The most important geotcchnical parameter for soil liners or caps for waste facilities is hydraulic conductivity. In typical practice, the vertical conductivity normally governs the barrier effect. Regulatory agencies are increasingly requiring in situ tests in addition to laboratory- tests to verify hydraulic conductivity. Many different in situ tests have been proposed; seven have some usage. These are discussed with reference to their suitability for liner/cap evaluation. Of these, the Sealed, Double-Ring Infiltrometer and the Two-Stage Borehole test have received widest acceptance and use; each has already been used on 50 to 100 projects. Both tests are examined in detail, including theoretical basis, field procedures, and data reduction. Reasons for their preference over other methods are discussed.

The Sealed Double-Ring Infiltrometer measures one-dimensional infiltration from which vertical hydraulic conductivity is determined. Testing times are comparatively long but a larger volume of soil is tested, 1000 to 3000 times that typically tested in the laboratory. It is the better of the two methods for testing thin liners (<50 cm). One test is adequate for all but the most poorly constructed test media.

The Two-Stage Borehole method involves three-dimensional infiltration, from which both vertical and horizontal conductivities are determined. A typical study involves multiple tests, the total volume of soil affected by a 5-test group being 250 to 1000 times that typically tested in the laboratory. Testing times are comparatively short, and the method is the better of the two for evaluating variability in hydraulic conductivity with depth, on slopes, and where horizontal conductivity is needed. A five-test group is adequate for all but the most poorly constructed test media.

Comparative results are presented from 15 projects. The two preferred methods yield comparable values; both have been successful in verifying well-built barriers and identifying those with defects.