HomeMy WebLinkAbout112-Soil Corrosivity Report
April 24, 2019
Kent Merselis
Barnard Investments, LLC
701 Gold Avenue
Bozeman, MT 59715
e-mail: kent.merselis@barnard-inc.com (Issued via email only)
Re: Soil Corrosivity Sampling and Testing
Nelson Meadows – Bozeman, MT
Dear Mr. Merselis:
A few weeks ago we were retained to perform soil corrosivity sampling and testing across the proposed
Nelson Meadows site to determine if the on-site soils are corrosive to the ductile iron pipe (DIP) water
main infrastructure. This additional investigation work was being mandated during the City of Bozeman
approval process and needed to the follow the testing and analysis standards in the DIPRA-Corrpro
Design Decision document. This letter summarizes the site conditions, our soil corrosivity sampling and
test results, our analysis of the results, and the corresponding corrosion protection recommendations
provided in the above-referenced document. Several attachments are provided as part of this letter.
These include maps showing soil and groundwater conditions, 2018 groundwater monitoring data, our
soil corrosivity sampling plan, the lab testing results, and our analysis of the data. In order to keep this
letter as short and concise as possible, most of the information is shown in tables.
Provided below is our scope of work for our evaluation:
• Reviewed soil conditions in our March 12, 2018 geotechnical summary letter for the project site.
• Reviewed the DIPRA-Corrpro Design Decision document.
• Reviewed 2018 groundwater data collected by Morrison Maierle.
• Prepared soil corrosivity sampling and testing plan.
• Determined water main invert depths based on Morrison Maierle’s design drawings.
• Dug 10 test pits along the proposed water main alignments in Royal Wolf Way and Prince Lane.
• Sampled silt/clay and sandy gravel soils from the water main invert depth to a height of 2.0’.
• A total of 6 silt/clay and 4 sandy gravel samples were collected and submitted for testing.
• Moisture content testing was conducted in the AESI lab on all 10 samples.
• Energy Labs in Helena conducted the geochemical lab testing on all 10 samples.
• Geochemical testing included: resistivity, chlorides, pH, sulfide ions, and redox potential.
• Analyzed site conditions and lab data according to DIPRA-Corrpro document.
Soil Corrosivity Sampling and Testing
Nelson Meadows – Bozeman, MT
Project: 18-021
April 24, 2019
Page 2
SUBSURFACE CONDITIONS
In February 2018, we dug 12 test pits across site (TP-1 through TP-12). All pits were backfilled with 10-
foot PVC monitoring wells, which were named MW-1 through MW-12. The project site is underlain by
pretty normal soil conditions for the Bozeman area, including about 1.0-foot of topsoil, an intermediate
layer of native silt/clay (variable thick), and native sandy gravel at depth. The shallowest gravels are in
the middle of the site adjacent to the creek/wetland corridor, while the gravels are deeper along the
east and west sides. Another soil variation is that the silt/clay soils are wetter and softer in the east half
of the site and much drier and stiffer in the west half. See Figure 4 (attached) from our March 2018
geotechnical summary letter for an exhibit that shows the gravel depth across the site.
In 2018, Morrison Maierle monitored groundwater depths in the 12 monitoring wells. They collected
data from March 30 through August 16. In all wells, seasonal high water (shallowest depth) occurred on
April 27. Attached to this letter is a map showing the monitoring well locations and the seasonal high
water depth at each well, along with a spreadsheet of all of their 2018 groundwater data.
Provided in Table 1 is a summary of the soil conditions that were observed in the test pits, along with
the seasonal high groundwater measurement from April 27.
