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1.
Abstract

Analytical solutions for the steady-state seepage field in a finite seabed under homogeneous and isotropic conditions with a drained circular lined tunnel are developed. A numerical model is established using the COMSOL Multiphysics software to verify the solutions presented in this study, and strong agreement is found. A parameter analysis considering the seabed thickness, coefficient of permeability, tunnel burial depth and lining thickness is presented. The results indicate that the error induced by ignoring the thickness of the seabed cannot be neglected when the tunnel is close to the bottom of the seabed.  相似文献   

2.
Abstract

Collapse is a major threat in tunnel construction. How to predict collapse risk accurately and timely is a complicated problem. In this paper, a non-linear mathematical model is applied to obtain the potential risk indices for a submarine tunnel using statistical analysis based on previous submarine tunnels, such as the Seikan tunnel and Xiang’an tunnel. Rough set theory is used to screen risk indices, determine each indicator value, and classify risk factors. Traditional weight calculation methods that are overly dependent on expert experience and other subjective factors are optimized and improved. Based on the frequency of each risk factor, the objective weight value of each risk index is determined according to weight back analysis theory. The ideal point method is used to calculate the collapse risk level. Predictions made by this new method are consistent with the actual tunnel collapse risk levels. This new method provides theoretical and technical basis for effectively predicting tunnel collapse.  相似文献   

3.
Circulation and hydrology of Manukau Harbour   总被引：1，自引：1，他引：0  
Abstract

Current meter and current drogue measurements made over tidal periods show that the circulation in Manukau Harbour is mainly tidal, with strongest flows within the inner harbour in the four main channels. In the entrance channel, peak tidal speeds reach 2.25 m.s?1 at the surface, and 0.6 m.s?1 near the bottom. Salinity and temperature observations show that the water is nearly homogeneous with depth in summer. A residence time of 22 d is calculated, assuming the small horizontal salinity contrast is maintained by freshwater inflow and evaporation.  相似文献   

4.
Abstract

A nonlinear pile‐soil interface model incorporated in a boundary element analysis is presented to simulate both the static and cyclic behavior of piles embedded in cemented and noncemented calcareous sediments. Based on the soil parameters derived from model test data, theoretical predictions are made for a few field tests. Finally, theoretical solutions are obtained for a full‐scale hypothetical pile embedded in homogeneous and layered calcareous sediments.  相似文献   

5.
Abstract

A relatively large submarine slide (slump block) and apparent unstable surficial sediments undergoing creep have been delineated in bathymetric and seismic reflection profiles along the U.S. Atlantic continental margin northeast of Wilmington Canyon. A downslope core transect was made over selected areas to assess the geotechnical properties of the sediments associated with the slide. Sediments are predominantly silty clays and clayey silts rich in illite, with lesser quantities of feldspar, kaolinite, chlorite, quartz, and smectite minerals. Surficial sediments (cored up to 12 m) upslope from the slump block reveal typical variations in the mass physical properties with core depth. Shear strength and wet unit weight show a steady increase with depth below the mudline commensurate with a decrease in water content. In contrast, surficial sediments downslope overlying the slump block generally have low shear strength and relatively high variability in other mass physical properties with core depth. Chemical evidence of slumping (as defined by the sulfate ion content) is not apparent in the pore waters collected from the upper 10 m of sediment. No important relationships are obvious among the physical and chemical properties, specifically the carbonates or complex solids of iron and manganese oxides or hydroxides. Sediment failure in the form of a major submarine slide appears to have been a significant deformational process during the geological past (late Pleistocene). Creep and associated deformational features recorded in the surficial sediments are presumably a result of recent geological processes.  相似文献   

6.
Abstract

The Modified Cam Clay (MCC) model is extended based on S-shaped compression so that the quantitative inaccuracies and the qualitative errors of the model associated with both low and high stress levels can be removed. The following modifications are made: (i) a material constant r, the spacing ratio, is introduced; (ii) the yield surface is modified for r?≠?2; and (iii) the ratio of the elastic compression index to the virgin compression index is assumed to be constant. The compression and shearing behavior of reconstituted clays for p′ < ∝ can be successfully described. Following the same method, the proposed complete S-shaped compression curve can be implemented to many existing models, removing errors of the models at the extremities of stress level and improving the performance of the models for different stress levels with one set of values of the model parameters.  相似文献   

7.
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8.
Abstract

This article discusses a possible alternate foundation system for a tension leg platform (TLP) in deep water. It was developed through Tecnomare S.p.A. for Agip S.p.A. during a feasibility study of a site located in the southern Adriatic Sea having a water depth of 827 m. The system is a combination of a pile‐gravity foundation, with relatively short (?20 m) steel piles of very large diameter (6 to 12 m) called “superpiles,”; which are closed at the top and open at the bottom. The superpiles are installed in soft soil under the effect of self‐weight and active suction. Permanent tension of the TLP tendons is equilibrated by self‐weight only; tension due to wave action is equilibrated by the weight of the soil inside the superpiles. In fact, a pulsating tension applied to the superpiles generates a suction in the pore water that tends to keep the soil plug inside the cylinder and prevents the cylinder as a whole from being extracted from the (impervious) foundation soil.

