Amend_0002_-_Geotechnical_Report.pdf

REPORT OF GEOTECHNICAL ENGINEERING SERVICES

MIIN Visitor Contact Station (PMIS 159776) Minidoka Internment Camp Hunt, Idaho

For FFA Architecture and Interiors, Inc.

May 13, 2015

GeoDesign Project: NatPark-2-02 FFA Project: 08.12.09.03

15575 SW Sequoia Pkwy, Suite 100 l Portland, OR 97224 l 503.968.8787 www.geodesigninc.com

May 13, 2015

FFA Architecture and Interiors, Inc.

520 SW Yamhill Street, Suite 900 Portland, OR 97204

Attention: Ms. Barbara Clement

Report of Geotechnical Engineering Services MIIN Visitor Contact Station (PMIS 159776)

Minidoka Internment Camp Hunt, Idaho

GeoDesign Project: NatPark-2-02 FFA Project: 08.12.09.03

GeoDesign, Inc. is pleased to submit our report of geotechnical engineering services for the proposed improvements to the Minidoka National Historic Site (MIIN) in Hunt, Idaho. The site is located northwest of the intersection of Hunt Road and S 1450 E. Our services for this project were conducted in accordance with the consultant agreement dated September 30, 2014.

We appreciate the opportunity to be of service to you. Please call if you have questions regarding this report.

Sincerely, GeoDesign, Inc.

Brett A. Shipton, P.E.

Principal Engineer cc: Mr. Danielle Pruett, KPFF Consulting Engineers (via email only)

NAK:BAS:kt

Attachments

One copy submitted (via email only)

Document ID: NatPark-2-02-051315-geor.docx

© 2015 GeoDesign, Inc. All rights reserved.

NatPark-2-02:051315

TABLE OF CONTENTS PAGE NO.

1.0 INTRODUCTION 1

2.0 SCOPE OF SERVICES 1

3.0 SITE CONDITIONS 2

3.1 Geologic Setting 2

3.2 Surface Conditions 2

3.3 Subsurface Conditions 3

3.4 Infiltration Testing 4

4.0 CONCLUSIONS AND RECOMMENDATIONS 4

5.0 DESIGN 5

5.1 General 5

5.2 Shallow Foundations 5

5.3 Seismic Design Considerations 7

5.4 Floor Slabs 7

5.5 Retaining Structures 8

5.6 Pavements 9

6.0 CONSTRUCTION 10

6.1 Site Preparation 10

6.2 Construction Considerations 12

6.3 Permanent Slopes 12

6.4 Excavation 13

6.5 Drainage 14

6.6 Materials 15

6.7 Erosion Control 18

7.0 OBSERVATION OF CONSTRUCTION 18

8.0 LIMITATIONS 18

REFERENCES 20

FIGURES

Vicinity Map Figure 1 Site Plan Figure 2

TABLE OF CONTENTS PAGE NO.

APPENDIX

Field Exploration A-1 Laboratory Testing A-1 Exploration Key Table A-1 Soil Classification System Table A-2 Rock Classification System Table A-3 Boring Logs Figures A-1 – A-11 Consolidation Test Results Figure A-12 Atterberg Limits Test Results Figure A-13 Summary of Laboratory Data Figure A-14 SPT Hammer Calibration

ACRONYMS

1 NatPark-2-02:051315

1.0 INTRODUCTION

GeoDesign, Inc. is pleased to submit this report of geotechnical engineering services for the proposed improvements to the Minidoka National Historic Site (MIIN) in Hunt, Idaho. The site is located northwest of the intersection of Hunt Road and S 1450 E. Figure 1 shows the site relative to existing topographic and physical features. Figure 2 shows the existing and proposed site layout (overlay) and our approximate exploration locations. Acronyms used herein are defined at the end of this document.

Based on our communications with Ms. Barbara Clement of FFA Architecture and Interiors, Inc.

and review of preliminary development plans, we understand the overall site development includes rehabilitation and an approximately 3,705-square-foot addition to the existing 2,305-square-foot, one-story Warehouse #5 building. The building will be used as a visitor contact station. The concrete slab footprint of the original building is approximately 48 feet by 112 feet. Other improvements include compacted gravel pads for roadways, pathways, and parking lot and concrete slabs for the entry plaza, pedestrian sidewalks, and north plaza, as well as utilities and septic drain fields.

Foundation loads were not available at the time of this report; however, based on our experience with similar structures, we anticipate that the column and continuous footing loads will likely be less than approximately 50 kips and 3 kips per lineal foot, respectively. We anticipate that the design floor slab loads will be light, less than 100 psf. We anticipate that only minor cuts and fills will be required to achieve final grades.

2.0 SCOPE OF SERVICES

The purpose of our geotechnical engineering services was to characterize site subsurface conditions and provide geotechnical engineering recommendations for use in design and construction of the proposed development. Our scope of work included the following:

Coordinated and managed the field investigation, including private and public utility locates, and scheduling subcontractors and GeoDesign field staff.

Conducted a visual reconnaissance of the site by geotechnical staff.

Reviewed readily available geologic maps, literature, and well log records for the site vicinity.

Drilled 11 borings to depths between approximately 3 and 14 feet BGS using a truck-mounted drill rig and hollow-stem auger drilling methods.

Conducted four infiltration tests in four borings at depths ranging between approximately 3 and 5 feet BGS at the proposed septic drain field locations.

