Exhibits A to G revised Aug 25 KO Approved.pdf

Sacramento District 1325 J street, Sacramento, CA, 95814

Exhibit A: Site Location

Site Location

Sacramento, CA, 95814

Exhibit B: Project Area Site Map

L2PDRBCJ

Rectangle

Sacramento, CA, 95814

Exhibit C: Site Access

Glendora Ridge Entry Gate

Access Road: Paved

Project Area: Refer to Exhibits D and E

Washout Location

L2PDRBCJ

Underline

Sacramento, CA, 95814

Exhibit D: Haul Route

Processing/Staging Area:

Refer to Exhibit E

Haul Route: Paved

Haul Route: Unpaved

Sacramento, CA, 95814

Exhibit E: Processing Area & Staging

Decking Area:

50 feet x 85 feet

Chip Broadcast Area:

.7 Acres

Contractor Equipment Storage: .1 Acres

Exhibit F: Washout Area Photographs and Draft Forest Service Geotechnical Report

Sacramento District

Sacramento, CA, 95814

TANBARK FLAT

ARBORIST TREE INVENTORY

December 20, 2024

Prepared for:

U.S. Army Corps of Engineers

Contracting Division 1325 J Street

Sacramento, CA 95814

Contract No. W9123824P0057

Prepared by:

1547 Los Angeles Avenue, Ste. 110 Ventura, CA 93004

(805) 535-4448 www.wildscaperestoration.com

Exhibit G

June 5, 2026

D

Big Dalton Road Landslide Repair Project Agreement Number: 26-ENG-PSWx-3857 Big Dalton Road (NFSR #1N14.1), MP 0.1 Pacific Southwest Research Station San Dimas Experimental Forest Glendora Area, Los Angeles County, California

Prepared by:

USDA Forest Service Enterprise Program Engineering Group

Prepared for:

USDA Forest Service Pacific Southwest Research Station

PRELIMINARY GEOTECHNICAL AND ENGINEERING

REPORT

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

Mission Statement

The mission of the U.S. Forest Service is to sustain the health, diversity, and productivity of the Nation’s forests and grasslands to meet the needs of present and future generations.

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

1. INTRODUCTION

1.1. OVERVIEW

1.2. PURPOSE

2. SITE CONDITIONS

2.1. GEOGRAPHIC AND TOPOGRAPHIC SETTING

2.2. SOILS, GEOLOGY AND GEOMORPHOLOGY

2.3. SURFACE CONDITIONS

2.4. SUBSURFACE CONDITIONS

3. PRELIMINARY GEOTECHNICAL CONCLUSIONS AND RECOMMENDATIONS

3.1. PRELIMINARY GEOTECHNICAL ASSESSMENT OF THE BIG DALTON ROAD LANDSLIDE

3.2. GEOTECHNICAL RECOMMENDATIONS FOR ROAD REPAIR ALTERNATIVES

4. LIMITATIONS

APPENDIX A – SITE MAPS

FIGURE A-1 – VICINITY MAP

FIGURE A-2 – SITE MAP

APPENDIX B – GEOMORPHIC MAPS

FIGURE B-1 – NRCS WEB SOIL SURVEY MAP

FIGURE B-2 – USGS GEOLOGIC MAP

FIGURE B-3 – LIDAR HILLSHADE IMAGE

FIGURE B-4 – 1952 AERIAL PHOTOGRAPH

FIGURE B-5 – 1966 AERIAL PHOTOGRAPH

FIGURE B-6 – 1980 AERIAL PHOTOGRAPH

FIGURE B-7 – 2006 AERIAL PHOTOGRAPH

FIGURE B-8 – 2025 AERIAL PHOTOGRAPH

APPENDIX C – WILLIAMSON DRIVE PROBE LOGS

APPENDIX D – PRELIMINARY DESIGN ALTERNATIVES

FIGURE D-1 – ALT 1 PLAN SKETCH: ROCK FILL

FIGURE D-2 – FIELD-DEVELOPED CROSS-SECTION AND ALT 1 PROFILE SKETCH: ROCK FILL

FIGURE D-3 – ALT 2 PLAN SKETCH: 1H:1V REINFORCED SOIL SLOPE (RSS)

FIGURE D-4 – ALT 2 PROFILE SKETCH: 1H:1V REINFORCED SOIL SLOPE (RSS)

FIGURE D-5 – ALT 3 PLAN SKETCH: MECHANICALLY STABILIZED EARTH WALL (MSEW)

FIGURE D-6 – ALT 3 PROFILE SKETCH: MECHANICALLY STABILIZED EARTH WALL (MSEW)

APPENDIX E – PRELIMINARY COST ESTIMATES

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

1. INTRODUCTION

1.1. Overview

This Preliminary Geotechnical and Engineering Report summarizes the US Forest Service (USFS) Enterprise Program’s (EP’s) preliminary geotechnical evaluation and provides design alternatives for repair of the Big Dalton Road Landslide. The Big Dalton Road Landslide is located along Big Dalton Road (NFSR #1N14.1) at about Milepost 0.1, on the San Dimas Experimental Forest, near Glendora in Los Angeles County, California (approximate latitude/longitude: 34.20775, -117.77307). Although the San Dimas Experimental Forest is operated by the Pacific Southwest Research Station, Big Dalton Road is managed and maintained by the Angeles National Forest; the Pacific Southwest Research station and the Angeles National Forest are jointly referred to as the Partner in this report. The location of the site relative to nearby geographic features is shown in the Vicinity Map, Figure A-1, in Appendix A.

