Construction Technology
1) Directional Drilling Process
For boreholes No.1 and No.2, a Φ110mm drill bit is used for hole - opening. The expected construction depth is 90m (determined according to the actual thickness of the overburden layer). Then, Φ150mm and Φ171mm reaming drill bits and high - strength spiral - groove reaming drill pipes are used in sequence to ream the hole to 90m. After that, a Φ168mm (with a compressive strength greater than 15MPa) geological casing is lowered and grouted for solidification. Finally, a Φ110mm drill bit is used to drill to the designed position.
For boreholes No.3 and No.4, a Φ171mm drill bit is used for casing - advancing drilling until reaching the intact bedrock, which is approximately 120m. Then, a Φ168mm geological casing is lowered and fixed. A Φ110mm drill bit is then used to construct to the designed hole depth.
2) The construction technology of using high - strength spiral - groove wire - line drill pipes for slag discharge, directional drilling and rotary drilling is adopted.
3)The drilling rig moves and relocates by means of crawler walking.
4) The test pressure of the casing is 15MPa. It is required that during the pressure test, the pressure should be stabilized for more than 30 minutes without leakage or pressure relief. A blowout preventer must be installed at the wellhead.
Grouting Technology and Process
(1) Grouting Technology
Considering the engineering effect and cost comprehensively, a composite grouting technology is planned to be adopted. Before grouting, the hole is washed with a large amount of clean water by a high - volume pump. Grouting is carried out in the order of thin cement slurry - thick cement slurry - Midwest composite grouting material slurry. First, a cement slurry with a water - cement ratio of 2:1 is grouted under high pressure to open the grouting channels, allowing the slurry to diffuse to the far end of the grouting curtain and seal the fine cracks at a long distance. Then, the water - cement ratio of the slurry is gradually increased to 1:1 to seal the medium - distance cracks. Finally, the Midwest composite grouting material is used to block water and reinforce the surrounding rock near the tunnel body.
Grouting Pressure: Considering the hydrostatic pressure and the diffusivity of the grouting material, the grouting pressure is controlled between 10MPa and 15MPa.
Diffusion Radius: Under normal fracture apertures and porosities, the effective diffusion radius of the slurry is not less than 6m.
Grouting Stop Conditions: When the grouting volume in the fault fracture zone reaches the designed value or the grouting pressure reaches 10MPa - 15MPa, and the pressure - holding time is not less than 30 minutes, the grouting can be stopped.
