Numerical Analysis of Mechanical Properties of Single and Group Grouted Micro Steel Pipe Piles

Literature Overview

The paper by Han Dongqing and Ge Chenhe, published in the Journal of Hebei University of Technology (2021, Vol. 50, No. 2), presents a displacement-loading-based numerical study on the mechanical behavior of grouted micro steel pipe piles under both single-pile and group-pile conditions. The research is supported by the Hebei Provincial Natural Science Foundation and addresses practical geotechnical engineering challenges encountered in highway infrastructure projects. The study systematically investigates the influence of pile length, pile quantity, pile spacing, arrangement pattern, and rock embedment depth on ultimate bearing capacity and group pile effect factors.

Core Technical Viewpoints

The authors adopt the displacement loading method to simulate the stress-strain response of micro steel pipe piles, which is particularly suitable for capturing the progressive failure mechanism in confined soil-rock mass. The fundamental finding is that single-pile ultimate bearing capacity increases monotonically with pile length, following a diminishing marginal return pattern. For group piles, the maximum axial force distribution follows the hierarchy: corner piles > edge piles > center piles, which is consistent with the well-known group interaction effect in deep foundation engineering.

The study identifies a critical pile spacing of S = 5D (five times the pile diameter) beyond which both bearing capacity and the group effect factor η stabilize. This threshold is practically significant because it provides a clear design guideline for pile layout optimization in constrained foundation zones.

Interpretation of Key Technical Points

Group Effect Factor and Bearing Capacity Relationship

The group effect factor η is defined as the ratio of the average single-pile bearing capacity in a group to that of an isolated single pile. The paper demonstrates that increasing the number of piles under constant spacing increases both total capacity and η, whereas increasing spacing at constant pile count reduces η while increasing total capacity. This apparent paradox is resolved by recognizing that the group effect factor is a relative measure, while total capacity is an absolute one.

Arrangement Pattern Comparison

The plum-blossom (quincunx) arrangement outperforms the rectangular arrangement by 4.7% in average single-pile bearing capacity and reduces the group effect factor by 10.2%. This is attributed to the more uniform stress distribution achieved by the staggered layout, which minimizes interference between adjacent piles.

Rock Embedment Depth

The influence of rock embedment depth on bearing capacity is found to be marginal, which suggests that for micro steel pipe piles, the skin friction along the soil segment dominates over the end-bearing contribution from the rock interface.

Engineering Practice Integration

In practice, the S = 5D criterion should be applied with caution in soft soil conditions where the stress overlap zone may extend beyond five diameters. For highway bridge abutment foundations, where group pile configurations are common, the plum-blossom arrangement is recommended when space permits, as it provides measurable capacity enhancement without increasing the overall foundation footprint.

The displacement loading approach used in this study has direct relevance to the quality control of pipe pile installation, as it mimics the actual loading behavior observed during static load testing. Engineers should ensure that the grouting process achieves full annular fill around the steel pipe to prevent void-induced premature failure, which would invalidate the assumptions underlying the numerical model.

Study Insights and Implications

This paper provides valuable quantitative data for the design of micro steel pipe pile foundations in highway engineering. The identification of S = 5D as the critical spacing is a practical contribution that can be directly incorporated into design checklists. However, the study is limited to numerical simulation without experimental validation, which is a notable gap. Future work should include centrifuge testing or full-scale load tests to calibrate the numerical parameters. From a metallurgical perspective, the steel pipe material properties (yield strength, strain hardening exponent) directly influence the pile-soil interaction behavior, and engineers should ensure that the grade of steel pipe used (typically Q235 or Q345 per GB/T 8163) matches the assumptions in the numerical model.