2-Appendix A Geotechnical Report 24OCT2025.pdf

Appendix A – Geotechnical Report

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PO Box 880 Midvale, Utah 84047

801.849.0055 gerhartcole.com

TECHNICAL MEMORANDUM

To: Joey Willardson, PE Consor North America, Inc.

986 West 9000 South West Jordan, Utah, 84088

From: Mathew McNeil, GIT Richard Buhler, PE

Reviewed by: Ryan Maw, PE, DGE Travis Gerber, PhD, PE

Date: June 23, 2023

Job Number: 21-1437

Subject: SLCDPU Beck Street Trunkline Rehabilitation Project

INTRODUCTION AND BACKGROUND

This technical memorandum (TM) summarizes findings from our field study and provides geotechnical recommendations for the design and construction for a sewer pipeline rehabilitation project for the Salt Lake City Department of Public Utilities (SLCDPU). We understand that during one of the early phases of work, the pipeline will be rehabilitated between 800 North near 300 West and 1800 North and Beck Street (see Figures 1 and 2).

The rehabilitation will include installing new manholes and approximately 7000 feet of new liner for the existing sewer pipeline in this area, and replacing approximately 1550 feet of existing 64-inch pipe with a new 66-inch centrifugally cast fiberglass reinforced polymer mortar (CCFRPM, Hobas-type) pipe. Consor Engineers (Consor) has asked Gerhart Cole (GC) to provide geotechnical data gathered from field and laboratory studies to help in the evaluation of subsurface conditions for the installation of the new manholes and replacement of the pipeline at three select locations within this phase of work. These three areas, which for the purposes of this document are referred to as Area 1, Area 2, and Area 3, are shown in Figure 2, and Figures 4a through 4c. We understand that the existing sewer pipeline in Area 2 may have been installed between 1900 and 1910.

GC completed a geologic hazard study for the project team in 2022 (see Technical Memorandum dated January 10, 2022). The previous memorandum included discussion of geologic hazards and the geotechnical conditions potentially affecting the proposed trunk line rehabilitation.

SCOPE OF SERVICES

Our scope of work was generally completed according to the proposal provided by GC to Consor on January 5, 2023 with the contract executed on March 15, 2023, under the terms of our cooperative reciprocal services agreement. The work completed includes:

Drilling four (4) test holes and performing two (2) cone penetration tests (CPT) at selected locations along the pipeline alignment.

Installing four (4) temporary piezometers.

Performing laboratory testing of select soil samples.

Reducing and interpretating data, as well as developing geotechnical design recommendations and construction considerations for select portions of the pipeline alignment.

Preparing this technical memorandum (TM).

The number and location of test holes was specified by Consor at the onset of our work.

The Cone Penetration Test (CPT) soundings were added at the suggestion of GC.

REGIONAL GEOLOGIC SETTING

The project site is located within the Basin and Range Physiographic Province, near the western slope of the Wasatch Mountain Range in the Salt Lake Valley. The Basin and Range is characterized by a series of alternating generally north-south trending, normal-faulted, narrow mountain ranges and semi-arid to arid alluvial/pluvial valleys formed from tectonic extension believed to have initiated during Early Miocene time (approximately 17 million years ago) and continues during present time. A large portion of the Basin and Range Province, including the project area, is part of a system of watersheds topographically restricted from draining into the ocean. Instead, drainage and groundwater accumulate within lakes and playas in the valley bottoms until it evaporates. The Great Salt Lake, located approximately five miles northwest of the project site, was formed by such processes.

During prehistoric times, the Salt Lake Valley was largely filled by the ancestral Lake Bonneville 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).

The majority of Quaternary deposits (the Quaternary period being from approximately 2.58 million years ago until present) shown on surficial geology maps of the project area consist of sediments deposited or reworked by Lake Bonneville, the modern Great Salt Lake, and the Jordan River. Lake sediments include near-shore beach, delta, spit, and bar deposits. In deeper water toward the center of the valley, deposits consist of finer grained sand, silt, and clay. Holocene deposits (post-Bonneville, about 10,000 years ago until present time) consisting of alluvium and flood plain deposits are located along the Jordan River and its tributaries, clay deposited where streams entered the Great Salt Lake, and marsh deposits are also found within the project vicinity (Van Horn, 1982).

SURFICIAL SITE GEOLOGY

Surficial geology along the study area is variable. At the southern end of the project at Area 1 with study points 23-TH-01 and 23-CPT-01, the surface geology is mapped as marsh deposits (mm) from the Holocene (Van Horn, 1982). These deposits are up to 4 meters thick, range from silt and clays to pebbly and sandy. Interbedding with fine sand in areas occurs and is locally saturated.

In Area 2 with study points at 23-TH-02, 23-CPT-02, and 23-TH-03, surface geology is indicated by Van Horn as being undifferentiated lake gravels (lg). Deposited during the Pleistocene and Holocene, this unit more specifically contains cobbles, gravels, and sand.

These deposits can range from 1 to more than 4 meters thick. Adjacent to these sites are several distinct units mapped as alluvial fan deposits (fgy) consisting of undifferentiated alluvium. These deposits are typically bouldery to clay soil types, up to 4 meters thick.

Area 3, in which Test hole 23-TH-04 is located, lies within mapped alluvial fan deposits (Van Horn, 1982). Van Horn’s mapping at each of these three project areas are presented in Figure 3a, with more detailed information regarding pertinent geologic units presented in Figure 3b.

GEOLOGIC HAZARDS

GC completed a geologic hazard study for the overall project in 2022 (see Technical Memorandum dated January 10, 2022). The reader is referred to this previous document for information regarding geologic hazards associated with the proposed trunk line rehabilitation.

