Blacktail_Mt._Road_Final_Geotechnical_Memorandum_GM37-16_Final.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

In Reply Refer To: HFL-17

TO: Mike Traffalis, Project Manager Michael Madar, Roadway Designer

FROM: Diann Morehouse, Geotechnical Engineer

DATE: March 24, 2017

Copies: Carolyn Sourek, Hydraulic Engineer

Benn Oltmann, Structural Engineer Steve Morrow, Environmental Specialist

Stephen Henry, Assistant Materials Engineer Nathan Jenks, Geotechnical Team Lead

SUBJECT: Final Geotechnical Recommendations Blacktail Road Surface Improvements Phase 2

MT FLATHEAD 917(2)

Geotechnical Memorandum GM37-16

Introduction

This memorandum provides geotechnical investigation results, analysis, and final geotechnical recommendations for the Blacktail Road Surface Improvements Phase 2 project in Flathead National Forest, Flathead County, MT. A project specific elevation datum is used for elevations provided in this memorandum. Figure 1, in Attachment A, shows the project location map.

Scope of Project

This project extends from the end of MT FLATHEAD 917(1) County section project (MP3.43±) at project Station 113+50 to about project Station 215+50 (MP 5.45±). There is a possibility of extending this project to a proposed snow park area located at approximately MP 7.0. The proposed project includes surface improvements from the existing gravel surface to a 24-foot asphalt concrete pavement surface and construction of two aquatic organism passage (AOP) culverts.

Blacktail Road

MT FS FLT 917(2)

Final Geotechnical Recommendations Page 2 March 2017

Geotechnical Investigation

The geotechnical investigation for this project consisted of a site reconnaissance, nine subgrade borings, and four deeper borings at two proposed AOP culvert locations. The subsurface investigation was completed in June 2016. Haz-tech Drilling Inc. from Meridian, Idaho, conducted the subsurface investigation. A CME 85 truck-mounted drill rig advanced the subgrade borings, which are generally, located every ½-mile, alternating in the center of each lane.

Subgrade borings were advanced with a 12-inch outside diameter (O.D.) solid stem auger and a 14-inch flat profile auger bit and augered to a maximum depth of 5 feet unless auger refusal is encountered. Deeper borings at the AOP culvert locations were advanced with an 8.25-inch outer diameter (O.D.) hollow stem auger though surficial and softer soils and a casing advancer system advanced the borings through soil and rocky materials. Samples of subsurface materials were obtained, when possible, at regular depth intervals using 1.5-inch inside diameter (I.D.) split spoon sampler in the deeper borings. No spilt spoon samples were obtained in the subgrade borings except one in SG16-05. One Shelby tube sample was collected in boring BT16-04.

Five subgrade borings (SG16-02 to SG16-06) were drilled within the current project limits and one subgrade boring (SG16-07) was drilled in the proposed extended section of the project. Three additional subgrade borings (SG16-08 to SG16-10) were drilled beyond the extended section of the project. Four deeper borings B16-01 to B16-04 were drilled at the two proposed AOP culvert locations at about stations 119+25 and 154+83. Attachment C contains plan view sheets showing the locations of the subgrade and deeper borings.

Samples from the borings were submitted to the WFLHD Materials Laboratory for testing. Tests performed includes gradation (AASHTO T 11/T 27), hydrometer (AASHTO T 88), Atterberg Limits (AASHTO T 89 /T 90), soil classification (AASHTO M 145/ASTM D 2487), natural moisture content (AASHTO T 265), and R-Value at 300 PSI Exudation (AASHTO T 190).

Project boring logs, containing logs for both the subgrade borings and the four deeper borings at the AOP culvert locations are contained in Attachment D. Laboratory test results are contained in Attachment E.

