# Study on Composite Fracture Characteristics and Hydraulic Fracturing Behavior of Hard Rock
## 5 True Triaxial Hydraulic Fracturing Test Verification  
### Test Scheme  
To investigate the fracture characteristics of hydraulic fractures, this study conducted true triaxial hydraulic fracturing tests. Considering that in true triaxial hydraulic fracturing tests, rock specimens need to have injection holes with a certain radius to ensure that the injection pressure acts on the pre-existing fractures, grooves were made on the 200 \(\times\) 200 \(\times\) 200 cubic specimens, as shown in Figure 10. This experiment was primarily conducted for the mechanical analysis of the hydraulic fracture propagation characteristics.  
Setting the crack angle, defined as the angle between the crack and the maximum horizontal principal stress, as \(\beta\), with \(\beta\) ranging from 0 to 90 degrees. The parameters for the true triaxial pre-existing crack hydraulic fracturing test are shown in Table 3.  
Figure 9: Influence of T stress on the \(p_{\rm b}\) and \(\theta_{\rm c}\) of hard rock. (**a**) \(r_{\rm c}=0.08\) mm; (**b**) \(r_{\rm c}=1\) mm.  
The upper end of the specimen was sealed by adding epoxy resin glue to the liquid injection pipe. After the lower end was sealed using the rubber plug, epoxy resin glue was injected into the specimen, as shown in Figure 11.  
In this study, the rock true triaxial fracturing simulation experimental device developed by Guizhou University (as shown in Figure 12) was used to simulate the hydraulic fracturing of the ore and rock in Copper Valley. This device is composed of three parts: a true triaxial loading system (including true triaxial loading bin, operation, and oil pressure control system), a pump pressure injection system (hydraulic injection pump), and a data acquisition system.  
This test followed the construction procedures for hydraulic fracturing at the copper mine site. The experimental steps were as follows: (1) specimen placement: place the specimen in the loading chamber, adjust the clamps to stabilize the specimen, connect the water injection pump valve, and seal the loading chamber cover; (2) triaxial loading: activate the servo triaxial pressurization system, uniformly load the three principal stresses of the specimen to predetermined values at a constant rate, and maintain stability for 1-2 h. This step is primarily for simulating the triaxial stress state of fractured rock due to  
Figure 11: Specim sealing method.  
Figure 10: Precast crack making.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**ID** & \(\sigma_{\textbf{z}}\) **(MPa)** & \(\sigma_{\textbf{H}}\) **(MPa)** & \(\sigma_{\textbf{h}}\) **(MPa)** & **Injection Rate (mL/min)** & \(\beta\) **(\({}^{\circ}\))** \\ \hline HF-1 & 12 & 9 & 6 & 20 & 0 \\ HF-2 & 12 & 9 & 6 & 20 & 15 \\ HF-3 & 12 & 9 & 6 & 20 & 30 \\ HF-4 & 12 & 9 & 6 & 20 & 45 \\ HF-5 & 12 & 9 & 6 & 20 & 60 \\ HF-6 & 12 & 9 & 6 & 20 & 75 \\ HF-7 & 12 & 9 & 6 & 20 & 90 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Hydraulic fracturing test scheme.  
hydraulic fracturing; (3) fluid injection: open the constant-flow water pump, set the injection flow rate and protective pressure, and achieve constant-flow water injection into the specimen until the water injection pump suddenly drops or is no longer rapidly increasing.