CO_ER_94_81_106(1)-1.Boulder_County.Left_Hand_Canyon.Final.2015._.pdf

BOULDER COUNTY

LEFTHAND CANYON DR.

Arapaho & Roosevelt National Forests

CO ER 94/81/106(1)

FINAL GEOTECHNICAL REPORT

Report No. CO-EX-0094-15-01

Geotechnical Services Branch

August 2015

Lefthand Canyon Dr., CO ER 94/81/106(1) August 2015

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 EXPLORATION AND LABORATORY TESTING PROGRAM

SECTION FOUR--ANALYSIS AND RECOMMENDATIONS

4.1 BRIDGE FOUNDATION DESIGN

Bearing Resistance and CRFB Design ............................................................. 9 4.1.1 Abutment Design ............................................................................................. 10 4.1.2 CRFB Requirements........................................................................................ 11 4.1.3

4.2 BOX CULVERT DESIGN

4.3 ROCKERY DESIGN

4.4 EMBANKMENTS

4.5 CUT SLOPES

4.6 CORROSION POTENTIAL

4.7 CONSTRUCTION CONSIDERATIONS

4.8 LIMITATIONS

SECTION FIVE--REFERENCES

TABLES

TABLE 1:- Summary of Seismic Parameters for Site Class B TABLE 2:- Resistance Factors φ for each Limit State TABLE 3:- Recommended Soil Parameters for Bridge Foundation Design TABLE 4:- Static Lateral Earth Pressures for Bridge Abutments TABLE 5:- Proposed Box Culverts TABLE 6:- Recommended Safety Factors for Static and Seismic Design of Rockeries TABLE 7:- Material Properties used in the Analysis for Rockeries TABLE 8:- Rockery Geometry TABLE 9:- A Summary of Calculated Factors of Safeties of Rockeries TABLE 10:- Summary of CRSP Analysis TABLE 11:- Rock Cut Slope Ranking by CRHRS

Lefthand Canyon Dr., CO ER 94/81/106(1) August 2015

FIGURES

FIGURE 1:- Vicinity Map FIGURE 2:- Bridge 1 Boring Logs Summary FIGURE 3:- Bridge 2 Boring Logs Summary FIGURE 4:- Bridge 3 Boring Logs Summary FIGURE 5:- Bridge 4 Boring Logs Summary FIGURE 6:- Preliminary Cut Slope Recommendations

APPENDICES

APPENDIX A - Field Exploration Program APPENDIX B - Laboratory Test Results APPENDIX C - Photos APPENDIX D - Rock Slope Mapping and Kinematic Analysis APPENDIX E - Colorado Rockfall Hazard Rating System (CRHRS) Field Measurements APPENDIX F - Colorado Rockfall Simulation Program (CRSP) APPENDIX G - Stability Analysis

Lefthand Canyon Dr., CO ER 94/81/106(1) 3 August 2015

SECTION ONE--INTRODUCTION

This report presents the results of the geotechnical investigation conducted for the Boulder County, Lefthand Canyon Drive, CO ER 94/81/106(1) project. The project involves reestablishing permanent access through the flood damaged areas of Lefthand Canyon Drive and includes improvements to the horizontal and vertical alignments, drainage including bridges and culverts, roadsides cut and fill slopes, and embankment armoring. Modifications to the roadway alignment include constructing rock cuts, where necessary, to create a consistent width. The project stationing and proposed alignment begins where Spring Gulch Crosses the roadway (MP 10.2) and extends to the intersection with US 36 (MP 0.0), as shown on Figure 1, Vicinity Map.

In September of 2013, a series of extreme rain events caused by a slow-moving cold front that stalled over the state of Colorado, clashing with warm humid monsoonal air from the south. This resulted in heavy rain storms and catastrophic flooding along Colorado's Front Range. These widespread storms dumped up to 17 inches of rain causing extensive flash flooding and damaging dams, roads, and bridges cutting off many residences from reaching their homes. Boulder and Larimer Counties were affected the worst and received the most damage. The damage was especially severe along roadways adjacent to streams and rivers, such as Lefthand Canyon Drive, where several segments of the roadway were washed out preventing access. The access through the damaged areas was reestablished by temporary repairs. Although the temporary repair was suitable for reopening the roadway to vehicle traffic, concerns remain regarding embankment stability, durability, and substandard roadway geometry.

This report summarizes the relevant findings from the geotechnical investigation and provides recommendations pertaining to the June 2015 Design Plans. The geotechnical investigation consisted of rock cut mapping, field subsurface exploration at proposed bridge locations, laboratory testing, correlation of available data, and geotechnical analysis. This design report draws interpretations from the following previously prepared reports.

• AECOM Technical Memorandum, “Recommendations for Retaining Wall and Pavement Construction, Lefthand Canyon Drive Station 70+00 to 364+00, Dated March 23, 2015.

• Yeh and Associates Inc, “Pavement Design Report, Boulder County Repairs. Left Hand

Canyon Drive, Boulder County, Colorado. Yeh Project Number 213-240 TO 2.1, Dated March 16, 2015.

Some important information presented in the previous reports is not repeated here; the reader is encouraged to read these report 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.

