Foundry Sand in Landfill Liners and Covers

Most foundry sands being disposed in landfills are green sands. The primary components of green sand are silica and bentonite (Stephens and Dunes 1982). Thus, green sands are sand-bentonite mixture, which makes them potentially useful as liner and cover materials (i.e., hydraulic barrier layers).

Construction of landfill liners and covers using foundry sand should be performed according to the same specifications used in the construction of clay barriers. Less bentonite comparative effort is required to obtain low hydraulic conductivity, when the bentonite content is higher than 6%. The hydraulic conductivity of foundry sands is not sensitive to compaction water content. The relationships between hydraulic conductivity and compaction water content at different comparative effort should be obtained in the laboratory, and an acceptable field compaction zone should be developed based on these relationship.

Research at the University of Wisconsin-Madison suggests that foundry sands having bentonite content > 6% and liquid limit > 20%, can be compacted to hydraulic conductivities = 1 x 10-7 cm/sec. Foundry sands having bentonite content > 6% can achieve low hydraulic conductivity when moderately compacted and without a significant addition of water.
Green sand is being used in the construction of landfill liners and covers in few states. Additionally foundry sand is widely used as daily cover by the landfill operators. So the waste management industry is familiar with foundry by-products. The beneficial reuse of foundry sand in the construction of landfill barrier layers should be investigated. Foundries having their own landfills can also profit from such use.

Based on the previous review, the following conclusions and set of guidelines were formulated to assist engineers and regulators in designing, constructing, or evaluating barrier layers constructed with foundry sands:

Foundry sands having liquid limit > 20%, plasticity index > 3%, or bentonite content > 6%, have an excellent chance to be beneficially used in barrier layer construction. These simple index properties should be used as a screening process before engaging in any hydraulic conductivity testing.

Since the hydraulic conductivity of foundry sands is not as sensitive to compaction water content and compaction effort as that of clayey soils, construction of barrier layers with foundry sands can be performed with less water addition and probably with less comparative effort. Before any design, the relationships between compaction water content and effort should be determined in the laboratory. These relationships can then be used to design acceptable field compaction criteria.

During construction of barrier layers using foundry sands, the degree of saturation at compaction should be > 60%. However, foundry sands with higher bentonite content can achieve low hydraulic conductivities even at lower degrees of saturation.

Barrier layers constructed with foundry sands are expected to be unaffected by freeze-thaw and desiccation. Therefore, barrier layers constructed with foundry sands will perform better than similar layers constructed with clayey soils. However, the resistance to freeze-thaw and desiccation should be evaluated on a case by case basis.
Barriers layer constructed with foundry sand are expected to resistant to permeation with brines and MSW leachates, in the short term. Compatibility tests are being performed to confirm such chemical resistance in the long term.