Geotechnical_Report_-_Added_for_Addendum_One.pdf

Geotechnical Engineering Report

Stoddard Road Widening Project

ACHD Project No. 522029

Meridian, Idaho for

Ada County Highway District c/o Kimley-Horn and Associates, Inc.

March 10, 2023

ACHD Project No. 522029 Meridian, Idaho for Ada County Highway District c/o Kimley-Horn and Associates, Inc.

March 10, 2023

412 East Parkcenter Boulevard, Suite 305 Boise, Idaho 83706 208.433.8098

ACHD Project No. 522029

Meridian, Idaho File No. 9082-033-00

March 10, 2023

Prepared for:

Ada County Highway District c/o Kimley-Horn and Associates, Inc.

1100 West Idaho Street, Suite 210 Boise, Idaho 83702

Attention: Tim Nicholson, PE

Prepared by:

GeoEngineers, Inc.

412 East Parkcenter Boulevard, Suite 305 Boise, Idaho 83706 208.433.8098

Kevin A. Russ, PE Geotechnical Engineer

Braydan P. DuRee, PE Associate Geotechnical Engineer

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Disclaimer: Any electronic form, facsimile or hard copy of the original document (email, text, table, and/or figure), if provided, and any attachments are only a copy of the original document. The original document is stored by GeoEngineers, Inc. and will serve as the official document of record.

March 10, 2023 | Page i File No. 9082-033-00

Table of Contents

1.0 INTRODUCTION

2.0 SCOPE OF SERVICES

3.0 FIELD EXPLORATIONS AND LABORATORY TESTING

3.1. Field Explorations

3.2. Laboratory Testing

4.0 SITE CONDITIONS

4.1. Regional Geology

4.2. Surface Conditions

4.3. Existing Pavement Section

4.4. Subsurface Soil Conditions

4.4.1. Fill

4.4.2. Native Soils

4.5. Groundwater Conditions

5.0 CONCLUSIONS AND RECOMMENDATIONS

5.1. Seismic Design

5.1.1 Seismic Design Parameters

5.1.2 Liquefaction Potential

5.1.3 Fault Rupture

5.2 Foundation Design

5.2.1 Summary

5.2.2 Subgrade Conditions and Soil Parameters

5.2.3 Bearing Capacity

5.2.4 Foundation Resistance to Lateral Loads

5.2.5 Resistance Factors

5.3 Lateral Earth Pressures and Backfill

5.4 Drainage

5.5 Infiltration Testing

5.5.1 Overview

5.5.2 Test Results

5.6 Pavement Design

5.6.1 Overview

5.6.2 Subgrade Soil

5.6.3 Traffic Loading

5.6.4 Design Life

5.6.5 Design Parameter Summary

5.6.6 Roadway Design Pavement Thickness

5.6.7 Multi-Use Pathway

5.7 Corrosion Considerations

5.8 Earthwork

5.8.1 Excavation, Grading and Subgrade Preparation

5.9 Materials

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5.9.1 Use of On-Site Soil

5.9.2 Imported Structural Fill

5.9.3 Fill Placement and Compaction Criteria

5.10 Weather Considerations

5.10.1 Paving

5.10.2 Construction Considerations

6.0 LIMITATIONS

7.0 REFERENCES

LIST OF FIGURES

Figure 1. Vicinity and Geology Map Figure 2A through 2D. Site Plans

APPENDICES

Appendix A. Field Explorations Figure A-1—Key to Exploration Logs Figures A-2 through A-10—Logs of Borings Figures A-11 through A-13-—Logs of Test Pits Appendix B. Laboratory Testing Figures B-1 and B-2—Sieve Analysis Results Figure B-3—Atterberg Limits Test Results Figures B-4 and B-5—Idaho R-value Test Results Appendix C. Site Photographs Figures C-1 through C-8—Site Photographs Appendix D. Bearing Capacity Plots Figure D-1—Bearing Capacity—Stoddard Rd Bridge over Ridenbaugh Canal Appendix E. Infiltration Testing Appendix F. Report Limitations and Guidelines for Use

March 10, 2023 | Page 1

1.0 INTRODUCTION

This report presents the results of our geotechnical engineering services in support of the Ada County Highway District’s (ACHD) Stoddard Road Widening project (ACHD Project No. 522029) located in Meridian, Idaho. The project includes widening Stoddard Road between Victory Road and Overland. The project location is shown in Figure 1, Vicinity and Geology Map. The project site relative to surrounding physical features is shown in Figure 2A through 2D, Site Plans.

The project includes:

■ Widening of Bridge No. 2085 over the Ridenbaugh Canal for pedestrian and bicycle facilities on both sides of Stoddard Road. The widening of the bridge is anticipated to match the existing foundation type, which consists of shallow spread footings bearing on soil.

■ Stoddard Road improvements will include:

Full-depth reconstruction of the Stoddard Road roadway structure to include hot-mix asphalt (HMA) for roadways per ACHD Policy Manual

Roadway widening of Stoddard Road, between W Grizzly Road and Victory Road, from two travel lanes to two travel lanes with a center median/turn lane.

6-foot-wide bike lanes on the east and west side of Stoddard Road from Kodiak Drive north to the existing bike lanes south of Overland Road.

10-foot-wide multi-use pathway on the west side of Stoddard Road from Victory Road north to the existing sidewalk south of Kodiak Drive.

Widening of the existing sidewalk to create a 10-foot-wide multi-use pathway from south of Kodiak Drive, to the intersection of Stoddard Road and Kodiak Drive, and then west along Kodiak Drive to the existing 10-foot-wide multi-use pathway on Kodiak Drive.

Pedestrian Crossing (PHB) across Victory Road at the intersection of Victory Road and Stoddard Road, on the west side of Stoddard Road.

■ Stormwater drainage improvements, including infiltration and subsurface drainage.

