GM 51-20 - SH-21 Wildlife Overpass - Final Foundations Recommendations (Rev 01B).pdf

158377846

M E M O R A N D U M Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661

DATE: November 10, 2021 (revision 1B)

TO: Lindsay Higa Project Manager

FROM: Robert Kraig Geotechnical Engineer

Eric Lim Sr. Geotechnical Engineer

SUBJECT: Geotechnical Memorandum GM 51-20 Rev 01B Final Foundation Recommendations SH-21 Wildlife Overpass at Cervidae Peak, MP 19.3

ID DOT 21(1)

Boise County, Idaho

INTRODUCTION

This memorandum provides geotechnical investigation results, rockfall analysis, and final foundation recommendations for the SH-21 Wildlife Overpass at Cervidae Peak, MP 19.3 Project. The project is located along State Highway (SH) 21 at MP 19.3, approximately 20 miles driving distance generally east of Boise, Idaho in Boise County, at approximate GPS coordinates 43.615704°N, 115.990785°W. Figures 1 and 2 show the Project Location and Site Vicinity, respectively. The recommendations appearing herein are based on the 95 percent plan set and review comments. A concrete arch structure is proposed with mechanically stabilized earth (MSE) walls functioning as wingwalls and abutments, and having precast concrete panels as the facing element. The final details of the arch structure, MSE walls, and related elements will include proprietary systems and, therefore, will be contractor-designed.

A Retaining Wall Selection Memorandum (GM 22-20) was prepared previously for the project.

If there is a substantive difference between this and the previous memorandum, the information provided in this memorandum supersedes that which appears in the previous memorandum.

Figure 1: Project Location (Google Earth capture)

Figure 2: Project Vicinity (Google Earth capture)

SCOPE OF PROJECT

A high number of wildlife vehicle collisions (WVCs) have been reported along the SH-21 corridor between MP 10 to MP 21, with the section between MP 19 and MP 20 having some of the highest number of collisions. A 150-foot long, precast concrete arch structure is proposed that will allow and encourage safe passage of wildlife over the highway, largely restoring a through-cut section of a local ridge. The arch structure will be approximately 52 feet wide and range in height between 24 and 27.5 feet high. The structure will be covered with soil and will extend from the cut slope located along the northeast side of the highway (as can be seen in Figure 3a) and over the highway to the southwest (foreground of the Figure 3a and left side of Figure 3b).

Figure 3a: View of project area facing east.

Figure 3b: View of project area facing northwest.

SUBSURFACE CONDITIONS

Subsurface investigation borings were conducted, and subsequent laboratory testing was performed on select samples collected during the investigation. The borings were drilled in July 2020 by Haz-Tech Drilling, Inc. out of Nampa, Idaho. The boreholes were drilled using a CME 85 truck-mounted drill rig using an automatic hammer system with a 140-pound weight falling over a 30-inch drop in general accordance with ASTM D1586. Standard 2-inch OD split spoon samplers were used to collect drive samples. The investigations included four (4) test hole borings (BH20-01 to BH20-04) at the proposed overpass section, which ranged in depth from approximately 30.2 feet to 50.3 feet below ground surface (BGS). Test hole locations are shown in Figure 4. Hole logging and draft boring log (gINT log) preparation were completed by the Idaho Transportation Department (ITD) and reviewed and finalized by the WFLHD project geotechnical engineer. All test holes were drilled in the shoulder off the paved driving surface.

Figure 4: Boring location figure showing approximate boring locations

N

BH-03

BH-04

BH-01

BH-02

Figure 5: Rock Core photo BH20-04 Runs 1-5 (30 to 49 feet BGS)

All test holes encountered non-plastic granular soils comprised predominately of silt and sand overlying relatively shallow, extremely weak (R0) granodiorite bedrock with the degree of weathering noted to be highly weathered to completely weathered. The completely weathered component was noted to have decomposed into poorly-graded sand. Bedrock contact was recorded between 2 feet to 10.0 feet BGS in the test holes, with the shallowest bedrock encountered in BH20-01 (4 feet) and BH20-03 (2 feet). Bedrock was encountered in BH19-02 and BH19-04 at 9.5 feet and 10.0 feet, respectively. An example of a photographed core sample is provided as Figure 5. The boring logs are provided in Attachment 1. Groundwater was not encountered in any of the test holes.

Laboratory tests were performed on selected representative samples to determine soil index properties. All laboratory testing was performed by the WFLHD. The testing program was performed to provide information to support the WFLHD pavement and geotechnical design teams during project development. Tests performed included Sieve Analysis (AASHTO T11 & T27), Soil Classification (AASHTO M145), Atterberg Limits (AASHTO T 89 & T90), Specific Gravity (AASHTO T100), and R Value testing (AASHTO T190) that was slightly modified by the WFLHD (WL190) method. Not all tests were performed on all samples. Pertinent test results will be included on the final boring logs. The laboratory test results are contained in Attachment A and summarized in Table 1.

Table 1: Material Testing Summary Subgrade Test Hole

Test Depth (ft)

Description USCS AASHTO PI P200

R Value

BH20-01 0 – 1 Silty Sand with Gravel

SM A-1-b(0) NP 13.5 ---

BH20-01 2.5 – 4.0 Silty Sand SM A-2-4(0) NP 29.2 72

BH20-02 5 – 6.5 /

7.5 - 9 Silty Sand SM A-1-b(0) NP 14.8 ---

BH20-03 0 – 0.5 Poorly graded Gravel with Sand

GP A-1-a(0) NP 2.4 ---

BH20-04 7.5 – 9.0 Silty Sand SM A-1-b(0) NP 19.1 --- Note: NP: Non plastic

The amount of fine-grain soils passing the #200 sieve (P200) ranged from 2.4 percent (by dry weight) in the surficial materials in BH20-03 to 29.2 percent (by dry weight) in BH20-01 between 2.5 and 4.0 feet BGS. All tested materials were found to be non-plastic. An R Value test was performed on the sample representing subgrade in BH20-01 and considered to be representative of the site. The test result produced an R Value of 72 indicating very good subgrade support qualities.

