B08--Attachment_3_-_GEOTECHNICAL_REPORT_(1).pdf

AE Project No: 29964.12.00

K-1 NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Contract # 140P2021D0001 | PMIS # CURE 318472 (N140) Basis of Design | 90% Detailed Design Development

Appendix K Geotechnical Reports (Electronic Only)

PEOPLE WHO MAKE IT HAPPEN

PEOPLE WHO MAKE IT HAPPEN

Deliverable: Geotechnical Investigation Project: Curecanti National Recreation Area, Replace Water and/or Wastewater System at Elk Creek and Lake Fork

Client: National Park Service Location: Denver, Colorado Contract No: 140P2021D0001 Task Order: 140P2024F0090 PMIS No: CURE 318472

Submittal: Final Date: August 27, 2024

Prepared by: Stanley Consultants-DOWL Joint Venture

AE Project No: JV45.000C1.02 i NPS | Glacier National Park, Replace Utility Systems (GLAC318705) NPS Order # 140P2024F0140 Project Program | Pre-Design Draft Submittal

TABLE OF CONTENTS

EXECUTIVE SUMMARY ....................................................................................................... I

1.0 INTRODUCTION

1.1 Purpose and Scope

1.2 Project Understanding

1.2.1 Existing Site Conditions

1.2.2 Proposed Construction

2.0 INVESTIGATION

2.1 Field Investigation

2.2 Laboratory Testing

3.0 SUBSURFACE CONDITIONS

3.1 Site Geology

3.2 Observed Soil Conditions

3.2.1 Existing Pavement

3.2.2 Granular Soils

3.2.3 Clayey Soils

3.2.4 Sandstone Bedrock

3.3 Groundwater

3.3.1 Groundwater Information Center Research

3.4 Seismicity

3.4.1 Faulting

3.4.2 Design Accelerations

3.4.3 Liquefaction

4.0 ENGINEERING ANALYSIS AND RECOMMENDATIONS

4.1 Foundation

4.1.1 Conventional Spread Footings

4.2 Lateral Earth Pressures

4.2.1 Seismic Earth Pressure

4.2.2 Coefficient of Friction

4.3 Slabs-on-Grade

4.3.1 Interior Slabs

4.3.2 Exterior Slabs

4.4 Drainage

4.5 Pavement Design

4.5.1 Flexible Pavement

4.5.2 Construction Considerations

4.5.3 Maintenance

4.6 Thrust Restraint

4.7 Horizontal Drilling Considerations

4.8 Earthwork

ii NPS | Glacier National Park, Replace Utility Systems (GLAC318705) NPS Order # 140P2024F0140 Project Program | Pre-Design Draft Submittal

4.8.1 Subgrade Preparation

4.8.2 Trench Excavation

4.8.3 Dewatering

4.8.4 Structural Fill

4.8.5 Compaction Requirements

4.8.6 Testing and Observations

4.8.7 Earthwork Volume Criteria

4.8.8 Cold Weather Construction

4.8.9 Wet Weather/Soil Construction

4.8.10 Geosynthetics

4.9 Soil Chemistry and Corrosion

5.0 LIMITATIONS

6.0 REFERENCES

APPENDIX A - EXPLORATION LOGS

FIGURES

Figure 1: Vicinity and Project Location

Figure 2: Exploration Locations

Figure 3: Surficial Geologic Map

TABLES

Table 1: Exploration Summary Table 2: Laboratory Tests Table 3: Documented Faults Table 4: Seismic Design Parameters Table 5: Foundation Design Parameters Table 6: Lateral Earth Pressures Table 7: Floor Slab Recommendations Table 8: Flexible Pavement Table 9: Thrust Block Support Table 10: Soil Suitability to HDD Table 11: Fill Specifications Table 12: Compaction Specifications Table 13: Soil Chemistry Test Results

APPENDICES

Appendix A - Exploration Logs

Appendix B - Photograph Log

Appendix C - Laboratory Test Results

EXECUTIVE SUMMARY

DOWL prepared this geotechnical report for the National Park Service (NPS). In this report, we specifically address the recommendations for CURE 318472 – Replace Water and Wastewater Systems at Elk Creek and Lake Fork in the Curecanti National Recreation Area. We identified the following geotechnical considerations:

• The basic soil profile on-site consists of three to 10 feet residual soils overlaying a sandstone bedrock.

• The residual soils range from sandy lean clay to poorly graded gravel. The sandstone is hard but is weathered and moderately to highly fractured.

• The soils at the Elk Creek site are suitable for the use of standard spread footing foundations and slabs-on-grade.

• The soil profile suggests that some areas of trench excavation may require jackhammering or blasting.

• The on-site soils are suitable for fill and backfill, except where structural fill is required.

This section is only a summary. Recognize that we do not provide details in this section. Read the report in its entirety for a comprehensive understanding of the items contained herein.

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

1.0 INTRODUCTION

1.1 Purpose and Scope

DOWL completed a geotechnical investigation for the proposed Replacement Water and Wastewater Systems at Elk Creek in the Curecanti National Recreation Area, Colorado. The scope of geotechnical services consisted of reviewing existing geotechnical and geological information, a site visit, field observations, subsurface exploration, laboratory testing, engineering analyses, and preparing this Geotechnical Report. The purpose of these services is to provide geotechnical-related recommendations for project planning and design.