2.0 SCOPE OF SERVICES

Our services have been completed in general accordance with the subconsultant agreement between Kimley-Horn and Associates, Inc. (Kimley-Horn) and GeoEngineers, Inc. (GeoEngineers) dated April 26, 2022. Our scope of services included:

1. Reviewing geologic maps available for the project vicinity and existing subsurface information from files provided by ACHD, Kimley-Horn, or publicly available sources.

2. Completing nine borings; installing two groundwater piezometer monitoring wells in completed boreholes; and completing three test pits with infiltration testing to characterize the subsurface soil and groundwater conditions.

3. Evaluating physical and engineering characteristics of the soils based on laboratory tests performed on samples obtained from the explorations.

4. Completing geotechnical engineering analysis and preparing this report.

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3.0 FIELD EXPLORATIONS AND LABORATORY TESTING

3.1. Field Explorations

Nine borings (GEI-B01-22 through GEI-B09-22), two piezometers (GEI-P01-22 and GEI-P02-22), and three test pits with infiltration testing (GEI-TP1-22 through GEI-TP3-22) were completed as part of our subsurface exploration program. The explorations were completed during the period of July 5 through July 8, 2022 at the locations shown in Figures 2A through 2D. Borings were advanced to depths of 6.5 to 36.5 feet below ground surface. Piezometers were installed to 17½ feet below ground surface in GEI-P01-22 and GEI-P02-22 to measure static groundwater levels. Test pits were advanced to a depth of 3 to 8.5 feet below ground surface, and infiltration tests were performed at the bottom of the test pit excavations. The exploration locations were estimated using recreational grade GPS units and were not surveyed by a state of Idaho-licensed surveyor. The exploration locations should be considered approximate, and the ground surface elevations are undetermined. Details of field explorations and logs of the explorations are presented in Appendix A, Field Explorations.

3.2. Laboratory Testing

Soil samples collected from the borings and test pits were transported to GeoEngineers’ office for further examination and laboratory testing to verify field classifications and to evaluate engineering properties of the soil. Selected samples were tested to determine moisture content, grain-size distribution (sieve analysis), fines content, Atterberg Limits, pH, resistivity, soluble sulfates, soluble chlorides, and R-value (Idaho T-8). A description of the laboratory testing and the test results are presented in Appendix B, Laboratory Testing, and are presented on the borings logs in Appendix A.

4.0 SITE CONDITIONS

4.1. Regional Geology

The site is in the central portion of the Western Snake River Plain. The Western Snake River Plain is a northwest trending structural basin bounded by high angle faults of Miocene and younger age. The basin separates the mountains of the Idaho Batholith on the northeast and the Owyhee Range on the southwest.

Sedimentary fine-grained lacustrine and floodplain deposits shed from adjacent uplands and basaltic and andesitic flows and ash deposited from regional volcanic activity fill the basin to depths exceeding 4,000 feet (Othberg 1994). Subsequent episodic erosion and deposition formed the Boise River Terraces by rapid fluvial downcutting followed by coarse gravel deposition, which may correlate to repeated periods of glacial climates. Fine-grained sediments deposited by the Bonneville Flood, known as slack-water deposits, and wind-blown deposits, loess, mantle the terrace surfaces.

The site is mapped as Gravel of Sunrise Terrace (Qsg) north of the Ridenbaugh Canal and Gravel of Amity Terrace (Qag) south of the Ridenbaugh Canal, which compose a sequence of terraces of the ancestral Boise River. The Gravel of Sunrise Terrace is the third terrace above the modern floodplain and consists of sandy pebble and cobble gravel with a thickness of approximately 13 meters (44 feet), mantled with 1 to 2 meters (3 to 6 feet) of loess (Othberg 1992). The Gravel of Amity Terrace is the seventh terrace above the modern floodplain and consists of sandy pebble and cobble gravel grading at depth to coarse pebbly sand with a thickness of approximately 10 meters (33 feet), mantled with 0.5 to 2 meters (2 to 7 feet) of loess (Othberg 1992). The subsurface explorations were generally consistent with the mapped geology. A geologic map of the area is presented in Figure 1.

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4.2. Surface Conditions

The Stoddard Road project has a north-south alignment between Victory Road and Overland Road intersections. Stoddard Road is primarily a two-lane HMA paved road with a center auxiliary turning lane north of Ridenbaugh Canal. Stoddard Road has gravel shoulders, detached sidewalks and attached sidewalks along the alignment. The Stoddard Road and Victory Road intersection has a stop sign to hold traffic on Stoddard Road, gravel shoulders and no dedicated turning lanes to accommodate traffic movement. The Stoddard Road and Kodiak Drive intersection, as well as the Stoddard Road and Overland intersection have concrete sidewalks, overhead traffic signals, pedestrian crossings and dedicated turning lanes to accommodate traffic movements. There are numerous underground utilities along Stoddard Road including gas, sewer, drainage, water, communications and irrigation. There are overhead power lines on the west side of Stoddard Road with drop-down buried lines providing power to various structures such as overhead lighting and traffic signals.

Ridenbaugh Canal crosses under Stoddard Road on a bend in the Ridenbaugh canal but generally crosses on an east-west direction approximately 600 feet north of the Victory Road intersection. There is an unnamed irrigation lateral that daylights just south of Idaho Power Company property on the west side of Stoddard Road. The site is relatively flat with a slight decrease in elevation towards the north. There is a relatively large grade change from the Victory Road intersection towards the Ridenbaugh Canal that is likely associated with the transition between the two mapped geologic units along the project alignment, the Gravel of Amity Terrace and the Gravel of Sunrise Terrace. Vegetation at the site includes landscaped grass areas and occasional sections of unmaintained landscaped areas. Photographs of the surface conditions at each exploration location, as well as photographs of soils samples collected are provided in Figures C-1 through C-8 in Appendix C, Site Photographs.

