Findings

Figures 1-4 illustrate the flow of water that occurred in the various test conditions. Figure 1 presents the flow rate when measured for all material types and specimen diameters in a condition of Plus 0 confine­ment (with shims equal to the thickness of the dry bentonite composite). Figures 2-4 show the same data with confinement at Plus 1/16, Plus 1/8, and Plus 3/16, respectively. A review of these figures indicates a significant difference in the performance of materials A, B, and C during the first half hour after exposure

FIG. 2—Plus 1/16 confinement.

to a hydrostatic head of 500 mm (19.7 in.). Also evident from review of the figures is that as the confine­ment is reduced (going from Plus 0 to Plus 3/16) there are fewer instances of bentonite stopping leakage during the test period.

For the tests reported in Fig. 1, concrete confinement is supported on shims equal to the dry thickness of the bentonite. Specimen numbers represent material type (A, B, or C) and diameter in inches (1, 2, 4, and 6.)

For the tests reported in Fig. 2, concrete confinement is supported on shims equal to the dry thickness of the bentonite plus 1.6 mm (1/16 in.). Specimen numbers represent material type (A, B, or C) and diameter in inches (1, 2, 4, and 6).

For the tests reported in Fig. 3, concrete confinement is supported on shims equal to the dry thickness of the bentonite plus 3.2 mm (1/8 in.). Specimen numbers represent material type (A, B, or C) and diameter in inches (1, 2, 4, and 6).

For the tests reported in Fig. 4, concrete confinement is supported on shims equal to the dry thickness of the bentonite plus 4.8 mm (3/16 in.). Specimen numbers represent material type (A, B, or C) and diameter in inches (1, 2, 4, and 6).

To illustrate how the flow rate changes as the diameter of the specimen changes, Fig. 5 shows combined data for all three material types. The results do not show a strong association between specimen size and the corresponding flow of water from the hole covered by the specimen.

Figure 6 shows how the flow rate changes as the confinement is reduced by increasing the thickness of shims that separate the bottom of the test box from the concrete pavers. The shim space dimension is the thickness of shims added to the base shim, which is the thickness of the bentonite. Material specimens of all sizes increased flow rate as the thickness of shims was increased.

Conclusions

Future efforts to develop standards for these bentonite composite sheets should anticipate the differences between materials. For example, specimens should be allowed to hydrate and stop leaking before hydro­static pressure is applied. The test report should include the time required to stop leakage with no addi­tional hydrostatic pressure.

The tightness of confinement is important to the sealing of bentonite composite sheet laps. Standards developed for application of bentonite composite sheets should set high standards for the flatness and smoothness of substrates. Material specifications may include tests to show the capacity of materials to overcome the challenge of irregular confinement.

In our test apparatus, with flat and parallel confining surfaces, the width of a specimen covering the test box hole was relatively unimportant to determining the rate at which bentonite stopped or slowed leakage. In this test apparatus even very small specimens were capable of stopping water flow. We caution against applying this to actual construction. It would be prudent to assume imperfect confinement—good in one spot but poor in another. Wider laps appear to offer a better likelihood of having a continuous line of effective confinement occur within the area of the lap.