CO.PFH.81-1(4).Tarryall_Creek_Rd.Tech_Memo.Geotech_Recs.2011.pdf

To: Micah Leadford, Project Manager, CFLHD, Lakewood CO

From: Khamis Y. Haramy, Senior Geotechnical Engineer, CFLHD, Lakewood CO

Through: Matthew DeMarco, Lead Geotechnical Engineer, CFLHD, Lakewood, CO

Subject: Geotechnical Recommendations for the Reconstruction of Tarryall Creek Road, Project CO PFH 81 1(4)

BACKGROUND

This memorandum provides geotechnical recommendations for the reconstruction and rehabilitation of a 16.8-mile long segment of the Tarryall Creek Road, Forest Highway (FH) 81.

Tarryall Creek Road is 41.8-miles long, lying mostly within the Pike National Forest in Park County, Colorado. The two-lane roadway begins at the intersection with US Highway 285 in the town of Jefferson, and continues southeasterly past the Tarryall Reservoir and through the town of Tarryall, eventually terminating at US Highway 24 west of the town of Lake George.

The Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) has been improving roadway geometry along this route since 1997. The first phase, constructed in 1997 under the project CO PFH 81-1(1), began on the southern end of the route at the intersection with County Road 31 and continued approximately 12 miles southeast to the intersection with US Highway 24. The second phase, constructed in 2007 under the project CO PFH 81-1(2), began at the northwest end of the road at the intersection with US Highway 285 and continued for approximately 8 miles to the southeast. The third phase, project CO PFH 81- 1(3), is currently under construction and is scheduled for completion in the fall of 2011. This phase begins at the termination point of the second phase (Station 790+68) and continued approximately 8.9 miles southeast to the intersection with County Road 23 (Station 1262+400).

The CO PFH 81-1(4) begins at the intersection with County Road 23 (Station 1262+40) and extends southeast for a distance of 16.8 miles, ending at Station 2151+50. The route is comprised of alternating segments of reconstruction (4R) and rehabilitation, restoration, and resurfacing (3R). Other improvements include grading, asphalt pavement, drainage, signing and striping, and other safety-related design necessary to meet current standards. Specifically, proposed improvements include roadway widening, horizontal and vertical alignment shifts through cuts and fills, retaining walls, slope rehabilitation, erosion control, drainage improvement, and other

Technical Memorandum Date: November 18, 2011 related features to meet current FHWA design standards. All recommendations provided in this memorandum are based on the 50% projects plans.

This memorandum provides recommendations for stable cutslope ratios, material quality and shrink/swell values for each proposed cutslope, and rockery walls located near Stations 1469+25, 1642+00, and 2095+80.

REGIONAL AND SITE GEOLOGY

Tarryall Creek Road is located in the South Park Valley, one of the three larger inter-montane valleys in Colorado. While Paleozoic and Mesozoic sedimentary rocks underlie much of South Park Valley, the Tarryall route is mostly underlain by decomposed to moderately weathered granite and granitic gneiss. Most of the route follows alongside water drainages that cross the road through existing culverts at several locations.

The route runs just east and parallel to the Elkhorn thrust fault. Although there is little information on this fault, it does not appear to have significant impacts on the route design.

Based on AASHTO –LRFD Seismic Design Guidelines, a peak horizontal ground acceleration of 0.05g and Class C soils were selected for this project. Per AASHTO recommendations, the horizontal ground acceleration at the site does not warrant detailed seismic analysis.

EVALUATIONS AND RECOMMENDATIONS

The proposed improvements for this segment of the roadway include several horizontal and vertical alignment shifts through cuts and fills. Wetlands and several historic structures exist along the alignment which creates major challenges for the designers during the alignment shifts.

Steeper and taller cutslopes exist along the first half of the alignment while the slopes along the second half are generally flatter and shorter.

CUTSLOPE EVALUATION

An evaluation of the proposed cutslopes to determine safe and stable slope ratios for the materials encountered was conducted by Central Federal Lands (CFL) engineers. Tarryall Creek Road transverses mountain terrains with cutslope heights ranging to 50-feet high, with moderate-to-steep slope ratios ranging from near vertical in bedrock to 1V:2.5H in soil matrix. Outboard fill ratios are locally as steep as 1V:1H. Several areas of the slopes are eroded and boulder size rocks have migrated down the slope by free falling, rolling, or sliding and have accumulated in the existing ditches. The cutslopes along this segment of the roadway are mostly composed of predominantly-to-moderately weathered granitic/gneissic bedrock. The physical and chemical weathering of the bedrock produced rock fragments of various sizes and shapes that accumulate on the slope surface. In areas of sufficient topographic relief and in steep cutslopes, the erosion process and gravitational forces cause the rock to move down to lower portions of the slope mixing with soil and forming colluvial materials. The characteristics of the colluvium are generally dependent on the bedrock sources and the weathering process. In most cases, however, the colluvial materials in the cutslopes consist of loose, incoherent, heterogeneous mixtures of soil materials and rock fragments. Colluvial deposits range to several feet in thickness and are somewhat unstable in nature requiring regular maintenance. As exposed in cutslopes, colluvial deposits are generally less than five feet thick and in some cases overlay weathered bedrock.

Highly weathered and highly fractured metamorphic (gneiss) and granitic bedrock also outcrops at various locations along Tarryall Creek Road. The metamorphic rock is generally blocky, contains several orthogonal joint sets, and tends to separate into discrete blocks, the size and shape of which is related to the orientation of the joints within each joint set. Road alignment in relation to discontinuity orientation does not appear to cause specific rock slope instability problems. Within granitic rock masses, large rounded boulders or corestones (up to 10 feet) also exist on some cutslopes. These boulders are generally stable unless erosion or slope excavation removes the decomposed soils from around the corestones and the boulders become more exposed and eventually unstable.

Table 1 provides a detailed summary of the materials that will be encountered at each cutslope and provides existing versus recommended stable slope ratios. The data is based on observations only, no drilling or test pit samples were taken. Recommended cut slope ratios are for stable slopes without soil or rock reinforcement.

