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1.
ABSTRACT

One-way cyclic loading is more typical for traffic loading and cyclic triaxial test has been recognized as a useful method for solving many engineering problems. Under traffic loading, the influence of variable confining pressure on cyclic behavior of natural organic clay subjected to cyclic traffic loading is rarely reported in the literature. In this study, a laboratory investigation on undrained cyclic behavior of natural organic clay is presented and conducted by cyclic triaxial apparatus. Tests are conducted by both constant confining pressure and variable confining pressure, to simulate the loading conditions induced by passing vehicles in actual engineering. Different stress levels are also considered in this study. By comparing between the results of constant confining pressure tests and variable confining pressure tests, it shows that the one-way cyclic behavior of organic clay is influenced significantly by variation of confining pressure, in terms of pore water pressure, permanent axial strain and stress–strain hysteretic loops.  相似文献   

2.
Abstract

The geological conditions of subsea tunnels are extremely complex, and a water inrush disaster is prone to occur when crossing a fault. Based on a geological analysis using cloud theory, analytic hierarchy process (AHP), and entropy weight theory, we aim to establish a normal cloud model with combination weight for water inrush risk evaluation in subsea tunnels. We select seven main factors, namely, the surrounding rock grade, rock integrity, overburden thickness, seawater thickness, tunnel section, permeability coefficient, and construction technology level as risk assessment indicators for the evaluation model. The risk is divided into five levels, and the numerical characters of the cloud model are calculated based on the standard of each risk level. The subjective weight and the objective weight are determined using AHP and the entropy weight method. The combination weight is calculated using the Delphi method. The comprehensive certainty degree is used to determine the risk level of water inrush. The model is applied to the Qingdao subsea tunnel to evaluate the water inrush risk crossing the F4-1 fault, and the results are basically consistent with the actual excavation conditions. This article provides a new idea for the risk assessment of water inrush in the subsea tunnel.  相似文献   

3.
This study documents the stratigraphic evolution of the Castagnola ponded turbidite mini-basin through analysis of a detailed base-to-top section measured in the central part of the basin. Vertical variations in facies characteristics, thickness ratio of mud cap vs. sandstone of event beds and net/gross are argued to be good proxies for pinpointing the stratigraphic transition from dominantly ponded deposition, where most of the flow is trapped by the confining topography, to a flow-stripping – dominated phase in which an increasingly large part of incoming flows can escape the basin by spilling over the enclosing topography. Thickness statistics of sandstones and mud caps of event beds from the case study show that in the initial stage of turbidite deposition only part of the mud of exceptionally large volume flows escaped the confining topography; as the basin was progressively infilled, nearly all inbound flows were affected by flow stripping, with part of the sand and most of the mud escaping the basin. In the latest recorded stage of deposition the abundance of by-pass features coupled with significant modification of the sandstone bed thickness population suggests that the turbidite system was no longer obstructed frontally, and could step forward onto a healed topography. In order to assess whether the documented trends of turbidite bed characteristics indicative of the ‘fill to spill’ transition could be recognised from wireline log data alone, synthetic logs were prepared by up-scaling the field data to resolutions typical of borehole geophysical log data. Vertical trends of average bed thickness and net/gross recognisable in the synthetic data suggest that the transition from ponded to spill-dominated situations should be resolvable in geophysical log data.  相似文献   

4.
Water inrush from completely weathered granite has presented a significant challenge for tunnel construction, suggesting the urgency and importance of revealing the water inrush mechanism. In this paper, a seepage erosion model is proposed to describe the water inrush. Assuming that completely weathered granite consists of solid grain phase, fluid phase, and fluidized grain phase, the three-phase interaction was constrained by mass balance equations, and the erosion of fluidized grains was described by a modified porosity evolution equation. The fluid flow is governed by a coupled Darcy–Brinkman/Navier–Stokes equation, which responds to the variation of the flow pattern in the evolution process. Then, the validity of the model has been proved, and the superiority has been studied by comparing with the previous models. The comparison results showed that the flow pattern has a significant impact on pore pressure, water velocity, and mutation time, and the proposed model can more accurately predict the development of velocity. Furthermore, this model was used to simulate the development of water inrush and achieved good results in predicting the direction, channel size, and whole evolution process of water inrush. The research findings from the paper can benefit tunnel engineering in the case of water inrush disasters.  相似文献   

