Geotechnical_Engineering_Report_22_April_2019_0002.pdf

Environmental Services Geotechnical Engineering Construction Materials Testing Special Inspections

2791 S Victory View Way Boise, ID 83709 (208) 376-4748 Fax (208) 322-6515 www.mti-id.com mti@mti-id.com

GEOTECHNICAL ENGINEERING REPORT

of

USGS Building 4 Reconstruction 230 North Collins Road

Boise, ID

Prepared for:

Slichter Ugrin Architecture 415 South 13th Street Boise, ID 83702

MTI File Number B190549g http://www.mti-id.com/ mailto:mti@mti-id.com

22 April 2019 Page # 2 of 28 b190549g_geotech

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Copyright © 2019 Materials

Testing & Inspection

TABLE OF CONTENTS

INTRODUCTION

Project Description

Authorization

Purpose

Scope of Investigation

Warranty and Limiting Conditions

SITE DESCRIPTION

Site Access

Regional Geology

General Site Characteristics

Regional Site Climatology and Geochemistry

SEISMIC SITE EVALUATION

Geoseismic Setting

Seismic Design Parameter Values

SOILS EXPLORATION

Exploration and Sampling Procedures

Laboratory Testing Program

Soil and Sediment Profile

Volatile Organic Scan

SITE HYDROLOGY

Groundwater

Soil Infiltration Rates

FOUNDATION, SLAB, AND PAVEMENT DISCUSSION AND RECOMMENDATIONS

Foundation Design Recommendations

Floor Slab-on-Grade

Recommended Pavement Sections

Flexible Pavement Sections

Pavement Subgrade Preparation

Common Pavement Section Construction Issues

CONSTRUCTION CONSIDERATIONS

Earthwork

Dry Weather

Wet Weather

Frozen Subgrade Soils

Structural Fill

Backfill of Walls

Excavations

Groundwater Control

GENERAL COMMENTS

REFERENCES

APPENDICES

Acronym List

Geotechnical General Notes

Geotechnical Investigation Test Pit Log

AASHTO Pavement Thickness Design Procedures

Plate 1: Vicinity Map

Plate 2A: Site Map

Plate 2B: Site Map

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INTRODUCTION

This report presents results of a geotechnical investigation and analysis in support of data utilized in design of structures as defined in the 2015 International Building Code (IBC). 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 structure 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.

Project Description

The proposed development is in the eastern portion of the City of Boise, Ada County, ID, and occupies a portion of the N½NW¼ of Section 11, Township 3 North, Range 2 East, Boise Meridian. This project will consist of demolition and construction of Building 4 with a new single-story building and development of a 60-stall asphalt parking lot to the south of Building 7 where a gravel storage yard currently exists. Total settlements are limited to 1 inch. Loads of up to 4,000 pounds per lineal foot for wall footings, and column loads of up to 50,000 pounds were assumed for settlement calculations. Additionally, assumptions have been made for traffic loading of pavements. MTI has not been informed of the proposed grading plan.

Authorization

Authorization to perform this exploration and analysis was given in the form of a written authorization to proceed from Mr. Jeff Geibel of Slichter Ugrin Architecture to Nick Stevens of Materials Testing and Inspection

(MTI), on 28 March 2019. Said authorization is subject to terms, conditions, and limitations described in the

Professional Services Contract entered into between Slichter Ugrin Architecture and MTI. Our scope of services for the proposed development has been provided in our proposal dated 27 March 2019 and repeated below.

Purpose

The purpose of this Geotechnical Engineering Report is to determine various soil profile components and their engineering characteristics for use by either design engineers or architects in:

Preparing or verifying suitability of foundation design and placement

Preparing site drainage designs

Indicating issues pertaining to earthwork construction

Preparing light and heavy duty pavement section design requirements

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Scope of Investigation

The scope of this investigation included review of geologic literature and existing available geotechnical studies of the area, 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 foundation materials.

