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A reclamation project was undertaken in Zhuhai, China, covering an area of 11.35 km2 with a clay-slurry thickness of 30 m. It is interesting to find a good reinforcement method, which is more suitable for strengthening such a large-scale dredged clay-slurry ground. Evidence from engineering practice shows that vacuum preloading or vacuum preloading combined with surcharge preloading method technology has certain advantages. However, further data is required to verify their performance in the field. This paper presents a case study using the conventional vacuum preloading and vacuum preloading combined with surcharge preloading method (vacuum pressure of 90 kPa, surcharge height of 3 m) to improve the thick clay-slurry ground for a subgrade engineering project in Zhuhai, China. A series of field monitoring, in-situ tests, and laboratory tests were performed to evaluate their treatment effect by analysis of vacuum degree, ground surface settlement, layered settlement, consolidation degree, lateral displacement, excess pore water pressure dissipation, and vane shear strength. The results showed that the ground treated via vacuum preloading combined with surcharge preloading method was more effective than conventional vacuum preloading. The vacuum degree of vacuum preloading combined with surcharge preloading method was slightly lower than that of conventional vacuum preloading, decreasing with depth. The settlement is increased by 55.56%, average consolidation degree is enlarged by 40.9%, lateral displacement can be augmented by 2.4 times, pore water pressure coefficient is less than 0.6, and undrained shear strength is amplified by 22.2%. These findings can be used to guide the design of the combined preloading method of the subgrade engineering project along the east coast of China. [ABSTRACT FROM AUTHOR]

AbstractThis case study proposed a novel electro-osmosis PRD vacuum preloading method to solve dredging sludge treatment issues: difficulty in draining from soil showing large volume, non-uniform settlement, and low strength. To verify the effectiveness of the new method, four kinds of physical model tests integrating particle image velocimetry (PIV) technique of traditional vacuum preloading (VP), prefabricated radiant drain vacuum preloading (PRD-VP), electro-osmotic vacuum preloading (EO-VP), and EO-PRD-VP methods are conducted. The water discharge, average surface settlement, pore water pressure, water content, and undrained shear strength after treatment, clogging range, relationship between clogging range and water discharge rate, and relationship between clogging range and average surface settlement are investigated. For those model tests, it is demonstrated that EO-PRD-VP method has the best advantage in volume reduction, uniform settlement, and strength improvement. Water discharge is enlarged by 13–33%. The differential settlement can reach 2.3 cm, decreased by 28–56%. The undrained shear strength can reach 12 kPa, increased by 1–2 times. In addition, the clogging range development is described, for the given water discharge rate and average surface settlement, clogging range of EO-PRD-VP method is the minimum. The empirical equations between clogging range and water discharge rate, clogging range, and average surface settlement are established to predict the clogging range, which can lay the foundation for developing the consolidation theory of EO-PRD-VP method. [ABSTRACT FROM AUTHOR]

The long-term deformation characteristics and damage degree of soft marine-deposited clay must be evaluated to safely design seawalls. According to abundant field evidence, soft marine-deposited clay tends to experience coupled dynamic loading and seepage loading. Determining the critical load and damage index for these complex loading conditions is imperative. With Tianjin marine-deposited clay as an example, we employed an improved cyclic triaxial instrument to analyse the cumulative plastic strain, water discharge and stress–strain relationship of the clay to understand its long-term deformation development. Based on shakedown theory, the critical cyclic stress ratio (CSR) was determined. A multi-factor damage variable method was proposed to evaluate the damage degree. Its comparison with a traditional mono-factor damage variable method illustrated that this method is effective for damage analyses of marine clay around seawalls under coupled dynamic-seepage loading. [ABSTRACT FROM AUTHOR]

The large amount of dredged soil produced annually by rivers, lakes, seas, and oceans affects the ecological environment. Dredged material such as soft clay, which has high fluidity and high water content, is often utilized for building ground in engineering practice. Conventional vacuum preloading can effectively treat such soil but has a deprived consolidation effect. To improve the treatment effect, in this study, prefabricated radiant drain (PRD) vacuum preloading technology is proposed innovatively by adding prefabricated horizontal drains (PHDs) based on a prefabricated vertical drain (PVD). And a four-way connector is designed to connect PVD and PHDs, PHDs are attached to PVD to transmit vacuum pressure. A series of laboratory tests are run to compare and analyze the differences between conventional PVD and PRD vacuum preloading. The water discharge, average surface settlement, water content, and undrained shear strength values are analyzed. Then the parametric analysis of PHD spacings and length-width ratios (characterizing PHD length) are investigated. Overall, with the new PRD method, the consolidation effect of soft clay ground with a water content of 80% is improved by approximately 15%. [ABSTRACT FROM AUTHOR]

