MIL-HDBK-1011/1
Oxisols are silty clays with high contents of oxides. They are
generally reddish brown to gray in color. The soils are relatively stable,
with low to medium expansion potential. With proper moisture conditioning and
compaction, oxisols are adequate as subgrades below lightly loaded structures.
Andisols and vertisols are suitable for agricultural use but do not
have sufficient strength and stability for construction use. For foundation
engineering purposes, andisols and vertisols are usually removed and replaced
with more stable subgrade materials. Lime stabilization is sometimes used to
improve the engineering characteristics of these soils.
2.3.4
Rocks. The most common rocks found in tropical regions are
intrusive rocks such as granite, igneous rocks such as andesite and basalt,
and sedimentary rocks such as sandstones and siltstones. In certain young and
active volcanic areas such as Hawaii, the Philippines and Indonesia, the lava
rock formations often contain large voids and cavities, such as lava tubes and
chambers which are difficult to detect. For important structures to be
constructed on lava formations, a thorough site investigation is required. If
lava tubes and chambers are found within the foundation construction zone,
collapse or fill them with appropriately engineered materials.
Depending on the degree of weathering, rocks can be excellent
sources of construction aggregates when they are quarried and crushed. When
andesites are used as concrete aggregate, the concrete has a lower compressive
strength than concrete made with basalt aggregates. Therefore, when using
andesitic rock aggregates for construction, the soundness, abrasion
resistance, and moisture absorption characteristics must be carefully studied
and trial batch mixes prepared and characterized before the work proceeds.
Intrusive and sedimentary rocks are often found in an extremely
weathered state. Except for occasional, less weathered boulders, these rocks
tend to behave as dense soils. Such rocks can be used as landfill for
embankments, but not as a base for pavement or as concrete aggregate.
2.3.5
Cut and Fill. In the tropics, where rainfall intensities are
typically high and the soils lack certain minerals, minimize and carefully
engineer massive alterations of the landscape by cut and fill. For grading
purposes, cut is preferred to fill. Limit cut and fill slope heights to a
maximum of 25 feet unless the work is properly engineered and controlled
during construction. The side slopes in cut and fill areas shall not be
steeper than a 2 horizontal to 1 vertical ratio. Generally, the flatter the
slope, the easier it is to construct, to induce natural vegetation growth, and
to maintain.
2.3.6
Soil Stabilization. The basic physical characteristics of tropical
soils can be modified and improved by various soil stabilization techniques as
described above. For large earthwork projects, it is recommended that a
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