Determination of in situ deformation modulus for cemented 

rockfill 

D. R. Tesarik, mechanical engineer, J. B. Seymour, mining engineer, 

and F. M. Jones, engineering technician 

Spokane Research Laboratory, National Institute for Occupational Safety and Health 

Spokane, WA USA 

As part of safety and stability studies at three underground mines in the United States, 

researchers installed earth pressure cells and embedment strain gauges in cemented rockfill to 

measure stress and strain changes as mining progressed and the rockfill was loaded. Data

from these instruments were used to calculate the in situ modulus of the rockfill for two of the

mines. A three-dimensional numerical model of a cemented rockfill cap was validated with 

data from extensometers anchored in the mine roof and sill; this model was then used to 

estimate deformation modulus at the third mine because the earth pressure cells and

embedment strain gauges recorded negligible changes. Rockfill binder used at these mines 

ranged from 4 to 8 per cent of the dry components by weight, the water-to-cement ratio was 

from 0.42 to1.0, and the largest aggregate size in the three mixes ranged from 5 to12 cm (2 to

4.7 in). The range of the calculated in situ deformation modulus values was large, but as

expected, it was bracketed by modulus values for cemented tailings and roller-compacted

concrete with similar cement contents and water-to-cement ratios. 

Dans le cadre d’études sur la sécurité et la stabilité de trois mines souterraines situées aux

États-Unis, les chercheurs ont installé des palpeurs de pression des terres et des extensomètres 

d’encastrement dans les remblais cimentés afin de mesurer les variations relatives à l’effort et

à la déformation des terrains durant l’exploitation minière et le chargement du remblai. Les 

données recueillies à l’aide de ces instruments ont servi au calcul du module in situ du remblai 

dans deux de ces mines. Le modèle numérique tridimensionnel de la semelle d’un remblai

cimenté a été validé à partir des données recueillies par les extensomètres ancrés dans la voûte 

et dans la semelle de ces mines; ce modèle a ensuite été utilisé pour estimer le module de

déformation à la troisième mine, étant donné que les palpeurs de pression des terres et les 

tensiomètres enfouis avaient relevés des variations négligeables. La matière d’agrégation de

l’enrochement utilisée dans ces mines comportait entre 4 % et 8 % des composantes sèches 

par poids; le rapport eau-ciment se situait entre 0,42 et 1,0; et la taille des agrégats les plus 

gros dans ces trois mélanges était de 5 cm à 12 cm (2 po à 4,7 po). L’écart entre les valeurs

calculées des modules de déformation in situ était grand, mais comme on s’y attendait, cet 

écart a été compensé par les valeurs modulaires des résidus cimentés et du béton tassé au

rouleau avec des teneurs en ciment et des rapports eau-ciment semblables. 

Als Teil der Sicherheits- und Stabilitätsstudien an drei Bergwerken in den Vereinigten Staaten

von Amerika installierten Forscher Erddruckgeber und Einbindungsdehnungsmesser in 

zementierter Steinschüttung, um Spannungs- und Dehnungsveränderungen während der 

fortlaufenden Bergarbeiten und der Belastung der Steinschüttung zu messen. Daten von

diesen Instrumenten wurden dazu verwendet, das In-Situ-Modul der Steinschüt-ung für zwei

Gruben zu berechnen. Ein dreidimensionales numerisches Modell einer zementierten

Steinschüttungsschwelle wurde mit den Daten von den im Grubendach und in der Schwelle 

verankerten Extensometern validiert; dieses Modell wurde dann dazu eingesetzt, um das

Deformationsmodul in der dritten Grube zu schätzen, da die Erddruckgeber und 

Einbindungsdehnungsmesser nur geringfügige Messabweichungen feststellten. Das in diesen

Gruben verwendete Steinschüttungsbindemittel variierte zwischen vier und acht Prozent an

Trockenkomponenten nach Gewicht, der Wasser-Zementfaktor betrug zwischen 0,42 und 1,0 

und die größte Zuschlagstoff-größe in den drei Mischungen schwankte zwischen fünf und 

zwölf Zentimetern. Die Reichweite der berechneten Werte des In-Situ-Deformationsmoduls

fiel etwas breit aus, aber es fand erwartungsgemäß eine Klammerung der Modulwerte 

zwischen den Zemen-trückständen und dem durch Walzen verdichteten Beton mit ähnlichem

Zementanteil und Wasser-Zementfaktor statt. 

