Draft Geotechnical Investigations Report Negrito.pdf

SOUTH FORK NEGRITO CREEK BRIDGE

NM FS 141(1)

CATRON COUNTY, NM

FINAL Geotechnical Report

Prepared by Federal Highway Administration

Central Federal Lands Highway Division

Geotechnical Services Branch March 2023 ii

SIGNATURE SHEET

Report prepared by: ___________________________________________________________

Brendan L. McGarity, E.I., Geotechnical Engineer

Aaron A. Baukus, Geotechnical Engineer Trainee

Report reviewed by: ____________________________________________________________

Devin T. Dixon, P.E., Geotechnical Engineer

Approved for distribution by: ____________________________________________________

James M. Arthurs, P.E., Ph.D., Acting Lead Geotechnical Engineer

Distribution

Electronic:

N:\NM\141(1)\Geotech\7_FinalDocs K:\TechServices\Geotech\1. Geotech Project Reports Project Management (PDF only) Project Development, Lead Designer (PDF only) Bridge (PDF only) Technical Services, Hydraulics (PDF only)

BRENDAN LEE

MCGARITY

Digitally signed by BRENDAN LEE

MCGARITY

Date: 2023.03.16 10:37:11 -06'00'

AARON ALGIRDAS

BAUKUS

Digitally signed by AARON ALGIRDAS

BAUKUS

Date: 2023.03.16 12:54:01 -04'00'

DEVIN T DIXON

Digitally signed by DEVIN T

DIXON

Date: 2023.03.16 11:17:41 -06'00'

JAMES M

ARTHURS

Digitally signed by JAMES

M ARTHURS

Date: 2023.03.16 17:36:14 -06'00'

Page | i

TABLE OF CONTENTS

SECTION ONE - INTRODUCTION

1.1 Background and Location

1.2 Scope and Purpose

SECTION TWO - GEOLOGY AND SEISMICITY

2.1 Regional Geology

2.2 Site Geology

2.3 Regional Seismicity

2.4 Seismic Design Parameters

2.5 Geologic Hazards

SECTION THREE - SUBSURFACE INVESTIGATION

3.1 Subsurface Investigation Program

3.2 Laboratory testing Program

3.3 Summary of Site Conditions

3.3.1 General Topography

3.3.2 Surface Reconnaissance

3.3.3 Subsurface Conditions

3.3.4 Groundwater

3.4 Rock Characteristics

3.4.1 Generalized Strength Criterion for Intact Rock

3.4.2 Rock Characterization

SECTION FOUR - ANALYSIS & RECOMMENDATIONS

4.1 Foundations

4.1.1 Bridge Foundation Selection

4.1.2 Site Characterization

4.1.3 Drilled Shaft Axial Resistance

4.1.4 Group Effects on Axial Resistance

4.1.5 Lateral Loads on Deep Foundations

4.1.6 Settlement

4.1.7 Field Testing

4.1.8 Scour Potential and Erosion

4.2 Abutment & Wingwall Design

4.2.1 Lateral Loads

4.2.2 Bearing Resistance

4.3 Permanent Earthworks

4.3.1 Embankment Construction

4.3.2 Temporary Cut Slopes and Shoring

4.3.3 Shrink/Swell Recommendations

4.4 Construction Considerations

4.5 Specifications

4.6 Disclaimer/Limitations Clause

SECTION FIVE - REFERENCES

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TABLES

Table 2.1 Summary of Nearby Seismic Source Faults Table 2.2 Summary of Seismic Parameters Corrected for Site Class C Table 3.1 Summary of Field Exploration Program Table 3.2 Summaries of Laboratory Index Test Results Table 3.3 Summary of Uniaxial Compressive Strength Results Table 3.4 Strength Parameters Calculated for Rock Based on Hoek Brown Criterion Table 4.1 Proposed Bridge Foundation Location Table 4.2 Drilled Shaft Resistance in Rock Table 4.3 LPILE Parameters2 Table 4.4 Lateral Earth Pressures for Bridge Abutments

PLATES

PLATE 1 Project Vicinity Map PLATE 2 Geology Map PLATE 3 Geology Map Legend PLATE 4 Geotechnical Boring Locations

APPENDICES

APPENDIX A Field Exploration Program APPENDIX B Laboratory Test Results APPENDIX C Photographs APPENDIX D Excavation Characteristics of Rock & Rippability Charts

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South Fork Negrito Creek Bridge, NM FS 141(1) March 2023

SECTION ONE - Introduction

1.1 BACKGROUND AND LOCATION

This report presents the results of the geotechnical engineering study and provides recommendations for the replacement of the South Fork Negrito Creek Bridge in Catron County, New Mexico. The project proposes to replace the existing South Fork Negrito Creek Bridge, which is located within the Gila National Forest. A project vicinity map is presented on Plate 1 of this report.

The South Fork Negrito Creek Bridge was originally designed by the Forest Service and constructed in 1959. The bridge is past its design life of 50 years. The superstructure consists of cast-in-place concrete T-Beams with a monolithic deck. The substructure consists of vertical concrete cantilever abutment walls on spread footings extending to approximately 24 feet deep, as indicated on the as-built drawings. Wingwalls are approximately 15 feet long cantilever concrete retaining walls on spread footings. The abutment foundation material is unknown but has been reported as stable for calculated scour conditions according to the most recent inspection report.

This inspection report, dated November 2018, did not report load rating values so it is unknown if the bridge meets the current load requirements. The original structural design also did not include seismic design considerations.

The bridge deck has areas of delamination, the curbs are completely delaminated and spalled, and the stringers have flexural cracks. The bridge inspection report, as noted before, indicates that the structural evaluation of the bridge is in poor condition. Furthermore, NM State Route 141 has the potential to be a main haul route and the expected increase in logging truck traffic will contribute to additional bridge deterioration. Therefore, replacement of the South Fork Negrito Creek Bridge was determined to be the desired option, as opposed to maintenance, due to the limited remaining lifespan of the existing bridge and new load requirements.

