Attachment 6 Geotechnical Evaluation Report_Rexford Lift Station.pdf

December 17, 2021 Project 21-4118G

Mr. Robert B. Morton, PE

Robert Peccia & Associates, Inc.

3147 Saddle Drive

Helena, Montana 59601

Via Email: BMorton@rpa-hln.com

Dear Mr. Morton:

Re: Geotechnical Evaluation, Proposed Lift Station, Rexford Bench Campground, Rexford, Montana

We have completed the geotechnical evaluation for the above-referenced project authorized by our

Subconsultant Agreement, dated November 11, 2021. The purpose of the evaluation was to assist Robert

Peccia & Associates (RPA) in evaluating the subsurface soil and groundwater conditions for the proposed lift station. The evaluation was completed in general accordance with our proposal to RPA dated March

11, 2021.

Summary of Results

One soil boring was completed at the location of the proposed lift station. The soil profile generally consisted of 3 1/2 feet of poorly-graded gravel overlying sandy silt that transitioned to silty sand, followed by poorly-graded fine to coarse sand to the termination depth of 25 1/2 feet below existing grade. Groundwater was encountered at a depth of 18.6 feet while drilling. We anticipate that ground water will likely be encountered during construction, especially when considering the current drought conditions.

Summary of Analysis and Recommendations

The bottom of the lift station is expected to bear at an elevation between 15 to 20 feet below existing grade. At this depth, the bottom of the lift station will bear on loose to medium dense silty sand or poorly-graded sand. When considering the high groundwater levels will require dewatering, we anticipate the sands will be susceptible to disturbance, and we recommend providing a 1 foot working platform beneath the base foundation. We recommend designing the lift station to counteract the anticipated buoyancy forces produced by the groundwater. For design, we recommend assuming groundwater could rise as high as 8 1/2 feet below the ground surface. The buoyancy forces can be resisted by the weight of the concrete in the lift station and the weight of the backfill placed above the foundation edges that extend beyond the perimeter of the sidewalls. Depending on the exact depth and location of the proposed lift station in relation to nearby buildings, shoring and/or underpinning may be required. The shoring should be designed by a qualified engineer.

Since groundwater was encountered at the time of our geotechnical exploration, the contractor should be prepared to dewater the excavation. We recommend groundwater be drawn down to a minimum depth of

2 feet below the anticipated bottom of excavation levels prior to excavation. The method of dewatering will need to be determined by the contractor based on their past experience, available equipment, and their dewatering specialist's recommendations.

2511 Holman Avenue

P. O. Box 80190 Billings, Montana 59108-0190 p: 406.652.3930; f: 406.652.3944 www.skgeotechnical.com

Robert Peccia & Associates December 17, 2021

Project 21-4118G Page 2

General

Please refer to the attached report for more detailed results of our fieldwork, engineering analyses, and recommendations.

Thank you for using SK Geotechnical. If you have any questions regarding this report, or require our services during the construction phase of this project, please call Greg Staffileno at (406)

652-3930.

Sincerely, Samuel A. Michalak

Engineering Geologist

Gregory T. Staffileno, PE

Senior Geotechnical Engineer

Attachment:

Geotechnical Evaluation Report

Table of Contents Description Page

A. Introduction

A.1. Project A.2. Purpose of this Evaluation A.3. Scope A.4. Documents Provided A.5. Location and Elevation

B. Results B.1. Log B.2. Site Conditions B.3. Soils B.4. Groundwater Observations B.5. Laboratory Tests

C. Analyses and Recommendations C.1. Proposed Construction C.2. Excavation and Dewatering C.3. Bearing Pressure C.4. Buoyancy C.5. Backfill C.6. Backfill Lateral Earth Pressure C.7. Lift Station Enclosure C.8. Settlement C.9. Seismic Considerations

D. Construction D.1. Excavation D.2. Dewatering D.3. Observations D.4. Moisture Conditioning D.5. Testing D.6. Cold Weather Construction

E. Procedures E.1. Drilling and Sampling E.2. Soil Classification E.3. Groundwater Observations

F. General Recommendations F.1. Basis of Recommendations F.2. Review of Design F.3. Groundwater Fluctuations F.4. Use of Report F.5. Level of Care

Professional Certification

Appendix Site Location Sketch Boring Location Photo Boring Location Sketch Geologic Map Descriptive Terminology Log of Boring Sheet ST-1 Laboratory Test Results

A. Introduction

A.1. Project

A new lift station is planned for the Rexford Bench Campground in Eureka, Montana. This lift station will replace an existing lift station and is to be located at the south end of the Boat Ramp Parking area.

