Laboratory investigation into the use of soundless chemical ...

3.1

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Rock characterization

A series of Brazilians tests were conducted to determine the tensile strength on the slabs (refer to Fig. 3 for test set-up). The Stanstead granite samples were cored from the granite slab and prepared in compliance with the ASTM D Standard. The results give a mean value of 7.5 MPa with a standard deviation of 0.33.

Brazilian test set-up on a Stanstead granite core

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3.2

The influence of borehole size on granite

The effect of three borehole sizes (25.4 mm (1ʺ), 31.75 mm (1.25ʺ) and 38.1 mm (1.5ʺ)) was investigated in a Stanstead granite slab with dimensions 152.4 mm (6ʺ)&#;×&#;152.4 mm&#;203.2 mm (6&#;8ʺ)&#;×&#;406.4 mm (16ʺ) (width × length × height). Each configuration was tested in duplicates to ensure repeatability and reliability. As shown in Fig. 1, the strain gauges were placed 38.1 mm (1.5ʺ) away from the borehole on each side, above and below borehole, and on the sides. The strains were measured until the first fracture appears, after which the strain gauges are potentially damaged due to localised splitting of the rock. The goal of the experiment is to identify the time of cracking using the measured strains in conjunction with timelapse photos of the slab.

Within a confined hole, radial pressure develops over time causing radial and tangential tensile stresses in the surrounding rock. A fracture is created at the weakest section along the inside surface of hole (Harada et al. ). The time of cracking was identified by plotting the measured strains over time.

When the rock is undamaged, its reaction to the SCDA pressure is linear. Therefore, if the expansive pressure exerted by the SCDA is also linear, the measured strains will also increase linearly. Consequently, the strain at which this linear increase terminates, or the onset of nonlinear behaviour, is interpreted as an indication that cracking has initiated. This point is referred to in this work as the &#;critical strain&#;. On the other hand, both the time of visual fracture initiation and visual fracture completion were recorded, where the time of crack completion represents the point in time at which the fracture reaches the free surface of the slab. The 25.4 mm (1ʺ) borehole is used as a base case to assess the time of fracturing with increasing borehole sizes (31.75 mm (1.25ʺ) and 38.1 mm (1.5ʺ)). As shown in Figs. 4, 5, 6, for a 1ʺ hole, the first fracture occurs at 18 h, 15 h, and 23 h for slabs M01, M02, and M03, respectively. Figures 7, 8 show the recorded strains for specimens G-SL1-100-500-M-02 and G-SL1-100-500-M-03. The strains for test G-SL1-100-500-M-01 were discarded. As summarized in Table 2, crack completion occurs after 21 to 27 h. As can be seen in Figs. 4, 5, 6, it takes on average 5 h after crack initiation for a crack to propagate to the free surface of the granite slab. As shown in Figs. 7, 8, the critical strain at which the linear behaviour ends for specimen G-SL1-100-500-M-02 and G-SL1-100-500-M-03 is 6 h and 19 h respectively. It is also apparent that the non-linear behaviour is exponential, suggesting that there has been significant brittle damage to the rock at those points. This is in accordance with the visual cracking at which the time of initial cracking for G-SL1-100-500-M-02 and G-SL1-100-500-03 occur at 15 h and 23 h respectively, both times at which the strains increase exponentially as shown in Figs. 7, 8 (SG2 was damaged at the beginning of the test and is therefore omitted from the results in Fig. 7). It is also shown that non-linear strain increase starts before cracking is visible on the slab surface, suggesting that visual identification of superficial cracking alone is not sufficient to determine the time at which fracturing initiates. This could be due to fracture initiating deeper in the block, or that the crack width is too small to capture with the camera.

Slab G-SL1-100-500-M-01 a At time t0 b Initial breakage at time t18 c Complete breakage at time t24

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G-SL1-100-500-M-02 a At time t0 b Initial breakage at t15 c Complete breakage at t21

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G-SL1-100-500-M-03 a At time t0 b Initial breakage at t23 c Complete breakage at t27

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Measured strains in specimen G-SL1-100-500-M-02

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Measured strains in specimen G-SL1-100-500-M-03

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Full size table

The delay between visible crack initiation and non-linear strain increase is also observed in all specimens with an increased borehole size of 31.75 mm (1.25ʺ). Specimens G-SL2-125-625-M-01 and G-SL2-125-625-M-02 show a TFC of 11.5 h and 16 h, respectively after SCDA is injected into the hole (refer to Figs. 9, 10 for visual cracking). The onset of non-linear behaviour for specimens G-SL2-125-625-M-01 and G-SL2-125-625-M-02 begins after 5 h and 8 h, respectively (refer to Figs. 11, 12 for strain values). The 31.75 mm specimens exhibited a faster reaction than the 25.4 mm where the time of initial cracking is decreased by an average of 5 h.

