Wetlands Geotechnical Tech Memorandum(Appendix D BOD OCT2018).pdf

Veterans Affairs Alameda Point

October 2018

Appendix D Geotechnical Technical

Memorandum

October 2018

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U.S. Department of Veterans Affairs | Alameda Point Project Wetland Mitigation Geotechnical Investigation and Recommendations Technical Memorandum

Technical Memorandum Date: Wednesday, August 01, 2018

Project: Alameda Point Project Wetland Mitigation

To: Dawn Edwards

CC: Matt Redington, P.E., Libby Mesbah, P.E.

From: Victor Crosariol, P.E. and Edwin Woo, P.E., G.E.

Subject: Geotechnical Investigation and Recommendations

1.0 Project Background

This technical memorandum presents the results of a geotechnical investigation conducted by HDR Engineering, Inc. (HDR) to support planning and design for onsite wetland mitigation as part of the overall Veterans Affairs (VA) Alameda Point Project. The wetland mitigation site is located in the southern portion of the former Alameda Naval Air Station (Alameda Airfield) on Alameda Point, in the City of Alameda, California, as shown on the Vicinity Map, Figure 1.

1.1 Project Background

The VA is proposing 113 acres of facilities development in the northern portion of an approximately 624 acre site at the Alameda Airfield. The balance of approximately 511 acres will remain primarily undeveloped as a California Least Tern conservation area. The southernmost portion of this conservation area (bordering San Francisco Bay) is proposed for construction and enhancement of up to 30 acres of onsite wetland mitigation to offset impacts to jurisdictional wetlands in the proposed development area. The onsite wetland mitigation will include creation of about 7.3 acres of new tidal salt marsh within a “wetland creation area” and enhancement of about 13.2 acres of existing tidal marsh/salina habitat within a “wetland enhancement area”. Recommendations in this geotechnical technical memorandum pertain to the wetland creation area.

1.2 Project Description

According to a design criteria memorandum by HDR’s ecological subconsultant, H.T. Harvey & Associates (HTH), the intent of the tidal marsh is to create 7.3 acres of high quality, self sustainable tidal salt marsh meeting the U.S. Army Corp of Engineering (USACE) definition of jurisdictional wetlands and other waters (HTH, 2017a). To meet this intent, the wetland creation area will consist of three primary components: 1) the tidal marsh creation area (tidal marsh), 2) a transition zone creation area (transition zone), and 3) a tidal inlet. The tidal inlet and portions of the transition zone are not included in the required 7.3 acres of tidal marsh. The total wetland creation area covers a footprint of about 12 acres, which includes approximately 8.0 acres of tidal marsh. The additional 0.7 acres of tidal marsh is included to account for design and construction uncertainties. Refer to Figure 2 (Site Plan) for a layout of the wetland creation area showing the primary project features described below.

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The tidal marsh will consist of a relatively flat marsh plain area with a dendritic slough channel network designed to promote an unmuted tidal regime that fully drains the marsh during each tide cycle. The majority of the marsh plain will be graded to about Elevation1 (El.) +6.4 feet.

The dendritic slough channel system will consist of small first order channels that flow into slightly larger second order channels that in turn flow into the main third order channel. The third order channel will be approximately 870 feet long with a 20 foot bottom width. It will meander down the middle of the tidal marsh at a uniformly gentle slope from about El. +4.5 feet at the top to El. of +2.2 feet at the tidal inlet where the channel discharges into the San Francisco Bay.

The transition zone creation area will consist of gently sloping vegetated areas grading up to the surrounding Alameda Airfield topography. According to HTH’s design criteria memorandum, the transition zone will improve habitat quality and provide resilience to future sea level rise by allowing for limited marsh transgression (HTH, 2017a).

The tidal inlet will consist of a riprap lined opening into the existing rock seawall (seawall) at the south edge of Alameda Airfield. To provide a smooth transition for flow into and out of the tidal marsh, the tidal inlet will include approximately 50-foot long riprap training dikes at each bank.

The top elevation of the training dikes will approximately match the top of the seawall at El. +10 feet.

An approximately 15 foot wide riprap “splash pad” is planned on the landside of the existing rock seawall to protect the ground surface against erosion from wave action overtopping the seawall.

The splash pad will also serve as an access road for vehicles and equipment to access the tidal inlet for maintenance purposes.

Refer to Figure 2 (Site Plan) for a layout of the project features described above.

1.3 Scope of Services

The purpose of our study was to evaluate the geotechnical aspects of the project. The scope of our services included:

• Compiling and reviewing available existing geologic data for the project area;

• Performing subsurface investigation consisting of exploratory borings and laboratory testing to obtain information on subsurface conditions within the project area;

• Performing engineering analyses of the available data to develop geotechnical conclusions, recommendations, and design criteria for the project, including:

o Site geology and seismicity;

o Soil and groundwater conditions encountered;

o Discussion of the potential for seismic hazards;

1 All elevations in this report are referenced to the North America Vertical Datum of 1988 (NAVD88).

hdrinc.com 1111 Broadway, Suite 1670, Oakland, CA 94607-4007 o Discussion and results of the stability analyses of the proposed tidal inlet configuration;

o Recommendations for earthwork including site grading, subgrade preparation for riprap, allowable fill materials, placement and compaction of fill, and suitability of onsite soil for use as fill;

o Discussion of construction considerations for grading and excavating the wetland creation area.