Our geotechnical engineering scope of work for this project included the initial site visit, drilling seven borings to depths ranging from approximately 10 to 35 feet below existing site grades, lab testing for soil engineering properties, and engineering analyses to provide foundation and construction recommendations.

1.2 Project Understanding

1.2.1 Existing Site Conditions

The Elk Creek water system consists of the following major elements: water supply wells, raw water conveyance piping, water treatment, treated water conveyance pipeline (to storage), one 200,000-gallon aboveground potable water storage tank, and primary and secondary water distribution piping.

Most of these features are in need of repair or replacement.

1.2.2 Proposed Construction

Planned work at Elk Creek generally consists of the following:

• Replacing the primary and secondary water distribution systems and improving the water treatment vault.

• Replacing the wastewater collection system, including a 1500-foot directional drill below an archeological feature west of the existing ponds.

• Replacing the full supervisory control and data acquisition (SCADA) system.

The National Park Service also plans to upgrade the electrical and mechanical systems as required to support infrastructure repairs and replacements. Additional cutting and patching of asphalt and concrete will be completed where utility installations intersect roadways and parking lots.

CURECANTI, COLORADO

GEOTECHNICAL INVESTIGATION

WWW.DOWL.COM VICINITY AND LOCATION MAP FIGURE 1

ELK CREEK - CURECANTI NATIONAL RECREATION AREA

VICINITY MAP

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LOCATION MAP

PROJECT LOCATION

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NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

2.0 INVESTIGATION

2.1 Field Investigation

Buckhorn Engineering performed fieldwork on May 5 and 6, 2024, which consisted of site observations and drilling of seven geotechnical borings throughout the project area. We present the boring locations in Appendix A. HRL Compliance Solutions advanced seven borings to depths ranging from 10 feet to 35 feet below the existing ground surface. Auger refusal was met on borings B-4 and B-7. Field personnel from Buckhorn Engineering recorded handheld locations and elevations at each boring.

Table 1: Exploration Summary

Boring Number

Drill Depth (feet)

Surface Elevation

(feet)*

Northing (feet)*

Easting (feet)*

Location

B-1 10 7,739 1,235,622 2,520,777 Northeast Campground Loop

B-2 10 7,745 1,233,348 2,521,985 Near Existing Tank

B-3 10 7,700 1,233,475 2,522,396 Along Roadway to Storage Yard

B-4 10 7,620 1,234,655 2,523,199 Near Staff Buildings

B-5 13 7,610 1,235,983 2,523,775 Near the Entrance

B-6 26 7,605 1,236,194 2,524,138 East of the Entrance

B-7 35 7,602 1,236,427 2,525,149 Near the Holding Ponds *Estimated from Handheld Device

HRL Compliance Solutions drilled the borings under the direction of a DOWL geotechnical engineer using a Diedrich D90 track-mounted drill rig equipped with a 4.25-inch O.D. solid stem auger and an HQ coring system. We conducted our field exploration referencing the following ASTM standards:

• ASTM D6151 Standard Practice for Using Hollow-Stem Augers for Geotechnical Exploration and Soil Sampling

• ASTM D1586 Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils

• ASTM D1587 Standard Practice for Thin-Walled Tube Geotechnical Sampling of Soils

• ASTM D3550 Standard Practice for Thick Wall Ring-Lined, Split barrel, Drive Sampling of Soils

• ASTM D2113 Standard Practice for Rock Core Drilling and Sampling of Rock for Site Investigation

The HQ3 rock core system consists of a triple tube core barrel with a diamond coring bit that is raised and lowered by a wireline system. Rotary drilling advances the coring. Water is circulated through the drill rod during coring to remove rock cuttings from the boring. The HQ3 core system recovers a 2.5-inch diameter core while being advanced into the bedrock using a 3.78-diameter diamond core bit. As the drillers recovered core samples, DOWL recorded the run length, recovery, and Rock Quality Designation (RQD) along with the rock type and associated geologic formation, the rock core’s field strength, weathering, fracture index, and structural parameters referencing DOWL’s Rock Classification Description presented in Appendix A.

We performed Standard Penetration Test (SPT) sampling using an automatic hammer and recorded the results on the boring logs. We have not corrected the SPT values on the logs for hammer efficiency, sampler type, overburden stress, etc. The resistance, or N-value, can be used to estimate the relative

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal density of granular soils and the relative consistency of cohesive soils. We provide the field N-value or resistance data on the exploration logs.

DOWL used a 2.5-inch outside diameter (O.D.), California sampler during the field investigation.

However, most correlations are based on the SPT which uses a 2.0-inch O.D. sampler and a 140 lb.

hammer with a 30-inch drop. DOWL corrected the blow counts using the method proposed by Lacroix and Horn (1973) to convert a nonstandard penetration resistance, N1, to the Standard Penetration Resistance, NSPT, as follows:

𝑁𝑆𝑃𝑇 = 𝑁1 (

2𝑖𝑛

𝐷1

12 𝑖𝑛

𝐿1

𝑊1

140 𝑙𝑏𝑠

𝐻1

30 𝑖𝑛 where: NSPT = “field” Standard Penetration Test blow count (uncorrected for other factors such as overburden pressure, groundwater levels, etc.); D1 = nonstandard sampler diameter; W1 = nonstandard hammer weight; and H1 = nonstandard hammer drop height. L1= nonstandard drive length. Because the hammer weight and drop height were nominally standard, the correction for the nonstandard sampler diameter is simplified as follows:

𝑁𝑆𝑃𝑇 = 𝑁1(2.0 𝑖𝑛 2.5 𝑖𝑛⁄ )2 = 0.64𝑁1

𝑁𝑆𝑃𝑇 = 𝑁1(2.0 𝑖𝑛 3.0 𝑖𝑛⁄ )2 = 0.44𝑁1

We provide exploration logs in Appendix A which include soil and groundwater conditions as well as SPT information. Stratification boundaries on the boring logs represent the approximate location of changes in soil types; in situ, the transition between materials may be gradual and may vary. In Appendix B we present photographs of the site conditions and samples obtained during drilling.

