Hydroconsolidation of Loess Soil

Loess soil lose its strength and suffer severe volume change when it get saturated. This behavior of loess makes it susceptible to supported foundation. The volume change is the most prominent structural and physical property of it. This collapse with contact of water is sometimes called hydroconsolidation, sometimes called hydrocompaction.

Montmorillonite present in loess soil as clay mineral has great sensitivity to moisture producing an unstable soil structure. Consolidation always associated with settlement which is unexpected in foundation design, at least up to certain tolerance. 

We are discussing about montmorillonite in our previous many posts; here we like to provide some information about its properties and structure. Details of it will be discussed in our upcoming post where we are going to provide clay mineralogy and soil structures, to explain different properties of soil at micro level.

The mineral structure of montmorillonite is such that exchangeable ions and water can easily take place between layers which can cause layers to separate. It is provided that montmorillonite is formed by sandwiching of silica sheets and octahedral sheets.

So we have idea about affinity of montmorillonite to water. This structure makes its susceptible to large volume change which can be determined by activity of clay soil as discussed in previous post.

The montmorillonite swells when it gains water into its lattice structure and reverse when water is removed from it due to seasonal or other causes. It has low internal friction and high plasticity when it is in moist state. Now a question may arise, under saturation montmorillonite swells, why does loess settle or collapse under saturation?

The answer is swelling behavior is valid when total swell mass has majority portion of clay (montmorillonite). But in loess there have little clay content that just coat silt particles and shows temporary cohesion resulting high strength before getting wet. 

When loess soil gains moisture, the bonds provided by clay content and also calcite (low content) becomes weak and dramatically reduce soil strength from its previous dry state.

So a localized collapse or consolidation is observed to undergo with a slight change in

-moisture content

-clay content

Thus, saturation of loess soil results consolidation of it under much lower stresses than that of dry. Thus in hydroconsolidation, loading is not only key factor for consolidation of soil, which is often less important contributor than moisture variation as this soil may collapse only under saturation even application of little or no loading.

Shear Strength Parameter of Loess Soil

Dear reader we have discussed about permeability of loess soil in previous post, here we will discuss about another engineering property of loess, shear strength. We will discuss about shear strength parameter- friction angle and cohesion here.

Being spread over many parts of the world, we have to learn about behavior of loess soil and shear strength is one of the most important properties that are required in designing foundation of building.

The shear strength parameters (cohesion and internal friction angle) depend on following factors of loess:

a. Clay content in loess

b. Moisture content

c. Initial density

It was reported by sheeler, 1968 that for a soil sample having moisture content below its saturation, the internal friction angles remain in the range of 28 degree to 36 degree.

Cohesive strength of soil usually falls between (0-70) psi. The higher value indicates soil sample of higher unit weight.

Sheeler also found that when clay content in loess is increased the cohesive strength is also increased.

The findings also followed that collapse nature of loess i.e. the shear strength of dry and wet loess soil have remarkable difference. Though with moisture content all types of soils lose their strength, the loess shows much more reduction in shear strength which definitely indicates its collapse nature. Thus a dry soil sample shows better resistance to shear failure for a certain load and greater cohesion too.

Permeability of Loess Soil

Dear reader we have discussed about factors that control permeability of soil in previous post. The factors are recalled here:

a. Size and shape of particles

b. Gradation of soil

c. Void ratio

d. Continuity

e. Soil structure etc.

Here we are concerned with loess soil. We will try to include some observation about loess soil, provided in last fifty years. These above factors are provided by Howe in 1961.

So permeability of soil varies with many local features. In case of loess vertical permeability is much greater than horizontal permeability. 

Sheeler, 1968

Under a specified load, after completion of consolidation, the vertical permeability is found in between (1x10-5 to 1x10-3) cm/sec. 

Bandyopadhyay, 1983

He worked with Peoria loess, Kansas; he concluded that vertical permeability was around 9x10-4 cm/sec which is much greater than horizontal permeability. The higher permeability associated by presence of vertical shrinkage and tubules joints in soil deposit with many other causes.

But determining the permeability of loess soil is how much realistic as the structure of loess collapse under saturation.

In 1961, Terzaghi expressed his view about loess; the permeability of this soil is quiet elusive as soil structure changes under saturation. Sheeler, 1968 also concluded that the structure becomes dense and break down to reduce permeability.

