ROCKFALL MANAGEMENT PROGRAM
TDOT MATERIALS & TESTS DIVISION, GEOTECHNICAL ENGINEERING SECTION
November 3, 2017
Geotechnical Engineering Section 6601 Centennial Boulevard Nashville, TN 37243-0360 T 615-350-4132 F 615-350-4128
TDOT Rockfall Management Program November 3, 2017
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TDOT Rockfall
Management Program
November 3, 2017
Tennessee Department of Transportation
Geotechnical Engineering Section
6601 Centennial Boulevard
Nashville, TN 37243-0360
615-350-4132
Robert Jowers, P.E.
Authors Lori Fiorentino, P.E.
Executive The purpose of this document is to standardize and centralize the Tennessee
Summary Department of Transportation Rockfall Management Program (RMP). TDOT takes
rockfall hazards risk seriously. The proposed five year rockfall mitigation plan is
included. The intent of the program is to identify, prioritize, and mitigate the rockfall
slopes having the highest risk. This document provides a brief history of the RMP, and
describes the processes that will be used to manage the rock slope hazard risk of
Tennessee highways. This document will be assessed and updated as required to
meet the demands of TDOT rockfall risk management.
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TABLE OF CONTENTS
SECTION PAGE
CHAPTER 1: GENERAL ............................................................................................... 1
1.1 Rockfall Hazards and Safety .......................................................................................... 1
1.2 The History of TDOT’s Rockfall Management Program ................................................. 1
CHAPTER 2: TDOT ROCKFALL MANAGEMENT PRIORITIES .................................. 3
2.1 Rockfall Hazards Rating Score ...................................................................................... 3
2.2 Rockfall Mitigation Construction Plans Preparation ...................................................... 5
2.3 5-Year Rockfall Mitigation Project Priorities ................................................................... 7
CHAPTER 3: MONITORING OF TDOT ROCKFALL SITES ......................................... 9
3.1 Reassessment of Hazardous Rock Slopes .................................................................... 9
3.2 Rock Slopes with No TRHRS Rating ............................................................................. 9
3.3 Rock Slope Monitoring .............................................................................................................. 9
APPENDICES
A Memorandum – Re: TDOT Rockfall Management Program and Project Development
B Description of Rockfall Mitigation Techniques
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CHAPTER 1: GENERAL
1.1 Rockfall Hazards and Safety
Roadway excavation cuts in rock are capable of releasing loose debris, and becoming a
rockfall. Roadways constructed alongside existing rock cliffs present a danger of rockfall, as
well. The rock debris released during a rockfall event can vary greatly in size and scope. Some
examples of the circumstances that can make rockfalls hazardous to the public are listed below:
1. Rock can strike a vehicle or person directly.
2. Rock can fall within the roadway and create an obstruction. In an attempt to avert the
rock obstruction, automobile collisions occur.
3. Traffic control plans for rockfall mitigation projects often must detour traffic onto non-
arterial roads of inadequate traffic capacity. This is significant due to the nature of the
rockfall mitigation work often occurs in rural mountainous areas where limited roadway
access already exists.
All three of these scenarios occur and have occurred in the state of Tennessee.
Due to the nature of our rugged, mountainous terrain, Tennessee has many hazardous
rockfall sites. As time passes, these rockfall sites can become more hazardous, and less stable
due to the continued weathering and freeze thaw cycles that the rock slopes endure. The
weathering cycles cause the rock mass to loosen and\or move along existing joints and
discontinuities, potentially allowing rock fall debris to be released.
1.2 The History of TDOT’s Rockfall Management Program
It can be said that the TDOT Rockfall Management Program (RMP) began in earnest
when the NHI course No. 13219 (1994) Rockfall Hazard Mitigation Methods was delivered to
TDOT Geotechnical Engineering Section staff in December, 2000. FHWA developed the
course encouraging state DOT’s to begin evaluating the rock slope hazard potential using a
standardized rating system. The data derived from the rating system would be used as a
decision tool to set priorities for prioritizing and funding rockfall mitigation projects.
