2018 Volume 127 Issue 3 Pages 391-407
Trenching surveys (excavating a ground surface and observing the exposed strata) are methods for immediately comprehending shallow subsurface stratigraphy, geological structure, and properties of strata. The most suitable survey method is presented for observing liquefied and fluidized strata under the ground, and for considering liquefaction countermeasures in an alluvial lowland where man-made strata are sporadically distributed and the groundwater table is generally shallow. Also presented is an outline of the procedure. To narrow the trench site, it is essential to analyze topographical data, historical record of liquefaction, and subsurface geology based on drilling data and groundwater table. Prior to excavating, sheet piles are driven and tightly arranged around the site. Then, riser pipes and a centrifugal pump are set into the trenching area to lower the groundwater level (the well-point method). During the process of dewatering and discharging water, environmental influences on surrounding areas should be considered. The depth of a trench in practice is 3 to 4 m or less. If it is necessary to obtain deeper strata, long specimens oriented via special sheet pile tool sampling can be used. After making observations of stratigraphy and structure of normal layers or liquefied and fluidized layers exposed on trench walls, it is useful to make relief peel specimens with a grout material to describe detailed liquefaction and fluidization features. It is proposed that exhibiting peel specimens is effective for understanding liquefaction and fluidization processes. Samples of liquefied strata for some examinations can be collected directly from trench walls.