CA ERFO NP DEVA 2022-1(2).Salt Creek Interpretive Trail.DRAFT-FINAL Geotechnical Memorandum.July 2024.pdf

To: Nate Allen, P.E., Project Manager/Construction Operations Engineer, CFLHD, Lakewood, CO

From: Devin T. Dixon, P.E., Lead Geotechnical Engineer, CFLHD, Lakewood, CO

Through: James M. Arthurs, Ph.D., P.E., Sr. Geotechnical Engineer, CFLHD, Lakewood, CO

Subject: Geotechnical Evaluations and Recommendations for the Salt Creek Interpretive Trail Replacement, Death Valley National Park, California, CA ERFO NP DEVA 2022-1(2)

1.0 PROJECT BACKGROUND INFORMATION

This memorandum presents the results of the geotechnical engineering study and provides recommendations for the replacement of the raised boardwalk (interpretive trail) along the Salt Creek tributary within Death Valley National Park, California. The project proposes to reconstruct the boardwalk trail which is located approximately 15 minutes west of the Furnace Creek Visitor Center off California State Route 190. A project vicinity map is presented in Figure 1.

The former Salt Creek Boardwalk Trail was constructed as a wooden ADA-accessible boardwalk loop that crisscrossed the creek and allowed visitors to view the endangered pupfish that have adapted to the unique habitat of Salt Creek. The boardwalk was a roughly half mile long signed interpretive trail with information and benches along the way. Heavy monsoon rains and severe flooding in August 2022 destroyed the boardwalk, altered the creek bed, covered the parking area in debris, and damaged the nearby vault toilet. The National Park Service has since closed and gated the existing access; however, visitors can still park vehicles at the access, walk around the gate, and hike to the Salt Creek Trail and other trails in the area.

1.1 January 2023 Scoping

A scoping trip consisting of a field review and follow-up office meeting was conducted on January 10th, 2023 with representatives from the Central Federal Lands Highway Division (CFLHD), National Park Service (NPS), and the U.S. Geological Survey (USGS). The purpose of the meeting was to broadly discuss elements of the project identified in the field review, address staff concerns

Technical Memorandum

Date: July 31, 2024 and information on the need for in-depth site investigations, discuss funding coordination, overarching priorities, and distribute roles and responsibilities.

The condition of the remaining boardwalk remnants was investigated during the field review. Few sections of the previous boardwalk had washed nearly one mile downstream of their former location and others were buried under flood debris. The boardwalk segments that remained relatively accessible and in-place were in generally fair to good condition. Concrete and structural steel elements showed corrosion from the abundance of evaporitic salts common to the region. No formalized plans of the previous boardwalk exist; however, typical section details of the existing bridge include an approximate out-to-out width of 6 feet with 4-inch-wide curbs. The superstructure consisted of 2x6 timber decking supported by 4x6 timber stringers. Pier supports were spaced at approximately 10 feet and were comprised of 4x6 timber posts supported on circular 12-inch diameter concrete shallow footings. At least one 25- to 30-foot-long bridge previously stood near the western end of the boardwalk but had reportedly been washed downstream. The former boardwalk will be carefully removed as part of this proposed project while taking into consideration the wilderness and sensitive nature of the environment. Access to the site is to be determined but based on the environmental sensitivity of the site it is to be assumed that any new construction may be limited to small vehicular traffic such as a skid steer loader or a small pickup truck. Structural components will be selected to be transportable assuming these smaller vehicle types.

1.2 Hurricane Hillary and Additional Damage

Death Valley National Park was further impacted by the remnants of Hurricane Hilary from roughly April 19th through 20th, 2023. The heavy rain led to flooding in many areas of the Park and resulted in a temporary Park-wide closure. This second flood event deposited at least two feet of sediment throughout the Salt Creek Boardwalk Trail area. Discussions were held with the Park soon after crews completed clearing and assessing travel ways to determine a path forward in terms of schedule and the overall design approach for the subject project. This resulted in developing the following action items:

1. Obtaining new LiDAR and/or aerial survey data in October or November 2023.

2. Revising the alignment based on the new survey in December 2023.

3. And moving the 50% PS&E submittal and field review to finalize the new alignment to January

2024.

These proposed schedule changes and work items were accepted by the Park Service and a change control was initiated in late September 2023. The boardwalk alignment was later modified at the 50% milestone to avoid paleontologically sensitive areas as identified by the Park in a report dated January 12, 2024. The length of the boardwalk was also modified around this time to ensure ERFO funding compliance.