4.3. Existing Pavement Section

The existing Stoddard Road consists of HMA pavement ranging in thickness from 3-¾ to 5 inches. Asphalt cores were not collected in the explorations through the HMA pavement as borings were advanced by augering through the HMA pavement. We observed approximately 6 to 27 inches of ¾-inch crushed aggregate base course. The base course consisted of dense gravel with variable amounts of silt and sand (GP, GP-GM, GM). We did not observe the presence of subbase in the explorations. A summary of the existing pavement section observed in each of the borings is shown in Table 1.

TABLE 1. SUMMARY OF PAVEMENT SECTION OBSERVED IN BORINGS

Boring ID HMA Thickness (in) Base Course

Thickness (in) Total Thickness (in)

GEI-B01-22 5 13 18

GEI-B02-22 4 6 10

GEI-B03-22 4 6 10

GEI-B04-22 5 10 15

GEI-B05-22 3-¾ 27 30-¾

GEI-B06-22 5 19 24

GEI-B07-22 5 19 24

GEI-B08-22 5 19 24

March 10, 2023 | Page 4

4.4. Subsurface Soil Conditions

Based on our explorations, subsurface conditions generally consist of fill and native alluvial deposits.

Descriptions of these deposits are provided below.

4.4.1. Fill

We encountered fill below the pavement section or in the shoulders of Stoddard Road in explorations GEI-B02-22 through GEI-B04-22, GEI-B09-22, GEI-P01-22 and GEI-TP01-22. The fill thickness ranged from 4 to 11 feet and thicker deposits encountered could be attributed to previous utility installations. The fill consisted of loose silty gravel with sand (GM), loose to medium dense clayey sand with gravel (SC), stiff silt with sand and gravel (ML) and very stiff sandy clay with gravel (CL). Laboratory testing of samples from the fill soils indicate moisture contents ranging from 11 to 30 percent, fines content (material passing the No.

200 sieve) ranging from 20 to 75 percent, and Atterberg limits ranging from a liquid limit of 31 to 38 and plasticity index ranging from 12 to 14.

4.4.2. Native Soils

Native soils were encountered in all explorations and consisted of interlayered clay, silt, sand and gravel deposits typical of alluvial deposits. This deposit consisted of a stiff to very stiff clay with varying amounts of sand and gravel (CL), medium stiff to hard silt with variable amounts of sand (ML), medium dense to very dense sand with variable amounts of silt and gravel (SP, SP-SM, SW-SM, SM) and a dense gravel with variable amounts of silt, sand and cobbles (GP, GM). Laboratory testing of samples from the native soils indicate moisture contents ranging from 4 to 27 percent, fines content (material passing the No. 200 sieve) ranging from 11 to 84 percent, and Atterberg limits ranging from a liquid limit of 33 to 34 and plasticity index ranging from 15 to 16.

4.5. Groundwater Conditions

Groundwater depths were recorded within borings at the time of exploration and measured within a standpipe piezometer installed in boring GEI-P01-22 and GEI-P02-22. A summary of the groundwater conditions is summarized in Table 2.

TABLE 2. SUMMARY OF GROUNDWATER CONDITIONS

Boring ID Depth to Groundwater (feet) Comments

GEI-B02-22 29

GEI-B03-22 29

GEI-B04-22 22

GEI-B08-22 19

GEI-B09-22 20

GEI-P01-22 >17.5 Piezometer installed dry

GEI-P02-22 >17.5 Piezometer installed dry

Groundwater levels are currently being monitored using a pressure transducer with a data logger installed within the standpipe piezometers in borings GEI-P01-22 and GEI-P02-22. The piezometers were dry when

March 10, 2023 | Page 5 they were installed on July 8, 2022 and remained dry through October 20, 2022—indicating groundwater is deeper than 17.5 feet during the monitoring period.

Depth to groundwater will vary depending on many variables such as seasonal precipitation, land use, water levels within the Ridenbaugh Canal, and local irrigation practices. Groundwater levels should be expected to fluctuate.

5.0 CONCLUSIONS AND RECOMMENDATIONS

5.1. Seismic Design

5.1.1 Seismic Design Parameters

AASHTO acceleration coefficients for design are based on the expected ground motion at the project site that has a 7 percent probability of exceedance in a 75-year period (approximate 1,000-year return period).

The site classification was based on relatively short borings by extrapolating the soil conditions to a depth of 100 feet. We recommend the seismic design parameters presented in Table 3 be used to design the bridge structure.

TABLE 3. AASHTO SEISMIC PARAMETERS

AASHTO Seismic Parameter Recommended Value

Site Class D

Seismic Performance Zone 2

Peak Ground Acceleration (PGA), bedrock Site Class B 0.079

Spectral Response Acceleration at 0.2-Second Period (SS), bedrock Site Class B 0.184

Spectral Response Acceleration at 1-Second Period (S1), bedrock Site Class B 0.066

Zero-period Site Factor, Fpga 1.6

Short Period Site Factor, Fa 1.6

Long-period Site Factor, Fv 2.4

Free-field Acceleration Coefficient (As), Amplified for Site Class C 0.126

Design Spectral Acceleration Coefficient at 0.2-Second Period (SDS), Amplified for Site Class C 0.295

Design Spectral Acceleration Coefficient at 1.0-second Period (SD1), Amplified for Site Class C 0.159

Notes:

Mapped values derived from the USGS online tool using Latitude 43.577267° and Longitude: -116.403789°

5.1.2 Liquefaction Potential

Liquefaction is a phenomenon where soils experience a rapid loss of internal strength as pore water pressures increase in response to strong ground shaking. The increased pore water pressure may temporarily meet or exceed soil overburden pressures to produce conditions that allow soil and water to flow, deform, or erupt from the ground surface. Ground settlement, lateral spreading and/or sand boils may result from soil liquefaction. Structures supported on or within liquefied soils may suffer foundation settlement or lateral movement that can be damaging to the structure.

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The evaluation of liquefaction potential is a complex procedure and is dependent on numerous site parameters, including soil grain size, soil density, site geometry, static stresses, and the design ground acceleration. Liquefaction potential of the site was evaluated by comparing the earthquake-induced cyclic stress ratio (CSR) to the soils cyclic resistance ratio (CRR) using methods by Idriss and Boulanger 2014 and Youd et al. 2001.

