PRESSURE, psi

Figure 23.—Contact pressure profile for base.

configurations where there was no contact at the tip of the
canopy. This difference in canopy resultant shifted the
base resultant slightly toward the rear by about 2 in at low
leg pressures,

Load development and contact pressure profiles on the
base were similar for these three canopy contact
configurations (see figure 27). Slightly higher toc loading
on the base was produced at high leg pressures for the
canopy configuration with tip contact. Load development
and contact pressure profiles for the canopy were
significantly different, since the contact configuration was
different for all three tests. As shown in figure 28, max-
imum contact loading was developed for the configuration
where a line of contact was established near the leg, since
little force was required at the rear of the canopy to
maintain equilibrium. The most uniform canopy loading

15

was established when the canopy contacts were equally
spaced from the leg connection.

Figure 29 shows load development for two-point canopy
and base contact. Maximum loads were developed at the
rear of the canopy and toe of the base. Total loading at
the rear of the canopy at 5,000 psi leg pressure was 475
tons. Distributed over the 180 in? of contact area provided
by the load cells, this translates into approximately
5,300 psi. Total load at the tip of the canopy was 117 tons,
which equates to 1,300 psi. Total load on the toe of the
base units was 235 and 200 tons, which equates to approxi-
mately 6,500 and 5,600 psi, respectively, for the 72 in? of
contact area, while loading at the rear of the base was 83
and 115 tons or 2,300 and 3,200 psi.

The test configuration with canopy contact over the first
80 in, beginning from the rear (approximately 50 pet of the
canopy area with no tip contact), produced a stress
concentration at the leg connection as shown in figure 30.
Maximum loads of 280 tons were developed in the two
load cells at this location, producing a contact pressure of
about 7,800 psi.

Horizontal shield loading is developed by horizontal
displacement of the canopy relative to the base. Since the
leg is inclined toward the face, the natural tendency is for
the canopy to be pushed toward the face. When this
motion is permitted without resistance from the roof, there
is no external force acting on the shield. When the
forward motion of the canopy is restrained by the roof,
this restraining force acts as a horizontal force on the
shield. Conversely, if the strata move toward the face and
sufficient friction is generated at roof and floor shield
interfaces, the canopy will be displaced backward toward
the gob, creating a horizontal force acting on the shield.

Hence, three horizontal displacements of the canopy
relative to the base are possible: (1) toward the face, (2)
no displacement, and (3) toward the gob. Each of these
conditions causes a change in the reactions developed at
the canopy-caving shield joint and lemniscate link forces,
which affect the load distribution on the canopy and base.
A comparison of canopy and base resultant force locations
for these conditions is shown in figure 31. Movement of
the canopy toward the gob (face-to-waste strata displace-
ment) caused the canopy resultant to move slightly forward
toward the tip where it acted closer to the leg in compar-
ison to the condition where the canopy was horizontally
constrained or where the canopy was permitted to freely
displace toward the face. A much greater effect was seen
on the base resultant because of the change in link forces.
The base resultant acted closest to the toe when the can-
opy was permitted to displace freely toward the face, and
moved progressively away from the toe toward the rear as
the canopy was horizontally restrained and then forced to


To simulate immediate roof conditions where the strata
deform under load, wood or rubber layers were placed
between the load cells and the platens. The contact area
on the canopy increases as the wood or rubber is com-
pressed from the forces acting on the shield. The contact
area increased from 40 pct for the rigid roof simulations
discussed above to about 90 pct for the wood and 100 pet
for the rubber contact materials. The location of the con-
tact force migrates from the ends (tip and rear) of
the canopy toward the leg as the canopy and simulated
roof material deform. In rigid roof simulations, the max-
imum contact force at the rear of the canopy also moves
inward toward the leg as the loading increases. A com-
parison of load development on the canopy for rigid and
compliant roof simulations is shown in figure 12. Max-
imum contact pressures developed on the canopy are sig-
nificantly higher for stiffer roof structures, since more
of the loading is concentrated on a smaller area of the
canopy (see figure 13).

The load distribution on the canopy is measured by the
24 load cells arranged in 2 lines of 12 cells along the
length of the canopy (see figure 10). Since the 24 load
cells define a contact area of about 10 pct of the total area
of the canopy, roof contact pressures as determined from
the load cell data are magnified by a factor of 10 (assum-
ing all load cells are in contact throughout the loading) in
comparison to full canopy contact over its entire surface.
Therefore, to provide a more realistic expression of roof
contact pressures, the load cell forces were distributed
over the entire surface of the canopy using a stress contour
software program. A description of the stress redistri-
bution technique is provided in the appendix. Several
observations are made regarding these pressure profiles:

Leg connection

Tip
Rear
80 : oT Tt
key igid

60 Leg pressures, 10° psi Rigid roof
40 o1 4

2 2 02 05

2 oo

fo

$

S

a

a

&

5

a

<

a

= 60 Compliant roof (rubber)
40
20

° 2 40 60 80 100 120 140 160
CELL LOCATION FROM CANOPY REAR, in

Figure 12.-Comparison of canopy loading for rild and
compliant roof simulations.

1. In general, the pressure increases nonlinearly from
the tip of the canopy to the rear as shown in figure 14. A

Leg connection

Rear Tip
120 T T T T T
90 Left side
2 60
os? J
S$ 0
+ 150 ‘ —.— ——
a 120 Right side
KEY
ZB 2% 4
2 D Rigid roof
uj 60 © Compliant roof (wood)
= x ‘4 Compliant roof (rubber)
° 20 40 60 80 100 120 140 160

CELL LOCATION FROM CANOPY REAR, in

Figure 13.—Maximum canopy contact loading for stiff and
compliant roof simulations.

ee

PRESSURE, psi



MEASURED LOAD, tons

KEY

ty Contact A

120
90
80
60

120
90
60
30

120
90
60
30

19

Canopy contact A f

x

1

1

1

1

l
Rear Leg

T T T T T T ty Contact B

Tip

Canopy contact B 4

if 4

n n mn 1 rn

Rear Leg

¥ . y " y ty Contact C

rs

Ti

Canopy contact C 4 T

4 y f
Y Canopy

20

|x

fl A fl 1
40. 60 80 100 120 140 60 Rear Leg
CELL LOCATION FROM REAR, in

Tip

MEASURED LOAD, tons

88sss

Figure 28—Maximum contact loading for controlled canopy resultant tests.

Leg connection:

Canopy loading

key
Leg pressures, 10° psi
at v4
02 05
a3

Y

x
i}

°

10 20 30 40 50 70 80 90 100
CELL LOCATION FROM REAR, in

Figure 29,Load development for two-point canopy and base contact.




