PULL-OUT TEST FOR
ROCI(BOLTS
lll'af.',t'
PULL-OUT
TEST TEST FOR ROC R OCI( I( BOLT BOLTS S - - - - - - - - - - _ ..
1.
__
...
INTRODUCTION
This technical document provides the method for conducting the pull-out test for rock bolts and rock anchors. ultimate capacities
Th
objective
this test method
a rock bolt anchor. This test method
is is
to measure the working and applicable
rock bolts end
to
anchored with cement grout or resin, (epoxy, polyester, and the like).
2.
ROCK BOLTS
A rock bolt is a short, low capacity reinforcement comprising a bar (or tube) fixed into rock and tensioned defined
in
Fi
1.
a predetennined load. Some
to
the components
of
a rock bolt are
Rock bolts are usually less than 6m long and rarely lon ger than than 10m
Their working load
is
typically between 150 and 200kN and they would normally be
formed from high yield steel bars with diameters up to 32mm. However, sometimes, working loads
up
to
300kN may be specified; typically these would be formed from
high yield steel bars or special steels having diameters up to 40mm.
2.
TYPES
The types
of
ROCK BOL rs
rock bolt commonly used for civil engineering works include:
(i) Mechanical bolts - typically typically these have a wedge shaped shell assembly which, when when expanded, expanded, a nchors
them
into the drill hole.
are fonned by inserting the the bar into a drill drill hole (ii) Cement groute d bolts - typicall y these are filled with grout.
(iii) Two-speed resin resin bonded bolts - with these these the bar is fixed (and then stressed) within
a fast setting resin at the distant end and subsequently bonded along the remainder length by a slower setting resin/cement grout. This type
its
rock bolt is used in ou
project.
Bar, tendon
CH
~ h , 1
n,
(hIlly bondoo ofter It r
f ~ ; . n g )
0,,11 hole
Fig. I Typical Rock Bolt
Rock bolts are used widely to improve the stability and load bearing characteristics rock mass. Often they ar
used to stabiljze relatively small blocks
rocks in cuttings,
slopes and underground excavations such as tunnels, caverns and mines. They can be used on their own or in conjunction with other SUpP0l1 systems such as shotcrete et
31
The proximal end
the bar may be threaded so that a nut and faceplate can be attached;
the plate may provide local support to the rock surface and allow the attachment of mesh reinforcement which may be required for a shotcre te finish. finish.
3.
DEFINITION OF PULL OUT TEST
A rock bolt is installed in the same manner and in the same material as its intended constr construct uction ion use Th bolt is pulie pulie hydr hydrau au ical ically ly an the displac displacemen emen measured concurrently. Th
th
head head is
bolt is pulled until the rock bolt system or rock fails. The
ultimate and working capacities of the bolt are calculated from the plot
load versus
displacement.
3. 1 TYPES Tw
types
PULL OU
TESTS
pullout tests should be conducted in the fiel
1. Basic verification pull-out test
2. Performance pull-out test
Th
main difference between the tw
pullout tests are in the test loads. In the basic
verification test, the test load is taken as 80 performance test, the test load taken as 133%
the yield load working load
the
the rock bolt.
Basic verification tests involve incremental loading and un loading These tests are conducted to find ou the capacity
the bolt.
'test rock bolts '.
the rock bolt and to find out the
grout length. This test is conducted in the test rock bolts and it will be conducted away from the original supporting structure. The tested bolts should not be used for production. The resin grout should be applied as per the design and the details are given in the drawings. Th main objective
41
,; ,c
<'
this test is to veri fy the length
the resin grout.
Pelformance Jests involve incremental loading performance test
is
'production rock bolt'. Th
used to verify rock bolt capacity, establish load-deformation behavior,
identify causes of rock bolt movement, and to verify that the actual un-bonded length is equal to or greater than that assumed test may also be used to assist
4.
the rock bolt design. Th
the interpretation
resu Its of
performance
the simpler proof test.