5.
Abstract

The study of the unsaturated soils is a very complex field to which several researches in laboratory and on site are directed these last years. An experimental study aims to quantify the influence of the Skempton coefficient B characterizing the degree of saturation on the behavior of the granular sand to the liquefaction resistance of sand. The study is based on undrained triaxial tests performed on natural Chlef sand carried out at an initial relative density Dr = 50% under a confining pressure of 50, 100 and 200?kPa for Skempton coefficient B?=?10 to 92%. The results of the tests show that an increase of confining pressure leads to an increase of the shear stress of the samples. For the lower value of the degree of saturation results of our findings indicate an increase of the resistance to liquefaction; our results are in good agreement with others observed in several research projects conducted with other sands. The increase of Skempton coefficient B induces an increase of pore water pressure and a decrease of the shear strength. The pore pressure (u) increases with an exponential manner with the increase of the degree of saturation; while the residual shear strength (Rs) decreases with logarithmic manner with the increase of degree of saturation (B).  相似文献   

6.
Abstract

It is important to understand the deformation mechanism of methane-hydrate specimens (MHSs) to avoid deforming the seabed during methane-hydrate production. For this purpose, discrete-element method (DEM) modelling is advantageous because it requires much less cost and effort compared to artificial specimens and in situ sediments. In this study, a method for generating DEM numerical simulation models of MHSs is proposed to study the deformation mechanism of MHSs. First, numerical models that consider the saturation of methane hydrate (SMH), following which the bi-axial compression of these models is simulated. The mechanisms controlling the shear strength of MHSs are verified and modified by investigating the stress–strain response behavior, crack-development process, and evolution of the void change rate (Echange) of MHSs. The increases of the peak strength and secant elastic modulus of the MHS with the increment of confining pressure follow parabolic relationships. Under different loading rates, the peak strength tends to increase parabolically with the increment of loading rate, while the relationship between the secant elastic modulus and loading rate is linear. Based on the testing results, empirical formulas of peak stress and elastic modulus are proposed for different confining-pressure and strain-rate conditions.  相似文献   

7.
Biological activity is known to influence sediment strength at bed–water interfaces. However, its precise effect on geomorphology and on bed composition is not known. This paper proposes a parameterization of sediment destabilizing and stabilizing organisms on three parameters that describe the erosion and mixing processes of the sediment bed, namely the critical bed shear stress, the erosion coefficient and the bioturbation coefficient. This parameterization is included in a 3D sand–mud morphodynamic model to form the sand–mud–bio model. The performance of the sand–mud–bio model is demonstrated by testing it on the Paulinapolder intertidal flat in the Western Scheldt estuary of The Netherlands. Model results show that biological influences on sediment strength result in significant morphological change and bed composition variations. Destabilizing organisms always cause a significant decrease in mud content in the bed and an increase of erosion. On the other hand, stabilizing organisms can, but do not necessarily, cause an increase of mud content and additional sedimentation.  相似文献   

8.
Abstract

Fluctuant marine and reservoir water levels are the main failure-inducing factors for embankment slopes. The soft embankment rocks, e.g., red-bed mudstone, eroded by the reservoir water level in the Three Gorges Reservoir area greatly influence the stability of the embankment slopes. In this study, unified strength theory was innovatively applied for damage evaluation and combined with the Weibull distribution to obtain the strength statistics of micro units. Additionally, one damage constitutive model and one damage evolution model considering the initial damage, strain softening and damage weakening were proposed. Then, a series of tests, e.g., modified cyclic wetting and drying test, triaxial compression test and modified numerical simulation test for reservoir embankment red-bed mudstone, were conducted to verify the feasibility of the proposed models. In addition, grey system theory was originally used to evaluate the effects of the Weibull distribution parameters (m and w) and the confining stress on the peak stress. Finally, the proposed model was tentatively applied to the modification of the limit failure height model of the bedded rock slopes. The verification implies that the proposed model results are consistent with the testing results, especially in the simulation of compaction, elastic deformation and strain softening and in the prediction of peak strength. The results from grey system theory analysis indicate that the micro unit strength parameter (w) has the most obvious effect on the strength. Moreover, the modified method based on the damage evolution model for calculating the limit failure height of the bedded rock slopes is conservative.  相似文献   

