116 7 Laboratory Soil Procedures for Kuwait Soil Taxonomy

The AASHTO group and subgroup classifications may be further modified by the addition of a group index value. The empirical group index formula was devised for approximate within-group evaluation of “clayey granular materials” and “silty-clay” materials.

The dominant AASHTO group classifications found in Kuwait and their inherent characteristics are:

A-2-4: Silty sand with a maximum silt content of 35 percent. The material is either non-plastic or has a low plasticity. The high sand content allows the soils to be classified as structural fill material and hence used for general engineering purposes (herein defined as construction and earthworks purposes).

A-1-b: This soil contains stone fragments and up to 25 percent fines (very fine sand, silt, and clay). They are either non-plastic or slightly plastic. The soil is suitable as general backfill for engineering purposes.

A-1-a: These soils are similar to those classified as A-1-b but are limited by the high percentage of stone fragments.

A3: Silty sand with a maximum of 10 percent silt. Usually non-plastic. This soil is suitable for structural and general backfill material.

A4: Silty soil with more than 35 percent fines. Moderately plastic. Generally, found near shore lines and within saline sabkha flats. This material is considered unsuitable for general engineering purposes in Kuwait.

A6: Clayey material with more than 35 percent fines. This material is highly plastic and is not suitable for general engineering purposes. Its extent is very limited in Kuwait.

A7: Similar to A-6 with high plasticity index. Includes expansive clays that are very rare in Kuwait. The material is considered not suitable for general engineering purposes in Kuwait.

7.25Soil Mineralogy

X-ray diffraction analysis (XRDA), differential thermal analysis (DTA) and infrared analysis (IRA) are the mostly used methods to analyze soil and clay samples for mineral assemblages. However, XRD is considered the most powerful technique to identify mineral species in soils. Therefore, XRD is to be adopted to establish whole soil and clay mineralogy of the soil samples collected from the control sections of the identified soil families and soil series. For whole soil mineralogy, the soil samples are ground in a pestle and mortar and filled into an aluminum holder and pressed for preferred orientation of the soil minerals to scan