Geotechnical_Report_GR16-16.pdf

Geotechnical Report Number GR16-16

David Evans and Associates, Inc.

Western Federal Lands Highway Division

Mirror Lake Trailhead Relocation

OR DOT 26(2)

Clackamas County, Oregon Task Order No. DTFH7015F19005

Prepared by

Oregon & Washington

August 11, 2016 Revised October 6, 2016 i

TABLE OF CONTENTS

INTRODUCTION

PROJECT DESCRIPTION

GEOLOGIC OVERVIEW

SUBSURFACE CONDITIONS

General

Soils

Groundwater

Infiltration

CONCLUSIONS AND RECOMMENDATIONS

UNIVERSAL TRAIL AND PROPOSED CAMP CREEK BRIDGE

Site Preparation

Site Grading

Structural Fill

Fill Settlement

Bridge Foundation Support

Seismic Considerations

Pavements

PROPOSED MIRROR LAKE TRAIL PARKING LOT

Site Preparation

Site Grading

Structural Fill

Fill Settlement

Retaining Walls

Pavements

PROPOSED SKI BOWL WEST PARKING LOT EXPANSION

Site Preparation

Site Grading

Structural Fill

Retaining Walls

Pavements

DESIGN REVIEW AND CONSTRUCTION SERVICES

LIMITATIONS

ii

TABLE OF CONTENTS

(Continued)

List of Figures Figure 1 Vicinity Map Figure 2 Site Map Figure 3 Site Geology Figure 4 Site Plan Figure 5 Surcharge Pressures Figure 6 Retaining Wall Drainage

Appendices Appendix A Field Explorations and Laboratory Testing Appendix B FHWA Laboratory Results Appendix C Mirror Lake Trail Geophysical Report

INTRODUCTION

At the request of David Evans and Associates, Inc. (DEA), GRI is providing geotechnical services to the Western Federal Lands Highway Division (WFLHD) for the relocation of the existing Mirror Lake Trailhead near Government Camp in Clackamas County, Oregon. The Vicinity Map, Figure 1, shows the general location of the site.

The purpose and scope of this supplemental geotechnical work is described in the May 25, 2016, WFLHD document, titled “Revised Statement of Work, DTFH7015D00002, Task Order No. DTFH7016F02009, Modification 1, Mirror Lake Trailhead Relocation, OR DOT 26(2).”

Elevations noted in this report reference the DEA project datum.

PROJECT DESCRIPTION

The existing Mirror Lake Trailhead is located on the south side of US 26 about two miles west of Government Camp, Oregon.

The proposed trailhead relocation project includes several main elements, which are shown on the Site Map, Figure 2, Site Geology, Figure 3, and the Site Plan, Figure 4. Proposed project elements include:

■ new turning lanes along US 26 and realignment of Glacier View Drive;

■ new Mirror Lake Trailhead (MLT) parking lot just west of the existing Ski Bowl West (SBW) parking lot and north of the Government Camp Sewer Treatment Plant, a retaining wall that will likely be required for the MLT parking lot;

■ expansion to the eastern end of the existing Ski Bowl West (SBW) parking lot; Ski Bowl is made up of two facilities, Ski Bowl West, which is on-site and Ski Bowl East located approximately 3/4 of a mile to the east and off the project site, see the Site Map, Figure 2 for locations. We understand the current plans call for a rockery retaining wall; however an oversteepend slope can be used in lieu of a retaining wall;

■ a new section of trail from the proposed MLT parking lot to the existing Mirror Lake trail; this section of trail will require a new bridge over Camp Creek; the section of the Mirror Lake Trail between the proposed MLT parking lot and the proposed bridge at Camp Creek is now known as the Universal Trail and will be paved; the portion of the proposed Mirror Lake Trail from the proposed Camp Creek Bridge to the existing Mirror Lake Trail is known as the Connecting Trail.

The proposed Connecting Trail is not part of this geotechnical investigation; and

■ removal of the existing Mirror Lake Trailhead parking lot and the existing MLT bridge over Camp Creek.

The Project Team intends to use the existing Oregon Department of Transportation (ODOT) pavement design information from the current Highway 26 rockfall and paving project for the new turning lanes along US 26. The proposed MLT parking lot will be constructed in an area with varying topography, ranging from about elevation 3,655 to 3,610 ft. The proposed MLT bridge will be constructed over Camp

Creek at about elevation 3,585 ft. The proposed Mirror Lake Trail will vary with existing topography between elevations 3,585 to 3,720 ft. The existing MLT parking lot is at about elevation 3,435 ft. The project area is located on the north side of the Tom Dick and Harry Mountain, which rises to an elevation of about 5,066 ft. See the Site Map, Figure 2, and the Site Geology, Figure 3, for proposed project locations.

GEOLOGIC OVERVIEW

The project is located southwest of Mt. Hood, a Neogene volcanic peak in the Cascade Range geologic province of Oregon. The project is located on the north side of the Tom Dick and Harry Mountain, which forms a prominent upland on the flank of the Mt. Hood Volcano.

