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1.
Prediction of Settlements of Soft Clay Subjected to Long-Term Dynamic Load   总被引：2，自引：0，他引：2  
—Presented is the numerical analysis of settlements of soft soil by a 2-D dynamic effective stressFEM method.The model based on the results of cyclic triaxial tests on the reconstituted soft Ariake clay isused to predict the wave induced excess pore water pressure and residual strain of soft clay.The settle-ments of two types of breakwaters on the soft clay under ocean wave load,a low embankment subjected totraffic load and the tunnel surrounded by soft clay in Shanghai subjected to locomotive load are calculatedas examples.  相似文献   

2.
Abstract

One method straightforwardly describing the creep degradation behavior of soft marine clay is proposed and applied to the embankment modeling. Based on the experimental phenomena, the evolution of creep coefficient of soft structured clay is identified comparing with reconstituted clay, and formulated using the creep coefficient of reconstituted clay and a creep-based structure parameter relating to the inter-particle bonding. The contributions of inter-particle bonding and debonding to creep coefficient are thus considered and the creep degradation behavior is then captured straightforwardly. The creep coefficient is extended to 3D and incorporated into a newly developed elasto-viscoplastic model to describe the creep degradation in a direct way. Based on the correlations, the liquid limit is adopted as the viscosity related input parameter. The model is derived using Newton–Raphson algorithm and implemented into a Finite Element code for coupled consolidation analysis. The general applicability on creep degradation of the model is validated by simulating 1D creep, 1D CRS (constant strain rate) and 3D undrained creep tests. Finally, the enhanced model considering creep degradation is applied and validated by simulating one test embankment and one test fill on marine deposited soft sensitive clays.  相似文献   

3.
Deep cement mixing (DCM) technique is a deep in-situ stabilization technique by mixing cement powder or slurry with soft soils below the ground surface to improve their properties and behavior. Some of DCM treated soft soil grounds are approximately in a plane-strain condition; for example, a fill embankment on DCM improved ground. In this study, a plane-strain physical model was created with instrumentation and used to investigate the bearing capacity and failure mode of a soft soil improved by an end-bearing DCM column group. This study focuses on the observed wedge-shaped shear failure of the model ground and attempts to give an account of the failure. Two different methods are used to calculate the bearing capacity of the model ground, and the computed values are compared with the measured ones. It is found that the simple Brom's method gives a better estimate of the bearing capacity of the present model ground. It is also found that measured data of pore water pressures at different locations in the soft soil indicate coupling between failure of columns and consolidation of the soft soil. This study has presented the first time that a wedge-shaped block failure was observed for pattern of DCM treated soil ground.  相似文献   

4.
Permeability characteristics of lime treated marine clay   总被引：1，自引：0，他引：1  
An attempt has been made to investigate the lime induced permeability changes in the permeability and engineering behavior of different lime column treated soil systems. Lime columns treated marine clay shows an increase in permeability up to a maximum value of 15–18 times that of untreated soil with time. The shear strength of the treated soil systems show an increment up to 8–10 that of untreated soil within a period of 30–45 days curing. In the case of lime injection systems, the permeability has been increased up to 10–15 times that of untreated soil, whereas the strength of the soil has been higher by 8–10 times that of untreated soil. Further, consolidation tests show a reduction in the compressibility up to 1/2–1/3 of original values. The test results revealed that both lime column and injection techniques could be used to improve the behaviour of underwater marine clay deposits.  相似文献   

5.
This paper has identified six major factors causing property changes in surrounding soils during and after installation of deep mixing columns: soil thixotropy, soil fracturing, cement penetration and diffusion, cementation, consolidation, and heating. Laboratory tests were performed to investigate the effects of soil thixotropy, soil fracturing, and cementation in a soft marine clay, Ariake clay. Laboratory tests were conducted to evaluate property changes in surrounding clays due to installation of deep mixing columns. Test results showed that an influential zone of property changes existed in surrounding clay ranging from the edge of the columns to the distance of about twice the radius of the columns. Within this influential zone, water content decreased as samples neared the columns, while pH values and electric conductivity increased. Test results also showed that undrained shear strengths of the surrounding clays decreased during mixing but regained after a 7-day curing period and continued increasing during 28 days in this study.  相似文献   

