CO FLAP US50(1) FINAL Geotechnical Report Signed 06.01.2020.pdf

U.S. Department of Transportation

Federal Highway Administration

LITTLE BLUE CREEK CANYON

CO FLAP US 50(1)

FINAL GEOTECHNICAL REPORT

Report No. CO-CX-0050-20-01

Geotechnical Services Branch June 1, 2020

U.S. DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

CENTRAL FEDERAL LANDS HIGHWAY DIVISION

LAKEWOOD, COLORADO

SIGNATURE SHEET

Report prepared by: __________________________________________

Dominic Monarco, Geotechnical Engineer

Report reviewed by: __________________________________________

Todd Hansen, P.E., Geotechnical Engineer

Approved for distribution by: __________________________________________

Marilyn Dodson, P.E., Lead Geotechnical Engineer

Distribution

Project Management Project Development, Lead Designer (2) Tech Services (2)

Little Blue Canyon, CO FLAP US 50(1) 1 May 2020

Table of Contents

SECTION ONE--INTRODUCTION

1.1 PROPOSED CONSTRUCTION

SECTION TWO--GEOLOGY AND SEISMICITY

2.1 GEOLOGY

2.2 GEOLOGIC HAZARDS

2.3 SEISMIC DESIGN PARAMETERS

SECTION THREE—FIELD EXPLORATION

3.1 FIELD EXPLORATIONS

2010- Yeh & Associates, Inc. Geotechnical Report 2020- Yeh & Associates, Inc. Geotechnical Report

SECTION FOUR-ANALYSIS AND RECOMMENDATIONS

4.1 RETAINING WALL DESIGN

4.2.1 Retaining Wall Design Methodology

4.2.2 GRS Retaining Wall Design Assumptions

4.2.3 GRS Retaining Wall Analysis Results

4.2.4 GRS Retaining Wall Preliminary Materials Recommendations

CUT SIDE ROCKERY DESIGN

4.2.5 Rockery analysis

EMBANKMENTS

4.2.6 Mechanically Placed Rock Embankment

CUT SLOPES

4.2.7 Rock Cut Structural Stability

4.2.8 Rockfall Catchment Area

4.2.9 Rock Cutslope Ratings

4.2.10 Draped Mesh

4.2.11 Controlled Blasting

4.2.12 Scaling

4.2.13 Rock Reinforcement and Mitigation

SECTION FIVE-CONSTRUCTION CONSIDERATIONS

SECTION SIX-- LIMITATIONS

Little Blue Canyon, CO FLAP US 50(1) 2 May 2020

TABLES

TABLE 1: - Summary of Seismic Parameters for Site Class A TABLE 2: - Summary of YA & Associates Borings Performed within the current CO US50(1) TABLE 3: - Assumed Foundation Soil Design Parameters Used in GRS Wall Design TABLE 4: - Critical GRS Sections TABLE 5: - Minimum Embedment Requirements Per Wall Height TABLE 6: - Results of GRS Internal Capacities and Global Stability Analysis TABLE 7: - Recommended Safety Factors for Design of Rockeries TABLE 8: - Material Properties used in the Analysis for Rockeries TABLE 9: - Minimum Required Rockery Geometry to Satisfy Safety Factors TABLE 10: - Minimum Base Rock Widths for Rockeries with a Given Height TABLE 11: - Rockery Base Width Requirements TABLE 12: - A Summary of Calculated Factors of Safeties for Rockeries TABLE 13: - Adjusted cutslope angles from YA 2019 TABLE 14: - Summary of CRSP Analysis for Proposed Cutslopes TABLE 15: - Rock Cut Slope Ranking by CRHRS TABLE 16: - Draped Mesh Locations TABLE 17: - Geotechnical-Related Special Contract Requirements

APPENDICES

APPENDIX A - GRS Wall Analysis Preliminarily Internal and External MathCAD APPENDIX B - GRS Wall Analysis Preliminarily Global Stability APPENDIX C - Rockery Wall Analysis MathCAD APPENDIX D - Colorado Rockfall Simulation Program (CRSP) APPENDIX E - Colorado Rockfall Hazard Rating System (CRHRS) Data sheets APPENDIX F - Excavation Characteristics of Rock and Rippability Charts

Little Blue Canyon, CO FLAP US 50(1) 3 May 2020

SECTION ONE--INTRODUCTION

This report presents the results of the geotechnical investigations conducted for the Colorado Department of Transportation, Little Blue Creek Canyon, CO FLAP US 50(1) project. The project is located between Gunnison, CO (~34 miles east) and Montrose, CO (~30 miles west) between M.P. 123 and 126.8. The route provides access to the Black Canyon of the Gunnison National Park, as well as access to Forest Service, BLM, and National Recreation Area Lands. To improve safety and vehicle capacity, CDOT has been improving the route by adding passing lanes and widening shoulders where feasible. This project is the last segment of US 50 to be reconstructed within Colorado.

This project will focus on improving the roadway geometry and cross sections through a 2.1-mile segment within the Little Blue Creek canyon (Segment 3, M.P. 123 to M.P. 125) and another 1.76 miles through the Windy Point road section to the new CDOT maintenance facility (Segment 4, M.P. 125 to M.P. 126.8). Roadway geometry improvements will increase roadway safety and stability through the construction of rockfall catchment areas, new guardrail, slope armoring, drainage improvements, rockeries, and minor bridge repairs. For consistency with previous projects within the region and for cost effectiveness, roadway widening will be accomplished mainly through rock slope and soil slope excavations and outboard retaining structures.

This report summarizes the relevant findings from the geotechnical investigation and provides recommendations pertaining to the October 2019 95% Design Plans. The geotechnical investigations consisted of rock cut mapping, field subsurface exploration at proposed wall locations, laboratory testing, correlation of available data, and geotechnical analysis. This design report draws interpretations from the following previously prepared reports.

• YA and Associates Inc, “Draft Final Geotechnical Assessment/ Investigation, US 50 Blue Creek Canyon Corridor, Gunnison County, Colorado. YA Project Number 28-139, Dated March 22, 2010.