Warranty and Limiting Conditions

MTI 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 the Client and Materials Testing and Inspection (“Consultant”). Use or misuse of this report, or reliance upon findings hereof, by parties other than the Client is at their own risk. Neither Client nor Consultant make representation of warranty to such other parties as to accuracy or completeness of this report or suitability of its use by such other parties for purposes whatsoever, known or unknown, to Client or Consultant. Neither Client nor Consultant shall have liability to indemnify or hold harmless third parties for losses incurred by actual or purported use or misuse of this report. No other warranties are implied or expressed.

Report Recommendations are Limited and Subject to Misinterpretation There is a distinct possibility that conditions may exist that could not be identified within the scope of the investigation or that were not apparent during our site investigation. Findings of this report are limited to data collected from noted explorations advanced and do not account for unidentified fill zones, unsuitable soil types or conditions, and variability in soil moisture and groundwater conditions. To avoid possible misinterpretations of findings, conclusions, and implications of this report, MTI should be retained to explain the report contents to other design professionals as well as construction professionals.

Since actual subsurface conditions on the site can only be verified by earthwork, note that construction recommendations are based on general assumptions from selective observations and selective field exploratory sampling. Upon commencement of construction, such conditions may be identified that require corrective actions, and these required corrective actions may impact the project budget. Therefore, construction recommendations in this report should be considered preliminary, and MTI should be retained to observe actual subsurface conditions during earthwork construction activities to provide additional construction recommendations as needed.

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Since geotechnical reports are subject to misinterpretation, do not separate the soil logs from the report. Rather, provide a copy of, or authorize for their use, the complete report to other design professionals or contractors.

Locations of exploratory sites referenced within this report should be considered approximate locations only.

For more accurate locations, services of a professional land surveyor are recommended.

This report is also limited to information available at the time it was prepared. In the event additional information is provided to MTI following publication of our report, it will be forwarded to the client for evaluation in the form received.

Environmental Concerns Comments in this report concerning either onsite conditions or observations, including soil appearances and odors, are provided as general information. These comments are not intended to describe, quantify, or evaluate environmental concerns or situations. Since personnel, skills, procedures, standards, and equipment differ, a geotechnical investigation report is not intended to substitute for a geoenvironmental investigation or a Phase

II/III Environmental Site Assessment. If environmental services are needed, MTI can provide, via a separate contract, those personnel who are trained to investigate and delineate soil and water contamination.

SITE DESCRIPTION

Site Access

Access to the site may be gained via Interstate 184 to Myrtle Street. Proceed southeast on Myrtle Street approximately 1.0 mile to its intersection with 3rd Street. From this intersection, proceed northeast 0.6 mile to

Washington Street. Turn southeast onto Washington Street and proceed 230 feet to Robbins Road. Continue

0.2 mile on Robbins Road to its intersection with Collins Road. The site is located to the east of this intersection and extends south along Collins Road to Garrison Road. Presently the site exists as a multiple building complex with associated paved parking lots and gravel storage yards. The location is depicted on site map plates included in the Appendix.

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 “Sandy Alluvium of Side-Stream Valleys and Gulches” as mapped by Othberg and Stanford (1993). Locally, these deposits are composed of medium to coarse sand interbedded with silty fine sand and silt and are mostly derived from weathered granite and reworked Tertiary sediments of the Boise Foothills. The thickness of this unit is variable. Because of the relative youthfulness of these deposits they contain only minor pedogenic clay and calcium carbonate.

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General Site Characteristics

This proposed development consists of approximately 11.5 acres of relatively flat and level terrain. Throughout the majority of the site, surficial materials consist of poorly graded gravel with silt and sand fills. Vegetation is limited to landscaping beds and consists of mature trees, bushes, and grasses.

Regional drainage is south and west toward the Boise River. Stormwater drainage for the site is achieved by percolation through surficial soils and runoff from impervious surfaces into existing infiltration facilities. From the northeast, intermittent off-site stormwater may drain onto the project site during flood events. Stormwater drainage collection and retention systems in the form of infiltration ponds exist along the western portions of the property. To the east of the project site are a series of detention basins that serve as flood control for numerous natural streams coming from the surrounding foothills.

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 -

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.

