Geotechnical_Engineering_Report_-_Eckert_Road_Boise_River_Bridge.pdf

Eckert Road, Ridenbaugh Canal Bridge Boise, Idaho Bridge Key No. 12806 Geotechnical Engineering Report

July 24, 2026 | Terracon Project No. 62235109B

Prepared for:

Local Highway Technical Assistance Council (LTHAC) 3330 W. Grace Street Boise, Idaho 83703

Ada County Highway District

(ACHD)

5800 N Meeker Avenue Boise, Idaho 83713

Geotechnical Engineering Report Eckert Road, Ridenbaugh Canal Bridge | Boise, Idaho July 24, 2026 | Terracon Project No. 62235109B | Bridge Key No. 12806

Facilities | Environmental | Geotechnical | Materials i

Table of Contents

230.01 Introduction

230.02 Field Exploration and Laboratory Testing

230.02.01 Borings

230.02.02 Field Tests

230.02.03 Geophysical Exploration

230.02.04 Laboratory Tests

230.03 Surface Conditions

230.04 Subsurface Conditions

230.05 Conclusions and Recommendations

230.05.01 General

230.05.02 Foundations

230.05.02.01 Shallow Foundations

230.05.02.02 Deep Foundations

230.05.03 Lateral Pressures and Backfill

230.05.04 Anchors

230.05.05 Drainage

230.05.06 Embankments

230.05.07 Erosion Control

230.05.08 Seismic Design

230.05.09 Construction

230.06 Appendices

230.07 Foundation Investigation Plat

230.08 Current Specifications and Minimum Testing Requirements

230.09 Special Provision Items

230.10 References

General Comments

Appendices Appendix A

Site Location Map Exploration Plan Boring Logs Photography Log

Appendix B Grain Size Distribution Graph Atterberg Limits Graph pH and Resistivity

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Appendix C LPILE Pile Analysis Results Axial Analysis Results Global Stability Analysis Results

Appendix D Supporting Information for Boring Logs Unified Soil Classification System

Facilities | Environmental | Geotechnical | Materials 1

Eckert Road, Ridenbaugh Canal Bridge

Bridge Key No. 12806 Boise, Idaho

Terracon Project No. 62235109B July 24, 2026

230.01 Introduction

The proposed project is replacing and realigning the Eckert Road bridges over the Ridenbaugh Canal (Bridge Key No.12805) and the Boise River (Bridge Key No. 12810) between Boise Avenue and Millbrook Way in Boise, Idaho. The intersection of Boise Avenue and Eckert Road will be redesigned as a roundabout as part of a companion project (ACHD Project No. 325043). This project includes improvements to the intersection of Eckert Road and the ingress/egress to Barber Park, an undercrossing for the Greenbelt northeast of the Boise River and retaining walls to support the new alignment of Eckert Road.

The purpose of this report is to provide geotechnical engineering recommendations for the replacement of the Ridenbaugh Canal Bridge (Bridge Key No. 12805), the retaining walls associated with the abutments and approach embankments for the Ridenbaugh Canal Bridge, and the retaining wall southwest of the Ridenbaugh Canal on the northwest side of Eckert Road. A Site Location Map showing the location of the structure is provided in Appendix A. Not included in this report are recommendations regarding the pavements, intersection improvements, the replacement of the Boise River Bridge (Bridge Key No. 12810), the retaining walls associated with the abutments and approach embankments for the Ridenbaugh Canal Bridge, and the retaining wall southwest of the Ridenbaugh Canal on the northwest side of Eckert Road. the undercrossing for the greenbelt northeast of the Boise River, and the retaining walls associated with the abutments and approach embankments for the Boise River Bridge

Currently, Eckert Road crosses over the Ridenbaugh Canal via a single-span bridge structure founded on timber piles that was constructed in 1954 and widened in 2003 and/or 2004. The widening was for a multimodal path and is founded on H-piles. The existing structure has an out-to-out width and length of approximately 44 and 71 feet, respectively.

Based on the Concept Plan dated August 2025 prepared by HDR Engineering (HDR) and discussions with HDR between contract authorization and the issuance of this report, the preferred alternative is to replace the existing structure in full with a new single-span bridge structure. The bridge structure will have an out-to-out width and length of about 70 to 62 and 89 feet, respectively. The bridge will shift to the down flow (northwest)

July 24, 2026 | Terracon Project No. 62235109B | Bridge Key No. 12806

Facilities | Environmental | Geotechnical | Materials 2 from the existing alignment roughly 24 feet. The new bridge will be supported by driven pile foundations at the abutments.

New embankments associated with the planned realignment of Eckert Road will be placed at each abutment. Wing walls attached to Abutment 1 (southwest abutment) will be approximately 9 feet in height. We understand the embankments, and slopes in front of and behind the walls will have a slope of 2(horizontal):1(vertical) or flatter. The road profile at the center line of SH-55 will be raised to finish grade elevation of approximately 2770 to 2772 feet. Embankment heights will vary from about 17 feet on the northwest side of the structure to no grade change on the southeast side of the structure.

The recommendations in this report are based the following publications:

■ AASHTO LRFD Bridge Design Specifications, 10th Edition, 2024

■ ITD Materials Manual, 2020.

■ ITD Roadway Design Manual, 2013

■ ITD Standard Specifications for Highway Construction, 2023, and the 2025

Supplemental Specifications.

