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Published (except as noted by red-line fonts) in the proceedings of the American Society of Civil Engineers "North American Water and Environment Congress '96" held in Anaheim, CA, June 1996
Federal Highway Administration's (FHWA) Turner-Fairbank Highway Research Center (TFHRC) J. Sterling Jones Hydraulics Research Laboratory conducted experiments to measure local scour around exposed pile groups for a variety of conditions including different spacings, different skew angles, different patterns and different exposures of a pile cap in the flow field. Two existing methods, one from FHWA's HEC-18 (Richardson et al, 1993) and one from the Summary of Pier Scour Equations used by the People's Republic of China (Gao Dongguang et al, 1994) were evaluated to the extent that they could be applied to the data. Since neither of these methods agreed well with the experimental data, a hybrid procedure based in part on concepts from these two methods was developed and is proposed for consideration.
Most of the pier scour research has focused on typical solid piers with very limited attention to the effect of pile groups when they are exposed to the flow field. In the simplest case, pile groups are capped above the water surface and only the pile group obstructs the flow field. In general cases, piles are capped below the water surface and the flow could be obstructed by three substructural elements including the pile group, the pile cap or footing and part of the pier. The HEC-18 method applies to the simple case but it does not apply for the general case. The Chinese method applies to the general case, but the summary report by Gao Dongguang et. al. does not describe how it was developed.
The study was conducted in 21.3 m long by 1.8 m wide tilting flume located in the Hydraulics Laboratory of the FHWA Turner-Fairbank Highway Research Center, in McLean, Virginia. The flume is equipped with a sediment recess located approximately midway from the upstream end. All experimental were clear water and run at threshold velocity of sediment motion. Incipient motion velocities were obtained by gradually lowering the flume tailgate to increase velocity while keeping flow depth constant by adjusting the flow rate. Incipient motion velocity was determined when upstream bed material began to be transported. The velocity readings were taken using 2D velocity probe once the flow became stable. A flow depth of 0.27 m was kept constant for most of the experiments. The bed material was a fairly uniform 0.28 mm non-cohesive sediment . The experiment duration was kept at 4 hours for most of the tests, but several 24 hour tests were conducted to determine the effects of the duration. Studies by other researchers such as Gosselin et al. (1994), Sumer et al. (1992), and Yanmaz et al. (1991) and Sheppard et al. (1994) Have shown that most of the scour occurs during the first three or four hours of a test. Although longer duration tests would have been desirable, the authors believe that practical comparative conclusions can be drawn from the 4 hour experiments since correction factors in this study were derived by normalizing scour depth with baseline scour depths that were measured with the same duration.
Five pile group patterns that included both uniform and staggered rows were tested for the simple pile obstruction only case. The uniform row patterns were also tested for the general cases where the pile cap and pier were also partially or fully submerged in the flow. A 3x3 symmetrical pile group with and without a pile cap is shown in figure 1 (a), (b), and (c).
There are numerous factors that could affect the scour depth, but the most important ones considered in this study were, spacing between the piles, skew angle of flow, and pile cap location in reference to undisturbed stream bed. Pile shape is a significant parameter, but only square shapes were used in this study. A pile cap that extended at equal distance from each side of the pier, and square pier that was twice the size of a single pile, as shown in fig. 1 (c), were used throughout the experiments.
The FHWA Pier Scour Equation
The local pier scour equation recommended by Federal Highway Administration (FHWA, Circular HEC-18) was selected as a frame of reference for this analysis. The equation is stated as:
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 Ks, 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
The spacing correction factor provided in the Chinese procedure, can be written as;
The above equation in its rearranged form can be written as;
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, Ka 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:
Figure 3 (a) and (b) show 3 x 5 symmetrical and 4 x 5 non-symmetrical 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 HEC-18 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 HEC-18 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 HEC-18 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, Kp, is plotted against (h1 /y) ratio, where h1 is the distance or height of pile group from the undisturbed stream bed, and y is the total depth of flow. The factor Kp on y-axis 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: h1 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, Kc, is plotted against the same (h1/y) ratio. The factor, Kc on y-axis is the ratio of scour depth from the suspended pier/pile cap to scour depth from an equivalent depth-weighted 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 Kc as a function of cap thickness, t, and h1/y but as figure 6 illustrates, there was very little correlation with cap thickness. Because of the poor correlation of Kc with "t" the envelop curve shown on the figure is suggested for Kc. 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;
A proposed procedure is being presented here based on the concept of HEC-18 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 HEC-18 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.
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.