5.1 Analysis of the anchoring performance:
The experimental data was arranged and observed, and the experimental phenomena was recorded, followed by acquisition of the drawing destruction mode and maximal anchoring force in different experimental groups, shown in Table 5–10 (The data marked as “*” is not considered).
Table 5 The test results by different anchor rod length.
Number
|
Destruction mode(kN)
|
Fracture load(kN)
|
Average(kN)
|
L8-1
|
GF deformation(2.3)→grouting cracking(6.8)→GF pull out(7.8)
|
7.8
|
7.95
|
L8-2
|
GF deformation(1.6)→grouting cracking(5.2)→GF pull out(5.4)
|
5.4*
|
L8-3
|
GF deformation(2.2)→grouting cracking(7.3)→GF pull out(8.1)
|
8.1
|
L12-1
|
GF deformation(2.3)→grouting cracking(7.6)→GF pull out(11.5)
|
11.5
|
10.63
|
L12-2
|
GF deformation(2.4)→grouting cracking(7.8)→GF pull out(9.8)
|
9.8
|
L12-3
|
GF deformation(2.0)→grouting cracking(8.1)→GF pull out(10.6)
|
10.6
|
L15-1
|
GF deformation(2.3)→grouting cracking(6.5)→grouting crushing(11.2)→GF pull out(15.5)
|
15.5
|
16.47
|
L15-2
|
GF deformation(2.5)→grouting cracking(7.1)→grouting crushing(13.6)→GF pull out(17.1)
|
17.1
|
L15-3
|
GF deformation(2.3)→grouting cracking(7.5)→grouting crushing(12.8)→GF pull out(16.8)
|
16.8
|
L20-1
|
GF deformation(2.3)→grouting cracking(7.5)→grouting crushing(12.3)→GF pull out(18.7)
|
18.7
|
18.55
|
L20-2
|
GF deformation(2.8)→grouting cracking(8.2)→grouting crushing(15.1)→GF pull out(15.6)
|
15.6*
|
L20-3
|
GF deformation(2.3)→grouting cracking(7.3)→grouting crushing(11.8)→GF pull out(18.4)
|
18.4
|
L30-1
|
GF deformation(2.6)→grouting cracking(7.8)→grouting crushing(11.5)→GF fracture(19.8)
|
19.8
|
19.83
|
L30-2
|
GF deformation(2.4)→grouting cracking(7.5)→grouting crushing(13.2)→GF fracture(19.6)
|
19.6
|
L30-3
|
GF deformation(2.2)→grouting cracking(8.2)→grouting crushing(12.8)→GF fracture(20.1)
|
20.1
|
Table 6 Test results of different anchor rod diameter.
Number
|
Destruction mode(kN)
|
Fracture load(kN)
|
Average (kN)
|
H1-1
|
GF deformation(2.5)→grouting cracking(8.0)→GF pull out(10.6)
|
10.6
|
11.16
|
H1-2
|
GF deformation(2.5)→grouting cracking(8.2)→GF pull out(11.7)
|
11.7
|
H1-3
|
GF deformation(2.2)→grouting cracking(7.5)→GF pull out(11.2)
|
11.2
|
H2-1
|
GF deformation(3.2)→grouting cracking(7.5)→grouting crushing(12.3)→GF pull out(14.2)
|
14.2
|
13.53
|
H2-2
|
GF deformation(3.5)→grouting cracking(7.1)→grouting crushing(12.6)→GF pull out(13.6)
|
13.6
|
H2-3
|
GF deformation(3.5)→grouting cracking(7.9)→grouting crushing(11.8)→GF pull out(12.8)
|
12.8
|
Table 7 The test results by different bore diameter.
