Appendix_C_-_Geotechnical_Assessment_Memo_-_University_Terrace_Park.pdf
PDF 252 KB Posted
- Attached to
- RFQu for Arroyo Seco Canyon Trail Stabilization
- Solicitation number
- RFQu No.PW-0308
- Issued by
- Santa Cruz County, San Francisco City, California
About this file
This is a geotechnical assessment memorandum prepared by Pacific Crest Engineering Inc. for the City of Santa Cruz Department of Public Works regarding slope instability and erosion at University Terrace Park in Santa Cruz, California. The assessment was conducted on April 7, 2026, following an on-site visit to evaluate a distressed area located on a paved trail approximately 1,150 feet south of the Meder Street trailhead. The affected area spans approximately 25 feet in length and extends 15 feet down to an active drainage channel, with the asphalt berm already compromised and slopes approaching vertical angles in certain locations. The City is evaluating three mitigation strategies: realignment of the path away from the over-steepened slope, construction of a retaining wall on the west side of the trail to maintain current alignment, or construction of a retaining wall on the east side with trail realignment. The assessment identifies critical site constraints including permitting challenges due to proximity to an active drainage channel, presence of a subsurface sewer line, and limited easement area with potential encroachment onto private property.
The geotechnical engineer recommends construction of a soldier pile or pin-pile retaining wall on the west side of the trail as the preferred mitigation approach, as it maintains the current trail alignment and eliminates encroachment risks while potentially allowing design above the drainage channel's high-water line to reduce permitting constraints. The other mitigation options are deemed either infeasible or problematic due to required setbacks from slope stability analysis, potential private property encroachment, exposure of the existing sewer line to future erosion, and the need for relatively tall walls. The assessment explicitly recommends that a comprehensive geotechnical investigation be conducted to substantiate initial opinions and suggests interim protective measures such as sandbags or temporary asphalt berms to divert surface runoff while permanent solutions are developed.
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G E O T E C H N I C A L | E N V I R O N M E N T A L | C H E M I C A L | M A T E R I A L T E S T I N G | S P E C I A L I N S P E C T IO N S
444 AIRPORT BLVD. , SUITE 106 | WATSONVILLE, CA 95076 | PHONE 831-722-9446 | WWW.4PACIFIC-CREST.COM
April 7, 2026 Project No: 26035
Tim Wong Assistant Engineer II City of Santa Cruz Department of Public Works 809 Center Street, Room 201 Santa Cruz, CA 95060
Subject: Geotechnical Assessment Memo University Terrace Park Landslide Santa Cruz
Dear Tim, As requested, we conducted at site visit on April 7, 2026, to observe and assess an area of erosion and slope instability adjacent to University Terrace Park in Santa Cruz. The distressed area is located on a paved trail that boarders the west side of the park and is approximately 1,150 feet south of the trail head at Meder Street. The distressed area is on the west side of the trail, where it appears that surface runoff has concentrated and cascaded onto the slope below the trail causing the slope to erode. This erosion and resulting slope instability was likely exacerbated by the constant erosion at the toe of the slope due to an active drainage channel. At the time of our site visit, the distressed area was approximately 25 feet long and extended approximately 15 feet down to the drainage channel below the trail. The asphalt berm along the trails edge has already been lost and the slope below the trail was nearly vertical in some locations.
It is our understanding that the City is in the process of considering mitigation strategies for the subject area. Strategies considered by the City include: 1) realignment of the path away from the over-steepened slope, 2) construction of a retaining wall on the west side of the trail that would maintain the current trail alignment, or 3) construction of a retaining wall on the east side of the trail allowing the trail to be realigned even further away from the over-steeped area. Site constraints include: 1) permitting challenges due to construction activities adjacent to an active drainage channel, 2) the presence of a sewer line beneath the trail, and 3) limited easement area and potential encroachment onto nearby private property. Site constraints 2 and 3 render the use of soil nails or tiebacks undesirable.
Given the over-steepened nature of the slope blow the trail at this location, any realignment effort would need to incorporate a quantitative slope stability analysis to determine the approximate location of the head scarp after the slope fails back to a stable gradient. Once the approximate location of the head scarp is determined, a minimum setback from the head scape is typically incorporated into the design. This encroachment into the trail due to the slope stability analysis would likely take up the
University Terrace Park - Landslide Project No. 26035 April 7, 2026 majority of the trail, therefore, a realignment effort without the incorporation of a retaining wall (mitigation option 1 above) is likely infeasible in our opinion.
It may be feasible to construct a retaining wall on the east side of the trail and realign the trail away from the over-steepened (mitigation option 3 above), but any such design would also need to incorporate the findings and requisite setback from the quantitative slope stability analysis. This process would push the trail and up slope retaining wall further east, resulting in a relatively tall wall that may encroach upon private property. This mitigation approach would also require the realignment trail and leave the exiting sewer line potentially exposed to future erosion/landsliding.
It is our opinion that construction of a soldier pile or pin-pile retaining wall on the west side of the trail (mitigation option 2 above) represents the preferred approach from our perspective. This option will maintain the current trail alignment, thus eliminating the potential for encroachment onto private property. The retaining wall would likely be similar in height to that of mitigation option 3. While this option likely poses the highest potential environmental permitting issues, it is likely that a wall can be designed completely above the high-water line for the drainage channel, thus decreasing the potential for permitting constraints.
This letter details our opinions based solely on our site observations. We recommend that a comprehensive geotechnical investigation be conduction in order to substantiate our initial opinions.
In the interim, we recommend that the City place sand bags or constructed a temporary asphalt berm along the western edge of the trail to divert surface runoff from cascading onto the slope below.
Pacific Crest Engineering Inc. appreciates the opportunity to provide our services and would be pleased to answer any questions. We can be reached at (831) 722-9446.
Sincerely, PACIFIC CREST ENGINEERING INC.
Matt Maciel, PE, GE President/Principal Geotechnical Engineer P.E. 82779, G.E. 3189
9/30/26
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