GM 02-23 ID ERFO FS 2019-1(1) Final Geotech Nez Perce Memo.pdf

158377846

M E M O R A N D U M Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661

DATE: January 9, 2023

TO: Curtis Jorgensen Project Manager

FROM: Todd Hansen Geotechnical Engineer

SUBJECT: Geotechnical Memo 02-23 Final Geotechnical Recommendations Nez Perce – Clearwater Repairs

ID ERFO FS NEZPR117 2019-1(1)

INTRODUCTION

This memorandum presents the final geotechnical recommendations and findings for the Nez Perce – Clearwater Repairs 2019 project. Geologic conditions were presented in “GM 20-22 Preliminary Geotechnical Recommendations”.

PROJECT DESCRIPTION

In April 2019, Forest Service roads within the Nez Perce and Clearwater National Forests sustained severe damage during widespread storm events that affected the region. To qualify for assistance under the Emergency Relief of Federally Owned (ERFO) Roads Program the Forest prepared preliminary Detailed Damage Inspection Reports and submitted them to the Western Federal Lands (WFLHD) ERFO Coordinator for consideration under that program. In Summer of 2022 the Western Federal Lands Highway Division (WFLHD) conducted a project scoping visit to investigate the sites for the level of damage and eligibility for the ERFO Roads Program and collect additional information. Seven sites are being repaired under the ERFO program, with six sites from 2019 and one site from a previous 2017 project. Major project work elements include construction of Special Rock Embankments (SRE), Reinforced Soil Slopes (RSS) and Mechanically Stabilized Earth (MSE) walls. One MSE wall is designed by the Government and the RSS and SRE repairs will be designed by the Contractor.

The sites proposed for repair under the project are listed in Table 1. The type of major work elements planned for the sites and the responsible party providing design is shown in the fourth column.

Table 1 – Proposed Repair Sites and Work Elements

Site Designation Latitude (north decimal degrees)

Longitude (east decimal degrees)

Repair (Design) (provided by Gov’t or Contractor)

FSR 354 MP 13.6

45.629603 -116.054797 Widening roadcut, rock buttress (Gov’t)

FSR 469 MP 0.8 45.827019 -115.813061 SRE (Gov’t) FSR 469 MP 2.8 45.853878 -115.804633 SRE (Gov’t) FSR 470 MP 2.36 46.123650 -115.599350 RSS (Contractor) FSR 470 MP 2.52 46.125361 -115.601350 RSS (Contractor) FSR 5503 MP 1.28 46.142219 -115.752419 SRE/RSS (Contractor) FSR 5503 MP 4.0 46.168333 -115.748028 MSE (Gov’t)

SITE CONDITIONS

Site conditions at the proposed repair sites were observed during the WFLHD site review in Summer 2022. Based on the general location of the sites and the photos in the DSRs, the general condition of the damaged sites can be described as roadways with generally steep to over-steep slopes located in mountainous terrain with difficult access and narrow corridors often common. Cause of failure is thought to be attributed to saturated slopes and possible internal erosion, surface erosion from sheet or concentrated flows caused (at least in some cases) by blocked culverts and ditches, or some combination thereof. Many of the undamaged adjacent fill slopes were observed to range from about 1V:1H (45 degrees) to approximately 1V:1.25H (38.7 degrees). Site photos showing damage are provided as Attachment 1. The photos contained in the attachment were obtained directly from the DSRs.

Surficial springs and evidence of shallow groundwater were observed during the site visit at FSR 470 MP 2.52 and FSR 354 MP 13.6. Groundwater conditions may vary from the observations due to seasonal variations but a reasonable and prudent Contractor should be prepared for shallow groundwater conditions at time of construction.

The surficial soils observed near the ditches and failed fill slopes during the 2017 and 2018 site visits by the WFLHD Geotechnical Engineer were generally silty sand with gravels and occasional large boulders, and where exposed the bedrock was field estimated to range from weak to strong rock.

SUBSURFACE CONDITIONS

No subsurface samples were obtained during the WFLHD site review in Summer 2022.

Subsurface materials were estimated based on visual reconnaissance of exposed areas and the information described below.

