25PW15 - Geotech Design- Parrot Creek Roseburg~1.pdf

GEOTECHNICAL DESIGN REPORT

PARROT CREEK CULVERT REPLACEMENTS

ROSEBURG, OREGON

For: Erik Ranger, P.E.

i.e. Engineering, inc.

809 SE Pine Street

Roseburg, OR 97470

By: THE GALLI GROUP

612 NW Third Street

Grants Pass, OR 97526

(541) 955-1611

02-6575-01

September 4, 2025

Table of Contents

Section Page

1.0 INTRODUCTION

2.0 SITE AND PROJECT DESCRIPTION

3.0 FIELD EXPLORATION

4.0 LABORATORY TESTING

5.0 SUBSURFACE CONDITIONS

5.1 SOIL

5.2 GROUNDWATER

6.0 GEOLOGIC HAZARDS AND SEISMIC DESIGN PARAMETERS

6.1 SITE GEOLOGY AND SEISMICITY

6.2 GEOLOGIC HAZARDS

6.3 SEISMIC DESIGN

7.0 CONCLUSIONS

8.0 GEOTECHNICAL RECOMMENDATIONS

8.1 SITE PREPARATION AND GRADING

8.1.1 Clearing, Grubbing and Stripping

8.1.2 Subgrade Densification

8.1.3 Subgrade Proofrolling

8.1.4 Dewatering

8.2 UTILITY AND SITE EXCAVATIONS

8.3 CUTS AND FILLS

8.3.1 Temporary Cut Slopes

8.3.2 Permanent Cut Slopes

8.3.3 Fill Slopes

8.3.4 Fill on Steep Slopes

8.4 FILL PLACEMENT AND COMPACTION

8.4.1 Beneath Structures and Roadways

8.4.2 On-Site Fill Re-Use

8.4.3 Non-Structural Fill

8.5 NEW CULVERT FOUNDATIONS

8.6 CULVERT BACKFILL AND ROAD PRISM RECONSTRUCTION

8.7 LATERAL LOAD RESISTANCE

8.8 RETAINING WALLS

8.8.1 Conventional Reinforced Concrete/CMU Walls

8.8.2 MSE Walls

8.8.3 Wall Loading Considerations

8.9 RETAINING WALL DRAINS

9.0 MATERIALS SPECIFICATIONS

10.0 GENERAL SITE DRAINAGE AND EROSION CONTROL

11.0 ADDITIONAL SERVICES AND LIMITATIONS

11.1 ADDITIONAL SERVICES

11.2 LIMITATIONS

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group

LIST OF FIGURES

Figure 1 Vicinity Map

Figure 2 Site Plan with Exploration Locations

Figure 3 Geologic Map

Figure 4 Fill on Steep Slope Cross-Section

Figure 5 Exterior Retaining Wall Drainage Cross Section

Figure 6 Typical Eco Block MSE Wall Drainage Cross Section

APPENDIX A: Boring Logs

APPENDIX B: Laboratory Test Results

GEOTECHNICAL DESIGN REPORT

PARROT CREEK CULVERT REPLACEMENTS

ROSEBURG, OREGON

1.0 INTRODUCTION

This report presents results of our geotechnical investigation and geologic hazard evaluation of the project areas for the proposed culvert replacements at four locations along Parrott Creek, in Roseburg, Oregon. The project site is located on four roadway crossings over Parrot Creek, in the southeast area of Roseburg, Oregon. Please see

Figure 1, Vicinity Map, for a more precise site location.

The purpose of this investigation and report was to evaluate the site area surface and subsurface conditions with a series of exploratory borings in order to provide geotechnical recommendations and a geologic hazard evaluation for design and construction of the four culvert replacements.

2.0 SITE AND PROJECT DESCRIPTION

The overall project site is an approximately 1,400-foot-long stretch of Parrott Creek, situated between SE Booth Avenue to the north and SE Marsters Avenue to the south.

The four planned culvert replacements, from upstream to downstream, east to west, are located at SE Ichabod, SE Giles, SE Kane, and SE Eddy Streets. This area of Roseburg is a long-established neighborhood, with most homes in the project area having construction dates from the early 20th century. The four culvert sites are located within a residential housing area, with some areas of dense vegetation immediately adjacent to the stream channel. Parrot Creek currently flows underneath the four subject road crossings, each utilizing 72-inch diameter corrugated metal pipe culverts. We understand a large storm event in March of 2025 caused high flood flows, which overtopped these four culverts. Three of the culverts were either severely damaged or entirely washed out during the March storm event. As such, SE Giles and SE Kane remained restricted to through traffic at the stream crossings, at the time of our field investigation on June 27, 2025.

We understand the proposed project to consist of replacing the four (4) existing CMP culverts with pre-cast, three-sided (open-bottom) box culverts, with embedded foundations for support. The new culverts will replace the current/previous 72-inch CMP culverts located beneath the subject 2-lane street surfaces. The existing culverts vary in length, as detailed in Table 1. We understand the city will consider the option of narrowing the street section and the culvert crossing to allow for shorter culverts.

Table 1- EXISTING 72-INCH CMP CULVERTS

Street Crossing Culvert Length

SE Ichabod St. 45 feet

SE Giles St, 37 feet

SE Kane St. 36 feet

SE Eddy St. 83 feet

We understand the replacement box culverts are anticipated to be designed and constructed in a similar fashion as the nearby existing culvert at the crossing of Parrott

Creek under SE Starner St. (located west of SE Ichabod and east of SE Giles Streets).

