Road Cut Excavation Recommendations Apache Trail AZ FLAP SR88(1).pdf

To: Dustin Robbins, Project Manager, CFLHD, Lakewood CO

From: Todd Hansen, Geotechnical Engineer, CFLHD, Lakewood CO

Review: Dominic Monarco, Geotechnical Engineer CFLHD, Lakewood CO

Subject: Road Cut Excavation Recommendations, Apache Trail, AZ FLAP SR88(1)

I. PROJECT INFORMATION

This geotechnical memorandum presents recommendations for excavation methods and anticipated ground conditions for the work to widen the existing Apache Trail road corridor. After the CFLHD geotechnical engineer’s site visit and review of the proposed Design construction limits, a total of 5 locations will require road cut excavation. Roadway excavation limits shown in Table 1 include the currently estimated volume in the AZ FLAP SR88(1) Plan Set and list anticipated method of construction. Methods of excavation and potential ground conditions are generally discussed in further detail below.

Technical Memorandum

Date: October 5, 2021

Table 1 – Roadway Excavation Summary

Stationing Side Max Existing Slope Height

(ft)

Excavation Volume (yd3)

Recommended Cut Slope

Ratio

Anticipated Method of Excavation

1267+00 – 1270+00

RT 23 1,125 1H:2V Rippable

1273+00 – 1275+00

RT 25 833 1H:2V Rippable

1288+67 – 1290+00

RT 30 358 1H:2V Rippable

1528+26 – 1532+00

LT 25 875 1H:2V Rippable

1655+50 – 1843+40

LT 20 4,962 1H:1.5V1 Drill & Blast

1 This anticipated cutslope angle may be steepened up to 1H:4V if rock conditions allow.

II. GEOLOGIC CONDITIONS

Based on a cursory field evaluation of exposed bedrock and the “Geologic Map of the Southwestern Part of the Tonto NF, Gila, Maricopa, Pinal and Yavapai Counties, AZ” (AGS, 2010), the Apache Trail corridor has been excavated through granite rock with overburden colluvium. As identified by the Arizona Geological Survey map the bedrock is classified as a “coarse-grained porphyritic granitoid, light brown to light gray porphyritic biotite granite with 2- 8cm diameter phenocrysts of potassium feldspar, plagioclase, quartz and biotite.” This rock weathers rapidly to decomposed granite (grus) and thin intrusive dikes are common. Regionally the rock formation is overlain by a welded ash tuff and a conglomerate deposit, and if these are present in the areas to be excavated the Contractor must evaluate how this may affect or impede the method of excavation. Mapped faults are present in the area south of Apache Lake and across the road alignment, which means localized zones of crushed or heavily fractured granite bedrock may be encountered during excavation. Though not mapped during the field investigation, the potential to encounter variable intact rock conditions requires a prudent and reasonable Contractor to be prepared with multiple methods of excavation should the need arise for different excavation methods.

III. ROADWAY EXCAVATION

Recommended slope ratios are based on the existing conditions at the observed cut faces and our previous experience with this type of bedrock material. In general, permanent cutslopes through weatherable, coarse-grained bedrock such as this decomposed granite (also referred to as weathered granitoid) can be advanced with conventional to heavy-duty construction equipment.

As excavations proceed there is potential to encounter large corestones. A corestone can be defined as a hard, unweathered boulder embedded within the weathered matrix of decomposed granite. If encountered, downsizing of corestones may be required to handle and transport. While downsizing of hard rocks has been achieved with blasting or mechanical breaker methods, it is the Contractor’s responsibility to determine appropriate means and methods.

Following the criteria presented below in Table 2, rippability of the weathered granitoid is estimated to range between the “Soft Rock” to “Hard Rock” description category. Determining rippability involves many factors, including equipment and tooling, and is the responsibility of the contractor. The bedrock excavation conditions for the rippable cuts will likely encounter harder, more durable material as the cut extends laterally into the hillside. Final cut slope angles may need to re-evaluated if unstable or fractured conditions are exposed. The CFL geotechnical engineer must be notified immediately if unanticipated conditions are encountered and the Contractor will need to determine if a revised cut slope needs to be flattened or steepened in order to maintain cut stability.

Slope rounding or break in slope angle along the top of the cuts is required only 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 oversteepend surface at the top of the cut.

Table 2 – Rock Hardness and Excavation Characteristics1

Rock Hardness

Description

Identification Criteria

Unconfined Compressive Strength

Seismic Compression (P-Wave) Velocity Excavation

Characteristics MPa psi m/s f/s

Very Soft Rock

Material crumbles under firm blows with sharp end of geological pick; can be peeled with a knife; too hard to cut a triaxial sample by hand. SPT will refuse. Pieces up to 3-c, thick can be broken by finger pressure.

