Introduction
The secant pile retaining wall is a deep excavation support structure that emerged in China in the 1990s. It primarily utilizes full casing drilling rigs (Benoto rigs) to form holes with casing protection, employing ultra-slow-setting concrete to allow adjacent reinforced concrete piles to interlock through casing cutting, forming a continuous wall with excellent waterproofing performance.
Development of Full Casing Drilling Rigs
The full casing drilling rig, also known as the Benoto rig, was developed by the French company Benoto in the early 1950s. Later, Japan, Germany, the UK, and Italy introduced and improved the technology, leading to continuous advancements in equipment and construction methods. By December 1997, Japan had produced 770 oscillating-type full casing rigs and 433 rotary-type full casing rigs. According to a 1993 survey by the Japan Infrastructure Association, the full casing method accounted for 26% of the 101,000 bored piles constructed by 31 contractors. In Hong Kong, the full casing method currently holds about 45% of the bored pile market share.
Introduction and Development in China
China began adopting the secant pile technology in the 1970s. By the mid-1990s, Kunming Jiecheng Pile Engineering Company developed the MZ series oscillating full casing drilling rig (grout pile machine) with pile diameters of 0.8m, 1.0m, and 1.2m. The MZ rig was widely used in deep excavation projects in Kunming, Wenzhou, Shenzhen, Beijing, Nanjing, Hangzhou, and Tianjin.
However, in dense water-bearing fine sand layers, the MZ rig faced challenges:
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The grab bucket struggled to extract soil due to equipment limitations.
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Extracted sand often leaked back into the hole due to groundwater pressure.
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The casing could not be advanced sufficiently, leading to “piping” risks.
To address these issues, a new method combining rotary drilling rigs with casing drilling rigs was developed, significantly expanding the applicability of secant piles.
Construction Features
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Steel Casing Wall Protection – Eliminates the need for slurry, reduces noise and vibration, and enhances environmental friendliness.
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Rotary Drilling for Soil Extraction – Enables construction in dense, water-bearing sand layers, extending the MZ rig’s applicability.
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Pre-Advancing Casing in Saturated Sand Layers – Prevents borehole collapse, ensures precise pile dimensions and verticality, and enhances interlock waterproofing.
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Water Injection for Counter-Pressure – Prevents sand inflow and bottom sediment issues, reducing defects like necking, pile breakage, and concrete segregation.
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Minimal Ground Disturbance – Ideal for urban construction near buildings and underground utilities.
Applications
The combined rotary and casing drilling method allows secant piles to be constructed in various soils, including:
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Miscellaneous fill (containing brick, stone, and concrete debris)
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Cohesive soil
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Sandy soil
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Weathered rock layers
Construction Principles
The method reduces soil-casing friction via hydraulic rotation, advancing the casing while maintaining a 2.5m lead over excavation. Water injection maintains internal pressure to prevent sand flow. Rotary drilling extracts soil to the bearing layer, followed by reinforcement placement (for RC piles) and underwater concrete pouring.
Pile Construction Sequence
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Primary Piles (A1, A2, etc.): Ultra-slow-setting concrete
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Secondary Piles (B1, B2, etc.): Reinforced concrete
Sequence: A1 → A2 → B1 → A3 → B2 → A4 → B3…
Construction Process & Key Points
1. Process Flow
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Guide wall construction
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Rig positioning and alignment
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Lowering and pressing the first casing section, checking verticality, and soil extraction
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Water injection and rotary drilling
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Final hole inspection
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Reinforcement cage placement (for B piles)
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Concrete pouring
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Rig relocation (repeat process)
2. Key Construction Steps
A. Guide Wall Construction
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Width: 3.5m, thickness: 0.3m
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Positioning holes: 2cm larger than pile diameter
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Elevation: ≥10cm above ground to prevent water ingress
B. Rig Positioning
After the guide wall curing, align the rig center with the pile position.
C. Casing Installation & Soil Extraction
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Press the first casing section (2.5–3.5m depth) while extracting soil.
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Maintain a 2.5m lead between casing tip and excavation face.
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Check verticality before installing subsequent sections.
D. Water Injection & Rotary Drilling
In dense sand layers, inject water to balance pressure and switch to rotary drilling.
E. Final Inspection & Concrete Pouring
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Clear loose soil, measure depth and verticality.
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For B piles, place reinforcement cage (±20cm tolerance).
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Pour concrete while withdrawing casing (keep casing tip ≥2.5m below concrete surface).
Key Technologies
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Water Injection Control
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Maintain water level ≥2m above groundwater to prevent sand flow.
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Ultra-Slow-Setting Concrete Control
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Slump ≤18cm to minimize “piping.”
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Maintain a 2.5m casing lead for sealing.
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Sand Pile Method for Joint Treatment
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Use sand piles at section ends, later replaced with concrete.
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Add jet grouting piles for waterproofing if needed.
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Labor & Equipment
Labor Organization (Per Shift)
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Drilling Team: 1 supervisor, 1 casing rig operator, 1 rotary drill operator, 1 crane operator, 6 assistants.
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Reinforcement Team: 20 workers for cage fabrication.
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Maintenance Team: 4 workers for equipment upkeep.
Materials & Machinery
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Ultra-slow-setting concrete: C20, 3-day strength ≤3MPa, slump 16±2cm.
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Equipment per zone: 1 rotary drill + 1 casing rig.
Quality Control
1. Pile Verticality (≤3‰)
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Check casing alignment before installation.
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Monitor verticality during drilling using inclinometers.
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Correct deviations using hydraulic adjustment, sand backfilling (A piles), or slow-setting concrete (B piles).
2. Concrete Quality
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Adjust the setting time based on construction pace.
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Ensure the mix design meets strength and workability requirements.
Safety & Environmental Measures
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Safety: Follow standard protocols for lifting, machinery, and electrical safety.
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Environmental: Proper spoil management to prevent pollution.
Case Study: Nanjing Metro Line 2
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Project: Suojie Station, 14 m-deep excavation (Grade 1 safety).
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Design: Φ1000@800mm secant piles (20mm overlap), max length 28m.
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Challenge: Dense, water-bearing fine sand layer at 6–8m depth caused casing deformation and sand inflow.
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Solution: Combined rotary drilling + water injection method stabilized construction, achieving 2 piles/day.
Conclusion
The secant pile method offers superior waterproofing, minimal ground disturbance, and high adaptability, making it ideal for urban deep excavation projects. The integration of rotary drilling enhances performance in challenging geotechnical conditions.