That SE Starner Street culvert was installed following a highwater event in December of

2005 and consists of a connected series of three-sided (open bottom) precast concrete bridge sections. The 3-sided precast bridge sections were each 5-foot-wide sections, with a rise of approximately 8 feet and a span width of 18 feet across the stream-channel. The precast bridge sections at SE Starner Street were supported on both sides by cast-in place footings. The culvert walls were embedded into the stream channel and secured to the installed footings. The replacement stream crossing on SE Starner also appeared to have eco-block MSE end wall support sections, and site drainage, erosion control, slope stability, and asphaltic concrete road repair improvements, as part of that project.

3.0 FIELD EXPLORATION

On June 27, 2025 our Staff Associate, Kristen Pierce and our drilling crew, visited the site to accomplish the subsurface investigation. A total of 3 exploratory borings were drilled, one at each culvert replacement location at SE Ichabod, Giles, and Kane Streets.

A fourth boring was also planned for SE Eddy Street. However, due to the high concentration of subsurface utilities where SE Eddy St. crosses Parrot Creek, the planned boring at that location could not be accomplished.

Boring B-1 was located on the west shoulder of SE Kane St. near the south bank of Parrot

Creek, situated west of the existing guard rail, extending to a total depth of 11.3 feet below the ground surface (bgs). Boring B-2 was located on the east shoulder of SE

Ichabod St, situated under the canopy of the trees that line the south bank of Parrot Creek and extended at a depth of 6 feet bgs. Boring B-3 was located on the east shoulder of SE

Giles St., near the existing mail box on the north bank of Parrot Creek and extended to a depth of 13.8 ft bgs. All exploration locations are shown on Figure 2, Site Plan with

Exploration Locations.

The drilling was accomplished with our ATV-mounted, solid-stem auger drill rig.

Borings were advanced with sample collection and testing being accomplished at various depths. Standard Penetration Testing (SPT) was accomplished in each boring. This entails driving a 1½ inch I.D., 2-inch O.D., steel split spoon sampler by dropping a 140-pound weight for a 30-inch drop. The total number of blows it takes to drive the sampler

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group the last 12 inches of an 18-inch drive is called the SPT N-value. The results are an indication of the relative density or consistency of the soil and can be correlated with soil strength and density parameters from testing on thousands of other projects.

Our field engineer identified the final exploration locations away from marked utilities, logged subsurface soils and water conditions and obtained soil samples for transport to our laboratory. Visual classifications of the soils were made in the field and are presented in Appendix A, Boring Logs, at the end of this report. The N-values shown in the logs are raw data from the field and have not been adjusted for sampling equipment type, sampler size, or overburden pressure. After drilling operations were completed, each of the boreholes were backfilled with on-site soils and bentonite pellets (where needed) and the surrounding area cleared of most soil/debris.

4.0 LABORATORY TESTING

Laboratory tests were performed on selected samples to measure soil index properties to provide a basis for estimating engineering properties. All soil samples collected during our investigation were tested for natural moisture content (ASTM D2216). The results are plotted on the Boring Logs, attached in Appendix A. In addition, three washed sieve analyses with hydrometer (ASTM D1140 and ASTM D7928), one Expansion Index

(ASTM D4829), and three Atterberg Limit tests (ASTM D4318) were conducted on samples taken at various locations and depths. Individual test results are presented in

Appendix B, Laboratory Test Results. A generalized summary of the laboratory test results is in presented in Table 2.

Table 2- SUMMARY OF LABORATORY TEST RESULTS

Boring, (Sample depth:

(feet bgs)

Soil Description Test Results

B-1 & B-3 (1.0 – 4.0)

Dark gray and brown sandy Clay

Expansion Index Test

EI = 86

(medium expansion potential)

B-2 (3.0 – 6.0)

Dark gray and brown sandy Clay

Atterberg Limit Test

LL PL PI

56 36 20

B-3 (5.0 – 6.5)

Brown sandy Silt, with clay

Atterberg Limit Test

LL PL PI

33 24 9

B-3 (7.5 – 9.0)

Brown sandy, clayey Silt

Atterberg Limit Test

LL PL PI

46 30 16

B-1

(5.0 – 6.0)

Dark brown sandy silty Clay with some gravel

Washed Sieve Analysis with Hydrometer

G S M C

9% 33% 21% 37%

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group

B-1

(8.0 – 9.0)

Dark brown sandy Gravel with some silt and clay

Washed Sieve Analysis with Hydrometer

G S M C

42% 38% 10% 10%

B-1

(10.0 – 11.3)

Dark brown sandy Gravel with some silt and clay

Washed Sieve Analysis with Hydrometer

G S M C

39% 37% 14% 10%

5.0 SUBSURFACE CONDITIONS

5.1 SOIL

In general, we encountered somewhat uniform subsurface soils and groundwater conditions during our subsurface investigation of the site. All borings encountered a 6- to

9-foot-thick layer of soil at the near surface that is predominantly Clay and Sand, with variable, lesser amounts of silt and gravel. Below this layer, in borings B-1 and B-3 (SE

Kane and SE Giles Streets) borings transitioned to gravelly Sands with approximately

20% fine-grained content to depths of 11 to 13 feet. However, Boring B-2 terminated at a depth of 6 feet, in very dense conditions immediately beneath the overlying clayey soil.

