STANDARD PENETRATION TEST (SPT) - PROCEDURE & ADVANTAGES

Standard Penetration Test

The standard penetration test is an in-situ test that is carried out to provide information on geotechnical engineering properties of soil. The test is carried out in borehole.The test will measure the resistance of the soil strata to the penetration undergone.The main purpose of the test is to provide an indication of the relative density of granular deposits such as sand and gravels. It can also be used to determine the confined compressive strength of cohesive soil. This test is widely used because it is a simple and inexpensive. 

Apparatus For Standard Penetration Test

1. Standard Split Spoon Sampler
2. Drop Hammer of weight 63.5 kg
3. Drilling Rig
4. Driving Head
5. Guiding Rod

Procedure For Standard Penetration Test

The test is conducted in bore hole by means of a standard split spoon sampler. Once the drilling is done to the desired depth, the drilling tool is removed and the sampler is placed inside the bore hole. The standard split spoon sampler 50.8 mm outer diameter and 35 mm inner diameter, is driven into the undisturbed soil at bottom of the bore hole by blows from a slide hammer with a weight of 63.5 kg falling through a height of 750 mm at the rate of 30 blows per minute, the sample is driven into the soil.

The number of blows of hammer needed for the tube to penetrate each 150 mm is counted. Further it is driven by 150 mm and the blows are counted. Similarly, the sample is once again further driven by 150 mm and the number of blows recorded. The number of blows recorded for the first 150 mm is not taken into consideration. The number of blows recorded for last two 150 mm intervals are added to give the standard penetration number (N). 

In case if the number of blows for 150 mm drive exceeds 50, it is taken as refusal and the test is discontinued. The standard penetration number is corrected for extensive correction and overburden correction.

Advantages Of Standard Penetration Test

• Test is simple and inexpensive.
• Able to penetrate dense layers, gravel etc.
• Test can be applicable for variety of soil conditions.

Disadvantages Of Standard Penetration Test

• Test is time consuming.
• The sample retrieved for testing is disturbed.
• The results are not very precise and highly reliable.

Vane Shear Test

Vane shear test is used to determine the undrained shear strength of cohesive soils especially soft clay. This test is done in laboratory or in field directly on the ground. This test is carried out with equipment consisting of a rod with vanes mounted to it that is inserted into the ground and rotated. This test was introduce by L Carlson and AW Skempton in 1948. This test give accurate results for soils of low shear strength i.e, less than 0.3 kg/cm2.

Apparatus Required

1. Vane Shear Apparatus
2. Vernier Callipers
3. Soil Specimen Container

Vane Shear Apparatus

Procedure Of Vane Shear Test

1. Clean the vane shear apparatus and apply grease to the lead screw for better movement off handles.

2. Take the soil specimen in container which is generally 75 mm in height and 37.5 mm in diameter.

3. Level the soil surface on the top and mount the container on the base of vane shear test apparatus using screws provided.

4. Lower the vane gradually into the soil specimen until the top of vane is at a depth of 10 -20 mm below the top of soil specimen.

5. Note down the reading of the pointer on circular graduated scale which is initial reading.

6. Rotate the vane inside the soil specimen using torque applying handle at a rate of 0.1°/sec.

7. When the specimen fails, the strain indicator pointer will move backwards on the circular graduated scale and at the point stop the test and note down the final reading of pointer.

8. The difference between initial and final reading is nothing but the angle of torque.

9. Repeat the procedure on two more soil specimens and calculate the average shear strength value.

10. Measure the diameter and height of vane using vernier caliper.

11. Sensitivity of given sooil sample is determined by repeating the above test procedure on remoulded soil which is nothing but soil obtained after rapid stirring of vane in the above test.

Sensitivity (St) = Undisturbed shear strength/Remolded shear strength.

Advantages Of Vane Shear Test

• This test is flexible.
• Vane shear test is quick and easy.

Disadvantages Of Vane Shear Test

• It cannot be conducted on the fissured clay.
• This test is not suitable for clays which contain sand or silt laminations in it.

WHAT IS SOIL CEMENT- TYPES, ADVANTAGES, DISADVANTAGES AND APPLICATIONS.

Soil Cement

Soil cement is a construction material, a mix of pulverized natural soil with small amount of Portland cement and water, usually processed in a tumble, compacted to high density. Hard, semi-rigid durable material is formed by hydration of the cement particles.

Soil cement is frequently used as a construction material for pipe bedding, slope protection and road construction as a sub base layer reinforcing and protecting the sub grade. It has good compressive and shear strength but is brittle and has low tensile strength so it is prone to forming cracks.

Soil cement mixture differs from Portland cement concrete in the amount of paste.While in Portland cement concretes the paste coats all aggregate particles and binds them together in soil cement the amount of cement is lower and therefore there are voids left, and the result is a cement matrix with nodules of uncemented material.

Types Of Soil Cement

1. Cement Modified Soil
2. Cement Treated Base
3. Soil Cement Base
4. Acrylic Copolymer

1. Cement Modified Soil (CMS)

Cement modified soil  is a sub base solution that blends cement and water with native (in-situ) soils to improve undesirable soil properties. The engineered mix forms a weather-resistant work platform for construction operations and a stronger, permanent sub grade for enhanced pavement support and capacity. Silt and clay soils, particularly when they are wet, can lead to construction problems. These soils can be soft, plastic, and difficult to compact. Cement modified soil is used to improve the engineering properties and construction characteristics of silt and clay soils by reducing this plasticity and enhancing the compaction and strength of the treated soil.

