YA 2010- Draft Final Geotechnical Report US50 03-22-10.pdf

Draft Final Geotechnical Assessment/Investigation US 50 Blue Creek Canyon Corridor

Gunnison County, Colorado

March 22, 2010

Project No. 28-139 CDOT Project Code 14933

Prepared for:

Muller Engineering, Inc.

777 S. Wadsworth Blvd. #4-100

Lakewood, CO 80226 Attn: Mr. A. Gray Clark, PE

Colorado Department of Transportation

Region 3 – Montrose Residency 2424 North Townsend Avenue

Montrose, CO 81401 Attn: Mr. Ron Alexander, PE

Prepared by:

Yeh and Associates, Inc.

5700 E. Evans Ave.

Denver, Colorado 80222 Phone: 303-781-9590

Fax: 303-781-9583 i

Table of Contents Page

1.0 Purpose and Scope of Study

2.0 Proposed Construction

3.0 Geologic Conditions

3.1 Station 301+28 to Station 574+00

3.2 Station 574+00 to Station 714+00

3.3 Station 714+00 to Station 767+00

4.0 Existing Roadway Geometry

4.1 Segments 1 and 2

4.2 Segment 3

4.3 Segment 4

5.0 Subsurface Investigation

5.1 Segment 1, Sta. 451+00 to 457+00 – Large Proposed fills

5.2 Segment 2, Sta. 500+00 to 510+00 – Unstable Embankment Fills

5.3 Segment 2, Sta. 519+00 to 535+50 – Unstable Slopes in Landslide Section 11

5.3.1 Time Domain Reflectometry

5.4 Segment 2, Sta. 548+00 to 569+00 –Embankment Fills and Proposed Cut

Slopes

5.5 Segment 3, Sta. 575+00 to 675+00 – Blue Creek Canyon, Fill Walls and Cut

Slopes

5.6 Segment 3, Sta. 683+00 to 690+00 – Proposed Cut Slope

6.0 Observed Field Conditions

6.1 Segment 1, Station 361+00 to Station 364+00

6.2 Segment 1, Station 374+00 to Station 376+00

6.3 Segment 1, Station 380+00 to Station 385+00

6.4 Segment 1, Station 406+00 to Station 406+50

6.5 Segment 1, Station 413+00 to Station 418+00

6.6 Segment 1, Station 419+50 to Station 420+50

6.7 Segment 1, Station 429+00 to Station 433+00

6.8 Segment 1, Station 436+00 to Station 438+00

ii

6.9 Segment 1, Station 440+00 to Station 442+00

6.10 Segment 1, Station 453+50 to Station 454+50

6.11 Segment 1, Station 471+00 to Station 473+00

6.12 Segment 2, Station 500+00 to Station 505+00

6.13 Segment 2, Station 519+00 to Station 535+50 (Landslide Section )

6.14 Segment 2, Station 560+00 to Station 562+00

6.15 Segment 2, Station 563+00 to Station 570+00

6.16 Segment 2, Station 582+00 to Station 585+00

6.17 Segment 3, Station 588+00 to Station 591+00

6.18 Segment 3, Station 591+00 to Station 600+00

6.19 Segment 3, Station 619+00 to Station 622+00

6.20 Segment 3, Station 626+00 to Station 627+00

6.21 Segment 3, Station 632+50 to Station 638+50

6.22 Segment 3, Station 638+50 to Station 640+50

6.23 Segment 3, Station 642+50 to Station 644+00

6.24 Segment 3, Station 646+50 to Station 647+50

6.25 Segment 3, Station 651+50 to Station 653+20

6.26 Segment 3, Station 653+20 to Station 654+25

6.27 Segment 3, Station 654+25 to Station 655+60

6.28 Segment 3, Station 655+60 to Station 656+50

6.29 Segment 3, Station 656+50 to Station 659+60

6.30 Segment 3, Station 659+60 to Station 661+50

6.31 Segment 3, Station 661+50 to Station 667+50

6.32 Segment 3, Station 666+10 to Station 666+60

6.33 Segment 3, Station 667+50 to Station 668+30

6.34 Segment 3, Station 669+50 to Station 670+50

6.35 Segment 3, Station 673+00 to Station 680+00

7.0 Analyses and Recommendations

7.1 Segments 1 and 2, General Cut Slopes (Excluding Landslide Sections at Sta.

500+00 – 510+00 and 518+00 – 534+00)

7.2 Segments 1 and 2, General Fill Slopes (Excluding Sections from Sta. 500+00 to 510+00, Sta. 518+00 to 534+00, and Sta. 560+00 to 562+00) iii

7.3 Segment 2, Unstable Slope and/or Embankment Fill Failure Locations (Sta.

500+00 to 510+00, Sta. 518+00 to 534+00, and Sta. 560+00 to 562+00)

7.3.1 Station 500+00 to 510+00 (Piling Location)

7.3.2 Station 560+00 to 562+00 (Pavement Distress/Embankment Fill Failure) . 45

7.3.3 Station 519+00 to 535+50, Landslide Complex

7.4 Segment 3, Blue Creek Canyon Fill Wall Recommendations

7.4.1 General Overview of Proposed Retaining Wall Systems

7.4.2 Design Considerations for Proposed Retaining Wall Systems

7.5 Segments 2 and 3, Cut Wall Recommendations

7.5.1 General Overview of Proposed Cut-Side Retaining Wall Systems

7.5.2 Design Considerations for Proposed Cut-Side Retaining Wall Systems

7.6 Segments 2 and 3, Blue Creek Canyon Cut Slopes

7.6.1 Rockcut Excavation

7.6.2 Overburden and/or Weak Bedrock Cut Slopes

7.6.3 Rockfall Potential from Outside of ROW

7.6.4 Rockfall Generated From Cut Slopes - Suspended Mesh

7.6.5 Drilling and Blasting

7.6.6 Rock Anchor Systems

7.7 Segment 4, General Cut and Fill Slopes (Sta. 696+00 to 767+00)

8.0 Segment 3 Pavement Subgrade

8.1 Segment 3 Existing Pavement Conditions

8.2 Subgrade Materials

9.0 Limitations

10.0 References

List of Figures

Figure 1. Map of project location, showing approximate extents of Segments 1 through

4. This report is relevant for all four project segments

Figure 2. Typical unstable slope near the start of the project (Approximate Sta.