We based the soil descriptions shown on the boring logs on field and laboratory testing referencing ASTM Standards D2487 or D2488. The stratigraphic contacts shown on the individual borehole logs represent the approximate boundaries between soil types. The actual transitions may be more gradual or abrupt. The soil and groundwater conditions depicted are only for the specific dates and locations reported and, therefore, may not necessarily represent other locations and times.

B-1

B-2

B-3

B-4

B-5

B-6

B-7

LEGEND

B-1 GEOTECHNICAL BORING LOCATIONS

CURECANTI NATIONAL RECREATION AREA, COLORADO

GEOTECHNICAL INVESTIGATION

WWW.DOWL.COM EXPLORATION LOCATIONS FIGURE 2

HIGHWAY 50

ELK CREEK

CAMPGROUND

GUNNISON RIVER

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NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

2.2 Laboratory Testing

Samples were transported to Buckhorn Engineering’s laboratory for testing. Representative field samples for laboratory testing were selected after visual examination of the soil and consideration of the design criteria. Buckhorn Engineering performed tests for index and engineering soil properties in Montrose, Colorado. Buckhorn Engineering’s Lab also completed the corrosion testing of select soil samples. Laboratory testing included:

Table 2: Laboratory Tests

Test Purpose

Natural Moisture Content

ASTM D 2216

Provides a measure of natural (in-situ) water content.

Atterberg Limits

ASTM D 4318

Provides an indicator of the plasticity and mineralogy composition of fine-grained soils.

Particle-Size Distribution

ASTM D 421

Provides a measure of grain sizes of the soils for classification and identification of physical characteristics.

Compressive Strength of Intact Rock

ASTM D 7012

Determines the strength of intact rock core specimens in uniaxial and triaxial compression.

Corrosion Tests (pH, Resistivity, and Soluble Sulfates)

To determine the potential for corrosive interaction of soils with concrete and metal.

In addition to the geotechnical tests discussed above, we sent four samples to Colorado State University for horticultural soil testing. We present all laboratory test results in Appendix C.

3.0 SUBSURFACE CONDITIONS

3.1 Site Geology

Elk Creek Campground site is part of the Curecanti National Recreation Area, which sits on the bank of the Blue Mesa Reservoir along the Gunnison River. The river runs along the south end of the Grand Mesa, Uncompahgre, and Gunnison National Forests in Colorado. There are zones of Quaternary Age sediment through the lake and drainage areas. There are small areas where the Tertiary Aged West Elk Breccia are mapped. There are a few areas of exposed Jurassic Age sedimentary rock along with the Black Canyon Schists, Precambrian in Age, that are exposed along most of the riverbank. (Hedlund & Olson, 1973)

The Quaternary sediment ranges from the alluvium deposits and talus/washout/colluvium deposits.

Most of these deposits are mapped along the riverbank and on the north side of the area along the highway. The exposed Tertiary Aged rock consists of the West Elk Breccia which is a volcanic breccia with a tuff breccia that are brown to gray with coarse angular clasts. The exposed sedimentary rocks of the Jurassic Age in the area are the Morrison Formation and the Junction Creek Sandstone. The Morrison Formation consists of mudstone and siltstone with very thin intercalated silty sandstone beds.

The Junction Creek Sandstone is white to brown and is medium to fine-grained. The exposed Black Canyon Schist is a quartz-biotite schist and gneiss. Throughout the schist on the geologic map are granitic dikes and sills. (Hedlund & Olson, 1973)

CURECANTI NATIONAL RECREATION AREA, COLORADO

WWW.DOWL.COM SURFICIAL GEOLOGY MAP FIGURE 3

PROJECT LOCATION

Hedlund, D.C. and Olson, J.C., 1973, Geologic map of the Carpenter Ridge quadrangle, Gunnison County, Colorado, U.S. Geological

Survey, Geologic Quadrangle Map GQ-1070, 1:24,000

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NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

3.2 Observed Soil Conditions

The generalized soil profile encountered at the proposed construction site consists of clayey sand and sandy clay overlying native sands weathered from the local sandstone formation. The clayey soils range in thickness from approximately ½ to 10 feet below the existing ground surface. The native sands primarily consist of medium-dense to very dense silty sands with gravel. The sands transition into sandstone bedrock with depth. In Appendix A, we present the exploration logs with lithology descriptions as well as other engineering properties. In the following paragraphs, we provide a general description of the soil strata.

3.2.1 Existing Pavement

During exploration, Borings B-1 and B-4 were drilled through the asphalt surfacing existing along the roadways. The existing asphalt thickness varied from 4.0 to 6.0 inches, with an average thickness of approximately 5.0 inches. Base course, visually classified as poorly graded gravel with sand and variable silt and clayey sand, was encountered in three of the seven borings. The base course thickness varied from 4.0 to 6.0 inches. The existing asphalt condition was generally fair. We did observe areas of delamination, longitudinal and transverse cracking, and corrugations.

