GeoTech Report used for T111.pdf

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GEOTECHNICAL ENGINEERING REPORT

of

Boise VA Primary Care and Mental Health Building

500 West Fort Street Boise, ID

Prepared for:

Spees Design Building 1414 Fourth Avenue Seattle, WA 98101

MTI File Number B201282g http://www.mti-id.com/ mailto:mti@mti-id.com

11 November 2020 Page # 1 of 35 b201282g_geotech

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Mr. Michael Hubbard Spees Design Building 1414 Fourth Avenue Seattle, WA 98101

(360) 223-8994

Re: Geotechnical Engineering Report Boise VA Primary Care and Mental Health Building 500 West Fort Street Boise, ID

Dear Mr. Hubbard:

In compliance with your instructions, MTI has conducted a soils exploration and foundation evaluation for the above referenced development. Fieldwork for this investigation was conducted on 14 August 2020 and from 3 to 4 September 2020. 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. MTI would be pleased to continue our role as geotechnical engineers during project implementation.

Additionally, MTI can provide materials testing and special inspection services during construction of this project. If you will advise us of the appropriate time to discuss these engineering services, we will meet with you at your convenience.

MTI appreciates this opportunity to be of service to you and looks forward to working with you in the future.

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

Respectfully Submitted, Materials Testing & Inspection

Nick Stevens, G.I.T. Reviewed by: Elizabeth Brown, P.E.

Staff Geologist Geotechnical Services Manager

Reviewed by: Monica Saculles, P.E.

Senior Geotechnical Engineer cc: Matt King, Blue NW (PDF Copy); Elena Cortez, Spees Design Building (PDF Copy); Corey Harelson, KPFF (PDF Copy) http://www.mti-id.com/ mailto:mti@mti-id.com SacullesM New Stamp

SacullesM Typewritten Text 11-11-20

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TABLE OF CONTENTS

INTRODUCTION

Project Description Authorization Purpose Scope of Investigation

SITE DESCRIPTION

Site Access Regional Geology General Site Characteristics Regional Site Climatology and Geochemistry

SEISMIC SITE EVALUATION

Geoseismic Setting Seismic Design Parameter Values

GEOLOGIC HAZARD ASSESSMENT

Regional Faults Historical Seismicity Seismically Induced Surface Rupture, Settlements, and Lateral Spreading Liquefaction Collapsible Soils

SOILS EXPLORATION

Exploration and Sampling Procedures Laboratory Testing Program Soil and Sediment Profile Volatile Organic Scan

SITE HYDROLOGY

Groundwater Infiltration Testing

FOUNDATION, SLAB, AND PAVEMENT DISCUSSION AND RECOMMENDATIONS

Foundation Design Recommendations Floor Slab-on-Grade Recommended Pavement Sections Flexible Pavement Sections Common Pavement Section Construction Issues

CONSTRUCTION CONSIDERATIONS

Earthwork Dry Weather Wet Weather Soft Subgrade Soils Frozen Subgrade Soils Structural Fill Backfill of Walls

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Excavations Groundwater Control

GENERAL COMMENTS

REFERENCES

APPENDICES

Warranty and Limiting Conditions Plate 1: Vicinity Map Plate 2: Site Map Plate 3: Foundation Cross Section Geotechnical Investigation Boring Log Geotechnical Investigation Hand Boring Log Geotechnical General Notes AASHTO Pavement Thickness Design Procedures Important Information About This Geotechnical Engineering Report

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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 northeastern portion of the City of Boise, Ada County, ID, and occupies a portion of the SW¼SW¼ of Section 2, Township 3 North, Range 2 East, Boise Meridian. This project will consist of construction of a two-story structure that is approximately 22,000 square-feet in size. Total settlements are limited to 1 inch. Loads of up to 6,000 pounds per lineal foot for wall footings, and column loads of up to 100,000 pounds were assumed for settlement calculations. Since actual loading information and details for the structure are unknown at this time, foundation recommendations provided in this report should be considered preliminary. Final recommendations can be provided once loading information becomes available. Additionally, assumptions have been made for traffic loading of pavements. Retaining walls are not anticipated as part of the project. 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. Michael Hubbard of Spees Design Building to Elizabeth Brown of Materials Testing and Inspection (MTI), on 10 August 2020. Said authorization is subject to terms, conditions, and limitations described in the Professional Services Contract entered into between Spees Design Building and MTI. Our scope of services for the proposed development has been provided in our proposal dated 15 July 2020 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.

