# Research on the Stress Characteristics of Initial Tunnel Supports Based on Active Load Adjustment
## 3 Design of the Load Adjustment System  
The above analysis reveals that a load adjustment layer can be set between the initial support of the tunnel and the surrounding rock. The load optimization adjustment system of the tunnel surrounding rock support system comprised a load adjusting unit (Figure 10), serial oil pipes, pressure equipment, anchor bolts, etc. (Figure 11).  
A certain number of load adjusting units were installed symmetrically on the arch frame of the tunnel, and the units were connected in series with high-pressure oil pipes to form a closed and connected loop. Different adjusting units had different load ratios (Figure 12). The optimization and adjustment of load in tunnel cross section can be achieved by controlling the position and load ratio of the adjusting units.  
Figure 9: Calculation results of bending moment of tunnel lining before and after load adjustment. (**a**) Bending moment of lining structure before implementation. (**b**) Bending moment of lining structure after the implementation of load adjustment.  
The adjustment system can apply prestress at the early stage. Subsequently, the load was automatically adjusted during the convergence process of the surrounding rock of the tunnel, and a load linkage between the adjusting units was achieved; finally, the anchor bolts were used to determine and maintain the final stress after the surrounding rock was basically stable. The hydraulic load adjustment system largely comprised three parts, including load adjusting unit, oil pipeline hose, and accessories. The accessories consisted of a high-pressure ball valve, pressure gauge, etc.  
Figure 11: Load adjustment system connection fittings. (**a**) High-pressure oil pipe; (**b**) ball valves; (**c**) pressure gauges.  
Figure 10: Assembly drawing of load adjusting unit.  
The load distribution ratio of the load adjusting unit was divided into three types, which were symmetrically arranged on the left and right sides. The load proportion was achieved by the change of the area of the oil cylinder at the bottom of the adjusting unit. Figure 11 presents the specific proportion distribution.  
The adjusting units were connected by high-pressure oil pipes, the opening and closing control was managed by a ball valve, and a pressure gauge was connected to monitor the pressure.  
The adjusting unit and the initial support I-beam of the tunnel were welded into one through the connecting plate. According to the overex excavation and underexcavation of the tunnel section, the screw was adopted to adjust the force-bearing try to contact with surrounding rock. The adjusting units in the same section were connected in series through oil pipes to ensure that the same hydraulic pressure exists in the system in order to optimize the stress experienced by the tunnel's lining using the load adjustment ratio.  
A total of 15 interconnected hydraulic cylinders were used. The respective cylinder represents a load adjustment point. Each cylinder had the same hydraulic pressure inside. Different cylinder piston areas were configured in accordance with the load adjustment ratio to form a cross-sectional load adjustment unit. The load adjustment unit was constructed together with the initial support, and the initial hydraulic pressure was applied at the first time after the completion of construction. The adjustment point was closely attached to the surrounding rock to bear the surrounding rock load. Subsequently, with the development of the convergence deformation of the surrounding rock of the tunnel, the load between the load adjustment points was automatically adjusted according to the preset proportion to minimize the bending moment, i.e., the stress optimization goal of the support structure.