604269001 R Geotech Eval.pdf

GEOTECHNICAL EVALUATION

PROPOSED BRIDGE STRUCTURES

MARSHALL GULCH TRAILHEAD & PICNIC AREA

CORONADO NATIONAL FOREST, ARIZONA

PREPARED FOR:

Morrison-Maierle, Inc.

3011 Palmer Street Missoula, Montana 59808

PREPARED BY:

Ninyo & Moore

Geotechnical and Environmental Sciences Consultants 1991 East Ajo Way, Suite 145

Tucson, Arizona 85713

March 7, 2014 Project No. 604269001

Geotechnical Evaluation March 7, 2013 Marshall Gulch Trailhead & Picnic Area Project No. 604269001 Coronado National Forest, Arizona

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TABLE OF CONTENTS

Page

1. INTRODUCTION

2. SCOPE OF SERVICES

3. SITE DESCRIPTION

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. Bedrock

6.3. Groundwater

7. GEOLOGIC HAZARDS

7.1. Faulting

7.2. Rockfalls, Landslides, and Debris Flows

8. CONCLUSIONS

9. RECOMMENDATIONS

9.1. General Earthwork

9.1.1. Excavation

9.1.2. Temporary Slope Stability

9.1.3. Shoring

9.1.4. Grading, Fill Placement, and Compaction

9.1.5. Borrow Material

9.2. Bridge Foundations

9.2.1. Bearing Resistance for Grade Beams

9.2.2. Resistance to Lateral Loads

9.2.3. Settlement

9.2.4. Seismic Design Considerations

9.3. Lateral Earth Pressures

9.4. New Pavements

9.5. Concrete Flatwork

9.6. Corrosion

9.7. Concrete

9.8. Site Drainage

9.9. Pre-Construction Conference

9.10. Construction Observation and Testing

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10. LIMITATIONS

11. REFERENCES

Tables Table 1 – Results of North Bridge West Abutment Analysis Table 2 – Seismic Design Parameters Table 3 – Requirements for Concrete Exposed to Sulfate-Containing Solutions

Figures Figure 1 – Site Location Figure 2A – Boring Location, North Bridge Figure 2B – Boring Location, South Bridge Figure 3 - Lateral Earth Pressures for Braced Excavation in Soil Figure 4 – Factored Bearing Resistance Chart, North Bridge East Abutment Footing Figure 5 – Factored Bearing Resistance Chart, South Bridge West Abutment Footing Figure 6 – Factored Bearing Resistance Chart, South Bridge East Abutment Footing

Appendices Appendix A – Boring Logs Appendix B – Laboratory Testing

604269001 R Geotech Eval 1

1. INTRODUCTION

In accordance with our proposal dated August 16, 2013, we have performed a geotechnical evaluation for two new bridge structures (North and South) over Sabino Creek in the Marshall

Gulch Trailhead & Picnic Area in the Coronado National Forest, in Pima County, Arizona. The purpose of our evaluation was to assess the subsurface conditions at the project site in order to formulate geotechnical recommendations for design and construction. This report presents the results of our evaluation, and our geotechnical conclusions and recommendations regarding the proposed construction.

2. SCOPE OF SERVICES

Our scope of services for this project generally included:

• Reviewing available topographic information, soil surveys, geologic literature, and aerial photographs of the project area.

• Conducting a field trip to the site for surface geologic reconnaissance and boring mark out.

• Depicting planned exploration locations on a map based on the information obtained from your office and submitting the map to your office for approval.

• Contacting Arizona Blue Stake for utility clearance prior to our exploration.

• Exploring the subsurface materials within the project limits by drilling, logging, and sampling four exploratory soil borings. The borings were located within or near the footprint of the proposed bridge structure abutments and extended up approximately 13 to 15 feet deep at the North Bridge location (B-1 and B-2) and to about 29 feet deep at the South Bridge location (B-3 and B-4). The borings were advanced with a two-wheel drive, truck-mounted drill rig equipped with hollow-stem augers, and NQ double-tube system rock coring equipment. The borings were logged by Ninyo & Moore personnel and representative samples were collected in the field and returned to our laboratory for further analysis and testing. The boring logs are presented in Appendix A.

• Conducting laboratory testing of selected samples for developing parameters for foundation design, roadway pavement design, and for earthwork factors during construction. Testing generally included in-situ moisture content and dry density, gradation, Atterberg limits, corrosivity characteristics (including pH, minimum resistivity, and soluble sulfate and chloride contents), and unconfined compressive strength of selected rock core specimens.

The results of the laboratory testing are presented in Appendix B.

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• Preparation of this report presenting our findings, conclusions, and geotechnical recommendations regarding the design and construction of the planned improvements.

3. SITE DESCRIPTION

The project site is situated in Section 31 of Township 11 South, Range 16 East of the Gila and

Salt River Baseline and Meridian, in Pima County, Arizona. The approximate location of the site is depicted on Figure 1. The two new bridge structures will be located within the Marshall Gulch

Trailhead & Picnic Area in the Coronado National Forest, just south of Summerheaven, Arizona.

The picnic area is located within the Mount Lemmon Range and can be reached from Tucson by heading generally north on Catalina Highway and Mount Lemmon Highway to Summerheaven, and continuing about one mile south on North Sabino Canyon Parkway. Specifically, Marshall

Gulch Trailhead and Picnic Area is located at the south end of North Sabino Canyon Parkway.