Based on information provided by the Partner, the Big Dalton Road Landslide occurred during a precipitation event in early 2025, after a wildfire burned the area in October 2024. The Big Dalton Road Landslide originated from the downslope edge of Big Dalton Road; the landslide currently narrows the safe travelway and threatens to further damage the road. It is understood that Big Dalton Road provides access to the Tanbark Flats Administrative Site, an important facility on the San Dimas Experimental Forest. The facility is currently planned to be renovated within the next several years, necessitating the use of Big Dalton Road for construction vehicles. Big Dalton Road also provides access for wildfire response. It is understood that the design vehicle for this repair is a standard dump truck and/or a Type 3 wildland firefighting truck.

1.2. Purpose

This preliminary evaluation was completed to provide preliminary geologic/geotechnical information based on review of available information, surface reconnaissance, and limited subsurface investigation, in order to provide geotechnical recommendations, design alternatives and preliminary cost estimates for repair of Big Dalton Road at the landslide site, to support alternative selection by the Partner. After alternative selection by the Partner, a second phase of this project (under a separate project charter) could include additional geotechnical evaluation (if needed), additional geotechnical recommendations specific to the selected alternative, Unmanned Aerial System (UAS) and/or traditional topographic survey, and/or development of a contract package which may include plans, specifications, and an independent government cost estimate (IGCE).

2. SITE CONDITIONS

2.1. Geographic and Topographic Setting

The Big Dalton Road Landslide is located on a steeply-sloping (70- to 100-plus percent grade), convergent drainage at the head of San Dimas Canyon, about 7 miles northeast of Glendora, in the San Gabriel Mountains in southern California. The site is located at about Elevation 3,335 feet. The general topographic layout of the site is shown on the Site Map, Figure A-2, in Appendix A. As shown on Figure A-2, 5-foot topographic contours were processed from raw lidar data available from the US Geological Survey (USGS, https://apps.nationalmap.gov/lidar-explorer/#/, CA LosAngeles 1 B23 acquisition, collected January 7, 2023 – January 4, 2024) using Esri ArcGIS Pro.

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

2.2. Soils, Geology and Geomorphology

2.2.1. NRCS Web Soil Survey

The National Resource Conservation Service (NRCS) Web Soil Survey (https://websoilsurvey.nrcs.usda.

gov/app/WebSoilSurvey.aspx) maps the project site as underlain by Trigo, Granitic Substratum- Exchequer families-Rock outcrop complex (Map Unit: 35), consisting of variable depths of loam, sandy loam and gravelly sandy loam, underlain by weathered grading to unweathered bedrock. The NRCS Web Soil Survey at the project site is presented in Figure B-1, in Appendix B.

2.2.2. Geologic Setting

The USGS (2006, Geologic Map of the San Bernardino and Santa Ana 30’ x 60’ Quadrangles, California, Open-File Report 2006-1217, Sheet 1 of 4) maps the project site as underlain by Proterozoic-age (about

2.5 billion to 541 million years old) Layered Gneiss (Map Unit: gn) metamorphic rocks, typically consisting of thinly and thickly layered, fine-grained gneiss. Geologic units consisting of granite, granodiorite, and quartz monzonite are also mapped in the area. The regional geologic map (USGS, 2006) at the project site is presented in Figure B-2, in Appendix B.

This area of southern California is seismically active. Based on mapping by the USGS (2006, Sheet 2 of 4) the San Dimas Canyon Fault and the San Gabriel Fault Zone are located within about 2 miles of the site, to the west and north, respectively. The San Andreas Fault is located about 20 miles northeast of the site. Based on review of data from the USGS Unified Hazard Tool (https://earthquake.usgs.gov/hazards/ interactive/), the largest contributor to seismic hazard at the Big Dalton Road Landslide is a large (up to about magnitude 8) earthquake from the San Andreas Fault about 20 miles away. Significant seismic hazard also comes from local (less than about 5 miles away), moderately large (up to about magnitude 7.5) earthquakes on the Sierra Madre, San Gabriel, Cucamonga, and Clamshell-Sawpit Faults.

2.2.3. Lidar Bare-Earth Hillshade Imagery Review

Lidar-based bare-earth hillshade imagery was processed from raw lidar data available from the USGS (https://apps.nationalmap.gov/lidar-explorer/#/, CA LosAngeles 1 B23 acquisition, collected January 7, 2023 through January 4, 2024) using Esri ArcGIS Pro. The project site is located in a steep, relatively smooth, sharply convergent drainage near a ridge crest. Other drainages in the area are similarly steep and convergent, though some contain sharp, undulating topography indicating near-surface and exposed bedrock, and/or historic erosion in local areas. Big Dalton Road is distinctly visible traversing the hillside and crossing the subject drainage, along with crossing other drainages. A series of primitive paths are also observed traversing the hillsides downslope of Big Dalton Road. Based on discussions with the Partner, it is understood that these paths were historically used by the Pacific Southwest Research Station to measure streamflow in drainages at specific elevations for research purposes.

In the drainage northeast of the project site, several erosional gullies are visible downslope of Big Dalton Road. Based on discussions with the Partner, it is understood that these erosional gullies are artificial, having formed during hydrology experiments previously completed by the Pacific Southwest Research Station. Otherwise, as a generality, we did not observe evidence of widespread landsliding in the vicinity of the project site, though evidence of sidecast fill sloughing can be occasionally observed elsewhere along area roads. The lidar-based bare-earth hillshade image is shown in Figure B-3, in Appendix B.

