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.

Comparison between Soil and Concrete

Soil is very complex and heterogeneous materials usually differs from traditional structural materials like concrete, steel etc. We have some/in many times great control over properties of concrete during production. Now-a-days development of concrete technology suppress many uncertainty in concrete production which leads to a finished product of desired properties. 

Careful mix design, mixing, transporting, compacting and curing leads to a more or less accurate product. Some computerized programs are now used to control batching optimizing water/cement ratio, proper weighing and sophisticated transporting and placing method bring great control over concrete production. Now we are preparing concrete in plant rather than in situ mixing thus providing extra bit of control over quality.

So far we have discussed about concrete; regarding soil we used to use it remaining in situ conditions. Construction of a foundation work involves large quantity of soil which is not practical or feasible economically to transport such enormous amount of soil from other suitable places as that of concrete (ingredient of concrete or unhardened concrete).

In some cases we are sometimes using some important techniques like grouting and improving soil with external reinforcement but that will not ensure the quality up to that of concrete as discussed above.

Concrete, we used in construction, can be inspected in laboratory before use. We try to provide similar stress condition, identical materials and identical moisture content (w/c ratio) of that will be delivered to site for placing for a project. In case of soil, we also perform some laboratory and field tests to have idea about behavior of soil. Some field tests are

-plate load test

-pile load test

-standard penetration test

-cone penetration test

-vane shear test

-some geophysical tests etc.

We have discussed some tests and some are to come in this blog. In case of laboratory test the main uncertainty results is collection of undisturbed sample. In many cases we have to make sample or use remolded sample. We have discussed that practically it is impossible to collect undisturbed sample.

Least undisturbed sample can be derived from block sampling but it is suitable for shallow depth. In borehole sampling, sample disturbance results a sample that don’t represent in-situ conditions.

Soil is unpredictable in nature which leads to change in properties not only with location but with also depth at a same place. The behavior of soil varies not only with types but with also conditions it is subjected. Its properties depends on -change in environmental condition

-drainage condition

-loading condition

In case of concrete, we deals with engineering properties like

-tensile strength

-compressive strength

-modulus of elasticity

We can design it with these properties which are known to us by testing or any other means. In contrast to concrete only two/three parameters cannot be used to characterize soil and many factors contain in its engineering properties. So to perform a design work elaborate testing of soil is required to determine its characteristics.

Formation of Soil

Soil is disintegrated or decomposed product of rock formed by weathering action. This disintegration may

-mechanical or physical disintegration

-chemical decomposition

Rock surface that is exposed to environment for a significant time, decomposed or disintegrated resulting small particles which is soil we are discussing. Geologic cycle that continues uninterrupted in nature produce such incidental material. Geologic cycle may consists of

-erosion

-transportation

-deposition

-upheaval of soil

Various mechanical and chemical processes degrade expose rocks and erosion products are carried by various transportation agencies like

-water

-wind

-glacier

-gravity

We will discuss about soils formed by transportation of each agencies.

This transportation agents picks erosion products to new locations and they get deposited resulting disturbance of equilibrium of earth forces which results large scale movement of earth and consequent upheaval. The process discussed above results rocks to expose further and geologic cycles are repeated.

But some types of soil remains in position of its formation which is known as residual soil. We have discussed about such soil in our previous post. The soil that transported from its original formation rock is called transported soil.

Depending on transportation agents, we can subdivide transported soil further into three categories:

a. Alluvial soil

b. Glacial soil

c. Aeolian soil

Sometimes there may have organic material, which may be subjected to organic decomposition resulting peat or other organic soils.

Hard Parent Rock and Properties of Residual Soil

We discussed about residual soil in our previous post. Here we will discuss about properties of residual soil in relation to parent rock.

Residual soils are found in the southern portion of United States, Hawaii and tropics. Properties of residual soil deposit are usually depend on the properties of their parent rock. We know that this soil remains in position of its formation rock.

If hard rocks like gneiss and granite are subjected to weathering, most of the products of weathering remains in parent location.

These deposits of residual soil usually have a layer of clayey/ silty clay in the top which rest upon sandy and or silty bottom layer. They are usually underlain by gradually varying partially weathered to a sound bed rock that is not weathered yet.

The depth of sound bedrock may vary within a wide range. This variation is sometimes observed within a short distance of mere few meters.

So engineering properties of such deep soil deposits very significantly with depth. The finer particles are found near top layer and larger fragments are in the greater depth.

Here we discussed about hard rock; in case of rocks that are chemically active the formation may vary with these. In the next post we will discuss about chemical rock like limestone.

Soil Retaining by Steel Sheet Pile

Dear reader we have already discussed about precast concrete and wooden sheet piles. Now we will learn about steel sheet pile.

The main advantage of steel sheet piles is they are reusable we can use these piles for several projects. They are lightweight i.e. easy to transport and thus facilitate handling.

Another advantage is they have higher driving resistance i.e. they can sustain high driving stress without distortion. So with lightweight, reusability and strength, these piles are the most appropriate choice in jobsite.

In United States the thickness of steel sheet piles are about (10-13) mm. But in Europe wider and thicker section is used. The sections that are used as steel sheet piles as follows:

a. Z

b. Deep arch

c. Low arch

d. Straight web sections

Regarding interlocking section of steel sheet pile may be

a. Ball and socket

b. Thumb and finger

Both interlocking arrangement provides watertight connections. We are providing necessary figures to understand the interlocking below:

The interlocking like ball and socket for Z section is shown below. We are providing table describing properties of section of section sheet piles that are provided by Bethlehem Steel Corporation. The allowable flexural stresses for designing steel sheet piles are shown below according to respective ASTM standard:

What does Soil Mechanics Mean?

