2026 B-117 Spring Creek Road Bridge Replacement

PRELIMINARY GEOTECHNICAL REPORT
MONTROSE COUNTY
SPRING CREEK ROAD BRIDGE B-117
OVER M&D CANAL
MONTROSE, COLORADO
June 2, 2023
Prepared for:
Mr. Keith Laube, P.E.
Public Works Director
Montrose County
63160 LaSalle Road
Montrose, CO 81401

TABLE OF CONTENTS
Introduction ............................................................................................................. 1
Construction Plans .................................................................................................... 1
Site Conditions ......................................................................................................... 1
Geology & Geologic Hazards ..................................................................................... 2
Soil Characteristics ................................................................................................... 2
5.1 Field Evaluation .................................................................................................... 2
5.2 Laboratory Testing ................................................................................................ 4
Engineering Analysis ................................................................................................. 5
6.1 Pavement Design Protection .................................................................................. 5
6.1.1 Traffic & Pavement Design Considerations ........................................................ 5
6.1.2 Design Parameters ......................................................................................... 6
6.1.3 Pavement Recommendations ........................................................................... 6
6.2 Hydraulic Analysis & Scour Protection ..................................................................... 8
6.3 Seismic Design Criteria .......................................................................................... 8
6.4 Liquefaction.......................................................................................................... 8
6.5 Foundation Engineering Analysis ............................................................................ 8
Conclusions & Recommendations ............................................................................... 9
7.1 General Design Considerations ............................................................................... 9
7.2 Foundation Soil Preparation ................................................................................... 9
7.3 Retaining Structures .............................................................................................10
7.4 Site Preparation and Grading ................................................................................10
7.5 Concrete .............................................................................................................12
7.6 Excavation and Shoring ........................................................................................12
8.0 Closing Considerations .............................................................................................13
8.1 Standard of Care and Interpretation of Subsurface Data ..........................................13
8.2 Use of This Report ...............................................................................................13
8.3 Retention of Samples ...........................................................................................14
Additional Services ............................................................................................................14
References .......................................................................................................................15
Montrose County Spring Cr Rd Bridge preliminary geotech report 6-2-23
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APPENDIX A - Project Maps ................................................................................................ A
APPENDIX B - Borehole Logs ............................................................................................... B
APPENDIX C - Laboratory Results ........................................................................................ C
APPENDIX D - Engineering Analysis .................................................................................... D
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INTRODUCTION
Buckhorn Engineering, Inc. (BEI) conducted an evaluation of subsurface and site conditions on
April 11, 2023, at the Spring Creek Road Bridge B-117 over the M&D Canal west of Montrose in
Montrose County, Colorado. Our services were performed at the request of Montrose County to
evaluate the site for replacement of the existing bridge. The evaluation consisted of a site
reconnaissance, drilling of two boreholes, logging and testing of representative materials found,
and analysis of available data. This preliminary report presents the findings of our evaluation
and our geotechnical engineering recommendations for site preparation, foundation soil
preparation, and management of drainage. Upon review by the County and design team, a final
geotechnical report will be produced.
CONSTRUCTION PLANS
The County would like to replace the existing bridge with a precast concrete bridge. According
to 30% drawings by BEI, the replacement bridge will span 40 feet and will have 14-inch deep
prestressed concrete slab girders with concrete abutments. The bridge design will comply with
the Colorado Department of Transportation's (CDOT) Standard Specifications for Road and
Bridge Construction (2021), the CDOT Geotechnical Design Manual (2021), the AASHTO LRFD
Bridge Design Specifications, 9th edition (2020), and the Montrose County Standards and
Specifications for Roads and Bridges (2022).
SITE CONDITIONS
The Spring Creek Road Bridge B-117 over the M&D Canal is located roughly 6 miles west of
Montrose and between 6000 Road and 5875 Road (see Vicinity Map, Map 1 in Appendix A). The
existing Spring Creek Road bridge is a steel girder bridge with concrete abutments and
guardrails. Deterioration of the concrete abutments and steel corrosion are the primary reasons
for the bridge replacement. The existing road surface is paved and the new road will also be
paved. The following photographs show the site when we drilled on April 11, 2023.
Photo 1 (left) is a view northwest (downstream) as BH#1 was drilled on the canal road
at the northeast side of the bridge and Photo 2 (right) is a view west while drilling
BH#2 at the southwest (upstream) side of the bridge.
The attached Site Map (Map 2 in Appendix A) is an aerial photograph that shows Spring Creek
Road, the existing bridge and canal, vegetative cover, the canal road, and the approximate
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locations of our two boreholes (BH#1 and BH#2). The M&D Canal flows to the northwest at this
site.