16

move toward the gob. The base resultant locations for relative to the base, the resultant was 41 in from the toe;
these three conditions are as follows: (1) for movement of and (3) for movement of canopy toward the gob, the
the canopy toward the face, the resultant was located 22 in resultant was 49 in from the toe.

from the toe; (2) for no horizontal movement of canopy

a a
wf g
2 =]
B 3
Ww Ww
4 a
a a
With horizontal loading Without horizontal loading

Figure 24.—Comparison of base contact pressure profile with and without horizontal loading.



2

effective design from the perspective of the loading distri-
bution on the base would be to have the device located
where it would not be in contact with the base-bearing
surface. One possibility would be to place a similar device
between the two base pontoons,

Unlike the canopy, where the resultant vertical force is
fixed in location at close proximity to the leg, the resultant

location on the base theoretically could be controlled if the
link forces could be controlled, This suggests that toe
loading might be reduced if the link forces could be re-
duced to cause the resultant force to act closer to the leg.
Additional research would help to evaluate this concept in
more detail.


18

°

CELL LOCATION FROM REAR OF BASE, in

100

Figure 27.-Load development for controlled canopy resultant tests.

@  fY Contact A
F T
=s f | q
ee K 1 canopy Ux,
ut Canopy contact Rear Leg Tip
Ww 4
ga Comparison of canopy - A
am 10 resultant locations ne 4 3 ty Contact B
a2 o 1 Fr & T
og i
g 65 TM—T ——— + TM—T —— g q | q
a 60 d e H Canopy x,
5° <a == Rear Leg Tip
aa 55 —— 4
SS solfem Leg position, ty Contact ¢
es Comparison of base }
wo 45 resultant locations 13 H f
40 n 1 1 1 L 1 n 1 1 4 x 1__Canopy xX
° ! 2 3 4 5 6 Rear Leg Tip
LEG PRESSURE, 10° psi o 46 188
Figure 26.—Comparison of canopy and base resultant force locations.
KEY
Leg pressures, 10° psi
of 4
o2 05
a3 96
ty Contact A
7
I
lS i
€ |
s |___Canopy x
= fe) 1 1 1 rf n L iat n L Rear Leg Tip
a
g 80 tY Contact B
g 60 i
a 40 f | 4
z 20 ! Canopy
a ) Rear Leg Tip
w 80 : : T r r 1 T T ty Contact ¢
60 4
40 7 if f
4 18
20 a _Canopy
i n n 7—— r 7 n Rear Leg Tip
10 20 30 40 50 60 70 80 90


13

40

—©-With horizontal loading

—©-Without horizontal loading
‘at 2,800 psi leg pressure

MEASURED LOAD, tons

0” 1 20 30 40 50 60 70 80 90
CELL LOCATION FROM REAR OF BASE, in

Too

MEASURED LOAD, tons

1oof — Without base-titting 4
device
80 Leg KEY 3 7
Pressures, |O” psi 4
60 of v4
40 4
20 4

 — Bi
20 30 40 50 60 70 80 90
CELL LOCATION FROM REAR OF BASE, in

100

Figure 19.—Comparison of base loading with and without
horizontal loading.

To provide more compliancy in the floor, 2.5 in of
tubber was used between the base and load cell on one
test. However, the rubber columns tended to shear from
the action of the leg forces pushing the base horizontally,
rather than compressing the rubber vertically as intended.
The end result was a reduction in horizontal force acting
on the base, which caused increased toe loading and
reduced pressure at the rear of the base.

To determine the effect of the canopy contact config-
uration on the base loading distribution, an effort was
made to control the resultant location on the canopy by
placing contact blocks at certain locations on the canopy.
However, as previously indicated, the resultant location on
the canopy must be in close proximity to the leg to main-
tain equilibrium. Hence, the base pressure was not signifi-
cantly affected by the changes in the canopy contact as
shown in figure 21. Since the location of the resultant
force on the canopy is dependent upon the canopy capsule
force, the capsule force was varied over its full range. As
shown in figure 22, the change in canopy capsule force
moved the location of the resultant force acting on the
base approximately 12 in. As the capsule force decreases,
the canopy resultant moves closer to the leg, causing the
base resultant to move closer to the toc.

An example of the base contact pressure for full-
contact loading is illustrated in figure 23. The pressure
profile is fairly linear between the rear and the toe when
the base-lifting device is removed and horizontal shield
loading is present. Peak toe pressures are typically in the
range of 400 to 500 psi for leg pressures approaching 6,000
psi, while pressures at the rear of the base are typically in

Figure 20.—Comparizon of base loading with and without
base-itting device.

the neighborhood of 50 to 100 psi. As previously indi-
cated, the absence of horizontal loading can affect these
contact pressures by about 75 pct. The pressure profile is
more nonlinear with higher toe loading when the horizon-
tal forces are eliminated. A comparison of contact pres-
sure profiles with and without horizontal loading is shown
in figure 24, Toe pressures of 700 to 800 psi were cal-
culated at leg pressures of about 3,000 psi with the base-
lifting device installed, and peak pressures at the initial
contact locations were in the range of 1,500 to 1,750 psi.
It is important to remember that these values are deter-
mined by distribution of the measured load cell forces over
the complete area of the base. If less than full contact
is attained, significantly higher contact pressures can be
developed.

Combined Canopy and Base Tests

A series of tests were conducted in which both the
canopy and base contact loads were measured. Five com-
binations of canopy and base contact configurations as
shown in figure 25 were evaluated. A primary purpose of
this test series was to evaluate the interaction of the
canopy and base contact conditions. Another objective
was to determine the canopy and base loading for two-
point contact on both components, since this is likely to be
the configuration that produces maximum loading. The
third objective was to simulate a reported common contact
configuration at the mine site where the shield was used,
that is, full base contact with full canopy contact except for
the sloped portion of the canopy tip. A final objective of


Figure 3.—Possible load distribution assuming elastic support
structure.

4. Tip distortion

Resultant
vertical force

Rotational
tendency

Rotational

tendency
Resultant
vertical force
y Immediate roof i

Stress concentration
at rear of canopy

Stress concentration

at toe of TE i

£ Immediate floor

A

Figure 4—Upward sloping of canopy tip to ensure tip contact.

acting in conjunction with the leg forces provide moment
equilibrium of the base. Since the rear link acts in oppo-
site direction to the leg, a resultant force forward of the
leg is possible. The location of the resultant force is
largely dependent upon the relative magnitude of the leg
and link forces.

Because of the relatively high load distribution at the
toe of the shield, which often causes the base to sink into
the floor, another trend in shield design has been the in-

corporation of a base-lifting device as shown in figure 6.”