Subsurface Conditions

All of the subgrade borings within the project area and proposed project extension were drilled to a depth of 5 feet. Due to the soft subgrade conditions in boring SG16-05, the subgrade auger was removed and a standard penetration test (SPT) was conducted through a hollow-stem auger between a depth of 5 and 6.5 feet. The subgrade borings encountered a gravel surfacing layer with a thickness ranging from 2.5 to 8.5 inches. A coarse (2 to 3-inch), poorly-graded gravel base material underlies the gravel surfacing layer with a thickness ranging from 8.5 to 19 inches.

SG16-08 subbase material thickness is 6 inches; however, this boring is outside the current project limits. Subgrade materials underlying the subbase are generally clayey gravel with sand, silty clayey gravel with sand, gravelly lean clay with sand, silty clayey sand with gravel.

All of the deeper borings at the AOP culverts were drilled to a depth of 51.5 feet except for B16- 02, which was drilled to a depth of 46.5 feet. All 4 borings were drilled from the existing roadway surface, and into the underlying material. Bedrock was not encountered in any of the deeper, AOP borings. Boring 16-01 was drilled near the outlet side of the proposed AOP culvert at Stoner

Final Geotechnical Recommendations Page 3 March 2017

Creek. Soils encountered in this boring include hard sandy clay and medium dense to very dense, silty sand, silty clayey gravel, and gravel with sand. Cobbles and boulders up to 4.5 feet were also encountered in this boring. Corrected ((N1)60) blow counted ranged from 7 to 67 blows per foot (bpf). A loose layer of silty sand was encountered between elevations 3641 and 3635 feet. No evidence of groundwater was encountered in boring but is assumed to be near stream elevation.

Boring 16-02 was drilled near the inlet side of the proposed AOP culvert at Stoner Creek. Soils encountered in this boring include stiff to hard sandy clay, and loose to medium dense silty clayey gravel with sand. Corrected ((N1)60) blow counted ranged from 10 to 44 bpf. A loose layer of silty, clayey gravel with sand was encountered between elevations 3636 and 3631 feet.

Groundwater was encountered between about elevation 3650 and 3646 feet.

Boring 16-03 was drilled near the outlet side of the proposed AOP culvert at the tributary of Stoner Creek near Station 154+83. Soils encountered in this boring include firm to hard sandy clay, and dense silty gravel with sand. Corrected ((N1)60) blow counted ranged from 11 to 93 bpf.

A firm layer of sandy clay was encountered between elevations 3848 and 3642 feet. No evidence of groundwater was encountered but is assumed to be near stream elevation.

Boring 16-04 was drilled near the inlet side of the proposed AOP culvert at the tributary of Stoner Creek near Station 154+83. Soils encountered in this boring include firm to hard sandy clay, and dense silty gravel with sand. Corrected ((N1)60) blow counted ranged from 18 to 100 bpf. A firm to hard layer of sandy clay was encountered between elevations 3848 and 3642 feet. No evidence of groundwater was encountered but it is assumed to be near stream elevation. Cobbles were also encountered in this boring.

Topography and Geology

Topography The project traverses mountainous terrain of the Salish Mountains. The road generally climbs continuously from about 3600 feet at the beginning of the project to about 4200 feet at the end of the project. The existing roadway grade ranges from 1.5 to 7 percent with two small sections that exhibit a slight descending grade. Near the middle of the project the road passes through a meadow with relatively flat topography. Approximately ten residents are located along the south side of the meadow adjacent to the road.

Site Geology The project is located between Flathead Lake to the east and the Salish Mountains to the west.

Flathead Lake is a remnant of the proglacial lake, Lake Missoula (approximately 10,000 years ago) and lies within the Rocky Mountain Trench. The project lies in an area mapped with bedrock consisting of Middle Proterozoic-aged Empire Formation (Ye), Main body of the Helena Formation (Yh), Revett Formation (Yr), and Spokane Formation (Ys). These formations are part of the Middle Proterozoic Belt Supergroup of quartzite and argillite rocks. Pleistocene-aged glacial and glaciofluvial surficial deposits (Qg) were mapped in the lower portion of the project.

See Figures 2 and 3 in Attachment A for geology maps of the area.

There are several mapped Quaternary aged faults in the vicinity of the project based on U.S.

