Select materials will normally be locally available, coarse-grained soils (classified G or S), although fine-grained soils in the ML and CL groups may be used in certain cases. Consider lime rock, coral, shell, ashes, cinders, caliche, and disintegrated granite when evaluating sources of select material.

Subbase Materials
Subbase materials may consist of naturally occurring, coarse-grained soils or blended and processed soils. Lime rock, coral, shell, ashes, cinders, caliche, and disintegrated granite may be used as subbases when they meet the requirements.The existing subgrade may meet the requirements for a subbase course, or it may be possible to treat the existing subgrade to produce a subbase. Do not admix native or processed materials unless the unmixed subgrade meets the LL and PI requirements for subbases.
A suitable subbase may be formed using material stabilized with commercial admixtures. Portland concrete, hydrated lime, fly ash, and bituminous materials are commonly used for this purpose. The plasticity of some materials can be decreased by adding lime or portland concrete, enabling them to be used as subbases.
base courses materials:
Natural, processed, and other materials are used for base courses. Descriptions of these materials follow.
A wide variety of gravels; sands: gravelly and sandy soils; and other natural materials such as lime rock, coral, shells, and some caliches can be used alone or blended as a suitable base course. Sometimes natural materials require crushing or removal of oversize material to maintain gradation limits. Some natural materials may be suitable for use as a base course by mixing or blending them with other materials.
Sand and Gravel. Many natural deposits of sandy and gravelly materials make satisfactory base-course materials. Gravel deposits differ widely in the relative proportions of coarse-and fine-grained material and in the character of the rock fragments. Satisfactory base materials can often be produced by blending materials from two or more deposits. Uncrushed, clean, washed gravel is normally not suitable for a base course because not enough fines are present. Fines act as a binder and fill the voids between coarser particles.
Sand and Clay. Natural mixtures of sand and clay are often located in alluvial deposits of varying thicknesses. Often there are great variations in the proportions of sand and clay from the top to the bottom of the deposit. Depending on the proportions of sand and clay, these deposits may also provide suitable base-course materials. With proper proportioning and construction methods, satisfactory results can be obtained with sand-clay soils.
Deposits of partly disintegrated rock that consist of fragments of rock, clay, and mica flakes should not be confused with sand-clay soils. The mica flakes make the deposit unsuitable for use as a base course. Mistaking these deposits for a sand-clay soil may result in base-course failure.
Processed Materials
Processed materials are made by crushing and screening rock, gravel, or slag. A properly graded, crushed rock base-course material produced from sound, durable rock makes the highest-quality base material. Existing quarries; ledge rock; cobbles and gravel: talus deposits; coarse mine tailings; and similar hard, durable rock fragments are the sources of processed materials.
Base courses made from processed materials can be divided into three general types: stabilized, coarse-graded, and macadam.

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