The base-lifting plates are extended by single acting hy-
draulic cylinders that lift the toes of the base during the
support advance. After the support is advanced, the plates
are retracted to rest against the bottom of the base, cre-
ating a discontinuity near the toe of the base. This ar-
rangement promotes two-point contact at the toe and rear
of the base similar to that produced on the canopy by the

Barczak, T. M., and R. C. Garson. Shield Mechanics and Result-
ant Load Vector Studies. BuMines RI 9027, 1986, 43 pp.

Figure §.—Rotational moment (top) causing high toe loading

(bottom) in two-leg shield design.

Hydraulic cylinder

Base-lifting device

Figure 6.—Base-lifting device.

sloping of the canopy tip. However, bases tend to have
higher bending stiffness than canopy structures and are
less likely to deform from loading. Therefore, deformation
of the floor is even more critical to establish full contact
along the length of the base. In hard floor conditions, the
presence of the base-lifting device can exaggerate the in-
herently high stress concentration at the toe of the base.
Fortunately, quite often there is debris or soft floor ma-
terial that compacts to conform to the base geometry to
alleviate the two-point contact loading.



INTRODUCTION

This work was done in support of the U.S. Bureau of
Mines’ goal to improve the health and safety of under-
ground coal mining through development of better roof
support systems,

Successful application of the longwall mining method
requires the use of high-capacity face support structures to
provide adequate ground control. State-of-the-art shield
supports provide capacities of up to 900 tons of (vertical)
supporting force. The use of these high capacities requires
correspondingly large canopy and base contact areas to
distribute the force to the mine roof and floor. Although
the resulting average force per area (contact pressure) is
relatively low (10 to 12 ton/ft? or about 140 to 170 psi for
the canopy and 30 to 40 ton/ft? or about 400 to 600 psi for
the base), the actual contact interaction causes unevenly
distributed and much higher contact pressures. When the
maximum of these distributions exceeds certain values,
local failures of the roof and floor might follow. Such fail-
ures could degrade strata stability and make support ad-
vancement difficult. To prevent this occurrence, a better
understanding of load distribution between longwall shields
and the contacting strata is required,

Assessment of a shield’s roof and floor contact pressure
can be made by some form of mathematical model or by
experimental means, The most commonly used mathemat-
ical model assumes a rigid structure yielding a linear
pressure profile along the length of the canopy or base as
shown in figure 12 However, this provides an erroncous
picture of the load distribution since the structures are
known to deform from loading. Unfortunately, numerical
models that have been developed to model the elastic re-
sponse of the shield components have met with only lim-
ited success, Hence, an experimental approach was chosen
for this study. The approach used was to employ a series
of hydraulic pressure cells distributed on the canopy and
base to measure the contact pressure under known loading
conditions provided by the Bureau’s 3-million-Ib load
frame.

The objective of this study was to evaluate the load
distribution acting on a shield for simulated underground

3Peng, S.S. Coal Mine Ground Control. Wiley, 1978, pp. 259-268.

conditions, This requires some consideration of the com-
pliancy of the roof and floor, since the deformation of the
strata can influence the contact area on the shield and the
corresponding canopy and base load distribution. The rig-
id steel platens of the load frame can simulate very stiff
strata, such as might be encountered with a strong sand-
stone or limestone structure, but the platens do not
simulate more compliant strata, such as shale or clay
formations, The compliancy of the immediate roof and
floor were simulated by wood or rubber layers placed
between the shield and the platens of the load frame.
Wood and rubber were chosen primarily for practical
reasons, but they do have properties similar to (broken)
tock in that they increase in stiffness as they are com-
pressed. Other parameters that were investigated in the
study included (1) contact configurations established by
the placement of the load cells on the canopy and base;
(2) amount of external horizontal force acting on the
shield; (3) applied vertical force; and (4) canopy capsule
force. The effects of a base-lifting device were also
investigated as part of this study.

Figure 1.—Uinear force distribution assuming rigid body
structure,



CANOPY AND BASE LOAD DISTRIBUTION ON A LONGWALL SHIELD

By Thomas M. Barczak' and David F. Gearhart?

ABSTRACT

This U.S, Bureau of Mines report examines the roof and floor contact pressure provided by the
interaction of a shield with the surrounding strata. Controlled forces were applied to an 800-ton two-leg
shield in the Bureau’s mine roof simulator; the distribution of forces acting on the canopy and base of
the shield were measured with 24 hydraulic pressure cells, Several influential factors that affect the load
distribution were investigated: (1) the magnitude of loading (leg pressure); (2) the profile of the canopy,
principally the upward-sloping tip; (3) the base-lifting device; (4) horizontal load acting on the shield;
and (5) compliancy of the immediate roof and floor. It was concluded that the shield does not develop
full canopy or base contact without deformation of the strata. Maximum contact pressures are devel-
oped at the rear of the canopy and on the toes of the base. Less than 10 pct of the available shield
capacity is developed at the canopy tip despite the design intention to ensure tip contact. Horizontal
loading reduced the toe pressure acting on the base by as much as 75 pct. The base-lifting device
exaggerated the inherently high toe loading, increasing the contact pressure by more than 200 pct.

Research physcist, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.
2Blectrical engineer, Schneider Services International, Pittsburgh, PA.


14

KEY

MEASURED LOAD, tons

Leg pressures, 10° psi

o1 v4

02 05

a3 6

ESES3 Contact area
J ty Contact A
4 i ;
J | A
4 | y
4 | Canopy ax
J =
Rear Leg Tip

| ty Contact B
7 T
4 1 ?
7 f | f Canopy
7 G
4 Hl x
) Rear Leg Tip
7 ty Contact C
4 T
7 f f Canopy xX
4 H x
| Rear Leg Tip

CELL LOCATION FROM REAR OF BASE, in

100

oe

Figure 21.—Effect of canopy resultant location on base loading.

key
Base resultants

—> Max capsule force

—> Zero capsule force

Bose resultant locations

Figure 22.-Etfect of canopy capsule force on base resultant.

the combined canopy and base series was to evaluate the
effects of face-to-waste strata movements on the canopy
and base loading distribution.

Using the scheme that was developed for the base
series of tests, an effort was made to vary the resultant
location on the canopy by varying the canopy contact
configuration. Seven pressure cells evenly spaced under
each base unit were used to simulate full base contact for
these tests, The resultant locations on the canopy and
base for these three tests are shown in figure 26, As seen
in the figure, the resultant location on the canopy migrated
to a position 2 to 3 in behind the leg at leg pressures
above 5,000 psi. With contact at the canopy tip, the can-
opy resultant was as much as 12 in closer to the rear of
the canopy at leg pressures below 1,700 psi than in other


Horizontal
shield
loading

Vertical shield loading

Figure 2.Horizontal and vertical forces acting on shield support.