The SPT tests resulted in field (uncorrected) N values in the soil overburden that ranged from 4 to 11, with a single outlier in BH20-01 of 30 where pieces of weathered granodiorite and wood were noted on the boring log, and all indicating apparent density of very loose to medium dense. The bottom of the loose surficial layer was encountered at approximate elevation 3237 feet in boring BH20-02 and elevation 3238 in boring BH20-04. All the SPT tests performed in the granodiorite resulted in N values greater than 100 blows per foot (bpf), thus meeting the criteria for “practical refusal.”

SITE SEISMICITY

Information from the U.S. Geological Survey (USGS) website was used to locate nearby faults. Based on the USGS database, there are at least 6 mapped latest Quaternary-aged (<15,000 years) faults and at least one historic-aged (<150 years) fault, as listed in Table 2 and shown in Figure 6. Note that for clarity all the nearby faults visible in the figure are not identified or listed in the table. The faults that are identified were shown to have Slip Rates of less than 0.2 mm/year, have Normal Slip Sense and classify under Fault Class A.

Approximate distances of the faults listed in the table range from 15 miles to 61 miles. The USGS shows older faults (>15,000 years) that are mapped nearby, which are not identified in the figure or listed in the table.

Table 2: Mapped Nearby Faults Fault Name Fault ID Age Line Type Big Flat 631 Latest Quaternary Moderately Constrained Boulder Front 639 Latest Quaternary Inferred Jakes Creek 630 Latest Quaternary Moderately Constrained Lost River 601 Historic Well Constrained Lone Pine 604 Undifferentiated Quaternary Inferred Sawtooth 640 Latest Quaternary Moderately Constrained Shirt Creek 625 Latest Quaternary Moderately Constrained Squaw Creek 632 Latest Quaternary Moderately Constrained Western Snake River Plain system

635 Undifferentiated Quaternary Moderately Constrained

Owyhee Mountains system

636 Undifferentiated Quaternary Well Constrained

Boulder front 639 Latest Quaternary Inferred Deadwood Reaves Creek system

605 Undifferentiated Quaternary Inferred

Figure 6: Excerpt from the USGS Quaternary Faults Database showing locations of nearby faults listed in Table 2

Big Flat, Jakes Creek, Shirt Creek and Squaw

Creek fault zones

Deadwood Reaves Creek fault system

Western Snake River Plain fault system

Owyhee Mountains fault system

Sawtooth fault

Boulder front fault

Lost River, and Lone Pine faults

Project Location

The procedures outlined in Section 3.10 of the 8th Edition of the AASHTO LRFD Bridge Design Specifications (2017) were followed to characterize the seismic hazard. These procedures use a design earthquake with a 7 percent probability of exceedance (PE) in 75 years, which equates to a return period of approximately 1000 years. Based on the predominance of rock and the standard penetration resistance in the top 100 feet of the soil profiled at the project site, we interpret the AASHTO seismic Site Class for the site as Site Class B (defined as rock with shear wave velocity between 2,500 ft/sec and 5,000 ft/sec). It can be noted that the borings performed for the project did not penetrate below 50 feet BGS.

The assumption is made that the bedrock encountered in the borings extends beyond 100 feet BGS. This is consistent with the NEHRP Site Class Map for the Boise Metro area (Phillips, 2011) in which the mountains around Highway 21 are mapped as Site Class B. Since this is a through cut through one of the mountain ridges, it is our opinion that the site will behave consistent with Site Class B parameters.

The USGS interactive deaggregation website was used to determine the peak ground acceleration and spectral accelerations at the project site for Site Class B rock. The spectral accelerations were then scaled using the site factors per AASHTO to represent the seismic accelerations at the ground surface. The results of this analysis are presented in Table 3.

Table 3: Summary of Seismic Parameters (7% PE in 75 Years) Design Parameter Value

Lat / Long (degrees) 43.615704° / -115.990785°

Site Class at Project Site (by Soil Type and Penetration Resistance in Top 100 feet of Soil Profile)

B

Horizontal Peak Ground Acceleration on Site Class B Rock

(PGA)

0.099

Horizontal Response Spectral Acceleration at a Period of 0.2 sec on Site Class B Rock (Ss)

0.235

Horizontal Response Spectral Acceleration at a Period of 1.0 sec on Site Class B Rock (S1)

0.076

Scaled Peak Ground Acceleration for Project Site, Site Class B (As)

0.099

Scaled Horizontal Response Spectral Acceleration at a Period of 0.2 sec for Project Site, Site Class B (SDS)

0.235

Scaled Horizontal Response Spectral Acceleration at a Period of 1.0 sec for Project Site, Site Class B (SD1)

0.076

Site coefficient for 0.2 second period spectral acceleration, Fa 1.000 Site coefficient for 1.0 second period spectral acceleration, Fv 1.000 AASHTO Seismic Zone 1

The values in the table were used to develop the 5-percent-damped-design response spectrum shown in Figure 7.

Figure 7: 5-Percent-Damped-Design Response Spectrum

LIQUEFACTION

Following the AASHTO procedures, this site was determined to be in Seismic Zone 1.

Liquefaction analysis is not required for the project and is low risk due to the lack of groundwater in the loose soil layers.

FOUNDATION ANALYSIS & DESIGN

Foundation Discussion A concrete arch structure was selected by the project design team to span the highway and support the corridor crossing. The subsurface conditions encountered in the subsurface investigation are suitable for shallow foundations provided the foundations or foundation fill extend down to the granodiorite bedrock encountered at the depths in Table 4 below.

Accordingly, we are providing geotechnical design parameters for use in the design of conventional shallow strip footing foundations. A conceptual illustration of a concrete arch structure is shown in Figure 8 and Figure 9. The footing elevations and estimated footing subgrade elevations are shown in Figure 10.