Later, a research project was commissioned to evaluate FHWA’s rockfall hazard rating
system on approximately two-thousand Tennessee rockfall sites that appeared to pose risk of
releasing rocks into the roadway. The rockfall site information was to be delivered as a
searchable inventory database. The research project began in 2001 and was completed in
2008, but was supplemented later to include a GIS interactive component. The research project
was well received. Figure 1 – Locations of “A” and “B” Sites and Table 1 – TDOT Regional
Hazardous Rockfall Sites illustrate partial findings. The information collected from that research
project was used to make priority lists, and also to monitor the risks of the individual rock slope
sites. TDOT could proactively identify rockfall mitigation projects to budget.
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TDOT Rockfall Management Program November 3, 2017
In 2007, TDOT programmed its first project described as a “Rockfall Remediation
Project.” Programmatically, the rockfall remediation was categorized within “Safety” projects.
Since then, TDOT has programmed twelve “Safety” contracts to remediate hazardous slopes
proactively, but dozens of other rock slopes have released rock into roadways during that time,
and were required to be mitigated reactively using emergency funding sources.
Figure 1
Locations of "A" and "B" Sites
Table 1
TDOT Regional Hazardous Rockfall Sites
“A” Sites “B” Sites
Region
(High to Very High Risk) (Moderate to High Risk)
1 582 629
2 288 166
3 100 188
4 2 1
In 2013, TDOT procured a rockfall engineering consultant to re-rate the most hazardous
rock slopes. The list became known internally as the “Top 46.”
After years of unreliable funding mechanisms for proactive rockfall mitigation projects,
the leadership of the Department made a commitment in 2016 to revamp the RMP, and begin
developing rockfall mitigation projects for purposes of letting to bidders and reducing risk
exposure due to rockfall hazards. Going forward, under TDOT’s database ‘PPRM’ the field
‘Project Type of Work:’ should be assigned “Mitigation – Rockfall.” Attachment A serves as the
authorizing document for the RMP.
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CHAPTER 2: TDOT ROCKFALL MANAGEMENT PRIORITIES
2.1 Rockfall Hazards Rating Score
In an effort to consistently evaluate the relative hazard “score” of rock slopes in a
standardized manner, FHWA introduced a Rockfall Hazard Rating System (RHRS). The RHRS
was introduced through an NHI instructional course offered to various state DOTs with rockfall
hazards. As a result of the NHI course, TDOT adopted the TDOT Rockfall Hazard Rating
System Field Sheet shown in Figure 2 that shall be used exclusively on TDOT rock slopes.
Each potential rockfall site is rated individually using the TDOT RHRS Field Sheet. The
field sheet gives eight different evaluation criteria. These criteria are an indication of the
individual rockfall site characteristics. Even though the geologic characteristics of a site may be
considered subjective between different raters, many of the other characteristics are
straightforward objective observations such as ADT, ditch width, slope height, curve data, and
site distance.
Using the TDOT RHRS Field Sheet, a total score is given relative to the hazardous
condition of the rock slope. These scores range from 0 to a maximum of 800. Higher scores are
indicative of greater rock slope hazard risks. Scores are further broken down by descriptive terms
of risk potential in Table 2.
Table 2
Rating Score Versus Risk Potential
Score Range Description
<200 Lower Risk Site
200 - 350 Moderate Risk Site
350 - 500 High Risk Site
>500 Very High Risk Site
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Figure 2
TDOT Rockfall Hazard Rating System Field Sheet
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2.2 Rockfall Mitigation Construction Plans Preparation
Guidance documents that are available for reference in roadway plans development
projects include, but are not limited to, the TDOT Standard Specifications for Roadway and
Bridge Construction, TDOT Design Guidelines, applicable TDOT Standard Drawings, and
relevant Special Provisions (SPs).
TDOT Special Provisions (SPs) listed in Table 3 have been developed for some of the
more common rockfall stabilization measures used in the industry. Please refer to these for
standardization, convenience, consistency, and clarity. The SPs contained in Table 3 should be
reviewed thoroughly and the basis of payment / unit item numbers thoroughly understood.
In case of discrepancy between the plans and the SPs, per Section 105.4 of the TDOT
Standard Specifications for Roadway and Bridge Construction, the Special Provisions
supersede.
Table 3
TDOT Special Provisions for Rockfall Mitigation
Designation Title
SP203E “Regarding Scaling and Trimming”
SP707D “Regarding Rockfall Slope Drape”
SP707E “Regarding Pinned Rockfall Slope Mesh”
SP707H “Regarding Rockfall Barrier Systems”
Within the SPs, the basis of payment section may include specific project pay item
numbers. These pay item numbers are provided in Table 4. Any information felt necessary to
clarify the use of an item should be appropriately footnoted within the estimated quantities
sheet.
