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Pier and Contraction Scour in Cohesive Soils (2004)

Chapter: Chapter 13 - Conclusions and Recommendations

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Page 111
Suggested Citation:"Chapter 13 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2004. Pier and Contraction Scour in Cohesive Soils. Washington, DC: The National Academies Press. doi: 10.17226/13774.
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Page 112
Suggested Citation:"Chapter 13 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2004. Pier and Contraction Scour in Cohesive Soils. Washington, DC: The National Academies Press. doi: 10.17226/13774.
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Page 112
Page 113
Suggested Citation:"Chapter 13 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2004. Pier and Contraction Scour in Cohesive Soils. Washington, DC: The National Academies Press. doi: 10.17226/13774.
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Page 113

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111 CHAPTER 13 CONCLUSIONS AND RECOMMENDATIONS 13.1 CONCLUSIONS 13.1.1 General The topic addressed is the prediction of the scour depth around a bridge pier founded in cohesive soils and subjected to water flow. The scour components considered are complex pier scour and contraction scour. The proposed method, which is based on 42 useful flume tests and 49 useful numerical sim- ulations, is a further development of the method formulated earlier for simple pier scour (cylindrical pier in deep water). 13.1.2 Erodibility of Cohesive Soils It is emphasized that erodibility is not an index but a rela- tionship or function between the water velocity (or, better, the shear stress at the water-soil interface) and the erosion rate of the soil. Erodibility is represented by the erosion function. Two important parameters help describe the erosion function: the critical shear stress and the initial slope of the erosion func- tion. It is found that, although the critical shear stress of a cohe- sive soil is not related to its mean grain size, the common range of critical shear stress values for cohesive soils (0.5 N/m2 to 5 N/m2) is comparable to the range obtained in sands. This explains why the maximum scour depth in cohesive soils is comparable to the one obtained in sands. The initial slope of the erosion function can be many times less than the one in sand (e.g., 1,000 times less) and, therefore, the scour depth can develop very slowly in some cohesive soils. There lies the advantage of developing a method that can predict scour depth as a function of time for a given hydrograph (cohesive soil) rather than a maximum depth of scour for a design flood (sands). This was the goal of this project. It also was found that the critical shear stress and the initial slope were not related to soil properties because the R2 of the regressions were all very low. To obtain the erosion function this study recommends using the Erosion Function Apparatus (EFA). 13.1.3 EFA This EFA was developed in the early 1990s to obtain the erosion function. A soil sample is retrieved from a bridge site using an ASTM standard, thin-wall steel tube (i.e., a Shelby tube); placing it through a tight-fitting opening in the bottom of a rectangular cross-section conduit; pushing a small protrusion of soil into the conduit; sending flowing water over the top of the sample at a chosen velocity; and recording the corresponding erosion rate. This is repeated for several velocities and the erosion function for each is obtained in that fashion. 13.1.4 SRICOS-EFA Method for Cylindrical Piers in Deep Water SRICOS stands for Scour Rate In COhesive Soils. Since the method makes use of the erosion function measured in the EFA, the method is referred to as the SRICOS-EFA Method. The SRICOS-EFA Method (program) gives the scour depth as a function of time for the period covered by the hydrograph for a given velocity hydrograph at a bridge, a given multilayered soil stratigraphy with an erosion function defined for each layer, and a given cylindrical pier in deep water (water depth larger than 1.6 times the pier diameter). The method is based on the calculation of two basic param- eters: the maximum depth of pier scour and the initial rate of scour. The maximum depth of scour is based on an equation obtained from flume tests and the initial rate is based on an equation giving the initial shear stress obtained from numeri- cal simulations. The initial rate of scour is read on the EFA erosion function at the corresponding value of the calculated shear stress. A hyperbola is used to connect the initial scour rate to the maximum or asymptotic scour depth and describes the complete scour depth versus time curve. Robust algo- rithms are used to incorporate the effect of varying velocities and multilayered soil systems. This earlier method was devel- oped by the authors under TxDOT sponsorship and was ver- ified by satisfactory comparison between predicted scour and measured scour at eight bridges in Texas. 13.1.5 SRICOS-EFA Method for Maximum Scour Depth at Complex Piers A set of flume experiments was conducted to study the maximum depth of scour for a pier including the effect of shallow water depth, the effect of rectangular shapes, the effect of the angle of attack on rectangular shapes, and the