Geological Survey, Quaternary Faults and Folds Database of the United States. Faults include

Final Geotechnical Recommendations Page 4 March 2017 several normal faults related to the Flathead Lake section of the Mission Fault, as well as the Swan Fault and South Fork Flathead Faults all east of the project. The Thompson Valley fault is located just west of the project.

Site Observations

During the project scoping trip on August 7, 2015 and subsurface investigation on June 1-3, 2016 the road surfacing appeared to be in good condition. Pictures of the project are contained in Attachment B at the end of this memorandum. Periodic shallow potholes of less than 4 inches deep and areas of slight wash boarding near sharp turns were noted. During the subsurface investigation, the left ditch exhibited standing water, up to 4 inches deep, between about Stations 158+00 and 160+50.

The road is built along the edge an ancient lake bed between about Stations 164+00 and 212+00 that developed during the Pleistocene-aged glaciation period. Some maps identify this area as Lost Lake.

Soils observed in cuts along the alignment consisted mostly of silty/clayey sand and gravel with some cobbles and a few boulders. Along the project, existing, stable, well-vegetated cut slopes have a maximum slope angle of about 27 degrees (about 1V:2H). Cut slopes located between the beginning of project (BOP) and Station 118+00 and between Stations 120+00 and 121+50 have a slope angle of about 33 to 34 degrees (about 1V:1.5H). These slopes are exhibiting signs of slow revegetation and minor raveling and erosion (Pictures 1 and 2). A rock buttress is located at the toe of the cut slope between Stations 120+00 and 121+50. It is assumed the buttress was constructed to mitigate a slope stability issue, although the specific reason for construction of the buttress is unknown.

The cut slope between stations about 140+00 and 147+00 is exhibiting a higher degree of raveling and erosion (Pictures 3 and 4). The current slope has a slope angle that ranges between 35 and 37 degrees (about 1V:1.4H to 1V:1.3H) with sparse vegetation, and rills up to 1.5 feet deep. This slope exhibits a slight slope angle change about two-thirds the way up the slope. From the slope angle change to the top of the slope there is more vegetation with less and shallower slope rills. A jersey barrier is located along the right shoulder between Stations 146+00 and 146+60. The purpose of this barrier is to reduce the amount of material coming down off the slope in to the road.

Fill slope ratios are generally between 1V:2H to 1V:1.5H along the alignment. Between the BOP and about Station 147+00 the fill slope ratios are close to 1V:1H in some areas. Jersey barrier is located along the left shoulder between Stations 122+60 and 126+90, between Stations 131+60 and 138+40, and between Stations 141+20 and 144+20 (Picture 5). It is assumed that the function of this Jersey barrier is to provide protection to roadway users at a location of a steep fill slope.

Geotechnical Recommendations

Subgrade The proposed final grade, after construction of the pavement section, will generally maintain the current roadway grade. The disadvantage to maintaining the existing elevation of the road surface

Final Geotechnical Recommendations Page 5 March 2017 is a risk of removing the existing surfacing section for the construction of the new pavement section and an encountering coarse (2 to 3-inch) poorly-graded gravel subbase layer. This coarse gravel layer is expected to be difficult to grade. The subgrade borings encountered the coarse gravel as close as 2.5 inches below the roadway surface with a thickness ranging from 8.5 to 19 inches deep. It is recommended to set the new pavement section over the existing surfacing section to minimize constructability issues. . Additionally, removal of the existing surfacing may result in reduced or poor support for construction equipment, especially under wet subgrade conditions.

It is anticipated the coarse (2 to 3-inch) gravel subbase extended the width of the road prism when the Department of Defense constructed the road in the 1950’s. It is suspected that over time the top width of the road has widened in areas due to side cast material and road surfacing migration, primarily towards the fill side. Due to this migration and widening there is a risk that the proposed pavement section will be constructed on less than desirable subgrade soils, soft shoulder materials, and/or frost susceptible soils. Risks include differential settlement, pavement distress, and frost heaves. Evidence of differential settlement and frost heaves were not observed in the field, but are much less noticeable in a gravel roadway surface than in paved surface. These risks were discussed with the project development team, and the risks were determined acceptable based on the current conditions of the roadway and scope of project. If during construction, poor subgrade soils are encountered it is recommended that the Geotechnical group be consulted for further recommendations.