• YA and Associates Inc, “Final Geotechnical Data Report, US 50 Little Blue Creek Canyon, Gunnison County, Colorado”. YA Project Number 218-112, Dated March 30, 2020.

Some important information presented in the previous reports is not repeated here; the reader is encouraged to read these reports in conjunction with this design report. With respect to measured and recorded data, and possible discrepancies between the previously published reports and this design report, this design report takes precedence.

1.1 PROPOSED CONSTRUCTION

To accommodate roadway geometry corrections and rockfall ditch widths the proposed construction will rely on both roadway excavation, (cutting into the existing slopes) and roadway embankment (accomplished by fill slopes and walls). In general, it is preferred to keep construction either in cut or in fill (not both) whenever feasible. However, the confined nature of the canyon with near vertical rock outcrops on one side, the Little Blue Creek below and the geometric realignment of the roadway for safety improvements it was determined that both fill sections and

Little Blue Canyon, CO FLAP US 50(1) 4 May 2020 cuts are required. Inboard cut walls in the form of rockeries are also proposed. Excavated material that meets the contract quality requirements will be used as fill material throughout the project as well as armoring and stabilizing the roadway embankment. Cut slope materials will generally include soil deposits (alluvium, colluvium, and residual soils derived from in-place weathering of bedrock), jointed weathered bedrock, and competent bedrock.

The design of stable roadway rock cut is reliant on good standard slope stability practices including;

(1) The implementation of presplitting techniques prior to slope excavation with production blasting.

(2) The installation of a minimum 10-foot wide catchment area along each cut to retain a sufficient amount of falling rocks and prevent them of reaching the roadway surface.

(3) The improvement of roadway geometry to assure better sight distance.

By increasing the ditch width and improving site distance for traffic the risk of vehicular accidents related to rockfall events will be significantly reduced. Due to the inherent unpredictability of rock excavation and designed rock cuts in excess of 180 vertical feet, reinforcement of the excavated rock mass using steel bar reinforcement and draped mesh systems are anticipated to be required.

Little Blue Canyon, CO FLAP US 50(1) 5 May 2020

SECTION TWO--GEOLOGY AND SEISMICITY

2.1 GEOLOGY

For a through discussion of the geology throughout the proposed corridor please refer to the Yeh and Associates (YA) Geotechnical Reports referenced in Sections 1 and 3.

2.2 GEOLOGIC HAZARDS

Geologic hazards exist from the natural environment and the existing roadway condition. Primary geologic hazards that exist in the vicinity of the project area are instabilities at cut and fill slope locations, rockfall, minor avalanche chutes, frost heave and swelling soils.

Slope instability and rockfall hazard exists throughout the roadway alignment and is likely to continue. Slope instability is generally a result of rock fragments dislodging from over-steepened colluvial soil slopes and elevated alluvial terraces, or closely jointed rock mass outcrops. Rockfall is primarily anticipated to originate from existing and proposed cut slopes and will occur during construction. Rockfall events during construction may originate from inside or beyond the project construction limits. Rockfall risk is particularly high in the wet winter and early spring months.

The newly constructed rock slopes will incorporate an engineered catchment ditch and draped rockfall mesh to significantly reduce the risk of rockfall since falling rocks will most likely be retained in the ditch.

Embankment slumps have occurred at various embankment fill locations requiring armoring, re-grading, and enhanced drainage. Such failures are generally attributed to over-steepened, marginally consolidated slopes.

2.3 SEISMIC DESIGN PARAMETERS

No known active faults were identified, that cross or are within the vicinity of the project, from a USGS Earthquake Hazard Program database search.

The American Association of State Highway and Transportation Officials (AASHTO) criteria for Recommended seismic response parameters are based on the (AASHTO) LRFD Bridge Design Specifications, 7th Edition, 2017, and represents horizontal peak ground acceleration (PGA) with 7 percent probability of exceedance in 75 years (approximate 1000-year return period). The 1000-year return period uniform hazard spectrum for the project (38.4144 latitude and -107.4070 longitude) was obtained in accordance with the AASHTO ground motion maps. As very hard rock and talus slopes are very near the surface within the canyon section on the project the site is classified as Class A according to site class definitions specified in Table 3.10.3.1-1 of AASHTO.

The recommended spectral acceleration coefficient values for probabilistic design is summarized in the following table.

Little Blue Canyon, CO FLAP US 50(1) 6 May 2020

Table 1: Summary of Seismic Parameters for Site Class A

Peak Seismic Ground Acceleration Coefficient, (As) 0.105

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDS) 0.206

Horizontal Response Spectral Acceleration at Period of 1.0 sec (SD1) 0.046

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 0.80

Site Factor at Short-Period of Acceleration Spectrum, (Fa) 0.80

Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 0.80

Based on the long acceleration coefficient SD1 value of 0.058, the site is assigned to seismic hazard Zone 1 in accordance with per AASHTO Section 3.10.3.2.

Little Blue Canyon, CO FLAP US 50(1) 7 May 2020

SECTION THREE—FIELD EXPLORATION

3.1 FIELD EXPLORATIONS

2010- Yeh & Associates, Inc. Geotechnical Report

This report, issued in 2010, presented the results for the geotechnical study conducted by Yeh & Associates (YA) for Muller Engineering on behalf of the Colorado Department of Transportation.

The report focused on the proposed widening of CO US50 between mile markers 117.5 and 126.5.

The subsurface explorations were conducted during multiple field visits between September 2007 and September 2008. The subsurface investigations included geotechnical borings and Time Domain Reflectometry (TDR) instrumentation for the CDOT Sand Dome Landslide project, an unstable roadway section of US50 directly to the West but outside of the proposed alignment of CO FLAP US50(1). A total of 50 geotechnical and pavement borings were drilled, of which 26 were advanced within the current project limits (Mile point 123 and 126.8). Additional data collected within the current project limits included two geophysical seismic refraction lines and kinematic structural mapping of existing roadway rock cuts for proposed cut slopes.

