Tom Stam and Kent von Maubeuge
As Bentofix® Geosynthetic Clay Liners are frequently used in areas subjected to freezing temperatures, a study was performed to evaluate the effects of numerous freeze-thaw cycles.
Research conducted by Zimmie1 as well as Chamberlain, Erickson and Benson2 indicates the tendency of compacted clay liner hydraulic performance to seriously degrade when subjected to several freeze/thaw cycles. The purpose of the study on the Bentofix® was to demonstrate that, because of the highly plastic nature of bentonite as well as it’s ability to absorb and discharge water, the hydraulic properties of the Bentofix® would not degrade under these conditions.
As most laboratory test programs have difficulty replicating the single direction freezing progression that occurs naturally (Diagram 1), it was a relevant concern when developing the test program utilized for the examination of a Bentofix® specimen. However, a progression of freezing from the surface, downward or a single dimensional freezing process, as opposed to freezing the sample from all directions or 3-D (Diagram 2), has been demonstrated in other soil freeze-thaw studies to not have any significant effects on the validity or consistency of the resultant test data (LaPlante & Zimmie ).
Single dimensional freezing was also considered to be very time consuming and difficult to replicate in the laboratory, while a multi-directional approach would facilitate a greater number of cycles that could be tested in a reasonable amount of time. It was therefore concluded that a simple multi-directional freezing approach was reasonable for the series of tests performed in this examination.
A 10 cm Bentofix® sample was prepared and assembled for testing in general accordance with ASTM D 5084 "Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter". The test conditions for each cycle of the test program were an effective confining stress of 5 psi, and a head pressure of 2 psi so that a correlation to standard index testing could be drawn.
For each cycle, the sample was hydrated for 48 hours, then permeated to steady state conditions in distilled water. Upon reaching steady state conditions, the sample was carefully removed from the permeameter, and placed into a standard commercial freezer. In order to minimize sample disturbance, the sample was left in the flexible membrane, between the porous stones, and weighted under a small load to closely replicate the test conditions. The results of the freeze-thaw cycles completed to date are presented in Table 1.
It appears that while most compacted clay materials are detrimentally
effected by repetitive freezing and thawing [1], the sodium bentonite component of the Bentofix® reacts differently than typical clayey materials. While a portion of the water in the bentonite freezes into ice lenses, the moisture is readily absorbed back into the bentonite upon thawing. It is interesting to note that when a trend line is applied, the performance of the Bentofix® appears to be gradually improving from already acceptable permeabilities. Current theories suggest that the cycling has the tendency to more effectively orient the clay platelets to inhibit vertical flow.
As the saturated bentonite has such a high initial moisture content, the slight loss in moisture during freezing would not appear to have any adverse effects on the frozen hydraulic conductivity of the sample, however, there is of course no way to measure this property as the aqueous based permeant would also be subject to freezing.
Similar tests were carried out at The Technical University of Munich_. Bentofix® GCLs were submitted to frost (24 h) – thaw (24 h) cycles (4 h at –5°C / 4 h at –10°C / 16 h at –15°C / 24 h at +5°C). The performance of Bentofix GCLs® was not affected due to frost/thaw cycles.
As the data from this test indicates, Bentofix® is resistant to the effects of repetitive freeze-thaw cycling. Based upon this series of tests, it would appear that Bentofix® is suitable for use in areas subjected to repetitive freeze-thaw cycling, and does not require frost protection measurements to be implemented.
