Vertical Bearing Capacity Calculation Methods for Large-Diameter Steel Pipe Piles

Literature Overview

This paper, published in Chinese Journal of Underground Space and Engineering in 2018 by Liang Chao and Liu Run from the State Key Laboratory of Simulation and Safety of Water Engineering, Tianjin University, addresses the vertical bearing capacity calculation of large-diameter steel pipe piles. The research was funded by the National Science and Technology Major Project (2016ZX05058-004-005). The paper appears in Volume 14, Issue 1, pages 169-175, and is classified under TU473.1 (pile foundations). The study is motivated by the increasing use of large-diameter steel pipe piles in offshore oil and gas exploration and offshore wind power engineering.

Background and Motivation

Large-diameter steel pipe piles have become increasingly popular in offshore engineering due to their simple structural form, convenient installation, and excellent bearing performance. However, as the diameter increases beyond conventional ranges (typically beyond 1.0-1.5 m), the bearing mechanism differs significantly from that of traditional small-diameter piles. The interaction between the pile and surrounding soil involves complex mechanisms such as soil arching, lateral soil pressure redistribution, and the transition from shaft resistance-dominated to end-bearing-dominated behavior.

Comparison of Calculation Methods

The authors systematically compared several theoretical and code-based methods for calculating the vertical bearing capacity of large-diameter steel pipe piles. The methods evaluated include:

Theoretical Methods

Method	Developer	Key Feature
Terzaghi's method	K. Terzaghi	Classical bearing capacity theory adapted for piles
Meyerhof's method	G.G. Meyerhof	Empirical approach with depth factors
Berezantsev's method	Berezantsev	Deep foundation theory with soil arching consideration

Code-Based Methods

Standard / Code	Application Area	Conservatism Level
GB 50007 (Technical Code for Building Pile Foundations)	Onshore building foundations	More conservative
Port Engineering Pile Foundation Code	Marine port structures	More conservative
API RP 2A / API 2A-WSD	Offshore oil and gas platforms	Less conservative
DNV-RP-C101 (DNV)	Offshore structures	Less conservative

Additional Method

Method	Description
Soil arching effect method	Considers the redistribution of soil pressure due to arching above the pile cap

Key Findings from Comparative Analysis

The comparative analysis revealed important differences among the various methods:

Terzaghi's and Meyerhof's methods tend to overestimate the vertical bearing capacity, producing relatively high calculated values that may not be conservative for design purposes.
Berezantsev's method and the soil arching effect method produce results that are relatively more accurate when compared with dynamic pile testing data.
Chinese codes (GB 50007 and Port Engineering Code) are more conservative than international standards (API and DNV), which may be attributed to different safety philosophies and the inclusion of additional safety factors.

Summary of Method Accuracy and Conservatism

Method	Accuracy vs. Dynamic Test	Conservatism
Terzaghi	Overestimates capacity	Non-conservative
Meyerhof	Overestimates capacity	Non-conservative
Berezantsev	Relatively accurate	Moderate
Soil arching method	Relatively accurate	Moderate
GB 50007	Conservative	High
Port Engineering Code	Conservative	High
API	Less conservative	Low
DNV	Less conservative	Low

Technical Interpretation

The differences among these methods arise from several fundamental factors:

Soil-pile interaction model: Different methods assume different mechanisms for soil-pile interaction, including elastic-plastic models, empirical correlations, and semi-empirical approaches.
Soil arching effect: For large-diameter piles, the soil above the pile cap can form an arch that redistributes vertical stress to the surrounding soil, reducing the effective end-bearing pressure. Methods that account for this effect (Berezantsev and the soil arching method) produce more accurate results.
Safety factors: Different codes apply different safety factors to the calculated bearing capacity, reflecting varying risk tolerances and regulatory philosophies.
Soil type and condition: The accuracy of each method depends on the specific soil conditions at the project site, and some methods may be more appropriate for certain soil types than others.

Influence of Pile Diameter on Bearing Mechanism

Pile Diameter Range	Dominant Bearing Mechanism	Recommended Method
Small (< 0.5 m)	Shaft resistance dominated	Standard pile design methods
Medium (0.5-1.5 m)	Transition zone	Berezantsev or soil arching method
Large (> 1.5 m)	End-bearing influenced by soil arching	Soil arching effect method

Engineering Practice Recommendations

Based on the comparative analysis, the following recommendations are provided for engineering practice:

For large-diameter steel pipe piles in offshore applications, the soil arching effect method or Berezantsev's method should be preferred over Terzaghi's or Meyerhof's methods for more accurate capacity estimation.
When using Chinese codes for onshore projects, engineers should be aware of the conservative nature of the results and consider whether the conservatism is appropriate for the specific project risk profile.
Dynamic pile testing (such as PDA or PILEDRIVE) should be used to validate calculated bearing capacities, as the study demonstrated that dynamic test results provide the most reliable benchmark for method comparison.
For offshore projects governed by API or DNV standards, the less conservative nature of these codes should be supplemented with additional site-specific geotechnical investigation and testing.

Key Questions and Reflections

The study raises several important considerations for practitioners. First, the applicability of these methods to different soil types (cohesive soils, granular soils, mixed soils) should be further investigated, as the bearing mechanism may vary significantly with soil type. Second, the effect of pile installation method (driven, bored, or jacked) on the bearing capacity should be considered, as installation-induced soil disturbance can significantly affect the pile-soil interaction. Third, the long-term settlement behavior of large-diameter steel pipe piles under sustained loads needs to be evaluated, as the bearing capacity calculations typically address only the ultimate limit state.

The growing use of large-diameter steel pipe piles in offshore wind power projects presents new challenges, as these piles are often driven to depths of 40-60 meters in varying soil profiles. The selection of an appropriate calculation method, validated by dynamic testing, is critical to ensuring both safety and economic efficiency in foundation design.

Summary and Study Insights

This study provides a comprehensive comparison of vertical bearing capacity calculation methods for large-diameter steel pipe piles, demonstrating that Terzaghi's and Meyerhof's methods tend to overestimate capacity, while Berezantsev's method and the soil arching effect method produce results closer to dynamic test data. Chinese codes are more conservative than API and DNV standards, reflecting different safety philosophies. Engineers designing large-diameter steel pipe pile foundations should select calculation methods based on the specific project context, validate results with dynamic pile testing, and consider the influence of soil arching effects that become increasingly important with larger pile diameters. The findings contribute to the development of more reliable and economical design practices for offshore and onshore foundation engineering.