1.3 Project Scope

The proposed boardwalk will be supported on deep foundation elements designed for scour at the recommended interval event as will be discussed in Section 4. Subgrade concrete work is expected to be limited to the boardwalk abutment. Seasonal limitations will be determined and construction during the midsummer will not be advisable due to extreme temperatures. The proposed boardwalk will be designed according to AASHTO LRFD Bridge Design Specifications, 9th Edition, 2020 and AASHTO LRFD Guide Specifications for Design of Pedestrian Bridges, 2009 with current interims. The boardwalk will be designed for pedestrian loading (and wind and stream flow) only, and ADA/ABA requirements will be considered as determined.

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

A hydraulics report has been prepared under separate cover by the CFLHD Hydraulics Engineer.

The hydraulics report includes discussion of flood impacts, such as scour and water surface elevations, and recommendations for embankment protection.

Figure 1 - Project Vicinity Map

2.0 GEOLOGY AND SEISMICITY

2.1 REGIONAL GEOLOGY

The project is located within the Great Basin 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 mountain ranges alternated with broad, flat basins.

The topography and dry climate create unique features and landscapes that include pediments, alluvial fans, bajadas, bolsons, Inselbergs, playas, mud flats, salt flats, lakes, sand dunes, canyons, and the Rio Grande Rift (NPS, 2020).

Holocene aged saline deposits that exist within the region include sand (Qs), slabby rock salt (Qsr), reworked rock salt (Qr), and rough silty rock salt (Qrsr). Other Holocene to Pleistocene aged surficial deposits include flood plain deposits (Qfp), fan and stream deposits (Qg1-Qg4), and deposits of Lake Manly (Qlm). Bedrock units within the region belong to the Funeral Formation and Furnace Creek Formation. These units are Pleistocene to Pliocene aged and include conglomerate facies (QTfc), basalt (QTb), and siltstone-sandstone facies (Tfs) (Wright & Troxel, 1993).

2.2 SITE GEOLOGY

Based on observations at the project site, the soil conditions appear to be alluvium from the existing streambed deposits. The project site is mapped as underlain by flood plain deposits (Qfp), fan and stream bed deposits (Qg1, Qg3-4), conglomerate facies (QTfc), and siltstone-sandstone facies (Tfs).

Conglomerate facies (Tfc) are also mapped in the vicinity of the site. These observations are consistent with the scoping trip observations as will be discussed in Section 3. The deposits and landforms are primarily a result of different types of fault activity, in addition to crustal sinking and some volcanic activity. The terrain can be described as a desert valley. A geology map of the project area and corresponding map legend are presented in Figures 2 and 3, respectively.

Figure 2 - Regional Geologic Map (Wright & Troxel, 1993)

Figure 3 - Description of Map Units (modified from Wright & Troxel, 1993)

2.3 REGIONAL SEISMICITY

Ten seismic hazard faults are located within 50 miles of the project. These faults are summarized in Table 1 and are primarily located west, north, and south of the project site. Seven of these faults have experienced deformation in the last 15,000 years or less. Known slip rates range from 0.00787 to 0.19685 inches per year (USGS, 2020a). The site-source distance using the Caltrans ARS Online (v3.0.2) tools was calculated to be approximately 14 miles with a mean magnitude of approximately 7. Based on the time of most recent deformation and distance to these faults, surface rupture at the boardwalk site is possible; however, large earthquakes that cause surface deformation along the Death Valley and Furnace Creek fault zones occur infrequently (Machette et al., 1999).

Table 1 - Summary of Nearby Seismic Source Faults

FAULT OR

FAULT ZONE

APPROXIMATE

DISTANCE

FROM CENTER

OF PROJECT

SLIP-RATE

CATEGORY

DIP

DIRECTION

AVERAGE

STRIKE

FAULT

LENGTH

TIME OF MOST

RECENT

DEFORMATION

(miles) (inch/year) (miles) (years) Northern Death

Valley Fault Zone; Grapevine

Mountains, Mesquite Flat-

Screwbean Spring, and Kit

Fox Hills Sections (Class A)

No. 141a, b, and c

1.48 0.03937 to

0.19685 V

N28°W N34°W N39°W

(for sections)

23.6 18.6 20.5

(for sections) <15,000

Towne Pass Fault, (Class A)

No. 97

6.8 <0.00787 45°-80° W N19°E 36.0 <15,000

Black Mountain Fault Zone;

Mustard Hills Transition Zone, Artists Drive, Copper Canyon, and Smith Mountain

Sections (Class A)

