*3.1. Experimental Apparatus and Procedures*

Three types of borehole protection pipes with internal support structures: "lineshaped", "Y-shaped", and "cross-shaped", are designed as shown in Figure 16. Two common borehole diameters of 100 mm and 160 mm are selected for experimental research.

**Figure 16.** Three internal support structures of the borehole protection pipe.

This experiment is based on the GCTS Rock Mechanics Test System as shown in Figure 17. The system is a hydraulic servo-mechanical system produced by GCTS Company in the United States, which meets the requirements of the ISRM triaxial rock test of the International Society of Rock Mechanics and the American standard ASTM D2664-04. It is mainly used to test the mechanical properties and seepage characteristics of rock, concrete, and coal under complex loading conditions. The test accuracy is high and the performance is stable.

**Figure 17.** Schematic of the GCTS Rock Mechanics Test System (1 Compression experimental apparatus; 2/3 Controller valve; 4 Control cabinet of confining pressure volume; 5 Control cabinet of pore pressure volume; 6 Radical compressive force sensor; 7 Displacement sensor; 8/9 Storage computer).

The experimental steps of the GCTS rock mechanics test system are as follows:

