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8 CHAPTER 1 INTRODUCTION 1.1 BRIDGE SCOUR on the basis of Atterberg Limits. Gravels and sands are typi- cally referred to as cohesionless soils; silts and clays are typ- Bridge scour is the loss of soil by erosion due to water flow- ically referred to as cohesive soils. ing around bridge supports. Bridge scour includes general scour and local scour. General scour is the aggradation or degradation of the riverbed not related to the presence of local 1.3 THE PROBLEM ADDRESSED obstacles. Aggradation is the gradual and general accumula- tion of sediments on the river bottom; one possible scenario This project deals with pier scour and contraction scour is the existence of slope failures upstream leading to the for- in cohesive soils (Figure 1.1). A previous project performed mation of spoils in the river, the erosion of these spoils under by the same team of researchers (Briaud et al., 1999, 2002a, higher velocities, followed by transport and deposition under and 2002b) began in 1990 and was sponsored by the Texas lower velocities at the aggrading location. Degradation is the Department of Transportation (TxDOT); it dealt only with gradual and general removal of sediments from the riverbed; pier scour in cohesive soils. In the TxDOT project, the piers one possible scenario is the man-made straightening of a river were cylindrical and the water depth was more than two course, a resulting increase in the water velocity, and the asso- times the pier diameter (deep water case). As part of the ciated increase in erosion. Local scour is the scour around TxDOT project, a new device to measure the erodibility of obstacles in the path of the water flow; it includes pier scour, soils--the Erosion Function Apparatus (EFA)--was devel- abutment scour, and contraction scour. Pier scour is the oped. The EFA test gives the erosion function for a soil and removal of the soil around the foundation of a pier; abutment became an integral part of the Scour Rate in Cohesive Soils scour is the removal of the soil around an abutment at the (SRICOS) Method to predict the scour depth as a function of junction between a bridge and embankment; contraction time when a cylindrical pier founded in a layered soil is sub- scour is the removal of soil from the bottom of the river due jected to long-term, deep water flow. In this NCHRP project, to a narrowing of the river channel created by the approach the SRICOS-EFA Method was extended to the case of com- embankments for a bridge. plex piers and contraction scour. Complex piers refer to piers with various shapes, flow attack angles, spacing between piers, and any water depth. Contraction refers to a narrow- 1.2 CLASSIFICATION OF SOILS ing of the flow channel by an embankment with a given Soils can be defined as loosely bound to unbound, natu- encroachment length, embankment width, and a given tran- rally occurring materials that cover the top few hundred sition angle. The input to the SRICOS-EFA Method is the meters of the Earth. By opposition, rock is a strongly bound, geometry of the piers and the contraction, the water velocity naturally occurring material found within similar depths or and water depth as a function of time over the life of the deeper. Intermediate geomaterials occur at the boundary bridge, and the soil erosion functions for the layers involved between soils and rocks. Classification tests and mechanical in the soil stratigraphy. The output is the scour depth as a properties help to distinguish between these three types of function of time during the life of the bridge. naturally occurring materials and to classify different cate- gories of soils. For soils, the classification tests consist of 1.4 WHY WAS THIS PROBLEM ADDRESSED? grain size analysis and Atterberg Limits (Das, 2001). The D50 grain size is the grain size corresponding to 50% of the soil Previously, the calculations for cohesive soils were based weight passing a sieve with an opening equal to D50. The first on the solution developed for cohesionless soils. Such an major division in soils classification is between large-grained approach was often overly conservative. Overly conserva- soils and fine-grained soils; large-grained soils have D50 tive scour depths led to foundations that were considered larger than 0.075 mm while fine-grained soils have D50 to be deeper than necessary and, therefore, more costly than smaller than 0.075 mm. Large-grained soils include gravels needed. The major difference between cohesionless soils and and sands that are identified on the basis of their grain size. cohesive soils is explained in the following description. Fine-grained soils include silts and clays that are identified Floods create peak velocities that last a few days. This length