3.2.2 Granular Soils

Granular soils were encountered and consisted of silty sand and silty gravel. The granular soils were encountered in borings B-4, B-6, and B-7. The granular soils are suitable for support of foundation elements or concrete floor slabs. The granular soils were medium dense to very dense in consistency and had a range of moisture of 1 to 13 percent with an average of 5 percent.

3.2.3 Clayey Soils

Clayey soils were encountered and consisted of clayey sand and sandy clay. The clayey soils were encountered in all borings except B-1, B-6, and B-7. The clayey soils are suitable for support of foundation elements or concrete floor slabs. The clayey soils were medium stiff to hard (medium dense to dense) in consistency and had a range of moisture of 6 to 15 percent with an average of 11 percent.

3.2.4 Sandstone Bedrock

A sandstone bedrock was encountered in all borings except B-2 and B-5. The sandstone was white to light brown with a fine-grained matrix and had a field hardness from soft to hard. The sandstone was observed as weathered to fresh throughout the project. The cores had a Rock Quality Designation (RQD) ranging from 0 to 79 percent, with the majority being below 30 percent. Most of the fractures were across weathered surfaces but there were also fresh breaks as well. The Unconfined Compression Test was performed on the core samples and produced numbers ranging from 6,000 to 16,000 psi. The sandstone is mapped as the Junction Creek Formation and is Jurassic in age.

3.3 Groundwater

We did not encounter groundwater in any of the borings during drilling; however, fluctuations of groundwater can occur due to seasonal moisture conditions, irrigation practices, changes in land use, and many other factors. Groundwater conditions may vary from those encountered at the time of the field investigation, depending upon the influence of these factors.

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

3.3.1 Groundwater Information Center Research

We researched the National Water Information System: Web Interface (USGS, 2024) website to estimate static water levels existing near the project area. The USGS data is for informational purposes only and contains historical well logs, some of which may not be accurate. Based on the USGS research, static groundwater levels range from 18 to 38 feet below the existing ground surface.

3.4 Seismicity

3.4.1 Faulting

The proposed development is located in an area of Quaternary faulting. Based on the USGS Quaternary Fault Fold Database (USGS, 2018) three (3) Quaternary faults exist in the project vicinity:

Table 3: Documented Faults

Fault Name Distance from ECC

(km)

Recent Earthquake

Published Slip Rate (mm/yr)

Fault Length (km)

Average Strike

Cimarron Fault 16 Undifferentiated Quaternary

Less than 0.2

57 N63°W

Cannibal Fault 36 Late Quaternary Less than 0.2

49 N20°W

Ridgway Fault 55 Undifferentiated Quaternary

Less than 0.2

24 N87°E

Available publications do not include documentation that these faults were offset during the Holocene (the last 15,000 years). Based on the distances to Quaternary faults, the potential for fault rupture at the site is low.

3.4.2 Design Accelerations

DOWL utilized site soil and geologic data, our knowledge of local geology, the project location, Minimum Design Loads, and Associated Criteria for Buildings and Other Structures (ASCE, 2021) and the National Earthquake Hazards Reduction Program (NEHRP) to estimate Seismic Site Classification of "C" at the project site. We queried the ASCE 7 Hazard tool website (ASCE, 2021) for seismic design parameters, which are provided in Table 4 and which are based on the soil conditions and project location:

Table 4: Seismic Design Parameters

Period (seconds) Modified Acceleration

Coefficient for Site Class C (g)

0.0 (peak) PGAM = 0.16

0.2 (short SDS = 0.21

1.0 (long) SD1 = 0.061

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

3.4.3 Liquefaction

Liquefaction is the partial or total loss of strength of soils that can occur during strong earthquake shaking of significant duration. Liquefaction is a process where high shear deformations result in a progressive build-up of pore water pressure. Because the seismic load occurs rapidly, the soil does not have time to drain, and the effective stress may be reduced to near zero, resulting in a temporary loss of shear strength. Earthquake-induced liquefaction generally occurs only under particular conditions, which include saturation, strong earthquake ground shaking for a long duration, and loose granular soil.

Liquefaction can also occur in silts and fine-grained soils. Typically, liquefaction occurs where the groundwater table is shallow (5 to 10 feet deep) and generally only at depths less than approximately 50 feet.

Loose, saturated soil materials such as sand, silt, and fine gravel may liquefy when exposed to seismic shaking. To be liquefiable, clay or silt (CL or ML) soil must have a plasticity index (PI) of less than 7, or if the soil classifies as a silty clay (CL-ML), the plasticity index must be less than 5 (Idriss, 2006). Based on the laboratory test results, both the embankment and the native sandy silt foundation soils are potentially liquefiable due to their gradation (silt and silty fine sand) and non-plastic characteristics.

The potential for liquefaction appears to be low in the area due to the lack of groundwater, the high density of the soil, and the shallow bedrock.

4.0 ENGINEERING ANALYSIS AND RECOMMENDATIONS

4.1 Foundation

Based on information from the subsurface exploration, laboratory testing results, and our analysis, the proposed vault improvements can be supported on a spread footing foundation system bearing on native soils or engineered fill. We provide specific recommendations in the following sections.