2. Cement Treated Base (CTB)

Cement treated base is a strong frost resistance base for a concrete or asphalt pavement wearing surface.Cement treated base is a mixture of aggregate materials or granular soils combined with measured amounts of Portland cement and water that hardens after compaction and curing to form a  durable paving material.

3. Soil Cement Base (SCB)

Soil cement base contains high amount of cement compared to cement modified base.It is commonly used as a cheap pavement base for roads, streets, airports and material handling areas. A seal coat is required in order to keep the moisture out. To avoid wear and tear a final coat of asphalt concrete is placed for pavement construction. 

4. Acrylic Copolymer

Acrylic copolymer is also called as rhino snot.It is a water-soluble acrylic polymer. This gains a property to penetrates the soil or sand mix and bind together.This is hence used instead of cement.When the whole mixture dries up, it become waterproof, UV resistant, solid bond which binds the soil together and reducing the dust. In higher concentration it creates a durable surface that can withstand heavy traffic, allowing existing soil to be used for roads, parking lots, trails and other heavy traffic areas.

Advantages Of Soil Cement

• Improve strength.
• Highly economical.
• Improve performance.
• Better weather resistant.
• Maintenance cost is less
• Reduce plasticity/cohesiveness.

Disadvantages Of Soil Cement

• Required proper supervision.
• Not suitable for some soil.
• Cracks may form in soil cement.
• Harmful for environment.
• Required more labor.

Applications Of Soil Cement

• Soil cement bricks.
• Airport pavements.
• Parking lots.
• City roads.

6 TYPES OF ROLLERS USED FOR CONSTRUCTION WORK

 Types Of Rollers

Rollers are the construction equipment used for the compaction of soil, sand, gravel and crushed stone etc.There are mainly 6 types of rollers:

1. Cylindrical Rollers
2. Sheepsfoot Rollers
3. Vibratory Rollers
4. Pneumatic Rollers
5. Smooth Wheeled Rollers
6. Grid Rollers

1. Cylindrical Rollers

Cylindrical rollers are light weight roller and are pushed by a person.The size varies but it is generally about 1 meter in diameter and about 1.5 meter long.These rollers are used mainly for small and private projects.It is made of iron, stone or concrete.The ground pressure generated by this type of roller is about 7 kg/cm2.

2. Sheepsfoot Rollers

Sheepsfoot rollers is also known as padfoot or tamping rollers.It consists of a drum having many round or rectangular shaped feet or lugs on it.It is available in different diameters and width of drum and different lengths and shapes of feet.Sheepsfoot rollers are good for compacting cohesive soils and silty clay in road construction work.

3. Vibratory Rollers

Vibratory rollers are most commonly used road rollers.This type of roller are fitted with one or two surface steel wheels 0.9 to 1.5 m in diameter and 1.2 m to 1.8 m in width.A vibratory roller is used for compacting granular base courses.It sometimes used for asphaltic concrete work.Self propelled vibratory rollers are now available weighing from 4 to 6 tonnes.

4. Pneumatic Rollers

Pneumatic roller has a number of rubber tires at the front and at the rear end. Empty spaces left in between the two tires that make 80% coverage area under the wheels. Pneumatic roller has the ability to exert contact pressure ranges from 500 - 700 Kpa. Pneumatic tired roller can be used for highways, construction of dams and for both fine grained and non-cohesive soils.It is also for smoothening of finishing bitumen layer on highways, roads, streets etc.

5. Smooth Wheeled Rollers

Smooth wheeled rollers consists of a large steel drum in front and one or two wheels or drum on the rear end.These are either tandem or three wheel rollers. Smooth wheel rollers are ideal for gravel, sand, ballast and surface dressings. They are not effective in compacting embankments and softer turf, but they are the most effective in compacting silty or sandy soils with the fewest passes. The weight of tandem roller varies from 2 to 8 tonnes and that of two wheeled roller varies from 8 to 10 tonnes. The ground pressure exerted by tandem rollers is about 10 to 17 kg/cm2

6. Grid Rollers

Grid rollers have a cylindrical heavy steel roller consisting of a network of steel bars coming together to form a grid like pattern. The grid may also be ballasted with concrete blocks for additional contact pressure. These rollers are ideal for compacting most coarse grained soils, since they provide little kneading action with high contact pressure. They are generally towed units and can operate at speeds between 5 and 24 km/hr and its weight varies between 5 tonnes net and 15 tones ballasted. 

WHAT IS PNEUMATIC CAISSONS AND ITS ADVANTAGES.

 Pneumatic Caissons

A pneumatic caisson is a rigid, inverted box with open at the bottom and close at the top. A working chamber is provided at its bottom to keep the caisson free of water and mud by use of compressed air. The design of a pneumatic caisson is similar to that of an open caisson is similar to that of an open caisson in many respect. The ultimate load carrying capacity, the design of walls, concrete seal and cutting edge are similar to that of open caissons. Pneumatic caisson is specially used at the place where it is not possible to construct the well. It is suitable for the depth of the water more than 12 m. However following differences should be clearly noted.

1. Working Chamber

The working chamber is made up of mild steel. It is about 3 m high and consists of a strong roof at its top.The chamber is air tight and the air in chamber is kept at a specified pressure to prevent entry of water and soil into it. The wall of the chamber should be thick and leak proof. To keep the frictional resistance low the outside surface of the walls are made smooth.

