Geotechnical_Report.pdf

GEOTECHNICAL INVESTIGATION

LIBERTY STREET IMPROVEMENTS

Liberty Street from Denton Street to Franklin Road

Boise, ID

PREPARED FOR:

Mr. Ismeal Medina Keller Associates, Inc.

131 Southwest 5th Avenue, Suite A Meridian, ID 83642

PREPARED BY:

Atlas Technical Consultants, LLC 2791 South Victory View Way Boise, ID 83709

September 3, 2025 B213462g

ACHD Project # 822009 & 222011

Atlas No. B213462g Page | i

Copyright © 2022 Atlas Technical Consultants

September 3, 2025 Atlas No. B213462g

Mr. Ismeal Medina Keller Associates, Inc.

131 Southwest 5th Avenue, Suite A Meridian, ID 83642

Subject: Geotechnical Investigation Liberty Street Improvements Liberty Street from Denton Street to Franklin Road Boise, ID

Dear Mr. Medina:

In compliance with your instructions, Atlas has conducted a soils exploration, pavement, and foundation evaluation for the above referenced development. Fieldwork for this investigation was conducted on January 13, 2022. Data have been analyzed to evaluate pertinent geotechnical conditions. Results of this investigation, together with our recommendations, are to be found in the following report. We have provided a PDF copy for your review and distribution.

Often, questions arise concerning soil conditions because of design and construction details that occur on a project. Atlas would be pleased to continue our role as geotechnical engineers during project implementation.

If you have any questions, please call us at (208) 376-4748.

Respectfully submitted, Jacob Schlador, PE Ethan Salove, PE Geotechnical Manager - Idaho Senior Geotechnical Engineer

9-3-2025

Jacob Schlador Idaho Stamp

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CONTENTS

1. INTRODUCTION

1.1 Project Description

1.2 Authorization

1.3 Scope of Investigation

2. SITE DESCRIPTION

2.1 Site Access

2.2 Regional Geology

2.3 General Site Characteristics

2.4 Regional Site Climatology and Geochemistry

3. SEISMIC SITE EVALUATION

3.1 Geoseismic Setting

3.2 Seismic Design Parameter Values

4. SOILS EXPLORATION

4.1 Exploration and Sampling Procedures

4.2 Laboratory Testing Program

4.3 Soil and Sediment Profile

4.4 Volatile Organic Scan

5. SITE HYDROLOGY

5.1 Groundwater

5.2 Soil Infiltration Rates

5.3 Infiltration Testing

6. SLOPES AND SETBACKS

7. LATERAL EARTH PRESSURES

7.1 Retaining Wall Backfill Materials

7.2 Retaining Wall Drainage

8. FOUNDATION AND SLAB DISCUSSION AND RECOMMENDATIONS

8.1 Foundation Design Recommendations

9. PAVEMENT DISCUSSION AND RECOMMENDATIONS

9.1 Flexible Pavement Section

9.2 Pavement Subgrade Preparation

9.3 Common Pavement Section Construction Issues

10. CONSTRUCTION CONSIDERATIONS

10.1 Earthwork

10.2 Dry Weather

10.3 Wet Weather

10.4 Soft Subgrade Soils

10.5 Frozen Subgrade Soils

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10.6 Structural Fill

10.7 Backfill of Walls

10.8 Excavations

10.9 Groundwater Control

11. GENERAL COMMENTS

12. REFERENCES

TABLES

Table 1 – Seismic Design Values

Table 2 – Groundwater Data

Table 3 – Lateral Earth Pressure Parameters

Table 4 – Gravel Equivalent Method Flexible Pavement Specifications

APPENDICES

Warranty and Limiting Conditions

Vicinity Map

Site Map

Geotechnical Investigation Boring Log

Geotechnical General Notes

Gravel Equivalent Method Pavement Design

R-value Laboratory Test Data

Important Information About This Geotechnical Engineering Report

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Copyright © 2022 Atlas Technical Consultants

1. INTRODUCTION

This report presents results of a geotechnical investigation and analysis in support of data utilized in design of structures as defined in the 2020 AASHTO LRFD Bridge Design Specifications.

Information in support of groundwater and stormwater issues pertinent to the practice of Civil

Engineering is included. Observations and recommendations relevant to the earthwork phase of the project are also presented. Revisions in plans or drawings for the proposed structures and pavements from those enumerated in this report should be brought to the attention of the soils engineer to determine whether changes in the provided recommendations are required.

Deviations from noted subsurface conditions, if encountered during construction, should also be brought to the attention of the soils engineer.

1.1 Project Description

The proposed roadway improvements are in the east-central portion of the City of Boise, Ada

County, ID, and occupies a portion of the W½SE¼ of Section 7, Township 3 North, Range 2 East, Boise Meridian. This project will consist of widening approximately 0.45 mile of Liberty Street and adding a 10-foot wide detached multiuse pathway. The Ridenbaugh Canal crossing will also be widened as part of the project. Retaining walls for the canal crossing are anticipated as part of the project. Total settlements are limited to 1 inch. Loads of up to 5,000 pounds per lineal foot for wall footings were assumed for settlement calculations. Atlas was informed by Mr. Ismael

Medina that a traffic index of 8 should be used in design of pavements. Atlas has not been informed of the proposed grading plan.