With respect to the moisture sensitivity of the subgrade materials, the relative consistency of a cohesive soil in the natural state can be defined by the liquidity index (LI). The LI of fine-grained soil is an indicator of how the soil will behave upon disturbance. A LI values less than 0 indicate the material will behave as a semi-plastic or solid material. LI between 0 and 1 indicates the natural moisture content is between the plastic limit and liquid limit. LI values greater than 1.0 indicate the material will behave like a fluid when disturbed. LI values from the soil samples tested range from less than 0 to 0.13. Although the laboratory results do not indicate high LI values with the tested material, if the material becomes wetted, such as with a rain event, these fine-grained soils, present throughout the project, are sensitive and may lose significant strength when disturbed.

Embankment Slopes The current design includes sliver fills and over-steepened fills in order to widen the embankment enough to support the proposed surfacing section. These areas have very steep existing creeping embankment slopes which will likely result in cracking and settlement of the pavement on the outboard side. Sliver and over-steepened fills should be avoided either by minor alignment shifts to catch at the crest, or by adding sufficient width in combination with flatter fill slopes to allow embankment construction with conventional earthmoving and compaction equipment.

In areas, where sliver and over-steepened fills cannot be avoided, mechanically placed special rock embankments are recommended. The base of the rock fill must be keyed into firm, competent soil or bedrock at the bottom of the slope, benched into the existing slope, and have a maximum slope ratio is 1V:1.25H. The recommended minimum thickness for a rock embankment is 4 feet but may need to be greater depending on the slope height and geometry at the location of placement. In areas where rock embankments are not feasible or cannot be supported directly on

Final Geotechnical Recommendations Page 6 March 2017 firm soil or bedrock, geotextile or geogrid reinforced soil slopes are recommended. Reinforced soil slopes can be constructed with the face nearly vertical, however, a sloping face is recommended in order not to create a drop off at the edge of the pavement structure. A geotechnical analysis is required based on the slope height and width to determine the strength and reinforcement requirements if it is determined that reinforced soil slope is necessary.

Cut Slopes Based on cross sections dated January 15, 2016, no cut slopes are shown. Between Station 147+00 and EOP, slopes can be cut with a slope ratio of 1V:1.5 H, if required, to reestablish the ditch line and are cut less than 5 feet high. Existing cut slopes between BOP and Station 147+00 are over-steepened, poorly vegetated, and are exhibiting considerable erosion. Material from these slopes are filling in the ditch and allowing material on the roadway. For the cut slopes between BOP and Station 147+00 greater than 5 feet in vertical height, it is recommended to lay these slopes back to a slope ratio of 1V:2H to facilitate revegetation. In areas where reducing the slope ratio is not feasible, a small cut-side gravity wall is recommended. Cost effective alternatives include a dry stacked rockery wall, ecology block wall or a gabion basket wall. The walls could possibly be designed to retain the base of the cut slope and collect raveling material at the top of the wall. Overtime the erosion may lessen and the slope will naturally revegetate. A geotechnical analysis is required to determine the height and configuration of the walls if it is determined that a cut-side wall is necessary.

Another possible mitigation is to leave the slopes the in their current state and shift the centerline slightly to the left to allow for additional room for material at the base of the slope. However, this would likely result in material continuing to ravel into the ditch thus requiring maintenance to clean the ditches. This mitigation may also require additional sliver fills on the outboard side of the road.

Open Bottom Arch Culverts Flathead National Forest has provided designs for two open-bottom arch, AOP culverts located at Station 119+20 and Station 154+80 with spans of 15 feet and 18 feet respectively. The planned length Station 119+20 culvert is 104 feet and Station 154+80 culvert is 84 feet. These two culverts will replace the two existing 6-foot CMP culverts at each location. Flathead NF culvert details show the arch culverts supported on strip footing, shallow foundations.

