164 Paper No. 99-1275 TRANSPORTATION RESEARCH RECORD 1690 Local pier scour has been the topic of numerous research studies on noncohesive alluvial materials. However, because of the complexities involved in cohesive-material scour, very little effort has been devoted to the study of scour around bridge piers situated in cohesive materials. The parameters that affect the scour mechanism at bridge piers located in sat- urated and unsaturated clayey soils are identified, and prediction equations for quantification of the effects of some of the cohesive properties on the resulting local scour are developed. The scour depth predictors developed from the analysis of the experimental data express pier scour in terms of the approach flow conditions, initial water content, compaction, and soil shear strength of the cohesive bed materials. These predictors are related to scour in noncohesive alluvial material to derive factors that express clay scour as a percentage of sand scour. At bridge locations where exact soil properties are unknown, these estimators may be used to provide bounds to scour estimations. The formation of local scour holes around bridge piers is almost an unavoidable problem in alluvial channel beds subjected to the ero- sive action of oncoming river flows. The design and construction of bridges that span alluvial channels require the knowledge or at least as accurate an estimate as possible of the maximum scour depth that might occur around the piers during the anticipated lives of bridges. Local pier scour is caused by the three-dimensional boundary-layer separation at the pier, which is characterized by a high level of turbu- lence and vorticity and which results in the erosion of bed material by the local flow structure. Briefly, when an obstruction such as a blunt- nosed pier is placed in a flow field, a system of vortices, which are identified as the downflow, the horseshoe, and the wake vortices, are locally developed around the pier. As reported elsewhere (1–5 ), this system of vortices is the basic mechanism of local scour. In the past, numerous experimental and analytical investigations of local pier scour were conducted in alluvial channels, and series of prediction equations were developed by researchers to estimate the maximum scour depth at bridge piers under different approach flow conditions, for different sediment sizes and gradations, and for different pier types and sizes. Extensive literature reviews on the topic can be found in several doctoral studies and research reports (6–8). Unfortunately, all of these studies have been confined to non- cohesive soils. Undoubtedly, this is not only due to the abundance of streams with these types of beds but is also because sand and gravel are easier to both characterize and model physically. The scour of cohesive materials is fundamentally different from that of noncohesive materials. It involves not only a complex mechan- ical phenomenon including shear stress and shear strength of soils but also the chemical and physical bonding of individual particles and the properties of the eroding fluid. Cohesive materials, once eroded, remain in suspension such that clear-water scour condi- tions always prevail. Along with the eroding fluid properties, as pointed out previously (9,10), the scour process in this environment is strongly affected by the amount of cohesive material present in the soil mixture as well as the type of mineral clay, initial water con- tent, soil shear strength, and compaction of the clay. In the past, sev- eral investigators (9,11–14) have analytically and experimentally studied the effects of soil compaction, flow conditions, physicochemi- cal properties of soil, and properties of pore fluid of soil and eroding fluid on the erosion rate of cohesive soils. In one of the earlier efforts (15), it was concluded that clays with high calcium and magnesium contents are more resistant to erosion than clays with high sodium contents. In another earlier study (16 ), it was stated that soils with higher plasticities, which indicates that they contain a higher per- centage of clay, are nonerosive in nature and that their suspensions are more rapidly flocculated, whereas the soils with lower plas- ticities are more erodible. It was found experimentally (9) that the montmorillonite clay has a higher critical shear stress than both illitic and kaolinitic soils for low values of sodium absorption ratio (SAR; in the range of 1 to 5) and high pore fluid concentrations. According to Alizadeh (9), the critical shear stress increases as the clay content in cohesive soils increases up to 20 percent. Alizadeh (9) reported that at higher values of SAR (50 to 60), the kaolinite clay has higher critical shear stress than montmorillonite clay. A scour depth predictor at culvert outlets was developed previously (13). The predictor related the depth of scour to the soil shear strength, the plasticity index, and the soil water content. It was con- cluded (14) that the erosion rate of calcium montmorillonite is about two times the erosion rate of sodium montmorillonite. The results of Shaikh’s experiments (14) showed that when calcium montmo- rillonite was treated with 0.88 percent sodium carbonate by dry weight, the erosion rate was reduced to the same order as that for sodium montmorillonite. In general, the rate of erosion equations developed by Shaikh indicate that the erosion rate decreases as the torvane shear strength and percentage of clay increase. An extensive review of the literature pertaining to local scour failed to produce any systematic experimental or field data collection program that has dealt with pier scour in cohesive materials. The objectives of this paper are to apply the knowledge on cohesive ma- terial scour gained in the past to the local pier scour and, specifically, (a) to study the effects of compaction, soil shear strength, and the approach flow conditions on pier scour in unsaturated cohesive soils; (b) to specify the influence of initial water content of saturated clay on pier scour; and (c) to develop scour prediction equations for unsat- urated and saturated cohesive soils to quantify the scour that may occur around circular piers. It is believed that the systematic investi- gation of local pier scour in cohesive materials is an important step in the evaluation of bridge safety, especially for bridges with limited service lives. Effects of Cohesive Material Properties on Local Scour Around Piers ALBERT MOLINAS, STERLING JONES, AND MAGDY HOSNY A. Molinas, Department of Civil Engineering, Colorado State University, Fort Collins, CO 80523. S. Jones, Turner-Fairbanks Highway Research Center, FHWA, 6300 Georgetown Pike, Reston, VA 22101. M. Hosny, Channel Maintenance and Weed Control Division, Water Research Center, Cairo, Egypt.