GATE Civil Engineering (CE) 2027 Test: Soil Types & Formations Free Online

Permeability of Gravel (>1) > Sand (10^-2 – 10^-4) > Silt (10^-4 – 10^-6) > Clay (< 10^-6)

Gravel has maximum permeability and clays have minimum permeability. Loess deposit has a loose structure and permeability is more than clay silt admixture.

Correct Answer :- d

Explanation : a) Loess is a deposit of wind-blown silt that blankets large areas of the continents. It is often light brown in color, consisting predominantly of quartz, feldspars, micas, and calcium carbonate.

b) Soil scientists define peat as organic soil with organic content of greater than 35%. To a geotechnical engineer, however, all0 soils with an organic content of greater than 20% are known as organic soil, while “peat” is an organic soil with organic content of more than 75%.

c) Soils deposited in riverbeds are known as alluvial deposits.The size of particles deposited in riverbeds depends on the speed of flow. If the flow of a river is strong, only large cobble-type material can get deposited.

d) Marl or marlstone is a carbonate-rich mud or mudstone which contains variable amounts of clays and silt. The term was originally loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under freshwater conditions. These typically contain 35–65% clay and 65–35% carbonate.The term is today often used to describe indurated marine deposits and lacustrine (lake) sediments which more accurately should be named 'marlstone'

Soil carried and deposited by river water are alluvial soils. Deposits made in lakes are lacustrine deposits. Wind transported soil are aeolian deposits. Drift is a general term used for the deposits made by glaciers. Deposits directly made by melting of glaciers are called till. Talus are colluvial soil deposited by gravity.

Silt size varies from 0.002 mm - 0.075 mm.

In the Indian Standard Soil Classification System (ISSCS) or BIS, soils are classified according to their grain size as boulder, cobble, gravel, sand, silt, or clay, as shown below in the tabulated form.

Lacustrine-Deposited at the bottom of lakes
Alluvial - Transported by running water
Aeolian - Transported by wind
Marine - Deposited in sea water

According to IS grain-size classification 

The specific Surface area is inversely proportional to grain size. Out of sand, silt, clay, and colloids, Sand has the highest grain size while colloids have the least grain size.

For coarse sand → 2 mm d ≤ 4.75 mm

For fine sand → 0.075 mm d ≤ 0.425 mm

For silt → 0.002 mm < d ≤ 0.075 mm

For clay → < 0.002 mm

Colloids are basically finer clay particles whose surface area is so high that its behaviour is controlled by specific energy rather than mass-energy.

∴ Increasing order of surface area ⇒ sand < silt < clay < colloids

Aeolian soil is a soil which is transported by wind and is mostly sand, thus, has high permeability.

Properties of Aeolian soil:

1) Aeolian soil are deposited by wind action.

2) It has a uniformly graded particle.

3) The void ratio and permeability of soil is high. 

4) They are non-plastic.

5) High compressibility and low density. 

The soils formed at a place may be transported to other places by agents of transportation, such as water, wind, ice and gravity.

1) Water transported Soils: Flowing water is one of the most important agents of transportation of soils. Swift running water carries a large quantity of soil either in suspension or by rolling along the bed.

The size of the soil particles carried by water depends upon the velocity.

All type of soils carried and deposited by water are known as alluvial deposits. Deposits made in lakes are called lacustrine deposits. Such deposits are laminated or varved in layers. Marine deposits are formed when the flowing water carries soils to ocean or sea.

2) Wind transported Soils: Soil particles are transported by winds. The particle size of the soil depends upon the velocity of wind. Soils deposited by wind are known as aeolian deposits.

Collapsible soil is those for which there is a decrease in volume on the addition of water. Loess is one such example. Loess is a silt deposit made by wind. These deposits have low density and high compressibility. The bearing capacity of such soils is very low.

3) Glacier-Deposited Soils: Glaciers are large masses of ice formed by the compaction of snow. As the glaciers grow and move, they carry with them soils varying in size from fine grained to huge boulders.

Drift is a general term used for the deposits made by glaciers directly or indirectly. Deposits directly made by melting of glaciers are called till. The soil carried by the melting water from the frint of a glacier is termed out-wash.

4) Gravity deposited soil: These are soils transported through short distances under the action of gravity. Colluvial soils such as talus have been deposited by the gravity. Talus consists of irregular, corase particles. It is a good source of broken rock pieces and coarse grained soils for many engineering works.

Bentonite is decomposed volcanic ash containing high percentage of clay mineral-montmorillonite.