2020- Yeh & Associates, Inc. Geotechnical Report

Muller Engineering was contracted by the FHWA to obtain additional subsurface data and information on the rock kinematic structure specifically within the narrows of Little Blue Creek Canyon. Muller Engineering then contracted YA to obtain this additional data. From October 1st through October 5th, 2018 YA observed 11 subsurface borings and subcontracted an unmanned aerial vehicle (UAV) to perform photogrammetric mapping of the slopes. Analysis and data processing for rock slopes, discontinuities and rock structure was performed by YA using geotechnical kinematic analysis modeling software.

A summary of the subsurface investigations and approximate stationing corresponding to the current FHWA 95% plans is summarized in Table 2 below.

Table 2: Summary of YA Borings Performed within the current CO US50(1) Alignment

Boring ID YA Report

October 2019 95% Approximate

Stationing

Offset from Proposed

Centerline (ft)

Total Depth

(ft)

Depth to Rock

(ft) Boring Intent

YA-23 2010 588+95 26 RT 32.5 27 Retaining Wall

P-01 2010 589+90 0 5 - Pavement Design

P-02 2010 599+90 13 RT 4.5 - Pavement Design

YA-24 2010 600+50 26 RT 13.4 7.5 Retaining Wall

YA-25 2010 609+60 21 RT 27.3 19.5 Retaining Wall

P-03 2010 609+90 4 RT 3 - Pavement Design

YA-26 2010 611+15 21 RT 29.2 21.3 Retaining Wall

UD-1 2020 619+60 10 RT 7.1 - Underdrain/Subexcavation

Little Blue Canyon, CO FLAP US 50(1) 8 May 2020

Boring ID YA Report

October 2019 95% Approximate

Stationing

Offset from Proposed

Centerline (ft)

Total Depth

(ft)

Depth to Rock

(ft) Boring Intent

P-04 2010 619+95 9 RT 4.5 - Pavement Design

UD-2 2020 620+75 10 RT 5.2 - Underdrain/Subexcavation

UD-3 2020 622+25 10 RT 11.5 - Underdrain/Subexcavation

Seismic Line 2 2010 626+55 to 627+70 14 LT - - Retaining Wall

YA-27 2010 629+70 8 LT 19.7 12 Retaining Wall

P-05 2010 629+70 5 RT 5 - Pavement Design

YA-28 2010 631+80 10 LT 40.1 - Retaining Wall

P-06 2010 640+05 16 RT 3 - Pavement Design

YA-29 2010 640+50 6 LT 33.7 28 Retaining Wall

MSE-4 2020 641+45 7 LT 36 26 Retaining Wall

YA-30 2010 644+25 9 LT 15.1 6 Retaining Wall

Seismic Line 1 2010 646+30 to 647+45 16 LT - - Retaining Wall

MSE-6 2020 646+95 12 LT 22 16 Retaining Wall

YA-06* 2010 649+35 23 RT 40.5 0 Rock Cut

P-07 2010 649+50 2 LT 0.7 0.5 Pavement Design

MSE-8 2020 649+95 21 LT 53.5 18 Retaining Wall

YA-07* 2010 650+15 26 RT 24.1 0 Rock Cut

MSE-8A 2020 652+15 4 RT 40.5 16 Retaining Wall

YA-32 2010 652+44 9 LT 33.1 28.1 Retaining Wall

YA-33 2010 655+80 10 LT 27.8 22 Retaining Wall

YA-34 2010 657+95 10 LT 50.5 44 Retaining Wall

YA-35 2010 659+90 6 LT 20.5 16.1 Retaining Wall

P-08 2010 660+00 14 RT 4 - Pavement Design

YA-36 2010 664+15 11 LT 38.6 28 Retaining Wall

P-09 2010 669+80 0 3 - Pavement Design

YA-37 2010 672+20 35 RT 20.6 - Retaining Wall

P-10 2010 679+95 0 2 2 Pavement Design

YA-38 2010 685+35 86 RT 33.1 13 Retaining Wall

LS-1A 2020 687+65 0 32 17.5 Alignment Shift

LS-3 2020 746+40 6 LT 22 - Alignment Shift

T-1 2020 794+95 15 LT 6 - Pavement Tie-in

T-2 2020 804+55 9 RT 6 - Pavement Tie-in

* Horizontal Boring

Little Blue Canyon, CO FLAP US 50(1) 9 May 2020

2019- Juniper Unmanned

Muller Engineering was contracted by the FHWA to obtain supplementary survey of the existing highway corridor. The engineering design survey data was delivered with the original 2010 Yeh report and an up-to-date base map was required. Juniper Unmanned was contracted to provide an unmanned aircraft system capable of performing a LiDAR (light detection and ranging) scan of the entire project corridor. Beginning the week of October 14, 2019, Juniper scanned selected areas along the approximate 4.3-mile alignment with a 500-ft buffer outside of the project centerline.

Analysis and data processing for developing the final survey digital terrain model was performed by FHWA with assistance from Juniper Unmanned.

Little Blue Canyon, CO FLAP US 50(1) 10 May 2020

SECTION FOUR-ANALYSIS AND RECOMMENDATIONS

This section provides analysis and recommendations for the fill side retaining walls, cut-side rockery design, rock and soil slope stability, and rockfall mitigation measures.

RETAINING WALL DESIGN

After a number of technical and constructability considerations, a Geosynthetic Reinforced Soil (GRS) Wall was selected as the preferred wall type. The primary reason for choosing the GRS wall was the reinforcement length can be shorter than a Mechanically Stabilized Earth (MSE) Wall requiring less of excavation by comparison. The close spacing of the reinforcements in a GRS wall preclude reinforcement pullout as a potential failure mechanism. This is because the backfill and reinforcement form a composite material. This also has the effect of reducing soil pressure on the facing elements to nearly zero.

GRS is similar to MSE in that it consists of a reinforced, high-quality backfill, with a facing element. However, in a GRS wall, the reinforcement spacing is much closer, typically less than 12 inches, compared to a typical 18- to 24-inch spacing in MSE walls. Concrete and soil have high compressive strengths and low tensile strengths, whereas steel and geosynthetic reinforcements have high tensile strengths. When reinforcements at close spacing are included in the soil mass, their tensile strength improves the strength of the soil and forms a composite material. This close reinforcement spacing forms a composite material, analogous to reinforced concrete.