The USFS provided two site investigation reports prepared by Allwest Testing, a geotechnical consultant. A November 20, 2019 report for the “NFSR 354 MP 13.6 Landslide” presents general geologic conditions and lab test data of bulk samples. Three test pits were excavated along the shoulder failures and samples were also collected from Adams Pit, a quarry located five miles to the north at latitude-longitude 45.642°, -116.033°. Test results of the roadway bulk samples indicate the soil at this site is a non-plastic silty sand with gravel, though moisture content may exceed optimum conditions for fill compaction. Groundwater levels can change and vary with seasonal conditions, such as drainage patterns and the presence of groundwater barriers, such as landslide shear surfaces or faults. Materials tested from Adams Pit classified as 3-inch minus or ¾-inch base course compatible.

A November 25, 2019 report for the “NFSR 469 MP 0.8 & 2.8 Landslides” identifies general geologic conditions and lab test data of bulk samples. Two test pits were excavated at each of the shoulder failures. Test results of the bulk samples indicate the soil at this site is a non-plastic silty sand with gravel, though if on site material was to be reused for backfill it would likely need to be dried out or mixed with imported fill to meet moisture requirements for compaction. While not directly observed by the CFT, groundwater conditions may vary and require provisions to reduce impact to construction activities.

As reported in the Allwest Testing consultations, the tested soils classfied as non-plastic. The fractions passing the “Silt/Clay Gradation” (#200 sieve) are 23.2 percent (FSR 354 MP 13.6),

20.3 percent (FSR 469 MP 0.8), and 20.8 percent (FSR 469 MP 2.8).

ANALYSES

Internal and external checks were completed on the Government-provided MSE wall design.

Sites with Contractor-designed structures were not analyzed due to the lack of survey information necessary to perform the full analysis. It will be the Contractor’s responsibility to provide these designs.

Soil properties used in the analyses were determined by back-calculation using RocScience SLIDE 7.0 with some caveats. Analyses were performed to find the back-calculated strength values of the native soil under the pre-failure condition; however, we could not back-calculate to a minimum safety factor of 1.0 without including a value of cohesion or apparent cohesion in the modeled soil. Because the test results showed the soils to be non-plastic, cohesion was not used in the analyses. The site conditions after failure did not appear to have changed

(or raveled) while standing at angles nearing 45 degrees from horizontal. The steep slopes and virtually unchanged condition suggest that there is some cohesion or apparent cohesion present in the native in-situ soil and an apparent cohesion of 100 pounds per square foot was used to develop a stable limit equilibrium analysis model for static conditions. With that said, the application of an apparent cohesion in the global stability model is applied only to match the modeled slope to existing observed conditions in the native cohesionless material.

MSEW 3.0 (Adama Engineering) was used for checking the internal and external stability analyses of the Government-provided design, and SLIDE 7.0 validated the global and compound stability analyses. These design tools allows us to specify a minimum strength and length of geosynthetic reinforcement.

DESIGN RECOMMENDATIONS

Site specific recommendations are shown below. The section numbers listed in parenthesis reference the parent specification found in either the Standard Specifications FP-14 or the Special Contract Requirements (SCRs). Project sites requiring geotechnical design include 3 SRE, 1 MSE, and 3 RSS structures. The MSE structure and two SRE are designed by the Government. Three RSS structures and one SRE will be designed by the Contractor.

• FSR 354, MP 13.6: Government-designed roadway excavation (Section 204) planned at this site to shift alignment into slope. A mechanically placed rock buttress (Section

252) will be placed from Station 101+20 to 101+80 at the base of the new cut slope.

The rock buttress should be lined underneath and against the cut slope with a geotextile filter, Class 1, Type A, non-woven to keep adjacent materials from infilling the rock buttress.

• FSR 469, MP 0.8 & 2.8: Government-designed SRE (Section 252) planned at both sites for shoulder stabilization. The SRE should be lined underneath and against the cut slope with a geotextile filter, Class 1, Type A, non-woven to keep adjacent materials from infilling the SRE.

• FSR 470, MP 2.36 & 2.52: Contractor-designed RSS (Section 261). The RSS face will be wire-faced backed with gabion rock with planned batter at 2V:1H with 18-inch vertical reinforcement spacing at both sites.

• FSR 5503, MP 1.28: Contractor-designed SRE base (Section 252) underlying Contractor-designed RSS (Section 261). The RSS face will be wire-faced backed with gabion rock with planned batter at 2V:1H with 18-inch vertical reinforcement spacing.

