NAMES OF SOME SOILS THAT ARE GENERALLY USED IN CIVIL ENGINEERING PRACTICE

Bentonite is a clay formed by the decomposition of volcanic ash with a high content of montmorillonite. It exhibits the properties of clay to an extreme degree.

Varved Clays consist of thin alternating layers of silt and fat clays of glacial origin. They possess the undesirable properties of both silt and clay. The constituents of varved clays were transported into fresh water lakes by the melted ice at the close of the ice age.

Kaolin, China Clay are very pure forms of white clay used in the ceramic industry.

Boulder Clay is a mixture of an unstratified sedimented deposit of glacial clay, containing unsorted rock fragments of all sizes ranging from boulders, cobbles, and gravel to finely pulverized clay material.

Calcareous Soil is a soil containing calcium carbonate. Such soil effervesces when tested with weak hydrochloric acid.

Marl consists of a mixture of calcareous sands, clays, or loam.

Hardpan is a relatively hard, densely cemented soil layer, like rock which does not soften when wet.

Boulder clays or glacial till is also sometimes named as hardpan.

Caliche is an admixture of clay, sand, and gravel cemented by calcium carbonate deposited from ground water.

Peat is a fibrous aggregate of finer fragments of decayed vegetable matter. Peat is very compressible and one should be cautious when using it for supporting foundations of structures.

Loam is a mixture of sand, silt and clay.

Loess is a fine-grained, air-borne deposit characterized by a very uniform grain size, and high void ratio. The size of particles ranges between about 0.01 to 0.05 mm. The soil can stand deep vertical cuts because of slight cementation between particles. It is formed in dry continental regions and its color is yellowish light brown.

Shale is a material in the state of transition from clay to slate. Shale itself is sometimes considered a rock but, when it is exposed to the air or has a chance to take in water it may rapidly decompose.

COMPOSITION AND STRUCTURE OF CLAY MINERALS

COMPOSITION OF CLAY MINERALS

The word 'clay' is generally understood to refer to a material composed of a mass of small mineral particles which, in association with certain quantities of water, exhibits the property of plasticity.

According to the clay mineral concept, clay materials are essentially composed of extremely small crystalline particles of one or more members of a small group of minerals that are commonly known as clay minerals.

These minerals are essentially hydrous aluminum silicates, with magnesium or iron replacing wholly or in part for the aluminum, in some minerals. Many clay materials may contain organic material and water-soluble salts.

Organic materials occur either as discrete particles of wood, leaf matter, spores, etc., or they may be present as organic molecules adsorbed on the surface of the clay mineral particles. The water-soluble salts that are present in clay materials must have been entrapped in the clay at the time of accumulation or may have developed subsequently as a consequence of ground water movement and weathering or alteration processes.

Clays can be divided into three general groups on the basis of their crystalline arrangement and it is observed that roughly similar engineering properties are connected with all the clay minerals belonging to the same group.

STRUCTURE OF CLAY MINERALS

Clay minerals are essentially crystalline in nature though some clay minerals do contain material which is non-crystalline (for example allophane). Two fundamental building blocks are involved in the formation of clay mineral structures.

They are:

1. Tetrahedral unit.

2. Octahedral unit.

The tetrahedral unit consists of four oxygen atoms (or hydroxyls, if needed to balance the structure) placed at the apices of a tetrahedron enclosing a silicon atom which combines together to form a shell like structure with all the tips pointing in the same direction. The oxygen at the bases of all the units lie in a common plane.

DEEP SEA CLAY BASICS AND TUTORIALS

DEEP SEA CLAY BASIC INFORMATION
What Are Deep Sea Clay? Deep Sea Clay Information

The clay materials formed in large parts of the deep sea and oceanic basins are, generally, quite distinct from terrigenous clays. This is because many such areas are so far removed from land that detrital terrigenous material becomes a minor, even insignificant, source of sediment.


