Factor Influencing The Shear Strength of Cohesionless Soil

The principal factors effecting the shearing strength of cohesion less soil are

1) Shape of particles

2) Gradation

3) Denseness

4) Confining pressure

5) Deviator stress

6) Intermediate principal stress

7) Loading

8) Vibration and repeated loading

9) Type of minerals

10) Capillary moisture

These parameters, stated above, are discussed below:

1) Shape of Particles

When other parameters is identical,angular shape particles of sand produce shear strength more than that of rounded shape particles.

2) Gradation

It is observed that a uniform graded sand have less shearing strength than that of well graded.

3) Denseness

When density increases interlocking increases to some extent. Consequently, the greater the denseness, the greater the strength.

Though the ultimate value of Ф′ is not affected by denseness, relative density(D_r) provides the value according to relation,

Ф′= 26^◦ +.2 D_r

4) Confining Pressure

Shear strength increases with an increase in confining pressure. But for the range of pressure in the common field problems, the effect of confining pressure on the angle of shearing resistance is not significance.

Shear Characteristics of Cohesionless Soil

Source of Shear strength of Cohesionless Soil

The friction between the particles cohesionless soil like sand and non-plastic silts provides shear strength. In case dense sands the friction as well as interlocking between particles also contributes to these strength.


Brittle Failure

Study on the stress-strain relationship shows that dense sands exhibits a relatively high initial tangent modulus. The stress reaches a maximum value at its peak at comparatively low strain and then decreases rapidly with an increasing strain and eventually becomes more or less constant.,


The dense sand shows initially a volume decrease in a drained test, but as the strain increases, the volume starts increasing.

In the case of dense sand, the specimen shows a clear failure plane and failure is known as brittle failure.


Plastic Failure

The stress-strain curve for a loose sand exhibits a relatively low initial tangent modulus. At large strains, the stress becomes more or less constant. The loose sand shows shows a volume decrease throughout increment of strains.

In the case loose sand, the specimen bulges (swell out) and ultimately fails by sliding simultaneously on numerous planes. The failure is known as plastic failure.


Determination of Failure Envelope

The failure envelope for dense sand can be derived either for the peak stresses or for the ultimate stresses. The value of the angle of shearing resistance (Φ^′) for the failure envelope for peak stress is considerably greater than for the ultimate stresses.

In the case of loose sands, as the peak stress and the ultimate stress are identical, there is only one failure envelope. The angle of shearing resistance in very loose state is approximately equal to the angle of repose. It should remember that the angle of repose is the angle at which a heap of dry sand stands without any support. It has been established that air-dry sand gives approximately the same value of Φ as the saturated sand. As it is easier to perform tests on dry sand, tests can be performed on dry sand instead of saturated sand.

If the failure envelope is slightly non-linear, a straight line may be drawn for the given pressure range and the angle of shearing resistance is taken as the slope of this line. The cohesion intercept, if any, is usually neglected.

Effect of Mixing Time on Properties of Concrete

The various components of a mix are proportioned so that the resulting concrete has adequate strength, proper workability for placing and cost effective. To achieve such properties the mixing should such that it produce an intimate mixture of cement, water, fine and coarse aggregate and suitable admixture of uniform consistency throughout each batch. The average strength of concrete increases with an increase in mixing time as it improve uniformity of mix.


Conventional Practice

In a construction site , it is common practice to mix concrete as rapidly as possible. But this time varies with type of mixture and basically it is not a function of time but of number of revolutions of the mixing. Generally, about 20 revolution are sufficient. As the manufacturer recommended certain speed of rotation, the number of revolutions and mixing time are independent.


Standard Value of Mixing


For a particular mixer, a relation between mixing time and uniformity is provided. Mixing less than 1 to 1.25 minutes result a significantly variable concrete but prolonging the time of mixing beyond these values render no significant improvement in uniformity. The length of mixing time also depends on quality of blending materials during charging of mixture.


The exact value of mixing time is also a function of size of mixture. A minimum mixing time of 1 minute for mixer size of 1 cum. yd (3/4 cum. m) and 15 additional seconds for each addition cubic yard giving satisfactory uniformity of mixing.

ASTM C 94-94


Mixing time is counted from the time when all the solid materials have been put in the mixer, and it is also required that all the water has to be added not later than after mixing time.

ACI 304R-89


Mixing time should count from the time when all the ingredient have been discharged into the mixer.