A study of shield mechanics indicates that the leg is the
dominant internal force acting on the shield canopy This
suggests that the location of the resultant external force
acting on the canopy is likely to be in close proximity to
the leg connection, since the canopy capsule force and the
reaction developed at the caving shield joint are of much
smaller magnitude than the leg forces. The distribution of
loading on the canopy is largely determined by the relative
length of canopy forward and rearward of the leg connec-
tion. Typically, considerably less loading is developed at
the tip of the canopy than at the rear, since the tip portion
is considerably farther from the leg and requires propor-
tionally less force to maintain moment equilibrium.

The distribution of forces is also dependent upon the
bending stiffness of the canopy. A perfectly rigid canopy
and base provide a linear distribution of loading as

SBarczak, T. M., and D. E. Schwemmer. ‘Two-Leg Longwall Shield
Mechanics. BuMines RI 9220, 1989, 34 pp.

illustrated in figure 1, while an elastic structure might
respond as shown in figure 3°in accordance with deforma-
tions produced by internal and external forces. To ensure
contact pressure at the canopy tip, there has been a recent
trend in shield design to slope the tip portion of the can-
opy upward with respect to the remainder of the canopy as
shown in figure 4.“For example, the shield utilized in this
study had a tip distortion of approximately 2 in. Initially,
this produces two-point contact at the tip and rear of the
canopy. Generally, for this type of design, elastic defor-
mation of the canopy does not produce full contact along
the length of the canopy. Hence for this type of canopy
design, deformation of the strata is necessary to establish
full canopy contact,

Since the canopy is cantilevered in front of the toe of
the base and the leg connection on the canopy is forward
of the leg connection on the base, there is a natural tend-
ency for a rotational moment (see figure 5) that produces
high toe loading in two-leg shield designs. The link forces


150 T T T T T T T
120F |
KEY
a 90F Leg pressures, 10° psi
§ a1 v4 J
2 60F o2 05
a L 43 96 4
4 30 Y
g ° : : i . : : f Contact configuration
2 150 : T -
w oooiog Note
= 20 j |oooloo
n ! Canopy | x
ES
gq 90 Rear Leg Tip
= 60
30 4 31415 1617 B19 Base
L j A oooio000
° 20 40 60 80 100 120 140 160 1234567
CELL LOCATION FROM REAR, in TT
Figure 30.—Load development in absence of canopy tip contact.
60 r r , r + DISPLACEMENT MODES
————
re) = 50 r resultant locations Waste-to-face 7]
we 454 —— Vertical only 4
5 ‘ ~~ Face-to-waste 7
Ze 40 7 Waste-to-face
2 w 35 4 horizontal displacement
S82 30 : :
oa
80 T T if
m4
2° 7 4 t
=< a Vertical displacement
3 6 60 4 only
HZ Comparison of base
weg 50 resultant locations ~| ——s
40 L 4
4 5 6
LEG PRESSURE, 10° psi ———
Face-to- waste
OT horizontal displacement

Figure 31.—Effects of horizontal displacements of the canopy on the canopy and base resultant force locations.


er REPORT OF INVESTIGATIONS/1992

Canopy and Base Load Distribution
on a Longwall Shield

By Thomas M. Barczak and David F. Gearhart

UNITED STATES DEPARTMENT OF THE INTERIOR

BUREAU OF MINES


Figure 7.—Mine roof simulator.

400 r

fe KEY

© Data point
— Linear regression
Steel plate 300 halle 4
l, Flatjack a sd =
6 pressure cell - Ls
Pressure uy 200 d
transducer ul
&
5
2
SIDE VIEW 100 d

Pressure
transducer

22
9
ic)

L 1
0024 0036 0048

DISPLACEMENT, in

L
012
PLAN

Figure 8.—Hydraulic pressure cell used to measure load
distribution. Figure 9.—Example of stiffness of pressure cell,


LOAD-MEASUREMENT DEVICES

Since load-measurement devices interface with the
immediate strata and the shield, they become an integral
part of the system and influence the resulting load distri-
bution. The load-measurement devices must be employed
in such a way that the entire load is transferred through
these devices to preserve force equilibrium, Under this
condition, the placement of the load cells alone defines the
contact configuration and contact area, which, as previ-
ously discussed, is a major factor influencing the magni-
tude and distribution of the forces acting on the shield.

The load cells should also be calibrated to preserve
the accuracy of the load measurements. Some information
should also be obtained pertaining to the stiffness of the
load-measurement devices. In areas where there is no

rotational degree of freedom within the shield component,
the relative stiffness of the load cells may influence the
resulting loading distribution. For example, when multiple
cells are closely placed between the leg and the rear of the
canopy, a cell(s) surrounded by stiffer cells tends to attract
less force. While force equilibrium is preserved, the distri-
bution of this force may be affected by this behavior. This
situation is most likely to occur when a rigid boundary
roof condition exists. Therefore, if several load-measuring
cells are employed, the relative stiffness of the load cells
should be similar. A more critical consideration in rigid
boundary roof conditions is that slight irregularities in the
surface of the canopy and base can produce localized areas
of high stress concentrations, particularly when very stiff
load cells are employed.

LABORATORY SIMULATIONS

Laboratory simulations were conducted in the Bureau’s
mine roof simulator. The simulator, shown in figure 7, is
capable of providing controlled forces or displacements in
both a vertical and horizontal direction to full-sized long-
wall shields. The biaxial load frame has a force capacity
of 3 million Ib vertically and 1.6 million Ib horizontally. A
total of 45 tests were conducted.

A two-leg 800-ton shield provided by Island Creek Coal
Co. was used for this study. This shield is a high-capacity
support representative of state-of-the-art two-leg shield
designs. It has a canopy contact area of 59.2 ft? and a
total base contact area of 24.6 ft? from two base pontoons.

TEST INSTRUMENTATION AND CALIBRATION

Hydraulic pressure cells were used to measure the load
distribution acting on the shield canopy and base. The
cells are constructed of two parallel 6-in by 6-in stainless
steel plates. The plates are welded along the circumfer-
ence and filled with oil to provide a flatjack arrangement
as shown in figure 8. The cells are fitted with a tube that
is connected to a pressure transducer for measurement of
the hydraulic pressure developed in the cell. Since the cell
is a closed system, deformation of the cell produced by
loading on the plate surfaces produces an increase in
hydraulic pressure.

Twenty-four pressure cells of this construction were
employed to measure the canopy and base load distribu-
tion on the longwall shield. Each cell was calibrated to a
force of 300,000 Ib. The calibrations were made in a stiff
1-million-Ib load frame. The accuracy of the load cells
ranged from 0.3 to 12.5 pet and averaged 2.4 pet for the
24 load cells. This equates to an error of 2.4 tons at 100
tons of measured load. An example of the stiffness of the

load cells is shown in figure 9. The stiffness is seen to
increase nonlinearly with increasing load, probably because
of some air in the cell system.