4.1.1 Conventional Spread Footings

The building foundation may be founded on conventional spread footings according to the parameters listed below.

• Remove any undocumented fill.

• Prior to placement of structural fill, proof roll subgrades to identify soft spots.

• Footing subgrades shall be inspected by a DOWL geotechnical engineer to verify that foundation conditions are similar to those encountered in the borings. Remove and replace soft or loose zones or zones of unsuitable material, if encountered, with structural fill.

• See Section 4.2 for lateral resistance design parameters.

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

Table 5: Foundation Design Parameters

Footing Design Criteria Recommendations Notes

Perimeter Footings Maximum Allowable Bearing Pressure

18 inches minimum width, 48 inches minimum below grade

Static Loads (Dead &Sustained Live):

2,500 psf

Transient Loads (Wind & Seismic):

3,000 psf

Interior Column Footings Maximum Allowable Bearing Pressure

Minimum width 18 inches square, 24 inches minimum below grade unless constrained by slab.

The resultant load is assumed to be in the middle 1/3 of the footing.

Static Loads (Dead & Normal Live):

2,000 psf

Transient Loads (Wind & Seismic):

2,650 psf

Maximum Settlement Estimate Total (in) Differential (in) Based on a two-foot square footing with a maximum load of 6 kips. ½ inch ¼ inch

Subgrade Preparation and Structural Fill

See Table 12: Compaction Specifications

Design uplift of shallow foundations from wind and seismic events using the weight of the foundation and soil above the footing. You can include soil resistance in the shape of a truncated pyramid above the foundation. The pyramid edges are defined by straight lines extending from the top of the footing on either side at a 2V:1H (vertical to horizontal) slope.

4.2 Lateral Earth Pressures

Design below-grade walls for building, landscape, and retaining walls and any structure retaining soil to resist both lateral earth pressures from the retained soil adjacent to the structure, as well as hydrostatic pressures from retaining water (if undrained, not recommended). Also, account for lateral surcharge loads from equipment, slopes, or vehicles adjacent to the walls in the structural wall design. We provide recommended lateral earth pressures for below-grade wall design, which are provided below.

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

Table 6: Lateral Earth Pressures

Lateral Earth Pressure Case Equivalent Fluid

Pressure (pcf)

Imported Structural Fill

At-rest - no wall movement (pcf) 60

Active - wall moves away from soil mass, (pcf) 35

Passive - wall moves into soil mass, (pcf) 540

On-Site Sandy Clay Soil

At-rest - no wall movement (pcf) 55

Active - wall moves away from soil mass, (pcf) 35

Passive - wall moves into soil mass, (pcf) 290

Residual Soil (Silty Sand)

At-rest - no wall movement (pcf) 55

Active - wall moves away from soil mass, (pcf) 35

Passive - wall moves into soil mass, (pcf) 375

• The above equivalent fluid pressures assume fully drained conditions and no hydrostatic forces acting on the wall.

• Construct below-grade walls, retaining walls, or other retaining structures with adequate drainage and waterproofing systems as specified by the Architect and Structural Engineer to reduce the potential for instability, leakage, or seepage.

• The retaining walls move away from or toward the soil to develop active and passive resistance, respectively. For walls that cannot tolerate movement, structurally design walls utilizing at-rest equivalent earth pressures.

• We based the above equivalent fluid pressures on the assumption that the surface of the backfill adjacent to the walls slopes down and away from the wall a minimum of 5 percent for 10 feet to provide drainage.

• Lateral surcharge pressures due to equipment, slopes, storage loads, etc., are not included in the above lateral earth pressure recommendations. Use the lateral earth pressure coefficient of 0.5, acting over the below-grade wall height, to estimate the lateral surcharge loads from equipment, adjacent foundations, and slopes behind and above walls.

4.2.1 Seismic Earth Pressure

We recommend using the Mononobe-Okabe approach to determine the additional earth pressures due to earthquakes. For the assumed unit weight of the retained earth at this project, 115 pcf, and the design peak horizontal ground acceleration, 0.18g, estimate the equivalent additional fluid (active) earth pressure acting on the wall: 35 pcf. We calculated this value using ½ the peak ground acceleration in the horizontal direction.

4.2.2 Coefficient of Friction

We recommend using a coefficient of friction of 0.45 between cast-in-place concrete and structural fill and 0.40 between cast-in-place concrete and the native sandy soil. The friction value may be combined with the passive pressure to resist horizontal loads.

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

4.3 Slabs-on-Grade

4.3.1 Interior Slabs

The native soils can be used to support the floor slabs. Compacted structural fill can also be used to support the floor slabs. Design the floor slabs using the recommendations in the table below.

Table 7: Floor Slab Recommendations

Description Value

Interior floor system Slab on-grade concrete.

Floor slab subgrade Scarify, moisture condition, and recompact at least 8 inches of on-site soil or structural fill placed and compacted in accordance with Section 4.8.5 of this report.

Base layer 4 inches of granular material is acceptable.

Basement Floor Slab Minimum of 8 inches of free-draining gravel.

Modulus of subgrade reaction 180 pounds per cubic inch (pci).

We also recommend that doweled joints be considered for the slab connections for slabs that will carry significant traffic. Subgrade areas that become soft, loose, wet, or disturbed or that cannot be re-compacted to structural fill requirements discussed above must be over-excavated as described in Section 4.8.