The proposed replacement structure type is assumed to be a simple span prefabricated steel structure on deep foundations. Deep foundations are proposed due to scour concerns and the higher costs anticipated with providing shallow foundations at the depth to rock (approximately 23 feet).

Additional materials, such as concrete, rebar, excavation, backfill, and dewatering, as well as possible increased construction time, would be required to construct shallow foundations.

Considerations related to the abutments for the South Fork Negrito Creek Bridge are presented in Section 4 of this report.

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1.2 SCOPE AND PURPOSE

The scope of work included a geotechnical investigation, analysis, and recommendations for bridge foundations for use in design and construction. This involved several tasks including field reconnaissance, subsurface sampling, laboratory testing, interpretation and correlation of field measurements, and geotechnical engineering analysis. Specifically, this investigation was conducted to determine the soil profiles at the bridge location and develop recommendations concerning bridge foundations, retaining structures, geologic hazards, and construction considerations.

We understand that a hydraulics report has been prepared under separate cover by the CFLHD Hydraulics Engineer. The hydraulics report includes discussion of flood impacts, such as bridge scour and water surface elevations.

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SECTION TWO - Geology and Seismicity

2.1 REGIONAL GEOLOGY

The project is located within the Mexican Highland Section of the Basin and Range Province. This province is part of the Intermontane Plateaus physiographic region. The Basin and Range Province extends from eastern California to central Utah and from southern Idaho to the state of Sonora in Mexico. This province is characterized by steep climbs up mountain ranges alternated with long treks across flat basins. Features and landscapes within this province include pediments, alluvial fans, bajadas, bolsons, Inselbergs, playas, mud flats, salt flats, lakes, sand dunes, canyons, and the Rio Grande Rift (NPS, 2020).

Surficial deposits include alluvium and colluvium: specifically, gravel, sand, and silt. Quaternary and Tertiary aged volcanic and volcaniclastic rock groups that exist within the region include the Gila Group, Santa Fe Group, rocks of Apache volcano, rocks of Eagle Peak volcano, rocks of Horse Mountain volcano, rocks of John Kerr Peak volcano, volcaniclastic rocks of the Spears Group, volcanic rocks of the Mogollon Group, and volcanic rocks of the Datil Group. The rocks and geologic features related to the aforementioned groups include basalt, andesite, dacite, rhyolite, tuff, volcanic agglomerate & conglomerate, latite, breccia, scoria, dikes, plugs, cinders, lava flows, and vents. Older units, Permian aged, include sedimentary rocks (Weber & Willard, 1959; Willard et al., 1961; Ratte, 2001).

2.2 SITE GEOLOGY

The soil conditions at the site are assumed to be alluvium based on the stream bed deposits. The project site is mapped as underlain by basaltic-andesite to dacite lava flows. This observation is consistent with the borings as will be discussed in Section 3 of this report. The deposits and landforms are volcanic in nature and the terrain can be described as mountainous and rugged. A geology map of the project area and corresponding map legends are presented in Plates 2 & 3.

2.3 REGIONAL SEISMICITY

No seismic hazard faults are located within 50 miles of the project. However, eight known Quaternary faults are mapped within approximately 50 miles of the bridge site. These faults are considered inactive and are summarized in Table 2.1. The Red Hills Faults and Unnamed Fault East of Mangas are located northwest and northeast of the project site; the Alma Mesa Faults, Unnamed Faults East of Alma, and Clifton Faults are located west and southwest of the project site; and the Mogollon Fault, Unnamed Fault south of Gila, and the Mockingbird Hill fault zone are located south of the project site. All fault systems are considered normal faults with slip rates less than 0.008 inches per year (USGS, 2022a).

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Table 2.1 Summary of Nearby Seismic Source Faults

FAULT OR

FAULT ZONE

DISTANCE

FROM

CENTER OF

PROJECT

SLIP-RATE

CATEGORY

DIP

DIRECTION

AVERAGE

STRIKE

FAULT

LENGTH

TIME OF MOST

RECENT

DEFORMATION

(miles) (inch/year) (miles) (years) Unnamed Faults

East of Alma, (Class A) No. 2011

13.8 <0.00787 W; E N12°W 7.5 <1,600,000

Alma Mesa Faults, (Class A) No. 941

23.6 <0.00787 E; SE; W;

NW N23°E 9.3 <1,600,000

Mogollon Fault, (Class A) No. 2012

27.9 <0.00787 65° W N37°W 9.3 <1,600,000

Mockingbird Hill Fault Zone, (Class A) No. 2013

37.1 <0.00787 W N18°E 4.3 <750,000

Red Hill Faults, (Class A) No. 2138

39.9 <0.00787 SE; NW N25°E 9.3 <1,600,000

Unnamed Fault South of Gila, (Class A) No. 2014

40.1 <0.00787 SW N52°W 2.5 <750,000

Unnamed Fault East of Mangas, (Class A) No. 2137

46.4 <0.00787 SW N23°W 1.9 <1,600,000

Clifton Faults, (Class A) No. 939

48.7 <0.00787 SW N49°W 8.7 <1,600,000

2.4 SEISMIC DESIGN PARAMETERS

Recommended seismic response parameters for the South Fork Negrito Creek Bridge project site design are based on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications, 8th Edition, 2017, and represents horizontal peak ground acceleration (PGA) with 7 percent probability of exceedance in 75 years (approximate 1,000-year return period). The 1,000-year return period uniform hazard spectrum for the South Fork Negrito Creek Bridge project site, located at 33.53014°N latitude and 108.58509°W longitude, was obtained in accordance with the AASHTO ground motion maps.