Table 1. Summary of Soil Conditions and High Groundwater in TP-1 – TP-12 (MW-1 – MW-12)
TP # TP LOCATION NATIVE
TOPSOIL
NATIVE
SILT/CLAY
NATIVE
SANDY GRAVEL
2018 SEASONAL
HIGH GROUNDWATER
1 East Side of Site 0.0’ - 1.0’ 1.0’ - 7.0’ 7.0’ - 12.0’ 6.1’
2 East Side of Site 0.0’ - 1.0’ 1.0’ - 8.0’ 8.0’ - 12.0’ 5.5’
3 East Side of Site 0.0’ - 1.0’ 1.0’ - 10.0’ 10.0’ - 15.0’ Dry @ 9.0’
4 East Side of Site 0.0’ - 1.0’ 1.0’ - 9.0’ 9.0’ - 12.0’ 6.3’
5 Middle of Site 0.0’ - 1.0’ 1.0’ - 7.0’ 7.0’ - 12.0’ 5.5’
6 Middle of Site 0.0’ - 0.75’ 0.75’ - 7.0’ 7.0’ - 12.0’ 4.9’
7 Middle of Site 0.0’ - 0.75’ 0.75’ - 6.0’ 6.0’ - 12.0’ 6.3’
8 Middle of Site 0.0’ - 0.75’ 0.75’ - 7.5’ 7.5’ - 12.0’ 7.1’
9 Middle of Site 0.0’ - 1.0’ 1.0’ - 6.0’ 6.0’ - 12.0’ 6.6’
10 Middle of Site 0.0’ - 0.75’ 0.75’ - 4.0’ 4.0’ - 12.0’ 3.9’
11 West Side of Site 0.0’ - 0.75’ 0.75’ - 10.0’ 10.0’ - 13.0’ Dry @ 9.0’
12 West Side of Site 0.0’ - 0.75’ 0.75’ - 10.0’ 10.0’ - 12.0’ Dry @ 9.0’
Notes: 1) All soil and groundwater measurements are depths below existing ground.
2) MW-1 was installed in TP-1, MW-2 in TP-2, and so on.
3) The 2018 seasonal high groundwater depths occurred across the site on April 27, 2018. This is shown in the table.
4) No groundwater was ever recorded in MW-3, MW-11, and MW-12. All wells were dry to a depth of 9.0’.
Soil Corrosivity Sampling and Testing
Nelson Meadows – Bozeman, MT
Project: 18-021
April 24, 2019
Page 3
SOIL CORROSIVITY SAMPLING AND TESTING
On April 12, 2019, we dug 10 additional test pits (TP-A through TP-J) across the site for the purpose of
collecting soil samples for soil corrosivity testing. Prior to the fieldwork, a sampling plan was prepared
based on the location of the roads and water mains and the proposed invert depth of the pipes. Our
plan is attached. Five pits (TP-A through TP-E) were along the Royal Wolf Way alignment and five pits
(TP-F through TP-J) were along the Prince Lane alignment. Morrison Maierle staked the roads at 100-
foot stationing so the planned test pit locations were easy to find. All samples were taken from a depth
range of 2.0 feet above the pipe invert down to the bottom of pipe elevation. A total of 10 samples
were obtained for testing. These included 6 silt/clay samples and 4 sandy gravel samples.
Provided in Table 2 is a summary of the soil and groundwater conditions observed in the test pits.
Table 2. Summary of Soil and Groundwater Conditions in TP-A – TP-J
TP # TP LOCATION NATIVE
TOPSOIL
NATIVE
SILT/CLAY
NATIVE
SANDY GRAVEL
GROUNDWATER
DEPTH
A Royal Wolf Way – Sta. 1+00 0.0’ - 1.0’ 1.0’ - 7.0’ 7.0’ - 7.5’ 7.5’
B Royal Wolf Way – Sta. 5+00 0.0’ - 1.0’ 1.0’ - 6.0’ 6.0’ - 9.5’ 8.5’
C Royal Wolf Way – Sta. 9+00 0.0’ - 1.0’ 1.0’ - 4.0’ 4.0’ - 6.0’ 5.5’
D Royal Wolf Way – Sta. 13+00 0.0’ - 1.0’ 1.0’ - 7.0’ 7.0’ - 12.0’ Dry @ 12.0’
E Royal Wolf Way – Sta. 22+00 0.0’ - 1.0’ 1.0’ - 8.5’ 8.5’ - 13.0’ 12.0’
F Prince Lane – Sta. 3+50 0.0’ - 1.0’ 1.0’ - 8.0’ 8.0’ - 12.0’ Dry @ 12.0’
G Prince Lane – Sta. 8+00 0.0’ - 1.0’ 1.0’ - 7.5’ 7.5’ - 12.0’ 11.5’
H Prince Lane – Sta. 11+00 0.0’ - 1.0’ 1.0’ - 5.0’ 5.0’ - 10.0’ 9.0’
I Prince Lane – Sta. 15+00 0.0’ - 1.0’ 1.0’ - 5.0’ 5.0’ - 9.0’ 7.0’
J Prince Lane – Sta. 20+00 0.0’ - 1.0’ 1.0’ - 9.5’ 9.5’ - 10.0’ 9.0’
Notes: 1) All soil and groundwater measurements are depths below existing ground.
