Underhand Cut and Fill Mining as Practiced in Three Deep Hard Rock Mines 

in the United States 

Ted J. Williams and Tom M. Brady 

Spokane Research Laboratory, NIOSH 

Doug C. Bayer 

Hecla Mining Company 

Mark J. Bren 

U.S. Silver Corporation 

Rimas T. Pakalnis 

University of British Columbia 

John A. Marjerison 

Stillwater Mining Company 

Rad B. Langston 

Consultant 

Abstract 

Underhand cut and fill mining methods are used at the Lucky Friday, Stillwater, and Galena mines in the western US 

in an effort to safely mine in difficult ground conditions and as a primary mining method where access below the ore 

is limited. The method has evolved at each mine to meet the unique situations associated with their milling procedure 

and mine layout. Details of the operational aspects of the backfilling and mining cycles at each mine are presented in 

this paper, accompanied by a summary of the mechanical properties of the backfill and its in-situ geomechanical 

behavior. 

1.0 Introduction 

The introduction of underhand cut and fill mining in the 1980s made hard-rock mining in 

hazardous ground conditions safer (Marcinyshyn, 1996). Underhand mining uses cemented mill 

tailings for backfilling the mined-out stope cut, making the mining process in the following cuts 

safer because the miners are always working beneath a reinforced cemented-backfill that will not 

fall during a rockburst. In 1980, Hecla Mining Company, in cooperation with the US Bureau of 

Mines’ (USBM) Spokane Research Center, conducted a test of underhand mining to recover a 
potentially rockburst-prone sill pillar at the Star Mine. The object of this test was also to protect 

miners from injuries caused by rock falling from the back (or roof) during a rockburst. This test 

demonstrated that underhand mining could work, but operational aspects of the method needed 

improvement. The areas in need of improvement were cemented backfill delivery and fill 

mat/support construction in the stope prior to backfilling. The cemented backfill delivery system 

used at the Star Mine delivered material to the stope in a medium-density slurry in which the fine 

cement particles were suspended in the excess water which was then decanted over the sand wall, 

resulting in inconsistent fill strengths. The sill mat construction technique was labor and material 

intensive. 

Hecla began another test titled “Lucky Friday Underhand Longwall” (LFUL) in 1985 which also 

involved the USBM and the University of Idaho (Werner 1990, Brackebusch 1994). In this test, 

Hecla developed a paste-type backfill with no free water so that cement fines would not be 

decanted from the stope as it was filled. This research developed the vertically-placed #7 

DYWIDAG bolt method, which is used for cemented mill-tailings backfill reinforcement in most 

mines today. This method reduced the time and cost of preparing the stope for cemented backfill 

significantly and made the underhand method a viable alternative to traditional overhand mining 

methods for steeply dipping veins. 

The underhand design differs from the overhand cut and fill design (Figure 1) in that it requires 

the vertical installation of DYWIDAG rock bolts in loose rock on the floor of the mined-out 

stope cut in a predetermined pattern prior to backfilling the cut with cemented mill tailings. 

Figure 1. Comparison of underhand and overhand mining methods. 

The 

next cut is mined beneath this fill, which becomes a reinforced concrete beam as the stope walls 

close upon it. The closure induces loads in the fill which are transferred to the vertical bolts as the 

fill deforms. The most important aspect of this method is the maintenance of a consistent fill 

quality; this includes percentage of solids, cement content, and placement. The proper 

combination of these factors is required so that the cemented tailings will develop enough 

strength to protect the miners working under it. 

The underhand method effectively mitigates ground-fall injuries because the backfill created is 

capable of withstanding the shock of a rockburst, as demonstrated by a 3.5 Richter magnitude 
rockburst that occurred with blasting at the Lucky Friday Mine, Figure 2.

Figure 2. Underhand stope survives 3.5 ML rockburst. 

 The reinforced 

cemented fill in the back survived with some bagging while the fly rock from the walls was 

restrained by the chain link installed along the stope walls. Prior to converting to underhand 

mining, the Lucky Friday Mine experienced three rockburst-related fatalities in four years. 