FIELD AND LABORATORY STUDIES

Between April 10 and April 14, 2023 four test holes and two CPT soundings were completed along the proposed alignment to a maximum depth of 30 feet. Study depths and approximate locations are indicated in Table 1. Three test holes and one CPT sounding were completed on Beck Street between Victory Road and 1600 North, placed at distinct locations of new manholes or where the pipeline is being replaced (See Figure 2), as directed by Consor. A fourth test hole and second CPT sounding were completed on 800 North, between 300 West and 400 West. Test holes and the CPT sounding on Beck Street were offset from their initial locations due to utilities in the area. All test holes and CPTs were located with a non-survey grade, hand-held GPS. Elevations were estimated based on publicly available LIDAR data (UGRC, 2020).

Surficial Site Conditions

This phase of the proposed Beck Street trunk line rehabilitation project extends from 800 North to approximately 1800 North in Salt Lake City, UT (See Figure 2). The pipeline alignment generally runs in a northeast-southwest trending orientation bounded by the Wasatch Mountains to the east and I-15 to the west. The proposed trunk line is approximately 1500 feet to 3000 feet east of I-15.

The locations of our field studies were relatively flat and paved. During our field studies it was discovered that Beck Street has up to 10 inches of concrete underlying the surface asphalt at the locations studied.

During the locating/clearing of utilities for the field studies through Blue Stakes, GC observed markings for buried power lines, sewer lines, fiber optic lines, crude oil lines, and high-pressure gas lines running northeast-southwest along the proposed trunkline alignment. The sizes and depths of these lines are not known. Other utilities may exist in the area. Due to utility conflicts along Beck Street, 23-TH-02, 23-CPT-02, and 23-TH- 03 locations were adjusted to the western northbound lanes of Beck Street. There were also overhead power lines near 23-TH-02, 23-CPT-02, 23-TH-03, and 23-TH-04.

Test Holes

All test holes were completed using a Simco 2800 HS/HT drill rig operated by A-Cache Corporation under subcontract to GC. All test holes were completed using hollow stem augering techniques. Soil samples were typically taken at continuous intervals of 2.5 feet for the full depth of each test hole. Standard penetration test (SPT) split-spoon samples were taken in coarser materials, and the blows required to drive the sampler 6 inches from a standard 140-pound hammer dropped from a height of 30 inches were recorded. The hammer energy efficiency was reported to GC as being 77.3 percent. Shelby tubes were pushed in finer grained materials in attempts to collect undisturbed samples for strength and/or consolidation testing. Soils found during drilling were logged in the field by a GC geologist.

Associated test hole logs are included in Appendix A and the approximate locations are plotted in Figures 2, 4a, 4b and 4c. Lines designating boundaries between different materials should be considered approximate; transitions between subsurface materials may be gradual or occur between sampling depths. Relative density and consistency descriptions used on the final test hole logs may deviate from typical criteria for a number of reasons, including reliance on other laboratory test results, visual observation, and/or manual manipulation of disturbed field samples, or the engineer’s judgement.

When drilling test holes 23-TH-02 and 23-TH-03, an odor suggesting potential hydrocarbon contamination was noted. The project team was alerted and, as subsequently described, chemical/environmental testing by a third-party laboratory was completed.

Temporary Piezometer and Groundwater Data

Temporary standpipe piezometers were installed in each of the test holes at a depth of 25 feet. Piezometers were constructed with the bottom 10 feet of each temporary piezometer being screened. The non-perforated riser pipe of each piezometer was sealed with cement bentonite grout to exclude surface water intrusion. Flush mount covers were used to seal out runoff and prevent tampering. GC returned to the project area four times after the field studies were completed between April 18, 2023 and June 5, 2023 to measure groundwater levels. Temporary piezometer construction details are summarized in Appendix A. A summary of groundwater observations can be found in Table 2a and is discussed in the Subsurface Conditions section of this TM.

Temporary piezometers were left in place for use by the project team. The piezometers will need to be properly abandoned during construction; the design team should include provisions for such within the project plans and specifications.

Cone Penetration Testing

The CPT soundings were advanced using a 25-ton truck-rig with a 15-cm2 piezocone (tip net area ratio of 0.80) operated by ConeTec of Salt Lake City, Utah, under subcontract to

GC. 23-CPT-01 was advanced to a depth of approximately 24 feet below site grade and 23- CPT-02 to a depth of approximately 30 feet. CPT data is summarized in Appendix B, and the approximate locations are plotted in Figures 2, 4a and 4b. CPT pore pressure dissipation tests were performed at 23-CPT-01 and 23-CPT-02.

A handheld, non-survey-grade GPS device was used at the time of the soundings to obtain the approximate latitude and longitude. Existing ground elevations were estimated based on these coordinates and publicly available LiDAR elevation data (UGRC, 2020).

When interpreting soil type from CPT results, it is important to recognize that the soil behavior type (SBT) shown on the logs reflects an interpretation of the soil behavior during undrained shear and do not reflect actual soil types. SBT found in the ConeTec CPT plots reflect the Robertson (2009 and 2010) relationships. It has been our experience that these SBT relationships often suggest more silt like behavior when compared with the actual soil types classified using laboratory testing. As such, proper use of CPT data requires skill, experience, and judgment.

Slug Testing

Hydraulic conductivities of the subsurface soils in the vicinity of the screened intervals of each well were evaluated in the field using a series of slug tests in each piezometer completed as part of this study. Slug testing consisted of lowering a weighted mass into the well to raise the water level and then allowing the water to dissipate back into the formation.

Once the level returned to equilibrium level, the slug was withdrawn, causing the water level in the well to drop. The level was then allowed to return to equilibrium before returning the slug to the well to repeat the process. Groundwater levels in the temporary piezometers were measured during slug testing with a logging pressure transducer, set to collect readings every 0.5 to 2 seconds.

Manhole Excavation Observation

GC was onsite to observe the excavation for a new manhole for the trunkline between June 6th and June 8th, 2023 along Beck St (See Figure 4b). We understand the trunkline was damaged by Dominion Energy’s Contractor in 2022 during installation of a new gas line.

