Prefeasibility Access Road Design-Revised
15
The inclusion of two 12 ft. driving lanes with 4 ft. shoulders provides for safe travel
of two way traffic. The 6:1 side slopes allow a driver who leaves the road an
opportunity to correct their path of travel without losing control. Constructing a flat
bottom ditch not only provides snow storage capacity, but improves sight distance
as well. This feature also provides a location which is out of the traveled way for
vehicles which may break down. The typical section also provides a minimum of 32
ft. clear zone for the safety of vehicles which leave the road.
Two additional sections will be used to reduce the area of impact in areas of large
cuts and fills. For areas with cuts over five feet, the flat bottom ditch will be
increased to 20 feet and the back slopes will be steepened to a maximum of 2:1.
Increasing the flat bottom ditch to 20 feet will provide adequate snow storage as
well as maintain sight distance. This typical section is shown in Exhibit D-3.
In areas of large fills, the section will include a 10:1 slope to the 30’ clear zone and
then will be allowed to steepen to 3:1 slopes until it meets existing ground. Guard
rail will be installed to stop errant vehicles prior to entering the area of 3:1 slopes
where control will not be able to be maintained.
Roadway horizontal and vertical geometry is important to the overall safety of the
road. The horizontal and vertical geometry of the road has been designed to meet
40 mph requirements. The signed speed limit will be determined after the
construction of road. It is expected that the signed speed limit will be 30 to 35 mph.
With the road being constructed in mountainous terrain, the grades which can be
maintained become important. Although the maximum allowable grade for a rural
collector at the proposed design speed is 11% according to AASHTO, a design grade
of 6% or less has been used as design criteria. This will ensure safe travel for the
loaded trucks as well as other users given the mountain terrain and winter
conditions.
The existing alignments of both Miller Creek Road and Warren Peak Road have areas
of concern for grades and curve radii. The proposed alignments will eliminate these
areas which do not meet the design criteria. Changes in alignment were required on
both routes to meet the design criteria. Permanent right of way and construction
easements will be required to be negotiated for these changes. The changes
required on the Warren Peak route will add substantial area that will be disturbed.
The Miller Creek route has less significant changes.
The design alignment for the Miller Creek Road does not have any areas that exceed
6% grade, and approximately 80% of the route is below the maximum grade
( maximum grade considered to be 5½ to 6%). The longest continuous stretch of
roadway that is at maximum grade is about 7,000’. The overall grade of the Miller
Creak Road route is much more gradual from the mine site to US HWY 14 than the
Warren Peak Road route.
