Zhenshun Hong

Estimation of Land Subsidence Based on Groundwater Flow Model

This article presents an approach for estimating land subsidence due to withdrawal of groundwater. The proposed method calculates the groundwater seepage in 3-D-condition and calculates the land subsidence one-dimensionally. The governing equation on groundwater seepage is based on the three-dimensional mass conservation law and the principle of effective stress. The land subsidence calculation method is derived based on the following assumptions: (1) displacements occur only in the vertical direction, and (2) in vertical direction the total stresses do not change. The governing equation is solved by numerical method, i.e., finite element method (FEM) in spatial discretization and finite difference method (FDM) in time series discretization. In FEM Galerkin method is adopted and in FDM, lumped matrix method is employed. The proposed method is calibrated via analyzing 1-D consolidation problem and the results are compared with those from Terzaghis one-dimensional consolidation theory and oedometer test. The proposed method is employed to analyze the consolidation of a soft layer due to withdrawal of groundwater from an aquifer under it. Moreover, this method is also applied to a field case of land subsidence due to groundwater pumping in a gas production field in Japan. The analytical results are compared with the field observed data. The results show that this approach simulates the field case well.

Sedimentation Behavior of Four Dredged Slurries in China

Sedimentation model tests were carried out on four clays in China for investigating the sedimentation behavior of slurries. The sedimentary process can be divided into fluid state and consolidation state. The consolidation state of slurries can be divided into Non-Terzahi soil and Terzaghi soil. The void ratio responsible for the intersect point of fluid state and consolidation state is determined as 8.6 times the void ratio at liquid limit based on the difference of settling type which can be divide into Consolidation settling and Zone settling with the initial void ratio increase. According to the relationship between the stable void ratio (at the end of consolidation) and the initial void ratio, the suggested value of void ratio at the transition point from Non-Terzaghi soil to Terzaghi soil is 2.8 times its value at liquid limit.

Void Ratio-Suction Behavior of Remolded Ariake Clays

A series of oedometer tests were performed on remolded or reconstituted Ariake clays. The test data can be well interpreted by a straight line in the bilogarithmic In f∼ log p plot. The suction pressure p 0 of remolded or reconstituted Ariake clays are determined by back extrapolation of the oedometer test curve to the initial void ratio e 0 . The void ratio at liquid limit e L is a powerful index of normalizing the relationships of initial void ratio e 0 and suction pressure p 0 for various remolded or reconstituted Ariake clays with different liquid limits. The normalized relationship between the normalized index e 0 /e L and suction pressure p 0 is responsible for sensitivity of one. The existing oedometer data of remolded or reconstituted soils in literature are also compiled to compare with the proposed unique relationship between e 0 /e L and p 0 .

Mitigation of levee failures using deep mixed columns and geosynthetics

Stability of levees is critical to the safety of human and structures, especially at high water levels. Levees may fail due to the existence of soft soil foundations or seepage of water through the levees or rapid drawdown. Deep mixing technology has been considered one of the good alternatives to solve foundation and seepage problems while geosynthetics can be used to stabilize slopes during rapid drawdown. Studies have shown that deep mixed columns and geosynthetics can increase the stability of highway embankments over soft soils. In those studies, however, no ponding water exists on either side of the embankment, which is not the case for levees. Experimental studies have shown that deep mixed columns under a combination of vertical and horizontal force could fail due to shear or tension/bending or rotation. A finite difference method, incorporated in the FLAC (Fast Lagrangian Analysis of Continua) Slope software, and a limit equilibrium method (specifically Bishops method), incorporated in the ReSSA software, were adopted in this study to investigate the stability of the levee with ponding water or under rapid drawdown. In this study, deep mixed columns were installed in continuous wall patterns, which were modeled as 2D deep mixed walls. Geosynthetic layers were modeled using cable elements with grout properties between geosynthetic and soil in the numerical analysis. Mohr-Coulomb failure criteria were used for the levee, the soft soil, and the deep mixed walls. The stability of a levee at different stages (end of construction, average service condition, high water surge, and rapid drawdown from the service condition and the highest water level condition) was examined. The study clearly demonstrated that the deep mixed walls can enhance the stability of the levee by providing shear/moment resistance and hindering seepage through the levee and geosynthetics can enhance the riverside slope stability of the levee by providing tensile resistance to the soil.

Gravitational Sedimentation Behavior of Sensitive Marine Ariake Clays

It has been well documented that natural marine Ariake clays are sensitive clays. In this study, extensive data of marine Ariake clays are obtained to investigate the gravitational compression behavior for sensitive clays. Analysis results indicate that the compression behavior of remolded Ariake clays is not different from that of other remolded/reconstituted soils. But natural Ariake clays do not follow the gravitational compression pattern reported by Skempton (1970) for natural sedimentary soils. At a given value of effective overburden pressure, the void ratios of natural Ariake clays are almost independent of liquid limits. Most natural Ariake clays lie above the sedimentation compression line proposed by Burland (1990). When the liquid limit is larger than 90% and the ratio of natural water content over liquid limit ranges 0.8–1.1, the natural Ariake clays lie around the sedimentation compression line. In addition, the natural Ariake clay with higher value of the ratio of natural water content over liquid limit lies above the natural Ariake clay with lower value of the ratio of natural water content over liquid limit. Salt removal is the most probable cause for such a phenomenon.

