MOD258(1) Blue Lake Geotech Report_Final.pdf

To: Michael Daigler, Project Manager, CFLHD, Lakewood CO

From: Todd Hansen, Geotechnical Engineer, CFLHD, Lakewood CO

Through: Marilyn Dodson, Lead Geotechnical Engineer CFLHD, Lakewood CO

Subject: Blue Lake Road Rockfall Assessment, Modoc County, California

CA FLAP MOD258(1)

I. PROJECT BACKGROUND INFORMATION

This report presents the rockfall stability findings following the geotechnical site visit to the Blue Lake Road project. The site is located in Modoc County, east of Likely, California, and includes the upper segment of Jess Valley Road/Modoc County Road 64 (CO RD64) and Blue Lake Road in the Warner Mountains of the Modoc National Forest. This heavily used recreation area is popular for forest access, great camping, good fishing and free-range cattle ranching. The intent of this trip was to investigate and classify the two identified rockfall sources along the upper segment of CO RD 64. The map in Figure 1 shows the general section of road included in this project.

A site investigation and partner agency meeting with Modoc County Road Department (MCRD) was performed July 24, 2020. Todd Hansen and Brendan McGarity with Central Federal Lands Highway Division (CFLHD) geotechnical group assessed the two rockfall slopes and discussed ongoing concerns with Mitch Crosby (MCRD Commissioner) and Zach Degraeve (MCRD engineer). Modoc County expressed concerns with rocks in the ditch at the two identified problem spots. These locations were identified by MCRD as ongoing concerns that the county is unable to fully address with their limited resources and manpower.

As stated by MCRD the county is under resourced but does what it can with available equipment and personnel to actively perform ditch inspection and maintenance. Based on conversations with local staff, the last time any ditch treatment was performed on CO RD64 was the fall of 2019 and more rocks had accumulated in the ditches since then. Therefore, Geotechnical Memorandum

Date: February 16, 2021 there is need to increase ditch capacity for rockfall storage as it permits more time between maintenance cleanings.

Figure 1 – Map of CO RD64 and Blue Lake Road.

II. GEOLOGIC CONDITIONS

The project is on the northwestern margin of the Basin and Range province, a large-scale tectonic spreading of continental crust that extends across Utah, Nevada and eastern California. Locally, the project is located within Modoc National Forest in northeast California, along the western slope of the Warner Mountain range. The Warner Mountain range trends north-south, extending 85 miles through northeastern California into southern Oregon. The range developed during the mid-Miocene after approximately 8 km of dip-slip

Blue Lake Road

Jess Valley Road/CO RD64 movement along the Surprise Valley fault on the east side of the range and flexure on the west (Egger and Miller, 2011). The range displays horst and graben topography and contains numerous ephemeral lakes. The area is predominantly underlain by Pliocene and Miocene volcanic rocks, with minor areas of older tertiary volcanics. Most of the volcanic rocks occur in layered sequences, but are discontinuous laterally, and may change significantly in thickness over short distances. Volcanic rocks within the area are generally composed of rhyolite, andesite, basalt, and pyroclastic rocks. Minor, nonmarine sedimentary rocks may be exposed, but are primarily composed of reworked volcanic debris from nearby eruptions (Grose et al., 2016). At the surface, glacial, stream, and lake deposits may be found scattered throughout the region. Volcanic activity has also occurred more recently, depositing younger flood basalt flows and pyroclastic rocks. This recent activity is largely located both to the west and north of the project area.

A. Site Geology

The project area is directly underlain by the Warner basalt, identified below in Figure 2 as Miocene volcanic basalt (Mvb). The Warner basalt was deposited approximately 14 to 16 Ma during the Miocene, and can be described as vesicular to very fine grained, dense, and locally platy (Grose et al., 2016). It can vary in thickness up to 600 meters and generally dips 15° to the west. Similarly, the areas to the north and south of the project are underlain by Pliocene aged basaltic rocks (Pvb) dated between 5.333 – 2.58 Ma. These basalt flows originated from shield volcanoes and may contain an abundance of olivine (Gay and Aune, 1958). Lastly, the area to the west, and a minor area to the north, are underlain by Pliocene nonmarine sedimentary rocks. These rocks consist of diatomaceous sandstone, shale, tuff, pumiceous sandstone, shale, and siltstone (Gay and Aune, 1958). The region is still tectonically active.