• Preparing this geotechnical memorandum presenting the results of our field exploration, laboratory testing, discussion of geotechnical considerations, and geotechnical recommendations.

2.0 Subsurface Exploration and Laboratory Testing

HDR’s geotechnical subsurface investigation program included advancing 10 hollow stem auger borings, designated Boring B-01 to B-10, to obtain information on subsurface conditions within the wetland creation area. Borings B-01 to B-03 were advanced immediately adjacent to the back of the seawall. Borings B-04 and B-05 were advanced 7 and 10 feet north of Borings B-01 and B-02, respectively. These closely spaced borings pairs (i.e. pairs B-01/B-04 and B02/B-05) were aligned perpendicular to the seawall to help identify the approximate depth and lateral extent of the buried land-side portion of the seawall. Borings B-06 through B-10 were located on or adjacent to the airfield runway/taxiway, between 140 to 550 feet north of the seawall, to characterize the soil conditions beneath the proposed tidal marsh.

Cal Engineering & Geology (CE&G) was retained by HDR to facilitate the drilling program, including coordinating the pre-drilling activities and logging the soils encountered in the borings.

Prior to performing subsurface investigations, CE&G procured the required Alameda County Public Works Agency (ACPWA) drilling permit and contacted Underground Service Alert (USA) to check for the presence of underground utilities.

Exploration Geoservices, Inc. (EGI) advanced the borings on Monday, March 27, 2017 and Tuesday, March 28, 2017 using a truck-mounted Mobile B-53 drill rig. Hollow-stem auger drilling was performed which provides a continuously cased hole to protect against collapse during drilling and sampling. Soil samples were collected at approximately 1.5-foot to 5-foot intervals. Disturbed samples were obtained by driving either a Standard Penetration Test (SPT) split-barrel sampler without liners or a Modified California split-barrel sampler with 6-inch long liners. Resistance blow counts were obtained with both Modified California and SPT samplers by dropping a 140-pound automatic trip hammer through a 30-inch free fall. The test borings were sealed upon completion with cement grout in accordance with the ACPWA drilling permit conditions.

Soil samples collected from the borings were initially classified and described by a CE&G field geologist in general accordance with ASTM D2488. Soil samples were transported to CE&G’s geotechnical laboratory for selected testing and confirmation of classification. Field soil descriptions were updated as needed based on laboratory testing results in accordance with hdrinc.com 1111 Broadway, Suite 1670, Oakland, CA 94607-4007

ASTM D2487. The following laboratory testing was performed on samples collected from the borings: moisture content and density (ASTM D2216), sieve analysis (ASTM D422), sieve analysis plus hydrometer (ASTM D422), Atterberg Limits (ASTM D4318).

The approximate locations of the borings are shown on Figure 2. The locations of the explorations were determined by hand-held GPS and tape measuring from existing site features and are accurate only to the degree implied by the method used. Logs of the borings are presented in Attachment A. The results of the laboratory tests are presented on the boring logs at the appropriate sample depths and/or in Attachment B.

3.0 Geologic Setting

3.1 Regional Geology

The project site is located in the eastern portion of the San Francisco Bay Area in the Coast Ranges geomorphic province of California, which is characterized by northwest-southeast trending valleys and ridges. These valleys and ridges are controlled by folds and faults that resulted from the collision of the Pacific and North American plates, subduction of the Pacific Plate beneath the North American Plate, and subsequent strike-slip faulting along the San Andreas Fault zone and the plate boundary fault systems. Bedrock underlying the region is primarily of the Franciscan Complex, characterized by a diverse assemblage of sandstone, shale, chert, greenstone and mélange.

Geologic formations in the San Francisco Bay Region range in age from Jurassic (190 to 135 million years ago) to recent Holocene (less than 11 thousand years ago). The Franciscan Complex is the oldest, and underlies younger surficial deposits throughout the San Francisco Bay Region. Following deposition, the Franciscan rocks were regionally uplifted and, in the process, extensively faulted and folded.

The Bay Area has experienced several episodes of uplift and faulting during late Tertiary time (about 25 to 2 million years ago). This produced a series of northwest-trending valleys and mountain ranges, including the Berkeley Hills, the San Francisco Peninsula and the intervening San Francisco Bay. Uplifted areas were eroded, and as a result, Pleistocene and recent marine sediments were deposited in the San Francisco Bay and stream and marshland sediments were deposited in low-lying areas adjacent to the Bay.

3.2 Regional Seismicity

Geologists and seismologists recognize the San Francisco Bay Area as one of the most active seismic regions in the United States. Active faults extending through the Bay Area have produced 11 large (moment magnitude, Mw 6.0 or greater) earthquakes over the last two centuries that have damaged buildings and other infrastructure. Most recently, the August 24, 2014 Mw 6.0 South Napa Earthquake caused extensive damage to the local built environment.

The faults causing such earthquakes are part of a system of faults along the boundary of the Pacific and North American plates and locally include the San Andreas, Calaveras, and Hayward-Rodgers Creek faults. The major fault in the system is the San Andreas Fault that extends for at least 450 miles along the coast of California.