SITE DESCRIPTION

Site Access

Access to the site may be gained via Interstate 184 to Myrtle Street. Proceed southeast on Myrtle Street approximately 0.6 mile to its intersection with 5th Street. From this intersection, proceed northeast for 0.7 mile to Fort Street. Continue northeast onto VA Hospital Loop. The site occupies the north corner of the existing park area along VA Hospital Loop. 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.

General Site Characteristics

The Boise VA Campus is approximately 52.5 acres in size. The site to be developed is approximately 1.5 acres in size and located in the north central portion of the campus. Currently, the proposed location of development consists of a portion of an approximately 9-acre park in the center of the campus. The park is generally flat and level, with mature trees located sporadically along the perimeter. A majority of the site is covered with grassy landscaping. A concrete walking path borders the southern edge of the proposed building site. Surrounding the proposed building site are existing VA buildings. A majority of the buildings are masonry in construction and multi-level. However, some structures are single-story and wood construction.

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Regional drainage is south and west toward the Boise River. Stormwater drainage for the site is achieved by percolation through surficial soils. The site is situated so that it is unlikely that it will receive any stormwater drainage from off-site sources. Stormwater drainage collection and retention systems are not in place on the project site, but are installed along the VA Hospital Loop in the form of curb, gutter, and drop inlet.

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.

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. See the Geologic Hazards Assessment section for discussion of hazards resulting from potential earthquake motions. 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 0.192 is appropriate for the project site based on a Site Class D.

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).

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Seismic Design Values Seismic Design Parameter Design Value

Site Class D “Stiff Soil” Ss 0.315 (g) S1 0.107 (g) Fa 1.548 Fv 2.374

SMs 0.487

SM1 0.253

SDS 0.325

SD1 0.169

GEOLOGIC HAZARD ASSESSMENT

This section provides an assessment of the geologic hazards for the site, including the potential for surface fault rupture, liquefaction, seismically induced settlements, lateral spreading, and collapsible soils.

The hazard evaluation methodology involved one or two steps. First, the potential for occurrence of each type of geologic phenomenon is assessed. If there is a potential for a phenomenon to occur, the second step is to assess whether the phenomenon likely will result in a significant hazard for designated structures. For this evaluation, a significant hazard is defined as one that results in structural damage and threatens life-safety.

Regional Faults

The subject site is located along the margin of the Boise Foothills. Per a map titled Geologic Map of the Boise Valley and Adjoining Area, Western Snake River Plain, Idaho (Othberg and Stanford, 1993), numerous Tertiary Period normal faults associated with the Boise Front fault system are mapped along the margins of the valley.

Tertiary faults experienced movement between 1.6 and 66 million years ago. None of the faults associated with the Boise Front fault system are known to be active. No faults are mapped on the project site, and no evidence of faulting was observed during the subsurface exploration. Based on research, the closest fault noted by MTI is a northwest trending normal fault that is mapped approximately ½-mile to the northeast of the project site.

Historical Seismicity

According to the USGS Earthquakes program, 29 earthquakes have been recorded within approximately 50 miles of the project site with magnitudes ranging from 1.4 to 3.7. All of these earthquakes occurred in the mountain ranges to the north, northeast, southwest, and southeast of the project site within seismic zones that are not related to the Boise Front fault system. The closest earthquake occurred on 11 December 2013 approximately 22 miles to the northeast of the project site, and had a magnitude of 2.2.

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Seismically Induced Surface Rupture, Settlements, and Lateral Spreading

Earthquakes generally are caused by a sudden slip or displacement along a zone of weakness, termed a fault, in the Earth’s crust. Surface fault rupture, which is a manifestation of the fault displacement at the ground surface, usually is associated with moderate to large-magnitude earthquakes (magnitudes of about 6 or larger) occurring on active faults having mapped traces or zones at the ground surface. The amount of surface fault displacement can be as much as 10 feet (3 meters) or more, depending on the earthquake magnitude and other factors. The displacements associated with surface fault rupture can have devastating effects on structures and lifelines situated astride the zone of rupture.

