錢營(yíng)孜煤礦2.4 Mta新井設(shè)計(jì)含6張CAD圖.zip
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英文原文
Effect of grout properties on the pull-out load capacity of fully groute rock bolt
A. K?l?c, E. Yasar*, A.G. Celik
Department of Mining Engineering, C? ukurova University, 01330 Balcal?, Adana, Turkey
Received 30 October 2001; received in revised form 30 April 2002; accepted 24 May 2002
Abstract
This paper represents the result of a project conducted with developing a safe, practical and economical support system for engineering workings. In rock engineering, untensioned, fully cement-grouted rock bolts have been used for many years. However,there is only limited information about the action and the pull-out load capacity of rock bolts, and the relationship between bolt–grout or grout–rock and the influence of the grout properties on the pull-out load capacity of a rock bolt. The effect of grout properties on the ultimate bolt load capacity in a pull-out test has been investigated in order to evaluate the support effect of rock bolts. Approximately 80 laboratory rock bolt pull-out tests in basalt blocks have been carried out in order to explain and develop the relations between the grouting materials and untensioned, fully grouted rock bolts. The effects of the mechanical properties of grouting materials on the pull-out load capacity of a fully grouted bolt have been qualified and a number of empirical formulae
have been developed for the calculating of the pull-out load capacity of the fully cement-grouted bolts on the basis of the shear strength, the uniaxial compressive strength ofthe grouting material, the bolt length, the bolt diameter, the bonding area and the curing time ofthe grouting material.
Keywords: Rock bolt; Grouting materials; Bolt pull-out load capacity; Bolt geometry; Mortar
1. Introduction
In rock engineering, rock bolts have been used to stabilise openings for many years. The rock bolting system may improve the competence ofdisturbed rock masses by preventing joint movements, forcing the rock mass to support itself (Kaiser et al., 1992). The support effect of rock bolt has been discussed by many researchers(e.g. Hyett et al., 1992; Ito et al., 2001; Reichert etal., 1991 and Stillborg, 1984). Rock bolt binds together a laminated, discontinued, fractured and jointed rock mass. Rock bolting not only strengthens or stabilizes ajointed rock mass, but also has a marked effect on therock mass stiffness (Chappell, 1989). Rock bolts perform their task by one or a combination of several mechanisms. Bolts often act to increase the stress and the frictional strength across joints, encouraging loose blocks or thinly stratified beds to bind together and act as a composite beam (Franklin and Dusseault, 1989). Rock bolts reinforce rock through a friction effect,
through a suspension effect, or a combination of two.For this reason, rock bolt technique is acceptable for
strengthening ofmine roadway and tunnelling in all type ofr ock (Panek and McCormick, 1973).
Generally rock bolts can be used to increase the support of low forces due to the diameter and the strength ofthe bolt materials. They enable high anchoring velocity to be used at closer spacing between bolts.Their design provides either mechanical clamping or cement grouting against the rock (Aldorfand Exner,1986).
Anchorage system ofr ock bolt is normally made of solid or tube formed steel installed untensioned or tensioned in the rock mass (Stillborg, 1986). Rock bolts can be divided into three main groups according to their anchorage systems (Franklin and Dusseault, 1989; Aldorf and Exner, 1986; Hoek and Wood, 1989;Cybulski and Mazzoni,1989). First group is the mechanically anchored rock bolts that can be divided into two groups:slit and wedge type rock bolt, expansion shell anchor.They can be fixed in the anchoring part either by a wedge-shaped clamping part or by a threaded clamping part. Second group is the friction-anchored rock bolts that can be simply divided into two groups: split-set and swellex. Friction-anchored rock bolts stabilise the rock mass by friction ofthe outer covering ofbolt against the drill hole side. The last group is the fully grouted rock bolts that can also be divided into two groups: cement-grouted rock bolts, resin grouted rock bolts.
A grouted rock bolt (dowel) is a fully grouted rock bolt without mechanical anchor, usually consisting of a ribbed reinforcing bar, installed in a drill hole and bonded to the rock over its full length (Franklin and Dusseault, 1989). Special attention should be paid to cement-grouted bolts and bolts bonded (glued, resined) by synthetics resins for bolt adjustment. Grouted bolts fix the using of the coherence of the sealing cement with the bolt rod and the rock for fastening the bolts.Synthetic resin (resined bolt) and cement mortar (reinforced-concrete bolt) can be used for this type rock bolt.These bolts may be anchored in all type of rock.
Anchoring rods may be manufactured of several materials such as ribbed steel rods, smooth steel bars, cable bolts and other special finish (Aldorfand Exner, 1986).
