Status of NCHRP Rock Scour Project

Jeffrey R. Keaton, MACTEC Engineering and Consulting, Inc., Su K. Mishra, Ayres Associates, Inc.

ABSTRACT:

The essence of National Cooperative Highway Research Program (NCHRP) Project 24-29 is geotechnical site characterization in scour-relevant terms for use by hydraulic engineers. The project goal is to develop guidelines for evaluating scour at bridge foundations on rock that can be integrated with the procedures of FHWA HEC-18. Rock scour in natural open channels appears to be related to five processes: 1) dissolution of soluble rocks, 2) abrasion of degradable rocks, 3) quarrying and plucking of jointed rocks, 4) cavitation, and 5) physical and chemical weathering that prepares rock masses and surfaces for subsequent scour. The definition of ‘rock’ for scour purposes is just as problematic as the definition of rock for other engineering applications. The physical properties of rock material can range from strong soil to much better than the best concrete. Two benchmark materials are being considered for rock in the context of scour: concrete and mortar. Rock exposed in channels that has characteristics of moderately good concrete probably is sufficiently resistant to hydraulic forces that it might be unscourable during the life of a conventional bridge. Rock with characteristics less that of mortar might be highly susceptible to scour when exposed to the normal range of stream flow during the life of a bridge. Quantifying the rate of rock scour is a challenge because it probably is governed by a threshold loading condition (velocity, hydraulic shear stress, or stream power) below which no scour occurs, but above which scour losses accumulate. NCHRP Project 24-29 is beginning field, laboratory, and modeling studies to refine the approach to quantifying rock scour at bridge sites.

 

INTRODUCTION

A number of bridges throughout the United States may be founded on erodible rock. Rock erosion processes include gradual dissolution by chemical weathering; disintegration and wearing away by impact and abrasion of bedload and suspended load particles; jacking and plucking of blocks of hard, jointed rock; and cavitation. Soft rock formations may scour rapidly during a single flood event, whereas hard rock formations may show no observable evidence of erosion after decades of floods. Geotechnical properties of most rock materials are not sufficiently well understood for the rock formations to be considered “scour-resistant”, let alone to define the time-rate of scour in susceptible formations. State departments of transportation (DOTs) are required by the Federal Highway Administration (FHWA) to evaluate scour at bridge sites and protect bridge structures from failure. Hydraulic engineers thus may be forced to consider all rock formations as if they were cohesionless sediments for the purpose of estimating scour depths. In many cases, this approach may be overly conservative, with large predicted scour depths that result in excessive foundation costs for new bridges and/or expensive retrofitting of existing bridges.

 

Click here to request a copy of this article in its entirety