Table 4: Foundation Excavation Depths

Boring No. Bedrock Depth (ft) Bedrock Elevation (ft)

BH20-01 4 3238

BH20-02 10 3237

BH20-03 2 3243

BH20-04 10 3238

Figure 8: Conceptual Arch Structure (South Portal)

Figure 9: Conceptual Arch Structure

Figure 10: Footing and Estimated Suitable Subgrade Elevations

Table 5: Summary of Factored and Unfactored Loads per Abutment

Load Type

Assumed Unfactored Load (ksf)

Strength I Limit State

Service I Limit State

Load Factor

(γ) Factored Load (ksf)

Load Factor

(γ)

Factored Load (ksf)

Dead load of superstructure and substructure (DC)

8 1.25 10 1.0 8

Foundation Loads Estimated foundation loads were provided by the WFLHD Structural Group in an email dated 9/30/2020 and are summarized in Table 5. Total estimated factored load per side of the concrete arch, for the Strength I Limit State, is on the order of 10 ksf. Per the 9/30/2020 email, 10-foot-wide strip footings were assumed for a wall line load of 100 k/ft. Final design loads will be determined by the contractor-designed arch system and reviewed by WFLHD Structures and Geotechnical teams.

Table 6: Summary of Resistance Factors for Bearing Resistance and Sliding Method / Soil / Condition Resistance Factor

Bearing Resistance ϕb

Semi-empirical methods (Meyerhof, 1957), all soils 0.45 ϕb Footings on rock 0.45

Sliding ϕτ Precast Concrete placed on sand 0.90 ϕτ Cast-in-Place Concrete on sand 0.80 ϕep Passive earth pressure component of sliding resistance 0.50

Resistance Factors Per Table 10.5.5.2.2-1 of the AASHTO LRFD Bridge Design Specifications (AASHTO BDS), the resistance factors shown in Table 6 are used for Strength Limit States.

As discussed in the AASHTO BDS section 10.5.5.1, all resistance factors for Service Limit States shall be taken as 1.0, except as provided for overall stability in Article 11.6.2.3.

Bearing Resistance, Settlement, Sliding & Eccentricity The site is in a through-cut in rock with the subsurface investigation showing that extremely weak (R0) granodiorite bedrock was encountered at relatively shallow depths. Given the in-situ condition of the rock, the conservative assumption was to model it as a very-dense sandy gravel (phi = 38 degrees) for bearing capacity and settlement purposes. Based on that assumption the following analyses were performed assuming that the foundations would be placed directly on the weathered but intact “bedrock” with some areas of overexcavation and replacement of loose fill materials expected on the northwestern end of both foundations. The bearing resistance for shallow foundations was calculated using AASHTO BDS Section

10.6.2.4 Settlement Analysis for Service Limit State Design and Section 10.6.3 for Strength Limit State Design. Bearing resistance values for various footing widths are plotted in Figure 11. Select point values from Figure 11 are presented in Table 7. A reduction factor of

0.45 was used for the Strength 1 Limit State factored bearing resistance values.

Figure 11: Bearing Resistance Plots

Table 7: Select Bearing Resistance Values

B' (ft) Bearing Resistance at Strength I Limit State (ksf)

Ultimate (Nominal) Bearing Resistance (ksf)

5 21.227 47.171 7 25.414 56.476 9 29.557 65.681 11 33.654 74.787 13 37.706 83.792 15 41.714 92.698

Estimated settlement values for various effective footing widths are also plotted in Figure 11.

A footing length of 150 feet was assumed to develop the plots. Select point values from the plot are presented in Table 8. A reduction factor of 1.0 (no reduction – nominal) was used for these Service 1 Limit State settlement values. As discussed earlier, the depth that bedrock (analyzed as very-dense granular soil) was encountered in the borings ranged from 2 feet to 10 feet BGS. As can be seen in the table, effective footing widths, B’, between approximately 5 feet and 15 feet will provide estimated settlement between 0.5 inch and 1.0 inch when placed on the bedrock or foundation fill placed directly on the bedrock. All bearing resistance values were calculated based on an embedment depth of 4 feet (elevation varies).

Table 8: Select Bearing Resistance for Settlement

B' (ft) Bearing Resistance at Service I Limit State, ksf 0.5-inch Settlement 1.0-inch Settlement

5 7.041 14.082 7 5.951 11.901 9 5.248 10.496 11 4.747 9.494 13 4.367 8.733 15 4.065 8.130

It is our understanding that tolerable settlement should be limited to 1 inch, or less. However, we anticipate that final settlement tolerances will be determined by the Contractor’s Design Engineer and/or Concrete Arch Structure Fabricator/Supplier.

Geotechnical Foundation Design Parameters The nominal soil parameters for abutment foundation design are provided in Table 9. The soil parameters listed in Table 9 may be used to determine the lateral earth pressures exerted and sliding resistance of the foundations. The recommended values appearing in the table may be adjusted if it can be shown by the Design Consultant that other values are permissible.

Per AASHTO 3.11.5.4, kp is reduced by the reduction factor, R.

Table 9: Recommended Geotechnical Parameters (Unfactored) for Foundation Design Parameter Design Value Nominal and Factored Bearing Resistance See Figure 11 and Table 6 Design Subgrade and Preparation Notes Remove all loose soils above bedrock, estimated up to 10 feet below the existing roadway. Encountered bedrock logged as highly weathered to extremely weathered, extremely weak (R0), similar to very dense sandy gravel soil. Prepare foundation per FP-14, Section 208 with Special Contract Requirements for Foundation fill or lean concrete backfill.

Minimum Depth for Frost 2. 3.0 ft Minimum Footing Elevation for Scour NA Assumed design depth for bearing, Df 4.0 ft Soil Angle of Internal Friction of foundation soils, Ф’ 1. 38 deg Soil Angle of Internal Friction of backfill, Ф’ 1. 34 deg Moist Unit Weight, γ 1. 125 pcf Active Earth Pressure Coefficient for backfill, ka 1. 0.28 Active Equivalent Fluid Pressure 1. 35 psf/ft

Table 9: Recommended Geotechnical Parameters (Unfactored) for Foundation Design (cont.)