Possible large-scale, ancient landslide deposits along CO RD64 were identified during the site investigation and are previously unmapped. The evidence for possible slides in Figure 3 is supported by previous research of large-scale earthquake-induced landslides in the region.

These are dormant slides that were initiated by tectonic events millions of years ago and unless severe precipitation events (rain, flooding) or earthquakes occur, these slides do not pose a threat to reactivate into the road.

Figure 2 – Geologic map of the Alturas 30’x60’ Quadrangle (Grose et. al., 2016).

Site 1

Site 2

Figure 3 – Previously unmapped ancient landslides near Site 2 (USGS hillshade).

B. Seismicity

Numerous seismic source faults are mapped within 40 miles of the project area. The critical active faults to the project site are based on the Caltrans ARS Online tool and may be found in Table 1 (CalTrans, 2020; U.S. Geological Survey, 2020). The fault systems surround the project and vary in length from 21 to 57 miles. Fault systems include right lateral and normal faults, generally striking northwest. The Surprise Valley fault is the most active fault system, exhibiting a slip rate of 0.0276 inches per year, with a maximum moment magnitude of 7.2.

Recommended seismic response parameters for the Blue Lake Road rehabilitation site design are based on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications, 8th Edition, 2017, and represents horizontal peak ground acceleration (PGA) with 7 percent probability of exceedance in 75 years (approximate 100-year return period). The 1000-year return period uniform hazard spectrum for the Blue Lake Road Project site, located approximately at 41.235275°N latitude &

120.384342°W longitude, was obtained in accordance with the AASHTO ground motion maps.

Based on near-surface rock at the site, the project area is classified as Class B according to the site class definitions specified in Table 3.10.3.1-1 of AASHTO. The recommended spectral acceleration coefficient values for probabilistic design are summarized in Table 2.

Table 1 - Summary of Nearby Seismic Source Faults

FAULT OR

FAULT ZONE

MAXIMUM

MOMENT

MAGNITUDE

DISTANCE

FROM

CENTER OF

PROJECT

FAULT

PARALLEL

SLIP RATE

FAULT

LENGTH

AGE

(Mmax) (miles) (inch/year) (miles) (years) Likely fault 7 9.32 0.0118 57.17 <750,000

Likely fault -secondary trace

7 9.93 - - <750,000

Likely (unnamed fault)

6.7 10.30 - - <750,000

Surprise Valley

2011 CFM

7.2 14.06 0.0276 53.44 <15,000

Nelson Corral fault

6.4 10.15 - 21.13 <750,000

Table 2: Summary of Seismic Parameters Corrected for Site Class B

Factored Horizontal Peak Ground Acceleration (As) 0.209g Factored Horizontal Response Spectral Acceleration at Period of 0.2 sec

(SDS) 0.504g Factored Horizontal Response Spectral Acceleration at Period of 1.0 sec

(SD1) 0.188g

Site Factor at Zero-Period of Acceleration Spectrum (FPGA) 1

Site Factor at Short-Period of Acceleration Spectrum (FA) 1

Site Factor at Long-Period Range of Acceleration Spectrum (FV) 1

Based on the long acceleration coefficient SD1 value of 0.188, the project site is assigned to seismic hazard Zone 2 in accordance with Table 3.10.6-1 of AASHTO. Seismic hazard zones reflect the variation in seismic risk in different regions needing different requirements for design as depicted in Table 4.7.4.3.1-1 in AASHTO.

