I. Introduction: Precision Requirements for Pre-Support Construction

Tunnel Pre-Support is the critical step in tunnel engineering for controlling ground stability and deformation. Whether utilizing long-distance Tunnel Roof Support Pipes for a pipe umbrella system or shorter Forepoling Pipes for localized reinforcement, the quality of construction directly determines the effectiveness of the pre-support.

This guide details the full construction technology process—from drilling to grouting consolidation—for Steel Casing Support (Forepoling Pipes), ensuring the steel pipes are accurately placed and effectively fulfill their role in Tunnel Excavation Support.

II. Essential Material and Equipment Preparation

Successful pre-support construction begins with selecting the correct materials and equipment.

1. Support Pipes (Steel Casing Pipes): High-strength, sufficiently thick-walled Steel Casing Pipes must be chosen to withstand the thrust forces during drilling and the ground pressures. Specifications must strictly adhere to design requirements, as detailed in the core pillar article: 【Pillar-PipeUmbrella】 Tunnel Roof Support Pipes and Pipe Umbrella Systems: Steel Casing, Methods, and Underground Pre-Support Solutions.

2. Drilling Equipment: Select appropriate drilling rigs (e.g., rock drills or hydraulic rigs) based on pipe diameter and length. The rig must feature high-precision positioning and angle adjustment capabilities to ensure the accuracy of the drilling angle and trajectory.

3. Grouting Equipment: High-pressure grout pumps and corresponding mixing equipment are essential to ensure the grout material is adequately dispersed and rapidly consolidated.

III. Construction Procedure and Technical Highlights

The installation of Tunnel Roof Support Pipes is primarily divided into three critical stages: precise positioning, drilling/installation, and grouting consolidation.

A. Phase One: Precise Positioning and Drilling

1. Layout and Marking: Precisely mark the starting point and direction of the boreholes according to the design drawings. Ensure the drilling rake angle (typically $3^{\circ}$ to $5^{\circ}$ upward) and circumferential spacing meet the design requirements.

2. Drilling: Employ casing advancement or pre-cased drilling techniques to prevent borehole collapse in soft ground. Drilling speed and control must be adjusted strictly according to geological conditions.

3. Advance Length Control: Borehole length must be precise to ensure sufficient overlap between pipe segments, maintaining continuous load transfer for the Pipe Umbrella System.

B. Phase Two: Steel Pipe Installation and Connection

1. Pipe Insertion: Immediately after drilling, insert the Forepoling Pipes into the boreholes.

2. Pipe Jointing: Steel pipes typically use threaded connections or socket joints. Joints must be flat and secure to prevent separation under high grouting pressure. Welding reinforcement may be necessary for critical, long umbrella pipes.

3. Grouting Port Setup: The Tunnel Roof Support Pipe must have pre-drilled or site-set grouting ports. These holes (or a perforated pipe design) are central to the pipe's function as a Grouting Pipe, allowing the grout to be injected into the annular space.

C. Phase Three: Grouting Consolidation

Grouting is the critical step that allows the Tunnel Roof Support Pipe to perform its function effectively.

1. Grout Material: Use early-strength, micro-expansive cement grout or a cement-water glass double-liquid grout to ensure rapid setting and thorough filling of the annular space outside the pipe.

2. Grouting Technique: Employ an inside-out, segmented, or sequential grouting method. Grouting pressure must be strictly controlled to ensure the grout permeates the target area without causing hydraulic fracturing of the ground, which would undermine Ground Stabilization.

3. Grouting Effectiveness: Grouting ends when the design final pressure is reached or when the required volume per unit length is achieved. Post-grouting, the steel pipe and the surrounding ground form a rigid composite body, achieving Ground Stabilization.

IV. Site Safety and Quality Control Highlights

1. Borehole Accuracy Check: Inclinometers or other measuring tools must be used regularly to check the drilling trajectory and angle deviation, ensuring the pipe umbrella forms a structurally effective canopy.

2. Grouting Pressure Monitoring: Real-time logging of grouting pressure, volume, and time is crucial for Grout Effectiveness Evaluation (reference: 【Anchor: Grouting-QC】 Grouting Process, Materials, and Quality Control).

3. Environmental Management: Especially in water-bearing strata, additional water-stop measures must be taken to ensure grouting quality.

V. Conclusion: Technical Detail Ensures Success

The installation technology for Tunnel Roof Support Pipes and Steel Casing Support is the lifeline for safe excavation in weak ground tunneling. Through refined control and rigorous quality management in every step—from positioning and drilling to installation and grouting—engineering teams can successfully construct a robust Pipe Umbrella System, providing a safe and stable working environment for subsequent Underground Construction Support.