THE NATURE OF SOIL By Sarik Salim

The nature of Soil Soil is defined as a collection of mineral particles that was formed due to the weathering process of igneous rock and mixed with organic matter. The void between particles is filled with water and gas. Cementation among the particles is weak and is due to carbonation and oxidation between particles and or organic matter.

Cont’d Characteristics of soil in general are greatly influenced by the process of the soil formation. The structural arrangement of the particle may be loose, medium dense or dense depending on the packing of material. The size of the soil particles (100 mm to < mm) To classify a soil, one must know the Grain Size Distribution (GSD).

Cont’d Base on the particle size, soil can be grouped as: Course grained such as gravel and sands Fine grained soil such as silts & clays. Table 1.1 (in your note) shows the classification of soil based on particle size. The engineering behavior of coarse grained soil is affected by: Its grain size Its grain size distribution Sometimes by particle shape

Microscopic View of Soil Particles 5 d 50 =0.26 mmd 50 =0.30 mm d 50 =2.40 mm

Cont’d The grain size distribution of a coarse grained soil is generally determined by means of SIEVE ANALYSIS. Table 1.2 (in your note) shows the number and mesh opening sizes uses in the US and British standards.

The chemical process occurred during the process of soil formation resulted in the changes of mineral form due to action of: Water, Oxygen, and Carbon Dioxide The presence of minerals and the effect of water greatly influence the interaction between the mineral grains, and this effect their plasticity and cohesiveness.

The classification of fine grained soil is not only based on the grain size but also the plasticity. Plasticity of clays can be determined by Atterberg Limit test from which the values of Liquid Limit (LL) & Plastic Limit (PL) are obtained. The measurement of plasticity is donated by the Plasticity Index (PL) that is the range in which clay exhibit plastic behavior. PI = LL - PL

Examples of fine grained soil are silts & clays. Silt has low cohesion Silt is more affected by water than grain size distribution. Silt is relatively impervious, difficult to compact, easily to eroded. Clay highly affected by water and the presence of certain clay minerals. The permeability of clay is very low, it is difficult to compact when wet.

Clay subjects to expansion and shrinkage with changes in moisture. The properties of clay are influenced not only by the size and shape of particles, but also their mineral composition.

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).