The high-water-content sludge produced through dredging seriously restricts marine ecological environments. Vacuum preloading methods (involving prefabricated vertical drains (PVDs)) are widely used to dewater and consolidate sludge ground, but these methods require long treatment periods and have deprived reinforcement effects. To overcome the limitations of PVD vacuum preloading, a prefabricated radiant drain (PRD) vacuum preloading method is proposed is this study by adding a prefabricated horizontal drain (PHD) to the PVD framework. Physical model tests are conducted under PVD and PRD vacuum preloading conditions, and a large-strain three-dimensional finite element (FE) model is established and validated against the physical model test data. FE parametric analyses are also conducted to evaluate the key factors influencing the PRD, i.e., the PHD length and spacing. The results show that the reinforcement effect of PRD vacuum preloading is better than that of PVD vacuum preloading. The ground settlement effect increases as the PHD length increases and the PHD spacing decreases. The settlement and pore water pressure metrics show elliptical distributions with an increasing PHD length. Through this work, an applicable sludge ground treatment design procedure is suggested based on parametric analyses; these findings can provide a reference for designing sludge ground treatments in the future. [ABSTRACT FROM AUTHOR]

Newly dredged fills feature a high moisture content and a high floating mud content. Based on the conventional vacuum preloading (CVP) method, this article presents a new method of air-booster vacuum preloading (AVP) to reinforce newly dredged fills for different air-booster periods (45, 60 and 90 min). Compared with other methods, this new method features no sand cushion and adopts a self-developed air-booster pipe and a water–air separation device. A series of laboratory model tests were performed to explore the effect of the improved AVP method for the enhancement of newly dredged fills. A comparison of variables monitored during reinforcement (vacuum pressure, surface settlement, water discharge and pore-water pressure) and after reinforcement (water content, vane shear strength, SEM and MIP) indicates that the reinforcement effect of the AVP method is superior to that of the CVP method; the former can effectively alleviate the problem of prefabricated vertical drain (PVD) clogging, greatly reducing engineering costs and significantly shortening construction periods. [ABSTRACT FROM AUTHOR]

The zero load readings of cone resistance (qc) and sleeve friction (fs) measured by a piezocone (uCPT) shift with the ambient temperature. A method of correcting the effect of temperature on uCPT measurements in seabed sediments has been proposed for the case where there is no temperature sensor in the uCPT probe. This method is based on the assumption that the "actual" profile of fs of soft shallow seabed sediments linearly increases with depth, and a rate of increase of α = 0.2 kPa/m was obtained using measured ground temperatures and fs values in seabed sediments in Isahaya Bay, Japan. An "actual" fs profile can then be constructed using the measured fs value at the shallow surface and the value of α. Using the differences between the measured and the estimated "actual" fs profiles, the ground temperature profile can be obtained, and then the effect of temperature on the uCPT measurements can be corrected. The proposed method was used for temperature corrections on uCPT measurements in Isahaya Bay, Japan. The values of undrained shear strength (su) from the temperature-corrected uCPT measurements agree well with the laboratory measured values of su using the undisturbed soil samples. [ABSTRACT FROM AUTHOR]

In this study, settling tests were conducted to investigate the sedimentation and self-weight consolidation behavior of seafloor sediments from Isahaya Bay, Ariake Sea, Japan. During the tests, the density variations with depth and time were measured by a gamma-ray transmission radioisotope densitometer. The test results show that the settling process of the seafloor sediments can be classified into the flocculation stage, settling stage, and consolidation stage. The settling rate of the seafloor sediments in the settling stage is dependent on the temperature and initial water content, while the settling rate in the consolidation stage is independent of the temperature and initial water content. The density profile changes from a constant density profile to a linear density profile when the sedimentation process transitions to the self-weight consolidation process. The relations between the void ratio (e) and effective vertical stress (p') at very low pressures can be calculated from the measured density values, and this can be used for the analysis of the self-weight consolidation of seafloor sediments. For the seafloor sediments tested in this study, the undrained shear strength (su) values are almost the same when the density values are less than 1.14 g/cm3, and the su values increase linearly with an increase in density when the density values are in the range of 1.14–1.2 g/cm3. [ABSTRACT FROM AUTHOR]