At the bottom of this boring, we recovered pieces of freshly fractured basalt rock in the tip of the sample. It is likely that these basalt pieces were fractured from a cobble or boulder sized rock within the soil layer, and are not indicative of bedrock depth at this location. Nearby water wells typically describe less than 15 feet of clay sediments overlying the basalt bedrock. The bedrock extends to depths of over 400 feet bgs, and appears to contain some interbedded shale, mudstone, or sandstone sedimentary rock layers, (OWRD, 2025).

According to the USDA/NRCS soil survey, the mapped, near surface soil profile along

Parrott Creek in the project area is the Curtin Clay; with slopes of 3% to 12%. The

Curtin Clay is typically over 5 feet thick here (NRCS, 2024). The Curtin Clay is reported to be expansive, with a high percentage (60-95%) of fine-grained content and a Liquid

Limit (LL) ranging from 65-75 and a Plasticity Index (PI) in the range of 40-50 (NRCS, 2024). Our laboratory testing indicates the upper clayey soils at the project are expansive. The results of our lab testing on this upper clayey soil show a greater percentage of sand and lower plasticity than what is typical for the Curtin Clay, with fine-grained content of 55%, LL of 56 and PI of 20.

Please see more specific soils information in Appendix A – Boring Logs. Please note that the subsurface is depicted in distinct layers in the boring logs, while in nature soil may change more gradually. Soils/rock conditions may also change somewhat between the locations investigated.

5.2 GROUNDWATER

The upper soils encountered during our investigation were generally dry to damp. The borings were conducted on the creek overbanks, approximately 12 feet above the stream bed for borings B-1 and B-2, and approximately 7 feet above the stream bed for B-3.

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group

Free water (groundwater) was only encountered only in boring B-3 at a depth of 11.5 feet. Nearby water wells typically indicate water levels at greater than 15 feet bgs, with localized/perched static water levels often reported near the boundary of the upper clayey soil and the basalt bedrock (OWRD, 2025).

Groundwater levels do change due to seasonal rainfall and other climatic occurrences.

Due to this project requiring construction activities below the streambed, dewatering efforts are anticipated. Streamflow though Parrott Creek is highly seasonal and scheduling of the project construction will affect the level of necessary dewatering.

Dewatering and drainage recommendations will be provided in the following sections of this report.

6.0 GEOLOGIC HAZARDS AND SEISMIC DESIGN PARAMETERS

6.1 SITE GEOLOGY AND SEISMICITY

The project site is located in southwestern Oregon (Douglas County), in the City of

Roseburg. The site is within the South Umpqua subbasin of the Umpqua River Basin.

The site is approximately 7 miles southeast of the convergence of the North Umpqua

River and the South Umpqua River into the Mainstem Umpqua River. In addition to the

South Umpqua River, numerous smaller tributaries help drain the 1,800 square miles of subbasin within the over 5,000 square mile watershed of the Umpqua River Basin.

Parrott Creek, one of those tributaries, travels from its headwaters, southeast of the project area, and converges with the South Umpqua River northwest of the project area.

The project site lies in the western portion of the Roseburg East 7.5-minute USGS topographic quadrangle. The site is located on the southeastern margins of Oregon’s

Coast Range Physiographic Province. In geologic history, the Oregon Coast Range is considered an ancient volcanic island chain that collided with and accreted to the North

American continent 50 million years ago as a result of the subduction of the Farallon

Plate. The entire Oregon Coast Range overlies a convergent tectonic margin known as the Cascadia Subduction Zone (CSZ). The Juan de Fuca Plate, a remnant of the ancient

Farallon Plate, is currently being sub-ducted beneath the North American Plate at this tectonic margin. These tectonic forces have created many faults and folds throughout the region.

Erosion is also a major landscape-shaping force for the area. The city of Roseburg receives an average annual precipitation of over 30 inches, with November through

January typically being the wettest months and averaging approximately 5 inches of rainfall each month (NOAA, 2025). Heavy rainfall throughout the region contributes to the steep slopes and drainages in the Coast Range, as well as landslides. The basement complex of the southern region of the Coast Range consists of Paleocene to early Eocene

(66-50 million years ago [Ma]) submarine Siletz River Volcanics, formed as seamounts or oceanic plateaus (R.E. Wells, 2000).

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group

The mapped geologic unit at the project site is the submarine, pillow basalt lava flows of the Siletz River Volcanics. Turbidite sandstone, mudstone, and conglomerate interbeds within the Siletz River Volcanics are common and appear to be derived from the Klamath

Mountains (R.E. Wells, 2000). Within close proximity to the project area are mapped exposures of turbidite sandstone of the Roseburg Member of the Siletz River Volcanics.

Along Parrott Creek, upstream and downstream of the project site, younger (Quaternary, < 2.6 Ma) unconsolidated, mixed-grained, fluvial sediment deposits are mapped. These fluvial sediments are not mapped along the section of Parrott Creek that encompasses the project area. In addition, multiple mapped landslide deposits occur within close proximity (less than one mile) of the project area. Please see Figure 3: Geologic Map for more details. Nearby water wells typically describe less than 15 feet of clay sediments overlying basalt, with some interbedded sedimentary rock layers (shale, mudstone, or sandstone), to depths of over 400 feet bgs (OWRD, 2025).

The area around the project site is tectonically stable, with the only Holocene or

Quaternary faults mapped within a 70 km radius being the “unnamed faults near

Sutherlin” approximately 20 to 25 km north of the project area. These faults have been designated Class B faults because, while geologic evidence exists for fault scarps with possible evidence of Quaternary deformation, the faults have not been consistently mapped and it is possible these scarps were created by fluvial erosion rather than tectonic activity (USGS, 2022).