1.2 Authorization

Authorization to perform this exploration and analysis was given in the form of a written authorization to proceed from Mr. Donn Carnahan of Keller Associates, Inc. to Monica Saculles of Atlas Technical Consultants (Atlas), on December 22, 2021. Said authorization is subject to terms, conditions, and limitations described in the Professional Services Contract entered into between Keller Associates, Inc. and Atlas. Our scope of services for the proposed development has been provided in our proposal dated November 17, 2021 and repeated below.

1.3 Scope of Investigation

The scope of this investigation included review of geologic literature and existing available geotechnical studies of the area, review of available environmental reports, visual site reconnaissance of the immediate site, subsurface exploration of the site, field and laboratory testing of materials collected, and engineering analysis and evaluation of pavement materials and foundation materials.

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2. SITE DESCRIPTION

2.1 Site Access

Access to the site may be gained via Interstate 184 to the Franklin Road exit. Proceed east on

Franklin Road approximately 0.8 mile to its intersection with Liberty Street. The site occupies the

Liberty Street from Franklin Road north to Denton Street. The location is depicted on site maps included in the Appendix.

2.2 Regional Geology

The project site is located within the western Snake River Plain of southwestern Idaho and eastern

Oregon. The plain is a northwest trending rift basin, about 45 miles wide and 200 miles long, that developed about 14 million years ago (Ma) and has since been occupied sporadically by large inland lakes. Geologic materials found within and along the plain’s margins reflect volcanic and fluvial/lacustrine sedimentary processes that have led to an accumulation of approximately 1 to 2 km of interbedded volcanic and sedimentary deposits within the plain. Along the margins of the plain, streams that drained the highlands to the north and south provided coarse to fine-grained sediments eroded from granitic and volcanic rocks, respectively. About 2 million years ago the last of the lakes was drained and since that time fluvial erosion and deposition has dominated the evolution of the landscape. The project site is underlain by the “Gravel of Whitney Terrace” as mapped by Othberg and Stanford (1993). Sediments of the Whitney terrace consist of sandy pebble and cobble gravel. The Whitney terrace is the second terrace above modern Boise River floodplain, is thickest toward its eastern extent, and is mantled with 2-6 feet of loess.

2.3 General Site Characteristics

The site to be redeveloped consists of a 0.45-mile long stretch of Liberty Street. This area is bounded by Franklin Road on the south and Denton Street on the north. The roadway is currently a crowned two-lane roadway. Liberty Street is relatively flat with minor 2 to 3 feet elevation changes noted throughout 0.45-mile stretch of road. The Ridenbaugh Canal runs east to west through the southern end of Liberty Street. The project site is surrounded by residential properties and commercial developments.

No vegetation was noted on the project site; however, landscaping grass, bushes, and mature trees were noted adjacent to Liberty Street. Slopes along the Ridenbaugh Canal range from roughly 3 feet horizontal to 1 foot vertical (3:1) to 2:1.

Regional drainage is north and west toward the Boise River. Stormwater drainage for the site is achieved by sheet runoff across the pavement. The site is situated so that it is unlikely that it will receive any drainage from off-site sources. Stormwater drainage collection and retention systems are in place on the project site in the form of seepage beds.

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2.4 Regional Site Climatology and Geochemistry

According to the Western Regional Climate Center, the average precipitation for the Treasure

Valley is on the order of 10 to 12 inches per year, with an annual snowfall of approximately 20 inches and a range from 3 to 49 inches. The monthly mean daily temperatures range from 21°F to 95°F, with daily extremes ranging from roughly -25°F to 111°F. Winds are generally from the northwest or southeast with an annual average wind speed of approximately 9 miles per hour

(mph) and a maximum of 62 mph. Soils and sediments in the area are primarily derived from siliceous materials and exhibit low electro-chemical potential for corrosion of metals or concretes.

Local aggregates are generally appropriate for Portland cement and lime cement mixtures.

Surface water, groundwater, and soils in the region typically have pH levels ranging from 7.2 to

8.2.

3. SEISMIC SITE EVALUATION

3.1 Geoseismic Setting

Soils on site are classed as Site Class D in accordance with Chapter 3 of the AASHTO LRFD.

Structures constructed on this site should be designed per the AASHTO LRFD for such a seismic classification. Our investigation revealed low hazard potential resulting from potential earthquake motions including: slope instability, liquefaction, and surface rupture caused by faulting or lateral spreading.

3.2 Seismic Design Parameter Values

Seismic Design Parameter Values were obtained from USGS US Seismic Design Maps Web

Services. The AASHTO LRFD seismic design parameter values have been provided below.

Table 1 – Seismic Design Values

Seismic Design Parameter Design Value

Site Class D “Medium dense sand or stiff clay”

Site Modified Peak Ground Acceleration, PGAM

0.133

Ss 0.196

S1 0.069

Fa 1.600

Fv 2.400

SDS 0.313

SD1 0.165

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4. SOILS EXPLORATION

4.1 Exploration and Sampling Procedures

Field exploration conducted to determine engineering characteristics of subsurface materials included a reconnaissance of the project site and investigation by soil boring. Borings were located in the field by means of a Global Positioning System (GPS) device and are reportedly accurate to within ten feet. Borings were advanced by means of a truck-mounted drilling rig equipped with continuous flight hollow-stem augers. At specified depths, samples were obtained using a standard split-spoon sampler and Standard Penetration Test (SPT) blow counts were recorded. Uncorrected SPT blow counts are provided on logs, which can be found in the

Appendix. Delayed water level observations were made in open borings to evaluate groundwater levels. At completion of exploration, borings were backfilled with bentonite holeplug.

