Backfill depleted open-pit mines with lined landfills, tailings, and heap leach pads

By Allan J. Breitenbach

Abstract

Landfills, tailings impoundments, and heap leach pads are the largest and highest geomembrane-lined fill structures in the world. These lined structures require large areas for storage and containment of solid wastes, precious or base metal mill waste tailings, and ore heap fill materials.

Today, municipal waste landfills are considering the use of depleted open-pit mines in remote areas to allow for more efficient use of nearby city land space and natural resources. Lined tailings impoundments and leach pads could also be located within depleted mine pit excavations for reduced overall mine site disturbance.

This article discusses the recent historic use and primary engineering concerns and benefits in lining, backfilling, and operating depleted open-pit mine excavations for containment of solid waste, tailings, and ore heap fills.

Introduction

The major mine disturbance areas related to open-pit operations include the excavated mine pit limits, the surrounding mine waste dump piles from overburden (non-ore) stripping excavations, and the tailings impoundment or heap leach facilities.

Tailings impoundments and leach pads are typically lined. A partial or complete backfilling of any depleted mine pit areas with these lined facilities, where practical, would significantly decrease the overall mine disturbance areas resulting in lower reclamation costs at closure. The post-mining backfill of open-pit mine excavations with lined solid waste landfills is a relatively new concept as well, beginning in the late 1990s.

The steep pit wall excavation slopes and the natural groundwater conditions above the mine pit bottom limits are the two greatest engineering design challenges to consider in lining and backfilling an open-pit mine excavation. Recently, several mine pits have been lined and backfilled for solid waste landfill and tailings impoundment slurry disposal operations. Lined mine pit heap leach designs have been considered in the past; however, there are no known lined mine pit heaps being constructed currently. An example of an active open-pit mine with surrounding waste piles, tailing impoundment, and leach pad facilities is shown in Photo 1.

This article will present case history examples of recent lined and backfilled mine pits for solid waste landfill and tailings impoundment disposal, as well as the general engineering design considerations for potential backfilling of lined mine pits for waste disposal and ore heap leach operations.

Case histories of lined facilities in mine pits

History

Open-pit mines have historically been left in an open condition during operations to closure, unless unstable wall conditions warranted partial backfilling to complete the pit ore excavations.

In some cases, the pit bottom limits were partially backfilled to above the natural groundwater level, where practical, to prevent ponding of water at closure or to stabilize waste dump slopes around the pit wall limits. Most open-pit walls are constructed to a safety factor of 1.0 to extract as much ore from the ground with the least amount of stripping to expose the ore body.

The backfilling of mine pits with lined landfills, tailings impoundments, and heap leach pads, where practical, would significantly reduce the mine disturbance area and related reclamation closure costs. In addition, mine pit backfilling makes efficient use of the excavated storage space with full facility containment within the natural ground vs. constructing above-ground dams, site grading fills, and diversion channels for facility containment. Known case histories of lined mine pit facilities by this author are presented in this section.

Lined landfills for mine pit backfill

Lined landfill operations in the 1980s included numerous excavated cells constructed below ground level and lined with a geomembrane liner, a clay soil liner, or a combination of both geomembrane and clay liner as a composite liner system for the disposal of solid wastes.

Excavated slopes were generally flattened as required for placement of the compacted low permeability clay soil liner. The excavated cell side slopes were steepened in the 1990s to present day, where geosynthetic clay liners (GCL) began to be accepted as an equivalent or better replacement to the clay soil liner. A steepened GCL and geomembrane lined valley wall slope with a flatter conventional clayey soil and liner at the base of the steep slope are shown in Photo 2.

The first abandoned open-pit mine quarry excavation to be lined and backfilled with municipal solid waste was the Bristol Landfill in Bristol, Va. The open-pit quarry included near vertical bedrock walls more than 300ft (100m) high. A mine pit haul road ramp extended from the mine pit rim to the pit bottom for truck access and removal of excavated rock materials, until the mine operations ended sometime before 1990. The mine pit quarry was converted to a lined landfill operation by 1998, as shown in Photo 3.

The near vertical rugged rock quarry pit walls were the most extreme engineering challenge known to date for placement of a geomembrane liner system. The rock walls were pre-scaled of loose rock debris and covered with safety wire mesh screen in 1996 and 1997 to prevent rock falls during liner construction and to anchor the liner system. A layer of geotextile fabric and HDPE geomembrane liner were placed on the lower pit side walls with plans to extend the pit wall liner upward in phases to maintain fully lined conditions above the rising active landfill surface. The mine pit floor was backfilled with a low permeability clayey soil for a conventional landfill bottom composite liner and overlying leachate drainage system.