Number
|
Destruction mode(kN)
|
Fracture load(kN)
|
Average(kN)
|
D45-1
|
GF deformation(2.3)→grouting cracking(7.4)→GF pull out(11.5)
|
11.5
|
10.77
|
D45-2
|
GF deformation(2.1)→grouting cracking(7.8)→GF pull out(10.1)
|
10.1
|
D45-3
|
GF deformation(2.2)→grouting cracking(7.5)→GF pull out(10.7)
|
10.7
|
D75-1
|
GF deformation(2.4)→grouting cracking(6.3)→GF pull out(8.1)
|
8.1*
|
10.5
|
D75-2
|
GF deformation(2.5)→grouting cracking(6.5)→GF pull out(10.7)
|
10.7
|
D75-2
|
GF deformation(2.2)→grouting cracking(6.6)→GF pull out(10.3)
|
10.3
|
D80-1
|
GF deformation(2.3)→GF pull out(7.6)
|
7.6
|
7.63
|
D80-2
|
GF deformation(2.2)→GF pull out(8.2)
|
8.2
|
D80-3
|
GF deformation(2.7)→GF pull out(7.4)
|
7.4
|
Table 8 The test results by different grouting strength.
Number
|
Destruction mode(kN)
|
Fracture load(kN)
|
Average(kN)
|
S1-1
|
GF deformation(2.3)→grouting cracking(4.5)→GF pull out(7.3)
|
7.3
|
7.73
|
S1-2
|
GF deformation(2.5)→grouting cracking(3.7)→GF pull out(7.9)
|
7.9
|
S1-3
|
GF deformation(2.1)→grouting cracking(4.3)→GF pull out(7.7)
|
7.7
|
S2-1
|
GF deformation(2.1)→grouting cracking(7.5)→GF pull out(11.4)
|
11.4
|
11.43
|
S2-2
|
GF deformation(2.3)→grouting cracking(8.2)→GF pull out(12.1)
|
12.1
|
S2-3
|
GF deformation(1.7)→grouting cracking(7.1)→GF pull out t(10.8)
|
10.8
|
S3-1
|
GF deformation(2.1)→soil-part loosening(14.7)→grouting pull out(17.9)
|
17.9
|
17.53
|
S3-2
|
GF deformation(1.8)→soil-part loosening(14.1)→grouting pull out(17.5)
|
17.5
|
S3-3
|
GF deformation(1.9)→soil-part loosening(13.3)→grouting pull out(17.2)
|
17.2
|
Table 9 The test results by different surface state.
Number
|
Destruction mode(kN)
|
Fracture load(kN)
|
Average(kN)
|
R0-1
|
GF deformation(2.1)→GF pull out(4.7)
|
4.7
|
4.33
|
R0-2
|
GF deformation(2.3)→GF pull out(4.5)
|
4.5
|
R0-3
|
GF deformation(2.0)→GF pull out(3.8)
|
3.8
|
R30-1
|
GF deformation(2.0)→grouting cracking(7.5)→GF pull out(11.4)
|
11.4
|
11.06
|
R30-2
|
GF deformation(2.4)→grouting cracking(7.7)→GF pull out(10.6)
|
10.6
|
R30-3
|
GF deformation(2.1)→grouting cracking(7.2)→GF pull out(11.2)
|
11.2
|
R100-1
|
GF deformation(2.2)→GF pull out(7.3)
|
7.3
|
7.53
|
R100-2
|
GF deformation(2.0)→GF pull out(7.5)
|
7.5
|
R100-3
|
GF deformation(2.4)→GF pull out(7.8)
|
7.8
|
Table 10 The test results by different deployment angle of anchor rod.