The project area is underlain by several geologic units, see Figure 3. The proposed MLT parking lot, proposed bridge location over Camp Creek, proposed expansion to the SBW parking lot, and the eastern 3/4 of the proposed Mirror Lake Trail is underlain by Quaternary volcanic rocks consisting of pyroclastic flows and debris flows that erupted from the Mt. Hood Volcano (Ma et al, 2009). These volcanic rocks consist of sand-, gravel-, and boulder-sized material with variable cementation and hardness. The surface of the new trail area is commonly capped by an estimated 1- to 3-ft-thick surficial layer of soil and forest duff (organic material). The western 1/4 of the new trail is underlain by relatively hard, Quaternary andesite volcanic rock. The existing MLT parking lot and SBW parking lot are underlain by Quaternary andesite volcanic rock, pyroclastic flows, and debris flows.

SUBSURFACE CONDITIONS

General

Subsurface materials and conditions for the proposed MLT parking lot, proposed SBW parking lot expansion, proposed MLT retaining wall, and the proposed new Camp Creek Bridge were investigated between May 23 and July 7, 2016, with seven test pits, designated TP-1 through TP-7, three hand augers designated HA-1 through HA-3, and two resistivity surveys at the south abutment of the proposed bridge over Camp Creek. The explorations were advanced until refusal at depths of 1.5 to 11 ft at the approximate locations shown on the Site Plan, Figure 4. The field exploration program completed for this project is described in Appendix A. Logs of the explorations are provided on Figures 1A through 6A. The terms and symbols used to describe the soils encountered in the explorations are defined in Table 1A and the attached legend. Laboratory testing of the samples collected were performed by FHWA and are attached in Appendix B at the end of this report. The Mirror Lake Trail Geophysical Exploration Report can be found in Appendix C.

Soils

For the purpose of discussion, the materials disclosed by the explorations have been grouped into the following units based on their physical characteristics and engineering properties.

1. PAVEMENT

2. FILL

3. SAND

The following paragraphs provide a detailed description of these units and a discussion of the groundwater conditions at the site.

1. PAVEMENT. Test pits TP-3 and TP-4 were advanced in an area of recycled concrete. Underlying the recycled concrete, approximately 3 in. of asphalt concrete (AC) was encountered. The pavement is underlain by 6 to 8 in. of crushed rock base course.

2. FILL. Fill was encountered in explorations HA-1, HA-2, and TP-2 through TP-7, and to a maximum depth of 3 ft. The sand fill typically consists of brown to light brown silty sand, however, HA-1 and HA-2 encountered sand with trace gravel to gravelly. Scattered construction debris was observed in the sand fill.

The relative density of the sand fill is loose to medium dense based on visual observation. The natural moisture content of the sand fill ranges from 4 to 18%.

3. SAND. Sand soils were encountered at the ground surface in explorations HA-3, TP-1, and beneath the fill in test pits TP-2 through TP-7. The sand typically consists of red-brown to gray sand with variable silt, gravel, and cobble content. In general, the content of silt decreased with depth; ranging from silty near the ground surface to trace to some silt at depth. The sand ranged from fine to coarse grained and generally increased in grain size with depth. Boulders of various sizes were encountered within the sand, ranging up to 4 ft in diameter. Based on visual observation, the relative density of the sand is medium dense to dense.

The natural moisture content of the sand ranges from 5 to 30%.

Groundwater

Regional groundwater was not encountered within any of the explorations. The regional groundwater level in portions of the alignment nearest Camp Creek will likely be similar to the surface water level of the creek. During periods of extended wet weather, we anticipate groundwater may become perched within the upper silty sands.

Infiltration

Four open pit infiltration tests were conducted in test pits TP-3, TP-4, TP-6, TP-7, and two encased falling head infiltration tests were performed in hand augers HA-1 and HA-2 at depths ranging from 1.7 to 8 ft below the existing site grade at the approximate locations shown on Figure 4. Details regarding the infiltration test methods are provided in Appendix A. The average unfactored, field-measured infiltration rates are tabulated below.

Location Depth of Infiltration

Test, ft Average

Infiltration Rate, in./hr

TP-3 4.5 1

TP-4 5.5 1.5

TP-6 8 15

TP-7 3.3 12.5

HA-1 1.7 20

HA-2 2.7 10

The infiltration rates measured in the field should be divided by a factor of safety to obtain the stormwater system design infiltration rate. We recommend a minimum factor of safety of 2 for infiltration design using open pit and encased falling head infiltration tests.

CONCLUSIONS AND RECOMMENDATIONS

The explorations indicate the majority of the site is mantled with Quaternary volcanic rocks consisting of pyroclastic and debris flows that erupted from the Mt. Hood Volcano. The flows predominately consist of sands with variable silt, gravel, cobbles, and boulders. As a result of the boulders encountered within the explorations, it should be anticipated that excavation at the project site may be difficult. Limited fills, up to 3 ft, were encountered in explorations HA-1, HA-2, and TP-2 through TP-7 and generally consisted of silty sand. Although regional groundwater was not encountered within the explorations, we anticipate near the proposed new Camp Creek Bridge, groundwater will largely follow the surface water level of the creek.

Over the remaining portions of the site, we anticipate perched groundwater may develop in the upper silty sand soils.

The remaining sections of this report have been divided into three main sections based on the primary design elements. These sections include the Universal Trail & proposed Camp Creek Bridge, the proposed MLT parking lot, and the proposed SBW parking lot expansion. The following sections of this report provide our conclusions and recommendations concerning earthwork and design and construction of the retaining walls, bridge foundations, and pavements.