The results of analyses of the installation and in service behavior of superpiles are described on conventional principles of soil mechanics and related to a typical profile at the Adriatic site. Installation problems are briefly discussed; then the stability in service is examined to evaluate the capacity of superpiles in compression and tension and the associated factors of safety. Finally, considering the pore pressure gradient generated by the applied tension inside the soil plug, the flow rate and related deformations during storms and during the entire expected life of the TLP are evaluated.

The proposed superpile system appears to be feasible, simple to install, and economical. Further study and experimental research are justified to optimize the system.  相似文献   

9.
Abstract

The San Diego Trough Geotechnical Test Area, located about 24 km southwest of San Diego in a water depth of about 1.2 km, lies near the base of the Coronado Escarpment directly north of the Coronado Fan. A new bathymetric map delineates a shallow basin in the soft, highly plastic, clayey silts flooring the Test Area. Measurements of shear strength by vane and static cone pene‐trometer, and bulk density by nuclear densitometer, were made in place from the submersible Deep Quest. Sixteen short (< 1.6 m) gravity cores were collected from ships.

The geotechnical properties show little areal variation and generally change uniformly with depth within the 55 km2 Test Area. Silt is the predominant grain size, averaging about 62%. In‐place bulk density shows little change with increasing depth, values range from 1.23 to 1.26 Mg/m3; laboratory density values increase with depth, ranging from 1.30 to 1.52 Mg/m3 between the surface and a depth of about 1.1 m. The difference between the in place and laboratory values may indicate sampling densification of the cored sediment. Water content in the cores decreases uniformly within the range of 249 to 43% dry weight. Shear strength increases linearly with depth. The laboratory shear strength values are lower than the in place values, which range from 4 kPa at the surface to about 29 kPa at a depth of 3.27 m. Predictor equations relate Atterberg limits, bulk density, water content, and laboratory and in place shear strength to depth. Sedimentation‐compression e log p curves have an equivalent compression index of 1.5 to nearly 2. Excluding rurbidite layers and sampling disturbance effects, all cores indicate a uniform depositional environment in the surface to 1.6 m of sediment sampled. The geotechnical properties indicate that the sediments in the west central and southwest parts of the Test Area exhibit vertical heterogeneity due to thin silt‐sand layers, presumably of turbidity current origin, that originated from the Coronado Canyon.  相似文献   

10.
Abstract

The overlying rock thickness of a subsea tunnel controls its vertical line. It not only ensures the safety and stability of tunnel construction period and operation period, but also ensures the economy of subsea tunnel. In the current research, few papers give full consideration to the complex indicators of prediction the overlying rock thickness. However, in this study, a hybrid intelligent system was established to predict the overlying rock thickness of a subsea tunnel based on Qingdao Kiaochow Bay Subsea Tunnel, China. The sea depth, basic quality index of rock mass, soft soil layer thickness, permeability coefficient, and construction management level were selected as the main factors influencing the overlying rock thickness. Using the data obtained from project site exploration, objective weight factors were calculated using rough set theory, and subjective weight factors were calculated using the analytic hierarchy process. Furthermore, the combination of weights was obtained for each factor. Finally, the weight of influencing factors was incorporated into the extension model, and the overlying rock thickness of pending section was calculated. The results of overlying rock thickness prediction model are consistent with the actual value, indicating that the model has good engineering applicability and application value.  相似文献   

11.
Abstract

Ocean bottom bases (OBBs) have been installed on both sides of the axis of the Sagami Trough east of the Izu Peninsula, central Japan, as the first step toward long‐term geodetic and geophysical observations at the plate boundary (subduction zone). The OBB is a platform for seafloor measurements; otherwise it is difficult to find an appropriate place for precise seafloor measurements in the subduction zones. It is made of a nonmagnetic concrete block of size 1100 × 1100 × 500 mm. It was lowered from a ship using a winch wire and installed on a predetermined place with its position being monitored by an acoustic transponder system and a 30‐kHz bottom pinger with an accuracy of about 2 m.

It was confirmed later during the divings on board the submersible Shinkai 2000 that the OBB was installed on a flat mud bottom in normal condition. No change has been recognized in the installation condition in 3 years; the OBB is stable enough to be used for acoustic range measurements on the seafloor as well as for several geophysical measurements.

