Hi Kelly,
Based on our recent roadway and water line projects, Molalla mostly has poor draining, clay soils. At times we find areas that are black/blue in appearance which would be technically described as organic soils. These soils if discovered, must be excavated and removed entirely. Other areas appear to be old river beds with cobbles and boulders. However the majority of the time we are finding the clays.
In my experience in regards to bridge designs, I would correspond with a registered licensed geotech engineer in the preliminary design phase to core for soil samples. Based on those bore logs and soil samples the geotech would create a design cross-section for the footing foundations. I understand you do not have time to do this pre-exploration, however, if you have a licensed geotech engineer that can be on site during the excavation, he can determine when to stop excavating and determine what the design section of rock should be prior to forming the concrete footings.
As for documentation, I would like to see (1) photos of the excavation and backfill and (2) follow up geotech report that supports the in-field design for the footings.
Attached is the report I received regarding the path embankment. I don’t see that it directly addresses the bridge footing. You may ask Evan to see if the geotech is available and willing to be on site during the footing excavation.
Regards,
Jennifer Cline, P.E.
City of Molalla
Public Works Director
O: 503.759.0218
F: 503.829.3676
"From: Kelly Andrews [mailto:andrewsgirls@yahoo.com]
Sent: Monday, June 15, 2015 2:25 PM
To: Jennifer Cline
Subject: Fwd: Bear Creek - ROW Impact Study
Hi Jennifer,
When you have time, would you provide feedback to me on Evan's question regarding soil strength?
Thank you!
Kelly
Sent from my iPad
Begin forwarded message:
From: "Craig Loughridge" <pioneer@bybryson.com>
Date: June 15, 2015 at 2:10:22 PM PDT
To: "Andrews Kelly" <andrewsgirls@yahoo.com>
Subject: Fw: Bear Creek - ROW Impact Study
Hi Kelly... Please see e-mail below that I received from Evan Kristof over the weekend.
Regards... Craig
From: Evan Kristof
Sent: Sunday, June 14, 2015 1:58 AM
To: Craig Loughridge
Subject: Bear Creek - ROW Impact Study
Craig,
As requested earlier this week, I took a look at what would be required to shift the north side of the bridge to minimize/eliminate the fill that would extend past the east side ROW. I also took into consideration giving us some more breathing room by trying to keep the foundation further away from the edge of the stream bank - with the donated trailer's limited length, we would have been very close to the bank, making constructing the footings without erosion into the stream a challenge.
I think that ideally, we would want a clear distance (measured from the front of the footing to the front of the other footing) of 32'. With the preliminary footing sizes that I had designed, that would correspond to a span length (measured from center of bearing to center of bearing) of about 36'.
We may be able to get a shorter distance to work, but we would need to be careful about bank erosion during excavation and construction of the footings.
You had mentioned that you would pass along my info, but I I have not yet been contacted by the wooden bridge designer to hear about structure depth or footing size yet. In terms of footing design, my intent was originally to keep the allowable bearing pressure below 1000 psf, which is a generally conservative approach. It would be good to know what kind of documentation the city would need in terms of the soil strength. Has there been any public works (roads, bridges) in the same vicinity built recently? If so, then we might be able to look at geotech reports prepared for those and then take a conservative approach to help expedite the process.
I will be out of town next week, but intermittently available by phone and/or email. Let me know if you would like me to produce an example plan set with the proposed geometry that I discussed above.
Evan
--
Evan Kristof
Portland State University
Department of Civil and Environmental Engineering
1930 SW 4th Avenue #200
Portland, OR 97201
(503) 725-4231 (direct)