Soils and Foundations最新文献

The paper presents an iterative approach based on stochastic simulations and adaptive Kriging metamodels to perform reliability and safety assessments of soil slopes. Two new rules for adaptively selecting support points are proposed, considering an entropy learning function and the closeness to the failure domain defined by a limit state function. In addition, a stopping criterion is proposed based on root-mean-square and mean absolute percentage errors computed with cross-validation at the local level, focusing on regions where the uncertainties are relevant. Finally, the selection rules for support points and the error metrics are implemented in two benchmark problems with a low, moderate, and high probability of failure. Ultimately, the work leads to an adaptive Kriging strategy for slope stability assessment, offering: (1) a fair comparison with other strategies based on a significant number of realizations, (2) a stopping criteria based on a new local error metric, (3) an insight of the behavior across different magnitudes of the probability of failure, and (4) a new selection rule that reduces the total number of support points significantly. The proposed scheme is easily paired with commercial software to compute support points, resulting in an attractive tool for practitioners.

Surface settlement due to tunnel excavation is affected by several factors. However, no explicit mapping correlation exists between surface settlement and the main influencing factors. In this study, three tree-based methodologies, including classification and regression tree (CART), random forest (RF), and gradient boosting decision tree (GBRT), were implemented to predict the tunneling-induced surface settlement of the South Hong-Mei Road tunnel in Shanghai, where a large mix-shield was used. Thirteen influencing factors within three categories (tunnel geometry, geological conditions, and shield operation factors) were employed as input variables. Results show that the ensemble methods (RF and GBDT) provide superior performance over the single-tree model (CART). Moreover, GBDT has the highest level of prediction accuracy among the three statistical learning methods. The importance of influencing factors on the tunneling-induced surface settlement was probed. The tunnel geometry had the greatest effect on surface settlement. It is followed by the influencing factors in shield operation factors. Moreover, geological conditions were not as influential as the other influencing factors. The outcomes of this study may provide a reference for evaluating tunneling-induced surface settlement in other similar tunnel projects.

Based on previous studies on the seismic monitoring of isolated (IS) and non-isolated (NI) buildings supported by piled rafts with a DMW (deep mixing wall) grid, under similar soft-soil conditions during the 2011 Tohoku earthquake, the effects of kinematic and inertial forces on the piled rafts, for which the natural periods of the superstructures were significantly different, were investigated. To evaluate the degree of contribution of the kinematic and inertial forces on the maximum bending moments and incremental axial forces near the pile heads, the coefficients related to the kinematic and inertial forces were presented. The coefficients of inertial force were separated into those of the superstructure and raft inertias to consider the embedment effects. Furthermore, the mechanisms of the inelastic behavior of the settlement and the load transfer in the piled raft systems, observed during the principal motion, were discussed. Consequently, it was found that the ratio of the natural period of the soil-pile-structure system to the predominant period of the ground, T_s/T_g, plays an important role in the contribution of the kinematic and inertial forces to the maximum bending moments near the pile head. Moreover, the T_s/T_g values were seen to be closely related to the differences in the inelastic settlement and load transfer mechanisms of the piled raft systems.

Weathered Red mudstone is widely distributed in Sichuan basin. The compacted weathered red mudstone has been used as subgrade fill materials of high-speed railway in southwestern of China. Dynamic responses of such materials under cyclic loading are critical to long-term stability of subgrade. Shakedown concept is widely employed in characterizing the permanent deformation behavior of soils. According to the evolution of axial strain (Werkmeister’s theory) or unit dissipated energy (Tao’s theory) with loading cycles, the behavior of unbound granular materials can be classified into three categories: plastic shakedown, plastic creep and incremental collapse. However, both theories are more suitable for the unbound granular materials with some limitations when used to separate the plastic creep and incremental collapse behavior. To overcome the limitations of the current theories, 26 cyclic triaxial tests were conducted on a saturated fully weathered red mudstone (SFWRM) to study the evolution of axial strain and unit dissipated energy during cyclic loading. A clear dependency of axial strain, axial strain rate on the unit dissipated energy level under various cyclic stress states were observed. A new criterion which is based on the responses of unit dissipated energy with cyclic stress ratio, was proposed to determine the limit between plastic creep and incremental collapse. Comparing with Werkmeister’s criterion and Tao’s criterion, the proposed criterion showed a better performance in identifying the incremental collapse behavior of the SFWRM.

A cut slope in Rajamandala, Indonesia exhibited unexpected large-scale deformation during the excavation process which was revealed by the clinometers attached to the preventive piles. A geological survey indicated that the mudstone layer composing the slope had been folded by tectonic movement and was highly weathered and eroded at the surface. The above internal factors could have resulted in the significant deformation of the slope, although few studies have focused on the influence of folded structures and horizontal in-situ stress on the deformation behavior of cut slopes. Therefore, the objective of this paper is to clarify the influence of the folded structure and anisotropic in-situ stress state induced by tectonic movement on the deformation behavior of the aforementioned cut slope. To achieve this clarification, numerical analyses were performed based on the explicit finite difference method included in the FLAC2D software using different geological structures and in-situ stress states. Consequently, it was revealed that such folded structures can lead to larger displacement in cut slopes when compressive stress concentration occurs at the bottom of the fold. Moreover, an anisotropic in-situ stress state can result in shear failure at the foot of the mudstone and reproduce the displacement of piles that possess shapes as close as possible to those observed at the site. These analytical results confirmed that the folded structure and anisotropic in-situ stress state were the inevitable factors in the large deformation of the cut slope in Rajamandala, Indonesia.