The excavation for the new manhole was completed by Diamond S Company.

The total excavation was approximately about 16 feet by 14 feet by 11 feet in depth. The pavement consisted of approximately 9 inches of asphalt over 10 inches of concrete.

Underlying the pavement, the soil generally seemed to be pipe backfill material consisting of gravel with varying amounts of clay and sand in addition some cobbles and a boulder up to 20 inches were excavated. A few pockets of more sandy material were observed but they seemed to be somewhat limited in extent and thickness. In general, the gravel and cobbles with sand seemed to be free standing during excavation but the more sandy pockets were more prone to sluffing. Photographs of the excavation are provided in Appendix D.

The existing trunkline was exposed at a depth of about 5 feet and seemed to match the dimensions provided by Consor of 66 inches in diameter. The structure seemed to be more of an arch and the Contractor stated they were concerned it could be a brick structure and might not be very structurally sound. The outside of it seemed to be covered with grout or concrete (See Appendix D, Figure D-1).

The pipeline that had damaged the trunkline was observed at a depth of 7 feet (Figure D-2).

Groundwater was first observed seeping into the excavation at a depth of about 10 feet and was estimated to seep into the excavation at a rate of 0.1 to 0.25 gallons per minute (Figure D-4). After 24 hours the groundwater seemed to stabilize at a depth of about 10 feet (Figure D-5 and D-6). During excavation no pumping or control of the groundwater was observed.

Laboratory Testing

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

The testing performed included index property testing (moisture content, Atterberg limits, and grain-size distribution), 1-D consolidation testing, and unconsolidated-undrained (UU) triaxial shear strength testing.

We also performed a suite of electrochemical testing of two samples to help assess the soils’ ferrous corrosion and concrete degradation potential. These tests included evaluations of minimum resistivity, pH, soluble chloride, and soluble sulfate. Results of laboratory tests are summarized in Table 3 and also presented in Appendix C.

Due to odors detected during sampling, and in consultation with Consor, select soil samples with depths ranging from 2.5 to 20 feet in test holes 23-TH-02 and 23-TH-03 were taken to a third-party lab for hydrocarbon analysis. Results from this testing have been provided separately to Consor. The results of the testing are available from Consor. The complete nature and extent of this potential contamination or its impacts to the project are unknown to

GC.

SUBSURFACE CONDITIONS

Soils

General Materials found across all test holes consisted primarily of lean clays, fine-grained silty sands, sandy silts, and sandy gravels. Relative densities ranged from very loose to medium dense for sands and gravels, and consistencies ranged from medium stiff to stiff for clays and silts.

Area 1: 800 North (Test Holes 23-TH-01 and 23-CPT-01) Materials found in test hole 23-TH-01 for Area 1 contained 2.5 feet of asphalt and sandy gravel fill, underlain by a layer of sandy clay to 5 feet. From 5 feet to approximately 17.5 feet the material was sand, that ranged from silty to trace silt. At 17.5 feet the soil transitioned to gravel with sand to 26 feet, underlain by sand with silt to 27 feet. Materials found in CPT sounding 23-CPT-01, approximately 30 feet west of 23-TH-01, reflect a similar soil profile.

Note the CPT sounding reached refusal conditions at 23.9 feet.

Area 2: Beck Street (Test Holes 23-TH-02, 23-CPT-02, and 23-TH-03) Test holes and the CPT sounding advanced on Beck Street reflected variable soil conditions. Materials found in test hole 23-TH-02 for Area 2 contained gravelly sand from approximately 1.5 to 5 feet underlain by a 2.5-foot layer of sandy clay. At 5 feet the material changed to gravel with sand to approximately 17.5 feet. At 17.5 feet to the bottom of the test hole the material was sandy gravel with clay. Hence the soil profile is fairly granular at depth, and similar to that observed in Area 1, albeit coarser.

At 23-CPT-02, the sounding reflects a soil profile different than at test hole 23-TH-02.

Beginning near the ground surface (recognizing that the top two feet were not observed/measured, having be removed as part of the pavement penetration), soils consisted of sand to sandy silt from approximately 2 feet to 5 feet. From 5 feet to approximately 13.5 feet, the materials appear to vary between silty sandy to sandy silt, but could alternatively be relatively loose gravels. Unlike at test hole 23-TH-02, at a depth of approximately 13.5 feet, the soil changes to clay before changing to becoming siltier at approximately 22.5 feet. This material extends to the bottom of the sounding at 30 feet.

The materials found in 23-TH-03 consisted of clay with sand and gravel from approximately

1.25 feet to 5 feet. This clay layer is underlain by sandy gravel to a depth of about 15 feet Following this was silty sand to 17.5 feet. At 17.5 feet the material becomes fine grained with an organic silt layer to about 22.5 feet, followed by silt and elastic silt layers with some interbedded fibrous peat to the bottom of the test hole at 27 feet. The organic content of the soils was found to be approximately 20%. Moisture content tests ranged between 107% to 156%, and a consolidation test showed the material to be highly compressible with a compression ratio of 0.55.

At the manhole excavation the material observed is discussed in the Manhole Excavation Observation section. These cobbles and gravel materials are thought to be backfill material for the pipeline, and undisturbed native soils did not appear to be found.

Area 3: 1600 North (Test Hole 23-TH-04) Materials found in test hole 23-TH-04 for Area 2 contained approximately 9 inches of asphalt atop 9 inches of concrete. Materials found underlying roadway surfaces were gravel and some sand from 1.5 feet to 10 feet. This gravel contained some cobbles. From 10 feet to 15 feet sandy gravel with possible cobbles were found. Gravelly sand was found from 15 feet to 20 feet, to 25 feet the material transitioned back to a sandy gravel. From 25 feet to 27 feet the material was silt with sand.

