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Prediction of Scour Depth Around Complex Piers Using Classification Methods

Amini Baghbadorani, Danial | 2016

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 49079 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Ataie-Ashtiani, Behzad; Jamali, Mirmosaddegh
  7. Abstract:
  8. In this study, existing prediction methods of maximum scour depth around complex piers are studies using available experimental data and a new relationship for scour prediction is presented. Complex pier consists of a column, pile cap and pile group and is frequently used for foundation of bridge piers in hydraulic and marine structures. The results of 52 clear-water scour experiments on 4 CP models are reported which can be valuable to hydraulic design of a CP. The 4 models had different values of pile cap thickness and number of piles inline with the flow and were tested at different pile cap elevations. The duration of experiments ranged from 24 to 120 hours. Then a database of laboratory tests on CPs was compiled from available technical literature. The Excel file of database consists 428 experiments on CP and 155 experiments on column on top of a caisson. The database was splitted into several cases based on position of pile cap including pile group alone, pile group with debris raft, pile cap fully submerged, pile cap partially buried, and pile cap buried, and column alone. Then the performance of existing equations such as HEC-18 and FDOT were assessed based on the database. It is found that in terms of central model perormance indices, the best existing equations are HEC-18 and a correction of Coleman method, and Present Study equation has at least 10% less error than existing equations. A brief discussion about using single-pier time factors for time development of scour around CPs is given and the limitation of time factors for case of buried pile cap is identified. Scour depth around pile groups is also studied in this thesis. Although scour around pile groups can be considered as a special case of CP scour, many researchers treat this topic separately, and therefore a different section is devoted to maximum local scour around pile groups. A dataset of 365 laboratory tests for scour-hole depth (SHD) around pile groups (PGs) under unidirectional aligned flow is compiled and the performances of the existing equations are evaluated on the dataset using several statistical indices. It is shown that the best available equation is a correction of HEC-18 equation with R2=0.58 and the poorest equation is the Florida's equation with R2=−0.4. All the available equation for the SHD estimations are based on the assumption that the estimated scour depth has reached the final equilibrium state. The test durations of the considered data ranged between 4 to 389 hours. A time factor (Kt) is proposed to take into account the temporal variation of the SHD around different pile groups (PGs). Among the dataset, 51 long duration experiments are scrutinized to show the temporal variation of scour depth toward equilibrium state. The time duration of experiments for these tests are up to 16 days. The proposed Kt factor for pile groups has a superior performance compared to existing time factors, which are applicable for single piers. Subsequently, the equilibrium scour depths are calculated by extrapolation of scour depths reported at the end of the experiments using the proposed Kt equation. The results showed that only 27 to 93 % of the equilibrium scour depths were obtained at the end of the experimental measurements. Finally, a new equation for prediction of equilibrium SHD around PGs is proposed which has 10% less prediction error than the existing equations. This comprehensive comparative study is a significant step forward in the correct estimation of current-induced SHD round PG foundations of hydraulic and coastal structures
  9. Keywords:
  10. Complex Pier ; Pile Group ; Bridge Pier ; Equilibruim Shape ; Scour Depth ; Model Tree

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