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Table 2.2 Summary of Seismic Parameters Corrected for Site Class C Horizontal Peak Ground Acceleration, (PGA) 0.073 g

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (Ss) 0.168 g Horizontal Response Spectral Acceleration at Period of 1.0 sec, (S1) 0.051 g

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.2 Site Factor at Short-Period Range of Acceleration Spectrum, (Fa) 1.2 Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.7

Factored Horizontal Peak Ground Acceleration, (As) 0.087 g Factored Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 0.202 g Factored Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.086 g

Seismic Zone Zone 1

Based on the long acceleration coefficient SD1 value of 0.086, the project site is assigned to seismic hazard “Zone 1” in accordance with Table 3.10.6-1 of AASHTO.

2.5 GEOLOGIC HAZARDS

Potential geologic hazards at the bridge site include those related to floods, earthquakes, and mass movement. Flooding, however, is the most common, widespread, and damaging of these geologic hazards and could pose a threat to the South Fork Negrito Creek Bridge. Much of New Mexico is prone to flooding caused by thunderstorms that happen year-round. Severe thunderstorms are most common during the monsoon season, especially in July and August. Flooding can cause many effects due to contact with flood waters such as extensive erosion, debris flows, and slope instability (NOAA, 2020).

Earthquakes, as discussed in the previous section, are another potential geologic hazard. Western New Mexico and the project site are located in an area of low potential for damaging earthquakes, although it is impossible to accurately predict the timing or location of future earthquakes. Another hazard related to earthquakes is liquefaction. Liquefaction is a phenomenon in which a cohesionless soil layer below the groundwater table loses a substantial amount of strength due to pore pressure generation resulting from ground shaking. Based on the subsurface conditions encountered at the project site and the low peak ground acceleration (PGA) indicated in Table 2.2, the risk of liquefaction is relatively low.

Mass movements are also common in New Mexico due to the steep mountainous and hilly terrain, heavy rains, and the fines content of the soil. These rock and soil movements include debris flows, landslides, and rockfalls. Although the bridge is located in terrain susceptible to mass movement, no indication of these events was observed during the site reconnaissance and are considered unlikely to impact this bridge site.

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SECTION THREE - Subsurface Investigation

3.1 SUBSURFACE INVESTIGATION PROGRAM

A subsurface investigation targeting the bridge site was performed by a Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) geotechnical engineer on September 24 & 25, 2020. The geotechnical subsurface exploration program consisted of drilling a total of two borings to depths of 31 and 21 feet deep. One boring was drilled near the southwest bridge abutment and another was drilled near the northeast bridge abutment. Both borings were drilled approximately 60 feet from their respective abutments due to drill rig access constraints and to meet the required width for one lane of vehicle passage. Hollow-stem augers were used to drill through the overburden soils and bedrock samples were recovered using HQ3 diamond core drilling. Standard penetration testing (SPT) and sample collection was performed at 5-foot intervals for each boring when possible. Grab samples of the upper subsurface were also collected. Subsurface conditions were logged, and representative samples were collected and transported to the CFLHD Materials Laboratory in Lakewood, CO, for physical property testing.

Logs of the explorations and boring locations are presented in Appendix A and Plate 4, respectively. Photographs related to this exploration can be found in Appendix C. A summary of the field exploration is provided in Table 3.1.

Table 3.1 Summary of Field Exploration Program

EXPLORATION

DESIGNATION LOCATION

APPROXIMATE

GROUND

ELEVATION (ft.)

TERMINATION

DEPTH (ft.)

DEPTH TO

GROUNDWATER

(ft.)

BH20-01

Southwest Abutment

STA 102+70,

13.8 ft. RT of centerline

7,453 31 18

BH20-02

Northeast Abutment

STA 104+24,

10 ft. LT of centerline

7,455 21 Not encountered.

Note: The exploration locations were estimated relative to existing features. Ground elevations were estimated from CFLHD survey data.

Additionally, a geophysical study was performed by a Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) geotechnical engineer to further define the subsurface profile near the bridge structure and to verify the seismic site class by collecting Vs30 data. Geophysical studies were completed on September 20, 2020. The geophysical study was performed using multi-channel analysis of surface waves (MASW) to determine the variability of subsurface shear wave velocities. The shear wave velocity provides a direct indication of the variation of stiffness of subsurface materials.

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MASW lines along the proposed bridge alignment were completed using 24 geophones at a regular spacing of 10 feet between geophones. A sledgehammer was used as the source to generate seismic energy.

Collected seismic data was not processed due to time constraints and thus not used in design. The unprocessed seismic data can be made available to the partners or contractor upon their request.

3.2 LABORATORY TESTING PROGRAM

Soil samples recovered from the borings by SPT were tested in the laboratory to support the field classifications and to provide an estimate of the engineering characteristics and mechanical properties of the soil. Laboratory tests included moisture content (AASHTO T 255), sieve analysis (AASHTO T 11 and T 27), classification (AASHTO M 145), and Atterberg limits (AASHTO T 89 and T 90). Soil corrosivity testing (AASHTO T 288 and T 289) was requested, but an insufficient amount of material was furnished to perform the test. When the necessary tests were completed, samples were classified using the Unified Soil Classification System (USCS) and AASHTO soil classification system. Results of the testing are summarized below in Table 3.2 and are presented in Appendix B.

Table 3.2 Summaries of Laboratory Index Test Results

BORING

NUMBER

SAMPLE

DEPTH

(ft)

PERCENT

GRAVEL

PERCENT

SAND

PERCENT

PASSING

#200

LIQUID

LIMIT

PLASTIC

LIMIT

USCS

CLASS.

AASHTO

CLASS.