112 effect of spacing between piers positioned in a row perpen- dicularly to the flow. The proposed equation for the maxi- mum depth of scour is in the form of the equation for the cylindrical pier in deep water with correction factors based on the results of the flume tests. Where Zmax(Pier) is the maximum depth of pier scour in milli- meters; Re is the Reynolds Number equal to VB′/v, V (m/s) being the mean depth velocity at the location of the pier if the bridge were not there; the K factors take into account the shallow water depth, spacing, shape, and angle of attack being considered through the use of the projected width B′ (m) in the calculation of the Reynolds Number. 13.1.6 SRICOS-EFA Method for Initial Scour Rate at Complex Piers A set of numerical simulations was performed to study the maximum shear stress around a pier including the effect of shallow water depth, rectangular shapes, angle of attack on rectangular shapes, and spacing between piers positioned in a row perpendicularly to the flow. The proposed equation for the maximum shear stress is in the form of the equation for the cylindrical pier in deep water with correction factors based on the results of the numerical simulations. where τmax(Pier) (kN/m2) is the maximum shear stress around the pier; Re is the Reynolds Number equal to VB/v, V (m/s) being the mean depth velocity at the location of the pier if the bridge were not there; B (m) is the pier diameter or pier width; ρ (kg/m3) is the mass density of water; the k factors take the shallow water depth, pier shape, pier spacing, and attack angle into account. 13.1.7 SRICOS-EFA Method for Maximum Contraction Scour Depth A set of flume experiments was conducted to study the depth of scour associated with the contraction of a channel including the effects of the ratio of the contracted channel width over the approach channel width, the contracted chan- nel length, and the transition angle. The proposed equation for the maximum depth of contraction scour is Where Zmax(Cont) (m) is the maximum depth of contraction scour; H1 (m) is the water depth along the center line of the Z K K H V gH gnH c max . . .Cont L HEC( ) = × −        ≥θ τ ρ1 90 1 49 01 1 0 5 1 1 3 τ ραmax . logPier w sh sp ( ) = −    k k k k V Re0 094 1 1 10 2 Z K K K Remax ..Pier in mm w sp sh( ) = ( )0 18 0 635 uncontracted channel after scour has occurred; VHEC (m/s) is the mean depth water velocity at the location of the pier in the contracted channel; τc (N/m2) is the critical shear stress of the soil; ρ (kg/m3) is the mass density of water; g (m/s2) is the acceleration due to gravity; n is the Manning’s Coeffi- cient (s/m1/3); and the K factors take the transition and the con- tracted channel length into account. Note that the parenthe- sis in the equation is a factored difference between the Froude Number and the critical Froude Number. Equations also are proposed for the uniform contraction scour depth as well as the location of the scour depths. 13.1.8 SRICOS-EFA Method for Initial Contraction Scour Rate A set of numerical simulations was performed to study the maximum shear stress around the contraction of a channel including the effects of the ratio of the contracted channel width over the approach channel width, the transition angle, the water depth, and the contracted channel length. The pro- posed equation for the maximum shear stress is in the form of the equation for the shear stress at the bottom of an open and uncontracted channel with correction factors based on the results of the numerical simulations. where τmax(Cont) (N/m2) is the maximum shear stress along the centerline of the contracted channel; γ is the unit weight of water (kN/m3); n is the Manning’s Coefficient (s/m1/3); V (m/s) is the upstream mean depth velocity; Rh (m) is the hydraulic radius defined as the cross-section area of the flow divided by the wetted perimeter; and the k factors take the contraction ratio, the transition angle, the water depth effect, and the contracted length into account. Equations also are proposed for the location of the maximum shear stress. 13.1.9 SRICOS-EFA Method for Complex Pier Scour and Contraction Scour in Cohesive Soils Once the equations were established, the SRICOS-EFA Method was assembled. Care was taken not to simply add complex pier scour and contraction scour to get total pier scour. Instead, advantage was taken of the fact that at the end of the maximum contraction scour, the velocity is at the crit- ical velocity and the maximum pier scour should be calcu- lated using the critical velocity of the soil and not the initial velocity in the contracted channel. In addition, the rules of accumulation due to the hydrograph and the multilayer sys- tem developed for the simple pier scour method were adapted for the complex pier and contraction scour method. The superposition and accumulation reasoning lead to the fol- lowing steps for the SRICOS-EFA Method for predicting the τ γθmax Cont c R c c H c L h( ) = ( )− − − − −k k k k n V R2 2 13