Lefthand Canyon Dr., CO ER 94/81/106(1) 4 August 2015

1.1 PROPOSED CONSTRUCTION

To accommodate hydraulic conveyance of future flood events, the project proposes replacement of four box culvert locations with new bridges, replacement of three corrugated metal pipe (cmp) culvert locations with three new concrete box culverts (cbc), new cut slope excavations and river embankment stabilization. In general, rather than reconstructing the roadway by encroaching back into the new stream channel, the project intends to shift the roadway into the cut slopes and use the rock excavated from the cut slopes to armor and stabilize 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 maintainable rock cuts relies on good standard slope stability practices including, (1) the implementation of presplitting controlled blasting 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, and (3) the improvement of roadway geometry to assure better sight distance. Increasing the ditch width and improving site distance for traffic, will significantly reduce the risk of rockfall exposure to the travelling public. Since limited rockfall mitigation and no bolting existed along the original route, it was decided that to design the new rock cuts in the same context. However, spot bolting and two area of draped mesh are anticipated to be required in areas of observed rock slope instability. For safety, rock catchment ditches and increased sight distance were also added.

For improved hydraulic capacity, several bridges, designated Bridge 1 through Bridge 4, are proposed in the final project plans to replace existing, smaller, less efficient, box culverts at Stations 231+45, 274+76, 386+34, and 436+52, respectively. The single-span bridges are designed to provide a 40- to 50-foot wide channel opening with vertical concrete abutments. A detailed layout of the proposed bridges with respect to site topography and boring locations are presented on Figure 2, Boring Log Summary.

The installation of concrete box culverts (cbc) are proposed to better pass flow events at Station 4+04 (Spring Gulch), Station 19+65 (Lefthand Creek placed within Lickskillet Road approach), and Station 471+67 (designated as Spruce Gulch). The existing cbc at Station 356+85 (tributary adjacent to forest service access road) is anticipated to be repaired.

Lefthand Canyon Dr., CO ER 94/81/106(1) 5 August 2015

SECTION TWO--GEOLOGY AND SEISMICITY

2.1 GEOLOGY

Lefthand Canyon is located along the Colorado Front Range of the Southern Rocky Mountain Province. The alignment from Station 0+00 to approximate Station 405+50 is underline by interlayered Precambrian age bedrock. The Laramide orogeny, a continental uplift with several episodes of faulting and volcanic activity, occurred approximately 70 million years ago.

During the Laramide orogeny, these bedrock materials were faulted upward forming the Rocky Mountains. This uplift also uplifted the overlying sedimentary layers, as encountered from approximate Station 405+50 to the end of the project near Station 553+50, consisting of a variety of sandstones, claystones, siltstones, and limestones commonly referred to as the Dakota Hogback.

The Rocky Mountains observed today have since been modified by glacial and fluvial processes that have eroded the bedrock materials, forming peaks and canyons with unconsolidated glacial, colluvial, and alluvial deposits.

Geologic conditions of the project site from, west to east, can be found on the United States Geological Survey (USGS) National Geologic Map Database http://ngmdb.usgs.gov/ngm-bin/ngm_compsearch.pl on the following maps:

• Gable, D.J., 1980, “Geologic map of the Gold Hill quadrangle, Boulder County, Colorado” U.S. Geological Survey, Geologic Quadrangle Map GQ-1525, scale 1:24,000.

• Wrucke, C.T. and Wilson, R.F., 1967, “Geologic map of the Boulder quadrangle, Boulder County, Colorado.” U.S. Geological Survey, Open-File Report OF-67-281, scale 1:24,000.

• Braddock, W.A., Houston, R.G., Colton, R.B., and Cole, J.C., 1988, “Geologic map of the Lyons quadrangle, Boulder County, Colorado” U.S. Geological Survey, Geologic Quadrangle Map GQ-1629, scale 1:24,000.

According to the geologic maps, bedrock from Station 0+00 to approximate Station 405+50 consists primarily of three interlayered bedrock units, the Boulder Creek Granodiorite, Silver Plume Granite, and Biotite Gneiss. The Boulder Creek Granodiorite is described as gray, pinkish grey to black and white, medium to course grained to very course grained, locally porphyritic rock with feldspar crystals several centimeters in length and well aligned. The Silver Plume Granite is characterized as pale-grayish-orange to pale-reddish-gray equigranular fine to medium grained biotite-muscovite-quartz-monzonite. The Biotite Gneiss is described as medium to dark grey, fine to medium grained gneiss and schist that is strongly foliated and moderately well layered with variable quantities of migmatite and pegmatite in semiconcordant stringers and lenses. All three of these rock types are cut by pegmatite dikes (bands of coarser, larger crystals) and by white quartz and feldspar veins.

From approximate Station 405+50 to Approximate Station 430+00, the primary outcropping bedrock formation within proximity to the roadway is the Fountain Formation; described as, pale-red, poorly sorted arkosic sandstone and conglomratic sandstone interstatified with pale-reddish-http://ngmdb.usgs.gov/ngm-bin/ngm_compsearch.pl http://ngmdb.usgs.gov/ngm-bin/ngm_compsearch.pl http://ngmdb.usgs.gov/Prodesc/proddesc_1073.htm http://ngmdb.usgs.gov/Prodesc/proddesc_8216.htm http://ngmdb.usgs.gov/Prodesc/proddesc_8216.htm http://ngmdb.usgs.gov/Prodesc/proddesc_1153.htm http://ngmdb.usgs.gov/Prodesc/proddesc_1153.htm

Lefthand Canyon Dr., CO ER 94/81/106(1) 6 August 2015 brown feldspathic sandstone.