Two different GRS wall facing alternatives have been evaluated in the preliminary engineering and analysis for the project:

1) Segmental Retaining Wall Units (SRWU) and

2) Precast Concrete Panels (Panel).

SRWU and panel facing were selected because these two facings are widely-used in industry, can be easily incorporated into the GRS system, and will allow for the broadest selection of facing appearance.

Due to the depth of excavation required for taller wall segments, temporary excavation shoring may be necessary for certain areas to maintain a minimum bench for traffic passage during construction. Variable subsurface conditions exist along the alignment, and shoring may be difficult in sections consisting of cobble and boulder layers. The contractor should be prepared for loose subgrade conditions that will make constructing temporary shoring difficult and time consuming. Using the GRS design methodology, the reinforced length can be reduced to 50 percent of the total wall height when the height of the wall is less than 25 feet. Reducing this dimension for sub 25-foot walls may help eliminate the need to shore excavations and therefore reduce costs and expedite construction. The possibility of reducing or eliminating the need for temporary shoring is the Contractor's responsibility and does not relieve the Contractor of responsibility to maintain minimum excavation stability requirements.

Little Blue Canyon, CO FLAP US 50(1) 11 May 2020

4.2.1 Retaining Wall Design Methodology

Design of a GRS retaining wall considers three general stability criteria: global, external, and internal. Global stability is related to the overall stability of the wall, adjacent improvements (such as roads), and slopes. External stability includes evaluation of sliding resistance, overturning resistance, and bearing resistance. Global and external stability analysis for GRS walls are evaluated in the same way as for other gravity retaining walls such as mass concrete walls, rockery walls, and MSE walls. Internal stability of the GRS wall includes checking strength of the reinforcement, overall load carrying capacity of the GRS mass, and connection strength of the facing elements. However, facing pressure in a GRS wall is considerably lower than in an MSE of the same height due to close reinforcement spacing (FHWA, 2018). The preliminary design presented in this report includes global, internal, and external stability checks for constructability and quantity estimating. The final design will be conducted by the Contractor based on final survey, proprietary wall elements, facing selection, and appropriate internal and external design checks.

Design analysis of the GRS wall generally followed recommendations provided in the AASHTO LRFD Bridge Design Specifications, 7th edition, and Design and Construction Guidelines for Geosynthetic Reinforced Soil Abutments and Integrated Bridge Systems (FHWA, 2018).

The first step of design was to evaluate the global stability of the wall system and determine an appropriate overall geometry (base width and toe embedment) to provide a satisfactory factor of safety. The Slide 7.0 software package was used to evaluate global stability. Slide 7.0 uses limit-equilibrium methods to determine factors of safety for slope stability. The Bishop Simplified method was used for this analysis.

Next, the overall geometry was checked for external stability considerations. The external stability calculations were adapted from the equations presented in the and GRS-IBS manual (FHWA, 2018). Finally, internal stability analysis was conducted to evaluate the appropriate geosynthetic reinforcement strength to use in the wall. Internal and external stability design calculations were performed using the MathCAD software package and can be seen in Appendix A and the preliminary internal stability capacity results can be seen in Table 6,

4.2.2 GRS Retaining Wall Design Assumptions

Material properties for the on-site soils and GRS wall backfill are critical parameters for design.

Material property values for the on-site soils were initially estimated from Table 5-15 of FHWA NHI-06-088 Soils and Foundations – Volume 1, and adjusted based on available boring logs and laboratory tests to best represent existing conditions. Properties for reinforced and retained soils are based on experience and testing in fills with similar index properties. Final soil and bedrock parameters used in design are summarized in Table 3.

Little Blue Canyon, CO FLAP US 50(1) 12 May 2020

TABLE 3 – Assumed Foundation Soil Design Parameters Used in GRS Wall Design

Material Unit Weight (pcf) Friction Angle (degrees) Cohesion (psf)

Reinforced Soil 125 38 100

Retained Soil 130 32 0

Foundation Soil:

Wall #1Near Sta. 630+90

Wall #2b- Near Sta. 647+10 Wall #3- Near Sta. 653+30 Wall #5- Near Sta. 664+90

125 38 100

Foundation Soil:

Wall #2a-Near Sta. 641+20 Wall #4- Near Sta. 659+00

125 36 100

Internal stability analysis for the 8-inch reinforcement spacing assumed a biaxial geosynthetic with a minimum ultimate tensile strength (Tult) of 4,800 pounds per foot and strength at 2 percent strain (T2%) of 2400 pounds per foot. For 12-inch spacing it is assumed that stronger geosynthetics will be required. Other biaxial geosynthetics could be used than those shown in the preliminary design calculations, (Appendix A) provided they have strengths greater than the calculated loads. Multiple grades of the same type of geosynthetic could be considered to economize the design. A global reduction factor of 0.4 was applied to the ultimate strength to arrive at the factored strength used in design. This factor is recommended in FHWA (2018) for use with the woven geotextiles that are typically employed for GRS construction. If a geogrid product is used, the ultimate strength should be reduced by appropriate creep, durability, and installation damage factors, as well as the recommended AASHTO LRFD resistance factor of 0.9 for geosynthetic reinforcements.

The analyzed geometry for GRS wall design is based on 95% design cross sections. These cross sections depict some or all of the following conditions:

a. A roadway surface that is either crowned or super-elevated and sloping towards the cut side,

b. either a post-and-beam guardrail or concrete barrier located 4 feet behind the proposed wall face,

c. the foreslope between the guardrail and wall at a grade of 1.0(V):3.0(H), and

d. steep slopes below the toe of the wall.

Toe slopes below the walls range from 30 to 40 degrees. Exposed wall heights are between 3 and 35 feet. There is no discernable relationship between wall height and toe slope, meaning where the wall is the tallest the slope ratio is not necessarily the steepest.

Six critical sections were selected for analysis and are shown in Table 4 below. These sections were selected based on the following criteria:

1) Represent the tallest wall segment within the wall,

2) below the wall is the steepest foreslope section, or

3) a combination of 1 and 2.