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The 2014 Working Group on California Earthquake Probabilities (WGCEP) published an updated report evaluating the probabilities of significant earthquakes occurring in the Bay Area over the next three decades (Field et al, 2015). The WGCEP estimated that there is a 72 percent probability that at least one moment magnitude 6.7 or greater earthquake will occur in the San Francisco Bay region before 2044. This probability is an aggregate value that considers principal Bay Area fault systems and unknown faults (background values) including the potential for multi-fault ruptures. The principal active faults in the Bay Area include the San Andreas, Hayward-Rodgers Creek, Calaveras, and the San Gregorio faults. Earthquakes occurring along these faults are capable of generating strong ground shaking at the project site.

Table 1 summarizes the approximate distances between the site and the six closest mapped active or potentially active faults based on the 2008 update to the United States National Seismic Hazard Maps (NSHM) online Fault Parameter database2 (USGS, 2008). The online information is documented by the United States Geological Survey (USGS) Open File Report 2008–1128 (Petersen et al., 2008). We note that the 2008 NSHM Fault Parameter Database is not a complete list of bay area faults that could cause ground shaking at the site. The project site is not located within an Alquist-Priolo Earthquake Fault Zone.

Table 1. Regional Faults and Seismicity

Fault (segments)

1Approximate Distance from Site, mi (km) Direction from Site Maximum Moment

Magnitude 2Hayward-Rodgers Creek

(RC+HN+HS) 6.5 (10.5) Northeast 7.3 3North San Andreas;

(SAO+SAN+SAP+SAS) 11.6 (18.6) Southwest 7.9

San Gregorio Connected 15.8 (25.4) Southwest 7.5 Mount Diablo Thrust 16.5 (26.6) East 6.7

4Calaveras (CN+CC+CS) 16.9 (27.2) East 7.0 Green Valley Connected (includes Concord Fault) 19.7 (31.6) Northeast 6.8

1. Approximate fault distances are in reference to latitude 37.7774 degrees and longitude -122.3148 degrees

2. Hayward segments: RC = Rodgers Creek, HN = Northern Hayward, HS = Southern Hayward

3. San Andreas segments: SAO = Offshore, SAN = North Coast, SAP = Peninsula, SAS = Santa Cruz

Mountains

4. Calaveras segments: CN = Northern Calaveras, CC = Central Calaveras, CS = Southern Calaveras

Earthquakes on these or other active or potentially active faults (including unmapped faults) could cause strong ground shaking at the site. Earthquake intensities can vary depending upon the magnitude of the earthquake, the distance of the site from the causative fault, the type of materials underlying the site, and other factors.

2 The USGS 2008 United States National Seismic Hazard Maps Fault Parameter Database is not a complete list of bay area faults.

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4.0 Site Conditions

4.1 Surface Conditions

The wetland creation area is located in the southern portion of Alameda Airfield, which was constructed on an artificial island created by first constructing rock dike containment walls3 in the San Francisco Bay and then infilling between the walls with dredged materials. In about 1958 the airfield was expanded to the south using similar construction methods to accommodate the extension of Runway 31 and other improvements. The wetland creation area is within this southern airfield expansion area. The proposed wetland creation area is bounded by the rock seawall to the southwest and south, existing tidal marsh/salina habitat to the east, and portions of Alameda Airfield runway/taxiway to the north and northwest. Most of the project area is covered by asphalt concrete (AC) and concrete paving associated with the airfield runway. In the southern portion of the site, there are unpaved areas on each side of the Runway 31 approach section that are predominantly covered by ice plant. A line of pilings extend to the south into the bay along the axis of the runway which once supported runway approach lights. The adjacent unpaved areas were found to be soft, weak and unstable and were not able to support the truck-mounted drill rig without the use of mats.

Most of the site is relatively level. Based on review of the project survey performed by Michael Dequine and Associates, Inc. (MDA, 2017), site grades within the proposed wetland creation area range from about El. +9 feet in the southern portion of the site to about El. +11 feet within the runway area at the northern portion of the site. Site grades in the adjacent unpaved areas are typically about ½ foot to 1 foot lower than the paved areas.

The crest of the seawall ranges from about El. +8 feet to El. +11 feet, has an approximate width of 6 to 9 feet, and on the surface consists of primarily of large boulders that are several feet in diameter. Some cobbles and smaller boulders line the landside edge of the seawall or fill void spaces between the larger boulders. The waterside surface of the seawall is typically sloped at about 1.2H:1V (Horizontal to Vertical) to 1.3H:1V from the crest down to about El. 0 feet. Based on project bathymetry data4, the bayside slopes flatten to about 5H:1V below El. +0 feet down to about El. -3 feet to -6 feet.

4.2 Site Geology

The Alameda Airfield is located on manmade landfill, which is consistent with Graymer (2000) who mapped surficial deposits at the wetland mitigation site as being underlain by artificial fill (af). The fill consists of hydraulically placed material dredged from the bay in the early part of the 20th century5. The map shows Quaternary-age dune sand (Qds) about one mile east of the site, on the natural portion of Alameda Island. Blake et al. (2000) describes these deposits as

3 For the purpose of this geotechnical memorandum, potential historic rock containment dikes are not differentiated from the rock seawall.

4 Bathymetry data was collected by MDA’s subcontractor, Meridian Surveying Engineering, Inc., and the data was integrated into the project survey map.