230.02 Field Exploration and Laboratory Testing

230.02.01 Borings

Terracon drilled three borings (borings B-2, RW-13 and RW-14) to respective depths of about 91.5 to 30.4 feet below the existing ground surface (bgs). The boring B-2 was drilled on February 18, 2025, boring RW-13 was drilled on December 1, 2025, and boring RW-14 was drilled on November 26, 2025. The quantity, location, and depth of the explorations are summarized in the following table.

Table 1: Exploration Boring Descriptions

Boring Name

Northing / Easting (US Survey Feet)

Top of Boring

Elevation (feet)

Depth (feet)

Location

B-2 43.5654°N

116.1328°W

2771.3 91.5 North Abutment

RW-13

43.5644°N

116.1336°W

2792.4 30.7

Southern portion of Retaining Wall on west side of Eckert Road

RW-14

43.5648°N

116.1334°W

2781.4 30.4

South side of Bridge, northern portion of Retaining Wall on the west side of

Eckert Road

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The borings locations are shown on the Exploration Plan in Appendix A. The borings were drilled using a truck-mounted drill rig equipped with hollow-stem augers. A Terracon field geologist/engineer recorded logs of the borings during the drilling operations.

The boring locations were selected by Terracon based on site access, utility, and safety considerations. Latitude and longitude provided in the table above were obtained with a recreational grade GPS unit. After drilling, the project team interpolated boring elevations from the survey data collected for this project. Boring locations and elevations are provided on the boring logs in Appendix A. Elevations shown on the boring logs and the Foundation Investigation Plat are based on the North American Vertical Datum of

1988 (NAVD 88).

230.02.02 Field Tests

Disturbed samples were collected from the borings using a 2-inch-outside diameter split-spoon sampler following Standard Penetration Test (SPT) methods in general accordance with ASTM D1586. Encountered soils were visually classified at the time of drilling per ASTM D2488. At select depths, Terracon used a 3-inch-outside-diameter split-spoon sampler (ring sampler) to obtain a larger sample quantity of the native granular soils. In our engineering evaluation, the blow counts required to drive the larger 3-inch-outside-diameter split-spoon sampler were corrected using the Lacroix-Horn equation (ASTM STP-523, 1973) to account for the larger sampler size and represent standard SPT N-values. The boring logs show the uncorrected blow counts that were recorded in the field. In addition to SPT samples, relatively undisturbed samples were occasionally obtained by pushing a thin-walled, seamless steel tube with a sharp cutting edge hydraulically into the soil. Quantities, locations, and depths are presented on the boring logs in Appendix A.

The SPT N-value provides a reasonable estimate of the relative in-place density of non-cemented sandy type materials. However, the N-value only provides an indication of the relative stiffness of cohesive materials since the penetration resistance of these soils may be affected by the moisture content. Considerable care must be exercised in interpreting the N-value in gravelly soils, particularly where the size of the gravel particles exceeds the inside diameter of the sampling spoon.

An automatic SPT hammer was used to advance the split-spoon sampler in the borings performed for this project. Greater efficiency is typically achieved with the automatic hammer compared to the conventional safety hammer operated with a cathead and rope. Published correlations between the SPT values and soil properties are based on the cathead and rope method. The higher efficiency of the automatic hammer affects the standard penetration resistance blow count (N-value) by increasing the penetration per hammer blow over what would be obtained using the cathead and rope method. The

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Facilities | Environmental | Geotechnical | Materials 4 effect of the automatic hammer's efficiency has been considered in the interpretation and analysis of the subsurface information for this report.

230.02.03 Geophysical Exploration

Geophysical studies were not conducted at this site.

230.02.04 Laboratory Tests

Soil samples collected in the field were taken to the laboratory where soils were visually classified using the Unified Soil Classification System and in general accordance with ASTM D2488, or ASTM D2487 where laboratory data was available. The Unified Soil Classification System is described in Appendix D. Representative samples were selected for testing to determine the engineering and physical properties of the soil. The following table lists the tests performed and provides a brief description of the purpose of each.

Table 2. Laboratory Testing

Tests Conducted To Determine

Moisture Content (ASTM D2216) Moisture content of the soil sample.

Sieve Analysis (ASTM D6913) Particle distribution of the soil sample Percent Passing the No. 200

Sieve (ASTM D1140) Percent of clay/silt sized particles in the sample.

Atterberg Limits (ASTM D4318) Plasticity Index, Liquid Limit, and Plastic Limit of the soil sample.

pH (ASTM D4972) and Minimum

Resistivity (ASTM G187) Corrosion potential of metals in contact with soil.

Note: Laboratory tests were conducted in general accordance with their respective ASTM standards.

Results of the field and laboratory tests are generally summarized on the boring logs.

Tabular and graphical results of the ph, minimum resistivity, Atterberg limits and gradation analyses are included in Appendix B. The laboratory test data, along with the field information, were used to prepare the boring logs included in Appendix A.

230.03 Surface Conditions

The Ridenbaugh Canal crosses beneath Eckert Road parallel to ingress/egress to Barber Park and Eckert Bridge in Boise, Idaho and flows northwest. The proposed new bridge structure will be constructed in the same general location but shifted roughly 24 feet down flow (northwest). The area surrounding the project primarily consists of undeveloped land and laydown yard at the west abutment and Barber Park and the Boise River Bridge on the east abutment. The Nampa-Meridian Irrigation District headworks

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Facilities | Environmental | Geotechnical | Materials 5 structure is located just up flow (southeast) of the existing bridge structure, and was being reconstructed during our exploration services and is currently being reconstructed at the issuance of this report. At the project location, Eckert Road is paved with hot mix asphalt surfacing including across the concrete bridge deck. Overhead power and telephone lines run along the southeast side of Eckert Road and cross to the south side of the intersection of Eckert Road and Boise Avenue.