PROCEDURE FOR BASIC VERIFICATION VERIFICATION TEST
The objective In
in
in
th
test is to find out the failure mode and to calculate the bond length.
the ideal conditions, the rock bolt fails in rock grout interface or bolt grout interface
but many times the bolt may fail very high for most
the rocks.
in
tension also. Generally the rock/grout strengths are Th
roughness of the bore hole, rock bolt and the
interaction between them with grout are the dominant factors
strength
in th
the whole
rock mass system. Finding this in-situ strength is very difficult or not possible; hence the pull ou test may be used to find this property. In order to understand the field field condit ions and to assess the failure mechanism, the following verification test is required.
Th
test load taken as the 80
cycles. After the each cycles
the yield load orthe rock bolt. Th incremental loading, the load
unloaded to the alignment load before start the next cycle. Th
is
load applied
held for
in fOUf
minutes then
detailed procedure for the
test is given below. Table } shows the values of verification test load for various diameters
Table
rock bolts.
Values
Verification Test loads fo
Various Diameters
Characteristic yield load
.fi",
5IPagi.:
n/4
4J51JOOOO
Veri I1cation
80 %
Rock BoILS
test load' , ~ ' , "
mm
tonnes
tonnes
20
13.0
10.4
25
20.4
16.2
32
33.3
26.7
40
52.1
41.7
The first step in the verification test comprises applying app lying a nominal loa to the rock bolt. This load, termed as alignment toad, is typically not more than five percent 'o the test load
of
the bolt and its purpose is to ensure that the stressing and testing equipment are
properly aligned. Generally the alignment load is taken as 5kN (500 kgf) as per the recommendation stabilization
of
of
ISRM. The displacement measuring equipment is zeroed upon
the alignment load (AL).
During the firslload cycle, the load is raised to 25 percent of the test load and the incremental movement is recorded. The load should be applied in 10 equal increments and hold the load for 5 minutes. The load is then reduced back to the alignment load in tile same 10 equal decrements. In the second load cycle, again the load is raised to 50 percent
of
the test load and
the incremental movement is recorded. The load should be applied in 20 equal increments and hold the load for 5 minutes. The load is then reduced back to alignment load in the
same 20 equal decrements. In the third load cycle, again the load is raised to 75 percent
of
the test load and
the incremental movement is recorded. The load should be applied in 30 equal increments
and hold the load for 5 minutes. Th
load is then reduced back to alignment load in th
same 30equal decrements. After the third cycle, pull the bolt in the same increments as lIsed during the last cycle or in 500 lb (2.2 kN) increments, whichever is less, until the rock bolt system fails or the limit
the loading system (80%
Finally, pull th
yield load
bolt 0.5 in. (12.5 mOl) beyond the fai fai lure displacement. Record
the load at every 0.05 in. (1
0101).
This is the reverse process, the displacement is
measured and the loads should be noted for the each
5. Th
bolt) is reached.
mm displacement.
PROCEDURES FOR PERFORMANCE TEST first step in
perfonnance test comprises applying
nominal load to the rock bolt
tendon. This load, termed the alignment load, is typically not more than five percent the design load and its purpose is to ensure that the stTessing and testing equipment are properly aligned. Th displacement measuring equipment is zeroed upon stabilization of the alignment load (AL).
Th
test load is taken as
loads can be taken as 50%
133
the working load
the rock bolt. Th working
ie ld load of the steel rock bo It as per the BS 8081
The following table shows the values
characteristic yield load, working load and test
load load for for pel formance tes t for various diameters of rock bolts are given.
Table
Values
Performance Test loadsfor Various Diameters
Characteristic yield load J;",
71
11:/4 "'
989.
415/JOO()()
Working load
=50% of.f,}/1
Rock Bolts
Performance test load
133% ofT"
mm
tonnes
tonnes
tonnes
20
13.0
6.5
8.7
25
20.4
10.2
13.5
32
33.3
16.6
22.2
40
52.l
26.1
34.7
During the first load cycle, the load
incremental movement is recorded. The
In the.
and
raised
is
load
second load cycle, again the
the incremental movement
is
to 25
should
load is
be
percent
of
applied
in
raised
recorded. The
to 50
load
and the
5 equal increments.
percent
should
load
the test
be
of
the design load
applied
in
10
equal
increments.
