Dredger Fill Research Articles

Deformation and failure mechanism of rapid stabilization for dredger fill in road engineering

Foundation deformation and failure of a finite length double-layered foundation by rapid stabilization was studied with regard to dredger fill in road engineering. Deformation and bearing capacity theories were proposed based on the discontinuous deformation and the lower strength. In order to study the applicability of the theory, variations of deformation, earth pressure, and tension stress of the artificial crust were investigated using two centrifuge-modeling tests. The test results indicated that the punching shear failure through the artificial crust layer and general failure mode in the soft clay layer occurred first in the dredger fill foundation. As the load increased, the cracking failure of the artificial crust layer occurred suddenly. The maximum tensile stress value was approximately 13% of the unconfined compressive strength. The soil pressure at the bottom of the artificial crust was approximately 50% lower compared with the untreated situation, which showed that the load was spread effectively. By comparing theory with the results of the centrifuge test, the bearing capacity theory was applied successfully to the assessment of dredger fill in road engineering.

Effects of chemical conditions on the engineering properties and microscopic characteristics of Tianjin dredged fill

Chemical conditions have significant effects on the engineering properties and microscopic characteristics of the dredged fill. In this study, a series of laboratory tests were conducted to investigate the engineering properties and microscopic characteristics of the dredged fill under different chemical conditions. In addition, the micro-mechanism of the effect of chemical conditions on the engineering properties of the dredged fill is revealed. Under alkaline conditions, the undrained shear strength and structural yield stress increased by 50.33 and 145 kPa, respectively. The particle morphology was transformed from a granular and flaky structure to an agglomerated structure, and the surface-to-surface connection was the main form of intergranular connection. These changes reflect the enhancement of the intergranular cementation strength of the dredged fill by the generated insoluble substances and suggest that adding a certain amount of alkaline materials is an effective method to improve the engineering properties of dredged fill. Under acidic conditions, the undrained shear strength was significantly decreased, while the compressibility was notably increased. The cementation between soil particles was severely damaged due to the corrosion of hydrogen ions, leading to an increase in the number of large-diameter pores in the dredged fill and a deterioration of the microstructural characteristics. Under saline conditions, the undrained shear strength was reduced by 5 kPa, and the compressibility was increased. No distinct agglomerated structures were formed due to the dispersion of the internal particles of the dredged fill, and the soil skeleton effect was not notable.

Shear Behaviors of the Intersurface between Rice Straw Rope and Dredger Fill Silt: Experimental and Mechanism Studies

The further development of land reclamation, port waterway, and wharf construction brings about proper treatments of dredger fill silt, while huge amounts of rice straw set aside in China argument rational disposal every year. Therefore, rice straw is bundled up as ropes, which represent as drainage body and reinforcement, to make eco-friendly treatment for dredger fill silt. This paper investigates the mechanical properties and validity of rice straw rope as certain treating material of dredger fill silt through a series of pull-out test, mass loss test, and tension test on specimens with different water contents and dry densities. The results reveal that peak value of interfacial shear strength rises with the increase of normal stress at the same immersion time, and in particular, it rises by up to 250.0% when the normal stress is 40 kPa. The tensile force of rice straw rope increases slowly with the rise of tensile displacement, and the failure mode changes from brittle to ductile with the rise of immersion time, which witnesses first rapid back slow degradation trend. The proper interfacial shear strength, tensile force, and reasonable degradation rate of rice straw rope make it ideal in drainage and consolidation of dredger fill silt.

