Site Investigation and Sub-Soil Exploration MCQ Quiz - Objective Question with Answer for Site Investigation and Sub-Soil Exploration - Download Free PDF

Explanation:

Type of Sampler for Stiff to Hard Cohesive Soils

In geotechnical engineering, obtaining accurate samples of soil is crucial for determining its properties and suitability for construction projects. The choice of sampler is essential as it affects the quality and integrity of the soil sample. For stiff to hard cohesive soils and slightly cohesive sands, the Dension sampler is the most appropriate due to its design and functionality.

Analyzing the Given Options

1. "Dension sampler." (Correct Answer)

   • The Dension sampler is specifically designed for obtaining samples from stiff to hard cohesive soils and slightly cohesive sands. It ensures minimal disturbance to the soil structure, providing accurate and reliable samples for analysis.

2. "Scraper bucket sampler." (Incorrect Answer)

   • The scraper bucket sampler is typically used for collecting disturbed samples from softer soils and sediments. It is not suitable for stiff to hard cohesive soils as it may not penetrate adequately and can disturb the soil structure.

3. "Split spoon sampler." (Incorrect Answer)

   • The split spoon sampler is commonly used in the Standard Penetration Test (SPT) for obtaining disturbed samples from various soil types, including loose to medium-dense sands. It is not the best choice for stiff to hard cohesive soils as it can cause significant disturbance.

4. "Piston sampler." (Incorrect Answer)

   • The piston sampler is used for obtaining relatively undisturbed samples from soft to medium cohesive soils and fine-grained sediments. It is not suitable for stiff to hard cohesive soils due to its inability to penetrate and sample such hard materials effectively.

Stationary Piston Sampler: It is used to obtain an undisturbed sample of soft and sensitive clays.

Other Important Samplers:

Split Spoon Sampler: It is commonly used to obtain a disturbed soil sample. It is suitable to use for collecting samples even from saturated sands and other soft and wet soils as a spring-core catcher device attached forms a dome and retain the sample.

Open drive thin-walled Sampler: It is used to obtain an undisturbed sample of clay.

Rotary Sampler: It is used to obtain a bulk sample of larger size such as stiff soils, or stones.

Explanation:

Soil Exploration Methods

Soil exploration involves various methods to determine the properties and conditions of soil for construction and engineering projects. These methods help in assessing the suitability and safety of the site. Common methods include:

• Boring: This method involves drilling into the ground to extract soil samples for analysis.

• Probing: This method uses probes to determine the soil's consistency, density, and other properties.

• Trenching: This method involves digging trenches to directly observe soil layers and conditions.

Analyzing the Given Options

1. "Boring" (Method)

   • Boring is a common soil exploration method used to extract soil samples for testing and analysis.

2. "Probing" (Method)

   • Probing is a technique used to measure soil properties such as density and consistency.

3. "Compaction" (NOT a method of exploration)

   • Compaction is a soil improvement method, not an exploration method. It involves densifying soil to improve its load-bearing capacity.

   • Since compaction is not a method of soil exploration, it is the correct answer.

4. "Trenching" (Method)

   • Trenching is a method where trenches are excavated to observe and investigate soil layers directly.

Concept:

Area ratio:

It can be defined as the ratio of the maximum cross-sectional area of the cutting edge to the area of the soil sample.

The area ratio can be expressed as

\({A_r} = \frac{{D_2^2 - D_1^2}}{{D_1^2}}\)

where

D₁ = inner diameter of cutting edge

D₂ = outer diameter of cutting edge

Note:

For stiff formation (A_r)_max = 20%

Soft sensitive clay (Ar)max = 10%

Maximum thickness of cutting edge = \(\frac{D_2 \ - \ D_1}{2}\)

Calculation:

Given, D₂ = 70 mm 

Soil is stiff clay, So (Ar)max = 20%

\({A_r} = \frac{{D_2^2 - D_1^2}}{{D_1^2}}\)

\({0.2} = \frac{{70^2 - D_1^2}}{{D_1^2}}\)