DATA DOCUMENTATION

Test data are documented in several ways throughout
the report. A brief explanation of data documentation
follows.

1, Measured Load Profiles.—These graphs depict load
cell response as a function of location on the shield. Dif-
ferent curves are shown for 1,000-psi increments of leg
pressure, These graphs are useful in showing load transfer
to different portions of the canopy or base from deforma-
tion of the contact material (simulated roof or floor) or
deformation of the shield structure.

2, Resultant Loading.—The location of the resultant
force acting on the canopy or base is shown as a function
of leg pressure for each test. Analysis of these data shows
movement of the resultant external force along the length
of the canopy or base for different load conditions. While
the location of the resultant force is not generally the point
of maximum pressure and is not unique to one contact
configuration, it is determined by moment equilibrium of
the loading distribution and therefore provides some
indication of the nature of the loading. Changes in the
location of the resultant are indicative of a change in load
distribution, These curves are useful in determining a shift
in loading toward the front or rear of the shield.

3. Contact Pressure Profiles—Measured force from
each of the load cells is distributed over the entire area of
the canopy or base as an approximation of contact pres-
sure for full-contact load conditions. Force equilibrium is


ACKNOWLEDGMENTS

The authors wish to acknowledge Island Creek Coal
Co., in particular Rau Pothini, longwall coordinator, for
supplying the shield used in this study. This work was a
cooperative effort established between the Bureau, West

Virginia University, and Island Creek Coal Co. Gratitude
is also extended to Syd Peng, chairman, Department of
Mining Engineering, West Virginia University, for his
assistance in organizing this effort.

ASSESSMENT OF INFLUENTIAL FACTORS

An investigation of a longwall shield’s canopy and base
load distribution under controlled laboratory conditions re-
quires a fundamental understanding of influential factors.
These factors are divided into three general categories:
(1) boundary conditions, which describe the interaction of
the shield with the strata; (2) shield design, which de-
scribes the component constructions and overall shield
mechanics; and (3) load-measurement devices that are
used to measure the contact pressure.

BOUNDARY CONDITIONS

The interaction of a shield with the immediate roof and
floor imposes boundary conditions that control shield load-
ing. These boundary conditions are established by (1) the
contact configuration established with the immediate roof
and floor; (2) loading induced by the strata; and (3) the
deformation of the roof and floor in response to the shield
resistance.

The contact configuration established with the imme-
diate roof and floor can range from full contact over the
entire canopy and base surface area to isolated point con-
tact at a few locations, From analysis of force and mo-
ment equilibrium of the shield, it is readily seen that the
contact configuration largely determines the magnitude
and distribution of forces imposed on the shield by the
strata interaction.

After being actively set against the roof, the shield
becomes a passive support, and load is developed in re-
sponse to support and strata interactions that produce
displacements of the shield canopy relative to the base.‘
As illustrated in figure 2, both vertical and horizontal
forces can be imposed on the shield by roof-to-floor and
face-to-waste displacements of the strata or by resistance
of the strata to internal shield forces that produce motion
of the canopy relative to the base. A shield acted upon by
vertical forces has a different loading distribution on the

‘Barczak, T. M,, and D. E. Schwemmer. Horizontal and Vertical
Load Transferring Mechanisms in Longwall Roof Supports. BuMines
RI 9188, 1988, 24 pp.

canopy and base than a shield acted upon by both vertical
and horizontal forces does. Therefore, it is necessary to
control both vertical and horizontal shield loading in the
laboratory and to understand the source of this loading to
fully evaluate the distribution of forces transmitted to the
roof and floor.

Shield canopy and base components are not rigid struc-
tures and the immediate roof and floor are generally not
rigid foundations. Therefore, a rather complex boundary
condition is developed involving the interaction of the
support with the strata. The distribution of forces acting
on the shield is determined by the elastic deformation of
the shield components and the compliancy of the immedi-
ate strata in response to the shield resistance. Likewise,
the contour of the immediate roof and floor may not coin-
cide with the profile of the canopy and base, causing iso-
lated areas of contact. The capacity of the strata to
deform to establish more uniform contact with the canopy
and base significantly influences the resulting loading
distribution in these situations. Hence, a stiff competent
roof with a well-defined contact boundary produces a
much different loading distribution on the shield canopy
than a friable roof with a compressible layer of debris over
much of the canopy does. Likewise, a strong floor is more
likely than a weak floor to produce localized areas of high
stress concentration on the base. Soft floor materials, such
as clay, exhibit plastic deformation in response to base
loading and conform more easily to the profile and elastic
response of the base structure.

SHIELD DESIGN

The design of a shield is also an important considera-
tion in assessing shield loading and strata interaction. The
primary considerations are (1) the location of the leg
connection relative to the length of the canopy and base;
(2) the arrangement and loading of the lemniscate links;
(3) the profile and bending stiffness of the canopy and
base.


17

ty ty
T
ga | 240 24 23 2 21 20
g20 | 230 oo q oo No tip
10 220
a9 | as oodog contact
Os Canopy WolxX 2 lod 9 6 Canopy 4
i non 1 xy
Rear Leg Tip Rear Leg Tip
r'3-l4-15-716-17-18-19 p/3-14-15 16-17-1819}
ooomdoog oc omoog
Base Base
T 7 q 7
Dy Dp Frid, O, O, 0) Op cin,o,0/0
Rear Toe Rear Toe
ty ty
T
Di | a4 ow | 210
a20 023 oe | 220
dio |! O22 o5 | so
og ! gle o3 | 90
os ! Dll Canopy |x o4 Canopy ion] x
1 a it —
Rear Leg Tip Rear Leg Tip
13-14-I5-716-17-18-19: T
os 230
ogoalooa o4 ! Zo
Base Base
T 7. i io
Dy Op Ching Os Og Be tke
Rear Toe Rear Toe
ty
T
O2| 24
o20 023
O10 d22
o9 12
o8 ul Canopy x
! =>
Rear — Leg Tip
ri3-14-I5-716-I7- 18-19:
Oooloooa0 KEY
1 O Load cell designations
Base and locations
T
0, Op CF ity Os 060

Rear Toe

Figure 25.—Combinations of canopy and base contact configurations.



APPENDIX.—DISTRIBUTION OF MEASURED CONTACT FORCES

The software package used to process the data from the
pressure cells uses the inverse distance squared function to
distribute the measured forces over the unmeasured areas
of the canopy and base to estimate the actual contact
pressure distribution if measurements were available over
the entire canopy or base surface. In order to simulate
full contact load conditions and to preserve force equilib-
rium, the measured forces from the load cells must be re-
duced in magnitude in proportion to the area covered by
the load cells relative to the total canopy or base surface
area. Otherwise, the stress contour program would pre-
serve the measured forces at the load cell and by inter-
polating between load cells produce a total loading that
exceeds the actual total load applied to the shield, The
profile of loading is then determined by superpositioning
of all distributed forces from the load cells as determined
by the iaverse distance squared relationship.