G-SL2-125-625-M-01 a At time t0 b Initial breakage at t11.5 c Complete breakage at t17

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G-SL2-125-625-M-02 a At time t0 b Initial breakage at t16 c Complete breakage at t20

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Measured strains in specimen G-SL2-125-625-M-01

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Measured strains in G-SL2-125-625-M-02

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Compared to specimens, G-SL1-100-500-M-01, G-SL1-100-500-M-02, G-SL1-100-500-M-03, crack completion occurs slightly earlier between 17 and 20 h.

The sharp decrease in strains shown in Figs. 11, 12 is attributed to local stress relief due to cracking. When the strain gauges were not damaged, the observed relaxation corroborates with visual cracking.

With a 38.1 mm (1.5ʺ) SCDA injected hole, the time of breakage is reduced by half when compared to a 25.4 mm (1ʺ) hole. The TFC is also significantly decreased, at 7.5 h and 10 h for specimens G-SL2-150-600-M-01 and G-SL2-150-600-M-02 respectively. For both specimens, complete slab fracturing occurred relatively quickly after cracking initiated, in 2.5&#;4 h as shown in Figs. 13, 14, 15. Figure 15 shows the rapidly increasing strains after 7.5 h and quickly decreasing strains once the slab splits, indicating that the slab is fully relaxed, and all stresses have dissipated. Given that the strain gauges were not bisected by any cracks, the observed relaxation corroborates with the visual cracking. Figure 16 shows that at the time of visual breakage (11 h), a small decay in strains is also observed shortly after indicating a relaxation of strains.

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G-SL2-150-600-M-01 a At time t0 b Initial breakage t7.5 c Complete breakage t10

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G-SL2-150-600-M-02 a At time t0 b Initial breakage t11 c Complete breakage at t13

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Measured strains of specimen G-SL2-150-600-M-01

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Measured strains in specimen G-SL2-150-600-M-02

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Overall, the specimens under no loading showed that visual cracking is delayed according to the measured strains. The measured strains indicate tensile damage in the slab before any superficial crack is apparent. This suggests that crack initiation may have occurred deeper in the slab before superficial cracking, or that the crack width is too small to observe. Therefore, the strain measurements are deemed a reliable tool to detect cracking without physical monitoring such as using a timelapse camera.

3.3

Effect of SCDA on rock breakage under load

The effect of uniaxial loading on rock breakage with SCDA was tested by subjecting a slab with a single SCDA-filled hole to uniaxial compressive stress using a 200-tonne uniaxial loading frame. This part of the study aims to quantify the fracture propagation rate of granite subjected to uniaxial far-field pressure and radial pressure from the expansive cement. As shown in Table 1, a total of 3 specimens were tested under a uniaxial stress of 5 MPa. Such loading level was selected so that the tensile stresses around the hole do not exceed the tensile strength of the granite. All Stanstead granite slabs were 6ʺ × 8ʺ × 16ʺ, and three different hole locations were tested. All specimens were greased with a thin coat of MoS2 to reduce friction between the rock and the loading plate as shown in Fig. 2b. As the first part of the investigation shows a decrease in TFC with the increase of borehole diameter, it was decided to adopt only the largest diameter of 38.1 mm (1.5ʺ) for the second part as uniaxial loading condition was thought to delay fracturing. To confirm the trend, another size of 44.45 mm (1.75ʺ) hole was also tested. Three specimens were tested; specimen G-SL2-150-600-UT-L with a 38.1 mm (1.5ʺ) SCDA hole located in the upper third of the slab, specimen G-SL2-175-700-M-L with a 44.45 mm (1.75ʺ) SCDA hole located in the middle of the slab and specimen G-SL2-175-700-LT-L with 44.45 mm (1.75ʺ) SCDA hole located in the lower third of the slab. Regarding the positioning of the holes, the goal is to observe the influence of hole diameter and its position on the TFC and time of complete breakage, all while monitoring the longest fracture path. Due to the limited number of samples available, it was not possible to return to the smaller SCDA hole sizes. The adopted holistic approach deemed adequate. As shown in Fig. 1, strain gauges were fixed 38.1 mm (1.5ʺ) away above (SG1) and below (SG3), and to the right (SG2) and left (SG4) side of the borehole. Both timelapse photos and strains were recorded to detect time of initial cracking and crack completion. Crack completion was assessed based on the time that cracking below and above the borehole reached the metal loading plates. As shown in Figs. 18, 20, 22, the strains for SG2 (right gauge) and SG4 (left gauge) remain in constant compression indicating that all slabs remained fully loaded over the entire duration of the test. As shown in Fig. 17b, a 38.1 mm (1.5ʺ) SCDA-filled hole located in the upper third of the slab (Specimen G-SL2-150-600-UT-L) shows initial cracking after 7 h and crack completion shortly after in 10 h. Figure 17b&#;d shows that initial cracking propagates towards the greased upper metal loading plate end first and then to the lower plate 3 h later when complete breakage of the specimen has occurred.