5 Hydraulic fill information source: http://www.militarymuseum.org/NASAlameda.html hdrinc.com 1111 Broadway, Suite 1670, Oakland, CA 94607-4007

“undifferentiated” (Qu), which could include beach sand, marine deposits, artificial fill, alluvium, or landslide material.

4.3 Subsurface Conditions

Subsurface conditions within the limits of the proposed wetland creation area generally consist of hydraulic fill overlying either the existing rock seawall (in the portion immediately adjacent to the seawall) or alluvium. Asphalt concrete, where present, was found to range from about 1.5 to 3 inches in thickness underlain by up to about 11 to 15 inches of aggregate base. No borings were advanced on tarmac areas surfaced with concrete, so concrete thickness was not determined.

Based on Borings B-06 to B-10, the hydraulic fill observed inland (i.e. away from the existing rock seawall) generally consists of very loose to medium dense, fine- to medium-grained poorly-graded sand, poorly-graded sand with silt, and silty sand. These sandy soils were found to be loose to medium dense in the upper 5 to 6 feet thick zone, underlain by very loose to loose sand extending to about 12 to 18 feet deep, then underlain by medium dense sand to the contact with the underlying alluvium. Fines content (primarily silt) was found to range from about 1 percent to 32 percent based on grain size analysis testing. Traces of shell fragments were observed intermittently within the fill. Alluvium was encountered underlying the hydraulic fill at depths of about 21 and 24.5 feet in Borings B-10 and B-09, respectively. The alluvium was found to consist of stiff sandy silt to very dense fine-to medium-grained silty sand. This alluvium may be associated with the dune sand or undifferentiated geologic unit noted in Section 4.2 above.

Subsurface conditions adjacent to the seawall in Borings B-01 to B-05 are more variable than in the inland borings. The fill observed near the seawall was found to consist of both coarse-grained and fine-grained soils, except in Boring B-04 where fine-grained soils were not observed. The coarse-grained soils consist mostly of very loose to loose poorly-graded sand, poorly-graded sand with silt, and silty sand. Intermittent gravel layers with varying amounts of sand and silt were also encountered near the seawall. The fine-grained soils near the seawall were found to be highly variable and ranged from very soft to stiff silt to very soft to medium stiff lean and fat clay. Based on the two boring pairs that were advanced (B-01/B-04 and B-02/B- 05), the fine-grained layers appear to taper out after only a short distance north of the seawall.

For example, silt and clay layers were encountered in Boring B-01, which is nearest to the seawall; however, silt and clay were not encountered in Boring B-04, which was advanced just 7 feet northward of Boring B-01.

Drilling refusal was encountered at the assumed seawall contact in Borings B-01 to B-05 at depths from about 15 to 23 feet below existing site grades. Based on the depth to which drilling refusal was encountered in these borings, the land-side below-grade slope of the seawall was estimated at the boring locations to be as steep as about 0.7H:1V to 0.8H:1V along the upper portion of the wall and then flattening out to 1.1H:1V to 1.6H:1V near the bottom of the wall.

The below-grade seawall slopes are difficult to estimate and could be flatter or steeper in other areas away from the boring locations. The depth to the base of the seawall could not be determined from the borings performed.

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HTH performed a soils investigation at the site for the purpose of evaluating ecological suitability of the onsite soils to meet mitigation design goals (HTH, 2017b). Their investigation included excavating 12 test pits ranging in depth from about 7 to 9 feet below existing site grades.

Consistent with observations from our borings, HTH noted that the soil consisted of homogeneous medium-grained to very fine-grained sand. Laboratory testing from HTH’s investigation indicated fines content ranging from about 2 to 4% and organic matter ranging from 0.4 to 0.6% by dry weight. Also, portions of the onsite sandy soils were found to contain concentrations of the heavy metal cadmium above allowable screening levels, and HTH recommends that these soils be removed and replaced (HTH, 2017b). Limits for removal are discussed in their memorandum, and related geotechnical recommendations for earthwork are presented in Section 6.1 of this memorandum.

4.4 Groundwater Conditions

Given that the wetland mitigation site is immediately adjacent to the bay, groundwater levels are expected to be shallow (i.e. within a few feet below the ground surface). This is consistent with our subsurface investigation in which groundwater was encountered at the time of drilling at depths of about 4 feet in Boring B-07, 2 feet in Boring B-08, 3 feet in Boring B-09, and 3 feet in Boring B-10. We note that these borings may not have been left open for a sufficient period of time to establish equilibrium ground water conditions. Consistent with our borings, HTH noted in their soils investigation memorandum that they “found groundwater relatively close to the marsh plain design grade” which is planned to be at about El. +6.4 feet (HTH, 2017b).

Groundwater levels are likely subject to tidal influence, especially in areas immediately adjacent to the seawall. Fluctuations in the groundwater level could also occur due to changes in seasons, variations in rainfall, and other factors.

5.0 Analyses and Conclusions

5.1 Seismic Hazards

The wetland creation site is located in a seismically active region of California. Significant earthquakes in the Bay Area have been associated with movements along well-defined fault zones. Earthquakes occurring along any of a number of other Bay Area faults have the potential to produce strong ground shaking at the site. In addition to ground shaking, earthquakes have the potential to induce other phenomena that could damage the wetland creation area including: soil liquefaction, lateral spreading, slope deformations, ground cracking, and tsunamis. A qualitative discussion of the potential for some of these seismic hazards to impact the wetland creation site is provided below. We note that a detailed and quantitative evaluation of seismic hazards is beyond the scope of this geotechnical study. Additionally, it is our understanding that seismic hazards would not be mitigated due to their limited potential to impact human life safety, and because of the prohibitive cost of mitigating seismic impacts for projects of this type.