303+00)

Figure 3. Existing rock cut at approximate Sta. 649+00, standing at approximately 0.25H:1V iv

Figure 4. Existing slope adjacent to the roadway at approximate Sta. 752+00, approximately 2H:1V

Figure 5. Auger drilling with the track-mounted rig within the landslide section, September 2007

Figure 6. Top end of a TDR cable, with attachment for connecting to Time Domain Reflectometer

Figure 7. Boring YA-07 being drilled horizontally into the rock cut near Sta. 650+00. 18 Figure 8. “Half-moon” cracking at Sta. 415+00 Figure 9. Slope Failure with piling near Sta. 504+00 Figure 10. Landslide section looking south (Sand Dome is on left side of photograph).

Figure 11. Landslide section looking north. Blue Arcs Indicate Approximate

Headscarp Locations. Red Line Indicates Approximate Outline of Entire Landslide Complex above Roadway

Figure 12. Landslide section looking east, where localized landslide embankment movement has caused half-moon cracking at approximate Sta. 528+00.

(Sand Dome is on upper right side of photograph)

Figure 13. Pavement deformation at Sta. 560+00 – 561+00 Figure 14. Area of proposed sliver rock cuts, Sta. 591+00 – 600+00 Figure 15. Sta. 619+00 – 622+00, proposed cuts of 0.5H:1V. Existing slope is 1H:1V.

Figure 16. Sta. 626+00 – 627+00, existing colluvial slope with proposed cuts of

0.5H:1V Figure 17. Sta. 632+00 – 638+50 Figure 18. Sta. 638+50 – 640+50, existing cut is 1H:1V with planar failure features. . 30 Figure 19. Sta. 642+50 – 644+00, existing colluvial slope is about 1H:1V Figure 20. Sta. 651+50 to 653+20, existing colluvial slope standing at approximately

1H:1V Figure 21. Sta. 654+25 to 655+60, existing talus slope standing at approximately

1H:1V Figure 22. Sta. 655+50 to 656+50 Figure 23. Sta. 656+50 to 659+60, existing colluvial slope standing at approximately

1H:1V Figure 24. Sta. 659+60 to 661+50 consists of highly weathered and fractured bedrock such as that pictured here Figure 25. Sta. 661+50 to 667+50 consists of inter-bedded granite and metamorphic rock types Figure 26. Sta. 666+10 to 666+60, a piling wall has been placed to stabilize the embankment fill Figure 27. Sta. 667+50 to 668+30, existing colluvial slope standing at approximately

1H:1V Figure 28. Sta. 670+00 to 671+00, existing highly weathered bedrock forms 1H:1V slopes Figure 29. Sta. 673+00 to 680+00 consists of inter-bedded granite and metamorphic rock types. Potential toppling failures are evident in the photograph v

Figure 30. Sta. 500+00 to 510+00, Piling section exhibiting fill slope distress Figure 31. Example of a design incorporating Expanded Polystyrene (EPS) Fill for reducing driving loads Figure 32. Sta. 560+00 to 562+00, Pavement Distress Section Figure 33. Conceptual cross-section of landslide section at approximate Sta. 530+00 –

Existing Conditions Figure 34. Conceptual cross-section of landslide section at approximate Sta. 530+00 –

Rock Buttress Above and Below Proposed Roadway Figure 35. Conceptual cross-section of landslide section at approximate Sta. 530+00 –

Rock Buttress Below Roadway Figure 36. Generalized wall geometry for SMSE wall Figure 37. Typical rockery wall section, from the FHWA manual “Rockery Design and

Construction Guidelines,” 2006 Figure 38. Draped Mesh with Mesh Anchor Figure 39. Draped mesh with Mesh Anchor (Special). Draped mesh to be placed over top support rope and draped down slope Figure 40. Attenuator system at Georgetown along I-70

List of Tables Table 1. Correlation between N60 and relative density/consistency Table 2. Summary of cut slope recommendations for Segments 2 and 3, Sta. 563+00 to 680+00:

Table 3. Pavement Thickness Measurements

List of Appendices

Appendix A – Boring Logs and Legend Appendix B – Engineering Geology Sheets Appendix C – Summary of Laboratory Testing Results Appendix D – Photographs of Rock Core Appendix E – Seismic Refraction Survey Report Appendix F – Time Domain Reflectometry Data Appendix G – Selected Slope Stability Analyses Appendix H – Stereonet Projections of Scanline Survey Data

US50 Blue Creek Corridor Project No. 28-139 Draft Final Geotechnical Report March 22, 2010 Gunnison County, Colorado

1.0 Purpose and Scope of Study

This report presents the results of our geologic and geotechnical study for the proposed widening and resurfacing of US Highway 50 between MM 117.5 and MM

126.5 west of Blue Mesa Reservoir in Gunnison County, Colorado. Figure 1 shows the general location of the project site. This report is an update to the Draft Geotechnical

Assessment/Investigation Report submitted on December 1, 2008, and encompasses the entire project corridor (Segments 1 through 4 in other documents).

The purpose of our study was to provide a geologic assessment of the subsurface rock and soil conditions along the project alignment, to identify geologic constraints within the corridor, and to present recommendations for proposed retaining walls, cut slopes and landslide mitigation features. The geotechnical portion of the study was conducted in general accordance with our scope of work for CDOT Project

Number NH-0501-046, Project Code 14933.

A field exploration program consisting of geologic reconnaissance, a seismic refraction survey, and exploratory drilling at proposed retaining wall and rock cut locations, as well as in landslide and embankment fill failure locations. Seismic surveys were conducted on September 5th and 6th, 2007 at selected locations in the canyon.

