Scour Technology  Bridge Hydraulics  Culvert Hydraulics  Highway Drainage  Hydrology  Environmental Hydraulics 
FHWA > Engineering > Hydraulics > Research > Scour Around Exposed Pile Foundations 
Scour
Around Exposed Pile Foundations

Where:  d_{s}  =  scour depth 
K_{1}  =  shape factor (1.1 for square nose, 1.0 for round nose, 0.9 for sharp nose)  
K_{2}  =  angle of attack factor  
K_{3}  =  dune factor (varies from 1.0 to 1.3)  
B  =  effective pier width; and  
F_{R}  =  Froude number=V/Sqrt (g y) 
The recommended procedure for applying this equation to an exposed pile group is to assume a solid pier that has the dimensions of the pile group if the piles were packed to touch one another. This procedure was intended to be a conservative approximation if the piles are spaced at one or two pile diameters apart. This procedure was not, however, logical for very large pile spacings where the piles begin to act as independent obstructions to the flow. It was also not a logical way to account for the combined effects of a pile cap and the exposed pile group if they both act as flow obstructions. The scour depth from this procedure may be referred to as the equivalent solid pier scour depth.
Spacing between piles
Spacing between the piles is one of the most important factor in estimating local scour depth around pile group. The scour depth reduces, as the spacing between the piles increases due to less interference from the adjacent piles. As the distance between the piles decreases, the scouring process will be affected by two processes. First, the vortices created around the piles will interact with each other, and secondly, the flow will be accelerated due to contraction created by the adjacent pile [(Elliott et al. 1985)]. To study the effect of spacing between the piles, the ratio S/D of 1 to 9 was selected, where S is the center to center spacing of the piles and D is the diameter or width of a single pile. In figure 2, the spacing correction factor K_{s}, is the ratio of scour depth at a particular value of S/D to that of equivalent solid pier for the same pile group configuration. It can be seen that the scour depth decreases as the spacing between the pile increases, and reaches to scour depth of a single pile for S/D ratio of approximately nine or greater. A slight increase in scour depth was noted as S/D increased from one (piles touching) to two; otherwise the scour depth gradually decreased as the spacing increased. The results are consistent with the findings of Sheppard et al.(ASCE, 1995). The relation for best fit and envelope curves are derived as an exponential function of S/D ratio, and are given as
Where:  A  =  0.47 for the best fit equation 
=  0.57 for an envelop of the data. 
The spacing correction factor provided in the Chinese procedure, can be written as;
Where:  m  =  number of rows normal to the flow; 
D  =  diameter of a single pile;  
B_{m}  =  center to center dist between the outermost rows 
The above equation in its rearranged form can be written as;
Where:  S  =  center to center spacing of piles. 
If the relative spacing, S/D, for the piles is set to unity (i.e., piles touching),this equation would predict that the scour depth from a pile group would be 2.25 times the scour depth from a single pile, no matter what number of pile rows are there. This might be reasonable approximation for some pile groups, but it does not seem reasonable as a general predictor. Since this method was translated from Chinese to English, there is a possibility that some terms or coefficients were inadvertently omitted during the translation. We included the effect of number of rows in the Chinese spacing correction factor, and performed regression analysis on data collected in this study. The modified form of equation can be written as;
Angle of attack:
Studies by Laursen and Toch (1956, 1953), and Varzeliotis (1960) showed that the scour depth increases as the skew angle for a single pier increases. The rate of increase in scour depth due to angle of attack also varies for different shapes and width to length ratios. The experimental results from this study show that the skew correction for a group of square piles is reasonably close to the skew correction for a solid pier with the same overall width to length ratio. Figure 3 shows the effect of angle of attack on two pile group patterns for different pile spacing. In figure 3, K_{a} is the scour depth of the skewed pile group adjusted for pile spacing and normalized by the scour depth of an equivalent solid pier skewed at the same angle to the flow direction and can be written as:
Where:  d_{s(pg)}  =  scour depth around a skewed pile group; 
K  =  spacing correction;  
d_{s(ES)}  =  scour depth around an equivalent solid pier set at the same skew angle to the flow direction. 
Figure 3 (a) and (b) show 3 x 5 symmetrical and 4 x 5 nonsymmetrical staggered piles layout patterns respectively. Results show that the deviation from the equivalent solid is maximum when the skew angle is near 30 degrees, but the deviation is less for the staggered row pattern than for the straight row pattern. The differences between the skew corrections for symmetrical patterns and the staggered patterns are attributed in part to the way the piles became aligned as the skew angle changed. As the skew angle increased, the staggered piles got aligned to each other, and became fully aligned at a 45 degree angle of attack.
