general, the definite trends of biochar effects on water retention and fertility of soils have been confirmed. However, the biochar effects on hydraulic conductivity, particularly unsaturated hydraulic conductivity of soil-biochar mix remain unclear, making it difficult to understand water seepage in both agricultural and geo-engineered infrastructures in semi-arid regions. This study examines the unsaturated hydraulic conductivity function derived based on the measurements of soil water characteristic curves of soil with biochar contents of 0%, 5% and 10%. A new parameter “biochar conductivity factor (BCF)” is proposed to evaluate the inconsistency in reported biochar effects on soil hydraulic conductivity and to interpret it from various mechanisms (inter- and intra- pore space filling, cracking, aggregation, bio-film formation and piping/internal erosion). The impact of biochar content on unsaturated hydraulic conductivity appears to reduce as the soil becomes drier with minimal effect in residual zone. Qualitative comparison of near-saturated hydraulic conductivity with test results in the literature showed that the BCF is generally higher for smaller ratio of sand to fine content (clay and silt). Moreover, the particle size of biochar may have significant influence on soil permeability. Future scope of research has been highlighted with respect to biochar production for its applications in agriculture and geo-environmental engineering. Long term effects such as root decay and growth, aggregation and nutrient supply need to be considered. Graphical Abstract

Dynamic Response Analysis of Cable of Submerged Floating Tunnel under Hydrodynamic Force and Earthquake

A simplified analysis model of cable for submerged floating tunnel subjected to parametrically excited vibrations in the ocean environment is proposed in this investigation. The equation of motion of the cable is obtained by a mathematical method utilizing the Euler beam theory and the Galerkin method. The hydrodynamic force induced by earthquake excitations is formulated to simulate real seaquake conditions. The random earthquake excitation in the time domain is formulated by the stochastic phase spectrum method. An analytical model for analyzing the cable for submerged floating tunnel subjected to combined hydrodynamic forces and earthquake excitations is then developed. The sensitivity of key parameters including the hydrodynamic, earthquake, and structural parameters on the dynamic response of the cable is investigated and discussed. The present model enables a preliminary examination of the hydrodynamic and seismic behavior of cable for submerged floating tunnel and can provide valuable recommendations for use in design and operation of anchor systems for submerged floating tunnel

A New Analysis Method Based on the Coupling Effect of Saturation and Expansion for the Shallow Stability of Expansive Soil Slopes

Expansive soil is a kind of unsaturated soil that is rich in hydrophilic clay minerals. The shallow slope stability of expansive soil is one of the important research topics in geotechnical engineering. However, there are no suitable methods for analyzing the shallow slope stability of expansive soil. Hence, this paper proposes a new method based on a coupling effect of saturation and expansion for analyzing the shallow slope stability. Especially, the coupling effect of saturation and expansion is introduced in detail, and used to further study the shallow slope stability. With the described coupling effect and the infinite slope, a formula calculating the overlying load of the shallow soil is established by the symmetrical limited expansion along the slope and perpendicular to the plane. Moreover, a calculation model for the factor of safety is presented according to the limit equilibrium method. The experiments are designed to demonstrate the feasibility and effectiveness of the proposed analysis method for the shallow stability of newly excavated and newly filled expansive soil slopes by rainfall. In the present study, the moisture content and shear strength of the shallow expansive soil slope are investigated, and the factor of safety is calculated. The results also show that the initial moisture content has an important influence on the shallow stability in terms of the two expansive slopes previously mentioned

Three-dimensional consolidation theory of vertical drain based on continuous drainage boundary

To remedy the limitation that the conventional drainage boundary only considers two extreme cases of pervious and impervious boundaries, the consolidation theory of vertical drain is derived by applying the continuous drainage boundary, and its validity is also proven. Based on the obtained solutions, the excess pore water pressure and the average degree of consolidation under the continuous drainage boundary condition are analyzed, and the effect of the drainage capacity of the top surface, the smear effect and the well resistance on consolidation are explored. Furthermore, the practicality of this theory is also validated by the comparison with experimental data. Results confirm that the complete and continuous process of the ground top surface can be changed from no drainage to a complete drainage by adjusting the value of the interface parameter b. Higher value of the interface parameter b means a stronger water permeability of the foundation, resulting in a faster dissipation of excess pore water pressure and a faster consolidation. Meanwhile, the vertical drainage of the vertical drain cannot be neglected in calculation even though vertical drains are based on a horizontal seepage. Moreover, the smear effect and the well resistance play an important role on consolidation

Effect of vacuum removal on consolidation settlement under a combined vacuum and surcharge preloading

