Time Variation of Scour at Abutments

A semiempirical model is presented to compute the time variation of scour depth in an evolving scour hole at short abutments (abutment length/flow depth ? 1), namely the vertical wall, 45° wing wall, and semicircular, in uniform and nonuniform sediments under a clear water scour condition. The methodology developed for computing the time variation of scour depth is based on the concept of the conservation of the mass of sediment, considering the primary vortex system as the main agent of scouring, and assuming a layer-by-layer scouring process. For an equilibrium scour hole, the characteristic parameters affecting the nondimensional equilibrium scour depth (scour depth/abutment length), identified based on the physical reasoning and dimensional analysis, are excess abutment Froude number, flow depth—abutment length ratio, and abutment length—sediment diameter ratio. Experiments were conducted for time variation and equilibrium scour depths at different sizes of vertical walls, 45° wing walls and semicircular abutments in uniform and nonuniform sediments under limiting clear water scour conditions (approaching flow velocity nearly equal to the critical velocity for bed sediments). The present model corresponds closely with the data of time variation of scour depth in uniform and nonuniform sediments obtained from the present experiments and reported by different investigators.     

Protecting Vertical-Wall Abutments with Riprap Mattresses

This study addresses the design of riprap mattresses as a scour countermeasure near vertical-wall bridge abutments under clear water flow conditions. It specifically deals with the diameter of riprap, Dr50, the lateral extent of mattresses, w, and their thickness, t. Experiments were performed in a rectangular, sand-bed open channel using different abutment lengths, three riprap stone sizes, and two different sands. The minimum size of stable stones as well as the mattress dimensions depend on the ratio between the abutment length and the flow depth. New equations for the evaluation of Dr50 and w are suggested. The geometric properties of the scour holes which develop at the edge of riprap mattresses are similar to those reported in the literature for spill-through abutments. Although it is not possible to fully arrest scour by winnowing, the corresponding scour depth is negligible when the mattress layer thickness is at least 6Dr50.     

Effects of Time and Channel Geometry on Scour at Bridge Abutments

Experiments are described to investigate local scour at bridge abutments. The experiments were performed in a two-stage channel using abutments that extended different distances onto the floodplain including right up to the edge of the main channel (Melville, Type III). To ensure the largest scour depths the conditions on the floodplain upstream of the abutment were close to critical conditions for the bed material. The time evolution of the scour and the ultimate scour depth were measured. The time development of the local scour corresponded well with the theories of Ettema and Franzetti and the theory of Whitehouse for scour at horizontal cylinders in the marine environment. Melville has suggested that scour at abutments on floodplains can be approximated by scour in rectangular channels if an imaginary boundary is assumed, separating the flow in the main channel from that on the floodplain. The experimental results confirm the validity of Melville's suggestion for the configurations tested in the experiments.     

Parallel Walls as an Abutment Scour Countermeasure

Scour at bridge abutments can cause damage or failure of bridges and result in excessive repairs, loss of accessibility, or even death. To mitigate abutment scour, both clear-water and live-bed laboratory experiments in a compound channel were performed using parallel walls. Two types of parallel walls were tested: the first was made of a solid thin wood plate and the second was made of piled rocks. For solid parallel walls, a series of vertically oriented, rectangular, straight plates of different lengths attached to the upstream end of a wing wall abutment parallel to the flow direction were employed. Three velocities of 0.9, 1.5, and 2.3 times the incipient motion value for bed sediment movement were used. The bed material was sand with a mean diameter of 0.8?mm and a standard deviation of 1.37. All the plates were seated at the bottom of the compound channel bank slope and were even with the abutment face. It was found that straight plates thus situated are able to move the scour hole away from the upstream corner of the abutment. As the length of the plate increased, the scour at the abutment declined. It was found that a length of 1.6L, with L being the length of the abutment perpendicular to the flow, caused the scour to be eliminated at the abutment for a velocity ratio (U/Uc) of 0.9 (clear-water scour). Similarly, a 1.6L long wall can reduce the time-averaged scour depth at the abutment by 100% for a velocity ratio of 1.5, and 70% for a velocity ratio of 2.3. If the upstream end of the wall is anchored below the scour depth, this countermeasure would likely be feasible for situations where rock is expensive. For parallel rock walls, various values of wall length and protrusion length into the main channel were tested. It was found that a wall that does not protrude into the main channel and having a length of 0.5L minimizes scour at the abutment for all three different velocity ratios (0.9, 1.5, and 2.3).     