BH#2
Photo 3. View south (upstream)
shows the nature of the bridge, the
canal, and the locations of both
boreholes. Photograph was taken on
April 11, 2023.
M&D Canal
BH#1
GEOLOGY & GEOLOGIC HAZARDS
According to the Geologic Reconnaissance Mapof the Montrose West Quadrangle, Montrose
County, Colorado (Hail, 1986), the M&D Canal is mapped on Cretaceous Dakota Sandstone
formation (Kd). This formation is light gray to light brown quartzose sandstone, conglomeratic
sandstone, and conglomerate at the base. It contains thin beds of shale, carbonaceous shale,
and subbituminous coal. According to the Geologic Map of the Montrose 30' by 60' Quadrangle,
Southwestern Colorado (Steven and Hail, 1989), the Dakota Sandstone formation in this region
is 40 to 210 feet thick. It was formed in a coastal (marginal marine to shallow water marine)
environment. Due to the generally massive and well-cemented nature of the sandstone, it often
caps mesas and forms cliffs and benches at the margins of mesas.
SOIL CHARACTERISTICS
5.1 Field Evaluation
Two boreholes were drilled, with BH#1 (elevation 5,250.9 feet) at the northeast side of the
existing bridge and BH#2 (elevation 5,251.7 feet) at the southwest side (see Map 2). A Simco
2800 truck-mounted drill rig used 4.25-inch O.D. solid stem continuous flight augers in the
overburden soils and NQ coring in the formational sandstone. The holes were terminated at 20
feet in both borings. Our findings and recommendations are based on materials found within
these boreholes to the depths explored. Soil conditions may change between boreholes and
below these depths. Therefore, during construction, Buckhorn Engineering should verify soil
continuity and validity of our recommendations.
Soil samples were obtained at a depth of 5-feet in both borings using a Modified California (2.0-
inch O.D.) split spoon sampler to perform in-situ Standard Penetration Tests (SPTs) in general
accordance with ASTM Standard D-1586. The number of blows required to drive the sampler 18
inches in 6-inch increments was recorded (SPT "N" penetration resistance values). Since this is
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the depth in both borings where formational Dakota Sandstone was encountered, drilling
methods were converted to NQ wireline coring. In both borings, coring occurred from 5 to 20
feet. The soil, bedrock, and groundwater conditions were logged, and representative soil/rock
samples were tested in our laboratory. The stratigraphy found in the borings and summarized
laboratory results are represented in the attached Borehole Logs (Appendix B). The full
laboratory results are in Appendix C.
In the boreholes we encountered brown, soft to medium stiff, sandy lean clay to 5.1 to 5.5 feet
(see Photo 4). This soil was underlain by light brown, red-brown, gray, and white, moderately
weathered, extremely weak to weak sandstone with some thin layers of siltstone/shale to 10
feet. The cored rock from 5.5 to 10 feet had total core recovery (TCR) of 94-98% and rock
quality designation (RQD) was 24-27% (see Photos 5 and 6). At these depths, the rock has a
strength index of R0 to R2 (extremely weak to weak rock) and a weathering index of W3
(moderately weathered), as seen Borehole Log Key in Appendix B. Orange iron-staining and
wavy thin bedding was observed in the rock layers from 5.5 to 10 feet. Below 10 feet, the
sandstone became uniformly light brown/tan, more massive, harder, and less weathered. For
the two core runs from 10 to 20 feet in both borings, TCR was 94-100%, RQD was 38-62% in
BH#1 and 60-82% in BH#2, rock strength index was R2 to R4 (weak to strong rock), and
weathering index was W1/W2 (fresh to slightly weathered). Photos 5-7 show the differences in
the rock between 5.5-10 feet vs. 10-20 feet.
Groundwater was measured at 10 feet in BH#1 during drilling, but a reliable water level could
not be measured in BH#2 due to the use of water during coring. Groundwater was likely
traveling through the weaker and more weathered thin beds of siltstone in the upper 10 feet of
rock and then is perched on the harder sandstone found around 10 feet in both borings. Since
water had recently been introduced to the canal for the 2023 irrigation season, groundwater
measured may not be representative of the shallowest levels possible at this site. The following
photographs show the nature of the sandy clay overburden soils and the underlying Dakota
Sandstone formation.
Photo 4 (left) shows the nature of the sandy clay overburden soil at 5-5.5 feet in BH#1.
Photo 5 (right) shows the formational Dakota Sandstone in BH#1 from 5.5 to 15 feet. Note
the weathered and broken nature of the rock from 5-10.5 feet in BH#1 and then the more
uniform and massive sandstone from 10.5-15 feet.