As a result, the products of other processes make a more important contribution to the fine grained sediments that accumulate in these environments (Berger 1974). Globally, the most important of these are the minute skeletal components of microfossils which form a continuous pelagic rain from surface to deeper waters.

Their contribution to the fine grained sediment accumulating at the ocean floor depends upon the dynamic balance between the processes of their production in surface waters and their destruction by dissolution on their journey down through the water column following death of the organisms. The two most important biogenic components are calcareous and siliceous microfossils.

The calcareous microfossils include foraminifera and coccoliths composed of calcium carbonate (CaCO3) mainly in the form of calcite, whilst the silceous microfossils include diatoms and radiolaria composed of opaline amorphous silica (SiO2), in the form known as opal-A. The rate of production of these organisms in surface ocean waters depends on biological fertility.

Diatoms dominate in more fertile nutrient rich water whereas coccoliths dominate in less fertile regions. Since seawater is universally under saturated with respect to amorphous silica, most silica is dissolved and recycled
as the skeletons of opaline microfossils descend through the water column.

A further fraction arrives at the sediment water interface where more is dissolved but in regions of high productivity some is preserved and may accumulate. Thus the distribution of siliceous pelagic sediments mirrors the patterns of the most highly productive ocean waters such as in regions of oceanic divergence and upwelling where nutrient-rich deep ocean waters rise to the surface.

The fate of calcareous pelagic sediment is similar except that the degree of undersaturation of seawater with respect to carbonates increases with depth. This gives rise to a depth in the oceans, known as the Calcite Compensation Depth (CCD), below which calcite does not accumulate.

In the deepest parts of the Ocean basins (> 3500 m) below the CCD, sedimentation rates may be extremely slow and hydrogenous processes involving iron and manganese oxides take on an important role. Such areas accumulate deposits know as deep sea red clays (Glasby 1991).

Red clays are extremely fine grained with often more than 80% < 2 um in size. They cover about 30% of the ocean basins and are most prevalent in the Pacific Ocean. Most of the components of Pacific red clays are allogenic, the most important being aeolian dust.

Red clays accumulate very slowly with the highest rates of sedimentation coeval with Pleistocene glacial periods when aeolian dust production was at a maximum (Glasby 1991). Because of their fine grain-size and long term stability serious consideration has been given to using red clays as sites for radioactive waste disposal (Burkett et al. 1991).

SPECIAL PROBLEMS OF CLAY SOIL IN FOUNDATION OF BUILDING BASICS AND TUTORIALS

PROBLEMS OF CLAY WHEN PRESENT ON THE FOUNDATION OF BUILDING
What Are The Problems Of Clay As Foundation?


Special Problems of Clay Soils
The majority of clay soils can cause foundation problems as they slowly change in volume due to increases or decreases in water content. This change is related to the season with the ground expanding in the winter and
contracting in the summer.

This seasonal change, which may be in the order of + or - 30mm at ground level, can affect the clay to a depth of about a metre, with the ground below this level having a fairly stable moisture content.

Where clay soils contain trees the problem is more severe. Trees and heavy vegetation draw a considerable amount of water from the ground during the growing season.

A mature poplar takes up as much as 1000 litres of water per week. In long hot summers with little or no rainfall the tree will continue to draw moisture out of the ground and the clay will shrink.

This, of course, is in addition to the seasonal drying mentioned above. If buildings are sited near individual or groups of trees serious cracking in the walls can occur as a result of ground movement.

To prevent this movement from affecting strip foundations they must be deeper than the tree roots. An alternative, of course, is to site the buildings well clear of the trees.

Where trees have been removed from clay soils the opposite problem occurs. As the ground slowly regains moisture it will expand and this can continue for a period of up to 10 years.


The pressure that dry clay develops when reabsorbing moisture is likely to be greater than that imposed by the building load and upward movement of the structure will occur.

If houses are built on the site before this ground expansion is complete, cracking will occur in the walls and foundations; the swelling will be uneven because it will be concentrated around the removed tree.