The natural topography at the project site slopes down to the Ridenbaugh Canal. In the overall project area, the ground surface slopes down to the Boise River and downriver (to the northwest). In general, Eckert Road near the Ridenbaugh Canal is built up on embankments that are up to about 14 feet higher than the surrounding land. Trees, shrubs and grasses are growing along the canal banks. An aerial photograph showing the area near the existing bridge is shown on the Exploration Plan in Appendix A.

We understand the bridge abutments will be located outside the scour prism and scour is not anticipated. The bottom of the canal is roughly 2748 feet in elevation at the down flow end of the proposed bridge.

230.04 Subsurface Conditions

Subsurface conditions encountered at the boring locations are indicated on the boring logs included in Appendix A. Stratification boundaries shown on the boring logs represent the approximate locations of changes in the soil. In-situ, the transition between materials may be gradual.

Between Boise Avenue to Ridenbaugh Canal (RW-13 and RW-14): In RW-13, fill materials consisting of silt with gravel, sandy silt, and poorly graded gravel with silt and sand extend to about 10 feet (about 2777.5 feet elevation) below the ground surface (bgs). The density of these fills has consistencies and relative densities that correlate to medium stiff to stiff and loose to very dense, respectively. Underlying the fills and at the surface of RW-14, are medium to very dense gravels with sand and varying amounts of silt, cobbles and boulders extending to the bottom of our explorations about 30½ feet bgs (about 2761.7 to 2751 feet elevation).

Between Ridenbaugh Canal and the Boise River (B-2): Boring B-2 was drilled in the roadway and encountered asphalt, roughly 2½ inches thick, and underlying the asphalt was fill materials including poorly graded gravel with silt and sand, and silty gravel with sand to about 16½ feet bgs (about 2754.8 ft elevation). Underlying the fill materials was a thin layer (about ½-foot) of sandy silt was encountered. Below the sandy silt were very dense gravels with varying amounts of silt, sand, cobbles and boulders extending to about 49 feet bgs (about 2722.3 feet elevation). Underlying the gravels were dense to very dense silty sand, hard sandy fat clay extending to about 58 feet bgs (about 2713.3

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Facilities | Environmental | Geotechnical | Materials 6 feet elevation) This layer was followed by very dense sands extending to the bottom of our exploration about 91½ feet bgs (about 2679.8 feet elevation.

The borings were monitored for the presence of groundwater. Groundwater was encountered at about 22 feet bgs while drilling in B-2. Fluctuations of the groundwater level may occur due to variations in the amount of precipitation, out flow from upriver reservoirs, flood control measures, runoff, irrigation, and other factors not evident at the time of exploration. The evaluation of these factors was beyond the scope of this report.

We anticipate frost depths of 24 inches, according to local regulations. According to the NRCS Web Soils Survey, the potential for frost action of the near surface soils in the project area is low to moderate and the potential for corrosion of concrete is low to moderate and steel is low to high.

230.05 Conclusions and Recommendations

230.05.01 General

Based on conversations with HDR, we recommend that spread foundations used to support the cast-in-place (CIP) retaining wall west of Eckert Road between Boise Avenue and the Ridenbaugh Canal.

For the Ridenbaugh Canal Bridge, different foundation options were discussed with HDR.

Due to construction considerations, driven piles are the recommended foundation to support the Ridenbaugh Canal Bridge.

230.05.02 Foundations

230.05.02.01 Shallow Foundations

Spread foundations used to support the Cip retaining wall extend to a minimum depth of 36 inches below the finished grade. To create a uniform bearing material, we recommend placing and compacting a minimum of 12 inches of Aggregate for Untreated Base (Type B) below the spread foundations The Aggregate for Untreated Base (Type B) should extend to undisturbed medium dense to very dense native granular soils and be compacted per Class A compaction. Granular Subbase is recommended for structural backfill except where Aggregate for Untreated Base (Type B) is specified.

Table 3. CIP Retaining Wall – Shallow Foundations

Description Criteria Foundation type Conventional spread footings.

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Description Criteria

Bearing material

Minimum of 12 inches of Aggregate for Untreated Base (Type B) supported on undisturbed medium dense to very dense native granular soils.

Minimum embedment depth 36 inches

Minimum footing width 60 inches

Scour Depth Not applicable for this structure

Nominal (ultimate) net bearing capacity See chart below.

Recommended resistance factor to be used with the nominal bearing capacity for strength limit state design 1 0.45

Presumptive bearing pressure for settlement less than 1 inch

See chart below.

Recommended resistance factor to be used with the nominal bearing capacity for service limit state design 1 1.0

Ultimate coefficient of friction to resist sliding 2 Cast-in-place footings: 0.65

Recommended resistance factors when designing resistance to sliding 1 Cast-in-place footings: 0.80

1. Recommended resistance factors are based on LRFD Bridge Design Specifications.

2. Ultimate friction coefficient for sliding is C multiplied by tanփ as defined in Section

10.6.3.4 of the LRFD Bridge Design Specifications.