SEISMIC SITE EVALUATION

Geoseismic Setting

Soils on site are classed as Site Class D in accordance with Chapter 20 of the American Society of Civil

Engineers (ASCE) publication ASCE/SEI 7-10. Structures constructed on this site should be designed per IBC requirements for such a seismic classification. Our investigation did not reveal hazards resulting from potential earthquake motions including: liquefaction and surface rupture caused by faulting or lateral spreading.

Incidence and anticipated acceleration of seismic activity in the area is low.

Seismic Design Parameter Values

The United States Geological Survey National Seismic Hazard Maps (2008), includes a peak ground acceleration map. The map for 2% probability of exceedance in 50 years in the Western United States in standard gravity (g) indicates that a peak ground acceleration of 0.21 is appropriate for the project site based on a Site Class D.

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The following section provides an assessment of the earthquake-induced earthquake loads for the site based on the Risk-Targeted Maximum Considered Earthquake (MCER). The MCER spectral response acceleration for short periods, SMS, and at 1-second period, SM1, are adjusted for site class effects as required by the 2015 IBC.

Design spectral response acceleration parameters as presented in the 2015 IBC are defined as a 5% damped design spectral response acceleration at short periods, SDS, and at 1-second period, SD1.

The USGS National Seismic Hazards Mapping Project includes a program that provides values for ground motion at a selected site based on the same data that were used to prepare the USGS ground motion maps. The maps were developed using attenuation relationships for soft rock sites; the source model, assumptions, and empirical relationships used in preparation of the maps are described in Petersen and others (1996).

Seismic Design Values

Seismic Design Parameter Design Value

Site Class D “Stiff Soil”

Ss 0.315 (g)

S1 0.106 (g)

Fa 1.548

Fv 2.374

SMs 0.487

SM1 0.253

SDS 0.325

SD1 0.169

SOILS EXPLORATION

Exploration and Sampling Procedures

Field exploration conducted to determine engineering characteristics of subsurface materials included a reconnaissance of the project site and investigation by test pit. Test pit sites were located in the field by means of a Global Positioning System (GPS) device and are reportedly accurate to within ten feet. Upon completion of investigation, each test pit was backfilled with loose excavated materials. Re-excavation and compaction of these test pit areas are required prior to construction of overlying structures.

In addition, samples were obtained from representative soil strata encountered. Samples obtained have been visually classified in the field by professional staff, identified according to test pit 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. MTI recommends that these logs not be used to estimate fill material quantities.

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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 on the accompanying logs located in the Appendix. The laboratory testing program for this report included:

Atterberg Limits Testing – ASTM D4318 and Grain Size Analysis – ASTM C117/C136.

Soil and Sediment Profile

The profile below represents a generalized interpretation for the project site. Note that on site soils strata, encountered between test pit 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 Sandy

Alluvium of Side-Stream Valleys and Gulches. Poorly graded gravel with silt and sand fill materials were encountered at ground surface. These materials varied from brown to dark brown and generally exhibited moisture contents of dry to slightly moist. Fills were noted to be loose to medium dense. Fine to coarse-grained sand, fine to coarse gravel, and 12-inch minus cobbles were present throughout. Silty sand fill materials were encountered below poorly graded gravel fills. These silty sand fill materials were generally dark brown, dry to slightly moist, loose to medium dense, with fine to coarse-grained sand. Some intermittent fine to coarse gravel was noted throughout these fills.

Silty sand sediments were encountered beneath fill materials. These sediments were light brown, dry to slightly moist, and medium dense. Fine to coarse-grained sand were present throughout this horizon. At depth, poorly graded sand with silt and gravel sediments were exposed. Poorly graded sands were tan, dry to slightly moist, and medium dense to dense. Fine to coarse-grained sand, fine to coarse gravel, and 8-inch minus cobbles were noted throughout. Excessive caving was noted throughout this horizon during excavation.

Competency of test pit sidewalls varied little across the site. However, moisture contents will also affect wall competency with saturated soils having a tendency to readily slough when under load and unsupported.

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.

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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.

