Loess: An Aeolian Soil Deposit

Dear reader in our last post we have discussed about dune sand as an Aeolian deposit. Here we will discuss about another wind transported soil, loess. We have introduced this soil in our previous post regarding collapsible soil.

The distribution of grain size is as usual uniform more or less like dune sand. But they are distinguished form dune deposit as it consists of silt or particles size identical to silt.

This soil shows cohesion which is mainly a contribution of coating of clay over particles of silt or similar size grains. Thus this soil remains stable unlike dune deposit; but there have a condition. This is soil must be in unsaturated condition more precisely a dry soil.

Some chemical leached with rain water also precipitated in such soil which contributes to cohesion to some extent. These soils have collapsible nature. When it gets saturated, the stability stated above collapsed, as binding strength remains between soil particles is lost.

Being collapse soil, careful interpretation of behavior of loessial deposits is conducted before foundation construction over them. These soils are found in many states of United States as listed below:

a. Iowa

b. Missouri

c. Illinois

d. Nebraska

These deposits are also observed in Mississippi and Tennessee along the river Mississippi

Typical Properties of Dune: An Aeolian Soil Deposit

We have discussed wind transported soil in our previous post. We know that sand blown by wind takes shape like dune. When a dune is formed, wind forces to blow sand over crest of it.

Sand particles as usual roll down along slopes from crest which produces a natural compaction in one side and loose deposit in other side. We are making clear this fact here. In the windward side, a sand deposit of compacted state is found and the opposite scenario are found in leeward side where roll over sand produce loose deposit relative to windward.

Dunes are found in many states of United States. We are listing some regions where dunes are found:

a. Lake Michigan (eastern and southern shores)

b. California (southern coast)

c. Atlantic coast

d. Along coast of Washington and Oregon

e. Rocky and alluvial planes of western USA.

Typical properties of sand of such deposits are as follows:

1. We know that particle size of sand is controlled by wind velocity i.e. it provides a sorting action of having same size particles at particular location. So distribution of particle size is uniform.

2. As discussed above wind velocity is key factor controlling grain size over path from source to particular distance. As wind carries bulky particles lesser distance than small one, the grain size generally decreases with distance.

3. As discussed above the sand in windward is more compacted than leeward. So relative density is all affected. In windward side, the relative density are sometimes found (50-65) % which may be decreased to (0-15) % on leeward side.

Significance of Activity of Soil

In United States, large parts of Africa and India there have severe problems associated with expansive potential of clay soil. The three clay minerals we concern mainly in foundation engineering are:

a. Kaolinite

b. Illite

c. Montmorillonite

If we can identify the mineral structure of clay soils, we can predict their expansion and shrinkage potential. 

Many lighter building in the regions of expansive soil, subjected to severe settlement in the regions stated above of the world; even before they were left for operation.

Montmorillonite is the main mineral element of black cotton soil. The mineral structure of clay is indicative to amount of water and form of their presence in clay minerals. Water can combine separated mineral units to form a sheets stakes over another unit.

Activity of clay soil defines its water retaining capacity and we know that swelling and shrinkage of soil depends on the water within soil. Thus both phenomena of volume change depend on activity of soil. Soils are classified as three types as follows:

a. Inactive soil

b. Normal soil

c. Active soil

Inactive soil

Soil having activity less than 0.75 is considered as inactive soil.

Normal soil

Soil having activity in between 0.75 and 1.25 are taken as normal soil.

Active soil

Soil that has activity greater than 1.25 is considered as active. Identifying clay minerals depending on activity:

Montmorillonite:

Soil having activity greater than 4 have significant amount of montmorillonite.

Kaolinite:

Soil having activity less than 1 has mainly kaolinite

Illite :

Soil having activity in between 1 and 2 have significant amount of illite.

Thus activity furnishes information about effect and type of mineral in clay soil. We conclude following points about activity:

a. Activity remains constant for a soil from specific origin. In such case, plasticity index reduces as quantity of clay fraction reduces and vice versa.

b. In case of highly active clay minerals like montmorillonite, shows great increase in plasticity index under small quantity of clay fraction.

