Attch 4_120107 GeoTech Report.pdf

GEOTECHNICAL EVALUATION

RPR FOUR ELEVATORS TO ADA

USAFA- FALCON STADIUM

2169 FIELD HOUSE DRIVE

COLORADO SPRINGS, COLORADO

PREPARED FOR:

Merrick & Company

5755 Mark Dabling Boulevard, Suite 245 Colorado Springs, Colorado 80919

PREPARED BY:

Ninyo & Moore

Geotechnical and Environmental Sciences Consultants 6001 South Willow Drive, Suite 195 Greenwood Village, Colorado 80111

August 31, 2016 Project No. 501165001

August 31, 2016 Project No. 501165001

Mr. Lucas Souhrada, PE Merrick & Company 5970 Greenwood Plaza Boulevard, Greenwood Village, Colorado, 80111

Subject: Geotechnical Evaluation

RPR Four Elevators to ADA USAFA- Falcon Stadium 2169 Field House Drive Colorado Springs, Colorado

Dear Mr. Souhrada:

In accordance with your authorization and our proposal dated March 25, 2016, Ninyo & Moore has performed a geotechnical evaluation for the above-referenced site. The attached report presents our methodology, findings, and conclusions regarding the geotechnical conditions at the project site and provides geotechnical engineering recommendations for the proposed development.

We appreciate the opportunity to be of service to you during this phase of the project.

Sincerely, NINYO & MOORE

Derek Magnuson, PG Project Geologist

Brian F. Gisi, PE Principal Engineer

DWM/BFG

Distribution: (1) Addressee (via e-mail)

8/31/2016

USAFA- Falcon Stadium August 31, 2016 Colorado Springs, Colorado Project No. 501165001

501165001R i

TABLE OF CONTENTS

Page

1. INTRODUCTION

2. SCOPE OF SERVICES

3. SITE DESCRIPTION AND BACKGROUND REVIEW

4. PROPOSED CONSTRUCTION

5. FIELD EXPLORATION AND LABORATORY TESTING

6. GEOLOGY AND SUBSURFACE CONDITIONS

6.1. Geologic Setting

6.2. Subsurface Conditions

6.2.1. Fill

6.2.2. Colluvium

6.2.3. Dawson Formation

6.3. Groundwater

7. GEOLOGIC HAZARDS

7.1. Faulting and Seismicity

7.2. Expansive Soils

7.3. Compressible/Collapsible Soils

7.4. Liquefaction Potential

8. CONCLUSIONS

9. RECOMMENDATIONS

9.1. Demolition

9.2. Earthwork

9.2.1. Excavations

9.2.2. Site Grading

9.2.3. Re-Use of Site Soils and Rock

9.2.4. Fill Placement and Compaction

9.2.5. Imported Soil

9.2.6. Settlement Monitoring

9.2.7. Utility Installation

9.2.8. Temporary Cut Slopes

9.3. Spread Footing Foundations

9.4. Earth Pressures and Retaining Walls

9.5. Concrete Flatwork

9.6. Corrosion Considerations

9.6.1. Concrete

9.6.2. Buried Metal Pipes

9.7. Scaling

9.8. Frost Heave

9.9. Construction in Cold or Wet Weather

9.10. Site Drainage

501165001R ii

9.11. Construction Observation and Testing

9.12. Plan Review

9.13. Pre-Construction Meeting

10. LIMITATIONS

11. REFERENCES

Tables Table 1 – 2012 International Building Code Seismic Design Criteria Table 2 –Lateral Earth (Equivalent Fluid) Pressures

Figures Figure 1 – Site Location Figure 2 – Boring Locations Figure 3 – Retaining Wall Drainage Detail

Appendices Appendix A – Boring Logs Appendix B – Laboratory Testing

501165001R 1

1. INTRODUCTION

In accordance with your request and authorization, we have performed a geotechnical evaluation for the proposed elevator addition at the United States Air Force Academy (USAFA) Falcon Sta-dium located at 2169 Field House Drive in Colorado Springs, Colorado. The approximate location of the site is depicted on Figure 1.

The purpose of our study was to evaluate the subsurface conditions and to provide design and construction recommendations regarding geotechnical aspects of the proposed project. This re-port presents the findings of our subsurface exploration, results of our laboratory testing, conclusions regarding the subsurface conditions at the site, and geotechnical recommendations for design and construction of this project.

2. SCOPE OF SERVICES

The scope of our services for the project generally included:

Review of referenced background information, including aerial imagery, published geologic maps, previous geotechnical evaluations, in-house geotechnical data, and available topo-graphical information pertaining to the project site and vicinity.

Walking geologic reconnaissance, mark-out of the boring locations at the project site, and notification of Utility Notification Center of Colorado (UNCC) of the boring locations prior to drilling.

Obtaining a dig permit through the USAFA to perform the subsurface investigation.

Drilling, logging, and sampling of four (4) small-diameter exploratory borings within the project site to depths ranging from 19.4 to 30 feet below ground surface (bgs). The boring logs are presented in Appendix A. Boring locations are presented on Figure 2.

Performance of laboratory tests on selected samples obtained from the borings to evaluate engineering properties including in-situ moisture content and dry density, Atterberg limits, gradation and percent materials finer than the No. 200 sieve, shear strength, and soil corro-sivity characteristics (including pH, resistivity, sulfate content, and chloride content). The results of the in-situ moisture content and dry density laboratory testing are presented on the boring logs and the remainder of the laboratory testing results is presented in Appendix B.

