Flexible  pavements  are  generally  satisfactory  for runway interiors, taxiways, shoulders, and overruns. Rigid pavements or special types of flexible pavement, such as tar rubber, should be specified in certain critical operational  areas. MATERIALS Select  materials  will  normally  be  locally  available coarse-grained soils, although fine-grained soils maybe used in certain cases. Lime rock, coral, shell, ashes, cinders,  caliche,  disintegrated  granite,  and  other  such materials  should  be  considered  when  they  are economical. Subbase Subbase  materials  may  consist  of  naturally occurring  coarse-grained  soils  or  blended  and  processed soils.  Materials,  such  as  lime  rock,  coral,  shell,  ashes, cinders, caliche, and disintegrated granite, maybe used as  subbases  when  they  meet  area  specifications  or project  specifications.  Materials  stabilized  with commercial admixes may be economical as subbases in certain instances. Portland cement, cutback asphalt, emulsified asphalt, and tar are commonly used for this purpose. Base  Course A wide variety of gravels, sands, gravelly and sandy soils, and other natural materials such as lime rock, corals, shells, and some caliches can be used alone or blended  to  provide  satisfactory  base  courses.  In  some instances, natural materials will require crushing or removal of the oversize fraction to maintain gradation limits. Other natural materials may be controlled by mixing  crushed  and  pit-run  materials  to  form  a satisfactory  base  course  material. Many  natural  deposits  of  sandy  and  gravelly materials  also  make  satisfactory  base  materials.  Gravel deposits  vary  widely  in  the  relative  proportions  of coarse and fine material and in the character of the rock fragments. Satisfactory base materials often can be produced  by  blending  materials  from  two  or  more deposits. Abase course made from sandy and gravel] y material has a high-bearing value and can be used to support   heavy   loads.   However,   uncrushed,   clean washed  gravel  is  not  satisfactory  for  a  base  course because the fine material, which acts as the binder and fills  the  void  between  coarser  aggregate,  has  been washed away. Sand and clay in a natural mixture maybe found in alluvial deposits varying in thickness from 1 to 20 feet. Often  there  are  great  variations  in  the  proportions  of sand  and  clay  from  the  top  to  the  bottom  of  a  pit. Deposits of partially disintegrated rock consisting of fragments of rock, clay, and mica flakes should not be confused  with  sand-clay  soil.  Mistaking  such  material for sand-clay is often a cause of base course failure because of reduced stability caused by the mica content. With  proper  proportioning  and  construction  methods, satisfactory  results  can  be  obtained  with  sand-clay  soil. It is excellent in construction where a higher type of surface is to be added later. Processed materials are prepared by crushing and screening  rock,  gravel,  or  slag.  A  properly  graded crushed-rock  base  produced  from  sound,  durable  rock particles makes the highest quality of any base material. Crushed rock may be produced from almost any type of rock that is hard enough to require drilling, blasting, and crushing. Existing quarries, ledge rock, cobbles and gravel,  talus  deposits,  coarse  mine  tailings,  and  similar hard, durable rock fragments are the usual sources of processed materials. Materials that crumble on exposure to air or water should not be used. Nor should processed materials be used when gravel or sand-clay is available, except when studies show that the use of processed materials will save time and effort when they are made necessary by project requirements. Bases made from processed  materials  can  be  divided  into  three  general types-stabilized,  coarse  graded,  and  macadam.  A stabilized base is one in which all material ranging from coarse to fine is intimately mixed either before or as the material is laid into place. A coarse-graded base is composed of crushed rock, gravel, or slag. This base may  be  used  to  advantage  when  it  is  necessary  to produce crushed rock, gravel, or slag on site or when commercial aggregates are available. A macadam base is one where a coarse, crushed aggregate is placed in a relatively thin layer and rolled into place; then fine aggregate or screenings are placed on the surface of the coarse-aggregate  layer  and  rolled  and  broomed  into  the coarse rock until it is thoroughly keyed in place. Water may be used in the compacting and keying process. When water is used, the base is a water-bound macadam. The  crushed  rock  used  for  macadam  bases  should consist of clean, angular, durable particles free of clay, organic  matter,  and  other  objectional  material  or coating.  Any  hard,  durable  crushed  aggregate  can  be used, provided the coarse aggregate is primarily one size and the fine aggregate will key into the coarse aggregate. Other  Materials In a theater of operations where deposits of natural sand and gravel and sources of crushed rock are not available,  base  courses  are  developed  from  materials that  normally  would  not  be  considered.  These  include 3-18