Comparison of Soil with Steel

Soil is very complex in nature and shows distinct behavior other than traditional construction materials. We have discussed about concrete in the last post. Here we will discuss about steel to compare with soil.

Steel is mainly iron alloyed by carbon to change its properties. Other than carbon it may contain

• Manganese

• phosphorous

• sulfur

• Silicon

Sometimes other alloying elements can be added intentionally to have enhanced properties of steel like nickel, titanium, boron, molybdenum, chromium etc.

The carbon and alloying element discussed above render hardening properties to steel. So by adjusting amount of these elements (one or more) and controlling form of presence in steel, the quality of steel can be altered in terms of

• Tensile strength

• Ductility

• Hardness

As an example, with carbon of greater content produce harder steel but having less ductility. The carbon content greater than 2.1 % produce cast iron that is harder and have malleability of null.

So steel is accurately controlled manufactured product whose properties can be tuned with varying alloying element. We have greater control over steel than concrete as discussed in last post.

Before using, we can evaluate its engineering properties accurately in laboratory or from manufacturer information which represent actual condition in which it remains in concrete or other materials. In case soil, we can never accurately simulate the actual in-situ conditions; though modern triaxial test apparatus can simulate many in-situ environments in single arrangement.

We cannot remove unexpected soil from project site as it involve huge soil transportation; in some cases we bound to relocate many structures considering geotechnical and geological conditions. Sometimes compensated foundation is used to suppress unexpected soil from project site when architectural and other requirements seek a number of basements.

Steel is a homogeneous material although its length in contrast to soil which heterogeneous in both horizontal and vertical direction even within few feet.

Steel shows precise modulus of elasticity rather than a soil sample (where shear modulus is important). With modulus of elasticity, compressive and tensile strength, we can easily design a section, but in case of soil we need elaborate testing program to have not accurate but near to actual value. Even under these numerous testing large safety factor is essential in foundation engineering.

Soils Transported with Water

We already learned that soil is a weather product of rock that may or may not be transported to another location. We have also learned about residual soil and transported soil. As water is included in this topic, definitely it is transported soil.

Of many transporting agents, water is one of the most dominating agents in transporting soil to new location. Swift flowing water usually carries a significant amount of soil and this carrying is done by either

-in suspension

-by rolling them alone bed

Water erodes hills through which it run and deposits erosion products on the valleys.

The size of particles carried by flowing water depends on velocity of current. Very swift water often can carry large size particles like boulders and gravel is very common.

When velocity of water decreases water cannot carry coarse particles and get deposited. The remaining relatively finer particles are transported further to downstream and they also get deposited when velocity is decreased significantly.

At confluences, the velocity reached almost zero and finer particles remains in flowing water get deposited forming a delta. The receiving bodies of the confluences may be

-lake

-sea

-ocean

The deposition that made in a lake is known Lacustrine deposits. When receiving bodies are sea or ocean the deposition is called Marine deposits. But all soils that are transported by and subsequently deposited from water are called Alluvial deposits.

Chemical Rocks and Properties of Residual Soil

In the previous post we have discussed about residual soil derived from hard rock. There mostly physical or mechanical processes are involved in weathering. Now we will discuss about some rocks where chemical weathering processes are observed.

Some chemical rocks are:

-limestone, made up mainly of calcite (CaCO₃)

-chalk, made up mainly of dolomite [Ca Mg(CO₃)₂] mineral

-dolomite also contains dolomite of heavy concentration

The main action that makes weathering is dissolving soluble materials present in such rocks. The ground water present in this rock deposit does the task removing soluble fraction from rocks leaving behind insoluble fraction.

Like hard rock, these soils derived from chemical rocks have also a gradually varying property, more identical particles remains near bed rock. There usually have some chemical changes as some fraction is removed by dissolving.

These soils that derived from weathering of rocks like limestone are in gray color in most cases. Although weathering is happened uniformly, the depth of intrusion of weathering may greatly vary.

These soils just above bedrocks may be in normally consolidated state. In case of, supporting a heavy foundation that is subjected heavy loads, there may have large settlement due to consolidation of these soils.