Parameter Design Value At-Rest Earth Pressure Coefficient, k0 1. 0.44 At-Rest Equivalent Fluid Pressure 1. 55 psf/ft Coefficient of Passive Earth Pressure , kp3 9.5 Reduction Factor, R ≈ 0.62 Reduced Passive Earth Pressure Coefficient, R*kp 1, 3.

5.9

Passive Equivalent Fluid Pressure 1. 700 psf/ft Friction angle, δ, between precast concrete and subgrade (Foundation Fill) 22 deg

Nominal coefficient of sliding, tan δ, between precast concrete and subgrade (Foundation Fill) 0.40

Friction angle, δ, between cast-in-place concrete and subgrade (Foundation Fill) 26 deg

Nominal coefficient of sliding, tan δ, between cast-in-place concrete and subgrade (Foundation Fill) 0.49

Note: The above values assume horizontal backfill and are unfactored.

1. Values are for structural backfill per standard specification 704.04.

2. Based on information obtained from the Boise County Planning and Zoning Department

3. Per AASHTO BDS section 3.11.5.4.

Footing Size Recommendation Based on the reported loading and the analyses provided in this memorandum, it has been shown that settlement will control footing size selection. Reference to Table 8 shows that a cast-in-place strip footing, 150 feet long, with a base width between 5.0 feet and 15.0 feet is anticipated to result in total settlement between 0.5 inch and 1.0 inch. Embedment depth is assumed at a minimum of 4 feet BGS. It is our understanding the external checks for sliding and eccentricity under the appropriate limit states will be performed by the Contractor’s Design Engineer and/or Concrete Arch Fabricator/Supplier, which may result in additional footing size requirements.

Foundation Backfill Discussion and Recommendations We anticipate that continuous spread footings will support the arch structure. The bedrock contact below the footings is anticipated to be irregular with loose fill soils expected at some locations, particularly at the northwestern end where it appears that an lower original grade may have been filled in. This situation, if left unaddressed, would increase the likelihood of differential settlements if the footings were placed on rock in some locations and on loose soil in other locations. To reduce settlements and decrease the potential for differential settlement, all soils should be removed from above the bedrock and the footings may be placed directly on the bedrock, which in some areas could result in excavations that are 6 feet deep, or more. Alternately, the footing elevations could be designed at a set elevation, and the loose soils over-excavated to the top of suitable bedrock then backfilled with Foundation fill consisting of rock cap backfill material per special contract requirement 704.01, or lean concrete backfill meeting the requirements of special contract requirement 614 (modified to be similar to ITD Class 15 concrete.) Lean concrete backfill typically has the advantage of being strong enough to excavate with little widening needed outside the footing edges, the ability to resist high bearing capacities, and no compaction and density testing in deep trenches needed. However, it is our understanding that lean concrete may be difficult to place and hold at the design footing grade of 2.32 percent. Rock cap backfill should be strong, durable, open-graded crushed rock that is properly compacted (modified to be similar to Class 1 Rock Cap material per ITD standard specifications). This option should provide a balance between adequate support, lower cost, lower testing requirements (provided a suitable method compaction specification is used) and is anticipated to be a readily obtainable material in the area. A conceptual detail showing a rock cap or lean concrete backfill is presented in Figure

12. Perform work for the trenching and backfilling in accordance with Section 208 in the FP- 14 and special contract requirements.

Figure 12: Conceptual Illustration of a Footing Overexcavation and Special Backfill

Given the condition of the in-situ rock and assuming the rock trenches are constructed as described, we estimate that the settlements would be similar to those that would occur in the in-situ rock and that the total settlements should be one inch, or less, as discussed under the Bearing Resistance, Settlement, Sliding & Eccentricity discussion of this memorandum.

Please note that excavated weathered granodiorite material is friable, extremely weak rock and not considered suitable for Rock Cap backfill. Special contract requirements will require durability testing on Rock Cap backfill.

Wing Walls If MSE Walls are used for wingwalls, free draining material such as select granular backfill (FP-14 Subsection 704.08) should be used. The fines content (i.e., percent passing the No.

200 sieve) of the select granular backfill, should be limited to 8 percent to be considered relatively free-draining. Recommended soil parameters for design of MSE wingwalls are listed in Table 9. The recommended values may be adjusted if it can be shown by the Contractor’s Design Engineer and/or Fabricator/Supplier that other values are permissible or appropriate.

Table 9: Recommended Soil Parameters for Design of MSE Wing Walls

Design Material Type

Design Soil Parameter Internal angle of friction, ɸ

Unit weight, Pcf ɣ

Cohesion, c psf, c

Select granular backfill 1

(reinforced zone) 34 125 0

Unclassified backfill 2

(retained zone) 31 120 0

Foundation Soil 3 38 125 0

1. Meeting the requirements of 704.08

2. Meeting the requirements of 704.06

3. Assumes the use of Foundation Fill meeting the requirements of 704.01

PRELIMINARY ROCKFALL ANLALYSIS

NOTE: Rockfall protection is not included in the current construction project, but this section is included for potential future use. A series of rockfall trajectory analyses were performed by WFLHD Geologist, Nick Farny, utilizing cross sections from the 30% design and RocScience RocFall 8.0 software to approximate the bounce height and the total kinetic energy of rockfall events at a variety of locations on and around the proposed wildlife overpass.

Rockfall trajectory analysis was modeled along two stations, and data from the analyses was collected at the three locations listed below and shown in Figure 13.

1. At the edge of the existing roadway (Station 72+35)

2. Passing over the wingwall of the proposed arch (also Station 72+35)

3. Along the top of the proposed overpass (Station 72+40)

Figure 13: Overview of the rockfall trajectory analyses locations on an image from the 30% plan set.

The rockfall simulations were created using information from site photos, including the rock block size (6 inch and 2-foot diameter rocks are used in the models) and surface roughness.