III. ROCKFALL SITE INVESTIGATION

Two rockfall sites are located along CO RD64 in route to Blue Lake Road. Rockfall sources, Site 1 and Site 2, were first identified during the October 2017 scoping trip with input from MCRD to the CFL team. An overview of the rockfall sites on CO RD64 are shown in Figure

4. The Modoc County Road Department routinely cleans out rock and debris from ditches below the road cut slopes, which they have observed as unstable. Modoc County does not feel comfortable continuing to clean up the ditches under these cut slopes and expose their personnel to potential rockfall. Additionally, the County was unsure if the rocks and debris are originating from the cut slope next to the road or from a source higher above. It appeared that rocks and debris in the ditch had only come from the road cuts, but further investigation by CFL geotechnical engineers identified additional possible rockfall sources above.

Figure 4 – Rockfall evaluation locations, Site 1 and Site 2, on CO RD64.

Site 1

A. CO RD64 Rockfall Site 1

Rockfall Site 1, located approximately between Station 3+25 to 5+00, was measured to a vertical height of 115-feet, with multiple benches able to provide possible rock sources. Based on site observations, the rock in the ditch appears to be coming primarily from the 37-ft long slope directly above the ditch. No markings or impacts were observed on the slope from above the closest outcrop, which indicates the upper section of the slope is more stable than the rock cut. Figure 5 and 6 below presents the approximate limits of the Site 1 rockfall area and character of the rockfall source area.

The ongoing unstable slope conditions are present in the road cut because when this section of the road was excavated, the toe of a paleolandslide overlying an ancient stream gravel deposit was exposed (Figure 6). This paleolandslide deposit is an inactive feature as no evidence of apparent slope movement or reactivation of the landslide was observed. In this location, the entrained slide debris includes large rocks within an easily erodible fine-grained sediment matrix that can become saturated during extreme storm events. Under intense precipitation or heavy snowmelt there is potential for the block-in-matrix landslide deposit to become saturated, leading to increased chances of large rock blocks becoming destabilized and falling into the ditch.

While no evidence of rockfall from the upper terraces was observed, there is potential that additional rock may occasionally reach the ditch from higher sources. The 66-ft long flatter grade slope above the rock cut is not as likely to produce a rockfall event because it is already close to the angle of repose for a rock talus debris field. If continued monitoring of the upper slope identifies new areas of concern or conditions change since our field investigation, the CFL geotechnical engineers should be notified for further assessment.

Figure 5 – Plan view of Rockfall Site 1.

Figure 6 – Estimated limits of paleolandslide overlying gravel alluvial deposits, Site 1.

B. CO RD64 Rockfall Site 2

Rockfall Site 2, located approximately between Station 6+50 and 9+00, consists of a massive talus slope underneath a tall basalt cliff, which has formed by the repeated rollout of large rockfall events. The CFL geotechnical team assessed both the cliff highwall and the talus debris field as possible sources of rockfall, taking measurements of each in the field to assess the possible drop height and distances to the road. Modoc County expressed concern of a possible rockfall reaching the road and hitting traffic or workers performing maintenance.

Based on discussions with the County there was no recorded or anecdotal history of a rockfall ever being witnessed or reaching the roadway. The CFL geotechnical engineers also had the good fortune to meet with a local ranch owner and pioneer descendant who anecdotally shared the community had not heard of any cases of a rockfall blocking the road since it was first established as access for the Jess Valley ranches.

All rock in the ditch appears to have come from the 46-ft long road cut that currently is approximately at a 1V:1H slope. Figure 7 and 8 below presents the approximate limits of the

Site 2 rockfall area and character of the rockfall source area. The rock in the talus slope shown in Figure 8 has originated from the basalt cliffs above, but based on our observations no high-sourcing rockfall has recently reached the roadway. This determination is based on field observations of the weathering stained rocks at the base of the talus slope rocks with no marks indicative of impacts. Within the talus field are pockets of vegetation growth, indicative of soil development and historically stable conditions. Several hundred feet from the road, at the top of the talus slope we did document a few fresh, unweathered rocks which could only have sourced from an active rockfall. The size and location of these recent rockfall is evidence of the inability of rockfall from the basalt cliff to reach the roadway across the long and hummocky talus field.

Talus rocks range in size, averaging 3-ft minimum dimension (equivalent of 1-yd3), with the largest measured boulder at 6’x8x’12’ (21-yd3). Additionally, spring water was observed flowing out of a wooden box culvert at the base of the cut and locals indicated a pipe was used to drain the spring into the ditch. This spring likely contributes to slope instability at this location.