Construction sludge, generated from tunneling and piling, is typically in a liquid state. It can be improved via physical treatments, such as dehydration, and/or chemical treatments, using stabilizers, in order to to be recycled as construction material. To adjust the strength of sludge, chemical treatments are often preferred. However, chemical treatments frequently result in alkali leaching. Methods to reduce alkalinity by curing the alkaline sludge under CO₂ gas at a certain concentration have been proposed in Japan. In recent years, technologies that utilize CO₂ to improve the quality of cementitious material have received considerable attention in terms of carbon capture. Therefore, the effects of stabilizers on the CO₂ fixation capacity of alkaline sludge during pH neutralization were investigated in this study. Accelerated carbonation and carbonate content measurement tests were conducted to detect the CO₂ content fixed in alkaline sludge specimens treated with various stabilizers. The test results showed that the fixed maximum CO₂ content per gram of dry mass of sludge, (m_CO2)^max, increased with the calcium oxide (CaO) content of the stabilizer(s) per gram of dry sludge, C_CaO. However, the rate of increase in (m_CO2)^max with C_CaO was significantly affected by the type of stabilizer used. In the case of quicklime (QL), the ratio of (m_CO2)^max to C_CaO was approximately 0.5, whereas, in the cases of fly ash (FA) and steel slag (SS), the ratio was approximately 0.25. The ratios for biomass ash and paper sludge ash were between that for QL and that for FA and SS. Detailed analyses of the test results suggest that the CaO content per gram of stabilizer(s) in the sludge, C*_CaO, can provide an estimate of the fixed maximum amount of CO₂ per gram of stabilizer(s) in the sludge, (m*_CO2)^max. However, other factors, including the amount of water-soluble Ca, should be considered for a precise evaluation. Additionally, the experimental results showed that the decrease in pH owing to neutralization increases with the increasing C_CaO. However, the type of stabilizer did not significantly affect the relationship between the degree of CO₂ fixation and the degree of neutralization.

Promoted by the international Helin Airplane project where sandy loess is widely distributed, this study investigates the feasibility of improving the mechanical properties of sandy loess by using lignin, an environmentally friendly material. Considering the climatic conditions in the distribution area of sandy loess, such as strong evaporation and large diurnal temperature difference, extensive experiments including curing test, wet-dry cycle test, freeze–thaw cycle test, triaxial test, XRD test and SEM test are conducted to determine the strengthening effect of lignin on sandy loess. Test results show that 21 days are enough for lignin-modified sandy loess to complete curing and obtain maximum compressive strength, which increases with the increase of lignin content. Lignin has insignificant effect on the resistance of sandy loess to wet-dry cycles, but it can significantly improve the resistance to freeze–thaw cycles. Lignin causes significant alterations on the stress–strain relationship and mechanical response of sandy loess by mainly changing the cohesion while maintaining the frictional angle unchanged. SEM results show that lignin gradually plays the role of cementation, bonding, and fiber reinforcement with increasing content, and it mainly affects fine-grained soils while it has less effect on large particles. Good agreement between different experiments is obtained.

A soil investigation was conducted in peatlands in Japan, using two types of tube sampling methods, and the quality of the samples was evaluated. Unlike clays, peats easily lose water by compressive actions involved in sampler intrusion and core extrusion, leading to a significant loss of pore water. It was shown that this effect could be minimized by careful thin-wall sampling with a stationary piston, whereas open-drive sampling without a piston led to the significant compression of the sample core. The consequence of this disturbance on the in-situ behavior was interpreted through an examination of the mechanical properties of peat samples subjected to the “strain paths” expected during sampling in laboratory tests. By observing the deformation characteristics through a series of constant-rate-of-strain consolidation, unconfined compression, triaxial compression, and bender element tests, it was demonstrated that, contrary to soft clays, fibrous peats with high initial permeability potentially lead to the underestimation of the settlement during construction and the overestimation of strength due to inappropriate sampling. These results can be explained by the dominance of densification by disturbance over soil structure degradation. It was also noted that the sample quality varied considerably within the sampling tube.

Roads on peat grounds are subject to stringent requirements in terms of controlling the residual settlement during their service period in order to minimize the post-construction maintenance work. This paper addresses the applicability of the Finite Element Method (FEM) by incorporating a time-dependent model for peats to an evaluation of the residual settlement. The applicability of isotach viscoplasticity, used to describe the peats’ behavior under variable-rate compression, was confirmed in laboratory tests. Long-term consolidation test results and a model simulation revealed the particular importance of two mechanisms in the residual settlement in peat grounds, namely, the stress-dependency of the coefficient of consolidation, c_v, and the viscosity-induced secondary consolidation. A model peat ground, capturing these mechanisms, was implemented for the FEM and applied to a case history of a high-standard road construction in Hokkaido, Japan. The viscoplastic parameters determined from standard oedometer tests successfully reproduced the observed long-term settlement in a peat-dominated section of the expressway. The analysis was extended to address the settlement in the peat ground with the installation of prefabricated vertical drains (PVDs). The results indicate that the delayed primary consolidation due to the stress-induced c_v reduction can be significantly alleviated, while the settlement arising from the secondary consolidation over a longer term is little affected by the PVDs. These insights point to the importance of untangling the two mechanisms of long-term settlement for the sake of making accurate predictions of the residual settlement in different time scales.