9.
The behaviors of granular material and influencing factors under complex dynamic loading are studied by more and more researchers with particle flow method. Only the strain-controlled loading has been generally used in the current study, although this method was not consistent with the practice of engineering in many situations. In this article, stress-controlled dynamic simulation tests were carried out with particle flow method, which were used to study the collapse characteristics of silt under mutation of principal stress orientation. The tests were performed by PFC2D. The simulation results and the laboratory real tests’ results had a high degree of similarity, particularly in the collapse strain and vibration times. It was very useful to forecast the silt's critical failure state. Based on the verification data, the effects of confining pressure and cyclic shear stress ratio on the collapse characteristics of silt were studied further. With the increase of cyclic shear stress ratio, the deviator strain amplitude increased and the required vibration times gradually reduced to achieve the same strain level. Under the same dynamic shear stress ratio and vibration times, the initial dynamic elastic modulus slightly increased with the increase of initial confining pressure, and the variation range of final collapse deviator strain was small. In the analysis of micro-structural evolution, the redistribution of internal stress of sample was revealed during cyclic loading. With the increase of vibration times, the development of distribution gradually stabilized, and then the high shear stress appeared in some connected regions. On that stage, the particle system developed to instability and failure. The PFC simulation results confirmed that the collapse state was the critical stage to trigger the liquefaction of silt.  相似文献   

10.
ABSTRACT

The lateral deflection of a cylindrical diaphragm wall and the associated ground movement induced by deep excavation are analyzed by performing site instrumentations and numerical analyses in the coastal area of Korea. Wall lateral deflection, rebar stress, and pore water pressure were measured and analyzed in eight directions. Variations of soil properties with the decrease of confining pressure are compared by performing various in situ tests before ad after excavation. To calculate the wall lateral deflection accurately, the effects of small strain nonlinearity, confining pressure, and the hysteresis loading/unloading loop developed during excavation are considered in the proposed numerical analysis. By comparing numerical results with measured ones, the importances of considering small strain nonlinearity and confining pressure reduction in the nonlinear (FEM) are emphasized. Also, the effects of wall stiffness on the performance of cylindrical diaphragm walls are studied for future similar excavation in the coastal area.  相似文献   

11.
Abstract

In this article, the dilatancy of calcareous soil is studied systematically based on triaxial consolidation drainage shear tests, and the difference in dilatancy between calcareous soil and siliceous soil is also investigated. It was found that: ① Calcareous soil experience obvious dilated deformation. Dilatancy tendency increases with increasing related density and decreases with increasing confining pressure. ② The volumetric strain rate initially increases from negative to positive. After it reaches a maximum, there is a small decrease in the volumetric strain rate, but it is still greater than zero, and the stress-strain curves are of softening type. ③ For the same condition, the dilatancy deformation of calcareous sand begins later than that of siliceous sand, and the volume compression before dilatancy is also larger for calcareous sand. ④ The critical state alone cannot accurately describe the entire deformation process of soil, and it is proposed that the phase transformation state be added to the standard method used to assess soil dilatation and contraction. ⑤ Based on the statistical analysis of experimental data, mathematical relationships were established between void ratio, relative density, and effective confining pressure of phase transformation state and critical state, respectively.
  • Highlights

  • Reports results from a well-designed experiment that includes a good amount of samples and data.

  • Effects of relative density and effective confining pressure on deformation mode and mechanical properties of calcareous sand are evaluated.

  • The difference in dilatancy between calcareous sand and siliceous sand was compared

  • The phase-transformation state and critical state were compared with the axial strain, volumetric strain and deviatoric stress.

  • Using phase-transformation void ratio and critical void ratio to describe the whole deformation process of calcareous sand is proposed.

  • The mathematical expressions of phase-transformation void ratio and critical void ration were given, respectively.

