PO Box 880 Midvale, Utah 84047

801.849.0055 gerhartcole.com

TECHNICAL MEMORANDUM

To: Robert Rousselle, PE, LEED-AP Ensign Engineering and Land Surveying 45 Sego Lily Drive, Suite 500 Sandy, Utah, 84070

From: Dan Billings, PE Travis Gerber, PhD, PE

Reviewed by: Ryan Cole, PhD, PE, BCGE

Date: December 18, 2024

Job Number: 23-1699

Subject: GSL Marina Water Line Replacement Geotechnical Engineering Services

INTRODUCTION AND BACKGROUND

We understand that the State of Utah (DFCM, Owner) intends to construct a new culinary water line between the new main supplying the new state correctional facility and the Great Salt Lake Marina. The new water line replaces the line currently supplying culinary water to the marina, albeit along a different alignment. The proposed alignment is approximately 8 miles long and generally follows the I-80 northside frontage road in Salt Lake County, UT.

See Figure 1 for project vicinity. We understand the nominally 4-inch diameter HDPE pipe will have a typical burial depth on the order of 4 to 5 feet below site grade. The pipeline will potentially require four trenchless crossings, currently planned to be installed using horizontal directional drilling (HDD) methods. The project includes construction of a new 200,000-gallon water storage tank to be located where the old access road to the Marina and Silver Sands Beach meets the Frontage Road. This tank will be accompanied by a booster pump station.

We understand that the Ensign Engineering and Land Surveying (Ensign; Design or Project Engineer) -led team was selected by DFCM to provide the design for the project, and Ensign has requested that Gerhart Cole (GC) provide geotechnical services in support of its efforts.

SCOPE OF SERVICES

The objective of GC’s work was to develop design recommendations and construction considerations for the planned water line (pipeline), tank, and booster pump station. Our scope of work, generally performed in accordance with our original proposal dated September 14, 2023 and subsequently modified July 9, 2024, includes:

• Performing field studies including the drilling of 9 hollow stem auger (HSA) and/or mud rotary test holes and 16 cone penetrometer test (CPT) soundings.

• Laboratory testing of select soil samples.

• Performing analyses and developing geotechnical design recommendations as well as construction considerations for the project.

• Preparing this technical memorandum (TM).

• Developing project-specific specification for HDD trenchless construction.

Our scope of work does not address any potential environmental contamination issues that may or may not be associated with this site.

REGIONAL GEOLOGIC SETTING

The project is located within the Great Salt Lake topographic zone west of the Wasatch Range, which occupies a large, north trending, deep, sediment-filled structural basin. The Great Salt Lake Basin and Wasatch Front are located at the eastern margin of the Basin and Range Physiographic Province, a region characterized by a series of generally north-trending mountain ranges separated by similarly trending valleys.

Much of the present topography and geomorphic features are a result of east-west crustal extension that began about 15 to 17 million years ago, and continues today (Hecker, 1993).

The crustal extension has resulted in the uplift of the mountain ranges (including the Wasatch Range) along range-front normal faults that separate the mountains from valleys.

For the past 15 million years, the area has been a region of internal drainage, and lakes of varying sizes have occupied the basin throughout most of this time (Currey et al., 1984).

Dissolved salts tend to become concentrated in these areas. The Great Salt Lake, located just north of the pipeline alignment, reflects these conditions.

During prehistoric, yet geologically recent times, the Salt Lake Valley was largely filled by Lake Bonneville (ancestor to the Great Salt Lake) which stabilized at several “stands” or “benches,” that ring local valleys along the mountain fronts. During Lake Bonneville’s existence, finer grained lacustrine materials were deposited within the lake with typically coarser alluvial and fluvial soils intruding from the margins. These processes were a continuation of similar ones occurring during even older antecedent lake cycles within the basin (Lund, 1990). As such, sediments that have been deposited and/or reworked by lakes occupying the Bonneville basin dominate the near-surface geology along the pipeline alignment. During the time that lakes existed in the basin, coarse-grained sediments were deposited in the shore and nearshore environments creating beaches, deltas, spits, and bars. In deeper water, toward the center and norther parts of the Salt Lake valley, deposits generally consist of finer grained sand, silt, clay, and marl (Oviatt and Miller, 1997).

SURFICIAL SITE GEOLOGY

Surficial mapping of surficial geology along the pipeline alignment has been performed by Tooker and Roberts (1971) at the west end of the project (including the area of the GSL Marina, which was then referred to as the “County Boat Harbor”), by McKean et al. (2019) generally north and west of the Kennecott Utah Copper / Rio Tinto (KUC/RT) tailings impoundment, and by McKean and Hylland (2019) at the east of the project. Clark et al.

(2020) also illustrates this geology at a coarser resolution on a single map of the Tooele 30’ x 60’ quadrangle. See Figures 3a through 3c for this mapping and Figure 4 for abbreviated descriptions of the units near the pipeline alignment. Additional discussion of these units is provided later in this document under the heading of “Subsurface Conditions.”

FIELD AND LABORATORY STUDIES

Test Holes

The field study included drilling nine test holes (24-TH-01, 24-TH-05, 24-TH-16, 24-TH-18, 24-TH-20, 24-TH-21, 24-TH-22, 24-TH-24, and 24-TH-26, with numbers generally increasing from west to east along the alignment, and test holes corresponding to -07 and - 14 not having been drilled due to uncertainty about the location of the existing water line).