Groundwater (Areas 1 through 3) Groundwater was found in test holes 23-TH-01, 23-TH-02, and 23-TH-03, between approximately 5.3 and 11.8 feet below existing grade during drilling. Groundwater was not found in 23-TH-04 during drilling but it was observed in the temporary piezometer.

Groundwater levels were estimated during drilling based on drilling conditions and feedback as well as field measurements using a water level indicator. After completion of drilling, temporary piezometers were installed as described in the Temporary Piezometer and Groundwater Data section of this document. Temporary piezometers were developed using a submersible pump or hand bailer, to remove fluids or material that entered the wells as a result of drilling activities. Slug tests were also completed after drilling and the temporary piezometer development was completed. Groundwater and temporary piezometer data along with measurements are summarized in Table 2a.

The depth of the water table can also be inferred from pore pressure dissipation testing (PPDT) completed as part of a CPT sounding. In the case of 23-CPT-01, the calculated depth of the water table (based on tests performed at depths of about 15 feet and 24 feet) is between 4.4 and 5.1 feet. In the case of 23-CPT-02, the calculated depth of the water table (based on test performed at a depth of about 30 feet) is 4 feet. The results of this PPD testing are summarized in Table 2b.

It should be recognized that groundwater levels are not static, but change in response to precipitation, runoff, irrigation, nearby water bodies, etc. The Contractor should verify water levels prior to commencing work.

Slug Test Data Reduction and Hydraulic Conductivity Evaluation As discussed above, slug testing was performed in water bearing zones composed of sands and gravels found in test holes 23-TH-01, 23-TH-02, and 23-TH-03. Slug test data was evaluated after the methods of Hvorslev (1951) as presented by Hydrogeology Field Manual (Weight, 2008). We note that groundwater was generally confined to gravel layers in the test holes. The flow characteristics of these layers dominate the response measured in the wells.

The slug test hydraulic conductivity data shows a range of hydraulic conductivity from very permeable (1x10-2 cm/s) to moderately permeable soils (3x10-5 cm/s). This testing would indicate the gravels and sands in Area 1 and near 23-TH-02 in Area 2 might be less permeable than the soil types would typically suggest, the variances perhaps due to elevated fines content, or differences in either hydraulic gradient or stratigraphic continuity not readily discernable from the test. The hydraulic conductivity data analysis outputs are included for reference in Table 4.

GEOTECHNICAL PIPELINE DESIGN RECOMMENDATIONS

This section should not be considered a complete specification; it provides general guidelines for the planned pipe. Recommendations contained in this section apply to the new 66-inch, CCFRPM Hobas-type sewer pipe in Area 2. Deviations from these assumptions should be discussed with GC, and the recommendations revisited.

With respect to the design and installation of buried pipe, we recommend that the pipe manufacturer’s recommendations be followed, together with the following guidance. This would include following the provisions of American Water Works Association [AWWA] Manual M45 for Fiberglass Pipe Design.

We understand that pipeline segments may be constructed according to SLCDPU’s specifications, which we understand reflect American Public Work Association [APWA] Utah Chapter’s 2017 Manuals for Standard Specifications and Plans. In the event of conflicting recommendations / specification requirements, the more rigorous of the two should be implemented.

As a structure, the performance of buried pipe reflects the properties of both the pipe material and the soil that surrounds it. The soil used in a pipe installation is generally divided into three zones, based on function and location within the trench: bedding, pipe-zone backfill, and trench-zone backfill.

Bedding and Pipe Zone Fill Fill in pipe zone and bedding/haunch areas should consist of imported, well-graded sand and gravel with maximum particle size of 1-1/2 inches and 5 to 15 percent non-plastic fines.

(However, it should be noted that compaction in the haunch areas of the pipe may be more easily obtained if the maximum particle size is limited to 3/8-inch or less.) Open graded , uniformly sized gravels should be avoided due to risk of migration of fine-grained native soils. Squeegee and pea gravel-types of materials should also not be used. Pipe zone fill should be placed in maximum 8-inch loose lifts and compacted to 95 percent of maximum dry density (MDD) as determined by ASTM D1557 and to a moisture content within 2 percent of the optimum moisture content (OMC).

Controlled Low-Strength Material (CLSM or Flowable fill) is an acceptable alternative and offers several advantages to granular bedding and pipe zone backfill, including:

Resistance to water flow, thereby reducing dewatering requirements and eliminating the need for seepage collars within the CLSM zone.

Uniform pipe support, especially in haunch areas.

Higher strength material than granular backfill, which offers additional pipe support in the soft native soils.

Pipe zone fill should be placed on firm, undisturbed subgrade. If trench bottom is disturbed by construction or upward flow of groundwater, disturbed soil should be removed and replaced with pipe zone fill as described above.

If soft to very soft soils are found in the trench bottoms or at foundation bearing layers subgrade stabilization will be required. Subgrade stabilization can be accomplished by forcing cobble rock into the subgrade to form a firm, unyielding surface. Cobble rock should consist of 3 to 12-inch rock pushed into the subgrade with the excavator bucket, roller, or tracked equipment. Rock should be placed as necessary to establish a firm working surface.

Alternatively, the bottom of the trench can be over-excavated and bedding material wrapped in a separation geotextile (“burrito wrap”) can be used. The geotextile should have suitable strength to provide confinement during material placement and densification. Details regarding the thickness (depth) of the over-excavation and the use of any geotextile will need to be based on conditions exposed during trench excavation.

Any voids formed in the process of subgrade stabilization, or during the movement of shoring/shielding within the trench (pipe and/or trench zones), should be filled with compacted pipe zone backfill.

The trench width should be wide enough to compact pipe zone materials around the pipe;

we recommend a minimum trench width of the pipe diameter plus 12 inches on each side of pipe. If the trench is wider, pipe zone fill should be placed and properly compacted across the full trench width.

