SSA_-_GSA_GeoTech_Report_110316.pdf

Geotechnical Subsurface Exploration Program SSA/GSA E-Vault – Building 94

Denver Federal Center Denver, Colorado

Prepared for:

E2 Optics 76 Inverness Drive East, Suite A

Englewood, Colorado 80112

Attention: Mr. James Carter

Job Number: 16-3682 November 2, 2016

TABLE OF CONTENTS

Page

Purpose and Scope of Study

Proposed Construction

Site Conditions

Subsurface Exploration

Laboratory Testing

Geologic Setting

Subsurface Conditions

Engineering Seismicity

Foundation/Floor System Overview

Foundation System

Floor System

Water Soluble Sulfates

Soil Corrosivity

Lateral Earth Pressures

Project Earthwork

Excavation Considerations

Utility Pipe Installation and Backfilling

Closure and Limitations

Locations of Test Holes ..................................................................................... Figure 1

Logs of Test Holes ............................................................................... Figures 2 and 3

Legend and Notes ............................................................................................. Figure 4

Axial Capacity Reductions ................................................................................... Figure 5

Lateral Capacity Reductions ................................................................................ Figure 6

Summary of Laboratory Test Results ................................................................ Table 1

Summary of Soil Corrosion Test Results .............................................................. Table 2

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Job No. 16-3682 Ground Engineering Consultants, Inc. Page 1 of 34

PURPOSE AND SCOPE OF STUDY

This report presents the results of a subsurface exploration program performed by

GROUND Engineering Consultants, Inc. (GROUND) for the proposed improvements to

Building 94 within the Denver Federal Center that is located near the intersection of Main

Avenue and 8th Street in Denver, Colorado. Our study was conducted in general accordance with GROUND’s Proposal No. 1607-1293, dated July 18, 2016.

Field and office studies provided information regarding surface and subsurface conditions, including existing site vicinity improvements and groundwater. Material samples retrieved during the subsurface exploration were tested in our laboratory to assess the engineering characteristics of the site earth materials. Results of the field, office, and laboratory studies for the proposed facility are presented below.

This report has been prepared to summarize the data obtained and to present our conclusions and opinions based on the proposed construction and the subsurface conditions encountered. Design parameters and a discussion of engineering considerations related to construction of the proposed facility improvements are included herein.

PROPOSED CONSTRUCTION

We understand that proposed construction will consist of screen walls, approximately 10 to 14 feet in height that will surround several equipment pads to support large mechanical units. Additionally, several ground-supported duct banks are also anticipated. No below grade levels are planned for construction. We assume underground utilities will also be constructed. Final grading plans and maximum loads were not provided at the time of this report preparation. We also assume that the finish floor elevation of the proposed equipment pads and mechanical units will be near/at the finish floor elevation of the existing building. Therefore, based on visual observations, it appears minimal cuts and fills (up to approximately 2 feet) will be necessary to generally facilitate the majority of the proposed construction. The project site is shown in Figure 1.

If proposed construction, including the anticipated site grading and structural loading, differs from those described above, or changes subsequently, GROUND should be notified to re-evaluate the information in this report.

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SITE CONDITIONS

At the time of our exploration, the project site consisted of an existing single-story building. The project site surrounding the existing building consisted of short to medium grasses and weeds and deciduous trees. An existing asphalt-paved area is located on the southeastern side of the existing facility. Underground utilities were also associated with the project site. Based on correspondence with the Client, the existing facility is believed to be constructed on a shallow foundation system consisting of spread footings with a slab-on-grade floor system. We understand no observed distress has been noted as being associated with the existing building. The general topography across the site was relatively flat to gently sloping with slopes up to approximately 2 percent generally descending towards the south and east with steeper slopes, up to approximately 10 percent descending toward the building. The project site is bordered by undeveloped land to the east and north, Main Avenue to the south, and an existing facility to the west.

Man-made fill was encountered in some of the test holes at the time of drilling. The exact extents, limits, and composition of any man-made fill were not determined as part of the scope of work addressed by this study and should be expected to potentially exist at varying depths and locations across the site.

SUBSURFACE EXPLORATION

The subsurface exploration for the project was conducted on October 11, 2016. A total of eight (8) test holes were drilled with a truck-mounted drill rig, advancing continuous flight auger to evaluate the subsurface conditions as well as to retrieve soil and bedrock samples for laboratory testing and analysis. The test holes were drilled at/near locations suggested by the Client along the length of the proposed screen wall alignment and within the mechanical pads. No test holes were drilled for use in pavement design. The

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Job No. 16-3682 Ground Engineering Consultants, Inc. Page 3 of 34 test holes ranged from approximately 20 to 31 feet below existing grades. A representative of GROUND directed the subsurface exploration, logged the test holes in the field, and prepared the soil and bedrock samples for transport to our laboratory. Soil cuttings from the test holes during drilling operations were collected in 55-gallon drums and left on-site for GSA disposal (as indicated in the GSA permit). Test holes were backfilled with sand to the surface of the test hole grade.