Descriptions and illustrations of selected rockfall mitigation techniques, including the
techniques listed in Table 3, are included in Appendix B. Selection of a rockfall mitigation
technique for a particular project is dependent upon a number of factors, including but not
limited to the hazard score, project design life, availability of right-of-way, cost of construction,
roadway closure impacts, etc. Because site conditions and other factors (e.g., funding,
manpower) can vary widely, any one method, or a combination of methods, may be most
appropriate for a particular project.
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Table 4
Rockfall Mitigation Pay Item Numbers
COMMON ROCKFALL MITIGATION ITEM NUMBERS
ITEM NO. DESCRIPTION UNIT
PAY ITEMS FOR SP203E "REGARDING SCALING AND TRIMMING"
203-01 ROAD & DRAINAGE EXCAVATION (UNCLASSIFIED) C.Y.
203-11 SCALING AND TRIMMING S.Y.
PAY ITEMS FOR SP707D "REGARDING ROCKFALL SLOPE DRAPE"
707-02.41 ROCK ANCHOR, TYPE I EACH
707-02.42 ROCK ANCHOR, TYPE II EACH
707-02.43 ROCK ANCHOR, TYPE I L.F.
707-02.44 ROCK ANCHOR, TYPE II L.F.
707-10.05 ROCKFALL DRAPE (TYPE I) S.Y.
707-10.06 ROCKFALL DRAPE (TYPE II) S.Y.
707-10.07 ROCKFALL DRAPE (TYPE III) S.Y.
707-10.21 ROCKFALL DRAPE TYPE IV (A) S.Y.
707-10.22 ROCKFALL DRAPE TYPE IV (B) S.Y.
PAY ITEMS FOR SP707E "REGARDING PINNED ROCKFALL SLOPE MESH"
203-40.01 ROCK ANCHORS (FOR PINNED MESH) EACH
203-40.02 ROCK ANCHORS (FOR PINNED MESH) L.F.
707-02.41 ROCK ANCHOR, TYPE I EACH
707-02.42 ROCK ANCHOR, TYPE II EACH
707-02.43 ROCK ANCHOR, TYPE I L.F.
707-02.44 ROCK ANCHOR, TYPE II L.F.
707-10.05 ROCKFALL DRAPE (TYPE I) S.Y.
707-10.06 ROCKFALL DRAPE (TYPE II) S.Y.
707-10.07 ROCKFALL DRAPE (TYPE III) S.Y
PAY ITEMS FOR SP707H "REGARDING ROCKFALL BARRIER SYSTEM S"
707-10.01 ROCKFALL FENCE (TYPE I) L.F.
707-10.02 ROCKFALL FENCE (TYPE II) L.F.
707-10.03 ROCKFALL FENCE (TYPE III) L.F.
707-10.04 ROCKFALL FENCE (TYPE IV) L.F.
707-10.08 WIRE MESH (DESCRIPTION) S.Y.
OTHER USEFUL ROCKFALL M ITIGATION PAYMENT ITEM NUMBERS
203-01 ROAD & DRAINAGE EXCAVATION (UNCLASSIFIED) C.Y.
203-01.01 ROAD & DRAINAGE EXCAVATION (SLIDE AREA) C.Y.
203-01.07 RD & DRNG EXCV (ACID PRODUCING- OFF SITE C.Y.
203-01.08 RD & DRNG EXCV (ACID PRODUCING- ON SITE C.Y.
203-01.09 ACID PRODUCING MATERIAL HAUL & TIP FEE TON
203-01.11 PRESPLITTING OF ROCK EXCAVATION S.Y.
203-01.12 ORIENTED PRESPLITTING OF ROCK EXCAVATION S.Y.
203-02.01 BORROW EXCAVATION (GRADED SOLID ROCK) TON
203-02.02 BORROW EXCAVATION (GRADED SOLID ROCK) C.Y.
203-40.02 ROCK ANCHORS (DESCRIPTION) L.F.
203-40.03 ROCK ANCHORS (DESCRIPTION) L.F.