1
Introduction

Rockfill deformation modulus is a required

component of mine design, but large-scale tests to

determine this value may be impractical, and results of 

laboratory tests may not adequately represent the

heterogeneity of the rockfill resulting from aggregate 

segregation when the rockfill is placed (Reschke, 1993). 

This problem can be addressed by placing instruments 

in the rockfill to measure mining-induced stress and 

strain. As part of a safety and stability assessment of 

backfill mining methods, engineers from the National

Institute for Occupational Safety and Health (NIOSH) 

installed earth pressure cells, embedment strain gauges,

and vertical fill extensometers in cemented rockfill at 

the Cannon, Buick, and Turquoise Ridge mines (Figure 

1) with the assistance of staff from the mines. 

Backfill binder ranged from 4 to 8 per cent of the dry 

components by weight, the water-to-cement ratio was 

from 0.42 to1.0, and the largest aggregate size in the

three mixes ranged from 5 to12 cm (2 to 4.7 in) (Table

1). In situ deformation modulus values were calculated

from stress changes measured by earth pressure cells

and strain changes measured by embedment strain

gauges or vertical backfill extensometers at the Cannon

and Buick mines. A calibrated three-dimensional,finitedifference code was used to determine the deformation 

modulus for the backfill at the Turquoise Ridge Mine. 

The authors hope that the range of in situ deformation

modulus values obtained from these three case studies 

and previous research by others will serve as a practical 

guide for future backfill mix design, particularly if an 

instrumentation program is not planned or before 

instrument readings are available. 

Figure 1.─Location of Cannon, Buick, and Turquoise

Ridge mines in the United States. 

The factors affecting rockfill modulus values are 

water content, cementitious properties and content, 

aggregate particle-size distribution, aggregate strength,

water quality, age, and degree of compaction. For

mixtures having similar water and cement contents, the 

modulus of rockfill (Table 2) is likely to be greater than 

the modulus of cemented tailings (Table 3) and less than

the modulus of roller-compacted concrete (RCC) (Table

4). Previous research supports this hypothesis.1

 

1

Except for tangent deformation modulus values between 20

and 50 per cent measured in cemented tailings at the Lucky 

Friday Mine by Williams et al. (2001).

Table 1.─Components of backfill mixes for the Cannon, Buick and Turquoise Ridge mines, 

percentage of total dry components 

Mine Coarse

aggregate 

Fine aggregate Cement Flyash Binder 1

Ww/

2

Wb

Cannon Minus 5.1-cm (2-in) 

granitic alluvium, 55 

Alluvial sand, 

39.5 

5.5 NA 5.5 1.0

Buick Minus 12.7-cm (5-in) 

crushed dolomite, 96 

NA 4 NA 4 1.0 (approx.)

Turquoise 

Ridge 

Minus 5.1 (2-in) crushed 

waste rock, 93 

NA 5.8 1.95 7.8 0.42

1

 Ww = Weight of water in mixture. 2 Wb = Weight of binder in mixture. 

2
Table 2.─In situ deformation modulus values for cemented rockfill

Mine Coarse aggregate Fine aggregate 1

Binder Age, 

days

Modulus average or 

range, MPa (psi) 

Reference 

Kidd Creek Minus 15-cm (6-in) 

crushed andesitediorite 

- 5.0 28 2,280-3,380

(330,686-490,227) 

Yu 1995 

Mt. Isa Minus 30-cm (11.8-

in) crushed Kennedy

Siltstone 

Deslimed 

tailings 

6-8 - 280 (40,610) Gonano and 

Kirby, 1977; 

Thomas et al.

1976 

1

Percentage of total dry components. 

