Interference of Scouring action between Pier and Abutment: Primary Approach

: Presented here are the results of a limited experimental program dealing with the problem describing the scour , which is formed around bridge pier neighboring abutment . To achieve this the results shown , generally, that the scour depends on a distance interaction between pier and abutment . A modification factor deals with this distance presented as a multiplicative factor used with an empirical predictive formula of scour depth around bridge pier .


Introduction:
The present work aims to show the effect of pier-abutment on local scour which formed around pier located at a specified location ( i.e neighboring the abutment) .This phenomenon which was not investigated before , where investigators through different approaches invented a variety of equations for estimation of depth and manner of scour around piers only or abutment only .
Many bridges over the world are destroyed or failed during a case of flow situations (Breusers,et al. 1977) some failure were due to a damage in foundation of last pier neighboring abutment .Accordingly due to necessity of a realistic design investigating the interference manner of local scour is extremely needed .
Analyzing the scour process by using a limited laboratory data to get the empiric coefficient used to improve one of a predictive formula which extracted from previous investigators .It should be noted that this formula deals with a local scour around pier without any interlocking with abutment .

Test
Facilities and Experimental Procedures: The experiments were performed at the hydraulic laboratory in the university of Technology , Baghdad .The experiments were conducted in closed system , waterrecerculating flume that measured 2m long , 0.61m wide , and 0.2m deep .The discharge measured by control panel which contains a pointer moving on calibrated dial to express the flow rate in l/s.In the present study the maximum discharge used 3.3l/s .
The bottom of the working section is filled with sand at a thickness 8cm and its median diameter is 0.575mm , according to this, the critical shear velocity V c * is 0.01726m/s , which determined using the shield s diagram (Hendersen,1966) .

Interference of Scouring Action between Pier and Abutment: Primary Approach
The flow depth used is 3cm , so that the approach velocity , V is 0.18033m/s .The clear water scour regime occur for flow velocities up to the threshold velocity for general bed movement , V c , this is called a critical velocity which can be determined from logarithmic velocity profile , its value is , V c = 0.24416 m/s .At this situation the experiments were run below the incipient motion velocity for sediment size undertaken , where the flow intensity , V/V c =0.739 .
A total of 7 experiments were performed , two of the test are referred to pier only and abutment only, however , the five experiments are presented to show the effect of spacing between pier and abutment on scour around pier and its pattern .The size and allocation of model used are shown in fig.
The distance or spacing between pier and abutment denoted X , is used with five different runs at the same flow conditions , 30cm, 21cm, 16cm, 11cm, 9cm, and 3cm .The run at X=30cm , represent , pier located at mid point of working section only .When using just the abutment model in experimental run , the distance X considered equal zero .However , the first spacing ,i.e.30cm , referred that just the pier model were in test .

Results:
The extracted experimental data are illustrated in Table(1) .To be in practical approach , it must be emphasized to employ the dimensionless term to express , the spacing ratio between pier and abutment .Because the scour depth were mainly normalized with pier diameter , b accordingly the spacing ratio will be normalized with pier diameter to be in consistent with a predictive equation .However , the distance ratio denoted as , X/b , used as a deterministic value within an empirical equation to give a distance coefficient , used as a multiplicative factor with a selected predictive formula .Refering to Table( 2) , the values of distance ratio X/b and distance coefficient are plotted in fig.
(2) , to show the best regression analysis curve with R=0.858 .The scatter around regression line appears uniform and the trend of curve is in consistency with data .The regression provides experimental coefficient that may be useful to modify the following predictive formula (Maatooq,1999) .
The above formula used as recommended by Maatooq(1999), to calculate the equilibrium scour which is formed around bridge pier for clear water with all ranges of flow depth .
If using distance coefficient as a modification factor , Eq.(1) will be in the following form : It should be noted that the K dis can be taken from fig.
(2) or from the following empirical equation : 3) Eng.&Tech.Vol.26,No.52008Interference of Scouring action between Pier and Abutment: Primary Approach *Dept.ofBuilding and construction Eng.Univ. of Tech.565 The above developed formula were examined with K dis values which extracted from experiments to show the reliability of it for usage as a modification factor to determine the equilibrium scour depth around bridge pier located neighboring abutment .
It can be seen from fig.
(3) , the Eq.( 3) is overpredicted for more than 70% of the data undertaken , that is refer to the reliability of model Eq.(3) as a modification factor .It should be refer that the present study can be considered as a primary conception to this problem .

Interference of Scouring Action between Pier and Abutment: Primary Approach Recommendations:
The present work is concerned with the local scour which is formed around bridge pier neighboring abutment dealing with this problem .The description of scour as it has been resulting from limited experimental program , shows a dramatic complexity of the problem .Accordingly , an extended experimental program must be used to arrive to a more reliable deterministic modification coefficient which can be considered as a universal form used to determine the scour depth around pier neighboring abutment .To achieve this the following recommendations can be suggested for further studies : 1