G-SL2-150-600-UT-L a At time t0 b TFC at t7 c Breakage at t7.5 d Complete breakage at t10

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As shown in Fig. 18, the measured strains for SG1 (strain gauge above the borehole) and SG3 (below the borehole) experience a sudden change in strain rate at 6 h leading to a TFC of 7 h.

Measured microstrain of specimen G-SL2-150-600-UT-L

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This will help estimate an L/phi ratio to design the hole spacing in practical applications where uniaxial loading conditions are present. Figure 19b shows a 1.75ʺ SCDA-filled hole located in the middle of the slab (G-SL2-175-700-M-L). Superficial cracking initiation is shown after 6.8 h and crack completion after 9.8 h. Compared to G-SL2-150-600-UT-L, the time of initial fracture is very close and does not significantly differ from the time of complete fracturing. Similarly, the fractures above the borehole propagated to the greased upper plate before the crack below the borehole has reached the metal plate. As shown in Fig. 20, there is a sharp increase in the strain rate at 6 h before visual cracking is detected.

G-SL2-175-700-M-L a At time t0 b Initial breakage t6.8 c Breakage at t9 d Complete breakage t9.73

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Measured strains in specimen G-SL2-175-700-M-L

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This shows that the visual behavior of cracking is once again delayed relative to the measured strains by 1 h. However, a different fracture pattern is observed with the 1.75ʺ SCDA filled hole located in the lower third of the granite slab (G-SL2-175-700-M-L).

In this case, Fig. 21b shows initial cracking superficial cracking apparent after 7 h, in accordance with specimens G-SL2-150-600-UT-L and G-SL2-175-700-M-L. However, the crack fully propagated after 12 h, which is significantly longer than what was observed for the latter two specimens. Initial fracturing is also in accordance with the measured strains in Fig. 22 where an increase in strain is observed after 7 h. The fracture pattern differs from specimens from the two other specimens tested: G-SL2-150-600-UT-L and G-SL2-175-700-M-L.

G-SL2-175-700-LT-L a At time t0 b Initial breakage c Breakage at t7 d Complete breakage at t12

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Measured strains in specimen G-SL2-175-700-LT-L

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As shown in Fig. 21c, cracking first propagates towards the lower metal plate and then propagates slowly towards the greased upper metal loading plate. The delay in fracture completion can therefore be related to the non-greased bottom plate offering increased frictional resistance, resisting the separation of the two slab halves.

As shown in Fig. 21c, cracking first propagates towards the lower bearing plate and then propagates slowly towards the greased upper loading plate. The delay in fracture completion can therefore be related to the non-greased bottom plate offering increased frictional resistance, resisting the extension strain that causes slab separation. Overall, Figs. 17, 19, 21 show that all fractures propagate in the direction of the major principal stress, unlike the unloaded slabs where fracturing generally propagated towards some or all the nearest faces. Since the loaded specimens had directional cracking as opposed to the unloaded specimens, optimal borehole spacing can be estimated. The ratio of the length of fracture over the SCDA hole diameter (Φ) is an indicator for maximum hole spacing. As each SCDA can generate a fracture length Lf, the spacing, S, between two SCDA holes is calculated as S&#;=&#;2Lf/ Φ. As shown in Table 3, the illustration for each specimen depicts which end of the granite slab is used to measure the length of fracture. The length Lf was selected based on the longest crack path measured from the borehole center. As the specimens have different hole configurations, it should be noted the Lf/Φ ratio reported in Table 3 is not a direct comparison between the specimens to judge their performance. Rather, it is used to help delineate the required SCDA borehole spacing in practice. As can be seen in Table 3, the Lf/Φ ratio for the loaded granite can be up to 6.4 to 7.3. As the top surface is greased, it acts as a line of symmetry. Thus, for practical applications, it can be claimed that the maximum allowable spacing between two SCDA boreholes is 2Lf/Φ, or 12.8 to 14.6Φ to achieve complete fracturing. According to work done by Gomez and Mura (), a proposed spacing between SCDA holes of 8Φ is suggested (Gomez and Mura ), however, for unloaded specimens.

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