Obtained disturbed soil and rock samples for laboratory testing at selected depths from the explorations.

Classified the material encountered in the explorations. Maintained a detailed log of each exploration. Observed groundwater conditions in the explorations.

Completed the following laboratory tests:

Fifteen moisture content determinations in general accordance with ASTM D 2216 Five particle-size analyses in general accordance with ASTM C 117 and ASTM D 1140

2 NatPark-2-02:051315

One Atterberg limits test in general accordance with ASTM D 4318 One consolidation test in general accordance with ASTM D 2435

Provided recommendations for site preparation, grading and drainage, stripping depths, fill type for imported material, compaction criteria, trench excavation and backfill, use of on-site soil, and wet/dry weather earthwork.

Provided recommendations for design and construction of shallow spread foundations, including allowable design bearing pressure and minimum footing depth and width.

Provided recommendations for preparation of floor slab subgrade.

Recommended design criteria for retaining walls, including lateral earth pressures, backfill, compaction, and drainage.

Provided recommendations for permanent and temporary slopes.

Provided recommendations for construction of sidewalks and compacted gravel pads for on-site access roads, pathways, and parking areas.

Provided recommendations for seismic coefficients as prescribed by the 2012 IBC, and evaluated the liquefaction and lateral spreading potential of site soil.

Provided this report summarizing the results of our geotechnical evaluation.

3.0 SITE CONDITIONS

3.1 GEOLOGIC SETTING

The site is located within the Snake River Plain, which represents a large, bow-shaped topographic depression within southern Idaho. Current understanding is the Snake River Plain was formed from the movement of the North American Plate over a hotspot, now located beneath Yellowstone National Park. Resulting volcanism produced extensive lava flows from numerous shield volcanoes that filled the Snake River Plain with basalt (Matthews et al., 2006).

Geologic mapping by Kauffman et al. (2005) indicates that the near-surface geology of the site is mapped as basalt of Rocky Butte. The Pleistocene-aged Rocky Butte basalt was extruded by a shield volcano related to the volcanism of the development of the Snake River Plain. Surface topography of the basalt flow since eruption has since been filled and smoothed by loess deposition. Thin and weakly developed soil caliche is common overlying the basalt. Several miles south of the site, the Rocky Butte basalt has been scoured by catastrophic flood waters of the Bonneville flood (Kauffman et al., 2005).

3.2 SURFACE CONDITIONS

The site is located northwest of the intersection of Hunt Road and S 1450 E in Hunt, Idaho. The site is currently occupied by multiple single-story buildings, sheds, and concrete slabs from collapsed or demolished buildings. The western half of Warehouse #5 has been demolished and removed; the concrete floor slab remains in place. The existing concrete slabs are cracked and uneven. Septic drain fields are present near the “Herrmann House” at the northwest site margin and the two residences located south of Warehouse #5. Site access roads, parking, and walking trails are surfaced with gravel. The site is vegetated with grass, small shrubs, and a few mature trees. Site topography near Warehouse #5 and surrounding structures is near flat. Gently to moderately steep slopes are present north of Warehouse #5. A water canal runs south of Hunt Road. At the time of our site investigation, water was not present in the canal. We understand

3 NatPark-2-02:051315 that an existing water well with an underground storage tank is located south of Warehouse #5.

During our site reconnaissance, we observed large cobbles and boulders, up to approximately 4 feet in diameter, at the ground surface.

3.3 SUBSURFACE CONDITIONS

3.3.1 General

Our field investigation consisted of drilling 11 borings (B-1 through B-11) to depths of approximately 3 to 14 feet BGS. The approximate exploration locations are shown on Figure 2.

Exploration descriptions, logs of the drilled borings, and laboratory testing results are provided in the Appendix of this report.

Our explorations generally encountered surficial fill or root zone underlain by silt and clay with variable proportions sand. This unit is typically carbonate-cemented and underlain by weathered basalt to the maximum depth explored. The following sections summarize the subsurface units encountered.

3.3.2 Fill

Surficial fill was encountered in borings B-1, B-6, B-7, and B-11 to depths of approximately 0.5 to

1.5 feet BGS. In general, the fill consists of medium dense gravel with varying amounts of sand, silt, and trace organics. The moisture content of one gravel fill sample measured 5 percent at the time of our exploration.

3.3.3 Root Zone

All explorations (except for B-1, B-7, and B-11) encountered an approximately 2- to 3-inch-thick root zone.

3.3.4 Silt, Clay, and Sand

Below the fill or root zone, all explorations encountered medium stiff to hard, brown to brown-white silt and clay with varying amounts of sand, trace to no gravel, weathered basalt fragments, and organic matter. Also, layers of medium dense to very dense, silty sand with variable gravel and weathered basalt fragments were encountered in boring B-2 at the ground surface and at a depth of approximately 8.5 feet BGS. Borings B-2, B-3, B-7, B-9, and B-11 were completed in silt or clay and silty sand deposits at depths ranging between approximately 5 and 10 feet BGS. This unit is typically carbonate-cemented. This was verified by reaction to hydrochloric acid (10 Mol).

Generally, this soil unit is continuous throughout the site. Thickness of this unit ranges from approximately 2 feet to more than 10 feet. Elevations of the top of the unit range from approximately 3,959 feet in boring in boring B-10 to 3,970 feet in borings B-2, B-8, and B-9. The base elevation of the unit ranges from approximately 3,952 feet in boring B-3 to 3,966.5 feet in boring B-8. At the time of our explorations the moisture contents of the silt or clay and silty sand soil generally varied between 9 and 30 percent. Atterberg limits tests of a selected sample indicated low plasticity. Fines content analyses indicated fines content between 72 and 85 percent for the native silt and clay soil.

