1642 research outputs found
Sort by
Shallow foundations for the support of vertical-wall bridge abutments: interaction between riprap and contraction scour
Get PDFThe database from flume experiments focused on the performance of riprap layouts based on field installations and FHWA HEC-23 design guidelines against clear-water abutment scour combined with Computational Fluid Dynamics (CFD) is used to investigate how flow fields at single span bridge openings, dominated by flow contraction, adjust in response to variations of bed roughness and cross-section geometry due to riprap installations. These adjustments increase bed shear stress magnitudes on the unprotected erodible bed leading to underestimated contraction scour depths therefore creating instability, and ultimately causing edge failure of the riprap. Based on the combined physical/numerical modeling approach an edge failure-resistant riprap layout is proposed. Furthermore, the CFD approach provides an insight into shear stress magnitudes within a nonuniform bed roughness in the bridge opening, and a comprehensive flowdepth-riprap interaction model to define limits for “hydraulically narrow” bridge openings that might be prone to edge failure of the scour protecting riprap
Experimental identification of the dominant fabric in widely graded soils
Get PDFTo estimate the vulnerability of widely graded soils to internal erosion, the soil structure has to be analysed. Based on the theory of the bimodal fabric, each particle size distribution (PSD) has a unique diameter that divides the PSD into particles of the soil skeleton and finer mobile particles. The particle diameter, which divides the PSD into these two fractions, is defined as the separation point (dT) and part of the soil skeleton. In this study the Sequential Fill Test (SFT) is used to identify the mobility of particle fractions and the soil matrix.
This study is an extension of a series of descriptions of this particular test method. The paper is focused on a widely graded PSD, which is prone to internal erosion. This study demonstrates the change of the soil matrix by variation of the amount of the fine particles. The soil matrix of the investigated PSD does not change until the pore volume is completely filled with mobile particles. If the mobile particles filled completely the pore volume of the coarse soil skeleton, they become also part of the soil skeleton. This matrix is called dual matrix. With further increase of the amount of fine particles, the coarse skeleton is replaced by a new skeleton, where all particles belong to the soil skeleton.
For the assessment of a selected PSD, which is prone to internal erosion, it is important to identify the amount of fine fractions, which can stabilize an internally instable dominant fine matrix (i.e. building a dual matrix)
An experimental study of live-bed scour at circular pier in covered flow
Get PDFLive-bed scour at a bridge pier in covered flow was experimentally investigated. The laboratory campaign also comprehended a clear-water run and preliminary tests (without the pier) that were performed to characterize the pattern of the bed-forms developing in the channel. Fixed-point measurements of the bed elevation were obtained for all the runs with a laser distance sensor. The maximum scour depth and the equilibrium time decreased for increasing flow velocity; these results are in agreement with the typical depictions of the process for open-channel flows. It must be however noted that equilibrium conditions were not achieved in the clear-water run. An analysis of the dune properties (length, height) was performed based on the results of the preliminary tests, and a comparison with literature predictors showed variable levels of agreement. The fluctuations of the scour depth after equilibrium were compared to the oscillations of the bed elevation related with the bedforms. Additional experiments in different conditions and with longer duration are presently un-der design
Experiments on submerged slope erosion under unsteady water head
Get PDFThe unsteady property of seepage among the slope soil may induce higher erosion. The research results which were carried out by previous scholars were usually based on constant water head while the slope erosion under unsteady water head has not been specially studied. A new setup was proposed in this paper to study the submerged slope erosion under unsteady water head. Using the proposed setup, a series of tests were conducted by contrasting slope erosion conditions with constant water head and with unsteady water head. These tests were purposed to represent the scouring process of unsteady slope soil detachment under periodic upward seepage loading, and to reveal the mechanism of periodic seepage on slope erosion. The measurements focused on the changing profiles of unsteady slope soil and threw light on the details of different seepage conditions, particularly with respect to terrains of seepage formation during the tests. According to the experimental analysis in different seepage conditions of water head, performance of the proposed apparatus was confirmed and the results show that periodic upward seepage would accelerate the erosion, and the effects of periodic seepage loading on slope erosion were quantified in detail. The results can be introduced into the theories of slope erosion to consider the effect of periodic upward seepage on slope soil detachment, and be applied in the erosion protection