Within the project area, Sabino Canyon Parkway traverses a relatively undeveloped natural mountain landscape of the Coronado National Forest, generally parallel to and west of Sabino

Creek. North Sabino Canyon Parkway is a two-lane asphalt concrete (AC) paved roadway providing vehicular access to the picnic area. The roadway ends at the south (lower) parking area, where it becomes a connecting trail. The parking areas are partly paved with a significantly distressed AC pavement or chip seal coat, and partly gravel-surfaced. The Sabino Canyon

Parkway roadway pavement is in a relatively good condition. Other existing improvements include trailhead kiosks and restroom facilities.

The Picnic Area currently includes two other parking areas located along the west and east sides of Sabino Creek (upper parking area). A low water crossing with two arch culverts provides access to the upper parking area on the east side. The western edge of this area along Sabino

Creek is protected by a 3 to 4 feet high stone retaining wall with partially damaged concrete toe armoring. Flowing water was observed in Sabino Creek during our field exploration.

Formational material and rock outcrops were observed on the slopes and in the creek bed in some areas within the vicinity of the project area. Relatively thick vegetation with pine and spruce forest is present on adjacent stream terrace deposits.

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According to the Mount Lemmon, Arizona-Pima County, 7.5-Minute United States Geological

Survey (USGS) Topographic Quadrangle Map (2011), the site lies at an average elevation of approximately 7,520 feet relative to mean sea level (MSL). The natural terrain in the project vicinity is generally rugged, mountainous, and generally slopes from the north to the south.

Seven topographic maps (between 1957 and 2002) from the Historic Aerials website

(Nationwide Environmental Title Research [NETR]) were reviewed for this project. North

Sabino Parkway was depicted on the maps dated 1959 and later, and the picnic area was depicted on the map dated 1984. The maps depicted the roadway alignment generally similar to the current alignment. We also reviewed aerial photographs from the period between 1996 and 2013 using Google Earth™. The photographs reviewed showed the roadway alignment and portions of the picnic area developments similar to their present condition.

4. PROPOSED CONSTRUCTION

We have reviewed a set of preliminary project plans entitled Marshall Gulch Road Plan and

Profile, prepared by Morrison-Maierle, Inc. (not dated). The proposed construction will include two new bridge structures over Sabino Creek, referred to as the North Bridge and the South

Bridge. The North Bridge will replace the existing low water crossing. The North and South

Bridge structures will be approximately 80 feet and 66 feet in length, respectively, and will be single-span structures constructed of precast reinforced concrete members. The bridge widths will be approximately 32 feet. We understand that grading and excavations will be performed for the bridge foundations, which will consist of a reinforced concrete grade beam on the order of

3.5 feet in width and approximately 32 feet long. We understand that the bridge foundations will be protected against scour by grouted rip rap. However, details of such protections were not available for our review.

This project will also include construction of a loop road, new parking areas, and other recreational facilities. The vertical alignment of the existing North Sabino Parkway is not expected to change. However, cuts up to about 10 feet are anticipated for the loop road at the

604269001 R Geotech Eval 4 approaches to the South Bridge and within the proposed parking areas on the east side of Sabino

Creek. Our understanding is that new fill embankments are generally not anticipated for this project. However, no cross-sections of the proposed loop road were available for our review.

Our scope of services for this project did not include evaluation of the cut slopes. We recommend that additional geotechnical exploration be performed to evaluate and provide slope configurations.

5. FIELD EXPLORATION AND LABORATORY TESTING

On November 18 and 19, 2013, Ninyo & Moore conducted a subsurface exploration in order to evaluate the existing subsurface conditions and to collect soil and rock core samples for laboratory testing. Our evaluation consisted of drilling, logging, and sampling four bridge borings, denoted as B-1 through B-4, with a CME-75 truck-mounted drill rig. Borings B-1 and

B-2 were advanced using hollow stem auger (HSA) drilling techniques and NQ double-tube system rock coring techniques at the approximate abutment locations of the North Bridge to depths of about 13.3 feet and 15.4 feet below the ground surface (bgs), respectively. Bedrock was encountered at a depth of about 6 feet in these borings. Borings B-3 and B-4 were advanced near the approximate abutment locations of the South Bridge to approximate depths of 28.8 feet bgs using HSA drilling techniques. The location of boring B-4 was moved approximately 50 feet to the north of the proposed east abutment due to difficult access. The approximate locations of the borings are provided on Figures 2A and 2B for the North and South Bridges, respectively.

Ninyo & Moore personnel logged the borings in general accordance with the Unified Soil

Classification System (USCS) and American Society for Testing and Materials (ASTM) D 2488 by observing cuttings and drive samples. Collected ring samples were trimmed in the field, wrapped in plastic bags, and placed in cylindrical plastic containers to retain in-place moisture conditions. Similarly, the Standard Penetration Test (SPT) and bulk samples were sealed in plastic bags to retain their approximate in-place moisture. Bulk samples were also collected from

604269001 R Geotech Eval 5 the cuttings and placed in large plastic bags. Rock cores were placed in wax-coated cardboard core boxes in sequence of the coring process. The boring logs are presented in Appendix A.

Soil and rock samples collected from the exploration locations were transported to the Ninyo &

Moore laboratory for geotechnical laboratory analyses. For the soil samples, the laboratory analyses included in-situ moisture content and dry density, gradation, Atterberg limits, and corrosivity characteristics (including pH, minimum electrical resistivity, and soluble sulfate and chloride contents).