2. Air Shaft

An air shaft is a vertical passage which connects the working chamber with an air lock at the top.The shafts are made up of steel tubes and the joints of the tubes are provided with rubber gaskets to make them leak proof.It provides an access to the working chamber for workmen and also used to transport the excavated materials to the ground surface. In large caissons two separate air shaft are provided one for the passage of workmen and other for the transportation of materials.

3. Air Lock

An air lock is a steel chamber provided at the upper end of the air shaft above water level.The purpose of provided an air lock is to permit the workmen and materials to go in or to come out of the caisson without releasing the air pressure in the caisson. The steel chamber of air lock is provided with two airtight doors, one of which opens to the shaft and the other opens to the outside atmosphere.

Advantages Of Pneumatic Caissons

• Excavation and pouring of concrete is done in the dry.
• The soil can be inspected and the soil samples can be taken if required.
• Bearing capacity of soil can be determined by conducting in-situ test in the working chamber.

Disadvantages Of Pneumatic Caissons

• It is very expansive.
• Large number of manual work required which increase the cost.
• The penetration depth below water table is limited to 30 m to 40 m.
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WHAT IS SITE INVESTIGATION

Site Investigations

Site investigation or sub surface explorations are done for obtaining the information about subsurface conditions at the site of proposed construction.Site investigation consist of determining the profile of the natural soil deposits at the site, taking the soil samples and determining the engineering properties of the soils.It also includes in-situ testing of the soils.

Site investigation are performed to obtain information on the physical properties of soil and rock around a site to design earthworks and foundations for proposed structure and for repair of distress to earthworks and structures caused by subsurface conditions.This type of investigation is called a site investigation.Additionally site investigations are also used to measure the thermal resistivity of the soils or backfill materials required for underground transmission lines, oil and gas pipelines, radioactive waste disposal and solar thermal storage facilities.

Site investigation are generally done to obtain the information that is useful for one or more of the following purposes:

• To determine the bearing capacity of the soil.
• To select the type of foundation for a given structure.
• To select the depth of the foundation  a given structure.
• To select the suitable construction techniques.
• To determine the ground water level and properties of water.
• To predict the lateral earth pressure against retaining walls.
• To estimate the probable maximum and differential settlements.
• To investigate the safety of the existing structures and to suggest the remedial measures.

Stages Of Site Investigation

1. Site Reconnaissance 
2. Preliminary Explorations
3. Detailed Explorations

Site Reconnaissance

Site reconnaissance is the first step in a site investigation.It includes a vist to the site and to study the maps and other relevant records.It helps in deciding future programme of site investigation, scope of work, methods of exploration to be adopted, type of sample to be taken and the laboratory testing and in-situ testing.

Preliminary Exploration

The main purpose of preliminary exploration is to determine the depth, thickness, extent and composition of each soil stratum at the site.The depth of the bed rock and the ground water table is also determined.The preliminary explorations are generally in the form of a few borings or test pits.Tests are conducted with cone penetrometers and sounding rods to obtain information about the strength and compressibility of soils.

Preliminary site exploration is carried out for small projects, light structures, highways, airfields, etc. The main objective of preliminary exploration is to obtain an approximate picture of sub-soil conditions at low cost. It is also called general site exploration.

Detailed Explorations

The main purpose of detailed explorations is to determine the engineering properties of the soils in different strata.It includes an extensive boring programme, sampling and testing of the sample in a laboratory.Detailed exploration is preferred for complex projects, major engineering works, heavy structures like dams, bridges, high rise buildings, etc. A huge amount of capital is required for a detailed site exploration hence, it is not recommended for minor engineering works where the budget is limited. For such type of works, data collected through preliminary site exploration is enough.

In this stage, numerous field test such as Vane shear test, plate load test and permeability tests are conducted to determine the properties of the soils in natural state.

TYPES OF SOIL SAMPLE

 Soil Sample

Soil sample are obtained during sub-surface exploration to determine the engineering properties of the soils and rocks.Soil sample are generally classified into two categories:

1. Disturbed Sample
2. Undisturbed Sample

Disturbed Sample

Disturbed sample are the sample in which the natural structure of the soil gets disturbed during sampling.However, these samples represent the composition and the mineral content of the soil.Disturbed samples can be used to determine the index properties of the soil, such as grain size, plasticity characteristics, specific gravity.Disturbed soil sample do not retain in-situ properties of the soil during the collection process.Engineers do not consider these sample to be  representative of underground soils except for geotechnical testing that do not rely on the structure of the soil itself.Scientist commonly test disturbed soil samples for soil type and texture, moisture content nutrient and contaminant analysis, among other evaluations.

Undisturbed Sample

Undisturbed sample are the samples in which the natural structure of the soil and the water content are retained.However, it may be mentioned that it is impossible to get truly undisturbed sample.Some disturbance is inevitable during sampling, even when the almost care is taken.Even the removal of the sample from the ground produces a change in the stresses and causes disturbances.

Undisturbed samples are used for determining the engineering properties of the soil, such as compressibility, shear strength and permeability. Some index properties such as shrinkage limit can also be determined.The smaller the disturbance, the greater would be the reliability of the results.

Undisturbed soil sample retain the structural integrity of the soil and have a high recovery rate within the sample.Collecting a perfectly undisturbed sample is difficult and the samplers may contain a small portion of undisturbed soil at the top and bottom of the sample length. Undisturbed samples allow an engineer to determine the geotechnical properties of the strength, permeability, compressibility and fracture patterns among others.