The combined vacuum and surcharge preloading technique is extensively used to accelerate the consolidation process of subsoils. The effect of vacuum pressure is often considered as a loading/unloading cycle of mean effective stress, such that elastic rebound occurs after vacuum removal, which cannot explain the observed postconstruction settlement in the field. In this study, the stress state of subsoils subject to vacuum and surcharge preloading is analyzed and decomposed into two components: (a) geostatic consolidation at a different depth, and (b) loading/unloading in the minor principal stress direction. A series of consolidated drained triaxial tests is conducted to simulate the soil behaviour after vacuum removal. Results show that the contribution of unloading in the minor principal stress direction outweighs the magnitude of elastic rebound after vacuum removal, and hence continued settlement dominates. A field case for highways is provided to further demonstrate the proposed mechanism

Mapping soil nail loads using Federal Highway Administration (FHWA) simplified models and artificial neural network technique

This study compiles a broad database containing 312 measured maximum soil nail loads under operational conditions. The database is used to re-assess the prediction accuracies of the default Federal Highway Administration (FHWA) nail load model and its modified version previously reported in the literature. Predictions using the default and modified FHWA models are found to be highly dispersive. Moreover, the prediction accuracy is statistically dependent on the magnitudes of the predicted nail load and several model input parameters. The modified FHWA model is then recalibrated by introducing extra empirical terms to account for the influences of wall geometry, nail design configuration, and soil shear strength parameters on the evolvement of nail loads. The recalibrated FHWA model is demonstrated to have much better prediction accuracy compared to the default and modified models. Next, an artificial neural network (ANN) model is developed for mapping soil nail loads, which is shown to be the most advantageous one as it is accurate on average and the dispersion in prediction is low. The abovementioned dependency issue is also not present in the ANN model. The practical value of the ANN model is highlighted by applying it to reliability-based designs of soil nails against internal limit states.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Laboratory investigation of pore pressure dissipation in clay around permeable piles

Excess pore water pressures induced by pile driving could have detrimental effect in the project, especially on the construction sequence and ground settlement. Measures that can effectively accelerate dissipation of pore pressures are therefore valuable. One way to reduce the pore pressure is the use of permeable piles. This paper presents a series of model-scale laboratory tests conducted on piles with drainage holes around the pile circumference. It is found that permeable piles could accelerate soil consolidation significantly, and the beneficial effect are more apparent for group pile tests. An approximate influencing zone of permeable piles was derived in this study based on the experimental observations.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Accuracy assessment of default and modified FHWA simplified models for estimation of facing tensile forces of soil nail walls

The accuracy of the default Federal Highway Administration (FHWA) simplified model for estimation of facing tensile forces for soil nail walls under in-service conditions was evaluated using a large number of measured long-term and short-term facing tensile force data collected from the literature. The estimation accuracy was quantified by the mean and coefficient of variation (COV) of the bias where bias is defined as the ratio of measured to calculated facing tensile force. Based on the available data, the default FHWA simplified model equation was found to overestimate long- and short-term facing tensile forces by about 15% and 23% on average, respectively. The corresponding spreads in estimation accuracy expressed as the bias COV were about 43% and 67%. Undesirable correlations between bias values and calculated facing tensile forces using the default FHWA simplified model equation were detected. A modified FHWA simplified model was proposed to improve the on average accuracy, reduce the spread in estimation accuracy, and remove the hidden correlations noted above. In addition, the modified equation has fewer empirical coefficients than the current formulation (i.e., four versus five). The facing tensile force equations developed in this study is a contribution to the design of facing of soil nail walls within the current FHWA soil nail wall design framework.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

A New Analysis Method Based on the Coupling Effect of Saturation and Expansion for the Shallow Stability of Expansive Soil Slopes

Expansive soil is a kind of unsaturated soil that is rich in hydrophilic clay minerals. The shallow slope stability of expansive soil is one of the important research topics in geotechnical engineering. However, there are no suitable methods for analyzing the shallow slope stability of expansive soil. Hence, this paper proposes a new method based on a coupling effect of saturation and expansion for analyzing the shallow slope stability. Especially, the coupling effect of saturation and expansion is introduced in detail, and used to further study the shallow slope stability. With the described coupling effect and the infinite slope, a formula calculating the overlying load of the shallow soil is established by the symmetrical limited expansion along the slope and perpendicular to the plane. Moreover, a calculation model for the factor of safety is presented according to the limit equilibrium method. The experiments are designed to demonstrate the feasibility and effectiveness of the proposed analysis method for the shallow stability of newly excavated and newly filled expansive soil slopes by rainfall. In the present study, the moisture content and shear strength of the shallow expansive soil slope are investigated, and the factor of safety is calculated. The results also show that the initial moisture content has an important influence on the shallow stability in terms of the two expansive slopes previously mentioned