UNIVERSAL TRAIL AND PROPOSED CAMP CREEK BRIDGE

Site Preparation

The project site is mantled with a layer of duff ranging in thickness from 8 to 18 in. The duff soils are generally fine-grained and contain a large percentage of organic matter. Prior to construction, the organic rich duff soil should be stripped from the proposed Universal trail alignment. It should be anticipated that deeper stripping and grubbing will be necessary where trees and shrubs are located within the proposed Universal trail alignment. The stripped materials are not suitable for use as compacted structural fill.

Following stripping or excavation to subgrade level, the resulting subgrade within the area of the improvements should be evaluated by a qualified geotechnical engineer, and soft soils or areas of unsuitable material should be overexcavated and replaced with select granular backfill (as per FHWA FP-14, 704.08). See the structural fill section below for additional details. As previously discussed, up to 3 ft of fill was encountered at the ground surface at several locations on site. It should be anticipated that some overexcavation may be necessary due to the presence of fill.

Site Grading

Final grading across the project should provide for positive drainage of surface water away from exposed slopes to minimize erosion. Prior to placing pavement base course aggregate, subgrade should be sloped to a minimum 0.5% slope to aid in drainage. Temporary excavation slopes should be made no steeper than about 1H:1V (Horizontal:Vertical), and permanent cut and fill slopes constructed with native materials should be no steeper than 2H:1V. It should be anticipated that boulders will be encountered within excavations and cut slopes. Where boulders are encountered on an exposed finish slope face, an experienced geotechnical professional should be contacted to evaluate boulder stability and the necessity of removal. In general, structural fills should extend a minimum horizontal distance of 1 ft beyond the limits of the improvements such as the edge of new pavements.

Our experience indicates the near surface silty sand soils are somewhat sensitive to moisture content.

When these soils are in excess of 4 to 5% of their optimum moisture content, they typically become weak and unstable when disturbed and remolded by wheeled construction traffic. Based upon our experience with similar soils, we anticipate the optimal moisture content would be between 8 and 15%. We anticipate the near surface soils within the limits of the Universal Trail will likely be above optimum at the time of construction. For this reason, we recommend a 12-in.-thick granular work pad be constructed on the Universal Trail to limit disturbance to the underlying subgrade. We recommend the granular work pad consist of select granular backfill, (per FP-14, section 704.08).

Structural Fill

Embankment Fill. We anticipate the height of fills will vary across the site and may be as thick as 15 ft, but will typically be less than 10 ft. In general, on-site or imported, organic-free soils in accordance with FP-14 Section 204.10 and approved by the geotechnical engineer may be used to construct structural fills. The on-site sand soils typically contain boulders and cobbles. All materials greater than 6 in. in diameter should be removed or reduced in size prior to placing as embankment fill.

Construction of fills should be in conformance with FP-14, Section 204. Approved organic-free soils used to construct structural fills should be placed in 12-in.-thick lifts (loose) and compacted using a segmented-pad roller to a density not less than 95% of the maximum dry density as determined by AASHTO T99. The moisture content of soils at the time of compaction should be controlled to within 2% of optimum. Some aeration and drying of the on-site silty sand soils may be required to meet the recommended compaction criteria. Fill placed in landscaped areas should be compacted to about 90% of the maximum dry density as determined by AASHTO T99.

Select Granular Backfill. Imported or onsite granular material used to construct work pads should be in conformance with FP-14, Section 704.08 and consist of granular material, such as sand, sandy gravel, or fragmental rock with a maximum size of up to about 3 in. with not more than 5% passing the No. 200 sieve (washed analysis). Select Granular Backfill should be placed in lifts up to about 12-in.-thick lifts (loose). All lifts should be compacted to at least 95% of the maximum density as determined by AASHTO T99. If the lifts are compacted with hand-operated equipment, the lifts will likely need to be reduced to 6 to 9 in. thick (loose).

Base Course Aggregate. Base Course Aggregates for construction of the Universal Trail pavement base course should meet the requirements of Section 703-2, Type C material and consist of crushed aggregate having a maximum size up to about 1 in. with not more than 5% passing the No. 200 sieve (washed analysis). Pavement base course should follow the guidelines in FP-14, Section 301. Pavement base course should be placed in lifts up to 6 in. (loose) and compacted to a minimum of 95% of the maximum density as determined by AASHTO T-180.

Fill Settlement

We anticipate fills will be required for construction of the proposed MLT parking lot and proposed new Camp Creek Bridge abutment. We understand final project grading plans are still in preparation; however we anticipate maximum fill heights of approximately 15 ft from existing grade. In general, soils encountered are granular with variable fines contents. Due to access limits, physical explorations around Camp Creek were limited. Non-intrusive resistivity surveys were performed on the south side of Camp Creek. Surveys indicated the upper 5 ft of soils had lower resistivity and underlain by a minimum 10-ft-thick, high resistivity layer. Layering was interpolated as loose soil with forest biomass at the ground surface and underlain by dense, openwork granular soils. We anticipate that settlement due to the placement of embankment fill will be less than about 1 in., if constructed on granular soils. The majority of this settlement will likely occur during construction as the embankment fill is placed.

Bridge Foundation Support

General. The maximum combined dead and live service loads on each of the bridge abutments of the bridges is unknown at this time, however we anticipate maximum loads of approximately 200 kips per bent. We anticipate the bridges can be supported on conventional spread footing foundations established in firm, undisturbed granular soils or structural fill and constructed in accordance with requirements outlined in FP-14, Section 550. Depending on the actual location of the bridge abutments, scour may need to be considered. Scour depths of Camp Creek were evaluated by the project structural engineer, DEA, and are included in the project plans. The following parameters were utilized for our design of the proposed bridge.