In 1925 soil mechanics was established by Dr. K. Terzaghi. This term was coined when he published a book Erdbaumechanik in German which was on the subject we are discussing. He defined soil mechanics as (we are simplifying this here):

Soil mechanics deals with sediments and other accumulation of solids in unconsolidated form produced by chemical and mechanical degradation of rocks. There may have organic constituents. The engineering problems with such materials is analyzed or solved by applying laws of the mechanics and hydraulics.

So Soil mechanics is thus a branch of

-Mechanics which encounters soil response under forces

-Hydraulics that encounters water flow through soil mass

Soils consist of solids but of discrete particles which don’t have strong bond like solid but they are not free particles like fluid. So these materials falls between solid and fluid and their behavior also follow this trend.

Thus it is not unexpected that both fluid mechanics and solid mechanics contribute in soil mechanics. As soils have an inherent particulate system, such system exists in soil mass and soil mechanics is sometimes termed as particulate mechanics.

Soil mechanics gives theoretical basis to analyze problems of geotechnical engineering and engineering geology. These principles are also related to

• geophysical engineering

• hydrology

• coastal engineering

• soil physics etc.

Factors Controlling Tolerances in Concrete Construction

We were discussing about tolerances in concrete construction. In the previous post, we have listed factors that influence tolerances; now we will discuss each factor:

a. Structural function and strength requirements

The structure should be strong and sufficient safety is provided which reflect design assumptions. The shape and size should be accurate enough for the job to satisfy design and construction requirements.

b. Esthetics:

The structure should satisfy expectation of owner and designer as well satisfying all requirements of appearance.

c. Economic feasibility:

The cost of construction or any precast/prestressed products depends largely on tolerance limit. Construction method is also equally important. It is obvious that tight tolerance associated with higher cost involvement.

d. Relationship of components

The tolerances provided in construction may be for individual part or for global structure. In case of individual tolerance, it depends on following terms of adjacent parts:

1. Materials of adjacent units

2. Connection and joint details

3. Applicability of tolerances into critical dimensions

e. Construction techniques

The feasibility and practicability of specific tolerance, in this regard, depends on

-available craftsmanship

-available technology

-available materials

f. Material properties

In prestress member, camber is a well know term and shrinkage is a global term in concrete members. In determining deflection in design stage difficulties arisen by these two term and should be considered while determining tolerances.

g. Compatibility

Compatibility between architectural details and expectation of finish should be fixed based on expected construction method. The relation between two should include in the tolerances in concrete construction.

h. Job conditions

This factor includes job conditions and situations. Clear specifications should be provided from design office where sensitive tolerances are required that represents requirements of project site.

i. Measurements

Some bench marks and control points are provided as a reference to verify tolerances of different parameters. This are achieved by mutual understanding and they are maintained undisturbed until final acceptance and completion of project.

What is Tolerance of Concrete Construction?

Dear reader we are discussing this topic according to ACI-117, a commentary on tolerance requirements of both materials and construction for concrete structures. We will discuss about tolerance of concrete structure throughout our upcoming post providing both graphical and written interpretation. No construction or erection can provide exactly straight, level and truly vertical perfection. We are lucky that such degree of perfection is also not necessary. We have some tolerances.

Tolerances can be defined as- an acceptable variation in location and dimension which provides parameters to both designer and contractor to perform a particular work.

This conveys a performance expectation from designer to contractor depending on which design is performed for a project. Tolerances encounter design assumption and requirements of project. Here we are concerned with tolerances of concrete construction.

Tolerance depend on

-normal needs

-construction techniques

-common practices

If the values of tolerances vary, cost involvement may be increased or decreased.

The degree of perfection (tolerances) depends on several factors like:

a. Structural function and strength requirement

b. Esthetics

c. Economic feasibility

d. Relationship of components

e. Construction techniques

f. Material properties

g. Compatibility

h. Job conditions

i. Measurement

We will discuss about each factors in the next post and tolerance of materials (reinforcement and concrete), foundation alignment, building alignment, erection tolerance of precast member etc. in our upcoming post.

Simplified Method for Determining Ground Water Level in Soil Exploration

Bearing capacity of soil and settlement of foundation depends significantly on the location of ground water level (GWL). We will provide three cases of location of ground water table that influence foundation bearing capacity in our upcoming Post. During exploration of soil, we can determine level of ground water conveniently but may have some error.

In every exploration program, we can simultaneously determine the water level but in some especial cases we have to make exploratory boring only for determining water level. The purposes are like determination of artesian ground water.

Generally GWL can be determined by allowing water to reach equilibrium level in to the borehole. This will produce correct and simplified general procedure for determining GWL. Now we have to known how much time is required to reach this equilibrium.

Sandy soil:

We know that permeability of this soil is very high and equilibrium level is achieved very early. This is at most few hours after boring is done.

Clay soil:

The permeability of this fine grained soil is very low and it takes time to reach equilibrium. In most cases, it may take few weeks to be stabilized.

Depth of general water level is recorded by using a tape or any other measuring means like chain lowering into borehole.

Sometimes piezometer is used to measure depth of GWL. This method is used when accurate measurement is essential in geotechnical exploration. A piezometer used for this purposes, consists essentially of plastic standpipe which is attached to perforated pipe or any type of porous tip.

This tube is kept in position with packing of

-gravel

-coarse sand

It is usually sealed with impermeable clay (like bentonite)