The Use of Slag Cement in CB Seepage Cutoff Walls for Levees

摘要

A cement-bentonite (CB) wall is a common remediation measure for levees to mitigate underseepage conditions. It is commonly selected in areas where increased strength is needed, such as when the wall is located near the waterside levee toe or across features such as bridges, utilities, and roadways. To achieve the desired low permeability, granulated blast furnace slag cement has become a frequently used constituent in the self-hardening slurry. Thus, slag cement-Portland cement-bentonite (SCCB) walls have gained increasing popularity in levee projects. The use of slag cement results in both reduced hydraulic conductivity and increased strength. However, higher proportions of slag cement have a disadvantage of increasing brittleness of the wall. The State of Practice has evolved to using SCCB walls in levees without an in-depth understanding of the properties of SCCB and its long term performance. Considering the need for an improvement in the understanding of SCCB cutoff wall properties and their specifications, a mix design testing program has been implemented for recent United States Army Corps of Engineers (USACE) and Sacramento Area Flood Control Agency (SAFCA) projects in the Sacramento and Natomas areas in California. The mix design program evaluated 11 mix designs and has provided valuable information on the strength, stress-strain relationships, and hydraulic conductivity properties of SCCB walls with different slag cement/Portland cement and cement/water ratios. The slag proportions in total cement contents (slag cement + Portland cement) were varied between 65 percent and 91 percent and cement (slag cement + Portland cement)/water ratios were varied between 0.10 and 0.2 with a constant bentonite content of 5 percent of total weight. Available testing results were evaluated for hydraulic conductivity, strength, strain at peak stress, failure mechanisms, water loss with time, and raw material costs. The results of this testing program, recent research studies, and recent local project experience were used to develop considerations for specification criteria such as minimum and maximum limits of slag cement proportions, minimum cement/water ratio, minimum bentonite content, minimum strength at 3-days and maximum strength at 28-days.