Several attachments as well as a photograph of the area show the site.

A.2. Purpose of this Evaluation

The purpose of this geotechnical evaluation was to assist Robert Peccia & Associates (RPA) in designing foundations for the proposed lift station through evaluating subsurface soil and groundwater conditions.

A.3. Scope

On March 11, 2021, we submitted a proposal to perform the work. An executed Subconsultant

Agreement, dated November 11, 2021, authorized our work. Our scope of services was limited to:

• Coordinating the locating of underground utilities near the boring location, although RPA personnel performed a private utility locate as well, which was critical to avoid power and water lines servicing a nearby building.

• Conducting one standard penetration test boring to a depth of 25 1/2 feet in the proposed lift station area.

• Returning the samples to our laboratory for visual classification and logging by a geotechnical engineer.

• Conducting laboratory tests including moisture contents, Atterberg limits (plasticity), gradations, and corrosion tests.

• Analyzing the results and formulating recommendations for earthwork, foundations, and construction.

• Submitting a geotechnical evaluation report containing a log of the boring, our analysis of the field and laboratory tests, subsurface soil and groundwater conditions, and recommendations for earthwork, lift station foundation, recommended lateral earth pressures, compaction, and reuse of on-site soils.

Project 21-4118G Page 2

A.4. Documents Provided

The following documents were provided for our use:

• Rexford Bench Campground Forcemain, prepared by RPA, dated November 2021

• Rexford Bench Complex Meeting Notes, prepared by RPA, dated January 13, 2021

A.5. Location and Elevation

The boring location was selected by RPA and our drill crew, and is shown on the Boring Location Sketch in the Appendix. The location is show on the Boring Location Photo in the Appendix. RPA indicated the surface elevation of the boring to be 2471.79 feet.

B. Results

B.1. Log

A Log of Boring sheet indicating the depth and identification of the soil strata encountered, penetration resistances, laboratory test data, and water level information is attached. It should be noted that the depths of contacts between soil strata are only approximate. These contacts may be more transitional than indicated on the log and the depth to contacts may vary between the boring location and final location of the lift station.

Geologic origins presented for each stratum on the Log of Boring sheet are based on the soil types, blows per foot, and available common knowledge of the depositional history of the site. Because of the complex glacial and post-glacial depositional environments, geologic origins are frequently difficult to ascertain. A detailed evaluation of the geologic history of the site was not performed.

B.2. Site Conditions

The site of the proposed lift station is generally located at the south end of the Rexford Bench Boat Ramp

Parking area, approximately 100 feet to the east of the access road to the boat ramp. The site slopes gently to the north, away from the existing Kamloops Loop Campground and towards Lake Koocanusa.

The location is adjacent to an existing bathroom building and just outside the paved portion of the parking area.

B.2.a. Geologic Setting

According to the Geologic and Structure Maps of the Kalispell 1º and 2º Quadrangle, Montana, and

Alberta and British Columbia (dated 2000), the project site is situated in an area of Pleistocene-age glacial and fluvioglacial deposits. A portion of this map is attached. As glaciers in the area melted and retreated to the north, large quantities of gravel, sand, silt, and clay were deposited beyond the ice margin. These sediments were transported by the glacial meltwater and deposited in settings such as outwash plains, Project 21-4118G Page 3 braided rivers, and glacial lakes. The transport energy of the meltwater in each setting controlled the grain size, sorting, and grading of the resulting deposits. Often, abrupt changes in vertical and horizontal characteristics are observed in glacial and fluvioglacial sediments as a result of rapid and frequent shifts in the hydrologic conditions of the melting glaciers. Underlying the glacial sediments at the site are

Proterozoic age rocks of the Shepard and Snowslip Formations. A high angle, concealed, fault is mapped in the vicinity of the project area. A detailed geologic evaluation of the site was not included as part of our scope of services.

B.3. Soils

Boring ST-1 was performed for the lift station and encountered about 3 1/2 feet of poorly-graded gravel overlaying sandy silt. This sandy silt transitioned to silty sand at approximately 8 1/2 feet below existing grade. The relative proportion of silt to sand continued to decrease with depth within the fluvioglacial deposits until the boring was terminated in poorly-graded sand at a depth of 25 1/2 feet. Lenses of elastic silt were observed throughout the sand strata.

Penetration resistances recorded in the gravel ranged from 4 to 6 blows per foot (BPF), indicating a loose to very loose relative density. Penetration resistances recorded in the sandy silt ranged from 16 to

20 blows per foot (BPF), indicating a stiff to very stiff consistency. Penetration resistances recorded in the sand layers ranged from 7 to 16 blows per foot (BPF), indicating a loose to medium dense relative density.

