39.45-H75 Geotech Memo.pdf

2487 Industrial Blvd #1, Grand Junction, Colorado 81505 T: (801) 501-0583 ~ info@geostrata-llc.com

Copyright 2025 GeoStrata 1 Rapid Creek Slope Angle Memorandum

MEMORANDUM

To: River City Consultants c/o Craig Rothluebber, P.E.

crothluebber@rccwest.com

From: GeoStrata

Scott W. Richards, PE, PG

Date: July 1, 2025

Subject: Rapid Creek Slope Angle Discussion

Introduction

We understand based on communications with River City Consultants that the current design for the Rapid Creek Roadway Realignment and Overpass Project consists of constructed slopes up to a 2V:1H slope ratio (approximately 63 degrees). GeoStrata previously completed a geotechnical investigation for the proposed project, the results of which are provided in a report titled “Final

Report, Geotechnical Investigation, Rapid Creek Roadway and Drainage Improvements, Palisade, Mesa County, Colorado” dated February 12, 2025. According to the geotechnical recommendations provided in our final report, a minimum slope angle for described Type C soils is 1V:1.5H (approximately 34 degrees). It has been requested that GeoStrata evaluate existing conditions such that constructed slopes be allowed to exceed this recommendation to a 2V:1H slope in construction. We understand that a slope stability evaluation is not requested at this time for retaining wall design or proposed construction.

A representative of GeoStrata visited the project site on June 30, 2025, to observe existing slope conditions in areas of proposed construction where slopes would be altered according to plan sheet

C19 provided on June 30, 2025. Existing conditions in these areas where the concrete box culvert would be installed along the Rapid Creek, and roadway realignment would alter the northwestern hillsides were observed to be 2V:1H or greater, and in some areas approached near vertical conditions. In general, hillsides consist of clast supported material exposed in slope faces which is likely the reason for slope steepness greater than typical Type C soil slopes as recommended by

OSHA. Additionally, shallow bedrock is likely on adjacent side slopes as observed on the northwest hillside, west of the project site, contributing to the apparent stable nature of the over steepened hillside.

Summary

Based on our site observations, it is our opinion that constructed hillsides of 2V:1H or less are feasible for construction with the understanding that increased risks are likely, which may include, but are not limited to, the following:

• Disturbing existing hillsides for construction may induce marginally stable conditions which may require rockfall mitigation for safe construction conditions and should be

Copyright 2025 GeoStrata 2 Rapid Creek Slope Angle Memorandum anticipated by the contractor during the bidding process. The responsibility of providing a safe work environment would be with the contractor.

• The contractor should be made aware of the existing utilities upslope and northwest of the northwest end of the project site where hillside modifications are proposed such that construction would not disrupt utility services.

• During and after construction, Mesa County should be aware of the need for continued maintenance from, and possible risk from, intermittent rockfall which would be consistent with what existing conditions have produced.

• The evaluation herein was based on observations and our experience with similar conditions and not from slope stability evaluations which would require a subsequent field study and additional costs as explained in our geotechnical engineering report and comes with inherent risks from assumptions.

The contractor may elect to utilize existing boulders at the project site and surrounding vicinity to create a ‘fit’ clast supported slope for additional support and added aesthetics.

A summary attachment of photos depicting items discussed in this memorandum is also provided.

We trust this provides you with the information you need at this time. If you have any further questions or require more information or clarification, please contact us.

Sincerely, Scott W. Richards, P.E., P.G.

GeoStrata (Grand Junction)

2487 Industrial Boulevard #1

Grand Junction, Colorado 81505

Attachments: Site Photographs from Site Visit, June 30, 2025.

Copyright GeoStrata, 2025

Plate A-01

Attachment 1 Rapid Creek Culvert Replacement Palisade, Mesa County, Colorado Project Number: 1812-002

Photo 01 showing existing conditions of Rapid Creek Road with yellow arrows indicating areas of proposed slope alteration. Existing slopes are greater than those planned which are up to 1.3 H to 1 V according to recent plans provided. The bottom arrow shows possible stacked boulders placed during previous roadway improvements which remain in place from the time constructed.

A-02

Core Photos Rapid Creek Culvert Replacement Palisade, Mesa County, Colorado Project Number: 1812-002

Photos 02, 03, and 04 showing clast supported slope material at the northwest end of the proposed construction site. Slopes in this area are approaching near vertical in apparent stable condition, however, possible clasts would likely pose a rockfall hazard during construction such at the one indicated by the yellow arrow, which is the same rock clast in both photos.

A-03

Core Photos Rapid Creek Culvert Replacement Palisade, Mesa County, Colorado Project Number: 1812-002

Photos 05 and 06. Top right photo showing boulder perched on hillside northwest of the project site as a rockfall potential during construction. Bottom right photo shows bedrock outcropping to the west which may indicate shallow bedrock in the northwest hillsides of the construction site facilitating apparent over steepened slopes.

A-04

Core Photos Rapid Creek Culvert Replacement Palisade, Mesa County, Colorado Project Number: 1812-002

Photo 07, looking east with the utilities cut in the northwest hillside (left in the photo) above the project site along Rapid Creek Road shown right in the photo. Arrow indicates water line manhole shown on Plate A06. Existing slopes shown exceed those planned for construction.

A-05

Core Photos Rapid Creek Culvert Replacement Palisade, Mesa County, Colorado Project Number: 1812-002

Photos 07, 08, and 09. Top right photo looking west from the waterline manhole of existing utilities on the northwest slope, upslope from the project site, looking down towards the road crossing of Rapid Creek. Top left photo shows water line running north east along the hillside cut. Bottom photo shows a view of utility cut into northwest hillside with Rapid Creek Road shown downhill to the right.

A-06

Core Photos Rapid Creek Culvert Replacement Palisade, Mesa County, Colorado Project Number: 1812-002

Photo 10, looking east showing typical rockfall likely to be encountered during and after construction with no mitigation to existing or proposed constructed hillside. The age of the utility cut is unknown to us at the time of this memorandum.