No. 142a, b, c, and d

6.8 <0.00787 to 0.19685

W; SW; NE

No. 142a

40°-75° W

No. 142 b, c, and d

N34°W N25°W N2°W

N23°W (for sections)

5.0 13.7 16.8 9.9

(for sections)

<15,000

Bare Mountain Fault, (Class A) No. 1079

23.4 <0.00787 50-70° E N2°W 11.2 <130,000

Panamint Valley Fault Zone;

northern Panamint Valley, Wildrose, southern Panamint

Valley, Brown Mountain Sections (Class A)

No. 67a, b, c, and d

26.2 0.03937 to 0.19685

V to 70°NE W

V; W

V; NE

(for sections)

N36°W N15°W N16°W N57°W

(for sections)

16.8 3.6

33.6 10.6

(for sections)

<15,000

Hunter Mountain-

Saline Valley Fault Zone;

Saline Valley and Hunter Mountain Sections,

26.4 0.03937 to 0.19685

V 20°

(for sections)

N37°W N58°W

(for sections)

19.3 26.1

(for sections) <15,000

FAULT OR

FAULT ZONE

APPROXIMATE

DISTANCE

FROM CENTER

OF PROJECT

SLIP-RATE

CATEGORY

DIP

DIRECTION

AVERAGE

STRIKE

FAULT

LENGTH

TIME OF MOST

RECENT

DEFORMATION

(miles) (inch/year) (miles) (years) (Class A)

No. 66a and b Ash Hill

Fault, (Class A) No. 233

29.4 Unspecified Not listed Not listed Not listed <15,000

Tin Mountain Fault, (Class A) No. 188

31.0 <0.00787 W N8°E 23.0 <130,000

Unnamed Faults West of Dry Mountain, (Class A) No. 499

41.5 Unspecified Not listed Not listed 21.1 <130,000

Tank Canyon Fault, (Class A) No. 71

47.9 Unspecified W Not listed 10.6 <15,000

2.4 SEISMIC DESIGN PARAMETERS

The following design values are for informational purposes since seismic analysis was not required for the proposed boardwalk structure. Recommended seismic response parameters for the Salt Creek Interpretive Trail project site design are based on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications, 9th Edition, 2020 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 Salt Creek Interpretive Trail project site, located at 36.59058°N latitude and 116.99072°W longitude, was obtained in accordance with the AASHTO ground motion maps.

Based on subsurface conditions encountered during the scoping trip, limited laboratory testing, and the USGS Vs30 online map viewer, the on-site soils can be assumed to be stiff. Therefore, the site is classified as Class D according to the site class definitions specified in Table 3.10.3.1- 1 of AASHTO. The recommended spectral acceleration coefficient values for probabilistic design are summarized in Table 2.

Table 2 - Summary of Seismic Parameters Corrected for Site Class D

Horizontal Peak Ground Acceleration, (PGA) 0.568 g Horizontal Response Spectral Acceleration at Period of 0.2 sec, (Ss) 1.307 g Horizontal Response Spectral Acceleration at Period of 1.0 sec, (S1) 0.56 g

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.0 Site Factor at Short-Period Range of Acceleration Spectrum, (Fa) 1.0 Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.5

Factored Horizontal Peak Ground Acceleration, (As) 0.568 g Factored Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 1.307 g Factored Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.841 g

Seismic Zone Zone 4

Based on the long acceleration coefficient SD1 value of 0.841, the project site is assigned to seismic hazard “Zone 4” in accordance with Table 3.10.6-1 of AASHTO. Seismic hazard zones reflect the variation in seismic risk in different regions needing different requirements for design as depicted in Table 4.7.4.3.1-1 of AASHTO.

2.5 Geologic Hazards

Potential geologic hazards at the boardwalk site include those related to earthquakes, extensive weathering, and mineral hazards. Earthquakes, as discussed in the previous section, could pose a threat to the Salt Creek Interpretive Trail. Southeastern California and the project site are in an area of moderate to high potential for damaging earthquakes, although it is impossible to accurately predict the timing or location of future earthquakes. Liquefaction potential is low since the site soils are relatively dense and mostly unsaturated. Seismic hazards can be mitigated by appropriate design analysis as specified in AASHTO. A seismic hazards map for the project vicinity is presented in Attachment 1.

Minerals-related hazards, such as those associated with saline deposits including sodium chloride, calcite, gypsum, and borax, are present in Death Valley National Park. The combination of sodium, chloride, and water can accelerate corrosion in metals. Corrosion control measures, such as galvanizing or applying coatings, should be considered with regards to the proposed boardwalk foundation elements.