Analysis of the Effectiveness of the Step Vacuum Preloading Method: A Case Study on High Clay Content Dredger Fill in Tianjin, China

As a solution to avoid the blockage of the drainage pipe by traditional vacuum preloading, step vacuum preloading (SVP) has been progressively studied. However, the effectiveness of this technique has yet to be systematically analyzed. In this study, an indoor model test was conducted in which vacuum pressure was applied in five stages (10, 20, 40, 60, and 80 kPa) to dredger soil with high clay content at a reclamation site in Binhai New Area, Tianjin, China. The extent of the consolidation effect of the soil was determined, and the effectiveness of the step vacuum preloading method to address drainage pipe blockage was evaluated. The results indicate that soil settlement increases at each stage of vacuum pressure treatment and the degree of vertical consolidation at each stage exceeds 90%. At the end of the treatment stage with vacuum pressure of 80 kPa, the weakly bound water was discharged. Dissipation of pore water pressure occurred in all stages. On the basis of these results, it is shown that SVP can efficiently reinforce dredger fill. Moreover, after SVP, the grain size of the soil and void ratio are still uniformly distributed. Regardless of their location from the drainage pipe, soil exhibits permeability coefficients within the same order of magnitude. The consolidation effect of soil in each stage and the increased drainage rate in the initial stage of vacuum preloading with 80 kPa indicate that the test in the current study can decrease the horizontal displacement of fine particles and can avoid drainage pipe blockage.

人工吹填土液化判别及地基处理方法分析

人工吹填土液化判别及地基处理方法分析

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 hypothesis A and hypothesis B. 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 sample Cα would increase firstly, followed by a small decrease with the increase of consolidation pressure generally. Cα would 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 the Cα would be affected little. Both compression index Cc and Cα of soft clay would reduce after preconsolidation, in which two parameters show an approximate linear relationship. The creep coefficient of soft clay under the geometric confinement is larger than that under the oedometer test Cαε. However, the trends of the relationship between the creep coefficient and loading are consistent regardless of the confinement conditions.

Laboratory experiments on HMC coupling mechanisms in innovative clean foundation treatments for Zn-contaminated dredger fills

Large-scale contaminated dredger fills have comprehensively resulted from human activities and geological deposition processes, and their disposal is a worldwide challenge. Innovative soil remediation coupling foundation treatment methods, namely, clean foundation treatment methods (CFTMs), were proposed and verified using a hydraulic–mechanical–chemical coupling triaxial testing system. The CFTM exploration triaxial tests on undisturbed clayey, silty, and sandy dredger fills showed that the critical injection significantly dilated soil volume even after the soil was vacuum pumped. Critical injection–vacuum soil flushing (CIVF), critical injection soil flushing (CIF), and vacuum soil flushing (VF) were proposed to perform clean foundation treatment for clayey silt, sandy silt, and silty sand of 1900–2300 ppm Zn. EDDS, HCl + CaCl2, and HCl were selected as the three chelating agents. Orthogonal tests on three factors (CFTM, soil type, and eluent) showed that CIF with 5:1 EDDS aq. of pH 3.8 was the best CFTM scheme for the three soil types at a depth of 2.5–10 m. CIF with HCl aq. of pH 3.8 also reached a high comprehensive clean foundation treatment efficiency for silty sand at a depth of 2.5 m. The deep depth and heterogeneous texture resulted in low Zn contamination extraction efficiency.

A Combination Method for Testing the Mechanical Properties of Ultra-Soft Fine-Grained Soils

Abstract With the consumption of land resources and a shortage of land space, the exploration and use of the ocean have increased. However, a new type of geotechnical engineering problem has been encountered in the development of marine engineering: the problem of ultra-soft soils, such as the dredger fills extracted from sediments in coastal reclamation projects and submarine landslides formed by the movement of sediments. Ultra-soft soils usually have very low strength with high water content, and flow easily. These features enable ultra-soft soils with high water content to exhibit physical properties of both solids and fluids. Reliably testing their mechanical properties presents new challenges for the traditional geotechnical test methods. To address these problems, a test method is proposed that combines a full-flow penetrometer test for soils and a rheological test for fluids. Kaolin samples with water content between 110 % and 157 % (approximately two to three times the liquid limit of kaolin) are tested as an example, and a detailed description and verification of the combination test method are presented. The results show that the combination test method can reliably measure the undrained shear strength of an ultra-soft soil at a wide range of shear strain rates in a short time, which is difficult for traditional methods. The combination test method proposed in this study provides a new reference for mechanical property testing of ultra-soft soils and hazard assessments related to ultra-soft soils.