Anticipated bulking (swelling) or shrinking of each cut materials is also listed in the table for design purposes. Materials bulk or shrink is the percent increase or decrease in volume of an earth material after it has been excavated and recompacted. Typically, intact rock bulks when excavated, mechanically broken, and recompacted while intact soil shrinks when excavated and recompacted. Catchment ditches widths of existing cutslopes were also measured and are shown in Table 1. In most cases, existing catchment areas were about 3 feet and appeared to have a sufficient width to retain a high percentage of falling debris. It is therefore recommended that a minimum of 3-foot wide catchment is included in the design at most cut location. In areas where proposed cutslope ratios are 2V:1H or steeper and proposed slope heights are greater than 30 feet (Station 1635+50 to 1642+00, Station1425+00 to 1427+25, and Station 1429+50 to1430+50), a 5-foot minimum catchment area is recommended. In steep rock cut areas where the 5-foot minimum catchment is not achieved we recommend slope draping. However, due to the weathered and jointed nature of the rock mass, loose colluvial materials overlaying bedrock, steep terrain, and slope heights, it is not feasible to construct catchment ditches to prevent rockfall debris in the roadway. Cut slope recommendations assume routine maintenance to keep a safe roadway conditions. The materials along the investigated cutslopes can be generalized into the following three categories:

Material 1: Colluvial gravel, cobbles and boulder size rock fragments in a silty sand soil matrix.

Material 2: Decomposed grading to predominantly decomposed granite and granitic gneiss.

Material 3: Predominantly decomposed grading to moderately to slightly weathered granite and granitic gneiss

Cutslopes are planned in both weathered bedrock and colluvial soil materials. Most of the cutslopes will be excavated only a few feet horizontally into the slopes. Design recommendations provided in Table 1 are to improve cutslope stability and reduce rock fall hazard while satisfying other design-specific project criteria. In general it is recommended that slopes in colluvial soils, categorized as Material 1, be cut at 1V:1.5H or flatter, where possible, and scaled of loose rock that may present safety hazards. This is possible on many of the lower slopes and should provide for vegetation. Slopes in Material 2 should be cut at a slope ratio from 1V:1.5H to 1V:1H depending on the weathering of the materials and the slope height. None of the existing slopes with slope ratios of 1V:1H or steeper have revegetated significantly. Slope cuts in Material 3 can be 1V:1H or steeper and may require blasting during excavation. In areas where rock blasting required for slope excavation, it is also recommended that pre- and post-blast condition surveys and ground motion monitoring (peak particle velocity) be conducted as nearby historic structures could be at risk from blast damage. Blasting should be in accordance with SCR for Section 205.

Refer to Table 1 for recommended cutslope rations for specific station intervals and shrink/swell factors for use in design.

Shallow cutslope ratios used to promote vegetation may also benefit from erosion control products such as erosion control blanket or guard bonded fiber matrix. Such products are biodegradable with a design life up to two years. A qualified erosion control engineer should be consulted for specific recommendations regarding slope protection measures.

TABLE 1.- Summary of Slope Recommendations for Soil and Rock Cuts

Begin Station

End Station

Existing Cut

Slope Ratio (Max)

Recommended Cut Slope

Ratio

Lithology Slope Height (Max)

Existing Rock fall catchment width

Swell/ Shrink

Material Type Number

1262+40 1262+85 NA. 1V:1.5H Brown silty sand NA. 5 ft -10% Material 1

1262+85 1264+50 NA. 1.33V:1H

Light yellow brown to gray granitic gneiss, fractured, moderately weathered

NA. 5 ft +15% Material 3

1264+50 1273+50 1V:1.5H 1 V:1.5H Brown silty sand with cobble to boulder 10 ft 5ft -10% Material 1

1273+50 1275+00 1 V:1H 1.33 V:1H

Light orange brown to gray granitic gneiss, fractured, moderately weathered

18 ft 5ft +15% Material 3

Begin Station

End Station

Existing Cut

Slope Ratio (Max)

Recommended

Existing Rock fall catchment width

Swell/ Shrink

Material Type Number

1275+00 1276+00 1 V:1.5H 1 V:1H

Light yellow brown to gray granitic gneiss, fractured, predominantly decomposed to moderately weathered

15 ft 3ft +10% Material 3

1276+00 1278+00 1 V:1.5H 1V:1.5H Brown silty sand with cobble to boulder size rock fragments <10 ft 3ft -10% Material 1

1278+00 1278+75 1 V:1H 1.33 V:1H

Light orange brown to gray granite knob, fractured, moderately weathered

15 ft 3ft +15% Material 3

1278+75 1280+60 1 V:1.5H 1 V:1.5H Brown silty sand with cobble to boulder size rock fragments <10 ft 3 ft -10% Material 1

1280+60 1281+00 1 V:1.5H 1.33 V:1H

Light orange brown to gray granite knob, fractured, moderately weathered

<10 ft 3 ft +15% Material 3

1281+00 1283+00 1 V:2H 1 V:1.5H Brown silty sand with cobble to boulder size rock fragments <10 ft 3 ft -10% Material 1

1283+00 1285+75 1 V:1.5H 1V:2H

Light orange brown to gray granitic gneiss, fractured, predominantly decomposed to moderately weathered

13 ft 3 ft +10% Material 3

1285+75 1287+50 1V:2.5H 1 V:1.5H

Brown silty sand with cobble size rock fragments overlying decomposed granite

13 ft 3 ft -10% Material 1

1287+50 1302+50 1 V:1.5H 1 V:1H

1V:1.5H Better

Light brown orange decomposed granite 20 ft 3 ft +10% Material 2

1304+00 1307+50 1 V:1.5H 1.33 V:1H

Light brown orange granite knob, fractured, moderately weathered with some decomposed dikes

15 ft 3 ft +15% Material 3

1307+50 1309+50 NA 1 V:1.5H Brown silty sand with small rock fragments NA 3 ft -10% Material 1

1309+50 1311+50 NA 1V:1H

Light brown orange granite knob, fractured, moderately weathered

NA 3 ft +10% Material 3

1319+00 1323+00 1V:1.5H 1V:1H

Light brown orange granite, fractured, predominantly decomposed

12 ft 3 ft +10% Material 2

1323+00 1338+50 1V:1.5H 1V:1.5H Brown silty sand with cobble to boulder size rock fragments 12 ft 3 ft -10% Material 1