Compilation and analysis of the data obtained.

Preparation of this report presenting our findings, conclusions, and geotechnical recommen-dations regarding design and construction of the project.

501165001R 2

3. SITE DESCRIPTION AND BACKGROUND REVIEW

The approximate location of the site is depicted on Figure 1. The proposed new elevators are to be located in the southwest portion of the existing Falcon Stadium. Falcon Stadium is situated at the eastern extent of an east-west trending ridge with a crest sloping gradually downhill eastward from the Rampart Range. Based on our field observations, the vicinity of Falcon Stadium generally slopes eastward. The site is bounded to the west by a wooded hill rising to the west and to the north, east, and south by stadium parking lots. The parking lots are followed by Academy

Drive to the north, Stadium Boulevard to the east and a wooded area to the south. Rampart

Reservoir is located approximately 6.4 miles southwest of the site. Much smaller Non-Potable

Reservoir Number One is located approximately 0.5 miles east of the site.

The existing Stadium was constructed in 1962 in a topographic bowl below the foot of the

Rampart Range of the Rocky Mountains. The stadium field is at an elevation of approximately

6,621 feet above mean sea level (MSL). The stadium contains three plaza levels with the lower level, middle level, and upper level plazas at elevations of approximately 6,647 feet above MSL, 6,673 feet above MSL, and 6,713 feet above MSL, respectively. According to our aerial imagery review, the stadium has existed similar to its current condition since September of 1999 or earlier.

The proposed project includes the addition of a pair of elevators and associated stairways in vegetated, steep, southeast-facing slopes between the lower level plaza and middle level plaza, as well as between the middle level plaza and the upper level plaza.

4. PROPOSED CONSTRUCTION

Based on our review of a concept elevator plan by Merrick & Company dated August 2016, the project includes the design and construction of 2 new elevators to provide ADA access between the Gate 3 lower level plaza area and the Gate 5 middle level plaza area, and 2 new elevators to provide ADA access between the Gate 5 middle level plaza area and the Gate 7 upper level plaza area. We assume that the entrances and exits from the elevators will generally match elevations of the existing lower level, middle level, and upper level plazas. The project will also include

501165001R 3 removal of existing landscaping areas, construction of new stairways adjacent to the new elevators, and construction of new plaza segments adjacent to the proposed elevators and stairs.

Grading plans were not provided at the time of preparation of this report. We assume the proposed construction will require minimal material cuts into the existing sloped landscape areas for any proposed elevator pits. However, we assume the development of the proposed elevators, stairways, and new plaza levels will involve design and construction of retaining walls retaining material fill thicknesses up to 30 feet. The fill will be placed on slopes in the existing landscaping areas, bringing the areas to grade below the proposed new plaza segments.

5. FIELD EXPLORATION AND LABORATORY TESTING

On August 3, 2016, Ninyo & Moore conducted subsurface exploration at the site to evaluate the existing subsurface conditions and to collect soil samples for laboratory testing. The evaluation consisted of the drilling, logging, and sampling of four small-diameter borings using a truck-mounted Failing SS10 drill rig equipped with 4-inch diameter solid-stem augers to depths of ap-proximately 19.4 to 30 feet bgs. The approximate locations of the borings are presented on

Figure 2. Relatively undisturbed and disturbed soil samples were collected at selected intervals.

The sampling methods used during the subsurface evaluation are presented in Appendix A.

Soil samples collected during the subsurface exploration were transported to the Ninyo & Moore laboratory for geotechnical laboratory analyses. Selected samples were analyzed to evaluate en-gineering properties including in-situ moisture content and dry density, Atterberg limits, gradation and percent materials finer than the No. 200 sieve, shear strength, and soil corrosivity characteristics (including resistivity, pH, sulfate content, and chloride content). The results of the in-situ moisture content and dry density tests are presented on the boring logs in Appendix A.

Descriptions of the laboratory test methods and the remainder of the test results are presented in

Appendix B.

501165001R 4

6. GEOLOGY AND SUBSURFACE CONDITIONS

The geology and subsurface conditions at the site are described in the following sections.

6.1. Geologic Setting

The site is located at the United States Air Force Academy below the foot of the Rampart

Range, within the Colorado Piedmont section of the Great Plains Physiographic Province.

The Laramide Orogeny uplifted the Rocky Mountains during the late Cretaceous and early

Tertiary Periods. Subsequent erosion deposited sediments east of the Rocky Mountains, including the Dawson Formation in the area. As a result of regional uplift approximately 5 to

10 million years ago, streams down-cut and excavated into the Great Plains forming the

Colorado Piedmont section (Trimble, 1980).

The majority of the stadium is mapped by Thorson et. al (2001) as being underlain by the

Upper Cretaceous and Paleocene-age facies unit one of the upper part of the Dawson

Formation. The southeastern and eastern portions of the stadium are mapped as being underlain by Holocene and late Pleistocene colluvium. Thorson et. Al (2001) describes facies unit one of the upper part of the Dawson Formation as cliff-forming, white to light gray, cross-bedded or massive, very coarse arkosic sandstone, pebbly arkose, or conglomerate; beds of massive, light gray to light brownish gray; very thick-bedded, homogenous clayey sandstone; and lesser amounts of white to light tan, cross bedded, poorly-sorted, fine- to medium-grained feldspathic sandstone. The formation in the northwestern part of the Pikeview Quadrangle, to which Falcon Stadium belongs, contains numerous interbeds of 1- to 6-foot thick, light to dark red, paleosol. The formation reaches thicknesses of up to 1,200 feet. Thorson et. al (2001) describes the colluvium as mostly matrix-supported gravelly sand, silty clay, and clayey silt; usually coarser grained on steeper slopes and finer grained on lower slopes.

