The Collaborative International Dictionary of English v.0.48:

Earthwork \Earth"work`\, n.
   1. (Mil.) Any construction, whether a temporary breastwork or
      permanent fortification, for attack or defense, the
      material of which is chiefly earth.
      [1913 Webster]

   2. (Engin.)
      (a) The operation connected with excavations and
          embankments of earth in preparing foundations of
          buildings, in constructing canals, railroads, etc.
      (b) An embankment or construction made of earth.
          [1913 Webster]

WordNet (r) 3.0 (2006):

earthwork
    n 1: an earthen rampart

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "earthwork":
   abatis, advanced work, arch dam, backstop, balistraria,
   bamboo curtain, bank, banquette, bar, barbed-wire entanglement,
   barbican, barrage, barricade, barrier, bartizan, bastion,
   battlement, bear-trap dam, beaver dam, boom, breakwater,
   breastwork, brick wall, buffer, bulkhead, bulwark, casemate,
   cheval-de-frise, circumvallation, cofferdam, contravallation,
   counterscarp, curtain, dam, defense, demibastion, dike, ditch,
   drawbridge, embankment, enclosure, entanglement, escarp,
   escarpment, fence, fieldwork, fortalice, fortification, gate,
   glacis, gravity dam, groin, hydraulic-fill dam, iron curtain, jam,
   jetty, leaping weir, levee, logjam, loophole, lunette,
   machicolation, mantelet, merlon, milldam, moat, mole, mound,
   outwork, palisade, parados, parapet, portcullis, postern gate,
   rampart, ravelin, redan, redoubt, roadblock, rock-fill dam,
   sally port, scarp, sconce, seawall, shutter dam, stockade,
   stone wall, tenaille, vallation, vallum, wall, weir, wicket dam,
   work