1338+50 1341+50 1V:1H 1V:1.5H Flatter slope

Brown silty sand with cobble to boulder size rock fragments, very rocky

25 ft 3 ft -10% Material 1

1341+50 1345+00 1V:1H 1V:1H Flatten slope

Brown silty sand with cobble to boulder size rock fragments 10 ft 3 ft -10% Material 1

End Station

Existing Cut

Slope Ratio (Max)

Recommended

Existing Rock fall catchment width

Swell/ Shrink

Material Type Number

1345+00 1353+00 2V:1H 2V:1H

Light brown orange granite knob, very fractured, moderately weathered

30 ft 3 ft +10% Material 3

1353+00 1372+50 1V:1H 1V:1.5H Flatten slope

Brown silty sand with cobble to large boulder size rock fragments 18 ft 3 ft -10% Material 1

1372+50 1376+50 1V:1.5H 1.33V:1H

Light brown orange granite knob, very fractured, moderately weathered

<10 ft 3 ft +10% Material 3

1376+50 1383+00 1V:1H 1V:1.5H Flatten slope

Brown silty sand with cobble to large boulder size rock fragments 18 ft 3 ft -10% Material 1

1383+00 1385+00 1V:1H 1V:1H

Gray gneiss, fractured, predominantly decomposed to moderately weathered

20 ft 3 ft +10% Material 3

1385+00 1390+00 1V:1.5H 1V:1.5H

Brown silty sand with cobble to small boulder size rock fragments, very rocky

10 ft 3 ft -10% Material 1

1390+00 1393+00 1V:1.5H 1V:1.5H Brown silty sand overlying gray gneiss, moderately weathered

20 ft 3 ft -5% Material 1

1393+00 1420+00 1V:1H 1V:1.5H Flatten slope

Brown silty sand with cobble to large boulder size rock fragments, very rocky

27 ft 3 ft -5% Material 1

1420+00 1425+00 1V:1.5H 1V:1.5H Brown silty sand with cobble size rock fragments

10 ft 2 ft -10% Material 1

1425+00 1427+25 4V:1H 2V:1H Flatten slope

Gray gneiss, fractured, moderately weathered 32 ft 2 ft +15% Material 3

1427+25 1429+50 2V:1H 1V:1.5H Flatten slope

Brown silty sand with cobble to boulder size rock fragments 30 ft 2 ft -10% Material 1

1429+50 1430+50 2V:1H 2V:1H 1.33:1 Better

Gray gneiss, fractured, moderately weathered 25 ft 2 ft +10% Material 3

1431+00 1433+00 1V:1H 1V:1.5H Flatten slope

Brown silty sand with gravel size rock fragments 20 ft 2 ft -10% Material 1

1433+00 1437+00 1V:1H 1V:1.5H Flatten slope

Brown silty sand and rock fragments overlying gray to white gneiss, moderately weathered

22 ft 2 ft -5% Material 1

1437+00 1444+00 1V:1.5H 1V:1.5H Brown silty sand with gravel size rock fragments <10 ft 2 ft -10% Material 1

1444+00 1448+50 4V:1H 2V:1.5H Flatten slope

Brown silty sand with cobble to boulder size rock fragments 50 ft 2 ft -5% Material 1

1448+50 1450+00 4V:1H 2V:1H Gray to white gneiss, moderately to slightly weathered 50 ft 2 ft +15% Material 3

End Station

Existing Cut

Slope Ratio (Max)

Recommended

Existing Rock fall catchment width

Swell/ Shrink

Material Type Number

1450+00 1454+50 2V:1H 1V:1.5H Flatten slope

Brown silty sand with cobble to boulder size rock fragments 25 ft 2 ft -10% Material 1

1454+50 1458+50 1V:1.5H 1V:1.5H Brown silty sand with gravel size rock fragments 15 ft 2 ft -10% Material 1

1458+50 1462+60 1V:1.5H 1V:1.5H

Brown silty sand with cobble to boulder size rock fragments, very rocky

10 ft 2 ft -10% Material 1

1462+60 1469+00 1V:1.5H 1V:2H

Gray to white gneiss, fractured, predominantly decomposed to moderately weathered

12 ft 2 ft +10% Material 3

1470+00 1471+00 1V:1.5H 1V:1H

Light brown orange to gray granitic gneiss, predominantly decomposed to moderately weathered

<10 ft 2 ft +10% Material 3

1471+00 1484+00 1V:1.5H 1V:1.5H Granite outcrop

Brown silty sand with gravel to cobble size rock fragments 14 ft 2 ft -10% Material 1

1484+00 1486+00 1V:2.5H 1V:1H Light brown orange granitic gneiss, moderately weathered

<10 ft 3 ft +5% Material 3

1486+00 1499+00 1V:1.5H 1V:1.5H

Brown silty sand with gravel size rock fragments, some predominantly decomposed rock outcrops

<10 ft 3 ft -5% Material 1

1499+00 1505+50 1V:1.5H 1V:1.5H

Brown silty sand with gravel to very large boulder size rock fragments

<10 ft 3 ft -10% Material 1

1505+50 1509+00 1V:1H 1.33V:1H Gray granitic gneiss, moderately weathered 20 ft 3 ft +10% Material 3

1509+00 1519+50 1V:1H 1V:1.5H Flatten slope

Light orange brown to brown silty sand with gravel to cobble size rock fragments

15 ft 2 ft -10% Material 1

1519+50 1520+50 1V:1.5H 1V:1.5H Light brown orange granitic gneiss, moderately weathered

12 ft 2 ft +10% Material 3

1520+50 1526+00 1V:1H 1V:1.5H Flatten slope

Brown silty sand with gravel to cobble size rock fragments 10 ft 2 ft -10% Material 1

1526+00 1528+00 NA 4V:1H

Light brown orange granitic gneiss, moderately weathered

NA none +15% Material 3

1528+00 1549+00 1V:1.5H 1V:1.5H

Brown silty sand with gravel to cobble size rock fragments, some granite outcrops

20 ft none -10% Material 1

1549+00 1551+00 2V:1H 1.33V:1H Light brown orange to gray granitic gneiss, moderately weathered 23 ft none +10% Material 3