Provided in Table 3 is a summary of the depth and composition of all 10 soil samples.
Table 3. Summary of Soil Sample Depth and Composition from TP-A – TP-J
TP # TP LOCATION DEPTH OF
PIPE INVERT
DEPTH RANGE
OF SAMPLE
SAMPLE
COMPOSITION
A Royal Wolf Way – Sta. 1+00 7.4’ 5.4’ - 7.4’ Silt/Clay
B Royal Wolf Way – Sta. 5+00 9.5’ 7.5’ - 9.5’ Sandy Gravel
C Royal Wolf Way – Sta. 9+00 5.5’ 3.5’ - 5.5’ Sandy Gravel
D Royal Wolf Way – Sta. 13+00 6.5’ 4.5’ - 6.5’ Silt/Clay
Soil Corrosivity Sampling and Testing
Nelson Meadows – Bozeman, MT
Project: 18-021
April 24, 2019
Page 4
E Royal Wolf Way – Sta. 22+00 6.7’ 4.7’ - 6.7’ Silt/Clay
F Prince Lane – Sta. 3+50 6.4’ 4.4’ - 6.4’ Silt/Clay
G Prince Lane – Sta. 8+00 7.3’ 5.3’ - 7.3’ Silt/Clay
H Prince Lane – Sta. 11+00 9.1’ 7.1’ - 9.1’ Sandy Gravel
I Prince Lane – Sta. 15+00 8.5’ 6.5’ - 8.5’ Sandy Gravel
J Prince Lane – Sta. 20+00 9.2’ 7.2’ - 9.2’ Silt/Clay
Notes: 1) All depth measurements are relative to below existing ground.
2) Pipe invert depths were calculated from Morrison Maierle’s design drawings.
All 10 samples were submitted to Energy Labs in Helena for geochemical testing. The analyses that they
conducted included resistivity, chlorides, pH, sulfide ions, and redox potential. A portion of each sample
was tested by AESI for moisture content. All testing results from AESI and Energy Labs are attached.
Provided in Table 4 is a summary of the lab testing for each of the 10 soil samples.
Table 4. Summary of Lab Testing Data from TP-A – TP-J
TP # SAMPLE
COMPOSITION
MOISTURE
CONTENT RESISTIVITY CHLORIDES pH SULFIDE
IONS
REDOX
POTENTIAL
A Silt/Clay 30.6 % 3470 ohm-cm 12 ppm 9.3 0.09 ppm 269 mV
B Sandy Gravel 16.4 % 9810 ohm-cm 3 ppm 8.9 0.05 ppm 336 mV
C Sandy Gravel 7.9 % 10,500 ohm-cm 4 ppm 8.8 0.16 ppm 294 mV
D Silt/Clay 20.6 % 2900 ohm-cm 9 ppm 8.8 0.07 ppm 265 mV
E Silt/Clay 22.8 % 4390 ohm-cm 6 ppm 8.7 0.05 ppm 287 mV
F Silt Clay 20.0 % 7300 ohm-cm 5 ppm 8.8 0.06 ppm 274 mV
G Silt/Clay 22.2 % 7320 ohm-cm 4 ppm 8.7 0.06 ppm 278 mV
H Sandy Gravel 10.6 % 8790 ohm-cm 4 ppm 8.8 0.21 ppm 251 mV
I Sandy Gravel 14.2 % 8200 ohm-cm 4 ppm 8.8 0.17 ppm 281 mV
J Silt/Clay 33.2 % 3380 ohm-cm 14 ppm 8.7 0.11 ppm 286 mV
Notes: 1) TP-A through TP-E were dug along the Royal Wolf Way alignment.
2) TP-F through TP-J were dug along the Prince Lane alignment.
HIGH GROUNDWATER EVALUATION
To make a determination if the water mains will be under the influence of groundwater, we compared
the pipe invert elevations at the test pit locations to the nearest 2018 seasonal high groundwater data.
Provided in Table 5 is summary of the pipe invert elevations and the 2018 high groundwater depths.
