Diaphragm Wall Construction and Excavation Techniques

Guide Wall Functions and Excavation Methods

The Role of the Guide Wall

• Locate and guide the excavation process.
• Support the excavation at the ground surface.
• Raise the level of the bentonite slurry (the pressure will increase the external pressure to compensate for the internal pressure).
• Support and guide the steel reinforcing cage.

Excavation Equipment and Tools

• Clamshell bucket or hydraulic grab: Used in soils.
• Boring bit: Used in rocks.
• Hydromill / Hydrofraise: Used in both soils and rocks.

Hydromill and Hydrofraise Technology

• Continuous drilling: The soil is removed as the process gets deeper into the ground.
• Two cutter drums: It penetrates concrete, though this causes erosion on the cutter drums and should be avoided.
• Reverse slurry circulation: Slurry is constantly screened and desanded.

Bentonite Slurry Properties and Management

It is essential to control the density of the bentonite slurry; if it is viscous, it performs better. It supports the excavation through its thixotropic properties: it becomes fluid when worked but gains strength when at rest. In permeable soils, when the bentonite slurry enters the voids of the soil, it gains strength.

Concrete Pouring and Panel Installation

It is important to keep the trench clean at the joint. The process involves alternating both primary and secondary panels and installing stop ends.

Concrete Pouring Procedures

• Bottom-up approach: Using tremie pipes.
• Displacement: Concrete displaces the slurry without mixing, as the density of bentonite slurry is lower than concrete.
• Continuous pouring: The bottom of the tremie pipe remains immersed in concrete.
• Final steps: Remove the upper part of the concrete and withdraw the stop ends.

Pile Wall Types and Construction

Generally, these are bored piles:

1. Soldier piles: Arranged as o-o-o-o-o.
2. Continuous piles: Arranged as ooooo.
3. Secant piles: Arranged as OoOoOo. We excavate the secondary piles first, then excavate the others. They must not touch the rebar of the first piles.

Sectional Walls and Braced Excavations

Wall by Section (Muro por bataches)

• Square panels: Typically 3m wide and 0.4–0.8m thick.
• Construction: Wall construction occurs during excavation.
• Support: An anchor is used to support each panel.
• Limitations: Not suitable below the Ground Water Table (GWT).

Braced Excavation (Entibación)

• Small trenches: Used for temporary works.
• Propping: Propping is required during excavation.
• Limitations: Not suitable below the Ground Water Level (GWL).
• Strength: No flexural strength, requiring the use of many props.

Support of Flexible Walls

• Non-propped or cantilevered walls.
• Props or struts: Used for stability and to reduce wall deflection.