203-40.04 ROCK ANCHORS (DESCRIPTION) L.F.
203-40.05 DRILLING L.F.
203-40.15 POST TENSION ROCK BOLT L.F.
203-40.16 ROCK BOLT PULL TEST EACH
203-40.17 ROCK DOWEL L.F.
610-12.01 HORIZONTAL DRAINS AND CASING L.F.
610-12.02 HORIZONTAL DRAINS L.F.
610-12.03 HORIZONTAL DRAIN OUTLET EACH
622-01.01 SHOTCRETE BUTTRESS C.Y.
622-01.02 STEEL REINFORCEMENT- SHOTCRETE BUTTRESS LB.
622-01.05 SHOTCRETE 6 IN THICKNESS S.F.
622-01.06 SHOTCRETE 12 IN THICKNESS S.F.
622-02.01 STEEL MESH & PLATES S.F.
805-05.01 LAUNCHED SOIL NAIL EACH
805-05.02 SOIL NAIL STABILIZATION L.F.
805-05.03 REINFORCED SHOTCRETE FACING S.F.
805-10.01 ROCK SLOPE WIRE MESH W/EROSION BLANKET S.F.
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TDOT Rockfall Management Program November 3, 2017
2.3 5-Year Rockfall Mitigation Project Priorities
Based on the data driven scores of the TRHRS (geology, ADT, ditch width, etc.), and
collaboration with key personnel of TDOT Regional Operations Division, the list of rockfall
mitigation projects contained in Table 5 captures the current TDOT project delivery priority, as of
the date of this document. Table 5 will be updated annually.
The project list is intended to be a delivery scheduling tool. Future funding mechanisms
could affect the project delivery schedule. Specific rockfall sites may be added, replaced, or
moved up in priority to accommodate changes in rockfall risks to the motoring public or in the
delivery schedule.
Input from the TDOT Regional Operations Division regarding rockfall sites that pose
risks will be sought annually. If there is ever dramatic, sudden increase in rockfall risks in the
state roadway system, contact the GES.
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CHAPTER 3: MONITORING OF TDOT ROCKFALL SITES
3.1 Reassessment of Hazardous Rock Slopes
The TDOT Geotechnical Section will keep the rockfall database inventory current.
Frequency of rock slope re-rating and evaluation shall be determined in accordance with
Table 6.
Table 6
Rockfall Database Inventory Reevaluation Frequency
TRHRS Score Range Reevaluate
<300 As Requested
300 - 500 Every 5 yrs.
>500 Every 3 yrs.
Upon review of these established slope rating frequencies, and considering there are
2,000+ monitored rockfall sites across Tennessee, it is observed that over 300 sites are
required to be re-rated annually to achieve the stated outcomes.
3.2 Rock Slopes with No TRHRS Rating
Even though there are over 2,000 rockfall sites monitored in the GES rockfall database,
it is not uncommon to be notified of a non-monitored rock slope asset that could pose risk.
Typical notifications arrive from the TDOT Operations Division - Maintenance forces, but
concerned motorists also provide notifications. Non-monitored rock slopes that appear to
present risk will be initially visited and preliminarily evaluated by GES, then a TRHRS score will
be initiated if required.
3.3 Rock Slope Monitoring
Rock slopes having higher risk scores relative to others may warrant continued field
monitoring. The level of warranted rock slope monitoring will be considered on a site by site
basis. Weekly visual monitoring may be adequate for some sites, while other sites could require
more sophisticated monitoring using advanced instrumentation.
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APPENDICES
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TDOT Rockfall Management Program November 3, 2017
Appendix A
Memorandum – Re: TDOT Rockfall Management Program
and Project Development
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Appendix B
Description of Rockfall Mitigation Techniques
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Decription of Rockfall Mitigation Techniques
For highway applications, the purpose of rockfall mitigation is to reduce the potential for rockfall
to occur (stabilization), or to control rockfalls once they occur, thereby reducing the potential for
rockfall to enter the roadway or other facility (protection). Because protection methods allow
rockfall to occur, periodic maintenance of the slope and catchment area is required.
Descriptions and illustrations of several rockfall mitigation techniques are presented in this
appendix.
