content). This quantity can be used in place of organic matter estimates by the Walkley–Black Organic C method.

Method 2: Organic carbon is mostly determined by wet digestion (Walkley 1947) method (6A1). The organic carbon is then multiplied by the Van Bemmelen factor of 1.724 to estimate percent organic matter based upon the assumption that soil organic matter contains 58% organic C.

7.24Engineering Data

The soils can be analyzed for their Atterberg limits and classified for engineering purposes according to the ASTM Unified Soil Classification System (USCS) (Gee and Bauder 1986), and the system adopted by the American Association of State Highway and Transportation Officials (AASHTO).

7.24.1 Atterberg Limits

The liquid limit and plastic limits are called Atterberg limits. The < 2-mm soil is further processed (<0.425 mm for Atterberg limits and 180 lm) so as to be in accordance with a standard method, e.g., Atterberg limits.

7.24.1.1 Liquid Limit (LL)

The liquid limit is the water content when soil starts to behave as a liquid. It is measured by placing soil sample in a standard cup and making a separation (groove) using a grooving tool. The cup is dropped till the separation vanishes. The water content of the soil is obtained from this sample to measure liquid limit. The LL is reported as percent water on a < 0.425-mm basis (40-mesh) (Fig. 7.4).

7.24.1.2 Plastic Limit (PL)

The plastic limit is the moisture content where the soil thread breaks apart at a diameter of 3.2 mm. A soil is considered non-plastic if a thread cannot be rolled out down to 3.2 mm at any moisture possible (Fig. 7.5).

7.24.1.3 Plasticity Index (PI)

Numerically, the Plasticity Index (PI) is the difference in the water content between the LL and the plastic limit (PL). The PI is reported as percent water on a < 0.425-mm basis.