Table 3.─In situ deformation modulus values for cemented tailings, percentage of total dry components 

Mine Fine 

aggregate 

1

Cement 1

Other 

binder 

1

Total 

binder

2

Ww/ 

3

Wb

Age, 

days

Modulus 

average or 

range, MPa 

(psi) 

Reference 

Lucky Friday Classified 

tailings 

10 - 10 1.49 15 681-5,095

(98,770-

738,967) 

Williams et 

al., 2001 

Garpenburg Deslimed 

tailings 

9 - 9 2.9 “Longterm” 200 (29,007) Krauland 

and Stille, 

1993 

Western Deep

Levels South

Deslimed 

tailings 

- Slagment, 

lime 

7 - - 30-110 (4351-

15,954) 

Gurtunca et 

al., 1993 

Chimo Paste - - - - 70-105 24-51 (3481-

7397) 

Hassani et 

al., 1998 

Mount Isa Deslimed 

tailings 

4 Copper 

reverbatory

furnace slag, 

6 

10 - >540 90-200 

(13,053-

29,008) 

Gonano, 

1977 

1

Percentage of total dry components. 2Ww = Weight of water in mixture. 3Wb = Weight of binder in mixture.

RCC differs from most rockfills because the paste 

volume fills, or nearly fills, aggregate voids to produce

a dense mixture; however, RCC generally has a maximum aggregate size of 7.62 cm (3 in) and unconfined

compressive strength (UCS) values in the same range as

some rockfills (American Concrete Institute [ACI],

1999). Reported UCS values obtained from laboratory 

specimens average 11.5 per cent lower than values

obtained from tests on cores retrieved from placed 

material (ACI, 1999). To account for a similar in situ

effect on the RCC deformation modulus, laboratory 

values were reduced by 11.5 per cent (Figure 2). 

Cannon Mine 

The Cannon Mine, located in central Washington,

USA (Figure 1), was a joint venture between Breakwater Resources, Vancouver, B.C., and Asamera 

Minerals, Calgary, AB, which operated the mine from 

1984 to 1994. Instruments were placed in two of the 

mine's gold-bearing ore bodies, the B-North and the BNeath, to monitor ground stability. These two ore 

bodies were developed between 61 and 313 m (200 and 

1028 ft) below the surface using an overhand benchand-fill mining method. Maximum dimensions of the 

shallower B-North ore body were 168 by 244 m (550 by 

800 ft) in plan view with stopes arranged in parallel 

panels across strike. Primary stopes 7.3 m (24 ft) wide 

were excavated and filled with cemented backfill in

3
Table 4.─Laboratory deformation modulus values for some roller-compacted concrete mixes (American 

Concrete Institute, 1999) 

Dam/project Cement, 

kg/m3

(lb/yd3

) 

Pozzolan, 

kg/m3

(lb/yd3

) 

1

Ww/

2

Wb Wb/

3

Wag

× 100 

Loading 

age, days

Compressive 

strength, 

MPa (psi) 

Deformation 

modulus, 

GPa (106

 psi)

Conception 90 (152) 0 1.20 4.4 28 6.76 (980) - 

Conception 90 (152) 0 1.20 4.4 90 8.62 (1250) 10 (1.40) 

Willow Creek 47 (80) 19 (32) 1.61 2.8 28 11.93 (1730) 13 (1.91) 

Willow Creek 104 (175) 0 1.06 4.5 90 6.89 (1000) 15 (2.20) 

Zintel Canyon 59 (100) 0 2.00 - 28 4.34 (630) 11 (1.54) 

Zintel Canyon 59 (100) 0 2.00 - 90 7.52 (1090) 15 (2.15) 

Zintel Canyon 59 (100) 0 2.00 - 365 10.69 (1550) 18 (2.57) 

Zintel Canyon 119 (200) 0 1.00 - 28 11.17 (1620) 16 (2.39) 

Zintel Canyon 119 (200) 0 1.00 - 90 14.69 (2130) 17 (2.47) 

Zintel Canyon 119 (200) 0 1.00 - 365 21.37 (3100) 23 (3.28) 

1

Weight of water in mixture. 2Weight of binder in mixture. 3Weight of aggregate in mixture. 

15.2-m- (50-ft-) lifts from the bottom to the top of the 

ore body. 