Standard penetration test results from our borings were used to evaluate the strength of the foundation soil for bearing capacity. The ultimate bearing capacity is shown as a function of effective footing width in the chart below. The resistance factor shown above should be applied to the bearing values presented.

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Figure 1: CIP Retaining Wall - Nominal (Ultimate, Unfactored) Bearing Capacity

vs. Effective Footing Width

The graph below shows the service limit state bearing resistance as a function of effective footing width. The graph was developed based on an estimated settlement of 1-inch due to the structural loads.

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Figure 2: CIP Retaining Wall - Pressure vs. Effective Footing Width

230.05.02.02 Deep Foundations

Due to the gravel layers with cobbles and potentially boulders encountered in the borings, we anticipate that piles may achieve refusal at relatively shallow depths. For this reason, we recommend the structure to be supported on H-piles driven through predrilled holes. The contractor should be prepared with materials and equipment to deal with the subsurface conditions that include boulders, cobbles, and groundwater.

We understand a single row of driven H-piles will be used to support the proposed bridge integral abutments. The H-piles used for this project should be HP14X117 pile sections with minimum yield strengths of 50,000 pounds per square inch (psi). Reinforced tips should be welded to the bottoms of the piles to reduce the potential for pile damage during driving into dense to very dense bearing soils with cobbles and potentially boulders.

Downdrag

It is our understanding that planned embankments will be less 17 feet in height with minimal settlement; therefore an embankment induced drag load (downdrag) component is not anticipated.

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Potentially liquefiable soils were not encountered within the borings. Due to the lack of potentially liquefiable soils below the pile caps, our calculations indicate that the downdrag load acting on each pile due to settlement of liquefied soil can be neglected in design of the piles.

Resistance Factors

Based on Table 10.5.5.2.3-1 of the LRFD Bridge Design Specifications if dynamic tests with signal matching (i.e., CAPWAP) are performed on at least two piles (one pile at each abutment), but on no less than two percent of the production piles, a resistance factor of 0.65 can be used for designing piles for the strength limit state. Additional information about the CAPWAP analysis is presented in Section 230.05.09 of this report.

As indicated in Section 10.5.5.3 of the LRFD Bridge Design Specifications, resistance factors for extreme limit state, including the design of foundations to resist earthquake loads, shall be taken as 1.0, except when considering uplift (when applicable), when the resistance factor should be 0.35. The service limit resistance factor is 1.0.

Minimum Pile Spacing and Pile Settlement

In accordance with Section 10.7.1.2 of the LRFD Bridge Design Specifications, piles should not be spaced closer than 2½ times the pile diameter.

Based on the pile capacity presented above, we estimate that settlement of a single pile will be 1 inch or less at the service limit axial load. Terracon will be available to estimate the settlement for the design pile groups when the Structural Engineer has determined their configuration and spacing.

Axial Resistance

Axial pile resistance was evaluated using the Nordland method for cohesionless soil layers, which composed the majority of the soil profile. The Alpha method was used to calculate the soil resistance for noncohesive soil layers. The analysis was performed using a spreadsheet developed by Terracon and validated in general accordance with Geoprofessional Business Association’s Software Validation Best Practices.

Results of the axial analyses of a HP14x117 including the soil profile and soil parameters used in our analysis, are presented below and in Appendix C.

The following pile elevations presented herein are based on the provided pile reactions, pile cap elevations, and settlement. A capacity of 385 kips was used to determine estimated pile tip elevation presented in Table 6. Based on the provided information, abutments are outside the scour prism. The project Structural Engineer should adjust

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Facilities | Environmental | Geotechnical | Materials 11 each elevation, as appropriate, once the pile cap and cutoff elevations have been finalized.

Because of variations in subsurface conditions, the required pile lengths may vary. When listing pile lengths and elevations on the plans, these values should be noted as estimates.

Ultimate (nominal) pile capacity versus depth are provided in the following plots. Due to the lack of liquefiable soils or soil susceptible to strain softening these values may be used for the extreme limit state. A geotechnical resistance factor of 0.65 should be applied to these values for use in strength limit state analysis.

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Figure 3. Nominal (Ultimate) Axial Pile Capacity vs. Depth (feet) –Abutment 1 (Southwest)

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Figure 4. Nominal (Ultimate) Axial Pile Capacity vs. Depth (feet) –Abutment 2 (Northeast)

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Lateral Capacity

The behavior of a single vertical pile under an applied lateral load was analyzed using the computer program LPILE. Based on information provided by the Structural Engineer, we analyzed an HP 14x117 pile with a fixed-head condition. Groundwater was modeled at 22 feet bgs. The lateral analyses were performed with an estimated applied service limit axial load of 180 kips. The lateral loads required to deflect the top of the pile a specified distance was evaluated along with the corresponding shear and moment within the pile. We analyzed deflections of 0.25 to 2.0 inches at 0.25-inch increments. The minimum tip elevation was based on the point of fixity as determined by the point where the deflection curve(s) indicate zero deflection plus five feet, as described in the Section

230.05.02.02 of the Materials Manual.

The results of the LPILE analyses, including lateral-load deflections and induced shear and slope displacement within the pile, are presented in Appendix C. Soil parameters used in the LPILE analyses are summarized in the following tables.

Table 4: LPILE Parameters for Piles - Abutment 1 (Southwest)

Depth Below the bottom of Pile cap1

Interval (Elevation), ft.