Clear-Water Scour at Abutments in Thinly Armored Beds

Experiments on local scour at short abutments (ratio of abutment length to approaching flow depth less than unity), namely vertical-wall, 45° wing-wall, and semicircular, embedded in a bed of relatively fine noncohesive sediment overlain by a thin armor-layer of coarser sediment, were conducted for different flow conditions, thickness of armor-layers, armor-layer, and bed sediments. The abutments were aligned with the approaching flow in a rectangular channel. The armor-layer and the bed underneath it were composed of different combinations of uniform sediments. In the experiments, the approaching flow velocities were restricted to the clear-water scour condition with respect to the armor-layer particles. Depending on the approaching flow conditions, three cases of scour at abutments in armored beds were identified. Effects of different parameters pertaining to scour at abutments are examined. The comparison of the experimental data shows that the scour depth at an abutment with an armor-layer in clear-water scour condition under limiting stability of the surface particles (approaching flow velocity nearly equaling critical velocity for the threshold motion of surface particles) is always greater than that without armor-layer for the same bed sediments. The characteristic parameters affecting the maximum equilibrium nondimensional scour depth (scour depth-abutment length ratio), identified based on the physical reasoning and dimensional analysis, are excess abutment Froude number, flow depth-abutment length ratio, armor-layer thickness-armor particle diameter ratio, and armor particle-bed sediment diameter ratio. The experimental data of clear-water scour condition in thinly armored beds under limiting stability of surface particles were used to determine the equation of maximum equilibrium scour depth through regression analysis. The estimated scour depths were in agreement with the experimental scour depths. Also, an equation of maximum equilibrium scour depth in uniform sediments was obtained.     

Use of Vanes for Control of Scour at Vertical Wall Abutments

Rock vanes are single-arm structures angled to the flow with a pitch into the streambed such that the tip of the vane is submerged even during low flow. Vanes have primarily been used in recent years for treatment of bank erosion in stream stability projects. These structures roll the water away from the eroding banks, thus limiting erosion of the channel banks. They have proven to be very effective treatments over a range of flow conditions. In this project, the effectiveness of vanes for preventing scour at single-span bridges with vertical wall abutments was evaluated based on laboratory experiments. The vanes were tested in small-scale experiments in a recirculating flume and subjected to a range of flow conditions, including bank full and a number of overbank flows, which were forced to return to the channel at the abutment. The results showed that the vanes were highly effective in moving the scour away from the abutment into the center of the channel under all flow conditions tested. Based on the experimental results, optimum design settings for the vane angle and height, most effective number of vanes, and distance upstream for placement of the first vane were determined.     

Allowable Bearing Pressures of Bridge Sills on GRS Abutments with Flexible Facing

Compared to geosynthetic-reinforced soil (GRS) retaining walls, GRS abutment walls are generally subjected to much greater intensity surface loads that are fairly close to the wall face. A major issue with the design of GRS abutments is the allowable bearing pressure of the bridge sill on the abutments. The allowable bearing pressure of a bridge sill over reinforced soil retaining walls has been limited to 200?kPa in the current NHI and Demo 82 design guidelines. A study was undertaken to investigate the allowable bearing pressures of bridge sills over GRS abutments with flexible facing. The study was conducted by the finite element method of analysis. The capability of the finite element computer code for analyzing the performance of GRS bridge abutments with modular block facing has been evaluated extensively prior to this study. A series of finite element analyses were carried out to examine the effect of sill type, sill width, soil stiffness/strength, reinforcement spacing, and foundation stiffness on the load-carrying capacity of GRS abutment sills. Based on the results of the analytical study, allowable bearing pressures of GRS abutments were determined based on two performance criteria: A limiting displacement criterion and a limiting shear strain criterion, as well as the writers’ experiences with GRS walls and abutments. In addition, a recommended design procedure for determining the allowable bearing pressure is provided.     

Constriction Effects in Clear-Water Scour at Abutments

The erosion process at a bridge abutment may be affected by the flow constriction when the abutment occupies a significant part of the flume width. We devised a specific experimental campaign to investigate the effect of the obstruction ratio (i.e., the ratio between the abutment length and the channel width) on the erosion depth: we performed homogeneous series of clear-water scour experiments in each of which the obstruction ratio was the only parameter that varied. The experimental results are presented in light of a dimensionless framework. It was found that the effect of the obstruction ratio on the time development of local scour depth may not be large; nevertheless, significant effect can be observed, even for relatively small values of the parameter during the earliest phases. The latter are important, for example, for the step-by-step modeling of the erosion development under unsteady flow conditions. We propose a simple equation to quantify the scour enhancement due to the flow constriction in clear-water abutment scour.     

