Hydraulic & Chemical Properties of Geosynthetic Clay Liners Exhumed from Dry Covers

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1 Hydraulic & Chemical Properties of Geosynthetic Clay Liners Exhumed from Dry Covers Craig H. Benson, PhD, PE, DGE Wisconsin Distinguished Professor and Chair Geological Engineering University of Wisconsin-Madison

2 Geosynthetic Clay Liners Rapid installation relative to compacted clay barrier 2

3 Geosynthetic Clay Liners For low hydraulic conductivity, Na bentonite granules must swell to from a gel (paste). Typical Cross-Section Gel must be maintained to retain low hydraulic (~ m/s) conductivity. Upper Geotextile Granular Bentonite Lower Geotextile If granules do not swell and form gel, higher hydraulic conductivity (>10-7 m/s).

4 Importance of Bound Cation Valence Na-Bentonite in DI Water (monovalent) crystalline + osmotic hydration Na-Bentonite in Calcium (Ca 2+ ) Rich Water (divalent) crystalline hydration only.

5 Wisconsin GCL Failure Leakage monitored with lysimeter (collection pan) directly beneath GCL

6 1000 Lysimeter 1 - Open Symbols 800 Lysimeter 2 - Solid Symbols Cumulative Percolation (mm) st Reconstruction 2 nd Reconstruction Lysimeter 2 2 nd Reconstruction Lysimeter 1 Composite GCL Data Time (years)

7 Dry Cover with GCL Barrier Only GCL exposed to wet-dry cycling with seasonal changes in meteorology. Bentonite interacts with pore waters in adjacent soils, potentially replacing Na with divalent cations like Ca and Mg. 7

8 Wet-Dry Desiccation Cycling & Hydraulic & Hydraulic Conductivity Conductivity of GCLs Hydraulic Conductivity (m/s) DI DI-CaCl 2 Tap-CaCl 2 CaCl Number of Wetting Cycles

9 Cover Profiles GCLs in Soil Covers 150 mm sandy lean clay 600 mm silty sand 800 mm silty sand 900 mm wellgraded sand with silt and gravel 800 mm silty sand Control GM GCL 150 mm sand GCL clay GCL clayey sand silty sand Site N Site D Site S Site O

10 Free Swell and Exchange Complex Free Swell (ml/2g) Earthen Composite Earthen Earthen Site N Site S Site D Site O New Ca GCL Bentonite Mole Fraction of Na in Exchange Complex Earthen Composite Earthen Earthen Site N Site S Site D Site O New GCL - Bentonites transformed from Na to Ca-Mg bentonite - Even GCL overlain by geomembrane has Ca-Mg bentonite (diffusion of Ca & Mg from underlying subgrade?)

11 10-5 Hydraulic Conductivity (m/s) Typical New GCL Site S Site N Site D Site O GCL+GM GCL only 11

12 Importance of Water Content Hydraulic Conductivity (m/s) Field Condition Site N Site S Site D Site O Hydraulic Conductivity (m/s) Laboratory Drying Tests - Field Samples Dry w/ Overburden Dry w/o Overburden Field Condition In Situ Water Content (%) Gravimetric Water Content (%) - Hydraulic conductivity tied to water content of GCL, even though complete exchange of Mg/Ca for Na. - Drying wetter specimens resulted in large increase in hydraulic conductivity.

13 Exhumed GCL where Na has been replaced by Ca and Mg. Remnant Granule Loss of swell precludes healing of desiccation cracks. Some granules visible.

14 Exhumed GCL showing cracks and remnant granules.

15 Key Points Cation exchange (Ca/Mg for Na) occurred in ALL GCLs (with and without overlying geomembrane). Na bentonite in all GCLs converted to Ca/Mg bentonite. GCLs with low water content had high hydraulic conductivity (with and without geomembrane). Desiccation or lack of hydration. Preferential flow thru cracks or around remnant granules. GCLs that retained higher water content had low hydraulic conductivity. 15

16 Landfill Final Cover GCL-GM Composite Barriers Is Site S an anomaly? Does GM protect the GCL? Does cation exchange still occur? Do GCLs in composites retain low hydraulic conductivity? 16

17 Sites for Exhumations Site B Site E Site F Site A 17

18 Swell Index (ASTM D 5890) Typical Na Bentonite Swell Index (ml/2 g) Composite GCL GCL-only 10 Typical Ca Bentonite 5 A B E-01 E-02 F-03 F-05 S D N O Site 18

19 Bound Monovalent Cation Fraction Mole Fraction Monovalent Cations, X m Composite GCL GCL-only Typical New GCL 0.0 A B E-01 E-02 F-03 F-05 S D N O Site 19

20 Exhumed Water Content 250 Exhumed Water Content (%) Composite GCL GCL-only 0 A B E-01 E-02 F-03 F-05 S D N O Site 20

21 Hydraulic Conductivity (ASTM D 5084) Hydraulic Condcutivity (m/s) Composite GCL GCL-only A B E-01 E-02 F-03 F-05 S D N O GCL Source Typical New GC 21

22 SW Hydraulic Conductivity & Exhumed Water Content 10-6 Hydraulic Conductivity (m/s) Lower K Higher K GCLs with higher water content had low hydraulic conductivity, even if Ca/Mg replaced Na Exhumed Gravimetric Water Content (%) 22

23 Importance of Subgrade Water Content Exhumed GCL Water Content (%) Lower K Higher K Subgrade Soil Water Content (%) GCL water content increases with subgrade water content. 9% is threshold for hydraulic conductivity corresponding to crystalline vs. osmotic hydration. 23

24 Lessons Learned GCLs beneath geomembranes undergo cation exchange due to Ca and Mg in subgrade and from dissolution of calcite in the bentonite. Become Ca-Mg bentonites. Hydraulic conductivity of GCLs in composite barriers varies significantly with hydration condition. Adequate subgrade water content (>9%) required to ensure osmotic swell and low hydraulic conductivity of the bentonite. Low hydraulic conductivity even if cation exchange occurs provided bentonite undergoes osmotic swell prior to exchange. Simple message get the GCL wet and keep the GCL wet 24

25 Practical Implication Percolation Rates from Covers with Geomembrane-GCL Composite Barrier Location of Study Duration of Study (yr) Average Annual Precipitation (mm/yr) Average Percolation Rate (mm/yr) Apple Valley, CA Boardman, OR Cassville, WI Geomembrane-GCL barriers are very effective hydraulic barriers in dry covers. 25

26 Questions? 26