6.
This paper has identified six major factors causing property changes in surrounding soils during and after installation of deep mixing columns: soil thixotropy, soil fracturing, cement penetration and diffusion, cementation, consolidation, and heating. Laboratory tests were performed to investigate the effects of soil thixotropy, soil fracturing, and cementation in a soft marine clay, Ariake clay. Laboratory tests were conducted to evaluate property changes in surrounding clays due to installation of deep mixing columns. Test results showed that an influential zone of property changes existed in surrounding clay ranging from the edge of the columns to the distance of about twice the radius of the columns. Within this influential zone, water content decreased as samples neared the columns, while pH values and electric conductivity increased. Test results also showed that undrained shear strengths of the surrounding clays decreased during mixing but regained after a 7-day curing period and continued increasing during 28 days in this study.  相似文献   

7.
Based on the vertical equal strain assumption, there are many consolidation theories of soft soil incorporating vertical drains. Because the influence of many factors is often ignored and the flow of the pore water usually happens in three-dimensional directions, analytical solutions could not be accurate enough. Because of the amount of computer storage and the computing time needed, two-dimensional finite element analysis is used to model the stone column improving the soft foundation. The embankment load is transferred to a column because of the soil arching effect. When the embankment is constructed, the settlement accounts for 64% of the total settlement, which shows that the column can reduce the post-construction settlement. The stone column's bulge is confined to a length about one to two times the diameter of the stone column away from its top, and the column at the toe would bears the shear stress to some extent.  相似文献   

8.
The effectiveness of constructing a geogrid-reinforced and pile supported embankment on soft ground to reduce differential settlement has been studied by pilot scale field tests and numerical analysis. Three-by-three pile groups with varying pile spacing were driven into a layer of soft ground, and a layer of geogrid was used as reinforcement over each pile group. Further, a 2-D numerical analysis has been conducted using the computer program FLAC 2D. The mechanisms of load transfer can be considered as a combination of embankment soil arching, geogrid tension, and stress transfer due to the difference in stiffness between pile and soft ground. Based on the pilot scale field tests and results of numerical analysis, we find that the geosynthetic reinforcement slightly interferes with soil arching, and helps reduce differential settlement of the soft ground. Also, the most effective load transfer and vertical stress reduction at the midspan between piles occurs when the pile cap spacing index D/b (D: pile cap spacing, b: diameter of pile) is 3.0.  相似文献   

9.
This article reports on a series of small-scale, plane strain, 1 g physical model tests designed to investigate the bearing capacity and failure mechanics of end-bearing soil-cement columns formed via Deep Mixing (DM). Pre-formed soil-cement columns, 24 mm in diameter and 200 mm in length, were installed in a soft clay bed using a replacement method; the columns represented improvement area ratios, ap, of 17%, 26%, and 35% beneath a rigid foundation of width 100 mm. Particle Image Velocimetry (PIV) was implemented in conjunction with close-range photogrammetry in order to track soil displacement during loading, from which the failure mechanisms were derived. Bearing capacity performance was verified using Ultimate Limit State numerical analysis, with the results comparing favorably to the analytical static and kinematic solutions proposed by previous researchers. A new equation for bearing capacity was derived from this numerical analysis based on the improvement area ratio and cohesion ratio of the soil column and ground model.  相似文献   

10.
The effectiveness of constructing a geogrid-reinforced and pile supported embankment on soft ground to reduce differential settlement has been studied by pilot scale field tests and numerical analysis. Three-by-three pile groups with varying pile spacing were driven into a layer of soft ground, and a layer of geogrid was used as reinforcement over each pile group. Further, a 2-D numerical analysis has been conducted using the computer program FLAC 2D. The mechanisms of load transfer can be considered as a combination of embankment soil arching, geogrid tension, and stress transfer due to the difference in stiffness between pile and soft ground. Based on the pilot scale field tests and results of numerical analysis, we find that the geosynthetic reinforcement slightly interferes with soil arching, and helps reduce differential settlement of the soft ground. Also, the most effective load transfer and vertical stress reduction at the midspan between piles occurs when the pile cap spacing index D/b (D: pile cap spacing, b: diameter of pile) is 3.0.  相似文献   

