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Rotary Drilling Rig Construction Technology for Bored Piles in Ultra-High Strength Rock Layers

Introduction

In today’s construction environment, super high-rise buildings are becoming increasingly common, with greater heights, requiring larger foundation diameters and higher bearing capacities. As the most common foundation type for super high-rise buildings, the construction quality and progress of bored piles directly affect building safety and the overall project schedule. Among various hole-forming techniques for bored piles, rotary drilling has emerged as an advanced, efficient, and rock-penetrating method with broad application prospects.

Project Overview

Geological Conditions

  • Significant undulation and inclination of rock surfaces (typical single pile rock surface elevation difference of 3m, maximum 6m)

  • Bedrock is composed of Yanshanian intrusive coarse-grained granite with largely unchanged structure and slightly developed weathering fractures

  • Slightly weathered rock with high uniaxial compressive strength (90- 130MPa)

  • Shallow, slightly weathered rock surface (pile penetration depth into slightly weathered rock ranging 2.5-14.6m)

Pile Foundation Characteristics

[Detailed specifications of pile foundations would be included here]

Comparative Analysis of Hole-Forming Technologies

[A technical comparison table of common hole-forming methods in rock layers would be presented]

Equipment Selection

Based on advanced drilling data and core sample strength test reports, we determined:

  • The project site features extremely hard rock with deep pile penetration into the rock

  • Requires at least an SR420 rotary drilling rig or percussion drilling equipment

  • A tight project schedule makes low-efficiency percussion drilling unsuitable

  • SR420 test drilling showed the capability to reach a moderately weathered rock layer, but proved slow or ineffective in slightly weathered rock

Construction Process Flow

[A step-by-step construction process diagram would be included]

Construction Methodology

1. Survey Layout and Mud Preparation

  • Use sodium bentonite fully dissolved in water (calcium bentonite may form calcium carbonate precipitates, affecting mud quality)

  • Mud pit capacity should be 1.2-1.5 times the volume of simultaneously constructed pile holes

  • Prefabricated mud pits are recommended for easy mobility and reuse

  • Maintain mud specific gravity at 1.15-1.3 and viscosity above 23s

2. Rig Positioning and Calibration

  • Reinforce the working surface with brick slag or hardening (reserve the pile hole position during hardening)

  • Adjust the mast and drill pipe angle after positioning

  • Use the crosshair method for hole centering and activate the positioning system for memory

  • No movement allowed after centering; drill arm angle must remain fixed

3. Casing Installation

  • Casing diameter is typically 20cm larger than the pile diameter

  • Casing depth should penetrate through soft soil and sand layers (preferably reaching a strongly weathered layer)

  • Compact backfill around the casing to prevent sinking

4. Drilling Process

  • Strongly weathered layer and above: SR420 rig with sand bucket

  • Moderately weathered rock: SR420 rig with bullet-shaped barrel drill in quincunx pattern

  • Slightly weathered rock: SR630 rig with roller cone barrel drill in quincunx pattern

  • Maintain 2-3 drill bits of each specification per rig with a dedicated repair area

Mechanical Interlocked Quincunx Drilling:

  1. SR630 and SR420 coordinated drilling (SR420 handles casing installation, moderately weathered rock, and lower-strength slightly weathered rock; SR630 focuses on high-strength slightly weathered rock)

  2. 2.5m barrel drill for edge cutting (large diameter prevents direct rock removal but effectively prevents hole deviation)

  3. 1.2m barrel drill creates symmetrical quincunx pattern (4 holes) for direct rock extraction

  4. 2.5m barrel drill cleans edges and breaks remaining rock

  5. 2.5m sand bucket removes debris

  6. Cyclic drilling (advanced depth determined by rock breaking length and lifting weight – typically 1.5-1.8m for 1.2m drill bit)

  7. Drilling efficiency metrics: [Specific data would be included]

5. Final Hole Inspection

  • Rotary drilling allows easy assessment of bottom rock conditions

  • Inspection requires participation from the owner, supervisor, contractor, and survey team

6. Hole Cleaning

Cleaning Methods:

  • Conventional: direct circulation cleaning, airlift reverse circulation cleaning

  • Innovative: slurry slag flotation, mechanical cleaning

High-Performance Slurry Preparation:

  • Premium sodium bentonite artificial slurry

  • Mixing ratio: water:bentonite: soda ash = 1000:80-120:1.5-2

  • Target specific gravity: 1.15-1.3; viscosity >23s

After final drilling:

  • Use a sand bucket for multiple cleanings (3-5 times) to completely remove small rock fragments

  • Requires skilled operators to ensure thorough cleaning

  • Under static balance, fine sand particles remain suspended in slurry without sedimentation

7. Reinforcement Cage Installation

  • Strengthen lifting points with welding for secure hoisting

  • Prevent cage collision with the hole walls to avoid collapse

8. Sediment Thickness Measurement

Three-Stage Four-Point Measurement Method:

  1. After hole completion

  2. After cage installation

  3. Before concreting

Measure four equidistant points (N, S, E, W) using a measuring rope:

  • All three measurements at all four points must show sediment thickness <50mm with consistent values

  • If pre-concreting measurement fails requirements, re-clean with the drill pipe

9. Pile Formation Inspection

  • Maintain a 30-50cm distance between the tremie pipe and the pile bottom

  • Initial tremie embedment depth ≥1m after first pour

  • Normal pouring: tremie embedment 2-6m

  • Core sample test results: sediment thickness 5mm, 10mm, 5mm (three samples)

Conclusion

This comprehensive construction methodology for rotary drilling in ultra-high-strength rock layers demonstrates how proper equipment selection, precise operational techniques, and rigorous quality control measures can overcome challenging geological conditions to ensure the successful completion of bored pile foundations for super high-rise buildings. The systematic approach from drilling to final inspection guarantees both construction efficiency and foundation quality.

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