Since the program preserves the measurement at the
location where it is recorded, a smooth distribution is
generally not achieved. The peaks and valleys that appear
in the stress distribution plots are an artifact of the
distribution process. Low values in close proximity to

large values appear as dimples below the surface, while the
larger value is distributed around the lower value. Like-
wise, large values appear as peaks. Hence, the actual con-
tact pressure is likely to be between the smooth surface of
the distribution shown in the contour plots and the peaks
and valleys.

The equation for the inverse distance squared function is

where  Z = neighboring measured point,
d = distance,
and n = number of Z elements,

INT.BU.OF MINES,PGH.,PA 29561


CONTENTS

Page
Abstract 1
Introduction .. 2
Acknowledgments . 3
Assessment of influenti: 3
Boundary conditions ... 3
Shield design ...... 3
Load-measurement devices 6
Laboratory simulations. .... 6
Test instrumentation and calibration 6
Data documentation ......... 6
Data analysis and test results . 8
Canopy tests . 8
Base tests ... 11
Combined canopy and base tests B
Summary and conclusions .. . 21
Appendix—Distribution of measured contact forces . . B
ILLUSTRATIONS
1. Linear force distribution assuming rigid body structure ... 2
2. Horizontal and vertical forces acting on shield support ... 4
3. Possible load distribution assuming elastic support structure. . 5
4, Upward sloping of canopy tip to ensure tip contact .... 5
5. Rotational moment causing high toe loading in two-leg shiel 5
6.. Base-lifting device ... 5
7. Mine roof simulator . . 7
8. Hydraulic pressure cell used to measure load distribution . . . 7
9, Example of stiffness of pressure cell ............0..seeeeeee 7
10. Canopy contact configurations for canopy load distribution tests 8
11. Contact area established against rigid roof................. 8
12. Comparison of canopy loading for rigid and compliant roof simulations 9
13, Maximum canopy contact loading for stiff and compliant roof simulations 9
14. Contact pressure profile for canopy . 9
15. Free-body diagram of shield canopy . 10
16. Movement of canopy resultant as shield loading increases . 10
17. Migration of load development at rear of canopy for rigid roof simulation n
18. Comparison of canopy resultant locations with and without horizontal loading . 12
19, Comparison of base loading with and without horizontal loading . . B
20. Comparison of base loading with and without base-lifting device 13
21. Effect of canopy resultant location on base loading .. . 4
22, Effect of canopy capsule force on base resultant . 14
23. Contact pressure profile for base .............. 15
24. Comparison of base contact pressure profile with and without horizontal loading . 16
25. Combinations of canopy and base contact configurations ... 7
26, Comparison of canopy and base resultant force locations . 18
27. Load development for controlled canopy resultant tests .. . 18
28, Maximum contact loading for controlled canopy resultant tests . 19
29. Load development for two-point canopy and base contact . 19
30. Load development in absence of canopy tip contact ....... . 20
31. Effects of horizontal displacements of the canopy on the canopy and base resultant force locations ..... 20


10

more detailed examination shows that the minimum pres-
sure develops in the region where the canopy begins to
slope upward, creating an inflection point of minimum
pressure in the pressure profile. From this inflection point
the pressure rises slightly to the tip and increases at a
much faster rate while progressing to the rear of the
canopy.

2. There is relatively little contact pressure developed
at the canopy tip compared with the rear area of the can-
opy, regardless of the contact configuration, Two things
are responsible for this behavior: (1) Since the tip of the
canopy is 2.25 times as far from the leg as the rear edge
of the canopy, only 44 pct of the force at the rear of the
canopy is required at the tip of the canopy to maintain
equilibrium (assuming a rigid canopy with point loading at
the front and rear ends); and (2) Because of the geometry
of the canopy with the tip inclined upward, more contact
is developed at the rear of the canopy than near the tip of
the canopy as loading increases, which further reduces the
forces required forward of the leg to maintain moment
equilibrium.

3. When full contact is established along the length of
the canopy, contact pressures of about 50 to 75 psi can be
expected at the tip of the canopy at leg pressures above
6,000 psi and pressures of about 300 to 350 psi at the rear.
For stiff immediate roof boundaries where full contact is
not established over the entire canopy, localized areas of
pressure approaching 1,000 psi may develop, typically near
the rear of the canopy.

4, Tests were also conducted in which the sloped
portion of the canopy was not in contact with the load
frame. This contact configuration may be caused by a
layer of debris on the flat portion of the canopy or by the
presence of cavities in the immediate roof that prevent tip
contact. The pressure profile for this configuration is
similar to the full-contact configuration where tip loading
was present, except the load is more uniformly distributed
with higher loads in front of the leg than occurred in the
full-contact configuration.

A free-body diagram of the canopy as shown in fig-
ure 15 indicates that there are three internal forces acting
on the canopy: (1) leg force, (2) canopy capsule force,
and (3) canopy-caving shield joint force. Since the caving
shield assembly has very little vertical stiffness and the
area of the canopy capsule is small, these components de-
velop little force and the leg becomes the dominant force
acting on the canopy. Therefore, the resultant external

force induced by the roof contact must be near the leg
connection to maintain moment equilibrium.

The resultant consistently migrates toward the leg as
loading increases from zero leg pressure, and generally is
positioned slightly rearward of the leg as shown in fig-
ure 16~as the pressure approaches yield pressure. The
tests indicate that the resultant begins significantly rear-
ward of the leg and migrates forward toward the leg for
stiff flat roof conditions, while for softer roof conditions,
as simulated by the wood or rubber interface material, the
resultant begins forward of the leg and migrates rearward
to a point slightly past the leg. This difference in the
direction of resultant load migration is explained by exam-
ination of the load distribution. As shown in figure 17,~
load development at the rear of the canopy in the stiff roof
simulation is confined initially to small contact areas that
progress toward the leg as leg pressures increase. This

Fy

\S |

Py

— > P,

Internal forces:
L = leg force
C = canopy capsule force
Px, Py = caving shield joint reaction

External forces:
F,= resultant canopy force

Figure 15.—Free-body diagram of shield canopy.