11.
The results of one-dimensional compression tests conducted on undisturbed specimens of Jiangsu soft marine clay is presented. Because of its high in situ void ratios and natural water content, Jiangsu soft marine clay displays high values of both the virgin compression index, Cc, and the secondary compression coefficient, Cα. The laboratory data indicates that the value of the ratio Cα/Cc for Jiangsu soft marine clay is constant. However, neither Cα nor Cc are constant: they both depend upon the natural water content (or void ratio) and thus are also dependent on the deformation (or compression) of Jiangsu soft marine clay. Settlement analyses show that the secondary settlement of Jiangsu soft marine clay is a significant component of the field settlement. The concept of a constant value for Cα/Cc is used to predict the secondary settlement of a surcharged embankment founded upon Jiangsu soft marine clay. The predictions are in agreement with the limited post-construction field measurements of the embankment settlement.  相似文献   

12.
Abstract

Pile foundation is the most popular option for the foundation of offshore wind turbines. The degradation of stiffness and bearing capacity of pile foundation induced by cyclic loading will be harmful for structure safety. In this article, a modified undrained elastic–plastic model considering the cyclic degradation of clay soil is proposed, and a simplified calculation method (SCM) based on shear displacement method is presented to calculate the axial degradated capacity of a single pile foundation for offshore wind turbines resisting cyclic loadings. The conception of plastic zone thickness Rp is introduced to obtain the function between accumulated plastic strain and displacement of soil around pile side. The axial ultimate capacity of single piles under axial cyclic loading calculated by this simplified analysis have a good consistency with the results from the finite element analysis, which verifies the accuracy and reliability of this method. As an instance, the behavior of pile foundation of an offshore wind farm under cyclic load is studied using the proposed numerical method and SCM. This simplified method may provide valuable reference for engineering design.  相似文献   

13.
Compressibility behaviour of lime-treated marine clay   总被引：2，自引：0，他引：2  
The necessity to tap natural marine resources from the ocean beds represents a considerable challenge for the construction of offshore structures on weak marine deposits. The use of lime to improve the behaviour of soft clays is not new. The present investigation examines lime-induced changes in the compressibility of marine clay. The test results indicate a reduction of 1/2 to 1/3 in the compressibility of the soil system within 30 to 45 days of treatment. The formation of various cementation compounds due to soil–lime reactions improves the soil characteristics with time. The results encourage the application of lime column and lime injection techniques to improve the engineering behaviour of soft marine clayey deposits. However, one has to be cautious in applying the lime technique to marine clays that contain sodium sulfate.  相似文献   

14.
A centrifugal model test was performed for an embankment backfilled with lime-stabilized soil on an undisturbed marine clay foundation. During the test, in-flight photographs were captured, settlements were measured by displacement sensors, and displacement contours were obtained from the markers installed on the front face of the model foundation. These test data were analyzed and discussed in this paper. The test results show that the embankment was stable at 2 m height but ruptured during the loading from 2 to 4 m height. The ratio of the maximum horizontal displacement increment to the ground settlement increment at the embankment centerline suddenly increased during the loading from 4 to 6 m height, indicating the failure of the foundation. This result is in agreement with the observation of the centrifugal test and the calculated Terzaghi ultimate bearing capacity under an undrained condition. Considering the brittle behavior and low tensile strength of the lime-stabilized soil, it is recommended that the lime-stabilized soil should only be used for a low embankment with a height less than 2 meters.  相似文献   

15.
A remarkable combined dry jet mixing (DJM) and prefabricated vertical drains (PVDs) method was used to enhance the performance for soft ground improvement. In the combined method, PVDs are first installed and then DJM columns are installed between the PVDs at larger spacing. This combined method improves the effectiveness of the ground improvement and creates a more economical solution. This paper presents a case study of the combined method for marine clay improvement in Lianyugang of China. The excess pore water pressure in soils created by the installation of DJM columns with and without PVDs was monitored and compared. In situ standard penetration tests were conducted in the DJM columns and before-and-after piezocone penetration tests were performed in soils surrounding the columns. Long-term settlement monitoring under embankment loading was carried out for the ground improved by combined method and DJM method alone. The field tests, settlement monitoring and economic benefit analysis results demonstrated that the DJM-PVD method is technologically sound and cost-effective as compared with the conventional DJM method.  相似文献   