Figure 7 – Plan view of Rockfall Site 2.

Figure 8 – Existing 1V:1H cut slope above ditch (rockfall source area) with large talus field above.

IV. ROCKFALL MODEL ANALYSIS

Rockfall modeling analysis was performed to estimate maximum possible extent of a rockfall event impacting the proposed roadway. The use of catchment areas to contain and restrict rockfall from reaching the roadway is one of the best and most effective rockfall protective measures compared with slope treatments. Both evaluated sites have undersized catchment ditches which are meant to buffer and collect falling debris. This analysis evaluated the proposed alignments at both rockfall sites using the Colorado Rockfall Simulation Program version 4.0 (CRSP) and results are presented in Appendix A. CRSP accounts for irregularities in the existing slope profile utilizing the actual slope’s cross section in the analysis. This feature allows for greater accuracy in the results as it captures potential launching features attributing to decreased percent rockfall containment. The ditch configuration is based on the proposed cross section with a sloping grade pitched away from the pavement.

Appendix A displays the CRSP analysis for the percent of rockfall retained in the ditch for proposed cut slope and roadway alignment options. The percent retained assumes rockfall is sourcing locally from the cut slope directly above the road and not from higher sources.

Analysis evaluated spherical rocks with 1-ft, 2-ft, 3-ft, and 4-ft diameters.

CRSP models were developed utilizing field observations that spherical rocks with 1-ft, 2-ft, 3-ft, and 4-ft diameters could fall from the mid-slope and base of the proposed cuts. Greater detail on the CRSP methodologies and material parameters used in the analysis are given in Appendix A. When interpreting the CRSP results, recognize that spherical rocks are not found at this site and that during construction the larger boulders can be removed or downsized.

Performing this upfront mitigation work will significantly reduce the chances of rockfall sourcing from the proposed cut slope and improve overall site safety.

V. ROCKFALL SLOPE MITIGATION OPTIONS

A. CO RD64 Rockfall Site 1 Mitigation Options

The existing road cut at this location has an approximate 1V:1H slope with 37-ft length dipping into the ditch. Above this is another terrace slope which can produce rockfall but displayed no clear indications of recent events that sourced from a higher elevation. The ditch below the cut slope was measured in the field, ranging from 6 to 8-ft wide and consistently 1-ft deep. This ditch is currently filled with rockfall debris which is not regularly cleared due to safety considerations.

In the existing road cut the exposed paleolandslide deposit contains angular cobbles and boulders supported in a matrix of fine-grained sand and silt, which is easily erodible during rain or snowmelt events. The landslide overlies a terrace deposit of imbricated gravels from the South Fork Pit River, and the slide likely occurred after the river undermined the slope causing it to fail and bury the stream gravel deposit. Based on our observations of the existing road cut in a stable condition and the high probability of further slope destabilization if the road cut is expanded, the recommendation is for realignment away from the existing slope.

Extending the cut slope into the hillside will require additional slope stabilization measures such as a pinned-mesh system or rock reinforcement such as bolts or dowels. The nature of the terraced slope would require stabilization of both the excavated slope and the bench above, to ensure no further destabilization could occur above the intended project limits. Excavation of the rock would be difficult, likely using specialty equipment and blasting techniques to advance the new road cut. Therefore, maintaining the cut slope in its current configuration and moving the road alignment away from the cut to increase ditch capacity is recommended as a cost-effective and best engineering value approach. Minor scaling and removal of large, destabilized rocks on the existing cut slope can be incorporated into the road realignment construction, using conventional excavation equipment anticipated for the project.

Figure 9 – Site 1 debris sourcing from slide deposit directly above ditch.

B. CO RD64 Rockfall Site 2 Mitigation Options

The road cut at this location has an approximate 1V:1H slope with 46-ft length dipping into the ditch. The ditch below the cut slope was measured in the field, ranging from 3 to 4-ft wide and 1 to 2-ft deep. This ditch is not sufficiently sized to perform as a rockfall debris catchment area even for the small identified immediate cutslope source.