Lined tailings impoundment for mine pit backfill

Since the 1980s, several underground mines have been backfilled with tailings backfill for economic, safety, or mine closure reasons. Tailings backfill in completed underground mine workings included paste or thickened tailings materials mixed with cement and other stabilizing additives, which reduced the required amount of tailings stored in above-ground impoundment facilities.

Historically, numerous open-pit mines, natural lakes, and seacoast areas have been backfilled with unlined tailings disposal as well. More mines are adopting the use of compacted earth and rock fill dams with geomembrane liner systems for tailings disposal with long-term containment and improved protection of baseline groundwater conditions. Lined tailings impoundment containment within mine waste piles has been common practice at several open-pit mines in Nevada since the early 1990s. However, open-pit mines have not historically been used for lined tailings impoundment disposal until recently. An example of a conventional above-ground lined tailings impoundment contained by compacted earth fill dams in the mid-1980s is shown in Photo 4.

The first geomembrane-lined mine pit backfilled with conventional tailings was the El Valle mine pit located in Asturias in northern Spain. The gold mine pit was depleted of ore adjacent to other ongoing nearby active mine pit operations by 2003.

In 2004, the bottom portion of the 500–1,700ft (152–518m) deep mine pit was backfilled to above the existing groundwater conditions with a low permeability clayey waste rock site grading fill in preparation for geomembrane liner placement. The clayey mine waste materials were taken from local mine stripping operations to expose the deeper ore materials. The El Valle tailings impoundment at startup of tailings disposal operations is shown in Photo 5. A conventional above-ground, lined tailings impoundment is shown in Photo 6.

The clayey site grading fill in the mine pit bottom limits allowed for dry construction liner installation above the existing or dewatered mine pit ground water levels. Sufficient compacted clayey subgrade fill was placed adjacent to the steep pit walls at startup to allow for perimeter access roads and future lined tailings impoundment expansion raises.

A woven geotextile fabric was placed between the geomembrane liner and the clayey rock and soil subgrade to cushion the liner from puncture on the occasional larger, cobble-sized rocks. The tailings impoundment liner consisted of a 60-mil (1.5-mm) HDPE liner. Tailings disposal within the geomembrane lined impoundment commenced in 2005 with conventional slurry tailings disposal.

Lined leach pads for mine pit backfill

This author is aware of only one lined mine pit leach pad operation, to date, that was constructed within the open-pit limit. The lined pad was constructed in 1984 on a relatively small scale as a pilot test pad in southwestern New Mexico, USA. According to mine personnel, the test pad was located in a depleted side pit wall bench area within a larger copper mine pit limit and lined with 80-mil (2.0- mm) HDPE liner.

The geomembrane liner was covered with about 5 million tons of low-grade run-of-mine ore dump fill and included sufficient area downhill of the lined pad limits for gravity leach solution drainage to an external lined process pond sump.

Since the 1984 test pad construction, no known lined and backfilled mine pit leach pads have been constructed at the bottom of depleted open-pit mine excavations. However, several copper mines in New Mexico and Arizona are considering this option, particularly where mine conditions indicate it is economic to construct for both operations and closure.

Liner design considerations for in-pit backfilling

General

Primary engineering concerns in lining, backfilling, and operating a depleted open-pit mine for containment of waste fill or ore heap materials include:

• installation and protection of the liner below natural groundwater conditions.
• stabilizing any steep pit rock wall slopes that are near a safety factor of 1.

Major benefits of backfilling with a lined facility:

• overall reduction in required liner area for storage of materials.
• minimal risk of spills with the elimination of above-ground containment dams and watershed diversions (particularly in high seismic earthquake zones).
• significant reduction in overall mine disturbance areas for less reclamation and closure costs.

The in-pit liner containment becomes more practical and cost effective if included early in the operation plans to allow use of nearby stripped mine waste materials for bottom pit site grading preparation and steep pit wall stabilization for liner placement.

The tailings impoundments and leach pad facilities are generally associated with open-pit mining operations and located in close proximity to the excavated mine pit and stripped non-ore mine waste pile limits. As the open-pit mine is developed, some mine sites have depleted open-pit ore zone pockets or multiple open-pit sites in close proximity to each other that may be amenable to lining and backfilling for tailings disposal or ore heap leaching.

The overall steep pit wall slopes of 35–55 degrees with benches in most hard rock mining operations create an engineering challenge for liner systems. Waste rock materials from continued mine overburden stripping operations can provide economic site grading fill to stabilize the floor and pit walls for dry geomembrane liner installation with the liner system protecting the underlying groundwater conditions.