16.
The behavior of single piles subjected to negative skin friction in soft soil was conducted by analyzing the results from full-scale long-term field measurements and three-dimensional (3D) numerical analyses. A skin friction coefficient (α and β coefficients) of the instrumented piles is back-calculated at different degrees of consolidation (U) of soft marine clay. Back-calculated β-values ranged from 0.15 to 0.35 for clay, and from 0.30 to 0.55 for sand, respectively. In addition, back-calculated α-values ranged from 0.1 to 0.3 for coated pile, and from 0.2 to 0.8 for uncoated pile when undrained shear strength of the soft clay was about 30–60 kPa, respectively. Moreover, this study describes behavior of a pile based on full-coupled 3D finite element (FE) analysis. The appropriate parametric studies needed for verifying the pile-soil interaction with consolidation are presented in this paper. Compared to the results from the measurements, it is shown that the computed results are capable of predicting the pile-soil behavior under consolidation. The major parameters that influence the pile behavior are discussed for different soil-pile conditions.  相似文献   

17.
Lateral flow of soft ground occurs when embankment filling is performed on reclaimed land of foreshore. If a utility pipe is buried in soft ground undergoing the lateral flow, severe damage to the pipe can be caused. A pile-supported embankment has been used to support embankment to minimize lateral flow of the soft soil by transferring the embankment loads through the piles to the firm layer. To prevent damage to the buried pipe subjected to lateral force of the soft ground, full-scale field experiments on the piled embankment were conducted for 70 days. The test results show that the piled embankment effectively reduces both the settlements of the ground and the lateral displacements of the buried pipe. Although additional load was not imposed on the embankment after finishing embankment filling, the settlement and lateral displacement of soft ground simultaneously increased. This coupling behavior was observed at the toe of the embankment and the back of the buried pipe. To quantitatively evaluate the coupling effect of the movement, the coupling area (CA) was coined and analyzed with the efficiency of load transfer. The efficiency evaluated by the CA was in good agreement with the efficiency by the soil arching mechanism.  相似文献   

18.
Abstract

Rapid industrial growth and increasing population has resulted in the discharge of wastes into the ocean, wastes which ultimately reach the seabed and contaminate the marine sediments. The soil-contaminants interaction, and their associated physico chemical properties with sediments control the behavior of marine clays. Marine clay deposits of low strength and high compressibility are located in many coastal and offshore areas. There are several foundation problems encountered in these weak marine clays. In this study, experimental work was carried out in the laboratory to stabilize soft marine clays using the lime column technique. Also the lime-induced effects on the physical and engineering behavior of marine clays in sulfate-contaminated marine environment was investigated. Consolidation tests indicate that compressibility of the lime-treated samples was reduced to 1/2–1/3 of the virgin soil after 45 days treatment. The test results also suggest that the lime column technique can be conveniently used to improve the behavior of contaminated marine clay deposits.  相似文献   

19.
This article presents a case history of determination of effective depth of prefabricated vertical drains (PVDs) under embankment loading on a very soft clay deposit in central China, near Jiujiang, Jiangxi Province. The height of the embankment was 5.3 m and construction time was about one year. The PVDs were installed to a depth of 8.5 m at a spacing of 1.5 m in a triangular pattern. Field observations and the finite element method (FEM) were employed to analyze the performance of the soft deposit during embankment construction. The influential depth of the embankment loading was evaluated based on settlement, excess pore pressure, and stress increase in subsoil, both from the observed data and FEM analysis. The effective PVD depth was determined in the following ways: (1) the depth of 5% subsoil settlement of surface settlement; (2) vertical stress increase in subsoil of 25% in-situ stress; and (3) consolidation time/PVD depth relation by FEM. Based on the analysis, the effective depth of PVDs was determined to be between 10 and 12.8 m for this field case.  相似文献   

20.
Lime migration studies in marine clays   总被引：3，自引：0，他引：3  
Weak marine clay deposits are present in several regions of the world and they are imposing severe problems for structures founded in these deposits. The use of lime to improve the properties of these soft deposits is not new. In the present investigation, a test programme was carried out to study the migration of lime into the marine clay from the installed lime columns and lime slurry injected points. The formation of various cementation compounds due to soil-lime reactions were identified by X-ray diffraction studies and the attributed changes occurring in the engineering properties of the soil systems were studied. The test results indicate that a sufficient amount of lime is diffused into the soil systems with time and the presence of excessive sodium ions in sea water does not retard the effective penetration of lime into the soil. It is suggested that both the lime-column and lime-injection techniques can be conveniently used to improve the behaviour of soft marine clayey deposits. For weak marine clays under large depths of water, the lime-injection technique is better suited.  相似文献   

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