Mitigation Method: Cut Back Slope
Mitigation Category: Stabilization
This technique can be used to “shift” the rock slope further from the roadway. Blasting is used
to cut the slope further from the existing roadway, thereby creating additional rockfall catchment
area at the base of the slope (Figure 1). Another common technique is to flatten the existing
slope to a more stable configuration. These techniques frequently require purchase of additional
right-of-way. Other considerations may include utility relocation and traffic maintenance during
construction.
Clay
Existing Slope
Proposed Slope
Shaley Limestone Bedrock Proposed Existing Existing
Additional Catchment Roadway
Catchment Area
Area
(Source: TDOT)
Figure 1
TDOT Rockfall Management Program Appendix B
November 3, 2017 Page B-2
Decription of Rockfall Mitigation Techniques
Mitigation Method: Rock Bolting
Mitigation Category: Stabilization
Rock bolting may be used to stabilize a large
unstable piece of rock in place, rather than
bringing it down using blasting or other
mechanical means. Rock bolts are long
anchor bolts which are installed through the
large unstable piece rock and into the
massive bedrock behind the unstable rock,
thereby attaching the unstable piece to the
larger rock mass. Rock bolts are typically
installed in a series with design lengths and
spacing specified in the project plans.
Figure 2 depicts installation of rock bolts.
(Source: Transportation Research Board, Rockfall Characterization and Control, 2012)
Figure 2
Mitigation Method: Shotcrete
Mitigation Category: Stabilization
Shotcrete (a concrete or mortar that is spray-
applied) may be applied to the face of highly
erodible rock units (such as shale), Limestone
particularly when the highly erodible unit
underlies a more competent rock unit (such as
limestone or sandstone). Exposure to air and
water accelerates the weathering process,
and the shotcrete protects the highly erodible
rock from exposure. Shotcrete provides
limited structural reinforcement but can help
retain small rock pieces. Drains are installed Shotcrete
through the shotcrete face to prevent buildup
Drains
of water pressure behind the shotcrete. The
lifespan of this type of treatment varies, but Limestone
future reapplication of shotcrete should be
expected.
(Source: TDOT)
Figure 3
TDOT Rockfall Management Program Appendix B
November 3, 2017 Page B-3
Decription of Rockfall Mitigation Techniques
Mitigation Method: Scaling
Mitigation Category: Stabilization
Scaling is the removal of overhanging, protruding, loose, and unstable rock from an existing
slope. Scaling may be selected as the only treatment for a particular project. However, scaling is
always performed following selected stabilization measures, such as cutting back the slope, and
prior to installation of other stabilization measures, such as slope drape, pinned mesh, rockfall
fence, and shotcrete.
Hand-scaling (Figures 4-6) is performed by experienced personnel hanging from ropes or
working from a basket on a crane, and using pry bars, hydraulic splitters, air bags, or chemical
expanders (expansive grout).
Mechanical scaling is performed using power equipment, such as trackhoes. Another common
mechanical scaling method is dragging a heavy object, such as a blasting mat, crawler track
(Figure 7), or slusher (Figure 8), across the slope, or up and down the slope, to abrade loose
rocks.
Personnel are hand-scaling from ropes.
(Source: FHWA Rockfall Hazards Mitigation Methods Participant Workbook, March 1994)
Figure 4
TDOT Rockfall Management Program Appendix B
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Decription of Rockfall Mitigation Techniques
Mitigation Method: Scaling (Continued)
Personnel are hand-scaling from a basket
mounted on a telescoping boom vehicle.
(Source: FHWA Rockfall Hazards Mitigation Methods Participant Workbook, March 1994)
Figure 5
In Figure 6, personnel are positioning an air pillow in the crack between a large piece of
potentially unstable rock and the intact rock face behind it. Once in position, the air pillow will be
inflated, thereby dislodging the potentially unstable rock.
Air Pillow Air Pillow
(Source: YouTube, WSDOT Rock Scaling on Sherman Pass)
Figure 6
TDOT Rockfall Management Program Appendix B
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Decription of Rockfall Mitigation Techniques
Mitigation Method: Scaling (Continued)
Crawler tracks (e.g., tracks found on a
bulldozer or an excavator) are being dragged
across the rock face with a crane to abrade or
slough larger loose rocks. Final hand scaling
may be required to remove smaller rocks.
