Current methodology for scour prediction around bridge foundations considers rock as either "erodible" or "non-erodible." Equations for scour in sand are used to predict the scour depth in erodible rock. As a consequence, predictions of scour in rock frequently overestimate the extent and depth of scour so that founding elevations for piers and abutments on rock are established at levels that require very expensive and difficult excavation.
Some rock types degrade very rapidly to produce a highly erodible combination of soil and rock fragments (e.g., compacted shales absorb moisture, swell, and disintegrate). Available scour prediction methods do not permit differentiation among rock types that behave in fundamentally different ways. Scour in fractured or degradable rock is affected by the properties of intact pieces of rock, as well as by the discontinuities in the rock strata. Hydrodynamic forces caused by highly turbulent flow around bridge piers and abutments may remove weakened layers to expose relatively intact rock. These new surfaces may deteriorate in-between flow events and then be susceptible to erosion during the next flood event. Other rock types decompose very slowly so that during the design life of a bridge, intact pieces of rock remain essentially unaltered. Scour in such rock types takes place by plucking, dislodgment, and displacement of rock fragments, especially when large pressure differences are generated around piers and abutments.
An improved methodology for estimating the rate and design depth of scour in rock over the service life of a bridge is needed. Guidelines that address design issues as well as site-investigation sampling and testing protocols are needed to assist practitioners in applying the methodology. Also, construction guidelines are needed to promote practices that minimize the potential for scour in rock.
The objectives of this research are to develop: (a) a methodology for estimating the time rate of scour and the design scour depth of a bridge foundation on rock, and (b) design and construction guidelines for application of the methodology.
Accomplishment of the project objective will require at least the following tasks.
Critically review the literature from foreign and domestic sources pertaining to scour of bridge foundations on rock (see Special Note B). This review should include information on how to quantify hydraulic shear stresses caused by turbulent flow systems around bridge piers and abutments.
Survey state and federal agencies to
(a) determine the various practices used for estimating the extent and depth of bridge foundation scour in rock;
(b) identify bridges experiencing significant scour in rock. Select sites suitable for further investigation in Phase II based on the availability of relevant data (e.g., time rate, scour history, hydraulic data, geotechnical properties, and pier configuration) for these bridges;
(c) determine geotechnical site-investigation sampling and testing protocols used to ensure an accurate assessment of rock characteristics; and
(d) determine best practices currently used for construction that minimize the potential for scour in rock.
Analyze the information collected in Task 1 and 2 to determine the controlling variables for rock scour based on substructure geometry and on climatic, hydraulic, and geologic characteristics. Propose a preliminary methodology for determining the rate and design scour depth of a bridge foundation on rock and preliminary sampling and testing protocols. (4.
Submit an interim report and an updated, detailed work plan for completing Phase II of the research. The interim report should document the results of Tasks 1 through 3 and include the updated work plan as a separate appendix. Meet with the NCHRP panel to discuss the interim report and Phase II work plan. Phase II will not begin until the interim report and work plan are approved by the NCHRP.
Investigate bridge sites approved by the NCHRP panel during the interim meeting and collect data needed for the development and validation of the Task 7 methodology and the Task 8 guidelines. Evaluate the data and determine areas where additional data are needed to effectively conduct Tasks 7 and 8. (6.
Conduct laboratory, field, and/or modeling studies as needed to
(a) supplement the Task 5 field data,
(b) support the development and validation of the Task 7 methodology, and
(c) support the development of the Task 8 guidelines.
(7.) Develop a methodology for determining the time rate of scour and the design scour depth over the expected life of the structure for bridge foundations on rock. Validate the methodology and refine as necessary. (8.) Develop design and construction guidelines for applying the Task 7 methodology. The design guidelines should include site investigation sampling and testing protocols. The construction guidelines should include best practices that minimize the potential for scour in rock. (9.) Submit a final report documenting the entire research effort. The final report should include the recommended design and construction guidelines as separate appendices.
The final report for Project 24-29 has been published as NCHRP Report 717 Scour at Bridge Foundations on Rock and is available with this link.