Tuesday, 9 January 2018

Damages in Earthquake

Damages Due To Earthquakes

Earthquake is a natural catastrophe that may instantly kill or incapacitate a large number of people, cause huge destruction to structures, and weaken the buildings reducing their useful life. The damage is the maximum close to the epicenter, the point from where the vibrations are initiated. Different types of damages can occur due to earthquakes, and these are discussed in succeeding paragraphs.

Liquefaction

Liquefaction starts with the forceful shaking of the soft wet soils, and rearrangement of its grains, due to which the soils start functioning as liquids. The load of structures is transmitted to the wet soils that may be changed into quicksand. The material that has been liquefied, may loose its bearing strength due to the excessive weight of the structures above it, and produce landslides. Consequently, the fluid pressure of the liquefied region may cause tilting or breaking of walls, failure of basement floors, and if the foundations are weak severe damage to the structures may occur. Any items or materials above the liquefied soil may be submerged into the soft soil. The liquefied region may itself also go downwards into the earth, and in the process bury anything on it.

Ground Shaking

The magnitude of ground shaking at a particular location will determine the earthquake damage. The extent of ground shaking will depend upon the scale of an earthquake, distance from the epicenter, and nature of the material. When the earthquakes are great, the amplitude is large, duration is more, and the area is vast. The amplitude of ground shaking at a site depends upon its distance from the epicenter of the earthquake, and it decreases with the increase in distance. Similarly, motions are of low frequency when these are located at greater distances. The ground motion frequency is a significant feature that determines the extent of damage to the structures, and the nature of construction that can be affected.

Structural Hazards

Earthquakes are a severe structural hazard that causes vibrations in the structures due to the ground shaking. If the structures are weak, or extremely rigid to withstand severe vibrations, then these may collapse. The tall buildings may experience extreme vibrations due to their height, and may fall down or into each other. Other destructive effects on structures due to an earthquake are sliding away from their foundations, and their horizontal or vertical movements that may make the structures unsafe.

Other Hazards

Other hazards that may cause earthquake damage include fire that can be started on the rupturing of power or gas lines, and severe losses may occur. In addition, bricks, rocks, trees may fall, sewage may enter water supplies and drinking of such water can cause serious diseases, and failure of transportation and means of communication may hinder rescue efforts. Furthermore, valuable records held by business concerns and governmental offices may be lost creating serious difficulties.

Preventive Measures

Preventive measures may reduce the destructive effects due to earthquakes, but may not completely eliminate the risk of damages. If a building is not properly designed to withstand earthquakes, it will be exposed to greater risks of structural damage. Suitable fixing of the structure with the foundation, and among the different constituents of the structure, is important for earthquake resistance. Structures that are not properly connected with the foundations may be shifted during an earthquake.

 

Maximum Dry Density And The Optimum Moisture Content 

INTRODUCTION

For any soil, for a given amount of compactive effort, the density obtained depends on the moisture content. At very high moisture contents, the maximum dry density is achieved when the soil is compacted to nearly saturation, where (almost) all the air is driven out.

This test is done to determine the maximum dry density and the optimum moisture content of soil using heavy compaction as per IS: 2720 (Part 8 ) – 1983.

APPARATUS

(i) Cylindrical metal mould – it should be either of 100mm dia. and 1000cc volume or 150mm dia. and 2250cc volume and should conform to IS: 10074 – 1982.

(ii) Balances – one of 10kg capacity, sensitive to 1g and the other of 200g capacity, sensitive to 0.01g

(iii) Oven – thermostatically controlled with an interior of noncorroding material to maintain temperature between 105 and 110'C

(iv) Steel straightedge – 30cm long

IS Sieves of sizes – 4.75mm, 19mm and 37.5mm

PREPARATION OF SAMPLE

A representative portion of air-dried soil material, large enough to provide about 6kg of material passing through a 19mm IS Sieve (for soils not susceptible to crushing during compaction) or about 15kg of material passing through a 19mm IS Sieve (for soils susceptible to crushing during compaction), should be taken. This portion should be sieved through a 19mm IS Sieve and the coarse fraction rejected after its proportion of the total sample has been recorded. Aggregations of particles should be broken down so that if the sample was sieved through a 4.75mm IS Sieve, only separated individual particles would be retained.