The results of the in-situ moisture and density testing are presented on the logs in Appendix A. A description of each test method and the remainder of the laboratory results are presented in

Appendix B.

The laboratory analyses of the rock samples included unconfined compression tests on rock cores. These compressive strength tests were performed in general accordance with ASTM test method C-39 and C-42, which are typically used by Arizona Department of Transportation

(ADOT) to estimate the compressive strength of rock cores for their projects. Varying results may be observed if other test methods are used. Ninyo & Moore prepared the rock specimens using capping compound (as opposed to grinding the sample ends flat). If the capping compound fails before the sample breaks, the surface area over which the compressive force is applied can be greatly reduced and tensile forces can be introduced into the sample ends. Together these effects can lead to specimen failure at lower stresses. The capping compound was found to have an unconfined compressive strength of around 8,000 pounds per square inch (psi). For samples prepared in this manner, measured failure stresses higher than 8,000 pounds psi are suspect and should be considered a minimum.

6. GEOLOGY AND SUBSURFACE CONDITIONS

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

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6.1. Geologic Setting

The project site is located in the Sonoran Desert Section of the Basin and Range physiographic province, which is typified by broad alluvial valleys separated by steep, discontinuous, subparallel mountain ranges. The mountain ranges generally trend north-south and northwest-southeast. The basin floors consist of alluvium with thickness extending to several thousands of feet.

The basins and surrounding mountains were formed approximately 10 to 18 million years ago during the mid- to late-Tertiary. Extensional tectonics resulted in the formation of horsts

(mountains) and grabens (basins) with vertical displacement along high-angle normal faults.

Intermittent volcanic activity also occurred during this time. The surrounding basins filled with alluvium from the erosion of the surrounding mountains as well as from deposition from rivers. Coarser-grained alluvial material was deposited at the margins of the basins near the mountains.

Within the Basin and Range province, mountain ranges composed of rocks formed from igneous rock followed by specific metamorphic processes are also present. Mount Lemmon is an example of what is known as a “metamorphic core complex” (Force, 2001). The deformation was a response to stretching of continental crust in the southwestern United

States about 25 million years ago, predating the later formation of the Basin & Range. Rocks within Sabino Canyon, because they were theoretically at considerable depth in this crust

(about 6 miles below the surface) at the time, were hot enough for many of the constituent mineral grains in the rocks to deform in plastic or ductile manner. Such deformation allowed this deeper portion of crust to respond to stretching by delamination into thinner packages.

The platy rocks so deformed are called mylonitic gneisses, and partially define a metamorphic core complex.

The bedrock geology in the vicinity of the site is mapped as units of the Precambrian-age

(greater than 570 million years old) Oracle Granite consisting of two informal subunits

(USGS, 1976). The first informal subunit, “Y”, is Precambrian Oracle granite composed of

604269001 R Geotech Eval 7 generally light colored, porphyritic coarse grained quartz monzonite, and the second informal subunit “Yo” is Precambrian Oracle granite composed of generally dark colored biotite rich augen gneiss. A later geologic map (Force, 1997) provides more detailed geologic descriptions of map units in the Marshall Gulch area. The Eocene-age (47 to 49 million years old) Lemmon Rock Pegmatite-Aplite is mapped at the base of the mountains in Sabino Creek. This pegmatite is described as porphyritic with megacrysts of feldspar and intergrown quartz with garnet and mica.

The surficial geology of the area generally consists of relatively thin soils developed on underlying bedrock, consisting of colluvium, alluvium, and recent stream channel deposits within and adjacent to Sabino Creek. These deposits are described as colluvium and alluvial stream deposits, and generally are composed of varying amounts of clay, silt, sand, gravel, cobbles, and boulders (McKittrick, 1988, and Force, 2001).

6.2. Subsurface Conditions

Our knowledge of the subsurface conditions at the project site is based on our review of background documents; field explorations, and laboratory test results. The following paragraphs provide a generalized description of the materials encountered in our borings.

More detailed descriptions are presented on the boring logs in Appendix A.

6.2.1. Fill

Fill soils were encountered at the surface of borings B-3 and B-4 and extended up to about 3 feet bgs. The fill generally consisted of loose to medium dense silty sand with gravel.

6.2.2. Colluvium

Colluvial soils were encountered at the surface of our borings B-1 and B-2, and extended to an approximate depth of about 6 feet bgs. In our borings B-3 and B-4, colluvial soils were encountered under the fill, and extended to approximate depths of

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5.5 and 12 feet bgs, respectively. The colluvial soils generally consisted of loose to very dense, silty sand with gravel. The dark color of these deposits may indicate possible organic matter. However, organic matter content was not part of our scope of laboratory testing for this project.

6.2.3. Bedrock

Decomposed to weathered bedrock consisting of Oracle Granite subunits was encountered underlying the colluvium in our borings B-2 through B-4. Lemmon Rock

Pegmatite-Aplite was encountered underlying the colluvium in boring B-1. The bedrock in borings B-1 and B-2 transitioned from weathered to fresh and called for rock coring to achieve depths to competent bedrock. The rock coring for B-1 started at a depth of

8.5 feet and the rock coring for B-2 started at a depth of 11 feet.