Cemented backfill with minus 5.08-cm (2-in) 

aggregate was end-dumped from an upper heading until

the excavated stope was completely filled (Figure 3) 

(Baz-Dresch, 2002). The backfilled stopes served as 

mucking levels for the stope at the next higher level. 

Completed stopes ranged from 9 to 35 m (30 to 115 ft) 

in height and up to 107 m (350 ft) in length, depending 

on their location in the ore zone. The upper levels of 

the primary stopes were filled tight to the back with 

cemented fill and secondary stopes were backfilled with 

cemented fill or waste rock (Seymour et al., 1999), 

depending on operational factors such as access 

maintenance.

The B-Neath ore body was a narrow, downward 

extension of the B-North deposit separated by a shear 

zone of very poor quality rock. Maximum dimensions 

of this ore body were 131 m (429 ft) thick, 152 m (500 

ft) long, and 61 m (200 ft) wide. The same mining 

method was used in the B-Neath ore body as in the BNorth (Tesarik et al., 1993). 

Backfill Instruments in the B-North Ore Body 

Earth pressure cells with vibrating-wire transducers

capable of reading 6.89 MPa (1000 psi) were placed in

the backfilled primary stopes to monitor stress changes 

when adjacent secondary pillars were mined. The 22.9-

cm- (9-in-) diam pressure cells (Geokon,2

 1996) in 

 2

Mention of specific products or manufacturers does not 

imply endorsement by the National Institute for Occupational 

Safety and Health. 

stopes D45 and D55 (Figure 4) were cast directly in 

backfill. Aggregate larger than approximately 2.5 cm (1 

in) was removed by hand to minimize the chance that

the aggregate would point-load the instrument. The 

earth pressure cell in stope D50 was cast in the stope 

using minus 6.35-mm (0.25-in), screened backfill inside 

a 1-yd 3

 wooden form. After the backfill had hardened,

the form was removed, and the remainder of the stope 

was backfilled. 

Vertical backfill extensometers equipped with 

vibrating-wire displacement transducers were assembled in stopes D45 and D55 to measure backfill strain 

over distances of 5.3 and 8.5 m (17.4 and 27.8 ft), 

respectively. Fabricated at the Spokane Research

Laboratory (SRL), the extensometers consisted of a 

steel base plate and head assembly connected to sections

of 5.1-cm- (2-in-) diam steel pipe (Figure 4) (Tesarik et 

al., 1991). 

Embedment strain gauges were also placed near the

earth pressure cells to measure vertical strain, but were 

not used in modulus calculations because the gauges did 

not record strain at the same time that the earth pressure 

cells recorded load or because the time between

readings was too long. To reduce the effect of creep,

modulus was calculated using stress and strain changes 

over a maximum time interval of 2 weeks. The 

accessible instruments were read manually with a 

Geokon GK-403 vibrating-wire readout box (Geokon, 

1998) approximately every week. 

4
s

Figure 2.─Deformation modulus values for cemented

tailings, cemented rockfill, and roller-compacted 

concrete. 

Backfill Instruments in the B-Neath Ore Body 

The primary stopes in the B-Neath ore body were

also instrumented with vertical backfill extensometers,

embedment strain gauges, and earth pressure cells

(Figure 5). Maximum ranges on the earth pressure cells

were 3.45 or 6.89 MPa (500 or 1000 psi). The 

embedment strain gauges (Geokon model VCE-4210) 

were 24.4 cm (10 in) long with 5.1-cm- (2-in-) diam 

steel platens at each end and a measurement range of 

0.635 cm (0.25 in) in compression (Geokon, 1990). 

Before placing the earth pressure cells and embedment

strain gauges approximately 1.5 m (5 ft) below grade in

the stope, they were cast in forms with a backfill mix 

having a maximum aggregate diameter of 1.3 cm (0.5

 

Figure 3.─Grain-size distributions for aggregate used 

in the cemented rockfill at Cannon Mine. 

in). When the instruments were placed in the stope, the 

forms were removed, and the instruments were covered

with backfill using a front-end loader. The newly 

placed backfill was left to harden before the stope was

filled to the planned elevation. 

