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Estimation of Scour Depth at Bridge Abutments Nchrp 24-20

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Estimation of Scour Depth at Bridge Abutments Nchrp 24-20 NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM DRAFT FINAL REPORT ESTIMATION OF SCOUR DEPTH AT BRIDGE ABUTMENTS NCHRP 24-20 Robert Ettema, Tatsuaki Nakato, and Marian Muste The University of Iowa Iowa City, Iowa 52242 USA Subject Areas Highway and Facility Design • Bridges, Other Structures, and Hydraulic and Hydrology •Soils, Geology, and Foundations • Materials and Construction Research Sponsored by the American Association of State Highway and Transportation Officials in Cooperation with the Federal Highway Administration TRANSPORTATION RESEARCH BOARD WASHINGTON, D.C. January 2010 www.TRB.org ESTIMATION OF SCOUR DEPTH AT BRIDGE ABUTMENTS EXECUTIVE SUMMARY Problem Statement The project described herein comprised a comprehensive investigation conducted to meet the overall goal of developing a comprehensive method for estimating abutment scour in compound channels. The method is intended to be readily used by civil engineers, while being cognizant of the diverse combinations of abutment alignment and design, pier proximity, compound-channel form and orientation, and variations in erodibility of channel boundaries. The investigation reveals abutment scour to entail substantially more complex processes than envisioned at the project’s conception, or portrayed in the literature on abutment scour. Consequently, though the project leads a practicable design method, more work is needed to verify aspects of the method. For example, the project’s findings show new insights into the geotechnical nature of abutment scour, a major scour aspect heretofore neglected. The insights show that geotechnical failure of the approach embankment commonly limits scour development and depth. Hydraulic processes commence scour, but geotechnical instabilities of channel banks and abutment embankment commonly limit scour extent. The complexities and uncertainties attendant to abutment scour normally require the design practitioner to apply a safety factor when using an estimation of scour-depth obtained from a general method for estimating scour depth. The project, however, is limited to the objectives listed below and does not expressly address values of safety factor. Objectives To produce a practical approach for estimating abutment scour, the project pursued the following series of objectives: ii 1. Delineate the general features of the flow field in the vicinity of abutments and their approach embankments in compound channels; 2. Explain quantitatively how variations in compound-channel geometry and roughness, as well as in abutment shape, alignment, and extent, influence the flow field in the vicinity of an abutment; and, 3. Obtain qualitative insight into, and formulate, the relationships between abutment scour and – i. Abutment flow field; ii. Variations in the material comprising the compound channel and the abutment; and, iii. Pier proximity near an abutment. 4. Use the findings from the foregoing objectives to develop a set of scour-depth estimation methods, or an overall approach, readily useable by engineers designing bridge foundations. The project fulfills the first three objectives, and partially fulfills the fourth. An overall design approach is proposed and detailed. However, the results of work associated with the first three objectives reveal abutment scour to be considerably more complex than envisioned when the objectives were set forth. Abutment Forms and Construction The project considered the following common forms and construction features of abutments: 1. Two common abutment forms, spill-through and wing-wall. Spill-through abutments comprise an abutment column at the end of an unconfined earthfill embankment, whereas wing-wall abutments comprise an abutment column formed as a vertical wall confining the end of an earthfill embankment. Variations exist for each common form; iii 2. Most abutment columns are supported by piles (circular or H) or sheet-piling extending into a floodplain or channel bed; 3. The compacted earthfill embankment approaching the abutment structure is erodible and subject to geotechnical instabilities. A portion of the embankment surrounding spill- through abutments may be riprap protected, and the riprap protection may include an apron; 4. The floodplain (often extensively comprising cohesive soils) may be much less readily eroded than the main-channel bed; and, 5. Frequently, the first pier of a multi-span bridge is located close to a bridge abutment. Prior studies of abutment scour focused on the simpler and perhaps idealized situations of scour that did not adequately account for important facets of abutment construction. Commensurately, the prior relationships and guidelines apply to simplified