B.4. Groundwater Observations

Groundwater was encountered at a depth of 18.6 feet below existing grade during and immediately upon completion of drilling. We anticipate the groundwater level at the site will fluctuate seasonally with snow melt and other hydrologic factors. If precise knowledge of the groundwater level is required, we recommend a monitoring well be installed to better evaluate groundwater levels over time.

B.5. Laboratory Tests

The results of the laboratory tests are presented on the boring log in the Appendix of this report. The results are also discussed in more detail below.

B.5.a. Moisture Content. Moisture content tests were obtained for the majority of the penetration test samples and are shown on the attached Log of Boring sheet. The moisture contents ranged from about

4 percent to 15 percent. The higher values were obtained within the sandy silt layer and below the observed groundwater level at the time of drilling.

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B.5.b. Classification Tests. Classification tests consisting of sieve analysis and Atterberg limits were performed on the thin wall sample obtained from the boring taken from 8 to 9 feet and a split spoon sample obtained from 14 to 15 1/2 feet. The results of these tests are summarized in Table 1 below.

Table 1. Summary of Laboratory Tests

Boring Depth

(feet)

Atterberg Limits ASTM

Classification P200

(%) LL PL PI

ST-1 8 to 9 16 13 3 ML 59

ST-1 14 to 15 1/2 NP NP NP SM 14.2

B.5.c. Corrosion Tests. Corrosion tests were performed on the sandy silt sample obtained from 8 to

9 feet, and the results are attached. The sample was tested for sulfates to evaluate the sulfate exposure to reinforced concrete. The result of the sulfate test was 0.0023 percent. The American Concrete Institute

(ACI) indicates this to be a negligible sulfate exposure. Even so, we recommend considering the soils to be a moderate exposure environment, and using cement meeting both Type I/II requirements to provide moderate sulfate resistance.

C. Analyses and Recommendations

C.1. Proposed Construction

The proposed lift station will likely consist of a 5-to-8-foot circular or rectangular concrete wet well bearing on an integral mat foundation approximately 15 to 20 feet below finished grade. The proposed lift station will replace an existing lift station, located around 100 feet to the south of the new lift station location. The proposed lift station will collect wastewater from the Rexford Bench Campground to the south and the picnic/beach areas to the north and pump it to the City of Rexford for treatment. The new lift station will likely have a greater capacity than the existing lift station, allowing it to handle additional wastewater from nearby comfort stations and host sites. The force main that connects to the existing lift station consists of 3-in galvanized pipe, however, it is understood this piping will likely be replaced as part of the sewer system renovation project.

C.2. Excavation and Dewatering

C.2.a. Depth. The mat foundation of the lift station is planned to bear between 15 and 20 feet below the existing grade. These depths are suitable for frost protection.

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C.2.b. Dewatering. Groundwater was observed during drilling and will need to be drawn down prior to and during excavation. While ground water was observed at 18.6 feet below existing grade, groundwater will likely fluctuate throughout the year due to rain, snow melt runoff, nearby lake water levels as well as other numerous hydrologic factors not apparent at the time of our fieldwork. Higher groundwater levels are likely, especially when considering the ongoing drought conditions. We recommend the contractor dewater the excavation a minimum of 2 feet below the bottom of the excavation, prior to excavating.

Dewatering prior to excavating results in less hydrostatic water pressures on the subgrade, reducing the risk of the subgrade becoming unstable and disturbed.

C.2.c. Subgrade. At a depth between 15 and 20 feet, the lift station near Boring ST-1 will bear on medium dense silty sand or poorly graded sand. It is our opinion these soils, will generally be suitable for support of the lift station mat foundation. To provide a more stable working platform, we recommend subexcavating 1 foot below the planned bearing elevation with a smooth bladed bucket and placing

6-ounce nonwoven geotextile fabric on the subgrade, followed by Tensar BX-1200 Geogrid, or equivalent. The geofabric and geogrid should extend laterally at least 2 feet beyond the footprint of the wet well foundation. A leveling course of 1 1/2-inch minus crushed base course can then be placed above the geogrid to bring the working platform to the planned elevation for the bottom of the mat foundation. The gravel base should be compacted to a minimum of 95 percent of its standard Proctor density.

C.3. Bearing Pressure

We recommend the lift station pad be designed for a net allowable bearing pressure of 2,000 pounds per square foot (psf). (Net allowable bearing pressure is defined as that bearing pressure in excess of the final minimum overburden pressure.) This bearing pressure includes a factor of safety of at least 3.0 against bearing capacity failure.

