## 2 RSR Measuring Technique
### In Situ Implementation Method  
The implementation procedures of the RSR method for in situ stress measurement include the following three parts:  
(1) Preparation. This step, including measurement location selection, measurement device checking and the sensing column grouting. As the RSR method is applied in the rock masses showing time-dependent behavior, the burial depth and rock mass quality of the measurement locations should be considered firstly. According to research on the concept of deep underground engineering and rock mass classification [19], it can be suggested that, in general, for the RSR method, the burial depth should exceed 600 m and the rock mass class should be in the range from the third grade to the fifth grade. The device checking should ensure that all of the instrumentation is in good order and satisfies the measuring requirements. The grouting material for the sensing column are the same as that used for borehole grouting. During grouting, the angles between the six sensor faces with the axes and the initial frequencies should be recorded. (2) Field measurements. The method can be conducted in the field after the preparations have been completed and the steps described in detail below have been implemented:  
1. Hole drilling. Sensors holes in the surrounding rock, usually are drilled from underground roadways, tunnels, chamber, etc., until the set depth is reached (Figure 5a). To facilitate sensing column installation and grout flow, the drilled hole is downward and has an appropriate inclination angle of 1-3\({}^{\circ}\). Additionally, the sufficient space of holes should be retained to accommodate the sensing column. The appropriate diameter of the drilled hole is 130 mm per diameter of the sensing column and commonly-used drills. After drilling, the hole should be flushed quickly with water to remove residual debris. However, it is worth noting that the duration time of flushing should not be too long to prevent rock falling due the water-softening effect. 2. Transducer installation. The connecting device pre-grouted with the sensing column is connected to the goniometer slot and the initial angle displayed by the goniometer is recorded when the \(y\)-axis is horizontal (Figure 2). Using the push rod and rollers, the sensing column is  
Figure 4: Explosion-proof data acquisition instrument (**(a)**. 3D comprehensive stress transducer; **(b)**. data acquisition instrument). Figure 3: Sensor installation equipment (**(a)**. Slot; **(b)**. goniometer; **(c)**. roller; **(d)**. pushrod). inserted incrementally into the preset measurement location and the final displayed angle is recorded (Figure 5b). After this, the push rods, rollers, and goniometer will be withdrawn. The above steps are repeated when more than one test point is required. As the attachment between the connection device and slot is free-sliding in axial direction of the drilled hole, the pushing rods cannot be arbitrary pulled out to prevent the separation of connection before the sensing column is in place. If it has to be in special case, it should be conducted slowly using the cables of the goniometer and sensing column to keep the consistency of the rotation angle during the installation. * Hole grouting. The grouting pipe is pulled to the place nearby the sensing column, sealing the hole, leaving open the injection and vent holes. The drilled hole is filled with pumped grout and slurry materials, so that the rock mass will be in close contact with the sensing column (Figure 5c). [PERSON] et al. [20] investigated the relationship between grouting time and recovery stress and found that the longer solidification time of slurry materials, the smaller the recovery stress of the sensing column will eventually be. In order to ensure complete filling with the grouting slurry, the bore of the hole should be sealed to prevent leakage and the grouting pressure is appropriately controlled at 0.2-0.5 MPa.