The rock quality designation (RQD) for the Oracle Granite of 68 is generally considered fair quality rock while the RQD for the Lemmon Rock Pegmatite-Aplite of 15 is generally considered very poor quality rock. This indicates that variable bedrock conditions are to be expected at the site.

6.3. Groundwater

Groundwater was not encountered in the borings. According to the available well information in the Arizona Department of Water resources (ADWR) database, the depth to groundwater in the vicinity of the site is approximately 3 feet bgs. However, this information is somewhat dated as it was recorded in 1948. Few existing wells are present near the project location. Groundwater levels can fluctuate due to seasonal variations, flows in the creek, groundwater withdrawal or injection, and other factors. Depending on the time of year that construction is performed, groundwater may be a constraint to the construction of the project, as stream flows in the creek, as well as subsurface perched water in deeper excavations, should be anticipated.

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7. GEOLOGIC HAZARDS

The following section provides a discussion regarding potential geologic hazards such as faulting, rockfalls, landslides, and debris flows.

7.1. Faulting

The site lies within the Sonoran zone, which is a relatively stable tectonic region located in southwestern Arizona, southeastern California, southern Nevada, and northern Mexico

(Euge et al., 1992). This zone is characterized by sparse seismicity and few Quaternary faults. Based on our field observations, review of pertinent geologic data, and analysis of aerial photographs, Quaternary faults are not located on or adjacent to the property. An unnamed, buried fault trace is depicted in Sabino Canyon through the project area on the geologic maps referenced in this report. However, according to Force, 1997, this fault is described as Pre-Eocene, which means it is more than 58 million years old.

The closest Quaternary fault is the Whitlock Wash Fault, located about 19 miles to the northeast of the site (Pearthree, 1998), is situated along the east side of the northern San

Pedro Valley. The Whitlock Wash Fault is a series of north-northwest striking normal faults that dip to the east. Recent movement along this fault was approximately 1.6 million years ago during the Early Pleistocene epoch. The slip-rate category of this fault is less than 0.2 millimeters per year (Pearthree, 1998).

Seismic parameters recommended for the design of the proposed improvements are presented in Section 9.2.4.

7.2. Rockfalls, Landslides, and Debris Flows

Rockfall and landslide hazards have the potential to be present within Marshall Gulch and

Sabino Canyon (Force, 2001). Some mapped alluvial fan deposits may in fact be unrecognized landslides and/or rockfall debris with resulting deposits of colluvium of poorly understood or unknown age (Force, 2001). Within mountain ranges of significant and

604269001 R Geotech Eval 10 relatively steep topography, and having mountain stream reaches of relatively restricted area, debris flows can occur during heavy precipitation events. These flows consist of sudden flooding accompanied by flows of water, mud, rocks, including cobbles and boulders, and organic debris including trees that can damage, or might bury, infrastructure.

Our review of digital imagery and maps for the project area did not reveal areas of active landsliding or rockfall, and these hazards are not considered to be a constraint to the current project. For bridges or culverts that are planned to cross the Sabino Creek drainage, consideration might be given to the risk of debris flows in their design, and/or possible increased maintenance and repair rates at these structures relative to other bridge installations not located within restricted canyon settings such as exist at Sabino Creek.

8. CONCLUSIONS

Based on the results of the limited subsurface evaluation, laboratory testing, and data analysis, it is our opinion that the proposed construction is feasible from a geotechnical standpoint, provided that the recommendations of this report are incorporated into design and construction of the proposed project, as appropriate. Geotechnical considerations include the following:

• The on-site near-surface fill and colluvial soils should generally be excavatable to shallow depths with conventional earth moving construction equipment in good working condition.

However, denser gravelly/cobbly soils and bedrock should be expected in the excavations for the bridge foundations. These soils and bedrock may result in slower excavation rates and may call for more aggressive excavation techniques (i.e. may include blasting, rock sawing, pneumatic hammering, heavy duty hoe ram, etc.).

• Based on the results of the field and laboratory evaluations, we recommend that the proposed bridge structures be supported on shallow foundations placed on a zone of moisture-conditioned and compacted structural backfill that extends 2 feet or more below the bearing surface or on competent bedrock. There should not be a foundation transition between bedrock and soil within the footprint of the foundations. Based on the results of our study, some of the on-site soils will not be suitable for re-use as structural backfill.

• Pavements and exterior concrete flatwork should also be founded on a zone of moisture-conditioned and compacted subgrade that extends 6 inches or more below the bearing

604269001 R Geotech Eval 11 surface or on competent bedrock. In addition, the subgrade soils within 2 feet below the bottom of the pavement base course should have an average R-value of 30 or more.

• We estimate an earthwork (shrinkage) factor of approximately 10 to 20 percent for the on-site soils and a bulking factor of 5 to 10 percent for bedrock materials.

• Groundwater was not observed in the borings. Based on nearby well data, the groundwater table is estimated to be as shallow as 3 feet bgs. Depending on the time of year that construction is performed, groundwater may be a constraint to the construction of the project, as flows in the creek, as well as subsurface perched water in deeper excavations should be anticipated.

• There are no known geologic hazards located at or immediately adjacent to the project site.

• Corrosivity test results indicate that subgrade soils at the site are generally mildly corrosive to ferrous materials, and the sulfate content of the soils present a negligible sulfate exposure to concrete.

