U-Shaped Steel Pipe Roof Support System in Soft Rock Tunneling

Literature Overview

This paper by Zhang Xianghuan from Hunan Baoshan Nonferrous Metals Mining Co., Ltd., published in Hunan Nonferrous Metals in 2025, Volume 41, Issue 2, pages 9-12, describes the application of a combined support system using U-shaped steel arch supports with advance pipe roof grouting and subsequent shotcrete application in soft rock tunnel excavation. The work documents practical engineering experience in ensuring safe and efficient tunneling operations in challenging geological conditions.

Geological Conditions and Support Challenges

Soft rock formations present unique challenges for underground excavation due to:

Challenge	Description	Impact on Support Design
Low uniaxial compressive strength	Typically < 20 MPa	Requires high load-bearing capacity
High plastic deformation	Time-dependent convergence	Requires deformable support system
Poor self-supporting ability	Rapid deformation after excavation	Requires immediate support installation
Groundwater influence	Reduces rock strength further	Requires waterproofing integration
Stress redistribution	High stress concentration near opening	Requires stress-relief measures

The U-shaped steel arch support system addresses these challenges through its inherent deformability, which allows controlled convergence while maintaining structural integrity.

Support System Configuration

The combined support system described in the paper consists of three integrated components:

U-Shaped Steel Arch Support

The U-shaped steel arches provide the primary load-bearing structure. Key design parameters include:

Material grade: Q235 or Q345 structural steel
Section dimensions: Typically 25U or 29U profiles
Arch spacing: Determined by geological conditions, typically 0.8-1.2 m
Arch height: Matched to tunnel profile dimensions
Connection method: Pin-and-plate connections allowing controlled deformation

The U-shaped profile provides high moment resistance with relatively low weight, making it suitable for soft rock conditions where rapid installation is critical. The open profile allows shotcrete to fill the interior cavity, creating a composite action between the steel arch and the concrete lining.

Advance Pipe Roof Grouting

The pipe roof system provides超前 support ahead of the excavation face. The system includes:

Pipe diameter: Typically φ108-φ159 mm seamless steel pipes
Pipe length: 10-15 m extending ahead of the excavation face
Grouting material: Cement-based grout with water-cement ratio of 0.8-1.0
Grouting pressure: 0.5-1.5 MPa depending on rock permeability
Pipe arrangement: Circular pattern with overlap between adjacent pipe roof sections

The pipe roof grouting consolidates the surrounding rock mass, improving its self-supporting ability and reducing the load on the U-shaped steel arches. The grouted zone effectively creates a reinforced shell around the tunnel perimeter.

Shotcrete Application

Following the installation of the U-shaped steel arches, shotcrete is applied to:

Fill the cavity between the arch and the surrounding rock
Provide immediate confinement to the surrounding rock
Create a waterproofing layer
Distribute loads uniformly across the arch surface

The shotcrete thickness is typically 80-150 mm, applied in layers with each layer not exceeding 50 mm thickness.

Support System Performance

The combined support system demonstrated effective performance in the following aspects:

Controlled deformation: The deformable U-shaped arches allowed controlled convergence while maintaining structural stability.
Load distribution: The pipe roof grouting reduced the load on individual arches by distributing stresses over a larger rock mass volume.
Construction safety: The advance pipe roof provided protection during excavation, reducing the risk of roof falls.
Construction efficiency: The modular U-shaped arch system allowed rapid installation, minimizing the unsupported time.
Cost-effectiveness: The combined system balanced material costs with long-term structural performance.

Engineering Practice Considerations

Based on the documented experience, several practical considerations emerge:

Geological mapping: Detailed geological mapping ahead of excavation is essential for adjusting support parameters to local conditions.
Monitoring integration: Real-time monitoring of convergence, arch deformation, and grouting effectiveness should be integrated with the construction process.
Sequential installation: The sequence of pipe roof installation, excavation, arch installation, and shotcrete application must be carefully controlled to maintain stability at each stage.
Quality control of grouting: Grouting quality directly affects the effectiveness of the pipe roof system. Grouting parameters should be optimized through trial grouting and adjusted based on rock permeability.
Interface treatment: The connection between the pipe roof and the U-shaped arches should be designed to transfer loads effectively while accommodating differential deformation.

Study Insights

This engineering case study demonstrates the practical value of combined support systems in challenging soft rock conditions. The integration of advance support (pipe roof grouting), deformable primary support (U-shaped steel arches), and secondary confinement (shotcrete) represents a rational approach to soft rock tunneling. The key insight is that no single support method is sufficient for soft rock conditions; rather, a multi-component system that addresses different aspects of ground stability is required. Engineers should approach soft rock tunneling with a systems engineering mindset, considering the interaction between different support components and the sequential construction process. The documented improvements in the support process reflect the iterative nature of engineering practice, where field experience informs design refinement.