The CSR was calculated using a free-field Acceleration Coefficient (As) of 0.126g. An earthquake magnitude of 7.0 was used for our analysis which represents the magnitude of the Squaw Creek Fault which is the highest contributing fault based on the USGS deaggragation plots. The CRR was calculated based on SPT blow counts encountered in the borings. The factor of safety against liquefaction is determined by dividing the CRR by the CSR. Liquefaction hazards, including settlement and related effects, were evaluated when the factor of safety against liquefaction was calculated as less than 1.2.

The analysis indicates that the site soils have low risk of seismic liquefaction (factor of safety greater than 1.2) and liquefaction settlement estimates were negligible for each boring evaluated. Since the site soils have a low risk of liquefaction, it is our opinion that lateral spreading risk is also low. In our opinion, seismic liquefaction and lateral spreading soil strength loss and soil movements do not need to be considered in design.

5.1.3 Fault Rupture

We reviewed the USGS (2014) online Quaternary Faults database, Idaho Geological Survey’s (IGS) Miocene-Quaternary Fault Map database and published maps available from IGS relative to seismic history and faulting in the project vicinity. A summary of faults mapped within about 20 miles of the project site are summarized in Table 4. We also included the Squaw Creek fault since it is the closest active fault to the site.

Based on our observations, our understanding of local geology and the site location with respect to the nearest known faults, it is our opinion the probability of damaging fault rupture on the site is low and does not warrant fault trenching or specific design considerations.

TABLE 4. SUMMARY OF NEARBY MAPPED FAULTS

Fault Name Distance to Site Age Probability of

Activity

Western Snake River Plain Fault System 0.0 to 20.0 miles Lesser Quaternary (<130 thousand years) Low

Squaw Creek Fault 26 miles Major Holocene (<10,000 years) Moderate

5.2 Foundation Design

5.2.1 Summary

The proposed widening of Bridge No. 2085 over the Ridenbaugh Canal and wing walls (if included) will be supported on shallow foundations which will match the existing shallow foundations that are supported on native sand and silt deposits. As-built drawings of Bridge No. 2085 provided by the client indicate the bottom of footing elevation to be 2649.45 for both the north and south abutments. The as-built drawings of the Ridenbaugh Canal show a 5-inch-thick concrete canal liner 3 feet above the existing bottom of

March 10, 2023 | Page 7 footing. We recommend the proposed widening of the bridge foundations be protected by matching the existing concrete canal liner.

5.2.2 Subgrade Conditions and Soil Parameters

The soil parameters used in our analyses are presented in Table 5 and are based on the conditions observed in our borings, geotechnical laboratory testing results, and our professional judgment.

TABLE 5. RECOMMENDED SOIL PARAMETERS FOR DESIGN

Parameter Proposed Granular Fill Existing Fill Soils Native Soils

Unit Description Granular Embankment

Fill (6-inch-minus Uncrushed Aggregate)

Very loose to loose sand and gravel and stiff silt Cemented Silt and Sand

Total Unit Weight (pcf) 130 120 120

Friction Angle, φ 34° 30° 34°

Cohesion, c (psf) 0 0 0

Young’s Modulus, E (ksf) 1 1,500 500 1,200

Poisson’s ratio, υ 2 0.35 0.30 0.40

5.2.3 Bearing Capacity

Bearing capacities for the shallow foundation of the proposed bridge and wing walls were computed using the Munfakh, et al., 2001 method from Section 10.6.3 of the AASHTO LRFD Bridge Design Specifications.

The settlement bearing resistance values (Service Limit State) were computed using the elastic half-space method from Section 10.6.2.4.2 of the AASHTO LRFD Bridge Design Specifications.

The factored shallow foundation bearing resistance recommendations for the bridge structure foundations are presented in Figure D-1 in Appendix D, Bearing Capacity Plot. The figure includes values for Service, Strength and Extreme Limit States as well as nominal (unfactored) resistance values. The bearing resistance for the Service Limit State are provided for 1 inch of settlement. We anticipate that the settlement will occur as the loads are applied and long-term settlement is expected to be negligible, provided the foundations are supported as described above.

The plot in Appendix D are based on an effective footing width which should be determined using the following equation:

B’ = B – 2ey

Where:

B’ is the effective footing width B is the actual footing width ey is the eccentricity along the footing width

The eccentricity is measured from the center of the footing to the location of the resultant reaction force along the width of the footing (y-axis). Refer to Section 10.6.1.3 of the AASHTO LRFD Bridge Design Specifications for additional information regarding eccentricity.

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The bridge foundations should have a minimum width of 36 inches and extend a minimum of 24 inches below the lowest adjacent final grade for frost protection and have a bottom of footing elevation matching the existing bridge shallow foundations. All previously placed undocumented fill should be removed below the foundations. Fill soils were observed to extend 12 feet below grade in Boring GEI-B02-22 and

7.5 feet below grade in Boring GEI-B03-22. Deeper embedment might be necessary for scour protection if recommended by the hydraulics engineer or based on the structural design of the foundation.

5.2.4 Foundation Resistance to Lateral Loads

Lateral loads may be resisted by friction at the base of the foundations in accordance with Section 10.6.3.4 of the AASHTO LRFD Bridge Design Specifications. For foundations supported as recommended above, the nominal (ultimate) sliding resistance between soil and foundation may be computed using a coefficient of

1.0 (cast-in-place concrete) or 0.8 (pre-cast concrete) applied to the total vertical force and the tangent of the internal friction angle of drained soil as defined in equation 10.6.3.4-2 of the AASHTO LRFD Bridge Design Specifications.

It also may be possible to utilize passive resistance against the footing edges; however, we recommend passive resistance not be used unless lateral displacement sufficient to mobilize passive resistance can be tolerated. The nominal (ultimate) passive resistance may be computed using an equivalent fluid density equal to 460 pcf provided adequate foundation movement to generate full passive resistance is achieved.