9. RECOMMENDATIONS

The following sections present our geotechnical recommendations for the proposed design and construction. If the proposed construction is changed from that discussed in this report, Ninyo &

Moore should be contacted for additional recommendations.

9.1. General Earthwork

The following sections provide our earthwork recommendations for this project. In general, the earthwork specifications contained in the current edition of the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-03, issued by the

United States Department of Transportation, Federal Highway Administration (FP-03

Standard Specifications), as amended by the Forest Service Standard Specifications (FSSS), are expected to apply, except as noted.

9.1.1. Excavation

Structure excavation and backfill should be performed in accordance with Section 208 of the FSSS and 209 of the FP-03 Standard Specifications.

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Our evaluation of the excavation characteristics of the on-site materials is based on the results of our exploratory borings, our site observations, and our experience on similar projects. In our opinion, excavation of the on-site soils can be accomplished to the assumed earthwork depths with conventional earthwork equipment. However, denser gravelly/cobbly soils and bedrock should be expected in the excavations for the bridge foundations. These soils and bedrock may result in slower excavation rates and may call for more aggressive excavation techniques (i.e. may include blasting, rock sawing, pneumatic hammering, heavy duty hoe ram, etc.). The contractor should be prepared to encounter competent bedrock in the excavations associated with this project. Competent bedrock is generally characterized by relatively hard, solid rock with a lack of fractures, joints, clay seams, or other discontinuities. Competent bedrock should not exhibit signs of physical or chemical weathering and should not be friable or prone to sloughing.

During construction, competent bedrock should be evaluated by the geotechnical engineer by observing the exposed excavation surfaces.

9.1.2. Temporary Slope Stability

Sidewalls for temporary excavations should not be anticipated to stand near vertical without sloughing. Therefore, the sides of excavations and trenches for this project should be stabilized in order to minimize damage to adjacent facilities resulting from vertical or lateral movement of the soil. The sides of the excavation may be stabilized by sloping back the sides and/or by using bracing. However, the trench sidewalls may be difficult to stabilize due to the presence of low cohesion granular soils, which could have a potential to caving and sloughing during excavation, especially if the soils are wet or saturated. Additionally, vibrations caused by nearby traffic or construction equipment could accelerate sloughing.

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

Administration (OSHA) regulations, for employees working in an excavation that may

604269001 R Geotech Eval 13 expose employees to the danger of moving ground. For planning purposes and according to OSHA soil classifications, a "Type C" soil should be considered for excavation in soil for this project. In general, temporary slopes above the water table and excavations in competent “Type C” soil should be inclined no steeper than 1.5:1

(Horizontal:Vertical [H:V]). Upon making the excavations, soil classification and excavation performance should be evaluated in the field by Ninyo & Moore in accordance with the OSHA regulations.

Temporary excavations that encounter groundwater seepage or surface runoff may need shoring or may be stabilized by placing sandbags or gravel along the base of the seepage zone. Excavations encountering groundwater seepage should be evaluated on a case-by-case basis. Flatter slopes or bracing should be used if sloughing or raveling is observed. 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.

Details for open-cut slopes and shoring based on soil type and groundwater conditions are provided in the latest amended OSHA regulations. These details apply to temporary open-trench excavations up to 20 feet deep. Trenches over 20 feet deep or in areas where seepage is encountered should be designed by the contractor’s engineer based on project-specific geotechnical analyses.

9.1.3. Shoring

Laying a slope back at an inclination of 1.5:1 (H:V) may not be feasible due to conflicts with existing site improvements. Therefore, it may be preferable to shore or brace the excavations than to utilize open cuts. Temporary earth retaining systems will be subject to lateral loads resulting from earth pressures. Shored or braced trench excavations in soils may be designed using the parameters on Figure 3. Due to the presence of loose, low cohesion soils, the excavations may not stand open long enough to install the trench boxes. The contractor should be prepared to deal with these soil conditions and plan

604269001 R Geotech Eval 14 accordingly. Once installed, some sloughing is possible at the ends of the trench box;

therefore, any loose material should be removed prior to backfilling of the trench.

The earth pressure values provided on Figure 3 assume that spoils from the excavation or other surcharge loads will not be placed above the excavation within a 1:1

(Horizontal:Vertical) plane extending up and back from the base of the excavation. If spoil piles are placed closer than this to the braced excavation, the resulting surcharge loads should be considered in the bracing or trench box design. We recommend that an experienced structural engineer design the shoring system. The shoring parameters presented in this report should be considered as guidelines.

9.1.4. Grading, Fill Placement, and Compaction

Vegetation and demolition debris from any existing structures, pavements, and other developed areas should be removed from the site in accordance with Section 201 of the

FP-03 Standard Specifications. Demolition debris and obstructions that extend below finish grade, if present, should be removed from the site in accordance with Section 203 of the FP-03 Standard Specifications. Excavation for roadway embankment should be performed in accordance with Section 204 of the FP-03 Standard Specifications.

We recommend that new grade beams be supported on 2 feet or more of adequately moisture-conditioned and compacted structural backfill material. Structural backfill should comply with the provisions of Sections 208, 209, and Section 704.4 of the FSSS and FP-03 Standard Specifications, as applicable. Suitable structural backfill should be a free draining granular material free of excess moisture, muck, frozen lumps, roots, sod, and other deleterious material. Structural backfill material should not have particles larger than 3 inches, a liquid limit not exceeding 30, and the percentage of materials passing the No. 200 sieve should not exceed 15. Based on the results of our explorations, some of the on-site soils will not be suitable for structural backfill without treatment, such as screening and/or blending with other materials. Additional field sampling and laboratory testing should be conducted by the contractor prior to

604269001 R Geotech Eval 15 construction to evaluate the suitability of on-site soils for re-use as structural backfill.

Unsuitable materials should be disposed of off-site or in non-structural areas.