Approximate locations are shown in Figures 2a through 2g. Test holes were typically drilled using hollow-stem auger (HSA) methods with a CME 850 drill rig operated by ConeTec under subcontract to GC. Test hole 24-TH-01 was drilled using both HSA and mud rotary methods with CME 850 (“D6”) and CME 55 (“D8”, tracked) drill rigs, respectively. Test holes were drilled to depths ranging from approximately 12 to 77 feet. Test holes were completed by backfilling them with soil cuttings, except 24-TH-01 which was backfilled with grout (no temporary piezometers were installed). Test holes in the roadway required pavement coring and, upon completion, the holes were topped with a concrete plug. Additional information about the test holes can be found in Table 1.

Test holes were logged in the field by a GC engineer. Soil samples were typically collected at select intervals using the Standard Penetration Test (SPT) with a standard 2-inch outside diameter, 1-3/8-inch (inside diameter) “split-spoon” sampler and an automatic trip hammer (although in a few instances, a modified California (MC) sampler with a 2-1/2-inch outside diameter and a 1-7/8-inch inside shoe diameter). The hammer energy efficiency of the CME 850 drill rig was measured in November 2023 to be 85.9 percent, and the hammer energy efficiency of the CME 55 drill rig was measured in September 2022 to be

74.1 percent. SPT samples were typically driven 24 inches or to practical refusal. The number of hammer blows required to advance the sampler in 6-inch increments was recorded in the field, with the sum of the second and third 6-inch intervals constituting the SPT blowcount or “N-value.” Relatively undisturbed samples were obtained via thin-walled Shelby tubes hydraulically pushed into fine-grained materials. Soil samples were initially classified using visual classification methods and drilling behavior. Where appropriate, field classifications were refined by laboratory testing.

Logs of the test holes are presented in Appendix A. When reviewing the logs, we direct the reader to the Legend to Soil Descriptions, found in the front of Appendix A. Lines designating boundaries between different materials shown on the logs should be considered approximate; transitions between subsurface materials may be gradual or occur between sampling depths. Subsurface conditions at the site may differ laterally and with depth from those indicated at specific study point locations. Specific conditions found in the test holes are discussed in the Subsurface Conditions section of this TM.

CPTs

The field study included pushing 16 cone penetration test (CPT) soundings (24-CPT-02 (near the marina), 24-CPT-03 (at the marina), 24-CPT-04, 24-CPT-06, 24-CPT-08, 24-CPT-

09, 24-CPT-10, 24-CPT-11, 24-CPT-12, 24-CPT-13, 24-CPT-15, 24-CPT-17, 24-CPT-19,

24-CPT-21, 24-CPT-23, and 24-CPT-25, with numbers generally increasing from west to east along the alignment). Approximate locations are shown in Figures 2a through 2g. The CPT soundings were advanced to depths ranging from 2 to 101 feet. Multiple CPT soundings refused on dense material or buried pavement. In some cases of shallow (premature) refusal, additional attempts were made to advance the soundings (resulting in soundings 24-CPT-03A, 24-CPT-03B, and 24-CPT-04A). The soundings were advanced/pushed by ConeTec under subcontract to GC. Pore pressure dissipation (PPD) testing was performed at select depths to obtain information about groundwater conditions.

Shear wave velocity measurements were also made in conjunction with sounding 24-CPT-

02 at the tank site. All soundings except 24-CPT-02 were allowed to self-close; 24-CPT-02 was backfilled with grout. Soundings made in the roadway pavement were patched with asphalt cold patch. Additional information about the CPT soundings can be found in Table 1.

Logs of the CPT soundings are presented in Appendix A. The CPT provides a near continuous measurement of penetration resistance, side friction, and dynamic pore-pressure within a soil profile to assess stratigraphy more precisely than with traditional test hole methods. The CPT, however, does not provide a soil sample. Subsurface conditions may differ laterally and with depth from those indicated at specific study point locations. When interpreting soil type from CPT results, it is important to recognize that the soil behavior type (SBT) index shown on the logs is not the same as a soil classification. It has been the experience of some local engineers that the Robertson (2010) correlation which is frequently used to define soil behavior type often indicates that local fine-grained materials are more silty and/or sandy than expressed in terms of the unified soil classification system. Hence, a material identified as a silty clay or even a silt based on SBT may actually be a lean clay.

Proper use and interpretation of CPT data requires skill, experience, and judgment.

Laboratory Testing

Select samples obtained during our field studies were tested in our geotechnical laboratory.

Testing included index property testing (moisture content, Atterberg limits, and grain-size distribution) and six suites of electrochemical tests to help assess the steel corrosion and concrete degradation potential for the soil. Undrained unconsolidated (UU) strength testing and one-dimensional consolidation testing was also performed on select fine-grained samples. Results of laboratory tests are summarized in Table 2, with additional test data presented in Appendix B.

It should be kept in mind that relative to lab testing results for this project, when looking at particle (grain) size distributions of recovered samples, the results may under-represent the presence of gravels due to the limited size of the sampler.

SUBSURFACE CONDITIONS

Soils

As mentioned previously, surficial geologic deposits along the pipeline alignment have been mapped by Tooker and Roberts (1971) at the west end of the project (including the area of the GSL Marina, which was then referred to as the “County Boat Harbor”), by McKean et al.