Passive resistance should be reduced as shown in Chart 1 in the Recommended Lateral Earth Pressures section of this report for movement less than approximately 0.05 times the embedment depth. This value is appropriate for foundation elements placed directly against native soil or surrounded by adequately compacted 6-inch-minus Uncrushed Aggregate per ISPWC 801.

5.2.5 Resistance Factors

We recommend the resistance factors presented in Table 6 be used for design of shallow foundations. The bearing resistance chart presented in Figure D-1 incorporate the recommended resistance factors.

TABLE 6. RESISTANCE FACTORS—SHALLOW FOUNDATIONS

Limit State

Resistance Factor

Bearing Resistance Sliding Resistance Passive Resistance

Strength 0.45 0.90 (pre-cast)

0.80 (cast-in-place) 0.45

Service 1.0 1.0 1.0

Extreme 1.0 1.0 1.0

5.3 Lateral Earth Pressures and Backfill

Embankment fill and wall backfill material used in conjunction with this project should be free from organic and deleterious materials and should conform to 6-inch-minus Uncrushed Aggregate per ISPWC 801. The lateral earth pressure values for fill material are summarized in Table 7.

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TABLE 7. RECOMMENDED LATERAL EARTH PRESSURES

Parameter

Value 1

Horizontal Backslope 3H:1V Slope 2 2H:1V Slope 2

Backfill Unit Weight (γ) 130 pcf 130 pcf 130 pcf

Backfill Soil Friction Angle (∅) 34º 34º 34º

Soil-Wall Interface Friction Angle (δ) (active only) 11º 11º 11º

Active Earth Pressure Coefficient (Ka) 0.26 0.33 0.40

At-Rest Earth Pressure Coefficient (Ko) 0.44 0.58 0.64

Passive Earth Pressure Coefficient (Kp) Movement Toward Upward Slope Movement Toward Downward Slope

3.54 6.85 1.90

10.02 1.34

Seismic Earth Pressure Coefficient (KAe) (Active Condition) 3 0.30 0.38 0.51

Seismic Earth Pressure Coefficient (Koe) (At-Rest Condition) 4 0.52 0.68 0.97

Notes:

1 All values are nominal (unfactored) and should be reduced based on anticipated wall movement per the chart below.

2 GeoEngineers should be consulted to revise lateral earth pressure coefficient and equivalent fluid density values if back-slopes with different slope inclinations are planned.

3 Based on a horizontal acceleration coefficient equal to half the peak ground acceleration (0.5 x As) and assumes that wall is allowed to move or translate during design seismic event.

4 Based on a horizontal acceleration coefficient equal to the peak ground acceleration (As).

Rigid structures should be designed using at-rest earth pressures. Active, static and seismic earth pressure coefficients should be used for walls that are free to rotate or translate. If wall movement will be less than about 0.001H (where H is the height of the abutment wall) under static conditions, then at-rest static earth pressures should be used. The passive pressure coefficients in Table 7 should be reduced for wall movements less than about 0.05H using the graph shown below. Corresponding factors should be applied (multiplied) by the lateral earth pressures presented in Table 7 above.

At Rest

Passive, Kp

Active Ka

At Rest

Recommended Lateral Earth Pressure Rotation Factors

March 10, 2023 | Page 10

In accordance with AASHTO LRFD Bridge Design Specifications, Section A11.3.1, the combined effect of seismic and static lateral earth pressures can be calculated using a triangular earth pressure distribution.

The effective equivalent fluid density can be calculated using the seismic earth pressure coefficient multiplied by the unit weight of the soil. The resultant force is applied at a point equal to H/3 above the bottom of the wall. This earth pressure includes both static and seismic portions. Therefore, static earth pressure should not be included separately. This seismic earth pressure is based on the Mononobe-Okabe method presented in AASHTO LRFD Bridge Design Specifications.

The lateral earth pressures assume the backfill soils are properly drained and do not include hydrostatic pressures that can develop if groundwater or surface water accumulates behind the retaining structures.

Hydrostatic forces should be included in the design, if appropriate.

Where applicable, a traffic surcharge load should be included immediately behind the retaining structures in accordance with AASHTO LRFD Bridge Design Specifications, Section 3.11.6.4. We recommend that a unit weight of 130 pcf and the appropriate (active or at-rest, depending on stiffness of the wall) lateral earth pressure coefficient presented in Table 7 be used to calculate the live load traffic surcharge (LS). Additional surcharge pressures resulting from loads applied at the ground surface should also be included in the design of buried structures walls. Such earth pressures may be generated by surface surcharge loads, point loads, line loads, and strip loads. Lateral surcharge pressures for uniform surface surcharge loads may be calculated by multiplying the uniform surcharge by the active or at-rest lateral earth coefficient presented in Table 7. GeoEngineers should be consulted if other surface surcharge loads adjacent to the walls are expected.

Backfill material for the structure should be compacted to at least 90 percent of the maximum dry density (MDD), except for within 2 feet of the pavement section which should be compacted to 95 percent of the MDD. Over-compaction of buried structures should be avoided because increased compactive effort will result in lateral pressures higher than those provided in this report. Heavy compaction equipment or other construction loads should not be allowed within a distance of one-half the height of walls. Hand-operated or lightweight compaction equipment, such as a vibrating plate, should be utilized within a distance of 3 feet of the structure and the fill lift thickness should be reduced to a maximum loose lift thickness of 6 inches.

5.4 Drainage

Drainage measures should be implemented to collect and divert any surface runoff from the roadway and prevent saturation of the backfill material. In addition, the top of the bridge should be sloped to allow water that infiltrates into the embankment fill above the structure to drain.

The backfill around the bridge structure should consist of Uncrushed Aggregate per ISPWC 801. This material is expected to be non-expansive, relatively well-draining, and in our opinion, will provide adequate drainage behind the bridge structure abutments.