The information and foundation recommendations below are based on LRFD methodology. The strength and service limit states provided follow the 2014, 7th edition of the AASHTO Bridge Design Specifications with the 2016 interim revisions. Design loads for the AOP, structural arch corrugated steel pipe (CMP) culvert were not provided and are therefore not included in the analysis of the strength limit state within this memorandum. It is assumed that due to the light weight of the culvert and reduction of the existing in-place materials displaced by the new culverts, that this will generally compensate for the new backfill material.

Scour elevations at both culvert locations were provided by the WFL Hydraulics Group as contained in Aquatic Organism Passage Hydraulic Report (Sourek, November 2016), including a recommendation that footing elevations be placed below scour depth. Based on the scour information, and geotechnical considerations, Table 1 shows the recommended footing elevations, and the soil properties used in analysis.

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Table 1: Recommended footing elevation soil parameters used in footing analyses.

Culvert

Recommended Footing

Elevation (ft.)

Moist Unit Wight, γm

(pcf)

Internal Friction Angle, ϕf degrees

Cohesion, C

(psf)

Station 119 Inlet 3638 120 32 0 Station 119 Outlet 3627 120 32 0 Station154 Inlet 3837 100 26 100 Station 154 Outlet 3834 100 26 100

The difference in footing elevation between the inlet and outlet at the Station 119 culvert is about 11 feet. The footing elevation should be stepped to account for the elevation difference across the length of the footing. A maximum step height of 4 feet is recommended. Boring B16-02 encountered loose material near bottom of the proposed culvert elevation. This material may be required to be over-excavated during the construction of the footings. The project CO should inspect the subgrade prior to pouring the culvert footings.

Backfill around the culvert is expected to meet the requirements of the Federal Lands Highway Standard Specifications (FP14), Subsection 704.04 Structural Backfill. Table 2 lists the estimated soil parameters for the expected backfill. The earth pressures values are based on Rankine earth pressure equations. It is generally assumed that compacted backfill creates an at-rest lateral earth pressures behind unyielding walls. Where a slight movement at the top of the wall (0.002 x wall height) is realized, active lateral earth pressures behind the wall develop. The design values shown in Table 2 do not include load and resistance factors.

Table 2: Soil Parameters for Structural Analysis

Parameter Design Value Moist Unit Weight, γm 125 pcf Angle of Internal Friction (Fill), ϕf 34 degrees Angle of Internal Friction (Retained), ϕf 30 degrees Active Earth Pressure Coefficient, Ka 0.28 At-Rest Earth Pressure Coefficient, Ko 0.44

Based on the soil testing results, it does not appear the material that will be excavated for the two AOP culverts will meet the requirements for Structural Backfill as stated in FP14 due to the elevated fines content.

It appears from the 95% design that the embankment slopes around the AOP culverts will be constructed as steep as 1V:1.5H. It is recommended that if the embankment slope is constructed as steep as 1V:1.5H that material having an angle of internal friction of at least 34 degrees is used in embankment construction.

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Factored bearing resistance graphs for the AOP footing widths are shown in Attachment F. The factored presumptive bearing resistance (service limit state) and the factored bearing resistance (strength limit state) are listed by width of foundation (B). It is recommended that the minimum width of the foundation is 2 feet. For the factored strength limit state values, a resistance factor φb, of 0.45 was applied to the nominal bearing resistance value, qn. For the factored service limit state value, the recommended nominal bearing resistance value was obtained from Table C10.6.2.6.1-1, Presumptive Bearing Resistance for Spread Footing Foundations at the Service Limit State in AASHTO LRFD Design Specifications, 7th Edition with 2016 Interim Revisions manual. A service limit state resistance factor, φb, of 1.0 was applied to the nominal bearing resistance value, qn.