PROCEDURE

(A) Soil not susceptible to crushing during compaction

 1. A 5kg sample of air-dried soil passing through the 19mm IS Sieve should be taken. The sample should be mixed thoroughly with a suitable amount of water depending on the soil type (for sandy and gravelly soil – 3 to 5% and for cohesive soil – 12 to 16% below the plastic limit). The soil sample should be stored in a sealed container for a minimum period of 16hrs.

 2. The mould of 1000cc capacity with base plate attached, should be weighed to the nearest 1g (W1 ). The mould should be placed on a solid base, such as a concrete floor or plinth and the moist soil should be compacted into the mould, with the extension attached, in five layers of approximately equal mass, each layer being given 25 blows from the 4.9kg rammer dropped from a height of 450mm above the soil. The blows should be distributed uniformly over the surface of each layer. The amount of soil used should be sufficient to fill the mould, leaving not more than about 6mm to be struck off when the extension is removed. The extension should be removed and the compacted soil should be levelled off carefully to the top of the mould by means of the straight edge. The mould and soil should then be weighed to the nearest gram (W2).

 3. The compacted soil specimen should be removed from the mould and placed onto the mixing tray. The water content (w) of a representative sample of the specimen should be determined.

 4. The remaining soil specimen should be broken up, rubbed through 19mm IS Sieve and then mixed with the remaining original sample. Suitable increments of water should be added successively and mixed into the sample, and the above operations i.e. (2.) to (4.) should be repeated for each increment of water added. The total number of determinations made should be at least five and the moisture contents should be such that the optimum moisture content at which the maximum dry density occurs, lies within that range.

(B) Soil susceptible to crushing during compaction

Five or more 2.5kg samples of air-dried soil passing through the 19mm IS Sieve, should be taken. The samples should each be mixed thoroughly with different amounts of water and stored in a sealed container as mentioned in Part (A)

(C) Compaction in large size mould

 For compacting soil containing coarse material upto 37.5mm size, the 2250cc mould should be used. A sample weighing about 30kg and passing through the 37.5mm IS Sieve is used for the test. Soil is compacted in five layers, each layer being given 55 blows of the 4.9kg rammer. The rest of the procedure is same as above.

REPORTING OF RESULTS

 Bulk density Y(gamma) in g/cc of each compacted specimen should be calculated from the equation..

 Y(gamma) = (W2-W1)/ V

where, V = volume in cc of the mould

The dry density Yd in g/cc

Yd = 100Y/(100+w)

The dry densities, Yd obtained in a series of determinations should be plotted against the corresponding moisture contents,w. A smooth curve should be drawn through the resulting points and the position of the maximum on the curve should be determined. A sample graph is shown below:

 The dry density in g/cc corresponding to the maximum point on the moisture content/dry density curve should be reported as the maximum dry density to the nearest 0.01. The percentage moisture content corresponding to the maximum dry density on the moisture content/dry density curve should be reported as the optimum moisture content and quoted to the nearest 0.2 for values below 5 percent, to the nearest 0.5 for values from 5 to 10 percent and to the nearest whole number for values exceeding 10 percent.

Atterberg Limits - Plastic Limit 

INTRODUCTION

The plastic limit is one of 5 limits developed by A. Atterberg, a swedish scientist.  The plastic limit is one of the most commonly performed of the Atterberg Limits along with the Liquid Limit.  These 2 tests are used internationally to classify soil

The plastic limit is defined as the moisture content at which soil begins to behave as a plastic material.  A plastic material can be molded into a shape and the material will retain that shape.  If the moisture content is below the plastic limit, it is considered to behave as a solid, or a nonplastic material.  As the moisture content increases past the plastic limit, the liquid limit will be approached.  The liquid limit is defined as the moisture content at which the soil behaves like a liquid.

EQUIPMENT

Soil sample,Mixing dish,425 micron Sieve and pan Spatula ,Glass plate,Water.

PROCEDURE

 1. Obtain equipment outlined above for the Plastic Limit test.

 2. Weigh 3 metal moisture content containers and record the weights.  Keep track of the containers and their weights.

 3. Using the soil provided or your own sample of dry material, pulverize about a handful of it using the small soil pulverizer. The pulverizer breaks the material up into particle sizes that will pass the 425micron sieve in accordance with the ASTM standard for this test. Any material not passing through the pulverizer can be discarded.