Little Blue Canyon, CO FLAP US 50(1) 13 May 2020

A minimum embedment of 5 feet and toe bench width of 6 feet was initially selected to improve global stability and bearing resistance at the tallest wall segments.

Table 4: Critical GRS Sections

Station Wall # Proposed Height Toe Slope

(Ft) (Degrees) 630+90 1 29 37 641+20 2a 30 34 647+10 2b 38 39 653+30 3 33 39 659+00 4 22 36 664+90 5 18 38

Table 5 was then established to reduce the required embankment at a given wall height. The total wall height at each section was then determined based on the minimum embedment required to achieve the required bench width for the toe slope ratio.

Table 5: Minimum Embedment Requirements Per Wall Height

Height Embedment (1)

(Ft) (Ft)

≥ 30 5

>22 to 29 max 4

>0 to 22 max 3

(1) Min bench width from base of wall to slope face= 4 feet.

FHWA (2009) recommends that the vehicle impact load from a post-and-beam railing system be considered in design of an MSE wall by distributing the load between the top two layers of reinforcement. This assumes a reinforcement spacing of 2 feet and implies distribution of the load to the top 5 feet of reinforcement (approximately equal to the typical installation depth of a post-and-beam system). Due to the tighter reinforcement spacing planned for the GRS wall, the guardrail load was distributed among all layers within 5 feet of the top of wall. Since the post portion of the railing will pass through these layers, the impact load will also transfer to them.

Finally, a 250 pound per square foot surcharge load was applied across the roadway to account for the traffic live load in accordance with the AASHTO LRFD specifications.

As discussed in Section 2 of this report, the project is classified as Seismic Performance Zone 1, in accordance with Table 3.10.6-1 of AASHTO. Therefore, we anticipated that seismic loads would not control design and the Extreme Event I limit state was not considered during the design analysis.

Little Blue Canyon, CO FLAP US 50(1) 14 May 2020

TABLE 6: Results of GRS Internal Capacities and Global Stability Analysis (1)

STA.

E X

IS

T

SI

ST

IN

G

S L

O

PE

F A

C T

O R

O F

SA

FE

T Y

W A

L L

PR

O

PO

SE

D H

E

IG

H

IN

E R

N A

L S

T A

B

IL

IT

H

E

IG

H T

A N

A L

Y Z

D

R

IN

F.

L E

N G

T H

(3

FO

U

N D

A T

IO

N

S O

IL

S (2

GLOBAL FACTOR OF

SAFETY

GRS STABILITY CAPACITY TO

DEMAND RATIO (CDR)

ST

A

IC

(S

U R

C H

A R

G E

SE

IS

M

IC

X

T E

R N

C

PA

C

IT

E R

N

PA

C

IT

Y

P

G

IV

N

SP

C

G

(Ft) (Ft) (Ft)

So il

Ty pe

(2

0.

5H

7H

5H

7H

Sl id in g

O ve rt ur ni ng

B ea ri

Sv =8 in ch es

Sv =1 in ch es

630+90 1.47 1 29 30 0.7H 1 1.45 1.65 1.38 1.56 2.0 2.6 13.5 1.8 1.3

641+20 1.39 2a 30 30 0.7H 2 1.27 1.37 1.18 1.27 1.9 2.6 9.8 1.8 1.3

647+10 1.28 2b 38 40 0.7H 1 1.29 1.53 1.22 1.48 2.1 2.7 13.4 1.7 1.6

653+30 1.23 3 33 35 0.7H 1 1.14 1.32 1.06 1.21 2.1 2.6 13.5 2.4 1.4

659+00 1.30 4 22 25 0.5H 2 1.30 1.32 1.20 1.20 1.4 1.3 7.2 2.1 1.2

664+90 1.41 5 18 20 0.5H 1 1.33 1.35 1.23 1.24 1.4 1.3 10.0 2.3 1.3

NOTES

(1) Soil Parameters

Retained Soil

Unit weight:130 pcf, Friction Angle: 32 Deg, Cohesion: 100 psf (2) Foundation Soils Type

Sandy-Gravels/Cobbles/Boulders Unit weight:125 pcf, Friction Angle: 38 Deg, Cohesion: 100 psf

Reinforced Soil

Unit weight:125 pcf, Friction Angle: 38 Deg, Cohesion: 100 psf

Sandy/Silty-Gravels/Cobbles/Boulders Unit weight:125 pcf, Friction Angle: 36 Deg, Cohesion: 100 psf

(3) Reinforcement Length Reinforcement length of 0.5H for walls 25 feet or shorter taller than 25 feet are 0.7H

Little Blue Canyon, CO FLAP US 50(1) 15 May 2020

4.2.3 GRS Retaining Wall Analysis Results

Global stability was analyzed using the Bishop simplified method. Results are attached to this report and summarized in Table 6 above.

The global stability analysis indicated that walls greater than 25 feet will require a reinforcement length equal to 70% of the design height while walls shorter than 25 feet in height require reinforcement lengths 50% of the design height. This ratio was used to define the minimum reinforcement length analyzed for internal and external stability. Although shorter wall sections were not analyzed directly, a minimum reinforcement length of 8 feet is recommended in accordance with the AASHTO LRFD specifications. The factored bearing resistance for the GRS wall was evaluated in accordance with AASHTO LRFD Bridge Design Specification.

The contractor is required to provide the proposed wall section and materials submittal prior to initiating design procedures well before wall construction for review by the FHWA geotechnical engineer to ensure the design requirements are understood.

4.2.4 GRS Retaining Wall Preliminary Materials Recommendations

The following preliminary materials recommendations should be considered when preparing cost estimates and project specifications or Special Contract Requirements.

• SRWU/Precast panel faced GRS walls shall meet the requirements of Sections 257, 262 and LRFD Bridge Design Specifications

• Facing Aesthetics (Color, texture, pattern)

• GRS backfill should be base coarse or a similar well-graded backfill material with a maximum particle size of 1.5 inches. Section 703.05, Class C or D material is acceptable, with the adjustment of the maximum particle size. A revision to Section 704 will be provided for the backfill specification.

• Geosynthetic reinforcement should meet the requirements of Section 714.04.