113 not primarily cohesive. Nevertheless, these comparisons give an indication that the SRICOS-EFA Method may not be lim- ited to cohesive soils. Indeed, the fact that the method is based on site-specific testing of the erosion function permits incorporating the soil behavior directly in the predictions. 13.1.11 Future Hydrographs and Scour Risk Analysis A novel technique was presented on generating future hydrographs. Indeed, since the SRICOS-EFA Method pre- dicts the scour depth as a function of time, it is necessary to input into the program the hydrograph over the design life of the bridge. The proposed technique consists of using a past hydrograph (from a gage station, for example), preparing the frequency distribution plot for the floods within that hydro- graph, sampling the distribution randomly and preparing a future hydrograph for the required period that has the same mean and standard deviation as the measured hydrograph. This process is repeated 10,000 times and, for each hydro- graph, a final scour depth (the depth reached at the end of the design life of the bridge) is generated. These 10,000 final depths of scour are organized in a frequency distribution plot with a mean and a standard deviation. That plot can be used to quote a scour depth with a corresponding probability of occurrence, or better, to choose a risk level and quote the cor- responding final depth of scour. 13.1.12 Example Problems A set of example problems was presented to help the reader become more familiar with the SRICOS-EFA Method. Some examples are performed by hand calculations; some use the SRICOS-EFA computer program. 13.2 RECOMMENDATIONS It is recommended that 1. The proposed method be incorporated in the next ver- sion of HEC-18; 2. The SRICOS-EFA Method program be transferred to a Windows™ environment; 3. The project be continued to solve abutment scour, the last major unsolved scour problem in cohesive soils; and 4. A set of short courses be offered across the country to teach the new method and the corresponding program. scour depth at a complex pier in a contracted channel. This step-by-step procedure has been automated in a computer program. 1. Collect the input data: velocity and water depth hydro- graph, geometry of the pier and of the contracted chan- nel, erosion functions of the soil layers. 2. Calculate the maximum contraction scour depth for the ith velocity in the hydrograph. 3. Calculate the maximum complex pier scour depth using the ith velocity in the hydrograph at the pier location if there is no contraction scour in Step 2, or the critical velocity for the soil if there is contraction scour in Step 2. 4. Calculate the total pier scour depth as the total of Step 2 and Step 3. 5. Calculate the initial maximum shear stress for pier scour using the ith velocity in the hydrograph. 6. Read the initial scour rate corresponding to the initial maximum shear stress of Step 5 on the erosion func- tion of the soil layer corresponding to the current scour depth. 7. Use the results of Steps 4 and 6 to construct the hyper- bola describing the scour depth versus time for the pier. 8. Calculate the equivalent time for the given curve of Step 7. The equivalent time is the time required for the ith velocity on the hydrograph to scour the soil to a depth equal to the depth scoured by all of the veloci- ties occurring prior to the ith velocity. 9. Read the additional scour generated by the ith veloc- ity starting at the equivalent time and ending at the equivalent time plus the time increment. 10. Repeat Steps 2 to 9 for the (i + 1)th velocity and so on until the entire hydrograph is consumed. 13.1.10 Verification of the SRICOS-EFA Method Several full case histories were identified for verification but none could satisfy the requirements necessary to verify the method developed. Some did not have enough details on the observed scour depth, some turned out not to be made of cohesive soil after drilling, some did not have a gage station nearby. It was decided to compare the maximum scour depth for pier and contraction to existing databases. These data- bases were mostly in sand, however, and included those col- lected by Mueller (pier scour), Froehlich (pier scour), and Gill (contraction scour). The comparisons between the predicted and measured scour depths are very satisfactory although it is not clear whether they should be or not since the soils were

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 516: Pier and Contraction Scour in Cohesive Soils examines methods for predicting the extent of complex pier and contraction scour in cohesive soils.

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