Trench Zone Fill Due to all the work being completed within existing roadways, the trench zone backfill should consist of well-graded, non-plastic sand and gravel with maximum particle size of 2 inches with less than 30 percent fines. However, clean drain rock or pea gravel-types of materials should not be used. Trench zone backfill should be compacted to 95% of MDD as determined by ASTM D1557 and to a moisture content within 2 percent of OMC.

Other Trench and Backfill Considerations Based on our field and laboratory studies, it is our opinion that the coarse-grained soils found in all three areas will likely meet the requirements of the trench zone backfill material with minimal processing (apart from moisture conditioning). Additionally, some of the coarse-grained soils could meet the requirements of pipe zone backfill material with some processing. Fine-grained silts, clay, or organic silts will likely not be able to be reused for bedding, pipe zone or trench backfill and should be disposed of in coordination with Consor and or the Owner.

To control the potential flow of water through more pervious granular backfill materials, consideration could be given to the use of seepage control/cutoff collars in the design for pipeline installations below the water table. Spacing should be limited to manageable pipe segment lengths to reduce current or future potential dewatering/groundwater management needs. Sizing of seepage collars in this application is not prescriptive but should be large enough to cutoff seepage water in the pipe trench from one section to another. Seepage collars could be constructed of concrete, flowable fill, or other methods capable of maintaining positive contact with the pipeline and sufficiently impermeable to prevent transmission of water along the pipe bedding. We recommend a maximum spacing of 500 feet and that seepage collars extend the full width of the pipe trench and to the base of the pipe bedding.

It is our understanding that the planned new pipes will be constructed near or below the groundwater table, with the pipe expected to be fully submerged in the design condition.

Uplift forces due to pipe buoyancy should be considered in both dewatering and pipeline installation/backfill plans, both during construction and thereafter.

Corrosion

Various criteria have been established by different organizations as to whether or not a soil is chemically aggressive relative to ferrous metals. The U.S. Federal Highway Administration has established the criteria summarized in Table 5 for buried steel based on minimum resistivity of saturated soil. From the data provided in the table and based on this criteria, it appears that the onsite soils gravelly soils are moderately corrosive, while the fine grained soils found in 23-TH-03 are considered very corrosive. We recommend that the services of a corrosion engineer be retained to further assess corrosion potential and provide the design of any needed corrosion mitigation measures.

Various criteria have been established by different organizations as to whether or not a soil is chemically aggressive relative to concrete. The American Concrete Institute (see ACI 318R) has established the criteria in Table 6 for concrete in contact with soil, based on sulfate attack. These criteria suggest the site soils have a negligible concrete degradation potential. The ACI category for the concrete degradation potential is S0, meaning that no special type of cement would be required for concrete in contact with site soils.

EARTHWORK RECOMMENDATIONS AND CONSTRUCTION CONSIDERATIONS

Construction Considerations

Excavation and Shoring As a minimum, all excavations should comply with Occupational Safety and Health Administration’s (OSHA) construction standards [Standard for Excavations, 29 CFR Part 1926, Subpart P], as well as applicable state and local regulations. The Contractor should rely upon his own methods to determine and maintain safe and stable slopes during construction, subject to his 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.

We understand the pipeline replacement along Beck Street will require excavation on the order of 10 to 17 feet below the ground surface. The excavations of the manholes will require excavations on the order of 10 to 20 feet. Given the work being within existing roads, and the potential for shallow groundwater, sloping excavations will probably not be feasible or allowed. It is noted that the project extents cover a large area and subsurface conditions are shown to vary significantly within our limited study locations. In Area 1, where groundwater was found to be shallow and flowing sand was observed during drilling, support of the excavation should be provided at all times to minimize the potential of sluffing or undermining of the existing roadways. This could be utilized with a slide rail system or temporary shoring utilizing sheet or soldier piles.

In Area 2, our study showed varying soil conditions with fine grained silts and sands found near 23-CPT-02 and more coarse-grained soils in and above the pipe zone at 23-TH-02 and 23-TH-03. The manhole excavation also presented coarse-grained soils which seemed to be pipe backfill material. The manhole excavation was open cut to a depth of about 11 feet and only some minor sluffing of the excavation sides was observed. Trench boxes were placed after the excavation was completed. We recommend that if trench boxes are used, the boxes be placed in contact against the excavation sides. Any gaps between the trench box walls and excavation side walls be filled in immediately to reduce the potential for slumping or sluffing of the excavation sidewalls which could compromise the roadway or adjacent utilities. Coarse-grained soils which might be stable during initial excavations are prone to becoming unstable with time as they dry.

In Area 3, trench boxes could also potentially be used due to the coarse-grained soils and the groundwater being below the proposed excavation depths. The Contractor should be prepared for varying soil conditions and groundwater depth during construction and have varying methods readily available to provide excavation support. Preconstruction review of the Contractor’s excavation methods as a design submittal, together with onsite construction observation by the Geotechnical Engineer, is recommended.

Dewatering Across our three study areas, groundwater has been measured at depths between 5 and 25 feet in temporary piezometers. For the pipeline replacement in Area 2, groundwater was measured between about 5 feet and 12 feet below the ground surface. This reach of the pipeline alignment might require mobile sumps, deep wells, and or vacuum well points due to the variability of soils and groundwater depths. At Area 1, where the groundwater was found at 5 feet below the surface, dewatering wells could be required to successfully dewater the excavation for the new manhole. For the manhole at Area 3, groundwater has been measured below the expected excavation depth and dewatering might not be required.

We recommend that groundwater levels should be maintained at least 2 feet below excavation grade prior to placement of fill materials in order to provide stability to the trench bottom and facilitate compaction. We recommend the temporary piezometers installed as part of this study be utilized to help monitor the effectiveness of dewatering efforts.

Installation of additional piezometers will likely be beneficial. We have assumed temporary piezometers will be removed as part of excavation activities. Any temporary piezometer located outside of planned excavation should be abandoned with a lean grout as part of Project completion.

