text: FM 7-100.1________________________________________________________________________________
  SURVIVABILITY
  10-31. Preparing fortified positions is a task for engineers in both the offense
  and defense. Fortified positions increase weapons effectiveness and protect
  personnel, weapons, and materiel. Engineers give priority to digging in CPs
  and key components of the OPFOR’s combat power. Fortification preparation
  combines and uses to best advantage the terrain’s protective properties, local
  construction materials, and engineer excavation equipment. The C3D meas-
  ures discussed above, under Support to Information Warfare, also contribute
  to survivability.
  OFFENSE
  10-32. In preparation for offensive action, the primary use of field fortifi-
  cation is in the preparation of assembly areas. Even there, the tasks of
  preparation typically exceed the capability of engineers in the limited time
  available. Consequently, the preparation of assembly areas becomes a
  shared responsibility involving all available personnel and equipment of
  all branches.
  10-33. Normally, the OPFOR locates assembly areas far enough from enemy
  forces to deny the enemy ground observation and to lessen direct-fire effects.
  It uses field fortification in a way that allows a smooth and protected move-
  ment of troops and supplies in and out of the assembly areas.
  DEFENSE
  10-34. When the OPFOR is transitioning to the defense and preparing com-
  plex battle positions or sanctuary areas, advance engineer deployment allows
  better use of terrain features and constructed fortifications. Engineers also
  have more time to construct or improve routes for movement of troops and
  supplies and to conceal forces and caches or short-duration storage facilities.
  In most cases, engineer units must concentrate their effort on only the most
  important parts of the AOR.
  10-35. The full preparation of defensive positions involving entrenchments,
  communications trenches, positions for tanks and infantry vehicles, and pro-
  tected CPs is a labor-intensive process. It often exceeds the capability of pure
  engineer units. Consequently, the OPFOR’s approach is to use all available
  personnel and equipment. Units of all arms and services receive training in
  preparing field fortifications and emplacements.
  COUNTERMOBILITY
  10-36. Creating engineer obstacles and carrying out demolition activities are
  significant engineer functions in all phases of combat. The obstacle plan is
  tailored and integrated into the overall operation plan. Engineer obstacles in-
  clude any actions taken to inflict losses and to delay and impede enemy
  movement. In the offense, obstacles protect flanks, disrupt counterattacks,
  and strengthen captured positions. In the defense, engineer obstacles may
  strengthen the defense, disrupt enemy operations, and cover gaps.