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Rotary Drilling Rig Construction Technology for Bored Piles in Strongly Developed Karst Formations

Introduction

Karst formations are created by long-term chemical dissolution of soluble rocks such as limestone. These formations feature dissolution grooves, slots, and caves, with particularly uneven cave roofs and floors exhibiting significant height variations. The rock layers often consist of alternating hard and soft strata, semi-rock semi-soil sections, semi-rock semi-dissolved zones, and inclined rock surfaces. Constructing bridge bored piles in strongly developed karst formations presents significant challenges due to complex geological factors that complicate drilling and pile formation. This article, based on an engineering case study, introduces construction techniques for bored piles in such formations and outlines corresponding construction methods and precautions.

1. Project Overview

The Jinghe Grand Bridge on Section 3 of the Beijing-Shanghai High-Speed Railway is located in Tengzhou City, Shandong Province. The bridge spans 11.445 km with the following structural layout:

  • 20 × 24 m simply supported double-track box girders

  • 328 × 32 m simply supported double-track box girders

  • 2 × (32 + 48 + 32 m) continuous double-track box girders

The foundation design for all piers and abutments consists of bored piles:

  • 1.0 m diameter: 1,422 piles, total length 44,272.77 m

  • 1.25 m diameter: 1,538 piles, total length 83,527.39 m

  • 1.50 m diameter: 108 piles, total length 14,325.16 m

Pile No. 203 (1.50 m diameter) has a maximum length of 85.5 m.

2. Geological Characteristics

The Jinghe Grand Bridge primarily traverses limestone formations with a monoclinal structure (dip angle: 30°∠12°). The bridge site exhibits well-developed joint fissures. Key geological sections include:

  • DK580+229～DK582+100:

    • Karst cave encounter rate: 76.9%

    • Strongly developed karst zones: 90–100%

  • DK582+100～DK583+100:

    • Karst cave encounter rate: 60.8%–73.7%

    • Beaded (chain-like) cave distribution

    • Pier 203 contains up to 13 beaded caves (height: 2–8 m)

    • Pier 286 features a cave height of 25.5 m

  • DK583+100～DK586+100:

    • Karst cave encounter rate: 66.1%–71.9%

  • DK586+100～DK587+100:

    • Karst cave encounter rate: 42.5%

    • Beaded cave development

Soil caves in the bridge area result from subsurface erosion and negative-pressure suction caused by surface and groundwater infiltration into overlying strata.

3. Bored Pile Construction Technology

3.1. Selection of Drilling Rig Type

Given the varied geology (clay, silty clay, medium-coarse sandy soil, marl, limestone), mast-type impact drilling rigs were selected. These rigs allow real-time adjustment of bit lifting height (stroke) based on rock hardness and deliver high-impact energy.

3.2. Drilling Sequence

In strongly developed karst zones, understanding geological conditions is critical. The project combined drilling exploration and geophysical prospecting to determine:

  • Overburden thickness

  • Soil/karst cave locations, quantities, and fill conditions

Key principles for the drilling sequence:

  • For piers with unequal pile lengths: longer piles first, shorter piles later

  • For the same pier:

    • Caves ordered from deep to shallow, multiple (beaded) to fewer

    • Edge/corner piles first to seal caves and block channels

3.3. Mud Retaining Wall

High-quality clay was used to prepare the slurry. For challenging piles (frequent mud loss/collapse), bentonite was added to enhance wall stability. When caves contained fine sand or fluid fills, 10%–20% cement was mixed into the slurry to improve impermeability.

3.4. Artificial Borehole Wall

For cave penetration, a mixture of clay blocks (with straw), crushed stone (15–25 cm), and gravel (<8–9 cm) was injected. The drill’s impact compacted these materials into karst fissures, forming a stable artificial wall.

Optimal mix ratio:

  • Clay 0.4: Crushed stone 0.6 (with optional straw)

Critical considerations:

  • Poor-quality artificial walls may collapse under concrete pressure, leading to pile breaks.

  • During concrete pouring:

    • Monitor elevation closely to detect slow declines.

    • Overpour by 1.5–2.0 m above design height.

    • Wait 0.5–1.0 hours after stabilization before withdrawing the casing.

3.5. Beaded Cave Mud Loss Treatment

Piers 203 (13 beaded caves) and 286 (25.5 m cave height) faced severe interconnected caves (encounter rate: 66.1%–71.9%). To address cross-cave mud loss:

  1. Pour underwater concrete to seal caves (2–3 m above the cave roof).

  2. Allow 1–2 days for curing before resuming drilling.

This method proved effective for Piers 289# and 290#, ensuring smooth hole completion.

Conclusion

The Jinghe Grand Bridge project demonstrates that adaptive drilling sequences, optimized slurry mixes, and artificial wall techniques are essential for success in karst formations. Key takeaways:

  • Prioritize geological surveys to guide drilling order.

  • Use bentonite/cement-enhanced slurries for unstable zones.

  • Ensure artificial walls are compacted to withstand concrete pressure.

  • For beaded caves, underwater concrete sealing is highly effective.

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