The overburden soils encountered along the project primarily consist of alluvial, colluvial, and talus deposits. Alluvium can be characterized primarily as sands and gravels with varying percentages of silt, cobbles and boulders. In addition, alluvial terraces were observed throughout the canyon on several different levels with some terraces being a significant rock fall hazard due to the primarily rounded boulders in a cohesionless matrix. Colluvial soils can generally be described as poorly sorted clay, silt, sand, and gravel with cobbles and boulders. Boulders larger than 10-foot in diameter are commonly present in the slopes and stream channel.

Bedrock and overburden soil type anticipated to be encountered by station in proposed cut slopes and can be found in the description column of Figure 6, Cut Slope Recommendations. A description of the soil and rock encountered from the borings is discussed in more detail in Appendix A.

2.2 GEOLOGIC HAZARDS

Geologic hazards exist from the natural environment, existing roadway condition, and proposed construction. Primary geologic hazards that exist in the vicinity of the project area are slope instability at cut and fill slope locations, rockfall, flash floods, and to lesser extent seismic events.

Slope instability and rockfall hazard exist throughout the roadway alignment and is likely to continue. Slope instability is generally a result of boulders/rock fragments dislodging from oversteepened 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 likely to occur during construction. Rockfall risk is particularly high in the wet winter and early spring months. The newly constructed rock slopes will incorporate a catchment ditch to significantly reduce the risk of rock falls since falling rocks will most likely be retained in the ditch.

Slumps and potouts have occurred at various embankment fill locations requiring armoring, re-grading, and enhanced drainage. Such failures are generally attributed to oversteepened slopes or erosion that occurred during the flood events.

Flash Floods have attributed to significant roadway damage in this canyon necessitating this project to reestablishing permanent access. Flooding can occur at any point during the year and may occur during construction. Exposure to flood events should be lessened by conducting excavations for bridge foundations, culverts, embankments, and channel work during periods of dry weather. The contractor should maintain a flood evacuation and mitigation plan during construction. See the hydraulic recommendations report prepared by the CFLHD Hydraulic discipline for full analysis and results.

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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, 2014, 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 Bridge 3 (40.10954 latitude and -105.31698 longitude) was obtained in accordance with the AASHTO ground motion maps. Bridge foundations are anticipated to be founded on competent bedrock or concrete riprap extending to bedrock;

therefore the site is classified as Class B 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 and can be used for all four proposed bridge locations.

Table 1: Summary of Seismic Parameters for Site Class B

Peak Seismic Ground Acceleration Coefficient, (As) 0.065

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDS) 0.136

Horizontal Response Spectral Acceleration at Period of 1.0 sec (SD1) 0.035

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.0

Site Factor at Short-Period of Acceleration Spectrum, (Fa) 1.0

Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.0

Based on the long acceleration coefficient SD1 value of 0.035, the site is assigned to seismic hazard Zone 1 in accordance with Table 3.10.6-1 of AASHTO.

Lefthand Canyon Dr., CO ER 94/81/106(1) 8 August 2015

SECTION THREE—FIELD EXPLORATION

3.1 FIELD EXPLORATION AND LABORATORY TESTING PROGRAM

CFL geotechnical personnel conducted a subsurface investigation during June 22 through July 1, 2015, to characterize the subsurface materials at each bridge foundation location. The subsurface exploration program consisted of drilling a total of 11 borings at the 4 proposed bridge locations.

Boring locations are shown on Figure 2, Boring Log Summary. Subsurface conditions were logged and representative samples were collected and transported to the CFLHD Materials Laboratory in Lakewood, CO, for physical property testing. More detailed descriptions of the work performed, borings logs, and laboratory test data are contained in Appendices A and B, respectively.

Subsurface exploration site photos are contained in Appendix C.

CFL geotechnical personnel also conducted rock slope discontinuity mapping and rockfall hazard ratings from May 28 through June 10, 2015, of proposed rock cuts to evaluate rockfall mechanisms and rock cut stability. Window mapping was conducted to characterize the discontinuities and estimate the shear strength (dip angle and direction, estimation of surface roughness, observation of infilling materials, estimation of persistence and spacing). The Colorado Rockfall Hazard Rating System (CRHRS) was used to evaluate the existing rockfall hazard on the roadway and aid in mitigation selection for the proposed alignment. Program Results of the rock slope mapping and kinematic analysis are presented in Appendix D. Results of the CRHRS are presented in Appendix E.

Lefthand Canyon Dr., CO ER 94/81/106(1) 9 August 2015

SECTION FOUR--ANALYSIS AND RECOMMENDATIONS

This section provides analysis and recommendations for the bridge foundations, box culvert design, rockery design, roadway embankment armoring, rock and soil slope stability, and rockfall.

4.1 BRIDGE FOUNDATION DESIGN

Spread footings extending to bedrock or placed on concrete riprap foundation block (CRFB) extending to bedrock are recommended for the foundation at the bridge abutments. Scour elevations were not provided at the time of this report; however, it is assumed that the Extreme Event II Limit State (check flood) scour elevation extends to bedrock. Competent bedrock is anticipated to be encountered within favorable depths typical for cast-in-place spread footings design heights for all abutments except Bridge 3, Abutment 1. At this location, a CRFB extending to bedrock is recommended. The CRFB is a shallow foundation that consists of a rectangular-shaped excavation filled with large concreted riprap (≥class 8).