abutment situations, such as an abutment placed in a straight rectangular channel, and can only be extended with considerable uncertainty to actual field conditions. Flume Experiments Extensive laboratory flume experiments were carried out with spill-through and wing-wall abutments. The variable erodibility of floodplain and embankment at bridge sites were simulated by means of tests utilizing the following arrangements that bracket them: 1. The floodplain and the embankment are fixed, whereby they are taken to be practically erosion resistant relative to an erodible main-channel bed; 2. The floodplain is as erodible as is the main channel bed, and the embankment is erodible but riprap-armored; and, 3. The floodplain, main-channel bed, and the embankment are essentially equally erodible. All are formed of the same non-cohesive sediment as the main-channel bed. iv The experiments entailed observation and measurement of abutment flow fields, scour processes, and scour bathymetry. The ranges of parameters varied for the experiments were constrained by the flumes, and time, available. The ranges were selected so as to encompass the main trends anticipated for abutment scour development. Numerical Experiments To broaden the range of insight into the flow field around abutments, the flume experiments were augmented with numerical experiments conducted using a depth-averaged numerical model of flow around a spill-through abutment. The numerical model, FESWMS-2D, simulated two- dimensional, depth-averaged flow around abutments in compound or rectangular channels; in this regard, an abutment set well back on a floodplain was treated as in effect being in a rectangular channel. The insights included information on distributions of flow velocity, unit discharge, and bed shear stress for varying lengths of abutment embankment. Additionally, information on flow vorticity is obtained. The numerical values of unit discharge extend those obtained from the laboratory flume. Scour Conditions and Observations Abutment form and layout in a channel develop a flow field essentially equivalent to flow through a short contraction. Consequently, the principal features of scour can be described as follow: 1. Abutment scour is essentially a form of scour at a short contraction. Accordingly, scour is closely influenced by flow distribution through the short contraction and by turbulence structures generated, and dispersed, by flow entering the short contraction; 2. For many, if not most abutments, abutment presence contracts flow non-uniformly across a bridge waterway. However, in situations of short embankments, adjoining relatively wide channels, flow contraction scour decreases in accordance with two limits: (i) If channel width is constant and embankment length decreases, scour depth at the abutment approaches zero; or, v (ii) For a full abutment form of constant length in a channel of increasing width, scour depth at the abutment approaches a limiting value associated with scour around an abutment in a very wide channel. This scour depth may be estimated approximately in terms of local flow contraction around the abutment itself. This second limit can be difficult to simulate by means of hydraulic models replicating the full form and usual construction of actual spill-through and wing-wall abutments, because most laboratory flumes are insufficiently wide; 3. Provided that the approach embankment of an abutment does not breach, so that flow passes through it, abutment scour principally develops as a local amplification of contraction scour associated with flow through a long contraction; 4. Abutment scour typically entails hydraulic erosion followed by geotechnical failure of the main-channel bank and earthfill embankment around the abutment column. Normally, an abutment column is a pier-like structure built from concrete or steel, around which an earthfill embankment is formed. The column transfers load from the bridge deck to the foundation in a similar manner as does a pier; and, 5. Abutment scour may involve three distinct scour conditions, herein termed Scour Conditions A, B, and C. These scour conditions were observed in the flume experiments and as well as at actual bridge sites: Scour Condition A occurs as scour of the main channel portion of a compound channel; Scour Condition B is scour of the floodplain, and occurs for abutments set well back from the main channel; and, Scour Condition C is a scour form that develops when breaching of an abutment’s embankment fully exposes its abutment-column structure such that scour develops at the abutment column as if it were a pier. The report extensively illustrates and examines these scour conditions. vi Scour Trends The flume experiments conducted during the project produced the scour trends described below for Scour Conditions A, B, and C. The trends are discussed in terms
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