Sedimentary characteristics of coral mud in the South China Sea during the reclamation process

Coral mud is a form of calcareous ooze composed of fine-grained coral debris distributed as interlayers due to particle sorting during the formation of reclamation reefs in the South China Sea. To analyze the effect of coral mud on the stability of reef foundations formed by dredger fill, in this paper, the effects of initial concentrations, initial settlement heights, and salinity levels on the sedimentary characteristics of coral mud are discussed based on laboratory simulation tests. Comparative tests of marine sedimentary soil with similar particle compositions and of terrestrial silty clay with similar plastic indexes were carried out, and the sedimentation velocity characteristics of coral mud were identified at the microscopic level. The experimental results show that reducing initial concentrations and initial settlement heights are conducive to the coral mud filling process and that a medium salt content has no significant effect. The sedimentation rate of coral mud is roughly 3–7 times that of common soil, and the time spent in the isokinetic sedimentation stage is only 1/3 that of common soil. Microscopic studies show that the settling velocity of clay in water is proportional to the sphericity of flocs. According to our experimental results, the relationship between the sedimentation velocity and initial concentrations of coral mud is obtained and can be used to guide the design of similar calcareous soil projects across the globe and of dredger filling projects in particular.

Particle obstruction and crushing of dredged calcareous soil in the Nansha Islands, South China Sea

The bearing capacity of the dredged calcareous soil in the Nansha Islands, South China Sea typically exceeds that of terrigenous dredger fill. Furthermore, the shear strength of calcareous soil exceeds that of siliceous sand under identical conditions. It is mainly caused by the interparticle obstruction between calcareous soil particles. To explore the obstruction mechanism of calcareous soil particles, a series of triaxial tests were performed on calcareous soils under both consolidated undrained (CU) and consolidated drained (CD) conditions, respectively. Particle shape characteristics and particle crushing were also investigated to study the formation mechanism of obstruction force. Test results indicate that irregular particle shape is the prerequisite for the emergence of particle obstruction in calcareous soils. A simple obstruction-dilatation model was developed to describe the relationship among particle obstruction, dilation and particle crushing. The obstruction force varies greatly during the shearing process, which is mainly influenced by particle size and stress level. Under low confining pressure, particle obstruction results in dilatation and increases internal friction angle. In contrast, obstruction causes particle breakage, increases the apparent cohesion, and ultimately decreases the effective internal friction angle under high confining pressures. The particle obstruction of calcareous soil noticeably improves shear strength. Therefore, the effect of particle obstruction on strength should be considered in coral reef engineering design work.

Distribution and origination of zinc contamination in newly reclaimed heterogeneous dredger fills: Field investigation and numerical simulation.

Heavy metal elements, including Zn, Cd, As, Ni, Cu, Pb and Cr, were detected in soils (no deeper than 75 m) from newly reclaimed zones of Shanghai, China. The Zn concentration exceeded soil quality limits. The Zn contamination was tested in both dredger fills and sedimentary layers (①3-3, ②3, ④ and ⑤1-1). However, it was not detected in layer ⑤1-2-⑨. PCA and HCA analysis show that exogenous Zn probably was the contaminant source of dredger fills before the fills were dredged from the neighboring waters. Stochastic heterogeneity of the dredger fills affects the Zn-depollution remarkably. Numerical simulations show both acid precipitation and widespread drainage channels in the zones contributed to Zn-decrease in the dredger fills no deeper than 1.2 m. Acid rainstorms work better than acid constant precipitation in Zn-remediation for layers below 0.4 m. To remove Zn contamination in deep dredger fills, un-consolidation of the fills should be utilized.