Grouted bolts are widely used in mining for thestabilisation oftunnelling, mining roadway, drifts and shafts for the reinforcing of its peripheries. Simplicity of installation, versatility and relatively low cost of rebars are further benefits of grouted bolts is comparison to their alternative counterparts (Indraratna and Kaiser,1990).
Bolts are self-tensioning when the rock starts to move and dilate. They should therefore be installed as soon as possible after excavation, before the rock has started to deform, and before it has lost its interlocking and shear strength.
Although several grout types are available, in many applications where the rock has a measure ofshort term stability, simple Portland cement-grouted reinforcing dowels are sufficient. They can be installed by filling the drill hole with lean, quickly set mortar into which the bar is driven. The dowel is retained in up holes either by a cheap form of end anchor, or by packing the drill hole collar with cotton waste, steel wool, or wooden wedges (Franklin and Dusseault,1989).
Concrete grouted bolts use cement mortar as a bonding medium. In drill holes at minimum of 15。Below the horizontal plane, the mortar can simply poured in,whereas in raising drill holes (roof anchoring ) various design ofbolts or other equipment is used to prevent the pumped mortar from flowing out (Aldorfand Exner,1986).
The load bearing capacity off ully cement-grouted rock bolts depends on the bolt shape, the bolt diameter,the bolt length, rock and grout strength. The bond strength off ully cement-grouted rock bolts is primarily frictional and depends on the shear strength at the bolt–grout or grout–rock interface. Thus any changes in this interfaces shear strength must affect the bolt bond strength and bolt load capacity.
This laboratory testing program was executed to evaluate the shear strength effect on the bond strength of the bolt–grout interface of a threaded bar and the laboratory test results confirm the theory.
2. Previous solutions
The effectiveness of a grouted bolt depends on its length relative to the extent ofthe zone ofoverstr essed rock or yield zone. The shear and axial stress distributions of a grouted bolt are also related to the bolt length because equilibrium must be achieved between the bolt and the surrounding ground (Indraratna and Kaiser, 1990).
Bearing capacities ofcement-gr outed rock bolts (Pb) and their anchoring forces are a function of the cohesion of the bonding agent and surrounding rock, and the bolting bar. The ultimate bearing capacity of the bolt (Pm) is expressed as follows (Aldorfand Exner, 1986):
(1)
where kb, safety coefficient (usually kbs1.5); C1, cohesion of the bonding material on bolting bar, ld, anchored length of the bolt, ds, bolt diameter.
(2)
where dv, drill hole diameter; C2, cohesion of the bonding material with surrounding rock (carboniferous rocks and polyester resins C2=3 MPa).
(3)
where C3, shearing strength of the bonding material.The maximum (ultimate) bearing capacity of the bolt (P m) will be the lowest value from P1mto P111m.
Bearing capacities ofall type bolts must also be evaluated from the view point of the tensile strength of the bolt material (Pms), which must not be lower than the ultimate bearing capacity resulting from the anchoring forces of bolts in drill holes (Pm). It holds that
Pms〉Pm (4)
where Pms, the ultimate bearing capacity o fthe bolt with respect ofthe tensile strength ofthe bolt material; Pm, the ultimate bearing capacity ofthe bolt.
3. Laboratory study
3.1. Experiments
The pull-out tests were conducted on rebars, grouted into basalt blocks with cement mortar in laboratory. The relations between bolt diameter (db) and pull-out load of bolt (Pb) (Fig. 2), bolt area (Ab) and pull-out load of bolt (Pb) (Fig. 3), bolt length (Lb) and pull-out load of bolt (Pb) (Fig. 5), water to cement ratio (wyc) and bolt bond strength (tb) (Fig. 7), mechanical properties of grout material and bolt bond strength (tb) (Fig. 9,Figs. 10 and 11), and curing time (days) and bolt strength (Figs. 12 and 13) were evaluated by simple pull-out test programme.
The samples consisted ofr ebars (ranging 10–18 mm diameters two by two) bonded into the basalt blocks.These basalt blocks used have a Young’s modulus of 27.6 GPa and a uniaxial compressive strength (UCSg) Of 133 MPa. Drilling holes which were 10 mm larger than the bolt diameter, having a diameter of 20 –28 mm for installation of bolts, were drilled up to 15–32 cm in depth. The bolt was grouted with cement mortar. The grout was a mixture ofPortland cement with a water to cement ratio of0.34, 0.36, 0.38 and 0.40 cured for 28 days. In order to obtain different grout types that have different mechanical properties, siliceous sand N100 mm;
500 mmM and fly ash N10 mm; 200 mmM were added in a proportion of10% ofcement weight and white cement with a water to cement ratio of 0.40. The sand should be well graded, with a maximum grain size of 2 mm (Schack et al., 1979). The Young’s modulus of the grouts was measured during unconfined compression tests and shear strength was calculated by means ofring shear tests.