BH 20-01 0-1 20 35 45 32 20 SC A-6 (2)

BH 20-01 6-6.5 52 33 15 39 26 GM A-2-6 (0)

BH 20-01 14-15.5 51 38 11 - - - -

BH 20-01 19-20.5 33 50 17 - - - -

BH20-02 1.5-2 45 26 29 38 19 GC A-2-6 (1)

- An insufficient amount of material was furnished to determine the liquid limit and plasticity index. Therefore, soil classification could not be determined.

Soil test results indicated a range of material types including clayey sand with gravel and silty/clayey gravel with sand, classifying as A-2-6 to A-6 by AASHTO and SC, GM, and GC by

USCS.

Compressive strength of rock (ASTM D 7012) was conducted on intact samples of rock core recovered from the borings. Results of the unconfined compressive strength (UCS) testing are presented in Table 3.3.

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Table 3.3 Summary of Uniaxial Compressive Strength Results

BORING NUMBER SAMPLE DEPTH (ft) RQD1(%) LENGTH / DIAMETER UCS (psi)

BH 20-01 20 50 2.5 18,180

BH 20-02 16 95 2.4 7,240

BH 20-02 19.5 95 2.4 4,110

1Rock Quality Designation for core run from which sample was taken.

3.3 SUMMARY OF SITE CONDITIONS

The project site can be divided into two critical sections: the southwest bridge abutment and the northeast bridge abutment. This section presents the results of the surface reconnaissance and subsurface exploration.

3.3.1 General Topography

The general topography of the project site is characterized by steep mountains and lengthy deserts that encompass southeastern Arizona and southwestern New Mexico. Rock outcroppings are present in the area surrounding the bridge site. Thick vegetation typical of both ponderosa pine and pinyon-juniper woodland forests is abundant at the project site. The South Fork Negrito Creek flows under the South Fork Negrito Creek Bridge from south to north from its headwaters west of Negrito Mountain near Route 159 to the confluence with North Fork Negrito Creek located approximately 7 miles downstream of the project site.

3.3.2 Surface Reconnaissance

The terrain can be described as rolling, forested, and partly mountainous. Vegetation including junipers, small and large pines, sunflowers, and native grass species heavily cover much of the project site. Fractured plate-shaped and blocky rock outcroppings protrude along the face of a hilly slope adjacent to the northeast bridge abutment. The slope appears to be roughly between a 1:1.5 to 1:1 (V:H) slope ratio and extends several hundred feet high to the top of the ridgeline. Near vertical rock cuts, roughly 15 to 20 feet high, are found at the base of the aforementioned slope and line the north edge of the road past the bridge while traveling west. Rock outcroppings, similar to those previously described, were also observed along the north bank of the channel upstream of the bridge. The rock type is consistent with the volcanic geology discussed in Section 2. Much of the exposed rock faces are fine grained, have a vesicular texture, are at least slightly weathered, and show signs of natural fracture patterns. Neither bridge abutment is recognizably founded on bedrock by means of visual observation.

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Rounded cobbles, boulders, and rock fragments, some measured to be roughly 3 to 3.5 feet in diameter and larger, were observed in the river channel beneath and surrounding the bridge. Moss covered rock embankment was noted along the southeast side of the southwest bridge abutment.

Moderately dense forest vegetation lines both banks of the creek and within the channel itself as the creek was dry at the time of the reconnaissance. Alluvial soils and sediments encompass the bridge site. A corrugated metal pipe culvert (CMP) roughly 2.5 to 3 feet in diameter crosses beneath the road approximately 30 feet northeast of the northeastern bridge abutment. A communications line also runs parallel to the road just south of the bridge. The buried cable connects to two above ground cable boxes located south of the bridge near the southwest and northeast bridge abutments. This communications line was marked with flagging at the time of the site investigation. An unnamed Forest Service Road intersects with NM State Route 141 roughly 260 feet southwest of the bridge site. This gravel road extends to the southeast and runs parallel to the creek channel for an unknown distance. Lastly, the gravel surface of NM State Route 141 was measured to be roughly 6 to 8 inches thick near the bridge site.

3.3.3 Subsurface Conditions

The subsurface conditions at the southwest abutment of the South Fork Negrito Creek Bridge were investigated by drilling boring BH20-01. The ground surface surrounding the boring location consisted of typical alluvial soils: namely silt, sand, and some gravel. The boring encountered clayey sand with gravel to a depth of approximately 2 feet. The material then transitioned to a silty gravel with sand to a depth of approximately 18 feet. Additional clay content was encountered in this layer at approximately 10 feet in depth along with an about 6-inch diameter boulder. A roughly 1-foot-thick layer of clayey sand with gravel was identified beneath the gravel layer and extended to approximately 19 feet in depth. Cobbles and boulders composed of basaltic-andesite were then encountered to a depth of approximately 23 feet. Rhyolite bedrock was encountered beneath the cobbles and boulders and extended to the termination depth of approximately 31 feet. This rock was red and black, highly vesicular (15-50%), moderately to highly weathered, low RQD (0 to 7%), and classified as medium strong rock (R3) by field identification methods.

The subsurface conditions at the northeast bridge abutment were investigated by drilling boring BH20-02. Similarly, the ground surface consisted of silt, sand, and varying amounts of gravel. The boring encountered a clayey gravel with sand to a depth of approximately 2 feet. Cobbles and boulders composed of basaltic-andesite were then encountered to a depth of approximately 16 feet.

Rhyolite bedrock was encountered beneath the cobbles and boulders and extended to the termination depth of approximately 21 feet. This rock was red and black, highly vesicular, slightly weathered, high RQD (95%), and classified as medium strong rock (R3) by field identification methods and unconfined compressive strength (UCS) testing.

