Evaluation of the SRICOS-EFA Method for Predicting Scour in Hawaiian Rivers

Rahimnejad, Reza
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[Honolulu] : [University of Hawaii at Manoa], [May 2016]
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Nearly 60% of bridge failures in the country are due to scour. The Hawai‘i Department of Transportation (HDOT) has a vested interest in the integrity of bridges and their foundations during heavy floods. It is known that many of the older existing bridges in Hawai‘i were not designed for scour. Therefore, it is critical to have an accurate assessment of their scour potential. In addition, it is also critical to obtain accurate scour predictions when designing new bridges. An underestimate of the scour depth could lead to potential risk of bridge failure while an overestimate can increase the cost of the new bridge construction unnecessarily. In Hawai‘i, scour calculations are traditionally performed based on the Richardson and Davis equation (1995) where the only soil parameter required is the mean particle size, D50. Using this method for cohesive soils is known to lead to overestimated scour depths since the scour depth is inversely proportional to D50, and cohesive soils have very small particle size. Inter-particle electrical forces exist in cohesive soils which cause cohesive soils to erode slower than granular soils. The SRICOS (Scour Rate In COhesive Soils) method accounts for the time-dependent nature of scour in silts and clays. It requires erodibility testing on soil samples using an Erosion Function Apparatus (EFA) and will generally result in smaller scour depths that can lead to savings in bridge construction. The main objectives of this research were to: (1) Obtain undisturbed soil samples from 5 water channels on the island of Oahu to perform EFA and soil testing; (2) Propose a method to define the critical shear stress and evaluate factors affecting its magnitude; (3) Develop a model to predict an EFA curve for a cohesive soil in Hawai‘i based on some common soil parameters; and (4) Examine the applicability of the Pocket Erodometer Test (PET) to Hawai‘ian cohesive soils. The main contributions from this research are summarized as follows: A cohesive soil was reconstituted in the laboratory under 4 different consolidation pressures resulting in 4 different water contents, void ratios, unit weights and consolidation stresses and then tested using the EFA. It was found that the critical shear stress increased with decreasing water content, decreasing void ratio, increasing unit weight and increasing consolidation stress. That the soils can be considered normally consolidated also suggests that the critical shear stress increases with undrained shear strength. The mathematical properties of the EFA curves for 33 cohesive soil samples were studied. Based on these curves, it was found that by plotting the log of the shear stress versus the scour rate, the curves approximate a hyperbola very closely. It is also proposed that the critical shear stress be estimated as the intercept of the hyperbola on the shear stress axis with some constraints. A model was developed to predict an EFA curve using common soil parameters and the hyperbolic model. Three parameters are needed to fully define this hyperbola. Four explanatory variables (soil parameters) are required to define the three hyperbolic model parameters. They include water content, liquid limit, plasticity index and activity, all of which are easily measured in the laboratory. Use of this model in the SRICOS EFA method to estimate scour depth can result in less scour and in significant foundation cost savings. Scour depths at five water channel sites were estimated using HEC-18 and the SRICOS EFA method. It was found that the SRICOS method always resulted in lower scour depths than HEC-18. The PET was used to derive the erosion categories for 33 cohesive soil samples from the five water channel locations and then compared to those from the EFA. It was found that the current PET erosion criterion that separates the medium and high erodibility categories should be increased from 15 to 28 mm to increase the reliability of the method. With this revised criterion, the reliability of the PET for Hawai‘i soils and for soils that were used to develop this criterion improved. It was also found that Briaud et al.’s (2012) erodibility criteria based on soil classification is not very reliable.
Ph.D. University of Hawaii at Manoa 2016.
Includes bibliographical references.
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Civil & Environmental Engineering
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