However, as with most of western Oregon, the project site is affected by the CSZ, an active subduction zone off the Oregon coast considered capable of Magnitude 8.5 or greater earthquakes. The offshore surface expression of the CSZ, near the base of the continental slope, is approximately 160 kilometers from the project site. Though, USGS tectonic models estimate the eastern, down-dip seismogenic zone of the CSZ to be as close as 45 kilometers from the project area. Average recurrence intervals for such

"great" earthquakes, as determined by recent investigations, range between 300-600 years. The last great earthquake was interpreted to be approximately 300 years ago

(Goldfinger, et al., 2012). The design earthquake for the proposed structures at the project site is one originating from the CSZ. Predictive models of a M9.0 originating from the CSZ may result in ground motions at the project site that will produce strong to very strong perceived shaking and damage that is negligible in buildings of good design and construction; but slight to moderate in well-built ordinary structures and considerable damage in poorly built or badly designed structures (USGS, 2017).

6.2 GEOLOGIC HAZARDS

Flooding. The project area is outside of both the 100-year and 500-year FEMA designated floodplains for the South Umpqua River (HazVu, 2018). However, Parrott

Creek has not been evaluated by FEMA for flood mapping. Other than the previously discussed high water/flooding events from earlier in 2025, and in 2005, there is no readily available documentation concerning the frequency or severity of Parrott Creek flood events. The South Umpqua River crested at 29.5 ft and 23.8 ft for the 2025 and 2005

(respectively) flood events, with the designated flood stage at the Roseburg gauge being

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group

22 feet (Douglas County Oregon, 2025). Therefore, risk of damage at the site from flooding is considered high.

Landslides / Slope Instability. The project site is located at the valley floor, which is relatively flat to mildly sloping at 5% or less, but the valley walls are moderately steep, sloping at 10% to 25%. The Statewide Landslide Information Database for Oregon has not mapped any historic landslides, landslide deposits or scarps in the immediate vicinity of any of the project site and has mapped the site as having low to moderate landslide susceptibility, with the hillside to the southwest having a high susceptibility approximately 300 feet away from the nearest culvert replacement. (SLIDO, 2024).

Slopes in the project area are currently stable, but improper site development can lead to slope instability and introduce a risk of damage. Therefore, in our professional opinion, the risk of landslide that could impact the project site is low, while the risk of slope instability throughout construction of the project is considered moderate.

Note: Recommendations for site grading and proper methods of cut-and-fill construction have been provided in the following sections of this geotechnical report. It is essential these recommendations be followed in order to minimize slope instability during excavations for the project.

Expansive Soils. An Expansion Index test (ASTM D-4829) was conducted on one combined clayey soil sample obtained during the subsurface investigation at the project site, from 1 ft to 4 ft bgs at locations B-1 and B-3. The composite soil sample had a tested EI50 value of 86. ASTM’s classification system considers EI50 values ranging between 51-90 having medium potential for expansion. Therefore, it appears that expansive soils are present within the upper 4 to 8 feet of the subsurface across the project site. These expansive soils can cause shrink/swell related movements if exposed to alternating wetting and drying conditions and should be properly mitigated as part of the site development. Recommendations for preparation of the foundation subgrade for structures are provided in the geotechnical design sections of this report, and must be followed in order to reduce the risk of adverse impacts to the development due to expansive soil.

Liquefaction and Lateral Spread. The project is underlain by medium stiff to very stiff Clays containing minor to significant amounts of sands in the upper 11 feet. Free groundwater was encountered at 11.5’ deep on B-3 during our site exploration.

According to Oregon HazVu, the site location is not susceptible to liquefaction (HazVu, 2018). Given the soils composition and anticipated depth to groundwater, liquefaction and lateral spread is not considered to be a potential hazard at the site.

Ground Rupture. There are no active Quaternary faults that have been identified at or around the project site. The nearest active fault is located approximately 15 miles north of the project site. Therefore, the risk of damage to the development at the site due to ground rupture is considered very low.

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group

Ground Shaking. Project structures, retaining walls, and fills should be designed according to the Oregon Structural Specialty Code (OSSC; 2022). Based on our field reconnaissance and subsurface SPT data conducted at the project site, the seismic site class and the seismic design category for the project site are designated as D. The probabilistic peak horizontal ground acceleration, PGA, for an earthquake with a 7% probability of occurring in a 75-year period is 0.26g for the project site (see Section 6.3, below, for more information). The PGA value can be used with an appropriate seismic coefficient in pseudo static analysis for design of the pertinent structural components of the proposed development.

Seismic Ground Amplification or Resonance. No hazardous amplification or resonance effects from seismic waves have been associated with the soil subsurface conditions in the project area. The risk of damage at the site from unexpectedly severe shaking due to seismic wave amplification is low.

Tsunami and Seiche. The project site is located approximately 50 miles inland, and is therefore not subject to inundation from a tsunami. There are no dams or large bodies of water located above this site. Therefore, the risk of damage to the site is zero due to hazard from seiche or seismic-induced flooding.

6.3 SEISMIC DESIGN

The American Association of State Highway and Transportation Officials (AASHTO), the Federal Highway Administration (FHWA) and the Oregon Department of

Transportation (ODOT) have developed specific seismic design criteria for bridges.