6.2. Subsurface Conditions

Our understanding of the subsurface conditions at the project site is based on our field ex-ploration and laboratory testing, review of published geologic maps, historic aerial imagery, 501165001R 5 and our experience with the general geology of the area. The following sections provide a generalized description of the subsurface materials encountered. More detailed descriptions are presented on the boring logs in Appendix A.

6.2.1. Fill

Approximately 7 inches of concrete were encountered at the surface in the borings.

Fill materials were encountered below the concrete in Borings B-2, B-3, and B-4, and generally extended to depths ranging from approximately 14 to 14.5 feet bgs in

Borings B-2 and B-3, respectively, and to the termination depth of approximately 30 feet bgs in Boring B-4. The fill materials encountered at the site were generally com-posed of red, reddish yellow, and brown to dark brown, moist to wet, clayey fine to coarse clayey sand with varying amounts of gravel.

Based on the results of the laboratory testing, selected samples of the fill had in-place moisture contents between approximately 4.7 and 9.3 percent and dry densities between approximately 117.6 and 130.3 pcf.

6.2.2. Colluvium

Colluvial deposits were encountered in Boring B-2 below the fill and extended to a depth of approximately 24.5 feet bgs. The colluvium generally consisted of reddish brown to brown, wet, medium dense, clayey fine to coarse sand.

Based on the results of the laboratory testing, selected samples of the colluvium had in-place moisture contents between approximately 7 and 14.7 percent and dry densities between approximately 118.9 and 127.6 pcf.

6.2.3. Dawson Formation

The Dawson Formation was encountered below the concrete in Boring B-1, the col-luvium in Boring B-2, and the fill in Boring B-3, and extended to borings’ termination depths of approximately 19.4 to 29.3 feet bgs. The Dawson Formation generally consisted of reddish brown to brown, yellow, red, and yellow to red, dry to

501165001R 6 moist, weakly to strongly cemented, fine to coarse grained sandstone with varying amounts of clay and silt.

Based on the results of the laboratory testing, selected samples of the Dawson

Formation had in-place moisture contents between approximately 3.6 and 5.4 percent and dry densities between approximately 110 and 112.2 pcf.

6.3. Groundwater

Groundwater was not encountered in the borings during drilling. Installation of groundwater monitoring wells was not within the scope of our geotechnical investigation. Groundwater levels can fluctuate due to seasonal variations in precipitation, irrigation, groundwater with-drawal or injection, and other factors. Based on our understanding of the project, groundwater is not anticipated to be a constraint during construction.

7. GEOLOGIC HAZARDS

The following sections describe potential geologic hazards at the site including faulting and seismicity, expansive soils, compressible/collapsible soils, and liquefaction potential.

7.1. Faulting and Seismicity

Historically, several minor earthquakes have been recorded around the Colorado Springs ar-

ea. Based on our field observations and our review of readily available published geological maps and literature, there are no known active faults underlying or adjacent to the subject site. The fault closest to the project site is the Rampart Range Fault.

The Rampart Range Fault lies approximately 3.4 miles west of the site (USGS & CGS, 2013). The fault trends north-south along the eastern side of the Front Range. The Rampart

Range Fault is approximately 28½ miles long and dips to the west (Widmann, 1997). Dick-son (1986) excavated and logged two trenches on the section of the fault that extends through the southwest corner of U.S. Air Force Academy property, south of the Colorado

Springs filtration plant. Trenching investigations indicated that the last displacement on this fault occurred between 600 thousand years and 30-50 thousand years before present. There-

501165001R 7 fore, the probability of damage at the site from seismically induced ground surface rupture from this fault is considered to be low.

Design of the elevator structures should be performed in accordance with the requirements of the governing jurisdictions and applicable building codes. Table 1 presents the seismic design parameters for the site in accordance with the 2012 International Building Code guidelines and adjusted maximum considered earthquake spectral response acceleration pa-rameters evaluated using the United States Geological Survey (USGS, 2016) ground motion calculator (web-based).

Table 1 – 2012 International Building Code Seismic Design Criteria

Seismic Design Factors Value

Site Class D Site Coefficient, Fa 1.6 Site Coefficient, Fv 2.4 Mapped Spectral Acceleration at 0.2-second Period, Ss 0.216 g Mapped Spectral Acceleration at 1.0-second Period, S1 0.059 g Spectral Acceleration at 0.2-second Period Adjusted for Site Class, SMS 0.345 g Spectral Acceleration at 1.0-second Period Adjusted for Site Class, SM1 0.141 g Design Spectral Response Acceleration at 0.2-second Period, SDS 0.230 g Design Spectral Response Acceleration at 1.0-second Period, SD1 0.094g

Horizontal peak ground accelerations for the site were calculated from the 2008 Probabilis-tic Seismic Hazard Assessment prepared by the USGS National Seismic Hazard Mapping

Project (USGS, 2008). The horizontal peak ground accelerations for the site that have a 10, 5, and 2 percent probability of being exceeded in 50 years are 0.030g, 0.051g, and 0.096g, respectively. These ground motion values were calculated for "firm rock" sites, which corre-spond to a shear-wave velocity of approximately 2,500 feet per second in approximately the top 100 feet bgs. Different soil or rock conditions may amplify or de-amplify these values.

