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8CHAPTER 1 INTRODUCTION 1.1 BRIDGE SCOUR Bridge scour is the loss of soil by erosion due to water flow- 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 obstacles. Aggradation is the gradual and general accumula- tion of sediments on the river bottom; one possible scenario is the existence of slope failures upstream leading to the for- mation of spoils in the river, the erosion of these spoils under higher velocities, followed by transport and deposition under lower velocities at the aggrading location. Degradation is the gradual and general removal of sediments from the riverbed; one possible scenario is the man-made straightening of a river course, a resulting increase in the water velocity, and the asso- ciated increase in erosion. Local scour is the scour around obstacles in the path of the water flow; it includes pier scour, abutment scour, and contraction scour. Pier scour is the removal of the soil around the foundation of a pier; abutment scour is the removal of the soil around an abutment at the junction between a bridge and embankment; contraction scour is the removal of soil from the bottom of the river due to a narrowing of the river channel created by the approach embankments for a bridge. 1.2 CLASSIFICATION OF SOILS Soils can be defined as loosely bound to unbound, natu- rally occurring materials that cover the top few hundred meters of the Earth. By opposition, rock is a strongly bound, naturally occurring material found within similar depths or deeper. Intermediate geomaterials occur at the boundary between soils and rocks. Classification tests and mechanical properties help to distinguish between these three types of naturally occurring materials and to classify different cate- gories of soils. For soils, the classification tests consist of grain size analysis and Atterberg Limits (Das, 2001). The D50 grain size is the grain size corresponding to 50% of the soil weight passing a sieve with an opening equal to D50. The first major division in soils classification is between large-grained soils and fine-grained soils; large-grained soils have D50 larger than 0.075 mm while fine-grained soils have D50 smaller than 0.075 mm. Large-grained soils include gravels and sands that are identified on the basis of their grain size. Fine-grained soils include silts and clays that are identified on the basis of Atterberg Limits. Gravels and sands are typi- cally referred to as cohesionless soils; silts and clays are typ- ically referred to as cohesive soils. 1.3 THE PROBLEM ADDRESSED This project deals with pier scour and contraction scour in cohesive soils (Figure 1.1). A previous project performed by the same team of researchers (Briaud et al., 1999, 2002a, and 2002b) began in 1990 and was sponsored by the Texas Department of Transportation (TxDOT); it dealt only with pier scour in cohesive soils. In the TxDOT project, the piers were cylindrical and the water depth was more than two times the pier diameter (deep water case). As part of the TxDOT project, a new device to measure the erodibility of soilsâthe Erosion Function Apparatus (EFA)âwas devel- oped. The EFA test gives the erosion function for a soil and became an integral part of the Scour Rate in Cohesive Soils (SRICOS) Method to predict the scour depth as a function of time when a cylindrical pier founded in a layered soil is sub- jected to long-term, deep water flow. In this NCHRP project, the SRICOS-EFA Method was extended to the case of com- 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- ing of the flow channel by an embankment with a given encroachment length, embankment width, and a given tran- sition angle. The input to the SRICOS-EFA Method is the geometry of the piers and the contraction, the water velocity and water depth as a function of time over the life of the bridge, and the soil erosion functions for the layers involved in the soil stratigraphy. The output is the scour depth as a function of time during the life of the bridge. 1.4 WHY WAS THIS PROBLEM ADDRESSED? Previously, the calculations for cohesive soils were based on the solution developed for cohesionless soils. Such an approach was often overly conservative. Overly conserva- tive scour depths led to foundations that were considered to be deeper than necessary and, therefore, more costly than needed. The major difference between cohesionless soils and cohesive soils is explained in the following description. Floods create peak velocities that last a few days. This length
of time is usually sufficient to generate the maximum scour depth in cohesionless soils. This means that only the peak velocity needs to be used in the calculations of scour depth for cohesionless soils and that such a scour depth is the maximum scour depth for that velocity. Typically, the velocities used are the 100- and 500-year flood velocities. In cohesive soils, scour and erosion rates can be 1,000 times slower than in cohesionless soils and a few days may generate only a small fraction of the maximum scour depth. Therefore, for cohe- sive soils it becomes necessary to consider the rate of ero- sion and the cumulative effect of multiple floods. 1.5 APPROACH SELECTED TO SOLVE THE PROBLEM The approach selected to solve the problem of predicting the scour depth versus time for complex piers in a contracted channel and for a given velocity hydrograph was based on a combination of existing knowledge review, flume tests, 9 numerical simulations, fundamental principles in method development, and verification of the method against avail- able data. The review of existing knowledge avoided dupli- cation of effort and helped in establishing a solid foundation. The flume tests gave the equations for the maximum scour depth and the influence of various factors. The flume tests also gave a calibration basis for the numerical simulations. These numerical simulations were used to generate the equa- tions for the maximum initial shear stress at the initiation of scour. The method was assembled by linking the calculated initial erosion rate (given by the numerical simulation results and the results of the EFA test) to the calculated maximum scour depth (given by the flume tests results) through the use of a hyperbolic model. The multiflood hydrograph and mul- tilayer soil were included through simple accumulation algo- rithms. Verification was based on comparison with existing databases as well as performing calculations for sample cases and evaluating the reasonableness of the results based on experience. A A A - A Figure 1.1. Typical bridge with potential contraction and pier scour.