The vertical backfill extensometers were installed in

BX-sized diamond-drill holes with the hole collars near 

the other instruments. The instrument design used in

the B-North ore body was revised to eliminate personnel 

exposure to the ribs of high, open stopes, reduce 

installation time, and prevent damage to the instrument

during backfill placement (Tesarik et al., 1993). 

Laboratory Deformation Modulus Values 

The design UCS of 8.3 MPa (1200 psi) was based on 

laboratory tests of 15.2- by 30.5-cm- (6- by 12-in-) 

cylindrical specimens. The corresponding laboratory

modulus was equal to 4031.4 MPa (584,700 psi) (BazDresch, 2002). To help quantify in situ strength, 15-cm- 

(6-in-) diam samples were cored from two stopes where

the backfill had cured for over 30 days. Average UCS

for these specimens was 5.65 MPa (820 psi); the 

corresponding deformation modulus was 1862 MPa 

(270,060 psi) (Brechtel et al., 1989). To determine if 

specimen size influenced backfill properties, SRL 

conducted three UCS tests on 45.7- by 91.4-cm- (18- 

by 36-in-) cylindrical specimens of the same backfill 

5
Figure 4.─Cross section of Cannon Mine’s B-North 

ore body with instrument locations. 

mix that had been placed in the stopes. Average UCS 

for these larger specimens was 4.1 MPa (600 psi), and 

average tangent modulus at 50 per cent of peak stress 

was 2251 MPa (326,500 psi), or 56 per cent of the value

obtained from 15.24-cm- (6-in-) diam laboratory 

specimens. 

In Situ Deformation Modulus Values 

The smallest stress change used to calculate

deformation modulus was 6.89 kPa (1 psi), which is the 

resolution of the 6.89-MPa (1000-psi) earth pressure

cell. These data were then averaged for each set of 

backfill instruments (Table 5). Data from the earth 

pressure cells in stope D50 were combined with 

readings from the vertical fill extensometers in stope 

D55 because the earth pressure cell at the top of stope 

Figure 5.─North-south cross section of Cannon Mine’s 

B-Neath ore body with instrument locations. 

D55 was not functioning when the backfill in stope D55 

exhibited the largest strain change. This change 

occurred when pillar D57, which was adjacent to stope 

D55 and the last secondary pillar in the B-North ore 

body, was mined. 

Average calculated in situ modulii were 29, 53, and 

64 per cent of the laboratory values from handconstituted 15.2-cm (6-in), hand-constituted 45.7-cm 

(18-in), and cored 15.2-cm (6-in) in diam specimens, 

respectively. Deformation modulus calculated from the 

cumulative stress-versus-strain plot for data recorded 

during the extraction of pillar D57 was 596.0 MPa 

(86,440 psi) (Figure 6). The data exhibit a high linear 

relationship, with the square of the correlation

coefficient (r2

) equal to 0.98. 

6
 

Table 5.─In situ backfill modulus values calculated from instruments installed at Cannon Mine 

Instruments Max. range of

EPC,1

 MPa 

(psi). Length of

VFX,2

 m (ft) 

Stope Ore

body

Elevation, 

m (ft) 

Age of

backfill, 

month3

Max stress

measured by 

EPC, kPa 

(psi) 

No. of 

data

pairs 

Average

modulus, 

MPa (psi)

St. dev., MPa

(psi) 

EPC 

VFX 

6.89 (1000) 

5.30 (17.4) 

D45 

D45 

BNorth 238 (780) 

229 (750) 

1.5-3.5 124 (18) 9 334.4

(48,500)

275.1 (39,900)

EPC 

VFX 

6.89 (1000) 

8.47 (27.8) 

D50 

D55 

BNorth 226 (740) 

213 (700) 

9.3-10.8 2586 (375) 7 607.4

(88,100)

549.5 (79,700)

EPC 

VFX 

6.89 (1000) 

13.59 (44.6)

X80 

X80 

BNeath 152 (500) 

152 (500) 

5.0-10.6 834 (121) 2 4424.4

(641,700)

1869.2

(271,100)

EPC 

VFX 

6.89 (1000) 

9.75 (32.0) 

X86 

X86 

BNeath 110 (360) 

110 (360) 

7.0-8.0 200 (29) 3 2906.0

(421,500)

708.1 

(102,700)

All 21 1182.5

(171,500)

1510.6

(219,100)

1

 Earth pressure cell. 2 Vertical fill extensometer. 3 Approximate age of backfill when stress and strain measurements were 

recorded. 