End Station

Existing Cut

Slope Ratio (Max)

Recommended

Existing Rock fall catchment width

Swell/ Shrink

Material Type Number

1551+00 1556+50 1V:2H 1V:1.5H Brown silty sand with gravel to cobble size rock fragments

<10 ft 3 ft -10% Material 1

1556+50 1558+25 1V:1.5H 1.33V:1H

Light brown orange to gray granitic gneiss, fractured, moderately weathered

14 ft 3 ft +10% Material 3

1558+25 1569+00 1V:2.5H 1V:1.5H Brown silty sand with gravel to boulder size rock fragments

<10 ft 3 ft -10% Material 1

1569+00 1571+00 NA 1.33V:1H

Light brown orange granitic gneiss, predominantly decomposed to moderately weathered

NA 3 ft +10% Material 3

1571+00 1578+00 NA 1V:1.5H Brown silty sand with gravel size rock fragments NA 3 ft -10% Material 1

1578+00 1582+00 1V:1.5H 1V:1.5H Orange brown silty sand with gravel size rock fragments

20 ft 3 ft -10% Material 1

1582+00 1583+50 NA 1.33V:1H

White to light brown orange granitic gneiss, fractured, moderately weathered

NA 3 ft +10% Material 3

1583+50 1588+00 NA 1V:1.5H

Brown silty sand with gravel to cobble size rock fragments

NA 3 ft -10% Material 1

1588+50 1589+00 NA 1V:1H

Gray granitic gneiss, predominantly decomposed to moderately weathered

NA 3 ft +5% Material 3

1589+00 1591+25 NA 1V:1.5H

Brown silty sand with gravel to cobble size rock fragments

NA 3 ft -10% Material 1

1591+25 1592+75 1V:1H 1.33V:1H

White to light brown orange granitic gneiss, fractured, moderately weathered

18 ft 3 ft +10% Material 3

1592+75 1593+50

NA

Over steepened

1V:1.5H Brown silty sand with gravel to small boulder size rock fragments NA 3 ft -10% Material 1

1593+50 1595+00 NA 1V:1H

Gray granitic gneiss, highly fractured, moderately weathered

NA 3 ft +10% Material 3

1595+00 1607+00 1V:3H 1V:1.5H Brown silty sand with gravel to cobble size rock fragments

<10 ft 3 ft -10% Material 1

1607+00 1619+00 1V:1H 1V:1H

Light brown orange to gray granitic gneiss, moderately weathered and soil mix

20 ft 3 ft +5% Material 3

1619+00 1623+00 NA 1V:1.5H

Brown silty sand with gravel to cobble size rock fragments

NA 3 ft -10% Material 1

1623+00 1626+00 1V:1H 1.33V:1H Gray to light brown orange granitic gneiss, fractured, moderately

20 ft none +10% Material 3

End Station

Existing Cut

Slope Ratio (Max)

Recommended

Existing Rock fall catchment width

Swell/ Shrink

Material Type Number weathered

1626+00 1629+50 1V:1H 1V:1.5H Flatten slope

Brown silty sand with gravel to cobble size rock fragments 25 ft 3 ft -10% Material 1

1629+50 1632+50 2V:1H 2V:1H 1.33:1 Better

Gray to light brown orange granitic gneiss, fractured, moderately weathered

24 ft 3 ft +10% Material 3

1632+50 1635+50 1V:1H 1V:1.5H Flatten slope

Brown silty sand with gravel to cobble size rock fragments 20 ft 3 ft -10% Material 1

1635+50 1642+00 4V:1H 2V:1H Avoid sliver cut

Gray to light brown orange granitic gneiss, fractured, moderately weathered

45 ft 3 ft +10% Material 3

1644+00 1661+50 1V:1H 1V:1H Avoid sliver cut

Gray to light brown orange granitic gneiss, highly fractured, moderately weathered

24 ft 3 ft +10% Material 3

1661+50 1672+00 1V:1H 1V:1.5H Flatten slope

Brown silty sand with gravel to cobble size rock fragments 24 ft 3 ft -10% Material 1

1672+00 1677+00 1V:1H 1V:1.5H Flatten slope

Yellow brown silty sand with cobble to small boulder size rock fragments

18 ft 3 ft -10% Material 1

1677+00 1689+00 1V:1H 1V:1.5H

Light brown orange decomposed to predominantly decomposed granite

18 ft 3 ft 0% Material 2

1689+00 1691+00 4V:1H 1V:1H Light brown orange granite, highly fractured, moderately weathered

15 ft 3 ft +15% Material 3

1691+00 1740+00 1V:1H 1V:1H Avoid sliver cut

Light brown orange decomposed to predominantly decomposed granite

25 ft 3 ft +5% Material 2

1741+85 1744+50 2V:1H 1V:1.5H Or flatter

Flatten slope

Light brown orange granite, predominantly decomposed to moderately weathered

24 ft 3 ft +10% Material 3

1744+50 1754+00 1V:1H 1V:1.5H Flatten slope

Light brown orange decomposed granite to residual soil 26 ft 3 ft 0% Material 2

1754+00 1755+00 1V:1.5H 1V:1H Light brown orange predominantly decomposed granite

15 ft 3 ft +5% Material 2

1755+00 1769+00 1V:1H 1V:1.5H Flatten slope

Light brown orange decomposed granite to residual soil 23 ft 3 ft -5% Material 2

1769+00 1770+50 1V:1H 1V:1H Light brown orange predominantly decomposed granite

20 ft 3 ft +5% Material 2

End Station

Existing Cut

Slope Ratio (Max)

Recommended

Existing Rock fall catchment width

Swell/ Shrink

Material Type Number

1770+50 1777+50 1V:1.5H 1V:1.5H Light brown orange decomposed granite to residual soil

15 ft 3 ft -10% Material 2

1777+50 1790+00 1V:1H 1V:1H

Gray to light brown orange predominantly decomposed to moderately weathered granite

22 ft 3 ft +5% Material 3

1790+00 1808+00 1V:1.5H 1V:1.5H Brown to light brown orange DG to residual soil 12 ft 3 ft -10% Material 2

