Lesson 1/Learning Event 2
TABLE 4. ASSIGNED CBR RATINGS FOR BASE COURSE MATERIALS
ments. Satisfactory base materials often can be produced by blending materials from two or more
deposits. 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. A base
course made from sandy and gravelly material that meets the requirements given in base course
requirement has a high bearing value and can be used to support heavy loads.
Sand-clay. Sand and clay in a natural mixture may be 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 top
to 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 due to the mica content. With proper
proportioning and construction methods, satisfactory results can be obtained with sand-clay. It is
excellent in stage construction where a higher type of surface is to be added later.
Stabilized soil mixtures. The stabilization of soils by various methods is discussed in detail in EN
5453 and TM 5-530. All principal types of stabilized soils mixtures can be used as base courses beneath
bituminous wearing surfaces.
Processed materials. 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 proc-