The TRD Method for In Situ Mixed Vertical Barriers

The conventional slurry trench methods of constructing low-permeability vertical barriers have been widely used, namely, soil-bentonite (SB), cement-bentonite (CB) and slag-CB. More recently, in situ mixing methods to construct vertical barriers, such as Deep Soil Mixing (DSM) and the Trench Remixing and Deep wall method (TRD) have found increased usage. The TRD method was has been widely employed in Japan for nearly 15 years but has only recently been used in the United States. The TRD method is a one-phase process that involves the simultaneous, full-depth cutting and mixing of in situ soils with additives to create, in place, a continuous soil mixed wall. A blend of cementitious materials (granulated ground blast furnace slag and Portland cement) and slurry are injected as the milling/cutting proceeds horizontally resulting in continuous vertical mixing of in situ soils with the injected materials, without creating the open trench typical of other walling methods. To date, three TRD projects have been completed in the United States. The focus of this paper is the most recent project completed on the Herbert Hoover Dike (HHD). Other projects include 1) the first TRD project in the United States where the method was used to construct a test cell under saline ground water conditions for use as a soil mixed barrier wall to prevent the intrusion of salt water into fresh ground water aquifers and 2) a cutoff wall to prevent leakage of secondary treated sewage from a containment pond into surrounding ground water.