C.4. Buoyancy

Considering ground water was encountered during drilling, we strongly recommend designing the lift station to counteract anticipated buoyant forces. For design, we recommend assuming groundwater could rise as high as 8.5 feet below the ground surface, or elevation 2463.3. The buoyant forces can be resisted by the weight of the concrete in the lift station and the weight of the backfill placed above the foundation edges that extend beyond the perimeter sidewalls. Assuming a mixture of on-site poorly graded sand, silty sand, and sandy silt are used as backfill, we recommend assuming a moist unit weight of 120 pounds per cubic foot (pcf) for backfill above the maximum water level and a submerged unit weight of 58 pcf below the maximum water level. This value has not been factored, and an appropriate factor of safety will need to be provided.

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If a mat foundation is placed beneath the lift station and extends beyond the perimeter walls, backfill placed above the mat can help resist the buoyancy forces. We recommend assuming the resistance to buoyancy is equal to the weight of the concrete in the lift station walls and mat foundation plus the weight of a wedge of soil above the mat foundation extending beyond the foundation walls. This wedge is formed by planes extending upward and outward from the top edges of the mat at an angle of 20 degrees from vertical. The submerged and moist unit weights indicated above can be used.

C.5. Backfill

It is our opinion the on-site soils can be used as backfill, but compaction will be difficult. If some settlement and regrading of the site is acceptable, the backfill may be placed in 4- to 8-inch loose lifts and compacted to 95 percent of its maximum dry density determined in accordance with ASTM Method of

Test D 698 (standard Proctor) within 2 percent of optimum moisture content. We anticipate well compacted backfill will still result in 1 to 2 inches of settlement. If the backfill is poorly compacted, the settlement will be much more, likely several inches or more. If this amount of settlement and regrading are unacceptable, we recommend flowable fill be used from the bottom of the excavation to a depth of

10 feet below existing grade, elevation 2462. At this point, the on-site soils may be used as backfill around the lift station above this elevation, so long as they are placed in 4- to 8-inch loose lifts and compacted to a minimum of 95 percent of its standard Proctor maximum dry density within 2 percent of its optimum moisture content.

C.6. Backfill Lateral Earth Pressure

We anticipate the backfill around the lift station will consist of a mixture of the on-site sand and silt.

Depending on the desired level of settlement, some of the backfill may also be the flowable fill previously mentioned. We recommend using at-rest equipment fluid pressures for design. Above elevation 2465.5, we recommend using an equivalent fluid pressure of 58 pounds per square foot per foot (psf/ft) of depth for the moist soils. Below elevation 2465.5, we recommend using 100 psf/ft of depth for submerged soils.

The values indicated above do not include a factor of safety. An appropriate factor of safety should be included when designing below grade walls to resist lateral earth forces.

C.7. Lift Station Enclosure

Specifics of the lift station design were not available at the time of this report. However, depending on design, the lift station may feature an above-ground enclosure. We assume the enclosure will bear on a cast concrete base which in turn will bear both on the concrete walls of the wet well and in part on the surrounding soils. Differential movement should be anticipated between the portion bearing on the wet well and the portion bearing on the surrounding soils. Additional reinforcement should be provided in the slab and foundations to better accommodate this differential movement.

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C.8. Settlement

As previously mentioned, the settlement of the lift station will depend on the method of backfill used. If settlement and regrading is acceptable, backfill compacted to 95 percent of its standard Proctor and plus or minus 2 percent of its optimum moisture content in 4- to 8-inch loose lifts will likely result in settlement of 1 to 2 inches if this backfill is well compacted. Poorly compacted backfill will likely result in several inches or more. If the lift station is backfilled using a combination of flowable fill to the depth of the top of the influence sewer followed by well compacted on-site soils compacted to 95 percent using

4- to 8-inch loose lifts, then we expect settlement to be closer to 1-inch total.

C.9. Seismic Considerations

Based on the results of our soil borings and review of available geologic information, we recommend using a "Stiff soil profile, Site Class D," as defined by the 2012 International Building Code (IBC) for design.

D. Construction

D.1. Excavation

Due to the depth of the lift station excavation, we anticipate the excavation will likely need to be conducted by a tracked excavator working from the surface. All excavations should be performed in accordance with OSHA requirements. The borings indicate the soils in the sidewalls of the lift station excavation will be Type C, sandy soils. All earthwork and construction should be performed in accordance with OSHA guidelines. Shoring of the excavation may be required, which may require design by a licensed engineer. Unsuitable material such as organics or other deleterious material should be separated and wasted, if encountered.

