24 T T T T T
2b 4
z
~ 20 4
uw
@
SF
ira
16 b Initial impact from _|
_-~ leading edge of
14 |_fmugk colurpn
0 2 4 6 8 10 12

IMPACT TIME, sec

« Figure 4, Impact forces on chute support structure.

70 350
60 300
z z
7 50 250
wi
S 8
& a0 200 &
2 fo)
ir
30 1 Key 150 |
$ Actual f ray
D2» — Actualtorce | 495 Q
o ——— Model force $
10 50
0 0
0 20 40 60 80 100 120 140
IMPACT TIME, sec
Figure 5. Comparison of actual and computer-

generated loads from 14 dumps of material.

situation made it extremely difficult to maintain the
instruments, but valid field data are being obtained
and cvaluated nonetheless. In addition, a mine-ready
data collection system has been proven in an extreme-
ly harsh environment. This system will accommodate
the anticipated wide range of strain levels to a high
degree of accuracy.

2.3 Computer modeling

Computer modeling aims at simulating dynamic
and static forces measured in the field on the chute
gate, as well as overall particle flow phenomena and
the potential for hang-ups. Of particular interest are
the dynamic effects of large boulders or chunks of
material falling through an cmpty ore pass. The
modeling research will also evaluate adequacy of
existing closed-form solutions of static and dynamic
load factors tor chute and gate structural designs, as

493

Figure 6. Load-cell washers installed on one side of
control gate support bolts.

discussed in the introduction.

Computer modeling involves the application of the
two- and three-dimensional particle flow codes PFC”
and PFC”? (Itasca Consulting Group, Inc., 1995a,
1995b). PFC* was used to simulate flow through the
ore pass and truck chute instrumented in the initial
ficld test. The simulation computed results from 40.
loads dumped from an LHD containing about 7 mt of
material each for a total weight of about 280 mt. The
rate of increase in gate loads dropped dramatically
after about one-third (10 to 15) of the LHD loads had
been delivered. The accumulated control gate forces
resulting from the first 14 loads are compared to the
actual results from the field test in Figure 5. Analysis
indicates that dynamic loads were a factor on the
control gate only during the first three to five dumps,
both in the computer model and in the field test.
However, the magnitude of force varied by a factor of
about 5 times. This discrepancy may be associated
with uncertainty in selecting the stiffness characteris-
tics of the particles and walls, as well as the shape of


36 ee ee ee
KEY

30 —— Mock-up drop -}
z test no. 3
25 —— PFCTM Model |
uw
220 4
oO
1s 4
Q
S10 4
<
Es ;
a

0 4

4 [re

0 05 10 15 20 25 30
TIME, sec

Figure 2. Results of mockup drop tests compared to
particle flow code data.

ohm strain gages were positioned in a T-configuration
on each beam.

2.2 Field tests

Field tests are underway in mines in Idaho and
Montana. The goal of these tests is to determine
actual static and dynamic loads and validate reduced-
scale tests results and computer models. The initial
field test was in a deep silver mine in northern Idaho,
The ore pass and chute and gate systems in this mine
contain offset boreholes and doglegs that direct falling
material to the floor of the ore pass and absorb the
initial impacts from the falling column of ore or
waste. The centerline of the gate assembly was offset
2.4 m from the longitudinal axis of the ore pass so that
falling ore did not directly hit it

Thirty-two strain gages were welded on eight 3.8-
cm bolts that were supported by brackets fastened to
the rock mass to support the assembly. Figure 3
shows the truck loadout chute during instrumentation
of the bolts. Four gages were mounted on each bolt at
90° angles, as in the mockup. Measurements of ten-
sile strain produced total vertical force on the
structure as material was dumped into the ore pass.

‘The test series consisted of 14 loads of damp waste
rock from load-haul-dump (LHD) units at 1.53 m* per
load. Twelve of the dumps ranged from 2,270 to
2,730 kg of material; two of the dumps (8 and 9) were
material from cleaning up the drift and weighed about
270 kg cach. The total weight of material dumped in
the ore pass was in excess of 27,300 kg.

