Introduction
Scour generally refers to the erosion of streambed or bank material due to flowing water. According to AASHTO LRFD (2020) commentary clause C3.7.5, scour is the most common cause for highway bridge failures in the US. The Commentary to the Canadian Highway Bridge Design Code, CSA-S6:19.1 (2019) Clause C1.9.1.3 also mentions that most failures of bridges and open-footing culverts are caused due to hydraulic effects, primarily scour and erosion. It is imperative for a bridge engineer to have a basic understanding of the types of scour, such as General Scour, Contraction Scour, Local Scour, etc. Total scour is the sum of all such conditions, as applicable.
General scour refers to the movement of geomaterials in the riverbed that is not due to the local obstacles present at a bridge site. Contraction scour, in a natural channel or a bridge site comprises the removal of material from the bed and banks across the entire channel width. This scour component occurs due to the flow area contraction at the bridge site causing velocity and shear stress increase on the stream bed. The contraction may be due to roadway embankment construction to reduce the bridge length or from a natural narrowing of the stream channel. On the other hand, local scour is caused due to material removal around piers, abutments, and embankments. Such obstructions cause an acceleration of flow and induce vortices causing local removal of the geomaterial in the stream bed.
Figure 1: General and Local Scour (Adapted from Figure C1.3 of CSA-S6:19.1)
General Approach
It is noteworthy that scour is not a load effect in and of itself; however, it has the potential of significantly altering the force effect consequences on the bridge structure. It is good practice to use deep foundations for cases where scour is present and avoid use of shallow footings altogether. We note that the Canadian Highway Bridge Design Code, CSA-S6:19 (2019) allows the use of shallow foundations as long as specific measures are taken to protect the spread footings against scour. In contrast, the British Columbia (BC) Ministry of Transportation and Infrastructure’s (BC MOTI) Bridge Standards and Procedures Manual, Volume 1 – Supplement to CHBDC S6:19 (2022) requires piled foundations for abutments and pier subjected to potential scour. A variance from this philosophy requires approval from the Ministry.
Design for Scour
Per AASHTO LRFD, the design objective for both Extreme I and II load combinations is life safety or non-collapse of the structure. The joint probability of effects such as blast loading (BL), earthquake (EQ), vehicular collision (CT), vessel collision (CV), ice load (IC), etc., is extremely low. Therefore, these loads are to be individually combined with permanent dead, live, and water (WA) loads, as appropriate. In terms of considering these load effects including scour, AASHTO LRFD recommends not considering contraction and local pier scour unless site-specific conditions dictate otherwise. However, general scour should be considered. An alternative per AASHTO LRFD is to consider one-half of the total scour in combination with BL, EQ, CT, CV, IC.
We emphasize that scour design is a multi-disciplinary exercise involving hydraulics, geotechnical, and structural engineers, working as a cohesive team. Detailed recommendations for scour and seismically induced liquefaction are provided in the Caltrans Seismic Design Criteria (SDC 2.0 - 2019) document. The Caltrans SDC requires consideration of both the occurrence and non-occurrence conditions for pile design in soils susceptible to scour and/liquefaction. The occurrence condition corresponds to 100% scour and the assumption that liquefaction will occur (if applicable). The non-occurrence condition implies that neither scour nor liquefaction will happen. The three conditions to be considered for the occurrence situations per SDC 2.0 are as follows:
Case 1 – Potentially Liquefiable Layer Only (No scour)
With no scour involved but a potential for liquefaction in the upper 50ft from the ground surface, the pile analysis must consider liquefied soil stiffness springs for the liquefiable layer and a reduced soil stiffness for the soil above the liquefiable layer. For potential liquefaction at deeper levels, site-specific design criteria need to be developed.
Case 2 – Scourable Layer Only
If the soil is susceptible to scour only (consideration to be given to general and contraction scour) with no liquefaction, soil springs from the scourable layer or any layers above it must be removed from the pile analysis.
Case 3 – Combination of Potentially Liquefiable and Scourable Layers
If the liquefiable layer is below the scourable layer, pile analysis will use liquefied soil stiffness for the liquefiable layer and no soil springs for the scour layer. If the liquefiable layer is above the scourable layer, lateral analysis shall be performed assuming no soil springs for both layers. See Figure 2 below for reference:
It may be argued that in case of a stiff, non-liquefiable crust present above liquefiable layers in the upper 50 ft (Case 1), using softer soil springs could underestimate the passive pressure-induced demands on the piles and the pile cap. Similarly for Case 3 with the liquefiable layer above the scourable layer, while the proposed combination may result in conservative demands for the inertial loading, this assumption could lead to lower kinematic demands if the area is likely to experience lateral spreading during a seismic event. Above discussion shows that combination of scour and liquefaction can be complex in certain circumstances. Close coordination between the various experts is therefore imperative for quantifying the scour and liquefaction-related effects and coming up with a robust design solution.
References:
AASHTO LRFD Bridge Design Specifications, 9th Edition (2020)
British Columbia Ministry of Transportation: Bridge Standards and Procedures Manual, Volume 1, Supplement to CHBDC S6:19 (July 2022)
Caltrans Seismic Design Criteria Document Version 2.0 (April 2019) and October 2019 Interim Revisions
Canadian Highway Bridge Design Code, CSA-S6-19 (2019)
Commentary to The Canadian Highway Bridge Design Code, CSA-S6-19.1 (2019)
Authors:
Saqib Khan, P.Eng., SE, M.A.Sc., is Principal Engineer at Spannovation
Lalinda Weerasekara, Ph.D., P.Eng. is Principal Geotechnical Engineer at ECORA
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