Technical Challenge
Whether called a core trench, cut-off wall, vertical barrier, or containment curtain, the premise remains essentially the same: create a vertically-oriented hydraulic obstruction to prevent the lateral migration of water (and/or contaminant). In the context of an earthen dam rehabilitation, such an installation is intended to control surface/basin water and groundwater seepage through and under the existing berm. For environmental applications, a trench may be installed down gradient of a known contaminant source (e.g. a landfill) to prevent the migration of contaminated water off-site or up gradient of a contaminated zone to prevent the flow of non-contaminated groundwater through the impacted area.
Conventional geotechnical methods vary, but typically involve the excavation of a narrow trench, generally three to five feet wide, that is keyed into a low-permeable native substratum (e.g. clay) to form a continuous barrier. Clayey soils or engineered slurry backfill of low hydraulic conductivity are then placed into the trench to complete the cut-off.
Traditional backfills range from unaltered clayey soils to bentonite-based liquid slurries and grouts to soil-cement-bentonite mixtures. Inherent challenges with these most commonly used materials include:
Physical Consistency
Field blending of multiple physical “ingredients” can be time-consuming and logistically challenging, even with specialized equipment. Uniform mechanical compaction of soil backfills typically requires a staged approach in thin lifts with repeated testing lifts.
Shrinkage
Most clayey soils and blends involving cement-based additives contract if given the opportunity to dry, and they do not have the swell potential to rebound when rehydrated. Bentonite-based formulations added in suspension (e.g. in a hydrated state) have already expanded, so rather than swelling in-place, they are also prone to shrinkage over time.
Structural Stability
Many backfills must be extensively and uniformly compacted (mechanically) to achieve the desired results. Because of the low percentage of solids, particularly with slurries and grouts, other backfills can migrate out of the trench even as they are being pumped into position (particularly in strata with large voids and high hydraulic recharge).
AquaBlok® Solution
In many cases, a backfill material with more structural strength is desired, particularly where subsurface flow rates are high, surrounding soils are less stable, or where loads must be supported (e.g. atop earthen dams or dikes). At 80% solids by weight, AquaBlok behaves much like the crushed stone aggregate that makes up its core composition.
As a ready-mixed blend of high-grade, Wyoming-derived sodium bentonite composited over individual stones, the AquaBlok product balances extremely low permeability (10-8 to 10-9 cm/s) with consistent structural integrity at a known and consistent ratio. Stable barriers can be achieved with as little as a 4-inch trench thickness, depending on trench depth and surrounding soil characteristics. However, the typical AquaBlok cut-off wall is designed to be 12- to 24-inches thick to accommodate standard equipment and to achieve required trench depths. Even at these dimensions, the reduction in trench width compared to conventional methods minimizes site disturbance, reduces material handling quantities, and lowers installation costs. No field blending or specialized conveyance equipment is required, adding to the ease of installation. Material can be simply gravity dropped into a trench, even if water is actively filling the trench.
Once the material is in place, significant hydration takes place in hours and essentially full hydration and swell can take several days depending on availability of source water. As the clay expands to fill internal pore space between the individual stones, it also swells laterally to self-key and form a durable bond with a trench side wall or water control structure surfaces (e.g. concrete, sheet pile, etc.).
Equipment Needed
Long-reach excavator and radial trenchers (e.g. Ditch Witch®) are the most commonly utilized machinery for excavation of the trench. Material can either simply be gravity-dropped from its shipping bag, placed using a payloader bucket, or conveyed into a trench from a stone slinger.
Material Needed
A dry bulk density of 90-lbs/ft3 along with the trench dimensions – length (L) x depth (D) x width (W) are all that are necessary to calculate material need.
Example: A vertical cut-off wall that is 25-feet long (L) x 10-feet deep (D) x 1-foot wide (W) = 250 ft3 @ 90-lbs/ft3 = 22,500 lbs or 11.25 tons or 9.4 bulk bags (at 2,400-lbs/bulk bag)
Many engineers and contractors will add a safety factor to account for variation in trench width (for the example described above, +5% would add 1,125-lbs to the 22,500-lbs estimated for a total of 23,625-lbs). At 2,400-lbs per bulk bag, the material need for this example would be just shy of ten (10) bulk bags.
Tips & Observations
Required depth and existing soil conditions drive trench width. Generally, trenches less than 5-feet deep can be constructed by trenchers capable of achieving 6- to 12-inches in width, trenches 6- to 12-feet deep are typically excavated using 12- to 18-inch wide buckets, and trenches 13-feet deep or deeper are often designed for 18- to 36-inches width.
If native soils are less stable and there is an increased risk of vertical slumping of the trench side walls during construction (resulting in an increase in the average trench width), then a larger safety factor (e.g. 10 to 15-percent) can be added to account for the increased material need.
Cut-off barriers should be cut into sound, low-permeability soils on the ends and base of the trench whenever possible to restrict water migration around the vertical wall.
Barriers can always be extended (in length) if seepage is experienced beyond the limit of the existing trench. New excavation can simply tie into the existing material and the product will hydrate together to form an uninterrupted bond.
Blending material with native soil is not recommended for this application.
Mechanical compaction will not hurt product performance but is not typically necessary, neither in a dry nor hydrated state.
Except in arid zones, the AquaBlok does not need to by wetted once placed in the trench. It will draw sufficient moisture from its surroundings. If the product is intended to be 12-inches thick (or thicker), and the AquaBlok is being added into a dry environment, it should be wetted to best ensure that the internal material has the opportunity to hydrate.
Despite the fact that AquaBlok hydrates in all directions, increasing adhesive and frictional force against the trench side walls, the material has not been known to mound (or slump) on the surface.
In many cases, the top 6- to 12-inches of a trench can be backfilled with topsoil to allow for re-vegetation without negatively impacting performance.
From the Field
Tontogony Creek Lagoon Cutoff Wall (Tontogony, OH)