FOUNDATION DESIGN PARAMETERS

Effective Friction Angle: 34 Effective Cohesion (psf): 0

Unit Weight (pcf): 120

Spread Footings. We anticipate bridge abutment foundations will be designed in accordance with the AASHTO LRFD manual. In our opinion, spread footings established in the near-surface alluvial soils or structural fill can be designed for a nominal bearing resistance of 6 ksf for the strength limit state. For the service limit state, we recommend a nominal bearing resistance of 2.0 ksf to limit footing settlement to less than 1 in. Spread footings should be at least 2 ft wide and embedded at least 2 ft below the lowest adjacent grade. In addition, for the strength and service limit state, the bottom of spread footings should be established below the estimated depth of scour or behind a plane extending upward at 11/2H:1V from the toe of the existing stream bank, whichever is lower. If the proposed bridge is to be constructed utilizing the Extreme Limit State, foundations will need to be embedded a minimum of 5 ft below the depth of scour. We recommend the bridge footings be underlain by a minimum 6-in.-thick layer of compacted crushed rock meeting requirements for crushed rock base as outlined in FP-14, Table 703-2, Class C. The crushed rock should have a maximum size of about 1 in. and should extend at least 12 in. beyond the edges of the footing. To facilitate dewatering of the deepest footing excavation with sump pumps during construction, we recommend the crushed rock at this location meet the requirements of FP-14, Table 703-A, be open graded, and have a maximum size of about 2.5 in. As an alternative, material in conformance with FP-14 Section 704.01 may also be used for the foundation fill. It may be preferable to cap the open-graded crushed rock with 2 in. thickness of crushed rock base outlined above to facilitate grading of the surface. A non-woven Class 1, Type A, geotextile separation fabric in conformance with FP- 14, Table 714-1 should be placed on the bottom of the excavation before placement of the drainage/stabilization layer. If boulders are encountered within 6 in. of the footing base, we recommend they be overexcavated and backfilled with select granular backfill (FP-14, 704.08). Local areas of softer or organic rich subgrade may require deeper overexcavation and should be evaluated by a qualified geotechnical engineer.

In accordance with the AASHTO LRFD design manual, resistance factors must be applied to the nominal bearing resistance, nominal sliding resistance, and nominal passive resistance for spread footing design.

The recommended resistance factors for footing design are summarized in the following table.

RESISTANCE FACTORS FOR SPREAD FOOTING DESIGN

Spread Footing Resistance Factors

Strength Limit State

Service Limit State

Bearing Resistance: 0.45 1.00 Sliding Resistance (Cast-in-Place Concrete): 0.80 1.00

Passive Component of Sliding Resistance: 0.50 1.00

Due to the potential for stream bank erosion, we recommend the foundation design for bridge foundations established below the anticipated elevation of scour not rely on the passive pressure of the embedded portion of the abutment/footing in front of the creek to resist sliding. However, the embedded portion of the abutment/footing in the longitudinal direction away from the creek can be used to provide lateral resistance. Foundations established above the scour elevation can use passive resistance in all directions.

If the passive earth pressure against the embedded portion of the bridge abutment/footing is used to resist sliding, we recommend computing this resistance on the basis of an equivalent fluid having a unit weight of 250 and 500 pcf (nominal passive resistance) for the service and service limit states. This design passive earth pressure would be applicable only if the abutment footing is cast neat against undisturbed soil, and/or if backfill for the footings and the abutment is placed as granular structural fill.

All spread footing excavations should be made with excavators equipped with a smooth-edged bucket and observed by a geotechnical engineer or engineering geologist. Areas of soft or otherwise unsuitable material encountered at subgrade elevation should be removed and replaced with compacted crushed rock in accordance with the Structural Fill section of this report. If groundwater is encountered in the footing excavation, it should be removed prior to placing concrete.

Seismic Considerations

Seismic Forces and Site Class. We understand the proposed new Camp Creek Bridge is to be designed to withstand seismic loading in accordance with the 2014 AASHTO Guide Specification for LRFD Seismic Bridge Design (AASHTO SBD) except as modified by the BDDM. Based on the AASHTO SBD, bridges shall be designed for “low probability of collapse, but may suffer significant damage and disruption to service” in response to a 1,000-year return interval earthquake (7% probability of exceedance in 75 years).

The AASHTO SBD uses the 2002 USGS seismic hazard maps to evaluate earthquake-induced peak bedrock acceleration (PBA). Based on the 2002 USGS seismic hazard maps, the 500- and 1,000-year return interval PBA for the site is 0.19 and 0.27 g, respectively. For seismic analysis, the AASHTO SBD methodology develops the design-level spectra using two parameters at spectral periods of 0.2 and 1.0 second (Ss and S1). The bedrock spectral response parameters were obtained from the U.S. Geological Survey (USGS) Uniform Hazard Response Spectra Curves for the coordinates of 45.3023°N latitude and 121.7744°W longitude. The 0.2- and 1.0-second parameters identified for the site are 0.33 and 0.14 g for a 500-year return interval seismic event and 0.50 and 0.22 g for a 1,000-year return interval seismic event, respectively. Site coefficients Fa and Fv are then used with a site class designation to adjust the bedrock spectral accelerations to ground surface spectral accelerations. Based on review of the soil profile at the bridge abutments, we recommend designing the bridges for Site Class D. The recommended values of Fa and Fv are summarized below.