Heaving and flowing soils were observed in 23-TH-01 (Area 1) below site grade and suggest the potential for challenging dewatering and flowing soil conditions. Such conditions should be anticipated and addressed by the Contractor. Given the extent of the work relative to the limited field studies completed and in consideration of the overall length of the project, the Contractor should be prepared for adverse dewatering conditions.

Also, due to the high compressibility potential of the organic fine grained soils and peat found in test hole 23-TH-03, there is an elevated risk of subsidence (i.e., ground settlement) during dewatering operations in Area 2. The Contractor should provide for mitigation measures (including limiting the length of trench dewatered at one time, as well as keeping the duration of dewatering as short as possible) to reduce this potential. The Contractor should monitor the area for ground settlement and the conditions of potentially impacted infrastructure below, during, and immediately after dewatering and construction is complete.

The Contractor is responsible to perform their own dewatering evaluation consistent with their means and methods of construction.

OTHER ITEMS

As described previously, hydrocarbon odor suggesting potential contamination was noted when drilling test holes 23-TH-02 and 23-TH-03. The complete nature and extent of this potential contamination is unknown to GC. Project specific considerations and impacts on construction activities including the handling, use, and/or disposal of soils are being addressed/coordinated by Consor and/or the Owner. Contaminated soils, if any, should generally not be used as backfill.

Our scope of work for this project did not include an evaluation of potential geoenvironmental hazards such as soil and/or groundwater contamination and disposal, nor the potential for hazardous materials along the subject pipeline alignments. The equipment, techniques, and personnel used to conduct a geoenvironmental investigation differ from those used for geotechnical and geologic engineering.

ADDITIONAL SERVICES

Subsurface conditions are inherently variable. It is important that we observe subsurface materials and conditions exposed at the site during construction, thereby taking advantage of opportunities to recognize potentially differing site conditions and reduce the risk of unanticipated and/or adverse outcomes. We also recommend that we review project plans and specifications for compatibility with our assessments and recommendations.

Additional recommended services include, but are not necessarily limited to:

Development of geotechnically related project specifications.

Review of final plans and specifications.

Review of geotechnically related Contractor submittals.

Observations during construction related to activities such as excavation, backfilling, and compaction.

Consultation as may be required during construction.

Additional information regarding such services can be obtained from our office.

LIMITATIONS

The assessments, considerations, and recommendations included in this document are based on limited field study and laboratory testing, as well as our understanding of the project’s design and manner of construction. If the project’s design or manner of construction changes, or if conditions are found that are different from those described, we should be notified immediately so that we can make revisions as necessary.

This document was prepared solely for the use of the addressee for the specified project and may not contain sufficient information for other parties or uses.

We represent that our services are performed within the limitations prescribed by our Client, in a manner consistent with the level of care and skill ordinarily exercised by other professional consultants under similar circumstances. No other representation expressed or implied, and no warranty or guarantee is included or intended. We do not assume responsibility for the accuracy of information provided by others.

FIGURES

Figure 1 Vicinity Map Figure 2 Site Map Figure 3a Geologic Map Figure 3b Description of Geologic Units Figure 4a Field Studies Locations – Area 1 (South) Figure 4b Field Studies Locations – Area 2 (Central) Figure 4c Field Studies Locations – Area 3 (North)

TABLES

Table 1 Field Studies - Test Hole Locations Table 2a Groundwater Summary Table 2b Pore Pressure Dissipation (PPD) Testing Summary Table 3 Laboratory Test Results Summary Table 4 Slug Test Results Table 5 Corrosion Potential Criteria for Steel and Test Results Table 6 Degradation Potential Criteria for Concrete (after USBR and ACI) and Test

Results

APPENDICES

Appendix A Test Hole Logs Appendix B Cone Penetration Test Results Appendix C Summary of Laboratory Test Results Appendix D Manhole Excavation Photographs

REFERENCES

Lund, W.R., Christenson, G. E., Harty, K. M., Hecker, S., Atwood, G., Case, W. F., Gill, H.

E., Wallace Gwynn, G. J., Klauk, D. R., Mabey, W. E., Mulvey, D. A., Sprinkel, B. T., Black, B. D., Nelson, C. V. (1990). Geology of Salt Lake City, Utah, United States of America. Environmental and Engineering Geoscience ; xxvii (4): 391–478.

Robertson, P.K., 2009, “Interpretation of cone penetration tests – a unified approach”, Canadian Geotechnical Journal, Volume 46: 1337-1355. DOI: 10.1139/T09-065.

Robertson, P.K., 2010. Soil behavior type from the CPT: an update. 2nd International Symposium on Cone Penetration Testing, CPT’10, Huntington Beach, CA, USA

Utah Geospatial Resource Center [UGRC], 2018. 2018 Northern Utah LiDAR Elevation

Data, collected in 2018, bare earth DEMs. Accessed on April 25, 2023 from https://gis.utah.gov/data/elevation-and-terrain/2018-lidar-northern-utah/

Van Horn, R. (1982). Surficial geologic map of the Salt Lake City North quadrangle, Davis and Salt Lake Counties, Utah: U.S. Geological Survey, Miscellaneous Investigations Series Map I-1404.

Weight, W.D. (2008). Hydrogeology Field Manual (2nd ed.). The McGraw-Hill Companies, New York, NY.

Location ID Date Completed Latitudea Longitudea Elevation

(ft)b Total Depth

(ft) Drilling/Advancement

Method Backfill Method

23-TH-01 4/10/2023 40.78680 -111.90100 4245 27 HSA 2" Temporary Piezometer 23-TH-02 4/11/2023 40.79630 -111.90590 4246 27 HSA 2" Temporary Piezometer 23-TH-03 4/13/2023 40.79819 -111.90823 4242 27 HSA 2" Temporary Piezometer 23-TH-04 4/13/2023 40.80203 -111.91305 4250 27 HSA 2" Temporary Piezometer

23-CPT-01 4/10/2023 40.78674 -111.90108 4244 24 CPT Bentonite Chips 23-CPT-02 4/13/2023 40.79738 -111.90727 4242 30 CPT Bentonite Chips

Notes:

a) Latitude/Longitude data established by Gerhart Cole via hand held GPS.

b) Elevations obtained from publicly available LiDAR data (UGRC, 2018).