The resolution of seafloor range measurement can be improved by two orders by using phase measurement techniques with the aid of pulse compression. Precise acoustic range measurement of the order of 10?5 is feasible under the following conditions: two‐way measurements between the two OBBs installed on the slope facing each other with angles larger than 1.5°. Correction is necessary for the effect of long‐term temperature variation.  相似文献   

12.
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   

13.
Abstract

The composition and properties of glacigenic sediments in the southwestern Barents Sea are described based on data from 33 shallow boreholes (< 143 m below seabed) and 11 seabed cores (<4.2m below seabed). The cores are tied into a regional seismostratigraphic framework, illustrating the relationships between different boreholes.

A massive, muddy diamicton (silty, sandy clay with scattered gravel) is found in nearly all cores. Average clay content (<2 pm) of this lithology is about 38%, but varies between about 25% and 50%. Short intervals of finely laminated, waterlain sediments or gravelly sand are cored in a few occasions. A high content of sand and gravel in the cores from near the Norwegian coast shows an influence of sediment input from the mainland, while material eroded from sedimentary rocks dominates farther offshore.

The data presented on physical properties include undisturbed and remolded undrained shear strength, natural water content, bulk density, compressional sound velocity (P waves), Atterberg consistency limits, effective preconsolidation pressure, and consolidation coefficient.

Prediction of overconsolidation from seismic mapping of erosional surfaces is confirmed by the borehole cores. High compaction is found both in Weichselian and older deposits, with a general increase in compaction toward the east as well as toward shallower water. Cores that are “underconsolidated” at their present burial depth are also reported.

The average compressional sound velocity is about 1780 m/s for the borehole cores, 1550 m/s for the seabed cores, and increases with increasing shear strength and consolidation. Both horizontal and vertical sound velocities are measured in several cores, and although the data have a considerable scatter, a slightly aniso‐tropic sound velocity is indicated.  相似文献   

14.
Abstract

A giant submarine slump, encompassing a 91‐km by 26‐km block, occurring on the continental slope offshore Iquique, Chile, was identified during a SeaMARC II survey. Utilizing SeaMARC II side‐scan imagery, bathymetry, and seismic reflection data, five morphostructural zones of the slump were identified: the fissured zone, scar zone, tensional depression, central block, and front zone. The fissured zone was developed on the crown of the slump; the scar zone is characterized by scars with the crescent‐shaped slip surfaces and throws ranging from 200 m to 50 m. The tensional depression zone is marked by an area voided by mass slumping, while the central block morphology was formed by uplift. The front zone is comprised of both compressional and tensional subzones. The compressional subzone is characterized by a relative topographic low, on the middle slope, whereas the extensional subzone is characterized by a convex pattern of alternated ridges and hollows, which may represent the debris of the slump on the lower slope. The formation of the slump was strongly influenced by the subduction of the Nazca plate beneath the Chile continental margin, which resulted in the subsidence of the continental slope with a resultant increase in the slope gradient and pore‐water pressure in the sedimentary layers. Slump formation was further facilitated by the development of a complex fault system associated with the subduction and by the triggering effect of earthquakes in the area.  相似文献   

15.
Abstract

Sands and silty sands are the predominant surficial soils of continental shelves. Cohesive fine‐grained soils are typical off the mouths of large rivers, near bays and estuaries, and in basins located on the shelf. The stratigraphy of shelf soils is very poorly known for most engineering purposes, except in the vicinity of the Mississippi Delta.

Vibratory coring is the most common method of sampling shelf sands to depths of about 13 m; greater soil depths are sampled by borings often using drilling and wireline sampling tools. Employment of self‐contained or wireline static cone penetrometers to obtain in situ measurements of sands has not been as common in the United States as in Europe. Dynamic piston corers are the most common samplers in cohesive soils, but rotary and hydraulically activated incremental corers are becoming available for marine use. Self‐contained or wireline vane shear devices and static cone penetrometers are used for the in situ testing of cohesive soils, and the latter device is also used for cohesionless soils. Dynamic cone penetrometers have been developed and have had limited experimental use at sea. In situ electrical resistivity and nuclear‐transmission and backscatter probes have been used in cohesive soils to obtain bulk‐density and water‐content measurements and for stratigraphic correlation. Acoustical properties of cohesive and cohesionless soils have been measured by in situ probes and have been estimated from results of geophysical surveys made on ships that are under way.

Environmental hazards to the foundations of offshore structures include earthquakes, wave‐induced loading and scour, and burrowing animals. Reported bottom‐current velocities on the United States continental shelf appear to have maximums of about 0.5 m/s under fair‐weather conditions and greater than about 5 m/s under hurricane conditions. Cyclical loading of the seafloor induced by storm waves appears to be a major hazard to soil stability in some areas.