Spread footings will be designed to have adequate geotechnical and structural resistances, tolerable settlements, and tolerable lateral displacements by CFL Bridge Engineer. Spread footing foundations were analyzed in accordance with Section 10.6 of the American Association of State Highway and Transportation Officials (AASHTO) probabilistic Load and Resistance Factor Design (LRFD), 7th edition, 2014. Resistance factors used for analysis for each limit state are provided in Table 2.

TABLE 2:- Resistance Factors φ for each Limit State

Bearing Resistance and CRFB Design 4.1.1

Although spread footing are recommended to be founded on bedrock, or CRFB extending to bedrock, estimated retained soil strength parameters, shown in Table 3, were used to determine the presumptive nominal bearing resistance (Qn) of 10 kips per square foot (AASHTO Table C10.6.2.6.1-1) to limit vertical deformations to one-inch. From review of the unconfined compressive strength tests presented in Appendix B, this approach is considered conservative.

TABLE 3:- Recommended Soil Parameters for Bridge Foundation Design

MATERIAL

DESCRIPTION

UNIT WEIGHT

ɣ, pcf

COHESION

c, psf

FRICTION

ANGLE

φ, deg

Sandy GRAVEL with cobbles and boulders 130 0 38

LIMIT STATE BEARING SHEAR RESISTANCE TO

SLIDING

PASSIVE PRESSURE

RESISTANCE TO SLIDING

Strength I 0.45 0.80 0.50 Service I 1.00 1.00 1.00

Extreme Event I, II 1.00 1.00 1.00

Lefthand Canyon Dr., CO ER 94/81/106(1) 10 August 2015

At the Strength I Limit State, bearing resistance was calculated as greater than the presumptive Service Limit State value of 10 ksf. Sliding resistance between the placed riprap and the supporting bedrock was evaluated with a resistance factor of 0.8 and a roughness coefficient of

0.78. Passive pressure was neglected due to the scour potential of the material in front of the foundation block.

Based on AASHTO Table 4.7.4.3.1-1, no seismic analysis is required for a single span bridges that fall in a seismic zone 1. Scour elevations were not provided at the time of this report; however, it is assumed that the Extreme Event II Limit State (check flood) scour elevation extends to bedrock, which is not considered susceptible to scour. The CRFP will behave similarly to the typical embankment armoring section during a flood event.

Abutment Design 4.1.2

Abutments and wing walls should be designed to resist lateral earth pressures and other applicable lateral loads in accordance with AASHTO. Lateral earth pressure is influenced by the strength of the abutment backfill, the presence or absence of water, and the ability of the abutment or wall to move in response to lateral loads. Other loads, such as live loads, construction loads, and soil compaction loads should also be considered in the design.

Unbalanced water behind an abutment or wall adds significant lateral pressure and should be avoided by using free draining gravity outlets for water. Abutment and wing wall backfill should consist of structural backfill.

The coefficient of at-rest earth pressure should be used for design if the abutment is so restrained that it cannot be expected to rotate (deflect at the top) 0.002 times the wall height. Where deflection of the abutment can be expected, a coefficient of active earth pressure should be used for wall design. Active and at-rest lateral earth pressures of properly placed and compacted select granular backfill and unclassified borrow above the water table are presented in Table 4. The values are unfactored loads and assume that the surface of the soil slope behind the wall is horizontal.

Table 4:- Static Lateral Earth Pressures for Bridge Abutments

Backfill Type

Assumed Backfill Parameters Earth Pressure

Case Unfactored

Equivalent Fluid Pressure (pcf)

Structural Backfill c = 0 psf ᵠ = 34 deg.

γ = 125 pcf

Active 35

At-Rest 55

Unclassified Borrow c = 0 psf ᵠ = 30 deg.

γ = 125 pcf

Active 42

At-Rest 63

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At the Strength I Limit State, lateral loads imposed on foundations may be resisted by the development of friction between the footing and supporting bedrock or CRFB. A resistance factor of 0.8 and a friction coefficient of 0.78 may be used for cast in place concrete. Passive pressure should be neglected due to the scour potential.

The subsurface conditions at the base of the spread footing and foundation block will not be known until excavation is completed to the design elevations. A CFL Qualified Geotechnical Representative must observe the bottom of all foundation excavations prior to footing or CRFB placement in order to evaluate the supporting capacity and potential scour of the foundation material. Adjustments may be recommended to the CO, as necessary, for bedrock, boulder obstructions, or loose material prior to construction of foundations.

CRFB Requirements 4.1.3

The CRFB is to be a rectangular-shaped column of concrete riprap meeting a minimum material gradation equal to or greater than Class 8, as specified in the contract documents and FP-14.

Construct the CRFB from the bottom up with the following dimensions:

• Base width (minimum) = 14 feet

• Top width (minimum) = 14 feet

• Height (estimated) = 13 feet (varies based on depth to bedrock, 15 feet max)

• Length (typical) = 58 feet (abutment length plus 2 feet on either end)

Concrete coverage is assumed to be approximately 25 percent, which binds the riprap with sufficient concrete coverage, but also allows for permeability within the block to prevent pore water pressure buildup behind the foundation block. The consistency of the concrete is specified in the contract documents (Section 601- Minor Concrete Structures, FP-14) with a maximum 5-inch slump and minimum 3,000 psi compressive strength after 28-days. These properties may be adjusted by the CO in the field during placement to ensure the consistency allows for concrete free flow in between the riprap matrix without full coverage. A rough surface at the base of the foundation is ideal for increased frictional resistance and stability at the base of the foundation block. Place Geotextile Class 1, Type C, nonwoven (subsidiary) filter material at the back of the excavation to prevent piping of materials into the riprap.