The test set-up is illustrated schematically in Fig. 1 and the procedure is explained below:
● After filling prepared grout mortar into the hole, bolt is inserted to the centre ofdrilling hole.
● After curing time, the rebars in the rock were axially loaded and the load was gradually increased until the bolt failed.
● The bond strength (tb) was then calculated by dividing the load (Pb) by surface area (Ab) ofthe bolt bar in contact with the grout.
● Pull-out tests were repeated for various grout types,bolt dimensions and curing times.The influence of the bolt diameter and the bond area on the bond strength ofa rock bolt can be formulated as follows (Littlejohn and Bruce, 1975):
(5)
where tb, ultimate bolt bond strength (MPa); Pb, maximum pull-out load ofbolt (kN); db, bolt diameter (mm); l , bolt length (cm); pdb lb ,bonded area (cm2).
Fig. 1. Pull-out test set-up ofr ebar.
3.2. Analysis of laboratory test results
3.2.1. Influence of the bolt material
Bolt diameters of10, 12, 14, 16 and 18 mm were used in pull-out tests. Typical results are represented in Table 1, Figs. 2 and 3. The most important observations
were:
（1）The maximum pull-out load (Pb) increases linearly with the section of the bolt while embedment length was constant.
Fig. 2. Influence of the bolt diameter on the pull-out load of bolt.
Fig. 3. Influence ofthe bond area on the pull-out load ofbolt.
（2）Bolt section depends upon bolt diameter. The relation between bolt diameter and bolt bearing capacity can be explained as follow empiric formulae (Fig. 2).
= （6）
（3）The values ofbolt bond strength were calculatedbetween 5.68 and 5.96 MPa (Table 1).
Bolt lengths of15.0, 24.7, 27.0, 30.0 and 32.0 cm were used in pull-out tests as seen in Fig. 4. Typical results are represented in Table 2, and Figs. 5 and 6.The most important observations were:
(1) The pull-out force of a bolt increases linearly with the embedded length ofthe bolt.
（7）
(2) Maximum pull-out strength ofa bolt is limited to the ultimate strength ofthe bolt shank.
Table 1
Influence of the bolt diameter on the bolt strengt
(mm)
(mm)
()
()
(kN)
(MPa)
10
12
14
16
18
20
22
24
26
28
24.2
24.2
24.2
24.2
24.2
76
91
106
122
137
43.2
54.7
59.2
73.6
81.6
5.68
6.01
5.58
6.03
5.96
Rock: Basalt; curing time: 35 days; w／c＝0.40; ＝8.15 MPa.
Fig. 4. The pull-out test set-up of different bolt length.
Fig. 5. The relationship between bolt length and pull-out load.
Fig. 6. The relationship between bolt bond area and pull-out load.
Table 2
Influence of the bolt length on the bolt bond strength
(cm)
()
(KN)
(MPa)
15.0
24.7
27.0
30.0
32.0
57
93
102
113
121
44.3
72.8
79.0
90.2
91.7
7.77
7.83
7.75
7.98
7.58
Rock: Basalt; db＝12 mm; curing time: 21 days; tg: 10.4 MPa; UCSgs＝5.5 MPa; Eg＝7.54 GPa.
3.2.2. Influence of grouting material
The water to cement ratio should be no greater than 0.40 by weight; too much water greatly reduces the long-term strength. Because, part ofthe mixing water is consumed by the hydration ofcement used. Rest of the mixing water evaporates and then capillary porosities exist which results in unhomogenities internal structure of mortar . Thus, this structure reduces the long-term
strength by irregular stress distribution (Neville, 1963;Atis；1997). To obtain a plastic grout, bentonit clay can be added in a proportion ofup to 2% of the cement weight. Other additives can accelerate the setting-time,improve the grout fluidity allowing injection at lower water to cement ratios, and make the grout expand and pressurize the drill hole. Additives, ifused at all, should
be used with caution and in the correct quantities to avoid harmful side effect such as weakening and corrosion（Frannlin and Dusseault,1989）.
The water to cement ratio (w/c) in grouting materials considerably affects pull-out strength of bolt. As seen in Table 3, UCSg and shear strength (tg) ofgr out in high wyc ratio show lower values whereas in low w/c ratio higher values. The ratio between 0.34 and 0.40 presents quite good results. Although the wyc ratio of 0.34 gives the best bond strength, groutibility (pumpability) decreases and a number of difficulties in application appear. In high w/c ratio, the pumpability of grouting materials into the drilling hole is easy but the bond strength of bolt decreases (Figs. 7 and 8).