AASHTO guide specifications state that seismic effects for box culverts and buried structures need not be considered, except when they are subject to unstable ground conditions (e.g., liquefaction, landslides, and fault displacements) or large ground deformations (e.g., in very soft ground). The State of Oregon, designates a single culvert span (or the sum of closely spaced culvert spans) of 20 feet or more shall be defined as a bridge and included in the National Bridge Inventory (NBI). For design purposes, ODOT processes large culverts (those with a diameter or span of 6 feet or greater) as bridges

(Oregon Department of Transportation, April 2024). It is assumed that the replacement culverts for this project will have spans greater than 6 feet, which places all culvert designs for this project under ODOT’s “large culvert” designation.

ODOT has adopted the FHWA load and resistance factor design criteria for bridges of considering the maximum potential ground motion from an earthquake with 7% probability of occurrence in 75 years. The earthquake parameters for the project area are based upon established values and methodology in AASHTO as modified by the ODOT

Bridge Design Manual (Oregon Department of Transportation, April 2024). The seismic parameters were obtained partly from the online USGS Seismic Design Web Services, per AASHTO Guide Specifications. The project site has been mapped by the State of

Oregon with a National Earthquake Hazard Reduction Program (NEHRP) seismic site class of B (Madin, Francyzk, Bauer, & Azzopardi, 2021). The project site was analyzed using the SPT values obtained at each boring location, in lieu of the average material

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group shear wave velocity in the upper 30 meters (100 ft) [Vs30] of the subsurface. Utilizing the

SPT value analyses method, the project site is also assigned a seismic site class of D.

The seismic design parameters recommended for use in this project are listed in Table 3.

Table 3: RECOMMENDED DESIGN ACCELERATION PARAMETERS

7.0 CONCLUSIONS

Based on our field investigation of the subsurface, laboratory testing and office review, the soils conditions at the site are suitable for the proposed development, provided the recommendations of our report are incorporated in the design and construction of the project.

Parrott Creek Culvert Replacement (02-6575-01)

Roseburg, Oregon Latitude: 43.195715°

Longitude: -123.348858°

Site Class (based on SPT) D

PGA, mapped horizontal peak ground acceleration 26% of g 0.2596 g

Ss, mapped short period (0.2 second) spectral acceleration 54% of g 0.5395 g

S1, mapped long period (1.0 second) spectral acceleration 25% of g 0.2466 g

Site Factor Fpga (Table 1.17.2.4-1 [ODOT], replaces AASHTO Table 3.4.2.3-1) 1.3404

Site Factor, Fa (Table 1.17.2.4-2 [ODOT], replaces AASHTO Table 3.4.2.3-1) 1.3684

Site Factor, Fv (Table 1.17.2.4-3 [ODOT], replaces AASHTO Table 3.4.2.3-2) 2.1068

As, Design Peak Ground Acceleration

(AASHTO equation 3.4.1-1) 30.2% of g 0.3479 g

Design Spectral Acceleration SDS, Short Period

(AASHTO equation 3.4.1-2) 72.6% of g 0.7383 g

Design Spectral Acceleration SD1, Long Period

(AASHTO equation 3.4.1-3) 47.8% of g 0.5195 g

T0, in seconds, for construction of design response spectrum (AASHTO equation 3.4.1-5),

0.14 s

Ts, in seconds, for construction of design response spectrum (AASHTO equation 3.4.1-6),

0.70 s

Seismic Design Category SDC (AASHTO Table 3.5-1)

SD1 ≥ 0.5

SDC = D

During the site investigation, borings encountered moderately expansive Clayey soils to depths ranging from approximately 5 feet to 11 feet below ground surface. These surficial clayey soils encountered at the site will generally provide poor bearing capacity support and could result in shrink-swell related movement if not properly mitigated.

However, the underlying gravelly Sand to sandy Gravel soils and/or weathered basalt bedrock will provide adequate support. Recommendations for removal and replacement of these soils with crushed rock backfill or mitigation of expansive soils related problems for the project are provided in the following sections of this report. We have also provided recommendations for the site preparation, excavations, cut and fill slopes, subgrade preparation and backfilling operations, conventional foundation support, retaining walls, drainage, and erosion control.

CAUTION: Expansive Soil

8.0 GEOTECHNICAL RECOMMENDATIONS

The following sections of this report provide proper site preparation and grading recommendations, subgrade preparation recommendations, and foundation support to mitigate expansive soil shrink-swell issues and reduce settlement related problems. This report also provides slope stability and retaining wall recommendations as well as general recommendations for site drainage and erosion control.

8.1 SITE PREPARATION AND GRADING

In general, the culvert replacement sites all consist of the existing roadway and fill prism soils and rock over the existing buried culverts and stream channel. The roadway shoulders and embankment fill slopes which extend down to the channel and overbank areas are typically covered with grassy vegetation and some mature trees. We understand the existing AC and base rock layers at each of the culvert replacement sites will be demolished and removed.

Subgrade preparation should also include removing all culvert backfill soils and rock layers within the extents of existing stream channel to expose the existing native creek bed and bank soils or fill subgrade soils conditions. Portions of these culvert replacement sites may also require debris removal from the existing road prism fill slopes and clearing, grubbing and stripping of organic materials from these slopes and associated stream banks, in order to construct the new culverts and stream beds and for backfilling for road prism reconstruction. All organic debris and construction debris and related items must be removed. Therefore, normal methods of clearing, grubbing, and stripping for removal of organic materials and subgrade soil preparation will apply.

