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Grouting Pipe Installation and Construction Guide

Grouting Pipe Setup and Fabrication

Grouting pipes are typically made of Φ30mm–Φ50mm steel pipes. The bottom end is hammered into a pointed shape, and several small holes are drilled approximately 30mm from the tip. Thumbtacks are inserted into these holes (acting as check valves) and wrapped with insulating tape and adhesive tape.

The steel pipe can serve as a reinforcement cage’s main bar, connected via threaded couplers or externally welded short sleeves.

Q1: How to Choose a Grouting Pipe?

  • (1) According to the Low-Pressure Fluid Conveyance Steel Pipe Standard, Φ50–Φ32 steel pipes should have a minimum thickness of 3.25mm.
  • (2) The Pile Foundation Technical Code permits the use of ultrasonic testing pipes as grouting pipes. Thus, Φ50 steel pipes are recommended to ensure probe accessibility.

Q2: Can Flange Connections Be Used for Steel Pipes?

Flange connections are not sufficiently reliable and are not recommended.

Optimal Grouting Time After Pile Construction

Grouting should begin 7–15 days after concrete pouring (once initial setting occurs).

  • Too early: Risks damaging the pile structure.
  • Too late: May obstruct grouting channels.
    (Final strength should be ≥ 1.2 × design grouting pressure based on test samples.)

Q: Does Grouting After 7–15 Days Meet Standards?

  • Pile Foundation Technical Code (6.7.5): Grouting should start ≥2 days but ≤30 days after pile formation.
  • Highway Bridge Construction Code (8.5.6): Grouting should occur 3–7 days post-concrete pouring at ≥75% design strength or after integrity testing.
    Thus, 7–15 days is an experience-based recommendation.

Grouting Pipe Unblocking (Pressure Test)

Before grouting, a hydrostatic pressure test should be conducted to clear grouting channels and improve injectability.

  • Test medium: Clean water (not cement slurry).
  • Water volume: ~0.6 m³.
  • Unblocking pressure: <8 MPa.

Key Notes:

  1. Avoid cement slurry for unblocking—use water to prevent clogging.
  2. For long pipelines, install an additional pressure gauge at the relief valve to prevent pressure discrepancies.
  3. Apply pressure abruptly but gradually—sudden 8 MPa spikes may damage pipes or concrete.

Grouting Pump Selection

The rated pump pressure should be 1.5× the max grouting pressure, with adjustable flow and dual-fluid injection capability.

  • Rated pressure: 6–15 MPa
  • Flow rate: 30–100 L/min
  • Pressure gauge range: 1.5–2.0× pump pressure

Q1: Is a 30–100 L/min Flow Rate Excessive?

Higher flow rates allow flexibility in emergencies.

Q2: Why Use Dual-Fluid Grouting?

For unexpected ground conditions (e.g., quick sealing with sodium silicate).

Q3: Why 1.5× Max Pressure?

Prevents blockages due to high concrete strength or insufficient debris clearance.

Grout Mix Ratio

Per Pile Foundation Technical Code (6.4.7):

  • Saturated soil: 0.45–0.65 water-cement ratio (add plasticizers for low ratios).
  • Unsaturated soil: 0.7–0.9 (loose gravel/sand: 0.5–0.6).

Key Strategy:

  • Start thin (better penetration) → End thick (prevent overuse, ensure pressure control).

Grout Volume Control

The total grout volume is a primary control factor. Per standards:

“Grouting stops when both volume and pressure meet design requirements.”

If volume isn’t specified, confirm billing terms:

  • Actual quantity billing: Measure strictly.
  • Lump-sum billing: Optimize mix and pressure to avoid waste.

Grouting Pressure Guidelines

Permissible pressure depends on:

  • Soil density, permeability, stress, depth, and grouting sequence.
    • High permeability (e.g., sandy soil): <4 MPa
    • Low permeability (e.g., clay): 4–10 MPa

Intermittent Grouting Technique

To maximize grout retention in the pile base:

  • Pause intervals (determined via pressure tests).
  • Short piles: If grout surfaces, wait hours for curing before resuming.

Grouting Sequence

Design/Inspector Requirements Often Include:

  • Same cap piles: Grout simultaneously (challenging per bridge codes).
  • Group piles:
    • Center → Outward: Better injectability.
    • Outward → Center: Better effectiveness.

Key Note:

Adjacent piles cannot be drilled simultaneously (per bridge codes). Adjust the drilling order to comply with the grouting sequence rules.

Final Recommendations:

✔ Use Φ50 steel pipes for reliability.
✔ Grout 7–15 days post-pouring (balance strength & injectability).
✔ Test with water first, apply pressure cautiously.
✔ Optimize pump specs (pressure ≥1.5× requirement).
✔ Adjust mix ratio based on soil conditions.
✔ Follow sequential grouting for best results.

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