Countermeasure Toe Protection at Spill-Through Abutments

An experimental study of scour countermeasures for spill-through abutments situated on the flood plain of a compound channel is reported. The purpose of the study was to determine the variations in the scour hole geometry under clear water conditions by varying the compound channel and abutment geometries, and to determine the extent and type of scour countermeasure toe protection provided. This approach avoids one of the inherent difficulties in conducting scour countermeasure experiments—that is, the subjectivity of determining whether the countermeasure used in the experiment is a success or a failure. Riprap and cable-tied block countermeasures are incorporated. The results show that for most cases, as the countermeasure apron width (i.e., the extent of toe protection) is increased, the scour hole is deflected further away from the abutment and reduces in size. However, for abutment and compound channel configurations where the scour hole forms close to the main channel bank, the scour hole increases in size as the apron width is increased. The results also show that cable-tied block mats allow the scour hole to form closer to the abutment than equivalent riprap aprons and result in deeper scour holes. A suggested design methodology for the extent of apron protection is presented. The method is an improvement on the current, rather-simplified practice of providing aprons of fixed width equal to twice the flow depth.     

Geobag Performance as Scour Countermeasure for Bridge Abutments

This paper presents observations and data from a sequence of laboratory experiments conducted to evaluate geobags as a countermeasure to protect bridge-abutment foundations from failure attributable to scour of the alluvial-river channel in which they are placed. Geobags comprise geotextile cloth bags filled with local sediment or concrete. The experiments focused on the performance of geobags placed as an apron around pile-supported wing-wall abutments retaining erodible embankments, and subject to live-bed and clear-water flow conditions. Though an apron of geobags is shown to substantially reduce or eliminate scour immediately at the abutment, the apron must be formed flexibly of linked geobags. Moreover, a performance concern is that the apron may shift scour to a location flanking or downstream of the apron, and in so doing imperil a nearby pier or riverbank. The experiments indicate the importance of protecting the embankment region beneath and immediately behind the abutment’s pile cap. Live-bed conditions proved to be the more critical for abutment protection, owing to the capacity of dunes to destabilize geobags around the edges of the apron. Design guidelines are given and include using current riprap configurations for sizing and placing geobags.     

Simulating the Behavior of GRS Bridge Abutments

Geosynthetic-reinforced soil (GRS) bridge-supporting abutments are similar in principle to GRS retaining walls, except that GRS abutments are typically subjected to a much higher area load, and that the loads are close to the wall face. The GRS abutment technology is relatively new, but it has great potential, and it has been gaining some popularity in recent years. This paper describes the finite element analyses of two full-scale loading tests of GRS bridge abutments referred to as the “National Cooperative Highway Research Program (NCHRP) experiment.” The analysis was carried out using the computer program Dyna3d, developed at the Lawrence Livermore National Laboratory. The finite element analysis of the NCHRP experiment will help with the understanding of the complex behavior of GRS structures in general, and the behavior of GRS bridge abutments with modular block facing in particular. The analysis of the two full-scale loading tests allows the loading conditions that are of greatest concern in the design of the bridge abutments to be examined rationally. The analysis shows that the performance of a GRS abutment, resulting from the complex interaction among the various components, while subject to a service load or a limiting failure load can be simulated in a reasonably accurate manner. In addition, a parametric study was conducted to investigate the performance of the modular block facing GRS bridge abutments subjected to live and dead loads from a bridge superstructure. This study investigated the performance of the GRS bridge abutments as they are affected by backfill properties, reinforcement stiffness properties, and reinforcement vertical spacing.     

Local Scour and Riprap Stability at an Abutment in a Degrading Bed

The paper reports on an experimental investigation concerning two important issues: (1) local scour and (2) riprap stability at a 45° wing-wall abutment in a degrading river bed of noncohesive sediment. The abutment considered was short (that is abutment length/flow depth <1). From the experimental observations, no influence of abutment inclusion on bed degradation was evident, as bed profiles with and without abutment were quite identical apart from the immediate vicinity of the abutment. Total scour depth at an abutment is found to be the maximum abutment scour depth in addition to the reduction of bed elevation due to bed degradation. The maximum abutment scour depth can be estimated from the equation given by Kandasamy and Melville in their 1998 paper. For scouring time beyond 24?h, the local abutment scour depth remains independent of time. In a degrading bed, the bed forms cause edge failure of the riprap at an abutment when the dunes propagate over the riprap layer. Initially, the dune height is significant causing the maximum damage of riprap layer. As the flow velocity reduces, the resulting bed-shear stress diminishes with the degrading bed and gradually the formation of dunes ceases. An additional experiment reveals that the damaged riprap layer is significantly vulnerable against a subsequent flood accompanied by large dunes.     