8.1.1 Clearing, Grubbing and Stripping

All roadway areas proposed for the new pre-cast three-sided box culvert structures, embankment structural fill and AC pavements shall have all existing AC and/or surficial base rock removed and be cleared and grubbed of all trees, stumps, brush, and other debris and/or deleterious materials. The areas of each culvert replacement site designated for development shall then be stripped and cleared of all vegetation, sod, and organic topsoil. The stripped materials and unacceptable organic soils removed must be hauled from the site or stockpiled for use in landscape areas only (such as landscape mounds).

This material shall not be used in structural fill or trench backfill.

If encountered, abandoned utility lines, storm drains, underground tanks or other items which provide void space beneath the surface must be removed or effectively plugged.

Movement of surface and/or groundwater through these old conduits can create the potential for piping of soils (the removal of soil fines by water seeping into the void spaces or through conduits), resulting in subsidence of the surface or settlement of structures and paved areas.

Additional stripping/removal of the existing road prism fill soils will be necessary to facilitate over-excavation on both sides and below the existing culverts. Over excavation should be accomplished to remove ALL expansive soils underneath the new culvert and end wall foundations, and laterally, at least two feet from the structures. If kept relatively clean of organic and manmade debris, the existing crushed rock fill materials beneath the

AC may be stockpiled and used for site grading and backfill of the new culverts. (See

Sections 8.4.2 and 8.6 of this report for details).

Holes or depressions resulting from the removal of the existing culverts or other underground obstructions or excavations through the old fill that extend below the finish subgrade and will be beneath culvert foundations and roadways, shall be cleared of all loose material and dished to provide access for compaction equipment. These areas shall then be backfilled and compacted with approved structural fill, as described later in this report.

It is recommended that grubbing and stripping of the site, debris removal and backfill and compaction of depressions below finish subgrade be observed by the geotechnical engineer or representative from The Galli Group.

8.1.2 Subgrade Densification

After removal of all vegetation, organic soil, and deleterious materials, and when the subgrade has been cut to grade, the exposed subgrade soils throughout the site shall be redensified by numerous passes with a heavy vibratory roller.

Note: Much of the work on these culvert replacement sites will likely consist of relatively narrow (3 to 6 feet wide) trenches for strip footings, located adjacent to excavation sidewalls and/or cut slopes. Therefore, a backhoe or excavator with a “Ho-

Pac” (hydraulic vibratory plate compactor) attachment may be utilized for subgrade redensification.

This densification shall be accomplished under all areas of the site for proposed improvements, including culvert structure/foundation support, retaining walls, and new/simulated stream beds/bank areas, etc. Redensification shall be discontinued if it starts to “pump up” the subgrade. Care must be taken to not over vibrate and disturb the subgrade soils.

8.1.3 Subgrade Proofrolling

The exposed subgrade soils throughout the site which will support structures, roadways, structural fills, and other improvements shall exhibit a smooth, firm and unyielding surface. After grubbing, stripping, (over-excavation where required) and redensification, all subgrade soils shall be proofrolled with a loaded to partially loaded dump truck, water truck or large heavy roller (no vibration) under the observation of a representative from

The Galli Group. Proofrolling shall be discontinued if it appears the operation is pumping moisture up to the surface or otherwise disturbing the in-place soils. When proofrolling, a successful test is when the tires of a loaded or partially loaded truck do not deflect the soils more than ⅜ inch. When a proofroll is not feasible, then observing Hoe-

Pac compaction of the subgrade, with heavy down pressure, will be sufficient for density verification.

Note: During the wetter winter and spring months it is likely that the subgrade of most foundation areas located within or near the stream bed and banks will become wet and will be easily disturbed under construction traffic. This could cause significant trouble in attaining compaction and will cause disturbance during proofrolling. We strongly recommend our representatives observe this overexcavation and proofrolling and terminate the proofrolling if this is damaging the subgrade.

Where subgrade soils are disturbed or do not demonstrate a firm, unyielding condition when proofrolled, the soil shall be redensified, or aerated and redensified, or replaced with imported granular fill. The imported fill material shall be compacted to a minimum of 95 percent of the maximum dry density as determined by ASTM Test Method D-698

(Standard Proctor). All soft and/or unstable areas must be over-excavated and backfilled with granular structural fill.

Note: This proofrolling and subgrade verification shall be accomplished on the exposed subgrade of over-excavated areas as well as the layers of structural fill and the finish subgrade surfaces.

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group

8.1.4 Dewatering

As noted earlier in the report, groundwater was encountered at one of the culvert replacement sites (B-3, SE Giles St). The groundwater level was approximately 4.5 feet beneath the surface water level in Parrott Creek, at the time of our field investigation.

Groundwater was not encountered at the SE Ichabod and SE Kane Street locations due to encountering very dense conditions shallower than the height of the streambanks (~ 12 feet). Given the locations of these sites and the nature of the project, it is likely that groundwater levels will rise to excavation subgrade levels, or that surface water could flow into the footing excavations, if left unprotected.

Dewatering efforts can be minimized with effective surface water diversion. If construction is performed in the dryer months, surface water diversion and dewatering efforts will be minimized. Dewatering can likely be achieved by installing a series of sumps around the perimeter and throughout interior portions of the excavations.

Alternatively, or additionally, cut off trenches or French drains could be installed around the footing excavation locations. This way, groundwater can be intercepted by these drains then redirected downstream of the excavations. The contractor is responsible for removing water from excavations and providing a stable subgrade prior to construction.