BITUMINOUS STABILISATION AND TYPES

Bituminous Stabilisation

Bitumens are non-aqueous system of hydrocarbon that are soluble in carbon di-sulphide.Bitumen stabilisation is generally done with asphalt as binder.In any inorganic soil which can be mixed with asphalt is suitable for bituminous stabilisation. In cohesionless soils, asphalt binds the soil particles together and thus serves as a bonding or cementing agent.In cohesive soils, asphalt protects the soil by plugging its voids and water proofing it.It helps the cohesive soil to maintain low moisture content and to increase the bearing capacity.The amount of bitumen required generally varies between 4-7% by weight.The actual amount is determined by trail.

There are four types of soil bituminous stabilisation:

1. Soil Bitumen
2. Sand Bitumen
3. Oiled Earth
4. Water Proofed Clay Concrete

1. Soil Bitumen 

Soil bitumen is a water proof, cohesive soil system.The best results are obtained if the soil satisfies the following criteria:

• Plastic Limit Less than 18%
• Liquid Limit Less than 40%
• The maximum size of the particle should not be greater than one-third the compacted thickness of the soil-bitumen.

The quantity of bitumen varies from 4 - 7% of the dry weight.

2. Sand Bitumen

This bitumen stabilised cohesionless soil system.The sand should be free from vegetal matter or lumps of clay.The sand may require filler for its mechanical stability.However, it should not contain more than 25% minus No.200 sieve material for dune sand and not more than 12% in case in other types of sand.The amount of bitumen varies from 4 - 10%.

3. Oiled Earth

In this system, a soil surface consisting of silt-clay material is made water proof by spraying bitumen in two or three applications.The bitumen penetrates only a short depth into the soil.The amount of bitumen required is about 5 litres per square meter of the soil surface.

4. Water Proofed Clay Concrete

A soil possessing a good gradation is water proofed by a uniform distribution of 1 - 3% of bitumen in this system.Soils of three different gradations have been recommended.  

Factors Affecting Bituminous Stabilisation

1. Types Of Soil

Bituminous stabilisation is very effective in stabilising sandy soils having little or no fines.

2. Amount Of Asphalt

The quality of bitumen-stabilised soil improves with the amount of asphalt upto a certain limit.

3. Mixing

The quality of bitumen-stabilised soil improve if the soil is mixed thorough.

4. Compaction

The dry density of bitumen soil depends upon the amount and type of compaction.It also depend upon volatile content.

Chemical Soil Stabilisation

In chemical soil stabilisation, soils are stabilised by adding different chemicals.These chemicals react with the soil which in return cause its structure to be changed.The chemical seal the space between particles, leaving no room for water to penetrate through.Among the most common chemicals that are used for soil stabilization include sodium chloride, calcium chloride and sodium silicate.The main advantages of chemical stabilisation is that setting time and curing time can be controlled. Chemical stabilisation is however generally more expansive than other types of stabilisation. The following chemicals have been successfully used:

1. Calcium Chloride
2. Sodium Chloride
3. Sodium Silicate
4. Chrome Lignin
5. Polymers
6. Other Chemicals

1. Calcium Chloride

Calcium chloride is an inorganic compound, a salt with the chemical formula CaCl2.When calcium chloride is added to soil it causes colloidal reaction and amend the characteristics of soil water.When calcium chloride is dissolved it reduces the loss of moisture from the soil.It also reduces the chances of frost heave, as the freezing point of water is lowered.This method is very effective for stabilisation of silty and clayey soil which lose strength with an increase in water content.

2. Sodium Chloride

Sodium chloride is commonly known as salt with the chemical name NaCl.Sodium chloride is mixed with the soil either by the mix-in place method or by the plant-mix method.It should not be applied directly to the surface.When sodium chloride is added to the soil, crystallisation occurs in the pores of the soil and it forms a dense hard mat with the stabilised surface.

3. Sodium Silicate

Sodium silicate are colorless glassy or crystalline solids or white powders.Sodium silicates, as well as other alkali silicates have been successfully used for soil stabilisation.Sodium silicate gives strength to soil when it react with it and make the soil impervious.This method of stabilisation is relatively inexpensive but its long term stability is doubtful.

4. Chrome Lignin

Lignin is an organic substance binding the cells,fibers and vessels which constitute wood and the lignified elements of plants.Chrome lignin is formed from black liquor obtained during sulphite paper manufacture.Sodium bicarbonate or potassium bicarbonate is added to sulphite liquor to form chrome lignin.It slowly polymerises into brown gel.

5. Polymers

Polymers are long-chained molecules formed by polymerising of certain organic chemicals called monomers.Polymers may be natural or synthetic.When a polymer is added to a soil reaction takes place.Polymers mainly affect the aggregation and strength of soils through their interactions with fine clay particle.Coating of adsorbed polymers on clays can increase their steric stabilisation by preventing clay particles from approaching each other as closely.

6. Other Chemicals

Apart from the above mentioned chemicals some other chemical are also used for soil stabilisation which are as fallows:

 

A. Some water proofer such as alkyl chloro silanes, siliconates amines and quaternary ammonium salts have been used for water proofing of soils.

B. Dispersant such as sodium hexa-metaphosphate, are used to increase electrical repulsion and to cause dispersed structure.

C. Coagulating chemicals such as calcium chloride and ferric chloride have been used to increase the electrical attraction and to form flocculated structure in order to improve the permeability of the soil.