1808+00 1812+00 1V:1.5H 1V:1H

Light brown orange decomposed to moderately weathered granite

<10 ft 4 ft +10 Material 3

1812+00 1823+00 1V:1.5H 1V:1.5H Avoid sliver cut Light brown orange DG 15 ft 4 ft 0% Material 2

1823+00 1832+00 1V:2H 1V:1.5H Brown residual soil (silty sand) <10 ft 3 ft -10% Material 1

1832+00 1892+00 1V:1.5H 1V:1.5H Avoid sliver cut

Brown residual soil (silty sand) with sporadic harder rock outcrops 12 ft 3 ft -10% Material 1

1892+00 1895+50 NA 1V:1.5H Brown to light brown orange DG NA 3 ft 0% Material 2

1895+50 1914+00 NA 1V:1.5H Brown silty sand with gravel size rock fragments NA 3 ft -10% Material 1

1914+00 1950+00 1V:1H 1V:1.5H Flatten slope

Light brown orange to gray decomposed to predominantly decomposed granite.

Rock outcrop at944 to

17 ft 3 ft 0% Material 2

1950+00 1957+00 1V:3H 1V:1.5H Brown silty sand with gravel size rock fragments <10 ft 3 ft -10% Material 1

1957+00 1960+00 1V:2H 1V:1H Light brown orange predominantly decomposed granite

<10 ft 3 ft 0% Material 2

1960+00 2032+00 1V:1H 1V:1.5H Flatten slope

Brown silty sand with gravel size rock fragments with sporadic harder rock outcrops

18 ft 3 ft -10% Material 1

2032+00 2034+00 1V:1.5H 1.33V:1H Dark gray to light brown orange granite, moderately weathered

<10 ft 3 ft +10% Material 3

2034+00 2039+00 1V:2H 1V:1.5H

Brown silty sand with gravel to boulder size rock fragments with sporadic harder rock outcrops

<10 ft 3 ft -10% Material 1

2039+00 2062+00 1V:1.5H 1V:1H

Light brown orange to gray decomposed to moderately weathered granite

<10 ft 3 ft +10% Material 3

End Station

Existing Cut

Slope Ratio (Max)

Recommended

Existing Rock fall catchment width

Swell/ Shrink

Material Type Number

2062+00 2079+00 2V:1H 2V:1H

1.33V:1H Better

Light brown orange to gray moderately weathered granite 14 ft 3 ft +15% Material 3

2079+00 2093+00 1V:1.5H 1V:1H Light brown orange DG 14 ft 3 ft 0% Material 2

2093+00 2096+00 1V:2.5H 1V.33:1H Light brown orange to gray moderately weathered granite

<10 ft 3 ft +15% Material 3

2096+00 2110+00 1V:1.5H 1V:1H Light brown orange DG with sporadic harder rock outcrops

<10 ft 3 ft 0% Material 2

2110+00 2135+00 2V:1H 2V:1H Light brown orange to gray moderately weathered granite

18 ft 3 ft +15% Material 3

2135+00 2143+00 1V:1H 1V:1.5H Flatten slope

Light brown orange decomposed granite mixed with brown silty sand and rock fragments

25 ft 3 ft -10% Material 2

2143+00 2156+00 1V:1.5H 1V:1H Avoid sliver cut

Light brown orange to gray decomposed to moderately weathered granite

18 ft 3 ft +10% Material 3

2156+00 2159+43 NA 1V:1.5H

Easy Grading Brown soil NA 3ft -10% Material 2

MATERIAL SHRINK / SWELL FACTORS

Anticipated material shrink/swell for cut material is also listed in the table for design purposes.

Materials shrink or swell is the percent decrease or increase in volume of an earth material after it has been excavated and recompacted. Shrink/Swell factors are based on experiences from previous projects and published data for similar materials. To accurately determine shrink/swell factors for soil/rock units is complex. Factors that impact earthwork quantities are top soil stripping, clearing and grubbing requirements, boulder stripping, compaction and construction practices. Typically, intact rock swells when excavated, mechanically broken, and recompacted, whereas intact soil shrinks when excavated and recompacted. Refer to Table 1 for recommended cutslope ratios for specific station intervals and shrink/swell factors for use in design.

CATCHMENT DITCHES

Catchment ditch widths of existing cutslopes were also measured and are shown in Table 1. In most cases, existing catchment areas were about 3 feet and appeared to have a sufficient width to retain a high percentage of falling debris. It is therefore recommended that a minimum of 3-foot-wide catchment be included in the design at most cut locations. In areas where proposed cutslope ratios are 2V:1H or steeper, and proposed slope heights are greater than 30 feet (Station 1635+50 to 1642+00, Station1425+00 to 1427+25, and Station 1429+50 to1430+50), a 5-foot minimum catchment area is recommended. In steep rock cut areas where the 5-foot minimum catchment is not achieved we recommend scaling and slope draping from top of the cut to 5 feet above roadway grade elevation. Due to the weathered and jointed nature of the rock mass, loose colluvial materials overlaying bedrock, steep terrain, and high slope heights, it is not feasible to construct catchment ditches with widths to prevent all rockfall debris from reaching the paved surface. Therefore, cutslope recommendations assume routine maintenance to keep a safe roadway condition. Experiences show that rockfall frequencies may increase for a short period following a rock cut. Slopes generally stabilize after couple years.

Cutslope and catchment evaluations consider local precedence and engineering judgment based on observations, experiences and past performance of slopes in the project vicinity. Based on the geological structure in rock cuts, and the fact that most cut depths are within 20-feet existing cut heights/slope ratio do not change appreciably and therefore, significant increase of risk from higher rockfalls are not anticipated.

A specific area of concern is between Station1689+50 and 1690+00. This cutslope mostly consists of highly fractured bedrock existing at a slope ratio of 4V:1H with almost on rockfall catchment area. As shown in the 50% plans, this slope possesses a high safety risk to the public from rock falls. The following alternatives are recommended for this site:

(a) Flatten the slope to a minimum slope ratio of 1.33V:1H with a minimum 5-foot catchment ditch,

(b) Scale and drape the entire slope segment with wire mesh anchored on top of the slope in accordance with SCR Section 651 or,

(c) Shift horizontal alignment a minimum of 5 feet to move away from the cut and to minimize impacts.