Fa and Fv SITE COEFFICIENT RECOMMENDATIONS

Return Interval Fa Fv

500-yr 1.53 2.24

1,000-yr 1.40 1.96

Other Seismic Considerations. In terms of potential seismic-related hazards in the area, we anticipate the risk of liquefaction, lateral spreading, settlement, or ground subsidence for the site is low. Based on the location of known and mapped faults in the area, we anticipate the potential for fault rupture or displacement at the site is absent, unless occurring on a previously unknown or unmapped fault. The risk of tsunami and seiche at the site is absent.

Pavements

We anticipate the paved Universal Trail will support pedestrian traffic and occasional pickup truck traffic for maintenance. Fill placed beneath the path areas should consist of embankment fill and meet the specifications noted above in the Structural Fill Section. To limit impacts to soil strength associated with aging, subgrade soils in cut areas should not be recompacted, and the subgrade should be protected from disturbance during the construction.

Based on our experience with similar projects, subgrade soil conditions, and the 1993 AASHTO Guide for Design of Pavement Structures, we recommend the following pavement sections.

RECOMMENDED PAVEMENT SECTIONS

Crushed Rock Base (CRB)

Thickness, in.

AC

Thickness, in.

Pedestrian Paths / Trails 6 3

Crushed Rock Base (CRB)

PCC

Thickness, in.

Pedestrian-Only Sidewalks 4 5

The recommended pavement sections should be considered minimum thicknesses, and it should be assumed that some maintenance will be required over the life of the pavement (15 to 20 years). The above sections do not consider regular vehicular traffic. If regular use is required by this equipment, we should be contacted to re-evaluate the above pavement sections. The section is based on the assumption that pavement construction will be accomplished during the dry season and after construction of other improvements has been completed. If wet-weather pavement construction is considered, it will likely be necessary to increase the thickness of CRB course to protect the fine-grained subgrade from disturbance during construction. In addition, if construction of pavement occurs during wet weather conditions, we recommend placement of a woven Class 1, Type A, stabilization geotextile fabric in conformance with FP- 14, Table 714-1 over the subgrade prior to placement of the CRB. The indicated sections are not intended to support extensive construction traffic, such as dump trucks and concrete trucks. Pavements subject to construction traffic may require repair.

For the above-indicated sections, drainage is an essential aspect of pavement performance. We recommend all paved areas be provided with positive drainage to remove surface water and water within the base course. This will be particularly important in cut sections or at low points within the paved areas, such as at catch basins. Effective methods to prevent saturation of the base course materials include sloping the subgrade to drain, providing weep holes in the sidewalls of catch basins, subdrains in conjunction with utility excavations, and separate trench drain systems. To provide quality materials and construction practices, we recommend the pavement work conform to the Oregon Department of Transportation standards.

Ambient temperature can have a critical effect on the behavior of pavements. Because of the higher elevation of the project and generally lower temperatures, in accordance with the Superpave Mix Design Manual by the Strategic Highway Research Program, we recommend the mix design consist of PG 64-28 asphalt binder, Level II, 1/2 in. dense pavement. Additionally, frost heave was evaluated for the pavements and accounted for in the design using guidelines provided in the 1993 AASHTO design manual and Oregon Department of Transportation Superpave Binder Implementation, SPR 353.

Prior to placing base course materials, the subgrade should be evaluated by a qualified geotechnical engineer and roadway areas should be proof rolled with a fully loaded 10-cy dump truck. Any soft areas identified should be overexcavated to firm ground and backfilled with compacted structural fill.

Provided the pavement section is installed in accordance with the recommendations provided above, it is our opinion that the paved entry road will support infrequent traffic by an emergency vehicle having a gross vehicle weight (GVW) of up to 75,000 lbs. For the purposes of this evaluation “infrequent” can be defined as once a month or less.

PROPOSED MIRROR LAKE TRAIL PARKING LOT

Site Preparation

The project site is mantled with a layer of duff ranging in thickness from 8 to 18 in. The duff soils are generally fine-grained and contain a large percentage of organic matter. Prior to construction, the organic rich duff soil should be stripped from the proposed parking lot footprint. It should be anticipated that deeper stripping and grubbing will be necessary where trees and shrubs are located within the proposed parking lot footprint. The stripped materials are not suitable for use as compacted structural fill.

Following stripping or excavation to subgrade level, the resulting subgrade within the area of the improvements should be evaluated by a qualified geotechnical engineer, and soft soils or areas of unsuitable material should be overexcavated and replaced with select granular backfill (FP-14, 704.08).

See the structural fill section below for additional details. As previously discussed, up to 3 ft of fill was encountered at the ground surface at several locations on site. It should be anticipated that some overexcavation may be necessary due to the presence of fill.

Site Grading

Final grading across the project should provide for positive drainage of surface water away from exposed slopes to minimize erosion. Prior to placing pavement base course aggregate, subgrade should be sloped to a minimum 0.5% slope to aid in drainage. Temporary excavation slopes should be made no steeper than about 1H:1V (Horizontal:Vertical), and permanent cut and fill slopes constructed with native materials should be no steeper than 2H:1V. It should be anticipated that boulders will be encountered within excavations and cut slopes. Where boulders are encountered on an exposed finish slope face, an experienced geotechnical professional should be contacted to evaluate boulder stability and the necessity of removal. In general, structural fills should extend a minimum horizontal distance of 1 ft beyond the limits of the improvements such as the edge of new pavements.