Multi-scale geotechnical features of dredger fills and subsidence risk evaluation in reclaimed land using BN

Shanghai has the largest newly reclaimed area in China’s coastal regions, and dredger fills are universally distributed in reclaimed lands there. Multi-scale geotechnical properties of dredger fills in region-scale (>1 km2), site-scale (1 m2–1 km2), mesoscale (1 cm2–1 m2), and microscale (1 µm2–1 cm2) were studied mainly through cone penetration investigation, borehole investigation, and microscopic inspections. Dredger impact craters, soil structures, and soil textures were investigated qualitatively and quantitatively to identify the dredger fills. Ultimate land subsidence since 1 July 2018 and land subsidence velocity characterized by the land subsidence from 1 July 2018 to 1 January 2019 were evaluated in Shanghai’s typical reclaimed regions. Afterwards, Bayesian network including sixteen variables was proposed for land subsidence risk assessment. Surface subsidence, land subsidence velocity, and comprehensive land subsidence were the most sensitive to data point location and average initial void ratio of underlying strata. These three risk variables were also highly correlated to the thickness of the dredger fill layer and the reclamation time. This finding indicates that variances of the four independent variables (i.e., data point location, average initial void of underlying strata, land reclamation time, and thickness of dredger fill layer) significantly decide variances of the three risk levels.

Vibration propagation of subgrade built on treated dredged fill

Saturated soft clay subgrade in Wenzhou was reinforced by a combination of vacuum preloading and surcharge preloading using different loading materials. Using the principle of similarity, embankments were erected on samples of the soft clay subgrade in a large model box using three different materials (slag, coarse aggregate and sand). A vibration transmission test was conducted on the samples. The test results were used to analyse three factors that majorly affect the transmission of dynamic loads. The relationship between the vibration velocity and the distance from the vibration source was also investigated and used to derive a modified vibration attenuation equation. An increase in subgrade compressive stiffness was found to be helpful to decrease the vibration amplitude. In addition, the attenuation of the vertical vibration velocity was observed to accelerate with increasing embankment height and width.

Laboratory Measurement and Analysis of the Deteriorated Layer Permeability Coefficient of Soil-Cement Deteriorated in a Saline Environment.

The deterioration of soil-cement in a saline environment leads to a reduction in strength and an increase in permeability. Effective methods of determining the deteriorated layer permeability coefficient of soil-cement are currently lacking. A laboratory test method for measuring the permeability coefficient of the deteriorated layer was proposed using the modified permeability coefficient testing apparatus. According to the proposed method, the permeability coefficient of the deteriorated layer could be obtained after testing the permeability coefficient of the soil-cement specimen in acuring room and testing the equivalent permeability coefficient and deterioration depth of the soil-cement specimen in a deteriorated environment. Using the marine dredger fill from Jiaozhou Bay as a case study, the deteriorated layer permeability coefficients of soil-cements with different cement contents were tested. It turned out that the permeability of the deteriorated layer increases with age. At the beginning of the curing age, higher cement content led to a smaller permeability coefficient of the deteriorated layer of soil-cement. As the curing age increased, the deteriorated layer permeability coefficient of the soil-cement with higher cement content increased. The evolution of the permeability coefficient of a deteriorated layer with age can be formulated as the Logistic function. This study provides support for anti-permeability designs of soil-cement structures in saline environments.

Effect of Polyacrylamide on Improvement of Dredger Fill with Vacuum Preloading Method

AbstractDredger fill possesses undesirable engineering properties due to high water content, compressibility, and low permeability, causing insufficient consolidation and long treatment periods usi...

Vacuum preloading combined with multiple-flocculant treatment for dredged fill improvement

As dredged slurry has high water content and low shear strength, and mainly consists of fine-grained particles, clogging frequently occurs around prefabricated vertical drains (PVDs) during vacuum preloading, often leading to insufficient consolidation and poor geotechnical properties of soil. To address this issue, this paper proposes a method of multiple-flocculant treatment combined with vacuum preloading for improving dredged slurry. This is achieved by pre-treating the dredged slurry with lime and anionic polyacrylamide (APAM) prior to vacuum preloading. In this method, the additions of lime and APAM in a certain quantity allow small particles to form larger-sized mass, i.e. they flocculate, thereby increasing the void ratio and permeability. This results in the formation of a new natural soil that is easier to be consolidated by vacuum preloading. To verify the effectiveness of the proposed method, the optimum ratio of lime content to APAM content was preliminarily determined by performing a deposition experiment. A vacuum preloading test was carried out after the pre-treatment with lime and APAM. The experimental results show that with the help of APAM, the shear strength and consolidation efficiency further improve in comparison to the slurry treated with only lime. Finally, the optimum ratio of lime to APAM contents for vacuum preloading was determined as 2:0.1.