March 10, 2023 | Page 11

5.5 Infiltration Testing

5.5.1 Overview

We completed infiltration testing within test pits GEI-TP1-22 through GEI-TP3-22. The tests were completed in general accordance with the in-situ small-scale Pilot Infiltration Test (PIT) method described in in Section 8200, Appendix B of the ACHD Policy Manual.

The test pits were excavated to between 3.5 to

8.5 feet below ground surface and more than 3 feet above observed groundwater in soil classified as sand with silt, (SP-SM), silty sand (SM) and gravel with sand (GP). The infiltration test pit dimensions were between 6 and 7 feet long by 3 and 3.5 feet wide, with an infiltration surface area of between 18 and 24.5 square feet, which meets the requirements described in the ACHD Policy Manual of 12 to 36 square feet for the base of the test pit.

A vertical measuring rod was placed on the floor of the test pit as a visual reference for monitoring water levels during testing. A piezoelectric pressure transducer was secured on the outside of the bottom of the measuring rod to provide accurate water level records in 15-second intervals throughout the duration of the test. Water was introduced into the test pit using a hose and perforated bucket diffuser to reduce disturbance to the test pit bottom.

A photograph of our typical test setup is presented above.

After the test set up was complete, we completed a series of falling head infiltration tests. The test pit was filled to maintain a water head of about 14 inches, and the water level was allowed to drop approximately 2 inches before the pit was refilled to the initial level again. At the conclusion of the falling head test series, we added water to the test pit at a rate that maintained approximately 12 inches of water in the pit. The flow rate, recorded in gallons per minute, was calculated by recording the time to fill a 6-gallon bucket occurring every 15 minutes over a 1-hour period. The water was then shut off and the pit was allowed to drain for 1 hour and the water level was recorded. At the conclusion of the test, the test pit was backfilled.

Additional details for each infiltration test is included in Appendix E, Infiltration Testing.

5.5.2 Test Results

Infiltration rates were calculated based on the falling head method trendlines during the pre-soak period and the flow rate required to maintain 12 inches of water in the test pit for 1 hour after the pre-soak period.

The results of the falling head tests conducted during the pre-soak period were compared to the measured infiltration rates obtained during the test period and were similar to those recorded during the pre-soak period. Our measured and recommended design infiltration rates for the test are presented in Table 8.

Photograph 1. Infiltration test in Test Pit GEI-TP2-22

March 10, 2023 | Page 12

TABLE 8. MEASURED AND CALCULATED INFILTRATION RATES AT TEST LOCATIONS

Test Pit ID Test

Depth Soil Material at Test

Depth Measured Infiltration Rate (inches/hour)

Maximum Design Infiltration Rate (inches/hour)

GEI-TP1-22 4.5’ Sand (SP-SM) 2.0 1.0

GEI-TP2-22 3.5’ Silty Sand (SM) 1.3 0.65

GEI-TP3-22 5’ Sand/Gravel (SM/GP) 3.5 1.75 Note:

1 The rates presented in this table are reflective of the test locations only and assume a minimum of 3 feet of separation from the seasonal high groundwater elevation and an infiltration window backfilled with filter sand to hydraulically connect the stormwater facility to the native alluvial granular soil unit.

5.6 Pavement Design

5.6.1 Overview

We understand Stoddard Road will be widened and full-depth reconstruction of the roadway structure with HMA pavement. A 10-foot-wide multi-use pathway will be constructed on the west side of Stoddard Road from Victory Road north to the existing sidewalk south of Kodiak Drive.

5.6.2 Subgrade Soil

Subgrade soil conditions encountered along the Stoddard Road alignment are described in the Subsurface Conditions of this report. Two laboratory R-value test were conducted on bulk samples obtained from borings GEI-B03-22 and GEI-B05-22. The test indicated an R-value of 28 and 56, respectively. Based on the test results, we used a value 28 for the R-value to conservatively approximate the native subgrade soils along the project alignment.

5.6.3 Traffic Loading

A Traffic Index (TI) of 8.0 was used for flexible pavement design of Stoddard Road for a 20-year design life, as specified by ACHD.

5.6.4 Design Life

Flexible HMA pavement was designed assuming a 20-year design life for Stoddard Road.

5.6.5 Design Parameter Summary

A summary of the design input parameters for flexible pavement evaluation using the Gravel Equivalency method is provided in Table 9.

TABLE 9. FLEXIBLE PAVEMENT THICKNESS DESIGN INPUT PARAMETERS

Design Parameter Design Value Notes

Fl ex ib le

Pa ve m en t

Flexible Design Parameters

R-value 28 Laboratory R-value test result.

TI 8.0 TI provided by Ada County Highway District.

Climate Factor 1 Figure 510.04.01.1 in ITD Materials Manual

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Design Parameter Design Value Notes

Aggregate Base R-value 80 Standard Input parameter

Subbase R-value 60 Standard Input parameter

Substitution Ratios

Plant Mix Aggregate Base

Subbase

2.0 1.0

0.85 Standard Input parameters

5.6.6 Roadway Design Pavement Thickness

Our recommended pavement section for Stoddard Road is presented in Table 10. We recommend a Type III subgrade separation geotextile be placed below the Subbase (HMA pavement section) and native subgrade soils. This pavement section meets the recommended pavement section thickness for Residential Collectors.

TABLE 10. RECOMMENDED PAVEMENT SECTION

Surfacing HMA Thickness (inches) ¾-inch Crushed Aggregate

Base Thickness (inches) 6-inch-Minus Uncrushed

Aggregate Subbase

HMA, SP-3, PG

64-28 3 6 12

5.6.7 Multi-Use Pathway

We understand ACHD is considering either Portland cement concrete (PCC) or HMA surfacing for the proposed pathway.