Between September 18th and 24th, 2007, Yeh and Associates, Inc. observed and logged

6 geotechnical soil borings that were drilled between Station 518+00 and Station

534+00 within the large landslide complex. Between June 25th and July 18th, 2008, Yeh and Associates, Inc. observed and logged 28 geotechnical borings at proposed retaining wall locations, potential slope instability locations, and proposed rock cut locations. Between September 29th and October 3rd, 2008, Yeh and Associates, Inc.

observed and logged an additional 6 geotechnical borings that were drilled within the large landslide complex, and 10 pavement-related borings. A total of 50 geotechnical and pavement borings were drilled. Rock and soil samples were obtained during each of these field exploration segments, and were examined by the project personnel.

Representative samples were tested in our laboratory to determine the classification and engineering characteristics of the on-site materials.

Gunnison County, Colorado

This project has been divided into four segments based on existing site conditions and proposed construction. The segments generally have very different and distinct geological characteristics that vary greatly within the project corridor. They are generally divided into the following segments however the limits of the segments have varied during the course of the project and may vary in the FOR or Final Advertisement

Sets but the project stationing should be unchanged:

• Segment 1, Sta. 353+00 – 491+00

• Segment 2, Sta. 491+00 – 585+00

• Segment 3, Sta. 585+00 – 696+00

• Segment 4, Sta. 696+00 – 767+00

See Figure 1 for locations of these segments. From the beginning of the project, Station 301+28.22, to Station 353+00, only pavement overlays are proposed at this time.

This report summarizes the data gathered, the results of our analysis, and our recommendations based on the proposed construction, site reconnaissance, seismic refraction survey, geotechnical subsurface investigation, and results of laboratory testing. General cut and fill slope recommendations and a discussion of the geotechnical engineering considerations are included in this report.

Figure 1. Map of project location, showing approximate extents of Segments 1 through 4. This report is relevant for all four project segments.

2.0 Proposed Construction

Proposed work includes resurfacing and widening US 50 to address safety and to improve vehicle capacity. Shoulders are to be upgraded where possible, and passing

Project start Sta. 301+28.22

Project end Sta. 767+00

Colorado

End Seg. 2/ Begin Seg. 3 Sta. 585+00

End Seg. 3/ Begin Seg. 4 Sta. 696+00

Begin Segment 1 Sta. 353+00 End Seg. 1/

Begin Seg. 2 Sta. 491+00

End Seg. 4

Denver

Gunnison

US50

Grand Junction

Gunnison County, Colorado lanes are to be constructed in some areas, requiring both cut and fill slopes. Slope stabilization, drainage improvements, and the addition of “z” slopes, side slopes, and clear zones may be required in various places throughout the project limits. Rock cuts and retaining fill walls will be required within Segment 3 (Blue Creek Canyon).

Landslide mitigation options are provided within the large landslide complex between

Sta. 518+00 and 534+00.

3.0 Geologic Conditions

3.1 Station 301+28 to Station 574+00

This part of the project area, comprising all of Segment 1 and most of Segment

2, lies within a broad valley of Cretaceous sedimentary rocks and Quaternary deposits, with Tertiary volcanic rocks, including the Blue Mesa Tuff and West Elk Breccia, forming a large plateau high to the north called Fitzpatrick Mesa. The bedrock beneath the roadway throughout this section of the project is mapped as Mancos Shale by Hansen

(1971), and many ancient and recent landslide features influence the topography and the roadway. Slopes in the area are gently inclined and “hummocky” due to widespread slope instability. Many small drainage channels and irrigation canals carry water throughout the project area, and groundwater seepage and surface water are both abundant.

The Mancos Shale (Km) is composed primarily of dark gray shale with occasional thin layers of limestone, which can weather into clay. The shale will generally have low to high plasticity and typically exhibits low strength characteristics.

Numerous landslide and slump features affecting the highway occur in the Mancos

Formation. Regionally the area has been mapped with hundreds of small and large landslides and slump features.

Typically the Mancos Shale is not stable for cut or fill slopes that are placed steeper than 2H:1V for wet conditions. In the event the Mancos Shale is saturated, it will typically form slopes of 3H:1V or flatter. Figure 2 depicts a typical unstable slope near the start of the project.

Figure 2. Typical unstable slope near the start of the project (Approximate Sta. 303+00).

3.2 Station 574+00 to Station 714+00

This section of the project corridor, comprising about 1,100 feet of Segment 2, all of Segment 3, and about 1,800 feet of Segment 4, is generally within Blue Creek

Canyon. The bedrock is mapped by Hansen (1971) as pre-Cambrian migmatitic gneiss with intrusions of quartz monzonite and abundant large and small granite pegmatites.

Slopes adjacent to the roadway are steep, with bedrock outcrops along the majority of this section. Big Blue Creek parallels the highway, and is joined by Little Blue Creek at approximate Station 616+00, to form Blue Creek. The creek follows the canyon to the north towards the Gunnison River, while the highway climbs out of the canyon beginning at approximate Station 662+00.

The migmatitic gneiss is generally light to medium gray, well-foliated, and moderately hard. It is frequently intruded with large sections of orange-pink, massive to slightly fractured quartz monzonite, as well as white-pink, very hard granite pegmatites.

These rock exposures pose some rockfall hazard, particularly where the rock is highly fractured or weathered.

Head Scarp

Slump

Gunnison County, Colorado

Figure 3 depicts an existing rock cut standing at approximately 0.25H:1V. In areas where the cut slope consists of colluvial soil overlying deeper bedrock, the slopes are appear to be stable at approximately 1H:1V slopes.

Figure 3. Existing rock cut at approximate Sta. 649+00, standing at approximately 0.25H:1V.

3.3 Station 714+00 to Station 767+00

The eastern section of the project, from the eastern end of the horseshoe curve to Windy Point Cut, comprises the majority of Segment 4. This section lies on gently sloping hills, with bedrock consisting of a member of the Tertiary-aged West Elk

Breccia. This rock is described by Hansen (1971) as latitic to andesitic tuff breccia, as much as 800 feet thick. The rock generally exhibits medium to high strength characteristics. No landslide features or pavement distress areas were observed within this section of the project. No groundwater seeps were visible during field reconnaissance, and the material appears to be well drained. This material generally supports existing cut slopes of 2H:1V. Figure 4 depicts a typical existing slope adjacent to the roadway near station 752+00.