Numerical simulations of bedrock erosion around the bridge
Get PDFThere are bedrock erosion problems in western Taiwan. A bridge or an instream structure in the river often affects the sediment transport, and then leads to severe bed change and local scour problems. Bedrock channels lack a continuous cover of alluvial sediments over the long term. When the bed armor layer flushed away, it makes the bedrock exposed, and then increases the channel incision. There are significant bedrock erosion problems in the nearby area of Chungcheng Bridge in Touchien River, Taiwan. However, hydraulic models of past few decades often aimed at the sediment transport of alluvial channel. The numerical simulation of bedrock erosion is an important issue for the planning of stable countermeasures. In this study, a two-dimensional mobile-bed model, called CCHE2D is adopted to simulate the bedrock erosion process in the nearby area of Chungcheng Bridge. A stream power with flow shear stress method of bedrock incision rate formula is established in the model. The results show that the model has the capability of simulating flow field and morphological changes in study reach
Scour below marine pipelines due to random waves on mild slopes
Get PDFThis paper provides a practical stochastic method by which the maximum equilibrium scour depth beneath a marine pipeline exposed to random waves on mild slopes can be derived. The approach is based on assuming the waves to be a stationary narrow-band random process, adopting the Battjes and Groenendijk (2000) wave height distribution for mild slopes including the effect of breaking waves, and using the empirical formulas for the scour depth on the horizontal seabed by Sumer and Fredsøe (1996). Results will be presented and discussed by varying the seabed slope and water depth. An approximate method is also proposed, and comparisons are made with the present stochastic method. Generally, it appears that the approximate method can replace the stochastic method
Seabed preparation design and construction for the Malampaya Phase 3 Depletion Compression Platform
Get PDFThe Depletion Compression Platform (DCP) was conceived by Shell Philippines Exploration B.V. (SPEX) to provide additional gas compression to account for the future expected decrease in well pressure from the Malampaya field. The Malampaya project is very important to the ongoing prosperity of the Philippines, with the exported gas feeding three power stations which have a combined output providing up to 45% of the power needs of Luzon, the largest and most populous island in the Philippines.
During the concept phase of the project SPEX selected Arup’s ACE Gravity Base Structure (GBS) as the substructure solution for the DCP. The DCP substructure was designed to be supported on four linked regular hexagonal pad footings with dimensions of 18 m x 20.8 m x 4m deep. These pad footings were founded on individually prepared support pads composed of engineered rock fill material of approximately 1 m in thickness.
It is standard practice to include a larger sized scour blanket around the footing perimeter to prevent local erosion, undermining and loss of support. However, during design development we were challenged by the project team to omit this element altogether by selecting a founding engineered rock fill material which was itself suitably sized to prevent scour.
Given there is no analytical approach available to accurately determine the local flow enhancement around the apices of the individual hexagonal pad footings at seabed level, Computational Fluid Dynamics (CFD) modelling and Wave Tank Testing (WTT) were employed. The effect of the existing adjacent Concrete Gravity Structure (CGS) was also considered as part of this assessment. The results of the CFD modelling and WTT was used to calculate a minimum size range for the engineered rock fill which is capable of resisting the enhanced flow velocities in the 100-year return period cyclonic storm event.
Following calculation of the required size of engineered rock fill material, this information was presented to the potential seabed preparation Contractors for agreement on the achievable surface profile. The agreed local surface profile of the seabed preparation support pads was then used to assess the structural performance of the pad footing. Once structural performance was shown to be adequate this confirmed that the dedicated larger sized scour protection layer could be omitted. This yielded savings to the project by minimising material procurement, vessel modification and offshore working time.