We recommend using Standard Drawing SD-709 from the ISPWC for the multi-use pathway project. ISPWC Standard Drawing No. SD-709 requires a minimum PCC pavement thickness of 4 inches and a minimum ¾-inch-minus crushed rock base course thickness of 2 inches for sidewalks and curb ramps. The standard plan requires the thickness be increased to 5 inches of PCC pavement over 4 inches of ¾-inch-minus crushed rock base course at driveway approaches. Consideration should be given to using the driveway section (5 inches PCC over 4 inches of Base) if maintenance vehicles will periodically access this portion of the path.

ACHD does not have a standard plan for HMA pavement for sidewalks or pathways. FHWA recommends a minimum asphalt section of about 2.5 inches HMA over a minimum base course thickness of 4 inches (FHWA, 2013). FHWA recommends the thickness be increased to 3 inches of HMA over 4 inches of aggregate base if the pavement will support maintenance vehicles periodically. In our opinion these sections are suitable for design of the pathway if HMA is the preferred surfacing material.

It should be noted that these pathway pavement sections are susceptible to frost heave and damage from tree roots. If frost damage is not acceptable, additional consideration should be given to placing 1.0 feet of Subbase below the Aggregate Base. Subbase used for frost protection should contain less than 5 percent passing the U.S. No. 200 sieve. The Subbase should provide frost protection for at least 75 percent of the 24-inch design frost depth below the ground surface. The AASHTO Guide for Planning, Design and Operation of Pedestrian Facilities provides guidance for tree root damage protection of pathways (e.g. tree placement, tree selection, root barriers, etc.).

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5.7 Corrosion Considerations

Corrosion susceptibility of foundation elements to exposed to soil and groundwater is influenced by soil/water pH, soil electrical resistivity, soluble chloride and soluble sulfate levels.

Steel foundation elements corrosion potential is increased by relatively lower soil/water pH, by lower soil electrical resistivity, and by higher chloride levels. Table 11 below provides a qualitative summary of steel corrosion potential.

TABLE 11. STEEL CORROSION POTENTIAL BASED ON PH, ELECTRICAL RESISTIVITY, AND CHLORIDE

CONTENT OF SOIL

Corrosivity Potential pH Range1 Electrical Resistivity Range2

(ohm-cm) Soluble Chloride Range3

(ppm)

Mild 7 – 10 >10,000 <100

Moderate 5.5 – 7 2,000 – 10,000 100 – 500

High/Severe 4 – 5.5 1,000 – 2,000 >500

Very High/Very Severe <4 0 – 1,000

Notes:

1 Based on Clayton, Design and Technical Service Manual, 9th Ed., 2013, https://foundationtechnology.com 2 Based on Corrosion Basics: An introduction, 2nd Ed. (2006), as cited by https://corrosion-doctors.org 3 Based on inferences from US DOT Publication No. FHWA-NHI-09-087, November 2009 ppm = parts per million

Sulfate content of soil and water is primarily an indicator of corrosion potential concerns for reinforced concrete foundation elements. Elevated sulfate levels increase the risk and potential severity of sulfate attack on buried concrete. Table 12 below provides the exposure categories determined by ACI 318-19, and qualitative summary of concrete corrosion potential.

TABLE 12. CONCRETE CORROSION POTENTIAL BASED ON SULFATE CONCENTRATION

Concrete Exposure Class1 Water-Soluble Sulfate in Soil (ppm)

S0 SO4 <1,000

S1 1,000 ≤ SO4 ≤ 2,000

S2 2,000 ≤ SO4 ≤ 20,000

S3 SO4 >20,000

Notes:

1 Table 19.3.1.1, ACI 318-19 (2019)

Laboratory tests were performed on one (1) representative sample to provide data for corrosion considerations. The laboratory test results specific to corrosion are presented in Table 13.

TABLE 13. CORROSION LABORATORY TEST RESULTS

Boring Depth (ft) Soil/Rock Classification pH Resistivity (ohms-cm)

Soluble Sulfate (ppm)

Soluble Chloride

(ppm)

GEI-B02-22 2.5–4.0 Silty gravel with sand (GM) 8.9 1,090 68 212

Notes:

ppm = parts per million https://foundationtechnology.com/ https://corrosion-doctors.org/

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The water-soluble sulfate (SO4) test results indicate a concrete exposure class S1 based on Table 19.3.1.1 of ACI 318-19. Based on these results, standard Type I/II cement is acceptable.

5.8 Earthwork

Earthwork should be completed in accordance with the ISPWC and ACHD supplementals to the ISPWC.

Additional geotechnical recommendations are provided below.

5.8.1 Excavation, Grading and Subgrade Preparation

Initial site preparation and earthwork activities will include removal of the existing pavement. In our opinion, site soils can be excavated using conventional excavation equipment such as backhoes, excavators or dozers. If earthwork activities cause excessive subgrade disturbance, replacement with structural fill will be necessary. A greater depth of disturbance of the subgrade soil may be expected if site preparation work is conducted during periods of wet weather when the moisture content of the soil exceeds optimum.

To provide uniform subgrade conditions, we recommend moisture conditioning (if necessary) and compacting the exposed subgrade prior to placing new subbase, base course and HMA. Where the existing base course is preserved, it should be moisture conditioned and compacted to at least 95 percent of the maximum dry density (MDD) as approved by the engineer prior to placing new base course material or pavement. We recommend that a proof-roll of the subgrade be observed by a representative from our firm prior to placing new fill, as needed, and prior to paving. Proof-rolling can be completed using a piece of heavy tire-mounted equipment such as a loaded dump truck. Soft or yielding areas observed during proof-rolling will require overexcavation and replacement with structural fill.

Wet, soft, loose or pumping soil identified during proof-compaction of the subgrade material should be removed and replaced at the direction of the engineer.

5.9 Materials

Materials used for site grading are classified as structural fill for the purpose of this report. Structural fill material quality varies depending upon its use as described below. For this project, structural fill may be used to meet grades for pavement reconstruction areas and/or to repair poor subgrade soil conditions prior to placement of new HMA pavement.

5.9.1 Use of On-Site Soil

In general, on-site granular (sand and gravel) soils may be suitable for use as borrow material provided it meets the requirements of ISPWC and ACHD supplemental specifications.