Figure 4. Existing slope adjacent to the roadway at approximate Sta. 752+00, approximately 2H:1V.

4.0 Existing Roadway Geometry

The existing alignment in Segments 1 through 4 traverses rolling and steep terrain vegetated with shrubs, grasses, and small trees. Most of the project area consists of undeveloped, federal and private land.

4.1 Segments 1 and 2

The existing highway gains approximately 850 feet in elevation between the start of the project, at Station 301+28, and Blue Mesa Summit, at Station 498+00, with an average grade of 4% through this section. It then slopes downward at an average grade of 5% on the approach to Blue Creek Canyon, which begins at approximately

Station 585+00.

In Segments 1 and 2 of the project, the roadway has many sharp curves and is located mostly on fills or cuts, with one fill section of significantly larger size from Sta.

405+00 to Sta. 430+00 (at the “S” curve). Existing cuts are in clayey soil, most likely derived from the Mancos Shale (see Regional Geology), and range from approximately

2H:1V to 3H:1V. Existing embankment fills in this section generally range from 2H:1V to 2.5H:1V. The source for the fill materials is unknown, however our subsurface investigation indicates they are most likely derived from the adjacent cutslopes.

Gunnison County, Colorado

Steeper cut and fill slopes (i.e. 1.5H:1V) are present, but they generally appear to be less stable than the 2H:1V slopes. The extents of a large, circular-shaped landslide feature is affecting the existing highway in Segment 2 between approximate Sta.

519+00 and Sta. 535+50 for approximately 1,600 feet. Multiple head scarps are located above the road.

4.2 Segment 3

The Blue Creek Canyon section from Station 585+00 to Station 685+00 parallels

Blue Creek downstream at an average grade of 1% before following a small tributary for about 0.6 miles, up and out of the canyon at a grade of 5%. The roadway then rises out of the canyon on a sharp “horseshoe” curve ending at Station 696+00, the end of

Segment 3.

In Segment 3, the roadway has fairly straight sections broken by sharp curves, and is located on fills and cuts, with one very large cut in rock located at a very sharp curve, with a design speed of 35 mph, near Sta. 650+00. Existing fill slopes in this segment are steep to moderately inclined, ranging from 1H:1V to 1.5H:1V. The source for the fill materials is not known, but is likely to be derived from nearby colluvial slopes or from soils in Segments 1, 2, and 4, and is also most likely not particularly thick.

Existing cut slopes are generally steep in rock outcrops and moderately inclined in colluvial soil areas, ranging from 0.25H:1V to 1H:1V. A few rockfall deposits from past events are evident within this segment, but overall the rockfall occurrence appears to be low to moderate relative to other sites in the state. The shoulder is very narrow or nonexistent in this segment, and there are many areas with rock cuts within a few feet of the edge of pavement.

4.3 Segment 4

The roadway remains mostly level from Sta. 696+00 to the project end near

Windy Point Cut, at Station 767+00. In Segment 4, the roadway has curves with a 40 mph design speed, and is built on small cuts or fills. Typical cut/fill slopes in this area are moderately inclined, with existing slopes near the roadway ranging from 3H:1V in soil to 1.5H:1V in one small exposed rock cut slope. No unstable slope features are

Gunnison County, Colorado apparent within this roadway section, indicating that a 2H:1V cut or fill will be constructable in this area without encountering slope stability issues. Current roadway design calls for 2H:1V cut and/or fill slopes in this area; if nothing steeper is planned, no slope reinforcement or retaining structures are expected to be necessary.

5.0 Subsurface Investigation

Subsurface conditions within the project area were explored in four stages due to right of way and access issues at various time periods from September 5th, 2007 to

October 3rd, 2008. These stages included (in chronological order):

• September 5th – 6th, 2007 – Seismic refraction survey field data was collected at 6 seismic line locations in Segments 2 and 3.

• September 18th – 24th, 2007 – 6 borings in Segment 2 landslide area, instrumentation installed

• June 25th to July 18th, 2008 – 28 borings for proposed cuts, fill walls, pavement distress locations, and an additional slope instability location.

• September 29th to October 3, 2008 – 6 additional borings in Segment 2 landslide area, and 10 shallow pavement borings in Segment 3.

See Appendix A for Boring Logs and Appendix B for Engineering Geology

Sheets showing borehole locations. A discussion of subsurface conditions, organized by Segment and Station, is below.

Soil samples were collected typically at 5-foot intervals for each boring, using a

California sampler or a standard split spoon sampler, driven by a 140-lb hammer with a

30-inch stroke. A safety hammer, utilizing a cathead and rope, was used during the sampling operations in June and July of 2008. At other times, an automatic hammer was used to drive the sampler. This procedure is similar to the Standard Penetration

Test (ASTM D1586). The number of blows required to drive the sampler 12 inches, or a fraction thereof, constituted the N-value as shown on the Boring Logs. For cohesionless soils, N-values can be correlated to the relative density of the material; for cohesive soils, N-values can be correlated to the consistency of the material. This correlation is summarized in Table 1 below.

Gunnison County, Colorado

Cohesionless Soils Cohesive Soils N60 Corresponds to N60 Corresponds to Approx. SU (psf)

0 to 4 Very loose 0 to 2 Very soft Less than 250 4 to 10 Loose 2 to 4 Soft 250 – 500

10 to 30 Medium dense 4 to 8 Medium stiff 500 – 1000 30 to 50 Dense 8 to 15 Stiff 1000 – 2000

> 50 Very dense 15 to 30 Very stiff 2000 – 4000 31 to 60 Hard > 4000 > 60 Very hard

Table 1. Correlation between N60 and relative density/consistency.