4.2 BOX CULVERT DESIGN

New concrete box culverts (CBC) are proposed to pass the 10 year flood event and 100 year flood with a 1 foot maximum overtopping depth. The installations of the CBCs are proposed as presented in Table 5. Box culverts are anticipated to be founded on unconsolidated Silty SAND with gravel, cobbles, and boulders. Material properties were estimated for foundation design based on observations during the field investigation and known empirical strength characteristics of like materials. A soil unit weight (γ) of 130 pcf, internal friction angle (φ) of 36, and cohesion intercept

(c) of 0 are the selected design values for box culvert foundations.

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Table 5:- Proposed Box Culverts

Station Box Culvert Size (H:V in Feet) Description

4+04 10:8 Crosses mainline, designated Spring Gulch tributary, embedded a minimum of 1 foot for fish passage

19+65 10:6 Crosses Lickskillet Road approach, Left Hand Creek 471+67 8:5 Crosses Mainline, designated Spruce Gulch

The culvert, head wall, and wing walls can be supported on spread footings designed for a nominal bearing resistance of 8,000 pounds per square foot (psf) provided that up to 1.0 inch of settlement is acceptable, the slope below the wall does not exceed 1V:4H, and the minimum footing with is 18 inches. The base of the footing should be located below the depth of frost potential, which is 30 inches based on local practices. Appropriate resistance values should be applied as shown in Table 2.

Active and at-rest lateral earth pressures of properly placed and compacted select granular backfill and unclassified borrow above the water table are presented in Table 4. The values are unfactored loads and assume that the surface of the soil slope behind the wall is horizontal. Unbalanced water behind a wall adds pore pressure and should be avoided by using structural backfill (FP-14, Section 704.04) against backfilled structures and assuring a free draining gravity outlet for captured water.

Unclassified borrow (FP-14, Section 704.06) may be used in the retained zone beyond the culvert footings. Below the mean canal level, the walls must also be designed for the full hydrostatic condition. Wing-wall design should consider surcharge loading due to traffic and construction equipment.

Lateral loads imposed upon structures may be resisted by the development of friction between the base of the footings and the supporting soils. A nominal friction factor of 0.72 may be used in design for cast-in-place footings established on Silty SAND and gravel. The appropriate resistance friction factor should be applied as shown in Table 2.

Because the box culverts are located in a natural drainage channels, surface water and groundwater is expected requiring dewatering and/or water diversion.

4.3 ROCKERY DESIGN

Proposed rockery locations are not fully identified at the time of this report; however, possible rockery locations to retain soil cuts are identified in the description column of Figure Figure 6, Cut Slope Recommendations. 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 projects as context sensitive solutions preserving scenic, aesthetic, historic and environmental resources, while maintaining safety and mobility.

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The Rockery was designed in accordance with the guidelines provided in the Central Federal Lands publication, “Rockery Design and Construction Guidelines”. No geotechnical investigations were conducted at the 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 and seismic 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 6.

TABLE 6:- Recommended Safety Factors for Static and Seismic Design of Rockeries

Mode of Failure Static Factor of Safety

Seismic Factor of Safety

Sliding 1.5 1.1 Overturning 2.0 1.5

Bearing 3.0 2.0

Rockeries were designed for two cases; the first case assumes a maximum back slope ratio of 1V:5H, with a driveway traffic surcharge of 150 pcf. The second case assumes a maximum back slope ratio of 1V:1.5H with no surcharge loads. Assumed material properties used in the design of the rockeries are presented in Table 7. Rockery geometries and the calculated rockery design factors of safety for each wall analysis are shown in Table 8 and Table 9. Rockery analysis

TABLE 7:- Material Properties used in the Analysis for Rockeries

Retained Soil Friction Angle, θ

Retained Soil Unit Weight, ɣs

Soil Cohesion, C

Sliding Friction Factor, μ

Rockery Face Unit Weight, ɣr

Horizontal Seismic

Coefficient, Kh

Vertical Seismic Coefficient, Kv

36 degrees 135 pcf 0.0 ksf 0.73 145 pcf 0.018 0.0

TABLE 8:- Rockery Geometry

Case Minimum

Base Width, B

Maximum Height, H

Minimum Embedment, Df

Backslope Angle, ß

Surcharge (psf)

Face Batter, V:H

Back Cut Inclination, V:H

1 6.0 ft 12.0 ft 1.5 ft 11 degrees 150 4:1 8:1

2 6.0 ft 10.0 ft 1.5 ft 34 degrees 0 4:1 8:1

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TABLE 9:- A Summary of Calculated Factors of Safeties for Rockeries

Case Factors of Safety Sliding Factor of Safety

Overturning Factor of Safety

Maximum Bearing Pressure

Bearing Capacity Factor of Safety

Static 2.1 2.2 3.0 ksf 9.3

Seismic 1.9 2.0 3.2 ksf 8.6

Static 2.3 3.5 1.8 ksf 15.6

Seismic 1.9 2.7 2.2 ksf 12.6

Rockery design assumes all flow is contained on the river side of the Lefthand Canyon Drive roadway embankment and that the river will not generate scour at the base of a wall located on the in-board side of the roadway. Hydraulic analysis should verify this assumption when rockery locations are identified.