A representative sample of the widely scattered engineering and scientific literature of continental shelf marine geotechnics and geotechnically related subjects has been made to aid marine geologists, geotechnologists, and other specialists.  相似文献   

16.
Abstract

A closure on the recent discussion regarding the review on the behavior of helical piles as a potential offshore foundation system by the Authors is presented. Installation torque, uplift capacity, installation effects, model pile, installation speed and results proposed by Discussers are commented.  相似文献   

17.
Abstract

The groundwater levels can have a significant impact on structures and infrastructures of coastal areas. The simulation of the hydraulic behavior of coastal aquifers is very important due to its specific boundary conditions. In this study, the groundwater level in the Bandar-e-Gaz coastal aquifer in the Gorgan Gulf region located in northern Iran is simulated using the MODFLOW mathematical model. The modeling of the coastal aquifer behavior carried out in two calibrations (steady and unsteady states). The validation periods (unsteady state), the mean absolute error (MAE) and mean bias error (MBE) were used to evaluate the results. The values of MAE and MBE criteria for the steady state were 0.31 (m) and –0.08 (m), while the average of these criteria in the unsteady state in calibration period were 0.46 and –0.39, and in the validation period were 0.42 (m) and –0.31 (m), respectively. The mentioned results confirm the precision of the model for the entire simulation period, they also indicate that the simulation has a limited underestimation behavior. Moreover, it shows that the precision of the results has a negligible variability, which means the simulation also has considerable reliability. The highest and lowest amount of error in the winter and summer seasons are in accordance with the lowest and highest seawater levels, and it shows the significant sea effect on the hydraulic behavior of the coastal aquifer.  相似文献   

18.
Abstract

Subbottom profiling (3.5 kHz) in the uppermost slope of the outer shelf of the northwestern Aegean Sea has shown downslope slumping and intense sediment deformation as well as a succession of microslumping within the Late Pleistocene delta sequences. Postdeposition bottom erosion by currents and the overconsolidated sediments (Cu/Po > 1) indicate the termination of deformations a long time ago. Deformation processes are estimated to have been active 18,000–10,000 B.P. Large‐scale active downslope deformations with clockwise sediment block rotation have affected the area as well. These longer‐period deformations are associated with regional neotectonics and older unconformities (i. e., Plio‐Pleistocene) as potential slide planes.

Geotechnical properties reflect mainly textural variations and locally (within sapropelic layers) high organic matter content. Relatively high values of shear strength were measured (5–29 kPa) with intermediate sensitivities (2–5), whereas relatively low values of water content (33–81%) were found along the five selective sediment cores.

Infinity slope stability analysis revealed that the slopes in the study area are most stable, although theoretical evaluations (Cu estimated from linear regression analysis) indicate relative instability for the potential glide plane at 20 m depth. The outer shelf is covered by compact relict sands, and their gentle and low‐angle (<0.2°) slopes are the most stable region of the investigated area.  相似文献   

19.
Abstract

This article presents a study of liquefaction resistance behaviour of sand using a cyclic triaxial test. The site investigation was performed, and frozen undisturbed specimens were taken from the Izumio site in Osaka, Japan. According to the evidence in 1995 Kobe Earthquake, the first two sand layers are vulnerable to undergo liquefaction. The effect of deviatoric stress on liquefaction resistance was focused on in this study. The excess pore pressure ratio, hysteresis loop, and effective stress path from the cyclic triaxial tests were reported. A multispring element model was employed to simulate the soil behaviour under cyclic loading. The results showed that applied deviatoric stress could influence the liquefaction resistance of sandy soil samples. The plots of the cyclic stress ratio versus the number of cycles to generate liquefaction known as a liquefaction resistance curve can be then constructed and compared with other sands.  相似文献   

20.
Abstract

Estimation of shoreline change using satellite images is considered as a very effective method because the coastline is found highly dynamic. This study focuses to develop a methodology to detect shoreline changes using satellite imageries obtained from Google Earth platform. The study was carried out in north-east coastline of Jaffna in Sri Lanka. Shorelines from 2002 to 2017 were delineated on the multi-temporal satellite images in the Google Earth software by visual interpretation and change was detected using Digital Shoreline Analysis System in ArcGIS. Tidal variation, digitizing error, and geometric errors were considered to calculate the uncertainty. Mean End Point Rate, mean Shoreline Change Envelop, mean Net Shoreline Movement, and mean Weighted Linear Regression Rate were used as main shoreline change statistics. Result shows that there is net shoreline accretion of 6.13?±?8.74 m with an annual rate of deposition of 0.5?m/year. During the study period, 76.12% of the observed shoreline is found accreted while the 23.88% of the shoreline is eroded. Mean Uncertainty of the shoreline is 3.73?±?0.59 m. The study revealed that the satellite images from Google Earth platform can be used for time series analysis of shorelines after appropriate corrections.  相似文献   

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