Groundwater

During this field investigation, groundwater was not encountered in test pits advanced to a maximum depth of

16.2 feet bgs. Soil moistures in the test pits were generally dry to slightly moist throughout. In the vicinity of the project site, groundwater levels are controlled in large part by seasonal runoff. Maximum groundwater elevations likely occur during late spring to early summer runoff season.

During previous investigations performed in May 2013, February 2014, March 2017, and April 2018 within approximately 0.1-mile to the north, south, and east of the project site, no evidence of groundwater was noted within numerous test pits advanced to depths as great as 16.0 feet bgs.

Based on evidence of this investigation and background knowledge of the area, MTI estimates groundwater depths to remain greater than approximately 15 feet bgs throughout the year. This depth can be confirmed through long-term groundwater monitoring.

Soil Infiltration Rates

Soil permeability, which is a measure of the ability of a soil to transmit a fluid, was not tested in the field. Given the absence of direct measurements, for this report an estimation of infiltration is presented using generally recognized values for each soil type and gradation. Of soils comprising the generalized soil profile for this study, silty sand sediments usually display rates of 4 to 8 inches per hour. Poorly graded sand and gravel sediments typically exhibit infiltration values in excess of 12 inches per hour.

It is recommended that infiltration facilities constructed on the site be extended into native poorly graded sand with silt and gravel sediments. Excavation depths of approximately 3.9 to 4.9 feet bgs should be anticipated to expose these poorly graded sand with silt and gravel sediments. Because of the high soil permeability, ASTM

C33 filter sand, or equivalent, should be incorporated into design of infiltration facilities. Actual infiltration rates should be confirmed at the time of construction.

FOUNDATION, SLAB, AND PAVEMENT DISCUSSION AND RECOMMENDATIONS

Various foundation types have been considered for support of the proposed structure. 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.

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Considering subsurface conditions and the proposed construction, it is recommended that the structure be founded upon conventional spread footings and continuous wall footings. Total settlements should not exceed

1 inch if the following design and construction recommendations are observed.

Foundation Design Recommendations

Based on data obtained from the site and test results from various laboratory tests performed, MTI recommends the following guidelines for the net allowable soil bearing capacity:

Soil Bearing Capacity

Footing Depth

ASTM D1557

Subgrade Compaction

Net Allowable

Soil Bearing Capacity

Footings must bear on competent, undisturbed, native silty sand sediments, poorly graded sand with silt and gravel sediments, or compacted structural fill. Existing fill materials and demolition debris must be completely removed from below foundation elements.1 Excavation depths ranging from roughly

2.1 to 2.8 feet bgs should be anticipated to expose proper bearing soils.

Not Required for Native

Soil

95% for Structural Fill

2,000 lbs/ft2

A ⅓ increase is allowable for short-term loading, which is defined by seismic events or designed wind speeds.

1It will be required for MTI personnel to verify the bearing soil suitability for each structure at the time of construction.

The following sliding frictional coefficient values should be used: 1) 0.35 for footings bearing on native silty sand sediments and 2) 0.45 for footings bearing on native poorly graded sand with silt and gravel sediments and granular structural fill. A passive lateral earth pressure of 373 pounds per square foot per foot (psf/ft) should be used for silty sand sediments. For compacted poorly graded sand with silt and gravel sediments, a passive lateral earth pressure of 454 psf/ft should be used. A passive lateral earth pressure of 496 pounds per square foot per foot (psf/ft) should be used for compacted sandy gravel fill.

Footings should be proportioned to meet either the stated soil bearing capacity or the 2015 IBC 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, MTI recommends continuous footings be suitably reinforced to make them as rigid as possible. For frost protection, the bottom of external footings should be 24 inches below finished grade.

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Floor Slab-on-Grade

Uncontrolled fill was encountered throughout the site. MTI recommends that these fill materials be excavated to a sufficient depth to expose competent, native soils or to a minimum depth of 1½ feet below finished subgrade. All debris associated with demolishment of the existing building must be completed removed. If fill materials remain after over-excavation, the exposed subgrade must be compacted to at least 95 percent of the maximum dry density as determined by ASTM D1557. MTI personnel must be present during excavation to identify these materials.