D.2. Dewatering

Groundwater was encountered at a depth of about 18 1/2 feet while drilling. We expect this could be several feet or more higher, depending on the time of construction. We recommend the contractor be prepared to dewater the excavation a minimum of 2 feet below the bottom of the excavation. Actual methods of dewatering will need to be determined by the contractor based on their past experience and available equipment. The deeper sands are likely highly transmissive, requiring significant dewatering.

Also, piping of the sands must be prevented during the dewatering.

D.3. Observations

We recommend excavations of the lift station be observed. These observations should be performed by a geotechnical engineer or an engineering assistant working under the direction of a geotechnical engineer.

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The purpose of these observations is to evaluate if the subgrade soils are similar to those encountered in the borings and suitable for the proposed construction.

D.4. Moisture Conditioning

On-site soils range from dry to wet of optimum and will need to be moisture conditioned to a moisture contact near optimum. This can be accomplished by mixing the soils and/or spreading the soils out to dry or add moisture.

D.5. Testing

We recommend density tests of fills and backfills placed around the lift station. Samples of proposed backfill and fill materials should be submitted to our testing laboratory at least three days prior to placement on the site for evaluation and determination of their optimum moisture contents and maximum dry densities.

D.6. Cold Weather Construction

If site grading and construction is anticipated during cold weather, we recommend good winter construction practices be observed. All snow and ice should be removed from cut and fill areas prior to additional grading. No fill should be placed on soils that have frozen or contain frozen material. No frozen soils should be used as fill.

Concrete delivered to the site should meet the temperature requirements of ASTM C 94. Concrete should not be placed on frozen soils or soils that contain frozen material. Concrete should be protected from freezing until the necessary strength is attained. Frost should not be permitted to penetrate below footings bearing on frost-susceptible soil since such freezing could heave and crack the footings and/or foundation walls.

E. Procedures

E.1. Drilling and Sampling

The penetration test boring was performed on November 11, 2021, with our CME 75 HT truck mounted drilling rig. Sampling for the boring was conducted in accordance with ASTM D 1586, "Penetration Test and Split-Barrel Sampling of Soils." Using this method, we advanced the borehole with hollow-stem auger to the desired test depth. Then a 140-pound hammer falling 30 inches drove a standard, 2-inch OD, split-barrel sampler a total penetration of 1 1/2 feet below the tip of the hollow-stem auger. The blows for the last foot of penetration were recorded and are an index of soil strength characteristics.

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E.2. Soil Classification

The drill crew chief visually and manually classified the soils encountered in the borings in accordance with ASTM D 2488, "Standard Practice for Description and Identification of Soils (Visual-Manual

Procedures)." A summary of the ASTM classification system is attached. All samples were then returned to our laboratory for review of the field classifications by a geotechnical engineer. Representative samples will remain in our office for a period of 60 days to be available for your examination.

E.3. Groundwater Observations

About ten minutes after taking the final sample in the bottom of a boring, the driller probed through the hollow-stem auger to check for the presence of groundwater. Immediately after withdrawal of the auger, the driller again probed the depth to water or cave-in. The boring was then backfilled.

F. General Recommendations

F.1. Basis of Recommendations

The analyses and recommendations submitted in this report are based upon the data obtained from the soil boring performed at the location indicated on the attached sketch. Often, variations occur between borings, the nature and extent of which do not become evident until additional exploration or construction is conducted. A re-evaluation of the recommendations in this report should be made after performing on-site observations during construction to note the characteristics of any variations. The variations may result in additional foundation costs, and it is suggested a contingency be provided for this purpose.

It is recommended we be retained to perform the observation and testing program for the site preparation phase of this project. This will allow correlation of the soil conditions encountered during construction to the soil boring, and will provide continuity of professional responsibility.

F.2. Review of Design

This report is based on the design of the proposed structure as related to us for preparation of this report.

It is recommended we be retained to review the geotechnical aspects of the designs and specifications.

With the review, we will evaluate whether any changes in design have affected the validity of the recommendations, and whether our recommendations have been correctly interpreted and implemented in the design and specifications.

F.3. Groundwater Fluctuations

We made water level observations in the boring at the times and under the conditions stated on the boring logs. These data were interpreted in the text of this report. The period of observation was relatively short, and fluctuation in the groundwater level may occur due to rainfall, flooding, irrigation, spring thaw, drainage, and other seasonal and annual factors not evident at the time the observations were made.

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Design drawings and specifications and construction planning should recognize the possibility of fluctuations.

F.4. Use of Report

This report is for the exclusive use of Robert Peccia & Associates to use to design the proposed structure and prepare construction documents. In the absence of our written approval, we make no representation and assume no responsibility to other parties regarding this report. The data, analyses, and recommendations may not be appropriate for other structures or purposes. We recommend parties contemplating other structures or purposes contact us.

