Slurry / diaphragm walls are very rigid walls offering significant resistance to bending from lateral loads as well as being very water resistant.

Slurry Walls were introduced into the North American market in 1962. Bencor has been at the forefront of using this technology, utilizing both conventional and specialized techniques to construct deep concrete slurry walls in rock and difficult soil conditions.

The term “Slurry Wall” is used to refer to the construction of a wall beneath the ground surface using a slurry fluid to stabilize the sidewalls of the trench from collapsing during the excavation before being filled with concrete.

Diaphragm Walls refers more specifically to a slurry wall built for some type of structural support. Since most Diaphragm Walls are usually thick (>2.5 feet) and built of reinforced concrete, they are very rigid walls offering significant resistance to bending from lateral loads as well as being very water resistant. Diaphragm Walls are ideal for lateral earth support for deep excavations and are often incorporated as the permanent building basement walls. This creates additional savings for the project by eliminating the need for temporary earth support and extensive dewatering.

Since reinforced walls are designed and constructed as a series of panels, there is no practical limit to their overall length. Individual panels usually range in sizes between 15 to 30 feet in length, with standard widths ranging between 24, 30, 36 and 48 inches. Depths of the wall have exceeded 200 feet regularly. Possible shapes of walls, depending on the purpose of the wall, include right angles, polygonal, T-shaped and cross-shaped walls.

Excavation stability is maintained by the slurry. Slurry is a powdered bentonite or polymer mixed with water. When a homogeneous mix has been attained, the slurry is sent to storage tanks for full hydration (required only when using bentonite) and storage. When introduced into a trench the bentonite slurry tends to permeate the soil, forming an impervious “filter cake.” Vice-versa when polymer slurry is used, a membrane barrier is formed on the excavation’s surface. Slurry hydrostatic pressure and the slurry properties, insure the excavation stability.

Slurry Walls can be categorized into two different types: Structural, and Non-structural.

Structural (Diaphragm Wall) 

For a structural diaphragm wall, slurry is replaced with concrete and some type of reinforcement for resisting structural loads. Structural Diaphragm Walls are used for:

  • Deep Basement Walls for buildings, parking garages
  • Permanent Retaining Walls
  • Rigid Earth Retention System for Deep Excavation Support
  • Circular Tunnel Access Shafts / CSO shafts
  • Top-Down building construction methods
  • Load Bearing Elements (Barrettes)

Non Structural (Slurry Wall / Cutoff Wall) 

For a non structural slurry wall, slurry is left in place to self harden or replaced with plastic (or low strength) concrete. Non Structural Slurry Walls are used for:

  • Groundwater Cutoff Walls
  • Environmental Cutoff Walls
  • Ground Improvement Systems
  • Dam and Levee Rehabilitation

Typical Construction Procedure

  • Develop Detailed Engineering Design
  • Site Preparation
  • Guide Wall Construction to provide accurate slurry wall alignment
  • Excavation of panels or load bearing elements using clam shell grab and/or hydromill depending on depth / soil conditions
  • Desanding to separate the cuttings and maintain slurry properties
  • Quality Assurance/Quality Control (QA/QC), Verticality Control & Verification; procedures include slurry & concrete testing, verifying panel depth and verticality using industry standard tools and built-in equipment instrumentation, along with real time monitoring
  • Reinforcing element placement by lowering rebar cages or steel beam assemblies into slurry panel
  • Concrete placement into trench via tremie pipe
  • QA/QC Post Installation (perform concrete coring)

Excavation Equipment

Clam Shells (Cable Grabs) 

This equipment is suitable for soft, medium dense to dense soil & boulders (in combination with chisels), to suggested depths of up to 120ft (37m).

Hydraulic Clam Shells (Hydraulic Grabs) 

This equipment is suitable for hard soil and stiff clays, to suggested depths of up to 260ft (80m).

Hydromills (Cutters)

This equipment is suitable for nearly all soils and rock types. Hydromills can use different types of cutting wheels depending on the soil and rock conditions. Achieving large depths is possible (>500ft (150m)), while maintaining very accurate verticality (<.3%) which is essential for proper closure at depths of hundreds of feet.

The application of the hydromill technique for the construction of large projects with deep slurry walls and cut-off walls offers the following advantages over the conventional clamshell techniques:

  •     Economy        
  •     Output    
  •     Reliability
  •     Environmental Compatibility

The hydromill continuously removes soil from the bottom of the trench, breaks it up, and mixes it with the bentonite slurry in the trench.  The slurry, heavy with soil particles, is pumped through pipes to the desanding plant where it is cleaned and returned to the trench.