Capacity Evaluation of Exterior Sacrificial Shear Keys of Bridge Abutments

Shear keys are used in bridge abutments to provide transverse support for the superstructure. The damage observed on bridge abutments in the aftermath of the 1994 Northridge Earthquake prompted the revision of the design of shear keys. As part of this revision, experimental and analytical work was conducted to investigate the seismic behavior of exterior shear keys in bridge abutments designed in accordance with current guidelines and to investigate shear keys designed for damage control. The latter work was aimed at providing guidance for seismic design of shear keys to act as structural fuses that would limit the input force in the abutment piles. Ten shear keys were designed and built at 1:2.5 scale of a prototype abutment design provided by Caltrans. The study concluded that a smooth construction joint should be considered at the interface of the shear key–abutment stem wall to allow sliding shear failure. A mechanism model was developed for capacity evaluation of shear keys with sliding shear failure. The results of the experimental program and development of the simple analytical model for capacity evaluation of exterior shear keys are presented in this paper.     

Optimum Design of Bridge Abutments under Seismic Conditions: Reliability-Based Approach

The paper focuses on the reliability-based design optimization of gravity wall bridge abutments when subjected to active condition during earthquakes. An analytical study considering the effect of uncertainties in the seismic analysis of bridge abutments is presented. Planar failure surface has been considered in conjunction with the pseudostatic limit equilibrium method for the calculation of the seismic active earth pressure. Analysis is conducted to evaluate the external stability of bridge abutments when subjected to earthquake loads. Reliability analysis is used to estimate the probability of failure in three modes of failure viz. sliding failure of the wall on its base, overturning failure about its toe (or eccentricity failure of the resultant force) and bearing failure of foundation soil below the base of wall. The properties of backfill and foundation soil below the base of abutment are treated as random variables. In addition, the uncertainties associated with characteristics of earthquake ground motions such as horizontal seismic acceleration and shear wave velocity propagating through backfill soil are considered. The optimum proportions of the abutment needed to maintain the stability are obtained against three modes of failure by targeting various component and system reliability indices. Studies have also been made to study the influence of various parameters on the seismic stability.     

Large-Scale Flume Tests of Riprap-Apron Performance at a Bridge Abutment on a Floodplain

Laboratory tests using a large-scale model of a spill-through bridge abutment led to important findings about the performance of a riprap apron as an abutment scour countermeasure. Riprap stone is widely used for protecting side slopes of embankments against erosion, and several design guidelines are available in the literature. In contrast, only a few guidelines exist for the design of a riprap apron around an abutment. These guidelines focus only on the armoring effect of riprap, neglecting other effects. This study shows that apron performance involves several mechanisms: armoring the bed, dissipating large-scale turbulence shed from the abutment, reducing the peak unit discharge, reducing the average shear stress, and shifting the scour region away from the abutment. Together, these mechanisms substantially reduce the maximum scour depth. The test findings are compared with those from a much smaller model of riprap-apron performance.     

Integral Abutment-Backfill Behavior on Sand Soil—Pushover Analysis Approach

This paper presents a study on the behavior of the abutment-backfill system under positive thermal variation in integral bridges built on sand. A structural model of a typical integral bridge is built, considering the nonlinear behavior of the piles and soil-bridge interaction effects. Static pushover analyses of the bridge are conducted to study the effect of various geometric, structural, and geotechnical parameters on the performance of the abutment-backfill system under positive thermal variations. The shape and intensity of the backfill pressure are found to be affected by the height of the abutment. Furthermore, the internal forces in the abutments are found to be functions of the thermal-induced longitudinal movement of the abutment, the properties of the pile, and the density of the sand around the piles. Using the pushover analysis results, design equations are formulated to determine the maximum forces in the abutments and the maximum length of integral bridges based on the strength of the abutments. Integral bridges with piles encased in loose sand and oriented to bend about their weak axis, abutment heights less than 4?m, and noncompacted backfill are recommended to limit the magnitude of the forces in the abutments.     