Samples of the subsurface materials were retrieved with a 2-inch I.D. California liner sampler. The sampler was driven into the substrata with blows from a 140-pound hammer falling 30 inches. This procedure is similar to the Standard Penetration Test described by ASTM Method D1586. Penetration resistance values, when properly evaluated, indicate the relative density or consistency of soils. Depths at which the samples were obtained and associated penetration resistance values are shown on the test hole logs.

The approximate locations of the test holes are shown in Figure 1. Logs of the exploratory test holes are presented in Figures 2 and 3. Explanatory notes and a legend are provided in Figure 4. GROUND utilized the Client-provided site plan indicating existing features, etc., and Google Map imagery to approximately locate the test holes.

LABORATORY TESTING

Samples retrieved from our test holes were examined and visually classified in the laboratory by the project engineer. Laboratory testing of soil and bedrock samples obtained from the subject site included standard property tests, such as natural moisture contents, dry unit weights, grain size analyses, swell-consolidation testing, unconfined compressive strength, and liquid and plastic limits. Water-soluble sulfate and corrosivity tests were completed on a selected sample of the soils as well. Laboratory tests were performed in general accordance with applicable ASTM and AASHTO protocols.

Results of the laboratory testing program are summarized in Tables 1 and 2.

GEOLOGIC SETTING

The subject parcel lies within the Denver Basin geologic province that consists largely of a sequence of sedimentary rock formations deposited and preserved in a structural depression in north-central Colorado. In the general project area, these sedimentary

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Job No. 16-3682 Ground Engineering Consultants, Inc. Page 4 of 34 rocks dip eastward at low angles (less than 10 degrees, typically) and are overlain by a variety of surficial deposits including alluvial (stream-laid) sediments, eolian (wind-blown) materials and colluvial (slope-wash) deposits.

According to available geologic maps (Lindvall, 19781) the overburden deposits underlying the project area are mapped as Verdos Alluvium (Qv), from the Upper

Pleistocene Era, generally consisting of light brown to reddish brown, poorly sorted, stratified gravel containing lenses of clay, silt, and sand. These generally alluvium materials are underlain by the Denver and Arapahoe Formation (Tkda) from the

Paleocene to Upper Cretaceous Eras generally consisting of claystone, siltstone, sandstone, and conglomerate.

SUBSURFACE CONDITIONS

The subsurface conditions encountered in the test holes generally consisted of topsoil

(approximately 2 to 4 inches thick) underlain by man-made fill and/or sand and clay/silt.

These materials were underlain by sandstone/siltstone bedrock encountered at depths

1 Lindvall, R.M, 1978, Geologic Map of the Fort Logan Quadrangle, Jefferson, Denver, and Arapahoe Counties, Colorado: U.S. Geological Survey, Miscellaneous Investigation Series, Map GQ-1427

Approximate Project Site

Qv af

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Job No. 16-3682 Ground Engineering Consultants, Inc. Page 5 of 34 ranging from approximately 9 to 21 feet below existing grades that extended to the test hole termination depths ranging from approximately 20 to 31 feet below the existing grades.

It also should be noted that coarse gravel, cobbles and boulders are not well represented in samples obtained from small diameter test holes. At this site, therefore, it should be anticipated that gravel and cobbles, and possibly boulders, may be present in the fill and native soils, as well as comparably sized fragments of construction debris, even where not included in the general descriptions of the site soil types below.

Man-Made Fill was composed of sands and clays, was medium to highly plastic, fine to coarse grained, slightly moist to very moist, and light brown to brown in color.

Delineation of the complete lateral and vertical extents of any fills at the site, or their compositions, however, was beyond our present scope of services. If fill soil volumes and compositions at the site are of significance, they should be further evaluated using test pits.

Sand and Clay/Silt were interbedded, low to highly plastic, fine to coarse grained, loose to dense/medium to hard, slightly moist to very moist, occasionally calcareous, and light brown to brown in color.

Sandstone and Siltstone Bedrock (Comparatively Unweathered Bedrock) were interbedded, low to highly plastic, fine to coarse grained, medium hard to very hard, moist, occasionally iron-stained, and brown to olive in color.

Swell-Consolidation Testing suggested a potential for swell and consolidation in the tested on-site materials. Swells ranging from approximately 0.1 to 3 percent and consolidations ranging from approximately 0.3 to 0.6 percent were measured upon wetting under various surcharge pressures (see Table 1).

Groundwater was encountered in the Test Hole B-6 at a depth of approximately 25 feet below existing grade at the time of drilling. Due to safety and the backfill instructions in the GSA permit, the test holes were backfilled immediately following drilling.

Groundwater levels can be expected to fluctuate, however, in response to annual and

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Job No. 16-3682 Ground Engineering Consultants, Inc. Page 6 of 34 longer-term cycles of precipitation, irrigation, surface drainage, nearby rivers and creeks, land use, and the development of transient, perched water conditions.