FIGURE 1:- Station 1689+50 Rock Cut

SLOPE SCALING AND EROSION CONTROL

Where differential weathering of the slope surface produces a rockfall hazard, as may be the case in some areas along the alignment, scaling can be used as a temporary solution. Scaling can be done by machine or hand to remove potential unstable boulders and detached blocks, thereby reducing the rockfall hazard and maintenance following construction. It is recommended that a minimum of 100 scaling crew hours be budgeted in the project. Slope scaling should be done on slopes with potential rockfalls and will be determined by a geotechnical engineer during construction. Scaling should be conducted in accordance with SCR for Section 623.

Shallow cutslope ratios used to promote vegetation may also benefit from erosion control products such as erosion control blanket or guard bonded fiber matrix. Such products are biodegradable with a design life up to two years. A qualified erosion control engineer should be consulted for specific recommendations regarding slope protection measures.

BROW SLOPE ROUNDING

A portion of Tarryall Creek Road existing cutslopes have a brow or erosional scarp at the top, where the slope has resisted erosion because of strength contributions by the overlaying root system. As erosion continues on the slope below, the brow will reach a height that exceeds the strength of the root-reinforced material and the brow will fail. Removing the brow and rounding the slope where it catches the natural topography can mitigate this hazard. It is therefore recommended that weathered/eroded top sections of existing and cutslopes are rounded when possible or within equipment reach. Rounding should be minimized to the extent possible to take advantage of the existing vegetation root mass for erosion control. Erosion control matting should also be installed at these rounding locations to promote revegetation. Slope matting quantities should be estimated based on existing slope length and slope ratios. Slope crest (brow) rounding should be in accordance with SCR for Section 623.

FIGURE 2 Station 1592 Typical Cutslope Brow

FIGURE 3:- Station 1672 Typical Cutslope Brow

EMBANKMENTS

The rehabilitation and reconstruction of Tarryall Creek Road requires extensive embankment work and drainage improvements. In general, fill slopes on the outboard side of the roadway are generally stable at existing slope ratio and no retaining structures will be required to support the roadway. It is recommended that fills be constructed, at a slope ratio of 1V:2H or flatter, using on site excavated materials (unclassified borrow) in accordance with Section 704-06. Fill materials must be clean and free of organic debris with rock fragments size less than 24 inches.

Fills must be placed in thin horizontal lifts with benching not exceeding two feet loose thickness (or compatible with the type of compaction equipment being used. Granular materials should be adjusted to within 2 percent of optimum moisture. Fill should be compacted to a minimum of 95 percent of standard Proctor maximum dry density.

EMBANKMENT EROSION PROTECTION

Several locations were identified for embankment erosion protection with Class 3 riprap rundown blanket. Embankment erosion was mainly caused by surface water runoff that has created erosional channels along the downslope and encroaching on the roadway shoulder.

Special rock embankment for bank armoring is recommended to rehabilitate eroded embankments and to prevent further erosion. Special rock embankment with a minimum bench width of 10 feet should be installed in accordance with Section 252 near approximate stations listed in Table 2.

TABLE 2:- Areas recommended for Special Rock Embankments

Begin Station End Station Length, (ft)

Width,

(ft) Riprap Class

1443+50 1443+60 5 10 Class 3 1681+00 1681+25 10 25 Class 3 1685+50 1685+70 40 20 Class 3 1695+50 1695+60 35 10 Class 3 1696+50 1697+10 20 60 Class 3 1866+50 1866+75 20 15 Class 3

ROADWAY DISTRESS

The existing asphalt roadway suffers moderate to severe cracking, pavement depressions, potholes, and localized subgrade failures at several locations along the alignment. These areas are considered to have a factor of safety close to failure and may be experiencing creep. Most of these failures are caused by inadequate drainage allowing for material oversaturation, weak subgrade with high fines content, thin/old pavement, or freeze/thaw during the spring months.

Roadway distress observed along the alignment is likely a combination of all three factors.

FIGURE 4:- Typical Roadway Distress, Station 1672

FIGURE 5:- Typical Roadway Distress, Station 2040

Subgrade stabilization is recommended as a mitigation method to improve roadway distress areas. Subgrade stabilization is a repair technique that uses granular backfill compacted in lifts with geotextile reinforcement in accordance with Section 204.11(b). Because of the existing pavement condition it is difficult to determine exact locations of deep patches along the alignment. It is recommended that several areas estimated at a total of 150-feet long be included in the bid documents for subgrade stabilization and should be used as directed by the CO.

Stabilization techniques are not a substitute for a proper constructed roadway; however, they are an economical solution for reducing pavement distress and slow reduce settlement. In roadway distressed areas, a minimum of 2 feet of subgrade materials should be excavated with a 2 percent sloping base and replaced with granular free draining backfill materials (Section 704.03) reinforced with biaxial geogrid (Section 207) layers placed at one foot intervals. Sub-excavation should either be full width of the roadway or half width depending on the areas. Drainage should also be improved to prevent water of penetrating the roadway subsurface.

Local springs under the pavement may exist and may require draining and/or stabilization prior to placement of roadway base and pavement. In areas where several springs or water seeps may exist beneath the paved surface, a blanket underdrain should be installed to improve drainage and prevent roadway damage. A granular blanket underdrain is recommended to be installed underneath the paved surface between station 1620+00 and 1690+00. This area borders a flat wetland on the left side and a steep rock cut on the right allowing the surface water to infiltrate and remain beneath the roadway surface and causing severe damage (potholing) during the freeze/thaw cycle. The blanket underdrain should consist of 6-inches of select granular fill placed under the roadway across the entire width of the roadway prism and daylights on the downslope side of the roadway. The select granular fill should be encapsulated by a Type I-A geotextile. An alternative geomembrane material (such as roadrain) may also be used instead of the granular blanket underdrain.