Comparison of existing procedures and proposed procedure for exposed piles only
Figure 4(a) and (b) presents the comparison of existing and proposed procedures for HEC18 and Chinese methodologies with 65 lab measured scour depths for simple pile groups with no pile cap in the flow field. Figure 4(a) illustrates that both methods over predicted almost all data points, including the measurements with skewed pile groups which had not been tested previously. The over prediction for large pile spacing was expected from HEC18 procedure, since it does not account for spacing between the piles. The Chinese procedure which is more logical over predicted by a greater margin. Figure 4(b) illustrates the improvement when the HEC18 procedure is modified to include a pile spacing correction using Ks from equation (2), and projected width accounting for skew angles and when the Chinese procedure is used with the modified equation (5) for the pile spacing parameter. Both procedures then predicted scour depths reasonably close to observed scour depths.
Components of Composite Pile Foundation
In the general case, the flow obstruction is a composite of a pile group stubbed up into the flow field and pier/pile cap suspended down into the flow field The hypothesis we used in planning the experiments and presenting the results was that we could measure or compute two components of scour  one component for the pile group stub and one for the suspended pier/pile cap  and add the two components to predict the scour caused by the whole obstruction.
Data was then collected on each component at various pile cap locations and thicknesses. The results are shown in figure 5 and 6. In figure 5, the pile stub factor, K_{p}, is plotted against (h_{1} /y) ratio, where h_{1} is the distance or height of pile group from the undisturbed stream bed, and y is the total depth of flow. The factor K_{p} on yaxis is the ratio of scour depth from pile group stub at a certain distance from the stream bed to the scour depth for a simple full depth pile group which was discussed previously. An expression that fits the data in figure 5 is:
Where: h_{1} and y are described above.
Similarly, experiments were run with the pier/pile cap suspended into flow at the same distances where measurements were taken for the pile group stub. In figure 6, the pier/pile cap factor, K_{c}, is plotted against the same (h_{1}/y) ratio. The factor, K_{c} on yaxis is the ratio of scour depth from the suspended pier/pile cap to scour depth from an equivalent depthweighted average width pier extending down to touch the stream bed. The depth weighted average width approach was adopted from the Chinese procedure. We expected to develop equations for K_{c} as a function of cap thickness, t, and h_{1}/y but as figure 6 illustrates, there was very little correlation with cap thickness. Because of the poor correlation of K_{c} with "t" the envelop curve shown on the figure is suggested for K_{c}. A better correlation for different (t/y) ratios may be obtained when more data becomes available.
The total depth of flow from composite pile foundation then can be calculated from the equation given below;
Where:  d_{s}  =  the total scour depth for the composite pile foundation, 
K_{p}  =  correction factor for the pile group stub,  
d_{s(pg)}  =  the scour depth for a full depth pile group of the same pattern,  
K_{c}  =  the correction factor for the suspended pier/pile cap from figure 6 and  
d_{s(e)}  is the scour depth from an equivalent depthweighted average width pier that extends to the undisturbed stream bed. 
Proposed Procedure
A proposed procedure is being presented here based on the concept of HEC18 and Chinese methodology. The proposed procedure steps for estimating local scour depth around pile group with or without pile cap situation are given below.
Comparison of Proposed and Chinese procedures for composite pile foundation
A series of 21 experiments were conducted to measure scour around a composite pile foundation as a whole. We tested our hypothesis by comparing the scour depth computed from the two components as described by the above proposed procedure to the scour depths measured for the whole composite pile foundation. The results are presented in figure 7 and are compared to the results from the Chinese procedure under the same conditions. The proposed procedure predicted the measurements relatively well; whereas the Chinese procedure over predicted them considerably.
There is a need for more data with a wide range of variable values to support these results. Considerable judgement should be applied if the recommended procedure is used for situations not tested in this study.
Conclusions and Recommendations:
Neither of the existing procedures tested in this study accurately predicted scour around pile groups. The HEC18 procedure can be modified to predict scour around simple pile groups with no cap in the flow field by applying corrections for pile spacing and skew angle. A new procedure based on adding components of scour is proposed for the general composite pile foundation with the pile cap below the water surface.
More data are needed to develop this procedure into a fully implementable tool for bridge scour evaluations. Until more data becomes available engineering judgement must be exercised if the proposed procedure is used beyond the conditions of pile patterns and pile cap thickness ratios that were included in the experiments.
Endnotes
^{1} Graduate Research Fellow from George Washington University, FHWA, TFHRC, McLean, VA. Currently with RUST Environmental and Infrastructure, Fairfax, VA.
^{2} Research Hydraulic Engineer, FHWA, TFHRC, McLean, VA.
References
Kornel Kerenyi
Office of Research, Development and Technology
2024933142
kornel.kerenyi@dot.gov