Technischer Bericht NTB 85-24
Schrumpfversuche an Bentonit-Proben
Like other cohesive soils, bentonites exhibit shrinkage effects, i.e. when a wet sample is dried out the sample volume decreases even below the saturation limit; this can lead to cracks and therefore to a decrease in heat transport through the material. The shrinkage of samples of the bentonites MX-80 and Montigel identified by Nagra as potential backfill materials has been investigated. For this purpose the amount of shrinkage has been determined on uniaxially and isostatically compacted samples under conditions close to those expected in a repository as well as on samples initially at the liquid limit. The shrinkage limit is at a water content of about 14 %, resp. 18 %, the shrinkage s' expressed as a fraction of total sample volume amounts to about 9 % for MX-80 and to ll % for Montigel Pulver, as determined for samples initially at the liquid limit. The shrinkage s' of the compacted samples with water content w when shrinkage begins between 6 % and 12 % and corresponding dry density pd between 1.5 Mg/m3 and 2.0 Mg/m3 follows the equation
s' = s w ? d / ? w
where the value of s, as determined experimentally, is s = (36 + 4)%.