7.2. Expansive Soils

One of the more significant geologic hazards in Colorado is the presence of swelling clays in bedrock or surficial deposits. Moisture changes to bedrock or surficial deposits containing swelling clays can result in volumetric expansion and collapse of those units. Changes in

501165001R 8 soil moisture content can result from rainfall, irrigation, pipeline leakage, surface drainage, perched groundwater, drought, or other factors. Volumetric change of expansive soil may cause excessive cracking and heaving of structures with shallow foundations, concrete slabs-on-grade, or pavements supported on these materials. Construction on soils known to be po-tentially expansive could have a significant impact to the project.

A review of a Colorado Geological Survey map delineating areas based on their relative po-tential for swelling in the Denver area by Hart (1973-1974) indicates the native soil materials in the site vicinity typically have low to moderate swell potential.

Laboratory testing for the swell potential of the on-site material was not performed for our geotechnical analysis of the proposed project due to the primarily granular nature of the site materials. Based on our experience and the presence of relatively sandy materials encoun-tered in the subsurface exploration, it is our opinion that the on-site soils do not have swelling characteristics sufficient to present a significant impact to the proposed project.

7.3. Compressible/Collapsible Soils

Compressible soils are generally comprised of soils that undergo consolidation when ex-posed to new loadings, such as fill or foundation loads. Soil collapse (or hydrocollapse) is a phenomenon where soils undergo a significant decrease in volume upon an increase in mois-ture content, with or without an increase in external loads. Buildings, structures, and other improvements may be subject to excessive settlement-related distress when compressible soils or collapsible soils are present.

Laboratory testing was not performed to characterize the consolidation characteristics of the fill, colluvium, or Dawson Formation materials. Based on the results of our subsurface ex-ploration, we do not anticipate significant consolidation settlement within the overburden soils or Dawson Formation resulting from the anticipated new loadings.

7.4. Liquefaction Potential

Liquefaction is a phenomenon in which loose, saturated soils lose shear strength under short-term (dynamic) loading conditions. Ground shaking of sufficient duration results in the

501165001R 9 loss of grain-to-grain contact in potentially liquefiable soils due to a rapid increase in pore water pressure, causing the soil to behave as a fluid for a short period of time.

To be potentially liquefiable, a soil is typically cohesionless with a grain-size distribution generally consisting of sand and silt. It is generally loose to medium dense and has a rela-tively high moisture contents, which is typical near or below groundwater level. The potential for liquefaction decreases with increasing clay and gravel content, but increases as the ground acceleration and duration of shaking increase. Potentially liquefiable soils need to be subjected to sufficient magnitude and duration of ground shaking for liquefaction to occur. Based on the absence of groundwater to the depths explored during our subsurface exploration and the low ground motion hazard, liquefaction is not considered a hazard at this site.

8. CONCLUSIONS

Based on the results of the subsurface evaluation, laboratory testing, and data analyses, it is our opinion that the proposed project is feasible from a geotechnical standpoint, provided the rec-ommendations presented herein are implemented and appropriate construction practices are followed. Geotechnical design and construction considerations for the proposed project include the following:

Fill materials were encountered at this site to depths of generally 14 to 30 or more feet bgs.

The fill materials consist of clayey fine to coarse sand.

Colluvium was encountered below the fill in Boring B-2 and extended to a depth of approx-imately 24.5 feet bgs. Based on the field exploration, laboratory test results, and background review, the colluvial deposits consist of medium dense, clayey fine to coarse sand.

Dawson Formation bedrock was encountered below the concrete of the Lower Level Plaza in Boring B-1, the colluvium in the Middle Level Plaza in Boring B-2, and the fill materials in the Middle Level Plaza in Boring B-3. The Dawson Formation bedrock was composed of weakly to strongly cemented, sandstone.

Up to 30 feet of new fill is anticipated for construction of the new plaza levels. Settlements due to the weight of the new fill should be allowed to occur before placement of concrete for the plaza levels.

501165001R 10

The on-site soils should generally be excavatable to the anticipated removal depths with moderate to heavy-duty earthmoving or excavating equipment in good operating condition.

Site soils generated from on-site excavation activities that are free of deleterious materials, and do not contain particles larger than 6 inches in diameter, can generally be used as engi-neered fill during site grading provided they are moisture-conditioned and compacted as recommended in this report.

Groundwater was not encountered during our subsurface exploration. While groundwater levels can fluctuate due to seasonal variations in precipitation, irrigation, groundwater with-drawal or injection, and other factors, ground is not anticipated to be a constraint during construction.

Based on our laboratory data, the sulfate content of the tested soils presents a negligible risk of sulfate attack to concrete.

Based on our laboratory data, the on-site materials have moderate resistivity characteristics and low chloride content, and could potentially be moderately corrosive to ferrous metals.

Therefore, special consideration should be given to the use of heavy gauge, corrosion pro-tected, underground steel pipe or culverts, if any are planned. As an alternative, plastic pipe or reinforced concrete pipe could be considered. A corrosion specialist should be consulted for further recommendations.