3.0 FIELD EVALUATION

3.1 SURFACE EXPLORATION AND LABORATORY TESTING

To evaluate the field conditions, a CFLHD team visited the project site and conducted limited field measurements and observations during the scoping trip in January 2023. This team included Nate Allen (Project Manager), Darin Lockhart (former Highway Design Engineer), Devin Dixon (Geotechnical Team Leader), David Bernard (former Geotechnical Engineer), Karl Eikermann (Supervisory Structural Engineer), and Luis Calderon (Hydraulics Team Leader). The CFLHD team met with representatives of the National Park Service (Death Valley National Park, Interior Regions 8/9/10/12, and Denver Service Center) and the U.S. Geological Survey Southwest Region.

Field observations focused on evaluating the existing boardwalk conditions and soil properties.

Evaluations and recommendations presented in this memorandum are based on visual observations and the limited measurements taken during the site visit. Due to limited ERFO project time and budget constraints to expedite project delivery and reestablish visitor access as well as environmental concerns (i.e., pupfish habitat), no subsurface investigations were conducted.

However, a sample of the overburden soil material was collected from the project area by the CFL Geotechnical Engineers and shipped to the CFLHD lab for testing.

Results of the classification testing are summarized below in Table 3 and are presented in Attachment 2. The analysis in this report was based on the lab testing results and presumptive properties and experience from similar repairs. The sample was collected north of the remaining boardwalk and approximately 68 feet west of the washed-out parking area, where desert valley sand material overlies geologic units of the Furnace Creek Formation. This material is primarily flood plain deposits and likely has a high salinity. The sample was tested for chemical properties and corrosive potential with results shown in Table 4. Based on these tests, the site soils are assumed to be corrosive according to chloride and sulfate concentrations, pH, and the California Transportation Department’s definition of a corrosive environment. Furthermore, the reported resistivity value was less than 1,500 ohm-cm which indicates the presence of high quantities of soluble salts and a higher propensity for corrosion according to Caltrans guidelines (Caltrans, 2021). Additional engineering property test results are presented in Table 5.

Table 3 - Classification Testing Results

FIELD

NUMBER

LOCATION

DESCRIPTION

APPROXIMATE

LAT/LONG

PERCENT

SAND

PERCENT

PASSING

PERCENT

MOISTURE LL PI USCS AASHTO

Bulk-1

North of existing boardwalk, ≈ 68 feet west of parking area

36°35’25.8” N

116°59’27.2” W 78 6 3 NV NP SP-

SM A-3(0)

Table 4 - Corrosivity Testing Results

FIELD NUMBER SAMPLE DEPTH RESISTIVITY PH SULFATE ION CONTENT CHLORIDE ION CONTENT (feet) (ohm-cm) (ppm) (ppm)

Bulk-1 0 - 1 86 9.2 1,905 4,361

Note: From Caltrans 2021, Section 6.1- For structural elements, the Department considers a site to be corrosive if one or more of the following conditions exist for the representative soil and/or water samples taken at the site: Chloride concentration is 500 ppm or greater, sulfate concentration is 1,500 ppm or greater, or the pH is 5.5 or less.

Table 5 - Additional Engineering Property Testing Results

FIELD NUMBER SAMPLE DEPTH

CORRECTED OPTIMUM

MOISTURE

CORRECTED MAXIMUM

DRY DENSITY

FRICTION

ANGLE

(feet) (%) (pcf) (°)

Bulk-1 0 - 1 11.2 116.4 35.6

3.2 SUMMARY OF SITE CONDITIONS

This section presents the results of the surface reconnaissance and discussions regarding anticipated subsurface conditions.

3.2.1 General Topography

The general topography of the project site is characterized by arid desert terrain, flood-carved canyons, spring-fed pools, and seasonal plant life. Pickleweed plants border the banks of Salt Creek and endangered pupfish inhabit the flowing waters during the winter. Further site-specific observations are discussed in Section 3.2.3.

3.2.2 Average Temperatures and Precipitation

Table 6 shows climate data for the period from 1911 to 2016 for the Death Valley, California (042319) weather station (WRCC, 2023).

Table 6 - Climate Data Summary

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC ANNUAL

Avg. Max. Temp.

(°F) 66.3 73.4 81.4 89.8 99.9 109.5 116.0 114.1 106.2 92.8 76.6 65.1 90.9

Avg. Min. Temp.

(°F) 39.2 46.0 53.8 61.6 71.9 80.9 87.6 85.6 75.5 61.7 48.0 38.3 62.5

Avg. Total Precip.