D₁ = 63.9 mm

Maximum thickness of cutting edge = \(\frac{D_2 \ - \ D_1}{2}\)

= \(\frac{70 \ - \ 63.9}{2}\) = 3.05 mm 

Explanation:

Lime Stabilization of Soil

• Hydrated lime is used to stabilize highly plastic soils such as black cotton soils. The volume of lime required for such stabilization is about 4-6 % of the volume of soil.
• In lime stabilization, the liquid limit decreases but the plastic limit and shrinkage limit increases. Therefore, with the increase in the value of the plasticity index of soil, the quantity of lime is required for stabilization.
• Soil becomes more friable and workable as the optimum moisture content (OMC) increases and the maximum dry density (γd­) decreases.
• The strength and durability of the stabilized soil is higher than the ordinary one.
• Lime increases the unconfined compressive strength as high as 60 times and the modulus of elasticity also increases substantially. This is an experimental result. However, this is due to a large increase in cohesion with small increases in the angle of shearing resistance.

Types of sample:

Undisturbed sample:

If the soil structure, moisture content & mineral content remains the same while sampling, the sample is called an undisturbed sample.

Disturbed sample:

These are two types

a) Representative sample:

If only soil structure gets modified but moisture content & mineral content remains the same while sampling, the sample is called a representative sample.

b) Non-representative sample:

If all the 3 i.e. soil structure, mineral content, moisture content get modified by sampling, samples are known as non-representative samples.

Degree of disturbance:

The degree of disturbance of the sample collected by various methods can be expressed by a term called the area ratio, which is given by

\({{\rm{A}}_{\rm{r}}} = \frac{{{\rm{D}}_0^2 - {\rm{D}}_i^2}}{{{\rm{D}}_i^2}}\)

D_i = inner diameter of cutting edge

D_o = outer diameter of cutting edge

A soil sample generally can be considered undisturbed if the area ratio is less than or equal to 10%. The area ratio for the standard spoon sampler is nearly 10% and for Shelby, tubes are 13.5 %.

Concept:

Soil Sampling:

The fundamental requirement of the sampling tool is that on being forced into the ground it should cause as little displacement, remolding, and disturbance as possible.

The degree of disturbance is controlled by three features of its design as follows:

• Cutting Edge
• Inside wall friction
• Non-return Valve

​

Cutting Edge:

A cutting edge embodies the following features:

a) Inside Clearance(Ci)

It is given by the following formula

\({C_i} = \frac{{{D_s} - {D_c}}}{{{D_c}}}\)

where D_s = inside diameter of sampling tube, D_c = inside diameter of cutting edge

For undisturbed sampling, it should lie between 1% and 3%.

b) Outside Clearance(Co)

It is given by the following formula

\({C_o} = \frac{{{D_w} - {D_T}}}{{{D_T}}}\)

where Dw = outside diameter of cutting edge, D_T = outside diameter of the sampling tube.

The Outside clearance should not be greater than the inside clearance.

c) Area Ratio(A_r)

It is calculated by the following formula

\({A_r} = \frac{{D_w^2 - D_c^2}}{{D_c^2}} \times 100\)

where D_w = outside diameter of cutting edge, D_c =  inside diameter of cutting edge

It should be kept as low as possible consistent with the strength requirements of the sample tube. Its value should not be greater than about 20% for stiff formations while for soft sensitive clays should not be greater than 10%.

Types of sample:

Undisturbed sample:

• If the soil structure, moisture content & mineral content remains the same while sampling, the sample is called an undisturbed sample.
• It is used for tests on shear, consolidation, and permeability and can also be used for tests like a disturbed sample.
• Obtained by using sharp-edged thin wall tubes or cylinders.

