Section 8: Scour

Overview

Incorporate the effects of scour in the determination of shaft and pile penetration. Design the foundations so that the penetration and resistance remaining after the design scour events satisfies the required nominal axial and lateral resistances. Both reduced geotechnical resistances and increased unsupported length in the columns must be accommodated for in design.

Scour at the foundations is not a force effect. However, scour can change the substructure conditions and topography and alter the consequences of force effects acting on the structure and foundations. AASHTO LRFD Bridge Design Specifications Article 2.6.4.4.2 requires changes in foundation conditions resulting from the design flood be evaluated at the strength and service limit states. Foundation condition changes from the check flood are to be considered and evaluated at the extreme event limit state.

Refer to the

TxDOT Scour Analysis Guide

for background to aid when determining total scour on any specific design flood and check flood for design purposes.

Design the foundations to resist debris loads occurring during flood events in addition to the loads applied from the structure.

LRFD Design

Use the same resistance factors when evaluating conditions with scour at the strength limit state as those used without scour. Do not include the axial resistance of the material lost due to scour in the shaft resistance.

Include on the plans the unfactored resistance to be achieved during construction for the unscoured bridge condition. This resistance will be the minimum target to achieve during dynamic analysis of pile installation.

Driven Piles and Scour

Design pile foundations such that the pile penetration after the design scour events satisfies the required nominal axial and lateral resistance. At pile locations where scour is predicted, the nominal axial resistance of the material lost due to scour should be determined using a static analysis. The piles will need to be driven to the required nominal axial resistance plus this nominal skin friction resistance that will be lost due to scour.

∑ηiγiQ_i ≤ ϕR_n

Nominal resistance needed (R_n) in the static final condition after compensating for design scour must be greater than the factored loads (∑ηiγiQi). Include an additional resistance factor (ϕdyn) for the driving resistance checks during construction based on the dynamic method used:

R_n ≥ (∑ηiγiQ_i) / ϕdyn

Normal pile driving resistance achieved during construction (R_ndr) includes the skin friction (side resistance) contribution that would be lost in the scour zone:

R_ndr = R_n + R_scour

R_ndr = Nominal (ultimate) resistance during pile driving, dynamically evaluated

R_n = Nominal (ultimate) resistance needed in the final static condition

R_scour = Unfactored skin friction which must be overcome during driving in scour zone (kips)

Q_p = (∑ηiγiQi) = factored load per pile (kips)

ϕdyn = resistance factor

D_est. = estimated pile length needed to obtain desired nominal resistance per pile (ft)

Scour Coding, Inspection, and Countermeasures

The Bridge Division establishes program requirements and provides geotechnical subject matter expertise for the determination of soil characteristics to be used for scour analyses and for the phases of scour evaluation that occur after a scour analysis: bridge inspection documentation, screenings, assessments, and scour countermeasures. This guidance can be found in the

TxDOT Scour Evaluation Guide

Stone Protection at Bridges

Protecting abutments and piers at bridges is beneficial in limiting the effects of scour. Use flexible armoring (i.e. stone protection) for wet crossing structures. Concrete riprap, due to its rigidity, masks problems. Consequently, voids can form under them and eventually undermine the pavement or approach slab. Guidance on the use of stone and sizing and thickness to specify can be found in Chapter 11 of the

TxDOT Scour Evaluation Guide

In the plans Stone Protection should be specified and called out as follows (on each abutment side or location of placement):

Riprap (Stone Protection) XX in.
(where XX is the size in inches)

Thickness = YY in.
(where YY is the appropriate thickness)