Experimental study of local scour around circular bridge piers in cohesive soils

Local scour at bridge piers is considered to be one of the most critical problems in bridge pier design, where such design requires an estimate of the maximum scour at a pier during the anticipated life of the bridge. Many earlier investigations concentrated on one type of bed materials: non-cohesive soils. An extensive review of the literature pertaining to local scour failed to produce any systematic experimental or field data collection program dealing with pier scour in cohesive materials.;In this dissertation, three sets of flume experiments were conducted in the Hydraulics laboratory at the Engineering Research Center of Colorado State University.;The first set (39 experimental data points) concentrates on the effect of clay content on pier scour in sandy soil. Mixtures of 0, 5, 10, 20, 30, and 40 percent clay content by dry weight were used.;The second set (39 experimental data points) investigates the effect of compaction on pier scour in unsaturated clay. Samples of clay at 15% and 20% initial water content were compacted at 58, 65, 73, 80, and 87 percent of compaction.;This dissertation presents the first systematic investigation of local pier scour located in the mixtures of cohesive and non-cohesive soil as well as in cohesive soil. The non-cohesive soil used in this study was medium size sand with mean grain size of 0.55 mm. The cohesive soil used was classified by the X-ray diffraction test as mostly Montmorillonite mineral clay and also considered medium plasticity clay according to the unified soil classification system (USCS).;The last set (33 experimental data points) studies the effect of initial water content (IWC) on pier scour in saturated clays. Samples of saturated clay at 32, 35, 38, 40, and 45 percent initial water content were used in the study.;The data obtained from the present experiments were used in developing relationships and charts for estimating the scour depth as a function of clay content, compaction, vane shear strength, initial water content, and Froude number.;The results of this study indicate that: (i) as the presence of clay content in sandy soil increases up to 40 percent, the scour depth decreases; (ii) the scour depth in unsaturated cohesive soils decreases with increasing degree of compaction; (iii) the scour depth decreases as the initial water content of the saturated cohesive soils decreases.;The application of the developed equations are limited to the soils similar to those used in this study. Therefore, further studies should be undertaken to cover other types of cohesive soils in order to develop more general equations for predicting the scour depth at bridge piers.