No known or reported geologic hazards are reported underlying, or adjacent to, the site.

Based on the driven sample blow-count values at the site, and the low ground motion hazard (relatively low ground accelerations), the likelihood or potential for liquefaction is consid-ered to be negligible and therefore not a design consideration.

9. RECOMMENDATIONS

Based on our understanding of the project, the following sections present our geotechnical rec-ommendations for design and construction of the proposed elevators and plaza levels.

Grading plans were not available at the time of this report. It is important that Ninyo & Moore be notified and given an opportunity to reevaluate our recommendations once this information be-comes available and prior to bidding the project for construction.

9.1. Demolition

The subject project may include demolition of existing flatwork, seating, landscaping, and other site improvements. All remnants from demolition activities should be removed from the site. Demolition of the existing improvements should include rerouting, removal, or in-

501165001R 11 place abandonment of underground utilities. Utilities should be adequately capped or rerouted at the project perimeter in accordance with the requirements of the governing authorities and the recommendations of the geotechnical consultant at the time of demolition. Abandoned underground utility pipes under proposed improvement limits should be removed from the site, or, if the pipes are left in place, they should be filled with flowable fill, such as grout or controlled low strength material (CLSM). The contractor should take adequate precautions when conducting excavations associated with the demolition activities to reduce the potential for damage to existing utilities that are to remain in service.

If structural components of existing underground utility pipes will be left in place under the proposed building footprints, these should be discussed on a case-by-case basis with the project Geotechnical Engineer and the design team. Additional site specific evaluations and data analysis may be needed to address these conditions.

9.2. Earthwork

The following sections provide our earthwork recommendations for this project. We antici-pate the site grading may consist of material fills up to 30 feet.

9.2.1. Excavations

Our evaluation of the excavation characteristics of the on-site materials is based on the results of the subsurface exploration, our site observations, and our experience with similar materials. The on-site fill and colluvial soils may generally be excavated with moderate to heavy-duty earthmoving or excavation equipment in good operat-ing condition. Strongly cemented zones are anticipated when excavating in materials of the Dawson Formation. Excavation in these materials may require the use of heavy ripping or rock breakers.

Equipment and procedures that do not cause significant disturbance to the excavation bottoms should be used. Excavators and backhoes with buckets having large claws to loosen the soil should be avoided when excavating the bottom 6 to 12 inches of ex-cavations as such equipment may disturb the excavation bases.

501165001R 12

The fill and colluvial deposits are susceptible to variations in moisture-content. As a result and depending on the time of year construction occurs, wet or saturated soils may be encountered after periods of heavy or prolonged precipitation. These soils may soften under the action of light equipment and foot traffic. Where encountered, drying or overexcavation of these materials is recommended. If the subgrade be-comes disturbed, it should be compacted or removed and replaced before placing additional backfill material.

Stabilization methods should be provided by the grading contractor, as needed, and may include the use of geogrids, geotextiles, pushing oversized rock into the sub-grade, and/or chemical stabilization. The subgrade stabilization methods proposed should be discussed with the Geotechnical Engineer prior to implementation. The stabilization method selected should consider the effects of such stabilization on fu-ture utility installation and/or repair work.

Groundwater was not encountered during our subsurface investigation. However, groundwater levels can fluctuate due to seasonal variations in precipitation, irriga-tion, groundwater withdrawal or injection, and other factors. Based on our understanding of the project, groundwater is not anticipated to be a constraint to the proposed construction.

The contractor should provide safely sloped excavations or an adequately construct-ed and braced shoring system, in compliance with Occupational Safety and Health

Administration (OSHA, 2005) guidelines, for employees working in an excavation that may expose employees to the danger of moving ground. If material is stored or equipment is operated near an excavation, stronger shoring should be used to resist the extra pressure due to superimposed loads.

9.2.2. Site Grading

Prior to grading, the ground surface in proposed improvement areas should be cleared of any surface obstructions, existing pavements, debris, topsoil, organics (in-cluding vegetation), and other deleterious material. Material generated from site

501165001R 13 stripping should not be incorporated into engineered fill, but may be stockpiled for re-use as landscaping material or other non-structural material. Topsoil-contaminated fill materials should not be used during site grading.

Materials generated from clearing operations should be removed from the project site for disposal (e.g. at a legal landfill site). Obstructions that extend below finish grade, if present, should be removed and resulting voids filled with compacted, engineered fill or CLSM.

The exposed subgrade materials should be firm and unyielding prior to fill place-ment. Subgrades should be evaluated by our representative during the excavation work. Additional recommendations specific to the site conditions encountered may be provided at the time of construction. The project budget should include additional cost associated with the removal and replacement of unsuitable subgrade material.

Subgrade materials that are disturbed during grading should be moisture conditioned and re-compacted according to the recommendations provided in this report.

Where fill is placed on slopes steeper than approximately 5:1 (horizontal:vertical), benches should be constructed into the overburden soils (natural or man-made) or formational material to minimize the plane of weakness between the new fill and existing subgrades.

The proposed elevators may be supported on shallow foundation systems consisting of spread-footings bearing on 2 or more feet of engineered fill extending to suitable overburden deposits or directly on intact sandstone. The engineered fill limits should extend laterally 12 or more inches beyond the footing limits.