• Geocomposite sheet drain, Type 1 per Section 714.02.

• Drainage pipe should be 6-inch diameter HDPE, PVC, or ABS pipe per Section 708.

• Geotextile separation fabric should be non-woven, Class 1, Type C per Section 714.01.

For estimating purposes, it is assumed that the GRS wall will be bid as a single item based on the area of the finished face by the square foot.

The Contractor should prepare the final wall design in accordance with Section 257 of the project SCRs and submit to CFLHD for review prior to beginning wall construction.

CUT SIDE ROCKERY DESIGN

Proposed rockery locations to retain soil cuts are identified between rock cuts. Rockeries are earth retaining gravity walls constructed of large interlocking, dry-stacked rocks typically placed without mortar or reinforcement. These structures rely on the weight, size, shape, and interface friction of the individual rocks to resist earth pressures and provide overall stability. Rockeries have been recently utilized on many FHWA projects as context sensitive solutions preserving scenic, Little Blue Canyon, CO FLAP US 50(1) 16 May 2020 aesthetic, historic and environmental resources, while maintaining safety and mobility.

The rockery walls were designed in accordance with the guidelines provided in the Central Federal Lands publication, “Rockery Design and Construction Guidelines” (FHWA 2006). No geotechnical investigations were conducted at the rockery wall sites; therefore, a conservative design approach was followed requiring both foundation and retained material property verification during wall excavation prior to rockery construction. In lieu of soil testing, presumptive soil strength properties for retained and foundation soils were assumed for observed on-site soils. Static lateral earth pressures were applied to the back of the rockery; however passive earth pressure was neglected at the toe of the wall. The design was checked for adequate factors of safety against sliding, overturning, and bearing capacity. The recommended factors of safety for static and seismic rockery design are presented in Table 7.

TABLE 7: - Recommended Safety Factors for Design of Rockeries

Failure Mode Static Factor of Safety

Sliding 1.5 Overturning 2.0

Bearing 3.0

Rockeries were designed for a maximum back slope ratio of 1.0(V):1.5(H) with no surcharge loads.

Assumed material properties used in the design of the rockeries are presented in Table 8. Rockery geometries for the critical section height of 15 feet to meet the minimum safety factor as presented in Table 7 above are shown in Table 9.

4.2.5 Rockery analysis

TABLE 8: - Material Properties used in the Analysis for Rockeries

Retained and

Foundation Soil Friction

Angle, θ

Retained and

Foundation Soil Unit Weight, ɣs

Retained and

Foundation Soil

Cohesion, C

Sliding Friction Factor, μ

Rockery Face

Friction Angle, θ

Rockery Face Unit Weight, ɣr

Rockery Face

Cohesion, 36 degrees 125 pcf 50 psf 0.73 45 degrees 150 pcf 0 psf

TABLE 9: - Minimum Required Rockery Geometry to Satisfy Safety Factors

Minimum Base Width, B

Maximum Height, H

Minimum Embedment, Df

Backslope Angle, ß

Surcharge (psf)

Face Batter, V:H

Back Cut Inclination, V:H

7.5 ft 15.0 ft 1.0 ft 35 degrees 0 4:1 8:1

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Constructing the rockery base with individual rocks wider than 5.5 feet will be too heavy for conventional earth-moving equipment to move and place according to specification and geometry shown in the plans. Therefore, the criteria as displayed in Tables 10 and 11 have been developed to assist in the constructability of the rockery walls.

TABLE 10: - Minimum Base Rock Widths for Rockeries with a Given Height

Total Wall Height (1)

(Ft)

Minimum Base Rock Width “B”

(Ft)

15 to > 13 7.5

≤ 13 to > 11 6.5

≤ 11 to > 9 5.5

≤ 9 to > 7 4.5

≤ 7 to > 5 3.5

≤ 5 to ≥ 3 3**

(1) Includes 1-foot Embedment ** Absolute Minimum Base rock width

TABLE 11: - Rockery Base Width Requirements

All Rockery Heights Height greater than 11 Feet: Heights less than or equal to 11 Feet

A maximum tolerance of 6 inches may be applied toward the total base rock width “B”. Use rock with an “L” Dimension that is no more than 1-foot less than base rock “B” dimension. Do not consecutively place base rocks with widths less than “B”.

A maximum of 2 rocks is permitted for a composite base width “B”. A ratio of 60:40, front rock: back rock is required. Offset joints in composite base rocks a minimum of 6 inches.

Do not consecutively place composite base rocks. A ratio of 60:40, front rock: back rock is required.

Given the criteria as outlined in Tables 7 through 11 factors of safety were calculated for sliding, overturning and bearing capacity and are presented in Table 12 below.

TABLE 12: - A Summary of Calculated Factors of Safeties for Rockeries (1)

Case Factors of Safety Sliding Factor of Safety

Overturning Factor of Safety

Bearing Capacity Factor of Safety

1 Static 1.5 2.7 5.8

(1) Total Rockery wall height = 15 feet

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EMBANKMENTS

Roadway reconstruction requires embankment stabilization associated with roadway realignment, widening, driveway approaches, and culvert replacement. It is recommended that proposed embankments be constructed at 1V:2H slopes or flatter to maintain slope stability and facilitate vegetation growth. As an alternative, embankments can be steepened when mitigation treatments such as reinforced soil slopes (RSS) or embankment armoring that also acts as a buttress. At the time of this report, no RSS locations were identified and are not included in this report, if soil slopes steeper than 1V:2H are required, a RSS design can be provided. Partially Grouted Riprap (PGR) is proposed by the CFL Hydraulics group in select locations. See hydraulics report for more detail.

4.2.6 Mechanically Placed Rock Embankment

Embankments located over wet areas near the horseshoe realignment, (Sta. 675+40 to 676+35 RT and Sta. 680+55 to 682+00 RT) should be constructed with mechanically placed rock embankment at least 5 feet below the final ground elevation and 10 feet above, (minimum of 15 total feet). This rock embankment will allow water to enter the toe of the embankment and exit freely without toe destabilization in the form of minor slumping which would eventually migrate upward to the roadway.

