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

Literature Overview

Su Shiding, Chen Zhangyu, and Sang Dengfeng (2021) from CCCC Fourth Navigation Engineering Research Institute published their findings in Port and Waterway Engineering (No. 2), investigating the settlement behavior of large-diameter steel pipe piles driven through calcareous sand layers into phyllite bedrock. By comparing high-strain dynamic testing (HSDT) Q-S curves with static load test (SLT) Q-S curves, the authors established clear correlations between geological conditions and pile settlement characteristics, providing practical guidance for pile foundation design in this specific geological setting.

Geological Setting and Pile Configuration

Geological Profile

The study area features a distinctive geological sequence:

Layer	Description	Thickness	Engineering Properties
Overburden	Soft clay/silt	5-15 m	Very soft, low bearing capacity
Calcareous sand	Medium-dense calcareous sand	2-20 m	Variable density, medium bearing capacity
Phyllite (strongly weathered)	Bedrock, strongly weathered	>10 m	Hard to very hard, high bearing capacity

The calcareous sand layer is of particular interest because its thickness and density vary significantly across the site, creating uncertainty in pile design.

Pile Specifications

Parameter	Specification
Pile diameter	1200-1800 mm
Wall thickness	14-22 mm
Steel grade	Q345 or Q370
Pile length	30-60 m
Driving method	Impact driving / vibratory driving
End bearing depth in phyllite	2-8 m

Q-S Curve Analysis and Settlement Classification

End-Bearing Pile Behavior

When the calcareous sand layer is thin (2-5 m) and the pile penetrates deeply into hard strongly weathered phyllite (>5 m), the Q-S curve exhibits end-bearing characteristics:

Load transfer mechanism: Primary resistance from pile tip bearing in phyllite; shaft friction in calcareous sand is secondary.
Settlement composition: Predominantly pile shaft compression (elastic shortening); minimal residual settlement.
Q-S curve shape: Relatively linear up to 80-90% of ultimate load, then sharp increase in settlement rate.
Ultimate settlement: Typically 15-30 mm at ultimate capacity.
Residual settlement: Less than 2 mm after load removal.

Friction Pile Behavior

When the calcareous sand layer is thick (10-20 m) and the pile terminates in softer strongly weathered phyllite (<3 m penetration), the Q-S curve shows friction-dominated characteristics:

Load transfer mechanism: Significant shaft friction in calcareous sand; pile tip resistance in phyllite is limited.
Settlement composition: Combination of pile compression and soil deformation at pile tip and along shaft.
Q-S curve shape: Gradual curvature throughout loading; no distinct linear-elastic region.
Ultimate settlement: 40-80 mm at ultimate capacity.
Residual settlement: 5-15 mm after load removal, indicating significant soil plastic deformation.

Comparison of HSDT and SLT Results

Agreement Analysis

Condition	HSDT vs. SLT Ultimate Capacity Agreement	Q-S Curve Shape Agreement
End-bearing type	Within 5-10%	Good agreement in elastic region; HSDT overestimates post-yield stiffness
Friction type	Within 10-20%	Acceptable agreement; HSDT may underestimate total settlement

The study confirms that HSDT is reliable for predicting the ultimate capacity of end-bearing steel pipe piles in this geological setting, while friction-type piles require more careful interpretation of dynamic test results.

Steel Pipe Pile Fabrication and Driving Considerations

Impact Driving Effects on Steel Pipe Integrity

The driving process subjects the steel pipe pile to repeated impact loads that can affect structural integrity:

Driving blow count: Excessive blows (>50 per meter in phyllite) can cause fatigue damage at weld seams and pile joints.
Pile head damage: Repeated hammer impacts can deform the pile head, requiring protective caps or driving shoes.
Joint integrity: Butt-welded or bolted pile joints must withstand cumulative impact loads. UT inspection of joints after driving is recommended for piles driven >40 m.
Residual stress: Driving introduces additional residual stresses that superimpose on fabrication residual stresses. For heavily driven piles, the combined residual stress state should be evaluated.

Weld Quality Requirements for Steel Pipe Pile Joints

Weld Type	Inspection Method	Acceptance Criteria
Butt weld (longitudinal)	100% UT, spot RT	No lack of fusion, no cracks, porosity ≤1 mm
Joint weld (splicing)	100% UT + 20% RT	Full penetration, no defects exceeding 2 mm
Driving shoe weld	100% MT/PT	No surface cracks, no undercut >1 mm

Practical Implications for Foundation Design

The study provides actionable guidance for engineers designing steel pipe pile foundations in similar geological conditions:

Geotechnical investigation priority: The thickness and density of the calcareous sand layer should be determined with high accuracy, as it directly influences pile classification and design approach.
End-bearing preference: Where geology permits, driving piles deeply into phyllite is preferred due to more predictable settlement behavior and lower residual settlement.
Dynamic testing applicability: HSDT can be used as a reliable alternative to SLT for end-bearing piles, reducing testing costs and project timelines.
Settlement allowance: Design settlement allowances should differ based on pile classification — 20-35 mm for end-bearing piles and 50-100 mm for friction piles.

Key Reflections

The research demonstrates that the interaction between steel pipe pile geometry, driving dynamics, and geological stratification creates complex settlement behavior that cannot be predicted by simplified analytical models alone. The Q-S curve comparison methodology provides a practical framework for characterizing pile behavior in the field. For steel pipe fabricators, the study reinforces the importance of maintaining consistent wall thickness and joint quality, as these factors directly influence the pile's load transfer efficiency and settlement performance under service conditions.