Advantages Of Chemical Soil Stabilisation

• It gives more strength to the soils.
• It improves the permeability of soil.
• It this method the setting time and curing time can be controlled.

Disadvantages Of Chemical Soil Stabilisation

• Require extra experienced labor.
• Chemicals should not be used directly to the surface.
• This method of stabilisation is more expensive relatively to other methods.

LIME STABILISATION - METHOD & BENEFITS

 Lime Stabilisation

Lime stabilisation is done by adding lime to the soil.It is very useful for stabilisation of clayey soils.When lime reacts with soil, there is exchange of cations in the adsorbed water layer and a decrease in plasticity of the soil occurs.The resulting material is more friable than the original clay and is therefore, more suitable as subgrade.Lime stabilisation improve the density and bearing capacity of the soil.Lime-soil stabilisation is useful to construct sub-base and base course for pavement.Lime treated soil is more suitable for warm regions where temperature is very high and for colder regions it is not suitable.

Quick lime is more effective as stabiliser than the hydrated lime, but the latter is more safe and convenient to handle.Generally the hydrated lime is used.It is known as slaked lime.Lime stabilisation is not effective for sandy soils because these soils can be stabilised in combination with clay, fly-ash or other pozzolanic materials.The ratio of fly ash to lime varies between 3 to 5.The fly ash is used about 10 - 20 % of the soil weight.

Method Of Mixing Lime Into Soil

The method of mixing lime into soils for stabilisation involves following steps:

• The soil which need stabilisation is scarified and pulverized by suitable equipment.

• Now add some amount of lime to the pulverized soil either the powder or slurry form and then mixed with suitable equipment.

• If lime powder is used then water is sprayed all over the soil.

• After spraying the water leave this mixture for 1 to 4 days. Lime-soil reaction is slow process and it required some time.

• Once the lime-soil reaction take place then spread the soil to required grade and compact it with rollers.

• When compaction is completed and the soil achieve the required dry density, the compacted lime-soil layer is allowed for curing for 1 week. 

• Finally after 1 week of curing the field tests are conducted to check water content and maximum dry density of compacted soil.

Factor Affecting Lime Soil Stabilisation

The following are the factors which affecting lime-soil stabilisation:

1. Soil Type
2. Lime Type
3. Lime Content
4. Compaction
5. Additives
6. Curing 

Benefits Of Lime Soil Stabilisation

• It is easy to mix with soil.
• It reduces the plastic properties of the soil when wetted.
• It sets slowly, the time interval between mixing and compacting is not critical.
• Cost of stabilisation is reasonably low.

WHAT IS SOIL SUCTION?

Soil Suction

Soil suction is considered as the summation of matric suction (ψ_{m) and osmotic suction (π).}

Soil suction is defined as the state of the soil when it is under reduced pressure.It is measured in terms of the height of the water column (h) suspended in the soil.

Soil Suction Formula

Soil suction can be represented as common logarithm of the height of the water column (h).The value of 'h' is substituted in centimeters.

 

Pf = log10 (h)

For example: The height of the water column is 100cm.Hence, the Pf value is given by:

 _{Pf = log10 (h)}

     _{= log10 (100)}

     _{= log10 (10^2)}

     _{= 2 x log10}

     _{= 2}

_{Similarly, for a height of column = 1 cm.}

Hence the Pf value is given by:

 Pf  = log10 (h)

 Pf   = log10 (1)

 Pf   = 0

Soil suction can also be represented in atm, kPa, kg/cm2 and bar. 

FACTOR AFFECTING SOIL SUCTION

The suction in soils depends mainly on the following factors:

1. Particle Size

In general the smaller the particle size, the greater is the soil suction.The soils with fine particles have a large number of small pores with small radii of menisci. It results in large capillary rise and hence greater suction.

2. Water Content

 Water content is also known natural moisture content, is the ratio of the weight of water to the weight of the solids in a given mass of soil.As the water content of a soil decreases, the soil suction increases and it attains the maximum value when the soil is dry.

3. Soil Structure 

Soil structure describes the arrangement of the solid parts of the soil and of the pore space located between them.The soil structure governs the size of interstices in the soils.As the soil suction depends upon the size of interstices, a charge in the soil structure affects the soil suction.

4. Denseness of Soil

As the denseness of a soil increases, generally soil suction increases.When the soil is loose, with a low density the pores are of large radius and the soil suction is low.

5. Temperature

A rise in temperature causes a reduction in surface tension (Ts) of the water.Consequently, the soil suction decreases as the temperature increases.

6. Dissolved Salts

The surface tension of water increases with an increase in impurities, such as salt.

7. Angle of Contact

The angle of contact between water and soil particles depends upon the mineralogical composition of soils.As the angle of contact (𝛉) increases, the soil suction decreases.The soil suction is maximum when the angle of contact is zero.

8. Wetting Cycle

As discussed in the preceding section, for the same water content, the soil suction is greater during the drying cycle than in the wetting cycle.

WHAT IS FROST ACTION IN SOIL?

Frost Action In Soil

Frost action in a soil is a phenomena that occurs in the winter and early spring climates.There are two type of frost action Frost heavy and Frost boil.Frost heavy is a phenomenon in which the water molecules present in the pores freeze during lower temperature which results in the expansion of the soil.Frost boil is the phenomenon of loosing of the soil when the  frozen soil starts to thaw.Frost boil occurs after the frost heavy process.