5.1 Segment 1, Sta. 451+00 to 457+00 – Large Proposed fills

Proposed fills between Sta. 451+00 and 457+00 are 6 to 16 feet thick, and are planned at a 3H:1V slope. A seismic refraction survey was completed using two-dimensional refraction tomography techniques. One survey line (Line 5) was completed, from approximately Sta. 450+60 to 470+90, parallel to and south of the existing roadway, between September 5th and 6th, 2007. This work was subcontracted to Zonge Geosciences, Inc., of Lakewood, Colorado, with the assistance of Yeh and

Associates personnel. See Appendix E for the seismic refraction report. Analysis of the data indicates weathered bedrock (Mancos Shale, based on published mapping and our subsurface investigation) 13 to 19 feet below the ground surface. Overburden soils in this area consist of gravelly and sandy clay.

5.2 Segment 2, Sta. 500+00 to 510+00 – Unstable Embankment Fills

A section of existing roadway between Station 500+00 and 510+00 has some unstable embankment fill materials, resulting in continued subsurface soil movement, deformed pavement, and increased maintenance activities. Steel piling was driven into the embankment on the fill side at an unknown date in an attempt to stabilize this area;

but appears to have limited effect. The general subsurface conditions within this area were explored with seismic refraction survey lines and with geotechnical drilling.

A seismic refraction survey was completed using two-dimensional refraction tomography techniques. Two survey lines (Lines 3 and 4) were completed, both from approximately Sta. 502+10 to 503+30, parallel to the existing roadway, on the cut

(north) side and fill (south) side. Data was collected between September 5th and 6th, Gunnison County, Colorado

2007. See Appendix E for the seismic refraction report. Analysis of the data indicates weathered bedrock 18 to 26 feet below the ground surface on the fill side, and 12 to 26 feet below the ground surface on the cut side.

Geotechnical drilling took place between June 25th and July 18th, 2008, utilizing a Viper GH-5 platform drill rig, owned and operated by Salisbury and Associates, Inc., of

Spokane, Washington, and utilizing wireline coring techniques to advance borings YA-

01 through YA-05. Refer to Appendix A for Boring Logs and Appendix B for

Engineering Geology Sheets showing boring locations.

A total of 5 borings were vertically advanced using BDBGM size casing (1.654 inch I.D.). Core samples were collected, logged, and photographed (Refer to Appendix

D, Photographs of Rock Core). Soil samples were collected at intermittent depths utilizing a standard split spoon sampler driven by a 140-lb “rope-and-cathead” hammer with a 30-inch stroke; this procedure is similar to the Standard Penetration Test (ASTM

D1586). See above for a discussion of the correlation between this procedure and the relative density or consistency of the material. Water levels were not measured during this portion of the field investigation due to the use of water in the wireline coring process; however slotted PVC standpipes were installed in selected boreholes for future water level monitoring. A summary of laboratory testing is provided in Appendix C.

In general, the subsurface conditions at this location consisted of 12 to 17 feet of gravelly clay, overlying fresh to weathered shale bedrock. No groundwater was encountered in any of these borings. The shale bedrock (Mancos Formation) was moderately weathered in all of the borings for 6.5 to 17 feet. Seismic refraction data correlates well for the cut side (boring YA-05), but indicates weathered bedrock at depths where competent rock was found in the boreholes.

5.3 Segment 2, Sta. 519+00 to 535+50 – Unstable Slopes in Landslide Section

A large, circular-shaped landslide feature is affecting the existing highway in

Segment 2 between Sta. 519+00 and Sta. 535+50 for approximately 1,600 feet.

Multiple head scarps are located above the roadway. The general subsurface

Gunnison County, Colorado conditions within this area were explored with a seismic refraction survey line and with geotechnical drilling.

A seismic refraction survey was completed using two-dimensional refraction tomography techniques. One survey line (Line 6) was completed, from approximately

Sta. 518+00 to 520+30, parallel to and south of the existing roadway. Data was collected between September 5th and 6th, 2007. See Appendix E for the seismic refraction report. Analysis of the data indicates weathered bedrock 12.5 to 40 feet below the ground surface.

Geotechnical drilling took place between September 18th and 24th, 2007, and between September 29th and October 3rd, 2008. In 2007 a rubber track-mounted

Diedrich D-50 drill rig equipped with hollow-stem augers and an automatic hammer was used to advance borings B-01 through B-07. In 2008, a rubber track-mounted CME 55 drill rig equipped with hollow-stem augers and an automatic hammer was used to advance borings YA-08 through YA-13. Refer to Appendix A for Boring Logs and

Appendix B for Engineering Geology Sheets showing boring locations.

A total of 12 borings were vertically advanced using hollow-stem augers. Soil samples were collected typically at 5-foot intervals for each boring using a California sampler driven by a 140-lb automatic safety hammer with a 30-inch stroke; this procedure is similar to the Standard Penetration Test (ASTM D1586). See above for a discussion of the correlation between this procedure and the relative density or consistency of the material. A summary of laboratory testing is provided in Appendix C.

Water levels were measured, and a total of 10 Time Domain Reflectometry

(TDR) instruments were installed and grouted into ten of the boreholes, for monitoring movement within the landslide. At least 3 readings have been taken from each instrument as of this report writing, including readings encompassing at least one wet cycle/season. Data from these instruments indicates movement in at least two boreholes; the remaining 8 boreholes either show no movement to date or are not capable of measuring deflection at this time due to damage from livestock, weather, and/or wildlife. See Appendix F for TDR readings taken in these 10 boreholes. See

Gunnison County, Colorado below for a discussion of TDR technology. Figure 5 depicts drilling with the hollow stem auger in September 2007.

Figure 5. Auger drilling with the track-mounted rig within the landslide section, September 2007.

In general, the subsurface conditions consisted of 7 to 44 feet of gravelly and sandy clay, overlying shale bedrock. The shale was moderately weathered in most of the borings, for 2 to 17 feet. Seismic refraction data correlates very well with bedrock depths in borings YA-08 and YA-13, on the western end of the landslide section where the seismic line was located.