However, in the 20 years since converting, a rockburst-related fatality has not occurred at the 

mine. In the Gold Hunter portion of the mine, underhand mining is now used by Hecla for 

production below the main access level. This allows for resource recovery without the expense of 

sinking the shaft. In 2001, as mining depths increased and ground conditions became more 

challenging, the Stillwater Mine also began using underhand cut and fill mining, consequently 

improving miner safety by reducing possible exposure to poor ground conditions and falling 

rock-related injuries—with the exception of the periods during initial sill development (Jordan et 

al., 2004). 

Also in 2001, personnel from the Spokane Research Laboratory of the US Bureau of Mines’ 

successor, the National Institute for Occupational Safety and Health (NIOSH), assisted the 

Galena Mine with the introduction of mechanized underhand cut and fill mining. The goal was to 

recover ore beneath the lowest level of the mine in an area where rockbursts had occurred and 

ground conditions were very poor. The initial stope was mined using the underhand method from 

the 5,500 level downward on a 70o

 dipping vein. When this stope proved successful, the mine 

began using the underhand method to recover other rockburst-prone sill pillars, which are 

typically conventional slusher-stopes with a 5-cap access raise or mechanized LHD stopes with 

ramp access. 

Due to the early efforts of the USBM and the present efforts of NIOSH, there are now twelve 

mines in North America that use the underhand method when mining in difficult ground 

conditions (Pakalnis et al., 2006). All of the mines use either cemented mill tailings or cemented 

rock fill for backfilling, (Tesarick et al., 2003). 

This paper discusses the similarities and differences between the underhand cut and fill mining 

methods used at the Lucky Friday, Stillwater, and Galena Mines in the northwestern US (Figure 

3). Each mine has developed its underhand mining system to accommodate the unique 

circumstances of their respective mining sites. This discussion includes descriptions of each 

mines’ backfill properties, delivery systems, stope preparation procedures for backfill, practices 
employed to mine the stopes under the backfill, and the geomechanics of the stope wall and 

backfill. 

Figure 3. Location of the Galena, Lucky Friday and Stillwater mines. 

2.0 Studies of Underhand Methodologies at Three Western Mines 

2.1 Backfill Properties and Delivery

The backfill properties vary significantly between the Galena, Lucky Friday, and Stillwater mines 

(Table 1). The Lucky Friday Mine uses a tailing paste fill with no free water, and 8–10% cement 

that develops 2,070 kPa (300 psi) of unconfined compressive strength (UCS) after curing for 

seven days. The fill is delivered to the stope at a rate of 118 t/hr (130 stph), or 64 m3/hr (85 

yd3

/hr) (Table 2). 

Similarly, the Stillwater Mine uses a total tailings paste fill with 10–12% cement that develops 

586 kPa (85 psi) of UCS after seven days. The lower strength of the Stillwater fill results from 

the large amount of -44 micron (-325 mesh) particles in the mill tailings. However, the fine 

particles allow the use of a lower velocity in the horizontal pipelines, 0.64 m/sec (2.1 ft/sec) 

versus 2.29 and 2.35 m/sec (7.5 and 7.7 ft/sec) at the other mines. This is because the fines hold 

the larger particles in suspension. 

Unlike the Lucky Friday and Stillwater mines, the Galena Mine uses a hydraulic fill which has 

free (excess) water. With the addition of 10% cement, the fill obtains a strength of 2,590 kPa 

(375 psi) after seven days. The excess water is decanted over the top of the sand wall after the 

cement has settled. Fines and cement retention behind the sand wall are critical to the Galena 

system. If too many fines and cement are lost, the resulting fill is weak. There are two 

explanations for this. One hypothesis suggests that the weakness results from an excess of intergranular space wherein the distance between particles is too great for the cement to bridge and 

maintain enough grain-to-grain bonds to produce adequate strength. Another possible explanation 

is that too much of the cement is washed over the wall and there is not enough left to develop 

sufficient bond strength. 
Table 1. Screen analysis of backfill material.

% of Tailings Passing Screen Size 

micron (mesh)

Screen Size 149 (100) 105 (140) 74 (200) 53 (270) 45 (325) 38 (400) 

Lucky Friday 79 65 52 40 34 27 

Stillwater 98 NA 70 62 61 NA 

Galena 75 64.9 49 34 NA 28 

Table 2. Backfill delivery system data.