Directly west of the oversteepened road cut there is an approximate 0.6V:1.0H stable slope with a power pole as shown in Figure 10. This stable slope demonstrates that the proposed road cut can be designed for an equivalent slope which would only require layback excavation sufficient to match the stable slope ratio. It is recommended the oversteepened road cut at STA 6+50 to STA 9+00 be laid back to match the power pole slope of 1V:2H to stabilize the cut above the ditch. This will bring the existing slope to a stable configuration, increase the ditch capacity for future rockfall and improve the sight distance around that road corner.

Removal of the overlying basalt talus boulders could require downsizing oversized rocks to use conventional equipment to excavate the road cut. Downsizing of rocks can be performed mechanically or with blasting techniques. The basalt rock is an igneous mafic deposit which means it has a high specific gravity and unconfined compressive strength. On site basalt samples were not tested for actual rock properties on this project. For constructability estimation the assumed values for intact fresh basalt rock are: density, 175 pounds per cubic foot and; compressive strength, 8,000 pound-force per square inch.

The largest rock measured in the talus slope is estimated to have a volume of 21-cubic yards, and while this size represents the observed extreme limit there is high potential for any excavation to encounter large boulders that must be downsized. The amount of rock downsizing is dependent on the anticipated equipment and large rocks encountered in the excavation. Methods to downsize rock include but are not limited to explosives, hydraulic breaker attachments on excavators or other methods proposed by a contractor.

Figure 10 – Flatter slope (power pole) west of oversteepened road cut shedding rocks.

VI. CONSTRUCTION CONSIDERATIONS

Roadway excavations are anticipated to encounter large, hard boulders (10-feet in diameter or larger) requiring carefully planned and uniquely adapted approaches such as blasting or mechanically breaking to downsize boulders. Controlled downsizing methods include but are not limited to non-detonating explosives, expanding grout, or hydraulic rock breakers. The contractor is responsible for making the final determination on the intact strength of encountered rock in order to evaluate equipment capabilities and constructability.

Though no subsurface investigation was performed there is potential for groundwater to be encountered. A spring at Site 2 was observed leaching into the ditch and based on conversations with local ranchers a pipe used to discharge water into the ditch at a higher rate.

This potential for groundwater or natural springs may create temporary unstable slope stability conditions during construction and temporary diversion of flow away from the work area may be required for water quality requirements or maintaining drainage of the construction site.

VII. LIMITATIONS

Subsurface exploration was not performed as a part of this work. Interpretation of surface and subsurface conditions is based on limited field reconnaissance and surface observations of soil and rock outcrops. The recommendations in this memorandum include interpretations developed by the Government in the process of preparing repair alternatives. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgement of the Contractor.

Rockfall and rockfall events are sporadic and unpredictable. Causes can include, but are not limited to, factors such as animal, human, construction, environmental, weather and/or earthquakes.

Because of the multiplicity of factors affecting rockfall dynamics, rockfall cannot be predicted and is not an exact science that guarantees the safety of individuals and property. However, by the application of sound engineering principles to a range of rockfall dynamics, the risk of injury and property loss can be reduced by the use of properly designed rockfall mitigation features in identified risk areas. Consequently, neither the rockfall hazard in terms of probability of a rockfall at any specific location, nor the risk to people or facilities form such events, is assessed in design.

VIII. SIGNATURES

Prepared by Todd Hansen, Geotechnical Engineer, CFLHD

Reviewed by Brendan McGarity, Geotechnical Engineer, CFLHD

Approved by Marilyn Dodson, Lead Geotechnical Engineer, CFLHD

IX. REFERENCES

Egger, A. E. and Miller, E.L. (2011) “Evolution of the northwestern margin of the Basin and Range: The geology and extensional history of the Warner Range and environs, northeastern California.” Geosphere; 7 (3): 756–773. doi: https://doi.org/10.1130/GES00620.1

Gay, T.E. and Aune, Q.A., California Geological Survey (CGS). (1958) “Reconnaissance geology of portions of the western half of the Alturas sheet, California.” Scale 1:62,500

Grose, T. L., Egger, A. E. and O’Neal, M.D. California Geological Survey (CGS). (2016) “Preliminary Geologic Map of the Alturas 30’ x 60’ Quadrangle, Lassen and Modoc Counties, California.” Scale 1:100,000.