  相似文献   

12.
In order to clarify the distribution and variation of silt and fluid mud in the Waiganmen shallow section of the 50000-ton intake channel of the Xiangshan Port, and to understand the influence of the channel excavation on the surrounding flow conditions and the strength of the backsilting, especially the impact of typhoon on the sudden silting of the channel, so as to demonstrate the feasibility and stability of the channel excavation. The fluid mud,hydraulic, sediment and topographic measurements were carried out in the study area, and the thickness of the fluid mud layers, tidal current, sediment and topographic data were obtained. Dual-frequency sounder, gamma-ray densitometer and SILAS navigational fluid mud measurement system were used to monitor the fluid mud, and the results were compared and verified. The adaptability and accuracy of the three methods were analyzed. The SILAS navigational continuous density measurement system and gamma-ray fixed-point fluid mud measurement are used to detect the density, thickness and variation of the fluid mud accurately. Based on the hydrological observation data,the process of erosion and deposition in excavation channel and its influence mechanism are analyzed, and the distribution characteristics and evolution law of siltation in engineering area are given in the form of empirical formula. The research shows that the super typhoon can produce large siltation, which results in sudden siltation of the channel. The tidal current is the main dynamic factor of the change of erosion and siltation of the excavation trench. Under the influence of reciprocating tidal current and excavation topography, the trial excavation trench is silted on the whole. There is fluid mud in the monitoring area of the trench, and the distribution of fluid mud is different in space. The thickness of the fluid mud at the bottom of the trench is generally larger than that outside the trench and the slope of the trench, and the siltation of the trench tends to be slow. The research results can provide scientific evaluation for channel excavation and maintenance, and support for the implementation of the project.  相似文献   

13.
More and more deep-seated long submarine tunnels are under construction, which greatly promotes the development of tunneling technology. The complex geological conditions and frequent geological disasters have become great challenges during submarine tunnel excavation. Among them, casualties and economic losses caused by water inrush are on the top levels in all kinds of tunnel geological disasters. Escape routes after water inrush from the working face during submarine tunnel excavation are investigated and optimized in the present study. Numerical simulations are performed using the FLUENT software to probe water flow characteristics after inrush. Two common cases of water inrush during double-line submarine tunnel excavation are researched. The variation rules of velocity and pressure in the tunnels after inrush are analyzed and discussed. The optimized escape routes are achieved. Finally, the water movement laws after inrush from the working face under different excavation situations are further discussed by comparing the two case studies. Water inrush of high velocity occurs on one working face of the double-line submarine tunnel, and the research results are as follows: (1) The velocity close to the tunnel side wall is the minimum, while it is the maximum in the middle position. (2) The pressure changes greatly at the intersection area of the cross passage and the tunnels. (3) The velocity and pressure nearby the working face without water inrush are both small. (4) The velocity at the high location of the cross passage is relatively small.  相似文献   

14.
The post-cyclic behavior of biogenic carbonate sand was evaluated using cyclic triaxial testing through a stress control method under different confining pressures between 50 to 600 kPa. The testing program included a series of isotropically and anisotropically consolidated, undrained triaxial compression and extension tests on samples of remolded calcareous Bushehr sand. Grading analyses (before and after each test) were used to examine the influence of particle breakage on post-cyclic behavior of Bushehr sand. The particle breakage commonly occurred in these soils even in lower values of confining pressure, yet there was not a clear correlation between the post-cyclic responses and particle breakage. Based on the present study, a concept is suggested for post-cyclic behavior of carbonate sand. It was observed that post-cyclic strength has a good correlation with cyclic stress ratio, type of consolidation, and value of residual cyclic strain. For all specimens, it is clear that the post-cyclic strength is greater than monotonic strength, irrespective of confining pressure and relative density.  相似文献   

15.
Yi Rui 《Marine Geodesy》2019,42(1):85-102
Submarine mudflow is one of the most serious geohazards. A new Mini-drum Centrifuge apparatus was developed for use. A series of experiments were performed to investigate the flow behaviour of submarine mud, based on the following aspects: (i) side view; (ii) change in basal pore pressure; (iii) change in water content and (iv) change in the Reynolds number. In this series of centrifuge tests in the subaqueous environment, the submarine mudflow transformed from a highly concentrated mudflow to a turbulent current at a water content of 113% to 121%. Accordingly, the Reynolds number Re, as the threshold of turbulent flow was about 1,150–2,250. Based on the water content measured before and after each test, the water entrainment during the flowing process was minimal for laminar flows, but large for turbulent flows. In addition, the evolution of basal pore pressure was found to be closely related to the regime of submarine mudflow: laminar or turbulent.  相似文献   

