Chapter five Soil Consistency and Classification

5.1 Introduction on an arbitrary basis, depending on the moisture content, the behavior of soil can be divided into four basic states: solid, semisolid, plastic, and liquid

the plastic limit : is the moisture content at the point of transition from semisolid to plastic state the liquid limit: is the moisture content at the point of transition from plastic to liquid state

These parameters are known as Atterberg limits.

I-Consistency

5.2 Liquid Limit (LL) Casagrande Method : Professor Casagrande standardized the test and developed the liquid limit device.

Hard Rubber

Brass Cup

Crank

Groove

5.2.1 Procedure: 1- Place a soil paste in the cup 2- A groove is then cut at the center of the soil pat with the standard grooving tool 3- By the use of the crank-operated cam, the cup is lifted and dropped from a height of 10 mm 4- The moisture content, in percent, required to close a distance of 12.7 mm (0.5 in.) along

the bottom of the groove after 25 blows is defined as the liquid limit. Remark: at least three tests for the same soil are conducted at varying moisture contents, with the number of blows, N, required to achieve closure varying between 15 and 35.

5- The relationship between moisture content and log N is approximated as a straight line. • The moisture content corresponding to N = 25 the liquid limit of the soil. • The slope of the flow line is defined as the flow index and may be written as:

5.3 Plastic Limit (PL) The plastic limit is defined as the moisture content in percent, at which the soil crumbles, when rolled into threads of 4.2 mm (1/8 in.) in diameter. • roll an ellipsoidal-sized soil mass by hand on a ground glass plate to reach the diameter

of the steel rod shown in figure • If the soil shows crumbles at this diameter, then weigh the soil sample when it is wet and

oven dried and determine the water content that is the PL • Repeat the procedure one more time, at least two samples are required

The plasticity index (PI) is the difference between the liquid limit and the plastic limit of a soil, or

Steel rod of diameter 4.2 mm

• Liquid limit and plastic limit of fine-grained soil are indicators of the nature of its plasticity.

• Liquid limit and plasticity index are required parameters for classification of fine-grained soils.

Liquidity index LI For scaling the natural water content of a soil sample to the Limits

LI >1 These soils, when remolded, can be transformed into a viscous form to flow like a liquid. LI < 1 Soil deposits that are heavily overconsolidated may have a natural moisture content less than the liquid limit.

II-Classification of Soil • Different soils with similar properties may be classified into groups and sub-groups according to their engineering behavior. • Classification systems provide a common language to express the general characteristics of soils Most of the soil classification systems are based on particle-size distribution plasticity

5.4 USDA Classification

This classification method is based on the particle-size limits

If the particle-size distribution of soil A shows 30% sand, 40% silt, and 30% clay-size particles, its textural classification can be determined by proceeding in the manner indicated by the arrows in Figure 5.1. This soil falls into the zone of clay loam.

For soil B of 20% gravel, 10% sand, 30% silt, and 40% clay, the modified textural compositions are

For example

On the basis of the preceding modified percentages, the USDA textural classification is clay. However, because of the large percentage of gravel, it may be called

gravelly clay.

Because textural classification systems do not take plasticity into account and are not totally indicative of many important soil properties, they are inadequate for most engineering purposes.

Two classification systems are commonly used by soils engineers that take into

consideration the particle-size distribution and Atterberg limits. They are: 1- The American Association of State Highway and Transportation Officials (AASHTO)

classification system 2- The Unified Soil Classification System(USCS).

5.5 AASHTO Classification System

The AASHTO system of soil classification was developed in 1929 as the Public Road Administration classification system. It has undergone several revisions, the present version is that of 1945. According to this system, soil is classified into seven major groups: Soils classified under groups A-1, A-2, and A-3 are granular materials of which 35% or

less of the particles pass through the No. 200 sieve. Soils of which more than 35% pass through the No. 200 sieve are classified under

groups A-4, A-5, A-6, and A-7. These soils are mostly silt and clay-type materials. This classification system is based on the following criteria:

1.Grain size a.Gravel: fraction of soil retained on sieve No. 10 (2-mm) b. Sand: fraction passing the No. 10 (2-mm) U.S. sieve and retained on the No. 200 (0.075 mm) c. Silt and clay: fraction passing the No. 200 U.S. sieve

2. Plasticity: • the term silty is applied when the fine fractions of the soil have a plasticity index of 10 or

less. • The term clayey is applied when the fine fractions have a plasticity index of 11 or more

To evaluate the quality of a soil as a highway subgrade material, one must also incorporate a number called the group index (GI) with the groups and subgroups of the soil.

GROUP INDEX (GI)

PI = plasticity index LL = liquid limit F200 = percentage passing through the No. 200 sieve

1. If the equation yields a negative value for GI, it is taken as 0. 2. The group index is rounded off to the nearest whole number (for example, GI 3.4 is

rounded off to 3; GI 3.5 is rounded off to 4). 3. There is no upper limit for the group index. 4. The group index of soils belonging to groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3 is always 0. 5. When calculating the group index for soils that belong to groups A-2-6 and A-2-7, use

Rules

Example

5.6 Unified Soil Classification System

Origin of USCS: This system was first developed by Professor A. Casagrande (1942) for the purpose of airfield construction during World War II. Afterwards, he modified it to enable the system to be applicable to dams, foundations, and other construction At present, it is used widely by engineers Two broad categories: (1) Coarse-grained soils that are gravely and sandy in nature with <50% passing through

#200 sieve. (2) Fine-grained soils with more than 50% passing through the #200 sieve.

Fine fraction percent passing No. 200 sieve Coarse fraction percent retained on No. 200 sieve

• Gravel fraction percent retained on No. 4 sieve • Sand fraction percent retained on No. 200 sieve