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GEOTECHNICAL DESIGN PROCEDURE FOR PREPARING ROCK SLOPE RECOMMENDATIONS

GEOTECHNICAL DESIGN PROCEDURE

GDP-13

Revision #1

GEOTECHNICAL ENGINEERING BUREAU

APRIL 2007

GEOTECHNICAL DESIGN PROCEDURE:

GEOTECHNICAL DESIGN PROCEDURE FOR PREPARING ROCK SLOPE RECOMMENDATIONS

GDP-13

Revision #1

STATE OF NEW YORK

DEPARTMENT OF TRANSPORTATION

GEOTECHNICAL ENGINEERING BUREAU

APRIL 2007

TABLE OF CONTENTS

I. INTRODUCTION3

A. Purpose3

B.Background3

II. PROCEDURE6

A.Procedure for Preparing Rock Slope Recommendations6

REFERENCES10

I. INTRODUCTION

A. Purpose tc "A. Purpose " \l 2

Rock slope recommendations are obtained through the Geotechnical Engineering Bureau’s Engineering Geology Section. The purpose of this manual is to inform both Regional and Consultant designers of the proper procedure for preparing rock slope recommendations.

B. Background

The Geotechnical Engineering Bureau provides rock slope recommendations which consider two major items in designing slopes for a proposed rock cut: stability and cost. Although rockfalls rarely cause injury and only occasionally cause property damage, when they occur they draw considerable public attention. Rock slope recommendations for creating a new slope or trimming an existing slope include cut angles and ditch catchment dimensions. Recommended cut angles are the steepest angle which will provide a stable, low maintenance slope. Catchment ditches prevent falling rock from reaching the roadway. Other methods of mitigating an existing slope may include rock scaling, rock bolts, slope mesh, buttresses, and rock fall barriers.

Rock slope recommendations are provided for new and existing cuts to accommodate new road alignments or changes in road template, or for existing unstable rock slopes to stabilize the slope and provide improved catchment. Rock slope recommendations are based upon an evaluation of rock cores, exposed bedrock, site conditions and cross sections. Cut angles are based upon rock type, bedding orientation, fracture orientation, and slope geometry. Occasionally, flatter cut angles will result in smaller excavation quantities. In some circumstances, cut angles can be steepened provided the slope can be properly reinforced.

Adequately designed rock catchment ditches will prevent most falling rock from reaching the road. Ritchie Ditch profiles are based upon the work of Arthur M. Ritchie, who conducted a study in 1963 sponsored by the US Department of Commerce, Bureau of Public Roads and the Washington State Highway Commission. The study produced a set of practical design criteria that could be used to size the width and depth of catchment area based on slope height and slope angle. The resulting ditch is flat bottomed and has a 1 V on 1 1/4 H foreslope. The steep foreslope is difficult to construct and maintain using typical construction methods and materials, and is deemed a non-recoverable slope which poses a hazard and must be protected using guide rail. The Ritchie Ditch profiles cover the widest range of slope heights and slope face angles, and are designed to achieve 85% rockfall catchment.

To expand on the Ritchie Ditch criteria, the Oregon DOT conducted an extensive research project, final report issued 2002, through a pooled fund effort funded by several State Transportation Departments, including NYSDOT, and the FHWA. The research project had three main goals:

1. Investigate how slope, catchment area and rockfall properties affect retention at the base of vertical, 4V:1H, 2V:1H, 1.33V:1H, and 1V:1H slopes; for slope heights of 40 ft. (12.2 m), 60 ft. (18.3 m), and 80 ft. (24.4 m); and catchment area slopes of flat-bottom, 1V:6H and 1V:4H.

2. Develop improved guidelines.

3. Provide flexibility in designing catchment areas that will retain percentages of rockfall ranging from 30% to 99%.

The Oregon Ditch design guide uses catchment area slopes that are considered recoverable. However, only a limited number of rock slope heights were considered in the study.

A third method of rock catchment design is the computer program Colorado Rockfall Simulation Program, which simulates rocks tumbling down a slope, predicts the statistical distribution of speed and bounce height, and can be used for locating and designing rockfall mitigation.

When providing ditch widths, the recommended width may have to be wider than what would be required for adequate catchment to allow for 5 ft. (1.5 m) of burden for presplit blasting. Presplit blasting is required for all slopes greater than 5 ft. (1.5 m) in height and 1V: 1H or steeper.

II.PROCEDURE FOR PREPARING

ROCK SLOPE RECOMMENDATIONS

Rock slope recommendations will be provided after a field meeting is held with the Regional Geotechnical Engineer, Regional or Consultant Designer, Regional Representatives and the Geotechnical Engineering Bureau Engineering Geologists. The intent of the field meeting is to provide an opportunity for a discussion on the aspects, specifically the constraints, of the project. This way, all individuals involved know the factors that contributed to the final rock slope catchment design, whether it is a Ritchie Ditch or some alternate section.

The following is the Procedure for Preparing Rock Slope Recommendations:

Scoping Phase

Step 1:

Prior to the scoping meeting, the Regional Geotechnical Group reviews the Initial Project Proposal and project limits and identifies the possibility of rock work.

Step 2:

A:

The Regional Geotechnical Engineer requests the Geotechnical Engineering Bureau, in writing, to review the project and provide a Rock Slope Location and Identification Report. The report will provide the locations of the existing rock slopes from the Geotechnical Engineering Bureau’s inventory as well as additional slopes within the project limits. In addition to this, the report will identify if there are stability concerns with any of these slopes. To provide the designer with a preliminary analysis, the stability of the existing slopes based on the existing alignment will be ranked between 1 (general instability problems, high potential for work) and 4 (stable slope, no work required). No recommendations will be included in the report.