CUT SLOPES

Recommended slope ratios are based on observed conditions at the existing cut face, investigation data, and anticipated slope height. In general, weathered bedrock slopes and combined soil and rock slopes can be cut at 1:1. Steeper cut slopes are recommended for intact rock based on structural analysis and rockfall potential. Cut slopes recommendations are based on the YA reports (2010 and 2020) and the revised LiDAR survey scans. While this report and the 95% CFL plans (October 2019)provide recommended cut slope ratios, the final cut slope angles are dependent on conditions encountered during construction. As blasting of the cut slope excavations advances they should be evaluated by the CFL geotechnical engineer and the Contractor’s geotechnical engineer or blasting representative.

4.2.7 Rock Cut Structural Stability

The majority of the bedrock observed consisted of gneiss, schist and migmatite with intrusive pegmatite veins. Discontinuities in the bedrock are varied in orientation and degree of weathering.

Rock slope stability is structurally controlled by the orientation, spacing, and persistence of discontinuities (joints, foliation planes, faults, and shear zones) within the rock mass; and the shear strength of the discontinuities (in-filling material, roughness, and seasonal pore-water pressure).

Potential rockfall sources resulting from differential erosion processes include colluvium, alluvial terraces, and residual soil deposits that could release individual rockfall events. Cut slope recommendations for soil and rock by station and structural geologic mapping data and kinematic analysis results are presented in the YA 2010 and 2020; rockfall and rock cut slope hazard rating analysis is presented in Appendix D and E respectively. Slopes with observed natural springs and seeps may require flattening during construction and should be evaluated by the geotechnical

Little Blue Canyon, CO FLAP US 50(1) 19 May 2020 engineer.

Cutslope recommendations provided in the YA (2010 and 2020) and revised survey cross sections were incorporated into the 95% plan set. Table 13 below are those stations where the proposed cutslope angle by YA was adjusted by CFLHD after further consideration to the following criteria:

1. Match existing cutslope angles, which are generally performing well.

2. Maintain consistent excavation burden for controlled blasting.

a. Minimizing cut slope heights by considering limited spot locations with minimal risk adjustments where higher (steeper) cutslope angles than those proposed by YA result in shorter rock cut heights and less rock excavation volume.

3. Minimizing final rock face surface area exposure by considering higher (steeper) cut slope angles than those proposed by YA at limited spot locations with minimal risk adjustments.

Less excavated surface area can reduce the rockfall risk in certain conditions based on the kinematic analysis, where there are no prominent daylighting or continuous discontinuity patterns.

4. Minimizing rockfall rollout based on CRSP analysis with certain cross section geometries.

Rockfall sourcing from lower (shallow angle) slopes has the potential to roll and build up angular momentum kinetic energy which can result in more modelled rocks reaching the travel lane (less rock retention in the ditch). Rockfall sourcing from steeper slopes (Vertical to 0.25H:1.0V) will have less chance to develop rotation falling into the ditch, with less likelihood of rolling out of the ditch.

TABLE 13: - Adjusted Cutslopes From YA 2020

Station to Station 2020 YEH Proposed cutslope (H:V)

FHWA Proposed cutslope (H:V)

650+50 - 651+50 0.25:1 0.33:1

666+40 - 667+70 0.25:1 0.50:1

4.2.8 Rockfall Catchment Area

A rockfall catchment area is defined as the area between the roadway edge of shoulder (HACP) and the base of a cut slope used to restrict most rockfalls from reaching the roadway. Refer to Appendix D for an illustration of the rockfall catchment area. 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. The Colorado Rockfall Simulation Program Version 4.0 (CRSP) was used to analyze the ditch catchment effectiveness of the ditch widths and cut slope geometries. 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. In select locations the ditch configuration includes a minimum 7-foot flat bottom to accommodate the drop inlet drainage structure. A flat bottom ditch is not the optimal slope for stopping rockfall but a minimum 10-foot 1V:4H sloping grade in the roadway clear zone to reach the shoulder is also included in the ditch design. This sloping grade combined with draped mesh and regular maintenance of the flat bottom ditch should not significantly reduce effectiveness of the rockfall catchment system

Little Blue Canyon, CO FLAP US 50(1) 20 May 2020

Table 14 displays the CRSP analysis for the percent of rockfall retained in the ditch for proposed cut slope ratios of 1V:0.25H, 1V:0.33H and 1V:0.5H. The percent retained assumes mitigation measures of draped mesh is utilized to reduce the effective rock free fall height to 40 feet from the ditch line.

CRSP models were developed utilizing field observations that spherical rocks with 2-foot diameters could fall from the top of the existing and proposed cuts. Greater detail on the CRSP methodologies and material parameters used in the analysis are given in Appendix D. The flat bottom ditch was not the controlling scenario in CRSP so the results in Table 14 represent the minimum assumed percent retained with a 10-foot 1V:4H sloping grade from the traveled way and does not include the added width of a flat bottom ditch. The improvement to the ditch containment from the existing rock slopes sections to the proposed rock cut slopes is estimated to be between 25% and 85%.

From the CRSP analysis, a 10-foot wide standard catchment ditch sloped downward from the roadway at 14 degrees (1V:4H) is recommended below proposed rock cut slopes. This ditch greatly increases the estimated percent of rockfall contained from the existing to proposed condition, as shown in Table 14. Routine ditch cleaning must be included CDOT’s maintenance plan to maintain a suitable catchment area.

TABLE 14: - Summary of CRSP Analysis for Proposed Cutslopes (1)

Proposed Cut Slope (V:H)

Analysis Points

Total Number of Rocks Passing Analysis Point

Percent Passing

Percent Retained in Ditch

1:0.25

Bottom of Cutslope 1000 100.0%

89.5% Edge of Ditch 105 10.5%

Center Line 1 0.1%

1:0.33

Bottom of Cutslope 1000 100.0%

85.8% Edge of Ditch 142 14.2%

Center Line 5 0.5%

1:0.50

Bottom of Cutslope 1000 100.0%

71.0% Edge of Ditch 290 29.0%

Center Line 8 0.8%

(1) Ditch: 10-Ft Gravel Sloped Ditch, 4-Ft shoulder. Dropping 1000 Spherical 2-Ft. Diameter Rocks. Assumed mitigation of draped mesh down to 40 feet from ditch line.