(in.) 0.31 0.51 0.28 0.13 0.06 0.04 0.12 0.15 0.19 0.10 0.18 0.18 2.24

Avg. Total Snowfall (in.) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Avg. Snow Depth (in.) 0 0 0 0 0 0 0 0 0 0 0 0 0

Note:

1) Period of Record: 06/01/1911 to 06/09/2016.

2) Percent of possible observations for period of record: Max. Temp.: 99.2% Min. Temp.: 99.0% Precipitation: 98.9%

Snowfall: 99.3% Snow Depth: 99.3%.

3.2.3 Surface Reconnaissance Observations (as noted during scoping)

The site is underlain by siltstone and sandstone facies of the Furnace Creek Formation derived from lake sediments. Bedrock outcroppings are a pale green clayey siltstone with fine sand and bedding that dip generally to the northeast. Rock outcroppings are exposed in approximately 10-to 30-foot-tall cliffs along the edges of the creek and form the bottom of the stream channels across most of the site. Slightly weathered to fresh bedrock is present in certain areas and is hard to the touch but can be peeled out of outcrops and broken by hand with moderate pressure. The bedrock is poorly cemented and where exposed disaggregates into silt and fine sand giving the outcrops a melted appearance. Blocks of the rock separate along vertical fractures and gently drop along the steep outcrops along the riverbank where they are slowly eroded into silt and sand by a combination of the wind, rain, and creek.

Flood plain deposits fill much of the stream valley at this site. These are generally silt and fine sand with some gravel from the nearby alluvial fans. These deposits are up to approximately 10 feet thick along the higher banks. Salt Creek is a braided stream through these deposits and is mainly constrained laterally by the heavy vegetation that takes root in the silt and sand and constrained vertically by the Furnace Creek bedrock below.

Most of the erosion from the 2022 floods occurred in the north stream banks in the eastern (non-loop) portion of the boardwalk. At these locations Salt Creek has washed out the banks moving the stream further to the north and closer to the bedrock outcrops/cliffs. This has created vertical banks approximately 10 feet high in the silt and sand flood plain deposits.

The boardwalk was originally set on cylindrical concrete footings approximately 2 feet height by about 1 foot diameter. Steel brackets were set through the entire depth of the footings to fasten the boardwalk to the footings. The original boardwalk was roughly 25 years old, and the footings showed various degrees of weathering. All the footings observed show weathering of a few millimeters of the cement all around the above-ground portion of the footing. The metal brackets were often rusted, and, in a few cases, the concrete portion of the footings were found fractured along the bracket where it had rusted inside the concrete. According to Park personnel some of the footings had separated from the boardwalk where the brackets had rusted through. In locations where the boardwalk was swept away by the 2022 flooding it appears that the floodwaters washed away the ground that supported the footings and the boardwalk sections acted like a raft floating in the higher water until the flow depth decreased. Several sections of the boardwalk are still in place with the footings still in the ground or with a minimal amount of the soil washed away from the footings.

Groundwater levels were not measured during the surface reconnaissance. However, most of the water entering the Salt Creek Valley is delivered by subsurface groundwater flow. Fluctuations in the groundwater level due to seasonal and climatic effects are expected and will likely follow water levels in the nearby springs.

3.2.4 Scour Potential and Erosion

Total scour depths were estimated by the assigned CFLHD Hydraulics Engineer. Scour depths were provided by the CFLHD Hydraulics Group in the Final Hydraulics Report dated July 2024 (CFLHD, 2024). The recommended total scour values for the boardwalk foundations was estimated to be between 1.0 and 6.0 feet for the scour design flood (50-year storm) and between

1.5 and 7.5 feet for the scour check flood (100-year storm) depending on the designated scour zone locations shown in Figure 22 and Table 6 of the Final Hydraulics Report. These scour depths are to be measured from the existing ground elevation at each helical pile location. The Salt Creek Interpretive Trail foundations will be designed for the estimated scour depths, and embankment protection is recommended along portions of the parking lot and roadway that interact with the channel. Please refer to the Final Hydraulics Report for more information.

4.0 ANALYSIS AND RECOMMENDATIONS

This section presents analysis and recommendations for the boardwalk foundations and construction considerations for the design and construction of the Salt Creek Interpretive Trail project. Based on discussions with the project team and the severe damage to all boardwalk structure components, the boardwalk will be replaced. Generalized subsurface profiles were developed based on field reconnaissance and surficial visual evaluation.

