Pull Out Test For Rock Bolts

PULL-OUT TEST FOR

ROCI(BOLTS

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PULL-OUT TEST FOR ROCI( BOLTS

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INTRODUCTION

This technical document provides the method for conducting the pull-out test for rock bolts and rock anchors. The objective of this test method is to measure the working and ultimate capacities of a rock bolt anchor. This test method is applicable to rock bolts end 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 to a predetennined load. Some of the components of a rock bolt are defined in Fig 1. Rock bolts are usually less than 6m long and rarely longer 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 of up to 300kN may be specified; typically these would be formed from high yield steel bars or special steels having diameters up to 40mm.

2.1 TYPES OF ROCK BOL rs The types of rock bolt commonly used for civil engineering works include: (i) Mechanical bolts - typically these have a wedge shaped shell assembly which, when expanded, anchors them into the drill hole. (ii) Cement grouted bolts - typically these are fonned by inserting the bar into a drill hole filled with grout.

(iii) Two-speed resin bonded bolts - with these the bar is fixed (and then stressed) within a fast setting resin at the distant end and subsequently bonded along the remainder of its length by a slower setting resin/cement grout. This type of rock bolt is used in our project.

Bar, tendon CH ~h,1

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(hIlly bondoo ofter It rf~;.ng)

0,,11 hole

Fig. I Typical Rock Bolt

Rock bolts are used widely to improve the stability and load bearing characteristics of a rock mass. Often they are used to stabiljze relatively small blocks of 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 etc

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The proximal end of 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 shotcrete finish.

3.

DEFINITION OF PULL OUT TEST

A rock bolt is installed in the same manner and in the same material as its intended construction use. The bolt is pu lied hydraul ically and the displacement of the bolt head is measured concurrently. The bolt is pulled until the rock bolt system or rock fails. The ultimate and working capacities of the bolt are calculated from the plot of load versus disp lacement.

3. 1 TYPES OF PULL OUT TESTS Two types of pullout tests should be conducted in the field

1. Basic verification pull-out test

2.

Performance pull-out test

The main difference between the two pullout tests are in the test loads. In the basic verification test, the test load is taken as 80 % of the yield load of the bolt. In the performance test, the test load taken as 133% of working load of the rock bolt.

Basic verification tests involve incremental loading and un loading of 'test rock bolts '. These tests are conducted to find out the capacity of 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. The main objective of this test is to veri fy the length of the resin grout.

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Pelformance Jests involve incremental loading of a 'production rock bolt'. The performance test is 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 in the rock bolt design. The resu Its of a performance test may also be used to assist in the interpretation of the simpler proof test.

4.

PROCEDURE FOR BASIC VERIFICATION TEST

The objective of the test is to find out the failure mode and to calculate the bond length.

In the ideal conditions, the rock bolt fails in rock grout interface or bolt grout interface but many times the bolt may fail in tension also. Generally the rock/grout strengths are very high for most of the rocks.

The roughness of the bore hole, rock bolt and the

interaction between them with grout are the dominant factors in the strength of the whole rock mass system. Finding this in-situ strength is very difficult or not possible; hence the pull out test may be used to find this property. In order to understand the field conditions and to assess the failure mechanism, the following verification test is required.

The test load taken as the 80 % of the yield load orthe rock bolt. The load applied in fOUf cycles. After the each cycles of incremental loading, the load is held for 5 minutes then unloaded to the alignment load before start the next cycle. The detailed procedure for the test is given below. Table } shows the values of verification test load for various diameters of rock bolts.

Table 1 Values of Verification Test loads for Various Diameters ofRock BoILS Characteristic yield load

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,Veri I1cation test load' 80 % of ,~',"

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 a nominal load to the rock bolt. This load, termed as alignment toad, is typically not more than five percent 'of 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 of ISRM. The displacement measuring equipment is zeroed upon stabilization of 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. The load is then reduced back to alignment load in the same 30equal decrements.

After the third cycle, pull the bolt in the same increments as lIsed during the last cycle or in 500 lbf (2.2 kN) increments, whichever is less, until the rock bolt system fails or the limit of the loading system (80% of yield load of bolt) is reached.

