Differentiate between disturbed and undisturbed sample and give 3 type of sampler for each sample.
Disturbed samples:
Disturbed samples are generally obtained to determine the soil type, gradation, classification, consistency, density, presence of contaminants, stratification, etc. The methods for obtaining disturbed samples vary from hand excavating of materials with picks and shovels to using truck mounted augers and other rotary drilling techniques. These samples are considered .disturbed. since the sampling process modifies their natural structure. Undisturbed samples:
Undisturbed samples are used to determine the in place strength, compressibility (settlement), natural moisture content, unit weight, weight, permeability, discontinuities, fractures and fissures of subsurface formations. Even though such samples are designated as .undisturbed, in reality they are disturbed to varying degrees. The degree of disturbance depends on the type of subsurface materials, type and condition of the sampling equipment used, the skill of the drillers, and the storage and transportation methods used. The disturbed and undisturbed sample can be differentiate in following basic types : Disturbed Sample Change in the stress condition
Undisturbed Sample No change due to disturbance of the soil structure
Change in the water content and the void ratio Disturbance of the soil structure
No change in void ratio and water content content
Chemical changes
No change in constituents and chemical properties No disturbance of the soil structure
Mixing and segregation of soil constituents
The 3 type of sampler for each sample : Disturbed Sample Split barrel sampler
Undisturbed Sample Thin wall Shelby sampler
Continuous auger
Piston sampler
Bulk sampler
Block sampler
Sampler For Disturbed Sample
Split Barrel Sampler • • •
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Used to obtain disturbed samples in all types of soils. Typically used in conjunction with the Standard Penetration Test (SPT), The sampler is driven with a 63.5-kg (140-lb) hammer dropping from a height of 760 mm (30 in). (AASHTO T206 and ASTM D1586), Available in standard lengths of 457 mm (18 in) and 610 mm (24in) Inside diameters ranging from 38.1 mm (1.5 in) to 114.3 mm (4.5 in) in 12.7 mm (0.5 in) increments. The 38.1 mm (1.5 in) inside diameter sampler is popular because correlations High area ratio disturbs the natural characteristics of the soil being sampled, thus disturbed samples are obtained. This corresponds to a relatively thick walled sampler with an area ratio – –
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(Hvorslev, 1949).
Figure (a) Split-Barrel Samplers: Lengths of 457 mm (18 in) and 610 mm (24 in); Figure (b) Inside diameters from 38.1 mm (1.5 in) to 89 mm (3.5 in).
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when the shoe and the sleeve of this type of sampler are unscrewed from the split barrel, the two halves of the barrel may be separated and the sample may be extracted easily. The soil sample is removed from the split-barrel sampler it is either placed and sealed in a glass jar, sealed in a plastic bag, or sealed in a brass liner. Separate containers should be used if the sample contains different soil types. Alternatively, liners may be placed inside the sampler with the same inside diameter as the cutting shoe. This allows samples to remain intact during transport to the laboratory. In both cases, samples obtained with split barrels are disturbed and therefore are only suitable for soil identification and general classification tests.
Continuous Auger
Soil augers and samplers are used in forestr y, environmental and agricultural applications, but are not limited to these uses. This equipment is commonly used for obtaining samples at or near the surface. With the use of extensions, augers have the capability to reach depths of up to 15 ft. However, it is recommended to use a power auger for depths greater than 5 ft. Augers can be used for sampling when there is not a need for a clean or undisturbed sample. Generally, augers are used to bore to a depth where a sample is to be obtained using a soil sampler or core sampler. Hand-operated augers are made from several materials including steel, aluminum, stainless steel, brass and wood. The auger material selected should not interfere with the tests that need to be completed on the samples. For example, galvanized and brass materials should never be used when agricultural samples are being tested for nutrient levels, since they can contaminate the sample with micronutrients and contribute to false test results. Using dirty and rusted equipment will also affect your results. Always clean your equipment after each use to prevent cross contamination and rusting.
Figure (c) typical continuous auger
Bulk sampler
These are disturbed samples obtained from auger cuttings or test pits. The quantity of the sample depends on the type of testing to be performed, but can range up to 50lb (25kg ) or more. Testing performed on the samples includes classification, moisture-density, limerock bearing ratio, and corrosivity tests. A portion of each sample should be placed in a scaled container for moisture content determination.
