NEW MEXICO STATE UNIVERSITY · 2017-02-01 · New Mexico State University (NMSU) ... xSlope inward...

NEW MEXICO STATE

UNIVERSITY

A - MOUNTAIN OBSERVATORY SERVICE ROAD

EROSION EVALUATION

FINAL REPORT OF FINDINGS

February, 2016

NMSU A-Mountain Observatory Service Road

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1.0 INTRODUCTION

The service road to the A-Mountain (Tortugas Mountain) is used to access NMSU’s observatory. The route is also used once a year by the faithful who ascend the mountain as a part of a religious celebration that has been ongoing for more than 100 years. The road is also used daily by hikers in the area.

New Mexico State University (NMSU) maintains the condition of the road and is currently gated at several locations as a means of protecting the facilities on A-Mountain. The area is owned by NMSU and the United State of America. Recent rain storms have eroded the road in several areas and the road condition and the continual maintenance has become a concern. NMSU is investigating options to provide a service road that does not require constant maintenance. Thusly, NMSU is pursuing a more permanent solution to the erosion issues.

The limit of this evaluation is from Station 2+00 to 19+00 as shown on Figure1.1 in Appendix A.


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2.0 EXISTING CONDITIONS

This service road is narrow, steep and is intended for intermittent use, therefore all-weather access is not a primary concern. The road width varies from 10’ to 16’ and in areas is more restrictive do to the erosion that has occurred. The average longitudinal slope or grade of the road is approximately 14%, see profile on Figure 2.1 in Appendix A.

The road cross slope, which plays a big factor in managing storm water run-off, varies from sloping toward the cut slope (inward) or away from the cut slope (outward), see Figure 2.2in Appendix A.

There is a section of the roadway which currently has guardrail protecting the fill slope side from vehicles driving off the edge. Portions of the existing guard rail is damaged and not anchored correctly, as shown in photo below.


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3.0 EXISTING DRAINAGE

The drainage basins are relatively small, less than 1 acre, see basins on Figure 3.1 in Appendix A. The service road meanders along the hill side which makes the drainage basin flow path steep. An evaluation of the approximate flow rates that would occur on these basins was needed to help in recommending the appropriate improvements. The rational method for computing flow rates was used to determine flow rate for a 100-year storm event.Because of the size and steepness of the basin a time of concentration of 10 minutes was used in our methodology. Few other aspects of this road design is more important than managing the surface drainage.

3.1 Drainage Basin Flow Rates

The drainage basins were defined in manageable segments in anticipation of placing culverts to outlet to the other side of the roadway. Conveying the drainage off the roadway at frequent locations helps control, roadbed erosion, maintain a stable road surface and


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reduce future maintenance and repairs. The flows rates calculated (Q=CIA) for these basins are as follows (see Appendix B):

Basin 1 has a Q100 = 0.72 cubic feet per second (cfs)Basin 2 has a Q100 = 1.05 cfsBasin 3 has a Q100 = 1.27 cfsBasin 4 has a Q100 = 1.03 cfsBasin 5 has a Q100 = 0.997 cfsBasin 6 has a Q100 = 1.33 cfsBasin 7 has a Q100 = 1.50 cfs

3.2 Existing Culverts

There are several existing 12” diameter or smaller culverts that convey storm water from a bar-ditch, against the hillside, beneath and across the roadway. These culverts are currently clogged and not working properly. When these culvert do not work correctly the water overtops the bar-ditch and crosses the road at a concentrated point, which is highlyerosive.


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3.3 Additional Culverts

Additional culverts should be considered to outlet flow more frequently. This will keep the roadside ditch from overtopping and reduce the erosion within the ditch. The basins described above should all have culverts placed at the downstream end of the basin.

All existing culverts should be replaced in favor of the larger 18” culvert for ease of maintenance. Total number of culverts are determined based on the capacity of the bar-ditch. Estimated total number of culverts needed would be seven (7).

In lieu of a continuous bar-ditch a drainage pipe can be placed, with open ends, along the road with outlets to cross the road that is priced in the repair with storm drain option.

4.0 ROAD

The existing longitudinal slope for this roadway varies, and on average is about 14%. At this grade water run-off velocities are more than ideal for managing erosion. The existing cross slope of the road varies from inward to outward and should be managed such that the road is self-draining. This refers to the purposeful shaping of the road from the cut side to the fill side of the roadway section. This allows surface storm water to flow across and off the roadway.

4.1 Road Slope or Grade

The longitudinal slope or grade of the roadway is established and major changes would require also require horizontal changes which is beyond the purpose of this evaluation. Design guide lines for this type of facilities are vague. Grades of 14% are acceptable for low volume use and if constructed out of crushed aggregate. NMSU, in some areas, has used asphalt millings which works well and could be used throughout the entire length.

4.2 Road Width

The minimum width of the roadway to be considered should be 12 to 14 feet for one-way traffic. The 14’ road width was modeled for quantities with a 2’ shoulder on the outward side. This additional 2’ also provides for some erosion loss off.

4.3 Road Side Slopes

The road cross slope or crown should be carefully considered to provide surface drainage of the roadway. The surface water from all sources must be conveyed off the roadway at frequent locations to control roadbed soil erosion. Options for road slope are as follows:

Slope inward towards the bar-ditch Slope outward toward the fill slope.Crowned so that surface drainage is split.


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All road-cuts and road-fills should have slopes that are stable for the particular soil and conditions. Given the rocky material cut slopes could be 1 horizontal to 1 vertical (1:1) and the fill slope could be as steep as 1.5 horizontal to 1 vertical (1.5:1). These slopes would also provide minimal impact to the overall roadway width.

