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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