4.1 Foundations

The destroyed boardwalk (interpretive trail) along the Salt Creek tributary within Death Valley National Park will be replaced with a new boardwalk consisting of the following elements:

pressure treated timber stringers, pier beams and rail elements; thermal modified decking; and helical piles for foundation support. The proposed boardwalk will be constructed from approximately STA 100+00 to STA 128+13.37 over the environmentally sensitive area. The proposed foundation system of helical piles will consist of 287 pier locations spaced a maximum of 10 feet apart for a total length of 2,813 feet.

4.1.1 Boardwalk Foundation Selection

A helical pile foundation system is proposed for the new boardwalk abutments and piers. Other options that were considered included micropiles, timber piles, fiberglass composite piles, drilled shafts, driven steel piles (H-piles), driven concrete piles, concrete piers, and cast-in-place concrete shallow footings. Favorable and unfavorable factors related to each of these foundation options are presented in Table 7.

Table 7 - Generalized Foundation Comparisons

FOUNDATIONAL

SYSTEMS PROS CONS

Helical Piles

Smaller and more accessible equipment requirements

Most cost effective Least environmental disturbance to site/easily transported to remote sites Can be battered (brace pile) to increase stability/added lateral resistance Can be installed in groundwater without casing Relatively quick installation Low noise and minimal vibrations Can be galvanized for corrosion resistance Eliminate concrete work

May limit boardwalk height due to stiffness limitations (limit on size)

Due to corrosive site, will need to design for additional thickness and corrosion protection

Difficulties installing in rock (although substantial competent rock is not expected at the DEVA site)

Micropiles

Larger sizes than helical piles allows for more stiffness and greater heights than helical piles

Minimal vibration and noise Doesn’t require a drilling platform Can be installed at any angle

Quite a bit more expensive than helical piles Require larger equipment, more site disturbance, more drill spoils.

Require grout for fill; possible environmental concerns.

Due to corrosive site, will need to design for additional thickness and corrosion protection Low lateral resistance May not meet seismic considerations (buckling)

Timber Piles

Inexpensive compared to concrete and steel

Easy to install Suitable for impact absorption

Small bearing capacity Prone to damage by hard driving Difficulty finding longer lengths of piles Cannot be driven through hard strata Will need to be treated to prevent decay and plant/animal attack (creosote oil) Requires larger pile driving equipment, more impact to site Need to be replaced more frequently (shorter service life)

Fiberglass Composite Piles

Light weight High capacity Ideal for difficult access areas Long service life, resistant to corrosion, suitable for marine applications and harsh environments

Cost effective

Will require larger pile driving equipment, more site disturbance

Noise and vibration from pile driving Transport considerations to site Availability

Drilled Shafts

Long term performance and strength, etc.

More expensive Larger equipment, more space to maneuver and install, more site disturbance Concrete considerations (i.e., sulfate attack)

Driven Steel Piles (H-Piles)

Manufactured in a variety of lengths and sizes

High structural strength

More expensive Require large equipment for installation, more site disturbance Noise and vibration from pile driving Corrosion considerations Transportation and installation considerations

Driven Concrete

Piles Durability Longevity

Higher cost Concrete considerations (i.e., sulfate attack)

FOUNDATIONAL

SYSTEMS PROS CONS

Large equipment, transportation considerations, more site disturbance, etc.

Concrete Piers (PermaTrak or similar system)

Cost effective

Scour considerations Construction/installation, equipment, transportation, concrete (i.e., sulfate attack), etc.

Cast-In-Place Concrete

Shallow Footings

Quick installation Fairly inexpensive Smaller construction equipment for installation (but more site disturbance than helical or timber piles)

Scour considerations Concrete concerns (uses sonotubes, but they are not intended to be permanent) Transportation, disturbance to site, etc.

A helical pile foundation system is feasible from a geotechnical perspective. Helical piles are an environmentally friendly deep foundation solution due to their lightweight and low impact of installation equipment. Based solely on hydraulic recommendations and assumptions stated in the Final Hydraulics Report (CFLHD, 2024), scour potential as measured from the thalweg was determined to be present at the boardwalk piers. Based on available data, it was determined by the combined team of Bridge, Hydraulics, and Geotechnical Engineers that helical piles would provide the best foundation system for this boardwalk.