Samples have been visually classified in the field by professional staff, identified according to boring number and depth, placed in sealed containers, and transported to our laboratory for additional testing. Subsurface materials have been described in detail on logs provided in the

Appendix. Results of field and laboratory tests are also presented in the Appendix. Atlas recommends that these logs not be used to estimate fill material quantities.

4.2 Laboratory Testing Program

Along with our field investigation, a supplemental laboratory testing program was conducted to determine additional pertinent engineering characteristics of subsurface materials necessary in an analysis of anticipated behavior of the proposed structures. Laboratory tests were conducted in accordance with current applicable American Society for Testing and Materials (ASTM) specifications, and results of these tests are to be found in the Appendix. The laboratory testing program for this report included: Atterberg Limits Testing – ASTM D4318, Grain Size Analysis –

ASTM C117/C136, and Resistance Value (R-value) and Expansion Pressure of Compacted Soils

– Idaho T-8.

4.3 Soil and Sediment Profile

The profile below represents a generalized interpretation for the project site. Note that on site soils strata, encountered between boring locations, may vary from the individual soil profiles presented in the logs, which can be found in the Appendix.

The materials encountered during exploration were quite typical for the geologic area mapped as

Gravel of Whitney Terrace. Asphaltic concrete with associated fill materials were encountered at ground surface throughout the site. These fill materials consisted of poorly graded gravel with sand fills with varying amounts of silt and were brown, light brown, and tan, slightly moist, medium dense to dense, and contained fine to coarse-grained sand and fine to coarse rounded gravel. In boring 2, silty sand with gravel fill materials were encountered beneath the surficial fills. Silty sand with gravel fills were brown to light brown, slightly moist, dense, and contained fine to coarse-grained sand and fine to coarse rounded gravel.

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Beneath the fill materials were lean clay soils with varying amounts of sand content. Lean clays were dark brown to brown, slightly moist to moist, soft to very stiff, and contained fine-grained sand. Below the lean clay soils in borings 2 and 4 were silty sand sediments. Silty sands were brown, slightly moist, dense to very dense, and contained fine to coarse-grained sand and fine gravel in boring 1. Weak to moderate calcium carbonate cementation was noted in boring 4. At depth throughout the site were poorly graded gravel with sand sediments. Poorly graded gravels with sand were brown to light brown, dry to slightly moist, dense to very dense, and contained fine to coarse-grained sand and fine to coarse gravel.

During excavation, boring sidewalls were generally stable. However, moisture contents will affect wall competency with saturated soils having a tendency to readily slough when under load and unsupported.

4.4 Volatile Organic Scan

No environmental concerns were identified prior to commencement of the investigation.

Therefore, soils obtained during on-site activities were not assessed for volatile organic compounds by portable photoionization detector. Samples obtained during our exploration activities exhibited no odors or discoloration typically associated with this type of contamination.

No groundwater was encountered.

5. SITE HYDROLOGY

Existing surface drainage conditions are defined in the General Site Characteristics section.

Information provided in this section is limited to observations made at the time of the investigation.

Either regional or local ordinances may require information beyond the scope of this report.

5.1 Groundwater

During this field investigation, groundwater was not encountered in borings advanced to a maximum depth of 20.4 feet bgs. Soil moistures in the borings were generally dry to moist throughout. Atlas has previously performed 6 geotechnical investigations within 0.25 mile of the project site. Information from these investigations has been provided in the table below.

Table 2 – Groundwater Data

Date Approximate Distance from Site (mile) Direction from Site

Groundwater Depth (feet bgs)

August 2007 0.02 West Not Encountered to 8.0

February 2008 0.14 Northwest Not Encountered to 15.4

March 2008 0.22 West Not Encountered to 16.0

September 2016 0.22 East Not Encountered to 14.2

April 2018 0.04 East Not Encountered to 15.0

February 2021 0.22 Southwest Not Encountered to 15.0

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Furthermore, according to United States Geological Survey (USGS) monitoring well data within approximately ½-mile of the project site, groundwater was measured at a depth of 27 feet bgs.

For construction purposes, groundwater depth can be assumed to remain greater than 20 feet bgs throughout the year. However, groundwater may be shallower in the vicinity of the

Ridenbaugh Canal during the irrigation season. Since this is an estimated depth and seasonal groundwater levels fluctuate, actual levels should be confirmed by periodic groundwater data collected from the piezometer installed in boring 2. If desired, Atlas is available to perform this monitoring.

5.2 Soil Infiltration Rates

Soil permeability, which is a measure of the ability of a soil to transmit a fluid, was tested in the field. For this report, an estimation of infiltration is also presented using generally recognized values for each soil type and gradation. Of soils comprising the generalized soil profile for this study, lean clay soils generally offer little permeability, with typical hydraulic infiltration rates of less than 2 inches per hour. Silty sand sediments usually display rates of 4 to 8 inches per hour;

though calcium carbonate cementation may reduce this value to near zero. Poorly graded gravel with sand sediments typically exhibit infiltration values in excess of 12 inches per hour. Infiltration testing is generally not required within these sediments because of their free-draining nature.