Mine Fill rate, t/h 

(stph) 

Fill rate, 

m3/h 

3

Pipe diameter, 

cm (in) 

Line velocity, 

m/sec (ft/sec)

Cement, 

% 

Slurry density, 

% solids by 

Lucky 

Friday 118 (130 ) 64 (85) 15 (6) in shaft,

10 (4) on level

0.98 (3.1) 

2.4 (7.7) 8 to 9 86% 

Stillwater 54 to 64 

(60 to 70) 39 (51) 15 (6) 0.64 (2.1) 10 to 12 73% 

Galena 73 (80) 7.6 (3) 2.3 (7.5 ) 10 75% 

2.2 Stope Preparation

After the stope has been mined, all three mines place 0.3–0.46 m (1–1.5 ft.) of broken rock, or 

prep muck, on the floor of the stope. The prep muck serves two purposes. The first is to prevent 

the fill damage that occurs during blasting by providing a blast cushion as mining processes 

proceed beneath the fill. The second purpose is as a base to drive the vertically oriented 

DYWIDAG reinforcement bolt in to. 

The mines use different strategies for installing the rock bolt-bearing plates that reinforce the 

backfill. At the Lucky Friday and Galena mines, the present practice is to install 15x15 cm (6x6 

in) rock bolt bearing plates that have DYWIDAG nuts welded to them in a predetermined pattern 

on top of the prep muck. At the Stillwater Mine however, the 1.8 m (6 ft) DYWIDAG bolts are 

driven 15 cm (6 in) into the prep muck. 

The Lucky Friday and Galena mines next lay chain link with a 5 cm (2 in) aperture on top of the 

prep muck and plates. At the Lucky Friday, the chain link is run lengthwise down the stope. 

Meanwhile, at the Galena, the chain link is run in overlapping sections from wall to wall and 

attached with 0.48 m (18 in) split-set bolts driven into the lowest row of split-set bolts in the 

stope wall (Figure 4). Then, 1.8 m (6 ft) DYWIDAG bolts are inserted through the chain link and 

screwed into the nut and plate below the chain link fence material. Another 15x15 cm (6x6 in) 

bearing plate and nut are then added to the top of the bolts and then the bolts are wired to each 

other and the walls to hold everything in position during backfilling. The bolts at the Stillwater 

Mine are placed in a pattern that is 1.2 m (4 ft) on center across the potential back and 1.8 m (6 

ft) on center along the length of the stope cut. Bearing plates are then added and wire mesh is 

installed from wall to wall once the fill is mined under during the next stope cut. 
Figure 4. Installation of chain-link fence and reinforcement bolts at the Galena Mine. 

To provide additional fill support in wide areas, the Lucky Friday and Galena mines place 

wooden stringers with 1.8 m (6 ft) bolts through them across the width of the stope. In areas 

wider than 3.7 m (12 ft), the Stillwater uses 15 x 7.6 cm (6 x 3 in) steel channels with bolts 

through them on 0.9 m (3 ft) centers. These channels are placed on 1.8 m (6 ft) centers along the 

length of the stope. Posts are then placed under the stringers and channels for additional support 

during the following cut. 

All three mines construct 2.1–2.4 m (7–8 ft) high wooden dams to retain the cemented fill in the 

stopes (Figure 5). 

Figure 5. Backfill retaining wall construction. 

In an effort to save time and money, the Lucky Friday constructs dams in the 

access slot by pushing waste rock against the chain link stretched between the walls. Each mine 

places a limit on the length of stope (backfill cell) that they will fill because it is imperative to 

completely fill the stope at one time to prevent the formation of cold-joints in the backfill. A 

cold-joint is the discontinuity created when a new cemented fill is poured on top of a previous 

cemented fill that has started to cure. Cold-joints severely reduce the integrity of the fill and have 

caused backfill collapses (Figure 6). 
Figure 6. Cold joint in section where backfill collapsed. 

