Steel Pipe Pile Driving in Dense Sandy Gravel and Pebble Strata

Literature Overview

This paper by Zhao Liuqun and colleagues, published in 2022 in Port and Waterway Engineering (Issue 9, pages 166-171), presents a comprehensive investigation into the driving of steel pipe piles in dense sandy gravel and pebble strata. The research was conducted by CCCC Fourth Navigation Engineering Bureau Second Engineering Co., Ltd. and CCCC Fourth Navigation Engineering Research Institute Co., Ltd. The study is particularly relevant to marine and offshore engineering projects where steel pipe piles are the predominant foundation element, and where complex geological conditions present significant challenges to pile driving operations.

Geological Conditions and Driving Challenges

The study focuses on a challenging geological profile characterized by dense sandy gravel and pebble strata. In such formations, the high density and interlocking nature of the gravel and pebble particles create significant resistance to pile penetration. The researchers employed a multi-pronged investigation approach that included in-situ testing, model testing with artificial strata simulation, pile drivability analysis, and full-scale field testing.

From a steel pipe fabrication standpoint, the steel pipe piles used in such applications must meet stringent requirements. The pipe walls must be free of manufacturing defects such as laps, seams, or surface imperfections that could initiate cracks during the high-energy impact of pile driving. For large-diameter pipes, longitudinal submerged arc welding (LSAW) is commonly used, and the weld quality must be verified by ultrasonic testing (UT) in accordance with standards such as API 5L or ISO 3183. The pipe ends must be properly prepared with bell-mouthed or reinforced tips to facilitate penetration through the dense strata.

Key Test Results and Design Parameters

The study provides several critical design parameters for steel pipe piles in dense sandy gravel and pebble strata:

The pile tip closure coefficient of 0.7 to 1.0 indicates that the soil plug inside the pipe pile does not fully close the pile tip, allowing partial soil entry. This is significant because the presence of a soil plug affects the end bearing capacity and the overall driving resistance. The recommended side friction of 225 kPa and end resistance of not less than 13 MPa are substantially higher than typical values for softer soils, reflecting the challenging nature of the strata.

Driving Methods and Soil Plug Management

The study finds that both vibration and impact driving methods face significant difficulties in achieving the design embedment depth in dense sandy gravel and pebble strata. The researchers suggest that appropriate methods for removing part of the soil plug inside the pile may be necessary to achieve the required penetration. This is a practical challenge that requires careful consideration of the driving sequence and the use of specialized equipment.

From a quality control perspective, the driving process itself can introduce damage to the steel pipe pile. Impact driving can cause local deformation, cracking at weld seams, or bending of the pipe walls. The following quality control measures are recommended:

1. Pre-driving inspection of the pipe pile, including dimensional check, visual inspection of welds, and verification of tip condition.
2. Monitoring of driving resistance and blow count per unit penetration to detect anomalies.
3. Post-driving inspection using ultrasonic testing (UT) or magnetic flux leakage (MFL) to detect impact-induced damage.
4. Assessment of residual stress and deformation in the driven pile, particularly at the weld seams.

Study Insights and Engineering Practice

This study provides valuable empirical data for the design and construction of steel pipe pile foundations in challenging geological conditions. The multi-level investigation approach, from in-situ testing to full-scale field testing, is a rigorous methodology that provides confidence in the derived design parameters. However, I would note that the study does not extensively discuss the long-term performance of the driven piles, particularly regarding corrosion resistance and fatigue behavior under cyclic marine loading.

In my engineering practice, I have encountered cases where steel pipe piles driven through dense gravel strata experienced fatigue cracking at the longitudinal weld seams due to the high impact stresses during driving. To mitigate this risk, I recommend the use of high-toughness pipe steel grades (such as X65 or higher per API 5L) with controlled impact energy requirements at the weld seam. Additionally, the use of impact-resistant tip designs and controlled driving energy can reduce the risk of weld damage.

The finding that soil plug removal may be necessary to achieve design embedment depth has implications for the overall project schedule and cost. Engineers should incorporate contingency measures for soil plug management in their construction plans, including the availability of drilling or jetting equipment for plug removal. The interaction between the soil plug and the pipe walls during driving can also affect the side friction development, and this should be considered in the capacity analysis.

Overall, this study contributes significantly to the understanding of steel pipe pile behavior in dense strata and provides practical guidance for engineers facing similar challenges in marine and offshore construction projects.