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Photo 6 (left) shows the cored sandstone from 15-20 feet in BH#1 and from 5.5 to 10 feet
in BH#2. Photo 7 (right) shows the cored sandstone from 10-20 feet in BH#2.
5.2 Laboratory Testing
Laboratory tests were performed on the representative soil and rock samples to evaluate the
range of physical and chemical properties (see Laboratory Results, Appendix C). Table 1 below
is a summary of the classification laboratory results. The results are also summarized on the
borehole logs (Appendix B).
Table 1
Summary of Laboratory Tests
Atterberg
Bore- Moisture Gradation (%) Unit
Depth Classifi- Limits UCS
hole Sample content Weight
(ft) cation (psi)
# (%) LL PL PI gravel sand fines (pcf)
BH#1 GS1 2-5 CL 20.2 33 15 18 2.8 32.6 64.6
BH#1 RS1 7.8 sandstone 137.2 663
BH#1 RS2 11.5 sandstone 126.6 3,225
BH#2 BS1* 0-4 CL
BH#2 DS2** 5-5.5 GP
BH#2 RS3 7.5 sandstone 140.0 1,993
BH#2 RS4 10.0 sandstone 125.4 2,873
*Standard Proctor and CBR tests were performed and are listed in the text
**Corrosivity Series tests were performed and are listed in the text
As seen in these results, the overburden soils to 5-5.5 feet classify as sandy lean clay (CL) with
moderate plasticity (Plasticity Index of 18). Unconfined Compressive Strength (UCS) tests were
performed on the sandstone at depths of 7.8 feet in BH#1 and 7.5 feet in BH#2 and they have
UCS values of 663 to 1,993 psi, respectively, which is very weak (R1) to weak (R2) rock. The
less weathered and more massive sandstone at depths of 11.5 feet in BH#1 and 10 feet in
BH#2 have UCS values of 3,225 to 2,873 psi, respectively, which is considered weak (R2) rock
(see Borehole Log Key in Appendix B).
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A series of geochemical tests were conducted on a sample of the soils from 5-5.5 feet in BH#2
(sample DS2). This soil is from the interface of the overburden sandy lean clay and the upper
zone of the weathered formational sandstone. The sample had a water-soluble sulfate
concentration of 0.80%, a chlorides content of 0.033%, an electro-conductivity of 3,640 S/cm,
and a pH of 8.2. The sulfate exposure is considered moderately corrosive by the American
Concrete Institute (ACI 2014). The electro-conductivity value is also on the high end of values
we see in the region.
ENGINEERING ANALYSIS
6.1 Pavement Design Protection
The primary purpose of a pavement section is to distribute concentrated wheel loads to the
subgrade in a manner such that the subgrade is not overstressed. Performance of the
pavement section is a function of subgrade strength and traffic loading. For purposes of
designing a pavement section, subgrade soil is represented by a soil support value (resilient
modulus) for flexible pavements (asphaltic concrete). Subgrade strength decreases when the
moisture content of the subgrade increases. Therefore, proper drainage, both surface and
subsurface, is essential for long-term pavement performance. Pavement design procedures are
based upon strength properties of the subgrade soil and pavement materials, along with the
design traffic conditions.
6.1.1 Traffic & Pavement Design Considerations
This site will serve passenger vehicles, pickup trucks, and various delivery and service trucks.
According to the Montrose County Website of 2020-2021 traffic counts, the ADT (Average Daily
Traffic) count for Spring Creek Road west of 6000 Road is 1,057 vehicles per day, which was
used for design of this road, and it is classified as an asphalt collector road. To determine the
equivalent single axle loads (ESAL's) used in our design, BEI first multiplied the average daily
traffic (ADT) by a lane factor to account for two-way distribution of traffic volume. As seen on
the attached ESAL Calculator for Asphalt Pavements, the ESAL's were calculated to be 390,872.
We rounded up to 400,000 ESAL's to be conservative. BEI used a lane factor of 0.6 in
accordance with Table 1.1 of the 2012 CDOT Pavement Design Manual (PDM). We assumed the
mixture of vehicle types presented in Table 2 (Traffic Loading) and applied equivalency factors
(per 2012 CDOT PDM Table 1.2) in order to convert the anticipated traffic mixture to an
equivalent number of 18-kip axle loads. We also assumed an annual growth rate of 2%.
Table 2: Traffic Loading
Percentage Trips per
Vehicle Type
of Traffic Day (ADT)
Passenger cars/pickup
90% 951
trucks
Delivery trucks - 2 axle,
5% 53
single unit
Combination trucks - multi-
5% 53
axle
TOTALS 100% 1,057
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6.1.2 Design Parameters
We used the pavement design parameters shown in Table 3 below.