Table 8 presents the preliminary bridge loads factored for the strength limit state as provided by the CFL Bridge Engineer in May 2024:

Table 8 - Pier Loading1

FOUNDATION

LOCATION

MINIMUM CENTRAL SHAFT

SIZE

(inches)

MAX. LOAD (AT TOP OF PILE)

SERVICE I

BRIDGE LOAD

PER PILE

(kips)

STRENGTH I

BRIDGE LOAD

PER PILE

(kips)

Pier 222 to 287 2 ⅞ 5.9 9.9 Pier 1 to 203 5 ½ 6.4 9.6

Pier 204 to 221 7 5.8 9.3 1 See Sheet S3 in the plans for foundation details.

4.1.2 Site Characterization

The subsurface profile was assumed to be entirely sand for analysis purposes. Groundwater was also assumed to be at the ground surface elevation. These assumptions are conservative, or, in other words, purposefully broad to account for the lack of subsurface data collected below the scour depth. As discussed in Section 3, the actual ground conditions encountered during construction may differ from this assumption.

4.1.3 Helical Pile Axial Resistance

Design analyses conducted using typical empirical methods based on an assumed SPT N-value of 21 for medium dense coarse-grained soil indicate that a nominal bearing pressure of approximately

9.2 kips per square foot (ksf) may be used for end bearing design. This design assumed two 10-inch diameter plates spaced 30 inches apart. Although LRFD resistance factors for helical piles are not presented in the code, it is recommended that a resistance factor of 0.50 be used for design.

This is based on the typical ASD factor of safety of 2.0 converted to a resistance factor of 0.50 (Perko, 2009). The contractor should select a helix configuration that meets criteria for loading and eases installation. Installation torque measurements should be performed to verify pile capacity during construction in accordance with manufacturer recommendations. Note that installation torque may not correlate well to capacity where piles refuse on boulders or bedrock.

These values and calculations were used for construction estimation purposes; the contractor is expected to determine the appropriate values and complete separate calculations for the design of the helical pile foundations based off the information provided.

The following procedure should be followed to meet the design requirements.

• All piles should be installed to the minimum depth required by the approved helical pile design and to the minimum depth required for lateral resistance presented on the bridge plans.

• If refusal is encountered, the pile may be terminated at that depth provided it meets the minimum embedment depth required for lateral resistance presented on the plans and the helical pile designer approves of the early termination.

• Where piles meet refusal prior to the minimum embedment depth required for lateral resistance, the holes should be pre-drilled such that the minimum embedment is achieved while also meeting axial requirements.

• Offsetting the helical pile and re-attempting to install may be considered with the approval of the bridge engineer.

• Shallow groundwater and flowing sands may be encountered, which could extend the depth of required piles.

These observations are for geotechnical axial resistance and lateral design calculations may control the final design (the greater depth of the two will control).

4.1.4 Helical Pile Lateral Resistance

Lateral load analysis has been performed, and is provided, by the assigned Central Federal Lands Structural Engineer using the software program LPILE developed by Ensoft, Inc. This program analyzes a single pile considering deflection as a function of design loads, foundation construction, and subsurface conditions. Table 9 provides preliminary LPILE input parameters for the foundation soils based on limited subsurface information and generally accepted engineering correlations.

Table 9 - LPILE Input Parameters

MATERIAL APPROXIMATE

ELEVATION

(feet)

LPILE

P-Y

MODEL

EFFECTIVE

UNIT WEIGHT,

γ’

(pcf)

FRICTION

ANGLE,

φ

(deg)

HORIZONTAL

SUBGRADE

MODULUS, KS

(lb/in3)

Poorly graded sand with silt

Existing ground to pile tip

Sand (Reese) 49.61 36 60

1 Groundwater assumed to be at the ground surface elevation.

4.2 Abutment Design

Abutments 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 to 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 adds significant lateral pressure and should be avoided by using free draining gravity outlets for water. Abutment 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 height. Where deflection of the abutment can be expected, a coefficient of active earth pressure should be used for design.

Active and at-rest lateral earth pressures of native materials and properly placed and compacted structural backfill above the water table are presented in Table 10. The values are unfactored loads and assume that the surface of the soil slope behind the abutment is horizontal.

Table 10 - 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 φ = 34 deg.

γ = 125 pcf

Active 35 0.67

At-Rest 55 0.67

Native Soil c = 0 psf φ = 36 deg γ = 112 pcf

Active 29 0.73

At-Rest 46 0.73

5.0 CONSTRUCTION CONSIDERATIONS

Specifications: All geotechnical recommendations stated above are incorporated into the SCRs to amend or are already incorporated within the FHWA Standard Specification for Construction of Roads and Bridges on Federal Highway Projects; known as FP-14.