2.2.4. Historical Aerial Photograph Review

As part of this evaluation, we reviewed historical aerial photographs from USGS EarthExplorer (https://earthexplorer.usgs.gov/) dated 1952, 1966, 1972, and 1980, and from Google Earth (various dates between 1994 and 2025). Big Dalton Road is visible in all aerial photographs in the set, indicating

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026 that it was constructed prior to 1952. The primitive paths mentioned in Section 2.2.3 are distinctly visible in 1952, becoming increasingly obscured by vegetation through the photograph set. The area of erosion in the drainage northeast of the site is visible as a bare area in the 1966 aerial photograph, becoming obscured by vegetation by 1972. Several bare areas suggesting past sidecast fill sloughs from Big Dalton Road are visible in the 1966 aerial photograph. Two bare areas in the drainage northeast of the site are visible in the 2006 aerial photograph, indicating natural (as opposed to sidecast fill related) landslides. The Big Dalton Road Landslide and the nearby wildfire burn scar are visible in the 2025 aerial photograph. The 1966, 1980, 2006 and 2025 aerial photographs are shown in Figures B-4 through B-7, in Appendix B.

2.3. Surface Conditions

2.3.1. General

We completed a geologic surface reconnaissance in the vicinity of the project site on January 27, 2026.

The weather at the time of our site visit was sunny with temperatures in the 40s and 50s. The weather preceding our site visit was seasonably warm and dry. The area surrounding the project site is characterized by moderately steep to very steep (about 70- to 100-plus percent grade) sharply undulating hillsides at the head of San Dimas Canyon. The hillsides are typically vegetated with dense low-lying brush. Big Dalton Road traverses across a series of drainage headwalls in the vicinity of the project site. Big Dalton Road varies between about 10 feet wide and 14 feet wide in the site area, contains numerous sharp curves, and is surfaced with asphalt in generally poor condition.

As a generality, the drainage headwalls surrounding the project site were dry at the time of our site visit, with no evidence of seasonal flow (no channels, evidence of groundwater seepage, etc.) at the elevation that Big Dalton Road crosses, though Tanbark Creek and other streams downslope of the site were flowing at the time of our site visit. Other than the Big Dalton Road Landslide described below, we did not observe evidence of adverse erosion or slope instability on the hillsides near the project site.

2.3.2. Big Dalton Road Landslide

The Big Dalton Road Landslide is characterized by an approximately 15-foot-wide, 30-foot-long, 6-foot-deep landslide scarp affecting the downslope (east) shoulder of Big Dalton Road. Candlestick-style construction cones were in place along the edge of the landslide to warn traffic of the potentially unstable and abrupt lane edge at the time of our site visit. Although the landslide has not yet affected/damaged the road prism, the precipitous drop at the road’s edge combined with the sharp inside curve in the road at the landslide has reduced the width of the safe travelway of the road; the road was passable by pickup trucks, but larger vehicles would be expected to struggle with the curve radius and safe travelway width.

The landslide scarp exposes sidecast fill related to original road construction, consisting of sand with silt and angular rock fragments, in a loose to medium dense condition. Downslope of the scarp, an approximately 1- to 2-foot-deep, 2- to 3-foot-wide scoured channel was observed extending about 70 feet further downslope, which gradually transitions to the pre-landslide condition, though angular rock fragments were observed at the surface at least 300 feet downslope of the scarp suggesting the landslide may have “ran out” (scoured and/or deposited) at least this far downslope. A photograph of the landslide headscarp and runout area, as observed from the road edge, is presented in Figure 1.

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

Figure 1: Photograph of the landslide headscarp and runout area, as observed from the road edge

As part of this evaluation, we developed a topographic cross-section using a clinometer and a measuring tape through the approximate center of the landslide, to aid in our evaluation and in our development preliminary design alternatives. The topographic cross-section is shown in Figure D-2, in Appendix D.

2.3.3. Bedrock Outcrop Mapping

Depending on the design alternative selected, engineering properties of bedrock (weathering grade, rock strength/hardness, fracture orientation/character, etc.) may be an important component in designing the repair for the Big Dalton Road Landslide. As such, we completed bedrock outcrop mapping in the vicinity of the project, in general accordance with the Federal Highways Administration (FHWA, December 2006) Soils and Foundations Reference Manual (Publication No. FHWA NHI-06-088) and the International Society of Rock Mechanics (ISRM, October 1977) Suggested Methods for the Quantitative Description of Discontinuities in Rock Masses. Depending on the design alternative selected, additional geotechnical investigation (such as soil borings and/or rock corings) may be needed, which may further refine characterization of the bedrock; thus, descriptions provided herein may be subject to change.

Bedrock outcrops in the area typically expose slightly weathered to unweathered (highly weathered in one location), strong to very strong (weak in one location) granite and granodiorite bedrock. Fractures were typically very closely to closely spaced, in poor to fair condition, and blocky/randomly oriented, though a preferential fracture set dipping about 50° to 80° generally easterly was observed in multiple outcrops. Additional details regarding rock mass conditions will be provided as needed as the design progresses.

2.4. Subsurface Conditions

We completed limited subsurface investigation at the project site on January 27, 2026, consisting of three Williamson Drive Probe (WDP) tests (referred to as WDP-1, WDP-2, and WDP-3 in this report) to depths of about 2 to 10 feet. WDP tests include driving a steel pipe into the ground with a hammer and recording blow counts which can be correlated to Standard Penetration Test (SPT) N-value, but does not include soil sampling so soil types typically cannot be readily determined from WDP tests.

Locations of the WDP tests, approximated using a hand-held GPS unit and by measuring from nearby physical features, are shown on Figure A-2. A brief description of the relative densities/consistencies encountered are given below. The WDP logs are presented in Appendix C. Elevations listed in the logs were approximated using LiDAR-based topographic data.

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

WDP-1 – WDP-1 encountered about 8 feet of loose/medium stiff soil (expected to be sidecast fill), underlain by about 2 feet of medium dense/very stiff soil (expected to be residual soil), before refusing (presumably on a cobble) at about 10 feet deep. Based on our observation of dry driving pipe, it is not believed that the WDP test encountered groundwater.