Frost Heavy In Soils

Frost heavy is an upwards swelling of soil during freezing conditions caused by an increasing presence of ice as it grows towards the surface, upwards from the depth in the soil where freezing temperatures have penetrated into the soil. This results in an increase in the volume of soil because when water is converted into ice there is about 9% increase in its volume.If the porosity of soil is 45% and soil is saturated, the expansion of the soil would be (0.09 x 45) = 4.05%. In other words, there would be a heavy of about 4 cm in every one meter thickness of the soil deposit.Due to frost heavy, the soil at the ground surface is lifted.

The frost have observed in most of the soils is much more than a heavy of about 4 cm per meter.This is due to the fact that when the ice lenses are formed in the soil due to freezing of water, the water film from the adjacent soil particles is also removed.The weight of overlying soil restrains vertical growth of the ice and promote the formation of lens-shaped areas of ice within the soil.The soils which are prone to frost action are mainly silts and fine sands.These soils have large capillary rise due to relatively fine particles.

Civil Insta

Frost Boil In Soils

A frost boil, also known as mud boils or mud circles are small circular mounds of fresh soil material formed by frost action and cryoturbation. After the occurence of frost heavy, if the temperature rises the frozen soil thaws and free water is liberated.Thawing process starts from the upper layer and moves downwards.The liberated water is trapped in the upper layer while the lower layers are still frozen.The strength of soil in upper layer is reduced due to its softening caused by an increases in water content.The process of softening of soil due to liberation of water during thawing is known as Frost boil.

Frost boils are among the most common features of patterned ground the pervasive process shaping the topology of soils in periglacial regions.They generally from regular patterns of polygons.Frost boils are a type of nonsorted circle and are characterized from other circle by barren centres of mineral soil and intercircle region filled with vegetation and peat.It is named after skin boils due to similarities in their formation processes although subsequent research has shown other methods of formation.

Frost boil affects the structures resting on the ground surface.The effect is more pronounced on highway pavements.A hole is generally formed in the pavement due to extrusion of soil and water under the action of wheel loads.Coarse grained soils are not affected much by frost boil, as the quantity of liberated water is small and that too is drained away quickly.The soils most prone to the softening effects are silty soils.These soils have low plasticity index and become very soft with a small increase in water content.

Civil Insta

8 WAYS TO CONTROL SOIL EROSION AT CONSTRUCTION SITES

Soil Erosion At Construction Sites

Soil erosion is a major concern for construction sites, especially if the project is very large.Most of time it is neglected by the Site Engineers or Supervisors.At construction sites it occurs due to heavy runoff due to rain or any other construction activities result in the washing away of soil, which is a slow process but if not rectify initially can result in delay of project which increases the overall cost of project.

Soil erosion can be caused by a variety of factors.Depending on this factor and the severity of the issue, solutions vary.The control of soil erosion helps preserve and protect the construction site and the structure.

Measures To Control Soil Erosion At Construction Sites.

1. Divide the project into sections.
2. Minimize disturbed area on the construction site.
3. Runoff water control method
4. Slope protection
5. Soil stabilization
6. Dewatering
7. Sediment control traps
8. Construction entrances

1. Divide The Project Into Sections

This is a very effective method to control soil erosion at construction sites.In this method working lands at site are divided into different sections.

2. Minimize Disturbed Area On The Construction Sites

During the preparation of a site for construction, it is advised to disturb only those area which are required  for the project.The remaining area of the site should be left undisturbed to protect the topsoil.

3. Runoff Water Control Methods

Some off the methods used to control the runoff water within and around the construction sites are as fallows:

• Geotextile: It is a commonly used for erosion control and soil improvement in a variety of construction projects such as road, pipelines and embankments.Depending on the application, geotextiles may have an open mesh weave, a warp-knitted structure or a closed fabric or nonwoven surface.The specific type of geotextile used is based on several criteria, including separation, filtration, drainage, reinforcement, sealing and protection.

• Soil Nail: Soil nail provides a resisting force against slope failures and offers relatively quick installation.The procedure involves drilling into the soil and placing steel bars into it.

• Riprap: It is a commonly used technique to protect soil from erosion in areas where there is a high degree of concentrated runoff. Riprap is a layer of very large stones interlocked together to act as a barrier on slopes that are unstable because of seepage problems or areas that are receiving a large concentrated flow.Large stone aggregate is poured onto geotextile membranes to keep the ground from being carried away by water.

• Turbidity Barriers: Turbidity barriers are useful in multiple ways, it can prevent soil erosion as well as water contamination.They are typically made of a geotextile membrane that floats and is anchored to the bottom of the water body with weights and used for sediment control.

• Articulated Concrete Blocks: Articulated concrete blocks are available in a thickness of wide variety and shapes.They are used as sloping structure on waterway embankments or manmade drainage systems.

• French Drains: It is a underground piping system called drain tile.The drain tile may also be performed to allow water to seep into the soil below the tile, while excess water travels to the exit point.French drains are installed with a slope of about 1 inch of drop per 10 feet of horizontal run.

4. Slope Protection

The slope protection can be protected by several methods in order to control soil erosion.Some of the method are as fallows:

• Silt Fencing: The purpose of silt fencing is to retain the soil on disturbed land at construction site.It is a temporary sediment barrier made of porous fabric.

• Fiber Rolls: It is a temporary erosion control and sediment control device used on construction sites to protect water quality in nearby streams, rivers, lakes and seas from sediment erosion.It is made of straw, coconut fiber or similar material formed into a tubular roll.Each fiber roll is installed on a horizontal contour level in shallow trenches 2 to 4 inches deep and fastened to the ground with wooden stakes.