Soil liquefaction is a phenomenon in which saturated (submerged), cohesionless soil experiences a temporary loss of strength due to buildup of excess pore water pressure during hdrinc.com 1111 Broadway, Suite 1670, Oakland, CA 94607-4007 cyclic loading induced by an earthquake. The soils most susceptible to liquefaction are loose, clean, saturated, poorly (uniformly) graded sand, and non- to low-plasticity silt or silty sand.

Denser soils are more resistant to liquefaction than looser soils, and soils with significant fines content are more resistant to liquefaction than clean sands. The hydraulic fill present at the site, consisting of very loose to medium dense sand to silty sand, generally meets the aforementioned characteristics for liquefiable soils. Historically, the Alameda Airfield experienced liquefaction during the 1989 Loma Prieta Earthquake (Mw 6.9) (Kayen et al., 1998).

Therefore, the onsite hydraulic fill soils should be considered highly-susceptible to liquefaction from a significant earthquake.

Kayen et al. (1998) indicated that widespread liquefaction during the 1989 earthquake damaged the existing runway pavements to the point of inoperability. Evidence of liquefaction manifested as sand boils, settlements, and minor lateral spreading. Liquefaction-induced settlements of up to 30 centimeters occurred over large areas in the west end of Alameda Airfield. Similar to effects observed from the 1989 earthquake, areas of the site could experience significant liquefaction-induced settlements and localized ground disturbance such as cracking and sand boils. Additionally, because the final wetland creation area will feature sloping ground and free slope faces at the slough channels, wide-spread ground deformations from liquefaction-induced lateral spreading are possible. With only gradually sloping ground, magnitude of lateral spreading deformations can sometimes be measured on the order of several feet.

Strong ground shaking could potentially induce slope deformations at the transition zones, the dendritic slough channels, and the tidal inlet. Slope deformations could be caused by or exacerbated by the presence of liquefiable soils.

Major earthquakes can generate tsunamis in large bodies of water such as the San Francisco Bay. Tsunamis are generally caused by earthquakes associated with subduction zones, the nearest of which is the Cascadia Subduction Zone (CSZ). The CSZ extends over 600 miles from Cape Mendocino on the Northern California Coast to Vancouver Island, Canada, and is thought to be capable of producing Mw 9.0+ “megathrust” earthquakes. The most recent great earthquake (Mw 8+) along the CSZ occurred in January 1700 and was estimated to be Mw 9 (Satake et. al 1996). The earthquake caused a tsunami that impacted areas as far away as Japan. Because of the lack of reliable information about the kind of tsunami run-ups that have occurred in the prehistoric past, there is considerable uncertainty over the extent of tsunami run-up that could occur along coastal San Francisco Bay. The Tsunami Inundation Map for Emergency Planning for Alameda County (CGS, 2009) shows the entire Alameda Airfield within the tsunami inundation area. Additionally, the wetland mitigation site grades will be between about 3 and 8 feet below existing site grades and will be open to bay through the tidal inlet.

Therefore, although tsunamis are rare, the wetland mitigation site would likely be impacted in the event of a significant tsunami inundating the San Francisco Bay. Tsunami impacts could include complete inundation of the wetland creation area, erosion damage to the wetland surface, and deposition of mud and deleterious debris.

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5.2 Tidal Marsh and Transition Zone Grades

The tidal marsh will be below El. +6.67, which corresponds to 0.3 feet above the Mean Higher High Water tidal datum (MHHW is at El. +6.37 feet). The majority of the marsh plain will be graded flat to about El +6.4 feet with limited areas that grade downward at 2 percent to provide drainage into the first and second order channels. The first, second, and third order channels will have invert elevations as low as +4.75 feet, +4.5 feet, and +2.2 feet, respectively. All of these areas will be graded into primarily very loose to loose sandy soils, which are erosive and exhibit relatively high permeability.

Given the nature of the soils, the near constant presence water from fluctuating tides and high groundwater, and the potential for seasonal and longer term climate changes, the marsh plain configuration should be expected to change over the life of the project. Potential changes could be caused by erosion or sediment deposition of soils within the marsh plain and the channels.

Changes to the marsh plain and the channels may be more pronounced early in the life of the project before the marsh vegetation is fully developed, including during construction (as discussed in Section 5.7). The vegetation root systems, once developed, will provide a stabilizing effect to the marsh plain and channel banks.

The slough channels are currently planned with 3H:1V side slopes which should be sufficient to limit the potential for significant slope movements. However, due to the tide cycles, seepage through the sandy channel banks, and surface runoff, the channels should be expected to reshape from their original geometry and meander from their original location over the life of the project, including during construction (as discussed in Section 5.7). Reshaping of the channels could manifest as surface sloughing and raveling, uneven slopes, rill erosion, depressions, or other surface irregularities. Evaluating short-term and long-term changes to the tidal marsh configuration, including stability of the channel banks, is beyond the scope of this geotechnical investigation. Changes to the channel configuration should not be considered detrimental to the project provided that primary ecological design goals are still met.