WDP-2 – WDP-2 encountered about 6 inches of loose/stiff soil (expected to be disturbed soil in the road shoulder), underlain by medium dense/very stiff soil (expected to be residual soil), before refusing (presumably on a cobble) at about 2 feet deep. Based on our observation of dry driving pipe, it is not believed that the WDP test encountered groundwater.

WDP-3 – WDP-3 encountered about 3½ feet of loose/medium stiff soil (expected to be sidecast fill), underlain by about 1 foot of medium dense/stiff soil (expected to be sidecast fill), underlain by dense to very dense/hard soil (expected to be residual soil), before refusing (presumably on a cobble) at about

5.7 feet deep. Based on our observation of dry driving pipe, it is not believed that the WDP test encountered groundwater.

3. PRELIMINARY GEOTECHNICAL CONCLUSIONS AND RECOMMENDATIONS

3.1. Preliminary Geotechnical Assessment of the Big Dalton Road Landslide

3.1.1. Contribution of Rainfall as a Landslide Trigger

Based on discussions with the Partner, the Big Dalton Road Landslide occurred during a heavy precipitation event in early 2025. Heavy rainfall can trigger landslides, either by severe surface erosion developing into a shallow landslide, or by an increase in porewater pressure in the subsurface soil that reduces its shear strength. Based on our evaluation and our experience and judgement, it is likely that prolonged heavy rainfall was the primary trigger of the Big Dalton Road Landslide.

3.1.2. Contribution of Altered Groundwater/Surface Water Regime to Landslide Susceptibility

As previously described, it is understood that the area surrounding the Big Dalton Road Landslide burned in a wildfire in fall of 2024. Wildfires can alter the groundwater regime, by reducing the amount of vegetation interception and transpiration; water that would previously fallen on vegetation instead potentially infiltrates and enters the earth as groundwater. Wildfires also alter the surface water regime, again due to the loss of vegetation interception and transpiration, as well as by creating water-repellant soils increasing runoff. Further, wildfires can leave debris that can be entrained in surface water, potentially resulting in increased erosion and debris flows. Based on our evaluation and our experience and judgement, it is likely that this mechanism contributed to the Big Dalton Road Landslide.

3.1.3. Contribution of Road Construction to Landslide Susceptibility

Original road construction practices may have additionally contributed to the Big Dalton Road Landslide.

Placement of over-steepened sidecast fill, potentially atop loose soil and/or organic debris, without proper compaction or with unsuitable fill materials during original road construction may have overloaded underlying soils, provided a flow path for groundwater, created voids in the soil as organic materials rots away, and provided a path for slope failure to propagate. Based on discussions with the Partner, other areas of sloughing side cast fill have been observed along Big Dalton Road in the past, further suggesting that this mechanism is indeed a contributing factor to the Big Dalton Road Landslide.

Improper surface water management may also be a contributing factor. It was observed that the road was insloped upgradient/northeast of the Big Dalton Road Landslide, and transitioned to outsloped near the landslide. It is likely that surface water drained over the Big Dalton Road landslide during the heavy rains that may have triggered the event, as described above.

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

3.2. Geotechnical Recommendations for Road Repair Alternatives

3.2.1. General

Three conceptual design alternatives for repair of Big Dalton Road in the vicinity of the landslide site are discussed in the following sections. Other options were considered, such as shifting the road into the cut slope; it was determined that this solution would result in an unsafe curve and is infeasible. Preliminary plan and profile sketches are presented in Appendix D. Preliminary cost estimates were compiled for each design alternative, as presented in Appendix E.

Based on our site observations and review of available information, all three alternatives are preliminarily assumed to be geotechnically feasible, provided that the preliminary geotechnical recommendations presented in this section are implemented. As discussed below, depending on the design alternative selected, additional geotechnical information may need to be collected to support full design. It is possible that geotechnical recommendations may change as new information is available.

Regardless of the alternative selected, Big Dalton Road will need to be shifted toward the fill/downslope side at the landslide site to create a more passable and safer curve for the design vehicle. For the purposes of this preliminary evaluation, it was expected that the inside edge of the road would need to shift 3 feet toward the fill/downslope side; the amount of shift needed could be further refined in future project phases.

The following geotechnical recommendations apply to all three design alternatives listed in the following sections.

Surface water should be properly managed to minimize dispersion of water into the landslide site.

Based on our preliminary evaluation, we expect surface water management to include insloping the road to a ditch, and a rolling dip on Big Dalton Road uphill/north of the landslide site.

It is understood that the ability to maintain the road is a priority for the Partner. For this reason, it is recommended to surface Big Dalton Road in the repair area with gravel, rather than replacing the asphalt surfacing.

Fill placed onto existing slopes steeper than 4H:1V should be “keyed” into place by constructing minimum 4-foot-wide horizontal benches. Fill subgrades should be adequately cleared and grubbed.

Fill should be compacted in multiple lifts to 95 percent of the maximum dry density according to AASHTO T-99, or as otherwise directed by the eventual geotechnical designer for final design.

Vegetation removal/earthwork should be limited to what is needed for the repair and construction.

Safe inclination of temporary cut slopes is entirely dependent on the nature of the material encountered. For project planning and budgeting purposes, it can be assumed that temporary cut slopes up to 4-feet high can be made near-vertical; temporary cut slopes above 4-feet high can be planned at 1H:1V. Temporary cut slope height and inclination must abide by Occupational Safety and Health Administration requirements, along with any local or state requirements, and is ultimately the responsibility of the contractor since the contractor is on site full time and can observe performance of temporary cut slopes.