3.3.5 Weathered Basalt

We encountered gray, vesicular, weathered basalt below the silt, clay, and sand in borings B-1, B-2, B-4, B-5, B-6, B-8, and B-10 to the maximum depth explored of between approximately 3 and

4 NatPark-2-02:051315

14 feet BGS. Boring B-2 encountered a layer of boulders, interpreted as weathered basalt, at depths between approximately 5 and 8.5 feet BGS. The weathered basalt is underlain by very dense, silty sand to the depth explored of approximately 10 feet BGS. The basalt was encountered as shallow as approximately 2 feet BGS in borings B-4 and B-5. Elevations of the top of the basalt range from approximately 3,952.5 feet in boring B-10 to 3,966.5 feet in boring B-8.

3.3.6 Groundwater

Groundwater was not encountered in our borings. Our review of Idaho Department of Water Resources water well logs indicates that static groundwater was encountered in the site vicinity at depths of 154 to 238 feet BGS. Perched water may be present on top of the weathered basalt during and after periods of precipitation. The depth to groundwater may fluctuate below the site in response to seasonal changes, changes in surface topography, irrigation, and other factors not observed in this study.

3.4 INFILTRATION TESTING

We understand that septic drain fields will be constructed for the proposed Visitor Contact Station and the existing Herrmann House buildings. The locations and configurations of the septic drain fields were conceptual at the time of this report. We conducted one infiltration test in borings B-2, B-3, B-5, and B-10 at depths ranging between approximately 3 and 5 feet BGS (approximate elevations of 3,954 to 3,966.5 feet). The infiltration testing locations and depths were based on correspondence with the project team. The infiltration tests were performed to evaluate the infiltration potential for the proposed septic drain fields. The infiltration rate in each boring was measured by embedding a 4.25-inch-inside diameter hollow stem auger into the base of the boring and filling the auger with water, allowing the area to saturate, and then measuring the drop in water with time. The tests were conducted under a hydrostatic head of between 24 to 35 inches. Representative soil samples were collected from below the infiltration test locations for grain-size distribution analyses. The results of the infiltration and laboratory testing are described in the “Infiltration System” section of this report and in the Appendix, respectively.

4.0 CONCLUSIONS AND RECOMMENDATIONS

Based on our research, explorations, testing, analyses, and other available information, it is our opinion that the proposed improvements can be constructed as proposed. We anticipate the following factors will have an impact on design and construction of the proposed improvements:

Infiltration rates were measured to be between 2 and 7.5 inches per hour.

Weathered basalt, cobbles, and boulders are present at shallow depths across the site.

Borings B-1, B-4, B-5, B-6, B-8, and B-10 encountered refusal to drilling on weathered basalt.

Construction considerations associated with the presence of shallow weathered basalt include the following:

Excavations can become difficult if not impossible with conventional equipment. Hard excavation conditions should be anticipated especially for septic drain fields and utilities.

During unusually wet periods, perched water may be present on top of the weathered basalt, and the earthwork contractor may need to use temporary dewatering techniques during construction.

5 NatPark-2-02:051315

The carbonate-cemented soil layers have the potential to degrade if the material becomes saturated. This degradation of the carbonate cementation bond could result in settlement under high building foundation loads. Laboratory testing indicated that the proposed foundation loads will not cause settlement.

Between 0.5 and 1.5 feet of fill was encountered within the proposed work area. However, due to the extensive site history, we anticipate areas of deeper fill may be encountered.

Actual fill composition and depth should be evaluated at the time of construction.

The surficial site soils are generally fine-grained and easily disturbed when moist. Careful attention will need to be paid to protect subgrades during construction in wet weather.

Our specific recommendations for site development are presented in the following sections and should be incorporated into the design and implemented during construction of the proposed development.

5.0 DESIGN

5.1 GENERAL

The following sections provide our design recommendations for the building. All site preparation and structural fill should be prepared as recommended in the “Construction” section of this report.

5.2 SHALLOW FOUNDATIONS

5.2.1 General

As previously discussed, the proposed work areas are underlain by three general soil/rock units.

The load-carrying capabilities of these layers are briefly discussed as follows:

Fill: Up to 1.5 feet of undocumented fill was encountered in the proposed improvement areas. Documentation regarding the fill placement and compaction is not available. Fill should not be relied upon to support building and wall foundations.

Silt or Clay and Sand: A 2-foot to more than 10-foot-thick layer of medium stiff to hard silt and clay with variable sand and medium dense, silty sand is present beneath the fill and/or root zone. These soils are capable of supporting anticipated building loads.

Weathered Basalt: Weathered basalt underlies the silt and sand with carbonate cementation.

This layer is capable of supporting heavy building loads.

Provided the site is prepared as recommended in the following sections of this report, it is our opinion that the proposed structural improvements can be supported on conventional spread footings bearing on native soil, weathered basalt, or new structural fill.

5.2.2 Footing Subgrade Preparation

Where footing excavations are deepened to reach an appropriate bearing layer, the excavation shall be backfilled with structural fill.