ENGINEERING SEISMICITY

According to the 2015 International Building Code® (Section 1613 Earthquake Loads), “Every structure, and portion thereof, including nonstructural components that are permanently attached to structures and their supports and attachments, shall be designed and constructed to resist the effects of earthquake motions in accordance with

ASCE 7, excluding Chapter 14 and Appendix 11A. The seismic design category for a structure is permitted to be determined in accordance with Section 1613 (2015 IBC) or

ASCE 7.” Exceptions to this are further noted in Section 1613.

Utilizing the USGS’s Seismic Design Maps Tool

(http://earthquake.usgs.gov/designmaps/us/application.php) and site latitude/longitude coordinates of 39.716138 and –105.124777 (obtained from Google Earth), respectively, the project area is indicated to possess an SDS value of 0.206 and an SD1 value of 0.097.

Per 2015 IBC, Section 1613.3.2 Site class definitions, “Based on the site soil properties, the site shall be classified as Site Class A, B, C, D, E or F in accordance with Chapter 20 of ASCE 7. Where the soil properties are not known in sufficient detail to determine the site class, Site Class D shall be used unless the building official or geotechnical data determines that Site Class E or F soil is likely to be present at the site”.

Based on the soil conditions encountered in the test holes drilled on the site, our review of applicable geologic maps, as well as our experience within the Project site vicinity, GROUND estimates that a Site Class D according to ASCE 7 (Table 20.3-1) could be anticipated for seismic foundation design. This parameter was estimated utilizing the above-referenced table as well as extrapolation of data beyond the deepest depth explored. Actual shear wave velocity testing/analysis and/or exploration to 100 feet was not performed. In the event the Client desires to potentially utilize Site Class C for design, according to ASCE 7, actual downhole seismic shear wave velocity testing and/or exploration to subsurface depths of at least 100 feet, should be performed. In the absence of additional subsurface exploration/analysis, a Site Class D should be utilized for design.

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FOUNDATION/FLOOR SYSTEMS OVERVIEW

Mechanical Equipment Pads

Based on our exploration and laboratory testing program, a potential for heave and consolidation exists within the on-site overburden materials. Additionally, existing man-made fill material appeared to be observed in some of the test holes to depths up to approximately 6 feet below existing grades. Deeper depths of man-made fill may exist on-site. It is unknown if these fill materials were placed in a properly moisture-density treated manner, and in the absence of test results/documentation, it should be assumed that they were not. We estimate that shallow foundations and floor systems/concrete slabs placed directly on the site soils could experience post-construction movements of approximately 1 to 1½ inches or more. For the least potential for total and differential movement (approximately ½-inch), the proposed mechanical pads should be supported on a deep foundation system consisting of drilled piers with a structural floor/slab system. Based on the nature of the proposed construction, a deep foundation system may not be as practical as a shallow foundation system and slab-on-grade floors. Drilled pier parameters are provided below in the event these systems are desired. A discussion regarding structurally-supported slabs can be provided upon request.

As an alternate (but not equivalent in performance) foundation/slab system, a shallow foundation/slab system consisting of spread footings and slabs-on-grade can be utilized for the proposed mechanical pads, provided a uniform fill thickness (fill prism) is constructed beneath and beyond the structure(s) in order to reduce (but not eliminate) the potential of movement, as indicated above. The fill prism should extend to a depth of at least 6 feet beneath the slab. The fill prism layer should extend laterally approximately 5 feet beyond the pad(s). Utilizing this option as well as other applicable suggestions provided in this report, GROUND anticipates potential movements on the order of 1 inch and differential movements on the order of ½ inch over a distance of 40 feet. Realized movements should be expected to exceed these estimates somewhat.

If the proposed mechanical pads have a higher tolerance for movement and provided the Owner is aware of the potential for increased movement (and associated distress), approximately 1½ inches or more, a shallow foundation system consisting of spread footings with a slab-on-grade floor system could be placed on the existing site materials.

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The upper 12 inches (or more) beneath slabs should be scarified, moisture-conditioned, and re-compacted in accordance with the Project Earthwork section of this report.

Realized movements may be more or less; movements exceeding 1½ inches should be expected locally.

Screen Wall

According to recent correspondence with the Client, we understand that an approximate

10 to 14-foot tall concrete masonry unit (CMU) screen wall will be constructed on the around the existing building/proposed mechanical pads. Typically, we understand that this wall system cannot accommodate much movement. In this regard, a deep foundation system should be used. Provided the Owner is aware of the potential for increased movement (and associated distress) of approximately 1½ inches or more, the screen wall could be founded on a shallow foundation system consisting of spread footings or drilled footings placed on the existing site soils. Actual movements may be more or less than that indicated above. To reduce the potential for movement to approximately 1 inch, a fill prism consisting of at least 3 feet should be placed below footings. As indicated, it has been GROUND’s experience that CMU walls are highly susceptible to distress/damage as a result of movement and the owner should understand that post-construction movements experienced by the foundation will likely result in cracking of the CMU system that may be significant in localized areas. If it is determined that these types of movements are not acceptable, a deep foundation system should be utilized.

Due to the proposed location (landscaped/irrigated area), the movement estimates provided above may be exceeded as a result of moisture infiltration affecting shallowly supported foundations/slabs.