Preliminary analysis of the results indicated that
the dynamic forces on the control gate assembly were

492

Figure 3. Instramenting support bolts at truck loadout
chute,

significantly less than anticipated. Figure 4 shows
typical impact forces measured on the chute support
structure during the third LIID dump. Impact forces
ranged from 1.06 to 1.33 of the static load on the
chute and gate assembly and were reduced significant
ly because the chute was offset from the ore pass.
This compared to an average impact value of 4.09
reported by Blight & Haak (1994) in tests in a vertical
ore pass where material struck the chute directly. An
offset chute design allows a cushion of material to pile
up at the base of the ore pass to absorb initial impacts
from the leading edge of the falling column of ore or
waste. The load response from all 14 dumps is shown
in Figure 5.

The second field test involved mounting weldable
strain gages on a chute and gate support structure at a
large underground platinum mine in south-central
Montana to determine axial strain. ‘These strain gages
were welded on eight support beams identical to those
used in the laboratory experiment, Eight load-cell
washers were installed to determine total force on the
gate assembly (Fig. 6).

Both the strain gages and load washers were
precalibrated in the laboratory on duplicate structural
members to yield direct axial force on the member. A
prototype single-channel data acquisition board was
developed to minimize the difficulties experienced in
the first ficld test and improve the capture of the wide
range of strain response from the ore pass structure.
During the experiment, the ore pass was consistently
hung up, and it was necessary to blast the chute at the
bottom of the ore pass to free the material, This



particles (clusters). Another factor difficult to model
are rock durability features and rebound during impact
(Larson et al., 1998).

Several difficulties remain in realistic modeling of
rock particle flow, such as determining shape func-
tions, damping factors, and the relative stiffness char-
acteristics of the rock particles and the confining
walls. The shape, rotation, and durability of each rock
affects rebound characteristics. Incorporating more
realistic rock particle shape and durability features
would improve rocklike characteristics during impact.

A variety of particle shapes can be developed by
modeling particles as an assemblage of balls. The
balls within the assemblage may be connected using
PFCTM contact and parallel bonds. A modeled particle
assemblage such as this would rebound more realisti-
cally or break into smaller pieces when striking a wall
or another particle in the model.

Another difficulty was the selection of a damping
constant. Without damping, a particle will bounce
indefinitely. In an actual rock flow, energy losses
occur when balls collide with each other or with the
walls of the ore pass. By using an appropriate
damping constant, the energy lost during collisions
can be modeled and rebound height controlled.

An alternative design approach, which may
accommodate the dynamic loading uncertainties ex-
perienced in the field, is currently under investigation.
The concept is based on “softening” the chute and
control gate assembly and/or the supporting structure
with the overall stiffness k,, by means of springs or
other components with the stiffness k, to dampen the
dynamic load. The resulting stresses in the bearing
members of the softened assembly with respect to
static stresses can then be expressed as

fae We{1+2h/8,K)!?

et )
where K = 1 +k/k,. Comparisons of equation 4 with
equation 3, where h>0, indicates a reduction in
stresses in the bearing members when the stiffness of
the softening components k, decreases.

3 SUMMARY

A state-of-the-art ore pass test facility has been
completed and is available to test various aspects of
ore pass design and particle flow characteristics. A
field-proven method of measuring and collecting data
has been developed that will accommodate the wide
range of structural responses to dynamic forces caused

494

by blasting during release of hang-ups. Comparisons
of field measurements and computer models indicates
that several difficulties remain’ before the dynamic
effects of material flow in ore passes and impact loads
on the gates can be accurately predicted. It has been
found that computer analyses significantly overesti-
mate dynamic impacts when the results of these
analyses are compared to impact loads measured in
field tests. Structural modifications using springs or
other stiffness-reducing methods should result in a
significant economy of design, reduction in material
costs, and increased safety of the control gate assem-
bly in ore passes.

REFERENCES

Aytaman, V. 1960. Causes of Hanging in Ore Chutes *
and Its Solution, Can, Min, J. 81:77-81.