 4. Put the soil into the mixing bowl and add enough water so that the sample can be easily molded into a ball.

 5. Obtain a ball about the same diameter as a nickel and place the ball on the glass plate.  Roll the ball into a thread of approximately 1/8 inch diameter. 

 6. If the thread crumbles before 1/8 inch diameter is reached, the sample is too dry and water must be added.  If the thread is easily rolled to 1/8 and even smaller, the sample is too wet and must be dried by working the soil with the hands.

 7. When the sample just begins to crumble at 1/8 inch diameter, this is the plastic limit.

 8. Immediately placed the thread of soil into a cup and obtain a wet weight of soil.  Place the soil in the oven and obtain the moisture content at the next lab meeting.

 9. Repeat test once more to obtain an average of 2 tests.

Atterberg Limits - Liquid Limit 

INTRODUCTION

The liquid limit is one of 5 limits developed by A. Atterberg, a swedish scientist. The liquid limit is one of the most commonly performed of the Atterberg Limits along with the plastic limit. These 2 tests are used internationally to classify soil

The liquid limit is defined as the moisture content at which soil begins to behave as a liquid material and begins to flow. The liquid limit is determined in the lab as the moisture content at which the two sides of a groove formed in soil come together and touch for a distance of 2 inch after 25 blows. Since it is very difficult to get this to occur exactly, we will run the test repeatedly until the groove closes 1/2 inch with over 25 blows and under 25 blows. We can plot these results as blow count versus moisture content and interpolate the moisture content at 25 blows from this graph.

EQUIPMENT

Soil sample, Metal Mixing Bowl,Small Spatula, Liquid Limit Device, Water, IS Sieve of 425 Micron, Weight Ballance.

PROCEDURE

1. Obtain equipment Required for the Liquid Limit test.

2. Weigh 3 metal moisture content containers and record the weights. Keep track of the containers and their weights.

3. Pass the soil sample through IS Sieve of 425 Micron.

4. Now take about 120gm soil sample passed through 425 micron sieve & put the soil into a metal mixing bowl and add enough water so that the sample has a creamy texture like smooth peanut butter.

5. Adjust the drop height of the liquid limit device to 10mm using the block end of the grooving tool. Measure from the block to where the bowl hits the block.

6. Place the wet soil sample in the liquid limit device as shown below. This should be done by first turning the crank so that the bowl is resting on the base. The soil should fill the bowl similarly to the way water would fill the bowl. The sample should be smoothed and curved somewhat towards the bottom of the bowl. The depth of the soil sample should be no deeper than the triangular extrusion on the end of the grooving tool.

7. When the soil sample as adequately placed in the bowl, use the grooving tool to cut a groove through the sample as shown below. The bottom of the brass cup should be seen.

8. At this point, turn the crank at a rate of 2 turn per second until the groove closes 1/2 inch, as shown below and keep track of the blow count. Record the blows on the data sheet and obtain a sample for a moisture content.

9. Repeat the test. If the blow count from the first try was greater than 25 blows, add some water and repeat. If less than 25 blow were obtained, add dry soil, mix extremely well, and repeat until a data point above and a data point below 25 blows is obtained.

Sieve Analysis Of Soil 

INTRODUCTION

Sieve analysis of different types of soil is necessary to know the percentage of material of different size in it. Each type of soil contains materials of different size & properties. By Sieve analysis we can break the soil sample in different size of particles & hence we can know the percentage of materials of different size. Each type of soil has a limit for each size of material, which can be checked by sieve analysis.

Sieve Analysis Of Soil For Granular Sub Base

APPARATUS

A set of IS Sieves for gradation of soil for GSB (75mm, 53mm, 26.5mm, 4.75mm, 75 micron, pan)

Weight Ballance

Observation Sheet

PROCEDURE

A Representative sample of soil will be taken for sieve analysis of soil.Take weight of Sample & note it on Observation sheet.Now Arrange the sieves in decreasing order.Sieve the soil sample with the sieves & note down the weight retained on each sieve on observation sheet.Calculate the % of weight retained on each sieve, passing of material on observation sheet

RESULT

If Your soil sample passed 100% from 75mm sieve then it means that your GSB is of grade-I

If Your soil sample passed 100% from 53mm sieve then it means that your GSB is of grade-II

If Your soil sample passed 100% from 26.5mm sieve then it means that your GSB is of grade-III

Check the desired limits for each size of material on observation sheet.