Exceptions


1) Many modern large mixer performs satisfactorily with a mixing time of 1 to 1.5 minutes.

2) In high speed pan mixers, the mixing time can be as short as 35 seconds.

3) When lightweight aggregate is used, the mixing time, less than 5 minutes, may hamper developments of strength for better result. Sometimes mixing of aggregate with water for 2 minutes is done first followed by 3 minutes mixing after cement is added.


Effect of Pro-longed Mixing Time

Generally, evaporation of water from the mix takes place resulting decrease in workability and increase in strength. The another effect is grinding of the aggregates, particularly if soft. The grinding makes aggregate more finer resulting lower workability. The friction effect also produces an increase in the temperature of the mix.

In case of air-entrained concrete, prolonged mixing reduces their content by about 1/6 per hour (depending on the type of air-entraining agent), while a delay in placing without continuous mixing causes a drop in air content by only about 1/10 per hour. On the other hand, a decrease in mixing time below 2 or 3 minutes may lead to inadequate entrainment of air.

Electrical Stabilization of Soil

Electrical stabilization of cohesive soil is performed using a process known as Electro-osmosis. A direct current (D.C) is supplied through clayey soil to migrate pore water to negative electrode (cathode). The strength of such soil is increased substantially as water is removed from it.

Electro-Osmosis

Electeo-osmosis is a method which drain water from cohesive soil with the help of direct current(D.C) . The cathode is a well point which collects the water, have to drained from the soil and discharges the water as in a conventional well-point system. 

Mechanism of Electro-Osmosis

Cation (positive ions) are formed in pore water when the dissolved minerals go into solution. These cations move towards the negatively charged surface of clay minerals to satisfy the electrical charge. As the water molecules act as dipoles, the cations also attract the negative end of dipoles. When the cations move to the cathode, they take with them the attached water molecules.

Installation of System

Anodes are in the form of steel rods located near toe of the slope of the excavation. Cathodes are in the form of perforated pipes, resembling well points, installed in the soil mass about 4~5 m away from the slope of cut. The electrodes are so arranged that the natural direction of flow of water is reversed and is directed away from the excavation. This arrangement is required to prevent sloughing of the slopes.

System Requirements

The system requires about 20~30 amperes of electricity per well at a voltage of 40~180v. The consumption of energy is between .5 to 10 KWh/Cum. of soil drained.

Significance of Electro-Osmosis

As considerable amount of water is removed from the soil mass, the strength properties are increased. It is also found that a small reversing of the direction of flow helps in increasing the stability of the slope even if there is no significant decrease in the water content of soil. So this process also increase the slope stability substantially.

Limitation of Electro-Osmosis

This process requires specialized and sophisticated equipment as well as electrical consumption of high amount is associated with this process. Thus it is a highly expensive drainage process compared with other method.

Suitability

This method should be used only in exception cases when other method can not be used. It is normally used to drain water in a cohesive soil of low permeability of

K=1 x 10^-5 to 1 x 10^-8 m/sec

Difference Between Magnitude and Intensity of Earthquakes

The stain energy released during an earthquake travels as seismic waves in all directions through the earth's layer, reflecting and refracting at each interface. These vibrations are more intense nearer the center of disturbance and these become feeble and ultimately die out, as the distance increased. Generally a common problem occur merging magnitude with intensity.

Magnitude:

Magnitude is a quantitative measure of the actual size of the earthquake. Based on maximum displacement, magnitude is determined and through magnitude energy released can be computed.


Intensity :

Intensity is a qualitative measure of the actual shaking at a location during an earthquake. Intensity is determined from three features of shaking -

1) perception by people and animal

2) performance of building

3) changes to natural surrounding.


Basic Difference

Magnitude defines size of a earthquake. As example, the size of an earthquake can be measured by the amount of strain energy released by the fault rupture. On the other hand, intensity is an indicator of the severity of shaking generated at given location. There is no doubt that the severity of shaking is much higher near epicenter than farther away.

This means that the magnitude of the earthquake is a single value for a given earthquake but different locations experience different degree of intensity for same quake.

Requirements For Frames in Regions of High Seismic Risk

In region of high seismic risk, structural frames proportioned to resist forces induced by earthquake motion should satisfy the requirements stated below :


1) Tensile steel ratio in flexural members should be well below balance steel ratio,Pb to ensure adequate rotation capacity at plastic hinges.