The backfill thickness should be measured from the bottom of the grade beam base and should be compacted by appropriate mechanical methods to 95 percent relative compaction as evaluated by ASTM D 698 and at a moisture content generally near its laboratory optimum. An earthwork (shrinkage) factor of approximately 10 to 20 percent for the on-site soils and a bulking factor of 5 to 10 percent for bedrock materials are estimated.

The foundations should not bear on any transitional materials (i.e. soil/bedrock contact), and should bear on a zone of structural backfill, as mentioned above, or on competent bedrock. As such, it is acceptable to have the proposed bridge foundations be founded on competent bedrock material. For this case, the bedrock should be evaluated by Ninyo

& Moore prior to placement of the structural concrete. If desired, the use of controlled low-strength material (CLSM) may be used to replace the soil in areas where a bedrock/soil transition is encountered.

Due to the limited nature of the subsurface exploration performed for the purpose of this study, we recommend that during the bridge foundation construction, the excavations be closely monitored by Ninyo & Moore to observe that the subsurface soil conditions encountered are consistent with the assumptions adopted in this report.

Following the foundation excavation as described above, and prior to the placement of structural backfill, the resulting exposed surface should be carefully evaluated by Ninyo

& Moore for the presence of soft, loose, or wet soils that were not removed as part of the excavation process. Based on this evaluation, additional remediation may be needed. This could include scarification and recompaction of the exposed surface.

We recommend that new pavements and flatwork be supported on moisture-conditioned and compacted subgrade soils extending 6 inches or more below the bottom of the aggregate base (AB) material. The subgrade should be compacted by appropriate

604269001 R Geotech Eval 16 mechanical methods to 95 percent or more relative compaction as evaluated by ASTM

D 698 at a moisture content generally near optimum. The subgrade prepared as described above should extend laterally one or more feet horizontally beyond the slab/pavement footprint. Furthermore, we recommend that pavement bear on imported or on-site soils with an average soil R-value of 30 or more, extending 2 feet or more below the bottom of the base material. This depth may be reduced if competent bedrock is encountered within the 2-foot depth.

9.1.5. Borrow Material

Imported fill, if utilized, should consist of granular material, which satisfies the requirements for structural backfill presented in Section 9.1.4. of this report. Import material in contact with ferrous materials should have a low corrosion potential

(minimum resistivity more than 2,000 ohm-cm, and chloride content less than 25 parts per million [ppm]). Import material in contact with concrete should have a soluble sulfate content of less than 0.1 percent. The geotechnical consultant should evaluate such materials and details of their placement prior to importation.

9.2. Bridge Foundations

Work and materials for construction of the proposed bridge foundation elements shall be in accordance with the FP-03 Standard Specifications and FSSS, and the recommendations outlined in this report. The following sections present our geotechnical recommendations for the proposed bridge foundations. Ninyo & Moore should be contacted for additional recommendations if the actual design details change from those detailed or assumed in our report.

Shallow foundations (grade beams) are recommended for support of the proposed new bridge structures over Sabino Creek. We understand that grade beams are being designed for this project by a specialty contractor. We recommend that the grade beams be constructed

604269001 R Geotech Eval 17 and installed in accordance with FSSS 553A and the recommendations outlined in this report.

For purposes of estimating the footing bearing resistance and settlements on rock, we evaluated the rock conditions encountered in our borings B-1 and B-2 in accordance with the Rock Mass Rating System (American Association of State Highway and Transportation

Officials [AASHTO] 10.4.6.4). As a result of this evaluation, poor rock quality conditions were used for the North Bridge abutments. For conservatism, dense to very dense soil conditions (decomposed rock) were used for both abutments of the South Bridge. Scour was not accounted for. We have assumed that the foundations will be adequately protected to the anticipated scour depths. The grade beam width and length were assumed to be 3.5 feet and

30 feet, respectively.

The assumed footing dimensions used in our analyses were based on preliminary design information provided by your office. A minor change in footing depth or length will not significantly alter the Service Limit Settlement Curves. However, a notable change in footing depth may have a significant effect on the Strength Limit Curve. We should be contacted in the event that recommendations are needed for different footing geometry or footing depths.

9.2.1. Bearing Resistance for Grade Beams

Bearing resistance was evaluated using the methods presented by Kulhawy & Carter

(1992a; 1992b) for jointed rock masses for the North Bridge west abutment and the

AASHTO bearing resistance formula for soil (AASHTO, Section 10.6.3.1.2) for the

North Bridge east abutment and both abutments of the South Bridge. A resistance factor of 0.45 in accordance with AASHTO was used. We have assumed that for the North

Bridge, the grade beams will be constructed at an elevation of 7,521 feet MSL. For the

South Bridge, the foundation elevations were assumed at 7,505 MSL and 7,510 MSL, for the west and east abutments, respectively. The assumed foundation embedment

604269001 R Geotech Eval 18 depth will be 2 feet below finished grade. The following table contains the results of our analysis for the North Bridge west abutment.