70 A a a
a
3 60 N -
<
Bae sok om ___ Leo pes
es SSS saya
$= wo Compliant roof +
Sr simulation
ES 30 Rigid roof 4
23 simulation
5
20) ds
g &
= 19 1 \ po
o 1 2@ 38 4 58 6 7

——_______ AVERAGE LEG PRESSURE, 103 psi

Figure 16.—Movement of canopy resultant as shield loading
Increases,



12

PRESSURE, psi

With horizontal loading

PRESSURE, psi

60 T T
set
sab
48 ----<---

44

Without > 3 T
horizontal loading ——" $= ~——_
fi f

‘With horizontal
loading |

1 1
Rear ~————> Tip

40

RESULTANT LOCATION
FROM CANOPY REAR, in

ce) | 2

3 4 5 6 7

AVERAGE LEG PRESSURE, 10° psi

Figure 18.—Comparison of canopy resultant locations with and without horizontal loading.

Tests were conducted to determine the effect of the
base-lifting device on the base loading distribution. The
base-lifting device is a 1.5-in-thick plate that is placed
under the toes of the base. It is hydraulically activated to
lift the toes of the base during support advance. Although
the device is inactive during the mining cycle, it promotes
two-point contact at the toe and rear of the base, which
causes greatly increased loading in these areas.

Since the base units are very stiff, the base does not
deform sufficiently to provide more uniform contact.
Hence, the resulting load distribution is highly dependent
upon the compliance of the immediate floor; toe loading
increases with increased floor stiffness. The compliance of
the floor was simulated by placing 3/4 in of plywood

between the load cells and the base. The 3/4 in of wood
was insufficient to permit full contact along the length of
the base. Hence, the resulting distribution was bimodal
with areas of higher stress at the toe and rear of the base.
However, the loading at the toe was typically an order of
magnitude greater than the loading at the rear of the base.
Maximum pressures at the toe of the support were on the
order of 7,500 psi when distributed over the 36 in? of load
cell area.

Figure 20 compares base load development with and
without the base-lifting device. For these particular tests,
maximum toe loading increased by 215 pct from 35 to 110
tons at 4,000 psi of leg pressure, while the load at the rear
of the base increased by an order of magnitude from
about 2 tons to 25 tons.


Library of Congress Cataloging in Publication Data:

Barczak, Thomas M.
Canopy and base load distribution on a longwall shield / by Thomas M. Barczak
and David F. Gearhart.
Pp. cm. — (Report of investigations; 9418)
Includes bibliographical references.
Supt. of Docs. no,: I 28.23:9418,
1. Mine roof control. 2. Dead loads (Mechanics). 3. Longwall mining. I. Gear-

hart, David F. Il. Title, II. Series: Report of investigations (United States, Bu-
reau of Mines); 9418,

TN23.U43 [TN288] 622 s—dc20 [622’.28] 92-5711 CIP



a

SUMMARY AND CONCLUSIONS

The distribution of internal shield forces to the im-
mediate roof and floor can be a critical clement in the
capability of a shield to provide effective ground control.
While shield roof and floor contact pressures are normally
rated assuming a uniform load distribution on the canopy
and base, underground utilization produces contact pres-
sures of much greater magnitude because the load distri-
bution is not uniform. The contact configuration and the
actual load distribution is largely dependent upon the com-
pliancy of the strata, which is difficult to simulate in the
laboratory.

Assuming the contact configuration established be-
tween the shield and the immediate strata is properly
simulated, the profile of the resulting load distribution
on the shield can be determined by placing an array of
load-measurement devices on the canopy and base. How-
ever, an accurate assessment of the magnitude of the con-
tact pressure is more difficult to obtain, since the load-
measuring instrumentation typically covers only a small
percentage of the total canopy and base contact area and
the magnitude is highly dependent upon this contact area.
The approach taken in this study was to distribute the load
measured from 24 hydraulic pressure cells over the entire
area of the canopy and base using a stress contour soft-
ware program. The difficulty in this approach is that most
stress contour programs are designed to interpolate be-
tween data points. Therefore, it is necessary to scale
measured loads in proportion to the percentage of meas-
ured contact area to preserve force equilibrium when
extrapolating to full-contact load conditions.

Because the canopy tip is sloped upward and a base-
lifting device is employed, the actual contact area that is
achieved underground with this particular shield design is
largely dependent upon the compliancy of the immediate
roof and floor. It is concluded that full contact on the
canopy and base is not achieved without deformation of
the strata, suggesting that high contact pressures are
developed at the ends of the canopy and base for very
competent (stiff) immediate roof and floor strata.

The loading distribution changes in both magnitude and
profile as the loading on the shield increases. The general
tendency is for initial loads to be concentrated at the ends
of the canopy and base, and then to migrate to other areas
as increased loading causes additional contact. For leg
pressures above setting pressures, the general load distri-
bution is a continually increasing pressure from the canopy
tip to the canopy rear and decreasing pressure from the
toe of the base to the rear of the base. A linear approxi-
mation can be used, but a nonlinear approximation is
more accurate, particularly when horizontal forces are
removed.

Relatively little force is developed at the tip of the
canopy compared with the rear of the canopy, largely due
to the fact that the tip is located farther from the leg than
the rear of the canopy is and partially due to the flexibility
of the canopy. This arrangement requires considerably
more force at the rear than is required at the tip to main-
tain moment equilibrium. The canopy capsule controls tip
loading, but only at low leg pressures, and is generally
ineffective at leg forces above setting pressure. Maximum
loads are developed at the rear of the canopy, with a mi-
gration of this maximum toward the leg as the immediate
strata deform.

The inclination of the tip promotes initial contact of the
tip and discourages contact between the tip and leg. If the
goal is to develop maximum tip loading, this design is war-
ranted since tip loading is maximized when the contact
pressure near the leg is limited. However, structurally this
configuration induces large amounts of bending strain that
cause high stress concentrations in the canopy for most
load conditions. Even with this design, the amount of
force generated at the canopy tip is rather small, about
10 pet of the total shield capacity for stiff roof conditions
and less than 5 pet for more compliant roof and floor
structures. The most efficient method to increase tip load-
ing is to shorten the distance from the tip to the leg, but
the capacity to do this is limited by the requirement to
minimize the exposed tip-to-face distance and the limi-
tation on extending the length of the base due to the
location of the face conveyor.

The canopy is cantilevered in front of the toe of the
base and the leg is inclined so that the leg connection on
the canopy is forward of the leg connection on the base.
This geometry induces a rotational moment that promotes
high load concentrations at the toe of the base. A face-to-
waste external horizontal load acting on the canopy re-
duces the concentration of toe loading. This horizontal
loading can be produced by face-to-waste movement of the
immediate roof or by frictional force developed at the roof
interface, which resists the natural tendency of the canopy
to be pushed toward the face by the leg forces. Hence,
friable strata that develop a layer of debris on the canopy
is more likely to produce high toe loading than more com-
petent strata that resist the shearing forces developed at
roof interface.