P-Y Curve Soil

Effective Unit

Weight (pcf)

Effective Internal Friction Angle

Undrained Cohesion, c (psf)

Lateral Subgrade Modulus k 2 (pci)

Strain Factor, ε50 2

0 - 13.7 3

(2763 - 2749.3) API Sand 125 29 --- Use Default Values

13.7 - 34 3

(2749.3 - 2729) API Sand 62 29 --- Use Default Values

34 - 40.7 (2729 - 2722.3)

API Sand 62 38 --- Use Default Values

40.7 - 44.7 (2722.3 - 2718.3)

API Sand 52 38 --- Use Default Values

44.7 - 49.7 (2718.3 - 2713.3)

Stiff Clay W/ Free Water

52 --- 4000 Use Default Values

49.7 - 83.2 (2713.3 - 2679.8)

API Sand 52 38 --- Use Default Values

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Depth Below the bottom of Pile cap1

Interval (Elevation), ft.

P-Y Curve Soil

Effective Unit

Weight (pcf)

Effective Internal Friction Angle

Undrained Cohesion, c (psf)

Lateral Subgrade Modulus k 2 (pci)

Strain Factor, ε50 2

1. Approximate depth below the bottom of pile cap.

2. The current version of LPILE, 2022.12.11 provides estimated default values of k and ε50 based on soil strength (where applicable), and these default values are recommended for the project.

3. Represents backfill (coarse aggregate for course, size No.1 per section 703.02.C of the Standard Specifications for Highway Construction) within predrilled hole.

Table 5: LPILE Parameters for Piles - Abutment 2 (Northeast)

Depth Below the bottom of Pile cap 1

(Elevation) Interval (ft.)

P-Y Curve Soil

Effective Unit

Weight (pcf)

Effective Internal Friction Angle

Undrained Cohesion, c (psf)

Lateral Subgrade Modulus k 2 (pci)

Strain Factor, ε50 2

0 - 7.7 3

(2757 - 2749.3)

API

Sand

125 29 --- Use Default Values

7.7 - 34 3

(2749.3 - 2723)

API

Sand

62 29

34 - 34.7 (2723 - 2722.3)

API

Sand

62 38 --- Use Default Values

34.7 - 38.7 (2722.3 - 2718.3)

API

Sand

52 38 --- Use Default Values

38.7 - 43.7 (2718.3 - 2713.3)

Stiff Clay W/

Free Water

52 --- --- Use Default Values

43.7 - 77.2 (2713.3 - 2679.8)

API

Sand

52 38 4000 Use Default Values

0 - 7.7 (2757 - 2749.3)

API

Sand

125 29 --- Use Default Values

1. Approximate depth below the bottom of pile cap.

2. The current version of LPILE, 2022.12.11 provides estimated default values of k and ε50 based on soil strength (where applicable), and these default values are recommended for the project.

3. Represents backfill (coarse aggregate for course, size No.1 per section 703.02.C of the Standard Specifications for Highway Construction) within predrilled hole.

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A reduction in lateral resistance should be applied if a group of piles has center-to-center spacings of 5B or less, where B is the pile diameter. The Structural Engineer should apply the appropriate P-multipliers presented in the LRFD Bridge Design Specifications Table 10.7.2.4-1 to account for the group effects that consider the pile spacing, group size, and load direction. Terracon can assist with this evaluation when pile group information is available.

Minimum Pile Depths / Highest Pile Tip

We have based our minimum pile depth based on the strong axis lateral capacity analyses and not terminating the piles in soft or medium stiff fine grained soils. Due to the gravel layers with cobbles and potentially boulders encountered in the borings, we recommend driving the H-piles through predrilled holes that are at least 34 feet in depth (minimum pile embedment depth below the bottom of the pile cap) and at least 4 inches greater than the H-pile diagonal dimension. The highest pile tip elevation is presented in Table 6.

Summary of Estimated Pile Elevations

The following table shows the estimated pile design elevations and pile lengths for each abutment. The values in this table are reported to the nearest whole integer. These elevations may change as design of the structure progresses. The project Structural Engineer should adjust each elevation, as appropriate, once the pile cap and cutoff elevations have been finalized.

Table 6: Pile Lengths (feet)

Pile Cap Location

(Boring ID)

Estimated Pile Length

Pile Cutoff

Elevation 2

Bottom of Pile Cap

Elevation

Estimated Pile Tip

Elevation1

Highest Pile Tip

Elevation3

Abutment 1 South (B-2)

45 2765 2763 2720 2729

Abutment 2 North (B-2)

45 2759 2757 2712 2723

1. Values estimated based on an ultimate, unfactored axial pile resistance of 385 kips.

2. Includes an estimated 2 feet pile reveal.

3. Values based on lateral capacity analysis. During construction, if the design axial capacity is achieved and pile driving refusal is encountered prior to the highest pile tip elevation, the design team should be notified to review the results and provide additional recommendations.

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Because of variations in subsurface conditions, the required pile lengths may vary. When listing pile lengths and elevations on the plans, these values should be noted as estimates.

230.05.03 Lateral Pressures and Backfill

Unfactored lateral earth pressures are presented in Table 7 for compacted Granular Subbase or ¾-Inch Type A or B Aggregate for Untreated Base backfill. As indicated in the generalized diagram below, the zone of backfill should extend out horizontally at least two feet from the base of the foundation and upwards at an angle of 60 degrees from horizontal or flatter for the active values to apply, upwards at an angle 45 degrees from horizontal or flatter for the at-rest values to apply, and upwards at an angle of 30 degrees from horizontal or flatter for the passive values to apply. Unless temporary shoring is used, flatter slopes than those listed above may be required during construction for excavation safety. Granular Subbase is recommended for structural backfill except where Aggregate for Untreated Base (Type B) is specified.