3.2.2. Alternative 1: Repair the Road with an Unreinforced Rock Fill

Alternative 1 includes repairing Big Dalton Road with an unreinforced rock fill. The unreinforced rock fill would need to be inclined no steeper than 1.5H:1V (horizontal to vertical); since the hillside downslope of the Big Dalton Road is inclined steeply (70-plus percent grade, gradually dropping to about 40 percent grade), the volume of rock fill material needed would be significant. Preliminary plan view and profile sketches are provided in Figures D-1 and D-2, in Appendix D. Preliminary geotechnical recommendations

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026 for the unreinforced rock fill alternative are provided below.

Rock fill should be constructed of relatively clean, coarse, angular rock fragments (3- to 6-inch rock or larger). Unreinforced fill consisting of finer materials should be inclined no steeper than 2H:1V.

If horizontal benches described in section 3.2.1 of this report are impractical along the length of the rock fill, at the very least, a shear key (a trench filled with rock) will be needed at the toe of the rock fill; details regarding the geometry of this shear key could be provided in a future project phase.

3.2.3. Alternative 2: Repair the Road with a Reinforced Soil Slope (RSS)

Alternative 2 includes repairing Big Dalton Road with a Reinforced Soil Slope (RSS). RSS systems consist of compacted fill embankments that incorporate the use of horizontal placement of geosynthetic reinforcement, at an inclination that is steeper than would be stable without reinforcement. Typically, RSS systems are used up to inclinations of about 1H:1V. Preliminary geotechnical recommendations for an RSS are provided below, for project planning and budgeting purposes.

Subsurface exploration consisting of geotechnical borings, with 10-feet of rock coring below the bedrock interface, should be implemented prior to final design of an RSS, to evaluate geotechnical design parameters and global stability. Details of the subsurface exploration program should be specified by (or at least collaboratively agreed upon with) the eventual designer. For project planning and budgeting purposes, it can be assumed that two (2) borings to 30-feet may be needed.

Assorted soil/rock laboratory tests will also be needed.

Alternatively, detailed geologic surface mapping, hand-dug test holes and hand-driven soil testing could be completed in conjunction with careful geotechnical observation of actual site conditions during construction, but the Partner must accept increased risk of minor to moderate changes at the time of construction, and some amount of increased risk of slope instability/RSS failure.

Detailed topographic survey of the RSS area should be completed prior to final design of the RSS. For project planning and budgeting purposes, based on site observations and lidar topography, it is assumed that the RSS would have to be about 25-feet high and 60 feet long.

Final geotechnical recommendations for the RSS will be provided at a later date by the eventual RSS designer. For project planning and budgeting purposes, it is expected that a 2-foot-wide horizontal bench will be needed in front of the toe of the RSS for embedment and bearing capacity requirements, and to enhance global slope stability.

Final geotechnical recommendations for the RSS will be provided at a later date by the eventual RSS designer. The RSS should be placed on undisturbed medium dense/stiff or better subgrade soils. The exposed RSS subgrade should be proofrolled in dry weather or probed in wet weather to evaluate areas of soft, loose, organic or otherwise unsuitable subgrade materials. If soft or loose soil conditions are encountered during proofroll/probing, the subgrade should be compacted (if compactible); if not compactible, additional mitigations may be needed (such as over-excavation and replacement, placement of reinforcing geosynthetic, etc.).

Final geotechnical recommendations for the RSS will be provided at a later date by the eventual RSS designer. The existing soils/rock will likely need to be benched to provide interlocking between the reinforced fill and the existing ground. It is possible that bedrock will be encountered during excavation of the Reinforced Fill Zone. If weathered bedrock encountered in the Reinforced Fill Zone is excavatable with conventional heavy construction equipment, it should be removed and benched as described in this section. Intact bedrock not excavatable with conventional heavy construction equipment should be left in place. If shallow intact bedrock is encountered, reinforcement may not extend deeply enough; anchoring reinforcement into rock may be needed.

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

Final gradation, soundness and plasticity index requirements for the reinforced fill zone of the MSE Wall will be provided at a later date by the eventual RSS designer. However, it may be expected that the reinforced fill may need to meet the requirements shown in Table 1 (from the Federal Highways Administration (FHWA, November 2009, Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Publication No. FHWA A-NHI-10-024). Other requirements for the reinforced fill may also apply, such as being reasonably free from organic, salvaged, or other deleterious materials, and being non-corrosive (though certain types of RSS systems may tolerate corrosive soils differently).

Table 1: Preliminary expected material gradation, plasticity and soundness requirements for the reinforced fill zone of a RSS system.

3.2.4. Alternative 3: Repair the Road with a Mechanically Stabilized Earth Wall (MSEW)

Alternative 3 includes repairing Big Dalton Road with a Mechanically Stabilized Earth Wall (MSEW).

MSEWs are retaining walls that use planar soil reinforcements (such as steel wire or geosynthetic mesh/strips, steel bar mats, or geotextile) between layers of controlled and specified fill, tied to a wall face, to provide mass to resist failure. MSEWs are common solutions for construction of fill slopes that need to be inclined steeper than 1H:1V.

Some proprietary MSEW systems (such as Hilfiker Retaining Walls® of Eureka, California) are designed to utilize local earth materials to provide mass to the structure, which are often more readily available in remote areas compared to concrete or other materials used to provide mass. These systems use welded wire mesh as reinforcement, and welded wire baskets packed with rock fill as facing. MSEW systems that utilize local earth materials are typically more cost-effective in remote areas, which may have logistical challenges for material sourcing. Preliminary geotechnical recommendations for project planning and budgeting purposes for a MSEW are presented below.

Subsurface exploration consisting of geotechnical borings, with 10-feet of rock coring below the bedrock interface should be implemented prior to final design of an MSEW solution, to evaluate geotechnical design parameters and global stability of the MSEW. Details of the subsurface exploration program should be specified by (or at least collaboratively agreed upon with) the eventual wall designer. For project planning and budgeting purposes, it can be assumed that two (2) borings to 30-feet may be needed. Assorted soil/rock laboratory tests may also be needed.