The surface area adjacent to the wall should ideally be graded to maintain positive drainage away from the wall. Additionally, the irrigation (sprinklers included) should be directed away from the wall to reduce moisture infiltration into the underlying foundation subgrade materials.

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FOUNDATION SYSTEMS

Deep Foundations

Geotechnical Parameters for Drilled Pier Design If proper design and construction as described below is implemented effectively, then post-construction, vertical foundation movements may be taken as ½ inch with similar differential movements over spans of about 40 feet.

Based on the results of the field exploration, laboratory testing, and experience, the design criteria presented below should be observed for a straight-shaft, drilled pier foundation system.

1) Drilled piers should bear in ‘comparatively unweathered’ bedrock underlying the site. For design purposes, ‘comparatively unweathered’ bedrock may be taken to be at and below depths ranging from approximately 9 to 21 feet below existing grades. For bidding purposes, this elevation may vary.

2) Drilled piers should be at least 18 inches in diameter and should be designed with a maximum length to diameter ratio of 30 to 1. The actual length to diameter ratios should be determined by the structural engineer.

3) Drilled piers should have a minimum length of 30 feet. The actual drilled pier lengths should be determined by the structural engineer based on loading, etc., with further increases in length possibly required by the conditions encountered during installation at each drilled pier location.

4) Drilled piers also should penetrate at least 10 feet into comparatively unweathered bedrock or 3 drilled pier diameters, whichever is greater.

5) Based on the bedrock penetration, minimum pier length and estimating the top of comparatively unweathered bedrock to range from approximately 9 to 21 feet below existing grade, drilled pier lengths of approximately 30 to 31 feet are anticipated to meet the geotechnical criteria. Actual drilled pier lengths commonly will be greater due to structural considerations, conditions in the drilled pier holes, actual comparatively unweathered bedrock depths, etc.

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6) Drilled piers bearing in comparatively unweathered bedrock may be designed for an allowable end bearing pressure of 30,000 psf.

The portion of the drilled pier penetrating comparatively unweathered bedrock may be designed for a skin friction value of 2,250 psf for the portion of the pier penetrating comparably unweathered bedrock. 100 percent of the skin friction may be used to resist both compressional loads and uplift.

7) Estimated settlement of properly constructed drilled piers will be low, on the order of ½-inch, to mobilize skin friction.

8) Drilled piers should be designed for a minimum dead load pressure of 5,000 psf based on drilled pier cross-section area.

Where minimum dead load cannot be applied, it will be necessary to increase the drilled pier length beyond the recommended minimum, even where the minimum bedrock penetration has been achieved or exceeded. This can be accomplished by assuming that skin friction on the extended zone acts in the direction to resist uplift.

9) Drilled piers should be reinforced as determined by the structural engineer. At a minimum, drilled piers should be reinforced for their full length to resist the tensile loading created by the swelling soils and bedrock. Tension may be estimated as an uplift skin friction of 875 psf applied to the upper 20 feet of the drilled pier.

Reinforcement design also should include any deficit between the dead load applied in design and the minimum dead load provided above.

10) A 4-inch or thicker continuous void should be provided beneath grade beams, drilled pier caps, and foundation walls to concentrate drilled pier loadings. The void space should be protected from backfill intrusion.

11) Penetration of comparatively unweathered bedrock in drilled pier shafts should be roughened artificially to assist the development of peripheral shear between the drilled pier and bedrock. Artificially roughening of drilled pier holes should consist of installing shear rings 3 inches high and 2 inches deep in the portion of each drilled pier penetrating comparatively unweathered bedrock and below a

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Job No. 16-3682 Ground Engineering Consultants, Inc. Page 11 of 34 depth of 20 feet, from top of pier. The shear rings should be installed 18 inches on centers.

The specifications should allow a geotechnical engineer to waive the requirement for shear rings depending on the conditions actually encountered in individual drilled pier holes, however.

12) Groups of closely spaced drilled piers placed to support concentrated loads will require an appropriate reduction of the estimated capacities. Reduction of axial capacity generally can be avoided by spacing drilled piers at least 3 diameters center to center. At this spacing or greater, no reduction in axial capacities or horizontal soil modulus values is required. Drilled pier groups spaced less than 3 diameters center to center should be studied on an individual basis to determine the appropriate axial capacity reduction(s). The settlement of closely spaced groups of drilled piers should be studied on an individual basis.

Linear arrays of drilled piers, however, must be spaced at least 8 diameters center to center to avoid reductions in lateral capacity when loaded in line with the array (parallel to the line connecting the drilled pier centers). Linear arrays of drilled piers spaced more closely than 8 diameters center to center should be studied to determine the appropriate lateral capacity reductions in that direction.

Refer to Figures 5 and 6 for additional information regarding reductions in lateral and axial capacity for closely spaced piers.

Drilled Pier Construction The following should be considered during the construction of drilled pier foundations.

13) The depth of comparatively unweathered bedrock should be determined in the field at each drilled pier location and may differ from other information provided herein.