(2019) generally north and west of the Kennecott Utah Copper / Rio Tinto (KUC/RT) tailings impoundment, and by McKean and Hylland (2019) at the east of the project. Clark et al.

(2020) also illustrates this geology at a coarser resolution on a single map of the Tooele 30’ x 60’ quadrangle. See Figures 3a through 3c for this mapping and Figure 4 for abbreviated descriptions of the units near the pipeline alignment.

For the purposes of this document, we divided the pipeline alignment into 7 reaches, beginning at the GSL Marina. Absent definitive alignment stationing at the time this document was prepared, we describe the approximate extent of reaches in terms physical landmarks. Descriptions of each reach are inherently general as natural variations preclude greater precision/specificity.

These reaches are delineated, following the pipeline alignment from the Marina to the east end of the project near 8000 West as follows:

• Reach 1: Developed area of the marina/boat harbor.

• Reach 2: Undeveloped area between Reaches 1 and 3.

• Reach 3: Existing boat storage area off of old North Temple Frontage Road where water tank and booster pump station are planned.

• Reach 4: From east end of Reach 3, along North Temple Frontage Road, to near the center of the SR-202 interchange at Saltair (near 24-CPT-08). While off the currently considered pipeline alignment, the present access road to the marina from the North Temple Frontage Road also generally lies within this reach.

• Reach 5: From east end of Reach 4, along North Temple Frontage Road, to end of bend in I-80 and North Temple Frontage Road at corner of KUC/RT tailings impoundment (near 24-TH-16)

• Reach 6: From the east end of Reach 5, along North Temple Frontage Road, to just east of 8800 West (Radio Tower Road).

• Reach 7: From east end of Reach 6, along North Temple Frontage Road, to east end of project near 8000 West.

We understand that the four trenchless crossings for the pipeline will be located at/near geotechnical study locations 24-CPT-12, 24-CPT-15, 24-TH-20, and 24-CPT-21/24-TH-21, where a notable waterway crosses under I-80 and the North Temple Frontage Road. In the absence of definitive alignment stationing at the time this document was prepared, we refer to each of these locations by the name of the proximately located test hole or CPT sounding.

Specific surface and subsurface conditions for each of the reaches and crossing locations follow. It should be recognized that the I-80 and North Temple Frontage Road embankments/roadways are constructed of fill materials overlying native materials. Large portions of these fills appear to have been placed under urgent conditions associated with flooding of the Great Salt Lake (which caused overtopping of the interstate and railroad) in the 1980s. While appearing to be primarily granular (and potentially including cobbles and boulder-sized materials), the fill may vary with location and consist of other soil types. A first order approximate of height/depth/thickness of fill materials may be obtained from examining cross-sections of the roadway and comparing the top of the roadway to the surrounding grade near the toe of the fill (excluding any grade change due to ditches or drainages that may be near the toe). It should be recognized that if the pipeline is located within the roadway prism and if the prism is built up high enough relative to surrounding grade, the pipeline trench may not extend into native materials. The amount of pipeline trench excavation which may be in fill embankment as compared to native soils can be better assessed once complete topographic survey information which ties to the surrounding native grade, together with plan and profile drawings for the pipeline, becomes available.

In very broad terms, it appears that materials found in the test holes and CPT soundings along the alignment tend to be coarser grained at elevations above 4219 to 4220 feet (being mostly sand and gravelly sands with occasional clayey sand layers). Below this elevation threshold, soils appear to transition to more fine-grained materials (composed primarily of clay with some silt and sand mixtures).

More specific descriptions at test locations are presented on the field study logs presented in Appendix A. Interpolation or extrapolation between specific study locations may not be representative; spatial variation should be expected.

Reach 1

This reach includes geotechnical study location 24-CPT-03. While Tooker and Roberts (1971) do not specifically map the marina/harbor as being associated with a particular geologic unit, Clark et al. (2020) map the reach as consisting of unit Qh, corresponding to “deposits and disturbed areas from human development.” Such deposits can be composed of variable materials, depending on source/origin materials. Data from our CPT sounding in this reach (which refused in dense materials at depths of less than 3 feet with several attempts) indicates that materials in the upper 3 feet are very granular and relatively dense.

We suspect portions of the fill will be quite granular, including cobbles and boulders used to construct over portions of the marina in what in certain years/season would otherwise be open water. Underlying native soils are likely similar to those in Reaches 2 and 4.

Reach 2

This reach does not explicitly include a specific geotechnical test hole or boring, but the native soils are expected to be generally similar to those in Reach 4, more particularly where sounding 24-CPT-04 is located. Remnants of the old access road and underlying fill embankment to the marina (boat harbor) are visible on aerial photography, and a portion of the pipeline trench excavation may be in this material.

Reach 3

This reach includes geotechnical study locations 24-TH-01 and 24-CPT-02. The small area is composed of 10 to 13 feet of generally granular fill (sands and/or gravels with varying amounts of the other) used to build up the boat storage area (and which also likely overlies a narrow embankment which served as part the former intersection of the boat harbor access road with the North Temple Frontage Road). Below this fill appears to be native soils similar to those in Reach 4. Asphalt was found at a depth of about 12 feet in the test hole. We understand this area was home to a couple of maintenance buildings which were destroyed by fire in 2010. While the fill (also referred to as undocumented fill) appears to be relatively dense for most of the fill’s thickness based on the test hole and CPT data, both test locations are rather centrally located in the fill area, and it is quite possible that less dense/compact materials are present away from the center. Less dense/compact fill presents elevated risks of adverse settlement when subject to structural loads such as would be expected from the tank planned for this area. Recommendations for addressing this risk are presented later in this report.

