Formatted Title:

Laboratory Examination of Amended Sand Cap Placement

Background/Objectives:

In an effort to reduce cap thickness and extend the life of sand caps, amendments such as granular activated carbon are being added to cap designs, particularly for areas where upward groundwater advection occurs. The long-term performance of an amended cap depends on coverage of the area with a minimum required loading (kg/m²) of amendment. The design amendment dosage to achieve the minimum loading at all locations is a function of the uniformity of the amendment concentration and amended cap layer thickness obtained by the placement method. The differences in the physical properties of sand and granular activated carbon promote separation of the capping media when placed as a cohesionless bulk mixture. Separation and layering of the media components can occur even when selecting their respective particle sizes to have the same settling velocity. The capping media momentum during placement operations can be sufficient to resuspend or displace capping media, particularly the low density activated carbon fraction, resulting in the transport of the more erodible fraction down gradient and the separation and layering of the media components. Achieving a uniform loading when separation and layering occur becomes more difficult when placing a thin amended layer. The objective of this laboratory investigation was to examine a range of placement operations to determine impacts of placement method on the distribution of amendment loading and to identify successful approaches to place an amended sand layer with lower overdosing requirements.

Approach/Activities:

Various capping media components were explored using settling tests of mixtures of various sand and anode coke particle sizes in 2-L graduated cylinders. The screening tests yielded the desired formation of a well-dispersed deposit when capping media components had very similar particle settling velocity. The selected mixture consisted of 25% by volume (about 12% by weight) anode coke having a particle size of coarse sand and 75% by volume sand composed of 75% medium sand and 25% fine sand. Instantaneous releases of 0.59 cubic feet (ft³) of capping media mixtures from the top of 2-foot (ft) and 10-ft tall, 12-inch (in.) diameter columns to form a 4-in. thick amended sand layer were examined to see the effects of confinement. Following the confined tests three placement methods (instantaneous dump, tremie and raining) were examined in a 6 ft x 6 ft x 6 ft tank filled with 5 ft of water to examine their performance without lateral confinement. The tests were conducted in the tank having either a flat, level clay layer installed on the bottom or a clay layer with a 25% slope to examine the impact of bottom slope on performance. In each test, the volume of wetted capping media placed was 1.2 ft³, sufficient to form a mound of capping media with a 2-ft diameter and a 4.5-in. thickness or with a 3-ft diameter and a 2-in. thickness. After each placement test up to 30 cores were collected, and the thickness of the capping media was measured in each core. In addition, the capping media in each core was analyzed for its percent coke and percent sand by weight to examine the uniformity of amendment content spatially. The distribution of coke vertically in the core was visually characterized.

Results/Lessons Learned:

The results showed separation and layering of sand and amendment occurred in all of the laterally unconfined placement tests performed in the large tank, regardless of placement method. Placement on a sloped sediment bed increased the degree of separation. Distributed placement such as raining or a moving tremie with gradual buildup of the amended sand layer thickness in a confined area provided greater uniformity in the amendment loading.

Menu

Jesseca Alexander