5.3 Infiltration Testing

Infiltration testing was conducted in general accordance with the Ada County Highway District

(ACHD) Policy Manual. The test location was presoaked prior to testing. Pre-soaking increases soil moistures, which allows the tested soils to reach a saturated condition more readily during testing. Saturation of the tested soils is desirable in order to isolate the vertical component of infiltration by inhibiting horizontal seepage during testing.

On January 13, 2022, testing was conducted within poorly graded gravel with sand sediments at a depth of 10.4 feet bgs in boring 3. A stabilized infiltration rate in excess of 8 inches per hour was obtained during testing. Per the ACHD Policy Manual requirements, the maximum design soil infiltration rate shall not exceed 8 inches per hour. Therefore, a design infiltration rate of 8 inches per hour should be used for the poorly graded gravel with sand sediments encountered at depth across the site. It should be confirmed that infiltration facilities are constructed on relatively silt free poorly graded gravel with sand sediments. Atlas recommends that all infiltration facilities be constructed in accordance with the local municipality requirements.

6. SLOPES AND SETBACKS

Slopes ranging from 3 feet horizontal to 1 foot vertical (3:1) to 2:1 were present along the banks of Phyllis Canal. Overall stability analysis of the designed foundation system should be performed in Accordance with the AASHTO LRFD Bridge Design Specification and should take into account embedment depth and setbacks from existing slopes (if applicable). This stability analysis is outside Atlas’ scope of work. However, Atlas is available to perform this analysis upon request.

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7. LATERAL EARTH PRESSURES

Retaining walls will be subject to lateral earth pressures. The magnitude of earth pressure is a function of both type and compaction of backfill behind walls within the “active” zone, and allowable rotation of the top of the wall. The active zone is defined as the wedge of soil between the surface of the wall and a plane inclined 30 degrees from vertical passing through the base of the wall. All clay soils must be completely removed from within the active zone. The following recommendations should be used when dealing with lateral earth pressures on a gravity block:

1) a sliding frictional coefficient of 0.35 is appropriate considering native silty sand sediments, and

2) a sliding frictional coefficient of 0.45 is appropriate considering native poorly graded gravel with sand sediments and granular structural fill under typical conditions.

7.1 Retaining Wall Backfill Materials

All fill materials used for the bridge approaches, embankments, and wall backfill must be free from organics and other deleterious materials and must meet the requirements for ¾-inch (Type I)

Crushed Aggregate in accordance with ISPWC Section 802 for Type 1 Crushed Aggregate Base.

Lateral earth pressures for the wall backfill are summarized in the table below and were calculated using Rakine theory. Soil parameters have been provided for a flat backslope. If alternate backslope angles are to be used, Atlas should be contacted to provide updated values. The following values are applicable under drained conditions.

Table 3 – Lateral Earth Pressure Parameters

Parameter Value1

Horizontal Backslope

Backfill Wet Unit Weight 135 pcf

Backfill Soil Friction Angle 34°

Active Lateral Earth Pressure Coefficient (Ka) 0.28

At-Rest Lateral Earth Pressure Coefficient (Ko) 0.44

Passive Lateral Earth Pressure Coefficient (Kp) 3.54

Active Seismic Earth Pressure Coefficient (Kae) 0.40

Equivalent Active Fluid Pressure 37 pcf

Equivalent At-Rest Fluid Pressure 57 pcf

Equivalent Passive Fluid Pressure 460 pcf 1All values are unfactored and should be reduced based on anticipated wall movement.

2Based on a horizontal acceleration coefficient of ½ peak ground acceleration and assuming the wall is allowed to move or translate during seismic event.

Active (static and seismic) pressures are appropriate under conditions where the walls are free to rotate or translate. If the wall movement will be less than roughly 0.001H (where H is calculated from the bottom of the pile cap to finished grade) under static conditions, at-rest pressures should be used. Passive pressure coefficients should be reduced for wall movements less than approximately 0.05H.

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The combined effects of seismic and static lateral earth pressures can be calculated using a triangular distribution in accordance with the AASHTO LRFD. The effective fluid density can be calculated using the lateral earth pressure coefficient multiplied by the unit weight of the soil.

A traffic surcharge should be incorporated in all areas where vehicular loads are expected to act on the walls within a distance of 0.5H in accordance with AASHTO LRFD Section 3.11.6.4. If other surcharge loads are anticipated to act on the walls, Atlas should be contacted to provide additional recommendations.

All backfill should be compacted to ITD Class A compaction standards. Heavy equipment should not be allowed within a distance of one half of the height of the wall. It is recommended that only small hand-operated compaction equipment be used for compaction of backfill within 3 feet of the wall (if applicable) in accordance with ITD 2023 Standard Specifications for Highway Construction

Section 210.03.

7.2 Retaining Wall Drainage

Atlas recommends that a drainage system be incorporated into the retained soil mass. The aforementioned soil unit weights and earth pressure coefficients assume that provisions are made that prohibit the buildup of hydrostatic pressures against the retaining wall structure. Such provisions may include geosynthetic and/or granular drainage media, weepholes, internal drainage piping, or combination thereof.

