9-Geotech Report 250 West SE Access Road.pdf

Applied GeoTech Project No. 2241680

June 5, 2026

City of St. George

61 South Main Street

St. George, Utah 84770

Attn: Cathy Hasfurther

Email: cathy.hasfurther@sgcity.org

Subject: Geotechnical Recommendations

250 West Street

St. George, Utah

AGEC Project No. 2241680

Applied Geotechnical Engineering Consultants, Inc., was requested to provide geotechnical recommendations for the proposed 250 West roadway from Black Ridge Drive approximately 675 feet to the turnout and where the recent reconstruction stopped. Tge site location is shown on Figure 1.

SCOPE OF WORK

Recently, the remainder of the SE Access Road was constructed that extends from the north end of the construction up the hillside to Tech Ridge. AGEC provided the overall geotechnical study for the geologic hazards and roadway construction for this portion of the roadway under a separate report under Project No. 2180295. Several documents were provided during the study including the Geotechnical Investigation Report, dated October

19, 2018, updated Geotechnical Report, dated September 8, 2023, Clarification of Grading

Recommendations, dated October 21, 2024, Wall Designs dated September 8, 2023. The information from the previous study was reviewed as a part of our evaluation.

During the previous study, the property was identified as being part of an overall larger landslide that has not shown activity in this vicinity. Several portions of the landslide have been triggered on property to the north from development grading and wetting of the underlying materials. The details of the landslide evaluation are provided in the referenced study above.

To investigate the subsurface along the roadway, AGEC drilled 5 borings at the approximate locations shown on Figure 2. The borings were drilled to a depth of approximately 10 feet and samples of the subsurface soils were obtained for laboratory testing. The field and laboratory results were used to evaluate and provide pavement and subgrade recommendations. The findings and recommendations are included in this report. AGEC has previously met with Alliance Consulting and provided our recommendations for the proposed roadway.

mailto:cathy.hasfurther@sgcity.org

250 SE Access Road Reconstruction

SITE CONDITIONS

The proposed roadway alignment extends off of Blackridge Drive to the south up a hillside with approximately 60 feet of elevation change or at a grade of approximately 10%. The existing roadway is 3 lanes with small shoulder areas or 45 feet width of asphaltic concrete.

There is curb and gutter on both sides and sidewalk on the west side. The east side sidewalk was removed during previous construction for installation of a power line. There are utilities buried within the roadway. The southern portion of the roadway has been cut into the hillside with slopes on both sides of the road. The west side is unretained and the east side has a poorly stacked rock protected slope covering the landslide talus materials.

The west side is fairly steep for approximately 20 feet and then moderately steep for 20 plus feet to the vacant commercial property to the west. This area has been previously graded and disturbed. The west side is a commercial property, currently under construction.

The existing roadway is in very poor condition with rutting; severe cracking and several areas have experienced significant heave from the underlying expansive clay.

SUBSURFACE CONDITIONS

AGEC has evaluated the subsurface conditions by drilling 5 borings at the approximate locations shown on Figure 2. he borings were drilled to depths of approximately 10 feet using solid flight augers.

The subsurface conditions generally consist of 3 to 6 inches of asphaltic concrete overlying

4 to 8 inches of untreated base course. The subgrade below the pavement section consisted of sandy lean clay on the upper portion of the slope with mudstone or “blue clay” within the lower 3 borings, B-3, B-4 and B-5. Basalt materials were occasionally encountered within the clay soil within Borings B-1 and B-2 and at approximately 8 feet within Boring B-

1 and 7 feet in Boring B-2.

Each of the materials are described below:

Asphaltic Concrete (AC): The asphaltic concrete varied from 3 to 6 inches in thickness. The thickness at each boring location is shown on Figure 3, Exploratory

Boring Logs. The asphalt was in very poor condition with severe cracking, rutting and surface heave through the majority of the roadway. The lower or southern section of the roadway showed significant heave.

Untreated Base Course: The untreated base course was moderately well compacted and moist. It appeared as a crushed type of material and varied from 4 to 8 inches in thickness. The base course thickness within each boring is shown on Figure 3, Exploratory Boring Logs.