ROCKERY DESIGN

This section provides recommendations for the design of two proposed rockeries along Tarryall Creek Road. Table 3 summarizes the proposed rockery locations and geometries. Fill rockery #W1 is to minimize impact of fill on trees and trail. This rockery will be installed on both side of a culvert. Cut rockery #W2 is planned to retain a driveway that leads to a historic building. A small diameter culvert installed under the driveway will empty onto the rockery face and is diverted away from the rockery in a paved ditch along the rockery foundation. Fill rockery #W3 is planned to prevent impacts to an existing residential driveway.

TABLE 3:- Location and Geometries of Proposed Rockeries

Wall Begin Station End Station Side Length (ft)

Maximum Height

(ft) Fill Rockery #W1 1469+25 1470+25 Left 100 12.0

Cut Rockery #W2 1642+00 1643+50 Right 150 6.7

Fill Rockery #W3 2095+80 2096+50 Left 70 9.0

Rockeries are earth retaining structures 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.

Rockeries were 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 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. Both rockeries were designed as gravity retaining walls and a surcharge load of 250 pounds per square foot was applied to simulate traffic loading. 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 the table below:

TABLE 4:- Recommended Safety Factors for Static and Seismic Design

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

Assumed material properties used in the design of the rockeries are presented in Table 5.

Rockery geometries and the calculated rockery design factors of safety for each wall analysis are shown in Table 6 and Table 7.Refer to Figure 6 for illustration of the parameters used in the tables and for design. A presumptive ultimate bearing capacity of the soils is estimated at 5 TSF (10 ksf)

TABLE 5:- Material Properties used in the Analysis for Both Rockeries

Φ Retained Soil

Friction Angle ɣs Retained Soil Unit Weight

C Soil

Cohesion μ Sliding Friction Factor ɣ r

Rockery Face Unit Weight

Kh Horizontal

Seismic Coefficient

Kv Vertical Seismic

Coefficient

Qs Vertical

Surcharge

37 degrees 125 pcf 50 ksf 0.8 140 pcf 0.05 0 250 psf

TABLE 6:- Rockery Geometry

B Minimum

Base Width

H Maximum

Height D

Depth at Toe ß

Backslope Angle

(V:H) Face Batter

(V:H) Back Cut

Inclination

2.0 ft 4 ft 1.0 ft 12 degrees 4:1 8:1

2.7 ft 6 ft 1.0 ft 12 degrees 4:1 8:1

3.4 ft 8 ft 1.0 ft 12 degrees 4:1 8:1

4.2 ft 10 ft 1.0 ft 12 degrees 4:1 8:1

4.8 ft 12 ft 1.0 ft 12 degrees 4:1 8:1

5.5 ft 14 ft 1.0 ft 12 degrees 4:1 8:1

TABLE 7:- A Summary of Calculated Factors of Safeties

Factors of Safety

Maximum Height

Sliding Factor of

Safety

Overturning Factor of

Safety

Maximum Bearing Pressure

Bearing Capacity Factor of

Safety Static 4 ft 2.5 2.2 1.1 ksf 9.1

Seismic 2.0 1.8 1.3 ksf 7.7 Static 6 ft 2.5 2.1 1.6 ksf 6.2

Seismic 2.0 1.6 2.0 ksf 5.0 Static 8 ft 2.5 2.0 2.1 ksf 4.7

Seismic 2.0 1.6 2.6 ksf 3.8 Static 10 ft 2.5 2.1 2.5 ksf 4.0

Seismic 2.0 1.6 3.2 ksf 3.1 Static 12 ft 2.5 2.0 2.9 ksf 3.4

Seismic 2.0 1.6 3.8 ksf 2.6 Static 14 ft 2.5 2.1 3.2 ksf 3.1

Seismic 2.0 1.6 4.2 ksf 2.3

FIGURE 6:- Illustration of the Parameters used in the design

LABORATORY TESTING

Representative bulk soil samples were obtained from different cut locations and tested in the laboratory. Laboratory tests included gradation/classification, Atterberg limits, and resistance (R-value). Table 3 summarizes the laboratory results.

TABLE 3.- Results of Laboratory Tests on Samples From Various Cuts

Sample Location Gradation Atterberg Classification R-Value

Begin Station

End Station Gravel Sand Fines Liquid

Limit Plasticity

Index AASHTO USCS

1338+00 1342+00 21.0% 72.4% 6.6% NV NP A-1-b SP-SM 78 1444+00 1448+00 28.0% 61.0% 11.0% NV NP A-1-b SP-SM 78 1578+00 1582+00 32.0% 55.0% 13.0% NV NP A-1-b SM 72 1744+50 1754=00 37.0% 57.9% 5.1% NV NP A-1-a SW-SM 78 1960+00 2032+00 40.0% 49.0% 11% NV NP A-1-a SP-SM 63

In general the materials classified as poorly graded sand and gravel with some silt and no plasticity (A1-a and A1-b in accordance AASHTO soil classification system AASHTO M-145 and as SP-SM in accordance with unified soil classification system ASTM D-2487). The percent of fines (passing the #200 sieve) in the tested samples averaged 9 percent. The resistance (R) value was also measured to determine the performance of the soils for use as subgrade or base for roadways under specified traffic loads. The results indicated R-values averaging 74 which indicates high resistance to plastic deformation of compacted materials. The materials were not tested for L.A. Abrasion and in most areas it is not expected to meet specifications for use as riprap or base.

Based on the laboratory results the excavated materials are suitable for use as unclassified borrow for embankment fill (Section 704.06) and backfill material for culverts (Section 704.03).

It may also be crushed and processed to meet the specifications for other uses along on the project.

If you have any questions please do not hesitate to call me at 720 963-3521.

______________________________________________ Date_________________

Khamis Y. Haramy

Geotechnical Engineer, CFLHD

Section 623. – GENERAL LABOR

Delete the text of this Section and substitute the following:

Description

623.01 This work consists of furnishing workers and hand tools for construction work, survey crews, slope scaling, slope crest overburden rounding, and/or furnishing qualified personnel to perform technical work ordered by the CO and not otherwise provided for under the contract.

623.02 Workers and Equipment. Furnish competent workers and appropriate hand tools for the work.

Obtain approval of the length of a workday and workweek before beginning the work. Keep daily records of the number of hours worked. Submit the records along with certified copies of the payroll.

623.03 Surveying Services. Furnish personnel, equipment, and material that conform to the requirements of Subsection 152.01. Survey according to Section 152.