Effect of Soil and Substructure Properties on Live-Load Distribution in Integral Abutment Bridges

This study is aimed at investigating the effect of soil–structure interaction and substructure properties at the abutments on the distribution of live-load effects in integral abutment bridge (IAB) components. For this purpose, numerous 3D and corresponding 2D structural models of typical IABs are built and analyzed under AASHTO live-load. In the analyses, the effect of various geotechnical and substructure properties such as foundation soil stiffness, considering and neglecting the effect of backfill, backfill compaction level, considering and neglecting the effect of wingwalls, abutment height and thickness, as well as number, size, and orientation of the piles are considered. The results from the 2D and 3D analyses are then used to calculate the live-load distribution factors (LLDFs) for the components of IABs as a function of the above-mentioned properties. The analyses results revealed that soil–structure interaction has a significant effect on the LLDFs for the abutment, but negligible effects on those for the girders and piles. Furthermore, the abutment height is observed to have a considerable effect on the LLDFs calculated for the abutment and pile moments. Moreover, the wingwalls are observed to have only a negligible effect on the LLDFs for all the IAB components.     

Nonlinear Soil–Abutment–Bridge Structure Interaction for Seismic Performance-Based Design

Current seismic design of bridges is based on a displacement performance philosophy using nonlinear static pushover analysis. This type of bridge design necessitates that the geotechnical engineer predict the resistance of the abutment backfill soils, which is inherently nonlinear with respect to the displacement between soil backfill and the bridge structure. This paper employs limit-equilibrium methods using mobilized logarithmic-spiral failure surfaces coupled with a modified hyperbolic soil stress–strain behavior (LSH model) to estimate abutment nonlinear force-displacement capacity as a function of wall displacement and soil backfill properties. The calculated force-displacement capacity is validated against the results from eight field experiments conducted on various typical structure backfills. Using LSH and experimental data, a simple hyperbolic force-displacement (HFD) equation is developed that can provide the same results using only the backfill soil stiffness and ultimate soil capacity. HFD is compatible with current CALTRANS practice in regard to the seismic design of bridge abutments. The LSH and HFD models are powerful and effective tools for practicing engineers to produce realistic bridge response for performance-based bridge design.     

Predicted and Measured Response of an Integral Abutment Bridge

This project examined several uncertainties of integral abutment bridge design and analysis through field-monitoring of an integral abutment bridge and three levels of numerical modeling. Field monitoring data from a Pennsylvania bridge site was used to refine the numerical models that were then used to predict the integral abutment bridge behavior of other Pennsylvania bridges of similar construction. The instrumented bridge was monitored with 64 gages; monitoring pile strains, soil pressure behind abutments, abutment displacement, abutment rotation, girder rotation, and girder strains during construction and continuously thereafter. Three levels of numerical analysis were performed in order to evaluate prediction methods of bridge behavior. The analysis levels included laterally loaded pile models using commercially available software, two-dimensional (2D) single bent models, and 3D finite element models. In addition, a weather station was constructed within the immediate vicinity of the monitored bridge to capture environmental information including ambient air temperature, solar radiation, wind speed and direction, humidity, rainfall, and barometric pressure. Laterally loaded pile models confirmed that inclusion of multilinear soil springs created from p-y curves is a valid approach for modeling soil–pile interaction within a finite element program. The 2D and 3D numerical models verified the field data indicating that primary accommodation of superstructure expansion and contraction is through rotation of the abutment about its base rather than longitudinal translation, as assumed in the original design of this bridge. Girder axial forces were suspected to be influenced by creep and shrinkage effects in the bridge superstructure. Pile strains were found to be well below strains corresponding to pile plastic moment. Overall, the 2D numerical model and the 3D numerical model predicted very similar behavior.     

Behavior of a Stiff Clay behind Embedded Integral Abutments

Integral bridges can significantly reduce maintenance and repair costs compared with conventional bridges. However, uncertainties have arisen in the design as the soil experiences temperature-induced cyclic loading behind the abutments. This paper presents the results from an experimental program on the behavior of Atherfield clay, a stiff clay from the United Kingdom, behind embedded integral abutments. Specimens were subjected to the stress paths and levels of cyclic straining that a typical embedded integral abutment might impose on its retained soil. The results show that daily and annual temperature changes can cause significant horizontal stress variations behind such abutments. However, no buildup in lateral earth pressure with successive cycles was observed for this typical stiff clay, and the stress–strain behavior and stiffness behavior were not influenced by continued cycling. The implications of the results for integral abutment design are discussed.