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

Detailed topographic survey of the MSEW area should be completed prior to final design of the MSEW. For project planning and budgeting purposes, it is assumed that the MSEW would have to be about 15-feet high and 60-feet long, based on site observations and lidar topography.

Final geotechnical recommendations for the MSEW will be provided at a later date by the eventual wall designer. For project planning and budgeting purposes, it is expected that a 4-foot-wide horizontal bench will be needed in front of the toe of the MSEW for embedment and bearing capacity requirements, and to enhance global slope stability.

Final geotechnical recommendations for the MSEW will be provided at a later date by the eventual wall designer. Due to the steep toe slopes, it is expected that in order to achieve bearing capacity and global slope stability requirements, the MSEW will need to be founded on competent bedrock.

The exposed MSEW subgrade should be observed by a qualified professional at the time of construction to evaluate the competency of subgrade materials. If needed, changes may be able to be made during construction to extend the wall to competent bedrock, by heightening the wall or pouring a concrete leveling pad to the depth of bedrock.

Final geotechnical recommendations for the MSEW will be provided at a later date by the eventual wall designer. The existing soils/rock will likely need to be benched to provide interlocking between the reinforced fill and the existing ground. It is likely that bedrock will be encountered during excavation of the Reinforced Fill Zone. If weathered bedrock encountered in the Reinforced Fill Zone is excavatable with conventional heavy construction equipment, it should be removed and benched as described in this section. Intact bedrock not excavatable with conventional heavy construction equipment should be left in place. If shallow intact bedrock is encountered, reinforcement may not extend deeply enough; anchoring reinforcement into rock may be needed.

Final gradation, soundness and plasticity index requirements for the reinforced fill zone of the MSE Wall will be provided at a later date by the eventual RSS designer. However, it may be expected that the reinforced fill may need to meet the following requirements (from the Federal Highways Administration (FHWA, November 2009, Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Publication No. FHWA A-NHI-10-024). Other requirements for the reinforced fill may also apply, such as being reasonably free from organic, salvaged, or other deleterious materials, and being non-corrosive (though certain types of MSE Walls may tolerate corrosive soils differently).

Table 2: Preliminary expected material gradation, plasticity and soundness requirements for the reinforced fill zone of a MSE Wall.

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

3.2.5. Summary of Road Repair Alternatives

As previously described, three conceptual design alternatives for repair of Big Dalton Road in the vicinity of the landslide site are discussed in the following sections. Based on our site observations and review of available information, all three alternatives are preliminarily assumed to be geotechnically feasible.

Depending on the design alternative selected, additional geotechnical information may need to be collected to support full design. It is possible that geotechnical recommendations may change as new information is available.

Table 3, below, presents our preliminary interpretation of advantages and disadvantages for each repair alternative. The ultimate decision for which repair alternative to select is the responsibility of the Partner.

Table 3: Preliminary Interpretation of Advantages and Disadvantages for the Repair Alternatives Advantage/ Disadvantage

Alternative 1:

Unreinforced Rock Fill

Alternatives 2:

Reinforced Soil Slope

Alternative 3:

Mechanically Stabilized Earth Wall

Limits of Disturbance Largest limits of disturbance longitudinally down the slope, and laterally to either side of the failure area; would also likely require a new access road to the toe.

Moderately large limits of disturbance longitudinally down the slope, and laterally to either side of the failure area; relatively small access ramp required from the existing road.

Smallest limits of disturbance, both longitudinally and laterally; relatively small access ramp required form the existing road.

Schedule Potentially the shortest design schedule, since little additional design work would be needed;

potentially the longest construction schedule, due to the large volume of rock fill needed to be placed.

Moderate design schedule, since some additional design work would be needed, and subsurface exploration may be needed; medium length construction schedule, due to moderate volume of soil excavated/placed.

Longest design schedule, since significant additional design work would be needed, including subsurface exploration; shortest construction schedule, due to smallest volume of soil excavated/placed.

Preliminary Cost Preliminarily expected to be the most expensive due to the large volume of material needed; rock fill has strict gradation requirements for coarse rock, so unlikely to be able to re-use excavated materials, and unlikely to be able to use local material sources.

Preliminarily expected to be moderately expensive; it should be noted that reinforced fill gradation requirements are less strict for RSS systems, so additional cost savings could more easily come from material re-use or local material sources.

Preliminarily expected to be moderately expensive; reinforced fill requirements are more strict than with RSS systems, so additional cost savings from material re-use or local material sources are less likely than with RSS systems.

Aesthetics Most invasive aesthetically, with a large rock fill embankment extending down the slope.

Least invasive aesthetically, as the structure can be revegetated to appear as a soil slope.

Moderately invasive aesthetically, as the MSE Wall facing does not typically revegetate.

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

4. LIMITATIONS

This evaluation was completed in accordance with generally accepted geotechnical engineering practices in the region where the work is conducted. It should be noted, however, that our geotechnical conclusions and recommendations are based on limited surface and subsurface information. The nature and extent of subsurface conditions may not become evident until the time of construction; this report should not be construed as a warranty of the subsurface conditions. As such, observation of actual subsurface conditions at the time of construction is integral to project success, and a contingency for unexpected conditions in the project budget and schedule should be included. Additional geotechnical observation/exploration may (or may not) improve the understanding of subsurface conditions.