We also contacted ITD project personnel to see if they could provide information on rockfall activity at the project site. Such information can be used to refine the models used in the analysis. At the time that the analyses were completed we had not received response from

ITD.

The models created for the analyses provide information on three aspects of the simulated rockfall at both the existing site and the proposed construction as listed below:

1. Bounce Height = the bounce height of rockfall events at the analysis location. This can be used to size the height of rockfall barriers.

2. Impact Energy = the total kinetic energy of rockfall events. This information is used to ensure that rockfall barriers will have the proper capacity to withstand rockfall impacts.

3. Barrier Height for X% Retention = the minimum height of a rockfall barrier to provide a certain percentage of rockfall retention. A typical goal is to retain (i.e. stop) 90% of all rockfall events.

A summary of the results of the rockfall trajectory analysis is provided in Table 10. Images of the models are included as attachments.

Table 10: Summary of Rockfall Trajectory Analyses

Station Location Bounce Height (ft) Impact Energy

(ft-lbs / kJ)

Barrier Height for X% Retention (ft)

Min Average Max Min Average Max 80 90 99

72+35

1. Edge of Existing Roadway

0.7 2.3 6 6.3 / <0.1

12,327 /

60,705 / 82 3 3.5 5.5

2. Over the Wingwall

0.3 1 2.2 7.7 / <0.1

3,523 /

18,678 / 25 1.1 1.4 1.9

72+40 3. Top of Overpass 0.25 0.8 1.5 10 /

<0.1 2,304 /

11,585 / 16 0.3 1 1.6

The analysis suggests that the existing roadway has an undersized ditch (approximately 9 to 11 feet wide). According to the Oregon Department of Transportation (ODOT) Rockfall Catchment Design Guide (RCAD), a slope like the one at the project site (approximately 70 vertical feet high, inclined at approximately a 1:1 with about a 6H:1V ditch foreslope) would require a 29-foot-wide ditch (min.) to intercept 90% of all rockfall events.

We observed that the existing roadway does have a steel guardrail along portions of the slope that is estimated to be approximately 2 feet tall that appears to be stopping some of the rockfall. Site photographs (not provided in this memorandum) show rocks up to 2 feet in diameter along the back of the guardrail, and as shown in Table 10 the models predict an average impact height of 2.3 feet. Based on the photographs it does not appear that the guardrail has sustained significant damage from rockfall impacts, which makes sense with respect to the low average rockfall impact energy of 17 kJ. However, the guardrail is still not an optimal rockfall barrier because rock can pass under the rail, and according to the analysis it is not high enough to stop 90% of all rockfall events (a barrier height of at least 3.5 feet would be needed). As an aside, 42-inch high cast-in-place concrete barriers were shown to withstand about 25 kJ under a single impact. (Musa, 2015).

The rockfall passing over the wingwall has a very low impact height (1-foot on average) and a relatively low impact energy (5 kJ on average). The rockfall passing over the top of the arch portal is even smaller with a 0.8-foot average impact height and a 3 kJ average impact energy. In both cases, the rocks must roll or bounce over a relatively large distance over flat ground before they reach the roadway, which slows them down significantly.

Based on the results of the analyses, the following summarizes the preliminary recommendations regarding rockfall at the project site:

• Installation of a barrier at the top of the overpass and the along the wingwalls to stop rock from falling into traffic. The height and capacity requirements of the barrier would be relatively low. An earthen or rock berm at least 2 feet high or a single 3-foot high gabion basket would likely be sufficient. Other options such as fences can be considered.

• Installation of a properly sized rockfall ditch and/or a rockfall barrier at the edge of the roadway at the toe of the existing slope. The existing ditch is undersized for the slope;

however, if RCAD is consulted, a minimum 21-foot-wide ditch (with a 4H:1V foreslope) to stop 90% of all rockfall would be needed. That would be much wider that the existing ditch and would likely require realignment of the roadway. As an alternative, options for rockfall barriers can be considered for placement at the edge of the road.

The rockfall barrier would not need to be large (at least 3.5 feet high would provide 90% retention) or very robust since rockfall events have only 17 kJ impact energy on average. The 30% plans show a standard steel guardrail to be installed along the approaches to the arch, which will stop some rock from entering the travel way, but as stated earlier, rock can pass beneath the rail. In addition, standard guardrail is also not designed to withstand rockfall impacts, and the guardrail may be undersized to provide 90% rockfall retention.

The above recommendations are preliminary and rockfall protection is outside the scope of the current design development. If implemented in the future, each option should be reviewed and revised based on potential changes to the design and any additional project-specific information that ITD may provide.

SLOPE EROSION IN VICINITY OF STATION 75+60 TO 77+60 AND GENERAL SLOPE

CONDITIONS

NOTE: Erosion mitigation/slope protection is not included in the current construction project, but this section is included for potential future use. Active slope erosion has been reported between approximate stations 75+60 to 77+60 as shown in Figure 14. The erosion activity at this location has reportedly become more of an issue following a recent area-wide fire that resulted in the loss of much of the vegetation on the area slopes. Note that this location is to the north of the proposed overpass structure and in that area the cross sections show that the road is sloped to the inside of the curve at the opposite side of the road. As can be seen in the figure the erosion appears to be limited to the upper portion of the fill slope and the erosion has advanced to the guardrail posts, possibly leading to a loss of lateral resistance.

The eroding section is in a through-fill section, with a slope ratio on the eroding side of approximately 1:1.4 (V:H). Until addressed, it is likely that the erosion will continue and lead to possible undermining of the road.

It is our understanding that ITD had expressed concern about the proposed wildlife barrier fence being proposed through this section, and whether the active erosion and overall condition of the slope might result in unfavorable conditions for the wildlife barrier and/or barrier footings during or following construction. Review of the 70 percent plan set shows that the planned location of the barrier is outside (downhill) of the active erosion zone. Since the proposed location of the wildlife barrier is beyond the limits of the active erosion, it is our opinion that there would be little to no immediate or short-term impacts on the fence.