14) Lenses or beds of relatively soft bedrock not suitable for foundation support may be encountered within the comparatively unweathered bedrock section, which may result in lengthening the drilled piers.

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15) Some bedrock beneath the site was medium hard to very hard. The pier-drilling contractor should mobilize equipment of sufficient size and operating capability to achieve the design lengths and bedrock penetration.

If refusal is encountered in these materials, a geotechnical engineer should be retained to evaluate the conditions to establish whether true refusal has been met with adequate drilling equipment.

16) Groundwater was encountered in Test Hole 6 at a depth of approximately 25 feet below existing grade at the time of drilling. Groundwater, combined with granular soils, often results in caving during pier installation. Seating of the casing in the upper layers of the bedrock may not create positive cutoff of water infiltration.

The contractor should be prepared to address this condition.

17) In no case should concrete be placed in more than 3 inches of water, unless placed through an approved tremie method. The proposed concrete placement method should be discussed during the pre-construction meeting by the Project

Team.

18) Where groundwater and unconsolidated soils and/or caving bedrock materials are encountered, the installation procedure of drilled piers can be a concern.

Commonly in these conditions, the drilling contractor utilizes casing and slurry during excavation of the drilled pier holes, which may adversely affect the axial and/or lateral capacities of the completed drilled piers. During casing withdrawal, the concrete should have sufficient slump and must be maintained with sufficient head above groundwater levels to displace the water or slurry fully to prevent the creation of voids in the drilled pier.

Because of these considerations, the drilling contractor should submit a written procedure addressing the use of casing, slurry, and concrete placement prior to commencement of drilled pier installation.

19) Drilled pier holes should be properly cleaned prior to placement of concrete.

20) Concrete utilized in the drilled piers should be a fluid mix with sufficient slump so that it will fill the void between reinforcing steel and the drilled pier hole wall, and

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Job No. 16-3682 Ground Engineering Consultants, Inc. Page 13 of 34 inhibit soil, water and slurry from contaminating the concrete. The concrete should be designed with a minimum slump of no less than 5 inches.

21) Concrete should be placed by an approved method to minimize mix segregation.

22) Concrete should be placed in a drilled pier on the same day that it is drilled.

Failure to place concrete the day of drilling may result in a requirement for lengthening the drilled pier. The presence of groundwater or caving soils may require that concrete be placed immediately after the pier hole drilling is completed.

23) The contractor should take care to prevent enlargement of the excavation at the tops of drilled piers, which could result in “mushrooming” of the drilled pier top.

Mushrooming of drilled pier tops can increase uplift pressures on the drilled piers.

24) Sonic integrity testing (sonic echo or cross-hole sonic) should be considered to be performed for an appropriate percentage of the drilled piers to assess the effectiveness of the drilled pier construction methods. Additional information on sonic integrity testing can be provided upon request.

25) Based on the data obtained for this study and our experience with similar sites and conditions, lateral load analysis using the Terzaghi method may take the values tabulated below for the modulus of horizontal subgrade reaction (Kh) to be characteristic of the soils and bedrock underlying the site, based on a simplified soil / bedrock profile. Resistance to lateral loads by deep foundations should be neglected in the upper 3 feet of soils, whether fill or native and within the fill materials.

Horizontal Modulus Subgrade Reaction (Kh) – Terzaghi Method

Material

Kh based on Foundation

Element Width / Diameter

1.5-Foot Diameter

Sands and Clays/Silts 60 tcf

Sandstone/Siltstone Bedrock

240 tcf

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Note that the Kh values tabulated above are dependent on deep foundation element width or diameter. For deep foundation elements greater than 1.5-foot in diameter, divide the Kh values by the corresponding pier diameters (measured in feet).

If “L-Pile” or a similar computer program is used for lateral analysis of the piles, recommended geotechnical parameters for input into that program are tabulated below for the same simplified soil / bedrock profile. These include, unit wet weights (γ'), angles of internal friction (), cohesion (c), for the earth materials, as well as values for strain at 50 percent of failure stress (50) and horizontal soil modulus (k). Again, resistance to lateral loads should be neglected in the upper

3 feet of soils, whether fill or native.

Note that below the water table (taken to be as shallow as 25 feet below existing grade) the unit weights must be adjusted for buoyancy by reducing the values by

62.4 pcf.

Estimated Geotechnical Parameters for Lateral Load Analysis

Soil / Bedrock

Material Parameter Recommended Value

Sands and Clays/Silts γ' 117 pcf c 500 psf

50 0.010 k 625 pci

Sandstone/Siltstone

Bedrock γ' 113 pcf c 5,000 psf

50 0.004 k 2,500 pci

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Shallow Foundations

Geotechnical Parameters for Shallow Foundation Design

1) Footings should bear on properly compacted, site-generated materials as discussed in the Foundation/Floor System Overview section.

Considerations for fill placement and compaction are provided in the Project

Earthwork section of this report.

2) Footings bearing on properly compacted, on-site materials, may be designed for an allowable soil bearing pressure of 2,000 psf for footings up to 5 feet in width based on an assumed maximum load of 50 kips. In the event the footing width is greater than 5 feet, or the maximum column load exceeds 50 kips, we should be notified to reevaluate these parameters.