6 NatPark-2-02:051315

Where imported granular backfill is used, the over-excavation shall extend laterally along all sides of the excavation at least one-half the depth of the excavation below the footing. The imported granular fill should be compacted and meet the specifications as discussed in the “Structural Fill” section of this report.

5.2.3 Dimensions and Capacities

Continuous wall and isolated spread footings should be at least 18 and 24 inches wide, respectively. The bottom of exterior footings should be at least 24 inches below the adjacent exterior grade and below the design frost depth. Interior footings should be embedded at least 12 inches below the base of the slab. Footings on or near steep slopes should be embedded so that a minimum of 10 feet of horizontal clearance exists between the toe of the footings and the adjacent slopes.

Footings bearing soil should be designed assuming an allowable bearing pressure of 3,000 psf.

Footings bearing on the weathered basalt may be designed assuming an allowable bearing pressure of 10,000 psf. These are net bearing pressures; the weight of the footing and overlying backfill can be ignored in calculating footing sizes. Also, the allowable bearing pressures apply to the total of dead plus long-term live loads and can be increased by one-half for short-term loads, such as those resulting from wind or seismic forces.

If the aforementioned load limits and design parameters are adhered to, then total foundation settlement should be less than 1 inch, with differential settlement between similarly loaded foundations of less than ½ inch.

5.2.4 Resistance to Sliding

Lateral loads on footings can be resisted by passive earth pressure on the sides of the structures and by friction on the base of the footings.

Our analysis indicates that the passive earth pressure for footings confined by the soil is 350 pcf, modeled as an equivalent fluid pressure. Where footings are confined by the deeper weathered basalt, the passive earth pressure for the basalt-embedded portion of the foundation element may be increased to 500 pcf. These values are not factored. Adjacent floor slabs, pavements, or the upper 12-inch depth of adjacent unpaved areas should not be considered when calculating passive resistance. In order to rely upon passive resistance, a minimum of 10 feet of horizontal clearance must exist between the face of the footings and adjacent down slopes.

For footings in contact with the silty or sandy soil, a coefficient of friction equal to 0.35 may be used when calculating resistance to sliding. This value may be increased to 0.40 for footings bearing on a granular pad or the deeper basalt unit.

5.2.5 Construction Considerations

All footing subgrades should be evaluated by the project geotechnical engineer or their representative to confirm suitable bearing conditions. Observations should confirm that the appropriate bearing layer has been exposed. Localized deepening of footing excavations may be required to penetrate unsuitable fill, loose or soft material, organics, and deleterious material.

7 NatPark-2-02:051315

If footing excavations are conducted during wet weather conditions, we recommend that a minimum of 3 inches of granular material be placed and compacted until well keyed at the base of the excavations. The granular material reduces subgrade disturbance during placement of forms and reinforcement and provides clean conditions for the reinforcing steel

5.3 SEISMIC DESIGN CONSIDERATIONS

5.3.1 IBC Design Parameters

Based on our investigation, the parameters in Table 1 should be used to compute seismic base shear forces if the building is designed using the applicable provisions of the 2012 IBC. We selected the site coefficient “C” based on the results of our explorations.

Table 1. IBC Seismic Design Parameters

Seismic Design Parameter Short Period

(T s = 0.2 second)

1-Second Period (T

= 1.0 second)

MCE Spectral Acceleration S s = 0.24 g S

= 0.09 g

Site Class C

Site Coefficient F a = 1.20 F v = 1.70

Adjusted Spectral Acceleration S

MS

= 0.29 g S M1

= 0.15 g

Design Spectral Response Acceleration Parameters S

DS

= 0.19 g S

D1 = 0.10 g

Design Spectral PGA 0.08 g

5.3.2 Liquefaction and Lateral Spreading

Liquefaction is caused by earthquake-induced ground shaking, which creates a rapid increase in pore water pressure that reduces the effective stress between soil particles to near zero.

Granular soil, which relies on interparticle friction for strength, is susceptible to liquefaction until the excess pore pressures can dissipate. In general, loose, saturated sand soil with low silt and clay content is the most susceptible to liquefaction. Silty soil with low plasticity is moderately susceptible to liquefaction under relatively higher levels of ground shaking. Based on the subsurface conditions encountered in our explorations, liquefaction is not possible under design levels of ground shaking.

5.4 FLOOR SLABS

Satisfactory subgrade support for building floor slabs supporting floor loads of up to 100 psf can be obtained on subgrade prepared as discussed in the “Site Preparation” and “Structural Fill” sections of this report. A minimum 6-inch-thick layer of compacted, imported granular material should be placed over the prepared subgrade to aid as a capillary break and provide uniform support. Imported granular material should meet the requirements for aggregate base rock in the “Structural Fill” section of this report. The imported granular material should be placed in one lift and compacted to not less than 95 percent of the maximum dry density, as determined by ASTM D 1557.

8 NatPark-2-02:051315

Slabs should be reinforced according to their proposed use and the structural engineer’s recommendations. Load-bearing concrete slabs may be designed assuming a modulus of subgrade reaction, k, of 150 psi per inch. This value assumes the floor slab is underlain by native soil or at least 1 foot of structural fill subgrade. If encountered, deleterious material and oversized debris, cobbles, boulders, and other deleterious material should be removed prior to compaction.

Vapor barriers beneath floor slabs are typically required by flooring manufactures to maintain the warranty on their products. Selection and design of an appropriate vapor barrier, if needed, should be based on discussions among members of the design team. We can provide additional information to assist you with your decision.