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Figure 5. Conceptual Limits for Wall Backfill

Table 7. Lateral Earth Pressures

Parameter

Fully Drained Condition Above

Groundwater 1

Soil Below Groundwater

Condition

Total Unit Weight, pcf 125 130

Friction Angle, Degrees 34 34

Parameter

Slope Above/Below

Wall 2

Pressure Coefficient

Equivalent Fluid Density (pcf)

Fully Drained Condition Above

Groundwater 1, 3

Soil Below Groundwater

Condition 3, 4

Horizontal 0.28 35 81

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Parameter

Slope Above/Below

Wall 2

Pressure Coefficient

Equivalent Fluid Density (pcf)

Fully Drained Condition Above

Groundwater 1, 3

Soil Below Groundwater

Condition 3, 4

Active Earth Pressure Condition

6.0(H):1(V) 0.29 36 82

4.0(H):1(V) 0.31 39 83

2.0(H):1(V) 0.41 51 90

At-Rest Earth Pressure Condition

Horizontal 0.44 55 92

6.0(H):1(V) 0.51 64 97

4.0(H):1(V) 0.55 69 100

2.0(H):1(V) 0.64 80 106

Passive Earth Pressure Condition

Horizontal 3.54 442 302

6.0(H):1(V) 3.29 412 285

4.0(H):1(V) 3.07 384 270

2.0(H):1(V) 1.92 240 192

1. To achieve and design on Fully Drained conditions, the recommendations in Section

230.05.05 should be followed.

2. Slopes reported in the table are above the wall for active and at-rest lateral earth condition, and below the wall for passive lateral earth condition.

3. These are ultimate values that assume compacted granular backfill with an estimated moist unit weight and an angle of internal friction as shown above.

4. For the active and at-rest earth pressures, the values presented for the soil-below-groundwater condition include water pressure. The passive earth pressure for the soil-below-groundwater condition does not include water pressure.

Lateral earth pressures on the walls due to seismic shaking for active conditions (i.e., for yielding walls allowed to rotate) were estimated based on the Mononobe-Okabe method.

Lateral earth pressures on the walls due to seismic shaking for at-rest conditions (i.e., for non-yielding walls restrained from rotation) were estimated based on the Wood (1973) method. For non-yielding walls (i.e., at-rest conditions), a seismic equivalent fluid pressure of 10.4 pcf for backfill should be applied. For retaining walls that are free to rotate (i.e., active conditions), a seismic equivalent fluid pressure of 4.9 pcf for backfill should be applied. These values are based on horizontal backfill above the walls.

For active conditions, the seismic pressure resultant load should be applied ½H above the base of the wall and at 0.6H above the base of the wall for at-rest conditions. The seismic pressure resultants are in addition to the lateral earth pressures for static conditions in Table 7.

We anticipate the proposed walls may be constructed with backfill that is either horizontal or upward sloping. Permanent backfill slopes should be constructed no steeper than 2(horizontal):1(vertical). The unfactored lateral earth pressure values presented above only apply to the specified slope inclination.

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Active earth pressures are applicable for a wall that is free to rotate. The amount of movement relative to the wall height to develop active earth pressures is presented in the LRFD Bridge Design Specifications Table C3.11.1-1. At-rest earth pressures are appropriate for a wall that is restrained at the top.

Some movement of the wall would be required to mobilize the full passive pressure.

Relative movements to develop passive resistance are provided in Table 3.11.1-1 of the LRFD Bridge Design Specifications. In accordance with Table 10.5.5.2.2-1 of the LRFD Bridge Design Specifications, the resistance factor for the passive earth pressure component of sliding resistance should be 0.50. As described in Section C10.6.3.4 of the LRFD Bridge Design Specifications, this resistance factor is based on the lateral foundation movement being less than what is needed to mobilize the full passive pressure. Passive pressures should not be used to resist the sliding of walls towards the canal or other downward slopes. The passive resistance should be neglected in the upper 2 feet of the backfill.

Lateral earth pressures should be adjusted for hydrostatic pressures, surcharge loads, sloping fill, live loads near the walls (including compaction equipment and traffic), and/or seismic loads as appropriate. The live load surcharge for vehicular loads is often modeled as an equivalent soil depth as presented in Tables 3.11.6.4.1-1 and 3.11.6.4.1- 2 of the LRFD Bridge Design Specifications. The unit weight for the modeled equivalent soil depth should be at least 125 pcf.

Fill, debris, and loose soil should be removed before placing backfill behind walls. ITD Class A compaction is recommended for backfill. Near the wall, wall backfill material should be compacted in accordance with Section 210.03.A of the Standard Specifications for Highway Construction.

230.05.04 Anchors

We do not anticipate that anchors will be used.

230.05.05 Drainage

Surface water runoff should be prevented from discharging or infiltrating behind or over the face of the structure, walls, or slopes. Surface water from the roadway should be collected and discharged to a safe location away from the structure and walls. Discharge areas should be protected with riprap underlain by a Riprap/Erosion Control Geotextile per Section 718.06 of the Standard Specifications for Highway Construction. For a discussion regarding erosion control, see Section 230.05.07 of this report.