Abstract To investigate the effects of different pore water environments on compressive behaviours of the deep-sea soft soil from South China Sea, four kinds of soil samples including natural sample with sea water or deionized water, remolded sample with sea water or deionized water were prepared, respectively. Then, a series of oedometer tests and scanning electron microscopy tests were performed on the above samples to examine their compressive behaviours and microscopic characteristics, respectively. Additionally, the microscopic pore distribution of natural sample with sea water was observed by mercury intrusion porosimetry tests. The effects of different water environments on macroscopic compressive behaviours and microscopic particle characteristics are analyzed. The deionized water environment increases the compressibility and decreases the consolidation coefficient for natural samples after structural breakage and remolding. The microscopic particle characteristics corresponding to the macroscopic compressibility are also qualitatively analyzed. In deionized water environment, soil particles exhibit higher roundness, more orderly and lower surface roughness than that in seawater environment. In this study, the deionized water environment has no obvious effect on the structural yield stress and secondary consolidation coefficients of deep-sea soft soil. [ABSTRACT FROM AUTHOR]

This paper describes a full-scale test on a very soft clay ground around 70,000 m2, which is conducted in Huizhou of Guangdong Province, China, to present a new method of vacuum preloading method. A novel moisture separator was developed, which can automatically regulate the vacuum pressure variation by changing the volume of the gas inside it. A large quantity of water drained by the proposed moisture separators can be directly used as a surcharge loading, which would shorten the ground improvement time and save costs as well. Three levels of silt-prevention prefabricated vertical drains were used in the treating process to accelerate the consolidation. In addition, the vacuum preloading method also included an effective radial drainage device which would strengthen the dredged soft clay fill in a deep layer. In the in situ test, tens of piezometers and settlement plates were installed to measure the variations of excess pore water pressures and settlement of two stages of observation points at different positions in the ground. The results show that the largest average consolidation settlement was 314.1 cm and made a saving of more than 66% in power consumption compared with traditional method. It demonstrates that this adopted method is an efficient, cost-effective, and environmentally friendly method for improving sites with low bearing capacity and high compressibility soils. [ABSTRACT FROM PUBLISHER]

The mechanical behavior of clay subjected to cyclic loading is important to consider in the design of the foundations of many types of structures that must resist cyclic loading, such as subgrades and offshore foundations, because clay undergoes greater settlement under cyclic loading than under static loading. The difference in settlement between these two loading patterns due to creep behavior is affected by the cyclic frequency and the cyclic stress ratio. This study investigated the effects of the frequency and cyclic stress ratio of cyclic loading on the creep behavior of a natural clay in China using stress-controlled triaxial tests. The assessed the following parameters: three frequencies, four cyclic stress ratios, and six vertical stresses. The test results indicate that the soft clay displays accelerated creep behavior under dynamic loads. A specific “limit frequency” (in this case, 0.2 Hz) and a “safe load” at which the strain of the soft clay increases very slowly were observed. The effect of the effective axial stress on the creep behavior increases with the increase in the cyclic stress ratio. Based on the tests, the critical cyclic stress ratio is 0.267 at a certain effective axial stress and frequency. [ABSTRACT FROM AUTHOR]

The accelerated rheological properties of natural soft clay are the external power and predisposing factors of geological and geotechnical disasters. To determine the changes in soil micro-structure for natural soft clay under accelerated creep condition, a detailed study has been carried out using consolidated undrained tri-axial tests under static and cyclic loads, as well as experimental data from mercury intrusion porosimetry and a scanning electron microscope. In the tri-axial tests, creep of the soft clay would be accelerated under dynamic loads, the sample would be destroyed faster, and the strain-rate would increase clearly along with the increase of the vibration frequency. Based on a comparison of micro-structure behaviors between the intact sample and the test soil samples under static and dynamic loads in the creep condition, an interpretation of the changes in soil micro-structure for natural soft clay under an accelerated creep condition was proposed. The research indicated that the large pores of the soil sample gradually changed into small pores, the large particles continuously changed into tiny particles, and the pore distribution gradually became uniform under dynamic loads. It was found that the fractal dimension of the pores decreased with the increase of the vibration frequency. The test results not only showed that the accelerated creep characteristics depended on the vibration frequencies of the applied dynamic loads, but the accelerated creep was actually an unceasingly self-adjusting and reengineering process of the internal structure in soil under dynamic loads. In addition, these results revealed that there was a close relationship between the vibration frequencies and the micro-structure parameters. [ABSTRACT FROM AUTHOR]