The pit wall stability would continue to improve, as lined backfill operations buttress and bury the exposed mine pit wall slopes. Each type of geomembrane lined facility has differing engineering concerns.

Lined landfill for in-pit backfill

Municipal solid waste landfills typically require a robust multiple liner system for leachate containment, collection, and recovery operations. A drain fill cover and daily solid waste soil cover or temporary synthetic geotextile cover are common for solid waste disposal.

Beginning 10-15 years ago, landfills are applying irrigated water or recirculated leachate flows to the top surface or by deep well injection to accelerate settlement, waste biodegradation, and methane gas collection (Breitenbach and Thiel, 2005). This may require multiple cells and graded gravity flow to sump-pump collection locations at the bottom of the landfill for recirculation throughout the life of the facilities.

Landfill liner systems prefer dry ground conditions with deep groundwater levels for no direct connection and transport of any leachate contamination away from the lined facilities. Therefore, most open-pit mines would require some type of continuous groundwater dewatering and monitoring system beneath an in-pit liner or the option of mine waste site grading fill to raise the liner subgrade above the seasonal high natural groundwater level conditions.

In general, lined solid waste landfill for in-pit backfill includes the following major engineering concerns:

• In general, lined solid waste landfill for in-pit backfill includes the following major engineering concerns:
• stable mine pit walls for liner stability and safe access to dispose of waste materials.
• protection of the liner system from differential subgrade settlement or puncture.
• protection of the liner system from overlying waste material placement and puncture with adequate bottom and side wall drain fill or geotextile cover.
• minimizing hydraulic heads on the bottom liner system with a leachate collection and recirculation sump pump system.
• designing deep sump well systems with redundancy and protection of the liner from the “pile driving effect” of vertical well down-drag forces during waste settlement (side wall wells along the liner slope are optional).

Lined tailings impoundment for in-pit backfill

Lined tailings impoundment may include storage of several types of tailings waste materials: conventional slurry disposal (about 45-55% solids to water by weight in a liquefied pipeline slurry discharge), thickened tails slurry disposal (about 60-70% solids to water for less water pool recirculation back to the plant), and other variations of dry filter and paste tailings transported by truck, conveyor, or positive displacement pipeline pumping disposal to the lined impoundment.

Pipeline slurry disposal around the perimeter of the lined impoundment is the most common practice in the mining industry with milled and depleted waste tailings generally consisting of fine grained sand, silt, and clay particles. The fine tailings materials form a perimeter sand and silt beach with settled solids forming a water pool in the interior. Decanted water from the water pool is recirculated back to the plant for reuse in operations.

Typically, lined tailings impoundment for in-pit backfill includes the following major engineering concerns:

• dry subgrade conditions for liner construction with any required dewatering to maintain the operational tailings beach and water pool surface above the subgrade groundwater level at all times to closure.
• liner subgrade backfill above bottom groundwater conditions or temporary dewatering is optional until the lined tailings backfill is raised above the groundwater level.
• stable mine pit walls for liner stability and safe access to dispose of waste materials.
• protection of the liner system from differential subgrade settlement or puncture.
• partial drain cover above the liner to minimize hydraulic heads on the bottom liner system (optional for maximizing tailings drainage and consolidation) with a bottom leachate collection and recirculation sump pump system.
• design the optional deep sump well systems with redundancy and protection of the liner from vertical well down-drag forces during waste settlement (side wall wells along the liner slope are optional).

Conclusions

Open-pit mines have seen recent use of the below-ground excavation limits for storage of solid waste landfill, tailings, and ore heap leach fills.

Primary engineering concerns in lining, backfilling, and operating a depleted open-pit mine for containment of waste or ore heap fill materials include:

• providing dry construction conditions for installation and backfilling of the liner system below the natural groundwater conditions.
• stabilizing typical 35-55 degree steep hard rock mining pit wall slopes that are near a safety factor of 1.

Major benefits of using lined facilities for mine pit backfill include:

• an overall reduction in the required liner area for storage of materials.
• minimal risk of spills (particularly in high seismic earthquake zones) with the elimination of above ground containment dams.
• significant reduction in overall mine disturbance areas for less reclamation and closure costs.

The in-pit containment becomes more practical and cost effective, if included in the operation plans for early use of nearby stripped mine waste materials for site grading preparation and pit wall stabilization during the life of the mine.

Allan Breitenbach, P.E., is a geotechnical engineer based in the Golden, Colo., office of Vector Engineering Inc.

References

Breitenbach, A.J. and Thiel, R.S. (2005), “A Tale of Two Conditions: Heap Leach Pad Versus Landfill Liner Strengths,” GRI-19 Geosynthetics 2005 Conference, Las Vegas, Nev.