As MSE walls will be constructed for the bridge approaches, surface drainage features should be incorporated into design to prevent salt-laden water from soaking into the

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MSE wall reinforced zones and/or sheet draining over the MSE wall faces. Such features could include paving the entire top surface up to all MSE wall faces, where applicable;

and providing raised curbs and drain inlets to collect surface runoff and direct it away from MSE walls.

The structure should be designed to resist hydrostatic pressures. Walls should either be designed to resist hydrostatic pressures or should include a drainage layer extending to appropriate outlet locations to reduce the potential for hydrostatic pressure on the walls.

The drainage layer should consist of drain rock composed of Drain Rock as defined in Section 711.01 Drain Rock of the 2025 Supplemental Specifications to the 2023 ITD Standard Specifications for Highway Construction. The Drain Rock layer should be a minimum of 30 inches thick and should be separated from other soils with a Drainage Geotextile. The Drain Rock layer should extend vertically from the base of the structure to within 3 feet of the ground surface. Weep holes in the walls may also be used in conjunction with the drainage layer to reduce the potential for hydrostatic pressures. As an alternative to Drain Rock, manufactured drainage panels installed in accordance with the manufacturer’s recommendations may be used.

230.05.06 Embankments

New embankments fill associated with the project vary in height across the length of the structure. Embankment heights along the south side of the Ridenbaugh Canal will range from existing grade to about 14 feet above existing grade. Embankments should be constructed with Granular Subbase and embankment slopes should be 2(horizontal):1(vertical) or flatter. Some sloughing and raveling of the soil slope faces may occur if soils are exposed to surface precipitation or runoff. To help reduce the potential for sloughing, erosion control measures, such as vegetation, mulching, or riprap described in Section 230.05.07 of this report, should be utilized. Class A compaction is recommended for construction of embankments. Prior to fill placement, existing undocumented fill soils, and disturbed soils should be removed from areas that will receive fill so that embankments are placed on undisturbed native or existing compacted embankment soils. Embankment fill placed on slopes steeper than 3(horizontal):1(vertical) must be keyed in per Section 205.03.F in the Standard Specifications for Highway Construction. For estimating purposes, the depth of stripping is assumed to be 6 inches. The depth of topsoil stripping is likely deeper where trees or other mature vegetation exists. Settlement of the embankments is estimated to be 1 inch or less. Due to the soil types encountered and based on test data from a nearby project, the majority of settlements (95 percent primary consolidation) are estimated to occur during or within approximately 1 month after construction.

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CIP Wall

As previously described, a CIP retaining wall will be constructed on the west side of Eckert Road between Boise Avenue and the Ridenbaugh Canal. Based on the anticipated height of new fill required to construct (up to about 10½ feet wall height included embedment) total embankment settlement is expected to be approximately 1 inch or less. Due to the typically granular nature of the soils encountered in the borings, we estimate this settlement will occur quickly, as the embankments are constructed. As the height of some of the CIP wall will vary across the wall’s length, the settlement will be differential across the length of the CIP wall. Settlement monitoring is not recommended for the CIP wall.

Analyses were performed to evaluate the global stability of the proposed wall. Our analyses are based on the assumption that the critical failure surfaces from slope stability analyses do not extend through the wall. The wall designer should check this assumption as part of the design by modeling the actual wall design in the stability analysis.

The results of our global stability analysis (static and pseudo-static) are included in Appendix C. The walls were modeled with 36 inches of embedment with a foundation toe key embedded an additional 12 inches. Scour is not anticipated for this retaining wall. Based on our global stability analyses, we estimate the proposed walls will have factors of safety of 1.5 or greater for static conditions and 1.1 or greater for the pseudo-static condition. If the walls are constructed with slopes behind the walls, we should be notified to revise these recommendations.

MSE Walls

As previously described, Contractor-designed MSE (panel and welded wire basket) retaining walls will be constructed as part of this project. Backfill behind the reinforced zone of MSE walls should consist of Granular Subbase. Material within the reinforced zone of MSE walls should consist of sand and gravel meeting the requirements of the MSE wall special provisions. The Contractor designing the MSE walls should determine the specific lengths and spacing of the reinforcing elements and any special requirements for MSE wall materials. Class A compaction is recommended for wall backfill.

The base of the MSE walls should be founded in accordance with the recommendations presented in Section 11.10.2.2 of the LRFD Bridge Design Specifications, but no less than 2 feet below the adjacent grade (or scour where applicable). If any portion of these MSE walls is founded above a slope, a minimum 4-foot-wide bench should be established in front of the MSE wall in accordance with LRFD Bridge Design Specifications.

The following table presents design parameters for the walls.

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Table 8: MSE Design Parameters

Material Moist Unit

Weight (pcf) Cohesion (psf)

Friction Angle (degrees)

Nominal Bearing Capacity (psf)

Reinforced Backfill * * * N/A

Retained Soil 125 0 34 N/A

Foundation Soil 1 – Undisturbed

Medium Dense to Very Dense Native

Granular Soils

125 0 34 See charts for

Foundation Soil 1 below

Foundation Soil 2 – Existing Fill, Fine Grained Soils and

Very Loose to Loose Granular Soils

115 0 28 See charts for

Foundation Soil 2 below

* To be determined by the wall designer.