We recommend that exterior slabs (e.g., patios, walkways, etc.) be structurally independent from the building foundations. Expansion joints should be provided between floor slabs and foundations. This will allow minor movement of the slabs to occur as a result of vehicular loading, tree root growth, seasonal soil shifting, and other factors, while reducing the potential for slab cracking around the perimeter. Interior slabs may be tied to the building’s foundation system.

All floor subgrades should be evaluated by a qualified geotechnical engineer or their representative to confirm suitable bearing conditions. Observations should also confirm that loose or soft material, organics, unsuitable fill, prior topsoil zones, and softened subgrades (if present) have been removed.

5.5 RETAINING STRUCTURES

5.5.1 Assumptions

Significant retaining walls are not proposed for the site; however, for preliminary planning purposes, our retaining walls design recommendations are provided below.

Our retaining wall design recommendations are based on the following assumptions:

(1) the walls consist of conventional, cantilevered retaining walls, (2) the walls are less than 8 feet in height, (3) the backfill is drained and consists of imported granular materials, and

(4) the backfill has a slope flatter than 4H:1V. Re-evaluation of our recommendations will be required if the retaining wall design criteria for the project varies from these assumptions.

5.5.2 Wall Design Parameters

Unrestrained site walls that retain native soil should be designed to resist an active earth pressure of 35 pcf. This value should be increased to 55 pcf where the walls are restrained against rotation. Where retained slopes are between inclinations of 4H:1V and 2H:1V, the designer may linearly interpolate between these active earth pressures. For the embedded building walls, a superimposed seismic lateral force should be calculated based on a dynamic force of 6H2 pounds per lineal foot of wall, where H is the average height of the soil supported by the wall in feet, and applied at 0.6H from the base of the wall.

If surcharges (e.g., slopes steeper than 2H:1V, foundations, vehicles, etc.) are located within a horizontal distance from the back of a wall equal to twice the height of the wall, then additional

9 NatPark-2-02:051315 pressures may need to be included in the wall design. Our office should be contacted for appropriate wall surcharges based on the actual magnitude and configuration of the applied loads.

The wall footings should be designed in accordance with the guidelines provided in the appropriate portion of the “Shallow Foundations” section of this report.

5.5.3 Wall Drainage and Backfill

The above design parameters have been provided assuming that drains will be installed to prevent buildup of hydrostatic pressures behind all walls. If a drainage system is not installed, then our office should be contacted for revised design forces.

A minimum 6-inch-diameter perforated collector pipe should be placed at the base of the walls.

The pipe should be embedded in a minimum 2-foot-wide zone of angular drain rock that is wrapped in a drainage geotextile fabric and extends up the back of the wall to within 1 foot of the finished grade. The drain rock and drainage geotextile fabric should meet specifications provided in the “Materials” section of this report. The perforated collector pipes should discharge at an appropriate location away from the base of the wall. The discharge pipe(s) should not be tied directly into stormwater drain systems, unless measures are taken to prevent backflow into the drainage system of the wall.

Settlement of up to 1 percent of the wall height commonly occurs immediately adjacent to the wall as the wall rotates and develops active lateral earth pressures. Consequently, we recommend that construction of flatwork adjacent to retaining walls be postponed at least four weeks after backfilling of the wall, unless survey data indicates that settlement is complete prior to that time.

5.6 PAVEMENTS

5.6.1 General

We understand that compacted gravel pads are proposed for roadways, pathways, and parking lot. Also, concrete slabs are proposed for the entry plaza, pedestrian sidewalks, and north plaza.

New compacted gravel pads and concrete slabs should be installed on competent subgrade or new engineered fill prepared in conformance with the “Site Preparation” and “Structural Fill” sections of this report.

5.6.2 Compacted Gravel Pads

We recommend the minimum sections shown in Table 2 be used for the compacted gravel pads.

Table 2. Minimum Compacted Gravel Pad Sections

Location Gravel Pad

(inches)

Pathway 4.0

Parking Lot 16.0

Roadway 22.0

10 NatPark-2-02:051315

We recommend that all gravel pads be underlain by firm, undisturbed native soil or structural fill over undisturbed native soil. The gravel pads should consist of imported granular material as described in the “Structural Fill” section of this report. The imported granular material should be compacted to not less than 95 percent of the maximum dry density, as determined by ASTM D 1557, or until well keyed, as determined by a qualified geotechnical engineer or their representative. We recommend that a qualified geotechnical engineer or their representative observe the prepared gravel pad subgrade before placing gravel pads.

In addition, a geotextile fabric should be placed as a barrier between the subgrade and granular material in parking lots and roadways. The geotextile fabric should meet specifications provided in the “Materials” section of this report.

We anticipate that periodic maintenance (i.e., grading and compaction) will be required for the compacted gravel pads.

5.6.3 Concrete Flatwork Construction Consideration

The site vicinity is subject to freezing temperatures during the winter months. Consequently, flatwork, such as sidewalks, is susceptible to damage from frost heave. The following three conditions are required for frost heave to occur:

Freezing temperatures Water Frost susceptible soil

The following measures can be taken to prevent frost heave:

Place clean, free-draining crushed rock directly beneath the flatwork to provide drainage and act as a capillary break to prevent water from accumulating within the freeze/thaw zone, based on gradation material that has less than 3 to 5 percent material finer than the U.S. Standard No. 200 Sieve by dry weight is suitable.

Seal joints in the concrete to prevent water from infiltrating into the surface due to precipitation or irrigation.

Provide adequate drainage to drain water from the freeze/thaw zone.

An additional measure that can be taken to prevent trip hazards associated with heave is to provide dowels between joints in the concrete and adjoining structures.

6.0 CONSTRUCTION

6.1 SITE PREPARATION

6.1.1 Demolition

Demolition includes complete removal of existing site improvements within 5 feet of areas to receive new pavements, buildings, retaining walls, or engineered fills. Underground vaults, tanks, manholes, foundation elements, and other subsurface structures should be removed in areas of new foundation elements. Utility lines can be completely removed or grouted full if left

11 NatPark-2-02:051315 in place. Soil disturbed during demolition should be removed and replaced in accordance with the recommendations in the "Structural Fill" section of this report.

Material generated during demolition should be transported off site for disposal or stockpiled in areas designated by the owner. In general, this material will not be suitable for re-use as engineered fill. However, concrete and base rock material may be recycled in accordance with the recommendations provided in the “Structural Fill” section of this report.

6.1.2 Grubbing and Stripping

Trees and shrubs should be removed from fill areas. In addition, root balls should be grubbed out to the depth of the roots, which could exceed 3 feet BGS. Depending on the methods used to remove the root balls, considerable disturbance and loosening of the subgrade could occur during site grubbing. We recommend that soil disturbed during grubbing operations be removed to expose firm, undisturbed subgrade. The resulting excavations should be backfilled with structural fill.

The existing topsoil zone should be stripped and removed from all fill areas. Based on our explorations, the average depth of stripping will be approximately 2 to 3 inches, although greater stripping depths may be required to remove localized zones of loose or organic soil.

Greater stripping depths (approaching 12 inches) may be anticipated in areas with thicker vegetation and shrubs. The actual stripping depth should be based on field observations at the time of construction. Stripped material should be transported off site for disposal or used in landscaped areas.

6.1.3 Undocumented Fill

Up to 1.5 feet of undocumented fill consisting of medium dense gravel with varying amounts of sand, silt, and trace organics was encountered in borings B-1, B-6, B-7, and B-11. However, due to the extensive site history, we anticipate areas of deeper fill may be encountered. Actual fill composition and depth should be evaluated at the time of construction. Due to the unpredictable strength properties of undocumented fill, there is increased potential for poor foundations, floor slab, and rigid pavement performance. Accordingly, we recommend the undocumented fill be excavated to undisturbed native soil in all structural areas (buildings, footings, rigid pavements, retaining walls) and replaced with structural fill.

Re-use of the excavated material may be possible, provided it is processed to remove debris and then placed and compacted as described in the “Structural Fill” section of this report. However, as a cost saving measure, the fill may be left in place in the proposed new gravel pads, rigid pavement, and slabs areas, provided that the top 1 foot of the fill is re-compacted as recommended in the “Structural Fill” section of this report. This assumes that a minor risk of future maintenance is acceptable.

6.1.4 Subgrade Preparation and Evaluation

Upon completion of stripping, and removal or re-compaction of undocumented fill, and prior to the placement of any fill, rigid pavement, or structural improvements, the exposed subgrade should be evaluated by proof rolling. The subgrade should be proof rolled with a fully loaded dump truck or similar heavy rubber-tired construction equipment to identify soft, loose, or

12 NatPark-2-02:051315 unsuitable areas. A member of our geotechnical staff should observe proof rolling to evaluate yielding of the ground surface. Soft or loose zones identified during the field evaluation should be moisture conditioned and compacted to an unyielding condition or excavated and replaced with compacted engineered fill (in conformance with the “Structural Fill” section of this report).

In areas to receive rigid pavement or concrete slabs, the top 12 inches of native soil subgrade should be compacted to at least 92 percent of its maximum dry density, as determined by ASTM D 1557, and until proof rolling indicates an unyielding, non-pumping subgrade is present.

If imported granular fill is used in building or compacted gravel pads and rigid pavement subgrade areas, then the subgrade should be compacted to at least 95 percent of its maximum dry density, as determined by ASTM D 1557.

6.2 CONSTRUCTION CONSIDERATIONS

The surficial silt/clay and sand soil at the site is easily disturbed during the wet season and when it is moist. If not carefully executed, site preparation, utility trench work, and roadway excavation can create extensive soft areas and significant subgrade repair costs can result. If construction is planned when the surficial soil is wet or may become wet, the construction methods and schedule should be carefully considered with respect to protecting the subgrade to reduce the need to over-excavate disturbed or softened soil. The project budget should reflect the recommendations below if construction is planned during wet weather.

Existing gravel surfaces should remain in place as long as possible to carry construction traffic.

If construction occurs when silty, wet soil is present, site preparation activities may need to be accomplished using track-mounted excavating equipment that loads removed material into trucks supported on granular haul roads. The thickness of the granular material for haul roads and staging areas will depend on the amount and type of construction traffic. Generally, a 12- to 18-inch-thick mat of imported granular material is sufficient for light staging areas and the basic building pad but is not expected to be adequate to support heavy equipment or truck traffic.

The granular mat for haul roads and areas with repeated heavy construction traffic typically needs to be increased to between 18 to 24 inches. The actual thickness of haul roads and staging areas should be based on the contractor’s approach to site development and the amount and type of construction traffic. The imported granular material should be placed in one lift over the prepared, undisturbed subgrade and compacted using a smooth-drum, non-vibratory roller.

In addition, a geotextile fabric should be placed as a barrier between the subgrade and imported granular material in areas of repeated construction traffic. The imported granular material and the geotextile fabric should meet the specifications in the “Structural Fill” and “Materials” sections of this report.

6.3 PERMANENT SLOPES

Permanent cut and fill slopes should not exceed 2H:1V. Fill slopes should be over-built by at least 12 inches and trimmed back to the required slope to maintain a firm face. Slopes that will be maintained by mowing should not be constructed steeper than 3H:1V. Access roads and pavements should be located at least 5 feet from the top of cut and fill slopes. The setback should be increased to 10 feet for buildings.

13 NatPark-2-02:051315

The slopes should be planted with appropriate vegetation to provide protection against erosion as soon as possible after grading. Surface water runoff should be collected and directed away from slopes to prevent water from running down the face of the slope.

6.4 EXCAVATION

6.4.1 General

As previously discussed, weathered basalt, cobbles, and boulders are present at relatively shallow depths throughout the site. The earthwork contractor should be prepared for these conditions during excavation of trenches or other deep cuts.

The native silt/clay and sand at the site should be readily excavatable with conventional grading equipment. The weathered basalt, cobbles, and boulders encountered below approximately 2 to

6.5 feet BGS will be more difficult if not impossible to excavate with conventional grading equipment. Most excavations will likely encounter the weathered basalt and cobbles/boulders unit. We recommend that project bid documents include a contingency for basalt, cobbles, and boulder removal, as well as the associated increased trench volumes for backfilling.

6.4.2 Temporary Slopes and Shoring

Temporary construction excavations in the native silt, clay, and sand soil for trenches adjacent to level ground (without adjacent structures) should stand vertical for short periods of time to a depth of approximately 4 feet, provided seepage is not observed in the sidewalls and surcharges are not present. Open excavation techniques may be used to excavate to depths between 4 and 10 feet BGS, provided the excavation is cut at a slope of 1H:1V and seepage or surcharges are not present. Temporary excavations deeper than 10 feet BGS or adjacent to slopes should be shored or laid back at an inclination of 1½H:1V or flatter.

If box shoring is used for trench excavations, it should be understood by the contractor that such shoring is used to protect workers and does not prevent caving. If trench excavations with box shoring are left open for extended periods of time, caving of the sidewalls may occur. The presence of caved material will limit the ability to properly backfill and compact the trenches once the utilities are installed. The contractor should be prepared to fill voids between the box shoring and the sidewalls of the trenches with sand or gravel before caving occurs.

Excavations within the influence area of building foundations may require shoring or underpinning of the existing foundations. The influence area of structural elements can be defined as a 1H:1V slope extending down from a 5-foot setback from the edge of a foundation element. A variety of shoring systems are available; consequently, we recommend the contractor be responsible for selecting the appropriate shoring. We should be contacted to provide appropriate shoring design parameters to the shoring designer.

The contractor should be aware of and become familiar with applicable local, state, and federal safety regulations, including current OSHA excavation and trench safety standards. Construction site safety, including the means, methods, and sequencing of construction operations, is the sole responsibility of the contractor. The information provided above is general in nature and should not be relied upon by the contractor during construction.

14 NatPark-2-02:051315

6.4.3 Dewatering

Our research indicates that static groundwater should be 154 feet or more below grade and will not be encountered in excavations.

If shallow surface/perched water seepage is present in excavations, we recommend placing stabilization rock at the base of the excavation and using a pump to remove the water.

The contractor should be responsible for temporary drainage as necessary to prevent standing water and/or erosion at the working surface.

6.5 DRAINAGE

6.5.1 Temporary Drainage

During mass grading at the site, the contractor should be made responsible for temporary drainage of surface water as necessary to prevent standing water and/or erosion at the working surface. During rough and finished grading of the building site, the contractor should keep all footing excavations and building pads free of water.

6.5.2 Surface

The ground surface at finished pads should be sloped away from their edges at a minimum 2 percent gradient for a distance of at least 5 feet. Roof drainage from the building should be directed into solid, smooth-wall drainage pipes that carry the collected water to the storm drain system. Trapped planter areas should not be created adjacent to roadways and structures without providing means for positive drainage (e.g., swales or catch basins).

6.5.3 Infiltration System

We understand that septic drain fields will be constructed for the proposed Visitor Contact Station and the existing Herrmann House buildings. The locations and configurations were conceptual at the time of this report. The infiltration tests were performed to evaluate the infiltration potential for the proposed septic drain fields. The results of our field infiltration testing and laboratory testing are presented in Table 3.

Table 3. Infiltration and Laboratory Testing Summary

Location Depth

(feet BGS)

Observed Infiltration Rate1 (inches per hour)

Fines Content2 (percent)

AASHTO

Classification

B-2 3.5 5.5 78 A-4

B-3 3.5 2.0 81 A-4

B-5 3.0 7.5 NA3 NA3

B-10 5.0 5.5 75 A-4

1. In situ infiltration rate observed in the field.

2. Fines content - material passing the U.S. Standard No. 200 Sieve.

3. Infiltration test completed within the weathered basalt unit.

15 NatPark-2-02:051315

The infiltration rates shown in Table 3 are short-term field rates and factors of safety have not been applied. For design of the proposed septic drain fields at the site, and based on the site-specific testing and soil classifications, we recommend that average in situ unfactored infiltration rates of 2 and 5 inches per hour for the silt/clay and the weathered basalt units, respectively, be used. These rates are field rates and proper correction factors should be applied to determine long-term infiltration parameters. The design engineer should determine the appropriate remaining correction factor values to account for maintenance, vegetation, siltation, and other factors.