Beus, M., & T.M. Ruff 1997. Development of a Mine
Shaft and Ore Pass Research Facility. NIOSH
Rep. Invest. 9637, 12 pp.

Blight, G.E., & B.G. Haak 1994, A Test on Model
Underground Ore Passees. Bulk Solids Handling
14(1):77-81,

Ferguson, G. 1991. Orepass Design Guidelines,
Report to Mining Research Directorate, Sudbury,
Ontario. Sudbury, ON: Bharti Engineering Asso.

Hambley, D.F, 1987. Design of Ore Pass Systems for
Underground Mines. CIM Bull, 80:25-30.

Tasca Consulting Group, Inc. (Minneapolis, MN),
1995a. PFC”? (Particle Flow Code in 2 Dimen-
sions), Version 1.1.

Itasca Consulting Group, Inc. (Minneapolis, MN),
1995b, PFC"? (Particle Flow Code in 3 Dimen-
sions), Version 1.1.

Janssen, H.A. 1895. Versuche uber Getreidedruck in
Silozellen. Zet. Ver. Deutsch. Ing. 39: 1045-1049.

Larson, M.K., S.R. Iverson, B.M. Stewart, & K.
Walker 1998. Preliminary Assessment of Particle
Flow Code as a Tool to Assess Ore Pass Safety.
Presentation, Rock Mechanics in Mining, Petrol-
eum, and Civil Works: 3rd N. American Rock
Mechanics Symp., Cancun, Quintana Roo, Mex-
ico, June 3-5. Paper 92. Ext. abs. in Intern. J.
Rock Mech. & Min. Sci. 35(4-5):533, Available
on CD-ROM from Elsevier, New York.


Rock Mechanics for Industry, Amadei, Kranz, Scott & Smeallie (eds) © 1999 Balkema, Rotterdam, ISBN 90 5809 052 3

Static and dynamic loads in ore and waste rock passes in underground mines

M.Beus & S.R. Iverson
Spokane Research Laboratory, National Institute for Occupational Safety and Health, Wash., USA

A. Dreschler & V.Scott
Department of Civil Engineering, University of Minnesota, Minneapolis, Minn., USA

ABSTRACT: This paper describes research to improve safety during transport of ore and waste in underground
mines. Field tests are underway in mines in Idaho and Montana. Strains measured on structural support
members in an ore pass provided information about total forces acting on the structure as material was dumped
into it. Results show that measured static loads were considerably less than actual total weight of the material
dumped and that dynamic loads were subject to many factors, such as effects of blasting to remove hang-ups.

Comparisons of measurements and computer results using a particle flow code indicated that several
difficulties remain before achieving realistic determinations and models of the dynamic effects of particle flow
in ore passes and impact loads on the gates. Impact loads were overestimated in computer analyses as compared
to loads measured in field tests. An alternative design approach based on softening the chute and control gate

assembly is being proposed.

1 INTRODUCTION

Hazards related to the operation of ore and waste rock
passes have been identified as a significant safety
problem in underground metal mines in the United
States. Such hazards include structural failures,
blocked gates, and water flow. Specifically, ore or
waste rock hang-ups can collapse spontaneously or
during freeing operations; the sudden release of hang-
ups is the single most important cause of serious acci-
dents, The dynamic loads induced by large falling,
blocks of ore or waste rock can weaken the chute and
gate structure, Blocked gates can result in spillage of
large volumes of material. Damage can also be
caused from an air blast as material is released. Water
flowing into an ore pass can result in catastrophic
muck flows and inundation.

Existing design standards for ore passes are
essentially rules-of-thumb based on simplified equili-
brium analyses, model experiments, empirical obser-
vations and experience. This approach tends to assign
high safety factors to the chute and gate structure so it
can withstand excessive static and dynamic loads.

Ore pass design has structural and functional com-
ponents, with one affecting the other and vice versa.
‘The structural components arc ore pass walls, liners,
timber lagging, and chutes and gates, which control

the flow of material. The functional component is
concerned with the flow, or lack thereof (hang-ups),
of ore and waste.