At the Galena Mine, a backfill cell is typically 18 m (60 ft) long and 2.4 m (8 ft) wide. Filling a 

cell to 2.1 m (7 ft) of depth requires 217 t (240 st) of dry sand and 21.7 t (24 st) of cement. 

Assuming the current cost of cement to be approximately $95.00 per t ($105.00 per st), the cost 

of filling one cell is $2,520. With 435 t (480 st) of ore were mined from a cell, the additional 

cement cost per metric ton mined is $4.76 ($5.25 per st) for underhand mining. Including all 

factors, the total cost for underhand mining is $3.18/t ($3.50/st) more than overhand mining. At 

the Lucky Friday Mine, the rockbolting material costs in underhand mining are less because there 

are 10 bolts in the overhand back versus 5-6 in the underhand back. The total cost for backfilling 

at the Lucky Friday Mine, including the related supply costs and the labor at the stope and batch 

plant is $3.88/t ($4.28/st) of ore mined. However, despite higher initial costs, underhand mining 

is a safer and cost effective method when repair costs are considered. 

2.3 Mining Practices

2.3.1 Backfill Stability 

Mining cannot proceed until the backfill is sufficiently stable. Each of the mines discussed in this 

study uses a slightly different backfill method but with the same end-goal. The Lucky Friday 

Mine cures the cemented tailings for three days before mining under it. At the Galena, the next 

mining cut beneath the fill does not proceed for a period of three to seven days until the fill 

reaches a UCS of approximately 2,070 kPa (300 psi). Due to the fact that the Stillwater Mine has 

a much lower strength fill, samples of each backfill pour are tested for their UCS strength at 

three, seven, and 28 days. The Stillwater Mining Company has developed graphs that detail 

required equivalent compressive strength compared with stope span widths. Mining does not 

proceed until the testing indicates that the fill has reached sufficient strength to match the span of 

the stope (Jordan et al., 2004). 

2.3.2 Drilling and Blasting 

Drilling and blasting are the most critical processes involved in mining beneath cemented 

backfill. Blast holes must be drilled with the highest degree of accuracy possible. Holes drilled 

into the fill or even the prep muck, if loaded and blasted, could severely damage the backfill. It is 

standard practice to prohibit drilling closer than 0.3 m (1 ft) to the prep muck at all three mines. 

The Galena Mine uses jackleg drills to install blast holes in conventional underhand stopes, and 

in the LHD stopes, a jackleg or jumbo is used, depending on the stope width. The Lucky Friday 
generally drills blast holes with a jumbo while the Stillwater uses jacklegs. Over blasting a 

breast-up or V-cut round could also damage the fill, which would necessitate additional ground 

support, a very costly additional expense. Breasting up with a modified V-cut is standard at all 

three mines but certain situations require a burn cut to avoid damaging the backfill. At the 

Stillwater Mine, the first five rounds under the backfill are limited to 1.2 m (4 ft) lengths with a 

burn cut. This practice also results in slowing the stope advance, which gives the fill more curing 

time. After this, 1.2, 1.8, or 2.7 m (4, 6, or 9 ft) rounds can be taken using either a V-cut or a burn 

cut (Figure 7). 

Figure 7. Drill patterns for blasting at the Stillwater Mine. 

2.3.3 Mucking 

Load/haul/dump units (LHDs), with capacities of 1.5 m3

 (2 yd3), are used for muck removal at all 

three mines. However, the Galena Mine also uses electric or compressed air powered slushers to 

pull muck to the ore chute in areas where underhand mining methods are used for sill pillar 

recovery in conventional stopes. 

2.3.4 Ground Support 

The Lucky Friday and Galena mines rarely install rockbolts for ground support in the back of 

underhand stopes during the mining cycle. This is because the chain link material installed during 

the sand fill prep cycle with the vertical DYWIDAG bolts is sufficient to prevent groundfall. The 

Lucky Friday Mine creates additional wall support in the Gold Hunter 30 vein by installing two 

rows of steel mats oriented horizontally along the north wall and chain link along the weaker 

south wall. The mats and chain link are anchored on 0.9 m (3 ft) centers along the wall with 1.2 

m (4 ft) split-set bolts and 15 x 15cm (6 x 6 in) bearing plates. On occasion, the back of the stope 

will be spot bolted with additional split-sets, depending on the width of the stope and the bolt 
pattern used in the fill prep. The Galena Mine follows a similar practice; the walls of the stope are 

supported by two rows of steel mats that are installed horizontally along each wall and anchored 

with 1.2 m (4 ft) split-set bolts on 0.9 m (3 ft) centers. However, if the crown area where the 

cemented fill contacts the stope walls is failing, 1.2 m (4 ft) split-sets with 30 x 46 cm (12 x 18 

in) headboards are installed to provide additional support. 