4.2.9 Rock Cutslope Ratings

The CFL geotechnical team rated 12 proposed rock cuts to be excavated to improve roadway geometrics, improve sight distance and increase rockfall ditch capacity. The Colorado Rockfall Hazard Rating System (CRHRS) was utilized during the rating survey. The following assumptions were made:

Little Blue Canyon, CO FLAP US 50(1) 21 May 2020

(1) The rockfall frequency scores for all the slopes were assumed to be yearly/seasonal as rockfall frequency data could not be obtained, and

(2) Climatic data, accident history, and average daily traffic counts were all kept consistent for all the rated slopes for continuity and accuracy.

The existing condition CRHRS score ranged from 579 to 975 with an average combined score rating of 677. By evaluating proposed cut slope heights, slope angles, ditch dimensions, sight distances, and slope condition improvements, a proposed condition CRHRS score was first estimated without structural mitigation (draped mesh). The proposed (unmitigated) condition CRHRS estimated score ranged from 453 to 669 with an average combined score rating of 518.

These results indicate an average 159-point improvement for the slopes analysis. Slopes were then evaluated with structural mitigation in the form of draped mesh. The draped mesh CRHRS score was estimated using identical parameters to the proposed condition, but with a reduced slope height of 40 feet across all slopes. The draped mesh CRHRS score ranged from 429 to 591 with an average combined score rating of 492. Draped mesh improvements from proposed conditions varied widely across slope IDs and is better evaluated on a case by case basis. A summary of CRHRS scores under all conditions is illustrated in Table 15, and a detailed discussion on the CRHRS parameters and the field data collected are included in Appendix E.

The station ranges displayed for the Slope ID’s in Table 15 are not an exact match to the proposed cut slope ranges due to the realignment and comparison of existing corridor rock slopes to that of the proposed rock cuts. The station ranges will be revised during a post-construction CRHRS rating when the actual slope conditions can be evaluated.

TABLE 15: - Rock Cut Slope Ranking by CRHRS an ki

Slope ID

Station Propose d Cut Slope

Length

(FT)

Proposed Cut Slope

Height

(FT)

CRHRS Combined Score

Start End

Existing Rock Slope or Cut Slope (1)

Propose d

Cut Slope

Proposed Cut Slope with Draped Mesh (2)

1 649+00 647+00 652+00 163 158 975 669 591

2 663+00 662+00 664+00 122 62 789 483 477

3 596+50 595+50 599+50 187 181 735 555 477

4 629+00 627+00 629+00 83 74 687 543 537

5 613+00 609+40 614+80 164 146 681 573 495

6 591+70 589+60 593+80 77 56 645 483 477

7 637+00 635+00 638+00 88 79 645 453 429

8 668+90 666+50 671+20 76 54 621 465 459

9 603+75 602+50 605+00 85 76 591 501 477

10 656+00 655+90 657+80 61 48 591 459 459

11 678+00 676+00 680+00 66 47 585 477 477

12 699+70 698+40 701+00 55 39 579 549 549

(1) Due to geometry of realignment, exact rock cuts and rock slopes were estimated based on current roadway location and does not align exactly with the proposed cut slopes.

(2) Reduced slope height to 40 feet maximum for draped mesh ratings.

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4.2.10 Draped Mesh

Draped mesh is recommended for those slopes defined in Table 16 below to provide surface control to highly weathered, fractured, jointed, raveling and eroding slopes. Draped installations suspend mesh from the top of a slope using grouted ground anchors, dowels or rock bolts. The mesh is free-hanging against the slope providing surface control for loose rock and talus which allows the material to gradually migrate down the slope behind the mesh as weathering and erosion processes occur. Draped installations are particularly beneficial on slopes subject to small-sized, high-volume rockfall resulting from highly weathered rock or colluvial deposits. In situations where loose rock, cobbles or boulders tend to accumulate in pockets behind the mesh, draped installations are easier for maintenance crews to clean behind and repair.

TABLE 16: Draped Mesh Locations

Station Side

Proposed Maximum Cut Slope Height

(FT)

Height of Mesh from The Bottom of Ditch

Line Start End

591+15 592+45 LT 56 40

596+10 597+00 LT

597+75 598+60 LT 40

602+80 604+25 LT 76 40

611+10 614+20 LT 146 40

627+20 628+50 RT 74 40

633+50 633+90 RT 50 40

635+90 637+20 RT 79 40

648+40 651+80 RT 158 40

656+45 656+70 RT 48 40

661+90 663+10 RT 62 40

The design criteria for the draped mesh is based on accepted engineering practices for designing rockfall mitigation systems. The wire mesh design is based on the Washington State Transportation Commission Report “Design Guidelines for Wire Mesh Cable Net Slope Protection, WA-RD 612.2.”

Modeling of the rockfall was completed using the Colorado Rockfall Simulation Program 4.0 developed by the Colorado Department of Transportation. Rockfall models were simulated based on the assumption of a maximum rock diameter of 2-feet (690 pounds) and a maximum rock impact velocity of 10 feet per second. The wire mesh is designed to hold smaller potentially unstable rock in place while contacting and covering the rock slope. The wire mesh is also designed to allow rocks to migrate between the mesh and rock cut face down into the ditch. The system is not considered a rockfall stabilization feature or a rockfall barrier system.

Periodic inspection and maintenance of rockfall mitigation features by CDOT is necessary to ensure the desired protection level is not degraded by impact damage which exceeds the design limits of the mesh system including, but not limited to, large debris flow events. Performing regular maintenance will prevent debris from accumulating and loading the mesh which can reduce the design capacity of the

Little Blue Canyon, CO FLAP US 50(1) 23 May 2020 mesh for a rockfall event.

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 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 from such events, is assessed in design.

4.2.11 Controlled Blasting

Blasting for rock excavation incorporates presplitting prior to production blasting to control overbreak at the excavation limits. Presplitting is recommended to provide slope stability by creating a fracture plane before any production blast holes detonate.