Commentary:

There may be areas where future rock slopes within a project’s limits are not evident yet are discovered during the progression of borings for a proposed cut. Step 3 may be relevant.

B:

The Regional Geotechnical Engineer requests the Geotechnical Engineering Bureau, in writing, to review the project and provide a Rock Outcrop Map. The request should include a general map identifying the location and boundary limits of the project. (Ideally, the map would be a Plan identifying the existing and proposed alignments).

Commentary:

If the Regional Geotechnical Engineer identifies that the project will require a Rock Outcrop Map, the requests for a Rock Slope Location and Identification Report and a Rock Outcrop Map may be combined.

To finalize the Rock Outcrop Map, an electronic version of the projects Plan, with identified survey datum, must be submitted to the Geotechnical Engineering Bureau.

Phase V

Step 3:

The Regional Geotechnical Engineer will schedule a field meeting with the Regional or Consultant Designer, Regional representatives and the Geotechnical Engineering Bureau Engineering Geologists to discuss the Rock Slope Location and Identification Report and the proposed work. To expedite site discussions and agreement on the final alternate, the memorandum from the Region requesting the field meeting should identify the constraints of the project. This will allow the Geologists to analyze alternatives prior to the field meeting.

Cross sections including all the project’s constraints (right-of-way restrictions, Regional project priorities, cost considerations, etc.) should be provided to the Geotechnical Engineering Bureau prior to the meeting so site specific recommendations can be discussed.

Site Discussions:

A. The Geologists will discuss rock slope and catchment requirements for each site.

B. The Designer will discuss the project’s constraints.

C. If the rock slope and catchment requirements are within the project’s constraints, recommendations will be made.

D. If the rock slope and catchment requirements are not within the project’s constraints, alternatives will be discussed.

E. Additional time may be required to develop additional alternates.

Additional discussions/ field meetings may be held, if warranted.

Step 4:

To document the alternatives discussed, and the reasoning for acceptance/rejection of each alternate, the Regional Geotechnical Engineer writes a memorandum outlining the minutes of the field meeting.

Step 5:

Based on the site discussions, constraints, slope analyses and after reaching verbal agreement with the Region, the Geotechnical Engineering Bureau will provide written Rock Slope and Catchment Recommendations for each site.

Step 6:

The Regional Designer provides the Regional Geotechnical Engineer with a copy of the Advanced Detail Plans. As part of their review, the Regional Geotechnical Engineer provides the Geotechnical Engineering Bureau with a copy of the Plans containing rock work for review.

Commentary:

If, during design, the development of the project produces changes that affect the rock slope catchment area, the Regional Designer shall notify the Regional Geotechnical Engineer. An additional meeting (Step 3) should be called to discuss the rock slope recommendations.

Step 7:

The Geotechnical Engineering Bureau provides the Regional Geotechnical Engineer with a written review of the Advanced Detail Plans which include the proposed rock work. The review is submitted to the Regional Designer who evaluates the comments received.

Phase VI

Step 8:

The Geotechnical Engineering Bureau provides the Regional Geotechnical Engineer a review of the Final Plans.

Construction

Step 9:

If, during construction, a situation arises where rock slope recommendations are required for an area omitted in the contract documents, the Regional Geotechnical Engineer will schedule a field meeting with the Engineer-In-Charge, Regional Designer, Regional representatives and the Geotechnical Engineering Bureau Engineering Geologists to discuss the field conditions.

Cross sections including all the project’s constraints (right-of-way restrictions, cost considerations, etc.) should be provided to the Geotechnical Engineering Bureau prior to the meeting so site specific recommendations can be discussed.

Site Discussions:

A. The Geologists will discuss rock slope and catchment requirements for each site.

B. The Engineer-In-Charge and Designer will discuss the project’s constraints.

C. If the rock slope and catchment requirements are within the project’s constraints, recommendations will be made.

D. If the rock slope and catchment requirements are not within the project’s constraints, alternatives will be discussed.

E. Additional time may be required to develop additional alternates.

Additional discussions/ field meetings may be held, if warranted.

Step 10:

Based on the site discussions, constraints, slope analyses and after reaching verbal agreement with the Region, the Geotechnical Engineering Bureau will provide written Rock Slope and Catchment Recommendations for each site.

REFERENCES

1. Rockfall Catchment Area Design Guide, Final Report, SPR-3(032), Oregon Department of Transportation, FHWA-OR-RD-02-04, January, 2002.

2. The Nature of Rockfall as the Basis for a New Fallout Area Design Criteria for 0.25:1 Slopes, Engineering Geology Group Oregon Department of Transportation, FHWA-OR-GT-92-05, September, 1994.

3. Evaluation of Rockfall and Its Control by Arthur M. Ritchie, Washington State Highway Commission, Highway Research Record, Vol. 17: 1963.

4. Rock Slope Rating Procedure, Geotechnical Engineering Bureau, Geotechnical Engineering Manual GEM-15, June, 1996.

5. Rock Slopes: Design, Excavation, Stabilization, FHWA-TS-89-045, September, 1989.

6. Colorado Rockfall Simulation Program Version 4.0 (for Windows), Sponsored by Colorado Department of Transportation, March 2000.

NEW YORK STATE DEPARTMENT OF TRANSPORTATION