Settlement Characteristics of Large-Diameter Steel Pipe Piles on Slate with Overlying Calcareous Sand Layer

Literature Overview

The paper by Su Shiding, Chen Zhangyu, and Sang Dengfeng (2021), published in Port and Waterway Engineering (No. 2), presents a comparative analysis of the load-settlement (Q-S) curves obtained from high-strain dynamic testing and static load testing for large-diameter steel pipe piles founded on slate with an overlying calcareous sand layer. The research was supported by the Guangzhou Zhujiang Science and Technology New Star Program. The study provides valuable insights into the settlement behavior of steel pipe piles in complex geological conditions, which is directly relevant to the design and quality control of large-diameter pipe pile foundations.

Core Technical Content

Geological Setting and Pile Configuration

The study area features a geological profile consisting of calcareous sand layers of varying thickness overlying strongly weathered slate (slate). The large-diameter steel pipe piles are driven through the calcareous sand layer into the slate layer. The pile diameters range from 800 mm to 1200 mm, with wall thicknesses of 14–20 mm, made from Q345 or Q355 grade steel. The pile length varies from 15 m to 35 m depending on the local geological conditions.

Q-S Curve Comparison and Settlement Classification

The key finding of the paper is the classification of pile settlement behavior into two distinct types based on the geological conditions:

Type I: End-Bearing Pile Behavior

When the calcareous sand layer is relatively thin (less than 5 m) and the pile penetrates sufficiently deep into the strongly weathered slate layer (more than 3 m), the Q-S curve exhibits end-bearing characteristics. The pile top settlement is primarily caused by pile shaft compression, and the residual settlement after unloading is small (less than 5 mm). The ultimate bearing capacity is governed by the end-bearing resistance of the slate layer.

Type II: Friction-Type Pile Behavior

When the calcareous sand layer is thick (more than 10 m) and the pile end rests on a relatively softer section of strongly weathered slate, the Q-S curve exhibits friction-type characteristics. Both high-strain and static load tests show that the soil resistance is fully mobilized, and the settlement increases dramatically upon reaching the ultimate load. The cumulative total settlement and residual settlement are large (exceeding 30 mm).

High-Strain vs. Static Load Testing

The comparison between high-strain dynamic testing and static load testing reveals that both methods can reliably predict the ultimate bearing capacity and the type of settlement behavior. However, the high-strain test tends to underestimate the residual settlement for friction-type piles due to the dynamic relaxation effect. For end-bearing piles, the high-strain test provides a more accurate prediction of the residual settlement because the end-bearing resistance is less sensitive to loading rate.

Welding and Fabrication Implications

Large-diameter steel pipe piles are typically manufactured as spiral-welded or LSAW welded pipes, and the quality of the welds directly affects the pile's structural integrity and load-bearing capacity.

1. Weld quality requirements: For large-diameter pipe piles used in marine and offshore environments, the longitudinal welds should be inspected by 100% radiographic testing (RT) or 100% ultrasonic testing (UT) in accordance with ISO 15614-1 or AWS D1.1. The acceptable weld quality level is UT level B or higher, with no defects exceeding 1.5 mm in equivalent flat-bottom hole diameter.
2. Impact of welding defects on pile performance: In the context of the Q-S curve analysis, welding defects such as lack of fusion or internal cracks can cause premature local buckling of the pile wall under compressive loading. This would manifest as an abnormal Q-S curve with a sudden increase in settlement at a load level significantly below the theoretical ultimate capacity. The paper's finding that the calcareous sand layer thickness has minimal influence on the ultimate bearing capacity implies that the pile wall integrity is the critical factor in determining the actual bearing capacity.
3. Pile driving damage assessment: The high-strain test also serves as a method to assess pile driving damage. Excessive driving impact can cause local yielding or buckling of the pile wall, particularly near the pile toe. For large-diameter piles (D > 1000 mm), the driving velocity should be limited to 0.3 m/s to prevent wall damage. Post-driving NDT should include 100% visual inspection of the pile head and 20% MT of the longitudinal weld for the upper 5 m of the pile.

Key Insights and Engineering Recommendations

The paper's most significant finding is that the ultimate bearing capacity of large-diameter steel pipe piles in this geological setting is primarily determined by the depth of penetration into the slate layer and the strength of the slate bearing layer, rather than by the thickness of the overlying calcareous sand layer. This has important implications for pile length design: increasing the pile length to extend through a thicker calcareous sand layer does not necessarily increase the bearing capacity if the pile end does not reach a sufficiently strong slate layer.

From a quality control perspective, the Q-S curve comparison methodology provides a valuable tool for verifying pile installation quality. Any significant discrepancy between the high-strain and static Q-S curves should trigger a detailed investigation of the pile condition, including underwater visual inspection (UVI) and close-up visual inspection (CUVI) of the pile surface.

The study also highlights the importance of geological characterization in pile design. The variability of the slate layer strength (from strongly weathered to moderately weathered) can lead to significant variations in pile capacity, even for piles of the same length and diameter. Detailed geotechnical investigation with sufficient borehole spacing is essential to avoid unexpected pile performance.

Reference Value

This paper provides practical guidance for the design and quality control of large-diameter steel pipe piles in complex geological conditions. The Q-S curve classification methodology can be applied to other geological settings with layered soil profiles. The welding quality requirements and NDT protocols outlined in this study should be incorporated into the quality assurance plan for any large-diameter pipe pile project in similar geological conditions.