Finally, pull the bolt 0.5 in. (12.5 mOl) beyond the 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 I mm displacement.

5.

PROCEDURES FOR PERFORMANCE TEST

The first step in a perfonnance test comprises applying a nominal load to the rock bolt tendon. This load, termed the alignment load, is typically not more than five percent of the design load and its purpose is to ensure that the stTessing and testing equipment are properly aligned. The displacement measuring equipment is zeroed upon stabilization of the alignment load (AL).

The test load is taken as 133 % of the working load of the rock bolt. The working load s can be taken as 50% of y ie ld load of the steel rock bo It as per the BS 8081 ~ 1989. The following table shows the values of characteristic yield load, working load and test load for pelformance test for various diameters of rock bolts are given.

Table 2 Values ofPerformance Test loadsfor Various Diameters ofRock Bolts

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Characteristic yield load

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Working load

11:/4 "'d2 * 415/JOO()() T w =50% of.f,}/1 \

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 is raised to 25 percent of the test load and the incremental movement is recorded. The load should be applied in 5 equal increments. In the. second load cycle, again the load is raised to 50 percent of the design load and the incremental movement is recorded. The load should be applied in 10 equal increments. In the third load cycle, again the load is raised to 75 percent of the design load and the incremental movement is recorded. The load should be applied in 15 equal increments. In the fourth load cycle, again the load is raised to 100 percent of the design load and the incremental movement is recorded. The load should be applied in 20 equal increments. In the fifth load cycle, again the load is raised to 120 percent of the design load and the incremental movement is recorded. The load should be applied in same increments as used for the last cycle or 500 lbf (2.2kN) increments.

After the fifth cycle, pull the bolt in the same increments as used during the last cycle or in 500 lbf(2.2 kN) increments, whichever is less, until the rock bolt system fails or the limit of the loading system (133 % of working load) is reached. Refer table 2 for tl1e various loads and the values for different diameters of rock bolts.

At the test load, the load is held for ten minutes prior to reducing the load to the lock~off 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 is to measure time-dependent (i.e., creep) movements of the rock bolt. This portion of the performance test is referred to as a creep test. During the creep test, the movement is more than 1 mm, and then follows the procedures given in the note 1.

5.1 Recording ofPerformance Test Data The magnitude of each load is determined from the jack pressure gauge. During creep testing, a load cell is used to ensure that the jack load remains constant.

The load-deformation data obtained for each load increment in a perfonnance test are plotted as shown in figure 2. Movement is recorded at each load increment, decrement and for the alignment load.

The total movement is measured consists of elastic movement and residual movement for verification test.

Elastic movements result from elongation of the tendon and elastic movements of the ground anchor through the ground. Residual movement includes elongation of the anchor grout and movement of the entire anchor through the ground.

The residual movement for a gIven increment of load is the movement that corresponds to the net "irrecoverable" movement that occurs upon application of a load increment and the subsequent relaxation of the load to the alignment load.

The elastic movement is 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 is an indicator of the stress-strain behav ior of tht:

ground~grout

bond in the anchor bond zone.

The elastic deformation should not be more than elastic deformation free (un-grollted) length of rock bolt at working load

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For perfonnance and proof tests, the measured total movement for the requ ired load hold at the test load should not exceed I mm between I and 10 minutes. If the movement is less than 1 mm for this period then the rock bolt 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 is considered acceptable with respect to creep.

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CAPACiTY

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WORKING CI\PACITY

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DEFl£CTlfJN .. Fig.2 Plotting Performance Test Data

Fu l'ther clarifiea tioDs and discussions:

Dr, R. VenugopalRao +91 991°368088

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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 of a load increment and the subsequent relaxation of the load to the alignment load.

The elastic movement is 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 is an indicator of the stress-strain behavior of the ground-grout bond in the anchor bond zone. The elastic deformation should not be more than elastic deformation free (un-grouted) length of rock bolt at working load

For performance and proof tests, the measured total movement for the required load hold at the test load should not exceed 1 mm between I and 10 minutes. If the movement is less than I mm for this period then the rock bolt is considered acceptable with respect to creep. If the movement is more than I 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 is considered acceptable with respect to creep.

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