Table 1: Field Study Locations SLCDPU Beck St. Trunk Line Rehabilitation

Table 2a: Groundwater Summary

4/11 4/13 4/18 5/2 5/15 6/5

23-TH-01 4245 15-25 10-25 0.9 5.3 -- 5.3 5.2 5.3

23-TH-02 4246 15-25 15-25 7.6 -- 8.3 7.4 4.65b 7.7

23-TH-03 4242 15-25 15-25 -- -- 11.8 10.8 12.2 12.3

23-TH-04 4250 15-25 15-25 -- 25.0 25.0 24.9 24.7 25.0

Notes:

a) Elevations obtained from publicaly available LiDAR data (UGRC, 2018).

b) Measured after rainstorm and water observed in well cap and could have raised groundwater depth.

Groundwater depth on date measured (2023) (feet) Location ID

Test Hole Ground Surface

Elev. (ft)a

Screened Depth

Interval (ft)

Sand Depth Interval (ft)

Location ID Date Measured

Ground Surface

Elevation (ft) a PPD Test Depth (ft)

PPD Test Equilibrium (ft)

Inferred Groundwater

Depth (ft)

Inferred Groundwater Elevation (ft) Comments

23-CPT-01 4/10/2023 4244 6.0 0.6 5.4 4238.6

23-CPT-01 4/10/2023 4244 15.0 9.9 5.1 4238.9

23-CPT-01 4/10/2023 4244 23.9 19.5 4.4 4239.6

23-CPT-02 4/13/2023 4242 23.0 19.0 4.0 4238.0

23-CPT-02 4/13/2023 4242 30.0 26.1 3.9 4238.1

Note:

a) Elevations obtained from publicly available LiDAR data (UGRC, 2018).

Table 2b: Pore Pressure Dissipation (PPD) Testing Summary

Table 3: Laboratory Test Results Summary

6 9 13 6 7 8 9 10 11 12 13 14 15 16

T es t H ol e

D ep th ft)

LL

P L

P I (

G R

A V

E L

(N o.

3"

S A

N D

(N o.

0-

N o.

4) F

IN

E

S

N o.

0) si lt

(0 .0

-0

.0 m m cl ay

0.

m m

-in m m

.5

-in

.5 m m

/4

-in m m

/8

-in

.5 m m

N o

.4

.7 m m

N o

.1 m m

N o

.2

.8 m m

N o

.4

.4 m m

N o

.6

.2 m m

N o

.1

.1 m m

N o

.2

.0 m m

R e si st iv ity

(o h m -c m p H

S o lu b le c h lo ri d e p p m

S o lu b le s u lfa te p p m

23-TH-01 5-7 ft 20.3 4 64 32 17 15 100 100 98 97 96 96 95 87 73 50 32

23-TH-01 7.5-9.5 ft 24.5 NP NP NP 0 74 26 100 100 100 100 100 99 98 95 86 58 26

23-TH-01 10-12 ft 25.7 1 68 31 17 13 100 100 100 99 99 99 98 97 92 68 31

23-TH-01 15-17 ft 5.9 1 67 32 18 14 100 100 100 99 99 99 98 97 92 66 32

23-TH-01 12.5-14.5 ft 23.4 3,300 8.9 44 37

23-TH-01 17.5-19.5 ft 10.4 62 29 9 100 100 82 55 38 31 27 24 20 14 8.6

23-TH-02 5-7 ft 41 24 17

23-TH-02 7.5-9.5 ft 12.8 61 20 19 10 9 100 100 62 47 39 33 30 27 25 22 19

23-TH-02 12.5-14.5 ft 20.2 44 28 28 17 11 100 100 85 69 56 49 46 43 39 35 28

23-TH-02 15-17 ft 15.9 51 29 20 11 9 100 100 83 61 49 40 35 31 27 24 20

23-TH-02 20-22 ft 13.3 50 32 18 100 100 76 60 50 43 37 33 28 24 18

23-TH-03 2.5-4.5 ft 24 16 8

23-TH-03 5-7 ft 7.5 46 33 21 100 100 76 66 54 47 42 38 34 29 21

23-TH-03 7.5-9.5 ft 7.1 48 30 22 12 10 100 100 87 64 52 45 40 36 32 28 22

23-TH-03 10-12 ft 64 24 12 100 100 100 52 36 27 24 21 19 16 12

23-TH-03 12.5-14.5 ft 14.9 45 35 20 100 100 95 73 55 45 40 37 33 27 20

23-TH-03 15-17 ft 600

23-TH-03 17.5-19.5 ft 73 64 9

23-TH-03 20-22 ft 156.7 28.2 72.4 87 75 12 20.6 Con, UU Test, Organic Content

23-TH-03 22.5-24.5 ft 28.4 32 24 8

23-TH-03 25-27 ft 107 67 47 20 0 40 60 100 100 100 100 100 100 94 86 78 70 60 20.4 Organic Content

23-TH-04 6.5-7 ft 9.2 0 49 51 30 21 100 100 100 100 100 100 98 94 86 71 51

23-TH-04 10-12 ft 6.9 40 36 24 100 100 85 68 60 54 50 47 43 36 24

M o is t /

S a t.

u n it w e ig h t ( p cf

Grain-Size Analysis (Percent Finer)

Other Testsa

O rg a n ic c o n te n t (

Corrosion M oi st ur e co nt en t

Atterberg Limits

D ry u ni t w ei gh t ( pc f)

Grain-Size

Table 3: Laboratory Test Results Summary

6 9 13 6 7 8 9 10 11 12 13 14 15 16

T es t H ol e

D ep th ft)

LL

P L

P I (

G R

A V

E L

(N o.