The perimeter transition zones between the existing Alameda Airfield topography and the tidal marsh will consist of vegetated slopes ranging from 10H:1V to 30H:1V. The elevation of the transition zones will be above the tidal marsh elevations (i.e., above El. +6.67). Therefore, these transition zones will typically be above water levels within the wetland creation area, except during periodic high water events (i.e., king tides and storm surges). Based on the relatively gentle slopes and grades being higher than typical water levels, these perimeter transition zones should remain relatively stable to allow vegetation root systems to develop.

Changes to grades within the transition zone due to erosion and deposition should be expected, but these will likely be less pronounced than in channels and lower tidal marsh areas.

5.3 Organic Soil Amendments

HTH performed a soils investigation at the site for the purpose of evaluating ecological suitability of the onsite soils to meet mitigation design goals (HTH, 2017b). They recommended that organic soil amendments be implemented to increase organic content to between 5 to 30 percent by dry weight. HTH’s ecological design criteria memorandum recommends that the hdrinc.com 1111 Broadway, Suite 1670, Oakland, CA 94607-4007 upper 6 inches of soil within the marsh plain and transition zone areas be homogenously mixed with composted organic matter (HTH, 2017a).

If potential erosion of the marsh plain early in the life of the project (as discussed in Section 5.2) is not acceptable, the project team could consider including a small amount of clayey material within the organic soil amendment. The inclusion of clayey soils would provide some additional level of erosion protection to the surface of the marsh plain, because clayey soils are more resistant to erosion than the onsite silty and sandy soils. HTH would need to be consulted to confirm if a clayey organic soil amendment would satisfy their ecological design criteria.

Detailed organic soil amendment recommendations are provided in HTH’s Wetland Mitigation and Monitoring Plan (MMP) (HTH, 2018).

5.4 Tidal Inlet Stability

Working with HTH, the HDR team developed a concept for the tidal inlet consisting of a riprap lined opening into the seawall at the south end of Alameda Airfield. HDR developed a two dimensional hydraulic model to evaluate various tidal scenarios and wetland basin configurations. Geometric characteristics were varied in the hydraulic models including channel bottom width, floodplain elevations, and tidal inlet width. Based on the hydraulic modeling and ecological design criteria provided by HTH (2017a), the 50 percent design level configuration consists of the following:

• A bottom width of 20 feet and 3H:1V side slopes corresponding to the geometry of the third order channel. The bottom and slopes of the tidal inlet will be lined with riprap.

• A tidal inlet invert elevation of +2.2 feet, which is about 1 foot below Mean Sea Level (MSL is at El. +3.22 feet).

• Riprap training dikes that will be about 50 feet long with a top width of 10 feet. The top elevation of +10 feet approximately corresponds with the top of the seawall. The training dikes are parallel to the third order channel where they intersect with the seawall. For the purpose of the stability analyses described below, the base of the training dikes are assumed to be at El. +6 feet.

Due to the relatively loose and soft onsite soils, we judge that the primary geotechnical concern for the tidal inlet is stability of the riprap lined slopes. The above described configuration was considered in our evaluation of tidal inlet slope stability. We selected an analysis cross-section perpendicular to the channel centerline, as shown on Figure 2 (Site Plan). The tidal inlet geometry analyzed is shown on the Figure 3 (Tidal Inlet Analysis Cross-Section).

Slope stability analyses were performed using the limit equilibrium software program SLOPE/W (GEO-SLOPE, 2016). The Spencer analysis method was used to satisfy both moment and force equilibrium (Spencer, 1967).

As discussed in Section 4.3, the fill near the seawall was found to contain intermittent layers of soft clay and silt that grade out to predominantly sandy soils just a short distance from the seawall. To account for this potential variability in soil conditions over short distances, relatively hdrinc.com 1111 Broadway, Suite 1670, Oakland, CA 94607-4007 conservative soil strength parameters were selected, and both undrained and drained conditions were analyzed, as described below:

• Case 1, Undrained Loading: The loading condition represents hydraulic fill material close to the seawall that may contain intermittent layers of soft fine-grained soils (i.e. silt or clay).

• Case 2, Drained Loading: The drained condition represents hydraulic fill material farther from the seawall that likely consists of predominantly loose to very loose coarse-grained soils (i.e. sand).

From a stability standpoint, we judge that the critical groundwater condition is one in which the groundwater table outside the channel is relatively high compared to the water level within the channel. This condition may be present when the water levels within the channel drain more quickly than the surrounding soils. This is similar to when the tidal basin drains back to the bay prior to the return of the next high tide cycle. The basin is configured to allow for complete drainage between the high tide cycles (i.e. an “unmuted condition”). To account for these conditions in our analyses, pore pressures were modeled using a static piezometric line with the following characteristics (shown on Figure 3):

• The water surface elevation (WSE) on the land-side of the training dike (i.e. away from the channel) was set at El. +6.4 feet corresponding to the tidal marsh plain elevation and approximately the MHHW tidal datum.

• Along the channel slope, the WSE follows the bottom of the riprap layer based on the assumption that the riprap layer drains instantaneously.

• Along the bottom of the channel, the WSE is at the tidal inlet invert elevation (El. +2.2 feet), representing complete drainage of the tidal marsh and third order channel.

Factors of safety against slope failure of the riprap lined tidal inlet slopes were found to be 1.53 for Case 1 (undrained loading) and 1.66 for Case 2 (drained loading). Although no specific factor of safety criteria has been adopted for this project, we consider these factors of safety to be adequate based on commonly accepted slope stability criteria. The results of these analyses are presented in Figures 4 and 5 for undrained (Case 1) and drained (Case 2) loading, respectively.

5.5 Riprap

The tidal inlet and training dikes will be composed of riprap underlain by 12 inches of aggregate bedding, geogrid, and geotextile. The splash pad will be composed of riprap underlain by a 6 inch aggregate filter layer. The purpose of the geotextiles and aggregate filters are to reduce the potential for finer onsite soils to migrate into the riprap. These geotextile and aggregate filter layers should be designed based on the riprap gradation and the gradation of the underlying soil. We judge that the 3H:1V side slopes of inlet channel and training dikes are sufficient to limit the potential for the riprap to move downslope overtime.

Although developing detailed riprap recommendations is beyond the scope of this geotechnical evaluation, we have the following recommendations:

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• The aggregate bedding or filter layers should be designed to provide a stable base for riprap over weak or unstable soils that may be encountered in the excavations.

Subgrade preparation recommendations are presented in Section 6.5.

• Riprap areas within the tidal inlet area should include stabilizing geogrid beneath the aggregate bedding layer. The geogrid should be composed of material with a specific gravity greater than that of seawater (i.e., greater than 1.03). One such non-buoyant material that is commonly available for geogrid reinforcement is polyester which has a specific gravity of 1.22 to 1.38 (Sarsby, 2006).

Further recommendations for riprap subgrade preparation and placement are presented in Section 6.5.

We conclude that the riprap splash pad can serve as an access road along the bay-side perimeter of the wetland creation area provided that the riprap is surfaced with a finer choke stone course (i.e., stones graded finer than the rip rap to provide a smoother surface course).

The choke stone course should be graded such that it provides sufficient access for rubber tired vehicles while maintaining the intended erosion protection of the splash pad.

5.6 Settlement

The majority of the wetland mitigation site will be modified by excavation (removal of soils), which will result in net unloading of the underlying soils. Therefore, long-term settlement at the wetland mitigation site is not a significant geotechnical concern. Portions of the training dikes at the tidal inlet may extend about 1 to 2 feet above existing site grades, but we judge that settlement of these dikes from this additional loading will likely be negligible. Although long-term settlement from static loading is not a concern, liquefaction-induced settlements are possible in the event of a significant earthquake, as discussed in Section 5.1.

5.7 Construction Considerations

Grading the wetland mitigation site will be complicated by the presence of shallow groundwater, tidal influence, and highly permeable sandy soils. The contractor should expect to encounter groundwater in any excavation extending deeper than about El. +8 feet. We anticipate that dewatering of the wetland mitigation site will be impractical due groundwater inflow from the highly permeable sandy soils and the proximity of the site to the bay. Excavation and grading will be further complicated by the presence of loose and soft soils likely to be encountered across the majority of the wetland creation area. As noted in Section 4.1, the ground surface of the unpaved areas was found to be soft and unstable under the weight of the truck-mounted drill rig during our geotechnical investigation. Additionally, recently graded transition slopes, marsh plain areas, and slough channels could experience erosion or other issues during construction due to tide cycles, seepage, and surface runoff (similar to issues discussed in Section 5.2).

To account for the above adverse site conditions during construction, the following measures could be implemented:

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• Phase construction so that work performed in the lower elevation areas of the site (i.e.

tidal inlet and slough channels) occurs during lower tides to limit the need to work in the wet.

• Phase construction so that the majority of the marsh plain and tidal inlet is constructed prior to removing portions of the seawall and opening the tidal inlet to the bay.

• Select equipment with low ground pressure to limit disturbance to the relatively soft and loose onsite soils or consider using mats to spread equipment loads.

• Develop temporary measures to direct surface runoff from Alameda Airfield away from the wetland creation area to limit erosion until vegetation root systems can develop.

• Temporarily monitor and repair slough channel and transition zone slopes that experience sloughing or erosion during construction. Repairs could include periodic regrading of slopes, replacement of eroded soils, reseeding of transition zones, or protecting trouble areas from surface runoff.

Dewatering with pumps may be feasible in limited areas depending on the size and depth of the excavation and the soil conditions encountered. However, the contractor should not rely on dewatering as a primary method to construct the project, because maintaining the majority of the site in a dewatered condition is likely not practical.

Specific recommendations for earthwork, temporary slopes, and shoring are presented in Section 6.

6.0 Site Work Recommendations

6.1 Site Preparation

The wetland mitigation site should be cleared of vegetation and obstructions. Excavated material may be reused as fill, provided it is broken up to meet the requirements described in Section 6.3, Fill Materials. Onsite soils containing the heavy metal cadmium should be removed and replaced, as recommended in HTH’s soil investigation memorandum (HTH, 2017b). The limits for removal should be shown on the construction drawings, and the specification should present procedures for proper disposal of the excavated material. Holes resulting from the removal of potentially contaminated soils or from the removal of underground obstructions extending below required grades or subgrades should be cleared and backfilled with suitable material compacted to the requirements described in Section 6.4, Fill Placement and Compaction.

6.2 Subgrade Preparation for Fill and Soil Amendments

After the completion of clearing and stripping within the tidal marsh and transition zone areas, the slough channel areas and subgrade in areas to receive fill, including soil amendments, should be prepared by grading the area to leave a smooth, even surface that is free from ruts.

Grading should be performed using methods that limit disturbance of the onsite soils. The subgrade in these areas should not be scarified. Fill should then be placed and compacted as described in 6.4, Fill Placement and Compaction.

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Subgrade preparation for the riprap section is discussed in Section 6.5.

6.3 Fill Materials

Soil fill placed at the site, including onsite soils, should meet the criteria in this section. Fill should contain no contaminants, debris, and other deleterious materials. The fill should contain no rocks or lumps larger than 4 inches in greatest dimension and contain no more than 15 percent larger than 2.5 inches. Larger building rubble within any existing fill, including brick, wood, concrete and masonry, if encountered, should be removed prior to reuse as fill. Onsite soil containing organic matter, except for roots greater than ½ inch, is acceptable for reuse as fill provided that the other requirements above are met. Existing concrete and baserock should not be reused as fill within the transition zones, tidal marsh, slough channels, or other areas where marsh vegetation is planned for wetland mitigation purposes. Existing concrete and baserock may be reused as fill beneath the splash pad areas provided that it contains no environmental contaminants and is broken up to meet the size requirements listed above.

Imported fill should have a liquid limit (LL) of less than 50, should have plasticity index (PI) not exceeding 25, and should contain no environmental contaminants or debris. These plasticity criteria (i.e. LL and PI criteria) do not necessarily apply to organic soil amendments. Refer to HTH’s MMP for specific soil amendment recommendations (HTH, 2018).

Developing detailed material recommendations for riprap, riprap filter, and access road choke stone is beyond the scope of this geotechnical evaluation.

6.4 Fill Placement and Compaction

Fill placed within the tidal marsh areas to recover required grades should be compacted to 85 to 90 of the soils maximum dry density in accordance with ASTM D698 (hereafter referred to as relative compaction). If relative compaction within the tidal marsh and transition zone areas exceeds 90 percent at measured locations, the area should be ripped and recompacted to within 85 to 90 percent relative compaction. Fill placed within splash pad, training dikes, the inlet, and any other areas to receive riprap should be compacted to at least 90% relative compaction. Examples of areas that may receive fill include holes to remove debris or potentially contaminated soils and any topographical low spots within the transition zone or splash pad areas. Fill material should be spread and compacted in lifts not exceeding eight inches in uncompacted thickness. In order to achieve satisfactory compaction of fill materials, the water content may need to be adjusted at the time of construction. This may require that water be added to soils that are too dry, or that aeration be performed in any soils that are too wet.

These placement and compaction recommendations do not apply to organic soil amendments.

Refer to HTH’s MMP for specific soil amendment recommendations (HTH, 2018).

6.5 Subgrade Preparation and Placement of Riprap Section

After the completion of clearing and stripping, soil exposed in areas to receive a riprap section (where “riprap section” includes riprap plus geogrid, geotextile, and aggregate bedding or riprap plus an aggregate filter) should be prepared by grading the area to leave a smooth, even hdrinc.com 1111 Broadway, Suite 1670, Oakland, CA 94607-4007 surface that is free from ruts and limits disturbance of the onsite soils. The subgrade should not be scarified or compacted. The riprap section should then be placed in a manner that limits disturbance to the underlying subgrade.

We note that in areas that are underwater, grading a smooth even surface prior to placement of the riprap section may be challenging. Grading tolerances can be widened in areas that are underwater. Geogrid and geotextile placement should be performed according to the manufacturer’s recommendations.

Aggregate bedding will be composed entirely of coarse gravel and cobbles, and compaction of these materials is not needed. Aggregate filters will be composed of fine to coarse gravel.

Compaction for aggregate filters can be achieved with at least four passes of a roller compactor to provide a firm and unyielding surface prior to the placement of the riprap.

After placement of geogrid, geotextile, aggregate bedding layers, or aggregate filter layers, riprap can be placed according to project specifications in wet or dry conditions.

6.6 Temporary Slopes and Shoring

All excavations that will be deeper than 5 feet and will be entered by workers should be sloped or shored for safety in accordance with applicable local, state, and/or federal safety regulations, such as the Occupational Safety and Health Administration (OSHA) guidelines (29 CFR Part 1926 Subpart P). The selection, design, and installation of the shoring system or temporary excavation slopes needed for the project should be made the responsibility of the contractor.

If temporary slopes are selected, the contractor should be aware that in no case should slope height, inclination, and excavation depths exceed those specified in the applicable regulations.

The contractor should determine temporary slope inclinations based on the subsurface conditions exposed at the time of construction. For planning purposes, onsite hydraulic fill soils should be categorized as OSHA Type C with temporary slope inclination of no steeper than 1.5H:1V. Flatter slope inclinations may be needed to maintain stability in some areas or circumstances, such as for excavations extending near or underwater.

If temporary slopes are left open for extended periods of time, exposure to weathering, rain, seepage, and tide cycles could have detrimental effects such as sloughing and erosion.