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20 Ton Chemical Rock Anchor Pullout Test
Prepared For:
MEE 443/488
Senior Design Project
University of Maine, Mechanical Engineering Department
Group:
Wind for the Rock
Members:
Brendan Owen, Justin Letteney, Stephen McGann
Testing Location:
Mosquito Mountain Granite Quarry
15-17 April 2008
Report Prepared By: Brendan L. Owen
INDEX
Complete Equipment List .................................................................................................................. 1
Introduction ...................................................................................................................................... 2
Background ..................................................................................................................................... 2-4
Test Design ...................................................................................................................................... 4-6
Hardware Calibration ...................................................................................................................... 6-7
Uncertainties ................................................................................................................................... 7-8
Difficulties Encountered/Adaptations Made .................................................................................... 8
Results ............................................................................................................................................. 8-9
Conclusion and Recommendations ............................................................................................... 9-10
References ...................................................................................................................................... XXX
Hilti HIT RE-500 Specs ............................................................................................................... APENDIX A
University of Maine
Wind For The Rock, Mech-Lab IV Spring 2008
Page 1
Equipment List
Item Manufacturer Model
Hydraulic Hand Pump Enerpac PH-46X
20 ton Hole-Jack Enerpac RCH 202
Load Cell FUTEK LLB500 – 30kip
Digital Multimeter Hewlett Packard 34401A
DC Power Supply Elenco Precision Quad Power XP-580
Torque Wrench Armstrong 64-084
Compressor Sears-Roebuck 102.17211
Generator Multiquip GA-6HZR
Manual Epoxy Dispenser Hilti MD2000
Epoxy Hilti HIT RE-500
1.25” Diamond Core Drill Bit Target Banner
Handheld Core Drill Hilti DD130
1”x13 C1018 Threaded Rod --- ---
University of Maine
Wind For The Rock, Mech-Lab IV Spring 2008
Page 2
20 Ton Chemical Rock Anchor Pull-Out Test University of Maine, Mechanical Engineering Senior Design Project
Spring Semester 2008
Introduction The senior engineering design group “Wind for The Rock” has designed a wind energy system
for the College of the Atlantic research station on Mount Desert Rock. As a part of this project
the group has chosen a 100 ft heavy duty triangular lattice tower to support up to six 400W Air-
X Marine wind turbines and an array of anemometers. The tower will have to either be anchored
directly to the local rock or to a cement foundation that is anchored to the local rock. After
consulting with a Hilti representative the group chose the chemical anchoring system HIT RE-
500 because it fulfilled the requirements of being of high strength and being the suggested
system for a saltwater environment. The spec sheet of HIT RE-500 is in APPENDIX A. The
drawback to the RE- 500 system was that it is intended for solid base concrete applications and
the representative could not make any statement as to the bonding strength of the adhesive epoxy
in a non concrete base material. This paper recounts the groups’ experience in conducting
chemical rock anchor pullout tests in granite, which was done to relate the bonding strength of
Hilti HTI RE- 500 adhesive epoxy to the anchoring depth in granite. The paper discusses the
calculations made, the testing procedure, the results and any difficulties encountered.
Background As a first step in designing the testing procedure the group made hand calculations to estimate
the expected load on a tower anchor bolt under static maximum load conditions. The maximum
recorded thrust load of the chosen wind turbine Air-X Marine by Soutwest Windpower is 150 lb
per turbine and the groups’ design calls for up to six such turbines [1]. The turbine loads were
assumed to act equally distributed at the top of the main legs of the tower in a horizontal
direction, see figure 1 for details.
Also considered is the drag force acting on the tower due to the wind. The turbine and the tower
are rated for a maximum wind speed of 110 mph and 120 mph respectively. Since there is little
difference between the two maximum ratings the group chose to assume that both the tower and
the turbine can withstand 120 mph. If 120 mph is assumed to be the wind speed at 100 ft then
the wind speed distribution along the height of the tower is given by,
University of Maine
Wind For The Rock, Mech-Lab IV Spring 2008
Page 3
���� � �� � � ������. 1�
where ��and �� are the reference wind speed and reference height respectively, and α is the
friction coefficient of the ground. In our case the reference wind speed and height are 120 mph
and 100 ft respectively and alpha is 0.1 for bare rock and water surfaces. Evaluating equation 1
showed that within the first 20 ft of tower the wind speed reaches roughly 85% of its maximum
value. Therefore, the group assumed that it was reasonable to model the wind load as a constant
wind speed of 120 mph along the height of the tower because it gives an added factor of safety.
