Underground Diaphragm Wall Construction: Techniques and Best Practices

Introduction

Underground diaphragm walls are a common construction method in municipal engineering, widely used in deep foundation pits for urban projects. After completion, they form a continuous reinforced concrete wall underground, serving multiple purposes such as water cutoff, seepage prevention, load-bearing, and retaining structures.

1. Construction Process of Underground Diaphragm Walls

1.1 Construction Preparation

Construction Roads

• Bearing Capacity Requirements: The foundation must support heavy equipment such as excavators, cranes, and concrete transporters, with a required bearing capacity of at least 100 kN/m².

• Road Specifications:

  • Width: 9–12 m (to accommodate 200–320T crawler cranes with a walking width of 7–9 m).

  • Thickness: 250 mm reinforced concrete (typically single-layer bidirectional reinforcement).

Guide Walls

Functions:

• Serves as a measurement reference and trenching guide.

• Stores slurry to stabilize trench walls.

• Supports the upper soil to prevent collapse.

• Bears construction loads (e.g., excavation machinery, rebar cage installation).

Construction Requirements:

• Minimum thickness: 200 mm.

• Embedment depth: ≥1.5 m (must penetrate non-fill layers by at least 200 mm).

• Common structural forms: Inverted “L” (for stable soil) or “[“-shaped (for weaker soil).

• For soft or loose soil, use soil replacement, trench wall reinforcement (e.g., jet grouting, TRD method), or deep guide walls.

1.2 Slurry Preparation

Functions:

• Wall stabilization.

• Cuttings removal.

• Cooling and lubrication of equipment.

Quality Control:

• Slurry mix design should account for geological and groundwater conditions.

• Performance indicators for fresh and recycled slurry must meet standards.

• Slurry storage capacity: ≥2× daily maximum trenching volume.

• The slurry circulation system includes mixers, storage tanks, pumps, pipelines, and desanders.

1.3 Trenching

Equipment Types:

• Grab Machines: Suitable for soils with SPT-N <40 (clay, sand, gravel).

  • Depth: Typically 60–80 m (sufficient for building projects).

  • Efficiency: 8–10 hrs for 35 m walls; ~20 hrs for 60 m walls.

• Impact Machines: Used in hard rock or boulder layers (low efficiency).

• Hydraulic Cutters (Hydrofraise):

  • Advantages: High precision (verticality 1‰–2‰), deep excavation (up to 150 m), and adaptability.

  • Limitations: Expensive, sensitive to obstructions (e.g., rebar, boulders).

Trench Division:

• Segment length: 4–6 m (must exceed grab machine’s horizontal reach of 2.8 m).

• For SPT-N >50 or rock UCS >3 MPa, use combined grab-impact or grab-cutter methods.

1.4 Joint Systems

Types:

• Flexible Joints: Pipe joints, hinge joints, cutter joints.

• Rigid Joints: H-pile joints, cross-steel plate joints, V-plate joints.

Comparison:

1.5 Rebar Cage Construction

Fabrication:

• Use welding or mechanical couplers (lap joints must stagger by 50%).

• HRB400 bars (or HRB335 bars ≥25 mm) require mechanical connections.

• Install lateral supports (2–3 rows) and vertical trusses (4–5 m spacing).

Lifting:

• Typical weight: 50T (60 m wall); 70T (with H-pile).

• Use 250–350T main crane + 100T auxiliary crane.

• Avoid segmented lifting to ensure stability.

1.6 Concrete Pouring

Requirements:

• Slump: 180–220 mm (underwater concrete).

• Aggregate size: ≤1/3 of rebar spacing or 1/6–1/8 of pipe diameter.

• Placement:

  • Dual-pipe method (spacing ≤3 m).

  • Initial embedment depth: ≥0.8 m; maintain 2–6 m during pouring.

Quality Checks:

• Test slump (3+ times per segment) and compressive strength (1 sample/100 m³).

• Conduct ultrasonic integrity tests (20% sampling) or core drilling.

1.7 Post-Grouting

Specifications:

• Use P.O.42.5 cement (water-cement ratio: 0.5–0.6).

• Grout pipes (≥3 per segment) should extend 200–500 mm below wall base.

• Start grouting after concrete reaches 70% strength.

• Termination criteria:

  • Grout volume meets design, OR

  • Pressure >2 MPa for 3 mins + 80% design volume.

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2. Structural Measures for “Integrated Wall” Systems

2.1 Wall Top Lowering

• Purpose:

  • Accommodate concrete overflow (300–500 mm above design).

  • Avoid conflicts with utility pipes.

  • Merge capping beams with wales to reduce costs.

2.2 Settlement Control

• Solutions:

  • Install edge piles near walls.

  • Use low-rebar or plain concrete in non-load-bearing sections.

  • Enhance connections with structural beams/slabs.

2.3 Leakage Prevention

• Techniques:

  • Rigid joints (e.g., I-shaped) + outer RJP/MJS jet grouting.

  • Internal brick/RC lining with drainage channels.

  • Waterstops at wall-slab joints.

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3. Safety & Environmental Compliance

3.1 Safety Measures

• Equipment: Operated by certified personnel.

• Trench Access: Restricted without proper safeguards.

• Crane Zones: Marked exclusion areas.

3.2 Environmental Protection

• Sludge Management:

  • Centralized storage.

  • No direct discharge into waterways.

  • Use sealed trucks for transport.

• Site Monitoring: Pre-construction surveys + real-time building/pipeline checks.

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Conclusion

Underground diaphragm walls are a versatile solution for urban deep excavations. Proper construction techniques, joint selection, and waterproofing ensure structural integrity and longevity, especially in “two-wall-in-one” applications. Adherence to safety and environmental protocols further guarantees project success.