8. FOUNDATION AND SLAB DISCUSSION AND RECOMMENDATIONS

Various foundation types have been considered for support of the proposed bridge. Two requirements must be met in the design of foundations. First, the applied bearing stress must be less than the ultimate bearing capacity of foundation soils to maintain stability. Second, total and differential settlement must not exceed an amount that will produce an adverse behavior of the superstructure. Allowable settlement is usually exceeded before bearing capacity considerations become important; thus, allowable bearing pressure is normally controlled by settlement considerations. Considering subsurface conditions and the proposed construction, it is recommended that the structure be founded upon conventional wall footings and wing walls.

At this time, Atlas has not been provided with a scour analysis for the proposed crossing. Atlas must be contacted if the results of the scour analysis will affect foundation bearing soils.

8.1 Foundation Design Recommendations

Atlas has not been informed of the anticipated depth of the foundations. The following figures show strength and service load bearing capacities for varying foundation widths. It is assumed that foundations will bear on competent, native poorly graded gravel with sand sediments at a depth of approximately 10 feet below existing roadway surface.

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Figure 1: Bearing Resistance – Strength Limit State

2 3 4 5 6 7 8 9 10 11 12 13 14 15

B E

A R

IN

G

R E

S

IS

T A

N C

E

K S

F

EFFECTIVE FOUNDATION WIDTH, B' (FEET)

Ultimate Bearing Resistance Strength Limit State (φ=0.45)

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Figure 2: Bearing Resistance – Service Limit State

2 3 4 5 6 7 8 9 10 11 12 13 14 15

B E

A R

IN

G

R E

S

IS

T A

N C

E

K S

F

EFFECTIVE FOUNDATION WIDTH, B' (FEET)

Service Limit State, 1.0" Settlement Service Limit State, 0.5" Settlement

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Footings should be proportioned to meet either the stated soil bearing capacity or the 2020

AASHTO LRFD Bridge Design Specifications minimum requirements. Total settlement should be limited to approximately 1 inch, and differential settlement should be limited to approximately ½ inch. Objectionable soil types encountered at the bottom of footing excavations should be removed and replaced with structural fill. Excessively loose or soft areas that are encountered in the footings subgrade will require over-excavation and backfilling with structural fill. To minimize the effects of slight differential movement that may occur because of variations in the character of supporting soils and seasonal moisture content, Atlas recommends continuous footings be suitably reinforced to make them as rigid as possible. For frost protection, the bottom of external footings should be 30 inches below finished grade.

9. PAVEMENT DISCUSSION AND RECOMMENDATIONS

As required by Ada County Highway District (ACHD), Atlas has used a traffic index of 8 to determine the necessary pavement cross-section for Liberty Street. Atlas has made assumptions for traffic loading variables based on the character of the proposed construction. The Client should review these assumptions to make sure they reflect intended use and loading of pavements both now and in the future. Atlas collected a sample of near-surface soils for

Resistance Value (R-value) testing representative of soils to depths of 2 to 5 feet below existing ground surface. This sample, consisting of lean clay soils collected from boring 3, yielded a R-value of less than 5; therefore, a R-value of 4 was used in design. The following are minimum thickness requirements for assured pavement function. Depending on site conditions, additional work, e.g. soil preparation, may be required to support construction equipment. These have been listed within the Soft Subgrade Soils section. Results of the test are graphically depicted in the

Appendix.

9.1 Flexible Pavement Section

The Gravel Equivalent Method, as defined in Section 500 of the State of Idaho Department of

Transportation (ITD) Materials Manual, was used to develop the pavement sections. ACHD parameters for traffic index and substitution ratios, which were obtained from the ACHD Policy

Manual, were also used in the design. A calculation sheet provided in the Appendix indicate the soils constant, traffic loading, traffic projections, and material constants used to calculate the pavement sections. Atlas recommends that materials used in the construction of asphaltic concrete pavements meet the requirements of the ISPWC Standard Specification for Highway

Construction. Construction of the pavement section should be in accordance with these specifications and should adhere to guidelines recommended in the section on Construction

Considerations.

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Table 4 – Gravel Equivalent Method Flexible Pavement Specifications

Pavement Section Component Roadway Section

Liberty Street

Asphaltic Concrete 3.25 Inches

Crushed Aggregate Base 6.0 Inches

Structural Subbase 17.0 Inches

Compacted Subgrade See Pavement Subgrade

Preparation section 1It will be required for Atlas personnel to verify subgrade competency at the time of construction.

• Asphaltic Concrete: Asphalt mix design shall meet the requirements of ISPWC, Section

810. Materials shall be placed in accordance with ISPWC Standard Specifications for Highway Construction.

• Aggregate Base: Material complying with ISPWC Standards for Crushed Aggregate Materials.

• Structural Subbase: Granular structural fill material complying with the requirements detailed in the Structural Fill section of this report except that the maximum material diameter is no more than 2/3 the component thickness. Gradation and suitability requirements shall be per ISPWC Section 801, Table 1.

9.2 Pavement Subgrade Preparation

Fill materials associated with the existing roadway were encountered throughout the site. Atlas recommends that the roadway be cut to subgrade elevation. If fill materials remain after excavation, the exposed subgrade must be compacted to at least 95 percent of the maximum dry density as determined by ASTM D698. The existing fill materials are not suitable for use as either the base or subbase components of the recommended pavement section. Once final grades have been determined, Atlas is available to provide additional recommendations.