In accordance with Sections 11.5.7 and 11.5.8 of the LRFD Bridge Design Specifications, a bearing resistance factor of 0.65 (strength limit state), 1.0 (service limit state) and

0.9 (extreme event limit state) should be used for MSE walls.

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For walls founded on level ground and on Foundation Soil 1, the nominal bearing capacity (as a function of effective footing width) is shown in the figure below.

Figure 6: MSE Wall on Foundation Soil 1 on Level Ground - Nominal (Ultimate, Unfactored) Bearing Capacity vs. Effective Footing Width

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For MSE walls founded on sloping ground up to 2(horizontal):1(vertical) and Foundation Soil 1, the nominal bearing capacity (as a function of effective footing width) is shown in the figure below.

Figure 7: MSE Wall on Foundation Soil 1 on Sloping Ground up to 2:1- Nominal (Ultimate, Unfactored) Bearing Capacity vs. Effective Footing Width

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For walls founded on level ground and Foundation Soil 2, the nominal bearing capacity (as a function of effective footing width) is shown in the figure below.

Figure 8: MSE Wall on Foundation Soil 2 on Level Ground - Nominal (Ultimate, Unfactored) Bearing Capacity vs. Effective Footing Width

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For MSE walls founded on sloping ground up to 2(horizontal):1(vertical) and Foundation Soil 2, the nominal bearing capacity (as a function of effective footing width) is shown in the figure below.

Figure 9: MSE Wall on Foundation Soil 2 on Sloping Ground up to 2:1 - Nominal (Ultimate, Unfactored) Bearing Capacity vs. Effective Footing Width

Based on the anticipated height of new fill required to construct and maximum MSE wall height up to about 17 feet included embedment, total settlement is expected to be approximately 1 inch or less. Due to the typically granular nature of the soils encountered in the borings, we estimate this settlement will occur quickly, as the embankments are constructed. As the height of some of the MSE walls will vary across the MSE wall’s length, the settlement will be differential across the lengths of these MSE walls. Settlement monitoring is not recommended for the MSE walls.

Analyses were performed to evaluate the global stability of the proposed walls. Our analyses are based on the assumption that the critical failure surfaces from slope stability analyses do not extend through the reinforced zones of an MSE wall. The wall designer should check this assumption as part of the design by modeling the actual wall design in the stability analysis, including the specific spacing, lengths, and strengths of the MSE reinforcement.

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The results of our global stability analysis (static and pseudo-static) are included in Appendix C. The walls were modeled with 24 inches of embedment. Scour is not anticipated for these retaining walls. Based on our analysis, the reinforcement for the Abutment 2 MSE walls with heights greater than 14 feet (including the embedment depth) should have a minimum length equal to 1.4 times the wall height (including the embedment depth) or eight feet, whichever is greater. For the Abutment 2 MSE walls with heights equal to or less than 14 feet (including the embedment depth) should have a minimum length equal to 0.7 times the wall height (including the embedment depth) or eight feet, whichever is greater. Based on our global stability analyses, we estimate the proposed walls will have factors of safety of 1.5 or greater for static conditions and

1.1 or greater for the pseudo-static condition. If the walls are constructed with slopes behind the walls, we should be notified to revise these recommendations.

230.05.07 Erosion Control

Erosion protection should be provided, as needed, to protect the embankments and structure foundations from erosion. Design of erosion protection and scour, including riprap, will be completed by HDR. Riprap should be sized for the appropriate hydraulic conditions, per the approved hydraulics report. Any ground surface disturbed during construction of the bridge and its foundation should be restored and erosion protection provided. As previously described, we understand abutments are outside the scour prism, therefore no scour is anticipated for our analysis. Roadway runoff should be collected and discharged away from bridge and embankment slopes to reduce the potential of erosion of the fill slopes. Erosion control measures, such as vegetation or mulching, should be taken in accordance with the ITD Best Management Practices (BMP) Manual to protect the slopes and drainage areas from the effects of precipitation and surface flows. Discharge areas and slopes within the canal should be protected with riprap underlain by a Riprap/Erosion Control Geotextile per Section 718.06 of the Standard Specifications for Highway Construction.

230.05.08 Seismic Design

Seismic ground motion parameters were developed for the project based on Section

3.10 of the LRFD Bridge Design Specifications and Section 630 of the Materials Manual.

The seismic parameters were obtained from the USGS Seismic Design Web Services for the 2009 AASHTO Guide Specification. These values are presented in the following table and are for an earthquake having a probability of exceedance of 7 percent in 75 years (approximately 1000-year return period).

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Table 9. Seismic Ground Motion Parameters

Ground Motion Parameter Value

Site Soil Classification 1 D

PGA, Peak Ground Acceleration Coefficient on Rock (Site Class B) 0.086 g

Ss, Horizontal Response Spectral Acceleration Coefficient at 0.2-sec Period on Rock (Site Class B)

0.203 g

S1, Horizontal Response Spectral Acceleration Coefficient at 1.0-sec Period on Rock (Site Class B)

0.07 g

FPGA, Site Factor for PGA, 1.6

FA, Site Factor for Ss 1.6

FV, Site Factor for S1 2.4

AS, Modified PGA for Project Site (Site Class D) 0.138 g

SDS, Modified Ss for Project Site (Site Class D) 0.325 g

SD1, Modified S1 for Project Site (Site Class D) 0.168 g

Seismic Performance Zone 2 2

1. The current version of the Materials Manual is based on the 9th Edition of the LRFD Bridge Design…