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)

What is Sample Disturbance in Soil Sampling?

During collection of soil samples from either shallow or deep location, there may have disturbance from small to large degree. When boring is of wash boring, a suspension of soil is collected; obviously this is a very disturbed sample. This method represents a soil that has almost lost all ambient properties (completely disturbed structure, altered natural moisture content etc); only provides information about changes in the layers.

With this complete disturbance, some methods provides less disturbed sample and there have some special method that provides least disturbed sample. There have some special method that provides nearly undisturbed sample. We have discussed about block sampling in our preceding post.

A soil sample may be disturbed in three consecutive stages either in any one stage or may be in combination of stages. These are:

a. Disturbance before sampling

b. Disturbance during sampling

c. Disturbance after sampling

Disturbance before sampling

During sampling generally we remove or cut out soils over the sample at desired depth. This method reduces vertical stress over the sample within a short duration reducing overburden pressure quickly. In some cases, soil layer beneath the sample may be deflected up for the same consequence of stress relief. In some sampling operation continuous collection of samples is applied depending on extent and requirement of soil investigation. In continuous sampling process, additional disturbance is observed due to shearing of previous sample by applying torsion to separate it from parent soil.

Disturbance During Sampling

During sampling the soil sample entered into the sampler by forcing them to take their position in sampler. We know there have some friction between sampler and soil and other mechanism that disturb the sample.

We can summarize this mechanism as below:

a. Disturbance by displacing soil by wall of sampler

b. Friction between sampler and sample

c. Adhesion between sampler and sample

Another factor that influences the disturbance is inside clearance between cutting shoe and sample.

Disturbance After Sampling

When a soil sample is collected carefully, least disturbing it which is practical during sampling and before sampling, may be disturbed by next steps of sampling process like

-transportation

-sealing

During transporting, in many cases, sample becomes disturbed. Sometimes in case of improper sealing of tube, moisture lose may be taken place.

In designing a soil sampler, the main criteria are that they don’t produce much disturbance. In determining disturbance of soil during sampling some parameters like ci, co, Ar, RQD and recovery ratio etc are used and specific limit of these parameters for different soil type are provided that will be discussed in upcoming posts.

What is Sampling of Soil?

Proper design of foundation of civil engineering structure, need sufficient knowledge about subsurface condition of proposed site upon which foundation is to be supported. Arrangement and orientation of underlying soil layers and physical properties of each layer, extent of each layer are the necessary information in deriving both bearing and settlement behavior of foundation soil. To obtain essential information like these, through laboratory and field testing, subsurface investigation is conducted. In this process soil samples are collected from shallow and deep strata. The method by which soil sample is collected is called sampling. Different types of sampler are used in this purpose depending on sample type and difficulties in collecting this.

The main methods of soil sampling are of two:

a. Block sampling

b. Borehole sampling

Block sampling:

In this technique, block samples are collected by hand cut. This is considered as the best techniques to collect samples in terms of sample quality i.e. this produce less disturbance to sample. In soil sampling, we all know the importance of undisturbed sample though it is a theoretical term.

In this technique surrounding soil of sample is excavated to collect sample and the sample is collected in a box. Being best quality sample, in many situations, however, it is not the best technique considering practicability and the most dominating factor economy.

Borehole sampling

This is widely adopted technique for collecting sample, especially form a considerable depth. A sampler is fitted to drill rod of boring apparatus is lowered through a dug hole in ground by removing soil using any of usual boring techniques.

Upon reaching to desired depth, the sampler is inserted in to soil either by

-pushing

-driving

After reaching sufficient soil sample into sampler, it is sheared at its base by rotating.

Variability of Shear Strength of Foundation Soil

Shear strength

Soil is a particulate or granular material. When a soil mass beneath foundation is subjected to excessive deformation, these particles allow rolling, sliding or rearrangement to some extent. In simple word shearing is happened. This capability of sliding to support a structure, its own weight (overburden pressure) and sustaining a slope is known as shear strength. This definition is for understanding the term shear strength easily; in engineering expression, shear strength is ultimate or maximum shear stress which a soil mass can sustain and this strength is always related to surface or plane of rupture.

In this blog our attempt is to simplify all civil engineering terms to understand easily. The shear strength that we are discussing can be measured both in laboratory or filed. In determining shear strength several factor have to be considered and strength value for different factor will be definitely different. We will discuss each test method according to respective standard.

 Now come to the point ‘variability’. The variability in shear strength due to flowing facts:

a. Soil is naturally a heterogeneous material and it is very rare to deal with a foundation that is supported on homogeneous soil.

b. Pore water pressure in relation to drainage facility

c. The nature and type of construction

d. Stress history greatly alter shear strength and its parameters c and φ especially in clay

e. Disturbance of soil structure

f. Various chemical and environmental action