3"

S A

N D

(N o.

0-

N o.

4) F

IN

E

S

N o.

0) si lt

(0 .0

-0

.0 m m cl ay

0.

m m

-in m m

.5

-in

.5 m m

/4

-in m m

/8

-in

.5 m m

N o

.4

.7 m m

N o

.1 m m

N o

.2

.8 m m

N o

.4

.4 m m

N o

.6

.2 m m

N o

.1

.1 m m

N o

.2

.0 m m

R e si st iv ity

(o h m -c m p H

S o lu b le c h lo ri d e p p m

S o lu b le s u lfa te p p m

M o is t /

S a t.

u n it w e ig h t ( p cf

Grain-Size Analysis (Percent Finer)

Other Testsa

O rg a n ic c o n te n t (

Corrosion M oi st ur e co nt en t

Atterberg Limits

D ry u ni t w ei gh t ( pc f)

Grain-Size

23-TH-04 12.5-14.5 ft 4.5 1,085 9 230 24

23-TH-04 15-17 ft 5.9 24 20 4 37 40 23 100 100 88 75 63 54 47 42 37 31 23

23-TH-04 20-22 ft 4.7 45 37 18 100 100 88 70 55 44 38 34 30 24 18

23-TH-04 25-27 ft 23.7 0 24 76 59 18 100 100 100 100 100 100 100 99 99 97 76

Note:

a) Abbreviations - Con - Consolidation test UU - Triaxial unconsolidated Undrained Compression test

Table 4: Slug Test Results

23-TH-01 15-25 10-25 5.3 3.1E-05

23-TH-02 15-25 15-25 8.3 4.9E-04

23-TH-03 15-25 15-25 11.8 1.1E-02

23-TH-04 15-25 15-25 -- N/A

Water not found, unable to perform slug test.

Screened Depth

Interval (ft)

Sand Depth

Interval (ft)

Ground Water Depth

(ft)

Hydraulic Conductivity

[K Value] (cm/s) Location ID Comments

Expected Corrosion Condition Minimum Resistivity (ohm-cm) Number of Specimens Very corrosive < 1,000 1 of 3

Moderately corrosive 1,000 to 5,000 2 of 3 Mildly corrosive 5,000 to 10,000 0 of 3

Slightly corrosive > 10,000 0 of 3

Table 5: Corrosion Potential Criteria for Steel and Test Results SLCDPU Beck Street Trunk Line Rehabilitation

Expected Degree of Attack (ACI Category) Water Soluble Sulfate (%)a Number of Specimens Negligble (S0) 0 to 0.1 2 of 2 Positive (S1) 0.1 to 0.2 0 of 2 Severe (S2) 0.2 to 2.0 0 of 2

Very Severe (S3) ≥ 2.0 0 of 2 Note:

a) 0.1% = 1,000 ppm.

Table 6: Degradation Potential Criteria for Concrete (after USBR and ACI) and Test Results

Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community, Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community

Figure 1

0 3,900 7,8001,950 Feet

SLCDPU Beck Street Trunk Line Rehabilitation

Vicinity Map

Explanation Site Vicinity

Site Vicinity

!A

!A

!A

!A

!A

Beck St.

I-15

800 N

1600 N

23-TH-04

23-TH-03

23-TH-02

23-TH-01

23-CPT-02

23-CPT-01

Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community, Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community

Study Area Map Figure 2SLCDPU Beck Street Trunk Line Rehabilitation ±

0 0.2 0.40.1 Miles

Explanation !AApproximate Test Hole Locations

Approximate Trunk Line Alignment

Area 1

Area 2

Area 3

!A

!A

!A

!A

Beck St.

I-15

800 N

23-TH-04

23-TH-03

23-TH-02

23-TH-01

23-CPT-02

23-CPT-01

Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community, Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community

Geologic Map Figure 3aSLCDPU Beck Street Trunk Line Rehabilitation ±

0 0.2 0.40.1 Miles

Explanation !AApproximate Test Hole Locations

Approximate Trunk Line Alignment

Note: Richard Van Horn, 1982, Surficial geologic map of the Salt Lake City North quadrangle, Davis and Salt Lake Counties, Utah.

I-1404. USGS. 1:24,000 Scale.

Geologic Units mm Marsh Deposit (Holocene) – Silt, clay, and clayey silt, locally pebbly and sandy, dark-gray, brown, and black; as much as 4 m thick. Locally contains abundant carbonized plant material and fragments of large gastropods and clams. At many places, the deposit contains a few to many holes approximately 1 mm in diameter. At places, interbedded with light-brown fine sand that may interfinger with adjacent alluvial fan deposits to the east. Locally water saturated; probably would be unstable during earthquakes and in steep-walled excavations.

lg Lake Gravels, undifferentiated (Holocene and Pleistocene) – Cobbly gravel and sand, light-gray;

thickness ranges from 1 to more than 4 m. At places, unconformities, fossil soils and silt beds are present within that unit. Steep slopes may be unstable.

fgy Undifferentiated Fan Deposits younger than Bonneville shoreline (Holocene and Pleistocene) – All areas where units are located are subject to sudden and violent flash floods and mudflows

1. See Figures 3 for geologic mapping (Van Horn, 1982 and 1987) near proposed improvements.

2. See Van Horn (1982 and 1987) for additional information on mapping; descriptions shown are units that are in the vicinity of the project

Notes:

Description of Geologic Units

Figure 3b

!A800 N

300 W

400 W

23-TH-01

23-CPT-01

Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community, Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community

Field Studies Map - Area 1 (South) Figure 4aSLCDPU Beck Street Trunk Line Rehabilitation ±

0 150 30075 Feet

Explanation !RApproximate Manhole Location

!AApproximate Test Hole Locations Approximate Trunk Line Alignment

!A

!R

Beck St.

23-TH-03

23-TH-02

23-CPT-02

Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community, Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS…