SOIL CLASSIFICATION

Sarik Salim

Soil Classification System

Soil classification system is important in geotechnical engineering because it provide systematic method of categorizing soil according to their probable engineering behavior.

By knowing the classification of the soil, an engineer will have a good idea on how to proceed with detailed site investigation and laboratory testing and subsequently with the design of foundation as well as the engineering situation both during and after construction.

Common soil classification systems used in civil engineering practice: British soil classification system (BS)

The Unified soil classification system (USCS)

The classification system proposed by AASHTO (American Association of State Highway and Transportation Officials).

British soil classification system (BS) The British soil classification system is shown

in table 1.3 (in your note). The plasticity chart shown in Figure (1.2). In this system, the soil is classified by group

symbols composed of main term shown in table 1.4

A group of symbol may consist of two to four letters for example

SW means well graded sand SCL means very clayey sand or clay of low plasticity

Cont’d The term fine is used when it is not required

to differentiate between Silts (M) and Clay (C).

Any cobbles or boulders retained in 63 mm sieve are removed before the classification test but the percentage should be determined.

The presence of these particles is indicated in the classification symbol joined by plus sign, for example:

GW+Cb means well graded gravel with cobbles

Example 1 Refer to the lecture note

The Unified Soil Classification System (USCS)

The unified soil classification method is introduced by Casagrande (1942).

This system is modified slightly and adopted by ASTM in 1985 (Table 5, in your note).

The USCS categorize the soil into two groups Coarse grained soil (gravel & sand) Fine grained soil (silt & clay)

Coarse grained soil

< 50 % passing from No 200 sieve. other considerations in this classification are:

The percentage of coarse fraction (retained on No 40 sieve).

Coefficient of uniformity (Cu) and coefficient of curvature (Cc) for the soil with < 12 % passing No. 200 sieve,

The symbol of the group G for gravel S for Sand

Fine grained soil

> 50% passing No. 200 sieve The symbol of the group

M for inorganic silt C for inorganic clay O for silt and clay that contain organic materials. Pt for peat, muck and other highly organic soils

The fine grained soil is further classified based on its plasticity index (PI)

cont’d

Other symbols used in this classification system: W for well graded soil P for poorly graded soil L for low plasticity clay H for high plasticity clay

Symbols such as GM, SM, GC, SC are used for soil that contain both coarse and fine particles.

Example 2 Refer to the lecture note

The AASHTO classification system Table 1.6 (in your note) shows the

classification system proposed by AASHTO (American Association of State Highway and Transportation Officials).

AASHTO is widely used in the field of highway.

This system classify the soil into seven main groups A1-A7.

Cont’d

Granular soil is classified into groups A-1, A-2 and A3, where < 35% of soil particles passing a No. 200 sieve.

Clay and silt categorized as group A-4 to A-7, where >= 35% passing No. 200.

AASHTO classification uses grain size distribution obtained from the Sieve Analysis and soil plasticity obtained from Atterberg limit test.

Gravels are soil fraction that pass the sieve with diameter of 75mm and retained in sieve No.10(2mm diameter)

Sand is the fraction of soil passing No.10 (2mm) sieve and retained in the sieve No.200 (0.075mm).

Silts and clay passes No.200 sieve Adjective silty is used when fine fraction of soil

have PI (Plasticity Index) <10 While Clayey is used when the fine fraction has P

>10 If boulders (Particle size >75mm) is found in the

soil, then they are not used in the analysis.

Group index (GI) is used to evaluate the quality of the soil as sub-grade material.

GI= (F-35)[0.2+0.005(LL-40)]+0.01 (F-15)(PI-10) Where, F the percentage of particles passing sieve NO.200 LL is the liquid limit PI is the plasticity Index Good quality soil for sub grade has low GI

Example 3 Refer to the lecture note