In lieu of the requirements of 19.8.20.2035 and 2036 NMAC,
the director may approve alternate methods of disposal of hard rock spoil,
including fill placement by dumping in a single lift, on a site specific basis,
provided the services of a registered professional engineer experienced in the
design and construction of earth and rockfill embankments are utilized and
provided the requirements of 19.8.20 NMAC and
19.8.20.2034 NMAC are met. For
19.8.20 NMAC, hard rock spoil shall be defined as rockfill consisting of at
least 80 percent by volume of sandstone, limestone, or other rocks that do not
slake in water. Resistance of the hard rock spoil to slaking shall be
determined by using a slake index and slake durability tests approved by the
director.
A. Spoil is to be transported
and placed in a specified and controlled manner, which will ensure stability of
the fill.
(1) The method of spoil placement
shall be designed to ensure mass stability and prevent mass movement in
accordance with the additional requirements of 19.
8.20 NMAC.
(2) Loads of noncemented clay shale and/or
clay spoil in the fill shall be mixed with hard rock spoil in a controlled
manner to limit on a unit basis concentrations of noncemented clay shale and
clay in the fill. Such materials shall comprise no more than 20 percent of the
fill volume as determined by tests performed by a registered professional
engineer and approved by the director.
B. Stability analyses.
(1) Stability analyses shall be made by the
registered professional engineer. Parameters used in the stability analyses
shall be based on adequate field reconnaissance, subsurface investigations,
including borings and laboratory tests.
(2) The embankment which constitutes the
valley fill or head-of-hollow fill shall be designed with the following factors
of safety.
Case Design Condition Minimum Factor of
Safety
I End of construction 1.5
II Earthquake 1.1
C. The design of a head-of-hollow fill shall
include an internal drainage system to the extent necessary to ensure continued
free drainage of anticipated seepage from precipitation and from springs or wet
weather seeps.
(1) Anticipated discharge from
springs and seeps and due to precipitation shall be based on records and/or
field investigations to determine seasonal variation. The design of the
internal drainage system shall be based on the maximum anticipated
discharge.
(2) All granular
material used for the drainage system shall be free of clay and consist of
durable particles such as natural sands and gravel, sandstone, limestone of
other durable rock which will not slake in water.
(3) The internal drain shall be protected by
a properly designed filter system.
D. Surface water runoff from the areas
adjacent to and above the fill shall not be allowed to flow onto the fill and
shall be diverted into stabilized channels which are designed to pass safely
the runoff from a 100-year, 24-hour precipitation event. Diversion design shall
comply with the requirements of Subsection F of
19.8.20.2011 NMAC.
E. The top surface of the completed fill
shall be graded such that the final slope after settlement will be no steeper
than 1v:20h (5 percent) toward properly designed drainage channels in natural
ground along the periphery of the fill. Surface runoff from the top surface of
the fill shall not be allowed to flow over the outslope of the fill.
F. Surface runoff from the outslope of the
fill shall be diverted off the fill to properly designed channels which will
pass safely a 100-year, 24-hour precipitation event. Diversion design shall
comply with the requirements of Subsection F of
19.8.20.2011 NMAC.
G. Terraces or other suitable controls, such
as berms, contour furrows, or micro-depressions, shall be constructed on the
outslope if required for control of erosion, or terraces for construction of
roads included in the approved postmining land use plan. Terraces shall meet
the following requirements:
(1) the slope of
the outslope between terrace benches shall not exceed 1v:2h (50
percent);
(2) to control surface
runoff, each terrace bench shall be graded to a slope of 1v:20h (5 percent)
toward the embankment; runoff shall be collected by a ditch along the
intersection of each terrace bench and the outslope;
(3) terrace ditches shall have a 5 percent
slope toward the channels specified in Subsection F of
19.8.20.2037 NMAC, unless steeper
slopes are necessary in conjunction with approved roads.