Sandy Lean Clay: The sandy lean clay is primarily soil mixed with occasional basalt cobbles or boulders and is anticipated to be talus from the overall landslide. The clay is generally medium plastic, and the amount of sand varies. The clay was generally mixed with various materials during the deposition process and varies from non-expansive to slightly expansive. The clay is reddish brown, moist and generally medium stiff. Laboratory test results are shown on Figure 3, Exploratory

Boring Logs and summarized on Table 1, Summary of Laboratory Test Results.

Basalt: The basalt was encountered within Borings B-1 and B-2. The basalt consists of cobbles and boulders mixed with the lean clay and is likely part of the talus from the landslide. At a depth of approximately 7 to 8 feet within Borings B-1 and B-2, the basalt was encountered and is likely larger cobbles or boulders that have deposited from the cliff above to the west.

Mudstone: The mudstone is part of the Petrified Forest Member of the Chinle

Formation and is known in the area as “blue clay”. Mudstone consists primarily of fat clay and is moderately to highly-expansive. The mudstone encountered was generally soft, moist and red to purple in color. Laboratory test results are shown on Figure 3, Exploratory Boring Logs and summarized on Table 1, Summary of

Laboratory Test Results.

PROPOSED CONSTRUCTION

The existing roadway is planned to be reconstructed for full width including the sidewalk and curb and gutter on both sides of the roadway. The construction will extend from

Blackridge Drive up the hill to the 2nd turnout where the original construction for the roadway recently ended. The subgrade is to be over-excavated and replaced with suitable materials. There will be a new storm drain installed, and the existing water and sewer piping will remain in place.

RECOMMENDATIONS

Based on the subsurface conditions encountered within the borings and previous discussions with City Engineers and Alliance Consulting, the roadway will be reconstructed, and we recommend that the existing pavement section be removed, the subgrade over-excavated to a depth

1. Subgrade Preparation: Due to the expansive clay soil/mudstone encountered through the roadway area and the past poor performance of the subgrade, we recommend that the subgrade preparation consist of a removal and replacement process. Based on our previous study and discussions, the depth of removal be

5 feet below subgrade elevation.

We recommend the exposed subgrade at the bottom of the over-excavation be scarified, moisture conditioned and recompacted to at least 90 percent of the maximum dry density as determined by ASTM D-698. The subgrade should not be allowed to dry out prior to the replacing of fill above.

Due to the remaining risk of surface heave, we recommend that the removed area may be replaced with low permeable fill materials for the bottom 3 feet of the replacement zone to reduce the wetting of the exposed mudstone at the bottom of the excavation. The material used as low permeable fill may consist of one of the following types of materials:

Option No. Liquid Limit (%) Percent Passing the No.

200 Sieve

1 50+ 15-20

2 30-50 20-40

3 10-30 40-80

These materials do not exist on site in a suitable classification. They may be used as a part of a mix to achieve the mixed condition. Use of the on-site clay materials may be considered and a mix of 2:1 (non-clay import to clay) has worked on previous projects. The majority of the on-site clay materials have liquid limits within option No. 2 and the mudstone would be option No. 1, thus a significant amount of low fines content would be needed to reduce the percent passing the No. 200 sieve. The maximum size should be 4 inches. If the on-site clay is used, it will need to be thoroughly mixed with the imported materials to provide a uniform type of material. AGEC should approve materials prior to use on site. AGEC should also observe the over-excavation prior to replacing materials.

We recommend that the sidewalk zone be replaced with the low permeable fill up to the base course.

The upper 2 feet of the replacement zone should be replaced with 2 feet of properly compacted fill material that meets the following:

Area Fill Type Soil Parameters

Upper 2 feet of replacement zone

Granular Fill -No. 200<35%, Maximum Size 4 inches

Liquid Limit < 30%

2. Pavement Section: Based on the listed traffic index, a 25 year design life, and

AASHTO design methods, the following pavement section is provided:

Flexible Pavement

Roadway

Traffic

Index

ROW

Asphaltic concrete

(inches)

Base Course

(inches)

250 West

3. Excavation/Earthwork

We anticipate that excavation of the onsite soils can be accomplished with typical excavation equipment. If excavation extends into the basalt caprock, difficulty will be encountered.

4. Grading Slopes and Trenches

The following table summarizes recommendations for excavation of trenches for slope construction.

Slope Condition

Maximum Slope*

(Horizontal:Vertical

Utility Trenches in On-site Soils (OSHA Soil Class C), Trenches in on-site bedrock, Soil Class A

1½:1

½:1

*Steeper trenches will require the use of shoring or a trench box and should be used to provide a safe work environment. Safe trench excavation is the responsibility of the contractor.