These values may be increased by ⅓ for transient loads such as wind or seismic loading. For larger footings, a lower allowable bearing pressure may be appropriate.

Compression of the bearing soils for the provided allowable bearing pressure is estimated to be 1 inch, based on an assumption of drained foundation conditions.

If foundation soils are subjected to an increase/fluctuation in moisture content, the effective bearing capacity will be reduced and greater post-construction movements than those estimated above may result.

This estimate of foundation movement is from direct compression of the foundation soils.

To reduce differential settlements between footings or along continuous footings, footing loads should be as uniform as possible. Differentially loaded footings will settle differentially.

3) Spread footings should have a minimum lateral dimension of 16 or more inches for linear strip footings and 24 inches for isolated pad footings. Actual footing dimensions should be determined by the structural engineer.

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4) Drilled footings should have a minimum diameter of 12 inches. Actual footing dimensions/sizes should be determined by the structural engineer.

5) Footings should bear at an elevation 3 or more feet below the lowest adjacent exterior finish grades to have adequate soil cover for frost protection

6) Continuous foundation walls should be reinforced as designed by a structural engineer to span an unsupported length of at least 10 feet.

7) Geotechnical parameters for lateral resistance to foundation loads are provided in the Lateral Earth Pressure section of this report.

8) Connections of all types must be flexible and/or adjustable to accommodate the anticipated, post-construction movements of the structure.

9) The lateral resistance of spread footings will be developed as sliding resistance of the footing bottoms on the foundation materials and by passive soil pressure against the sides of the footings. Sliding friction at the bottom of footings may be taken as 0.30 times the vertical dead load.

10) In order to reduce differential settlements between footings along continuous footings, footing loads should be as uniform as possible. Differentially loaded footings will settle differentially. Similarly, differential fill thicknesses beneath footings will result in increased differential settlements.

Shallow Foundation Construction

11) The contractor should take adequate care when making excavations not to compromise the bearing or lateral support for nearby improvements.

12) Footing excavation bottoms may expose loose, organic or otherwise deleterious materials, including debris. Firm materials may become disturbed by the excavation process. All such unsuitable materials should be excavated and the foundations deepened.

13) Foundation-supporting soils may be disturbed or deform excessively under the wheel loads of heavy construction vehicles as the excavations approach footing

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Job No. 16-3682 Ground Engineering Consultants, Inc. Page 17 of 34 bearing levels. Construction equipment should be as light as possible to limit development of this condition. The movement of vehicles over proposed foundation areas should be restricted.

14) All foundation subgrade should be properly compacted with a vibratory plate compactor prior to placement of concrete.

15) Fill placed against the sides of the footings should be properly compacted in accordance with the Project Earthwork section of this report.

SLABS-ON-GRADE

Geotechnical Parameters for Slab-on-Grade Floors

1) Lightly loaded slabs may be placed on compacted, on-site materials, as discussed in the Foundation/Floor Systems Overview section of this report.

2) An allowable subgrade vertical modulus (K) of 75 pci may be utilized for lightly loaded slabs supported by the on-site materials. This value is for a 1-foot x 1-foot plate; they should be adjusted for slab dimension.

3) The prepared surface on which the slabs will be cast should be observed by the

Geotechnical Engineer prior to placement of reinforcement. Exposed loose, soft, or otherwise unsuitable bedrock materials should be excavated to competent bedrock. Additional gravel may be necessary to achieve proper grades.

4) Slabs should be separated from all bearing walls, columns, and footings with slip joints, which allow unrestrained vertical movement. Slabs should not bear on footings or other foundation elements.

5) Joints should be observed periodically, particularly during the first several years after construction. Slab movement can cause previously free-slipping joints to bind. Measures should be taken to assure that slab isolation is maintained in order to reduce the likelihood of damage to walls and other interior improvements.

6) Interior partitions (if applicable) resting on floor/concrete slabs should be provided with slip joints so that if the slabs move, the movement cannot be transmitted to

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Job No. 16-3682 Ground Engineering Consultants, Inc. Page 18 of 34 the upper structure. This detail is also important for wallboards and door frames.

A slip joint, which will allow at least 2 or more inches of vertical movement, is recommended. If slip joints are placed at the tops of walls, in the event that the slabs move, it is likely that the wall will show signs of distress, especially where the slabs meet the exterior wall.

7) Concrete slabs-on-grade should be placed on properly prepared subgrade. They should also be constructed and cured according to applicable standards and be provided with properly designed and constructed control joints.

The design and construction of such joints should account for cracking as a result of shrinkage, tension, and loading; curling; as well as proposed slab use. Joint layout based on the slab design may require more frequent, additional, or deeper joints, and should also be based on the ultimate use and configuration of the slabs. Areas where slabs consist of interior corners or curves (at column blockouts or around corners) or where slabs have high length to width ratios, high degree of slopes, thickness transitions, high traffic loads, or other unique features should be carefully considered. The improper placement or construction of control joints will increase the potential for slab cracking. ACI, AASHTO, and other industry groups provide many guidelines for proper design and construction of concrete slabs-on-grade and the associated jointing.

