Vertical Bearing Capacity Analysis of Steel Pipe Post-Grouting Pre-Pressure Transfer Piles

Literature Overview

This paper by Bai Miaomiao, Tang Liyun, Zhang Shuyun, Yang Genshe, and Shen Yanjun from the College of Architecture and Civil Engineering, Xi'an University of Science and Technology, published in Chinese Journal of Underground Space and Engineering (2020, Vol. 16, Issue 4, pp. 1030-1039), presents a novel steel pipe post-grouting pre-pressure transfer pile method for foundation underpinning in complex fill soil conditions. The research was funded by the National Natural Science Foundation of China (41502298) and the China Postdoctoral Science Foundation (2017M613293XB).

Engineering Background and Problem Statement

The research originated from a practical engineering challenge: a residential building's podium structure suffered damage due to uneven settlement of complex miscellaneous fill soil foundations. The authors developed a novel steel pipe post-grouting pre-pressure transfer pile method specifically designed for poor-quality soil conditions where conventional underpinning methods are inadequate.

The key innovation combines three technologies:

1. Steel pipe pile installation (providing structural strength)
2. Post-grouting (enhancing soil-pile interface and soil mass)
3. Pre-pressure loading (pre-compressing the soil and pile system before transferring structural loads)

Bearing Capacity Mechanism Analysis

The authors conducted a detailed analysis of the pile-soil interaction mechanism, focusing on the grout diffusion range from the pile side and its effect on the surrounding soil mass:

The post-grouting process creates a hardened grout-soil composite zone around the pile, which significantly improves the interface friction and provides additional confinement to the surrounding soil mass.

Modified Bearing Capacity Formula

The authors proposed a modified bearing capacity formula for the steel pipe pre-pressure transfer pile that accounts for the grout diffusion effects:

The modification includes:

• An enhanced skin friction coefficient reflecting the grout-hardened interface
• A modified end-bearing factor accounting for soil improvement at the pile toe
• A consideration of the grout diffusion radius and its influence on the failure surface

Comparative Static Load Test Results

Three types of transfer piles were tested under identical field conditions:

The steel pipe post-grouting pre-pressure transfer pile demonstrated:

• Improved pile-soil contact interface through grout filling
• Enhanced soil stiffness in the surrounding zone
• Reduced pile settlement under the same loading
• Higher single pile bearing capacity
• Effective prevention of continued structural damage

Grout Diffusion and Soil Improvement

The post-grouting process creates a radial diffusion zone around the steel pipe pile. The grout penetrates the surrounding soil through pressure injection, creating a grout-soil composite zone that:

1. Increases the effective friction angle at the pile-soil interface
2. Provides additional confinement to the soil mass
3. Reduces the permeability of the surrounding soil
4. Creates a more uniform load distribution along the pile length

The diffusion radius depends on:

• Grout injection pressure
• Grout mix fluidity and setting time
• Soil permeability and structure
• Injection rate and duration

Pre-Pressure Loading Mechanism

The pre-pressure loading stage is a critical innovation in this method. Before transferring the structural loads to the new piles:

1. The pile system is loaded to a predetermined pre-pressure level
2. This induces controlled soil compression and pile settlement
3. The grout-hardened interface achieves its full mobilization
4. Subsequent structural loading results in minimal additional settlement

This pre-conditioning effect is analogous to the pre-loading concept in geotechnical engineering and significantly improves the long-term performance of the underpinning system.

Engineering Practice Considerations

From a steel pipe manufacturing and installation perspective:

1. The steel pipe must be designed to withstand the grout injection pressure without deformation
2. Grout injection ports must be precisely positioned and sealed after use
3. The pipe connection details must accommodate the pre-pressure loading without joint failure
4. Corrosion protection is essential for long-term performance in fill soils

Study Insights and Engineering Value

The combination of steel pipe, post-grouting, and pre-pressure loading creates a synergistic effect that is greater than the sum of individual components. The steel pipe provides the structural skeleton, the grout enhances the soil-pile interaction, and the pre-pressure pre-conditions the system for optimal service performance.

This method is particularly valuable for urban underpinning projects where space is limited, adjacent structures must be protected, and settlement tolerance is extremely low. The effectiveness in complex miscellaneous fill soil conditions expands the range of applicable ground conditions for steel pipe underpinning.

Future development should focus on optimizing the grout injection parameters through numerical modeling and full-scale testing, establishing design guidelines for different soil types, and developing monitoring protocols to ensure long-term performance.