Dear reader we will discuss about shear strength parameter of loess soil in our next post.

Specification for Alkali-Silica Reaction Due to Fine Aggregate in Concrete

Dear reader we are discussing this topic according to ASTM C33 which provides specification for aggregates for concrete. Dear reader we have discussed about deleterious substances present in fine aggregates as a constituents of concrete. Here we will discuss about impurities that results alkali-silica reaction.

The effects of alkali-silica reaction in concrete are beyond scope of this post. The concrete that is subjected frequently to following exposure, it is expected that no materials that produce deleterious reaction within concrete:

a. Wetting

b. Extensively exposed to atmosphere where high humidity is common

c. Extensively exposed to moist ground

These materials in fine aggregates react with alkalies present in cement and produce sufficient amount of expansion of mortar or concrete itself. Details explanation of alkali-silica reaction will be published very soon in upcoming posts.

But ASTM specified to use such fine aggregate that have sufficient amount of impurities to do that harm, only when cement used for concrete proportioning have alkali content less than 0.60 present. The amount is determined making equivalent to sodium oxide (Na2O+0.658K2O).

ASTM also allowed such aggregates when materials are included in mix design that can prevent or control alkali-aggregate reaction. Dear reader we have discussed about such materials in our posts regarding glass sand, where also alkali-silica reaction is the prime factor. We will provide mitigation measure according to ASTM C33 in next post.

Searching Faults: As Earthquake Geologic Evidence

Geologic evidence for earthquake hazard analysis is mainly focused on pointing out of faults. The techniques that are introduced by geologist are:

a. Remote sensing imagery (infrared photography)

b. Examining air photos

c. Reviewing published literature

d. Field reconnaissance with trench log

e. Boring and test pits

f. Geophysical 

Evidence of earthquake ground movement by rippling of rails

Reiter suggested following features for identifying faults:

1. Indication of fracturing on the surface and clearly noticeable fractures on the surface which includes:

a. Slickensides

b. Fault gauge

c. Fault breccias

We know these terms are new to you; the explanations are 

a. Disruption ground surface

b. Evidence of ground movement

c. Both sides of fault is grinded

2. We know geologic strata are an essential portion of geologic map. We can study on such strata and look for presence of

a. Dissimilar materials

b. Repeated or missing strata

c. Truncation of structures or strata

The rest features will be discussed in next part where we will include:

a. Geomorphic and topographic indicators

b. Secondary geologic characteristics

c. Remote sensing imagery

d. geophysical indicators

e. Geodetic indicators

In important and larger project with geotechnical investigation it is also done.

We are now discussing rest features as continuation of previous post as follows:

3. Geomorphic and topographic indicators. Land form is concerned in geomorphic indicator. These indicators include:

a. Offsets in drainage or stream 

b. Change or tilt in elevation of shorelines or terraces

c. Sag ponds which includes water accumulation near faults (strike-slip) in depression

d. Anomalous gradient in streams

e. Topographic scraps

f. Triangular corner or ridges

4. Secondary geologic characteristics:

These includes

a. Juxtapositions of hot springs

b. Changes in chemical composition, alignment and gradient of

–springs

-volcanic vents

5. Features of imagery found from remote sensing device which includes, contrast in

a. vegetation

b. topography

c. tone

6. Geophysical indicators which includes subsurface faulting. The indicators are

a. offsets on earthquake reflection horizons

b. difference in earthquake wave velocity

c. steep magnetic gradient or linear gravity

7. Geodetic indicators which include fault movement appeared as changes in distance or tilting from previous points in geodetic surveys.

Credibility of Soundness Tests of Concrete Aggregates

We have already learnt about many aspects of soundness and different test methods for determining soundness of aggregates for concrete construction. The tests we have and will discuss are:

a. Sulfate soundness (BS 812 and ASTM C88) test

b. Freezing and thawing test

c. Methylene blue test

The last two tests will be discussed later. ASTM C33 recommended to perform freezing and thawing test when aggregate fail to pass sulfate soundness test. Methylene blue test is conducted to identify expansive clay minerals remains in some aggregates.

 

There have also other tests to determine soundness. But no one produce reliable results that can actually measure soundness. ASTM C88 test measures reduction in aggregate size under formation of salt crystal into pores of aggregates. BS 812 also produces similar types of measure. They just produce a qualitative measures of soundness depending on which we cannot reject a stock of aggregates.