If the moisture content of the subgrade soils increases, some differential movement of a slab-on-grade floor system is possible. To reduce the effects of some differential movement, separate floor slabs from bearing walls and columns with expansion joints, which allow vertical movement. Use floor slab control joints to reduce damage due to shrinkage cracking.

If the floor coverings are sensitive to moisture, place a vapor retarder below the slab, underlain by four inches of clean drain gravel. A choker layer such as fine-concrete aggregate (ASTM C 33 sand) may be used to reduce the potential for drain gravel puncturing the vapor barrier.

4.3.2 Exterior Slabs

Exterior slabs-on-grade, exterior architectural features, and utilities founded on or in backfill or the site soils will likely experience some movement due to the volume change of the material. Damage from potential movement may be reduced by:

• Minimizing moisture increases in the backfill.

• Controlling moisture density during placement of the backfill.

• Designing for vertical movement between the exterior features and adjoining structural elements and designing control joints.

Exterior slabs are susceptible to frost action, which can generate substantial frost heave at certain times of the year. The potential for frost heave may not be acceptable at entries, bays, or other critical areas adjacent to the building that will be exposed to weather. One approach to providing partial frost protection would be to place and compact a minimum of 24 inches of aggregate base course beneath the slab. Alternatively, if partial frost protection is unacceptable, over-excavate and replace the native soil with aggregate base course to the anticipated frost depth (42 inches).

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4.4 Drainage

Drainage is critical to the structure's long-term performance. To reduce the potential for movement due to an increase in the moisture content of subgrade soil, we strongly encourage the implementation of the following recommendations:

• Per the 2024 IBC (ICC, 2017), slope the ground surface within 10 feet of the structure downward, a minimum of 5 percent away from the structure. Slope the ground surface beyond 10 feet of structures downward at least two percent away from the structure.

• Apron slabs and pavement may be used to reduce infiltration adjacent to structures further.

Aprons should consist of asphalt or Portland cement concrete pavement placed directly adjacent to the foundation stem walls. An elastomeric sealant should also be considered between aprons and foundation stem walls to further reduce the potential for moisture to infiltrate the area directly adjacent to the foundations. Slope apron slabs and pavement are a minimum of 2 percent, downward and away from the building.

• Install eve gutters, downspouts, and extensions so that they dispose of water at least 10 feet away from the structure.

• Do not irrigate within 10 feet of the building. Periodically inspect and flush irrigation systems to detect potential leaks and avoid saturation of foundation backfill.

• Remove or repair landscaping, curbs, or other barriers that impair drainage.

• Do not bury rain gutter discharge pipes because they can leak, which often goes undetected.

Seepage problems can also be caused by clogging, crushing, and poor grading of the pipes.

• Do not construct infiltration basins adjacent to or up gradient of the structures. If detention is required by statute, infiltration basins should be located down a gradient and at least 30 feet from the foundations.

4.5 Pavement Design

The primary purpose of a pavement section is to distribute concentrated wheel loads to the subgrade so that the subgrade is not overstressed. The performance of the pavement section is a function of subgrade strength and traffic loading. For purposes of designing a pavement section, subgrade soil is represented by a soil support value for flexible pavements (asphaltic concrete). Subgrade strength decreases when the moisture content of the subgrade increases. Therefore, proper surface and subsurface drainage is essential for long-term pavement performance.

Pavement design procedures are based on the strength properties of the subgrade soil and pavement materials and the design traffic conditions.

4.5.1 Flexible Pavement

Based on our design calculations, anticipated traffic, and field conditions, we recommend the pavement sections shown below for the proposed new pavement areas for each location.

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

Table 8: Flexible Pavement

Asphalt Surfacing (in)

Crushed Aggregate Course (CAC) (in)

Total Thickness (in)

3.0 6.0 9.0

4.5.2 Construction Considerations

• Remove unsuitable material, including soft and/or organic soil, as encountered. Over-excavate soil to a depth of 12 to 18 inches below the pavement surfacing materials.

• Base Course shall meet the requirements for Class 4-6 Crushed Base Course in Section 701, Aggregates, in Colorado Standard Specifications for Road and Bridge Construction.

• Asphalt shall meet the requirements in Section 401, Plant Mix Pavements - General, in the Colorado Standard Specifications for Road and Bridge Construction.

• Compact asphaltic concrete to at least 92% of its theoretical maximum Rice density (ASTM D2041).

• Compact all pavement materials (and subgrade) in accordance with Table 11 in Section 4.8.5.

• Sub-excavate any unstable areas and replace them with moisture-conditioned and compacted aggregate base.

• Place and compact structural fill-in level lifts, not more than eight inches in loose thickness, up to planned grade.

• Grade pavement such that surface water drains into the curb or storm drains at a minimum two percent slope.

4.5.3 Maintenance

• The pavement's life will depend on achieving adequate drainage throughout the section, especially at the subgrade.

• Surface and subgrade, crushed surfacing, and asphalt surfaces to slope at no less than two percent to an appropriate stormwater disposal system or other appropriate location that does not impact adjacent buildings of the properties.

• Maintain grades outside of paved areas to prevent water collection adjacent to the pavement.

• Seal cracks and perform surface maintenance on pavement surfaces every three to five years to reduce the potential for surface water infiltration into the underlying pavement subgrade.

• Water that ponds at the pavement subgrade surface can induce heaving during the freeze-thaw process, which can readily damage the pavement.