5. Pavement and Base Course Materials

Pavement and base course materials should meet St. George City requirements for gradation and quality.

6. Compaction

Compaction of materials placed at the site should equal or exceed the following minimum densities when compared to the maximum dry density as determined by ASTM D-1557 or ASTM D-698 as shown below::

Area Percent Compaction

Subgrade (bottom of over-excavation)

Replacement Fill

Utility trenches Backfill

Trench pipe zone

Base course

90*

*ASTM D-698

To facilitate the compaction process, the fill should be moisture conditioned to within 2 percentage points of the optimum moisture content as determined by ASTM D-1557 prior to placement.

Fill should be placed in loose lift thicknesses which do not exceed the capacity of the equipment being utilized. Generally, 8 to 12-inch loose lifts are adequate. Lift thickness should be reduced to 4-inches for hand compaction equipment.

7. Drainage

Positive site drainage should be maintained during the course of construction.

After construction has been completed, positive drainage of surface water away from roadway is critical. Storm drains, boxes etc, should be sealed to not allow leakage into the subsurface areas. A crack and joint seal program should be established to maintain proper drainage.

Our experience with pavements has also shown that the adjacent slopes should have positive drainage to not allow ponding or seepage of water into the subgrade soils.

8. Utility Pipes

There are existing utility pipes that will need to remain in place during the excavation process. The pipes should not be disturbed and the existing supporting materials should remain at least a 1:1 slope down both sides from the top of the pipe.

Pipe backfill should follow the same types of materials as described for the roadway above.

9. Limits of Over-Excavation

The overexcavation should extend at least 2 feet passed the improvements.

Where existing buried utilities exist on the west side of the roadway, the same criteria as noted in No. 8 above should be used.

LIMITATIONS

This report has been prepared in accordance with generally accepted geotechnical engineering practices in the area for the use of the client for design purposes. The conclusions and recommendations included within the report are based on the information obtained from the field and laboratory testing, the referenced reports and our experience in the area. Variations in the subsurface conditions may not become evident until excavation is conducted. If the subsurface conditions are found to be significantly different from those described above, we should be notified to reevaluate our recommendations.

APPLIED GEOTECHNICAL ENGINEERING CONSULTANTS, INC.

6/5/26

G. Wayne Rogers, P.E.

Attachments

P:\2024 Project Files\2241600\2241680- CS - SE Access Road @ Tech Ridge\250 West Geotech Work\Geotech Report 250 West SE Access Road.docx

APPLIED GEOTECHNICAL ENGINEERING CONSULTANTS, INC.

TABLE I

SUMMARY OF LABORATORY TEST RESULTS

250 W. Southeast Access Road Reconstruction Project Number 2241680

Sample Location Natural

Moisture

Content

Natural

Dry

Density

(pcf)

Gradation Atterberg Limits

Sample Classification Boring

No.

Depth

(feet)

Gravel

Sand

Silt/ clay

Liquid

Limit

Plasticity

Index

B-1 1 16 105 8 19 73 35 18 Sandy Lean Clay (CL)

B-1 2½ 13 107 76 34 18 Sandy Lean Clay (CL)

B-1 4 12 119 80 30 16 Sandy Lean Clay (CL)

B-2 1 19 92 1 18 81 41 23 Sandy Lean Clay (CL)

B-2 4 11 110 8 44 48 29 12 Sandy Lean Clay (CL)

B-2 6½ 12 112 84 32 16 Lean Clay (CL)

B-3 1 28 85 46 23 Mudstone Bedrock

B-3 2½ 27 86 56 29 Mudstone Bedrock

B-4 1 27 91 1 15 84 58 30 Mudstone Bedrock

B-4 4 23 79 83 52 29 Mudstone Bedrock

B-4 6½ 19 Mudstone Bedrock

B-4 9 19 Mudstone Bedrock

B-5 1 13 110 87 27 11 Mudstone Bedrock

B-5 2½ 22 4 18 78 46 48 Mudstone Bedrock

B-5 4 21 1 12 87 49 19 Mudstone Bedrock

B-5 6½ 19 Mudstone Bedrock

B-5 9 16 Mudstone Bedrock