As mentioned above, no active faults have been mapped in the vicinity of the site. Based on the absence of nearby active faults, is the opinion of MTI that the probability of the occurrence of seismically induced surface rupture, settlements, and lateral spreading is negligible.

Liquefaction

Liquefaction is a soil behavior phenomenon in which a soil located below the groundwater surface loses a substantial amount of strength due to strong earthquake ground shaking. Some types of soil tend to compact during earthquake shaking, inducing excess pore water pressure in the saturated soil, which, in turn, causes a reduction in strength of the soil. Recently deposited (i.e., geologically young) and relatively loose natural soils, and uncompacted or poorly compacted fills, are potentially susceptible to liquefaction. Loose sands are particularly susceptible. Loose silts also have potential for liquefaction. Dense natural soils and well-compacted fills have low susceptibility to liquefaction. Clayey soils, gravel sediments, and bedrock generally are not susceptible to liquefaction.

Possible consequences of liquefaction include vertical settlement, lateral displacement, loss of bearing capacity for foundations supported by soil that liquefies, increased lateral loading on structures retaining soil that liquefies, and flotation of lightweight structures embedded in soil that liquefies. Based on the minimal presence of groundwater it is the opinion of MTI that the probability of liquefaction at the project site is negligible.

Collapsible Soils

Collapsible soils undergo a sudden decrease in volume due to the addition of water into the soil structure and/or additional loading. Collapsible soils in arid climates are generally associated with alluvial fans produced by intermittent stream flow. The soils dry prior to subsequent deposition and do not become fully consolidated under the overburden stresses. Collapsible soils typically exhibit a loose, honeycomb structure. They typically have a low to relatively low unit weight, low degree of saturation, and a high dry strength. Wetting a collapsible soil results in the loss of capillary tension or the softening, weakening, and dissolving of cemented agents allowing the larger particles to slip past each other into a denser soil structure. Typically, collapsible soils are made up of fine-grained soils.

Collapse potential laboratory testing of onsite soils was outside of MTI’s scope of work for the project.

However, based on the soils conditions encountered onsite and experience with other projects in the vicinity of the site, it is MTI’s opinion that the potential for collapsible soils is negligible.

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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 soil boring. Borings were located in the field by means of a Global Positioning System (GPS) device and are reportedly accurate to within fifteen feet. Three 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 loose excavated materials and bentonite holeplug.

Two borings were advanced by hand auger. Hand borings were loosely backfilled with excavated soils.

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. MTI recommends that these logs not be used to estimate fill material quantities.

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 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 Sandy Alluvium of Side-Stream Valleys and Gulches. Clayey sand sediments were encountered at ground surface.

These sediments were brown, dry to slightly moist, and loose to medium dense. Fine to coarse-grained sand was noted throughout. Intermittent fine to coarse gravel was noted throughout this horizon in the hand borings.

Silty sands were encountered beneath surficial clayey sands. These sediments were light brown and dry to slightly moist. Relative densities commonly ranged from very loose to medium dense. Fine to coarse-grained sand was present in throughout these horizons. Intermittent fine to coarse gravel was noted throughout this horizon in the hand borings.

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Beneath the silty sands in borings 1 and 2, clayey sand sediments were exposed. Clayey sands were brown, dry, and loose to medium dense, with fine to coarse-grained sand. Underlying clayey sand sediments in boring 2 and at depth in boring 3, sandy lean clay soils were exposed. Sandy lean clays were light brown to brown, slightly moist, and medium stiff to very stiff. Fine to coarse-grained sand was noted throughout.

During drilling, 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.

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.

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 borings advanced to a maximum depth of

16.5 feet bgs. Soil moistures in the borings were generally dry to slightly moist throughout. Relatively deep groundwater is likely near the project site because of its topography and elevation above the Boise River.

MTI has previously performed 10 geotechnical investigations within 1,960 feet of the project site. Information from these investigations has been provided in the table below.

