Hydraulic Conductivity of Two Prototype Clay Liners

Two prototype liners were constructed at a site near Austin, Texas, using clays of low and high plasticity. The clays were compacted to 100% of standard Proctor density at a water content slightly wet of optimum using a sheepsfoot roller. The overall hydraulic conductivity (k) of each liner was determined by ponding water on the liners and measuring the rate of seepage. The field‐measured k's of the liners were surprisingly high ($4\times {10}^{-6}$ and $9\times {10}^{-6}\mathrm{cm}/s$). After water in the ponds was removed, laboratory permeability tests were performed on hand‐carved samples obtained from the liners, on samples obtained with a thin‐walled sampling tube, and on laboratory‐compacted samples. Field permeability tests were also performed with ring infiltrometers. The tests showed that: (1) Essentially all of the laboratory tests, even on undisturbed samples, produced a measured k that was approximately 1,000 times less than the field‐measured k; and (2) ring infiltration tests showed considerable scatter but the average k was close to the overall field‐measured k. The findings raise important questions about whether laboratory permeability tests on compacted clay are relevant to clay liners and reinforce previous suggestions that compacted clay liners may contain numerous hydraulic defects such as fissures, slickensides, zones of poor bonding between clods of clay, and zones of relatively poor compaction. The desirability of field permeability tests is evident from the results reported.