Settling behavior of clay suspensions produced by dredging activities in China

ABSTRACT Dredging activities in China produce a great deal of clay suspensions which are generally pumped into yards as filling materials to form reclaimed lands. Understanding the settlement behavior of clay suspensions during settling process is essential in engineering practice for meeting the design requirement on the storage capacity of reclaimed yards. To this end, this study performed 25 types of sedimentation tests on the clay suspensions with different liquid limits, initial water contents, and initial heights using different sizes of sediment columns. The experimental results indicate that the settlement–time curves of clay suspensions can be classified into three stages. It is found that the first stage can be ignored, and the sedimentation can be attributed to the second stage termed as settling stage in practice. Hyperbolic model is proposed for describing the settlement–time relationship during the settling stage, and a quantitative method of determining the settlement at the end of the settling stage is suggested. The changing trends of the settlement and the time at the completion of sedimentation are investigated with considering the effects of liquid limit, initial water content, and initial height.

Complete Compression Curves of Reconstituted Clays

AbstractUnlike that adopted in most existing compression models, the complete compression curve of reconstituted clays in the e − lnp′ space is S shaped. In this paper, an accurate S-shaped equation is proposed that is suitable for describing the complete compression behavior of reconstituted clays with p′ ≥ 0.01 kPa. The proposed equation is an elementary function and is ready for implementation into general stress and strain models. The proposed compression equation is validated by simulating the compression behavior of a variety of reconstituted clays.

Stability of Levees over Soft Soil Improved by Deep Mixing Technology

Stability of levees is critical to the safety of human and structures, especially at high water levels. Levees may fail due to the existence of soft soil foundations or seepage of water through the levees. Deep mixing technology has been considered one of the good alternatives to solve these problems. Studies have shown that deep mixed columns can increase the stability of highway embankments over soft soils. In those studies, however, no ponding water exists on either side of the embankment, which is not the case for levees. Experimental studies have shown that deep mixed columns under a combination of vertical and horizontal force could fail due to shear or bending or rotation. A finite difference method, incorporated in the FLAC (Fast Lagrangian Analysis of Continua) Slope software, was adopted in this study to investigate the stability of the levee with ponding water. In this study, deep mixed columns were installed in continuous wall patterns, which were modeled as 2-D deep mixed walls. Mohr-Coulomb failure criteria were used for the levee, the soft soil, and the deep mixed walls. The stability of a levee at different stages (end of construction, average service condition, and high water surge) was examined. The study clearly demonstrated that the deep mixed walls can enhance the stability of the levee by providing shear/moment resistance and hindering seepage through the levee.

Consolidation Calculating Method of Soft Ground Improved by DJM-PVD Combined Method

Dry Jet Mixed (DJM) and Prefabricated Vertical Drains (PVD) have been combined to improve soft ground in Huai-yan highway in China. A simplified calculating method of the ground improved by DJM-PVD combined method is proposed based on the assumption that the soil-cement columns are impermeable. The consolidation behavior of ground improved by the DJM-PVD combined method is compared with that of PVD improved ground. The results show that the stress concentration on the columns allows excess pore pressure in surrounding clay to dissipate more quickly than that of PVD improved ground. A case study is also presented for verifying the applicability of proposed method.

Geosynthetic Reinforcement for Riverside Slope Stability of Levees Due to Rapid Drawdown

Stability of levees depends on a number of factors. Riverside slope instability due to rapid drawdown after storms or other events is one of the factors contributing to possible failure of levees. Rapid drawdown would create difference in water head, which generates seepage force to destabilize the slope. Geosynthetics have been successfully used to stabilize natural and man-made slopes and should be able to be used for protecting the riverside slopes of levees as well. However, limited studies have been conducted so far for this application. In this study, geosynthetic layers were placed horizontally in the riverside slope to ensure its stability during rapid drawdown. The global stability of the levee is guaranteed by the installation of deep mixed walls in the core of the levee, which was investigated in another study. A finite difference method, incorporated in the FLAC (Fast Lagrangian Analysis of Continua) Slope software, was adopted. Geosynthetic layers were modeled using cable elements with grout properties between geosynthetic and soil. The riverside stability of the levee at different conditions (the average service condition, the high water surge, and drawdown from the service condition and the highest water level condition) was examined. This study clearly demonstrates that geosynthetics can enhance the riverside slope stability of the levee by providing tensile resistance to the soil.