Based on fluid mechanics the drag force acting on an object immersed in a moving fluid is given
by,
����� � �� � 12 � ������ � ������� � �� ! ���. 2� where rho and v are the fluid density and velocity, the area is the projected area of the
obstruction and �� is the drag coefficient of the lattice tower. According to ASCE-7 the drag
coefficient for a triangular lattice tower with flat-sided members can be estimated by,
�� � 3.4 � $� % 4.7 � $ ' 3.4 �Eq. 3� where δ is the solidity of one face and is given by,
$ *+ *, +!- � !� ! *+ . ./ �01*1!2 ,-2*0 3 !� ! �Eq. 4�
The drag coefficient for lattice towers with circular members is dependant on Reynolds number
and can be as much as ½ times that of flat-sided member towers [2] [3]. Since the chosen tower
is a combination of the two the group assumed that it is reasonable to calculate with the flat-
sided member equation without any modifications to generate a conservative estimate. Table 1
lists the calculated values relevant to the wind load.
Table 1 Calculated Wind Loading Values
Enclosed Area Of One Face 468.4 ft^2
Member Area Of One Face 87.5 ft^2
Solidity Of One Face 0.2
Drag Coefficient For Triangular Lattice Tower
accounting for shielding and interference between
members
2.6
University of Maine
Wind For The Rock, Mech-Lab IV Spring 2008
Page 4
Drag Force Due To Wind Loading 7971 lb, acting at 50 ft above the
ground
This load was modeled as equally distributed among the three main legs at 50 ft as shown in
figure 1. Based on the loads derived above and the model shown in figure 1 on the group
calculated that the worst case reaction for one anchor group is roughly 75000 lb. This means that
each of the 12 anchor bolts in the front leg will be subjected to a static load of roughly 18512 lb
in addition to the pretention load applied at installation.
Figure 1 Tower loads for hand calculations
Test Design In general the test was designed following an existing 890 kN rock anchor pullout test report by
Richard E. Mayo et al [4]. The result of the test was intended to establish a relationship between
University of Maine
Wind For The Rock, Mech-Lab IV Spring 2008
Page 5
the bonding strength of HIT RE-500 (see appendix A for specifications) and the embedded
length of 1” C1018 threaded rod in granite. The group and there advisor agreed on a set of four
test holes 1 – 2.5 ft in depth with an increment of ½ ft per hole. The specification sheet for RE-
500 called for a hole diameter of 1 1/8 inches for rod diameters of 1”. The holes were to be
drilled and subsequently filled completely with the epoxy after which the rods were to be slowly
threaded in. According to specification the cure time to full strength at ambient temperatures of
ca. 32 Fahrenheit is 50 hours so the group intended to set all rock anchors and let them cure for 2
days. As shown in figure 2, the free end of the threaded rod was to be inserted into a 20t Enerpac
hole jack which rested on a 2” thick steel plate for support. The nut that transmits the load from
the jack to the rod was to be pretension with a torque wrench set to 100 ft-lb which is equivalent
to ca. 1200 lb of tension. The described test setup would allow the group to theoretically apply
an upward force in the range of 0-40000 lb which well exceeds the calculated loads on a single
anchor bolt in the preceding section. The four holes were to be drilled using a 1 1/8 inch
diamond coated core drill bit and a hand held Hilti DD130 core drill. The test site was the
Mosquito Mountain granite quarry in Frankfort ME, owned and operated by Freshwater Stone.
The test began on the 15 of April and was finished on the 17 of April.