The construction of the seabed preparation support pads using the larger sized engineered rock fill was performed successfully within the tolerances established and agreed through early consultation with the potential seabed preparation Contractors
The development of automated spatial and temporal measurement system for lab-scale local scour
Get PDFThe spatial and temporal measurements of local scour around bridge piers provide the quantification of local scouring process. Most studies on a laboratory scale faced difficulties in obtaining a holistic local spatial variability around bridge pier, especially for continuous small time interval. Long experimental period, which could take up to few days, does not permit a consistent time interval spatial scour measurement due in particular to the physical constraints that exist under laboratory conditions. This study proposed an automated, cost-effective system which is capable of detecting changes in both spatial and temporal local scour. The system allows measurement to be made by using data recorder at an adjustable distance (± 0.1 mm) and angle (± 0.1˚) from the original position, which is programmed and controlled with an Arduino, which is an open source microcontroller with multiple capability of controlling electrical components such as motors and sensors. When the data recorder is in position, data is automatically captured and sequentially saved at a particular spatial interval. In this study, a web camera was used as a data recorder to capture images in the azimuthal plane for a one-hour interval. Images were captured for 30 seconds per measurement per position. The system was set up to monitor and measure the temporal and spatial local scour continuously in an 80-hour experiment. Results show that the location of maximum scour depth varied for different time intervals, and migrated from downstream to upstream of the pier. The rate of scour decreased as duration of experiment was increased. The system was able to provide a holistic view of both spatial and temporal variability in the development of local scour on a laboratory scale
Modification of an erodibility category limit for the pocket erodometer
Get PDFThe pocket erodometer test (PET) is an inexpensive and expedient way to derive the erosion categories of soils. The test involves impinging a regulated jet of water at the end of an undisturbed soil sample and measuring the depth of scoured hole after 20 jet applications. The depth of the scoured hole provides an indication of the erosion category. PET and companion erosion function apparatus (EFA) tests were per-formed on 33 cohesive soil samples from 5 different sites on the island of Oahu, Hawaii. In the EFA test, water is run over a Shelby tube of soil placed at the bottom of a flume. The rate of scour is measured under different flow velocities. PET and EFA tests data were used to develop a plot of the PET erosion depth versus EFA erosion category which revealed that a correlation clearly exists between PET erosion depth and EFA erosion category value, the PET erosion depth that separates medium and high erodibility categories should be revised and the erodibility criteria based on soil classification is not very reliable for Hawaiian fine-grained soils
Towards quantifying rate of scour using the Erodibility Index Method: case study
Get PDFThis case study presents relationships between total energy input to plunge pools and scour depth at three BC Hydro dams in British Columbia, Canada. Total energy is defined as the product of stream power and time. The relationship indicates the potential to develop a technique quantifying the rate of scour of rock using the Erodibility Index Method (Annandale 1995; 2006).
The scour assessment used the Erodibility Index Method to theoretically quantify scour extent and compare it to observed scour. The Erodibility Index is quantified using in-situ rock parameters including UCS, RQD, joint spacing, aperture, alteration, roughness, and orientation. A graph relating the Erodibility Index and threshold stream power (Annandale 1995) is then used to quantify the ability of the rock to resist the stream power of flowing water. The stream power of the flowing water was quantified using daily discharge records and dam spillway geometries for flip-bucket jets.
Numerically generated scour profiles were used to quantify total energy at the surface of the plunge pool and at depth over time. The total energy input was then correlated with both the modeled and surveyed plunge pool depth to develop the relationship.
The study showed a statistically significant semi-logarithmic relationship between both modeled and surveyed scour depth and total energy input. In both cases, the rate of plunge pool development decreases over time, and continued energy inputs are required to enact changes to depth.
The study revealed that correlations between calculated and observed scour profiles improved with the quality of geologic information and the certainty by which jet stream power and its decay could be quantified. The geologic information at one of the sites was incomplete and resulted in poor comparisons between observed and calculated scour. At the other two sites, where geologic information was more complete comparisons were more favorable