Survey and establish controls within the tolerances shown in Table 152-1, or within other tolerances as established by the CO.

Prepare field notes in an approved format. Furnish calculations. All field notes, supporting documentation, and calculations become the property of the Government upon completion of the work.

623.04 Office Technical Services. Furnish qualified engineering personnel experienced in highway construction and design, capable of performing in a timely and accurate manner.

Provide personnel with a minimum of NICET Level II certification in highway design and construction, or State (SHA) or industry certification-related design and construction equivalent to their intended responsibilities. Personnel with 2 years or more of recent job experience in the type of highway design and construction provided for under the contract may be used in lieu of certifications. Provide the names and relevant experience of all personnel. Furnish supporting tools and equipment (e.g., calculator, computer, and software, and appropriate and commonly-used drafting tools for the assigned task).

All calculations, notes, and supporting documentation become the property of the government upon completion of the work.

623.05 Slope Scaling. Remove loose or potentially unstable rock and soil debris and vegetation on the slopes adjacent to the roadway as directed by the CO.

(a) Submittals. Two weeks prior to commencing scaling activities, submit to the CO for approval a detailed slope scaling plan that includes the following:

1) Proposed scaling sequence and schedule for each rock slope to be scaled and slope crest overburden rounding;

2) Types of equipment and tools used for slope scaling, including a high-reach excavator capable of removing rock from a cut slope crest 50 vertical feet above the roadway;

3) Personnel Qualifications. The Foreman shall have a minimum of two years and scaling crew a minimum of one year demonstrated experience in rock scaling/crest slope rounding in similar capacities;

4) ) Rock and soil removal and disposal plan for materials generated from the scaling and slope crest preparation work;

5) Devices, measures, and procedures to protect the public, construction personnel, facilities/structures/utilities from danger or damage during the scaling activities;

and

6) Traffic control plan as specified in Section 156.

(b) Pre-Scaling Meeting. One week prior to commencing scaling activities, schedule an on-site pre-scaling meeting with scaling Forman An FHWA Geotechnical Engineer will be present at the meeting to identify slope scaling locations and specific rocks and boulders to be removed.

(c) Scaling Crew Requirements. Provide scaling crews. A crew is defined as one Foreman and two qualified scalers (if a crewmember must leave for any reason that member shall be replaced by a qualified replacement).

(d) Protection of Property. Provide devices, measures, and procedures to protect the public, construction personnel, and property/structures/utilities damage caused by scaling and slope crest rounding activities. Any injuries or damages caused by scaling activities are the sole responsibility of the Contractor.

(e) Sequencing. Begin slope scaling at the top of the cut slope and proceed downward towards the roadway.

623.06 Acceptance. Additional surveying services will be evaluated under Section 152. Hired technical services will be evaluated under Subsections 106.02 and 106.04. Slope scaling will be evaluated under Subsection 106.02.

Measurement

623.07 Measure the Section 623 items listed in the bid schedule according to Subsection 109.02 and the following as applicable.

Round portions of an hour up to the nearest half hour. Measure time in excess of 40 hours per week at the same rate as the first 40 hours.

For surveying services, the minimum field survey crew is two persons. Measure surveying service by the crew hour. Do not measure time spent in making preparations, performing calculations, plotting cross-sections and other data, and processing computer data, and other efforts necessary to successfully accomplish the ordered survey services.

Do not measure time for worker’s transportation time to and from the project site.

Measure office technical services by the hour as ordered by the CO for performing calculations, plotting cross-sections and other data, and processing computer data.

Measure slope scaling by the crew hour as ordered by the CO for slope scaling activities.

Work and materials associated with scaling activities, including protecting against danger and removing and disposing of spoil and materials associated with scaling activities are incidental to the scaling and will not be measured for payment.

Removal and disposal of materials generated by scaling and slope rounding, including equipment needed, loading/unloading, and transporting material is incident to the work and will not be measured for payment.

Payment

623.08 The accepted quantities will be paid at the contract price per unit of measurement for the Section 623 pay item listed in the bid schedule. Payment will be full compensation for the work prescribed in this section. See Subsection 109.05.

Section 205 – ROCK BLASTING

205.01 Description Delete and add the following:

The project will require carefully planned and uniquely adapted blasting approaches to achieve engineered road cuts that are both structurally sound and aesthetically pleasing.

Construct cut faces to have a natural, rather than artificial, appearance to the fullest extent practicable. Identify and use specific measures applicable to blasting that will enable the final cut faces to blend with the form, line, color and texture of the existing rock formations of the surrounding landscape. Controlled blasting will be required in order to utilize the existing geologic structure properly and to minimize back-break beyond the trim line.

Following are specific blasting objectives:

(1) Minimize blast damage beyond the trim line. Blast damage is defined to include widening and loosening of existing joints or foliation, displacement of blocks of intact rock, and creation of new fractures.

(2) Utilize the natural geologic bedding planes and joint structure in a predicted and controlled manner to form the final cut.

(3) Blast and scale to the trim lines to produce stable faces such that the subsequent rockfall and associated maintenance costs are minimized.

(4) Accomplish the creation of ledges and a rough, natural appearance by incorporation of these features in the blast plan, and not by overshooting and selective removal of damaged rock.

(5) Minimize obvious drill hole scars or machine scaling equipment scars in the final cut faces.

(6) Prevent damage to the natural environment outside the clearing limits from fly rock, operation of equipment, or other construction related causes in accordance with Subsection 107.02.

(7) Conduct all blasting, scaling, and associated bolting and cleanup work in accordance with Sections 108, 156, 260, and 261, ensuring traffic safety at all times.

(8) Prevent damage to the existing roadway wear surface from blasted or scaled rock through the use of temporary roadway protection measures.

Controlled blasting consists of the controlled use of explosives and blasting accessories in carefully spaced and aligned drill holes to produce specific free surfaces or shear planes in the rock along the predetermined excavation backslope. Controlled blasting techniques include cushion blasting or variations of cushion blasting, as approved by the CO, but do not generally include pre-splitting. Pre-splitting may only be used for temporary excavations in rock that will eventually be covered (e.g., wall backslope) or further excavated using cushion blasting techniques.