Groundwater Data

Date Approximate Distance from Site (feet) Direction from Site Groundwater Depth

(feet bgs) December 2008 340 North Not Encountered to 6.2

May 2012 625 Southeast Not Encountered 21.5 February 2007 700 Southeast Not Encountered to 26.5 March 2015 920 Southeast Not Encountered to 51.5

November 2015 1,100 Northeast Not Encountered to 9.2 December 2017 1,370 West Not Encountered to 31.5 December 2016 1,400 South Not Encountered to 16.5

April 2009 1,460 Southeast 30.5 April 2008 1,750 Southeast Not Encountered to 12.0 April 2019 1,960 South Not Encountered to 16.2

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For construction purposes, groundwater depth can be assumed to remain greater than 20 feet bgs throughout the year.

Infiltration Testing

Infiltration testing was conducted in general accordance with the City of Boise Stormwater Design Manual.

Test locations were 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.

Testing was conducted on 4 September 2020. Details and results of testing are as follows:

Infiltration Testing Results

Test

Location

Test

Depth

(feet bgs)

Soil Type

Stabilized

Infiltration Rate

(inches/hour)

Design

Infiltration Rate

(inches/hour)

HB-1 5.1 Silty Sand 6.0 3.0

HB-2 2.2 Silty Sand 6.0 3.0

Appropriate factors of safety have been applied to the stabilized infiltration rates achieved during testing to obtain the design infiltration rates listed above. The reason for the decreased infiltration rate is to account for long term saturation of the soils and the potential for less permeable soils to settle into the bottom of the infiltration facilities. Due to varying fines content in the soil horizon, addition testing or observation may be necessary during construction and/or once the design depth of the infiltration bed is known. MTI recommends that all infiltration facilities be constructed in accordance with the local municipality requirements. In the

Treasure Valley it is industry practice to install seepage bed facilities below parking lot areas. The pavement sections provided in this report are suitable for a seepage bed application.

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.

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. However, since actual loading information and details for the structure are unknown at this time, foundation recommendations provided in this report should be considered preliminary. Final recommendations can be provided once loading information becomes available.

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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 at least 1.5 feet of compacted structural fill reinforced with two layers of Tensar TX160 geogrid.

Geogrid reinforced fill must bear on native clayey sand or silty sand sediments.1 The exposed subgrade should be prepared as follows:

1. Six inches of structural fill (ISPWC Type 1 crushed aggregate base) should be placed over the native clayey sand/silty sand sediments and be compacted to at least 95% of the maximum dry density as determined by ASTM D1557.

2. A layer of Tensar TX160 geogrid should be placed over the compacted structural fill followed by 6 inches of compacted structural fill.2 This process should be continued until two layers of geogrid are in place. At least 6 inches of compacted structural fill should be placed over the top layer of geogrid.

Geogrid should extend a minimum of 2 feet beyond the footings on all sides. Geogrid should be overlapped a minimum distance of 24 inches between splices. See the attached Plate 3 for graphical representation of this system.

Not Required for Native Soil

95% for Structural Fill

2,000 lbs/ft2

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

2Structural fill must consist of ISPWC Type 1 crushed aggregate base.

A sliding frictional coefficient value of 0.45 should be used for footings bearing on granular structural fill. A passive lateral earth pressure of 349 pounds per square foot per foot (psf/ft) should be used for silty sand sediments. For compacted sandy gravel fill, a passive lateral earth pressure of 496 psf/ft should be used.

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 30 inches below finished grade.

Based on the soil types encountered onsite, foundation drains are not needed.

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Floor Slab-on-Grade

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 should be provided below slabs-on-grade to provide drainage and a uniform and stable bearing surface. 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 clayey sand soils 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. These have been listed within the Soft Subgrade Soils section.

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 Light Duty

Driveways and Parking Heavy Duty

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 Not Required Not Required 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.

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 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 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.

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.

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CONSTRUCTION CONSIDERATIONS

Recommendations in this report are based upon structural elements of the project being founded on a geogrid reinforced prism bearing on competent, native silty sand sediments, clayey sand 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.

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.

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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.

Soft Subgrade Soils

Shallow fine-grained subgrade soils that are high in moisture content should be expected to pump and rut under construction traffic. 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 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.

• 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. MTI is available to provide recommendations and guidelines at your request.

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

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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.

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.

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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…