(Source: FHWA Rockfall Hazards Mitigation Methods Participant Workbook, March 1994)
Figure 7
A slusher is a bucket-type device that has
cutting teeth on one edge, similar to a
trackhoe bucket. A cable and pulley system is
used to raise and lower the slusher on a slope,
dislodging loose rock and other debris.
Figure 8 depicts a slusher on its trip down the
slope.
(Source: TDOT)
Figure 8
TDOT Rockfall Management Program Appendix B
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Decription of Rockfall Mitigation Techniques
Mitigation Method: Trimming
Mitigation Category: Stabilization
Trimming is the process of removing overhangs of loose rock, unstable rock, or both, from a
slope by mechanical percussion (e.g., hydraulic rock breakers) and/or blasting methods.
Long-reach excavators (trackhoes) fitted with
hydraulic rock breakers (“hoe rams”) are
operating from the top of the slope (mid-photo)
and from the bottom of the slope (boom arm is
mid-photo, cab is to the right, out of photo) to
remove loose and protruding rock from the
slope.
(Source: FHWA Rockfall Hazards Mitigation Methods Participant Workbook, March 1994)
Figure 9
TDOT Rockfall Management Program Appendix B
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Decription of Rockfall Mitigation Techniques
Mitigation Method: Rockfall Slope Drape
Mitigation Category: Protection
Rockfall slope drape, typically consisting of wire mesh, cable net, or rope net, is draped across
a rock slope to prevent rock fall from entering the roadway. The slope drape hangs loose just
above the bottom of the slope, thereby directing rock fall to the catchment area at the base of
the slope where it is removed by maintenance forces.
A long rectangular panel of wire mesh is being
attached to ground anchors above the crest of
the slope and draped down the rock face with
the assistance of a helicopter.
(Source: TDOT)
Figure 10
TDOT Rockfall Management Program Appendix B
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Decription of Rockfall Mitigation Techniques
Mitigation Method: Rockfall Slope Drape (Continued)
The fellow in the orange shirt is placing
fasteners to connect the current panel of mesh
to the previous panel of mesh. Each panel will
be joined to its adjacent panels in this manner,
resulting in one continuous drapery panel
across the rock face. The completed mesh
drapery will allow any loose rock that falls from
the rock face to ravel down behind the mesh to
the catchment area (without bouncing out into
the roadway) for removal by maintenance
forces.
(Source: TDOT)
Figure 11
A rockfall slope drape project on SR-115 in
Blount County nearing completion.
(Source: TDOT)
Figure 12
TDOT Rockfall Management Program Appendix B
November 3, 2017 Page B-9
Decription of Rockfall Mitigation Techniques
Mitigation Method: Pinned Rockfall Slope Mesh
Mitigation Category: Protection
Pinned rockfall slope mesh takes the rockfall slope drape system and “pins” it to the slope using
anchors installed into the slope face. The anchors are installed on a pattern, and the anchor
length and location are indicated on the project plans. Unlike the free-hanging rockfall slope
drape, the pinned mesh retains pieces of rock that dislodge from the rock face. Depending upon
the size of the dislodged rock piece, it cannot fall any further than the closest downslope anchor
location. Drills suspended from cranes are typically required to drill holes in the rock face for
installation of the anchors.
Personnel are using a ‘wagon drill’ suspended
from a crane to drill holes in the rock face for
the anchors that will be used to pin the mesh
to the rock slope face.
(Source: TDOT)
Figure 13
The pinned mesh retains unstable rock
fragments between anchor locations.
(Source: Caltrans Rockfall Mitigation Design Packet))
Figure 14
TDOT Rockfall Management Program Appendix B
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Decription of Rockfall Mitigation Techniques
Mitigation Method: Rockfall Barrier Systems
Mitigation Category: Protection
Rockfall barrier systems are commonly referred to as rockfall fence. The rockfall fence is placed
between the base of the rock slope and the roadway (or other zone requiring protection) to
prevent fallen rock from entering the roadway or other protected zone. The height and location
of the rockfall fence are based in part upon the expected travel distance and bounce height of
the fallen rock.
Ring net rockfall fence placed at the bottom of
the rock slope prevents fallen rock from
entering the adjacent roadway.
(Source: TDOT)
Figure 15
Wire mesh rockfall fence placed at the bottom
of the rock slope prevents fallen rock from
entering the adjacent roadway.
(Source: TDOT)
Figure 16