8) Slabs should be adequately reinforced. Structural considerations for slab thickness, jointing, and steel reinforcement in floor slabs should be developed by the Structural Engineer. Placement of slab reinforcement continuously through the control joint alignments will tend to increase the effective size of concrete panels and reduce the effectiveness of control joints.

9) All plumbing lines should be carefully tested before operation. Where plumbing lines enter through the slab, a positive bond break should be provided. Flexible connections allowing 2 or more inches of vertical movement should be provided for slab-bearing mechanical equipment. Greater movements may occur depending upon the fill prism section selected by the owner and connections allowing greater movements incorporated.

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10) Moisture can be introduced into a slab subgrade during construction and additional moisture will be released from the slab concrete as it cures. Placement of a properly compacted layer of free-draining gravel, 4 or more inches in thickness, beneath the slabs should be performed. This layer will help distribute floor slab loadings, ease construction, reduce capillary moisture rise, and aid in drainage. The free-draining gravel should contain less than 5 percent material passing the No. 200 Sieve, more than 50 percent retained on the No. 4 Sieve, and a maximum particle size of 2 inches.

11) The Client/Project Team should review the American Concrete Institute’s (ACI)

Sections 301/302/360 for additional guidance regarding slab on grade design and construction. Vapor Barriers should meet applicable performance standards as stated in ASTM E 1745.

Slab movements are directly related to the increases in moisture contents to the underlying soils after construction is completed. The precautions and parameters itemized above will not prevent the movement of floor slabs if the underlying materials are subjected to moisture fluctuations. However, these steps will reduce the damage if such movement occurs.

WATER-SOLUBLE SULFATES

The concentrations of water-soluble sulfates measured in selected samples retrieved from the test holes were approximately 0.01 percent. Such a concentration of water-soluble sulfates represents a negligible environment for sulfate attack on concrete exposed to these materials. Degrees of attack are based on the scale of ‘negligible,’

‘moderate,’ ‘severe’ and ‘very severe’ as described in the “Design and Control of

Concrete Mixtures,” published by the Portland Cement Association (PCA). The Colorado

Department of Transportation (CDOT) utilizes a corresponding scale with 4 classes of severity of sulfate exposure (Class 0 to Class 3) as described in the published table below.

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REQUIREMENTS TO PROTECT AGAINST DAMAGE TO

CONCRETE BY SULFATE ATTACK FROM EXTERNAL SOURCES OF SULFATE

Severity of Sulfate

Exposure

Water-Soluble Sulfate (SO4)

In Dry Soil

Sulfate (SO4) In Water

(ppm)

Water Cementitious

Ratio (maximum)

Cementitious Material

Requirements

Class 0 0.00 to 0.10 0 to 150 0.45 Class 0

Class 1 0.11 to 0.20 151 to 1,500 0.45 Class 1

Class 2 0.21 to 2.00 1,501 to 10,000 0.45 Class 2

Class 3 2.01 or greater 10,001 or greater 0.40 Class 3

Based on these data GROUND, makes no recommendation for use of a special, sulfate-resistant cement in project concrete.

SOIL CORROSIVITY

The degree of risk for corrosion of metals in soils commonly is considered to be in two categories: corrosion in undisturbed soils and corrosion in disturbed soils. The potential for corrosion in undisturbed soil is generally low, regardless of soil types and conditions, because it is limited by the amount of oxygen that is available to create an electrolytic cell. In disturbed soils, the potential for corrosion typically is higher, but is strongly affected by soil chemistry and other factors.

A preliminary corrosivity analysis was performed to provide a general assessment of the potential for corrosion of ferrous metals installed in contact with earth materials at the site, based on the conditions existing at the time of GROUND’s evaluation. Soil chemistry and physical property data including pH, reduction-oxidation (redox) potential, and sulfides content were obtained. Test results are summarized on Table 2.

pH Where pH is less than 4.0, soil serves as an electrolyte; the pH range of about 6.5 to

7.5 indicates soil conditions that are optimum for sulfate reduction. In the pH range above 8.5, soils are generally high in dissolved salts, yielding a low soil resistivity

(AWWA, 2010). Testing indicated pH values of approximately 8.9 and 9.0.

Reduction-Oxidation testing indicated negative potentials: approximately -107 and -118 millivolts. Such a low potential typically creates a more corrosive environment.

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Sulfide Reactivity testing for the presence of sulfides indicated ‘trace’ results. The presence of sulfides in the site soils also suggests a more corrosive environment.

Soil Resistivity In order to assess the “worst case” for mitigation planning, samples of materials retrieved from the test holes were tested for resistivity in the in the laboratory, after being saturated with water, rather than in the field. Resistivity also varies inversely with temperature. Therefore, the laboratory measurements were made at a controlled temperature.

Measurements of electrical resistivity indicated values of approximately 1,126 and

14,265 ohm-centimeters in samples of the site earth materials.