• Do not allow inverted crowns at the subgrade or pavement surfaces without center concrete gutters designed to have asphalt overlap.

4.6 Thrust Restraint

It is understood that thrust blocks will be used throughout the project. Pipe thrust can be restrained by bearing against the soil through a thrust block or by friction between the pipe surface and the soil.

Thrust restraint may be required for any one of these conditions:

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

• Bends in the vertical plane

• Bends in the horizontal plane

• Valves

• Reducers

• Tees

In all cases, the design engineer must determine the direction and magnitude of the maximum expected pipe thrust.

The thrust block bearing against the soil must resist horizontal and vertical components of the thrust forces. Based on soil types observed during the investigation and procedures set forth by the Ductile Iron Pipe Research Association (DIPRA) and Montana Public Works (MPW), we calculated the following bearing capacities. For the reclaimed water transmission main, we assume that the pipe will be located five (5) feet below the ground surface. In town (Idaho and Washington Street alignments), we assume that the water mains will be 6.5 feet below the asphalt surfacing. DOWL has provided resistance per foot of depth for thrust blocks not located at the previously mentioned assumed depths. Please note that resistance per foot does not include support related to cohesion, and therefore, the allowable bearing value should be utilized as a starting point for any adjustments.

Table 9: Thrust Block Support

Soil Type Allowable Bearing

(psf) Resistance per Foot of

Depth (pcf)

Surficial Clays/Sands 950 40

Gravels 1,025 200

Rock 2,500 400

For the fully available passive earth pressure to develop, a certain amount of movement is required. In well compacted soil, this movement is typically insignificant; however, in less competent soil, the movement may be detrimental to the water main. For this reason, we applied a reduction factor of 0.85 to the above allowable bearing pressures.

We strongly recommend a Factor of Safety of 1.5 be applied during the sizing of the thrust blocks. We recommended that thrust blocks be placed against undisturbed earth or have backfill placed against the thrust block compacted to 97 percent of maximum dry density as determined by ASTM D 698. For thrust restraint by pipe friction, refer to the Ductile Iron Pipe Research Association criteria (DIPRA, Seventh Edition, 2016).

4.7 Horizontal Drilling Considerations

A horizontal boring is planned below the archeological feature west of the wastewater ponds between Borings B-6 and B-7. We have classified the suitability of the soil for directional drilling, referencing ASTM F1962-20 (Standard Guide for Use of Maxi-Horizontal Directional Drilling for Placement of Polyethylene Pipe of Conduit Under Obstacles, Including River Crossings), in Table 10.

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

Table 10: Soil Suitability to HDD

Boring Suitability Soil Description

6 generally suitable stiff clay

6 generally suitable mudstone

6 & 7 difficulties may occur in fractured rock;

massive rock is generally suitable sandstone

7 generally suitable dense silty sand

4.8 Earthwork

4.8.1 Subgrade Preparation

• Soil containing vegetation and organics (topsoil) extended approximately 6 to 8 inches below the existing ground surface in the locations explored. Remove soil containing vegetation and organics below planned improvements or structures.

• Prepare final native subgrades with smooth blade equipment. To maintain an undisturbed, native soil subgrade condition, the contractor must carefully plan and implement excavation to avoid disturbance. Scarify, moisture condition, and compact subgrade soil as specified in the table in Section 4.8.5.

• Grade the exposed subgrade surfaces to remove mounds and depressions, which could prevent uniform compaction. If unexpected fills or obstructions are encountered during site clearing or excavation, remove such features and clean the excavation prior to placing backfill and/or construction.

• Soil disturbance negatively impacts the soil's performance. Disturbed soil is not allowed below any structure or pavement, especially at footing or slab subgrades.

• Moisture condition and compact disturbed soil or fill placed to achieve site grades to the requirements in Table 12: Compaction Specifications.

• Remove pumping or rutting subgrade areas to depths between 12 and 18 inches or as directed by DOWL.

• Replace over-excavations with granular structural fill. Contact DOWL's geotechnical engineer to review and approve the exposed subgrade.

• Once prepared and approved by DOWL, it is the contractor's sole responsibility to protect subgrades from degradation.

• As mentioned in Section 3.2, we encountered zones of weathered bedrock. As the residual soil is removed, boulders or bedrock blocks may be encountered. Founding a building or retaining wall directly on rounded boulders or core stones will create a nonuniform foundation stiffness that may result in differential settlement and structural cracking. If core stones or competent bedrock are encountered, place at least a 6-inch layer of structural fill between the foundation element and the bedrock.

4.8.2 Trench Excavation

Based on the materials encountered in the soil borings, conventional earthmoving equipment should be capable of excavating most of the site soils. There are areas of the project where bedrock is at shallow

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal depths and will most likely be encountered during excavation. This may require additional excavation processes such as blasting or hydraulic hammer.

The following considerations should be recognized as pertinent to excavations:

• Subsurface conditions may vary from those shown on logs of exploration holes.

• The condition of structures adjacent to waterline excavation areas should be documented prior to construction.

Subsurface conditions identified in this report are based on information from a limited number of exploration holes and may vary between holes. Because the soils are primarily residual, the material characteristics should be maintained throughout the area. Additionally, past fill areas that the excavation may encounter may create exceptions to the soil conditions identified herein.