4.4 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 over 1V:2H are required, an RSS design can be provided. Riprap embankment armor is anticipated for new embankments adjacent to Lefthand Creek.

4.4.1 Riprap Embankment Armor

The riprap embankment armor and potential scour design was not complete at the time of this report. Final rip-rap armor design is the responsibility of CFLHD Hydraulic Services and should be reviewed for geotechnical stability. A preliminary analysis was conducted assuming a scour depth of 8 feet below the existing stream channel a concrete riprap meeting a minimum material gradation equal to or greater than Class 8, with a concrete coverage of approximately 15 percent.

This binds the riprap but also allows for sufficient permeability within the block to prevent pore-water pressure buildup behind the foundation block. A rough surface at the base of the foundation was assumed for increased frictional resistance and stability at the base of the foundation block.

Riprap embankment armor was evaluated to determine minimum dimensions from stability analysis for proposed slope geometry, foundation material, and groundwater conditions. Two slope scenarios are assumed, 1V:1H slope with a maximum height of 30 feet and 3V:1H slope with a maximum height of 24 feet. Analyses of the buttress were performed using gSTABLE7, two dimensional, limit equilibrium computer program from Gregory Geotechnical Software. The Simplified Bishop method of slices was used with retained soil parameters estimated as soil unit weight (γ) of 130 pcf, internal friction angle (φ) of 38, and cohesion intercept (c) of 0. Foundation soil parameters were estimated as soil unit weight (γ) of 140 pcf, internal friction angle (φ) of 42, and cohesion intercept (c) of 0. The recommended design for geotechnical stability of the analyzed scenarios consists of a minimum 6.5 foot top width and a 10 foot minimum base width of high friction angle material extending to bedrock or below scour depth.

Lefthand Canyon Dr., CO ER 94/81/106(1) 15 August 2015

4.5 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 combination soil and rock slopes can be cut at 1:1, highly weathered bedrock and non-saturated colluvium and alluvium can be cut at 1V:1.5H. Steeper cut slopes are recommended for intact rock based on structural analysis and rockfall potential as presented in Figure 6. Cut slopes recommendations are based on the preliminary CFL plans dated June 8, 2015, final cut slope angles are dependent on conditions encountered during construction and should be evaluated by the geotechnical engineer.

Rock Cut Structural Stability

The majority of the bedrock observed consisted of sandstone, granite, and gneiss with variable discontinuity orientations and variable weathering grades. Rock slope stability is structurally controlled by the orientation, spacing, and persistence of discontinuities within the rock mass (joints, foliation planes, faults, and shear zones); and the shear strength of the discontinuities (in-filling material, roughness, and pore-water pressure). Rockfall potential resulting from differential erosion processes consists of deposits of colluvium, alluvial terraces, and/or residual soil.

Structural geologic mapping data and kinematic analysis results are presented in Appendix D;

rockfall and rock cut slope hazard rating analysis is presented in Appendix E. Cut slope recommendations for soil and rock by station, including shrink/swell, potential rockery locations, and ditch type recommendations, are presented on Figure 3. Areas of observed springs and seeps encountered during construction may require flattening and should be evaluated by the geotechnical engineer.

Rockfall Catchment Area

A rockfall catchment area is defined as the area between the roadway edge of shoulder and the base of a cut slope used to restrict high percentage of rockfalls from reaching the 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. Colorado Rockfall Simulation Program Version 5.0 (CRSP) was used to analyze the ditch catchment effectiveness of the existing slope conditions compared to the proposed combinations of 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.

Slope stations 190+00 and 211+00 were chosen to be representative of the typical critical slope with respect to cut height, slope face angle, minimal ditch width and potential launching features.

From field observations it was assumed that spherical rocks ranging from 1 to 3 feet 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 F.

From the analysis, a 10-foot wide standard catchment ditch sloped 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 10. Routine ditch cleaning should be included in the county’s maintenance plan to avoid decreased rockfall retention.

Lefthand Canyon Dr., CO ER 94/81/106(1) 16 August 2015

TABLE 10:- Summary of CRSP Analysis

ST

A

TI

O N

EXISTING SLOPE GEOMETRY PROPOSED SLOPE GEOMETRY

EST.

IMPROVEMENT

IN

CONTAINMENT

M A

X S

LO

PE

A

G

LE

(D

EG

H

EI

G H

T O

F

PO

TE

N

TI

A

L R

O C

K

FA

LL

(F

T)

V

ER

A

G E

D

IT

C H

W

ID

TH

FO

R

ES

LO

PE

(V

:H

ES

T

C O

N

TA

IN

M

EN

T

M A

X S

LO

PE

G

LE

(V

:H

H

EI

G H

T O

F

PO

TE

N

TI

A

L R

O C

K

FA

LL

D

IT

C H

W

ID

TH

FO

R

ES

LO

PE

(V

:H

ES

T

C O

N

TA

IN

M

EN

T

190+00 65 43 < 2 Flat 27% 1:0.5 44 10.0 1: 4 97% 70%

211+00 68 55 < 2 Flat 4% 1:0.5 83 10.0 1: 4 90% 86%

Rock Cutslope Ratings

The CFL geotechnical team rated 18 proposed rock cuts to be excavated to gain roadway width and/or to improve sight distance. The Colorado Rockfall Hazard Rating System (CRHRS) was utilized during the rating survey. The following assumptions were made; (1) the rockfall frequency scores for all the slopes were assumed to be every 1 to 2 years as no reliable rockfall frequency data could be obtained, (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 489 to 876 with an average combined score rating of 687. By evaluating proposed cut slope heights asumeing a slope cut ratio of 1V:0.5H, no increased site distance, and smooth slope condition a proposed condition CRHRS score was estimated without structural mitigation (rock bolts or draped mesh). The proposed condition CRHRS estimated score ranged from 417 to 669 with an average combined score rating of 563, as illustrated in Table 11. These results indicate an average 124 point improvement for the slopes analysis. A detailed discussion on the CRHRS parameters and the field data collected are included in Appendix E.