Due to the limited changes in the embankment and roadway geometry, and the limited anticipated loads on the footings, the risk of settlement and/or differential settlement is relatively low. It is recommended that if any unsuitable soils are encountered during excavation that these materials are removed to a depth were suitable, firm materials are encountered and replaced with structural backfill.

Anticipate some excavated material to be saturated and likely above optimum moisture content for compactive efforts. Unsuitable organic material should be excavated and disposed. Boulders and cobbles were encountered in the borings and anticipated to be encountered in excavations.

Seismic conditions were not evaluated for these two AOP culverts since extreme event limit state does not control due to the relative magnitude of the loads. This is based on discussion in Section

12.5.3 of the AASHTO LRFD Design Specifications, 7th Edition with 2016 Interim Revisions manual.

Ditches The project cross sections show a minimal ditch depth. Adequate ditch depth that will contain ditch water below the top of subgrade is recommended. In the areas of active cut slope erosion, ditches will require regular ditch maintenance.

Shrink/Swell Excavated materials are anticipated to swell approximately 15% when excavated and placed in a loose condition, and shrink approximately 10% when compacted relative to existing in-place (in situ). Shrink swell factors are estimates and commonly vary by plus or minus 5%.

Excavations Excavations for the open bottom culverts will require dewatering in order to place the concrete footings.

Closing

Recommendations contained in this memorandum are based on observations made during site visits on August 7, 2015, and June 1-3, 2016. The stationing listed was determined by referencing nearby stakes and should be considered approximate.

Final Geotechnical Recommendations Page 9

Questions regarding this memorandum should be addressed to Diann Morehouse at (360) 619- 7609 or Diann.Morehouse@dot.gov.

References

Harrison, J.E., Cressman, E.R., and Whipple, J.W. (2000, map originally published in 1992) Geologic and Structure Map of the Kalispell 1°x2° Quadrangle, Montana, and Alberta, and British Columbia: A Digital Database, Miscellaneous Investigation Series Map I-2267, U.S.

Geologic Survey

Harrison, J.E., Griggs, A.B., and Wells, J.D., (2000, map originally published in 1986) Geologic and Structure Map of the Wallace 1°x2° Quadrangle, Montana, and Idaho: A Digital Database, Miscellaneous Investigation Series Map I-1509-A, U.S. Geologic Survey and U.S. Department of the Interior.

Attachments

A. Project Map B. Project Pictures C. Boring Location Plan View D. Boring Logs E. Laboratory Test Results F. Factored Bearing Resistance Graphs mailto:Diann.Morehouse@dot.gov

ATTACHMENT A

Project Maps

Final Geotechnical Recommendations March 2017

Figure 1: Project Location Map

BOP

EOP

End of extension

Project Location

Final Geotechnical Recommendations

Figure 2: Excerpt of the Geologic and Structure Map of the Kalispell 1° x 2° Quadrangle, Montana, and Alberta and British Columbia (Harrison, et al, 2000). The red dashed line is the approximate location of the project.

Figure 3: Excerpt of the Geologic and Structure Map of the Wallace 1° x 2° Quadrangle, Montana, and Idaho (Harrison, et al, 2000)

ATTACHMENT B

Project Pictures

Picture 1: View back on alignment at approximately Station 118+50.

Picture 2: View ahead on alignment at approximately Station 119+00.

Picture 3: View ahead on alignment at approximately Station 134+00

Picture 4: View back on alignment at approximately Station 134+00

Picture 5: View back on alignment at approximately Station 126+00.

ATTACHMENT C

Boring Locations

B H

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N

BEGIN PROJECT

S E

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u g u s t

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BORE LOCATIONS

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BORE LOCATIONS

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u g u s t

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SEE SHEET 3 FOR MATCHLINE

STATE PROJECT

NUMBER

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AERIAL VIEW

BORING LOCATIONS

ATTACHMENT D

Boring Logs

ATTACHMENT E

Laboratory Test Results

ATTACHMENT F

Factored Bearing Resistance Graphs