Soil samples
Soil samples are obtained during surface exploration or soil
exploration to determine the engineering properties of the soils and rocks.
Soil samples are generally classified into two categories -
1. Disturbed samples
2. Undisturbed samples
1. Disturbed samples -
These are the samples in which the natural structure of the soil gets disturbed during sampling. However, this 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 etc.
2. Undisturbed samples -
These 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 a truly undisturbed sample.
Undisturbed samples are used for determining the engineering properties of the soil, such as compressibility, shear strength and permeability etc.
Samplers - In foundation engineering, samplers are used to collect soil
samples at or near the ground surface. These samples are used to determine soil
properties like compressibility, shear strength, and permeability etc.
Sampling - In geotechnical
engineering, sampling is the process of removing small amounts of material to
characterize their variation and for laboratory testing.
Type of samplers-
Following are the various types of samplers :
- Split spoon sampler
- Scraper bucket sampler
- Shelby tube wall sampler
- Piston sampler
- Denison sampler
1. Split Spoon Sampler -
The most commonly used sampler for obtaining distributed
sample of the soil is the standard split spoon sampler.
It consisted mainly of the three parts-
Driving shoe made of tool steel about 75 mm long.
Steel tube about 450 mm long, split longitudinally in two
halves.
Coupling at the top of the tube about 150 mm long.
The inside diameter of the split tube is 38 mm and the
outside diameter is 50 mm.
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| Fig:- Standard Split Spoon Sampler |
The coupling head may be provided with a check valve and
four venting parts of 10 mm diameter to improve sample recovery. This sampler
is also used in conducting standard penetration test.
After the ball hole has been made, the sampler is attached
to the drilling rod and lowered into the hole. The sample is collected by
jacking or forcing the sampler into the soil by repeated blows of a drop
hammer, the sampler is then withdrawn.
The split tube is separated after removing the shoe and the
coupling and the sample is taken out. It is then place it in a container sealed
and transported to the laboratory.
2. Scrapper Bucket Sampler -
If a sandy deposit contains pebbles, it is not possible to
obtain samples by standard split spoon sampler or split spoon sampler fitted with
a spring core catcher.
The pebbles come in between the springs and prevent their
closure. For such deposits (Sandy deposits), a scraper bucket sampler can be
used.
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| Fig: Scraper Bucket Sampler |
Scraper bucket sampler consists of a driving point which is
attached to its bottom end. There is a vertical split in the upper portion of
the sampler. As the sampler is rotated the scrapping of the soil enter the
sampler through the split. When the sampler is filled with the scrapping it is
lifted.
A scraper bucket sampler can also be used for obtaining the
samples of cohesion less soils below the water table.
3. Shelby Tubes and Thin-walled Samplers -
Shelby tubes are thin wall tube samplers made of seamless Steel. The commonly used samplers have the outer diameter of either 50.8 mm or 76.2 mm. The bottom is sharpened and bevelled, which act as a cutting edge. The area ratio is less than 15% and the inside a clearance is between 0.5 to 3%. The length of the tube is 5 to 10 times the diameter for sandy soil and 10 - 15 times the diameter for clayey soil.
The diameter generally varies between 40 to 125 mm, and the thickness varies from 1.25 mm to 3.15 mm.
 |
| Fif:- Shelby tubes are thin wall tube sampler |
The sample tube is attached to the drilling rod. It is then pushed into the soil. Care shall be taken to push the tube into the soil by a continuous rapid motion without impact or twisting. The tube is turned 2 revaluations to shear the sample off at the bottom before it is withdrawn. The tube is taken out and its ends are sealed before transportation.
Shelby tubes are used for obtaining and undisturbed samples of clay.
4. Piston sampler -
A piston sampler consists of a thin wall tube with a piston inside. The Piston keeps the lower end of the sampling tube closed when the sample is lower the bottom of the hole. After the sampler has been lowered to the desired depth, the piston is prevented from moving downward by a suitable arrangement, which differs in different types of piston samplers. The presence of the piston prevents rapid squeezing of the soft soils into the tube and reduces the disturbance of the sample. A vacuum is created on the top of the sample which helps in retaining the sample. During the withdrawal of the sampler, the piston provides protection against the water pressure which otherwise would have occurred on the top of the sample.
 |
| Fig:- Piston sampler |
Piston samples are used for getting undisturbed soil samples
from soft and sensitive clays.
5. Denison sampler -
The Denison sampler is a double-walled sampler. The outer
barrel rotates and cuts into the soil. The sample is obtained in the inner
barrel. The inner barrel is provided with a liner. It may also be provided with
a basket-type core retainer.
The sampler is lowered to the bottom of the drilled hole. A
downward force is applied on the top of the sampler. A fluid under pressure is
introduced through the inner barrel to cool the coring bit when the outer
barrel rotates. The fluid returns through the annular space between the two
barrels. The rotation of the outer barrel is continued till the required length
of the sample is obtained.
The Denison sampler is mainly used for obtaining samples of
stiff to hard cohesive soils and slightly cohesive sands. However, it cannot be
used for gravelly soils, loose cohesionless sands and silts below ground water
table and very soft cohesive soils. The Denison sampler gives a sample 5½"
(140 mm) in diameter and 20 inches (508 mm) long.
DESIGN FEATURES AFFECTING THE SAMPLE DISTURBANCE -
(1) Area ratio -
The area ratio is defined as
Ar = (Maximum
cross-sectional area of the cutting edge / Area of the soil sample) * 100
Figure shows the lower portion of a sampler.
 |
| Fig:- Soil Sampler |
The area ratio
can be expressed as
Where,
D₁= inner diameter of
the cutting edge,
D₂= outer diameter of the cutting edge.
(2) Inside clearance -
The inside clearance is defined as
Ci
= (D3 - D₁)/ D₁ x 100
D3 = inner diameter
of the sampling tube.
The inside clearance allows elastic expansion of the sample
when it enters the tube. It helps is reducing the frictional drag on the
sample. For an undisturbed sample, the inside clearance should be between 0.5
and 3 percent.
(3) Outside clearance -
The outside clearance is defined as
Co
= (D2 – D4)/ D4 x 100
D₁ = outer diameter of the sampling tube.
For reducing the
driving force, the outside clearance should be as small as possible. Normally,
it lies between zero and 2 percent.
(4) Inside wall friction-
The friction on the inside wall causes disturbance of the
sample. The inside surface of the sampler should be smooth. It is usually
smeared with oil before use to reduce friction.
(5) Design of non-return valve -
The non-return valve provided on the sampler should be of
proper design. It should have an orifice of large area to allow air, water or
slurry to escape quickly when the sampler is driven. It should immediately
close when the sampler is withdrawn.
(6) Method of applying force -
The degree of disturbance depends upon the method of
applying force during sampling and upon the rate of penetration of the sampler.
For obtaining undisturbed samples, the sampler should be pushed and not driven.
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