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Assessment of Scour Potential of a Circular Pier in Silty Sand Using ISEEP
Caractérisation par ISEEP du potentiel d'érosion d'une pile circulaire dans un sable silteux
Kayser M., Gabr M.
Department of Civil, Construction and Environmental Engineering
North Carolina State University, Box 7908, Stinson Drive, Raleigh, NC 27695-7908; PH. (919)515-7904;
ABSTRACT: Work in this paper describes an approach for the assessment of soil scour potential through the use of an In Situ
Erosion Evaluation Probe (ISEEP) that is advanced by water jetting. Soil erosion parameters are assessed for silty sand in terms
of critical stream power, and therefore, critical shear stress, and detachment rate coefficient. Scour depth around a circular bridge
pier was computed using ISEEP data and compared with an empirical approach available in literature for estimating scour depth
in soil similar to the tested in the study. The application of the idea and the utility of this technique to assess scourability profile
are presented and discussed.
RÉSUMÉ: Le travail présenté dans cet article décrit une approche pour l’évaluation du risque d'affouillement d’un sol in situ en
utilisant une sonde d’érosion équipée de jet d’eau. Les paramètres d’érosion sont évalués pour les sables limoneux en fonction
d’une puissance critique et par conséquent en termes d’une contrainte de cisaillement critique et d’un coefficient exprimant le
taux de détachement. La profondeur d’affouillement autour d’une pile de pont a été calculée en utilisant les données issues de la
sonde. Elle a été comparée à celle issue de l’approche empirique pour un sol similaire au sol étudié. L’applicabilité de l’approche
proposée et son utilité pour l’évaluation du profil d’érodabilité sont présentées et discutées.
KEYWORDS: Bridges, Erosion, Foundation, In Situ, Pier, Probe, Scour, Shear, Soil
1
INTRODUCTION
The assessment of scour and erosion rates of soil profiles
supporting hydraulic structures and critical bridges is vital for
ensuring safe performance under normal flow conditions, as
well as the integrity of their foundation systems during and
after severe storms. Richardson and Davis (2001) highlighted
the importance of assessment of local scour around bridge
piers as it is one of the most common causes of bridge
failure. Several approaches ranging from simple steel
sounding rods to remote sensing have been developed to
assess scour depth after it has occurred. As presented by Lu
et al. (2008) the more sophisticated approaches, including
acoustic doppler and ground penetrations radars, have a high
cost and require frequent maintenance and repair. Even then,
these approaches do not provide an estimate of scour under
future storm events. Current techniques for providing such
information require either the removal of soil samples for
laboratory testing, in a device such as the Erosion Function
Apparatus (EFA) by Briaud et al. (2001), or limiting the
measurements to erodibility of the surface sediments.
Gabr et al. (2012) presented a prototype device, termed
ISEEP (In Situ Erosion Evaluation Probe), for assessment of
scour parameters with depth. ISEEP has been constructed as
simple stainless steel tubes fitted with truncated cone tip. The
cone-tipped vertical probe is attached to a digitally controlled
centrifugal pump that provides controllable and repeatable
water velocity at the tip, with sustained flow rate against any
induced back pressure. As the water jet is induced through
the cone tip, it mobilizes the soil particles. The test data are
analyzed using the stream power (bed shear stress multiplied
by the flow velocity) concept proposed by Annandale (2006)
to account for the nature of the flow conditions induced
during testing. The results from the tests are reduced to
provide critical shear stress (
c
) and a rate of scour per unit
shear stress (k
d
). These two values are used in conjunction
with the applied shear stresses (
applied
) per a given flow type
and as appropriate to the structure being analyzed, to
compute the scour rate (E) using the excess shear model as
follows (Annandale 2006):
) -
( k = E
applied
d
c
(1)
In this study, experimental work and analyses are
conducted, using ISEEP-estimated data, for evaluating
erosion parameters for a soil with 15% clay and 85% sand.
The soil is classified as silty sand according to the Unified
Soil Classification System. Tests are performed with
different jet velocities and critical stream power value (P
c
)
and the corresponding
c
, and k
d
are evaluated using the data
reduction scheme proposed by Gabr et al. (2012). An
example showing the computation of the scour depth around
a bridge pier using ISEEP-estimated data is presented. The
results are compared with values obtained using empirical
equations reported in literature, and the estimated scour depth
using both approaches is presented and discussed.
2 BRIDGE PIER SCOUR
The magnitude and geometry of local scour at bridge piers in
soil profiles with percent fines content have been
documented in literature (e.g. Hosny 1995, Molinas and
Hosny 1999, Briaud et al. 1999, 2001, and 2004). Hosny
(1995), and Molinas and Hosny (1999) proposed empirical
equations to assess scour depth for saturated and unsaturated
compacted soil with a percent fines that lends a degree of
cohesion to the soil. They reported that the scour depth
decreased as compaction density was increased for the
unsaturated “cohesive” soil conditions, and scour depth