The difference in rock quality at the two boring locations should be emphasized. The rhyolite rock cores recovered at the northeast abutment were only slightly weathered with 100% recovery and

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95% RQD. Whereas the rhyolite rock cores recovered at the southwest abutment were highly weathered with less than 40% recovery and 0% to 7% RQD. This difference in rock quality has substantial implications on rock socket length and total drilled shaft length at the southwest abutment compared to the northeast abutment. Drilled shaft analysis and rock socket length is discussed in detail in Section 4. Additionally, the elevation of the rhyolite bedrock differed by 9 feet between the two abutments: approximately 7,430 feet and 7,439 feet in elevation for borings BH20-01 (southwest abutment) and BH20-02 (northeast abutment), respectively.

3.3.4 Groundwater

Groundwater was encountered during drilling in boring BH20-01 at approximately 18 feet. No groundwater was encountered during drilling in BH20-02 due to switching to rock core tooling at a shallow depth and introducing water into the recovered core samples. Groundwater depth measurements were not taken after completion of the borings. Fluctuations in the groundwater level due to seasonal and climatic effects are expected.

3.4 ROCK CHARACTERISTICS

A natural rock mass is rarely a continuous, isotropic, homogeneous material. It can be highly variable and may be very difficult to characterize in a generalized manner. A rock mass is generally composed of intact or weathered blocks of rock separated by discontinuities such as joints or bedding planes. The discontinuity and weathering characteristics help establish the design recommendations for a foundation on rock. These characteristics include, but are not limited to, the rock lithology, strength properties, deformation properties, frequency of discontinuities (RQD), spacing of discontinuities, orientation of discontinuities, aperture, filling, condition, degree of weathering, and the groundwater condition. This information was observed at limited borehole locations and laboratory testing of select rock samples. The following sections discuss the rock strength properties and rock characterization based on the field investigation and lab testing results.

3.4.1 Generalized Strength Criterion for Intact Rock

Intact samples of rock core were taken from the South Fork Negrito Creek Bridge boreholes and laboratory testing, such as the uniaxial (unconfined) compression test, was performed. Table 3.3 summarizes the results of the UCS testing. In the field, the rock could be fractured with a single firm blow of a geological hammer and it was estimated that the rock corresponded with the description of “medium strong rock,” R3, as described in the FHWA Rock Description Guidelines.

The range of relative strength for R3 rock as listed in this reference is between 3,500 – 7,250 psi.

The rock also appeared to have slight to high weathering, depending on the boring.

Based on the aforementioned UCS testing, the rock strength for the South Fork Negrito Creek Bridge is conservatively assumed to be approximately 1,000 and 4,110 psi for the southwest and

Page | 11 northeast abutments, respectively. For the southwest abutment, the UCS test results were not used as no recoverable cores came from boring BH20-01 due to poor rock quality, and results from BH20-02 were considered unrepresentative of the subsurface profile at BH20-01. A reduced compressive strength value of 1,000 psi was conservatively chosen as the rhyolite bedrock at this location was highly weathered with very low RQD (0% to 7%). For the northeast abutment, the minimum UCS values was used to represent the rock at this location, and is within the range of R3, “medium strong rock,” as discussed in the previous paragraph.

3.4.2 Rock Characterization

The Geologic Strength Index (GSI) (Hoek and Marinos, 2000) system was used to characterize the rock. The GSI is a slight modification of the Rock Mass Rating system (RMR) (AASHTO, 2017) which is more appropriate for the bridge foundation evaluation at this site. The GSI is the algebraic sum of ratings assigned for the following rock mass properties:

Intact rock strength Rock Quality Designation (RQD) Discontinuities spacing Discontinuities condition Presence of groundwater

For this project, the GSI was estimated based on collected rock core samples. The rock composition was identified as rhyolite with slight to high weathering and blocky to disintegrated structure, depending on the abutment. The estimated GSI values given these assumptions are tabulated below in Table 3.4. Other pertinent rock properties include the Hoek-Brown constant, mi (estimated from Table 10.4.6.4-1 in AASHTO 2017), rock unit weight, γ, and the damage factor, D. These values were assumed to be 25 for rhyolite, 160 pcf, and 0.5 as an intermediate value for drilled shafts, respectively. For intact rock, the following equations were then used to determine the appropriate curve fitting coefficients:

m = m ∗ exp ∗

For GSI > 25: α = 0.5 + ∗ (exp − exp − ) s = exp GSI− 1009 − 3 ∗ D

A summary of the assumed rock properties and the Hoek Brown criteria calculations is presented in Table 3.4 below. These values are directly related to the bearing resistance discussed in subsection 4.1.3.

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Table 3.4 Strength Parameters Calculated for Rock Based on Hoek Brown Criterion Location GSI (psi)

SW Abutment 25 1,000 0.0000454 0.7029 25 0.531

NE Abutment 65 4,110 0.009 4.7219 25 0.502

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SECTION FOUR - Analysis & Recommendations

This section presents analysis and recommendations for the bridge foundation, abutment wingwalls, earthworks, and construction considerations for the design and construction of the South Fork Negrito Creek Bridge project. Based on discussions with the project team, a replacement of the existing bridge is the preferred alternative. Generalized subsurface profiles were developed based on field reconnaissance, surficial visual evaluation, and subsurface investigations.

4.1 FOUNDATIONS

The existing bridge superstructure will be replaced with a simple single-span structure. The existing northeast abutment is to remain in place whereas the southwest abutment is to be moved 40 feet back, extending the bridge length to 80 feet. Based on the in progress preliminary 95% design plans, the proposed abutment centerline station and cap elevation are shown in Table 4.1.

