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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition

Appendix D

Riprap Inspection Recording Guidance

To guide the inspection of a riprap installation, a recording system is presented in this appendix. This guidance establishes numerical ratings from 0 (worst) to 9 (best). Recommended action items based on the numerical rating are also provided (Lagasse et al. 2006).

A single-digit code is used as indicated below to identify the current status of the rock riprap regarding its condition compared to the design intent, and the immediacy of need for maintenance activities to return it to the design condition.

This guidance covers riprap installations that may be:

  1. located on stream banks for lateral stream stability purposes;
  2. placed against bridge piers or abutments for protection against scour at the structure;
  3. placed across the stream to provide vertical grade stabilization; or
  4. other applications in riverine environments (e.g., guide banks or spurs).
Code
Description
U
UNINSPECTABLE:
The riprap is uninspectable, due to burial by sediment, debris, or other circumstance. Until the condition of the riprap can be reliably determined, a plan of action should be developed that considers the degree of risk posed by potential failure of the installation.
9
THE RIPRAP INSTALLATION IS STABLE:
Riprap stones are angular to subangular with no evidence of deterioration or segregation of sizes; and the distribution of stone sizes and overall thickness of riprap layer conform to design specifications; and there is no evidence of displacement of individual stones.
8
THE RIPRAP INSTALLATION IS STABLE:
Riprap stones are angular to subangular with no evidence of deterioration or segregation of sizes; and the distribution of stone sizes and overall thickness ofriprap layer conform to design specifications; and some displacement ofindividual stones is evident, but only smaller sized particles significantly smallerthan the design d 50 size have moved.
7
THE RIPRAP INSTALLATION IS STABLE:
Evidence of some deterioration of stones due to surficial weathering (abrasion, freeze-thaw or wet-dry spalling); and stone shape is primarily subangular.
OR
A minor decrease in overall layer thickness is evident, and/or particledisplacement noted with displaced particles approaching the design d 50 size; and the geotextile or granular filter has NOT been exposed.
6
THE RIPRAP INSTALLATION HAS EXPERIENCED EROSION:
Individual stones are primarily subrounded in shape due to surficial weathering; and the distribution of stone sizes still exhibits a d 50 particle greater than theminimum allowable d 50 size.
OR
Minor decrease in overall layer thickness is evident; and some particles greaterthan the design d 50 size have been displaced; and the geotextile or granularfilter has NOT been exposed.
5
THE RIPRAP INSTALLATION HAS EXPERIENCED EROSION:
Similar condition as Code 6, except that the geotextile or granular filter hasbeen exposed in local areas, or around the periphery of the installation. Theinspector should attempt to identify whether stone displacement has occurreddue to gravity slump or slide, or by hydraulic forces.
4
THE RIPRAP INSTALLATION HAS EXPERIENCED SIGNIFICANT EROSION:
Individual stones are subrounded to rounded in shape due to significant deterioration, and the distribution of stone sizes exhibits a d 50 particle smallerthan the minimum allowable d 50 size.
OR

Significant decrease in overall layer thickness is evident in local areas; and some particles greater than the design d 50 size have been displaced; and the geotextile or granular filter has been exposed in local areas.
3
THE RIPRAP INSTALLATION IS UNSTABLE:
The riprap matrix consists primarily of stones smaller than the minimumallowable d 50 particle size; and the overall layer thickness is less than 50% ofspecification.
OR
A significant portion of the particles greater than the design d 50 size has beendisplaced, and the geotextile or granular filter has been exposed over morethan 20% of the installation area.
2
THE RIPRAP INSTALLATION IS UNSTABLE:
The riprap matrix consists almost entirely of stones smaller than the minimumallowable d 50 particle size; and the overall layer thickness is less than 2 particlesthick.
OR
Most of the particles greater than the design d 50 size has been displaced, and the geotextile or granular filter has been exposed over more than 50% of the installation area.
1
THE RIPRAP INSTALLATION IS ERODED AND CAN NO LONGER SERVE ITS FUNCTION. IMMEDIATE MAINTENANCE IS REQUIRED:
Most of the riprap matrix has been displaced or is missing; and native subgradesoil is exposed.
OR
Large patches or voids in the riprap matrix have been opened; and stones areno longer in contact with structural elements.
0
THE RIPRAP INSTALLATION IS ESSENTIALLY GONE AND SCOUR IS IMMINENT. IMMEDIATE MAINTENANCE IS REQUIRED:
The riprap has deteriorated to the point that it cannot perform its protectivefunction even in minor events.

Particle Size Distribution. During inspection, the existing particle size distribution should bedetermined and compared with the design particle size distribution to assess whether theriprap particles have deteriorated over time. NCHRP Report 568 (Lagasse et al. 2006) andDesign Guideline 4 provide guidance for determining particle size distribution in the field.

Recommended action.

Code U: The riprap cannot be inspected. A plan of action should be developed to determinethe condition of the installation. Possible remedies may include: removal of debris,excavation during low flow, probing, or nondestructive testing using ground penetrating radaror seismic methods.

Codes 9, 8, or 7: Continue periodic inspection program at the specified interval.

Codes 6, 5, or 4: Increase inspection frequency. The rating history of the installation shouldbe tracked to determine if a downward trend in the rating is evident. Depending on thenature of the riprap application, the installation of monitoring instruments might beconsidered.

Codes 3 or 2: The Maintenance Engineer's office should be notified and maintenance shouldbe scheduled. The cause of the low rating should be determined, and consideration given toredesign and replacement. Materials other than standard riprap might be considered.

Code 1 or 0: The Maintenance Engineer's office should be notified immediately. Dependingupon the nature of the riprap application, other local officials and/or law enforcementagencies may also need to be notified.

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Updated: 09/15/2011

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