Introduction
Rotary drilling piles have become increasingly popular in urban deep foundation pit support construction due to their advantages of fast construction speed, high precision, low noise, environmental friendliness, and convenient machine mobility. However, challenges such as significant soil disturbance, poor hole wall protection, excessive sediment thickness, and inter-pile cross-flow often arise during construction.
This article explores quality control measures for rotary drilling piles in deep foundation pit support construction, including optimizing drilling speed, adjusting slurry density, and improving construction sequencing to enhance hole-cleaning efficiency and increase the time interval between adjacent pile construction.
Project Case Study
1. Project Overview
The Science Avenue-Daguan Road Tunnel Project is located at the intersection of Yunxi Road, Science Avenue, and Daguan Road in Guangzhou. The 895-meter-long bidirectional six-lane tunnel was constructed using the open-cut method with supporting piles.
The foundation pit support system consisted of 607 bored piles with diameters of Φ1000mm and Φ1200mm, spaced 20cm apart, and lengths ranging from 10.5m to 24.7m. The geological conditions included layers of miscellaneous fill, fine sand, alluvial silty clay, medium sand, slope-wash silty clay, completely weathered granite, and highly weathered granite. The pile tips were embedded in clay or medium sand layers.
According to the survey, the silt soil was prone to seismic subsidence, and the saturated sand layer exhibited slight liquefaction (liquefaction index I1E = 0.19–4.77).
2. Rotary Drilling Pile Construction Process
The project utilized rotary drilling rigs, with construction techniques similar to traditional bored (percussion) piles except for the rotary drilling process.
Common Issues in Rotary Drilling Support Pile Construction
1. Significant Soil Disturbance and Poor Hole Wall Protection
Due to the high drilling speed and lack of slurry circulation, rotary drilling piles exhibit weaker hole wall protection compared to conventional bored or percussion piles. Collapse and necking are common in fill and soft soil layers. Excessive soil disturbance can also lead to cross-flow between adjacent piles.
Additionally, over-excavation is frequent, with concrete usage exceeding theoretical estimates by over 10%, compared to 5%–10% in conventional bored piles under similar geological conditions.
2. Excessive Sediment Thickness at Hole Bottom
Poor hole wall protection in rotary drilling piles increases the risk of excessive sediment accumulation due to deep cutting or wall collisions during drilling. Field measurements showed sediment thickness exceeding 50cm after initial cleaning.
While sediment thickness has minimal impact on support pile functionality, it complicates low-strain integrity testing and reduces bearing capacity, hindering the adoption of rotary drilling piles for large-diameter load-bearing applications.
3. Small Adjacent Pile Spacing Leading to Cross-Flow
In deep foundation pit support systems, small pile spacing is common to ensure soil stability. Traditional “alternate pile” construction methods minimize disturbance, but rotary drilling’s high soil disruption and weak slurry protection can still affect adjacent piles even after concrete setting.
Quality Control Measures for Rotary Drilling Pile Construction
1. Adjusting Drilling and Lifting Speed Based on Soil Conditions
Rotary drilling rigs offer rapid hole formation (up to 1m/min), but excessive speed increases soil disturbance and cross-flow risks. Recommended drilling speeds:
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Clay: 0.5–1 m/min
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Miscellaneous fill or sand layers: 0.3–0.5 m/min
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Hard-to-soft transition: Increase speed
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Soft-to-hard transition: Reduce speed
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Prone-to-shrinkage layers: Frequent reaming to prevent necking
Lifting speeds should also be controlled:
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Clay: 1–2 m/min
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Miscellaneous fill or sand layers: 0.5–1 m/min
2. Optimizing Slurry Density and Implementing Secondary Cleaning
High-quality bentonite slurry stabilizes hole walls by balancing groundwater pressure. Slurry density adjustments based on soil conditions enhance wall protection and reduce cross-flow.
Secondary cleaning (post-drilling and pre-concrete pouring) ensures that sediment thickness and hole quality meet standards.
3. Adopting the “Skip-Three-Pile” Construction Method
Traditional “alternate pile” methods require 48-hour intervals between adjacent piles, but rotary drilling’s high disturbance may still cause collapse or cross-flow.
The “skip-three-pile” method improves stability:
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Drill Pile 1, then skip three piles to drill Pile 5.
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After Piles 1 and 5 are concreted, drill Pile 3.
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Once Pile 3 is set, drill Piles 2 and 4.
This approach reduces concrete overuse (from 10% to 4.7%) and enhances pile integrity, with over 90% achieving Class I quality in low-strain testing.
Conclusion
Rotary drilling piles are widely used in deep foundation pit support due to their speed, low noise, and minimal slurry pollution. By optimizing drilling speed, slurry density, and construction sequencing, pile quality can be significantly improved.
However, unlike traditional bored piles, rotary drilling lacks standardized national or industry regulations. Establishing specific guidelines for parameter control will further promote its application in foundation pit support and other pile foundation projects.