• Mats: Mats laid on the soil surface to protect against the washout of soil due to action of rain and surface water.It is also often to provide surface protection when used with slope stabilisation and soil retention system such soil nailing, soil panels and soilcell.

5. Soil Stabilization

Soil stabilization is a physical or chemical treatment which increase or maintain the stability of a soil or improve its engineering properties.It can be achieved either temporary or permanently depending on the requirement of the project.The use of wood binders, mulch and blankets can act as a temporary measure while the permanent methods include seeding, channel stabilization, green buffer and planting. 

6. Dewatering

Dewatering is a term used to describe the action of removing groundwater or surface water from a construction site.Normally the dewatering process is done by pumping or evaporation.

7. Sediment Control Traps

Runoff water on construction sites can be reduced by employing sediment control traps or basin.These units help the sediments to settle before the water is discharged.The sediment basins are constructed with a capacity to store at least two year of stormwater.

 

8. Construction Entrances

A stable construction entrances mainly made up of crushed stone helps to reduce the amount of sediment getting carried away.This entrance required regular maintenance  and it is made at a length of 50 feet.

WHAT IS SOIL LIQUEFACTION - CAUSES AND EFFECTS

Soil Liquefaction

Soil liquefaction is a phenomenon in which saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during earthquake.Soil liquefaction occurs in a fully saturated soil.

Causes Of Liquefaction

The soil is a mixture of soil particles that are connected together.These particles are naturally rest upon each other due to gravity.Soil liquefaction occurs due to sudden and rapid load on the soil particles.The sudden water pressure leads to soil losing its cohesive strength.Once the soil loses its cohesion, it gets softened, weak and loses its solid properties that are converted to liquid properties.

Effects Of Liquefaction

1. Sand Boiling
2. Surface Landslides
3. Damage To Offshore Structures
4. Failure Of Dams And Retaining Walls
5. Failure Of Structure Under Earthquake

1. Sand Boiling

When liquefaction occurs below the surface that is fully compacted, the water pressure below the surface makes the water to break out like a bubble.These come out as boiling water.This is called as sand boiling.

2. Surface Landslides

The term landslide describe a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill or a combination of these.The failure of water carrying bodies can result in surface landslides.

3. Damage To Offshore Structure

Liquefaction is common in soil that is submerged.These conditions cause huge damage for the bridge construction, structures supporting submerged soil deposit.

4. Failure Of Dam And Retaining Wall.

The soils supporting Dams and Retaining walls undergoes liquefaction which results in the collapse of these structures.

5. Failure Of Structure Under Earthquake

Liquefaction takes place when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking.They can either split or lean bringing complete collapse of the structure.Past earthquake records have shown a huge failure of building structures due to liquefaction.When these types of hazards take place it can't provide enough time to evacuation, thus it result in a huge loss of life and property.

Method Of Reducing Soil Liquefaction Hazards

1. By Avoiding Liquefaction Susceptible Soils
2. Built Liquefaction Resistant Structures
3. Improve The Soil

1. By Avoiding Liquefaction Susceptible Soils

Avoid construction on liquefaction susceptible soils.There are various criteria to determine the liquefaction susceptibility of a soil.By characterizing the soil at a particular building site according to these criteria one can decide if the site is susceptible to liquefaction.

2. Built Liquefaction Resistant Structures

If it is necessary to construct on liquefaction susceptible soil because of space restrictions, favorable location, or other reasons, it may be possible to make the structure liquefaction resistant by designing the foundation elements to resist the effects of liquefaction.

3. Improve The Soil

Improving the soil involves mitigation of the liquefaction hazards by improving the strength, density and drainage characteristics of the soil.This can be done using a variety of soil improvement techniques.

CAUSES OF FOUNDATIONS FAILURE

 The foundations may fail due to the following reasons:

1. Unequal settlements of sub-soil.
2. Unequal settlements of masonry.
3. Lateral pressure on the wall.
4. Lateral movement of the sub-soil.
5. Sub-soil moisture movement.
6. Atmospheric action.
7. Weathering of sub-soil due to plants.

1. Unequal Settlements of Sub-soil

Unequal settlements of sub-soil is a major causes leads to foundation failure because it leads to cracks in the structural components and rotation thereof.Unequal settlement of sub-soil may be due to:

• Eccentric loading.
• Unequal load distribution on the soil strata.
• Non-uniform nature of sub-soil throughout the foundation.

2. Unequal settlements of masonry

The portion of masonry situated between the ground level and concrete footing has mortar joints which may either shrink or compress, leading to unequal settlement of masonry.Due to this the super structure will also have cracks which leads to unequal settlements of masonry.

3. Lateral Pressure on the Wall

The wall transmitting the load to the foundation may be subjected to lateral pressure from a pitched roof or an arch or wind action.Due to this, the foundation will subjected to a moment .If the foundation has not been designed for such a situation it may fail by either overturning or by generation of tensile stresses on one side and high compressive stresses on the other side of the footing.

4. Lateral Movement of Sub-soil

Lateral movement of sub-soil is applicable to very soft soil which are liable to move out or squeeze out laterally under vertical loads specially at the locations where the ground is sloping.Such a situation may arise in granular soils where a big pit is excavated in the near vicinity of the foundations.Due to such movement excessive settlements takes place or structure may even collapse. 