Table 1 – Results of North Bridge West Abutment Analysis

Bridge Structure Bearing

Elevation (ft)

Nominal Bearing Resistance, qult

(kips per square foot [ksf])

Factored Bearing

Resistance, qfactored (ksf)

North Bridge-west abutment 7,521 21.0 9.5

Factored bearing resistance charts for the North Bridge east abutment and both abutments of the South Bridge are provided on Figures 4 through 6 of this report. The factored net bearing resistance on the vertical axis corresponds to the equivalent “net” uniform

(Meyerhoff) stress on an equivalent footing with a calculated “effective” footing width

B’ based on load eccentricity (AASHTO, 2010). The weight of any soil above the footings should be added to the weight of the structure when calculating the total factored equivalent uniform vertical bearing pressure, qfactored. An estimated unit weight of 110 pounds per cubic foot (pcf) may be assumed for compacted soil density above spread footings. For computing the “net” equivalent uniform vertical bearing pressures, qfactored, the weight of concrete plus soil above base-of-footing level (to finished grade) times the appropriate load factor should be subtracted from qfactored. As an approximation, the difference in unit weights of concrete and soil can be neglected, i.e., use the soil density for the weight of material (concrete plus soil) above the base of the footing level.

9.2.2. Resistance to Lateral Loads

Recommended factored coefficients of friction for sliding resistance when analyzing lateral loads for spread footings founded on relatively competent rock which is undisturbed by effects of blasting or mechanical excavation and on sandy soil are presented below:

604269001 R Geotech Eval 19

• For rock-to-concrete shear for cast-in-place concrete (conservative assumption), a factored coefficient of friction of 0.56 (equivalent to φτ [tan φ] = 0.8 [tan φ] with φ = 35 degrees; and φτ from Table 10.5.5.2.2-1 in AASHTO) is recommended;

• For cast-in-place or precast concrete on sandy soil, a factored coefficient of friction of 0.46 (equivalent to φτ [tan φ] = 0.8 [tan φ] with φ = 30 degrees; and φτ from Table 10.5.5.2.2-1 in AASHTO) is recommended.

9.2.3. Settlement

Settlement of spread footings on rock was estimated for the North Bridge west abutment using the procedures outlined in Section 10.6.2.4.4. in AASHTO. The settlement was evaluated based on a poor quality rock mass. The elastic modulus of the rock mass (Em) of 273 kips per square inch (ksi) was evaluated based on Section

10.4.6.5 of AASHTO and an applied stress of 10 ksf (approximate factored bearing resistance estimated as presented in Table 1 of this report), the estimated total settlement is less than 0.25 inch.

For the North Bridge east abutment and both abutments of the South Bridge, the estimated settlements were calculated based on the Schmertmann method, as detailed in

Soils and Foundations – Volumes I and II, Samtani, N. C., and Nowatzki, E. A., Publications No. FHWA NHI-06-088 and FHWA NHI-06-089, Federal Highway

Administration, (FHWA, 2006), for the assumed footing dimensions and embedment.

Values of soil moduli used in the analyses described above were estimated using published sampler blow count correlations presented in Table 5-16 (FHWA, 2006).

Field blow counts for both SPT and ring samplers were adjusted to N160 values. For the

North Bridge east abutment, the underlying rock mass was assigned a conservative value of the elastic modulus. Estimated settlements for different effective footing widths are presented on the Factored Bearing Resistance Charts (Figures 4 through 6 in this report) for the North Bridge east abutment and both abutments of the South Bridge. The settlement curves indicate the service limit state for each abutment.

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The settlement estimates are based on the anticipated loading conditions and the available subsurface soil information. The majority of this settlement is anticipated to occur during construction. No appreciable long-term settlements are anticipated.

9.2.4. Seismic Design Considerations

Earthquake ground motion parameters were estimated based on AASHTO ground motion software (AASHTO, 2009). The site is located in a zone where the horizontal peak ground acceleration coefficient (PGA) of 0.073g has a 7 percent probability of being exceeded in 75 years. Short and long-period spectral acceleration coefficients, SS and S1, are 0.168 and 0.049, respectively. Additional seismic design parameters according to AASHTO (2010) are presented in Table 2.

Table 2 – Seismic Design Parameters

Parameter Value AASHTO LRFD Bridge Design Specifications

Site Class B Table 3.10.3.1-1 Fpga 1.0 Table 3.10.3.2-1 Fa 1.0 Table 3.10.3.2-2 Fv 1.0 Table 3.10.3.2-3 AS 0.073 Section 3.10.4.2 SDS 0.168 Section 3.10.4.2 SD1 0.049 Section 3.10.4.2

Seismic Zone 1 Table 3.10.6 -1

9.3. Lateral Earth Pressures

The abutments, wing walls, and retaining walls, if anticipated for this project, should be designed in accordance with AASHTO (2010) Section 5.5. Active earth pressure occurs when the wall moves away from the soil and the soil mass stretches horizontally, sufficient to mobilize its shear strength, and a condition of plastic equilibrium is reached. An outward lateral movement of about 0.001H (where H is the height of the wall) at the top of the wall is

604269001 R Geotech Eval 21 generally needed to mobilize the active earth pressure condition. For the design of cantilevered yielding walls with a drained granular backfill, we recommend the use of an equivalent fluid active earth pressure of 35 pounds per square foot (psf) per foot (psf/ft) of wall depth estimated in accordance with Sections 3.11.5.3 and 3.11.5.5 of AASHTO. The appropriate earth pressure load factor from Table 3.4.1-2 of AASHTO should be applied.