Number
|
Destruction mode(kN)
|
Fracture load(kN)
|
Average(kN)
|
A0-1
|
GF deformation(2.0)→grouting cracking(7.2)→GF pull out(10.3)
|
10.3
|
10.77
|
A0-2
|
GF deformation(2.2)→grouting cracking(8.1)→GF pull out(11.2)
|
11.2
|
A0-3
|
GF deformation(2.3)→grouting cracking(7.6)→GF pull out(10.8)
|
10.8
|
A10-1
|
GF deformation(1.8)→grouting cracking(6.3)→GF pull out(9.4)
|
9.4
|
8.97
|
A10-2
|
GF deformation(2.0)→grouting cracking(7.5)→GF pull out(10.2)
|
10.2
|
A10-3
|
GF deformation(2.1)→grouting cracking(6.6)→GF pull out(7.3)
|
7.3
|
A15-1
|
GF deformation(1.9)→grouting cracking(4.7)→GF pull out(7.1)
|
7.1
|
6.37
|
A15-2
|
GF deformation(2.3)→grouting cracking(5.1)→GF pull out(6.7)
|
6.7
|
A15-3
|
GF deformation(2.0)→grouting cracking(4.6)→GF pull out(5.3)
|
5.3
|
As evident from the experimental results, the destruction mode does not occur in isolation. Several destruction modes transpire during the drawing process. When the anchorage system suffers a large displacement, it is considered as the final destruction mode. Four main anchorage system destruction modes can be obtained: geo-filament anchor pullout (Fig. 12a), grout-soil damage (Fig. 12b), geo-filament fracture (Fig. 12c), soil-part loosening and grouting pullout (Fig. 12d).
By summarizing the destruction phenomena and the properties of each destruction stage, the following conclusions can be drawn:
a. The filament-grout interface damage emerges to be the primary destruction mode of the anchorage system. Only with the robust strength level of the grouting can the grout-soil interface damage be triggered; there exists complete exertion of the anchoring system force at this moment. Hence, the grout-soil interface damage appears to be an ideal destruction mode, However, it can lead to the destruction of the site itself, hence, such a destruction mode appears non-conducive for heritage protection.
b. When the anchoring length reaches the depth of about 3000 mm, the maximal anchoring force seems to be almost equal to the geo-filament’s breaking tension value, thus resulting in the geo-filament fracture.
c. The anchoring length (L), grouting strength (S), and surface state (R) are factors sensitive to the enhancement of the anchoring performance of the geo-filament bolt, while the geo-filament thickness (H), bore diameter (D), and anchorage angle (A) are non-sensitive towards the enhancement of the anchoring performance.
5.2 Relation between load-displacement (P-S):
By removing the undesired values in the experimental groups and averaging experimental results, the P-S load-displacement curves of L, D, H, R, S, and A can be attained (Fig. 14–17). According to the destruction characteristics of the anchoring system, each destruction stage can be distinguished by a certain color in the load-displacement curve.
a. The green area of the load-displacement curve illustrates the deformation stage of the geo-filament. The maximum load at this stage is expressed as Ni.
b. The yellow area of the load-displacement curve demonstrates the stage where the grouting crack or the pullout appears. The maximum load at this stage is expressed as Nj.
c. The red area of the load-displacement curves depicts the stage of geo-filament pullout or fracture. The maximum load at this stage is expressed as Nk.
(1) Influence of anchoring length L on the load-displacement curve.
The L-P-S curve (Fig. 13) establishes that an increase in the anchor rod length tends to significantly increase the anchoring force and can effectively control the displacement of the anchoring system under similar pulling force.
When the anchor rod length remains between 800~1500 mm, maximal anchoring force can be increased by increasing the anchor rod length. However, when the anchor rod length is between 1500~3000 mm, increasing the anchor rod length fails to effectively augment the maximal anchoring force. Moreover, when the anchor rod length is about 3000 mm, the maximal anchoring force appears to be extremely close to the fracture tension value of the geo-filament.
(2) Influence of geo-filament thickness H on the load-displacement curve.
The H-P-S curve is shown in Figure 14. It is evident that the thickness of H2 appears to be double as compared to H1, and the maximal anchoring force only tends to increase by 17.5%. Increasing the geo-filament’s thickness does not guarantee an effective enhancement in the maximal anchoring force. Moreover, this increase in thickness is also unable to provide an efficient control over the displacement. Therefore, changing H does not necessarily contribute to the effective improvement of the anchoring performance.
Due to the geometry of the geo-filament sheet, the strength of the soil in the anchorage system appears to be greater than that of the grouting, and an increase in the thickness (H) triggers a certain weakening effect on the integrity of the grouting. Through the experiment, it can be concluded that increasing the thickness of the geo-filament does not enhance the anchorage performance, but rather, it accelerates the crushing of the grouting.
(4) Influence of grouting body strength S on the load displacement curve.