Organic, loose, or obviously compressive materials must be removed prior to placement of concrete floors or floor-supporting fill. In addition, the remaining subgrade should be treated in accordance with guidelines presented in the Earthwork section. Areas of excessive yielding should be excavated and backfilled with structural fill. Fill used to increase the elevation of the floor slab should meet requirements detailed in the

Structural Fill section. Fill materials must be compacted to a minimum 95 percent of the maximum dry density as determined by ASTM D1557.

A free-draining granular mat (drainage fill course) should be provided below slabs-on-grade. This should be a minimum of 4 inches in thickness and properly compacted. The mat should consist of a sand and gravel mixture, complying with Idaho Standards for Public Works Construction (ISPWC) specifications for ¾-inch (Type 1) crushed aggregate. The granular mat should be compacted to no less than 95 percent of the maximum dry density as determined by ASTM D1557. A moisture-retarder should be placed beneath floor slabs to minimize potential ground moisture effects on moisture-sensitive floor coverings. The moisture-retarder should be at least 15-mil in thickness and have a permeance of less than 0.01 US perms as determined by ASTM E96.

Placement of the moisture-retarder will require special consideration with regard to effects on the slab-on-grade and should adhere to recommendations outlined in the ACI 302.1R and ASTM E1745 publications. Upon request, MTI can provide further consultation regarding installation.

Recommended Pavement Sections

MTI has made assumptions for traffic loading variables based on the character of the proposed construction.

The Client shall review and understand these assumptions to make sure they reflect intended use and loading of pavements both now and in the future. Based on experience with soils in the region, a subgrade California

Bearing Ratio (CBR) value of 8 has been assumed for near-surface silty sand sediments on site. 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.

Flexible Pavement Sections

The American Association of State Highway and Transportation Officials (AASHTO) design method has been used to calculate the following pavement sections. Calculation sheets provided in the Appendix indicate the soils constant, traffic loading, traffic projections, and material constants used to calculate the pavement sections.

MTI recommends that materials used in the construction of asphaltic concrete pavements meet 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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AASHTO Flexible Pavement Specifications

Pavement Section Component1 Driveways and Parking

No Truck Access

Driveways and Parking

Truck Access

Asphaltic Concrete 2.5 Inches 3.0 Inches

Crushed Aggregate Base 4.0 Inches 4.0 Inches

Structural Subbase 6.0 Inches 6.0 Inches

Compacted Subgrade See Pavement Subgrade

Preparation Section

See Pavement Subgrade

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

Asphaltic Concrete: Asphalt mix design shall meet the requirements of ISPWC, Section 810 Class III plant mix. 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.

Pavement Subgrade Preparation

Uncontrolled fill was encountered in portions of the site. MTI recommends that these fill materials be excavated to a sufficient depth to expose competent, native soils or to a minimum depth of 1½ feet below finished subgrade. If fill materials remain after over-excavation, the exposed subgrade must be compacted to at least 95 percent of the maximum dry density as determined by ASTM D698 for flexible pavements. MTI personnel must be present during excavation to identify these materials.

Common Pavement Section Construction Issues

The subgrade upon which above pavement sections are to be constructed must be properly stripped, compacted, 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 MTI 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. MTI anticipated that pavement areas will be subjected to light to moderate traffic. MTI does not anticipate pumping material to become evident during compaction, but subgrade clays and silts near and above optimum moisture contents may tend to pump. 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.

MTI recommends that rigid concrete pavement be provided for heavy garbage receptacles. This will eliminate damage caused by the considerable loading transferred through the small steel wheels onto asphaltic concrete.

Rigid concrete pavement should consist of Portland Cement Concrete Pavement (PCCP) generally adhering to

ITD specifications for Urban Concrete. PCCP should be 6 inches thick on a 4-inch drainage fill course (see

Floor Slab-on-Grade section), and should be reinforced with welded wire fabric. Control joints must be on

12-foot centers or less.

CONSTRUCTION CONSIDERATIONS

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

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

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. Mature trees, brush, and thick grasses with associated root systems were noted at the time of our investigation. 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 MTI 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.

MTI 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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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. 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.

