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These sodium bentonite clay granules are the same product that is often referred to as an "Engineering Grade Bentonite" the type that is extensively used for oil and geothermal drilling muds. Here the ratios of clay to water (and other additives) are closely monitored, however, for the sealing of ponds, streams, and generally preventing the passage of ground water the application is more of an art than an exact science.
The granules can be introduced direct into the ground, allowing for natural moisture to be taken up by the clay. For example low permeability vertical barriers in bunds and dams can be achieved by cutting a slit trench and simply pouring the granules into the excavation. (or, "sandwich" the granules between geosynthetic (weed) membrane to provide a highly effective clay barrier) Moisture in the ground will cause the granules to swell and form a seal. A vertical trench filled with bentonite can also be particularly effective to constrain root growth around newly planted trees, the bentonite barrier preventing damage to foundations and service ducts.
For natural ponds a detailed explanation of using the dry clay granules to seal a pre-formed hollow can be found on the internet at www.youtube.com/watch?v=7ltqNH7eoEM. The process involves mixing the granules into the sub-soil and then compacting the mix. The video explains the need to keep the sides of the pond relatively shallow and importantly (the part 2 video) clearly demonstrates the best way to fill the completed structure so as not to disturb the seal.
This method, or a variation where the clay granules are dry "pre-mixed" with excavated soils in a cement mixer, re-introduced into the pond shape and then compacted with a wacker plate (or hob nail boots like 18th century canal builders) does require dry weather to prevent the whole project degenerating into a sticky mess !
Unfortunately no absolutes can be given with regard to the proportion of granules to sub-soil for either an in-situ or a premix method, this needs to be established on site with the actual soils being excavated. However, when using granules a mix of one pack (20kg) worked into the top 50mm (2inch) of sub-soil should provide a seal for up to a square metre of typical "sandy clay" material.
Sodium Bentonite is also used in electrical and lightening earth pit and earth ring installations as a non corrosive linkage to natural soils. The clay holds moisture within its mineralogical structure and thus mitigates against seasonal variation in ground resistance levels.
Alternatively for some applications the granules can be mixed with up to 1.5 times their own weight of water to form a sticky and dense "puddling clay". Add water to the granules and let them hydrate for an hour or so, the clay can then be kneaded into shape. For example clients have sealed disused land drains using this method, and on older buildings provided a external vertical "damp course" against stonework and prevented the ingress of damp where utilities have been entered through masonry.
A single 20kg pack will produce in the order of 40ltr of sticky clay - sufficient to cover an area of two Sqm approximately 20mm thick.
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