9.3 Common Pavement Section Construction Issues

The subgrade upon which above pavement sections are to be constructed must be properly stripped, compacted (if indicated), inspected, and proof-rolled. Proof rolling of subgrade soils should be accomplished using a heavy rubber-tired, fully loaded, tandem-axle dump truck or equivalent. Verification of subgrade competence by Atlas personnel at the time of construction is required. Fill materials on the site must demonstrate the indicated compaction prior to placing material in support of the pavement section. Atlas anticipated that pavement areas will be subjected to moderate traffic. Subgrade clayey and silty soils near and above optimum moisture contents may pump during compaction. Pumping or soft areas must be removed and replaced with structural fill.

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Fill material and aggregates in support of the pavement section must be compacted to no less than 95 percent of the maximum dry density as determined by ASTM D698 for flexible pavements and by ASTM D1557 for rigid pavements. If a material placed as a pavement section component cannot be tested by usual compaction testing methods, then compaction of that material must be approved by observed proof rolling. Minor deflections from proof rolling for flexible pavements are allowable. Deflections from proof rolling of rigid pavement support courses should not be visually detectable.

10. CONSTRUCTION CONSIDERATIONS

Recommendations in this report are based upon structural elements of the project being founded on competent, native poorly graded gravel with sand sediments or compacted structural fill.

Structural areas should be stripped to an elevation that exposes these soil types.

10.1 Earthwork

Excessively organic soils, deleterious materials, or disturbed soils generally undergo high volume changes when subjected to loads, which is detrimental to subgrade behavior in the area of pavements, floor slabs, structural fills, and foundations. No vegetation was noted on the site at the time of our investigation; however, mature trees, brush, and landscaping grasses were noted adjacent to the roadway. It is recommended that organic or disturbed soils, if encountered, be removed to depths of 1 foot (minimum), and wasted or stockpiled for later use. However, in areas where trees are/were present, deeper excavation depths should be anticipated. Stripping depths should be adjusted in the field to assure that the entire root zone or disturbed zone or topsoil are removed prior to placement and compaction of structural fill materials. Exact removal depths should be determined during grading operations by Atlas personnel, and should be based upon subgrade soil type, composition, and firmness or soil stability. If underground storage tanks, underground utilities, wells, or septic systems are discovered during construction activities, they must be decommissioned then removed or abandoned in accordance with governing Federal, State, and local agencies. Excavations developed as the result of such removal must be backfilled with structural fill materials as defined in the Structural Fill section.

Atlas should oversee subgrade conditions (i.e., moisture content) as well as placement and compaction of new fill (if required) after native soils are excavated to design grade.

Recommendations for structural fill presented in this report can be used to minimize volume changes and differential settlements that are detrimental to the behavior of footings, pavements, and floor slabs. Sufficient density tests should be performed to properly monitor compaction. For structural fill beneath building structures, one in-place density test per lift for every 5,000 square feet is recommended. In parking and driveway areas, this can be decreased to one test per lift for every 10,000 square feet.

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10.2 Dry Weather

If construction is to be conducted during dry seasonal conditions, many problems associated with soft soils may be avoided. However, some rutting of subgrade soils may be induced by shallow groundwater conditions related to springtime runoff or irrigation activities during late summer through early fall. Solutions to problems associated with soft subgrade soils are outlined in the

Soft Subgrade Soils section. Problems may also arise because of lack of moisture in native and fill soils at time of placement. This will require the addition of water to achieve near-optimum moisture levels. Low-cohesion soils exposed in excavations may become friable, increasing chances of sloughing or caving. Measures to control excessive dust should be considered as part of the overall health and safety management plan.

10.3 Wet Weather

If construction is to be conducted during wet seasonal conditions (commonly from mid-November through May), problems associated with soft soils must be considered as part of the construction plan. During this time of year, fine-grained soils such as silts and clays will become unstable with increased moisture content, and eventually deform or rut. Additionally, constant low temperatures reduce the possibility of drying soils to near optimum conditions.

10.4 Soft Subgrade Soils

Shallow fine-grained subgrade soils that are high in moisture content should be expected to pump and rut under construction traffic. Throughout construction, soft areas may develop after the existing asphalt is removed and heavy rubber tired equipment drives over the site. In addition, areas where significant cracking has occurred will likely have soft subgrade soils because of moisture infiltration and will be prone to pumping and rutting. During periods of wet weather, construction may become very difficult if not impossible. The following recommendations and options have been included for dealing with soft subgrade conditions:

• Track-mounted vehicles should be used to strip the subgrade of root matter and other deleterious debris and used to remove the existing asphalt and to perform any other necessary excavations. Heavy rubber-tired equipment should be prohibited from operating directly on the native subgrade and areas in which structural fill materials have been placed. Construction traffic should be restricted to designated roadways that do not cross, or cross on a limited basis, proposed roadway or parking areas.

• Soft areas can be over-excavated and replaced with granular structural fill.

• Construction roadways on soft subgrade soils should consist of a minimum 2-foot thickness of large cobbles of 4 to 6 inches in diameter with sufficient sand and fines to fill voids. Construction entrances should consist of a 6-inch thickness of clean, 2-inch minimum, angular drain-rock and must be a minimum of 10 feet wide and 30 to 50 feet long. During the construction process, top dressing of the entrance may be required for maintenance.

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• Scarification and aeration of subgrade soils can be employed to reduce the moisture content of wet subgrade soils. After stripping is complete, the exposed subgrade should be ripped or disked to a depth of 1½ feet and allowed to air dry for 2 to 4 weeks. Further disking should be performed on a weekly basis to aid the aeration process.