2) Use of high strength steel (having limited ductility) should be avoided.


3) Lateral reinforcement in column must serve not only their usual functions of column (ties or spiral) but also serve as shear reinforcement to provide adequate resistance to horizontal forces.

4) As much as possible longitudinal beam reinforcement should be carried through beam- column joints without interruption.

But when required special attention must be paid to bar anchorage .

5) At least minimum amount of flexural reinforcement should be used in both top and bottom throughout the length of all beams to allow for

a) possible shifts in points of inflection

b) load combination not accounted in design.


6) At least minimum amount of web reinforcement should be used throughout length of all beams and closed-hoop reinforcement should be provided in regions which can subject to plastic hinging to improve rotational capacity.

Determination of Effective Span Length

Effective Span length: A structure is generally represented by a simple line diagram using a center line distances between column and between floor beams. But, In practical cases, the column may have considerable width as well as the beam may have considerable depth leaving some modification of the respective length of these members. As width and depth of column and beam respectively, is accounted in these center line diagram, the derived results (moment, shear, torsion etc.) due to self weight and superimposed loads, from analysis is not practical and reasonable one. These difference between their center line distances and clear distances raised concept of effective of effective span length which results in a practical substantially accurate value of the results.

Significance of effective span length:In analysis of Beam as well as column the effective span length provides effective and sometimes economical results.

Beam Analysis

It is usually assumed that the members are prismatic having constant moment of inertia between center

lines,but a beam intersecting a column have a moment of inertia about infinite between column face to column center line. This is due to the consideration that the depth of the beam greatly increased in that region. Thus consideration of actual variation in member depth in analysis produce increased support moment followed by decreased span moment. In addition, it is apparent that critical section for design for negative bending would be at the center line, through an effective depth of unlimited value is available, in the region of support, in the beam.  

Column Analysis

The variation of width and moment of inertia stated above is also applicable for columns.

Nature of Variation

Beam

The slope of moment diagram for the beam is quite steep in the region of the support producing substantial difference between the support center line moment and face moment. If a section is designed considering center line moment, a unnecessary large section will result. It is economical also desirable to reduce support moments by elastic analysis to account for the finite width of the supports.

The difference in moment between support center line and support face is
equal to (Val)/3

Where , l=center line distance between support,

a l= column width

a= a ratio to column width to c/c distance between support.

Column

It is observed that,in the case of columns, the gradient of moment curve is not as steep as that for beam. So the difference between center line moment and the moment at the top and bottom face of the beam is small and in most cases it is disregarded.

Method of Approaches to analysis

Structural designers frequently uses two methods to analysis the frames. These are

1) The structure is simplified using simple line diagram and a deduction of (Val)/2 is done from center line moment without adjusting for the higher stiffness with in the thickness width of the column. This method is less realistic.

2) This involves the consideration of a rigid link within the width of the column, connecting the column center line with clear span of the flexural member. In case of column analysis, the portion of the column within the depth of the beam can also be represented using a rigid link. These is both realistic and easy to implement in matrix analysis programs.

Consequence of This Concept

Consequence of the concepts of effective span is reduction in congestion in the beam-column joint location where it is often difficult to place concrete because of the high quantity of reinforcing steel from the flexural members framing into the column (usually from two different directions) and from the column itself. But, a somewhat higher percentage of reinforcement required at midspan usually causes little difficulty in concrete placement.

Effective span for simply supported and continuous beams are as follows:

1) Simply supported beams

The effective span of a simply supported beam shall be taken as the smaller of the distance between the centers of bearing, or the clear distance between supports plus the effective depth.
2) Continuous beams

If the width of support is less than 1/12 of the clear span, the effective span shall be taken as stated (1) above. If the supports are wider than 1/12 of the clear span or 600 mm, whichever is less, the effective span shall be as follows:

a) For end span with one fixed and the other continuous or for intermediate spans, the effective span shall be the clear span between supports.

b) For end span with one end free and other continuous, the effective span shall be equal to the clear span plus half the effective depth of the beam or the clear span plus half the width of the discontinuous support, whichever is less.

3) Monolithic frames

In case of monolithic frames, the effective span shall be equal to the distance between intersections of the center lines of the connecting members.
4) Cantilever beams


The effective length of the cantilever shall be taken as its length to the face of the supports plus half its effective depth except where it forms the end of a continuous beam where the length to the center of the support shall be used.