Diaphragm Walls in Deep Excavations

Diaphragm Wall Construction Methods for Deep Excavations

Diaphragm walls are among the most widely used retaining systems for deep excavations in urban environments and infrastructure projects. Their high stiffness, low permeability, and ability to limit ground movements make them particularly suitable for metro stations, underground structures, shafts, and deep basements constructed near existing buildings.

The construction of diaphragm walls requires specialized equipment, strict quality control, and a carefully planned sequence of operations. Because the wall is constructed panel by panel under slurry support, every stage of the process directly affects the structural integrity, verticality, and watertightness of the final system. Compared to other retaining systems, diaphragm wall construction generally requires larger working areas to accommodate cranes, slurry treatment plants, reinforcement cages, and excavation equipment.

Typical Diaphragm Wall Construction Sequence

1. Guide Wall Construction

Guide walls are shallow reinforced concrete elements constructed at the ground surface before trench excavation begins. Although temporary, they are essential for maintaining excavation alignment, panel geometry, and wall continuity.

Their primary functions include:

• Maintaining accurate horizontal alignment of the wall

• Supporting the upper soil layers during excavation

• Guiding the grab or trench cutter vertically

• Assisting in reinforcement cage positioning

• Providing support during tremie concreting operations

  
  

Guide walls are particularly important because the upper portion of the trench is often the least stable, especially as slurry levels fluctuate during excavation.

2. Initial Excavation and Slurry Preparation

Before full-depth excavation starts, the slurry circulation system must already be operational. A shallow starter trench is first excavated and immediately filled with bentonite or polymer slurry so the slurry pump remains fully submerged.

The supporting slurry stabilizes the trench walls by balancing soil and groundwater pressures throughout excavation.

In some projects, localized pre-excavation may also be required before guide wall installation to remove:

• Buried obstructions

• Existing foundations

• Utilities

• Old retaining structures

• Uncontrolled fill materials

Maintaining proper slurry levels throughout excavation is critical to prevent trench instability or collapse

.

Figure 1: Excavation with a small grab for a T-panel in the Central Artery Project, Boston, MA

3. Primary Panel Excavation

Primary panels are excavated first using mechanical grabs or trench cutters operating under slurry support.

Typical panel lengths range from approximately 2.5 m to 7.0 m depending on:

• Excavation equipment

• Soil conditions

• Wall thickness

• Project geometry

In favorable ground conditions, larger panels can be excavated using multiple bites. This method allows the construction of wider panels and more complex geometries such as corner panels and T-panels. Throughout excavation, continuous monitoring of trench verticality and stability is required to maintain acceptable construction tolerances and ensure proper continuity between adjacent panels.

4. Slurry Cleaning and Desanding

The supporting slurry must be continuously monitored and maintained within specified quality limits during excavation and prior to concreting.

The most important slurry parameters include:

• Density

• Viscosity

• Sand content

• pH

As excavation progresses, slurry is circulated through a desanding and regeneration plant to remove suspended particles and restore acceptable properties.

Proper slurry cleaning is essential before concreting because contaminated slurry can negatively affect:

• Concrete quality

• Bond performance

• Panel integrity

• Joint watertightness

Although fresh slurry may occasionally be introduced, slurry regeneration is generally more economical and environmentally efficient.

Figure 2: Diaphragm wall grab during panel excavation

5. Joint Construction Methods

Panel joints are critical elements in diaphragm wall construction because they influence both structural continuity and groundwater control.

Several joint systems are commonly used, including:

• Flat joints

• Circular stop-end joints

• Steel beam joints

• Grooved water-stop joints

Joint selection depends on:

• Contractor preference

• Excavation equipment

• Groundwater conditions

• Project performance requirements

Grooved joints with integrated water stops are often preferred because they improve interlock and reduce seepage potential. In North America, steel I-beams are also widely used as stop-end elements and water barriers. Simpler flat joints are generally less effective for projects with demanding groundwater control requirements.

6. Reinforcement Cage Installation and Tremie Concreting

Once the excavation reaches the design depth and the trench bottom is cleaned, the reinforcement cage is lifted and lowered into the slurry-filled panel.

Because cages are often extremely long and heavy, they must contain sufficient transverse and diagonal reinforcement to maintain stiffness during lifting and placement.

Adequate spacing must also be provided for multiple tremie pipes.

Tremie Concreting

Concrete placement is performed using tremie pipes extending to the bottom of the trench. The concrete is placed from the bottom upward, progressively displacing the supporting slurry.

To prevent contamination and segregation:

• The tremie pipe tip must remain continuously embedded in fresh concrete

• A minimum embedment of approximately 0.6 m (2 ft) is typically maintained

• Additional overpouring is commonly required to ensure complete slurry displacement

Poor tremie practices may trap slurry pockets within the wall, potentially leading to:

• Groundwater leakage

• Reduced durability

• Honeycombing

• Structural defects

• Localized blowouts

Such issues have contributed to major repairs and delays in several large infrastructure projects, including portions of Boston’s Central Artery / Tunnel Project (“Big Dig”).

Figure 3: Reinforcement cage lifting for diaphragm wall construction

7. Secondary Panel Excavation

After the primary panels gain sufficient strength, secondary panels are excavated between them to complete the continuous wall system.

When trench cutters are used, the cutter trims into the edges of adjacent primary panels to create a clean interface and improve continuity between panels.

After excavation:

1. The trench is cleaned

2. The reinforcement cage is installed

3. Tremie concreting is performed from the bottom upward

The alternating sequence between primary and secondary panels continues until the entire diaphragm wall perimeter is completed.

Construction Challenges and Quality Control

Successful diaphragm wall construction depends heavily on field quality control and experienced execution.

The most common construction challenges include:

• Trench instability

• Loss of slurry

• Excessive trench deviation

• Joint defects

• Reinforcement cage instability

• Poor slurry properties

• Concrete contamination during tremieing

• Groundwater leakage through joints

To minimize these risks, continuous monitoring is typically performed for:

• Slurry properties

• Verticality

• Panel depth

• Reinforcement cage alignment

• Concrete volume and tremie embedment

• Joint positioning

Strict quality assurance procedures are especially important in urban excavations where wall movements and groundwater inflow can directly affect adjacent structures and utilities.

Modeling Recommendations for Diaphragm Wall Analysis

Reliable diaphragm wall analysis requires realistic representation of soil behavior, groundwater conditions, construction sequencing, and soil–structure interaction.

Figure 4: Diaphragm wall types in DeepEX and example of a model with LEM and FEM analysis

Key modeling recommendations include:

• Define realistic soil stratigraphy, including weak layers and groundwater conditions

• Use appropriate constitutive models based on the required level of analysis (e.g., Mohr-Coulomb

  for preliminary studies and Hardening Soil models for advanced analysis)

• Simulate excavation and support installation sequentially to capture staged construction effects

• Properly model wall stiffness, interfaces, and support systems

• Include groundwater pressures and seepage effects, particularly for deep excavations below the water table

• Refine the numerical mesh near wall interfaces, excavation bottoms, and support connections

• Evaluate both structural capacity and serviceability performance, including wall deflections and ground settlements

• Validate numerical results using analytical methods, field data, or comparisons between LEM, beam-spring, and FEM approaches

• Perform sensitivity analyses for key parameters such as soil stiffness, groundwater level, and support stiffness

  
  

Advanced numerical tools such as the DeepEX platform allow engineers to combine staged excavation analysis, groundwater modeling, and soil–structure interaction within a unified workflow, improving both design efficiency and project reliability.

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