Soil Corrosivity Sampling and Testing
Nelson Meadows – Bozeman, MT
Project: 18-021
April 24, 2019
Page 5
Table 5. High Groundwater Evaluation for Pipe Invert Depth at TP-A – TP-J
TP # SAMPLE
COMPOSITION
DEPTH OF
PIPE INVERT
SEASONAL HIGH WATER
DEPTH AT NEAREST MW’s
GROUNDWATER REACHES
PIPE INVERT ELEVATION
A Silt/Clay 7.4’ 5.5’ - 6.1’ Yes
B Sandy Gravel 9.5’ 6.3’ Yes
C Sandy Gravel 5.5’ 4.9’ - 7.1’ Likely
D Silt/Clay 6.5’ 6.6’ - 7.1’ Likely
E Silt/Clay 6.7’ 6.6’ - Dry @ 9.0’ Likely
F Silt Clay 6.4’ Dry @ 9.0’ No
G Silt/Clay 7.3’ 6.6’ Yes
H Sandy Gravel 9.1’ 5.5’ - 6.6’ Yes
I Sandy Gravel 8.5’ 4.9’ - 5.5’ Yes
J Silt/Clay 9.2’ 6.3 - Dry @ 9.0’ Yes
Notes: 1) TP-A through TP-E were dug along the Royal Wolf Way alignment.
2) TP-F through TP-J were dug along the Prince Lane alignment.
3) All depth measurements are relative to below existing ground.
4) Pipe invert depths were calculated from Morrison Maierle’s design drawings.
DIPRA ANALYSIS AND RECOMMENDATIONS
The site condition and lab testing data was entered into Table 2 – Likelihood Score Sheet and Table 3 –
Consequence Score Sheet (from the DIPRA-Corrpro document) to get the likelihood and consequence
scores for the 10 samples. These scores were then entered into Figure 1 – DDM Two-Dimensional
Matrix to obtain the DIPRA corrosion protection recommendation per Table 1 – Design Decision Model.
See the attached DIPRA worksheets for each of the samples.
Provided in Table 6 is a DIPRA scores and recommendations for each of the 10 soil samples.
Table 6. Summary of DIPRA Scores and Recommendations from TP-A – TP-J
TP # SAMPLE
COMPOSITION
TABLE 2
LIKELIHOOD SCORE
TABLE 3
CONSEQUENCE SCORE
TABLE 1
DIPRA RECOMMENDATION
A Silt/Clay 18.5 0 Standard Shop Coat
B Sandy Gravel 13.5 0 Standard Shop Coat
C Sandy Gravel 11 0 Standard Shop Coat
D Silt/Clay 23.5 0 V-Bio Enhanced Poly Encsmnt
E Silt/Clay 18.5 0 Standard Shop Coat
F Silt Clay 8.5 0 Standard Shop Coat
Soil Corrosivity Sampling and Testing
Nelson Meadows – Bozeman, MT
Project: 18-021
April 24, 2019
Page 6
G Silt/Clay 13.5 0 Standard Shop Coat
H Sandy Gravel 11 0 Standard Shop Coat
I Sandy Gravel 11 0 Standard Shop Coat
J Silt/Clay 18.5 0 Standard Shop Coat
Notes: 1) TP-A through TP-E were dug along the Royal Wolf Way alignment.
2) TP-F through TP-J were dug along the Prince Lane alignment.
3) For likelihood scores of 1 to 20, the DIPRA recommendation is “as manufactured with shop coat”.
4) For likelihood scores of 21 to 40, the DIPRA recommendation is “V-Bio Enhanced Polyethylene Encasement”.
5) For likelihood scores of 41 to 50, the DIPRA recommendation is “V-Bio Poly Encsmnt w/ Zinc Coated Pipe”.
SUMMARY AND RECOMMENDATIONS
Based on the site’s soil conditions and the depth of the water mains, the native soils within the pipe
zone (pipe invert to 2.0’ above) will vary from silt/clay to sandy gravel, depending on location. As part of
our investigation, we collected and tested six samples of the silt/clay and four samples of the sandy
gravel. As expected, the silt/clay has a little higher corrosion potential to DIP water mains as compared
to the sandy gravel (which is not corrosive). But based on the testing and DIPRA analysis, the silt/clay
soils are “borderline” between being non-corrosive and needing polyethylene encasement for corrosion
protection. The main differences between the two soil types is that the silt/clay has a higher moisture
content and a lower resistivity value, both of which result in the silt/clay being more corrosive.