A soil mass that is neither stretched nor compressed is said to be in an at-rest state. If the wall is rigidly restrained, so that it does not rotate sufficiently to reach the active earth pressure condition, at-rest earth pressure conditions will exist. An equivalent fluid at-rest earth pressure of 54 pounds psf/ft estimated in accordance with Sections 3.11.5.2 and

3.11.5.5 of AASHTO should be used for the drained condition. The appropriate earth pressure load factor from Table 3.4.1-2 of AASHTO should be applied.

Retaining walls should also be designed to resist a horizontal earth pressures from vertical surcharge pressures induced by adjacent light loads, slab, or traffic loads plus any adjacent footing loads. Depending upon the load combination/limit state being evaluated, an appropriate load factor should be applied to the computed surcharge loads. Load factors for load combinations should be selected from Table 3.4.1-1 of AASHTO. Hydrostatic pressures, if any, should also be added to the lateral pressures, as appropriate.

Passive earth pressure occurs when the wall or foundation moves into the soil and the soil mass is compressed horizontally, mobilizing its shear strength. For below-grade portions of the walls with granular backfill (derived from on-site soils) in front of the toe of the wall, nominal passive resistance expressed in terms of an equivalent fluid passive earth pressure of 715 psf/ft was estimated in accordance with Section 3.11.5.4 of AASHTO, for the case of a vertical wall and horizontal backfill (Figure 3.11.5.4.1 of AASHTO). This value assumes that the ground is horizontal for a distance of 10 feet or more in front of the wall or three times the height generating the passive pressure, whichever is more. For calculating a factored passive resistance, a resistance factor of 0.50 should be used as specified in Table

10.5.5.2.2-1 in AASHTO. We recommend that the upper 12 inches of soil not protected by

604269001 R Geotech Eval 22 pavement or a concrete slab, or any soil subject to possible future scour or excavation, be neglected when calculating passive resistance.

Drainage should consist of free-draining granular material and could be accompanied by weepholes through the walls or corrugated, perforated pipe placed parallel to the wall or abutment bottom, wrapped in a filter fabric, and surrounded by 6 or more inches of a granular filter material (e.g., pea gravel). In lieu of the wrapped open-graded gravel, a geocomposite drainage mat attached to the wall and discharging to a drain pipe or weepholes may be considered.

9.4. New Pavements

The following recommended pavement sections were developed using the Pima County

Roadway Design Manual Chapter 3 (Roadway Manual) and the ADOT Preliminary

Engineering and Design Manual (PEDM). We assumed that the subgrade preparation recommendations outlined in this report will be employed. The new pavement sections are designed for a 20-year service life.

For new paved areas, we assume that asphaltic concrete sections will be constructed. The heavy duty pavement section was estimated to accommodate an average of about 200 vehicles per day, with 10 percent of recreational vehicles (RVs) and utility vehicles

(equivalent to medium trucks per the PEDM vehicle classification) and a growth rate of 3 percent. The calculated 18-kip equivalent single axle loads (ESALs) for the service life is estimated at approximately 82,000.

The pavement sections given below are assumed to bear on imported or on-site soils with an average soil R-value of 30 or more. For purposes of design and new construction, it is assumed that soils placed within 2 feet of the finished roadway subgrade will exhibit an R-value as indicated above. According to the PEDM, a resilient modulus of approximately

10,725 pounds per square inch (psi) was calculated based on the design R-value and an

604269001 R Geotech Eval 23 assumed seasonal variation factor of 3.5. This value should also be representative of engineered fill derived from on-site soils in the proposed pavement areas.

An asphalt pavement section consisting of 3 inches of AC per Section 404 of the FSSS over

8 inches of AB per Section 324 of the FSSS should be considered for the loop road, and/or heavy duty pavements. For the parking areas, or light duty pavements, an asphalt pavement section consisting of 2.5 inches of AC over 6 inches of AB should be adequate.

We recommend the underlying subgrade soils be prepared as described in Section 9.1.4 of this report. AB material should be compacted to a relative compaction of 100 percent or more of the maximum dry density, as evaluated by ASTM D 698, at a moisture content near optimum.

9.5. Concrete Flatwork

To reduce the potential manifestation of distress to any concrete flatwork (approach slabs) due to movement of the underlying soil, we recommend that such flatwork (if utilized for this project) be installed with crack-control joints at appropriate spacing as designed by the structural engineer. We recommend that concrete flatwork be supported on 6 or more inches of adequately moisture-conditioned and compacted subgrade as described in Section 9.1.4 of this report. Positive drainage should be established and maintained adjacent to flatwork.

9.6. Corrosion

The corrosion potential of the on-site materials was analyzed to evaluate its potential effect on the foundations and structures. Corrosion potential was evaluated using the results of laboratory testing of soil samples obtained during our subsurface evaluation that were considered representative of soils at the subject site.

Laboratory testing to evaluate corrosion characteristics consisted of pH, minimum electrical resistivity, and soluble sulfate and chloride contents. The pH and minimum electrical resistivity tests were performed on two samples in general accordance with Arizona Test

604269001 R Geotech Eval 24

236b, while sulfate and chloride tests were performed on four samples in accordance with

Arizona Test 733 and 736,…