TABLE 11:- Rock Cut Slope Ranking by CRHRS

RANKING ROCK CUT STATION

(ID)

STATION CUT SLOPE

LENGTH

CRHRS COMBINED SCORE

START END EXISTING PROPOSED

1 191+00 (023) 188+00 191+00 300 876 627 2 330+50 (013) 330+00 331+00 100 840 669 3 269+00 (021) 268+00 271+50 350 828 603 4 302+00 (017) 294+00 301+50 750 819 747 5 363+00 (008) 358+50 363+00 450 786 615 6 335+00 (012) 332+00 336+00 400 762 510 7 292+50 (018) 292+00 294+00 200 750 597 8 323+50 (014) 322+50 328+00 550 735 591 9 483+00 (001) 482+50 484+00 150 705 489 10 211+00 (022) 209+00 212+00 300 705 579 11 287+00 (019) 285+00 288+00 300 624 555

Lefthand Canyon Dr., CO ER 94/81/106(1) 17 August 2015

RANKING ROCK CUT STATION

(ID)

STATION CUT SLOPE

LENGTH

CRHRS COMBINED SCORE

START END EXISTING PROPOSED

12 343+00 (010) 342+50 344+00 150 623 551 13 383+00 (006) 381+50 383+00 150 618 597 14 348+00 (009) 346+00 348+00 200 576 513 15 318+50 (015) 317+00 318+50 150 570 507 16 399+00 (003) 398+00 400+00 200 537 483 17 368+00 (007) 363+00 369+00 600 519 483 18 404+00 (002) 404+00 406+50 250 489 417

Controlled Blasting

Blasting has been designed with presplitting prior to production blasting to control overbreak at the excavation limits. Presplitting was specifically used to provide slope stability by forming a fracture plane before any production blasting is conducted. Presplitting utilizes lightly loaded, closely spaced drill holes fired with 25 millisecond delays before the production blast to form a fracture plane across which the radial cracks from the production blast cannot travel. The fracture plane formed allows for steeper slopes with improved maintenance. Presplitting should be thought of as a protective measure to prevent rock overbreak and to keep the final face from being damaged by the production blasting.

The blasting agent used for presplitting generally contains considerable less powder-factor and higher gas-producing explosives placed as a small diameter charge in a large borehole (charge diameter/borehole diameter ratio of < 0.5). By using a small diameter charge in a larger diameter hole and delays between the boreholes, the gas pressures drop rapidly following the explosion due to expansion into a larger volume to produce a stable fracture and reduce the possibility of forming hairline cracks within the borehole walls. In massive strong rock, 100 percent of the holes produce half-cast. Success or failure of presplit blasts may be measured by the half-cast factor. Half-cast factors are the percentage of the total half-casts which are visible after the rock has been excavated.

When rock is severely jointed between blast holes and the joints intersect the cut face at less than 30-degree angle will cause fractures to intersect the jointing planes possibly causing large pieces of material to fallout from the face. Other geologic factors which affect the outcome of control blasting techniques are soft seams or mud seams. If the bench is intersected by numerous mud seams it is difficult to produce good results.

Scaling

Scaling of existing rock slopes is recommended at select locations not mitigated by new permanent cut slopes. Scaling is conducted to reduce the amount and frequency of rock fall debris on the roadway by removal of loose and unstable material from existing slopes. Scaling is anticipated to be achieved by hand and light tools. Approximately 80 crew hours are recommended to be included in the contact documents. Locations are to be determined during construction.

Scaling is only considered a temporary or short term rock fall mitigation technique to remove any loose material that could potentially dislodge. Routine scaling should be included in the county’s maintenance plan.

Lefthand Canyon Dr., CO ER 94/81/106(1) 18 August 2015

Rock Bolting

Recommended rock cut slopes were designed to be structurally stable; however, the rock mass is highly variable and is likely to vary in rock quality from the exposed outcrops mapped to the final cut face. Therefore, approximately 500 linear feet of tensioned rock bolts is recommended to be included in the contact documents to address localized rock instabilities as they become apparent.

Spot bolting or systematic (pattern) bolting is based on detailed slope stability mapping, kinematic analyses, and engineering judgment and should be evaluated by a CFL Qualified Geotechnical Representative during excavations.

Tensioned rock bolts consist of steel reinforcing bars grouted (cement or polyester resin) into small-diameter drillholes within a jointed, fractured rock mass – are considered “active” reinforcing elements providing a normal, confining component of compressive force across rock mass discontinuities. Relatively small normal loads applied across rock failure surfaces can result in enormous gains in rock mass shear strength, putting rock-on-rock frictional resistance to work for immediate enhancements to slope stability. Unlike massive earth retention projects requiring high-capacity ground anchor systems, rock reinforcement commonly works at surficial scales using much smaller reinforcing elements (e.g., No. 8-14 deformed bar) installed to manageable hole depths (10- 30 ft).