Table 3: Pavement Design Parameters
Design
Pavement Design Parameter Source
Value
Montrose County website for 2020-
ADT 1,057
2021 Traffic Counts
Montrose County Road and Bridge
Design life 20 years
Manual Section 5.24.A
Traffic growth rate 2% Assumed
Rounded up from calculations (see
ESAL's (calculated) 400,000
attachments)
Initial serviceability 4.2 AASHTO 1993 Section 1.3
Terminal serviceability 2.0 AASHTO 1993 Section 1.3
Reliability 75% AASHTO 1993 Table 2.2
Drainage coefficient 1.0 CDOT 2012 PDM Table 4.9
California Bearing Ratio (CBR) 4.0 Laboratory results
Structural Number (SN) required 2.84 Calculations (see attachments)
Flexible Pavement
Standard Deviation 0.44 CDOT 2012 PDM Table 3.1
Asphalt layer coefficient 0.44 CDOT 2012 PDM Table 3.2
Base layer coefficient 0.14 CDOT 2012 PDM Table 3.3
Subbase layer coefficient 0.10 CDOT 2012 PDM Table 3.3
Subgrade resilient modulus (Mr) 6,000 psi Calculations using CBR value
6.1.3 Pavement Recommendations
Based on our design calculations, anticipated traffic, and field conditions, the pavement design
results are presented in the attachments and our recommended flexible pavement section is
presented in Table 4. We analyzed the use of 12-inches of roadbase and achieved a structural
number (SN) of 3.44, which is greater than the minimum SN of 2.84 required for traffic and
subgrade conditions.
Table 4: Recommended Flexible Pavement Section
Asphalt Aggregate Base Subbase Structural
Pavement
Thickness Course Thickness Course Number
Section
(in) (ABC) (in) Thickness (in) (Sn)
Recommended 4 12 0 3.44
1. Pavement Construction Considerations
a. Moisture condition and compact the base course material as required to
accommodate any proposed modifications to the road profile.
b. Proof-roll the compacted base using a loaded dump truck or water truck. Proof-
rolling should be observed by a BEI representative and soft areas should be repaired
at our discretion.
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c. Excavate any unstable areas and replace with moisture conditioned and compacted
aggregate base. Place and compact structural fill in level lifts, not more than 6-
inches in loose thickness, up to planned grade.
d. Grade pavement such that surface water drains into roadside swales at a minimum
2% slope.
e. New Base Course shall meet the requirements for CDOT Class 6 Crushed Base
Course in Section 703, Aggregates, in Colorado Standard Specifications for Road and
Bridge Construction (CDOT 2022).
f. Asphalt shall meet the requirements in Section 401, Plant Mix Surfacing, in the
Colorado Standard Specifications for Road and Bridge Construction (CDOT 2022).
g. Compact base course to at least 95% of its maximum modified Proctor density
(AASHTO T180).
2. Pavement Site Preparation and Grading
a. Design and construction of the pavement should promote drainage away from the
paved areas. Where needed, ditches and culverts should either be constructed or
modified to accept road drainage to reduce the potential for erosion.
b. Compaction requirements of fill material: aggregate base course should be
compacted to 95% of modified Proctor (AASHTO T180) maximum dry density at
+2% of optimum moisture content.
c. A representative of BEI should be called out to the site to observe placement of
roadbase and base course and verify the compacted density. The County should
contact BEI in advance of any required excavations to discuss the specific testing
requirements, budget, and scheduling needed for these services.
d. All paving construction activities should be monitored and tested by a competent
civil/geotechnical engineering firm for compliance with the recommendations
contained in this report and with the specifications in the 2022 edition of the
Montrose County Standards and Specifications for Roads and Bridges.
e. Native subgrade soils in the region are corrosive to steel and concrete. Separating
any pavement surface from the native soils by placing pavement on granular
material (aggregate) and elevating the roadbed will help in keeping salts from
migrating up into the pavement. Even though these salts are not corrosive to
asphalt, the forces of salt growth can cause physical pavement damage over time.
3. Pavement Maintenance
a. The pavement's life will be dependent on achieving adequate drainage throughout
the section, especially at the subgrade.
b. Surface of final or finished road grade shall slope at no less than 2% to an
appropriate stormwater disposal system or other appropriate location that does not
impact adjacent buildings or properties.
c. Maintain grades outside of paved areas to prevent the collection of water adjacent to
the pavement.
d. Seal cracks and perform surface maintenance on all pavement surfaces every 3 to 5
years to reduce the potential for surface water infiltration into the underlying
pavement subgrade.
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