Our experience indicates the near surface silty sand soils are somewhat sensitive to moisture content.

When these soils are in excess of 4 to 5% of their optimum moisture content, they typically become weak and unstable when disturbed and remolded by wheeled construction traffic. Based upon our experience with similar soils, we anticipate the optimal moisture content would be between 8 and 15%. We anticipate the near surface soils within the limits of the proposed parking lot may have above optimum moisture contents at the time of construction. For this reason, where soft or wet soils are encountered, we recommend a 12- to 18-in.-thick granular work pad be constructed on the parking lot to limit disturbance to the underlying subgrade. We recommend the granular work pad consist of select granular backfill, (704.08).

Structural Fill

Embankment Fill. We anticipate the height of fills will vary across the site and may be as thick as 15 ft, but will typically be less than 10 ft. In general, on-site or imported, organic-free soils in accordance with FP-14 Section 204.10 and approved by the geotechnical engineer may be used to construct structural fills. The on-site sand soils typically contain boulders and cobbles. All materials greater than 6 in. in diameter should be removed or reduced in size prior to placing as embankment fill.

Construction of fills should be in conformance with FP-14, Section 204. Approved organic-free soils used to construct structural fills should be placed in 12-in.-thick lifts (loose) and compacted using a segmented-pad roller to a density not less than 95% of the maximum dry density as determined by AASHTO T99. The moisture content of soils at the time of compaction should be controlled to within 2% of optimum. Some aeration and drying of the on-site silty sand soils may be required to meet the recommended compaction criteria. Fill placed in landscaped areas should be compacted to about 90% of the maximum dry density as determined by AASHTO T99.

Select Granular Backfill. Imported or onsite granular material used to construct work pads and retaining walls should be in conformance with FP-14, Section 704.08 and consist of granular material, such as sand, sandy gravel, or fragmental rock with a maximum size of up to about 3 in. with not more than 5% passing the No. 200 sieve (washed analysis). Select Granular Backfill should be placed in lifts up to about 12 in.

thick (loose). All lifts should be compacted to at least 95% of the maximum density as determined by AASHTO T99. If the lifts are compacted with hand-operated equipment, the lifts will likely need to be reduced to 6 to 9 in. thick (loose).

Base Course Aggregate. Base Course Aggregates for construction of the parking lot pavement base course should meet the requirements of Section 703-2, Type C material and consist of crushed aggregate having a maximum size up to about 1 in. with not more than 5% passing the No. 200 sieve (washed analysis).

Pavement base course should construction should follow the guidelines in FP-14, Section 301. Pavement base course should be placed in lifts up to 6 in. (loose) and compacted to a minimum of 95% of the maximum density as determined by AASHTO T-180.

Fill Settlement

We anticipate fills will be required for construction of the proposed MLT parking lot. We understand final project grading plans are still in preparation; however we anticipate maximum fill heights of approximately 15 ft from existing grade. In general, soils encountered are granular with variable fines contents. We anticipate that settlement due to the placement of embankment fill will be less than about 1 in. if constructed on granular soils. The majority of this settlement will likely occur during construction as the embankment fill is placed.

Retaining Walls

We understand a retaining structures consisting of either a cast-in-place concrete or mechanically stabilized earth (MSE) wall will be constructed for the MLT parking lot at the project site. Based on conversations with the project civil engineer, DEA, we understand a wall is currently planned at the proposed MLT parking lot. MSE walls should be constructed in accordance with FP-14, Section 255. Cast-in-place cantilever walls should be constructed in accordance with FP-14, Section 258. We understand current plans call for walls up to about 12 ft in height from existing grade; however we understand final plans are still in preparation and that some locations may be as tall as 15 ft. Design parameters used in design of the walls are included in the following table.

FOUNDATION DESIGN PARAMETERS

Effective Friction Angle: 36 Effective Cohesion (psf): 0

Unit Weight (pcf): 120

Based on the material encountered in the explorations, we anticipate the majority of the walls will be founded in firm, undisturbed sand soils. If soft soils, boulders, or organic soils are encountered at foundation level, it will be necessary to overexcavate and replace the unsuitable materials with compacted crushed rock. To provide more uniform support, the walls should be founded on a minimum 6 in.

thickness of compacted crushed rock meeting requirements for crushed rock base as outlined in FP-14, Table 703-2, Class C. We recommend the toe of the retaining walls be provided with a minimum of 1-ft embedment or H/10, whichever is greater, where H is the height of the wall in feet. To evaluate sliding, we recommend a nominal value of 0.40 for the coefficient of friction for wall footings established in accordance with the above criteria. MSE retaining walls should be backfilled with select granular backfill within the zone of reinforcement.

Cantilever or gravity walls (including MSE walls) are free to yield by tilting, or deflecting, about the base and can be designed using a lateral earth pressure computed on the basis of an equivalent fluid unit weight of 35 pcf for completely drained, horizontal wall backfill. Walls supporting an upward slope of about 2H:1V should be designed on the basis of an equivalent fluid unit weight of 60 pcf. Additional horizontal pressure due to seismic loading may be estimated by assuming an equivalent fluid pressure of 3 and 7 pcf, for the 500- and 1,000-year hazard levels, respectively (Agusti and Sitar, 2013). Additional lateral pressures due to surcharge loadings in the backfill area can be estimated using the guidelines provided on Figure 5.