F.5. Level of Care

Services performed by SK Geotechnical Corporation personnel for this project have been conducted with that level of care and skill ordinarily exercised by members of the profession currently practicing in this area under similar budget and time restraints. No warranty, expressed or implied, is made.

Professional Certification

I hereby certify that this report was prepared under my direct supervision and that I am a duly Licensed Professional

Engineer under the laws of the State of Montana.

Gregory T. Staffileno, PE

Senior Geotechnical Engineer

License Number 10798PE

December 17, 2021

Appendix

Site Location Sketch

Geotechnical Evaluation

New Sewer Lift Station

Rexford Bench Campground

Rexford, Montana

Drawn by: SK Geo/Google Earth Date 11/24/2021

Project: 21-4118G

Scale: NTS FIGURE

Sheet 1 of 1 1

Boring Location Photo

New Sewer Lift Station

Rexford Bench Campground

Rexford, Montana

Photo by: SK Geo Date 11/24/2021

Project: 21-4118G

Sheet 1 of 1 2

Boring Location Sketch

New Sewer Lift Station

Rexford Bench Campground

Rexford, Montana

Drawn by: RPA Date 12/8/2021

Project: 21-4118G

Sheet 1 of 1 3

ST-1

Geologic Map

New Sewer Lift Station

Rexford Bench Campground

Rexford, Montana

Drawn by: SK Geo Date 12/08/2021

Project: 21-4118G

Sheet 1 of 1 4

ST-1

Descriptive Terminology

Standard D 2487

Classification of Soils for Engineering Purposes

(Unified Soil Classification System)

Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests A

Soil Classification

Group

Symbol Group Name B

Coarse-

Grained

Soils

More than

50% retained on No.

200 sieve

Gravels

More than

50% of coarse fraction retained on

No. 4 sieve

Clean Gravels

Less than 5% fines C

CU ≥ 4 and 1 ≤ CC ≤ 3 E GW Well graded gravel F

CU < 4 and/or 1 > CC > 3 E GP Poorly graded gravel F

Gravels with

Fines

More than 12% fines C

Fines classify as ML or MH GM Silty gravel F, G, H

Fines classify as CL or CH GC Clayey gravel F, G, H

Sands

50% or more of coarse fraction passes No. 4 sieve

Clean Sands

Less than 5% fines D

CU ≥ 6 and 1 ≤ CC ≤ 3 E SW Well graded sand I

CU < 6 and/or 1 > CC > 3 E SP Poorly graded sand I

Sands with

Fines

More than 12% fines D

Fines classify as ML or MH SM Silty sand G, H, I

Fines classify as CL or CH SC Clayey sand G, H, I

Fine-

Grained

Soils

50% or more passes the

No. 200 sieve

Silts and

Clays

Liquid Limit less than 50

Inorganic

PI > 7 and plots on or above

"A" line J CL Lean clay K, L, M

PI < 4 or plots below "A" line J ML Silt K, L, M

Organic Liquid limit – oven dried < 0.75

Liquid limit – not dried

OL

Organic clay K, L, M, N

Organic silt K, L, M, O

Silts and

Clays

Liquid limit

50 or more

Inorganic PI plots on or above "A" line CH Fat clay K, L, M

PI plots below "A" line MH Elastic siltK, L, M

Organic Liquid limit – oven dried < 0.75

Liquid limit – not dried

OH

Organic clayK, L, M, P

Organic siltK, L, M, Q

Highly Organic Soils Primarily organic matter, dark in color, and organic odor PT Peat

A

B

C

D

E

F

G

Based on the material passing the 3" (75 mm) sieve.

If field sample contained cobbles or boulders, or both, add "with cobbles or boulders, or both" to group name.

Gravels with 5 to 12% fines require dual symbols

GW-GM well-graded gravel with silt

GW-GC well-graded gravel with clay

GP-GM poorly graded gravel with silt

GP-GC poorly graded gravel with clay

Sands with 5 to 12% fines require dual symbols.

SW-SC well-graded sand with clay

SP-SM poorly graded sand with silt

SP-SC poorly graded sand with clay

CU = D60 / D10

CC = (D30)

2 / (D10 x D60)

If soil contains ≥ 15% sand, add "with sand" to group name.

If fines classify as CL-ML, use dual symbol GC-GM or

SC-SM.

H

I

J

K

L

M

N

O

P

Q

If fines are organic, add "with organic fines" to group name.

If soil contains ≥ 15% gravel, add "with gravel" to group name.