Reach 4

This reach includes geotechnical study locations 24-CPT-04, 24-TH-05, 24-CPT-06, and 24- CPT-08 (at Saltair). The pipeline alignment and North Temple Frontage Road follow along a type of “shoreline” which has been described as “alkali or mud” on USGS quadrangle topographic maps. Tooker and Roberts (1971) assign a description of “spring and marsh deposits” (unit Qsm) to the soils along this reach, which is more particularly described as “clay, silt, and sand that is variably organic-rich, calcareous, or saline”. According to Clark et

al. (2020), the soils grade easterly approaching Saltair into a generally similar unit they identify as Qlpm and describe as “Great Salt Lake and playa mud” comprised of “clay, silt, oolitic sand, and pelletal sand composing the bed of Great Salt Lake and some slightly higher adjacent areas” including “typical playa mud, which is subaerial in origin, and is washed in and/or derived from in-place weathering of the underlying deposits.” From our test holes and CPT soundings, it appears that the existing, near-surface subgrade is imported granular fill of sand and gravel, with an estimated thickness which could in places exceed 12 feet (see test hole log 24-TH-05).

Reach 5

This reach includes geotechnical study locations 24-CPT-09, 24-CPT-10, 24-CPT-11, 24- CPT-12, 24-CPT-13, and 24-CPT-15. Within the northeast side of the SR-202/Saltair interchange area (between 24-CPT-08 and 24-CPT-09), the native soils appear to become more coarsely graded as compared to Reach 4, and the alignment appear to follow along a high-level “shoreline.” These materials are identified by Clark et al (2020; mapped as unit Qlk) as “younger lacustrine carbonate-chip sand and gravel” (which are more particularly described as “composed of calcium-carbonate clasts, including ooids, pellets, and rounded, irregularly shaped flakes and chips, with some pebbles of local rocks”), whereas they are described by McKean et al. (2019; mapped as unit Qlos) as “lacustrine oolitic sand deposits” comprised of “oolitic sand, carbonate chips and clasts, and minor silt”. Per our CPT soundings, soils in this reach appear to be generally dense to very dense extending to depths of 6 or more feet, below which there are clays. As discussed, relative to Reach 6, a portion of the uppermost materials may be roadway import material, perhaps including an older, now buried, pavement section (as potentially suggested by the CPT tip stress signature at a depth of 4 feet in 24-CPT-10).

Reach 6

This reach includes geotechnical study locations 24-TH-16, 24-CPT-17, 24-TH-18, 24-CPT- 19, 24-TH-20, 24-CPT-21, 24-TH-21, 24-TH-22, and 24-CPT-23. Within this reach, the soils appear to become finer grained as compared to Reach 5. North of the KUC/RT tailings impoundment, much of the soils are composed of what has been mapped as unit Qlmy by McKean et al. (2019) and McKean and Hylland (2019), described as being “young/er lacustrine mud” of the Great Salt Lake. This mud is composed of silt, clay, and minor sand.

A few, much less extensive, deposits are also mapped within this reach, including unit Qal1 (“modern stream deposits”) along the original Lee Creek drainage, and unit Qldy (“young lacustrine and deltaic deposits”) toward the east end of the reach (McKean and Hylland, 2019). Unit Qal1 is comprised of very fine to medium sand, silt, and a minor amount of clay.

The composition of unit Qldy is more finely grained as discussed hereafter relative to Reach

7. Our test holes and CPT soundings within this reach indicate the soils in the upper 2-1/2 to 4 or more feet of the subsurface profile are sandier than may otherwise be inferred from the mapping. We believe this is a result of the roadbed having been built up relative to the surrounding grade. As stated earlier, we know that in response to flooding in the early to mid-1980’s, I-80 was raised. We do not know if the North Temple Frontage road within this reach was also raised, or to what height; however, it appears that it was raised, given the finding of gravels and a layer of asphalt in the depth range of 6 to 7-1/2 feet in test hole 24- TH-22. Asphalt was also found at a depth of about 4 to 5 feet in 24-TH-20. We anticipate the depth to buried pavement decreases toward the east, and a former, now buried pavement may be source of CPT shallow refusal in 24-CPT-23 (or this could also be the result of densely compacted granular fill and/or oversized aggregates).

Reach 7

This reach includes geotechnical study locations 24-TH-24, 24-CPT-25, and 24-TH-26.

Soils in this reach are primarily mapped by McKean and Hylland (2013) as unit Qldy, with lesser occurrences of units Qdd and Qlmy (the latter of which has been more fully described relative to Reach 6). Unit Qldy, “younger lacustrine and deltaic deposits,” is described as being “silty sand, clay, and marl” and “commonly more clay rich with very little silt and sand.”

Unit Qdd (“deltaic distributary channel-fill deposits”) corresponds to relatively recent deposits of silt with very fine to fine gained sand and clay, including natural levee deposits and correlating with distributary channels of deltas formed by the paleo-Jordan River (McKean and Hylland, 2013). Our test holes and CPT sounding are generally consistent with these descriptions. Given the location of 24-TH-26, it appears that the upper 7 feet of the profile may be fill associated with an existing pipeline installation. Unlike elsewhere, we understand that North Temple Frontage Road in this reach, while being overlain with an asphalt surface topping, is largely constructed of concrete.

Trenchless Crossing 24-CPT-12 (in Reach 5)

Based on data collected, it appears that the upper 12 feet of the subsurface profile (datum = roadway surface) at this location consists primarily of dense to very dense sands and possibly gravels. Deeper soils appear to consist of stiff clays with some interbedded sand/silt layers, becoming soft toward the bottom depth of the sounding at 25 feet (although some sand layers [such as the one at 17 to 19 feet] and stringers are present). CPT PPD testing suggests groundwater at the time of the study was located at a depth of about 15 feet. Given that granular soils tend to be more permeable, the upper part of the subsurface profile may present challenges with maintaining adequate drilling fluid/mud conditions within portions of the bore path.

Trenchless Crossing 24-CPT-15 (in Reach 5, near Reach 6; corner of KUC/RT Tailing Impoundment)

Based on data collected, it appears that the upper 6 feet of the subsurface is a medium dense to very dense sandy fill, below which is a soft to stiff lean clay extending down to a depth of about 25 feet. There may or may not be a previous, now buried, pavement section in this reach. At the time of the study, the groundwater level appears to have been 9 to 10 feet below the ground surface.

Trenchless Crossing 24-TH-20 (in Reach 6)

Based on data collected, it appears that the upper 6 feet of the subsurface is a medium dense to very dense sandy fill, below which is a soft to stiff lean clay extending down to a depth of about 22 feet. It appears that a buried pavement is located at a depth of about 4 to 5 feet. Moisture conditions transitioned from moist to wet, potentially indicating the depth of the water table, at a depth of about 16 feet.

Trenchless Crossing 24-CPT-21 / 24-TH-21 (in Reach 6; C-7 Ditch)

Based on data collected, it appears that the existing roadway rests atop 2 to 4 feet of dense to very dense sandy fill with gravel, below which is a buried asphalt pavement. Below this and some base course lies a lean to fat clay extending down to a depth of 17 feet, below which are layers of sand and clay. At the time of the study, moisture conditions transition from moist to wet at a depth of about 16 to 20 feet (together with a water level measurement of 18.4 feet), indicating the depth of the water table.

Groundwater

Groundwater was either found (or inferred) in most of the test holes and the deeper CPT soundings completed as part of this study. Groundwater levels were estimated during drilling based on drilling conditions such as heaving soils, soil moisture content, and/or field measurements using a water level indicator. Groundwater depths derived from the CPT PPD testing are presented in Table 1. In general, it appears that groundwater at the time of the field studies was located in the range of 9 to 18 feet deep (as compared to pipeline trench excavations anticipated to be nominally 5 feet deep).

It should be recognized that much of the ground along the pipeline alignment is intermittently (seasonably), if not perennially, wet. Standing water may be observed just beyond elevated roadways in the area. While land existing north of the alignment may be rightfully considered to be playas, there are also instances of seeps and springs (particularly immediately east of the Marina, which may or may not be underwater depending upon the level/stage of the lake). Also, it should also be recognized that the KUC/RT tailing pile exists as a means of dewatering the tailings, and as such, the pile constitutes a potential source of groundwater mounding/influx.

Groundwater levels are not static, changing in response to precipitation, runoff, irrigation, nearby water bodies, etc., as well as changes in seasons. The Contractor should verify water levels prior to commencing work. The Contractor should be prepared for areas with locally deeper excavations and/or relatively shallow and large quantities of groundwater within pipeline trenches.

SEISMICITY AND SEISMIC EFFECTS

The east end of the pipeline alignment is located approximately 3-1/2 miles to 4 miles west of the Granger Segment of the West Valley Fault Zone (WVFZ). Unlike most of the normal active faults along the Wasatch Front area, this fault is east dipping and antithetic to the Salt

Lake City Segment of the Wasatch Fault Zone. In concert with the Taylorsville Segment, the Granger Segment is considered capable of producing a magnitude 6.3 earthquake and has a long-term recurrence rate estimated to be about 4,500 years (WGUEP, 2016).

More significantly, while much of the ground shaking hazard in the Salt Lake Valley /along the Wasatch Front is dominated by the Wasatch Front Zone, the pipeline and particularly the tank planned for the marina area are impacted by the Antelope Island Segment of the Great Salt Lake Fault Zone (GSLFZ). The nearest trace of this fault is located less than 2000 feet from the tank site, with the fault being located at the north end of the marina, and extending north and east toward Antelope Island, and west and south toward Lake Point and the west side of the Oquirrh mountains. This active, west dipping normal fault is considered independently capable of producing a magnitude 7.0 earthquake (WGUEP, 2016). The long-term segment rupture rate of this fault is estimated to be on the order of 2,500 years

(WGUEP, 2016).

Seismic Site Class

Seismic site class was evaluated at the planned location of the water tank near the Marina and I-80 using shear wave velocities measured as part the advancement of sounding 24- CPT-02. The measured Vs100 value (weighted harmonic mean of the top 100 feet of the soil profile) was 634 ft. Absent any potential liquefaction, the corresponding seismic site class per ASCE 7-16 and IBC 2021 is initially “D.” This site class is not a default value.

However, per ASCE 7-16, “soils vulnerable to potential failure or collapse under seismic loading, such as liquefiable soils” are to be classified as Site Class F, irrespective of shear wave profile. Based on Youd et al.’s (2001) method of liquefaction triggering analysis applied to this same sounding, it appears that liquefaction is triggered in some discrete soil strata at depth (being zones at depth of approximately 15 to 16 feet, 40 to 42 feet, and 51 to 54 feet). While not prominent, it appears that because of the liquefaction, the site class is F.

We note that there was some evidence of liquefaction occurring in and around the Marina as a result of the 2020 Magna (magnitude=5.7) earthquake. The full extent of potential liquefaction caused by the event is difficult to assess due to the lack of detailed subsurface studies and the similar-appearing effects of other earthquake-related phenomena such as cyclic-softening of clays. Liquefaction-related effects and design considerations are discussed hereafter.

Ground Shaking - Design Response Spectra

Given the proximity of the project to the seismic sources, there is a moderately high to high ground shaking hazard for the project (although our focus in our scope work is the water tank and booster pump structures; we understand the pipeline itself will not be designed for seismic ground motions). Seismic design parameters (USGS, 2024a) consistent with the generalized acceleration response spectrum procedure (with 5% damping) of the 2021 International Building Code and ASCE 7-16 are shown for the water tank and booster pump structures in Table 3. Acceleration parameters presented in the table have not been adjusted to account for any particular occupancy category or seismic importance factor. The MCE-based geometric mean peak ground acceleration adjusted for site conditions (PGAm) provided in the table is used for geotechnical engineering assessments such as pseudo-static slope stability and liquefaction triggering analyses. In many areas, this value generally represents “geo-mean” ground motions having a 2 percent chance of exceedance in 50 years (i.e., 2PE50), and is different than the risk-targeted “PGA” value shown in the previous row of the table (which itself nominally corresponds to an estimated 1 percent probability of structural collapse with ground motions oriented in the maximum direction). The risk-targeted (structural) parameters would be used for structural design as outlined in the referenced Code.

As stated above, since the site has liquefiable soils, the seismic site class is ‘F’, which could require a site-specific response analysis. However, the referenced Codes provide an exception for short period structures (less than 0.5 s) which allows the use of Fa and Fv (without site specific study) from the site class absent any liquefaction, which in this case is site class ‘D’. These coefficients appear in the table as parenthetical values. Values for SDS and/or SD1 have not been provided in the table because a seismic response coefficient (Cs) approach (rather than Sa values from a response spectrum based on SDS and SD1) should be used in this particular situation when the IBC’s exception to performing a site-specific study is taken (see ASCE 7-16, Sections 11.4.8 and 12.8.1.1). SDS can be calculated from other values in the table (SDS = SS x Fa x 2/3), if desired. If needed, SD1 can similarly be calculated from values in the table (SD1 = S1 x Fv x 2/3).

With respect to earthquake magnitude for MCE-based seismic design, a deaggregation of the USGS’ probabilistic ground motion hazard analysis underlying its national seismic hazard maps (B/C boundary conditions) and the mapped seismic coefficients in building codes, at a structural period of zero (i.e., at peak ground acceleration, PGA), the modal magnitude-distance pair is 6.9 and 5.9 km (USGS, 2024b), representing about a 21% contribution to the seismic hazard, as calculated by the USGS’ binning method.

Liquefaction Potential

A potential concern in seismically active areas is the impact of liquefied soil on structures and other improvements. In our assessment of this hazard and as mentioned above, we have used Youd et al.’s (2001) method to assess triggering of liquefaction. In our analyses, MCE ground motions and the modal magnitude-distance pair from a deaggregation of the peak ground acceleration (PGA) were used to represent seismic demands. As reported above, at site of the water tank and booster pump station, liquefiable zones were found at depths of approximately 15 to 16 feet, 40 to 42 feet, and 51 to 54 feet.

Using the method of Yoshimine et al. (2006) to estimate the amount of liquefaction-induced settlement, we calculate (best-estimate) ground settlements at this location to be on the order of 1 to 2 inches for the design seismic demand. This settlement does not include any type of factor of safety apart from the definition of MCE ground motions, or any settlement from static (non-seismic) loading conditions. For project elements embedded below the water table, consideration should also be given to potential flotation/buoyancy when soils liquefy. The severity of this seismic uplift depends upon the location, base level of the structure relative to liquefiable strata.

In addition to vertical settlements, liquefaction can induce either lateral spreading or even cause stationary oscillations of the ground during ground shaking. To assess the magnitude of potential lateral spread, we used the commonly employed method of Youd et al. (2002).

This method is based on SPT blowcounts and uses soil parameters such as fines content and mean grain size. In using this method, we have converted the CPT penetration resistance to equivalent SPT blowcounts. Fines content and mean particle sizes of soil layers within the liquefiable zones were estimated from correlations as well as laboratory testing. This predictive method also considers the slope of the ground, the continuity and thickness of liquefiable strata, and the depth of that strata. In the case of the water tank and booster pump building location, it appears that liquefaction susceptible soils with density sufficient to spread are either too deep or are too thinly layered to present more than a minimal lateral spreading hazard.

GEOTECHNICAL DESIGN RECOMMENDATIONS AND CONSTRUCTION

CONSIDERATIONS – PIPELINE AND GENERAL EARTHWORK

Excavation

We understand from Ensign that pipeline construction will require excavations typically on the order of 5 feet below the ground surface (although locally deeper excavations may occur near conflicting utilities and other improvements). The sandy/gravelly soils in this depth may have a tendency to ravel if unsupported. Given the proximity of the work to roadways/ pavements which may be undermined and/or present destabilizing loading conditions, we are of the opinion that temporary shoring measures will need to be included for trench excavations.

As a minimum, all excavations should comply with Occupational Safety and Health Administration’s (OSHA) construction standards, as well as applicable state and local regulations. The Contractor should rely upon their own methods to determine and maintain safe and stable slopes during construction, subject to their construction procedures and those subsurface conditions more fully exposed during construction. Any disturbed material should be removed or recompacted prior to placing fill and/or concrete.

Access and Construction Equipment Trafficability

Given the generally low-volume nature of North Temple Frontage Road, we anticipate that work for the pipeline may take place without needing to provide an extensive widening of the existing roadway or needing a supplemental access road for construction equipment. It should be recognized that if such is needed, native soils adjacent to the roadway/pipeline alignment typically present poor to very poor support for vehicular and equipment traffic (especially when wet). If needed, GC can work with the project team to develop a subgrade stabilization plan to meet trafficability needs.

Dewatering

As discussed previously under the subheading of groundwater, based on groundwater levels observed at the time of the field studies relative to the typical pipeline trench excavation depth of 5 feet, we do not believe groundwater will be a major construction issue.

However, in areas of locally deeper excavation, excavations that reach the native soils, and/or near the tailings impoundment, the Contractor should be prepared for potentially large quantities of groundwater within open pipeline trenches. We recommend that assessments of groundwater potential be made prior to construction in such locations.

Subgrade Stabilization

Where soft to very soft soils are found in trench bottoms or at foundation bearing levels, subgrade stabilization will be required. Subgrade stabilization may initially be attempted by forcing cobble rock into the subgrade to form a firm, unyielding surface. Cobble rock should consist of 3 to 12-inch subrounded to rounded pieces of rock pushed into the subgrade with an excavator bucket, roller, or tracked equipment. Additional rock should be placed as necessary to establish a firm working surface. Nesting and the creating of voids between added rock must be avoided. It may also be necessary to use a separation geotextile with suitable strength to provide confinement during material placement and densification. Details regarding the use of any geotextile will need to be based on conditions exposed during trench excavation.

Geotechnical Pipeline Design Recommendations

While this section of this document provides general guidelines for the planned pipeline, it should not be considered a complete project specification. Recommendations contained in this section reflect a small diameter (less than 12-inch; but with an actual planned diameter of 4 inches for a majority of the pipeline) HDPE, steel, or ductile iron pipe. If this description is incorrect, the recommendations presented herein need to be revisited and potentially revised.

With respect to the design and installation of the buried pipe, we recommend that the pipe manufacturer’s recommendations be followed, together with the guidance/ recommendations presented herein, as well as guidance provided within American Water Works Association’s [AWWA] Manual M55 “PE Pipe – Design and Installation”, Second Edition (2020); Plastic Pipe Institute’s [PPI] “Handbook of Polyethylene Pipe”, Second Edition (2008), ASTM D2774 “Standard Practice for Underground Installation of Thermoplastic Pressure Piping” (2021), AWWA Manual M11 “Steel Pipe – A Guide for Design and Installation”, Fifth Edition (2018), and AWWA Manual M41 “Ductile Iron Pipe and Fittings”, Third Edition (2009), or more current standards.

Some considerations (not being/representing a comprehensive list) that should be made by the Project Design and Construction Teams when planning/designing the proposed pipeline include:

1. Along much of the pipeline alignment, there appears to be abandoned/ buried asphalt pavement (together with underlying base materials). This pavement could present excavation and drilling (HDD) difficulties.

2. Along much of the pipeline alignment, there appears to be fill materials below the current overlying and/or adjacent pavement surface. As stated previously, it appears that much of this may have been granular import material placed under urgent conditions in response to flooding of the Great Salt Lake in the 1980’s. It seems quite possible that portions of this fill may include (or even be primarily composed of) oversized (i.e., cobble- and boulder-sized) materials. Such would present excavation and potential shoring/ shielding challenges during construction.

3. Roadway embankments along some of the alignment may have been intended to function as dikes or levees relative to high waters of the Great Salt Lake, rather than only serving to raise the roadway above potential flood waters. Dike and levees are often designed in consideration of erosion potential (and armoring [or at least usage of select fill material] to mitigate said erosion) and permeability/ seepage. Evaluation of such considerations relative to the placement/ location of the pipeline is beyond the scope of this report. It should be recognized that placement of the pipeline in/near the lake-side slope of an embankment (rather than elsewhere away from the slope) does increase the likelihood that any dike/levee functions of an embankment will likely be adversely affected by pipeline construction.