• Alternative soil stabilization methods include use of geotextiles, lime, and cement stabilization. Atlas is available to provide recommendations and guidelines at your request.

10.5 Frozen Subgrade Soils

Prior to placement of structural fill materials or foundation elements, frozen subgrade soils must either be allowed to thaw or be stripped to depths that expose non-frozen soils and wasted or stockpiled for later use. Stockpiled materials must be allowed to thaw and return to near-optimal conditions prior to use as structural fill.

The onsite, shallow clayey and silty soils are susceptible to frost heave during freezing temperatures. For exterior flatwork and other structural elements, adequate drainage away from subgrades is critical. Compaction and use of structural fill will also help to mitigate the potential for frost heave. Complete removal of frost susceptible soils for the full frost depth, followed by replacement with a non-frost susceptible structural fill, can also be used to mitigate the potential for frost heave. Atlas is available to provide further guidance/assistance upon request.

10.6 Structural Fill

Soils recommended for use as structural fill are those classified as GW, GP, SW, and SP in accordance with the Unified Soil Classification System (USCS) (ASTM D2487). Use of silty soils

(USCS designation of GM, SM, and ML) as structural fill may be acceptable. However, use of silty soils (GM, SM, and ML) as structural fill below footings is prohibited. These materials require very high moisture contents for compaction and require a long time to dry out if natural moisture contents are too high and may also be susceptible to frost heave under certain conditions.

Therefore, these materials can be quite difficult to work with as moisture content, lift thickness, and compactive effort becomes difficult to control. If silty soil is used for structural fill, lift thicknesses should not exceed 6 inches (loose), and fill material moisture must be closely monitored at both the working elevation and the elevations of materials already placed. Following placement, silty soils must be protected from degradation resulting from construction traffic or subsequent construction.

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Recommended granular structural fill materials, those classified as GW, GP, SW, and SP, should consist of a 6-inch minus select, clean, granular soil with no more than 50 percent oversize

(greater than ¾-inch) material and no more than 12 percent fines (passing No. 200 sieve). These fill materials should be placed in layers not to exceed 12 inches in loose thickness. Prior to placement of structural fill materials, surfaces must be prepared as outlined in the Construction

Considerations section. Structural fill material should be moisture-conditioned to achieve optimum moisture content prior to compaction. For structural fill below footings, areas of compacted backfill must extend outside the perimeter of the footings for a distance equal to the thickness of fill between the bottom of foundation and underlying soils, or 5 feet, whichever is less.

All fill materials must be monitored during placement and tested to confirm compaction requirements, outlined below, have been achieved.

Each layer of structural fill must be compacted, as outlined below:

• Below Structures and Rigid Pavements: A minimum of 95 percent of the maximum dry density as determined by ASTM D1557.

• Below Flexible Pavements: A minimum of 92 percent of the maximum dry density as determined by ASTM D1557 or 95 percent of the maximum dry density as determined by

ASTM D698.

The ASTM D1557 test method must be used for samples containing up to 40 percent oversize

(greater than ¾-inch) particles. If material contains more than 40 percent but less than 50 percent oversize particles, compaction of fill must be confirmed by proof rolling each lift with a 10-ton vibratory roller (or equivalent) until the maximum density has been achieved. Density testing must be performed after each proof rolling pass until the in-place density test results indicate a drop (or no increase) in the dry density, defined as maximum density or “break over” point. The number of required passes should be used as the requirements on the remainder of fill placement.

Material should contain sufficient fines to fill void spaces, and must not contain more than 50 percent oversize particles.

10.7 Backfill of Walls

Backfill materials must conform to the requirements of structural fill, as defined in this report. For wall heights greater than 2.5 feet, the maximum material size should not exceed 4 inches in diameter. Placing oversized material against rigid surfaces interferes with proper compaction, and can induce excessive point loads on walls. Backfill shall not commence until the wall has gained sufficient strength to resist placement and compaction forces. Further, retaining walls above 2.5 feet in height shall be backfilled in a manner that will limit the potential for damage from compaction methods and/or equipment. It is recommended that only small hand-operated compaction equipment be used for compaction of backfill within a horizontal distance equal to the height of the wall, measured from the back face of the wall.

Backfill should be compacted in accordance with the specifications for structural fill, except in those areas where it is determined that future settlement is not a concern, such as planter areas.

In nonstructural areas, backfill must be compacted to a firm and unyielding condition.

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10.8 Excavations

Shallow excavations that do not exceed 4 feet in depth may be constructed with side slopes approaching vertical. Below this depth, it is recommended that slopes be constructed in accordance with Occupational Safety and Health Administration (OSHA) regulations, Section

1926, Subpart P. Based on these regulations, on-site soils are classified as type “C” soil, and as such, excavations within these soils should be constructed at a maximum slope of 1½ feet horizontal to 1 foot vertical (1½:1) for excavations up to 20 feet in height. Excavations in excess of 20 feet will require additional analysis. Note that these slope angles are considered stable for short-term conditions only, and will not be stable for long-term conditions.

During the subsurface exploration, boring sidewalls generally exhibited little indication of collapse;

however, sloughing of native granular sediments from boring sidewalls was observed. For deep excavations, native granular sediments cannot be expected to remain in position. These materials are prone to failure and may collapse, thereby undermining upper soil layers. This is especially true when excavations approach depths near the water table. Care must be taken to ensure that excavations are properly backfilled in accordance with procedures outlined in this report.