5.3.1 Time Domain Reflectometry

Time-domain reflectometry instruments for slope stability use a coaxial cable grouted in place within a vertical borehole, with an attachment at the top that can be connected to a Time Domain Reflectometer (Figure 6). This is an instrument commonly used in the communications field for identifying faults and breaks in long-distance cables. In geotechnical applications, if the cable shears due to slide movement, the instrument can detect the location (depth) of cable failure. The reflectometer transmits a voltage pulse through the cable, which is reflected from the other end of the cable, back to the transmitting end. The time it takes to receive this reflected signal is

Gunnison County, Colorado measured, so if the propagation velocity of the cable is known, the distance to the other end of the cable can be calculated. Coaxial communications cable is manufactured with a known propagation velocity; in this manner, the length of the cable can be measured.

If this length becomes shorter with subsequent measurements, in can be inferred that the cable has been “cut” short presumably by slope movement.

TDR technology detects discrete slide planes; thick zones of displacement are more difficult to locate, because the cable is stretched into tensile failure rather than shear failure. Simple bending of the cable, without damage, will not indicate movement.

In addition, if the cable becomes dislodged from the bottom of the hole, nothing will hold the lower end in place as the slide moves, and no shear movement can be detected.

For these reasons, grouting the cable properly becomes an important issue. The grout mixture should have similar shear strength and stiffness properties to that of the soil surrounding it.

There are several differences between TDR technology and standard inclinometers. TDR cable typically costs about $2 per foot of cable, plus the cost of installing the connector end (about $50); inclinometer casing costs approximately $5 per foot. When taking measurements, nothing is lowered into the borehole with TDR technology; therefore there is no risk of losing equipment down the hole. However, unlike inclinometers, TDR does not provide the operator with magnitude or direction of slope movement—only with failure depth. When grouted properly, TDR can measure shear zones as thin as 5-mm; this is approximately 1/120 that possible with an inclinometer.

Figure 6. Top end of a TDR cable, with attachment for connecting to Time Domain Reflectometer.

5.4 Segment 2, Sta. 548+00 to 569+00 –Embankment Fills and Proposed Cut Slopes

The general subsurface conditions within these areas were explored between

June 25th and July 18th, 2008, utilizing a Viper GH-5 platform drill rig, owned and operated by Salisbury and Associates, Inc., of Spokane, Washington, and utilizing wireline coring techniques to advance borings YA-18 through YA-20 and YA-39. Refer to Appendix A for Boring Logs and Appendix B for Engineering Geology Sheets showing boring locations. Borings YA-18, YA-19, and YA-39 were in the proposed fill area; boring YA-20 was in the proposed cut area.

A total of 4 borings were vertically advanced using BDBGM-size casing (1.654 inch I.D.). Core samples were collected, logged, and photographed (Refer to Appendix

D, Photographs of Rock Core). Soil samples were collected at intermittent depths utilizing a standard split spoon sampler driven by a 140-lb “rope-and-cathead” hammer with a 30-inch stroke; this procedure is similar to the Standard Penetration Test (ASTM

D1586). See above for a discussion of the correlation between this procedure and the relative density or consistency of the material. Water levels were not measured during this portion of the field investigation due to the use of water in the wireline coring

Gunnison County, Colorado process, however slotted PVC standpipes were installed in selected boreholes for future water level monitoring. A summary of laboratory testing is provided in Appendix C.

In general, the subsurface conditions in the proposed fill locations consisted of 4 to 7 feet of clayey gravel, overlying 3 to 27.5 feet of medium- to high-plasticity clay.

Bedrock was encountered in borings YA-18 and YA-19 at 8 feet and 29.5 feet below ground, respectively; no bedrock was encountered in boring YA-39, which was drilled to a total depth of 34.3 feet. Groundwater was encountered in boring YA-39 at a depth of

16.5 feet.

The subsurface conditions in the proposed cut location, explored in boring YA-

20, consisted of 42.2 feet of cobbles and gravel with a sandy clay matrix. No bedrock or groundwater was encountered in boring YA-20.

5.5 Segment 3, Sta. 575+00 to 675+00 – Blue Creek Canyon, Fill Walls and Cut Slopes

The general subsurface conditions at locations of proposed fill walls and cut slopes within Blue Creek Canyon were explored with a seismic refraction survey line and with geotechnical drilling.

A seismic refraction survey was completed using two-dimensional refraction tomography techniques. Two survey lines (Lines 1 and 2) were completed; Line 1 was from approximately Sta. 646+40 to 647+50, Line 2 was from approximately Sta. 626+70 to 628+00. Both of these lines were parallel to and west of the existing roadway, 14’ from the existing white line. Data was collected between September 5th and 6th, 2007.

See Appendix E for the seismic refraction report. Analysis of the data indicates weathered bedrock 12.5 to 20 feet below the ground surface for Line 1, and 11 to 24 feet below the ground surface for Line 2. .

Geotechnical drilling took place between June 25th and July 18th, 2008, utilizing a

Viper GH-5 platform drill rig, owned and operated by Salisbury and Associates, Inc of

Spokane, Washington, and utilizing wireline coring techniques to advance horizontal borings YA-06 and YA -07, and vertical borings YA-21 through YA-37. Refer to

Appendix A for Boring Logs and Appendix B for Engineering Geology Sheets showing boring locations.

Gunnison County, Colorado

In this section, a total of 18 borings were vertically or horizontally advanced using

BDBGM size casing (1.654 inch I.D.). Core samples were collected, logged, and photographed (Refer to Appendix D, Photographs of Rock Core). Soil samples were collected at intermittent depths utilizing a standard split spoon sampler driven by a 140-lb “rope-and-cathead” hammer with a 30-inch stroke; this procedure is similar to the

Standard Penetration Test (ASTM D1586). See above for a discussion of the correlation between this procedure and the relative density or consistency of the material. Water levels were not measured during this portion of the field investigation due to the use of water in the wireline coring process. PVC standpipes were installed in selected borings for future water level monitoring. A summary of laboratory testing is provided in Appendix C.