According to the DIPRA Decision Model, the likelihood score is the deciding factor for most projects on if
a soil is corrosive and to what extent corrosion protection is recommended. The consequence scores
are typically low and most often do no come into play. For this project, the four likelihood scores for the
sandy gravel samples were all between 11 and 14, which corresponds with a non-corrosive soil and no
need for any corrosion protection. Only one of the silt/clay samples had a score in the low to mid 20’s
(23.5), which corresponds with a DIPRA recommendation of V-Bio Enhanced Polyethylene Encasement
for corrosion protection (for scores from 21 to 40); but there were three silt/clay scores at 18.5, which is
right below the cut-off of where polyethylene encasement would be recommended. For all three of
these samples, a little lower resistivity value would have kicked them into the polyethylene encasement
recommendation. All scores in the silt/clay were far below 41, which is value at the lower end of the
range where zinc-coated pipe is recommended.
Based strictly on the DIPRA criteria, only one of the 10 samples warrants the use of V-Bio Enhanced
Polyethylene Encasement. All others are considered non-corrosive; and thereby no need for any type
of corrosion protection. With that said, three of the silt/clay samples were very near the lower limit
of where polyethylene encasement would be recommended. Given that most of the pipe zone soils
across the site will be silt/clay, it is probably a good idea to error on the side of conversancy and just
poly-wrap all pipe (including the piping in the sandy gravel). It makes no sense to just poly-wrap the
pipe in the silt/clay soils and eliminate it in the sandy gravel soils. This would be hard to monitor
during construction.
3/30/2018 4/6/2018 4/13/2018 4/19/2018 4/27/2018 5/4/2018 5/11/2018 5/18/2018 5/25/2018 6/1/2018 6/8/2018 6/15/2018 6/22/2018 6/29/2018 7/5/2018 7/13/2018 7/27/2018 8/16/2018
1 7.21 6.97 6.41 6.19 6.12 6.63 6.74 7.03 6.98 7.12 7.26 7.54 6.23 7.29 7.48 7.73 8.18 -
2 6.36 6.46 5.92 5.67 5.49 6.26 6.50 6.73 6.64 6.91 6.97 7.26 7.15 6.77 7.32 7.67 8.31 -
3 ------------------
4 6.36 6.70 6.36 6.36 6.28 6.86 7.02 7.23 7.11 7.40 7.50 7.53 7.55 7.71 7.86 ---
5 5.57 5.84 5.52 5.54 5.49 5.93 5.99 6.19 6.00 6.22 6.28 6.42 6.31 6.43 6.59 6.92 7.49 -
6 5.16 5.34 4.93 4.93 4.89 5.35 5.40 5.65 5.43 5.66 5.74 5.95 5.77 5.86 6.00 6.35 6.86 -
7 6.61 6.74 6.28 6.27 6.29 6.68 6.71 6.98 6.82 6.93 7.05 7.30 6.95 6.88 7.23 7.35 7.71 -
8 ---7.89 7.09 7.81 8.04 8.22 8.19 8.35 8.43 --8.54 ----
9 8.16 7.91 7.59 7.19 6.55 6.68 6.78 6.89 6.79 6.94 7.02 7.28 7.41 7.54 7.65 8.04 --
10 4.55 4.55 4.16 4.09 3.93 4.15 4.17 4.32 4.10 4.28 4.35 4.47 4.38 4.43 4.67 5.03 5.57 6.00
11 ------------------
12 ------------------
*NOTE:
During the Week of 6/15/2018 to 6/23/2018 Bozeman, MT received 1.83" of rain.
Red highlighted cells indicate the highest groundwater depth measured.
Cells that have a value of " - " means that no water was present within the monitoring well at the time of measurement.
Well
Value in each cell is the measurement from the ground level at each monitoring well to the static water level in each well.
Nelson Meadows - Weekly Water Levels (FT)
DATES
3.00
3.25
3.50
3.75
4.00
4.25
4.50
4.75
5.00
5.25
5.50
5.75
6.00
6.25
6.50
6.75
7.00
7.25
7.50
7.75
8.00
8.25
8.50
8.75
3/22/2018 3/29/2018 4/5/2018 4/12/2018 4/19/2018 4/26/2018 5/3/2018 5/10/2018 5/17/2018 5/24/2018 5/31/2018 6/7/2018 6/14/2018 6/21/2018 6/28/2018 7/5/2018 7/12/2018 7/19/2018 7/26/2018 8/2/2018 8/9/2018 8/16/2018 8/23/2018 8/30/2018Groundwater depth (feet)Date
Nelson Meadows Groundwater Depth
Well 1
Well 2
Well 4
Well 5
Well 6
Well 7
Well 9
well 10