Draped Mesh

Draped mesh is recommended from Station 329+00 to Station 330+00 and Station 394+00 to Station 396+00, to provide surface control to highly weathered, fractured, jointed, raveling and/or eroding slopes. Draped installations suspend mesh from the top of a slope using grouted ground anchors or rock bolts. The mesh lays loose against the slope providing surface control for loose rock and talus, allowing the loose 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 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 can be easily cleaned by temporarily lifting the mesh from the slope.

4.6 CORROSION POTENTIAL

An analytical test was conducted on selected samples collected from the site to determine if soils may have detrimental effects on concrete and buried metals. A combined soil sample from all four borings at various depths was tested for resistivity and pH chemical characteristics. The resistivity result (AASHTO T 288) is 15,250 ohm x cm and the pH result is 7.4. The resistivity and pH results do not indicate an aggressive soil environment; therefore, sulfates and chlorides were not tested. Standard corrosion protection measures should be implemented; however, sulfate resistant cement is not required.

Lefthand Canyon Dr., CO ER 94/81/106(1) 19 August 2015

4.7 CONSTRUCTION CONSIDERATIONS

Roadway excavations are anticipated to encounter hard bedrock and large boulders (10-feet in diameter or larger) requiring carefully planned and uniquely adapted blasting approaches.

Controlled blasting methods that utilize the natural geologic bedding planes and joint structure in a predicted and controlled manner to minimize back break beyond the trim line may be required. The contractor will be required to make the final determination on the rippability characteristics of encountered material based on equipment capabilities. Pre-blasting and post-blast surveys will be required for structures within 500 feet to blasting activities.

Groundwater is anticipated in excavations for foundations and rip-rap embankment armor.

Temporary diversion of stream flow away from the work area may be required to meet the water quality requirements specified in Section 157 of the FP-03. Dewatering and stabilization methods for foundation construction will depend on conditions encountered. The high potential of encountering large boulders and shallow bedrock will likely inhibit the construction of a conventional cofferdam without requiring special construction techniques.

It is recommended that all foundation excavations be inspected by a CFL geotechnical representative prior to the installation of any foundation element. If it is determined that adverse bedding conditions are encountered the Contractor will need develop shoring methods to stabilize any excavations that are deemed not stable.

Based on conditions encountered during the geotechnical investigation, it is not likely that a significant depth of topsoil will be present at the project site. For estimating purposes, it should be assumed that limited top soil will be stripped and stockpiled for re-use on the project. Topsoil is anticipated to be imported for the project.

4.8 LIMITATIONS

The recommendations in this report are based on the data obtained from exploratory borings, field review, and laboratory test results. The results of these explorations and tests represent conditions at the specific locations indicated. Subsurface variations across the site are likely and may not become evident until excavation is performed. The Analysis and Recommendations sections in this report include interpretations and recommendations developed by the Government in the process of preparing the design. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgment of the Contractor.

Lefthand Canyon Dr., CO ER 94/81/106(1) 20 August 2015

SECTION FIVE--REFERENCES

AECOM Technical Memorandum, “Recommendations for Retaining Wall and Pavement Construction, Lefthand Canyon Drive Station 70+00 to 364+00, Dated March 23, 2015.

ACI, 2005, “Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary

(318R-05)”.

American Association of State Highway and Transportation Officials (AASHTO), 2014, LFRD

BRIDGE DESIGN SPECIFICATIONS, 7th Edition.

Andrew, R.D., R. Bartingdale and H. Hume, 2011, “Context Sensitive Rock Slope Design Solutions,” FHWA-CFL/TD-11-002, available at www.cflhd.gov, January 2011.

Andrew, R.D., 1994. “The Colorado Rockfall Hazard Rating System,” Colorado Dept. of Transportation, report # CTI-CDOT-2-94.

Braddock, W.A., Houston, R.G., Colton, R.B., and Cole, J.C., 1988, “Geologic map of the Lyons quadrangle, Boulder County, Colorado” U.S. Geological Survey, Geologic Quadrangle Map GQ-1629, scale 1:24,000.

Central Federal Lands Highway Division (CFLHD), 2015, “Plans for Proposed CO ER

94/81/106(1) Boulder County, Lefthand Canyon Drive, dated June 2015.

Central Federal Lands Highway Division (CFLHD), 2015, “Plans for Proposed CO ER

94/81/106(1) Boulder County, Lefthand Canyon Drive, Status Set, dated August 2015.

U.S. Department of Transportation Federal Highway Administration, Plans for Proposed CO ER

94/81/106(1) Boulder County, Lefthand Canyon Drive, Preliminary June 2015 Plans.

Federal Lands Highways (FLH), 2007, “Technical Guidance Manual (TGM)”, A geotechnical supplement to the FLH Project Development and Design Manual.

Federal Highway Administration (FHWA), 2007, LRFD for Highway Bridge Substructures and

Earth Retaining Structures, Publication FHWA-NHI-05-094, dated January.

FHWA, 2008, Federal Lands Highway Project Development and Design Manual (PDDM), dated

March.

Gable, D.J., 1980, “Geologic map of the Gold Hill quadrangle, Boulder…