The S-P-S curve is shown in Figure 15. As the grouting body strength increases, the bonding strength of the grout-rod interface tends to be improved, and the anchoring force increases with the increase in the grouting body strength. Under the same load, the anchoring system’s displacement of S1 and S2 appears to be about twice of S3, indicating that the displacement can be effectively controlled by increasing the grouting strength.
(5) Influence of anchor rod surface state R on the load displacement curve.
The R-P-S curve (Fig. 16) illustrates that the maximal anchoring force of the anchor rod R0 without a rib is about 4.33 kN. The maximal anchoring force of R100 and R30 with a rib is about 7.53 kN and 11.0 6kN, respectively. The maximal anchoring force increases by 73.90% and 155.42% for the respective states after the rib is added. When the tension is greater than 3 kN, the displacement of ribbed anchoring system appears considerably lesser than that of the one without a rib. This indicates that the change of geo-filament surface state can effectively increase the ultimate anchoring force and control the displacement as well.
(6) Influence of anchoring deployment angle A on the load displacement curve.
The A-P-S curve (Fig. 17) exhibits that the anchoring force seems to undergo a reduction with the increase of the angle, and the deployment also increases under the same load. Since the load applied here does not reach the anchor rod just over a single action line, the anchor is unable to perform optimally. However, keeping in mind construction technology requirements, it is suggested that the layout angle of the anchor rod must be at 10° for it to perform to its fullest.
5.3 Strain distribution (\(\xi - L\)) characteristics of GF and the grouting interface:
The analysis of the destruction results of this experiment imply that the anchoring system is ineffective due to the bond-slip present between the geo-filament and the grouting interface, which causes the bolt to pull out.
In the P-S curve (Fig. 13–18), the bond-slip stage is shown as the red segment. The distribution in this stage is similar to that of the brittle failure. The anchoring system exhibits no evident deformation when the anchor rod is suddenly pulled out.
For the anchoring of the raw earth architectural ruins, it appears vital not only to protect the cultural relics while evading the destruction of the grout-soil interface, but also to allow the anchoring system to perform optimally, hence, the anchoring force design needs to have a certain safety reserve.
In accordance to the load displacement curve (P-S), the failure characteristics, and the strain distribution at the geo-filament-grout interface ( \(\xi - L\) ), and taking into account the particularity of anchoring of the raw earth architectural ruins, the calculation formula for the design value N of the anchoring force is given as shown in Equation (1) and (2).
N1 = Ni +(Nj - Ni) × 50% (1)
N2 = Nj +(Nk - Nj) × 30% (2)
Where \({N_i}\) is the extreme value of the geo-filament deformation stage, with the anchoring system in the elastic stage; \({N_j}\)is the extreme value when the grouting cracks or is pulled out, with the anchoring system in the plastic stage; and \({N_k}\) is the extreme value of the geo-filament pull-out stage, where the anchoring system remains in the bond-slip stage.
The specific values of \({N_i}\), \({N_j}\), \({N_k}\) are shown in Tables 5–10. The N values of different experimental lots (L, H, D, S, R, A) can be derived by sorting out the data, as shown in Table 11.
Considering the values of N1 and N2 in Table 11, the strain distribution curves of the geo-filament-grout interface under the action of N of each experimental lot (L, H, D, R, S, A) are extracted, as shown in Figures 19–24.
Table 11
The N values in different experimental lots.