If Atterberg limits plot in hatched area, soil is a

CL-ML, silty clay.

If soil contains 15 to 29% plus No. 200, add

"with sand" or "with gravel", whichever is predominant.

If soil contains ≥ 30% plus No. 200 predominantly sand, add "sandy" to group name.

If soil contains ≥ 30% plus No. 200 predominantly gravel, add "gravelly" to group name.

PI ≥ 4 and plots on or above "A" line.

PI < 4 or plots below "A" line.

PI plots on or above "A" line.

PI plots below "A" line.

Particle Size Identification Boulders .......................................... over 12"

Cobbles .......................................... 3" to 12"

Gravel coarse ......................................... 3/4" to 3" fine ........................................ No. 4 to 3/4"

Sand coarse ................................ No. 4 to No. 10 medium ........................... No. 10 to No. 40 fine ................................ No. 40 to No. 200 Silt ................................. No. 200 to .005 mm

Clay ................................ less than .005 mm

Relative Density of Cohesionless Soils very loose ..................................... 0 to 4 BPF loose ........................................... 5 to 10 BPF medium dense .......................... 11 to 30 BPF dense ........................................ 31 to 50 BPF very dense ................................. over 50 BPF

Consistency of Cohesive Soils very soft ....................................... 0 to 1 BPF soft ............................................... 2 to 3 BPF rather soft ..................................... 4 to 5 BPF medium ........................................ 6 to 8 BPF rather stiff ................................... 9 to 12 BPF stiff ........................................... 13 to 16 BPF very stiff ................................... 17 to 30 BPF hard ........................................... over 30 BPF

Moisture Content (MC) Description rather dry MC less than 5%, absence of moisture, dusty moist MC below optimum, but no visible water wet Soil is over optimum MC waterbearing Granular, cohesionless or low plasticity soil with free water, typically near or below groundwater table very wet Cohesive soil well over

OMC, typically near or below groundwater table

Drilling Notes Standard penetration test borings were advanced by 3¼" or 4¼" ID hollow-stem augers, unless noted otherwise. Standard penetration test borings are designated by the prefix "ST" (split tube). Hand auger borings were advanced manually with a 2 to 3" diameter auger to the depths indicated. Hand auger borings are indicated by the prefix "HA."

Sampling. All samples were taken with the standard 2" OD split-tube sampler, except where noted. TW indicates thin-walled tube sample.

CS indicates California tube sample. BS indicates bulk sample.

BPF. Numbers indicate blows per foot recorded in standard penetration test, also known as "N" value. The sampler was set 6" into undisturbed soil below the hollow-stem auger. Driving resistances were then counted for second and third 6" increments and added to get BPF.

Where they differed significantly, they were separated by backslash (/). In very dense/hard strata, the depth driven in 50 blows is indicated.

WH. WH indicates the sampler penetrated soil under weight of hammer and rods alone; driving not required.

Note. All tests were run in general accordance with applicable ASTM standards.

Laboratory Tests DD Dry density, pcf WD Wet density, pcf OC Organic content, %

LL Liquid limit PL Plastic limit PI Plasticity index

P200 % passing 200 sieve MC Natural moisture content, % MDD Maximum dry density (Proctor), pcf OMC Optimum moisture content (Proctor), % qu Unconfined compressive strength, psf UCS Unconfined compressive strength, psi qp Pocket penetrometer strength, tsf October 30, 2018

2468.3

2463.3

2454.3

2446.3

Elevation Reference:

Provided by RPA.

Thinwall: 8'-9'

LL=16, PL=13, PI=3

P200= 59.0%

SPT: 14½'-15½'

LL=NP,PL=NP,PI=NP

P200= 14.2%

4.0

NR

8.0

15.1

8.4

6.3

3.1

5.7

13.5

12.4

3.5

8.5

17.5

25.5

GP

ML

SM

SP

POORLY GRADED GRAVEL with SAND, fine-grained, trace roots, dark brown, moist, loose to very loose. (Alluvium)

SANDY SILT, non-plastic, pale yellow, moist, medium dense. (Fluvioglacial Deposits)

SILTY SAND, fine- to medium-grained, trace lenses of elastic silt, grayish brown, rather dry, medium dense to loose. (Fluvioglacial Deposits)

POORLY GRADED SAND, fine- to coarse-grained, trace lenses of elastic silt, very dark grayish brown, waterbearing, medium dense.

(Fluvioglacial Deposits)

END OF BORING

Water down 18.6' with 24' of hollow-stem auger in the ground.

Water not observed to dry cave-in depth of 2' immediately after withdrawal of auger.

Boring then backfilled.