Table 4.1 Proposed Bridge Foundation Location

FOUNDATION APPROXIMATE STATION TOP OF SHAFT ELEVATION*

SW Abutment 102+97 7,449.30 FT NE Abutment 103+77 7,450.06 FT

*As presented on the in progress preliminary 95% design plan set

4.1.1 Bridge Foundation Selection

A drilled shaft foundation system is proposed for the South Fork Negrito Creek Bridge. Spread footings were considered but determined impractical due to additional materials, time, and cost that would be required for construction. Additionally, spread footings would likely be less economical due to the large size of the associated excavation, along with a presumably large footing size to accommodate the structural load requirements. Driven piles were also considered but determined impractical due to the presence of cobbles and boulders that could cause pile damage during driving as well as the relatively short distance to bedrock potentially not meeting minimum length requirements.

A drilled shaft foundation is feasible from a geotechnical perspective. Drilled shafts provide a small footprint, support large foundation loads, and can provide lateral resistance at shallower depths than other deep foundation options. Scour potential was determined to be present at the abutments and is discussed in detail in the Final Hydraulics Report dated February 2021. Design recommendations for the drilled shafts are provided in this report.

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The following bridge loads factored for the strength limit state were provided by the CFL bridge engineer on November 14th, 2022:

Southwest & northeast abutments = 252 kips (per shaft; 3 shafts per abutment)

4.1.2 Site Characterization

The subsurface profile consists of sand, gravel, cobbles and boulders, and rhyolitic bedrock.

Subsurface profiles for individual abutments were selected to align with their corresponding boring logs for the axial resistance analysis. The subsurface profile for lateral analysis was simplified to “sand” and “strong rock” for modeling purposes in the LPILE software.

The groundwater elevation for both abutments was assumed to correspond with the thalweg elevation of 7,443 ft. The level of groundwater encountered in the boreholes during the subsurface investigation lies beneath the thalweg elevation; however, these elevations may not be representative of typical groundwater levels experienced year-round. The subsurface investigation was performed during September, which corresponds to the dry season and low groundwater levels. During the wet season, the water table will rise with the influx of moisture. The magnitude of rainfall within the wet season is quantified by a 100-yr design flood and is used by the Hydraulics engineer for scour estimations. Given the relatively low hydraulic conductivity of the overburden soil, the 100-yr design flood is likely to fully saturate the soil profile, thus elevating the groundwater levels to the thalweg elevation.

4.1.3 Drilled Shaft Axial Resistance

Drilled shafts for the South Fork Negrito Creek Bridge will be socketed into rhyolite bedrock.

Values for unconfined compressive strength (UCS) of competent bedrock in the analysis came from laboratory testing and engineering judgement. Intact core samples of the rhyolite bedrock layer were only recovered for the northeast abutment from boring BH20-02. A value of 4,110 psi was used for the northeast abutment analysis. Due to the relatively higher degree of weathering and fracturing of the rhyolite encountered in BH20-01, no samples were recovered meeting the L/D requirements for UCS testing. Due to this, a reduced UCS value of 1,000 psi was used for the southwest abutment analysis.

The drilled shaft axial resistance analysis was performed for 24-inch diameters at the abutments using the methodology presented in section 10.8.3.5.4 in AASHTO. A minimum estimated tip elevation for each foundation element has been used to calculate the nominal and factored side and tip resistance for various socket geometries at the strength limit state. This minimum tip elevation assumes a rock socket length equal to at least 150% of the diameter of the shaft. For the selected drilled shaft diameter of 24 inches, a minimum rock socket length of 13 feet and 4 feet is required to meet both axial and lateral resistance at the southwest abutment and the northeast abutment, respectively. The factored resistance for the strength limit state was calculated by applying a

Page | 15 resistance factor of 0.50 to the tip resistance and a resistance factor of 0.55 to the side resistance.

Recommended rock socket lengths, assumed tip elevations, and calculated resistances are summarized below in Table 4.2.

Table 4.2 Drilled Shaft Resistance in Rock

ABUTMENT TOP OF SHAFT

ELEVATION (ft)

ESTIMATED TIP

ELEVATION (ft)

ESTIMATED

SHAFT LENGTH

(ft)

SOCKET

LENGTH (ft)

FACTORED TIP

RESISTANCE

(kips) Southwest Abutment 7449.30 74171 30 13 260

Northeast Abutment 7450.06 7435 15 4 1083 1Estimated tip elevation extends approximately 5 feet beyond the termination depth of the exploratory boring. Calculations assume rock properties remain consistent with depth.

4.1.4 Group Effects on Axial Resistance

The resistance of a shaft group to the applied axial loads is not necessarily the sum of the axial resistance of individual shafts within the group. The zone of influence from an individual shaft in a shaft group may overlap with other shafts, depending on spacing. Historically, the efficiency of groups of drilled shafts has not been a concern as long as the center-to-center spacing between shafts is greater than three times the shaft diameter (3D) to avoid interference between adjacent shafts, assuming a single row shaft group configuration. An efficiency factor (η) should be applied for spacing less than 3D as shown in Table 10.8.3.6.3-1 of AASHTO. Based on the geometry of the chosen shaft system for the South Fork Negrito Creek Bridge, group effects are not anticipated (η = 1).

Besides the effect of overlapping zones of influence, effects of construction on ground conditions in and around the group can be significant. Excavated deep foundation elements in cohesionless soils tend to decrease the effective stress of the surrounding soils. Poorly controlled shaft construction methods can result in soil loosening during drilling and adversely reduce the lateral stress around other shafts within the group. Casing driven in advance of excavation may increase the relative density and effective stress of the surrounding soil and prevents caving of overburden material. If casing is used in construction, the possible effects on axial and lateral resistance may need to be considered. For this analysis, effects on axial and lateral resistance were not considered as it was assumed the casing would be removed or the temporary casing zone would be relatively small compared to the length of the shaft beneath this zone. If a significant amount of temporary casing or permanent casing along the entire shaft is to be used, the Geotechnical engineer should be informed to consider the effects on axial and lateral resistance and its impact on shaft length design.

4.1.5 Lateral Loads on Deep Foundations

Lateral load analysis will be performed by the bridge engineer using the software program LPILE developed by Ensoft, Inc. This program analyzes a single pile or shaft considering deflection as a

Page | 16 function of design loads, foundation construction, and subsurface conditions. Table 4.3 provides preliminary LPILE input parameters for the foundation soils based on available subsurface information and presumptive engineering correlations. We recommend that lateral support above the scour elevation defined in the CFLHD Hydraulics Report be neglected due to potential loss of material during the design flood event. It is also recommended that lateral support within five feet of the final road grade be neglected due to the potential loss of support from frost penetration or other shallow ground disturbance.

Table 4.3 LPILE Parameters2

APPROXIMATE

ELEVATION

(FT)

DEPTH BELOW

EXISTING GROUND

SURFACE (FT)

LPILE P-Y

MODEL

EFFECTIVE

UNIT WEIGHT

(PCF)

FRICTION

ANGLE

(DEG)

SOIL

MODULUS

(PCI)

Southwest Abutment (BH20-01)

7,453 – 7,451 0 to 2 Sand 116.0 30 25

7,451 – 7,443 2 to 10 Sand 122.0 34 90

7,443 – 7,435 10 to 18 Sand 59.61 34 60

7,435 – 7,434 18 to 19 Sand 53.6 30 125

7,434 – 7,430 19 to 23 Sand 72.6 35 125

TRANSITION TO ROCK PROPERTIES

LPILE P-Y

MODEL

EFFECTIVE UNIT

WEIGHT (PCF)

UNIAXIAL COMPRESSIVE

STRENGTH (PSI)

7,430 – 7,422 23 to 31 Vuggy Limestone

160 1,000

Northeast Abutment (BH20-02)

7,455 – 7,453 0 to 2 Sand 116.0 30 25

7,453 – 7,443 2 to 12 Sand 135.0 35 90

7,443 – 7,439 12 to 16 Sand 72.61 35 125

TRANSITION TO ROCK PROPERTIES

LPILE P-Y

MODEL

EFFECTIVE UNIT

WEIGHT (PCF)

UNIAXIAL COMPRESSIVE

STRENGTH (PSI)

7,439 – 7,434 16 to 21 Vuggy Limestone

160 4,110

1Water table assumed to be at thalweg elevation 2Neglect support above the scour elevation (7,439.5 feet)

Page | 17

Material properties provided are for single shafts and do not account for the reduced lateral resistance of shafts in a group. P-multipliers are a function of the number of rows of shafts and center-to-center shaft spacing in the direction of loading. P-multipliers are required even for a single row of shafts if the center-to-center spacing is less than 5 shaft diameters. P-multipliers are specified in Table 10.7.2.4-1 in AASHTO. When this analysis method is used, the resistances at the strength limit state as represented by the P-y curves should not be factored since they already represent the nominal conditions.

4.1.6 Settlement

A resistance factor of 1.0 for the service limit state is recommended to assess the ability of the foundation to meet the specified deflection criteria. Elastic settlements were estimated according to Article 10.8.2.2 (AASHTO). Elastic settlement will occur immediately and be essentially complete at the end of construction. Total settlement is estimated to be less than one inch at all locations based on the loads provided. Differential settlements, long term settlements, and shaft group settlement are not anticipated.

4.1.7 Field Testing

Steel crosshole sonic logging tubes (1.5-inch diameter steel tubes) should be installed in all drilled shafts prior to concrete placement for integrity testing. The recommended number of access tubes and tube spacing are dependent on the selected shaft diameter, typically one access tube per 12-inches of shaft diameter, with a minimum of three access tubes per shaft. The crosshole sonic logging tests should be conducted in accordance with ASTM D-6760, FP-14, and Special Contract Requirements (SCR) for quality assurance and quality control of the drilled shafts.

4.1.8 Scour Potential and Erosion

Contraction scour and local abutment scour depths were determined by the hydraulics engineer.

The total scour elevation was determined to be 7,439.9 feet. A detailed scour depth analysis is presented in Section 4.4.3 of the final hydraulics report.

Riprap is recommended for abutment protection per guidance from the hydraulics engineer. For additional information, please refer to the final hydraulics report.

4.2 ABUTMENT & WINGWALL DESIGN

4.2.1 Lateral Loads

Abutments and wing walls should be designed to resist lateral earth pressures and other applicable lateral loads in accordance with AASHTO. Lateral earth pressure is influenced by the strength of the abutment backfill, the presence or absence of water, and the ability of the abutment or wall to

Page | 18 move in response to lateral loads. Other loads, such as live loads, construction loads, and soil compaction loads should also be considered in the design.

Unbalanced water behind an abutment or wall adds significant lateral pressure and should be avoided by using free draining gravity outlets for water. Abutment and wing wall backfill should consist of structural backfill as specified in Section 704.04 of FP-14.

The coefficient of at-rest earth pressure should be used for design if the abutment is so restrained that it cannot be expected to rotate (deflect at the top) 0.002 times the wall height. Where deflection of the abutment can be expected, a coefficient of active earth pressure should be used for wall design. Active and at-rest lateral earth pressures of native materials, properly placed and compacted structural backfill, and unclassified borrow above the water table are presented in Table

4.4. The values are unfactored loads and assume that the surface of the soil slope behind the wall is horizontal.

Table 4.4 Lateral Earth Pressures for Bridge Abutments

Backfill Type Assumed Backfill

Properties Case

Unfactored Equivalent Fluid

Density (pcf)

Nominal Friction Factor

Structural Backfill c = 0 psf = 34 deg.

γ = 125 pcf

Active 35 0.67

At-Rest 55 0.67

Native Soil c = 0 psf = 35 deg γ = 122 pcf

Active 33 0.70

At-Rest 52 0.70

4.2.2 Bearing Resistance

Bearing resistance was not considered for abutment wingwalls. The wingwalls are planned to be cantilevered to the abutment structure and will not have their own separate foundation.