In our opinion, site retaining walls established in the near-surface granular soils can be designed for a nominal bearing resistance of 8,000 psf for the strength limit state. For the service limit state, we recommend a bearing resistance of 3,000 psf to limit footing settlement to less than 1 in. Because of the granular, non-plastic nature of the near surface soils, we anticipate the settlement will be limited to elastic deformation and the majority of the deformation will likely occur during construction.

Passive pressure on retaining walls to resist sliding can be estimated using an equivalent fluid having a unit weight of 250 and 500 pcf (nominal passive resistance) for the service and strength limit states, respectively. These design passive earth pressures would be applicable only if backfill around the base of the wall is placed as granular structural fill, and the ground surface in front of the wall is relatively flat. We recommend a nominal value of 0.40 for the coefficient of friction for walls established in accordance with the above criteria.

In accordance with the AASHTO LRFD design manual, resistance factors must be applied to the nominal bearing resistance, nominal sliding resistance, and nominal passive resistance for spread footing design.

The recommended resistance factors are provided below.

RESISTANCE FACTORS FOR SPREAD FOOTING DESIGN

Spread Footing Resistance Factors

Strength Limit State

Service Limit State

Bearing Resistance: 0.45 1.00 Sliding Resistance (Cast-in-Place Concrete): 0.80 1.00

Passive Component of Sliding Resistance: 0.50 1.00

We recommend placing a minimum 2-ft-wide granular rock backdrain adjacent to retaining walls for the entire height of the wall. The granular rock backdrain should consist of well-graded gravel or crushed rock of up to 2-in. size, with less than 2% passing the No. 200 sieve (washed analysis) and meet the specifications in FP-14, section 703.17. Drainage behind the wall facing can be collected by a perforated drain pipe located at the bottom of the granular rock backdrain. See Figure 6 for the wall drainage detail.

The lateral earth pressure criteria assumes the general wall backfill will consist of granular structural fill material compacted to about 95% of the maximum dry density as determined by AASHTO T99.

Compaction close to the wall face (within approximately 3 ft) should be accomplished with hand-operated vibratory plate compactors. Overcompaction of backfill could significantly increase lateral earth pressures behind walls and should be avoided.

Pavements

We anticipate the MLT parking lot will support predominately passenger vehicles with occasional heavy vehicles (snow plows) for maintenance. Fill placed beneath the parking lot should consist of embankment fill and meet the specifications noted above in the Structural Fill Section. To limit impacts to soil strength associated with aging, subgrade soils in cut areas should not be recompacted, and the subgrade should be protected from disturbance during the construction.

Based on our experience with similar projects and subgrade soil conditions, we recommend the following pavement sections.

RECOMMENDED PAVEMENT SECTIONS

Crushed Rock Base (CRB)

Thickness, in.

AC

Thickness, in.

Areas Subject to Occasional Heavy Truck Traffic

12 4

PCC

Thickness, in.

Areas Subject to Occasional Heavy Truck Traffic

6 7

Pedestrian-Only Sidewalks 4 5

The recommended pavement sections should be considered minimum thicknesses, and it should be assumed that some maintenance will be required over the life of the pavement (15 to 20 years). The above sections do not consider regular heavy truck traffic such as buses. If regular use is required by this equipment, we should be contacted to re-evaluate the above pavement sections. The section is based on the assumption that pavement construction will be accomplished during the dry season and after construction of other improvements has been completed. If wet-weather pavement construction is considered, it will likely be necessary to increase the thickness of CRB course to protect the fine-grained subgrade from disturbance during construction. In addition, if construction of pavement occurs during wet weather conditions, we recommend placement of a woven Class 1, Type A, stabilization geotextile fabric in conformance with FP-14, Table 714-1 over the subgrade prior to placement of the CRB. The indicated sections are not intended to support extensive construction traffic, such as dump trucks and concrete trucks. Pavements subject to construction traffic may require repair.

For the above-indicated sections, drainage is an essential aspect of pavement performance. We recommend all paved areas be provided with positive drainage to remove surface water and water within the base course. This will be particularly important in cut sections or at low points within the paved areas, such as at catch basins. Effective methods to prevent saturation of the base course materials include sloping the subgrade to drain, providing weep holes in the sidewalls of catch basins, subdrains in conjunction with utility excavations, and separate trench drain systems. To provide quality materials and construction practices, we recommend the pavement work conform to the Oregon Department of Transportation standards.

Ambient temperature can have a critical effect on the behavior of pavements. Because of the higher elevation of the project and generally lower temperatures, in accordance with the Superpave Mix Design Manual by the Strategic Highway Research Program, we recommend the mix design consist of PG 64-28 asphalt binder, Level II, 1/2 in. dense pavement. Additionally, frost heave was evaluated for the pavements and accounted for in the design using guidelines provided in the 1993 AASHTO design manual and Oregon Department of Transportation Superpave Binder Implementation, SPR 353.

Prior to placing base course materials, the subgrade should be evaluated by a qualified geotechnical engineer and roadway areas should be proof rolled with a fully loaded 10-cy dump truck. Any soft areas identified should be overexcavated to firm ground and backfilled with compacted structural fill.

Provided the pavement section is installed in accordance with the recommendations provided above, it is our opinion that the paved entry road will support infrequent traffic by an emergency vehicle having a gross vehicle weight (GVW) of up to 75,000 lbs. For the purposes of this evaluation “infrequent” can be defined as once a month or less.