A special rock embankment (SRE) foundation is also planned to provide a flat base for the RSS, fill in around rock outcrops or large boulders, and reduce or avoid rock excavation. The SRE should be lined underneath, against the cut slope, and overlain with a geotextile filter, Class 1, Type A, non-woven to keep adjacent materials from infilling the SRE.

• FSR 5503, MP 4.0: Government-designed MSE wall (Section 257) planned at this site.

The MSE wall will have welded wire facing backed with gabion rock and 18-inch vertical reinforcement spacing.

CONSTRUCTION CONSIDERATIONS

Based on the site visits and test data provided by others for this project, the Occupational Safety and Health Administration (OSHA) soil type is estimated to be Type C for the sites in this contract. A competent person as defined by OSHA should confirm this prior to opening temporary excavations.

It should be assumed that boulders of unknown maximum size will be encountered during excavations. It is possible that bedrock might be encountered at one (or possibly more) of the sites, especially the FSR 5503 MP1.28 Swan Creek site. If present, the Contractor must be prepared to excavate into bedrock of unknown strength. If competent bedrock is encountered, contact the CO and the project Geotechnical Engineer to discuss the possibility of shortening reinforcement lengths. Blasting will not be allowed on this project. No subsurface geotechnical investigation was performed for this project. Surficial boulders, cobbles and bedrock outcrops were observed at these various sites and nearby locations. The published geologic maps indicate the presence of metamorphic and igneous bedrock, which when observed in exposures could range from intact massive rock to boulders.

Constructability considerations for excavations are based on interpretation of the mapped geologic conditions. Excavations into unweathered, massive bedrock or corestones in weathered rock material will require specialty excavation equipment to advance. A corestone can be defined as a strong, unweathered boulder embedded within a matrix of weathered bedrock. If encountered, large rocks may need to be broken into smaller sizes to handle and transport. Final determination of bedrock relative intact strength and method of excavation is the responsibility of the Contractor.

Groundwater springs and shallow seeps were witnessed during our Summer 2022 site visit, and it is possible that groundwater seeps or springs may be encountered during construction depending on the time of year and recent weather activity. The likelihood of encountering springs or seeps might be reduced by timing construction to coincide with the dry season.

The project repair sites are in narrow corridors with possible difficult construction logistics.

Pass through construction methods of construction might be considered.

There is no Government-provided material source for this project. It is the Contractor’s responsibility to locate construction materials conforming to the contract requirements from a commercial or private source.

It is the Contractor’s responsibility to provide suitable designs for the Contractor-designed MSE walls on the project. Design the MSE walls in accordance with the Load and Resistance Factored Design (LRFD) procedures set forth in FHWA GEC 011 (Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes) and AASHTO LRFD Bridge Design Specifications.

Slope rounding along the top of the cuts is required to prevent the immediate erosion of material along the top of the cut. This is recommended in areas where overburden soil or bedrock raveling creates an oversteepened surface at the top of the cut.

Construction Equipment Considerations Some sections of forest roads evaluated on these site visits are not sized to pass standard-length semi-trailer transportation or large equipment. Selecting the proper equipment for performing work will require the Contractor to evaluate the accessibility limitations and the planned construction activity. At a minimum a reasonable and prudent Contractor will consider the following when evaluating how construction will be performed:

• Account for job site conditions when choosing equipment;

• Select correctly sized equipment for the site and task;

• Identify specialty equipment or attachments that may be necessary; and

• Ensure that selected equipment and supplies can transport to the site.

CLOSING

Please contact Todd Hansen at (360) 619-7903 or Todd.Hansen@dot.gov with any questions regarding this memorandum.

CC: Eric Lim, Geotechnical Doug Anderson, Geotechnical Kevin Parker, Design Geotechnical File

Attachment 1:

Site Photos

Figure 1 – FSR 354 MP 13.6

Figure 2 – FSR 354 MP 13.6

Figure 3 – FSR 469 MP 0.8

Figure 4 – FSR 469 MP 2.8

Figure 5 – FSR 470 MP 2.36

Figure 6 – FSR 470 MP 2.52. Note bedrock outcrop in cut and wet ditch.

Figure 7 – FSR 5503 MP 1.28

Figure 8 – FSR 5503 MP 4.0