Important structural design factors are the static
and dynamic loads that ore pass chutes and gates must
withstand. Blight et al. (1994) conducted tests on
model underground ore passes to determine factors
associated with static gate pressure and dynamic
loads. The effects of ore pass length, inclination, and
the presence or absence of doglegs, which absorb im-
pacts from the release of hang-ups, were determined.
The results indicated minimal change in static load
when the material column exceeded a depth about 1 m
above the gate, and that total static load and dynamic
load factors decreased significantly when the inclina-
tion was less than 70°. They also found that the pres-
ence of a dogleg had little effect on the static gate, and
that peak impact loads could exceed four times the
static load in vertical or near-vertical ore passes.
‘Their conclusions were that stalic loads on the gate of
an ore pass could be predicted accurately using
equations developed by Janssen (1895) for vertical or
inclined silos.

Simplified versions of the Janssen equations for
determining static pressure normal to the control gate
of a vertical or inclined silo are—

489


Rankin, J., & G. Tolley 1978, Fault Tree Analysis.
Mine Safety and Health Admin., 32 pp.

Timoshenko, S. 1934. Theory of Elasticity. Eng. Soc.
Monograph. New York: McGraw-Hill.

495


Oy = Oymaxl L-€TM pax a)

and Gym = YR/Ktan6, (2)
where oy = vertical pressure on chute gate of
silo or ore pass,
ONmax = Maximum vertical pressure,
= hydraulic radius (cross-sectional
area over perimeter),
K = ratio between lateral and vertical
pressure,
Y = unit weight of rock in ore pass,
é = wall friction angle,
and Zz = height of ore above chute gate.

Solutions to determine dynamic load factors are
typically based on solutions derived by Timeshenko
(1934) and found in most engineering handbooks. It
can be shown that impact stress (a,) produced in a
structural member resulting from the impact of a fall-
ing body from a height (h) is greater than the stress
(o,) and deformation (6,) produced by the same body
applied as a static load in the ratio of —

9
—! t4(t+2H/6,)!"
Oy, (3)

It is generally assumed that the energy losses of
material falling down an ore pass are very high and
that the dynamic load factor can be approximated by
a case of sudden loading (h = 0), which results in a
dynamic load factor of 2.

Functional design factors minimize malfunctions
in material flow such as hang-ups, piping, and water
inundation and were extensively reported (Hambley &
Pariseau, 1983). This research developed guidelines
for dimensional relationships between ore pass open-
ings and ore size to prevent hang-ups; proper sizing of
drawpoints, chutes, and feeders; inclined versus verti-
cal passes; proper branch and bend angles; ore pass

that a hang up in noncohesive ores is directly related
to ore pass diameter, ore particle size, and height of
ore has also been described (Aytaman, 1960). In
cohesive ores, additional properties, such as cohesion,
density, and internal friction angle, have to be con-
sidered (Hambley, 1987). Two distinct types of hang-
ups were recognized: those caused by interlocking of
large boulders that become wedged in the ore pass and
those caused by cementing of cohesive fines.

Some of these design relationships are summarized
in Table 1.

2 RESEARCH APPROACH

Research at the Spokane Research Laboratory of
the National Institute for Occupational Safety and
Health (NIOSH) to investigate ore pass hazards and
develop methods to improve safety has evolved into
three approaches: (1) reduced-scale laboratory tests,
(2) full-scale field tests, and (3) computer modeling of
particle flow. Current research concentrates on deter-
mining the static and dynamic forces acting on the
chute and control gate assembly (Beus et al., 1998).

2.1 Experimental test facilities

The laboratory facilities for ore pass research
consist of both reduced-scale and full-scale ore pass
and chute components. The reduced-scale hoist and
ore pass testing facility has recently been constructed
to facilitate testing and to validate computer models in
a controlled setting (Beus & Ruff, 1997). ‘This fully
automated facility utilizes a 18.3-m-high hoist tower
to simulate the headframe and shaft (Fig. 1). A S.S-
m-deep underground "shaft" lined with concrete
sections houses a loading pocket and measuring
cartridge. The hoist drum is operated by a digitally
controlled 37-kW de motor. The skip has the capacity
of 0.453 mt. The ore pass system is simulated by a

Table 1. Ore dimensions and hangup prevention (from Hambley ctal., 1984)

Dimensional requirements

Types of hang-ups prevented

Did>5
D > (2c/y)(1+1/)(1+sing)

. Interlocking arches.