In
the third load cycle, again the
the incremental movement
is
load is
raised
to 75
percent of the design load and
recorded. The load should be applied
in
15
equal
increments. In the fourth load cycle, again the and
the incremental movement
is
load is
raised to
recorded. The
load
100
percent
should
be
of
the design
applied
in 20
load
equal
increments. In the fifth load cycle, again the load and
the incremental movement
increments
as
is
is
raised
to 120
percent
of
the design
recorded. The load should be applied
used for the last cycle or 500 lbf (2.2kN) increments.
in
load
same
After the fifth cycle, pull the bolt cycle or
in
or the limit tl1e
in
the same increments as used during the last
500 lbf(2.2 kN) increments, whichever the loading system (133
is
less, until the rock bolt system fails
of working load) is reached. Refer table 2 for
various loads and the values for different diameters
At the test load, the load l o c k ~ o f f
is
rock bolts.
held for ten minutes prior to reducing the load to the
load or alignment load. During this ten minute load hold period, movements are
measured and recorded at 1,2,3,4,5,6, and 10 minutes. The purpose of this load hold to measure time-dependent (i.e., creep) movements performance test more than
is
in
the note
each load
testing, a load cell
is
1.
determined from the jack pressure gauge. During creep
is
used to ensure that the jack load remains constant.
The load-deformation data obtained for each load increment in
is
Performance Test Test Data
The magnitude
plotted as shown
th
referred to as a creep test. During the creep test, the movement
mm, and then follows the procedures given
5.1 Recording
rock bolt. This portion
th
is
figure 2. Movement
is
in
a perfonnance test are
recorded at each load increment, decrement
and for the alignment load. load.
The total movement
is
measured consists
elastic movement and residual
movement for verificatio n test.
Elastic movements result from elongation of the tendon and elastic movements the ground anchor through the ground. Residual movement includes elongation of the anchor grout and movement
the entire anchor through the ground.
The residual movement for a gIven increment
load
is
the movement that
corresponds to the net "irrecoverable" movement that occurs upon application increment and the subsequent relaxation
The elastic movement
is
a load
the load to the alignment load.
therefore the arithmetic difference between the total
movement measured at the maximum load for a cycle and the movement measured at the alignment load. Although not used for anchor acceptance, residual movement indicator
the stress-strain behav ior
tht: g r o u n d ~ g r o u t
bond
in
is an
the anchor bond zone.
The elastic deformation should not be more than elastic deformation free (un-grollted) length of rock bolt at worki ng load
IV(J/I
For perfonnance and proof tests, the meas ured total move ment for the requ ired ired load hold at the test load should not exceed
mm between
less than 1 mm for this period then th
rock bolt
is
and
10
minutes.
the movement
is
considered acceptable with respect to
creep. If the movement is more than 1 mm for the above specified period, the test load should be held for another 50 min and records the movements. This movement must be less than 2 mm, and then the rock bolt
lOIP:-lge
is
considered acceptable with respect to creep. creep.
o·
lJLTIMAH.
-.1'
_ 1 - - _
CAPACiTY
WORKING
CI\PACITY
-
-
-
-
-
~
~
-
-
-
-
-
-
.
_
-
-
-
DEFl£CTlfJN
Fig.2 Plotting Performance Test Data
Fu l'ther
Dr
R.
clarifiea tioDs an
VenugopalRao
+91 991°368088
11
j' :) ;)
discussions:
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The residual movement for a given increment of load
is
the movement that
corresponds to the net "irrecoverable" movement that occurs upon application increment and the subseq uent relaxatio
The elastic movement
is
a load
the load to the alignment load.
therefore the arithmetic difference between the total
movement measured at the maximum load for a cycle and the movement measured at the alignment load. Although not used for anchor acceptance, residual movement indicator Th
the stress-strain behavior
the ground-grout bond
in
is an
the anchor bond zone.
elastic deformation should not be more than elastic deformation free (un-grouted)
length
rock bolt at work ing load load
For performance and proof tests, the measured total movement for th required load hold at the test load should not exceed 1
mm
between I and
less less than I mm for this period then the rock bolt creep. If the movement should
be
is
·1
II
minutes. If the movement
is
considered acceptable with respect to
more than than I mm for the above specified period, the test load
held for another 50 min and records the movements. This movement must
less than 2 mm, and then the rock bolt
10
is
10
is
considered acceptable with respect to creep. creep.
be