1.7-3.0 246-435 450-1,200 1,475-3,935 Easy Ripping

Soft Rock

Can just be scraped with a knife;

indentations 1-mm to 3-mm show in specimen with firm blows of the pick point;

has dull sound under hammer.

3.0-10.0 435-1,450 1,200- 1,500

3,935-4,920 Hard Ripping

Hard Rock

Cannot be scraped with a knife; hand specimen can be broken with a pick with a single firm blow; rock rings under hammer.

10.0-20.0 1,450- 2,900

1,500- 1,850

4,920-6,070 Very Hard

Ripping

Very Hard Rock

Hand specimen breaks with a pick after more than one blow; rock rings under hammer.

20.0-70.0 2,900- 10,150

1,850- 2,150

6,070-7,050 Extremely Hard

Ripping or Blasting

Extremely Hard Rock

Specimen require many blows with geological pick to break through intact material; rock rings under hammer.

> 70.0 > 10,150 > 2,150 >7,050 Blasting

1Table from Weaver (1975).

A. Rippable Excavation Methods

Excavate using equipment capable of removing the material while preventing material from escaping outside the construction limits. Based on the available mapped geology and site review, the bedrock is expected to be rippable near the surface in areas identified for cut slope construction;

however, final estimation of bedrock rippability in determining constructability of excavations is the responsibility of the Contractor.

Data correlating seismic velocity to rippability is publicly available through equipment manufacturers and as published charts. The Contractor is responsible for selecting appropriate rippability data based on the seismic velocity of the rock and the anticipated equipment to be used in excavation. No site investigation was performed by the CFL geotechnical engineer and therefore no seismic data is provided in this memo. If requested by the Contractor, CFL can provide publicly-available rippability parameters for-information-only but the Contractor or the Contractor’s geotechnical engineer is responsible for obtaining and interpreting seismic velocity of the rock and selecting the final rippability parameters that will be used in construction.

Figure 1 – Example of rippable granite rock with colluvium overburden.

Figure 2 – Example of rippable granite rock with erosion rills present in slope.

B. Drill and Blast Excavation

Excavation into hard, intact rock slopes will require drilling and blasting techniques to advance the cut slope. If the blast design does not result in planned fragmentation of the rock then the Contractor will need heavy excavation equipment capable of breaking and transporting large intact rocks, or another method of rock downsizing readily available.

For economy of scale and maximizing ditch capacity for potential rockfall, the rock cut volume between Station 1528+026 and Station 1532+00 should be maximized as much as possible.

Current proposed cut slope in this rock is 1H:1.5V could be steepened up to 1H:4V, provided the rock conditions allow for this slope angle. Steepening up the back slope will permit additional cut width in the base and better sight distance around the corner.

Rock blasting will require production blasting techniques to create the final cut face. Presplitting is no allowed, to prevent half-cast marks left in the final cut face. Blasting methods that are expected to be used include cushion blasting or trim blasting. A pre-blast survey of historic or culturally sensitive resources will need to be performed prior to any blasting activity.

Figure 3 – Example of hard, blocky rock requiring drill and blast techniques.

IV. LIMITATIONS

Subsurface exploration was not performed as a part of this work. Interpretation of surface and subsurface conditions is based on limited field reconnaissance and surface observations of soil and rock outcrops. The recommendations in this memorandum include interpretations developed by the Government in the process of preparing preliminary repair alternatives. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgement of the Contractor.

V. SIGNATURES

Prepared by Todd Hansen, Geotechnical Engineer, CFLHD

______________________________________________ Date: 10/5/2021

Reviewed and approved by Dominic Monarco, Geotechnical Engineer, CFLHD

______________________________________________ Date: 10/5/2021

(for Marilyn Dodson, Lead Geotechnical Engineer, CFLHD)

VI. REFERENCES

U.S. Department of Transportation, Federal Highway Administration (FHWA). (2014). “Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14.”

Publication No. FHWA-FLH-14-001.

Arizona Geologic Survey (AGS). (2010) Compiled by Richard, S.M. “Geologic map of the southwestern part of the Tonto National Forest, Gila, Maricopa, Pinal, and Yavapai Counties, Arizona” Map DGM-76, scale 1:100,000.

Weaver, J.M. (1975). “Geologic Factors Significant in the Assessment of Rippability.” The Civil Engineer in South Africa (Die siviele ilngenieur in Suid-Afrika). Volume 17, Issue 12. p 313-316.