. Cohesive arches.

. Interlocking arches (drawpoints).
Hang-ups in transfer chutes

D_ = ore pass dimensions.
d iiameter of largest particle.
-ohesion of fines.

y =density of fines.

rt =ratio of opening length to width.
internal friction angle.

‘hute height.
hute width divided by width of outlet.



Figure 1.
facility.

Reduced-scale hoist and ore pass test

3.3. m in diameter corrugated culvert and an
instrumented control gate. The “ore pass” can be
inclined up to 25° off vertical in 5° increments to
study the effects of ore pass inclination. Ore and
waste material is loaded through a grizzly and loading
pocket system into the skip for hoisting. The skip
hoists the ore to the top of the headframe where it is
discharged into a hopper and a chute directed to the
top of the ore pass. The ore falls through the ore pass
into the chute and gate assembly and impact and static
load levels are acquired, The material is then is
dumped into the ground level grizzly and recycled
through the hoisting loop. Design of the ore pass,
chute, and control gate assembly is based on that
currently used in several mines.

In addition, to obtain accurate ficld data, full-scale
mockups are fabricated in which the actual structure
used at the field site is duplicated and calibrated in a
loading frame. The full-scale laboratory mock-up
using known loading conditions thus reproduces the
expected response of chute support structure and gate
forces in a mine, providing a calibrated system. These
mockup tests are also used to develop and evaluate

491

instruments schemes that can be easily and cheaply
implemented in the field.

In preparation for the initial field test, a full-scale
mockup of the support bolt assembly was constructed.
The support bolts were suspended on steel beams that
spanned stcel and reinforced concrete abutments. The
chute and gate were simulated with a rigid I-beam
welded to the chute support frame. The chute support
frame, I-beams, hanger bolts, and saddles are identical
to those used in the mine. Loads are applied using
two 180-mt hydraulic jacks. In the initial mockup,
four 350-ohm strain gages spaced 90° apart were in-
stalled longitudinally on the eight support bolts and
wired in series. The result is an electrically averaged
output signal through a 1,400-ohm effective resis-
tance. In calibration tests, this configuration was
shown to minimize the effects of bending and torsion-
al strains in the bolt and produce a true measurement
of axial strain. Long-term static load trends were
measured with Windows 95-based data acquisition
software and hardware at a scanning rate of up to 100
samples per second.

Static load tests on the full-scale mockup consisted
of applying load through a load distribution plate to
approximate the cross-sectional area of the actual
chute gate. The maximum force used for the structur-
al design of the chute gate was estimated to be 90 KN.
The support framework was designed to accommo-
date a total maximum force of 453 KN. A regression
analysis relating actual load measured on a test ma-
chine to computed load based on a summation of
strain from the eight bolts yielded a correlation coeffi-
cient (R’) of 0.995 and validated the measurement
approach.

After completion of the static tests, the hydraulic
cylinders and load distribution plate were removed.
Dynamic tests consisted of dropping a load of mine
waste rock having a bulk density of 1.7 g/cm’ from a
height of 183 cm into a 2.4-m-wide, steel-reinforced
container sitting on the mock-up chute assembly. A
front-end loader with a clam shell bucket was used to
drop the material into the container. After each drop,
the container and material were lifted and weighed.
Three tests were completed with material weighing
808, 1,000, and 1,238 kg. Results of test 3 indicated
a force at equilibrium of 14 kN and a peak impact
load of 31 KN. Figure 2 shows these results compar-
ed to a particle-flow computer model (section 2.3).
Peak impact forces were similar; however, static
forces were somewhat less than actually measured.

In preparation for the second field test, strain gages
were installed on identical sections of support beams.
The beam sections were then end-loaded in a test load
machine to yield axial load on the beam. Two 350-