Cut-off Walls. Over the next two months we will publish in this blog a discussion of the pros and cons of each of the major methods of constructing cut-off walls. These include:
1. Slurry Trench Method
2. Auger Soil Mixing Method (commonly referred to as the DSM or Deep Soil Mixing Method)
3. One-Pass Trench Method
4. TRD
5. Jet Grout Method
Each method offers certain advantages and disadvantages, and our industry has done itself a disservice by overselling a given method on the wrong site. Hopefully, this blog help facilitate discussion within our industry about the appropriate use for these technologies.
The soil bentonite slurry trench cutoff wall has been the gold standard for cutoff walls. The USACE and the EPA have tested, drilled, sampled, and evaluated the soil bentonite cutoff wall for use in dikes, dams and for pollution control. The soil bentonite slurry trench cutoff walls have had proven success since the early 1950’s, so we should examine the reasons for this success. What parameters do we need to build a good cutoff wall? We will look first at quality control.
Key quality control parameters for a soil bentonite slurry trench cutoff wall are:
• Gradation Range – Well graded or just 200 mesh requirement
• Organic content – 1%
• Mixing Control – Visual on 100% of mix
• Bentonite Content
• Key-In Verification
• Excavation Continuity
• Permeability
Gradation: Studies have shown that a well graded material has a high unit weight and a small amount of pore space. The high unit weight exhibits small consolidation (important under a dike or dam). The high unit weight also resists displacement from high head differentials. A well graded backfill with low PI clay will provide a backfill with the lowest change in permeability from chemical attack.
Organic Content: Normal requirement is less than 1% by weight organic material in backfill. The obvious reason is future breakdown of organic material leading to increase in permeability.
Mix Control: Visual observation on 100% of backfill material! There should be no unmixed lumps greater than 2” to 6” depending on specifications.
Bentonite Content: Control varies by project requirement. If backfill permeability is controlled by bulk clay than bentonite content can be shown by change of slump or by verification of bentonite used. If permeability reduction is driven by bentonite addition than batching controls and or methylene blue testing may be needed to control bentonite content.
Key-In Verification: Visual identification of top and bottom of key by excavation cuttings should be under the observation of a geotechnical engineer and or inspector. Control of exact key depth and quality are documented and archive samples of the key material obtained.
Excavation Continuity: Complete access to move excavation tools back and forth in trench to sound and sample all locations in excavation.
Permeability: Complete access to all mixed backfill and the ability to control all key parameters. Permeability results are easy to relate to gradation results and bentonite content.