At the Stillwater Mine, surface support is provided to the back and walls as mining progresses 

under the backfill by installing 40 Kg (88 lb) expanded metal mesh with a 7.6 x 20 cm (3 x 8 in) 

aperture that is held in place with 1.5 m (5 ft) split-set bolts on 0.9 m (3 ft) centers. The mesh is 

also pinned to the back with 15x15 cm (6x6 in) bearing plates and nuts on the exposed ends of 

the bolts that were pre-placed in the backfill. When these nuts are tightened, an axial load is 

generated in the bolt between the upper and lower plates. 

3.0 Geo-mechanical aspects 

The safety implications related to backfill stability make it the most important aspect of 

underhand mining. When underhand cut and fill mining was first implemented at the Lucky 

Friday and Galena mines, the USBM, and later, NIOSH, installed an extensive suite of 

geotechnical instruments in the first pour. The object was to document how the reinforced 

backfill responded to the procession of mining beneath it. Backfill pressure was monitored with 

earth pressure cells and extensometers were used to measure stope wall closure on the backfill 

(Williams et al., 1990 and 1992, Whyatt et al., 1992). The data indicated that the stope walls 

converged immediately on the fill as mining proceeded beneath the instrument locations, 

inducing up to 4.2 MPa (600 psi) loads on the fill. The fill reached its ultimate strength within 1 

cm (0.4 in) of wall closure and then started deforming as the walls continued to close. At the 

Lucky Friday and Galena mines, up to 10 cm (4 in) of wall closure was measured as the next cut 

was mined below it. In later tests, the vertical reinforcing bolts were instrumented with strain 

gages and load cells to determine the loads that were induced on them by the deforming backfill. 

The collected data indicate that the loads on the vertical #7 DYWIDAGs approach their 16,300 

kg (36,000 lb) yield strength (Williams et al., 2000). This load results from backfill deformation 

caused by wall closure and the 15x15 cm (6x6 in) bearing plates at the top and bottom of the 

bolts resisting this deformation. A cone of compression is established between the bearing plates 

at the top and bottom of the bolts which induces a tensile load in the vertical bolts (Williams et 

al., 2001 A&B). In effect, this makes the fill a reinforced concrete beam. The fill is stable as long 

as the cones of compression from adjacent bolts overlap. The fill between the bolts will start to 

fall out due to gravity but the chain link in the back will catch it. This process is referred to as 

bagging. Eventually enough falls and the whole fill fails. However, the fill failure is not 

problematic because it occurs after the stope has advanced several cuts down (figure 8). 
Figure 8 Geo-mechanics of underhand mining with cemented backfill. 

4.0 Summary 

The USBM, NIOSH, and the participating mines developed the underhand mining method as a 

safe alternative to the overhand mining method for rockburst-prone mines and mines with weak 

rock conditions. Despite major differences in their milling and backfill procedures, the Lucky 

Friday, Stillwater, and Galena mines have successfully implemented underhand mining methods 

that use cemented mill-tailings. Further, underhand mining has allowed these mines to safely 

mine rockburst-prone pillars and areas with extremely weak ground that could not be safely 

mined using other methods. Moreover, underhand mining has made it possible for the Lucky 

Friday and Galena mines to recover ore that was previously unrecoverable beneath the lowest 

levels of the mines. As such, any mine with ground control concerns should consider using 

underhand cut and fill mining with cemented mill tailings as a safe, cost-effective way of 

recovering resources from those areas. 

The findings and conclusions in this report are those of the author(s) and do not necessarily 

represent the views of the National Institute for Occupational Safety and Health. 
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