However, local disturbance impacts to the slope during construction of the barrier are of concern as the surface soils in the slopes appear loose and are reportedly/evidently prone to raveling and erosion. Additionally, it has been reported that erosion may be occurring in the vicinity of the culvert outfall and should be considered in determining placement of the barrier, which will require additional consultation with the design team and project partners. Refer to the final plan set for final planned location of the wildlife barrier. Precautions should be taken to minimize impacts to the slopes during construction of the wildlife barrier.

Concerning the area of active erosion discussed above, the options considered by the Cross Functional Team (CFT) to address it included 1) reconstructing the upper portion of the fill slope 2) protecting the slope with erosion resistant material such as riprap 3) installing a permanent erosion control product that would promote revegetation and 4) installing curb to redirect runoff. However, although the options are briefly discussed below, it is our understanding that addressing the erosion at this site is outside the scope of the current project but may be addressed in the future.

Reconstructing the upper portion of the fill slope was discussed but dropped from further consideration due to the level of effort, impacts and reach (temporary excavation to encroach into the travel way) and cost.

The options of protecting the slope with riprap rock (rock inlay) and installing permanent erosion control were both discussed and were considered likely to halt the active erosion, with both options being simpler to construct, have comparatively lower cost, and have lower environmental impact compared to reconstructing the slope.

The rock inlay option would be relatively simple to construct by benching into the existing slope approximately 25 to 30 feet downslope, excavating approximately 2 feet into the slope from the bench to the top of the slope, and lining the excavation with a geotextile filter fabric followed by placement of a 2-feet thick layer of angular class 1 riprap. Reestablishing vegetation would not be the primary goal with this option but could still be achieved somewhat if a medium that would allow growth were placed on the finished face and into the surface voids of the riprap (e.g., rock mulch as defined by ITD). The extent of the repair down the slope would be limited by the reach of an excavator operating from the road. This option may have a longer service life (semi-permanent) than the permanent erosion control option.

The permanent erosion control option would require placement of a pinned mesh system to retain topsoil for growth media that would in turn help to reestablish a vegetated slope face. A pinned mesh system product was used recently on a reinforced soil slope at Lost Trail Pass in northeastern Idaho near the Montana border, which is at a higher elevation and has a steeper slope face at approximately 1:1, and having a vegetated face that is reported to be establishing. The pinned mesh system option may or may not have a shorter service life than the rock inlay option.

Constructing curbing along the road edge with the purpose of preventing runoff over the road edge and onto the eroded area was also considered. However, as discussed earlier, the cross-sections at this location show the road is sloped to the inside of the curve and away from the side having the active erosion, and because of this, curbing would provide no value.

Figure 14: Eroded slope between approximate stations 75+70 and 77+60.

CONSTRUCTION CONSIDERATIONS

The subsurface investigation showed that bedrock was encountered between 2 and 10 feet BGS. The bedrock encountered was identified as granodiorite, extremely weak (R0), with the degree of weathering noted to be highly weathered to completely weathered. A photo of rock core was provided earlier as Figure 5. Given its apparent condition, we anticipate that the bedrock will be excavatable using standard rock excavating tools and methods. The excavated bedrock is not considered suitable for Foundation fill (Rock Cap backfill) in footing trenches due to its extremely weak rock strength.

Active erosion at upper slope. Note lack of bench behind the guardrail

Current location of wildlife barrier fence

Road sloped to inside of curve

Alternate location of wildlife barrier fence away from the culvert and future uphill erosion creep

Slight over-excavation may be needed into bedrock due to the varying upper condition. Limit excavations into bedrock to the depth sufficient to attain the footing elevations shown in the plans and for constructing a leveling course of foundation fill or lean concrete backfill.

Excavate to create a surface that is suitable for constructing the rock-filled trenches or lean concrete backfill according to Special Contract Requirements Section 208.

There are several stockpiles of material in a turnout approximately half a mile south of the project as illustrated in Figure 2. Volume of this material is estimated by the ITD at approximately 1,400 cu yd. The material has been stockpiled by the ITD as remnants from ditch clearing activities along the highway. It is reported to contain organics like woody material such as branches along with oversized boulders and other undesirable debris.

Several buckets of material were obtained from the stockpiles but were not considered to be representative of the material in its entirety and therefore were not tested in the laboratory.

Given that, this material is of unknown suitability.

SUMMARY

This memo was prepared to provide subsurface investigation results and geotechnical recommendations for the proposed SH-21 Wildlife Overpass at Cervidae Peak, MP 19.3 project in Boise County, Idaho. Professional judgments and recommendations are presented in this memo. They are based partly on evaluation of the technical information gathered and partly on the Geotechnical Team’s general experience with subsurface conditions in the area. It should be noted the borings might not represent potentially unfavorable subsurface conditions away from the borings. The observed water levels and/or conditions indicated on the logs are as recorded at the time of the exploration and described herein. The water levels and/or conditions may vary considerably, with location, season/time, changes in climate, rainfall, and other factors and are otherwise dependent on the duration of, and methods used in, and interpretations made from the exploration program.

If soil or rock conditions are encountered during construction that vary from those discussed in this memo, or if the nature of the design or project needs, project stationing, and/or configurations should change, the WFLHD Geotechnical Team should be notified immediately in order to evaluate what effects, if any, the changes have on the recommendations contained in this memo. The recommendations presented in this memo are applicable only to this specific project. These data are not to be used for other purposes.