New exterior flatwork should be placed on 12 or more inches of moisture-conditioned and compacted engineered fill extending to suitable native deposits.

Some shrinkage and bulkage should be anticipated when on-site soils are excavated, processed, and compacted. For planning purposes, an estimated shrinkage factor of up to approximately of 10 percent may be used for on-site colluvium.

501165001R 14

9.2.3. Re-Use of Site Soils and Rock

Clayey sand fill material was encountered below the upper level and middle level plaza areas to depths of approximately 14 feet to a depth explored of 30 feet bgs. The clayey sand fill material, as encountered in the borings, is anticipated to be suitable to be re-used as engineered fill. However, additional testing should be performed at the time of fill placement to check the suitability of the material.

Below the fill materials, colluvial deposits were encountered in Boring B-2 to a depth of approximately 24.5 feet bgs. Based on the laboratory test results and our general observations, it is our opinion that colluvium free of deleterious materials and organic matter, and does not contain particles larger than 6 inches in diameter, can generally be used as engineered fill provided it is moisture-conditioned and compacted as recommended in this report.

Below the fill materials and colluvial deposits, sandstone of the Dawson Formation was encountered in Borings B-1, B-2, and B-3 to the depths explored. We anticipate that sandstone materials will be pulverized during excavation operations. Based on the laboratory test results and our general observations, it is our opinion that pulverized sandstone material with greater than 50 percent passing the No. 4 sieve, free of deleterious materials and organic matter, and does not contain particles larger than 6 inches in diameter, can generally be used as engineered fill provided it is moisture-conditioned and compacted as recommended in this report.

Fragments of rock, cobbles, and inert construction debris (e.g., concrete or asphalt) larger than 6 inches in diameter may be incorporated into the project fills in non-structural areas and below the anticipated utility installation depths. A Geotechnical

Engineer should be consulted regarding appropriate recommendations for usage of such materials on a case-by-case basis when such materials have been observed during earthwork. Care should be taken to avoid nesting of oversized materials during placement. Recommendations provided in Section 203 of the current CDOT

Standard Specifications for Road and Bridge Construction should be followed during the placement of oversized material.

501165001R 15

9.2.4. Fill Placement and Compaction

Granular soils (on-site soils that classify as SC, SM, SP, SP-SM, or import soils) used as engineered fill should be moisture-conditioned to moisture contents within 2 percent of optimum moisture content. Engineered fill should be compacted to a relative compaction of 95 percent, or more, as evaluated by ASTM D1557.

Fill should be compacted by appropriate mechanical methods using vibratory compaction equipment. The optimal lift thickness of fill will depend on the type of soil and compaction equipment used, but should generally not exceed approximately

8 inches in loose thickness. Fill materials should not be placed, worked, or rolled while they are frozen or thawing, and should not be placed during poor/inclement weather conditions.

Compaction areas should be kept separate, and no lift should be covered by another until relative compaction and moisture content within the recommended ranges are obtained.

Use of CLSM should be considered in lieu of compacted fill for areas with low tolerances for surface settlements, for excavations that extend below the groundwater table and in areas with difficult access for compaction equipment. CLSM should be placed in lifts of 5 feet or less with a 24-hour or more curing period between each lift.

9.2.5. Imported Soil

Imported soil for use as engineered fill should have 40 or less percent passing the

No. 200 sieve, a very low swell potential (approximately 1 percent or less when wet-ted against a surcharge pressure of 500 psf when remolded at optimum moisture content), and a low plasticity index (approximately 15 or less).

Imported soil should not contain organic matter, clay lumps, bedrock (claystone, sandstone, etc.) fragments, debris, other deleterious matter, or rocks or hard chunks larger than approximately 3 inches nominal diameter.

501165001R 16

Import material in contact with ferrous metals should have low corrosion potential.

Import material in contact with concrete should have soluble sulfate content less than 0.1 percent.

We further recommend that proposed import material be evaluated by the project’s geotechnical consultant at the borrow source for its suitability prior to importation to the project site. Import soil should be moisture-conditioned and placed and compacted in accordance with the recommendations set forth in Section 9.2.4.

9.2.6. Settlement Monitoring

We recommend that a settlement monitoring program be implemented to monitor the rate of settlement of the mass earthwork and determine when the concrete can be placed for the plaza levels. We recommend that 4 to 6 settlement devices be installed within the proposed plaza areas. Placement of the monuments should be determined by Ninyo

& Moore. Ninyo & Moore should be retained to review data from the settlement moni-toring instruments. The settlement monuments should be installed within the top of the fill and should consist of No. 4 or larger rebar with a length of at least 5 feet, driven 2 or more feet into the fill for stability. Ninyo & Moore should review the settlement plate details and the proposed installation procedures prior to earthwork construction. Care should be taken not to disturb the monuments during the monitor-ing period.

The elevations of the settlement devices should be monitored in general accordance with the following schedule:

Precise elevation data will be necessary for accurate settlement analyses, and survey-ing services should be obtained accordingly. The monitoring data should be submitted to Ninyo & Moore for analysis and evaluation of when construction may proceed. A settlement period in the range of about 2 to 3 weeks is estimated. Actual settlement period will be a function of the pace of earthwork operations. These are only estimates based on our experience with projects of this type, the soil conditions observed, and the test results. Other factors affecting the effective drainage path and

501165001R 17 permeability of the soils may reduce or increase the time for the settlement to occur.