10.9 Groundwater Control

Groundwater was not encountered during the investigation and is anticipated to be below the depth of most construction. Excavations below the water table will require a dewatering program.

Dewatering will be required prior to placement of fill materials. Placement of concrete can be accomplished through water by the use of a treme. It may be possible to discharge dewatering effluent to remote portions of the site, to a sump, or to a pit. This will essentially recycle effluent, thus eliminating the need to enter into agreements with local drainage authorities. Should the scope of the proposed project change, Atlas should be contacted to provide more detailed groundwater control measures.

Special precautions may be required for control of surface runoff and subsurface seepage. It is recommended that runoff be directed away from open excavations. Silty and clayey soils may become soft and pump if subjected to excessive traffic during time of surface runoff. Ponded water in construction areas should be drained through methods such as trenching, sloping, crowning grades, nightly smooth drum rolling, or installing a French drain system. Additionally, temporary or permanent driveway sections should be constructed if extended wet weather is forecasted.

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11. GENERAL COMMENTS

Based on the subsurface conditions encountered during this investigation and available information regarding the proposed structure and roadways, the site is adequate for the planned construction. When plans and specifications are complete, and if significant changes are made in the character or location of the proposed structure, consultation with Atlas must be arranged as supplementary recommendations may be required. Suitability of subgrade soils and compaction of structural fill materials must be verified by Atlas personnel prior to placement of structural elements. Additionally, monitoring and testing should be performed to verify that suitable materials are used for structural fill and that proper placement and compaction techniques are utilized.

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12. REFERENCES

Ada County Highway District (ACHD) (2017). Ada County Highway District Policy Manual (August 2017).

[Online] Available: <http://www.achdidaho.org/AboutACHD/PolicyManual.aspx> (2021).

American Association of State Highway and Transportation Officials (AASHTO) (2020). LRFD Bridge

Design Specifications. Washington D.C.: AASHTO

American Society of Civil Engineers (2021). ASCE 7 Hazards Tool: Web Interface [Online] Available:

<https://asce7hazardtool.online/> (2021).

American Society for Testing and Materials (ASTM) (2017). Standard Test Method for Materials Finer than

75-μm (No. 200) Sieve in Mineral Aggregates by Washing: ASTM C117. West Conshohocken, PA: ASTM.

American Society for Testing and Materials (ASTM) (2014). Standard Test Method for Sieve Analysis of

Fine and Coarse Aggregates: ASTM C136. West Conshohocken, PA: ASTM.

American Society for Testing and Materials (ASTM) (2012). Standard Test Methods for Laboratory

Compaction Characteristics of Soil Using Standard Effort: ASTM D698. West Conshohocken, PA: ASTM.

American Society for Testing and Materials (ASTM) (2012). Standard Test Methods for Laboratory

Compaction Characteristics of Soil Using Modified Effort: ASTM D1557. West Conshohocken, PA: ASTM.

American Society for Testing and Materials (ASTM) (2013). Standard Test Methods for Resistance Value

(R-Value) and Expansion Pressure of Compacted Soils: ASTM D2844. West Conshohocken, PA: ASTM.

American Society for Testing and Materials (ASTM) (2017). Standard Practice for Classification of Soils for

Engineering Purposes (Unified Soil Classification System): ASTM D2487. West Conshohocken, PA: ASTM.

American Society for Testing and Materials (ASTM) (2017). Standard Test Methods for Liquid Limit, Plastic

Limit, and Plasticity Index of Soils: ASTM D4318. West Conshohocken, PA: ASTM.

Desert Research Institute. Western Regional Climate Center. [Online] Available: <http://www.wrcc.dri.edu/>

(2021).

International Building Code Council (2018). International Building Code, 2018. Country Club Hills, IL:

Author.

Local Highway Technical Assistance Council (LHTAC) (2017). Idaho Standards for Public Works

Construction, 2017. Boise, ID: Author.

Othberg, K. L. and Stanford, L. A., Idaho Geologic Society (1993). Geologic Map of the Boise Valley and

Adjoining Area, Western Snake River Plain, Idaho. (scale 1:100,000). Boise, ID: Joslyn and Morris.

U.S. Department of Labor, Occupational Safety and Health Administration. CFR 29, Part 1926, Subpart P:

Safety and Health Regulations for Construction, Excavations (1986). [Online] Available: <www.osha.gov>

(2021).

https://asce7hazardtool.online/

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WARRANTY AND LIMITING CONDITIONS

Atlas warrants that findings and conclusions contained herein have been formulated in accordance with generally accepted professional engineering practice in the fields of foundation engineering, soil mechanics, and engineering geology only for the site and project described in this report. These engineering methods have been developed to provide the client with information regarding apparent or potential engineering conditions relating to the site within the scope cited above and are necessarily limited to conditions observed at the time of the site visit and research. Field observations and research reported herein are considered sufficient in detail and scope to form a reasonable basis for the purposes cited above.

Exclusive Use

This report was prepared for exclusive use of the property owner(s), at the time of the report, and their retained design consultants (“Client”). Conclusions and recommendations presented in this report are based on the agreed-upon scope of work outlined in this report together with the Contract for Professional Services between…