HomeMy WebLinkAboutGeotechnicalSummary_032320211
Gary Fox
From:Lee Evans <levans@alliedengineering.com>
Sent:Tuesday, March 23, 2021 12:15 PM
To:Gary Fox
Cc:Erik Schnaderbeck
Subject:Lot 1, Annie Sub. - Geotech Summary
Attachments:Lot 1, Annie Sub. - Test Pit Locations - Google Earth - 03.19.21.jpg
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Good morning Gary –
Just wanted to give you a quick update on the geotech work on Lot 1, Annie Sub. (2305 Durston Rd). Erik S. had a good
day in the field on Friday (3/19). He dug 7 test pits across the site with a mini‐exc. provided by Townsend Backhoe (aka
Action Excavation). All TPs are backfilled with 10‐foot long, 4” PVC monitoring wells. The wells are capped and labeled
to match the TP designation.
See the attached TP location map showing the approx.. locations of the pits/monitoring wells.
The soil conditions are as expected based on on‐site well log and knowledge of NW side Bozeman conditions. No
issues… Shallow gravels and pretty deep groundwater.
Here is a summary of the soil and GW info from each pit:
• TP‐1 (NW Corner)
o 0.0’ – 1.0’: Native Topsoil
o 1.0’ – 3.5’: Silt/Clay
o 3.5’ – 9.5’: Sandy Gravel
o Dry at 9.5’: (No Groundwater)
o Well casing stickup is 1.5’ to 2.0’.
• TP‐2 (SW Corner)
o 0.0’ – 1.3’: Native Topsoil
o 1.3’ – 4.0’: Silt/Clay
o 4.0’ – 9.0’: Sandy Gravel
o Dry at 9.0’: (No Groundwater)
o Well casing stickup is 1.5’ to 2.0’.
• TP‐3 (South‐Central Area ‐ SW)
o 0.0’ – 0.8’: Native Topsoil
o 0.8’ – 2.8’: Silt/Clay
o 2.8’ – 9.0’: Sandy Gravel
o Dry at 9.0’: (No Groundwater)
o Well casing stickup is about 1.5’.
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• TP‐4 (North‐Central Area ‐ NW)
o 0.0’ – 1.0’: Native Topsoil
o 1.0’ – 2.3’: Silt/Clay
o 2.3’ – 9.3’: Sandy Gravel
o Dry at 9.3’: (No Groundwater)
o Well casing stickup is 1.0’ to 1.5’.
• TP‐5 (North‐Central Area ‐ NE)
o 0.0’ – 1.3’: Native Topsoil
o 1.3’ – 2.5’: Silt/Clay
o 2.5’ – 9.3’: Sandy Gravel
o Dry at 9.3’: (No Groundwater)
o Well casing stickup is about 1.25’.
• TP‐6 (SE Corner)
o 0.0’ – 2.5’: Random Fill (Silt/Clay w/ some gravels and concrete chunks)
o 2.5’ – 3.5’: Native Topsoil (Buried)
o 3.5’ – 9.3’: Sandy Gravel
o Dry at 9.3’: (No Groundwater)
o Well casing stickup is about 1.0’.
• TP‐7 (NE Corner)
o 0.0’ – 3.0’: Random Fill (Clayey Gravel w/ abundant 1”‐minus gravels and occasional
cobbles/boulders)
o 3.0’ – 4.3’: Native Topsoil (Buried)
o 4.3’ – 9.3’: Sandy Gravel
o Groundwater @ 9.3’ (Wet)
o Well casing stickup is 1.0’ to 1.5’.
In summary –
• About 1.0 to 1.5 feet of topsoil.
• Some random surface fill on east side (2.5’ – 3.0’ thick overlying buried topsoil).
• Native sandy gravels begin at depths of 2.5’ to 4.0’ in most areas.
• Groundwater on 3/19 was @ 9.3’ or deeper.
• Monitoring wells are installed in all 7 TPs.
Geotechnical Issues:
• No issues (due to shallow gravels and deep groundwater)
• Due to possible moister/softer road subgrade in some areas, we will likely recommend a Mirafi RS580i
geosynthetic for subgrade separation (available @ Core & Main).
Preliminary Geotechnical Recommendations:
• Crawl spaces will be OK due to deeper groundwater.
• We recommend WGM monitor the GW wells during the spring/early summer to verify high GW levels.
• Native sandy gravel @ 2.5’ to 4.0’ is the “target” bearing material for all foundations.
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• All building footings to bear on native gravel or on granular structural fill supported on “target” gravel.
• For crawl space foundations, we recommend mass exc. down to “target” gravel and placement of bldg. pad
structural fill section up to perimeter footing grade (as needed).
• Water and sewer main trench backfill will largely be native sandy gravel.
• The preliminary street section is as follows:
o 3” asphalt
o 6” base course
o 15” sub‐base course
o Mirafi RS580i geosynthetic
o Compacted subgrade
Hopefully, this keeps you going on your subdivision planning and design.
I’ll work on the geotechnical report over the coming weeks.
At some point, I would like to see your weekly GW monitoring data so I can incorporate it into the final geotech. report.
Please call or email with questions.
Thx, Lee
Lee S. Evans, PE
Civil/Geotechnical Engineer
Allied Engineering Services, Inc.
32 Discovery Dr., Bozeman, MT 59718 | Tel: 406.582.0221 | Fax: 406.582.5770
Cell: 406.599.9599 | Email: levans@alliedengineering.com | Web: www.alliedengineering.com