Figure 2 Test setup for rock anchor pullout test
University of Maine
Wind For The Rock, Mech-Lab IV Spring 2008
Page 6
Table 2 Hardware listings for figure 2 Component
Number Description
1 20t Enerpac hole-jack 2 10ksi Enerpac hydraulic hand pump 3 1” steel washer 4 1”-13 steel nut 5 1” C1018 threaded rod 6 HIT RE-500 Epoxy 7 2” steel base plate
Hardware Calibrations The first hand on task for the group was to calibrate a 30kip button type load cell that was then used to generate a hand pump psi vs. voltage curve. This allows for a correlation between the psi gauge reading and the applied force. The following section contains the calibration data and curves. The group calibrated the FUTEK 30kip load cell on a MTS tension/compression test rig. The results are shown in the following table and graph. Table 3 FUTEK 30kip Load Cell Calibration Data
Compression Load From MTS (Lb)
Voltage From
Load Cell (mV)
0 0.068
2000 1.328
4000 2.648
6000 3.962
8000 5.268
10000 6.629
12000 7.919
14000 9.286
16000 10.586
20000 13.259
25000 16.609
Figure 3 Load vs. Voltage curve for Futek 30kip cell
After the load cell was calibrated the group proceeded to calibrate the Enerpac hand pump and
hole jack. The load cell was set on the piston of the jack and then pressed against a hardened
y = 1509.x - 4.321
R² = 1
0
5000
10000
15000
20000
25000
30000
0 5 10 15 20
Loa
d (
lb)
Voltage (mV)
Load Cell Calibration FUTEC
30K
Load vs. Volt
Linear (Load vs.
Volt)
University of Maine
Wind For The Rock, Mech-Lab IV Spring 2008
Page 7
steel plate secured to a hydraulic press stand. The following tables and graph show the results of
this calibration step.
Table 4 Pressure and load cell voltages for hydraulic jack calibration Pressure Reading from Pump (psi)
Voltage From Load Cell (mV)
Average Run1&2 (mV)
Run 1 Run 2
0 0.068 0.068 0.068
1000 3.2 3.231 3.2155
2000 6.46 6.504 6.482
3000 10.073 9.977 10.025
4000 13.302 13.428 13.365 The data in table 4 lead to the following mV-pressure relationship
Figure 4 Voltage as function of pressure Uncertainties Table 5 on the following page lists the uncertainties that the group applied to the calibration data
and the torque setting on the Armstrong torque wrench. The calculations below show how the
group established the final uncertainty in the applied load as a function of the two uncertainties.
4*!3 � 1509 � 8*21!9 % 4.321 ' :� 1 ,0;*, ���. 5� 8*21!9 � 0.003 � :� 00<� % 0.049 ���. 6�
<���� � 34*!33:� 00<� � <>�?@@��? ' 34*!3
3:� 1 ,0;*, � <>�?A?B@��B ���. 7�
y = 0.003x - 0.049
R² = 0.999
0
2
4
6
8
10
12
14
16
0 1000 2000 3000 4000 5000
Vo
lta
ge
(m
V)
Pressure (psi)
mV vs. Pressure
mV vs. Pressure
Linear (mV vs.
Pressure)
University of Maine
Wind For The Rock, Mech-Lab IV Spring 2008
Page 8
<���� � ~ D 456 2/
In percent of theoretical full load this amounts to 1.14% error.
Table 5 Uncertainties associated with the procedure Source Value
Pressure Gauge Reading ± 100 psi Torque Wrench ± 3% of setting (@100 ft-lb = ±3 ft-lb)
Difficulties Encountered / Adaptations Made The first problem the group encountered was that it did not have enough RE-500 epoxy to be
able to fill the four depths agreed upon. After recalculating it was decided that the group would
drill 4 holes with depths of 6, 7, 8 and 10 inches. A second deviation from the original plan was
that the vendor supplying the drilling equipment mistakenly delivered a 1 ¼ inch core drill bit.
The group was informed by Hilti that such a small variation in hole diameter should not alter the
bonding strength significantly so it was decided to proceed with the 1 ¼ inch bit.
A third complication arose while drilling the holes.
The group was unable to drill the fourth hole with a
depth of 10 inches because the core drill bit had
been worn out to the point that it made drilling to
that depth impossible. The final testing was
conducted on the holes with 6, 7, and 8 inches of
depth.