Borings YA-06 and YA-07 were advanced horizontally into a very large existing rock cut near Sta. 650+00. Two drill rigs were utilized at the same time for this drilling operation, which required an all-day closure of the eastbound lane of US50. Traffic control maintained a one-lane, one-way road in the westbound lane. Horizontal coring presents special challenges; the maximum possible drill pressure is reduced because there is no gravitational component; it depends on the drill operator’s strength, and a well-anchored drill rig. For these reasons, the depth of these borings was limited by time, and by the hardness of the rock. These two borings were drilled starting in rock, and encountered hard to extremely hard migmatite, granite, and quartz pegmatite rock types for their full depths. Boring YA-07 especially encountered extremely hard rock which broke into small pieces from the coring process. Boring YA-06 was advanced further into the slope due the slightly less hard rock encountered. Figure 7 depicts boring YA-07 being drilled. The wood block in the lower right corner is anchored into the ground with 12-inch steel rods.

Figure 7. Boring YA-07 being drilled horizontally into the rock cut near Sta. 650+00.

In general, the subsurface conditions in the proposed fill wall locations throughout the rest of the canyon consisted of 6 to 44 feet of gravel with varying degrees of sand, clay, cobbles, and/or boulders, and also several boreholes with voids between cobbles and gravel. This material is most likely fill or shot-rock placed when the roadway was first constructed through Blue Creek Canyon, and overlies migmatitic schist or granite bedrock. Bedrock was encountered in 13 of the 16 vertical boreholes;

the average bedrock depth in these boreholes was 22 feet. Bedrock depths generally agree with the analysis of seismic refraction data discussed above. No bedrock was encountered in boreholes YA-21, YA-28, or YA-37.

5.6 Segment 3, Sta. 683+00 to 690+00 – Proposed Cut Slope

The general subsurface conditions within a proposed cut slope between Sta.

683+00 to 690+00 were explored between June 25th and July 18th, 2008, utilizing a

Viper GH-5 platform drill rig and utilizing wireline coring techniques to advance boring

YA-38. Refer to Appendix A for Boring Logs and Appendix B for Engineering Geology

Sheets showing boring locations.

Gunnison County, Colorado

One boring was vertically advanced using BDBGM-size casing (1.654 inch I.D.).

Core samples were collected, logged, and photographed (Refer to Appendix D, Photographs of Rock Core). Another planned boring was abandoned due to the closely-spaced trees nearby, which would have required removal. Soil samples were collected at intermittent depths utilizing a standard split spoon sampler driven by a 140-lb “rope-and-cathead” hammer with a 30-inch stroke; this procedure is similar to the

Standard Penetration Test (ASTM D1586). See above for a discussion of the correlation between this procedure and the relative density or consistency of the material. Water levels were not measured during this portion of the field investigation due to the use of water in the wireline coring process; however a slotted PVC standpipe was installed in the borehole for future water level monitoring. A summary of laboratory testing is provided in Appendix C.

In general, the subsurface conditions in the proposed cut slope location consisted of 10.2 feet of clayey sand with gravel, overlying 23 feet of extremely soft to medium hard, decomposed migmatitic schist bedrock. Much of this material was highly micaceous and crumbled easily to silty sand. No hard or particularly competent bedrock was encountered to the full depth of this borehole at 33.1 feet.

6.0 Observed Field Conditions

The following sections describe specific areas where geologic conditions have affected the existing pavement sections of the highway. Based on our site review, we have outlined specific sections of roadway that exhibit pavement distress, slope instability, difficult rock cuts, potentially unstable embankment fills or soil cuts, and/or other noticeable geotechnical conditions.

6.1 Segment 1, Station 361+00 to Station 364+00

Noticeable pavement cracking, both longitudinal and transverse, and deformation causing a rough roadway, is prevalent in this section of the roadway. In this case, this is most likely due to inadequate drainage of groundwater and surface water; a large seep is evident in the hillside next to this section.

Gunnison County, Colorado

6.2 Segment 1, Station 374+00 to Station 376+00

Embankment fill failure is evident from “half-moon” pavement cracking along the fill side within this section. Fill slopes are steep, lightly vegetated and approximately 15 feet in height.

6.3 Segment 1, Station 380+00 to Station 385+00

Extensive pavement cracking and deformation are causing a rough roadway in this section, very similar to 361+00 to 364+00 discussed above. Cracking is most likely due to inadequate sub-grade drainage of groundwater.

6.4 Segment 1, Station 406+00 to Station 406+50

A small slump was observed in the existing cut slopes. The existing cut is approximately 20 feet high and is steeper than 2H:1V.

6.5 Segment 1, Station 413+00 to Station 418+00

Embankment fill failure is evident from “half-moon” pavement cracking along the large fill section at the “S” curve here (Figure 8). Fill slopes are steep and are not heavily vegetated.

Figure 8. “Half-moon” cracking at Sta. 415+00.

6.6 Segment 1, Station 419+50 to Station 420+50

Pavement distress is located on outside corner indicating a subsurface drainage problem or poor sub-grade material.

6.7 Segment 1, Station 429+00 to Station 433+00

This section of the roadway is directly below hummocky, treeless ground indicative of creep-type landslide movement, although no recent movement is evident.

6.8 Segment 1, Station 436+00 to Station 438+00

Minor pavement distress is evident here, most likely due to the relatively steep slope angle (25 deg) of the existing fill embankment.

6.9 Segment 1, Station 440+00 to Station 442+00

Much like the section from 429+00 to 433+00, this section of the roadway is directly below hummocky ground indicative of creep-type landslide movement, although no recent movement is evident.

6.10 Segment 1, Station 453+50 to Station 454+50

Embankment fill type movement is evident from “half-moon” pavement cracking along the fill section. Fill slopes are relatively steep and are not heavily vegetated.

6.11 Segment 1, Station 471+00 to Station 473+00

Pavement distress and cracking is evident in this section, once again most likely due to the steep slope angle of the existing fill embankment.