Figure 6.─Vertical stress in stope D50 versus vertical 

strain in stope D55 measured during destressing of 

pillar D57. 

The range of in situ deformation modulus is very

large, with a coefficient of variation equal to 1.3.

Because of this large range (Table 5), the calculated in

situ modulus for the Cannon Mine bracketsthe modulus

values obtained from laboratory tests conducted on both 

hand-constituted and cored specimens. The lower limit

may be used for design purposes when a conservative 

estimate of subsidence is a primary design factor, 

whereas the upper limit may be more appropriate if 

limiting backfill stress is important. 

Buick Mine 

The Buick Mine, located in southeast Missouri, USA

(Figure 1), is one of seven mines owned and operated 

by The Doe Run Co. in a tabular, flat-lying deposit

called the New Lead Belt. The mine uses a room-andpillar mining method to extract lead, zinc, and copper 

ore from depths ranging from 335 to 366 m (1100 to

1200 ft). Pillar recovery is achieved by using from 2 t 

(2.2 st) of backfill per 1 t (1.1 st) of pillar ore mined to

no backfill in narrow areas with abutments or barrier 

pillars (Roberts et al., 1998). Area 5, with 10.7- m- (35-

ft-) wide rooms ranging from 14 to 19 m (46 to 62 ft) 

high and the section instrumented in this study, was the

first area in the New Lead Belt to use cemented backfill 

for pillar recovery. Backfill was spread throughout the 

test area with front-end loaders in 0.3- to 0.6-m (1- to 2-

ft) lifts and leveled with a wheeled dozer. The final 1-m

(3-ft) gap was closed using minus 5-cm (2-in) cemented

waste rock placed with a slinger truck. 

Backfill Instruments in Test Area 5 

Most of the backfill instruments were placed in the 

east-west backfill drifts when the fill was mid-height to 

the pillars, but some earth pressure cells and embedment strain gauges were also placed near the mine roof

(Figure 7). The maximum designed displacement on 

the embedment strain gauges was 0.635-cm (0.25-in) in

compression, and the maximum designed stress on the

earth pressure cells was 6.89 MPa (1000 psi). Prior to 

installing these instruments, they were cast in wooden 

forms using minus 0.64-cm- (0.25-in-) cemented 

aggregate and left to cure for several weeks. The forms

were removed before the instruments were secured in 

place with wet backfill, and the newly placed backfill

was allowed to cure at least 1 day before equipment was 

driven over the instruments. The vertical backfill

extensometers were similar to those used in the B-North 

ore body at the Cannon Mine (Tesarik et al., 1995). 

7
Figure 7.─Plan view of Buick Mine’s area 5 with 

instrument locations. 

In Situ Deformation Modulus Values 

Readings were recorded from the instruments every 

2 hr using a data acquisition system set up near the test 

area. The modulus of deformation was calculated for 

each 2-hr interval for stress changes greater than or

equal to 6.89 kPa (1 psi). The curing time of the backfill 

above the instruments ranged from 2.4 to 11 months

when the data were recorded. 

In situ modulus values calculated for the Buick Mine

(Table 6) are substantially higher than those calculated 

for the Cannon Mine despite a lower cement content.

The larger value for average backfill modulus at the 

Buick Mine may be due to the large size of the coarse

aggregate or less aggregate segregation and/or compacttion of the backfill by heavy equipment during the

spreading and leveling operations. As expected, the 

modulus was lower than reported modulus values for 

RCC (Table 4). With a coefficient of variation equal to 

0.79, the range of modulus of deformation values was 

not as large as that from the Cannon Mine. 