Figure (d) apparatus for buck sampler
Sampler For Undisturbed Sample
Thin Wall Shelby Sampler •
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To obtain relatively undisturbed samples of cohesive soils for strength and consolidation testing. Commonly, it has a 76 mm (3.071in) outside diameter & a 73 mm (2.875 in) inside diameter, Resulting in an area ratio of 9 percent. Vary in outside diameter between 51 mm (2.0 in) and 76 mm (3.0 in) typically come in lengths from 700 mm (27.56 in) to 900 mm (35.43 in), Larger diameter sampler tubes used when higher quality samples are required and sampling disturbance must be reduced. The thin-walled tubes are manufactured using carbon steel, galvanized-coated carbon steel, stainless steel,and brass. Carbon steel tubes the lowest cost tubes but are unsuitable if the samples are to be stored in the tubes for more than a few days or if the inside of the tubes become rusty, significantly increasing the friction between the tube and the soil sample. Galvanized steel tubes preferred in stiff soils carbon steel is stronger, less expensive galvanizing provides additional resistance to corrosion. Stainless Steel tubes preferred for offshore bridge borings, salt-water conditions, or long storage times • •
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Manufactured with a beveled front edge for cutting a reduced-diameter sample [commonly 72 mm (2.835 in) inside diameter] to reduce friction. Can be pushed with a fixed head or piston head. The following information should be written on the top half of the tube and on the top end cap: project number, boring number, sample number, and depth interval. We should also write on the tube the project name and the date the sample was taken. Near the upper end of the tube, the word "top" and an arrow pointing toward the top of the sample should be included. Putting sample information on both the tube and the end cap facilitates retrieval of tubes from laboratory storage and helps prevent mix-ups in the laboratory when several tubes may have their end caps removed at the same time. Both ends of the tube should then be sealed with at least a 25 mm (1 in) thick layer of microcrystalline (non-shrinking) wax after placing a plastic disk to protect the ends of the sample.
Figure (e) Shelby Tube Sealing Methods. (a) Microcrystalline wax (b) O-ring packer.
Piston Sampler • •
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Also known as an Osterberg or Hvorslev sampler. Particularly useful for sampling soft soils where sample recovery is often difficult although it can also be used in stiff soils. The piston sampler is basically a thin-wall tube sampler with a piston, rod, and a modified sampler head.
Figure (f) Piston Sampler. (a) Picture with thin-walled tube cut-out to show piston, (b) Schematic (After ASTM
Block Sampler Block sampling has traditionally involved the careful hand excavation of soil around the sample position, and the trimming of a regular-shaped block. This block is then sealed with layers of muslin, wax and cling film, before being encased in a rigid container, and cut from the ground. The process is illustrated in Figure (g). A similar process can be carried out in shafts and large-diameter auger holes.
Trial pits are normally only dug to shallow depths, and shafts and large-diameter auger holes tend to be expensive. Therefore block samples have not traditionally been available for testing from deep deposits of clay. In the past decade, however, there has been an increasing use of rotary coring methods to obtain such samples. When carried out carefully, without displacing the soil, rotary coring is capable of producing very good quality samples. When the blocks are cut by hand then obviously the pit will be air-filled, but when carried out in a borehole it will typically be full of drilling mud.
During the sampling process there is stress relief. At one stage or another the block of soil will normally experience zero total stress. This will lead to a large reduction in the pore pressures in the block. The soil forming the block will attempt to suck in water from its surroundings, during sampling, either from the soil to which it is attached, or from any fluid in the pit or borehole. This will result in a reduction in the effective stress in the block.
In addition, where block sampling occurs in air, negative pore pressures may lead to cavitations in any silt or sand layers which are in the sample. Cavitation in silt and sand layers releases water to be imbibed by the surrounding clay, and the effect will be a reduction in the average effective stress of the block.
Block sampling is an excellent method of ensuring that the soil remains unaffected by shear distortions during sampling, but samples obtained in this way may not (as a result of swelling) have effective stresses that are the same as those in the ground. Therefore the strength and compressibility of the soil may be changed. This should be allowed for either by using appropriate reconsolidation procedures, or by normalizing strength and stiffness, where appropriate, with effective stress.
Figure (g) Block Sampling In A Trial Pit