Prepared By:

Shane Markus, PE, LEG Randy Nagel Johanna Nugent Geotechnical Engineer Civil Engineering Technician Civil Engineer

Digitally signed by

SHANE MARKUS

Date: 2026.06.05 14:39:10 -07'00'

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

APPENDIX A – SITE MAPS

FIGURE A-1 – VICINITY MAP

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

FIGURE A-2 – SITE MAP

Landslide Headscarp

Landslide Runout

Big Dalton Road

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

APPENDIX B – GEOMORPHIC MAPS

FIGURE B-1 – NRCS WEB SOIL SURVEY MAP

FIGURE B-2 – USGS GEOLOGIC MAP

Big Dalton Road Landslide

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

FIGURE B-3 – LIDAR HILLSHADE IMAGE

FIGURE B-4 – 1952 AERIAL PHOTOGRAPH

Big Dalton Road Landslide

Artificial Erosion Area Sidecast Fill Sloughs

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

FIGURE B-5 – 1966 AERIAL PHOTOGRAPH

FIGURE B-6 – 1980 AERIAL PHOTOGRAPH

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

FIGURE B-7 – 2006 AERIAL PHOTOGRAPH

FIGURE B-8 – 2025 AERIAL PHOTOGRAPH

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

APPENDIX C – WILLIAMSON DRIVE PROBE LOGS

Location:

Elevation (feet)

Depth (start)

Depth (end)

Depth (change) Blows/Increment Notes Blows Per Foot

[D] (BPF)

Standard Pentration Test

Equivalent [N] (BPF) 3,333.00 0 0.5 0.5 6 0 12 5.65 3,332.50 0.5 1 0.5 3 0 6 3.30

3332 1 1.5 0.5 5 0 10 4.91

3331.5 1.5 2 0.5 14 0 28 10.91

3331 2 2.5 0.5 17 0 34 12.69

3330.5 2.5 3 0.5 12 0 24 9.68

3330 3 3.5 0.5 10 0 20 8.40

3329.5 3.5 4 0.5 8 WDP drive pipe broke upon attempted retreival; about 0 16 7.07

3329 4 4.5 0.5 7 8 feet of pipe was left in the ground. 0 14 6.37

3328.5 4.5 5 0.5 8 0 16 7.07

3328 5 5.5 0.5 11 0 22 9.05

3327.5 5.5 6 0.5 13 0 26 10.30

3327 6 6.5 0.5 10 0 20 8.40

3326.5 6.5 7 0.5 9 0 18 7.74

3326 7 7.5 0.5 10 0 20 8.40

3325.5 7.5 8 0.5 8 0 16 7.07

3325 8 8.5 0.5 15 0 30 11.51

3324.5 8.5 9 0.5 27 0 54 18.16

3324 9 9.5 0.5 34 0 68 21.72

3323.5 9.5 10 0.5 50 Bouncing; refusal presumably on a cobble 0 100 29.30

WDP-1 is located about 10 feet northeast of the failure area, on the downslope shoulder of the road

WDP-1

Location:

Elevation (feet)

Depth (start)

Depth (end)

Depth (change) Blows/Increment Notes Blows Per Foot

[D] (BPF)

Standard Pentration Test

Equivalent [N] (BPF) 3,332 0 0.5 0.5 10 0 20 8.40

3,331.50 0.5 1 0.5 43 0 86 26.06 3,331.00 1 1.5 0.5 50 0 100 29.30 3,330.50 1.5 2 0.5 50 Bouncing; refusal presumably on a cobble 0 100 29.30

WDP-2

WDP-2 is located directly behind (northwest) of the failure area, on the upslope shoulder of the road

Location:

Elevation (feet)

Depth (start)

Depth (end)

Depth (change) Blows/Increment Notes Blows Per Foot

[D] (BPF)

Standard Pentration Test

Equivalent [N] (BPF)

3325 0 0.5 0.5 10 0 20 8.40 3,324.50 0.5 1 0.5 10 0 20 8.40

3324 1 1.5 0.5 8 0 16 7.07

3323.5 1.5 2 0.5 5 0 10 4.91

3323 2 2.5 0.5 4 0 8 4.13

3322.5 2.5 3 0.5 7 0 14 6.37

3322 3 3.5 0.5 11 0 22 9.05

3321.5 3.5 4 0.5 13 0 26 10.30

3321 4 4.5 0.5 37 0 74 23.20

3320.5 4.5 5 0.5 56 0 112 31.99

3320 5 5.5 0.5 62 0 124 34.62

3319.8 5.5 5.7 0.2 60 Bouncing; refusal presumably on a cobble 0 300 68.73

WDP-3

WDP-3 is located about 5 feet south of the failure area, on the downslope shoulder of the road

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

APPENDIX D – PRELIMINARY DESIGN ALTERNATIVES

FIGURE D-1 – ALT 1 PLAN SKETCH: ROCK FILL

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

FIGURE D-2 – FIELD-DEVELOPED CROSS-SECTION AND ALT 1 PROFILE SKETCH: ROCK FILL

Approximate Existing Ground Surface from Field-Developed Cross-Section

Alternative 1 Approximate Proposed Ground Surface

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

FIGURE D-3 – ALT 2 PLAN SKETCH: 1H:1V REINFORCED SOIL SLOPE (RSS)

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

FIGURE D-4 – ALT 2 PROFILE SKETCH: 1H:1V REINFORCED SOIL SLOPE (RSS)

Approximate Existing Ground Surface

Alternative 2 Approximate Proposed Ground Surface

Approximate preliminary Reinforced Fill Zone (to be removed and replaced)

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

FIGURE D-5 – ALT 3 PLAN SKETCH: MECHANICALLY STABILIZED EARTH WALL (MSEW)

Preliminary Geotechnical and Engineering Report – Big Dalton Road Landslide Repair June 5, 2026

FIGURE D-6 – ALT 3 PROFILE SKETCH: MECHANICALLY STABILIZED EARTH WALL (MSEW)

Approximate Existing Ground Surface

Alternative 3 Approximage Proposed Ground Surface

Approximate preliminary Reinforced Fill Zone (to be removed and replaced)

TANBARK FLAT ARBORIST TREE INVENTORY

U.S. Army Corps of Engineers https://wildscaperestoration.sharepoint.com/Shared Documents/Staff/UG - Projects/UG c-USACE Tanbark Flat Tree

Survey/8 - Reports/20241219 Tanbark Tree Report - NM.docx

This page is intentionally left blank.

https://wildscaperestoration.sharepoint.com/Shared Documents/Staff/UG - Projects/UG c-USACE Tanbark Flat Tree

Survey/8 - Reports/20241219 Tanbark Tree Report - NM.docx

TABLE OF CONTENTS

Section Page

INTRODUCTION

PROJECT LOCATION

PROJECT SETTING

TREE SURVEY

4.1 Tree Survey Methodology

4.2 Post-fire Survival

4.3 Tree Results

ARBORIST STATEMENT

REFERENCES

Figures Page Figure 1. Tanbark Vicinity Figure 2. Mess Hall Figure 3. Barracks Figure 4. Assistants Quarters Figure 5. Office Lab Figure 6. Two Story House Figure 7. Pump House Figure 8. Gas House Figure 9. Carpenters Shop Figure 10. Staff House

Appendices Appendix A Data Table Appendix B Site Photographs

TANBARK FLAT ARBORIST TREE INVENTORY Page 1

Survey/8 - Reports/20241219 Tanbark Tree Report - NM.docx

INTRODUCTION

Wildscape Restoration, Inc. (Wildscape) has conducted a Tree Inventory at the Tanbark Flat Administrative Site within San Dimas Experimental Forest (SDWF) for the U.S. Army Corps of Engineers (USACE). This report includes a detailed inventory of 96 trees, assessments of tree health, and the collection of geo-referenced data.

PROJECT LOCATION

The project is located in the San Gabriel Mountains in the Angeles National Forest (ANF). The center of the project area is located at the following coordinates: latitude 34.205946 and longitude -117.762197.

The site is located in the U.S. Geological Survey (USGS) 7.5-minute Glendora quadrangle. The regional location and local vicinity maps are shown in Figure 1.

PROJECT SETTING

The San Dimas Experimental Forest was established in 1933 and covers 6,945 hectares (about 32 square miles) within the ANF. The Tanbark Administrative Site is comprised of historic buildings dating back to 1935 including office/lab buildings, barracks and a mess hall built by the Civilian Conservation Corps.

These buildings housed scientists and other personnel. The subject area of the survey is 8 acres.

TREE SURVEY

Wildscape’s arborist, Amanda Gibbs and biologist Mary Logan conducted site visits on November 20 and 21, 2024, to survey the 115 trees identified in the scope of work. These potentially hazardous trees are being surveyed due to their location, directly adjacent to structures and roads. The U.S. Forest Service Biological Science Technician Bonni Corcoran escorted the biologists and assisted in identifying which trees were to be assessed.

The following data was collected from each tree:

• Photographs of each tree were taken on the phone application “Solocator.”

• GPS location of each tree was collected on the Field Maps software and handheld Garmin GPS unit.

• Individual round metal tree tags with unique numeric identification for each tree were affixed on the north side of the trunk where feasible.

• The tree species was determined in the field.

• Diameter at Breast Height (DBH) was collected with a measuring tape.

• Height was estimated using a rangefinder and calculated with the tangent method.

• Canopy spread of each tree was measured by pacing on the north to south extent.

• A health rating was assigned using the scale below in Table 1.

• Identification of defects, pests, diseases, parasites, leaning angle, trunk scars, swelling, bleeding, frost damage, and foreign objects, etc. that affect the overall health and vigor of the tree.

• A final assessment determining whether the tree should be left in place, trimmed, or removed.

According to Borchert et al. (2002), the chances of Coulter’s and/or gray pine surviving a fire depends on the severity of the burn on the trunk plus the scorch of the canopy. The higher the burn goes up the trunk, and higher the percentage of canopy scorch will increase the likelihood of death over the next 36 months. The results of this article were considered when determining if a tree should be trimmed

TANBARK FLAT ARBORIST TREE INVENTORY Page 2

Survey/8 - Reports/20241219 Tanbark Tree Report - NM.docx versus removed during the survey. In addition, Ms. Corcoran explained how the stand was in poor condition prior to the fire due to severe drought conditions. While this is anecdotal evidence, the trees’ prior health may increase the likelihood of failure of the stand overall. If a tree is diseased or stressed, it is less likely to recover from a fire.

Using the information from the Brochert et al. article, the trees in this survey data set with trunks burned more than 2-3 meters up the trunk were slated for removal. If the canopy scorch ranged from 30%-50% or greater with a combination of a burned trunk, it was also slated for removal. If the canopy was only lightly touched by the fire, and only a few inches were burned from the base of the trunk, it was recommended that deadwood be trimmed.

Table 1. Tree Health Rating

Vigor Rating Description

1 Excellent – vigorous, healthy plant, no signs of necrotic or chlorotic leaves

2 Good – plant healthy with limited signs of vigorous growth; no fire damage observed

3 Adequate – plant health with no signs of vigorous growth, some necrosis or other damage may be present; light fire damage in either trunk or crown

4 Poor – low vitality, or main stem dead but basal sprouts emerging; significant fire damage

5 Dead – no evidence of live plant material or recovery

X Missing – no plant present

The survey started at the mess hall in the northern extent. The tree identification tags started at #101.

The first set of round metal tags are gold-colored and are numbered 101-125.The next set range from #131-200. The final two trees were numbered with tags #357 and #358.

On November 20, 2024, the mess hall, assistant’s quarters, and office/lab trees were tagged and assessed. On November 21, 2024, the two-story house, pump house, cottage/garage, staff house, carpenter’s shop, and gas house and guard house trees were completed.