16.
ABSTRACT

The behavior of loose anisotropically consolidated calcareous sand obtained from an island in the South China Sea was investigated under undrained monotonic and cyclic loading in a hollow cylinder torsional apparatus. The tests were conducted on specimens which consolidated under various initial effective confining pressures and consolidation stress ratios. The monotonic test results show that the failure and phase transformation line are essentially independent of the consolidation conditions, while the initial contractive tendency of the specimens decreases with an increasing consolidation stress ratio. During monotonic loading of the anisotropically consolidated specimens, a same major principal stress direction is observed at the constant stress ratio lines up to the phase transformation line, irrespective of initial effective confining pressure. The cyclic strength of the sand increases with an increasing consolidation stress ratio. Moreover, a pronounced stress dependence is observed in the sand with higher consolidation stress ratio. During cyclic loading, the generated excess pore water pressure presents considerable fluctuations. The normalized terminal excess pore water pressure is described as a function of consolidation stress ratio. The tests show that the particle shape, rather than particle crushing, plays an important role in the monotonic and cyclic behaviors of the calcareous sand.  相似文献   

17.
Abstract

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.  相似文献   

18.
冲绳海槽天然气水合物与地质构造的关系   总被引：7，自引：0，他引：7  
海底天然气水合物大多与通过切穿沉积盖层的断裂的上升烃类流体相关,这些高渗透带包括底辟和泥火山等侵入构造,所以海底断裂、底辟和泥火山等构造周围可能赋存天然气水合物;其次,高沉积速率和巨厚沉积层可使有机质迅速掩埋而保存起来,为天然气水合物的生成提供充足物源,因此,邻近陆坡河谷口的海底沉积扇也是天然气水合物赋存的有利地区;另外,由于陆坡区的水合物沉积层比海盆更容易受外界温压变化的影响发生失稳分解,诱发海底滑坡,所以滑坡与天然气水合物赋存之间的关系也非常密切。冲绳海槽邻近海域具有覆水深、沉积厚度大、沉积速率高和有机质丰富等有利赋存条件,目前的研究已经在该海域发现了天然气水合物赋存的地球物理证据BSR,因此,在现有研究基础上开展断裂、泥火山、海底扇、海底滑坡等与天然气水合物相关的构造研究,可以深入了解天然气水合物在不同地质构造中的分布特征与演化,为更精确地评估其资源潜力提供参考。  相似文献   

19.
Abstract

This article presents an experimental investigation on the dynamic consolidation (DC) drainage behavior of soft marine clays. A sinusoidal harmonic load with different frequencies was applied to simulate the DC method in which the conventional impact load was replaced by the cyclic load. Four geotextile-filter strips were used to form the side drainage channels simulating the wick drain method. A series of loading tests were conducted on soft soil specimens at different confining pressures (i.e., 20, 40, 70, and 100?kPa) and different vibration frequencies (i.e., 0, 0.5, 1, 1.5, 2, and 5?Hz). Test results showed that both confining pressure and frequency have significant influences on the drainage behavior of soft marine clay specimens. The magnitude of drainage volume consistently decreases linearly with increasing confining pressure. Compared to static loading condition, specimens under cyclic loading condition at different frequencies show a better drainage performance. Specimen at applied frequency of 1?Hz exhibits the maximum cumulative drainage volume due to the resonant effect.  相似文献   

20.
Abstract

The preloading method has been widely applied in land reclamation as a green and economical approach to ground improvement. To improve the consolidation behavior, the addition of lime generally improves soil performance. In this article, a modified oedometer equipped with a tactile pressure sensor and a bender–extender element is employed. The effects of lime treatment on the geotechnical properties of dredged mud are investigated; these properties include the coefficient of the earth pressure at rest (K0), settlement, drainage behavior, and evolution of the elastic wave velocity. The results reveal that the addition of lime not only influences the specimen’s deformation behavior but also influences its shear strength during and after loading. Combined with the evolution of the elastic wave velocity, the microstructural evolution of the specimen could be described and explained. It is concluded that a compressional wave is more sensitive to the change in the specimen’s void ratio and saturation. For K0, initially (<25?kPa), the higher the lime content is, the greater the K0 fluctuation is. Afterwards, the higher the lime content is, the smaller the K0 fluctuation is, and finally K0 slowly approaches approximately 0.2.  相似文献   

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