CT: The foundation preparation for the spillway structure consisted of stripping the overburden and exposing the bedrock. For the embankment sections, the soft, loose overburden was stripped to expose competent sands or gravels and a cutoff trench was excavated down to the rock surface. The cutoff trench is located under the crest of the dam and has a bottom width of 20 feet at the contact with bedrock. Near the left abutment, where the bedrock dips downward, the cutoff trench was extended to bedrock until the depth to rock exceeded 5 feet. The cutoff trench then became a key trench, excavated to a maximum depth of 5 feet.
S: USDI – http://www.usbr.gov/projects/Facility.jsp?fac_Name=Gray+Reef+Dam (last access: 2 December 2014)
N: 1. cutoff (n): 1640s, “act of cutting off,” also “portion cut off,” from verbal phrase cut off (late 14c.). Of rivers, from 1773; of roads, from 1806; of clothing (adj.), from 1840.
trench (n): late 14c., “track cut through a wood,” later “long, narrow ditch” (late 15c.), from Old French trenche “a slice, cut, gash, slash; defensive ditch” (13c., Modern French tranche), from trenchier “to cut, carve, slice,” possibly from Vulgar Latin *trincare, from Latin truncare “to cut or lop off” (see truncate).
2. A cutoff trench is a core located below the base of a dam or levee structure. The trench is filled with an impervious material, such as clay, to form a watertight barrier to prevent under-levee seepage.
S: 1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=cutoff&searchmode=none (last access: 2 December 2014); http://www.etymonline.com/index.php?allowed_in_frame=0&search=trench&searchmode=none (last access: 2 December 2014). 2. FEMA – http://www.fema.gov/media-library-data/1404150139578-ee3a6bc655a236dceba367adcfb8c982/FEMA_P312_Chap_8.pdf (last access: 2 December 2014).
GV: cut off trench, cut-off trench.