Disturbed sample:

• These are used for mechanical analysis, water content determination, index properties, compaction, and stabilization test
• These are two types

a) Representative sample:

• If only soil structure gets modified and moisture content and mineral content remain the same while sampling, the sample is called a representative sample.
• Used for determining specific gravity and atterberg limits

b) Non-representative sample or zero representative:

• If all the 3 i.e. soil structure, mineral content, moisture content get modified by sampling, samples are known as non-representative samples.
• Obtained by wash boring and auger boring

Explanation:

Method of exploration:

Various method of exploration are as follows:

• Geo-physical methods
• Open excavations
• Boring
• Sounding and penetration

Boring:

• Borings are generally used for depths greater than 3 m or when difficult groundwater conditions are met.
• The various method of boring is as follows.
• Auger boring:
  • Use in partially saturated sands, silts, and medium to stiff clays.
  • Highly disturbed sample for identification only.
  • Small depth of exploration. E.g. highways and borrow pit.
• Wash boring:
  • This method can be used below water table and is suitable for all types of soils except hard rocks.
• Percussion drilling:
  • This is the only method that is used in the bouldery and gravelly stratum.
• Rotary drilling:
  • This method is primarily meant for rocks, it is also used for clays and sands.​

Explanation:

• A thin-walled sampler is used for obtaining an undisturbed sample.
• Shelby tube samplers are thin-walled, hollow steel tubes, which are driven into the ground to extract a relatively undisturbed soil sample for use in laboratory tests used to determine density, permeability, compressibility, and strength.
• Piston samplers are thin-walled tube samplers, which are also used for collecting undisturbed samples in soft soil.

Important Points

Undisturbed sample: 

• An undisturbed sample is that in which the natural structure and properties remain preserved.
• These samples are used in size distribution, Atterberg's limits, consolidation parameter, coefficient of permeability, shear strength parameter.

Disturbed sample:

• A disturbed sample is that in which the natural structure of soils get partly of fully modified and destroyed.
• A thick-walled sampler is used for obtaining disturbed but representative samples.

Concept:

Soil samples are classified as disturbed and undisturbed samples.

Undisturbed samples: The soil samples in which original soil structure and mineral properties have not undergone any change are called undisturbed samples.

It is important in the identification of soil properties such as shear strength, water content, soil structure, consolidation properties, etc.

Disturbed sample: The soil samples in which original soil structure and mineral properties are destroyed during sampling operation are called disturbed samples.

They are used to study soil grain properties, in CBR test for highway construction, etc.

The extent of disturbance depends on cutting edge and inside wall friction of soil sampler.

The sampling tubes are of two types:-

a) Thin-walled sampler: These are used to obtain undisturbed samples. The sampler tube is pushed into the soil without any twisting.

b) Thick-walled sampler: These are used to obtain disturbed but representative samples. The sampler tube is twisted and pushed into the soil.

Important Points:

Direct excavations: It is a method of soil exploration. This method is generally used in small works and produces disturbed samples.

Augers- It is a boring method in which the boring is carried by an auger that is held vertically and pressed down shearing the soil around the auger.

Concept:

Lateral earth pressure is the pressure that soil exerts against a structure in a sideways, mainly horizontal direction. Since most open cuts are excavated in stages within the boundaries of sheet pile walls or walls consisting of soldier piles and laggings and since struts are inserted progressively as the excavation proceeds, the walls are likely to deform (as shown in the figure).

Little inward movement can occur at the top of the cut after the first strut is inserted. As the earth pressure magnitude increases due to an increase in effective stress while moving from top to bottom, so deflection of a sheet pile in a braced cut increases from top to bottom.

Concept:

Lime stabilization of Soil:

• It is the process of stabilizing soils by adding lime to reactive soils to generate long-term strength gain through a pozzolanic reaction.
• It is used to improve the strength, stiffness, and durability of fine-grained clayey soils.
• The addition of lime to soils produces a maximum density under a higher optimum moisture content than in the untreated soil, thus leading to flocculated or aggregated larger particle groups which are fairly stable.
• An increase in lime content causes a considerable reduction in swelling pressure and thus an increase in shrinkage limit and plastic limit. Subsequently, it decreases the liquid limit.
• Adding lime to soils increases soil resilient modulus shear strength substantially.