Table 3
The influence of the water to cement ratio on the bolt bond strength
w/c
(MPa)
(MPa)
()
(KN)
(MPa)
0.34
0.36
0.38
0.40
42.0
38.9
33.3
32.0
11.9
11.3
10.7
10.3
102
102
102
102
80.9
79.0
77.4
75.3
7.93
7.75
7.59
7.38
Rock: Basalt; db=12 mm.
Fig. 7. The influence of water to cement ratio on the bolt bond strength.
Fig. 8. The influence of water to cement ratio on the bolt pull-out load.
The bond strength of fully cement-grouted rock bolts is primarily frictional and depends on the shear strength at the bolt–grout or grout–rock interface. Thus any change in this shear strength of interfaces affects the bolt bond strength and load capacity. The influences of mechanical properties ofgr outing materials on the bearing capacity ofbolt can be described as follows:
(1) The uniaxial compressive and shear strength of the grouting materials has an important role on the behaviour ofr ock bolts. It was observed that increasing shear strength ofthe grouting material logarithmically increases bolt bond strength as shown in Table 4 and Fig. 9. The relation between grout shear strength and bolt bond strength was formulated as follows:
（8）
(2) Table 4 and Fig. 10 show that increasing grout compressive strength considerable increases the bond strength ofthe grouted bolts.
（9）
(3) In Fig. 11 and Table 4 show that there is another relationship between Young’s modulus ofgr out and bolt bond strength. Increasing the Young’s modulus increases bolt bond strength.
（10）
Table 4
Influence of the mechanical properties of the grouting materials on the bolt load capacity
注漿類型
(MPa)
(GPa)
(MPa)
(kN)
()
(MPa)
/
fly
fly
White cemen
5.30
12.84
17.74
20.80
22.94
31.60
30.58
33.33
37.72
32.01
33.33
38.94
42.00
1.15
2.74
2.96
3.39
3.79
6.22
4.89
5.25
6.63
7.40
8.05
9.12
9.30
2.04
4.99
6.22
7.95
9.17
6.73
7.34
8.05
8.15
10.30
10.70
11.30
11.93
16.53
43.75
55.28
57.59
59.84
55.45
58.15
56.01
58.15
75.26
77.39
78.99
80.87
84
84
84
84
84
83
83
83
83
102
102
102
102
1.94
5.20
6.63
6.83
7.14
6.73
6.32
6.73
7.03
7.34
7.54
7.75
7.95
0.95
1.04
1.07
0.86
0.78
1.00
0.86
0.84
0.86
0.71
0.70
0.68
0.67
Curing time: a1 day; b3 days; c5 days; d7 days; e14 days; f21 days.
Fig. 9. The relationship between grout shear strength and bolt bond strength.
Fig. 10. The relationship between UCS ofgr out materials and bolt bond strength.
Fig. 11. Changing ofbolt bond strength due to Young’s modulus ofgr out.
3.2.3. Influence of the curing time
An important problem in the application ofcementgrouted bolts is the setting time ofthe mortar, which strongly affects the stabilizing ability of bolt. Cementgrouted dowels cannot be used for immediate support because ofthe time needed for the cement to set and harden (Franklin and Dusseault, 1989).
In the pull-out tests, eight group of bolts having same length and mortar with a water to cement ratio of 0.4 were used for determining the effects of curing time on the bolt bond strength. Each group ofr ock bolt testing was performed after different setting times (Table 5). As can be seen in Figs. 12 and 13, the strength of bolt bond increases rapidly in 7 days due to curing time.However, the bond strength ofbolt continues to increase rather slowly after 7 days.Rock bolts may lose their supporting ability because of yielding of bolt material, failure at the bolt–grout or
grout–rock interface, and unravelling of rock between bolts. However, laboratory tests and field observations suggest that the most dominant failure mode is shear at the bolt–grout interface (Hoek and Wood, 1989). So,this laboratory study focussed on the interface between
rock bolt and rock and the mechanical properties of grouting materials.
Fig. 12. Changing ofpull-out load ofbolt due to curing time.
Fig. 13. Changing ofbolt bond strength due to curing time.
Table 5
The influence of the setting time on pull-out resistance
凝結(jié)時(shí)間
(days)
(mm)
(cm)
()
(KN)
（MPa）
1
3
5
7
14
21
28
35
12
12
12
12
12
12
12
12
24.2
24.2
24.2
24.2
24.2
24.2
24.2
24.2
91
91
91
91
91
91
91
91
17.64
43.75
56.83
71.25
75.48
76.55
78.46
80.06
1.94
4.79
6.22
7.85
8.36
8.46
8.66
8.77
Rock: Basalt; wycs0.40; dds22 mm.
4. Conclusions
The laboratory investigation showed that the bolt capacity depends basically on the mechanica
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