ASTM C33 provides limits of loss 10% and 15% for sodium sulfate and magnesium sulfate test respectively. Freezing and thawing test also produce a qualitative measure of soundness. Now why we are emphasizing on the word ‘qualitative’? This is due to:

We are ignoring presence of hydrated paste of cement surrounding aggregate on the behavior or performance of them under laboratory disruptive action. Therefore we require a satisfactory service record of concrete produced with identical aggregate from identical source under identical expected durability exposure.

How is Frost Damage Simulated in Soundness Testing for Concrete Aggregate?

Dear reader we have learnt about soundness test of concrete aggregate according to BS 812. Here we will learn about ASTM C88 Test Method for soundness test of aggregate for concrete. Here our main concern is about mechanism of disruption of aggregates during sulfate test for measuring soundness and its relation to frost damage in aggregates.

As in British standard Test for soundness, there also graded sample of aggregates are immersed in sulfate solution. The solution may be magnesium sulfate or sodium sulfate. After immersion this samples are dried in oven.

Under these immersion-drying cycles (several cycles are done) the aggregate get disrupted. Why and how are they disrupted, we will discussed later? The aggregate under disruption get disintegrated which results reduction in aggregate size.

Sieve analysis is done to determine reduction in size which defines how unsound the aggregates are. Now we will learn why aggregates are disrupted?

When immersion-drying cycles are done salt solution remains in pores of aggregate get crystallized while drying in oven. These crystallization and consecutive rehydration of sodium sulfate or magnesium sulfate produce disruptive effects on the aggregates. These actions are similar to action of ice in aggregate and when a aggregate sample shows sufficient resistance to such disruption of salt, it is believed to have adequate resistance to frost action.

Soundness Test of Concrete Aggregate According to BS812

So far we have learnt about detrimental impact of unsound aggregate. Now we have to learn how to determine soundness of aggregate for concrete. Here we try to measure the disruption of aggregates due to formation of crystals of salt. Here we are discussing about British test method, in the next post, we will discuss about ASTM C88 which also used to measure to account degree of soundness of aggregate.

In this test, graded aggregate are immersed in salt solution and then dried in several times. Under this process the aggregate sample subjected to disruption and we will measure the degree of disruption.

Here listing some conditions and requisite testing materials; those are as follows:

a. Graded aggregate means aggregate sizes between (10-14) mm.

b. The salt solution is of magnesium sulfate must be as saturated

c. Drying means oven drying at (105-110)⁰C

d. The immersion and drying cycles are of five times.

From the above discussion we have already understand what are doing in this test? But for your convenience we are summarizing these as:

Aggregate sizes of above limit are alternately immersed in salt solution and oven dried for five times at provided temperature. These aggregates break up under such operation and we will measure the proportions of breaking up.

Now soundness value is found as percentage which is determined by measuring proportions of aggregate having size greater than 10 mm after immersion-drying cycles to original mass of samples.

What is Plate Tectonics as Earthquake Terminology? What are Aseismic and Seismic Deformation?

Dear reader we have discussed about continental drift in the very beginning posts of our blog. We have also learnt about formation of Mount Everest and Andes earlier. This theory shows us that the massive continents are pushing across ocean floor and through seas by some forces.

Plate tectonic is involved with above theory. Here earth surface is considered as coalition of number of intact blocks termed as plate. These large plates move past each other by a driving force. These plates are

1. African plate

2. American plate

3. Antarctica plate

4. Australia-Indian plate

5. Eurasian plate

6. Pacific plate

Major tectonic plates with direction of movement showing other details as in legend

There have also 14 subcontinental plate also like

1. Caribbean

2. Cocos

3. Nazca

4. Philippine etc

These are also called microplates.

Microplates were broken from large plates (major plates) near boundaries of many plates. The deformations generated between plates are found to occur near narrow zones of their boundaries, some deformations are continuous and slow and some are sudden.

The slow and continuous deformation will not produce earthquake i.e. aseismic deformation and the sudden deformation produce earthquakes i.e. seismic deformation.

Seismic activity over the world showing epicenters of significant earthquake

According to this theory, as deformations mainly occur at plate boundaries, it is also expected to found epicenters of earthquake near plate boundaries. The figure provided above showing earthquake epicenters also supports this theory.