The actual depths and estimated infiltration rates can vary significantly from the values presented above. We recommend that the design infiltration values for the septic drain fields be confirmed by field testing completed during installation of the septic drain fields. The results of this field testing might necessitate that the septic drain fields be deepened or the system enlarged in other ways to achieve the design infiltration rate.

6.6 MATERIALS

6.6.1 Structural Fill

6.6.1.1 General

Structural fill includes fill beneath foundations, slabs, rigid pavements, any other areas intended to support structures, or within the influence zones of structures. Structural fill should be free of organic matter, debris, and other deleterious material and, in general, should consist of particles no larger than 3 inches in diameter. Recommendations for suitable fill materials are provided in the following sections.

6.6.1.2 On-Site Soil

The on-site native soils will be suitable for use as structural fill only if it can be moisture conditioned. During our exploration, we observed native soil that was wet as well as dry of optimum. Consequently, moisture conditioning will consist of drying or wetting the soil depending on the location and depth obtained.

When used as structural fill, native and pre-existing fill soil should be placed in lifts with a maximum uncompacted thickness of 6 to 8 inches and compacted to not less than 92 percent of the maximum dry density for fine-grained soil and 95 percent of the maximum dry density for granular soil, as determined by ASTM D 1557.

6.6.1.3 Imported Granular Material

Granular material for use as structural fill should be pit- or quarry-run rock, crushed rock, or crushed gravel and sand that is fairly well graded between coarse and fine, has at least two mechanically fractured faces, and has less than 5 percent by dry weight passing the U.S. Standard No. 200 Sieve.

Imported granular material should be placed in lifts with a maximum uncompacted thickness of 12 inches and compacted to not less than 95 percent of the maximum dry density, as determined by ASTM D 1557. During the wet season or when wet subgrade conditions exists, the initial lift should be approximately 18 inches in uncompacted thickness and should be compacted by rolling with a smooth-drum roller without using vibratory action until well keyed.

16 NatPark-2-02:051315

6.6.1.4 Stabilization Material

Stabilization material should consist of pit- or quarry-run rock, crushed rock, or crushed gravel.

The material should have a maximum particle size of 6 inches, have less than 5 percent by dry weight passing the U.S. Standard No. 4 Sieve, and have at least two mechanically fractured faces.

The material should be free of organic matter and other deleterious material. Stabilization material should be placed in lifts between 12 and 18 inches thick and compacted to a firm condition.

Where the stabilization material is used to stabilize soft subgrade beneath rigid pavements, gravel pads, or construction haul roads, a geotextile should be placed as a barrier between the soil subgrade and the imported granular material. The geotextile fabric should meet the specifications provided below for subgrade geotextiles. Geotextile is not required where stabilization material is used at the base of utility trenches.

6.6.1.5 Trench Backfill

Trench backfill placed beneath, adjacent to, and for at least 12 inches above utility lines (i.e., the pipe zone) should consist of well-graded granular material with a maximum particle size of 1½ inches and less than 10 percent by dry weight passing the U.S. Standard No. 200 Sieve. The pipe zone backfill should be compacted to at least 90 percent of the maximum dry density, as determined by ASTM D 1557, or as required by the pipe manufacturer or local building department.

Within roadway alignments, the remainder of the trench backfill up to the subgrade elevation should consist of well-graded granular material with a maximum particle size of 2½ inches and less than 7 percent by dry weight passing the U.S. Standard No. 200 Sieve. This material should be compacted to at least 92 percent of the maximum dry density, as determined by ASTM D 1557, or as required by the pipe manufacturer or local building department. The upper 3 feet of the trench backfill should be compacted to at least 95 percent of the maximum dry density, as determined by ASTM D 1557.

Outside of structural improvement areas (e.g., roadway alignments or building pads) trench backfill placed above the pipe zone may consist of general fill material that is free of organics and material over 6 inches in diameter. This general trench backfill should be compacted to at least 90 percent of the maximum dry density, as determined by ASTM D 1557, or as required by the pipe manufacturer or local building department.

6.6.1.6 Aggregate Base Rock

Imported granular material used as base rock for building floor slabs and rigid pavements should consist of ¾- or 1½-inch-minus material (depending on the application). In addition, the aggregate should have less than 5 percent by dry weight passing the U.S. Standard No. 200 Sieve. The aggregate base should be compacted to not less than 95 percent of the maximum dry density, as determined by ASTM D 1557.

6.6.1.7 Retaining Wall Select Backfill

Backfill material placed behind retaining walls and extending a horizontal distance of ½H, where H is the height of the retaining wall, should consist of select granular material consisting of pit-

17 NatPark-2-02:051315 or quarry-run rock, crushed rock, or crushed gravel and sand that is fairly well graded between coarse and fine and has less than 5 percent by dry weight passing the U.S. Standard No. 200 Sieve.

We recommend the select granular wall backfill be separated from general fill, native soil, and/or topsoil using a geotextile fabric that meets the specifications provided below for drainage geotextiles.

The wall backfill should be compacted to a minimum of 95 percent of the maximum dry density, as determined by ASTM D 1557.