Bearing Capacity Calculation and Time Effect of Large-Diameter Driven Steel Pipe Piles in Coral Reef Strata

Literature Overview

This 2020 paper published in the Journal of Xi'an University of Architecture and Technology (Natural Science) by Zhang Yanlin from Chongqing Water Conservancy and Electric Power Vocational and Technical College and Ma Xiaoqiu from Jilin Jianzhu University investigates the bearing capacity calculation and time effect of large-diameter driven steel pipe piles in coral reef limestone strata. The research was supported by the Jilin Jianzhu University Higher Education Teaching Research Fund (XJY201809). The study is motivated by a bridge project that is a landmark of China's "Belt and Road" initiative, where the pile foundations are embedded in weakly cemented, porous, and highly brittle coral reef limestone. Traditional pile foundation design methods and bearing capacity evaluation standards are found to be inadequate for this geological condition.

Geological Background and Challenge

Coral reef limestone presents unique challenges for pile foundation design. Unlike conventional limestone or sandstone, coral reef limestone is characterized by:

Weak cementation: The calcium carbonate cement between coral fragments is often incomplete, resulting in low cohesive strength.
High porosity: The biological structure of coral creates a highly porous matrix with interconnected voids.
High brittleness: The material exhibits brittle failure behavior with limited ductility.
Structural heterogeneity: The presence of coral fragments, aragonite cement, and void spaces creates a highly heterogeneous material.

These characteristics mean that the conventional assumptions underlying pile capacity calculations — such as the development of skin friction through soil-pile adhesion and the mobilization of end bearing through soil displacement — may not hold for coral reef limestone.

High Strain Dynamic Testing Program

The study conducted extensive high-strain dynamic testing (HSDT) on driven steel pipe piles. The HSDT method involves driving the pile using a heavy hammer and measuring the force and velocity at the pile head using piezoelectric force transducers and velocity transducers. The wave equation analysis of the measured signals provides estimates of the pile's bearing capacity, including the skin friction and end bearing components.

Comparison with Code Calculations

The test results were compared with the bearing capacity values calculated using conventional design codes. The comparison revealed a significant discrepancy: the measured bearing capacities were substantially lower than the code-calculated values. This discrepancy is attributed to the following mechanisms:

Soil structure degradation during driving: The impact energy from pile driving causes the coral reef limestone structure to be damaged, leading to shear contraction and softening. The damaged soil can no longer provide effective embedment resistance (嵌固力) and can only offer low friction resistance through a frictional mechanism.
Absence of dilatancy: Coral reef limestone, due to its structural nature, cannot develop the dilatancy (剪胀效应) that would normally lead to progressive increase in skin friction with depth. Without dilatancy, the skin friction does not increase with depth as expected, resulting in lower overall pile capacity.
Time effect insignificance: The conventional time effect, where pile capacity increases with time due to soil consolidation and stress redistribution, is found to be negligible in coral reef limestone. The measured capacity increased by only 16% over the observation period, compared to the much larger increases typically observed in clay soils.

Proposed Bearing Capacity Calculation Method

Based on the test results and the identified mechanisms, the authors propose a modified bearing capacity calculation method specifically applicable to steel pipe piles embedded in coral reef limestone. The key features of the proposed method include:

Reduced skin friction factor: The skin friction parameters are adjusted downward to reflect the structural degradation caused by pile driving.
End-bearing-dominated design: The method recommends selecting relatively stronger reef limestone as the bearing stratum and designing the pile to rely primarily on end bearing rather than skin friction.
Avoidance of excessive pile length or diameter: The method advises against blindly increasing pile length or diameter to compensate for the reduced skin friction, as this is inefficient and may not address the fundamental issue of soil structure degradation.

Engineering Practice Integration

Design Recommendations

Stratum selection: Engineers should prioritize embedding the pile tip in relatively stronger reef limestone layers rather than relying on skin friction along the pile shaft. This requires detailed geological investigation to identify competent bearing strata.
Pile diameter optimization: The proposed method suggests that increasing pile diameter may be more effective than increasing pile length for improving bearing capacity in coral reef limestone, as end bearing scales with cross-sectional area while skin friction is limited by the degraded soil structure.
Driving technique modification: The impact of pile driving on the surrounding soil structure is a critical factor. Reducing the driving energy or using alternative installation methods (such as auger-displacement piles) may help preserve the soil structure and improve skin friction development.
Testing requirement: Given the significant uncertainty in bearing capacity prediction for coral reef limestone, extensive in-situ testing, including high-strain dynamic testing and static load testing, is essential for each project.

Standards and Code Implications

The findings of this study highlight the limitations of existing pile design codes (such as GB 50007 Code for Design of Building Foundation) when applied to coral reef limestone. The code-calculated values may be unconservative for this geological condition, potentially leading to inadequate safety margins. The proposed calculation method provides a basis for developing site-specific design provisions or for modifying existing code provisions to account for the unique behavior of coral reef limestone.

Key Questions and Reflections

Several important questions arise from this study. First, the long-term stability of piles in coral reef limestone under sustained loads is not fully characterized. While the short-term time effect is found to be insignificant, the very long-term behavior (decades) may involve additional mechanisms such as chemical dissolution of calcium carbonate cement, which could further reduce bearing capacity. Second, the effect of seawater chemistry on the coral reef limestone surrounding the pile is not considered. In marine environments, the interaction between seawater and the limestone can lead to chemical weathering and progressive weakening of the bearing stratum. Third, the dynamic response of the pile-soil system during service loading, such as traffic loads on the bridge, is not addressed. The brittle nature of coral reef limestone may lead to sudden failure under dynamic loading, which is a concern for bridge safety.

Study Insights and Implications

This paper addresses a critical and under-studied problem in geotechnical engineering: the behavior of driven steel pipe piles in coral reef limestone. The extensive high-strain dynamic testing provides valuable empirical data that reveals the fundamental inadequacy of conventional design methods for this geological condition. The identification of soil structure degradation during driving as the primary mechanism for reduced bearing capacity is a significant insight that has direct implications for pile design and construction practice. The proposed calculation method offers a practical tool for engineers working on projects in coral reef regions, such as those along the "Belt and Road" initiative. The finding that end-bearing-dominated design is more appropriate for coral reef limestone is a key takeaway that should guide pile foundation design in similar geological settings. This work represents an important step toward developing geotechnical design methods that are tailored to the unique challenges of tropical and subtropical marine environments.