8.2 UTILITY AND SITE EXCAVATIONS

During the construction of the project, we anticipate excavations will be required for construction of foundations and for drainage/utility installations. Excavations will encounter the existing roadway fill and underlying native, stiff, sandy/silty clays. Deeper excavations will encounter medium dense to very dense, sandy Gravel, with some silt and clay. It is possible excavations will encounter weathered basaltic bedrock, which is assumed to be in rippable condition.

Excavations. Medium to large excavators should have no difficulty in removing the existing roadway, fill, and native soils materials, and excavating trenches into the native soils across most of the project sites to between 10 and 12 feet bgs. However, there is possibility of encountering marginally rippable, weathered bedrock. If this occurs, large excavators, and possible breaker attachments, could be necessary to excavate to the required depths for construction of the project.

Expansive soils are present at the project site and will require removal and replacement to mitigate shrink-swell related issues for the new culvert structures. Over excavation and removal of all expansive clayey soils must be accomplished under the culvert structures, and a minimum of two feet, laterally, beyond the vertical culvert walls.

Trench excavations during dry weather should stand in shallow trenches (less than 4 feet). Surface water runoff, seepage or wet weather and long-term dry weather may cause these upper soils to slough into the trench. Deeper trenches or excavations are required at surface water levels for the project; care must be taken to divert surface water flows from these excavations. Encountering the groundwater table and wet to saturated soils conditions are very likely. These excavations will require construction dewatering

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group of groundwater as well as some form of shoring measures (if vertical cut walls are greater than 4 feet) or flatter temporary cut slopes to perform construction operations effectively and safely at these deeper locations. It is essentially impossible to adequately place and compact structural fill if there is standing water in the excavation. Dewatering the foundation excavations until they are properly backfilled is necessary.

Note: Some excavations may encounter shallow volcanic rock or very large boulders.

The rippability of the underlying rock is unknown. Excavations which encounter rock or boulders may require the use of large excavators outfitted with narrow buckets and rock teeth. Drilling, hammering, or blasting may also be necessary.

8.3 CUTS AND FILLS

Permanent cuts and fills will be likely required for the proposed development at this site.

Temporary cuts and fills will be necessary during the process of removing and replacing the existing culverts and the onsite fill materials. These cuts and fills must be constructed at proper inclinations and be of the recommended materials to remain stable.

8.3.1 Temporary Cut Slopes

During dry weather, temporary cut slopes may be cut at 1.5H:1V or flatter in the upper clayey soils and the underlying sandy gravel. Temporary cut slopes in weathered volcanic rock may also be cut at 1.0H:1V. During wet weather, the contractor must be prepared to flatten temporary cut slopes to 2.0H:1V in soil. Temporary flattening of cut slopes in weathered volcanic rock is not necessary. Some minor sloughing of the face may occur.

Note: Please note, that while we have commented on the anticipated stability of the soil in trenches and cuts, we are not responsible for job site safety. The contractor is at all times responsible for job site safety, including excavation safety. We recommend all local, state, and federal safety regulations be adhered to.

8.3.2 Permanent Cut Slopes

Permanent cut slopes are possible on this project. All permanent cut slopes into the native materials shall be excavated at the following inclinations:

Native sandy/silty Clay 2.5H:1V

Native sandy Gravel 2.5H:1V

Fractured/weathered rock 1.5H:1V

Note: It is very important that surface runoff from above the cut slopes not be allowed to collect and run onto the cut slopes. Such heavy saturation from runoff can cause added surface sloughing of these areas and must be prevented.

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group

8.3.3 Fill Slopes

Permanent fill slope inclinations shall be constructed as follows:

Imported Angular Crushed Rock 1.75H:1V

Angular Clean Jaw Run Shale 2.0H:1V

Sandy Decomposed Granite 2.0H:1V

Not Recommended as Fill Under Structures

Native sandy/silty Clay 2.5H:1V

Native gravelly Sand/Sandy Gravel 2.5H:1V

All such fills shall be placed and compacted as Structural Fill as described below in

Section 8.4 of this report. In order to decrease surface sloughing and erosion (except crushed rock fill slopes), the outside edges of all fill areas must be overbuilt and then cut back to a compacted fill face. Loose soils on the face of the fill slopes must not be allowed.

8.3.4 Fill on Steep Slopes

All fills placed on existing site slopes steeper than 10% must incorporate additional precautionary measures. It is critical, in order to decrease long-term settlements beneath all fill areas and to maintain adequate strength/stability, that these fills be placed and compacted properly.

We recommend the following for the construction of all embankment fills placed on existing steep slopes at the site:

Key Trench. The toe of all fills placed on slopes must be keyed into the slope by use of a key trench. The depth of embedment should be 2 feet into densified native soils for fill slopes up to 8 feet high, 3 feet for fill slopes up to 15 feet high and 4 feet for fills over 15 feet high. The key trench should run parallel to contour lines, be wide enough to accommodate excavation and compaction equipment (9 to 10 feet minimum), and have the base flat or sloped back into the hillside somewhat. The key trench shall adhere to redensification and proofrolling recommendations prior to placement of structural fill.

Benching. The underlying native slope should be cut into flat benches (into undisturbed native soil) back up the slope above the key trench, prior to placement of the fill slope.

These benches should run parallel to the contour lines and be flat or tipped back slightly into the hillside.

Drainage. All noticeable seepage or wet zones observed during the keying and benching excavation process (or other areas which may seep during wet weather) should be provided with subdrains. At the discretion of the geotechnical engineer, at a minimum, the key trench and upslope end bench may require a subdrain section. Where wet conditions exist, the other benches may also require subdrain sections to remove subsurface flow from behind the new fill. Please note that fills placed on slopes typically

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group have a much lower lateral permeability than the native soils. Therefore, seepage through the native soil can become trapped behind these fills causing fill slope stability problems, if proper subdrains are not installed. We recommend that our engineer observe all keys and benches during construction to verify where subdrains should be installed.

