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

Literature Overview

This technical paper by Zhang Xianghuan from Hunan Baoshan Nonferrous Metals Mining Co., Ltd., published in Hunan Nonferrous Metals, Volume 41, Issue 2, 2025, pages 9-12, documents the application of a combined support system using U-shaped steel arches with advanced pipe roof (overcapping) and subsequent shotcreting in soft rock tunnel excavation. The paper presents a complete support technology developed through practical improvement and field validation.

Geological Conditions and Support Challenges

Soft rock formations present unique challenges in underground excavation:

Low uniaxial compressive strength (typically below 20 MPa)
High plasticity and creep potential
Poor self-supporting ability of the surrounding rock mass
Susceptibility to water damage and strength reduction upon exposure

In the specific case studied at Hunan Baoshan, the large cross-section roadway excavation encountered soft rock that required a support system capable of:

Withstanding high ground pressure and convergence
Accommodating deformation without failure (deformable support)
Providing immediate ground stabilization to prevent progressive deterioration
Maintaining structural integrity over the long term

Support System Components

Component	Function	Key Parameters
U-shaped steel arch	Primary load-bearing structure	Compressive strength per steel grade; deformation capacity
Pipe roof (overcapping)	Advance ground stabilization	Pipe diameter, length, spacing, grouting
Shotcrete layer	Surface sealing and confinement	Thickness, strength grade, mix design
Connecting bolts	Arch-to-arch load transfer	Bolt grade, spacing, torque

Combined Support Technology

The combined support system operates on the principle of staged ground stabilization:

Stage 1: Pipe Roof Installation (Advance Support)

Steel pipes are installed ahead of the excavation face through pre-drilled boreholes. The pipes are grouted to bond with the surrounding rock, creating a reinforced zone ahead of the working face. This advance support:

Prevents loosening of the ground ahead of the excavation
Reduces the span of unsupported roof
Provides immediate structural support to the overlying strata
Allows safe advancement of the excavation face

Stage 2: U-Shaped Steel Arch Installation

Following excavation and temporary stabilization, U-shaped steel arches are erected as the primary support structure. The U-shaped profile offers:

Excellent compressive strength through its I-beam cross-section
High bending stiffness in the vertical plane
Controlled deformation capacity through shear yielding of the web
Easy installation using bolted connections at the arch legs and crown

Stage 3: Shotcrete Application

After the steel arches are installed and the ground is stabilized, shotcrete is applied to:

Seal the rock surface and prevent moisture ingress
Provide additional confinement to the surrounding rock
Fill gaps between the arch and the rock surface
Create a continuous load-bearing composite structure

Performance Evaluation and Practical Improvements

The combined support system was validated through field performance monitoring:

Convergence measurements demonstrated controlled deformation within design limits
No structural failures were observed during the monitored period
Construction progress was maintained at acceptable rates
The system proved adaptable to varying ground conditions along the roadway

Key improvements developed during implementation included:

Optimization of pipe roof spacing based on actual ground pressure measurements
Development of a shotcrete mix with improved bond strength to the wet rock surface
Standardization of arch installation procedures to ensure consistent quality
Implementation of a monitoring protocol for early detection of support distress

Engineering Practice Implications

This case study demonstrates the effectiveness of a staged, combined support approach in soft rock tunneling. The technology is particularly applicable to:

Mining roadways in soft ground conditions
Infrastructure tunnels in weathered rock formations
Emergency support in existing tunnels experiencing ground pressure increases

The approach aligns with the modern philosophy of "supporting the rock rather than fighting it," allowing controlled deformation while maintaining structural integrity. The U-shaped steel arch provides the necessary deformability that rigid supports (such as concrete lining) cannot offer in high-pressure soft ground conditions.

The paper's practical value lies in its documentation of a complete, field-proven support technology that can be directly applied to similar geological conditions. The systematic approach of combining advance support, deformable primary support, and final sealing represents best practice in modern tunnel engineering.