Monitoring of these settlements will allow this to be observed, if it occurs, potential-ly allowing construction of the plazas to begin at an earlier date than anticipated. We anticipate at least two weeks of settlement data will be required before we can begin evaluation of eventual total settlements and refinement of our estimates for an effec-tive settlement time frame.

9.2.7. Utility Installation

The contractor should take particular care to achieve and maintain adequate compaction of the backfill soils around manholes, valve risers and other vertical pipeline elements where settlements are commonly observed. Use of CLSM or a similar material should be considered in lieu of compacted soil backfill in areas with low tolerances for surface settlement. This would also reduce the permeability of the utility trenches.

Pipe bedding materials, placement and compaction should meet the specifications of the pipe manufacturer and applicable municipal standards. Materials proposed for use as pipe bedding should be tested for suitability prior to use.

Special care should be exercised to avoid damaging the pipe or other structures during the compaction of the backfill. In addition, the underside (or haunches) of the buried pipe should be supported on bedding material that is compacted as described above. This may need to be performed with placement by hand or small-scale compaction equipment.

Surface drainage should be designed to divert the surface water away from utility trench alignments. Where topography, site constraints or other factors limit or preclude adequate surface drainage, the granular bedding materials should be surrounded by a non-woven geotextile fabric (e.g., TenCate Mirafi® 140N or the equivalent) to reduce the migration of fines into the bedding which can result in severe, isolated settlements.

501165001R 18

Development of site grading plans should consider the subsurface transfer of water in utility trenches and the pipe bedding. Sandy pipe bedding materials can function as efficient conduits for re-distribution of natural and applied waters in the subsurface. Cut-off walls in utility trenches or other water-stopping measures should be implemented to reduce the rates and volumes of water transmitted along utility alignments and toward buildings, pavements and other structures where excessive wetting of the underlying soils will be damaging. Incorporation of water cut-offs and/or outlet mechanisms for saturated bedding materials into development plans could be beneficial to the project. These measures also will reduce the risk of loss of fine-grained backfill soils into the bedding material with resultant surface settlement.

9.2.8. Temporary Cut Slopes

Temporary excavations will be needed for this project to construct the proposed structures and utilities. Based on the subsurface information obtained from our exploratory excavations and our experience with similar projects, we anticipate that the soil conditions and stability of the excavation sidewalls may vary with depth.

Soils with higher fines content may stand vertically for a short time (less than 12 hours) with little sloughing. However, as the soil dries after excavation or as the excavations are exposed to rainfall, sloughing may occur. Soils with low cohesion

(e.g., predominately sandy or gravelly material), may slough or cave during excavation, especially if wet or saturated.

The contractor should provide safely sloped excavations or an adequately constructed and braced shoring system, in compliance with OSHA regulations as mentioned in Section 9.2.1.

In our opinion, the site soils and rock should generally be considered a Type C soil when applying the OSHA regulations. For these soil conditions, OSHA recommends a temporary slope inclination of 1.5H:1V or flatter for excavations 20 feet or less in depth. Appropriate slope inclinations should be evaluated in the field by an

OSHA-qualified “Competent Person” based on the conditions encountered.

501165001R 19

9.3. Spread Footing Foundations

Perimeter footings should extend to 36 inches or more below the lowest exterior finished grade (for frost protection). Continuous wall footings should have a width of 18 inches or more and column footings should have a width of 24 inches or more. Footings should be re-inforced in accordance with the recommendations of the Structural Engineer.

Footings bearing on an improved subgrade overlying approved overburden soils (natural and man-made) as recommended in Section 9.2.2 may be designed using a net allowable soil bearing pressure of 3,000 pounds per square foot (psf) for static conditions. Footings bearing on intact sandstone material may be designed using a net allowable soil bearing pressure of

5,000 pounds per square foot (psf) for static conditions. The bearing capacity may be in-creased by one-third when considering loads of short duration such as wind or seismic forces. The foundations should preferably be proportioned such that the resultant force from design loads, including lateral loads, falls within the kern (i.e., middle one-third of the foot-ing base).

Uplift resistance can be developed from the weight of the footings, the effective weight of any overlying soil, and the weight of the supported structure itself. The effective unit weight of the soil can be assumed to be 120 pcf above the highest possible groundwater level and

60 pcf below. Soil uplift resistance may be calculated as the weight of the soil prism defined by a diagonal line extending from the perimeter of the foundation to the ground surface at an angle of up to 20 degrees from the vertical. Under large moment and/or shear loading, the effective size of the uplift soil prism may be reduced. An appropriate safety factor should be applied.

The bottom surface of foundation excavations should be compacted with hand-held dynamic compaction equipment (i.e., jumping jack, flat-plate vibrator) prior to placement of forms and reinforcing steel. The base of foundation excavations should be free of water and loose soil prior to placing concrete. Concrete should be placed soon after subgrade compaction to reduce bearing soil or rock disturbance. Should the soils at bearing level become excessively dry, disturbed, or saturated, the affected soil should be moisture conditioned and recompact-

501165001R 20

ed. It is recommended that Ninyo & Moore be retained to observe, test, and evaluate the soil and rock foundation bearing materials.

Based on the results of our subsurface exploration and laboratory testing, we estimate total and differential settlement of up to about 1 inch and 1/2-inch, respectively, may occur. Due to the amount of fill placed adjacent to the elevator cores for the new plaza construction, fi-nal connections between the plaza level and elevators should be delayed until settlements from the weight of the new fill have occurred.