Results As stated in the previous section the group was
forced to change the number and depths of the
anchor holes to 3 holes with depths of 6, 7 and 8
inches. The following section lists the results of this
final test setup.
The only setup that the group was able to bring to
complete failure was the 6” deep anchor and this
required repeated application of the full load that the
pump and jack were capable of. Although theoretically limited too 10000 psi, it was possible to
Figure 5 Core drilling granite, not an easy manual task
University of Maine
Wind For The Rock, Mech-Lab IV Spring 2008
Page 9
get the load up to 10500 psi which translates to ca. 52992 lb acting vertically on the anchor.
Accounting for the preload the total vertical force on the anchor was 54192 lb ±456 lb. Since the
group was not able to bring the anchors to complete failure by applying the full load once it was
decided to repeatedly apply the full load possible and see if and when the anchors failed. The
following table lists the cycling sequences and there results,
Table 6 Load cycles applied to anchors
Embeded Depth Loading Descriptions
6 in. Loaded 9 cycles to 10.5 ksi, retightened after 2 cycles (full loss of pretention at this point), lost all shear resistance in 9th cycle, complete failure
7 in.
Loaded to 3, 6, 9 ksi retightened to 100 ft-lb between each, loaded 5 cycles to 10.5 ksi then retightened to 100 ft-lb. Loaded 10 cycles to 10.5 ksi and then retightened, repeated 10 times total After a total of 100 cycles the anchor had lifted ca. 1/8 inches out of the hole.
8 in. Loaded 3 cycles to 10.5 ksi, retightened between each. After 3rd cycle no loss of pretension in bolt and no uplifting of the anchor. The group decided to stop here because the quarry closed.
Conclusions and Recommendations The test went well in the sense that the group is confidant that HIT RE-500 is in fact a possible
chemical anchoring system for our application. The group cannot make a reliable statement on
the ultimate bonding strength since we were
limited to the 20t hole jack but we think that,
given the suggested anchoring depths for the
selected tower of over 3 ft for, this system should
work.
A first sign of failure is indicated by the total loss
of the pretention that is applied. In the worst case,
the 6” hole depth, it was clearly visible that the
nut had been separated from the jack surface. In
the case of the 7” hole depth, the group was sure
that the bond was slowly failing because the
pretension was consistently lost after each
increment of 10 cycles. The setup was also
Figure 6 Anchor uplift, 7" depth
University of Maine
Wind For The Rock, Mech-Lab IV Spring 2008
Page 10
disassembled midway through the test and it was visible that the rod and epoxy were being
pulled out of the hole as can be seen in figure 6 on the preceding page. A second failure indicator
is the failing of the granite top surface layer in the vicinity of the anchor hole. Figure 7 shows
how the top layer of rock is beginning to fail.
Theoretically this would be visible around the base
of a tower leg.
Based on the groups observations a hole depth of 8”
in granite is the recommended minimum for a
vertical static load no greater 54kip with additional
consideration of the calculated uncertainty. Since
mounting a 100 ft tower with multiple turbines on it
is an expensive endeavor the group further
recommends that the tests should be repeated with a
more powerful jack in order to get a definite
ultimate bonding strength as a function of
anchoring depth. Preferably this second test should
be carried out on Mount Desert Rock since there is
no guaranty that the properties of MDR granite are
the same as those of Mosquito Mountain granite. Figure 7 Rock Surface Failure
Anchor failure indicator, top rock
layer is pealing off
References
[1] Southwest Windpower. “Air-X The New 400 watt turbine! Owner’s Manual”. 28 April
2008. http://www.windenergy.com/products/air_x.html.
[2] Holmes, John D. . “Wind Loading of Structures”. Spon Press. New York: 2003. ISBN 0-419-
24610-X (print edition)
[3] Holmes, John D. . “Antennas and open frame structures”. PowerPoint presentation.
Louisiana State University.
<http://www.hurricaneengineering.lsu.edu/CourseMat/03Lect23Antennas.ppt>
[4] Mayo, Richard E. et al (2003) . “100-Ton (890 kN) Rock anchor Pullout Test”. ASCE Research
Library. Available from: http://www.ascelibrary.org.
APPENDIX A : HIT RE-500 Adhesive Epoxy Specifications
