566
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
(2011) indicated that the estimation of time-dependent clear
water scour magnitude at bridge piers remains a challenge
during the limited duration of excessive flow as, for example,
in the case of a storm surge. Equation 3 proposed by Briaud
et al. (1999) provides an estimate of the
max
as a function of
the flow velocity at a round pier. However, in order to assess
the scour depth with time using ISEEP data, a reduction in
max
with the progression of scour depth is needed. While a
significant number of studies have been performed for the
assessment of maximum scour at piers, these approaches
were not specifically concerned with evolution of shear
stresses with time and flow field. Data presented by Briaud
et al. (1999) indicated a nearly linear relationship between
the
t
max
and
scour depth) /d (pier diameter), where
t
is
the shear stress with the progression of scour depth with a
minimum value of
critical
. An iterative approach is used to
estimate
t
since the maximum depth of scour is not known
a’priori.
The scour depth around a bridge pier is estimated for a
flow velocity range of 1.0 m/s to 2.0 m/s (Froude number
0.23 to 0.45) with a pier diameter of 1 m and a depth of flow
= 2 m. The computations are performed based on the ISEEP
data and compared with the values from the empirical
equation by Ansari et al. (2002), as the conditions for Ansari
et al. (2002) empirical equation are in agreement with the
percent fines in the test soil. Figure 4 shows scour depth,
normalized with respect to the pier diameter ratio, versus
time for different flow velocities. The scour depth from the
equation by Ansari et al. (2002) is within 4.6-8 D (D = pier
diameter) for a flow velocity range of 1.0 m/s to 2.0 m/s,
which is higher than values estimated using the ISEEP data.
A reason for the deviation can be attributed to the fact that
the maximum shear stress equation developed by Briaud et
al. (1999) was for clay, while the soil in this study is 85%
sand. Furthermore, the application of Ansari et al. (2002)
approach required the definition of the scour level in sand
first which can widely vary depending on the parameters
assumed.
6 SUMMARY AND CONCLUSIONS
The ISEEP approach developed by Gabr et al. (2012) was
used to provide parameters for evaluating scour potential
with time in a 15%-85% silty sand mixed bed. Soil erosion
parameters included critical shear stress and detachment rate
coefficient. Higher detachment rate was obtained for the silty
sand than the sand soil. Application of the ISEEP data to
assess magnitude of scour with time for a circular bridge pier
indicated a scour depth on the order of 1-6 m versus 4.6-8 m
estimated by an empirical equation in literature. The
difference in results is attributed to the difference in the
approach for scour computation and the limitations of
estimating the evolution of shear stresses with time and flow
field. In this case, the applicability of the empirical equation
was somewhat limited since the testing conditions deviated in
terms of soil type and moisture conditions.
7 ACKNOWLEDGEMENTS
This work is supported by the US Department of Homeland
Security under Award Number: 2008-ST-061-ND 0001. The
views and conclusions contained in this document are those
of the authors and should not be interpreted as necessarily
representing the official policies, either expressed or implied,
of the U.S. Department of Homeland Security.
8 REFERENCES
Annandale, G. W. and Parkhill, D. L. 1995. Stream Bank Erosion:
Application of the Erodibility Index Method. Proceedings of
International Water Resources Engineering Conference, 2,
1570-1574.
Annandale, G.W. 2006. Scour Technology: Mechanics and Practice.
McGraw Hill, New York.
Ansari, S.A., Kothyari, U.C. and Ranga Raju, K.G. 2002. Influence
of Cohesion on Scour around Bridge Piers.
Journal of
Hydraulic Research
, 40 (6), 717–729.
Briaud, J.-L, Ting, F. C. K., Chen, H. C., Gudavalli, R., Perugu, S.
and Wei, G. 1999. SRICOS: Prediction of Scour Rate in
Cohesive Soils at Bridge Piers.
Journal of Geotechnical and
Geoenvironmental Engineering
, 125 (4), 237–246.
Briaud, J.-L., Ting, F.C.K., Chen, H.C., Cao, Y., Han, S.-W. and
Kawk, K. 2001. Erosion Function Apparatus for Scour Rate
Predictions.
Journal of Geotechnical and Geoenvironmental
Engineering
, 127 (2), 105-113.
Briaud J.-L., Chen, H.-C., Li, Y., Nurtjahyo, P., Wang, J. 2004. The
SRICOS-EFA method for complex piers in fine grained
soils,” Journal of Geotechnical and Geoenvironmental
Engineering, ASCE, Vol 130, No. 11, p1180-1191.
Debnath, K. and Chaudhuri, S. 2010a. Laboratory Experiments on
Local Scour Around Cylinder for Clay and Clay–Sand Mixed
Beds.
Engineering Geology
, 111, 51–61.
Debnath, K. and Chaudhuri, S. 2010b. Bridge Pier Scour in Clay-
Sand Mixed Sediments at Near-Threshold for Sand.
Journal of
Hydraulic Engineering
, 136(9), 597-609.
Ettema, R., Constantinescu, G. and Melville, B. 2011. Evaluation of
Bridge Scour Research: Pier Scour Processes and Predictions ,
Contractor’s Final Report for NCHRP Project 24-27(01),
National Cooperative Highway Research Program, March
2011, 181.
Gabr, M., Caruso, C., Key, A. and Kayser, M. 2012. Assessment of
In Situ Scour Profile in Sand Using a Jet Probe. Journal article
accepted in ASTM Geotechnical Testing Journal, In Press.
Hanson, G. J. and Cook, K. R. 2004. Apparatus, Test Procedures,
and Analytical Methods to Measure Soil Erodibility In Situ.
Applied Engineering in Agriculture
, Vol. 20, No. 4, 455-462.
Hosny 1995. Experimental Study of Scour around Circular Piers in
Cohesive Soils. Ph.D. Dissertation, Colorado State University.
Kothyari, U.C., Garde, R.J. and Ranga Raju, K.G. 1992. Temporal
variation of scour around circular bridge piers.
Journal of
Hydraulic Engineering
, 118(8), 1091-1105.
Lu, J-Y., Hong, J-H., Su, C-C., Wang, C-Y. and Lai, J-S. 2008. Field
Measurements and Simulation of Bridge Scour Depth
Variations during Floods.
Journal of Hydraulic Engineering
,
134(6), 810-821.
Mehta, A. J. 1991. Review Notes on Cohesive Sediment Erosion. In:
N.C. Kraus, K.J. Gingerich, and D.L. Kriebel, (eds.), Coastal
sediment ’91, Proceedings of Specialty Conference on
Quantitative Approaches to Coastal Sediment Processes,
ASCE; pp.40– 53.
Molinas, A. and Hosny, M. 1999. Experimental Study on Scour
around Circular Piers in Cohesive Soil.
Publication No.
FHWA-RD- 99-186
, Federal Highway Administration, U.S.
Department of Transportation, McLean, VA.
Richardson, E.V., and Davis, S.R., 2001, Evaluating Scour at
Bridges (4th ed.)
,
Federal Highway Administration Hydraulic
Engineering Circular No. 18, FHWA NHI 01-001.