Refer to Figure 4 - Fill on Steep Slope Cross-Section, for a graphic representation and more details regarding these recommendations.

Note: Compaction of the fill being placed, even for landscape purposes as “waste” soils, is critical to its stability and to the stability of adjoining areas. Our personnel must inspect and verify the key and bench cuts, the drainage installation (if needed) and all fill placement and compaction that will support (vertically or laterally) any portion of the structures, or that are on slopes greater than 10%.

8.4 FILL PLACEMENT AND COMPACTION

8.4.1 Beneath Structures and Roadways

Structural fill is defined as any fill, placed and compacted to specified densities, used in areas that will be under structures, pavements, sidewalks, and other load-bearing areas.

Structural Fill Materials. Ideally, and particularly for wet weather construction, structural fill must consist of a free-draining, angular, granular material (non-expansive) with a maximum particle size of six inches. The material should be reasonably well-graded with less than 5 percent fines (silt and clay size passing the No. 200 mesh sieve).

During dry weather, any organic-free, non-expansive, compactable granular material, meeting the maximum size criteria, is acceptable for this purpose. Locally available crushed rock and jaw-run crushed "shale" have performed adequately for most applications of structural fill. The on-site crushed rock fill materials which are reasonably clear of organics and debris may be utilized as structural fill for certain applications for most areas beneath structural fill sections. See Section 8.4.2 for on-site undocumented fill reuse requirements and see Section 9.0 for structural fill materials specifications.

Note: It is the contractor's responsibility to understand the impending weather and plan for use of structural fill that will be capable of being compacted properly and remain stable under the expected construction traffic in all weather that could arise during the project construction.

Structural Fill Placement. Structural fill shall be placed in horizontal lifts not exceeding 8 inches loose thickness (less, if necessary, to obtain proper compaction) for heavy compaction equipment and four inches or less for light and hand-operated equipment. Each lift must be compacted to a minimum of 95 percent of the maximum dry density, unless otherwise specified, as determined by the Standard Proctor test, (ASTM D698/AASHTO T99). Mechanical means must be used for compaction of structural fill; compaction by "jetting" or water settling will not be allowed.

Beneath Footings. When structural fill is used beneath footings or other structural elements it must extend beyond all sides of such elements a distance equal to at least the total depth of the structural fill beneath the structural element in question for vertical support (i.e., for 2 feet of structural fill beneath footings, extend the fill at least 2 feet past all edges of the footing).

To facilitate the earthwork and compaction process, the earthwork contractor must place and compact fill materials at or slightly above their optimum moisture content. If fill soils are on the wet side of optimum, they can be dried by continuous windrowing and aeration or by intermixing lime or Portland Cement to absorb excess moisture and improve soil properties. If soils become dry during the summer months, a water truck must be available to help keep the moisture content at or near optimum during compaction operations.

Fill Placement Observation and Testing Methods. The required construction monitoring of the structural fill utilizing standard nuclear density gauge testing and standard laboratory compaction curves (ASTM D-698 specified) is not applicable to larger jaw run shale (2” or above) or larger crushed rock. The high percentage of rock particles greater than ¾’s of an inch in these materials causes laboratory and field density test results to be erratic and does not provide an adequate representation of the density achieved. Therefore, construction specifications for this type of material typically specify method of placement and compaction coupled with visual observation during the placement and compaction operations.

For these larger rock materials, we recommend the 8-inch lift be compacted by a minimum of 3 passes with a heavy vibratory roller. One “pass” is defined as the roller moving across an area once in both directions. The placement and compaction shall be observed by our representative. After compaction as specified above is completed, the entire area shall be proofrolled with a loaded dump truck to verify density has been achieved. All areas which exhibit movement or compression of the rock material under proofrolling shall be reworked or removed and replaced as specified above. When a proofroll is not feasible, such as within narrow strip footing excavations, then observing the Hoe-Pac work the top of the lift, with heavy down pressure, will be sufficient for compaction verification.

Nuclear Density Testing of Structural Fill. Field density testing by “nuclear” methods would be adequate for verifying compaction of 2-inch to ¾-inch minus crushed base rock, Decomposed Granite, and other materials 2 inches or smaller in size. Therefore, typical specifications would suffice. Testing shall be accomplished in a systematic manner on all lifts as they are placed. Testing only the upper lifts is not adequate.

8.4.2 On-Site Fill Re-Use

Acceptable structural fill must have a diameter not exceeding 6-8 inches, in order for adequate compaction to be achieved when using typical medium to large, single-drum

6575rpt Culvert Geotech Design- Parrot Creek Roseburg The Galli Group vibratory rollers for compaction of 8 to 10-inch, loosely placed lifts. Any on-site soil considered for re-use as structural fill must meet the material specifications outlined in

Section 9.0. Materials that do not meet these criteria may not be used as structural fill, but may be utilized as non-structural fill for the project. Large rock pieces must be separated and removed from any acceptable on-site material fill prior to reuse as structural fill. These large rock pieces may be utilized in the stream bank/bed/channel reconstruction. Alternatively, oversize pieces can be broken down into smaller sized particles to meet the maximum acceptable size, after which these materials can be placed and compacted as structural fill.

8.4.3 Non-Structural Fill

Any waste…