Application of image-pro plus in the shear strength and micro-structure of solidified soil mixed with fly ash

The connection between shear strength and micro-structure of solidified soil is to be done. However, most of the literature is only qualitative analysis between macro and micro. The purpose of this paper is to get the macro-micro relationship. The dredger fill is solidified by the different curing agent mixed with fly ash, and the shear strength index is measured by the direct shear test. The microscopic images are obtained by using Scanning Electron Microscope (SEM). The image is dealt with by using Image-Pro Plus (IPP), and the micro-structure parameters are deduced that include mean equivalent particle diameter Dp, mean equivalent pore diameter Db and plane void ratio e. The research shows that there is no linear relation between the internal friction angle and the micro-structure parameters; The cohesion has a good linear relation with three micro-structure parameters; Compared with the other two microstructural parameters, the function of the plane void ratio on the cohesion is greater.

Laboratory Tests for the Consolidation Property of Dredger Fill in the Shenzhen Area

In order to analyze the special engineering properties of the super soft dredger fill, experimental studies had been done on the physical-mechanical properties of that in Shenzhen, especially for its consolidation properties. The test results indicated that the super soft dredger fill had the properties of high water content, high void ratio and high clay content; When the confining pressure applied to the sample was less than 200kPa, the change of deformation and void ratio of it was more obvious; When the confining pressure applied to the sample was less than 80kPa, the chang of permeability coefficient of that was more significant.

Centrifugal test of a road embankment built after new dredger fill on thick marine clay

The construction of a 100-km road network is planned on a land reclamation area in the Oujiang Estuary in China. The embankment had a height of 4 m and a base width of 60 m. The reclamation area is newly filled by a 3-m dredger fill on a 48-m thick layer of marine clay. Estimation of the settlement of the future road network is difficult. To guide the construction of the road network, a 1/100-scale centrifuge model test was performed with a marine clay sample from the construction site to simulate the layered settlements and evaluate the drainage effect of prefabricated vertical drains in the dredger fill in the following 10 years. The results of the centrifuge modeling test are verified by 10-month in situ monitoring, which shows agreement between the centrifuge modeling test results and the in situ results. The test results indicate that additional time is needed to reinforce the newly added dredger fill by the surcharge preloading method to uplift the elevation of the reclamation area with dredger fill.

Mechanism test on inhomogeneity of dredged fill during vacuum preloading consolidation

First, this article presents a simulation experiment of hydraulic reclamation, and then a vacuum preloading (VP) test using the sedimentary soil obtained by the first experiment. In the VP test, the distribution and variation of different physico-mechanical parameters before and after the treatment were tested. According to the test results, the concept “non-Terzaghi soil” is proposed to explain the inhomogeneity and its mechanism rendered by “seepage separation.” And then a staged VP (SVP) drainage consolidation method has been introduced to improve the inhomogeneity and seepage curtain phenomena around prefabricated vertical drains (PVDs) during the consolidation. The test results demonstrate that the clogging problem around PVDs has been prevented and the consolidation efficiency has been promoted after the SVP test. It has been noticed that the cumulative drainage volume and the settlement displacement of SVP test were 27% and 24%, respectively, greater than that of VP test, and the soil tends to be more homogeneous. Moreover, it has been shown that the inhomogeneity degree of the permeability coefficient, unit weight, void ratio, water content, cohesion, internal friction angle, compression modulus, and the soil surface settlement in slurry after the SVP test were 3.10, 1.02, 1.03, 1.09, 2.30, 1.92, 1.19, and 1.02, whereas that after VP test was 397.27, 1.07, 1.40, 1.40, 4.74, 3.00, 1.76, and 1.22. Finally, the mechanism of SVP method has been discussed.