4. We understand that most of the pipeline alignment will be located beneath existing roadway pavements or graded shoulders. While certain levels of soil compaction are sufficient to provide acceptable levels of pipeline performance, higher levels of compaction are often needed to preclude adverse settlement of pipe and trench backfill beneath roadways and other such improvements/ hardscaping. It is in light of this that certain jurisdictions have backfill requirements that exceed minimums often prescribed by pipe manufacturers.

a. Absent more stringent compaction requirements of jurisdictional authorities, we recommend that sands and gravels used as backfill in open areas away from improvements and where there are not potential concerns for future settlement, we recommend that backfill be placed in maximum 8-inch loose lifts and compacted to at least 95 percent of maximum dry density (MDD) as determined by ASTM D698 (standard Proctor compaction effort) [or 90% of MDD as per ASTM D1557] and within 2 percentage points of optimum moisture content (OMC) as determined by the same standard.

b. In other areas where there may be concerns regarding future settlement impacts, for trench depths of less than 15 feet, we recommend that backfill be placed in maximum 8-inch loose lifts and compacted to at least 95 percent of MDD as determined by ASTM D1557 (modified Proctor compaction effort) and within 2 percentage points of OMC as determined by the same standard.

c. Care should be taken during compaction to not damage or misalign the pipe.

5. The smaller the diameter of a thermoplastic pipe, the smaller the maximum size of the backfill to be used near the pipe. For up to 4-, 8-, and 16-inch pipes, we recommend a maximum particle size of 1/2-, 3/4-, and 1-inch, respectively. Backfill in the pipe (initial backfill) zone and bedding/haunch areas should consist of imported, well-graded sand and gravel with maximum particle size previously stated and 5 to 20 percent non-clayey fines. Open graded gravels should be avoided due to the risk of migration of fine-grained native soils, as should other materials such as pea gravel and squeegee which tend to be present challenges if the pipe trench needs to be accessed/re-excavated in the future. Recommended pipe zone backfill can be used in the trench zone (final backfill) over the pipe zone, as can other soils consisting of similar well-graded sand and gravel (such as untreated base course [UTBC]), but with a maximum particle size of 3 inches. Controlled low-strength material (CLSM or Flowable Fill) may also be an acceptable alternative, provided it is excavatable by typical earth working equipment.

6. Based on the data at the specific project locations studied, it appears that much of the material within the anticipated trench excavation (5-foot depth) might be used as backfill meeting the above-described gradation. In some cases, oversized materials may need to be removed, or clayey soils excluded.

Pipe Buoyancy

Given the anticipated size of pipe (nominal diameter of 4 to 12 inches), the amount of cover (a reported minimum of 4 feet), and the relative depth to the water table, we do not believe pipe buoyancy will be a long-term design consideration. However, it is possible that floatation/ movement during construction may be an issue where groundwater could be found in excavations or in the area of the marina if lake levels become such that they intersect the pipeline trench.

GEOTECHNICAL DESIGN RECOMMENDATIONS AND CONSTRUCTION

CONSIDERATIONS – TRENCHLESS CROSSINGS

As stated previously, we understand that there will potentially be four trenchless crossings for the pipeline, located where existing waterways/culverts cross under I-80 and the North Temple Frontage Road at/near geotechnical study locations 24-CPT-12, 24-CPT-15, 24-TH- 20, and 24-CPT-21/24-TH-21. Horizontal directional drilling (HDD) was the trenchless installation method of choice selected by Ensign.

This section of this document should not be considered a complete specification; it provides general guidelines for the planned HDD pipeline crossings. Recommendations contained in this section reflect pressurized (non-gravity flow) HDPE pipeline of a small diameter (less than 12 inches for the HDD crossings; but with an actual planned diameter of 4 inches) which serves as the utility carrier pipe installed using mini- to possibly mid-HDD types of equipment (barring oversized particles being present). If this description is incorrect, the recommendations presented herein need to be revisited and potentially revised.

With respect to design and installation, we recommend that the pipe manufacturer’s recommendations be followed, together with the guidance presented herein, as well as guidance provided within American Water Works Association’s [AWWA] Manual M55 “PE Pipe – Design and Installation”, Second Edition (2020); Plastic Pipe Institute’s (PPI) “Handbook of Polyethylene Pipe”, Second Edition (2008; Chapter 12); and Municipal Advisory Board’s [MAB] “MAB Guidelines for Use of Mini-Horizontal Directional Drilling for Placement of HDPE (PE4710) Pipe in Municipal Applications”, Second Edition (MAD-7;

2020).

Some of the considerations (not being/representing a comprehensive list) that should be made by either the Project Engineer or HDD Contractor when planning/designing the proposed HDD work include:

1. Along much of the pipeline alignment, there appears to be abandoned/ buried asphalt pavement (together with underlying base materials). This pavement could present excavation and drilling (HDD) difficulties.

2. Along much of the pipeline alignment, there appears to be fill materials below the current overlying and/or adjacent pavement surface. As stated previously, it appears that much of this may have been granular import material placed under urgent conditions in response to flooding of the Great Salt Lake in the 1980’s. It seems quite possible that portions of this fill may include (or even be primarily comprised of) oversized (i.e., cobble- and boulder-sized) materials. Such would present excavation and potential shoring/ shielding challenges during construction. We recommend that once alignments are finalized, test pits be excavated by the Contractor near each end of each trenchless crossing to characterize subsurface conditions more thoroughly than could be accomplished during the field studies performed as part of the document.

3. Compatibility with site conditions, which in the case generally appear to be medium dense to dense sands and gravel (with possible oversized materials) at the top of the soil profile. In some cases, these granular materials overlie abandon/buried asphalt pavement. Deeper soils tend to be clayey, ranging from soft to stiff. More detailed information is provided elsewhere in this document.

4. The upper granular materials may be fairly permeable. This permeability may present issues of maintaining adequate quantities and circulation of drilling mud.