6.12 Segment 2, Station 500+00 to Station 505+00

Ongoing slope failure is occurring on the fill side of the roadway in this location

(See Figure 9). It appears that vertical H-piles were constructed as an attempt to mitigate the instability, but the piling has deflected downslope several feet, and movement continues to affect the roadway specifically between Station 502+50 and

504+50. See Section 5.2 above for a discussion of seismic refraction lines and

Gunnison County, Colorado geotechnical drilling Yeh and Associates, Inc. conducted in this location. See Section

7.3.1 for recommendations specific to this location.

Figure 9. Slope Failure with piling near Sta. 504+00.

6.13 Segment 2, Station 519+00 to Station 535+50 (Landslide Section )

The existing highway crosses a large landslide complex located between approximate Sta 519+00 to 535+50. Based on our site review it appears the landside is most active below the current roadway from approximate Sta. 521+50 to 523+50 and from approximate Sta. 526+00 to 529+00. The landslide appears active above the roadway from approximate Sta. 523+50 to 527+50. The slide has caused movement of the roadway, large pavement cracking, and has required continuous maintenance by

CDOT. Visible cracking was observed in the pavement between Station 526+00 and

528+00, but pavement overlays and patching have obscured many other sections. The roadway through this section is rough and has several transverse pavement drop-offs.

This landslide is very large, and is likely older than the roadway and therefore is not

Gunnison County, Colorado caused by the presence of the roadway. Within the upper portions of the landslide, the landowner maintains a small stock pond. This pond lies within a swamp-like area with numerous seeps that are likely contributing to slide movement. Trees in this area are not able to maintain their root systems, indicating significant movement. The ground throughout the slide is very hummocky, with small scarps and seasonal seeps visible. It is also likely the landslide complex moves intermittently in different sections. For example, the lower section likely fails and displaces down the slope between 20 and 60 feet. As a result of movement of the toe, another section midslope then fails and displaces down the slope, causing the section above to then fail, resulting in sequential slope failures progressing up the slope. Figure 10, Figure 11, and Figure 12 depict sections of the landslide area.

Figure 10. Landslide section looking south (Sand Dome is on left side of photograph).

Figure 11. Landslide section looking north. Blue Arcs Indicate Approximate Headscarp Locations. Red

Line Indicates Approximate Outline of Entire Landslide Complex above Roadway.

Figure 12. Landslide section looking east, where localized landslide embankment movement has caused half-moon cracking at approximate Sta. 528+00. (Sand Dome is on upper right side of photograph).

6.14 Segment 2, Station 560+00 to Station 562+00

From 560+00 to 562+00, this section has existing fill slopes at approximately

2.5H:1V. Noticeable pavement cracking, both longitudinal and transverse, and deformation causing a rough roadway, is prevalent in this section of the roadway. In this case, this is most likely due to inadequate drainage of groundwater and surface water; water-loving plants are prevalent, and areas of grass flattened by flowing water are also visible. Figure 13 depicts this area. Based on our subsurface investigation, the foundation soils in this area are wet, medium to high plasticity clays.

Figure 13. Pavement deformation at Sta. 560+00 – 561+00.

6.15 Segment 2, Station 563+00 to Station 570+00

Large cuts are proposed in this section. An archeological site has been identified in the area of the proposed cut; in order to avoid this site, a retaining wall has been proposed.

6.16 Segment 2, Station 582+00 to Station 585+00

This section has an area of shallow hard bedrock (a pegmatite), with colluvial cobble and soil slopes on either side. Large cut slopes are proposed in this section. The limits of rock and soil areas are difficult to predict accurately prior to construction, because no geotechnical borings were drilled in this area.

6.17 Segment 3, Station 588+00 to Station 591+00

This section has steep rock cliffs behind proposed rock and soil cuts. It appears that 0.5H:1V should be acceptable assuming bedrock is shallow in this section.

6.18 Segment 3, Station 591+00 to Station 600+00

In this section, steep sliver rock cuts are proposed. It appears that 0.5H:1V should be acceptable assuming bedrock is shallow in this section, however, it will be difficult to start the cuts and specialized blasting means and methods may be necessary to initiate the rock cut at the top since the rock forms steep pinnacles in places. This area is shown in Figure 14 below.

Figure 14. Area of proposed sliver rock cuts, Sta. 591+00 – 600+00.

6.19 Segment 3, Station 619+00 to Station 622+00

This section consists of colluvial soil and boulder slopes that appear to be at approximate 1H:1V slopes. We do not believe this section will stand at a 0.5H:1V slope. We recommend either laying the slope back at a 1H:1V or using a soil nail wall system. This area is shown in Figure 15 below.

Figure 15. Sta. 619+00 – 622+00, proposed cuts of 0.5H:1V. Existing slope is 1H:1V.

Gunnison County, Colorado

6.20 Segment 3, Station 626+00 to Station 627+00

This section consists of colluvial soil and boulder slopes with significant boulder talus fields above the cut. We do not believe this section will stand at a 0.5H:1V slope.

We recommend either laying the slope back at a 1H:1V or using a soil nail wall. This area is shown in Figure 16 below.

Figure 16. Sta. 626+00 – 627+00, existing colluvial slope with proposed cuts of 0.5H:1V.

6.21 Segment 3, Station 632+50 to Station 638+50

This section consists of inter-bedded granites, pegmatites and metamorphic bedrock (Figure 17, below).

Figure 17. Sta. 632+00 – 638+50.

6.22 Segment 3, Station 638+50 to Station 640+50

This section consists of existing planar failure mechanisms in the bedrock cut

(see Figure 18, below). The existing cut is approximately 1H:1V. We do not believe this section will stand at a 0.5H:1V slope. We recommend either laying the slope back at a 1H:1V or using a soil nail wall system.

Figure 18. Sta. 638+50 – 640+50, existing cut is 1H:1V with planar failure features.

6.23 Segment 3, Station 642+50 to Station 644+00

This section consists of colluvial soil slopes that are at an approximate 1H:1V slope (see Figure 19, below).