The base-lifting device exaggerates the inherently high
toe loading on the base. The design of the base-lifting de-
vice could be improved to eliminate the debris that builds
up between the lifting plate and the base structure. This
debris prevents the full retraction of the base-lifting device,
which further aggravates the toe loading problem. A more


Report of Investigations 9418

Canopy and Base Load Distribution
on a Longwall Shield

By Thomas M. Barczak and David F. Gearhart

UNITED STATES DEPARTMENT OF THE INTERIOR
Manuel Lujan, Jr., Secretary

BUREAU OF MINES
T S Ary, Director


preserved in the distribution process by distributing the
measured load cell forces in proportion to the arca of the
cells relative to the area of the canopy or base so that the
total force remains a constant. A description of the stress
contour method is provided in the appendix.

DATA ANALYSIS AND TEST RESULTS

The tests are categorized in three areas determined
by the employment of the load-measurement devices:
(1) canopy distribution studies, (2) base distribution
studies, and (3) combined canopy and base distribution
studies.

Canopy Tests

A total of 10 tests were conducted to measure the
loading distribution on the canopy. Parameters investi-
gated during this test series included (1) contact config-
uration, (2) stiffness of the roof boundary, and (3) hori-
zontal shield loading. Three different canopy contact
configurations, as shown in figure 10, were investigated.
Full base contact was utilized for all canopy tests. Wood
and rubber were utilized to simulate the compliance of the
immediate roof to assess the impact of strata deformations
on the canopy load distribution. Horizontal loads are de-
veloped by the platens of the load frame resisting the hori-
zontal motion of the canopy and base caused by the action
of the leg forces. By allowing the load frame platens to
freely displace in the horizontal direction, the horizontal
loading on the shield was eliminated for certain tests to
determine the effect of the horizontal loading on the
distribution of forces acting on the canopy.

The profile of the canopy causes the canopy tip and
rear to contact first, which causes initial load develop-
ment at the front and rear of the canopy. As shown in
figure 11, the canopy does not bend sufficiently to cause
contact over the full length of the canopy. In this test, the
shield reacted against the rigid steel platens of the load
frame to simulate a hard flat roof structure that does not
deform from the shield contact pressure. Contact was
established over about 40 pet of the canopy for leg
Pressures up to 4,500 psi. At this magnitude of loading,
there is no contact directly above the leg, which is located
approximately 4 ft from the rear of the canopy, and no
contact 2 ft from the tip of the canopy. Hence, there
exists a span of about 7.5 ft in the middle portion of the
canopy that never establishes contact under these load
conditions. While loading above 4,500 psi was not attem-
pted for these specific tests, the data suggest that full
canopy contact would not be attained when the shield is
reacting against a flat stiff surface even at yield leg
pressure.

Y
t
BEGHReoe Base
Oo0Gbooe oeoe
| Canopy
Rear Leg KEY Tip
Y DO Load cell locations
4 and designations
EEEETEPS EEEE
PEBELTTs)
‘Canopy
—~X
Rear Leg Tip
Y
{
AGEEXHEAB ww
inj} ayayaya a) contact
' Canopy
Rear Leg Tip

Figure 10.— Canopy contact configurations for canopy load
distribution tests.

key

1 Load cell (no contact
‘estabiished)

Load cell in contact
with platen

Figure 11.—Contact area established against rigid roof. Top,
‘exaggerated view of canopy distortion; bottom, load measuring
cells on canopy.


120 r
Cell 1

20 | [cen 2

60 4

150
120
90
60
30

Cell 3. Cell 4

MEASURED LOAD, tons

° 2 4 6 0 2 4 6
AVERAGE LEG PRESSURE, 10° psi

Position of load eels
oon

Figure 17.—Migration of load development at rear of canopy
for rigid roof simulation,

explains the movement of the resultant forward. In
contrast, load development. during the weak roof
simulation occurred over a larger area, but the load
development at the rear of the canopy increased at a faster
rate than at the tip of the canopy. This explains the
movement of the resultant toward the rear for weak
(compliant) roof conditions.

When the canopy was permitted to displace toward the
face without external resistance to eliminate horizontal
shield loading, slightly less contact load was observed in
front of the leg, and slightly higher loads were observed to
the rear of the leg. This caused the resultant location to
move toward the rear of the canopy. When the external
horizontal force is removed, the resultant tends to shift
toward the rear of the support to offset the moment
created from the line of action of the vertical forces acting
on the canopy and base (see figure 4). A comparison of
resultant locations and calculated pressure profiles with
and without horizontal loading is shown in figure 18.

i

Base Tests

A total of 22 tests were conducted to evaluate the
contact pressure on the base of the shield. The effect
of three parameters on the resulting load distribution were
evaluated: (1) horizontal loading, (2) the base-lifting de-
vice, and (3) the location of the resultant canopy force.
Full base contact using 24 load measuring cells (12 on
each base pontoon) were utilized for all tests, All tests
employed wood or rubber under the load cells to simulate
floor deformation. Rigid floor conditions were not
simulated,

Horizontal loading is typically generated from the
internal leg forces and the restraining frictional force
imposed by the immediate roof and floor. When the can-
opy is permitted to displace forward toward the face with-
out external resistance, no horizontal loading is generated.
Without horizontal loading, the resultant vertical force
acting on the base must be coincident with the resultant
vertical force acting on the canopy. Since the canopy re-
sultant is located near the leg connection, this places the
resultant vertical force acting on the base at about 75 in
from the rear of the base or 21 in from the toe.

The effect of an externally applied horizontal load is to
push the resultant further toward the rear of the base. If
the horizontal force is generated from internal leg forces,
the effect of the horizontal force on the base loading
distribution will be height dependent, since the leg changes
inclination with height. For the 80-in test height, the base
resultant with horizontal forces generated by the internal
leg force acted at about 65 in from the rear of the base.
Therefore, for this shield configuration, horizontal loading
moved the resultant approximately 10 in toward the rear
of the base, The resultant location starts near the leg and
migrates toward the toe as loading increases. The migra-
tion toward the toe is suppressed when horizontal loading
is present, causing the resultant to act closer to the leg.
A comparison of base loading distribution with and
without horizontal loading is shown in figure 19.

In summary, if the immediate roof and floor are
capable of restraining the natural tendency of the canopy
and base to displace horizontally from the action of the leg
forces, horizontal loading will be developed that will
reduce toe loading. On the other hand, if there is loose
debris on the canopy or under the base, the canopy and
base may not be restrained and toe loading will be greater.
The increase in toe loading caused by the lack of
restraining horizontal force varies depending on the
magnitude of the horizontal force developed and the shield
geometry. For the example shown in figure 19, toe
pressure was found to increase by about 75 pet when the
horizontal loading was completely eliminated.