5.9.2 Imported Structural Fill

Imported structural fill is to consist of sand and/or gravel and is to be free of debris, organic material, frozen soil or other deleterious material. We recommended the following materials be specified for this project:

■ We recommend imported structural fill and subbase (if used) be specified as 6-inch-minus Uncrushed Aggregate per Section 801 of ISPWC.

■ Base course below pavement should be specified as ¾-inch (Type 1) Crushed Aggregate per Section 802 of ISPWC.

■ Drain Rock should be specified as 3-inch-minus per Section 801 of ISPWC.

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5.9.3 Fill Placement and Compaction Criteria

We recommend backfill placed within the roadway prism be placed and compacted to 95 percent of the MDD in accordance with the ISPWC. Subbase and base course materials should be compacted in loose lifts not exceeding 9 inches when using heavy drum rollers. The actual thickness will be dependent on the structural fill material used and the type and size of compaction equipment. Each lift should be conditioned to the proper moisture content and compacted to the specified density before placing subsequent lifts.

We recommend a qualified representative observe and document the fill placement and compaction operations during construction. Soil conditions are to be evaluated by in-place density tests, visual evaluation, probing and proof-rolling of the structural fill and recompacted on-site soil as it is prepared to determine compliance with the contract documents and recommendations in this report.

5.10 Weather Considerations

5.10.1 Paving

HMA or PCCP should not be placed on wet, soft, muddy, or frozen subgrade or during rainfall, sand or dust storms or before imminent storms that might damage the construction. The engineer should have the discretion as to whether the surface and materials are dry enough to proceed with construction. We recommend the placement of HMA asphalt should follow ISPWC Section 810 Plant Mix Pavement and the ACHD Supplementals to the ISPWC. Concrete pavement should be designed and constructed following ISPWC Section 705 including ACHD supplementals and standard plans.

5.10.2 Construction Considerations

The native surficial soils could be moisture sensitive and easily disturbed, especially if earthwork is completed during periods of wet weather. During dry weather, the soils will: (1) be less susceptible to disturbance; (2) provide better support for construction equipment; and (3) be more likely to meet the required compaction criteria.

Based on the soil and groundwater conditions observed in our borings at the time of our field explorations, we do not expect that groundwater will be encountered for the anticipated excavation depths. If surface water is collected within excavations, it will be necessary to dewater. We anticipate that dewatering can likely be accomplished by pumping from sumps.

On-site fill encountered in some borings, has a moderate amount of fine-grained particles. This soil will be more sensitive to moisture. If the moisture content is more than 3 percent above optimum, the material is likely to become soft and unstable. Operation of equipment in these conditions can be difficult and the required compaction criteria will be difficult to achieve. If wet weather earthwork is unavoidable, we recommend the following:

■ Measures are to be taken by the Contractor to prevent surface water from collecting in the work areas, and to remove water should it collect within these areas. Examples of such measures include, but are not limited to, sumps and pumps or routing stormwater to a designated disposal area.

■ Earthwork activities are not to take place during periods of heavy precipitation.

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■ The site soils are not to be left uncompacted and exposed to moisture. Sealing the surficial soil by rolling with a smooth-drum roller prior to periods of precipitation should reduce the extent to which the soil becomes wet or unstable.

■ Subgrades that are prepared before inclement weather are to be reinspected to identify areas requiring repair. Any such areas are to be repaired at the direction of the geotechnical engineer.

■ Construction activities are to be scheduled so that the length of time that soil is exposed to moisture during wet weather is reduced to the extent practical.

6.0 LIMITATIONS

We have prepared this report for the exclusive use of Kimley-Horn and Associates, Inc., ACHD and their authorized agents for the Stoddard Road, Victory to Overland project in Meridian, Idaho. Kimley-Horn and Associates may distribute copies of this report to their authorized agents and regulatory agencies as may be required for the project. The data and report should be provided to prospective contractors for their bidding or estimating purposes, but our report, conclusions and interpretations should not be construed as a warranty of the subsurface conditions.

Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in the field of geotechnical engineering in this area at the time this report was prepared. The conclusions, recommendations, and opinions presented in this report are based on our professional knowledge, judgment and experience. No warranty or other conditions, express or implied, should be understood.

Please refer to Appendix F, Report Limitations and Guidelines for Use, for additional information pertaining to use of this report.

We appreciate the opportunity to provide these services. Please call if you have any questions regarding the content of this report or require additional information.

7.0 REFERENCES

American Association of State Highway and Transportation Officials. 1993. “AASHTO Guide for Design of Pavement Structures.”

Ada County Highway District. 2011. “Ada County Highway District Policy Manual.” Updated April 2017.

Ada County Highway District. 2017. “Supplemental Specifications to the ISPWC.”

Idaho Geologic Survey. 2004. Idaho Miocene – Quaternary Fault Database. Accessed August 2022 http://www.idahogeology.org/DrawOnePage.asp?PageID=73.

Idaho Standards for Public Works Construction. 2020. “Idaho Standards for Public Works Construction.”

Othberg, K. L. 1994. “Geology and Geomorphology of the Boise Valley and Adjoining Areas, Western Snake River Plain, Idaho, Idaho Geological Survey.”

http://www.idahogeology.org/DrawOnePage.asp?PageID=73

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Othberg, K. L., and Stanford, L. R., 1992, Geologic Map of the Boise Valley and Adjoining Area, Western Snake River Plain, Idaho, Idaho Geological Survey, Geologic Map Series, GM-18, Scale 1:1,00,000.

USGS, 2006, Quaternary fault and fold database for the United States, Available online at:

https://earthquake.usgs.gov/hazards/qfaults/. Accessed August 2022.

USGS U.S. Seismic Design Maps Online Tool: https://earthquake.usgs.gov/ws/designmaps/. Accessed August 2022.

https://earthquake.usgs.gov/hazards/qfaults/ https://earthquake.usgs.gov/ws/designmaps/

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