Soil Bentonite Slurry Walls have very low permeability

Explore the applications and advantages of using Soil Bentonite Groundwater Barriers.

**SB Slurry Trench Excavation**

Soil Bentonite (SB) slurry walls are the most common type of slurry wall. They have been in use in the United States since the 1940's with common use starting in the 1970's. Since then, literally thousands of these walls have been constructed for a variety of applications. For this type of wall, the permanent backfill is a blend of soil and bentonite that is placed in the excavated trench through the slurry. In the US, the SB technique is used far more frequently than any of the other methods.

Advantages of Soil Bentonite Slurry Walls

Compared to Other Barrier Wall Methods, Soil Bentonite Slurry walls offer the following advantages:

Low cost
High productivity
Very low permeability
Verifiable continuity and depth.
Excellent resistance to contaminated groundwater
Ability to easily flex with ground movements, even some earthquakes.
The slurry remains fluid, allowing time for penetrating difficult layers or obstacles.
Re-use of most of the excavated materials

Construction Method for Soil Bentonite Slurry Walls

The SB slurry wall is typically excavated with a long reach excavator under a bentonite water slurry. The slurry stabilizes the excavation and allows it to proceed to practically any depth, even well below the water table. Long reach excavators designed for slurry trenching are capable of depths up to around 90 ft (26 m). Beyond those depths, clamshell excavators may be used to go even deeper. Once the trench is completely excavated, a blend of soil excavated from the trench, dry bentonite, borrow soils, bentonite slurry and any other necessary additives are mixed at the surface and placed into the trench by a bulldozer or second excavator in a semi-fluid state, displacing the trench slurry forward. Once the backfill operation is complete, the SB backfill takes a slight set and acts like a soft clayey soil. A schematic of the process is below.

Backfill Blending for Soil Bentonite Backfill

SB backfill may be blended using a variety of different equipment. The most common and convenient method is to mix batches of backfill alongside the slurry trench using small excavators and/or bulldozers. The resultant mix should look like wet concrete (i.e. "slumpable") and is placed into the trench with an excavator.

**Placing SB Backfill in Slurry Trench**

Soil Bentonite Backfill Properties

The most important property of SB backfill is a low permeability. Typical values of SB backfill permeability are in the range of 10-6 to 10-8 cm/sec. A typical specification on an environmental project would require a permeability less than 1 x 10-7 cm/sec, but a levee or dewatering project may require a permeability less than 1 x 10-6. Either value is usually easily achievable with the right mix of materials. SB backfill has low strength and will remain soft, in the range of 300 psf (15 kPa), but this nearly always sufficient to maintain a vertical cut through the wall for subsequent installation of utilities and other light structures. For larger surface loadings like roads and structural foundations, it is usually recommended to remove the top few feet of the wall, dry as needed, and re-compact the soil over the softer SB to support larger loads.

When Soil Bentonite backfill is exposed to contaminants or aggressive groundwater, it normally performs fairly well. This is because most of the matrix is composed of inert and solid materials. Usually, the most important variables in SB mix design are bentonite content and grainsize distribution. Design mix studies will determine if contamination will affect the permeability enough to justify increased bentonite concentrations or other mix modifications.

**SB Backfill Relationship Between Fines Content and Permeability**

Learn More about Slurry Cut-Off Walls

Geo-Solutions has authored and participated in over 50 different papers on slurry walls. View our library to read and download several slurry cut-off wall technical papers.
Learn more from case studies on slurry walls in our Section on Case Studies.
Learn more and view pictures of our slurry wall equipment in our Section on Equipment.

Slurry Wall Sample Specifications

Read and download Geo-Solutions sample specs for soil bentonite slurry cut-off walls, cement bentonite slurry walls, soil-cement-bentonite slurry walls, composite slurry walls, bio-polymer slurry groundwater collector trenches.

Call Us Today

For help with slurry cut-off walls utilizing various types of bentonite slurry mixes, call us at 724-335-7273.