Jones, C.L., Higgins, J.D., & Andrew, R.D. (2000). Colorado Rockfall Simulation Program 4

U.S. Department of Transportation, Federal Highway Administration (FHWA). (2014).

“Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14.” Publication No. FHWA-FLH-14-001.

X. ATTACHMENTS

Appendix A: CRSP Analysis

APPENDIX A

COLORADO ROCKFALL SIMULATION PROGRAM

(CRSP)

D.1 INTRODUCTION

Colorado Rockfall Simulation Program 4.0 (CRSP) was used to assess the effectiveness of an increased ditch width with respect to rockfall retention capabilities. CRSP is an analysis program and numerical modeling tool for evaluating rockfall events and provides design criteria for rockfall fences, catchment ditches, catchment berms and other rockfall protection structures. CRSP uses the Discrete Element Method for dynamic model simulation using the equations of motion for an approximation of rock and slope interaction. CRSP allows the slope profile to be generated from slope survey data, including cross-sections, which produce representative slope conditions and accurate portrayals of rockfall dispersion, runout distance, trajectory, bounce height, and energy along the slope.

The CRSP software requires various, site specific rockfall parameters to be input to create an accurate simulation model. Mechanical properties of the slope, including hardness and roughness values were input into CRSP, based on observations and estimations made during field reconnaissance and as recommended in the CRSP user’s manual. Various points along the slope were analyzed to determine the characteristics of simulated rockfall, including velocity, bounce height, kinetic energy, and rollout distance. The analysis points are critical to determine the percent of rockfall passing each point. Falling rock parameters, including rock density, shape, and size were input into the program and were based on field observations and measurements along the critical section slopes.

D.2 DATA INPUT PARAMETERS

During field investigations, boundaries or zones were identified where the slopes composition changed with respect to rock fall size and shape, rock density, slope surface roughness, and slope hardness coefficient. The slope geometry was obtained from survey data of the existing and the proposed conditions.

D.2.1 Rock Size and Shape

Rock size and shape influence the degree to which slope surface roughness affects rockfall behavior.

Rockfall data shows that spherical or near spherical rocks move faster downslope than other irregular shapes. Large rocks tend to have longer runout because of their greater kinetic energy. Rock shapes can be generalized as “spherical”, “tabular”, or “discoidal” etc. to facilitate analysis. These shapes can have a large effect on the rock trajectory during a rockfall. Spheres generally represent a worst case scenario since they comprise the greatest mass for a given size and tend to accelerate faster and decelerate slower than angular rocks. Elongated shapes, such as cylinders, normally take a longer slope distance to reach high velocities and have shorter runout distances compared to spherical rocks. At high velocities, elongated rocks act much like spherical rocks because they are rotating end-over-end so the rock contacts the slope at just two points. During acceleration and deceleration, angular rocks tend to tumble and fall while spherical rocks roll without needing to overcome any undulation in their rock surface, resulting in a longer runout distance.

D.2.2 Rock Density

CRSP allows rock slope densities ranging from hard igneous rock to soft sedimentary rock to be input into the program. A density of 175 lb/ft3 was used for all slopes analyzed, assuming basalt rock.

D.2.3 Slope Surface Roughness

A range of slope surface roughness values from >3.0 to 0.1 can be applied to each zone. For this project site, the entire slope profile is characterized by accumulated rockfall debris. Higher elevation slopes contain the largest rocks because of inertial energy loss which creates a complicated slope, with surface roughness of 1.5. Down gradient of this slope segment the size of collected rocks grades smaller and surface roughness of 1.0 and 0.5 represented the lowest segment of the talus field and ditch. A slope roughness of 0.25 was used for the paved roadway surfaces.

D.2.4 Tangential & Normal Coefficients

The tangential coefficient of frictional resistance is the component related to the rock’s velocity parallel to the slope. This coefficient is influenced by both vegetation and slope material. Values of the tangential coefficient range from 0.50 for soft soil slopes to 1.0 for smooth hard surfaces and paving.

