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4 overburden and riprap. The gradation specifics depend on the character of the surrounding soils. Proper compaction is required for all layers. See Figure 2 for an example reverse filter. Geotextile, Geosynthetic Clay Liner, or Geomembrane - Similar to the clay blanket and reverse filter, engineered liners and fabrics can be used after excavation of the sinkhole. Native material should be placed over the liner or fabric. Liners and fabrics can be used in conjunction with reverse graded filters as another layer of protection. Grouting - Grouting is a repair option especially effective if widespread cracks, voids, and joints are found within the foundation or embankment. Grout is generally applied with low pool and under dry conditions so that the grout can set up. Various mixes are available for grouting and selection of properties is dependent on soil conditions, size of sinkhole, and environmental conditions. Mixtures with fluid-like consistency (low viscosity) take longer to set up but are likely to penetrate more deeply and into smaller voids. Thicker, more viscous mixtures are quicker to set up but may not be as effective at filling the void. A combination of mixes can be used to fill the smaller and deeper areas with fluid grout and then the larger void with more viscous grout. Depending on the size of the sinkhole, soil conditions, and the grout material, the grout can be gravity fed (pipe placed in hole for funneling) or injected under pressure. The hole and pipe are often flushed with water to remove debris and loose material prior to application of grout. When setup is complete, the area should be backfilled with native soil and rocks/riprap placed to protect the grout near the surface. There are two general families of grout: cement grouts and chemical grouts. Each family has primary grout sub-types. Primary types of cement grout include ordinary Portland cement (OPC) and ultrafine cements. Primary types of chemical grouts include silicates (typically sodium silicates), acrylic gels (acrylamide, acrylic, and acrylates), and polyurethane foams (hydrophilic or hydrophobic). Each grout type varies in appropriate application based on their unique characteristics including viscosity, set-up time, expansion, flexibility, strength, life span, and cost. Due to the wide array of grout products available and the complexity of selecting the appropriate grout mix, a qualified grouting engineer or contractor should be engaged to provide site-specific recommendations. Photo 4: Application of chemical urethane grout (Big Battlement Dam) Below is a list of grout types commonly used in geotechnical void-filling applications: • Acrylamide Grout – Highly impermeable and low viscosity. Changes from liquid to a solid in a controllable set-up time. Acrylamides generally have a long life span (100 years+). [5] • Polyurethane Chemical Resin Grout – Hydrophilic foams react with water to form an expansive flexible foam or non-expansive gel that hardens. It can be used for grouting applications where dry conditions are not possible or too costly. Hydrophobic expansive foams require little water to react and easily withstand wet/dry cycles. Polyurethane foam generally has a life span between 75 and 100 years. [5] • Epoxy Grouts – Suitable for underwater applications • Ultrafine Cementitious Grout – Cement grouts generally cost less than chemical grouts and have higher strength. Cement grouts have higher viscosity and faster set-up times, which can be controlled to a degree with water and additives. Life span is typically 100 years+. [5]