Excavations must conform to OSHA Standards for Excavations, 29 CFR Part 1926.652 Appendix B to Subpart P. Based on field observations and laboratory tests, the soil at the site classifies as OSHA Type B.

OSHA requires that Type B soil excavation slope angles not to exceed 1H:1V (horizontal to vertical). The nature and extent of subsurface variations and groundwater conditions between the boring locations may not become evident until construction. Evaluate soil conditions during construction by the contractor’s responsible person to comply with OSHA requirements. Temporary excavation slopes may be required for soil improvement excavations and utility trenches. Conduct excavations and shoring in accordance with OSHA standards. Do not allow surcharges within a horizontal distance equal to half the excavation depth. Construction vibrations can cause excavations to slough or cave. Ultimately, the contractor is solely responsible for site safety and excavation configurations.

4.8.3 Dewatering

Depending on the time of year and precipitation, groundwater may be encountered throughout the project alignment. Dewatering and control of groundwater and subsurface water during excavations will be critical to the successful completion of this project. Where and when necessary, dewater prior to making final excavations to reduce the potential for groundwater flow through excavations. Water flow through open excavations will soften and weaken subgrades and increase settlements. Dewater to lower groundwater a minimum of 1.5 feet below the planned excavation depth. Dewatering of clay soil has the potential to take long periods of time.

4.8.4 Structural Fill

Consider fill placed within the planned building footprint as structural fill. The on-site sands are suitable for use below the foundation, floor slabs, or exterior concrete flatwork. The on-site sands are also suitable for use as fill below pavements, the upper one foot of the exterior foundation wall backfill, utility trenches, and landscaped areas.

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

Table 11: Fill Specifications

Soil/Fill Product Allowable Use Material Specifications

Non-Structural Fill (Landscape Fill)

Any area that will not have structures (typically landscape areas)

• Soil classified as GM, GW, SM, SW, SC, CL, CH, or ML according to the USCS.

• Soil may not contain particles larger than 8 inches in median diameter.

• Soil must have less than three percent of deleterious substances such as wood, metal, plastic, waste, etc.

• Approved by Landscape Architect.

General Fill

• Site grading outside the building footprint.

• Utility backfill areas

• Non-structural fill

• Foundation walls backfill

• Soil classified as GP, GM, GW, GC, SP, SM, SW, SC, CL, or ML according to the USCS.

• Site soil must have less than three percent vegetation, organics, and debris.

• Soil may not contain particles larger than 6 inches in diameter.

• Soil must contain less than 3% (by weight) of organics, vegetation, wood, metal, plastic, or other deleterious substances.

Structural Fill

• General fill

• Over-excavations

• Soil improvements

• Retaining Wall backfill

• Soil classified as GP, GM, GW, SP, SM, or SP with at least 30 percent retained on a number 4 sieve and less than 15 percent passing a number 200 sieve.

• Soil may not contain particles larger than 2 inches in diameter.

• Soil must contain less than 3% (by weight) of organics, vegetation, wood, metal, plastic, or other deleterious substances.

Unsatisfactory Soil NONE

• Soil classified as MH, OH, CH, OL, or PT may not be used at the project site.

• Any soil type not maintaining moisture contents within 5% of optimum during compaction is unsatisfactory soil that must be moisture conditioned prior to disposal and replacement.

• Any soil containing more than 3% (by weight) of organics, vegetation, wood, metal, plastic, or other deleterious substances.

4.8.5 Compaction Requirements

Place fill material in lifts not exceeding eight inches in uncompacted thickness. Moisture condition and compact fill according to the table below.

NPS | Curecanti Replace Water & Wastewater Systems at Elk Creek and Lake Fork NPS Order # 140P2019F0280 Project Program | Final Submittal

Table 12: Compaction Specifications

Application Moisture Content (% of optimum) Minimum Compaction

Subgrade ±4 95% ASTM D698

Below Foundations ±4 98% ASTM D698

Below Slabs-On-Grade ±4 97% ASTM D698

Base and Subbase Courses ±4 97% ASTM D1557

Utility Trenches ±4 95% ASTM D698

Site Grading Fill ±4 95% ASTM D698

Foundation Backfill ±4 95% ASTM D698

4.8.6 Testing and Observations

We recommend the following compaction testing frequencies:

• Structural Fill below Footing and Subgrade - One compaction test every 50 linear feet (LF) of footing trench or 2 tests per wall line, whichever results in the greater number of tests, per each 1-foot lift of fill.

• Foundation/Retaining Wall Backfill - One compaction test every 100 LF of wall or 2 tests per wall line (interior and exterior sides), whichever results in the greater number of tests, per each 1-foot lift of backfill.

• Interior and Exterior Slab Subgrade - One compaction test every 1,000 square feet (sf) of slab area or two tests per slab area, whichever results in the greater number of tests, per 1-foot lift of fill.

• Pavements - One compaction test every 2,500 sf of pavement area on each subgrade, subbase, and base course layer as applicable, per each 1-foot lift of backfill.

• Trenches - One compaction test every 150 LF or 2 per trench, whichever results in the greater number of tests, per each 1-foot lift of backfill.

To verify that construction conforms to the intent of the specifications, we recommend that DOWL be retained to observe and record the following:

• Site preparation, including grubbing, stripping, excavating, and proof-rolling.

• Removal of topsoil and root zone beneath slabs and pavements.

• Interior and exterior slab subgrades.

• Excavations and sub-excavations…