Turquoise Ridge Mine 

The Turquoise Ridge Mine, owned by Placer Dome,

Inc., is a gold deposit located 45 km (28 miles) northeast of Golconda, Nevada, USA (Figure 1). Mineralization occurs at depths between 427 to 671 m (1400 to 

2200 ft) along the intersection of northwest- and 

northeast-trending shear zones (Berentsen et al., 1996). 

Drift-and-fill mining is used because of the poor ground 

conditions commonly encountered. To determine if a 

new mining method could improve productivity and

reduce the exposure of mine personnel to ground fall

hazards, a backfill span was tested 390 m (1280 ft) 

below the surface. A backfill cap consisting of six 

adjacent 3.8-m- (12.5-ft-) wide, 4.6-m- (15-ft-) high, 

and 30.5-m- (100-ft-) long cemented rockfill drifts was 

undercut by mining two outside drifts 4.3 m (14 ft) wide

and 4.6 m (15 ft) high. This left a central pillar that was 

retreat mined in sections to produce a 13.7-m- (45-ft-) 

wide, 30.5-m- (100-ft-) long room beneath the backfill. 

Laboratory Deformation Modulus Values 

In-stope backfill samples were collected in 15.2- by 

30.5-cm (6- by 12-in) cylindrical containers according

to specifications set by the American Society for 

Testing Materials (ASTM, 2000a) and placed in a 

climate-controlled room at SRL to cure. These specimens were tested 2.5 yr later to determine their 

laboratory deformation values at full cure. To meet end

parallelism test standards, the specimens were capped 

with sulphur (ASTM, 2000b). Average tangent modulus

between applied stresses of 2.1 and 4.8 MPa (300 and

700 psi) was 2264.2 MPa (328,400 psi) with an average 

UCS equal to 9.6 MPa (1390 psi). 

In Situ Deformation Modulus Values Calculated 

from Calibrated Numerical Model 

A three-dimensional numerical model of the backfill 

cap using the FLAC3D computer code developed by

Itasca Consulting Group (2002) was used to estimate in

situ deformation modulus. The host rock was not 

included in the model because instruments indicated

that the backfill cap had separated slightly from the 

mine roof and deformed as an elastic beam loaded by its

own weight. The modeled cap was composed of 0.3- by 

0.3- by 0.61-m (1- by 1- by 2-ft) bricks with the 0.61-m 

(2-ft) edges aligned parallel to the longest axis of the 

cemented drifts. The vertical boundaries and the bottom

of the model were rollered when gravity load was 

applied, then the sides were pinned to simulate shear

between the abutments and the backfill. To simulate

8
Table 6.─In situ backfill modulus values calculated from instruments at Buick Mine 

Instruments Maximum stress measured 

by EPC, kPa (psi) 

No. of data 

pairs 

Average modulus, MPa 

(psi) 

St. dev., MPa (psi) 

94EPC,1 94ESG2 552 (80) 5 3339.1 (484,300) 1729.2 (250,800) 

95EPC, 95ESG 276 (40) 7 4498.1 (652,400) 1221.1 (177,100) 

96EPC, 96ESG 586 (85) 6 10,884.8 (1,578,700) 4388.5 (636,500) 

102EPC, 102ESG 255 (37) 5 8244.8 (1,195,800) 2561.4 (371,500) 

103EPC, 103ESG 710 (103) 7 3935.5 (570,800) 2969.6 (430,700) 

104EPC, 104ESG 131 (19) 5 1280.4 (185,700) 208.9 (30,300) 

105EPC, 105ESG 434 (63) 9 1274.8 (184,900) 863.2 (125,200) 

94EPC, 93VFX 552 (80) 6 7149.9 (1,037,000) 2837.2 (411,500) 

94EPC, 103VFX 552 (80) 6 4077.6 (591,400) 3994.1 (579,300) 

95EPC, 96EPC,

3

104VFX 

276 (40) 5 6246.7 (906,000) 4437.5 (643,600) 

All of the above NA 61 5029.7 (729,500) 3959.7 (574,300) 

1

Earth pressure cell centered mid-height in the backfill on pillars 93, 94, 102, and 103. 2Embedment strain 

gauge centered mid-height in the backfill on pillars 93, 94, 102, and 103. 3

Average of readings from 95EPC 

and 96EPC was used. 