4.3 PERMANENT EARTHWORKS

4.3.1 Embankment Construction

Embankment construction is anticipated in relation to the construction of the proposed bridge structure and approach road reconstruction. Soils in the project area are typically clayey sand with gravel or silty/clayey gravel with sand. The existing embankment fills have been constructed between approximately 1V:1.5H to 1V:2H. The embankment areas should be constructed with traditional embankment construction methods. Construct permanent long-term embankments with a maximum slope ratio of 1V:2H to maintain slope stability and reduce potential for erosion.

Page | 19

4.3.2 Temporary Cut Slopes and Shoring

Temporary cut slopes are anticipated for construction of the proposed bridge and retaining walls.

Shoring may be desirable depending on preferences to limit areas of disturbance. Groundwater was encountered at 18 feet in depth during the subsurface investigation and may be encountered during excavation depending on the season, and dewatering could be necessary. Design and safety of this work is the responsibility of the contractor. The work shall be performed in a manner to minimize hazards and exposure to the public, construction personnel, and equipment. The subsurface investigation encountered material ranging from clay to boulder sized particles.

Cobbles and boulders were encountered while drilling. Boulders up to 3.5 ft in diameter were seen underneath the bridge.

4.3.3 Shrink/Swell Recommendations

On-site soils expected to be encountered within the project limits generally consist of clayey sand with gravel or silty/clayey gravel with sand. It is estimated that these soils will have a 10 percent shrink percentage, corresponding to a shrink/swell factor of 0.90. The recommended shrink/swell factor is based on a combination of standard tabled values for common materials in the FLH Technical Guidance Manual (2006) and experience with other CFLHD projects in similar materials.

4.4 CONSTRUCTION CONSIDERATIONS

Roadway Excavation: Excavate using equipment capable of removing the material while preventing material from escaping outside the construction limits.

Based on the subsurface investigation and surface geologic mapping, the rock encountered, primarily rhyolite, is expected to be rippable in the area surrounding the abutments of the Negrito Creek Bridge and up station of the structure; however, final determination of bedrock rippability is the responsibility of the contractor. Shear wave velocity of the geologic layers was collected by geophysical methods, but not processed. This unprocessed data may be made available to the contractor at their request. Based on Table D.1 in Appendix D, the rock encountered can be presumed to be between the hard and very hard rock descriptions. Additional information regarding the excavation characteristics of rock and rippability charts can also be found in Appendix D.

Evidence of definitive in-place bedrock was encountered from the subsurface investigation detailed in this report. Appropriate construction equipment should be mobilized to the site based on the rock information detailed above. The contractor is responsible for safety of excavations and shoring design.

Page | 20

Drilled Shafts: The contractor will be required to submit a drilled shaft construction plan as required in Section 565.04 of the FP-14 which includes the outlining of proposed methods to maintain borehole stability, manage the excavation of rock, concrete placement, and dewatering.

Rhyolite bedrock was encountered at all the exploration boreholes. A large difference in discernable rock quality at the boring locations was observed. The rhyolite bedrock at BH20-01 was highly weathered with very low RQD (0% to 7%). Rock cores recovered at this location were largely less than 4 inches in length along with angular to subrounded rock fragments. The rhyolite bedrock cores recovered in BH20-02 were only slightly weathered with 100% recovery and 95% RQD. Additionally, be prepared for cobbles and boulders ranging in size from approximately 12 to 42 inches. Loose subsurface materials were also encountered at shallow depths, which may warrant the use of temporary casing.

Groundwater was encountered at approximately 18 feet in depth during the subsurface investigation. Thus, it is anticipated that groundwater will be encountered at the foundation excavations. The contractor should be prepared for wet conditions and choose their construction method accordingly.

Dewatering: Dewatering may be needed depending on time of construction and the season.

Perform dewatering according to section 208.07 of the FP-14.

Slope Instability: Slopes could become unstable during construction operations due to precipitation, flooding events, or heavy loading on soft subgrade soils. Precautions should be taken if movement or cracking is observed. Seepage is a contributor to slope instability and similar dewatering and stabilization methods could be used to suspend slope movement.

4.5 SPECIFICATIONS

Special provisions were developed to be consistent with geotechnical recommendations stated above and were incorporated into the special contract requirements (SCR) to amend the FHWA Standard Specification for Construction of Roads and Bridges on Federal Highway Projects;

known as FP-14.

4.6 DISCLAIMER/LIMITATIONS CLAUSE

The recommendations in this report are based on the data obtained from exploratory borings, field review, and laboratory test results. The results of these explorations and tests represent conditions at the specific locations indicated. Subsurface variations across the site are likely and may not become evident until excavation is performed. The Analysis and Recommendations sections in this report include interpretations and recommendations developed by the Government in the process of preparing the design. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgment of the Contractor.

Page | 21

SECTION FIVE - References

American Association of State Highway and Transportation Officials (AASHTO), 2017, LFRD Bridge Design Specifications, 8th Edition.

Federal Highway Administration (FHWA), 2007, Geotechnical Technical Guidance Manual, dated May.

Federal Highway Administration (FHWA), 2008, Federal Lands Highway Project Development and Design Manual (PDDM), dated March.

Federal Highway Administration (FHWA), 2014, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects FP-14.

Federal Highway Administration (FHWA), 2017, Soil Description and Identification Guidelines, dated December.

Federal Highway Administration (FHWA), 2017, Rock Characterization Guidelines, dated December.

Federal Highway Administration (FHWA), 2021, “NM FS 141(1) Hydraulic Report,” dated February.

Haneberg, William C., 1992, “Geologic hazards in New Mexico - part 1,” New Mexico Bureau of Mines and Mineral Resources, New Mexico Geology…