Corrosivity Assessment The American Water Works Association (AWWA, 20102) has developed a point system scale used to predict corrosivity. The scale is intended for protection of ductile iron pipe but is valuable for project steel selection. When the scale equals 10 points or higher, protective measures for ductile iron pipe are suggested. The

AWWA scale (Table A.1 Soil-test Evaluation) is presented below. The soil characteristics refer to the conditions at and above pipe installation depth.

Table A.1 Soil-test Evaluation

Soil Characteristic / Value Points

Resistivity <1,500 ohm-cm ..........................................................................................… 10 1,500 to 1,800 ohm-cm ................................................................……......…. 8 1,800 to 2,100 ohm-cm .............................................................................…. 5 2,100 to 2,500 ohm-cm ...............................................................................… 2 2,500 to 3,000 ohm-cm >3,000 ohm-cm ................................................................................… 0 pH 0 to 2.0

2.0 to 4.0

4.0 to 6.5

6.5 to 7.5 ......................................................................................................... 0 *

7.5 to 8.5 >8.5

Redox Potential

< 0 (negative values) 0 to +50 mV ................................................................................................…. 4

2 American Water Works Association ANSI/AWWA C105/A21.5-05 Standard.

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+50 to +100 mV ............................................................................................… 3½ > +100 mV

Sulfide Content

Positive ........................................................................................................…. 3½ Trace .............................................................................................................… 2 Negative .......................................................................................................…. 0

Moisture Poor drainage, continuously wet ..................................................................…. 2 Fair drainage, generally moist ....................................................................… 1 Good drainage, generally dry

* If sulfides are present and low or negative redox-potential results (< 50 mV) are obtained, add three points for this range.

The redox potential of a soil is significant, because the most common sulfate-reducing bacteria can only live in anaerobic conditions. A negative redox potential indicates anaerobic conditions in which sulfate reducers thrive. A positive sulfide reaction reveals a potential problem caused by sulfate-reducing bacteria. Anaerobic conditions are regarded as potentially corrosive.

Based on a maximum possible score of 25.5 using the AWWA method, the value of 10 for the use of corrosion protection, and scores of approximately 10 and 20 in the on-site materials, the soil appears to comprise a low severity corrosive environment for buried metals.

If additional information are needed regarding soil corrosivity, the American Water Works

Association or a Corrosion Engineer should be contacted. It should be noted, however, that changes to the site conditions during construction, such as the import of other soils, or the intended or unintended introduction of off-site water, may significantly alter corrosion potential.

LATERAL EARTH PRESSURES

Structures which are laterally supported and can be expected to undergo only a limited amount of deflection should be designed for “at-rest” lateral earth pressures. The cantilevered retaining structures will be designed to deflect sufficiently to mobilize the full active earth pressure condition, and may be designed for “active” lateral earth pressures.

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“Passive” earth pressures may be applied in front of the structural embedment to resist driving forces.

The at-rest, active, and passive earth pressures in terms of equivalent fluid unit weight for the on-site backfill and CDOT Class 1 structure backfill are summarized on the table below. Base friction may be combined with passive earth pressure if the foundation is in a drained condition. The values for the on-site material in the upper 10 feet provided in the table below were approximated utilizing a unit weight of 117 pcf and a phi angle of

24 degrees.

Lateral Earth Pressures (Equivalent Fluid Unit Weights)

Material Type Water

Condition

At-Rest

(pcf)

Active

(pcf)

Passive

(pcf)

Friction

Coefficient

On-Site

Materials Drained 69 49 240 0.30

Structure Backfill

(CDOT Class 1) Drained 55 35 400 0.45

If the selected on-site soil meets the criteria for CDOT Class 1 structure backfill as indicated in the Project Earthwork section of this report, the lateral earth pressures for

CDOT Class 1 structure backfill as shown on the above table may be used. To realize the lower equivalent fluid unit weight, the selected structure backfill should be placed behind the wall to a minimum distance equal to the retained wall height.

The lateral earth pressures indicated above are for a horizontal upper backfill slope. The additional loading of an upward sloping backfill as well as loads from traffic, stockpiled materials, etc., should be included in the wall/shoring design. GROUND can provide the adjusted lateral earth pressures when the additional loading conditions and site grading are clearly defined.

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PROJECT EARTHWORK

The following information is for private improvements; public roadways or utilities should be constructed in accordance with applicable municipal / agency standards.

General Considerations: Site grading should be performed as early as possible in the construction sequence to allow settlement of fills and surcharged ground to be realized to the greatest extent prior to subsequent construction.

Prior to earthwork construction, vegetation and other deleterious materials should be removed and disposed of off-site. Relic underground utilities should be abandoned in accordance with applicable regulations, removed as necessary, and properly capped.

Topsoil present on-site should not be incorporated into ordinary fills. Instead, topsoil should be stockpiled during initial grading operations for placement in areas to be landscaped or for other approved uses.

Use of Existing Native Soils: Overburden soils that are free of trash, organic material, construction debris, and other deleterious materials are suitable, in general, for placement as compacted fill.