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.

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 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. MTI is available to provide further guidance/assistance upon request.

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.

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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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, test pit sidewalls generally exhibited little indication of collapse; however, sloughing of fill materials and native granular sediments from test pit 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.

Groundwater Control

Groundwater was not encountered during the investigation and is anticipated to be below the depth of most construction. Should the scope of the proposed project change, MTI 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 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.

GENERAL COMMENTS

Based on the subsurface conditions encountered during this investigation and available information regarding the proposed structure, 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 MTI must be arranged as supplementary recommendations may be required. Suitability of subgrade soils and compaction of structural fill materials must be verified by MTI 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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REFERENCES

American Association of State Highway and Transportation Officials (AASHTO) (1993). AASHTO Guide for

Design of Pavement Structures 1993. Washington D.C.: AASHTO.

American Concrete Institute (ACI) (2015). Guide for Concrete Floor and Slab Construction: ACI 302.1R.

Farmington Hills, MI: ACI.

American Society of Civil Engineers (ASCE) (2013). Minimum Design Loads for Buildings and Other

Structures: ASCE/SEI 7-10. Reston, VA: ASCE.

American Society for Testing and Materials (ASTM) (2013). 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) (2011). 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) (2010). Standard Test Methods for Liquid Limit, Plastic

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

American Society for Testing and Materials (ASTM) (2011). Standard Specification for Plastic Water Vapor

Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs: ASTM E1745. West

Conshohocken, PA: ASTM.

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

(2019).

Idaho Geological Survey. (2004). Earthquakes in Idaho 1872-2000. [Online] Available:

<http://129.101.67.129/livemaps/IdahoEpicenters/index.html> (2019).

Idaho Geological Survey. (2003). Miocene and Younger Faults in Idaho. [Online] Available:

<http://www.idahogeology.org/pdf/Maps_(M)/m-08-m.pdf> (2019).

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International Building Code Council (2015). International Building Code, 2015. Country Club Hills, IL: Author.

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

Construction, 2017. Boise, ID: Author.

National Oceanic and Atmospheric Administration. (2016). U.S. Earthquake Intensity Database: Web Interface.

[Online] Available: <https://www.ngdc.noaa.gov/nndc/struts/form?t=101650&s=35&d=35> (2019).

Othberg, K. L. and Stanford, L. A., Idaho Geologic Society (1992). 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>

(2019).

U.S. Geological Survey. (2016). Interactive Fault Map: Web Interface. [Online] Available:

http://earthquake.usgs.gov/hazards/qfaults/map/#qfaults> (2019).

U.S. Geological Survey (2019). National Water Information System: Web Interface. [Online] Available:

<http://waterdata.usgs.gov/nwis> (2019).

U.S. Geological Survey. (2009). Seismicity of Idaho 1990-2006. [Online] Available:

<http://earthquake.usgs.gov/earthquakes/states/Idaho/seismicity.php> (2019).

U.S. Geological Survey. (2011). U.S. Seismic Design Maps: Web Interface. [Online] Available:

<https://earthquake.usgs.gov/designmaps/us/application.php> (2019).

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APPENDICES

ACRONYM LIST

AASHTO: American Association of State Highway and Transportation Officials

ACI American Concrete Institute

ASCE American Society of Civil Engineers

ASTM: American Society for Testing and Materials bgs: below ground surface

CBR: California Bearing Ratio

D: natural dry unit weight, pcf

ESAL Equivalent Single Axle Load

GS: grab sample

IBC: International Building Code

IDEQ Idaho Department of Environmental Quality

ISPWC: Idaho Standards for Public Works Construction

ITD: Idaho Transportation Department

LL: Liquid Limit

M: water content

MSL: mean sea level

N: Standard "N" penetration: blows per foot, Standard Penetration Test

NP: nonplastic

OSHA Occupational Safety and Health Administration

PCCP: Portland Cement Concrete Pavement

PERM: vapor permeability

PI: Plasticity Index

PID: photoionization detector

PVC: polyvinyl chloride

Qc: cone penetrometer value, unconfined compressive strength, psi

Qp: Penetrometer…