9.4. Earth Pressures and Retaining Walls

Earth pressures are used to compute the lateral forces acting on reinforced concrete walls.

These pressures can be classified as at-rest, active, and passive. The direction and magnitude of the soil/wall movement just before failure affects the resulting pressure condition. At-rest conditions exist when there is no movement, such as for a restrained wall. Active stresses are exerted when the wall moves out and the soil moves toward the wall away from the soil mass, thereby mobilizing the shear strength of the soil. The active pressures are fully mobilized at horizontal movements of about 0.1 percent of the wall height for cohesionless soils, such as sands and gravels. Passive stresses exist when the wall moves toward the soil mass. Movement typically needed to mobilize passive pressures greatly exceeds that needed to mobilize active pressures. The passive pressures are, therefore, rarely fully mobilized and are often overestimated when used to compute resistance forces.

The recommended equivalent fluid pressures in Table 2 assumes the free-draining, granular backfill would exhibit an angle of internal friction () of 30 degrees and a unit weight of 120 pcf. The on-site soils would exhibit and internal friction angle (ϕ) of 28 degrees and a unit weight of 120 pcf. The parameters listed in the table below are for static conditions.

Table 2 –Lateral Earth (Equivalent Fluid) Pressures

Soil Condition

Active Pressure (pcf)

At-rest Pressure (pcf)

Passive Pressure (pcf)

Free-Draining Granular

40 60 360

On-Site Overburden 43 64 332

501165001R 21

The use of heavy compaction equipment adjacent to below-grade walls could result in lateral earth pressures well in excess of those predicted in Table 2. We recommend that the upper

24 inches of soil that is not protected by pavement or a concrete slab, be neglected when calculating passive resistance. For frictional resistance to lateral loads, we recommend that an ultimate coefficient of friction of 0.35 be used between soil and concrete. The ultimate coefficient of friction can be increased to 0.40 for foundations bearing directly on intact sandstone.

To limit long-term hydrostatic pressure behind the wall, we recommend that a drain be in-stalled at the foundation wall with a collection pipe leading to a reliable discharge. Based on the subsurface conditions, we recommend the pipe be sloped no less than 0.25 percent. A de-tail for the foundation drain may be found in Figure 3.

9.5. Concrete Flatwork

The new plazas should be 4 or more inches thick. The ground-supported flatwork will be subject to soil-related movements resulting from heave/settlement, frost, etc. Thus, where these types of elements abut rigid building foundations or isolated/suspended structures, dif-ferential movements should be anticipated. We recommend that flexible joints be provided where such elements abut the main structure to allow for differential movement at these lo-cations. Positive drainage should be established and maintained adjacent to flatwork. Water should not be allowed to pond on flatwork.

To reduce the potential manifestation of distress to exterior concrete flatwork due to move-ment of the underlying soil, we recommend that such flatwork be installed with crack-control joints at appropriate spacing as designed by the Structural Engineer.

9.6. Corrosion Considerations

The corrosion potential of on-site soils to concrete and buried metal was evaluated in the la-boratory using selected samples obtained from the exploratory borings. Laboratory testing was performed to assess the effects of sulfate on concrete and the effects of soil resistivity on buried metal. Results of these tests are presented in Appendix B. Recommendations re-

501165001R 22 garding concrete to be utilized in construction of proposed improvements and for buried metal pipes are provided in the following sections.

9.6.1. Concrete

The test for water-soluble sulfate content of the soils was performed using CDOT

Test Method CP-L 2104. The laboratory test results are presented in Appendix B.

The laboratory test results indicate that the on-site soils present low corrosivity potential to concrete. The percentage of water-soluble sulfates in water (sulfate content) measured was low. Based on Table 601-2 of the CDOT 2011 Standard

Specifications for Road and Bridge Construction, the on-site soils represent a Class 0 severity of sulfate exposure to concrete on a scale that ranges between Class 0 and

Class 3. Therefore, we recommend that the concrete used for this project should have a maximum water to cementitious material ratio of 0.45 and the cementitious materials should meet one of the below outlined requirements.

ASTM C 150 Type I, II or V ASTM C 595 Type IP ASTM C 1157 Type GU ASTM C 150 Type III cement if it is allowed, as in Class E concrete

The Structural Engineer should ultimately select the concrete design strength based on the project specific loading conditions. However, higher strength concrete may be selected for increased durability, resistance to slab curling and shrinkage cracking.

We recommend the use of concrete with a design 28-day compressive strength of

4,200 psi or more, for concrete slabs at this site. Concrete exposed to the elements should be air-entrained.

9.6.2. Buried Metal Pipes

The corrosion potential of the on-site materials was analyzed to evaluate its potential effects on buried metals. Corrosion potential was evaluated using the results of laboratory testing of samples obtained during the subsurface evaluation that were considered representative of soils at the subject site.

501165001R 23

The results of the laboratory testing indicate the on-site materials have moderate resistivity characteristics, low chloride content, are slightly alkaline, and have a low potential to be corrosive to ferrous metals. Therefore, special consideration is not necessarily warranted for the use of heavy gauge, corrosion protected, underground steel pipe or culverts, if any are planned. A corrosion specialist should be consulted for further recommendations.

9…