For question regarding this memorandum, please contact Robert Kraig at 360-619-7699 robert.kraig@dot.gov or Eric Lim at 360619-7826 eric.lim@dot.gov.

~~~~~~~~~~~End Memorandum~~~~~~~~~~~ mailto:robert.kraig@dot.gov mailto:eric.lim@dot.gov

REFERENCES

AASHTO, AASHTO LRFD Bridge Design Specifications, 8th Edition, 2017, American Association of State Highway and Transportation Officials, Washington, D.C.

Diann Morehouse, 2020, Retaining Wall Selection Memorandum, SH-21 Wildlife Overpass at Cervidae Peak, MP 19.3, U.S. Department of Transportation, Federal Highway Administration, Western Federal Lands Highway Division, Geotechnical Memorandum GM 22-20.

USGS, Seismic Design Web Service Documentation (Beta), Earthquake.usgs.gov, https://earthquake.usgs.gov/ws/designmaps/aashto- 2009.json?latitude=43.615704&longitude=-115.990785&siteClass=B&title=SH- 21%20Wildlife%20Overpass, accessed 11/17/2020

USGS, Quaternary Fault and Fold Database of the United States – Interactive Fault Map, Earthquake Hazards Program, United States Geologic Survey, U.S. Department of the Interior https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf , accessed November 2020.

Boise County’s Planning and Zoning Department Office Website, https://boisecounty.us/services/planning-and-zoning/, accessed 12/9/2020, Musa, Abdisa (2015) Evaluation of Concrete Barrier as Rockfall Protection, PhD Thesis, University of Akron.

Oregon Department of Transportation (2001) Rockfall Catchment Area Design Guide – Final Report, Report No. FHWA-OR-RD-02-04.

Phillips, William, 2011, NEHRP Site Class Map for the Boise Metro Area, Idaho, Idaho Geological Survey

Rocfall 8.0 (2020), Rocfall: https://www.rocscience.com/store/rocfall, accessed Dec 7, 2020.

RK/NF

Attachments: Attachment 1 – Subsurface Boring Logs & Test Results Attachment 2 – Preliminary Rockfall Models

CC: Y-Thao Truong, Designer

Doug Anderson, Geotechnical Team Lead Benn Oltmann, Bridge Functional Manager Chuck Dissen, Construction Operations Engineer Megan Chatfield, Materials Functional Manager Miranda Lang, Highway Design Manager

Nick Farny, Geotechnical Engineer Erin Chipps, Environmental Protection Specialist Matt Dillin, Hydraulics Engineer

Geotechnical File https://earthquake.usgs.gov/ws/designmaps/aashto-2009.json?latitude=43.615704&longitude=-115.990785&siteClass=B&title=SH-21%20Wildlife%20Overpass https://earthquake.usgs.gov/ws/designmaps/aashto-2009.json?latitude=43.615704&longitude=-115.990785&siteClass=B&title=SH-21%20Wildlife%20Overpass https://earthquake.usgs.gov/ws/designmaps/aashto-2009.json?latitude=43.615704&longitude=-115.990785&siteClass=B&title=SH-21%20Wildlife%20Overpass https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf https://boisecounty.us/services/planning-and-zoning/ https://www.rocscience.com/rocscience/products/rocfall

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Attachment 1 FLH Descriptive Terminology

Boring Log Legend

Boring Logs

Laboratory Test Results

10/04/2018

SOIL CLASSIFICATION CHART

Criteria for Assigning Group Symbols and Group Name Using Laboratory TestsA Group

Symbol Group NameB

COARSE-GRAINED

SOILS

More than 50% retained on No. 200 Sieve

GRAVELS

More than 50% of coarse fraction retained on No. 4 Sieve

Clean GRAVELS Less than 5% fines E

Cu ≥ 4 and 1 ≤ Cc ≤ 3C GW Well-graded GRAVEL D

Cu < 4 and/or 1 > Cc > 3 C GP Poorly-graded GRAVEL D

GRAVELS with fines More than 12% fines E

Fines classify as ML or MH GM Silty GRAVEL D,F Fines classify as CL or CH GC Clayey GRAVEL D,F

SANDS

Less than 50% retained on No. 4 Sieve

Clean SANDS Less than 5% fines I

Cu ≥ 6 and 1 ≤ Cc ≤ 3 C SW Well-graded SAND H

Cu < 6 and/or 1 > Cc > 3 C SP Poorly-graded SAND H

SANDS with fines More than 12% fines I

Fines classify as ML or MH SM Silty SAND F,H Fines classify as CL or CH SC Clayey SAND F,H

FINE-GRAINED

SOILS

50% or more passes the No. 200 Sieve

SILTS and CLAYS Liquid limit less than 50

Inorganic PI > 7 and plots on or above the “A” line J CL Lean CLAY K,L,M PI < 4 or plots below “A” line J ML SILT K,L,M

Organic Liquid limit – oven dried Liquid limit – not dried < 0.75 OL Organic CLAY K,L,M,N

Organic SILT K,L,M,O

SILTY and CLAYS Liquid limit 50 or more

Inorganic PI plots on or above “A” line CH Fat CLAY K,L,M PI plots below “A” line MH Elastic SILT K,L,M

Organic Liquid limit – oven dried Liquid limit – not dried < 0.75 OH Organic CLAY K,L,M,P

Organic SILT K,L,M,Q

HIGHLY ORGANIC

SOILS Primarily organic matter, dark in color, and organic odor PT PEAT A Based on material passing the 3-inch sieve.

B If field sample contains cobbles or boulders, add “with cobbles” or “boulders” or both, to group name.

C Cu = D60/D10 Cc = (D30)2 / (D10 x D60) D If soil contains ≥15% sand, add “with sand” to group name.

E Gravels with 5 to 12% fines require dual symbols:

GW-GM well-graded GRAVEL with silt GW-GC well-graded GRAVEL with clay GP-GM poorly-graded GRAVEL with silt GP-GC poorly-graded GRAVEL with clay

F If fines classify as CL-ML, use dual symbol GC-GM or SC-SM G If fines are organic, add “with organic fines” to group name.

H If soil contains ≥15% gravel, add “with gravel” to group name.

I Sand with 5 to 12% fines require dual symbols:

SW-SM well-graded SAND with silt SW-SC well-graded SAND with clay SP-SM poorly-graded SAND with silt SP-SC poorly-graded SAND with clay J If Atterberg limits plot in hatched area, soil is a CL-ML, silty

CLAY.

K If soil contains 15 to 29% plus No. 200, add “with sand” or “with gravel”, whichever is predominant.

L If soil contains ≥ 30% plus No. 200, predominantly sand, add “sandy” to beginning of group name.