SCALE:11/11/21 1" = 4'

LOCATION:

ST-1 page 1 of 1

L O G O F B O R I N G

Description of Materials 2471.8

DRILLED BY: E. Hollibaugh

WL

MC

21-4118G

METHOD: CME 75HT

ST-1

Symbol

BORING:

Depth

2511 Holman Avenue P. O. Box 80190

Billings, MT 59108-0190 Phone: 406.652.3930

Fax: 406.652.3944

DATE:

Remarks 0.0

Elev.

See Attached Sketch

BPF

PROJECT: 21-4118G

GEOTECHNICAL EVALUATION

New Sewer Lift Station Rexford Bench Campground Rexford, Montana qp (tsf)

B O

R

IN

G B

P F

W L-

M C

Q P

E

LE

V

8.

G

P J

L A

G N

N N

.G

D T

/1 7/

0 10 20 30 40 50 60 Liquid Limit (LL)

12/3/21

NP

NP

8.4% 5.7%

ML-CL

Project Number: 21-4118G

TW

Jar #7

8'-9' 14'-15½'

LL MC ClassificationBoring

CL

ML or OL

Legend Depth PL PI

MH or OH

CH

NP

P la st ic it y

In d ex

P I)

Sample No.

ST-1

ST-1

New Sewer Lift Station

P 200, %

Rexford, Montana

59.0 14.2

2511 Holman Avenue P. O. Box 80190

Billings, MT 59108-0190 Phone: 406.652.3930

Fax: 406.652.3944

ML

SM

Atterberg Limits Tests

Rexford Bench Campground

0.1110

Percent Passing U.S. Standard Sieve Size

Depth:

28.2

Sample No.:

1 1/2"

12.8

3/8"

#100

3" 1.5"

Sand

3/4"

Percent Gravel:

Percent Silt + Clay:

Gravel

8.4% coarse

Date Received: 11/24/2021Sample:

#4

New Sewer Lift Station

Sieve Analysis

ST-1

#4

#10 medium

#40 #100 #200

ML

#20

#40

12/3/21 coarse

Plasticity Index:

P er ce nt

P as si ng

Particle Size in Millimeters

Sieve Size

3"

59.0

SANDY SILT

8'-9'

TW

59.0 fine

Rexford Bench Campground

ASTM Group Name:

Percent Sand:

#200

#203/4" fine

#80

Liquid Limit:

Plastic Limit:

3/8"

Classification:

Moisture Content:

#10

Project Number: 21-4118G 2511 Holman Avenue

P. O. Box 80190 Billings, MT 59108-0190

Phone: 406.652.3930 Fax: 406.652.3944

0.1110

NP

Percent Passing U.S. Standard Sieve Size

Depth:

85.8

Sample No.:

1 1/2"

0.0

3/8" #100

3" 1.5"

Sand

3/4"

Percent Gravel:

Percent Silt + Clay:

Gravel

5.7% coarse

Date Received: 11/24/2021Sample:

#4

New Sewer Lift Station

NP

Sieve Analysis

ST-1

#4

#10 medium

#40 #100 #200

NP

SM

#20

#40

12/3/21 coarse

Plasticity Index:

P er ce nt

P as si ng

Particle Size in Millimeters

Sieve Size

3"

14.2

SILTY SAND

14'-15½' Jar #7

14.2 fine

Rexford Bench Campground

ASTM Group Name:

Percent Sand:

#200

#203/4" fine

#80

Liquid Limit:

Plastic Limit:

3/8"

Classification:

Moisture Content:

#10

Project Number: 21-4118G 2511 Holman Avenue

P. O. Box 80190 Billings, MT 59108-0190

Phone: 406.652.3930 Fax: 406.652.3944

2511 Holman Avenue P. O. Box 80190

Billings, Montana 59108-0190 p: 406.652.3930; f: 406.652.3944 www.skgeotechnical.com

Corrosivity of Soil

ASTM G162/G187, AASHTO T 88

Date: Project: 21-4118G Geotechnical Evaluation Rexford Branch Campground Sewer Lift Station Eureka, Montana

Client: RPA

Date sampled: 11/11/21 Date tested:

Sampled by: Drill Crew Tested by:

ST-1 8-9 6100 0.164 8.33 8.36 0.0023 NT NT

Remarks: ND = non-detect NT = not tested Sulfate result is E300.0 water soluble method from Energy Labs.

Sulfide (mg/kg)

Oxid- Reduc (mV)

December 17, 2021

Boring Depth (feet)

Resistivity (Ω•cm) Soil Box

Conductivity (m.mhos/cm)

Calculated pH

Marble pH

Sulfate (wt %)