Cal_Fire_Ramona_AAB_Geotech_Inv_Report_-_5-5-22.pdf

Consultants in the Earth & Material Sciences

16801 Van Buren Blvd., Bldg. B Riverside, CA 92504 Tel: 951.776.0345 Fax: 951.776.0395 www.aragongeo.com

PRELIMINARY GEOTECHNICAL INVESTIGATION

RAMONA AIR ATTACK BASE UPGRADE PROJECT

RAMONA, SAN DIEGO COUNTY, CALIFORNIA

FOR:

CALIFORNIA DEPARTMENT OF FORESTRY AND FIRE PROTECTION

23300 CASTLE STREET

RIVERSIDE, CALIFORNIA 92518

PROJECT NO. 4732-SF

MAY 5, 2022

Consultants in the Earth & Material Sciences

16801 Van Buren Blvd., Bldg. B Riverside, CA 92504 Tel: 951.776.0345 Fax: 951.776.0395 www.aragongeo.com

May 5, 2022 Project No. 4732-SF

California Department of Forestry and Fire Protection 23300 Castle Street Riverside, California 92518

Attention: Mr. Matthew McCleod, MPA, PE

Subject: Preliminary Geotechnical Investigation Report Ramona Air Attack Base Upgrade Improvements Ramona Airport, San Diego County, California.

Mr. McCleod:

In accordance with our proposal dated July 13, 2021 and your authorization, Aragón Geotechnical Inc. (AGI) has completed preliminary geotechnical and geological assessments for (1) A proposed barracks building for aircraft flight crews, and (2) Replacement of deteriorated concrete pavements in aircraft apron areas. The attached report presents in detail the findings, opinions, and recommendations developed as a result of surface inspections, subsurface exploration and field tests, laboratory testing, and quantitative analyses. Our scope included an infiltration feasibility assessment and opinion letter for stormwater BMPs, but excluded environmental testing for contaminants in soil, groundwater, or air at the site. Infiltration-related findings are in a separate report for the designer’s use in formulating a required preliminary water quality management plan.

Ten exploratory borings were drilled within the proposed construction area to characterize local soils and bedrock. Aircraft pavements were cored and field-measured for thickness.

Up to roughly 33 inches of granular fill soil and native clay was logged underneath pavement. In the barracks site, natural residual clay dominated the surficial soils. Both the apron and barracks sites were unexpectedly discovered to be underlain at shallow depths by a very old and thoroughly altered sedimentary sandstone unit, and not weathered granitic bedrock. The sandstone was very dense but easily drilled.

Geologic influences for building design include required structural measures to mitigate elevated risks of strong earthquake ground motions at the site. However, threats from other natural hazards including liquefaction, surface fault rupture, excessive settlement, California Department of Forestry and Fire Protection May 5, 2022 Project No. 4732-SF Page No. ii

Aragón Geotechnical, Inc.

gross instability or landsliding, seiching, induced flooding, and tsunami appear to range from very low to zero. Groundwater was not encountered in any boring to our maximum-explored depth of 26.4 feet, and will not be a factor in construction.

Findings indicated the site is suitable from a geotechnical viewpoint for a typical light-frame building. We have recommended that all surficial clay be removed and substituted with suitable granular engineered fill as mitigation for very high expansion potential. The average depth to competent sandstone below the barracks should be only about 2 feet below existing surfaces, however. Conventional shallow footings should provide adequate building support. Overexcavation is recommended when or if needed to supply at least 12 inches of engineered fill below all foundation concrete. The accompanying narrative report includes values for both vertical and lateral loads, recommendations for site earthwork, and prescriptive code values applicable to seismic design.

We preliminarily recommend that replacement airfield pavements consist of Portland cement concrete (12 inches), over cement-treated aggregate base (6 inches), over granular sand-clay base fill salvaged from existing onsite fill (12 inches). These dimensions are minimum thicknesses. Native clay must be spoiled whenever found below airfield pavement. The designated subgrade is either sandstone or weathered granitic bedrock. AGI relied upon FAA-recommended guidance contained in Advisory Circular 150/5320-6G, Airport Pavement Design and Evaluation, (June 7, 2021). The AC was our primary reference for materials test assignments and for structural section analyses.

Copies of recommended construction specifications are presented in an appendix to the investigation report.

Thank you very much for this opportunity to be of service. We welcome questions, and can be contacted through the convenience of email at www.aragongeo.com

Very truly yours, Aragón Geotechnical Inc.

Mark G. Doerschlag, CEG 1752 C. Fernando Aragón, P.E., M.S.

Engineering Geologist Geotechnical Engineer, G.E. No. 2994

MGD/CFA:mma

Distribution: (4) Addressee

Aragón Geotechnical, Inc.

TABLE OF CONTENTS

Page

1.0 INTRODUCTION

2.0 PROPOSED CONSTRUCTION

3.0 FIELD INVESTIGATION AND LABORATORY TESTING

4.0 SITE GEOTECHNICAL CONDITIONS

4.1 Previous Site Uses

4.2 Surface Conditions

4.3 Soil Map, Flooding, and Climate Reviews

4.4 Previous Airport Geotechnical Investigations

5.0 ENGINEERING GEOLOGIC ANALYSES

5.1 Regional Geologic Setting

5.2 Local Geologic Conditions

5.3 Slope Stability

5.4 Faulting and Regional Seismicity

5.4.1 Fault Rupture Potential

5.4.2 Strong Motion Potential

5.4.3 Secondary Seismic Hazards

6.0 CONCLUSIONS AND RECOMMENDATIONS – STRUCTURAL

6.1 General

6.2 Site Grading

6.3 Foundation & Floor Slab Design

6.4 2019 California Building Code Seismic Criteria

6.5 Pavement Design – Interior Driveways & Parking Stalls

6.6 Temporary Sloped Excavations

6.7 Trench Backfill

6.8 Soil Corrosivity

7.0 CONCLUSIONS AND RECOMMENDATIONS – AIRFIELD PAVEMENTS

7.1 Pavement Section Design Method

7.2 Recommended Structural Section Options

7.3 Reclaimed-Materials & Earthwork Volume Adjustments

7.4 Recommended Airfield Pavement Construction Protocols and Standards 42

8.0 CONSTRUCTION OBSERVATION

9.0 INVESTIGATION LIMITATIONS

10.0 CLOSURE

REFERENCES

Geotechnical Map Explanation & Subsurface Exploration Logs . . . . . . . . APPENDIX A Laboratory Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . APPENDIX B FAARFIELD Design Model and Sample FAA Specifications . . . . . . . . . . APPENDIX C

PRELIMINARY GEOTECHNICAL INVESTIGATION

RAMONA AIR ATTACK BASE UPGRADE PROJECT

CALIFORNIA DEPARTMENT OF FORESTRY AND FIRE PROTECTION

RAMONA, SAN DIEGO COUNTY, CALIFORNIA

1.0 INTRODUCTION

This report presents the results of preliminary soils engineering, pavement design, and geologic hazard evaluations conducted by Aragón Geotechnical, Inc. (AGI) for the referenced CAL FIRE project site, located at the San Diego County-owned Ramona

Airport. The air attack base (AAB) occupies roughly 6½ acres of leased land on the north side of the airport at 2498 Montecito Road; the latter is the sole access route to the CAL

FIRE facility and other aviation-related fixed-base operators (FBOs). Regional arterial thoroughfares near the airport include State Highways 67 and 78 to the east, and Highland

Valley Road to the southwest. Map coordinates are 33.040053 N x 116.912074 W at the approximate center of a proposed crew barracks building (this coordinate point was selected for seismological analyses). Situs per the Public Lands Survey System places the project in the NE¼ of Section 18 [projected], Township 13 South, Range 1 East (San

Bernardino Baseline and Meridian). Jurisdictional control for entitlements will be exercised by San Diego County. AGI anticipates further review of architectural and structural design of occupancy buildings by the Division of the State Architect.

Currently the Ramona AAB features three primary buildings used for storage, mainte-nance, and base personnel operations. Firefighting aircraft normally stationed at Ramona comprise Grumman S-2T turboprop air tankers and an OV-10A Bronco observation plane.

A package plant mixes, stores, and pumps Phos-Chek brand fire retardant to the aircraft.

Upgrades and additions to airway pavements, the retardant package plant and loading pits, crew living quarters, and stormwater management are proposed. The accompanying Site

Location Map (Figure 1) depicts the general location of the project on a 1:24,000-scale topographic quadrangle map. Although out-of-date with respect to existing airport improvements and surrounding growth of the Ramona community, the older map series was selected for clearer depictions of ground slope, drainage patterns, and historical off-site land uses and improvements.

Multiple objectives were targeted by AGI’s geotechnical design investigation. Determina-tions of the nature and engineering properties of the subsurface materials underlying the

AAB were of first importance. These data in turn were analyzed to affirm site suitability for the proposed barracks building, and to provide preliminary foundation design, grading, and

0 2000 4000 FT.

Reference: U. S. Geological Survey 7½-Minute Series Topographic Map, San Pasqual Quadrangle (1988).

SITE LOCATION MAP

CAL FIRE RAMONA AAB, RAMONA, SAN DIEGO COUNTY, CALIF.

PROJECT NO. 4732-SF DATE: 5/5/22 FIGURE 1

SITE

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construction recommendations. A second set of materials tests and comprehensive engineering analyses were geared to the proposed removal and replacement of deteriorated concrete airfield pavements. AGI’s objectives for this phase were to develop a recommended pavement structural section compatible with the aircraft wheel loads and annual duty cycles provided to AGI by CAL FIRE, and to create a guidance document concerning recommended construction specifications for materials, subgrade preparation, and minimum compaction for soil and aggregate pavement structural section elements.

Ramona AAB improvements will serve newly acquired C-130 firefighting tankers and their air crews in addition to the existing forces.

Our dual investigation scope included reconnaissance of the airport site and surrounding terrain, historical aerial photograph interpretations, reviews of airport investigation reports by others with a special focus on soil test data, geologic literature research, subsurface exploration, recovery of representative soil samples, laboratory testing, and the creation of this geotechnical design report. Later-authorized services included assessment of stormwater infiltration feasibility. As of this report’s issue date, infiltration field testing had been completed. The site was classified as “restricted for infiltration” in accordance with

San Diego County technical guidelines. AGI’s infiltration feasibility report has been issued under separate cover for the design civil engineer’s use in formulating a preliminary water quality management plan.

Geological assessments for the barracks building focused on risks posed by active earthquake faults, strong ground motion, liquefaction or other secondary seismic hazards, and groundwater. These were evaluated using published resources and site-specific quantitative analyses, plus conclusions drawn from field findings and local case-history experience. However, Phase I or Phase II environmental site assessments, well construction, percolation testing for onsite wastewater treatment system (OWTS) design, or contaminant testing of recovered samples were outside the scope of our investigation.

2.0 PROPOSED CONSTRUCTION

C-130 Barracks. CAL FIRE supplied a conceptual site development plan we referenced for project information and borehole locality selection. The scaled drawing (Sheet CS101) included spot elevations and topographic contours, existing AAB improvements, and the

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planned envelope of a cruciform barracks building sized at about 8,200 square feet. The finish floor elevation was not specified. Geotechnical assumptions for the project included one- or two-story light frame construction resting on perimeter and interior shallow strip foundations. A residential-duty concrete slab-on-grade floor was also assumed. A simple dual-pitch standing-seam metal roof system would be predicted. Light frame construction ordinarily imposes only modest foundation loads. Interior improvements were expected to be limited to ordinary sleeping, kitchen, and bathing/sanitation uses. The barracks would be augmented by doorway landing and walkway hardscape, and the provision of up to 7 paved automobile parking stalls just north of the structure.

Basements or other subterranean construction were not shown on the drawing and would be unlikely. Live water and gas utilities exist next to the structure site in the AAB lease area and will connect with the barracks via buried service laterals. According to the plans, an existing OWTS will not be modified nor will a new septic disposal field be required.

Rather, a plan note indicted the barracks will include a sewer lift station for both existing and future wastewater volumes. The ultimate destination for the effluent was not defined.

Airfield Pavement & Phos-Chek Package Plant. Ramona AAB currently features approximately 4.58 acres of Portland cement concrete (PCC) airfield pavement. At least

3 generations of pavement are present, plus many smaller repair areas. Almost all site

PCC pavements exhibit varying degrees of crack distress. The apron supports aerial operations of retardant loading and standby parking. Two connections to the airport’s taxiway are located along the southern edge of the concrete apron. Ramona Airport pavements beyond the CAL FIRE lease area are composed of asphaltic concrete.

It is our understanding that most or all of the existing PCC pavement is intended for replacement. About 0.7 acre of asphalt pavement beyond the current AAB eastern lease line could also be replaced with PCC surfacing, according to CAL FIRE. A new retardant package plant would be delivered to the AAB and relocated to a proposed spot at the north edge of the apron. The package plant will include skid-mounted tanks, mixers, pumps, and pipe manifolds. The manufacturer indicates that a curb-type containment stem wall roughly 6"W x 6"H would surround the plant. In the apron, three tanker loading pits would be served by a shallow subterranean concrete box channel possibly 24 inches wide and

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20 inches deep containing retardant supply and return lines, a water line, and possibly other small conduits. An aircraft-rated steel grating would cover the U-shaped channel.

During the course of our investigation, we recognized that construction means and methods, materials selections, cost, and sustainability goals would be important to optimizing a recommended pavement structural section suited to the aircraft loads and environmental conditions. AGI’s analytic approaches continually focused on these ideas:

Minimize the volume of material to be hauled from the site.

Minimize the amount of new or virgin materials (soil, aggregate, concrete) that have to be trucked to the airport.

Employ recommended selective grading where feasible to recover granular soils for engineered subbase use, while rejecting unfavorable clay soils that might require chemical modification to render them suitable under pavements.

Reuse and recycle demolition wastes on-site where feasible. The existing concrete pavement can be crushed, screened, and used for an FAA-compliant stabilized aggregate base course under new PCC pavement.

Identify PCC mix components, and optional placement methods, that should help minimize concrete degradation from Phos-Chek. Retardants based on ammonium phosphate and ammonium sulfate chemistries are moderately corrosive to concrete.

Some spillage is unavoidable during loading operations. Concrete surface erosion, raveling, and spalling are known negative effects.

We infer that raw cut-and-fill earthwork volumes required for the barracks and pavement replacement project phases will be very small. Maximum elevation changes from present surface grades in the flat building area are not expected to exceed a few inches. We would expect new airfield pavements to essentially mirror current elevation contours and slope directions, since the new construction will be required to match existing surfaces at

AAB building finish floors, taxiways, and next-door FBO hangars. Neither man-made slopes nor retaining walls are expected in the project.

3.0 FIELD INVESTIGATION AND LABORATORY TESTING

Subsurface geotechnical site characterization comprising 10 exploratory soil borings was completed by AGI on October 24, 2021 (9 holes) and April 18, 2022 (1 hole near prospective stormwater BMPs). There were no significant access impediments. AGI-

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selected drill sites were cleared of utility interference issues by notification to the 811

DigAlert service in advance of AGI’s work. All pavement sites were first perforated with a

10-inch-diameter diamond core barrel and the recovered core measured for average thickness. Drilling sites were preferentially placed to explore (1) Proposed retardant loading pit locations expected to incur maximum repetitive aircraft loads; (2) The relocated

Phos-Check package plant site; (3) Asphalt sections in the proposed lease expansion area, east side of the AAB; and (4) The barracks site in “untouched” native condition, and where a patch of newer PCC pavement is the standby station for the OV-10A observation plane.

Per DSA rules, at least two borings were required for the barracks. Collectively, the

Ramona AAB borings met a goal of spanning the entire improvement area to gauge the degree of geotechnical site variability. Soil boring locations and depths were deemed adequate to obtain data concerning (1) Soil type classifications, water contents, in-place densities, and settlement risks in light of local geological interpretations; (2) Continuity of layers or units across the base; and (3) Unit geological origins and a derivation of site

“stiffness” for earthquake engineering purposes.

The soil borings were drilled with a truck-mounted hollow-stem auger rig capable of driving and retrieving soil sample barrels. Borehole termination depths ranged from 5.0 to 26.4 feet. Predicted groundwater depths and onerous San Diego County permitting require-ments for even temporary explorations encountering groundwater forced the relatively shallow terminations. None of the borings were prematurely halted above their target depths by machine refusal. As expected, all borings encountered soils and soft rock that were amenable to drive-tube sampling, performed at 2-foot to 5-foot depth increments.

Close to existing grades where soil bearing capacity, subgrade modulus, and settlement potential would be the main items of concern, relatively undisturbed soil samples were recovered by driving a 3.0-inch-diameter “California modified” split-barrel sampler lined with brass rings. Deeper horizons in the barracks and stormwater BMP borings included

Standard Penetration Tests (SPTs) conducted using an unlined 2.0-inch O.D. split-barrel spoon. All sampler driving was done using rods and a mechanically actuated automatic

140-pound hammer free-falling 30 inches. Bulk samples of auger cuttings representative of shallow fill and native materials were bagged. All geotechnical samples were brought to AGI’s Riverside laboratory for assigned soils testing.

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Drill cuttings and each discrete sample were visually/manually examined and classified according to the Unified Soil Classification System, and observations made concerning relative density, constituent grain size, visible macro-porosity, plasticity, and past or present groundwater conditions. Basement rock was described in concordance with ISRM terminology for weathering, hardness, strength, and other characteristics based on drilling behavior and simple field tests. Continuous logs of the subsurface conditions encountered were recorded by a senior Engineering Geologist, and the results are presented on the

Field Boring Logs in Appendix A. The approximate locations of the borehole explorations are illustrated on the Geotechnical Maps (Plate Nos. 1 and 2 foldouts), located at the back of this report.

After drilling, tamped crushed-rock fill was placed in the open boreholes and the interstitial spaces filled with liquid cement-bentonite grout. A high-early-strength grout pack matching the PCC pavement section was installed in the AAB lease area. Compacted AC cold patch was used in the asphalt area at drill sites B-6 and B-7. Turf was reinstalled at the barracks building borings. AGI rated the pavement repairs as suitable for renewed air operations by day’s end.

“Undisturbed” samples were tested for dry density and water content. One-dimensional consolidation tests were conducted on selected barrel samples from the barracks site in order to evaluate settlement or collapse potential. The recovered bulk soil samples were evaluated for index and engineering properties such as shear strength, compaction criteria, expansion potential, CBR, and corrosivity characteristics. Discussions of the laboratory test standards used and the test results are presented in Appendix B.

4.0 SITE GEOTECHNICAL CONDITIONS

4.1 Previous Site Uses

AGI’s scope included limited historical research to ascertain changes to surficial conditions through time, and address known or possible geotechnical impacts to project design or construction. Digitized historical aerial photographs archived at the

U.C. Santa Barbara Geospatial Collections were downloaded and interpreted for evidence of past structures, land use, and for geological assessments of active faulting potential and geomorphology. Newer monoscopic imagery was reviewed in the Google Earth Pro web application. Finally, the on-line version of the U.S.

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Geological Survey Historical Map Collection was accessed for digital scans of topographic quadrangle sheets pre-dating the base map image used for Figure 1.

Reviewed historical sources are listed under “References” at the end of this report.

The oldest imagery we located dated to April, 1939. At that time, the entire airport property and vicinity included only open rangeland and tilled surfaces probably used for grain crops. It was not difficult to pick out exposed rock outcrops in the photos and match them to the same outcrops visible today. Tonal contrasts were evident in the 1939 photos that we could correlate to particular geological units found in borings.

We could not detect signs of any agricultural wells. Rural-land uses presumably continued on the future airport site until the U.S. Navy built an emergency dirt airstrip in 1943. In 1953 the undeveloped CAL FIRE AAB site was vacant, lightly disturbed barren ground. The airport facility was conveyed to San Diego County in 1956.

County websites state that the California Division of Forestry (CDF) opened its very first aerial firefighting base at Ramona in 1957. The rudimentary outpost included

Quonset huts and one small frame building fronting a dirt apron. None of the original structures have survived. By 1974, the base included a very small concrete apron that possibly still exists today underneath the retardant package plant. Other airport improvements included a reconfigured main runway and new parallel taxiway, plus the establishment of the first FBO just west of the CDF base.

Between 1974 and 1989 the air base received a significant expansion of the concrete-paved apron area. A single operations and maintenance structure (since removed) was interpreted. The barracks site remained bare dirt. Today’s collection of three modern office, maintenance, and storage buildings was built between 1996 and 2002. Additional PCC apron pavement was added during the same time span.

The new steel-reinforced pavement was distinguished by smaller panel sizes between sawn crack control joints. A small one-story frame structure (U.S. Forest Service-operated) was installed just north of the proposed barracks in 2003 or 2004. Other than replacement of older-generation fixed retardant tanks with the current skid-mounted package plant in 2017, the Ramona AAB site has had minimal changes for the last two decades.

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4.2 Ramona AAB Surface Conditions

The aircraft apron area features slightly concave-up slope gradients ranging between roughly 1.6 and 2.2 percent toward the south-southwest according to contours shown on a CAL FIRE topographic survey map. Total relief within the AAB lease area is about 9 feet. The proposed barracks site is flat and level, with a turf-grass surface except for the eastern end. Incident rainfall at the base would appear to drain via sheetflow to the southern edge of the lease area, where runoff would eventually be captured by several shallow unlined basins next to the airport’s main taxiway. Raised and grated inlets in the basins feed a buried storm drain.

As already noted, the majority of the airbase apron consists of two generations of

PCC pavement. The older generation comprises 20' x 20' plain concrete panels with deteriorated joint sealants. The panels commonly exhibit radial cracks that would be interpreted as caused by center heave. Vertically offset panel joins are common.

Many panels have been removed and replaced due to crack distress. Anecdotal reports from base personnel indicated that taxiing aircraft are accompanied by water jets squirting from construction joints after rain events. At the time of field studies, however, the pavement surfaces appeared clean and free of evidence for subgrade soil migration to the surface.

Post-1996 pavement is composed of a slightly thinner concrete section featuring sawn control joints on 10-foot centers each way and internal steel reinforcement composed of No. 3 deformed bars (spacing undetermined). The newer pavement also exhibits significant tension crack distress. Vertical displacements at cracks are usually undetectable, though. Cracks are preferentially oriented in a somewhat regular block-type pattern that might be governed by bar locations. The pavement cracks have been surface-reamed and filled with injected epoxy.

4.3 Soil Map, Flood Zone, and Climate Reviews

Soil Series Data. In preparation for airfield soil sampling, AGI reviewed Natural

Resources Conservation Service soil survey data of the airport vicinity. The number and locations of samples would potentially be influenced by differences in mapped soil types. NRCS data indicated three major soil series were present: Bonsall-

Fallbrook sandy loam (BnB); Placentia sandy loam (PeC); and a small zone of

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Fallbrook sandy loam (FaC2). The Bonsall-Fallbrook soil series is presumed to characterize the bulk of the AAB apron and barracks site, although it should be noted soil unit boundaries at the printed map scale are approximations only (image clip below). All three soil series are characterized by deeply weathered clay horizons that can range to more than 4 feet thick over parent materials. Fallbrook sandy loam

FaC2 composes residual soil over granitic basement rock. The other soil series plus another clay unit (Bosanko clay BsC on the image clip) have a distribution pattern that correlates with a concealed sedimentary formation. Weathered clay soils are notorious in the region for their low strength and high shrink-swell activity.

Flood Risk Category. Surface flooding is not reported by base personnel to be a known issue around the Ramona Airport area. The CAL FIRE facility does not receive offsite flows, lacks natural or man-made channels, and is not next to surface impoundments. Ramona Airport is categorized as flood zone X, or outside of mapped

100-year and 500-year floodplains on the relevant FEMA Flood Insurance Rate Map

Source: NRCS Web Soil Survey, March 2022.

http://websoilsurvey.nrcs.usda.gov/HomePage.htm

N

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(FEMA, 2012). Existing pavement gradients exceed 1 percent and direct surface runoff to basins equipped with storm drain relief. Inundations of the apron do not occur. Subgrade soil softening potential due to local ponding was thus eliminated as a threat to pavement section integrity, assuming adequate setbacks from site BMPs.

Ramona AAB Climate Regime. The Santa Maria Valley area enjoys a quintessential

Mediterranean climate of cool, wet winters and warm dry summers. The average precipitation is about 16½ inches per year, arriving mainly in January through March.

Agricultural production near Ramona includes frost-sensitive citrus, avocado, and wine grape crops indicative of temperate conditions. Although surrounding terrain is mountainous, the town and airport lie at elevations near 1400 feet AMSL, so snow is unusual and short-lived. A newsworthy snowfall of an inch or two in the area was reported by local media as recently as January 25, 2021.

Airport pavement design considerations must include potentials for soil freezing and ice lens development. Fine-grained soils are much more susceptible than granular soils to frost disruption. Stabilized subgrades like soil cement would be inadvisable where or if frost penetration can intercept the treated soils. The USDA reports that frozen-ground conditions generally require a “severe” freeze (<24 degrees Fahren-heit) and much longer durations than a few hours. The noted low-temperature threshold is very rare in Ramona. The Western Regional Climate Center reports that the average January minimum temperature from 1917-2018 was 38 degrees F. We do note, however, that the County of San Diego Building Code amendments assign a 12-inch frost line depth as the minimum requirements for buildings. The County-code frost line depth thus becomes one controlling variable in establishing a final barracks foundation design, even though local crops and USDA maps argue for frost-free soils year-round. Sensitive fine-grained subgrade soils are expected to be fully removed and replaced with highly compacted granular base courses. Freeze-thaw risk to new pavement should be nil, in our opinion.

4.4 Previous Airport Geotechnical Investigations

AGI reviewed exploration data and test results from studies by others done for

Ramona Airport in 2013 and 2019. The San Diego offices of Leighton Consulting, Inc., supervised pavement coring and limited subsurface explorations at 15 locations

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total (Leighton, 2019). The stated objectives were to measure existing pavement structural sections, and obtain California Bearing Ratio (CBR) test values for the underlying subgrade soils. The exploration sites were limited solely to asphalt-paved taxiways and the main runway, plus one infield boring. All were beyond the AAB lease area. A total of 7 CBR tests were done.

Borehole logs for the Leighton explorations situated at paved sites depicted ranges of 1.75" to 13.0" of asphalt over 2.0" to 23.0" of “aggregate base”. The latter actually comprised disparate materials listed variously as asphalt grindings, poorly graded gravel, or decomposed granite sand. Three sites had asphalt over Portland cement concrete over “aggregate base”. Subgrade soils were logged as silty sand fill (13 sites total, including all 12 sites examined in 2013), or clayey sand with gravel (1 site), or lean clay (1 site). Leighton’s soil boring depths did not exceed 5 feet.

4.5 Ramona AAB Subsurface Conditions

Eight out of ten AGI soil borings were started in paved surfaces. Layer thicknesses and approximate total depths to competent native materials are tabulated below.

Location Pavement Thickness

Subbase Thickness

Native Residual Soil Thickness

Native Competent Formation &

Depth

Boring B-1 7.0" older PCC 23" clayey sand none Arkose @ ~30"

Boring B-2 7.25" older PCC 29" clayey sand none Arkose @ ~36"

Boring B-3 6.75" newer PCC 12" clayey sand 13" lean clay Arkose @ ~32"

Boring B-4 6.75" newer PCC 6" clayey sand over 18" silty sand

10" lean clay Arkose @ ~41"

Boring B-5 6.0" newer PCC 6" clayey sand 6" lean clay Arkose @ ~18"

Boring B-6 4.0" hot mix AC 2-3" sandy gravel none Weathered granite @ ~7"

Boring B-7 3.75" hot mix AC 7" clayey sand none Weathered granite @ ~11"

Boring B-8 [Barracks] none – turf none ~24" lean clay Arkose @ ~24"

Boring B-9 [Barracks]

6" older(?) PCC 16" silty sand (DG) none Arkose @ ~22"

Boring B-10 [BMP adjacent] none – turf none ~18" clayey sand fill over ~30" lean clay

Arkose @ ~48"

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Careful examination of the holes revealed variations in fill depths and material qualities indicative of major differences in paved-area site preparations. Fill was markedly thicker under older PCC pavements versus newer PCC or asphalt.

Especially deep fill was not found, though, consistent with historical aerial images of minimal surface irregularities before the air base was built. Weak, natural lean-clay subgrade soil was present at each of the three newer-PCC exploration sites. Our conclusion was that relatively complete excavation and site export of weak soils was employed for the older pavement but not for the last-generation PCC surfaces.

Asphalt-paved areas seemed to represent mass-graded cuts, i.e., original grades were higher than as-built elevations and poor soils were automatically stripped away.

None of explorations encountered layers meeting a definition of “crushed aggregate base” per contemporary specifications (e.g., Greenbook, Caltrans, or FAA). AGI characterized almost all aggregate fill as an imported, yellowish-brown colored, pit-run sandy gravel base. Intact clasts were subrounded, sometimes weathered and friable, and often slightly oxide-stained. We hypothesized that the borrow source was a natural (unprocessed) older alluvium deposit derived from granitic bedrock. At exploration sites B-4 and B-9, substitution with locally derived decomposed granite sand was observed. Basal contacts of fill over native were everywhere sharp, clean, and free of buried organics.

Site residual soils are the product of extreme weathering and decomposition of the underlying parent materials. Sandy lean clay (USCS classification CL-OL) was logged at five borings. The barracks site had an essentially intact, naturally created residual clay horizon about 2 feet thick. Depending upon moisture, the clay ranged from stiff but plastic consistency to hard, desiccated, and cohesive. AGI laboratory tests indicated the clay is very highly expansive (Expansion index > 130), while CBR tests on clay reported by Leighton (2019) produced low values ranging from 1 to 3.

Virtually all of the Ramona AAB site was unexpectedly confirmed to be underlain by an (ostensibly) Quaternary or late Tertiary sedimentary unit classified as fine to medium grained arkosic sandstone. The sandstone was easily drilled, producing distinctly yellow to olive yellow-colored cuttings. Although grain outlines and even an occasional gravel-size fragment could be discerned in samples, all constituent

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feldspar grains were completely altered to clay. Internal layering was periodically visible along with graded beds sometimes topped with laminated claystone. Total depth of the sandstone was not determined, but would be more than 26 feet at the barracks and stormwater BMP sites.

Although geologically considered a consolidated rock formation, the arkose has the engineering characteristics of a dense soil. Terminology and classifications applicable to soils were used in drilling logs and for engineering analyses. Penetra-tion resistance for soil sampling tools in the non-porous and clayey arkose was high and usually in the range of “very dense” relative density.

Two exploration borings intercepted speckled, medium-grained, and highly weathered granitic bedrock considered typical of the Santa Maria Valley area. Plutonic bedrock would also be the predicted unit under arkosic sandstone. Easy drilling effort in granite was observed, and the auger borings produced quality sandy cuttings.

Section 5.2 (Local Geologic Conditions) and the drilling logs in Appendix A contain additional descriptions and interpretations of Ramona AAB soil and bedrock units.

4.6 Groundwater

None of AGI’s soil borings encountered groundwater to the maximum explored depth of 26.4 feet. Recovered soil samples lacked iron oxide mottles or stains that would be evidence for transient or seasonal soil saturation. AGI research also found no evidence for present-day or historical occurrence of rising water such as springs, seeps, or clustered phreatophytic vegetation anywhere within the Ramona AAB leasehold area.

The site is within the Santa Maria Creek groundwater subbasin. The basin is utilized for municipal and agricultural water supplies. Wells reportedly tap deep fractured-bedrock aquifers, although shallow perennial and ephemeral perched-water horizons would be predicted near major watercourses and topographic low spots.

Project-area water levels are believed to be approximately correlated to measured water surface elevations in several (former) environmental monitoring wells installed roughly 1,000 feet east of the proposed barracks building. Based on logged well

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readings, projections of calculated groundwater gradients, and AGI’s site-specific studies, depth to groundwater is estimated to be around 30 feet near the CAL FIRE project area. The project is upgradient of a small historical release of aviation fuels from former underground storage tanks at the Pacific Executive Aviation business at

2450 Montecito Road (Case T0607302420, Calif. WRCB, 2022). Active clean-up efforts including a soil vapor extraction system and pumped groundwater treatment plus re-infiltration via a deep gallery were successfully completed between 2009 and

2012. The County Department of Environmental Health determined that the site met case closure status by May 2014. Minor remaining contaminants were expected to naturally attenuate to below action levels by year 2017 to 2032. The FBO remains in operation.

In light of shallow depths to basement rock, zero on-site groundwater extraction, and the generally rural character of areas upgradient from the Ramona Airport boundary, we judge that groundwater should indefinitely remain below 26½ feet in depth.

Groundwater should not influence building design or construction. Future fluctuations in water elevations or the development of perched horizons remain possible, however, due to variations in precipitation, temperature, consumptive uses, recharge management, or land use changes in Ramona which were not present at the time observations were made.

5.0 ENGINEERING GEOLOGIC ANALYSES

5.1 Regional Geologic Setting

Ramona lies within the Peninsular Ranges Physiographic Province, one of 11 continental provinces recognized in California. The physiographic provinces are topographic-geologic groupings of convenience based primarily on landforms, characteristic lithologies, and late Cenozoic structural and geomorphic history. The

Peninsular Ranges encompass southwestern California west of the Imperial-

Coachella Valley trough and south of the escarpments of the San Gabriel and San

Bernardino Mountains. Most of the province lies outside of California, where it comprises much of the Baja California Peninsula. The province is characterized by youthful, steeply sloped, northwest-trending elongated ranges and intervening valleys.

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Structurally, the Peninsular Ranges province in California is composed of a number of relatively stable, elongated crustal blocks bounded by active faults of the San

Andreas transform system. Tectonic deformations and large earthquakes are mostly limited to the block margins. Exceptions are most notable approaching the Los

Angeles Basin, where compressive stress gives rise to increasing degrees of vertical offset along the transform faults and a change in deformation style that includes young folds and active thrust ramps. Ramona is located near the geographic center of the Santa Ana tectonic block, the longest sides of which are bounded by the active

Elsinore fault zone to the northeast and the Newport-Inglewood and Rose Canyon fault systems to the southwest. The Santa Ana block exhibits remarkable internal structural coherency over a length exceeding 160 miles. Minimal internal disruptions have in turn promoted the preservation of multiple Tertiary-age erosional surfaces atop basement rocks. The Santa Maria Valley is an excellent example of a deeply weathered surface that is sporadically punctuated by residual hard-rock outcrops.

The Peninsular Ranges structural blocks are dominated by the presence of intrusive granitic rock types similar to those in the Sierra Nevada, although the province additionally contains a diverse array of metamorphic, sedimentary, and extrusive volcanic rocks. In general, the metamorphic rocks represent the highly altered host rocks for the episodic emplacement of Mesozoic-age plutons of varying composition.

Parts of the province include thick sequences of younger marine and non-marine clastic sedimentary rocks of Mesozoic and Tertiary age, ranging from claystones to conglomerate. Pre-Quaternary sedimentary rocks are conspicuously absent from almost all of the interior San Diego County ranges, however, which are dominated by crystalline basement materials.

5.2 Local Geologic Conditions

The Ramona-area erosional surface is very slowly being dissected by headward progression of the incised San Dieguito River and its tributaries, including Santa Maria

Creek. Surrounding mountainous terrain features more erosion-resistant bold rock outcroppings. As noted above, however, weathered-rock horizons still harbor relict hard-rock masses and pinnacles. The soil map clip on page 10 features many visible hard granitic corestones (light-colored spots) poking up from highly weathered rock and soil near the airport.

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The arkosic sandstone penetrated by AGI soil borings appears to represent indurated alluvial fills in bedrock paleochannels. We interpret the airport deposits are laterally continuous with at least one mapped island of older sediments located about a half-mile to the south. Hernandez et al. (2007) placed an uncertain Tertiary-Quaternary age assignment on the unit (symbol QTf in Figure 2, next page). However, characteristics such as superposition directly on the older erosional surface and extreme felsic-grain alteration are near-identical to familiar Paleocene and Eocene formations found elsewhere on the Santa Ana tectonic block. We interpret the unit to be a basal, locally derived and finer-grained sequence of the well-known Ballena gravels. The unit age would thus be Eocene. Ballena strata represent one of the uncommon instances of older sediment preservation in the interior County. Fluvial gravel beds containing exotic clasts are perched on ridgetops east of Ramona, but have been mostly stripped away by Quaternary erosion within the Santa Maria Valley.

The maximum depth of the sandstone unit at the Ramona AAB site is unknown. We do know that at least one monitoring well log for the Pacific Executive Aviation FBO site hinted at a sandstone depth exceeding the well’s terminal depth of 30 feet.

Several boreholes had logged gray-colored cuttings (granite?) at around 20 to 24 feet, though. Granitic bedrock of the Japatul Valley pluton would be the expected basement material, based on surface exposures just north of Montecito Road and

AGI’s pavement-area sampling at drilling sites B-6 and B-7.

5.3 Slope Stability

The almost zero-relief site was found to be free of natural features associated with gross instability of slopes. The air base is also distant from the closest mountain terrain to the north. We judge landslide risks to be nil.

5.4 Faulting and Regional Seismicity

The project is situated in region of active and potentially active faults, as is all of metropolitan Southern California. Active faults present several potential risks to structures and people. Hazards associated with active faults include strong earthquake ground shaking, soil densification and liquefaction, mass wasting

(landsliding), and surface rupture along active fault traces. Four factors can be generalized as the principal determinants of seismic risk at a given location (p. 19):

0 2000 4000 ft.

Selected vicinity units:

af Man-made fill (Ramona Airport) Qya Unconsolidated younger alluvial valley fill deposits (late Holocene) QTf Indurated arkosic sandstone and cobble conglomerate [QTf-l] (queried Tertiary, probable Eocene)

Kjv-w Kjv Crystalline basement rocks of the Peninsular Ranges Batholith. Kjv-w is deeply weathered tonalite Kc assigned to the Japatul Valley pluton, interpreted as the oldest local granitic intrusion (Cretaceous).

Kcp Kwmd

Reference: Modified after Hernandez, et al., 2007. Scale is approximate.

VICINITY GEOLOGIC MAP

CAL FIRE RAMONA AAB, RAMONA, SAN DIEGO COUNTY, CALIF.

PROJECT NO. 4732-SF DATE: 5/5/22 FIGURE 2

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Distance to seismogenically capable faults.

The maximum or “characteristic” magnitude earthquake for a capable fault.

Seismic recurrence interval, in turn related to tectonic slip rates.

Nature of earth materials underlying the site.

5.4.1 Fault Rupture Potential

Surface rupture presents a primary or direct potential hazard to structures built across an active fault trace. Reviews of official maps delineating State of Californ-ia Earthquake Fault Zones and San Diego County Fault Hazard Management zones indicated the project site is distant from zones of mandatory investigation for active faulting (Calif. Geological Survey, 2022a; San Diego County, 2007;

2021). The closest active regional fault source with young surface displacements would be the Elsinore Fault in the Lake Henshaw area about 15.8 miles away at closest approach. Around two miles east of Ramona Airport is the concealed

Temescal Fault, a pre-Quaternary inactive feature that could be responsible for vertical throw between basal Ballena sedimentary deposits located east and west of the fault line. Temescal fault-line surface traces are limited to mountain terrain north of Ramona, where the fault separates disparate granitic rock types.

AGI’s aerial photographic interpretations did not suggest visible lineaments or other manifestations of fault-line topography related to active faults on or next to the site. Subtle features would have been obliterated by cultivation even in the oldest available imagery, however. The site area has also been heavily modified by airport construction. Nonetheless, we judge that chances for direct surface fault rupture affecting the project are extremely low.

5.4.2 Strong Motion Potential

All Southern California construction is considered to be at risk of experiencing strong ground motion during a structure’s design life. Although hundreds of historical micro-earthquakes have been recorded within a dozen miles of the project site, felt earthquakes are infrequent. The central Santa Ana tectonic block is relatively quiescent from an earthquake source perspective.

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Current and future probabilistic risk models for the Ramona area fundamentally assign the highest seismic risks from random (gridded point-source model) seismicity with imputed maximum-magnitude events of M5.8 to M6.1 at a 2,475-year return period. Random seismicity accounts for over three-fourths of the airport’s probabilistic risk. Large characteristic earthquakes along the Julian segment of the Elsinore Fault are the sole fault-line events contributing more than

5% to total hazard. Farther east, the San Jacinto Fault has significant historical seismicity, but at a distance of ~38 miles and greater, shaking hazard from this source is low. The mode-magnitude event for peak ground acceleration at a 2% in 50-year exceedance risk is a random Mw6.3 earthquake at an imputed distance of ~10 kilometers from the project (U.S. Geological Survey, 2022c; dynamic conterminous U.S. 2014 model).

The searchable ANSS Comprehensive Earthquake Catalog indicates about 59 events of local magnitude M4.5 or greater have occurred within 100 kilometers of the project since instrumented recordings started in 1932 (Figure 3, next page).

The map shows that almost all plotted earthquakes have occurred far to the east of Ramona AAB in the Colorado Desert and Anza areas. Several had local magnitudes greater than M6.0 and caused light structural damage near the earthquake epicenters. Peak ground accelerations at Ramona are not calculated to have exceeded 0.05g for any depicted seismic event, however.

Elsinore Fault Zone: This fault zone and related extensions trend northwesterly from the Sierra El Mayor region in northern Mexico to an indistinct terminus near

Whittier in Los Angeles County. Like other faults that parallel the better-known

San Andreas Fault, the Elsinore fault zone is one component part of the San

Andreas transform plate boundary. The predominant sense of offset in the zone is right-lateral strike-slip, although short segments are reported to exhibit mostly normal or reverse offset (Kennedy, 1977). Geomorphic expression of the zone is strongest in the 50-odd miles between Pauma Valley and Corona, where multiple parallel and right-stepping en-échelon fault strands bound deep sediment-filled subsiding basins. Recorded earthquakes are much fewer for the Elsinore fault zone than the San Jacinto or San Andreas faults, suggesting low mean slip rates and longer recurrence intervals between events. Only one significant

Reference: U. S. Geological Survey (2022b) real-time earthquake epicenter map. Plotted are 59 epicenters of instrument-recorded events from 1932 to present (3/15/22) of local magnitude M4.5 or…