Table 7.─Backfill properties used in numerical model of

backfill span at Turquoise Ridge Mine 

Young’s modulus, MPa (psi 1314.8 (190,700)

Cohesion, kPa (psi) 689 (100) 

Angle of internal friction, deg 44 

Tensile strength, kPa (psi) 614 (89) 

Specific weight, kg/m3

 (lb/ft3) 2146 (134) 

Poisson’s ration 0.2 

mining beneath the cap, the boundary condition was 

changed from rollered to free in the same sequence in

which actual mining took place. The age of the backfill 

when the cap was undercut ranged from 4.6 to 8 

months. 

The apparent cohesion and angle of internal friction

used in the model were values reported for grouted

rockfill containing similar constituents (Sainsbury et al.,

2001). Tensile strength was 10 per cent of the average 

UCS value obtained from 27 laboratory tests conducted

by Turquoise Ridge staff on 5.2- by 30.5-cm (6- by 12-

in) specimens cured for 28 days. Specific weight was

the average weight of the samples tested (Table 7). 

These samples were collected at the backfill plant from 

batches placed in all six drifts. An initial estimate of the

deformation modulus used in the model was 1379.0

MPa (200,000 psi). 

The numerical model was validated using relative 

displacements measured between borehole extensometer anchors at the top of the backfill and the bottom 

of the mine roof. These relative displacements were

reduced slightly using displacements from extensometer

anchors between 0.6 and 4.3 m (2 and 14 ft) in the mine 

roof to account for vertical strain in the overlying host 

rock (Figure 8). Measured relative rock displacement 

above 4.3 m (14 ft) was negligible. The initial estimate 

of the deformation modulus used in the model was reduced to 1314.8 MPa (190,700 psi) so that the slope of 

the regression line for measured versus calculated 

displacements was approximately equal to 1.0 (Figure 

9). This model-calculated in situ modulusis 58 per cent

of the average laboratory value. There were no calculated failures in the modeled cap. One of the extensometers indicated that there was localized inelastic

behavior in the backfill, but this was not considered significant enough to affect the general behavior of the cap. 

Conclusions 

• In the three mines studied during this investigation, 

higher levels of compaction and less aggregate segregation during backfill placement resulted in larger 

deformation modulus values for backfill mixes having 

similar water-to-cement ratios, aggregate content, and 

amount of binder. 

• Average in situ deformation modulus values of 

cemented backfill containing 4 to 6 per cent cement and 

with water-to-cement ratios ranging from 0.4 to 1.0

were 1182.5 to 5029.7 MPa (171,500 to 729,500 psi). 

This range could be used to bracket backfill stresses in

mine feasibility studies. 

9
Figure 8.─Elevation and plan views of extensometers 

and biaxial stressmeters installed in backfill cap and 

host rock at Turquoise Ridge Mine. 

• The range of in situ deformation modulus values for 

cemented rockfill was large, but bracketed by modulus 

values for cemented tailings and RCC with similar 

cement contents and water-to-cement ratios. 

• In situ deformation values were from 30 to 64 per 

cent of values obtained from specimens tested in the 

laboratory. 

Figure 9.─Numerically calculated displacement 

versus measured displacement in backfill cap at 

Turquoise Ridge Mine. 

Acknowledgments 

The authors wish to express their appreciation to 

Mark Mudlin and John Baz-Dresch, employed by 

Asamera Minerals at the Cannon Mine until its closure; 

Bill Lane, Tom Yanske, and Greg Sutton, The Doe Run 

Co.; and Rob Usher, Brian Simmons, Bob Schuler, Bert

Bellows, Chris Jacobsen, Rick Smith, and John Evans 

of the Turquoise Ridge Mine, Placer Dome, Inc. All

generously offered assistance in installing the backfill 

instruments, providing mine maps and mine sequence 

schedules, and supplying material property data. The 

authors would like to thank the surface and underground 

staff at these mines for assistance in transporting 

supplies, drilling holes for the instruments, stringing

instrument cables, and protecting the installed instruments with shotcrete or backfill. 

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