The normal coefficient of restitution is a measure of the change in the velocity normal to the slope after impact, compared to the normal velocity before the impact. The normal coefficient is determined by the rigidity of the slope surface and generally ranges from 0.10 for soft soil slopes to 1.0 for smooth hard surfaces and paving (Jones et al., 2000). A summary of the parameters used for analysis can be found in Table D1.

Table D1- Parameters for Analysis

Material Surface

Roughness Tangential Coefficient

Normal Coefficient

Rock 1.5 to 1.0 0.75 0.15

Ditch/Shoulder 0.50 0.75 0.20

HACP 0.25 0.95 0.90

D.2.5 Analysis Points

Multiple analysis points were used in each CRSP analysis. These analysis points were selected to determine the percent rockfall passing and ultimately retained within a certain zone, particularly the percent passing the inboard shoulder. This point was significant as it provided the retained percentage of a given ditch width’s ability to retain rockfall thereby reducing the percentage of rockfall passing into the traveled way. Table D2, as well as Figures D1 and D2, display the analysis points (AP) used in each CRSP model.

Table D2- CRSP Analysis Points

ANALYSIS POINTS

Existing and Proposed Condition

CRSP Nomenclature Corresponding Roadway Element

AP-1 Bottom of Cutslope

AP-2 Edge of Ditch

AP-3 Center Line

Figures D1 & D2 - CRSP Analysis Points, Existing & Proposed Conditions (typical)

D.3 CRSP Analysis Results

Given the aforementioned input data; multiple scenarios of the existing and proposed conditions were run through CRSP with potential rockfall sizes emanating from various heights on the slope. Tables D3 and D4 present the summary of the CRSP Analysis on the percent contained for the proposed slope treatment options.

Table D3- CRSP Results CO RD64 Site 1

Drop Height= 27 Feet, 8 Ft Ditch, 0-Ft shoulder. Dropping 1000 Spherical Rocks From Mid-Slope Source

Rock Source Area Rock Size

(dia.)

Analysis Points

Total Number of Rocks Passing Analysis Point

Percent Passing

Percent Retained In

Ditch Height on Upper Bound of Slope to Height on Lower Bound of Slope

Source Area Description

27 0 Rockfall from existing slope cut

Bottom of Cutslope

887 88.7%

49.8% Edge of Ditch 502 50.2%

Centerline 399 39.9%

27 0 Rockfall from existing slope cut

Bottom of Cutslope

692 69.2%

78.5% Edge of Ditch 215 21.5%

Centerline 125 12.5%

27 0 Rockfall from existing slope cut

Bottom of Cutslope

238 23.8%

98.7% Edge of Ditch 13 1.3%

Centerline 3 0.3%

27 0 Rockfall from existing slope cut

Bottom of Cutslope

12 1.2%

100% Edge of Ditch 0 0%

Centerline 0 0%

Table D4- CRSP Results CO RD64 Site 2

Drop Height= 33 Feet, 13 Ft Ditch, 0-Ft shoulder. Dropping 1000 Spherical Rocks From Mid-Slope Source

Rock Source Area Rock Size

(dia.)

Analysis Points

Total Number of Rocks Passing Analysis Point

Percent Passing

Percent Retained In

Ditch Height on Upper Bound of Slope to Height on Lower Bound of Slope

Source Area Description

33 0 Rockfall from existing slope cut

Bottom of Cutslope

955 95.5%

72.1% Edge of Ditch 279 27.9%

Centerline 193 19.3%

33 0 Rockfall from existing slope cut

Bottom of Cutslope

668 66.8%

96.9% Edge of Ditch 31 3.1%

Centerline 10 1.0%

33 0 Rockfall from existing slope cut

Bottom of Cutslope

303 30.3%

100% Edge of Ditch 0 0%

Centerline 0 0%

33 0 Rockfall from existing slope cut

Bottom of Cutslope

1 0.1%

100% Edge of Ditch 0 0%

Centerline 0 0%