B12_Geotechnical_Design_Description_0004.pdf

Hart Mountain NAR Grade Road Restoration Project

Geotechnical Design

Below is a summary of the important design decisions made with regard to the geotechnical design.

Roadside Stabilization

The loss of material on the downgradient side of the roadway is a direct result of surface erosion. Site visits and surface and subsurface investigations did not suggest that there is a deep-seated global stability or landslide problem present at the site. However, a global slope stability analysis of the roadway was performed in order to establish that the construction of a keyed riprap slope does not decrease the overall global stability of the roadway in the project AOI. The computer software program SLIDE2 by RocScience, Inc. was used to perform this analysis. Engineering properties of soil used in this analysis were developed based on back-calculation of existing conditions, soil and rock behaviors observed on-site during the field investigation, and engineering judgment. Results of this analysis indicate that under existing conditions the factor of safety against slope failure is less than is typically designed for in a new road, meaning that the calculated stability of the existing slope does not meet new construction standards. Results also indicate that constructing a keyed riprap slope does not affect the factor of safety against a failure of the slope.

Therefore, the keyed riprap slope solution should be viewed as a maintenance repair to reduce road narrowing at isolated locations and not an engineered solution to increase global slope stability. Alternatives to increase global slope stability would be outside of the project construction budget and were deemed outside of the project objectives.

Road Resurfacing

Road resurfacing material engineering parameters were collected from ODOT’s Oregon Standard Specifications for Construction (2018b), Dense Graded, 1”-0 aggregate in Section 02630.10.

Prior to placement of resurfacing material, the existing roadway needs to be cut a minimum of 2 inches below road surface corrugations to remove all “washboard” effects and prevent distress from quickly reflecting to the new roadway surface when traffic resumes. Following removal of road surface corrugations, a 3% minimum crown (or as required by the civil design drawings and specifications) should be graded into the roadway to allow for drainage away from the roadway.

Surfacing material should be placed to a uniform thickness, a minimum of 4 inches thick. Road surfacing material should then be compacted using appropriate equipment to 95% of the maximum density per AASHTO T 99 Standard Proctor Method A. Material should be moisture conditioned to assist compaction, and small quantities of fine aggregate can be added to coarse aggregate as appropriate to assist in achieving the required compaction. It is important to ensure that aggregate is uniformly mixed prior to compaction and that no pockets of coarse or fine material are present. Existing roadway material that is cut to remove washboards may be reused for final roadway surfacing if it meets the requirements specified above from ODOT (2018b).

Hart Mountain NAR Grade Road Restoration Project

Rock Cuts

As part of the low-visibility curve modifications, approximate Stations 9+00 to 10+00 will include a small rock cut into the hillslope for improving sight distances. The ground in this location consists of strong, slightly weathered and closely fractured basalt. The proposed cut is less than 10 feet high. It is recommended that this cut be excavated at an approximate batter of 1/2H:1V. Because of the limited cut height and consequent volume, this cut should be excavated using mechanical methods such as a pneumatic hammer and excavator bucket equipped with ripper teeth.