PROPOSED SKI BOWL WEST PARKING LOT EXPANSION

Site Preparation

The project site is mantled with a layer of duff ranging in thickness from 8 to 18 in. The duff soils are generally fine-grained and contain a large percentage of organic matter. Prior to construction, the organic rich duff soil should be stripped from the proposed parking lot footprint. It should be anticipated that deeper stripping and grubbing will be necessary where trees and shrubs are located within the proposed parking lot footprint. The stripped materials are not suitable for use as compacted structural fill.

Following stripping or excavation to subgrade level, the resulting subgrade within the area of the improvements should be evaluated by a qualified geotechnical engineer, and soft soils or areas of unsuitable material should be overexcavated and replaced with select granular backfill (FP-14, 704.08).

See the structural fill section below for additional details. As previously discussed, up to 3 ft of fill was encountered at the ground surface at several locations on site. It should be anticipated that some overexcavation may be necessary due to the presence of fill.

Site Grading

Final grading across the project should provide for positive drainage of surface water away from exposed slopes to minimize erosion. Prior to placing pavement base course aggregate, subgrade should be sloped to a minimum 0.5% slope to aid in drainage. Temporary excavation slopes should be made no steeper than about 1H:1V (Horizontal:Vertical), and permanent cut and fill slopes constructed with native materials should be no steeper than 2H:1V. It should be anticipated that boulders will be encountered within excavations and cut slopes. Where boulders are encountered on an exposed finish slope face, an experienced geotechnical professional should be contacted to evaluate boulder stability and the necessity of removal. In general, structural fills should extend a minimum horizontal distance of 1 ft beyond the limits of the improvements such as the edge of new pavements.

Our experience indicates the near surface silty sand soils are somewhat sensitive to moisture content.

When these soils are in excess of 4 to 5% of their optimum moisture content, they typically become weak and unstable when disturbed and remolded by wheeled construction traffic. Based upon our experience with similar soils, we anticipate the optimal moisture content would be between 8 and 15%. We anticipate the near surface soils within the limits of the proposed parking lot may have above optimum moisture contents at the time of construction. For this reason, where soft or wet soils are encountered, we recommend a 12- to 18-in.-thick granular work pad be constructed on the parking lot to limit disturbance to the underlying subgrade. We recommend the granular work pad consist of select granular backfill, (704.08).

Structural Fill

Embankment Fill. We anticipate the height of fills will vary across the site and may be as thick as 15 ft, but will typically be less than 10 ft. In general, on-site or imported, organic-free soils in accordance with FP-14 Section 204.10 and approved by the geotechnical engineer may be used to construct structural fills. The on-site sand soils typically contain boulders and cobbles. All materials greater than 6 in. in diameter and should be removed or reduced in size prior to placing as embankment fill.

Construction of fills should be in conformance with FP-14, Section 204. Approved organic-free soils used to construct structural fills should be placed in 12-in.-thick lifts (loose) and compacted using a segmented-pad roller to a density not less than 95% of the maximum dry density as determined by AASHTO T99. The moisture content of soils at the time of compaction should be controlled to within 2% of optimum. Some aeration and drying of the on-site silty sand soils may be required to meet the recommended compaction criteria. Fill placed in landscaped areas should be compacted to about 90% of the maximum dry density as determined by AASHTO T99.

Select Granular Backfill. Imported or onsite granular material used to construct work pads and retaining walls should be in conformance with FP-14, Section 704.08 and consist of granular material, such as sand, sandy gravel, or fragmental rock with a maximum size of up to about 3 in. with not more than 5% passing the No. 200 sieve (washed analysis). Select Granular Backfill should be placed in lifts up to about 12 in.

thick (loose). All lifts should be compacted to at least 95% of the maximum density as determined by AASHTO T99. If the lifts are compacted with hand-operated equipment, the lifts will likely need to be reduced to 6 to 9 in. thick (loose).

Base Course Aggregate. Base Course Aggregate used for construction of the parking lot pavement base course and foundation base course should meet the requirements of Section 703-2, Type C material and consist of crushed aggregate having a maximum size up to about 1 in. with not more than 5% passing the No. 200 sieve (washed analysis). Pavement base course should construction should follow the guidelines in FP-14, Section 301. Pavement base course should be placed in lifts up to 6 in. (loose) and compacted to a minimum of 95% of the maximum density as determined by AASHTO T-180.

Retaining Walls

We understand a rockery wall is proposed for the SBW parking lot expansion. Based on conversations with the project civil engineer, DEA, we understand the wall is currently planned for a height of 5 ft with a 2H:1V backslope up to about 15 ft in height. Rockery walls should be constructed in accordance with FP- 14, Section 252. The rockery should be sized based on the following recommendations. The rock selection should be based upon the total wall height, including embedment.

ROCKERY WALLS SPECIFICATIONS

Height, ft Min. Base Rock

Width, ft Min. Base Rock

Weight, lb Min. Facing Rock

Weight, lbs Min. Top Rock

Weight, lbs

4 and less 2.5 660 200 200

4 - 5 2.75 2000 660 200

5 - 6 3.0 2000 660 200

Based on the material encountered in the explorations, we anticipate the majority of the walls will be founded in firm, undisturbed sand soils. If soft soils, boulders, or organic soils are encountered at foundation level, it will be necessary to overexcavate and replace the unsuitable materials with compacted crushed rock. To provide more uniform support, the walls should be founded on a minimum 6 in.

thickness of compacted crushed rock meeting requirements for crushed rock base as outlined in FP-14, Table 703-2, Class C.