M If soil contains ≥ 30% plus No. 200, predominantly gravel, add “gravelly” to beginning of group name.

N PI ≥ 4 and plots on or above “A” line.

O PI < 4 or plots below “A” line.

P PI plots on or above “A” line.

Q PI plots below “A” line.

APPARENT DENSITY OF COARSE-

GRAINED SOIL

SPT N-value (blows per foot) Apparent Density

0 to 4 Very Loose

5 to 10 Loose

11 to 30 Medium Dense

31 to 50 Dense

> 50 Very Dense

CONSISTENCY OF

FINE-GRAINED SOIL

SPT N-value (blows per foot) Consistency

0 to 1 Very Soft

2 to 4 Soft

5 to 8 Firm

9 to 15 Stiff

16 to 30 Very Stiff

> 30 Hard

SOIL STRUCTURE TERMS

Stratified Alternating layers of varying material or color with layers > 1/4 inch (6 mm), note thickness and inclination.

Laminated(1) Alternating layers of varying material or color with layers < 1/4 inch (6 mm), note thickness and inclination.

Fissured(1) Breaks along definite planes of fracture with little resistance to fracturing.

Slickensided(1) Fracture planes appear polished or glossy, sometimes striated.

Blocky(1) Cohesive soil that can be broken down into smaller angular lumps which resists further breakdown.

Disrupted Soil structure is broken and mixed. Infers that material has moved substantially -landslide debris.

Homogeneous Same color and appearance throughout.

Lensed Inclusion of small pockets of different soil, such as small lenses of sand scattered through a mass of clay; < 1/4 inch (6 mm) note thickness.

(1) Do not use laminated, fissured, slickensided, or blocky for coarse-grained soils.

Descriptive Terminology for Boring Logs

Field descriptions of borings are based on the FLH Soil and Rock Description and Identification Guidelines that generally follow the Visual-Manual Procedure (ASTM D 2488). The soil classifications shown on the boring logs are based on laboratory tests (ASTM D 2487) when the two-letter group symbol follows the group name in parenthesis.

Standard Penetration Test (SPT): the SPT consists of driving a 2 in (50 mm) O.D. split barrel sampler a depth of 18 in (450 mm) or 24 in (600 mm) using a 140 lb (63.6 kg) hammer with a 30 in (750 mm) drop. The blow count is the number of blows recorded for each 6 inch (50 mm) increment.

The N-value is the total number of blows for the second and third increments. Note that the N-values shown on the boring logs do not include any corrections for non-standard sampler size, hammers, drill rods, etc.

ANGULARITY OF

COARSE-GRAINED SOILS

Angular

Sharp edges and relatively plane sides with unpolished surfaces

Subangular

Similar to angular description, but with rounded edges

Subrounded

Nearly plane sides, but will have well-rounded corners and edges.

Rounded

Smoothly curved sides and no edges

PARTICLE SIZE OF COARSE-GRAINED SOILS

Component Grain Size Limits

Boulders > 12” (> 300 mm)

Cobbles 3 – 12” (75 – 300 mm)

Coarse Gravel 3/4 - 3" (19 - 75 mm)

Fine Gravel #4 Sieve - 3/4" (4.75 - 19 mm)

Coarse Sand #10 - #4 Sieve (2.00 - 4.75 mm)

Medium Sand #40 - #10 Sieve (0.425 - 2.00 mm)

Fine Sand #200 - #40 Sieve (0.075 - 0.425 mm)

10/04/2018

GRAIN/CRYSTAL SIZE FOR ROCKS (MODIFIED AFTER WENTWORTH, 1972)

Grain Size Description Criteria Less than 0.003 inches (<0.075 mm)

Very fine grained Cannot be distinguished by unaided eye. Few to no mineral grains are visible with a hand lens.

0.003 to 0.02 inches (0.075 to 0.425 mm) Fine grained

Few grain/crystal boundaries are visible; grains can be distinguished with difficulty by the unaided eye but can be somewhat distinguished by hand lens.

0.02 to 0.08 inches (0.425 to 2 mm) Medium grained Most grain/crystal boundaries are visible; grains distinguishable by eye and with the aid of a hand lens.

0.08 to 0.2 inches

(2 to 4.75 mm) Coarse grained Grain/crystal boundaries are visible; grains distinguishable with the naked eye and hand lens.

Greater than 0.2 inches

(>4.75 mm)

Very coarse grained

Grain/crystal boundaries are Clearly visible; grains are distinguishable with the naked eye.

GRAIN SHAPE (FOR SEDIMENTARY ROCKS)

Description Characteristic

Angular Showing very little evidence of wear. Grain edges and corners are sharp. Secondary corners are numerous and sharp.

Subangular

Showing definite effects of wear. Grain edges and corners are slightly rounded off. Secondary corners are slightly less numerous and slightly less sharp than in angular grains.

Subrounded Showing considerable wear. Grain edges and corners are rounded to smooth curves. Secondary corners are reduced greatly in number and highly rounded.

Rounded Showing extreme wear. Grain edges and corners are smoothed off the broad curves. Secondary corner are few in number and rounded.

Well-rounded Completely worn. Grain edges or corners are not present. No secondary edges or corners are present. DEGREE OF WEATHERING

Term Description Grade

Fresh No visible sign of rock material weathering; slight discoloration on major discontinuity surface is possible. I

Slightly Weathered

Discoloration indicates weathering of rock material and discontinuity surfaces. All rock material may be discolored by weathering and the external surface may be somewhat weaker than in its fresh condition.

II

Moderately Weathered

Less than half the rock material is decomposed and/or disintegrated to a soil.

Fresh or discolored rock is present either as a discontinuous framework or as corestones. A minimum 2-inch (50 mm) diameter sample cannot be broken readily by hand across the rock fabric.

III

Highly Weathered

More than half of the rock is decomposed and/or disintegrated to soil. Fresh or discolored rock is present either as a discontinuous framework or as corestones.

A minimum 2-inch…