5. Sub-soil Moisture Movement

Sub-soil moisture movement is a major causes of failure of foundation on cohesive soil, where the sub-soil water level fluctuates.When water table drops down, shrinkage of sub-soil takes place.Due to this there is a lack of sub-soil support to the footing which leads to cracks in the buildings.

6. Atmospheric Action

The behaviour of foundation may be adversely affected due to atmospheric agents such as sun, wind and rains.If the depth of foundation is shallow, moisture movements due to rains or drought may cause trouble.If the building lies in a low lying area, foundation may even be scoured.If the water remains stagnant near the foundation it will remains constantly damp resulting in the decrease in the strength of footing or foundation wall.

7. Weathering of Sub-soil due to Plants

Sometime plants like small trees, shrubs is grown very near to the wall.The roots of these shrubs absorb moisture from the foundation soil, resulting in reduction of their voids and even weathering.Due to this the ground near the wall depresses down.If the roots penetrates below the level of footing, settlements may increase resulting in foundation cracks.

METHOD OF IMPROVING BEARING CAPACITY OF SOIL

Safe Bearing Capacity of Soil

Safe bearing pressure of soil is so low that the dimension of the footings work out to be very large and uneconomical.In such circumstance, it become essential to improve the safe bearing pressure, which can be done by the following methods:

1. Increase the depth of foundation
2. Compaction of soil
3. Drainage of soil
4. Confining the soil
5. Grouting
6. Chemical treatment
7. Replacing poor soil

1. Increase The Depth Of Foundation

It has been found that in cohesionless soil, the bearing capacity increases with the depth due to the confining weight of overlaying materials.At deeper depths the over budden pressure on soil is higher hence the soil is more compacted at deeper depth.This method is not economical because the cost of construction increases with increase in the depth.The method is useful only when better bearing stratum is encountered at greater depth.

2. Compaction Of Soil

It has been found that compaction of natural soil deposits or man-made fills result in the improvement of bearing capacity and reduction in the resulting settlements.Compaction of soil can be effectively achieved by the following mean:

A. Ramming Moist Soil

In this method  the foundation soil is moistened and then compacted with the help of hand rammer or mechanically operated frog rammers or vibratory rollers.The voids of the soil are very much reduced resulting in the reduction in settlements.

B. Flooding The Soil

In this method the bearing pressure of very loose sands can be increased by flooding the soil.This method is adopted to improve the bearing capacity of the soil especially sand dunes.

C. Vibration

Heavy vibratory rollers and compactors may compact the layer of granular soils to a depth of 1 to 3 m with this method the sandy soil can be effectively compacted, resulting in increased safe bearing power and decreased settlements when super-structure loads come on the soil.

D. Vibroflotation

Vibroflotation is a commercial method which combines the effect of vibration and jetting.A heavy cylinder vibroflot is inserted in the soil  while the cylinder vibrates due to a rotary eccentric weight.A water jet on the tip of the vibro flot supplies a large amount of water under pressure.As the vibro flot sinks, clean sand is added into a crater that develops on the surface. The method is very useful when foundation is required to support heavy loads spread over a greater area.

    

E. Compaction by Pre-Loading

This method is adopted where footing is founded on clayey soils which result in long term settlements.Pre-loading results in accelerated consolidation, so that settlements are achieved well before the actual footing is laid.The load used for this process is removed before the construction of the footing.

F. Using sand Piles

This method is adopted for sandy soils or soft soil because this is very useful for such soils.In this method hollow pipes are driven in the ground at close interval which results in compaction of  soil enclosed between the adjacent pipes.After that these pipes are removed and then sands are filled and ramming in these holes results in the formation of sand piles.

G. Rubble Compaction into the Soil

Rubble compaction into the soil is also a effective method of increasing the bearing capacity of soil.In this method, a layer of rubber 300 to 450 mm thickness is spread over the foundation level and rammed well.After ramming if this layer of rubble gets buried in the soil and another layer of 150 mm thickness is spread and rammed manually.

3. Confining The Soil.

In this method, the soil are enclosed with the help of sheet piles.This confined soil is further compacted to get more strength.This method is applicable for shallow foundations.

4. Drainage of Soil.

With the increase in percentage of water content in soil, the bearing capacity decreases especially when it is saturated.In case of sandy soil, the bearing capacity may reduce as much as 50% due to pressure of water content.Cohesionless soils can be drained by laying the porous pipes to a gentle slope over a bed of sand and filling the trenches above the pipes with loose boulders.These trenches subsequently should lead to the nearest well or any water body.

5. Grouting

This method is useful in loose gravels and fissured strata.Bores holes in sufficient numbers are driven in the ground and cement grout is forced through these under pressure.In this method, poor soil bearing stata is hardened by injecting the cement grout under pressure, because it scale off cracks, voids and fissures of the stata are thus filled with the grout, resulting in the increase in the bearing value.

6. Chemical Treatment

This method is adopted to increase the bearing capacity of soil with the help of certain chemicals in place of cement grout like silicates of soda and calcium chloride.These chemicals are injected with pressure into the soil.The chemical should be such that it can solidify and gain early strength.

7. Replacing the Poor Soil 

In this method the poor soil is first removed and then the gap is filled up by superior material such as sand, stone, gravel or other hard material.In order to do this, excavate a foundation trench of about 1.5 m deep and then fill the hard material is stage of 30 cm.Then compact the hard material at every stage.This method is useful for foundation in black cotton soils.