5.0 ROAD COSTS

Road construction costs are most influenced by the standard of the road build, particularly road width and type of surfacing, and the steepness of the terrain. Placing a road with cuts and fills on steep cross slopes greatly increases the time of construction, the amount of excavation and earthwork and the areas of clearing. Drainage is important factor in the frequency of future road maintenance. Costs associated with drainage improvements could be significant.

The existing roadway is to be remain in the approximate location eliminating much of the initial earthwork costs associated with developing a new roadway.

As a minimum the existing culverts and general shape of the roadway should be addressed. This includes culvert cleaning, re-grading the bar-ditch to provide capacity,repairing and re-anchoring the guard rail, shaping the roadway (cross slope) and platingthe surface with millings.

Cost are presented in three (3) options:

1. Regrade the road to the proper width to provide a minimum 12’ driving lane and 2’shoulder in the area for the guardrail, utilizing NMSU asphalt millings to surface the road. Clean the existing culverts and construct additional culverts to prevent drainage overtopping the roadway. Remove and salvage the guardrail W-beam and reinstall with steel posts.

2. Same as 1 with the addition of a longitudinal storm drain with open ends to collect runoff and discharge to the existing and new culvert crossing.

3. Regrade the road to the proper width to provide a minimum 12’ driving lane and 2’shoulder in the area for the guardrail. Construct 6” thick base course surface for the entire width and length of the roadway. Clean the existing culverts and construct additional culverts to prevent drainage overtopping the roadway. Remove existing guardrail and replace with all new guardrail system.

Cost are developed for each improvement in the order of constructability.


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1) Engineer's Opinion of Probable Construction Cost for Repair Guard Rail & Construction Additional Culverts

Item Description Unit Estimated Quantity

Estimated Unit Cost

Estimated Total Cost

1 Clean existing culverts LF 80 $50.00 $ 4,000

2 Bar-ditch Earthwork, unclassified excavation, complete in place CY 1000 $13.00 $ 13,000

3 Subgrade Prep, complete in place SY 2650 $4.00 $ 10,600

4 18" Storm Culverts with end sections, complete in place LF 75 $110.00 $ 8,250

5 Guard Rail repair and re-anchor, Station 2+50 to 16+50 LF 1400 $22.00 $ 30,800

6 Placement of asphalt millings from BOP to EOP SY 2650 $7.00 $ 18,550

Construction Sub Total $ 81,200 7 NMGRT Construction % 6.7500% $ 5,481 Construction Total $ 86,681

Soft Costs 8 Engineering & geotechnical $ 10,870 9 NMGRT Construction % 8.3125% $ 904 Soft Cost Total $ 11,774 Project Total $ 98,455


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2) Engineer's Opinion of Probable Construction Cost for Repair Guard Rail, Construct Storm Drain Pipe and Construct Additional Culverts

Item No Item Description Unit

Estimated Quantity

Estimated Unit Cost


1 Clean existing culverts LF 80 $50.00 $ 4,000

2 Bar-ditch Earthwork, unclassified excavation, complete in place CY 200 $13.00 $ 2,600


418" Storm Drain along road with open lnlets, complete in place LF 900 $110.00 $ 99,000


6 Guard Rail repair and re-anchor, Station 2+50 to 16+50 LF 1400 $22.00 $ 30,800

7Placement of asphalt millings from BOP to EOP SY 2650 $7.00 $ 18,550


Soft Costs 9 Engineering & geotechnical $ 19,730

10 NMGRT Construction % 8.3125% $ 1,640 Soft Cost Total $ 21,370

Project Total $ 202,632


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3) Engineer's Opinion of Probable Construction Cost for Replacement of Guard Rail and Construct New Culverts

Item No Item Description Unit

Estimated Quantity

Estimated Unit Cost


1 Earthwork, unclassified excavation, complete in place CY 3500 $13.00 $ 45,500



4 6" Crushed Aggregate Base Course, complete in place SY 2650 $18.00 $ 47,700

5 Guard Rail replacement, Station 2+50 to 16+50, complete in place LF 1400 $35.00 $ 49,000


Soft Costs 7 Engineering, Surveying, geotechnical $ 26,455 8 NMGRT Construction % 8.3125% $ 2,199 Soft Cost Total $ 28,654

Project Total $ 212,317

7.0 RECOMMENDATION

Controlling the drainage across the roadway will help with the erosion and a recommended cross slope of 3% continuous throughout. The cross slope is be sloped outward as shown in figure 7.1 of Appendix A. A well-defined drainage swale on the inward slope, or toe of the cut slope, will help to control off site concentrated flows from crossing the road. Culvert placement at the downstream end of each drainage basin, see figure 3.1 & 7.1 of Appendix A, is also recommended to remove the storm water from that side of the roadway.

A 12’ to 14’ wide roadway section is recommended with a 2’ wide shoulder on the outward slope for guardrail placement. The guardrail should be replaced or repaired and would require removal in order to provide a shoulder.

These recommendations could be completed as repairs or complete reconstruction depending on budgetary constraints.


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APPENDIX A – FIGURES

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Proposed Culvert LocationsFigure 3.1

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APPENDIX B - CALCULATIONS

NEW MEXICO STATE UNIVERSITY · 2017-02-01 · New Mexico State University (NMSU) ... xSlope inward...

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Transcript of NEW MEXICO STATE UNIVERSITY · 2017-02-01 · New Mexico State University (NMSU) ... xSlope inward...