Classification
|
N values (kN)
|
L
|
L8
|
L12
|
L15
|
L20
|
L30
|
N1
|
4.23
|
5.03
|
7.45
|
7.18
|
7.45
|
N2
|
7.35
|
8.67
|
13.71
|
14.00
|
14.69
|
H
|
H1
|
H2
|
—
|
—
|
—
|
N1
|
5.15
|
7.82
|
—
|
—
|
—
|
N2
|
8.88
|
12.26
|
—
|
—
|
—
|
D
|
D45
|
D75
|
D85
|
—
|
—
|
N1
|
4.89
|
4.42
|
2.40
|
—
|
—
|
N2
|
8.53
|
7.74
|
3.97
|
—
|
—
|
S
|
S1
|
S2
|
S3
|
—
|
—
|
N1
|
3.35
|
4.82
|
5.96
|
—
|
—
|
N2
|
5.24
|
8.75
|
14.78
|
—
|
—
|
R
|
R0
|
R100
|
R30
|
—
|
—
|
N1
|
2.13
|
2.20
|
4.82
|
—
|
—
|
N2
|
2.79
|
3.80
|
8.55
|
—
|
—
|
A
|
A0
|
A10
|
A15
|
—
|
—
|
N1
|
4.90
|
4.39
|
3.44
|
—
|
—
|
N2
|
8.57
|
7.45
|
5.27
|
—
|
—
|
(1) Design value N of anchoring force based on the anchor length L.
As evident from Figures 19–24, with the increase of anchoring length L, grout strength S, and geo-filament surface roughness R, the anchoring performance of geo-filament appears to be enhanced appreciably, among which, the anchoring length L seems to produce the most striking influence, and the variation of anchoring length L appears to be the most common contributor in practical application.
Figure 19 illustrates that the maximum transmission depth of anchoring force in the geo-filament anchoring system is about 2.2 m, and the stress of the anchor rod suffers significant changes in the central part. Combined with the practical experience and experimental data, the designed anchoring depth of the geo-filament bolt should not be more than 2 m, and the optimal anchoring length should be between 1.2 m and 1.5 m.
By comparing the strain distribution curves of the geo-filament anchor under the action of the design value N of the anchoring force, it seems reasonable to further determine the design value N of anchoring force based on the change of anchoring length L. Considering the safety reserve of the anchoring system and its performance, the calculation formula for N is shown in Equation 3.
(3)
(2) Grouting strength design value.
When the failure of the anchoring system occurs between the grouting and the site soil, the anchoring system performs optimally, and is an ideal location for failure. However, such destruction is detrimental to the site soil as well as non-conducive to the protection of earthen cultural relics. Hence, when designing the anchor rod, it should be given priority that if the damage occurs in the later usage stage, the damaged part appears between the grouting and the anchor rod, to ensure that the site itself is not damaged and remains intact.
The load of the geo-filament bolt is mainly transmitted by the bond stress (shear stress) between the bolt and the grouting body in the anchoring section. The destruction form of the grouting is in the form of shear failure.
By comparing the stress distribution at the interface of the geo-filament-grout in Figures 20–25 and the obtained data, it can be concluded that under the action of the design value of anchoring force N, the front-end L/5 segment of the anchor rod experiences about 45% of the anchoring force, and the damage of the anchorage system begins with the fracture of the end grouting. The impairment of the anchoring system begins with a crack in the front-end grouting of the anchor rod.
Therefore, the stress distribution set of the anchoring system under the action of N can be simply deduced as uniform distribution at the front-end L/5 and triangular distribution at the other parts. The simplified diagram is as shown in Figure 25.
In Figure 25, \({\tau _1}\)is the bonding stress between the geo-filament-grout interface and \({\tau _2}\) is the bonding stress between the grout-soil interface at the bolt front-end L/5 section. According to the balance of internal and external force, \({\tau _1}\), \({\tau _2}\)can be obtained as:
(4)
(5)
Where is the width of the geo-filament, and D depicts the bore diameter of grouting (Fig. 7). \({\tau _1}=\frac{{\pi D}}{{2d}}{\tau _2}\) can be obtained from Equations (4) and (5), \(\alpha =\frac{{\pi D}}{{2d}}\) is defined as the shear stress diffusion coefficient. The relationship of \({\tau _1}\) and \({\tau _2}\) can be written as:
(6)
Where we need to satisfy \({\tau _2} \leqslant \frac{{\tau _{2}^{{\hbox{max} }}}}{n}\), and is the safety factor. Therefore, the relation between \({\tau _1}\) and \({\tau _2}\) can be further written as:
(7)
Through comprehensive analysis, it can be concluded that the shear strength of the grouting should not be more than \(\frac{\alpha }{n}\) times that of the soil for the design of the geo-filament bolt.