Helical Piles: Construction of helical piles for the project will be complicated by several factors:

1) Site access,

2) Environmental concerns,

3) Granular soils below the water table (flowing sands),

4) Corrosive soils,

5) And seasonal limitations.

Based on these factors, the Contractor should assume loose granular material will be encountered at each helical pile and abutment location. Ground water depth should be expected to fluctuate seasonally. The contractor is expected submit a helical pile design and construction plan in accordance with Section 582 of the SCR and FP-14. We anticipate that the following items will be included in the plan:

1) Helical pile materials

2) Subcontractor qualifications

3) Proposed construction sequence and schedule

4) Pile installation and verification equipment

5) Shop drawings and design calculations

6.0 DISCLAIMERS/LIMITATIONS CLAUSE

Subsurface explorations were not performed as a part of this scope of work. Interpretation of surface and subsurface conditions is based on limited field reconnaissance and surface mapping of soil and rock outcrops. The Analysis and Recommendations section of this report includes 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 judgement of the Contractor.

Attachments (2)

Seismic Hazards Map (Project Vicinity)

Laboratory Test Results

Prepared by:

Devin T. Dixon, P.E.

Geotechnical Team Leader, CFLHD

Reviewed by:

______________________________________________ Date: July 31, 2024

James M. Arthurs, PhD., P.E.

Senior Geotechnical Engineer, CFLHD

Approved by:

______________________________________________ Date: July 31, 2024

Devin T. Dixon, P.E.

Geotechnical Team Leader, CFLHD

7.0 REFERENCES

American Association of State Highway and Transportation Officials (AASHTO), 2020, LFRD Bridge Design Specifications, 9th Edition.

California Department of Transportation (Caltrans), 2021, Corrosion Guidelines,” Version 3.2, dated May.

Central Federal Lands Highway Division (CFLHD), 2024, “CA ERFO NP DEVA 2022-1(2) Salt Creek Interpretive Trail Final Hydraulics Report,” dated July.

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(a), Soil Description and Identification Guidelines, dated December.

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

Machette, Michael N., Martinez, Carlos Ninci, Crone, Anthony J., Haller, Kathleen M., D’Addezio, Giuliana, 1999, “Geologic and Seismic Hazards Investigations of the Cow Creek Area, Death Valley National Park, California,” Open-File Report 99-155, U.S.

Geologic Survey (USGS), <https://pubs.usgs.gov/of/1999/0155/report.pdf>, Accessed April 2023.

National Park Service (NPS), 2020, “Series: Physiographic Provinces, Basin and Range Province,” updated April 15, <https://www.nps.gov/articles/basinrange.htm>, Accessed April 2023.

Perko, Howard A., 2009, “Helical Piles: A Practical Guide to Design and Installation,” John Wiley & Sons, Inc.

Reese, Lymon C., Wang, Shin-Tower, Vasquez, Gonzalo L., Arrellaga, Jose A., and Isenhower, William M., 2022, “LPILE v2022 Technical Manual,” Ensoft, Inc., dated February.

State of California (CA.gov), 2020, Caltrans ARS Online (v3.0.2), <https://arsonline.dot.ca.gov/index.php?>, Accessed April 2023.

U.S. Geological Survey (USGS), 2020(a), “Quaternary Fault and Fold Database of the United States,” Interactive fault map & database search, USGS Earthquake Hazards Program, <https://www.usgs.gov/natural-hazards/earthquake-hazards/faults?qt-science_support_page_related_con=4#qt-science_support_page_related_con>, Accessed April 2023.

U.S. Geological Survey (USGS), 2020(b), “Vs30 Map Viewer,” online map, USGS Earthquake Hazards Program, <https://earthquake.usgs.gov/data/vs30/>, Accessed April 2023.

U.S. Geologic Survey (USGS), 2020(c), “U.S. Seismic Design Maps,” USGS Earthquake Hazards Program, <http://earthquake.usgs.gov/designmaps/us/application.php> &

<https://earthquake.usgs.gov/ws/designmaps/aashto-2009.json?latitude=34&longitude=- 118&siteClass=C&title=Example>, Accessed April 2023.

Western Regional Climate Center (WRCC), 2023, “Death Valley, California Period of Record Monthly Climate Summary,” <https://wrcc.dri.edu/cgi-bin/cliMAIN.pl?ca2319>, Accessed March.

Wright, L.A., and Troxel, B.W., 1993, “Geologic map of the central and northern Funeral Mountains and adjacent areas, Death Valley region, southern California,” U.S. Geologic Survey, Miscellaneous Investigations Series Map I-2305, scale 1:48,000.

4.2 Abutment Design