Site Investigation and Sub-Soil Exploration MCQ Quiz in বাংলা - Objective Question with Answer for Site Investigation and Sub-Soil Exploration - বিনামূল্যে ডাউনলোড করুন [PDF]

Explanation:

(i) The sample disturbance depends on the design features of a sampler (cutting edge, inside wall friction and non-return valve) and the method of sampling.

(ii) Disturbance can be measure in terms recovery ratio, which is

\({L_r} = \frac{{{\mathop{\rm Re}\nolimits} {\mathop{\rm cov}} ered\ length\ of\ sample}}{{penetration\ length\ of\ sample}}\)

If, L_r = 1, Good recovery

L_r  > 1, Sample of swelled

L_r < 1, Sample of shrunk or compressed

High fissure soils means that the soils which have tendency to increase in volume when water is added and decrease in volume if water is removed.  They are also called Expansive soils.

1. Open Drive Thin-Walled Sampler:
   • This is a basic type of sampler. The tube is advanced into the soil by applying weight and turning it. It can provide relatively undisturbed samples of fine sands, silts, and clays.
   • The advantage of this sampler is its simplicity and low cost. However, in saturated conditions, it can cause considerable disturbance to the soil structure.
2. Standard Split-Spoon Sampler:
   1. This is the most common type of soil sampler and is used in Standard Penetration Testing (SPT).
   2. It consists of a split tube that can be opened lengthwise to retrieve the sample. It's driven into the soil by a hammer and provides a disturbed sample, which is suitable for identification purposes and providing a measure of density or strength in coarser soils. In finer, saturated soils, it can cause significant disturbance.
3. Stationary Piston Sampler:
   1. This sampler works under a principle that minimizes the disturbance of the soil sample.
   2. The piston is held stationary at the bottom of the sample tube while the tube is advanced into the soil. This way, the soil enters the tube with minimal disturbance.
   3. It's especially effective in conditions where the soil is soft and saturated, as it can minimize the remoulding of the soil and maintain the structure of the sample.
4. Rotary Sampler:
   • This method is used to obtain soil samples from hard, stiff, or rocky soils that cannot be sampled using the traditional sampling methods.
   • A drill bit is used to break down the soil and bring it to the surface.
   • However, this method is unsuitable for soft, saturated soils as it greatly disturbs the soil structure and is likely to cause soil liquefaction.

Explanation:

Soil stabilization is the process of improving the engineering properties of the soil and thus making it more stable.

Mechanical Stabilization:

• Mechanical stabilization is the process of improving properties of soil by changing its gradation.
• Two or more types of natural soil is are mixed to obtain a composite material which is superior to any of its components.
• To achieve desired grading, sometimes the soils with coarse particles are added or the soils with fine particles are removed.
• Mechanical stabilization is also known as granular stabilization.
• For the purpose of mechanical stabilization soil is subdivided into two categories: 1. Aggregates 2. Binders

Additional InformationSoil stabilization either achieved by mechanical, physical and chemical stabilization.

Physical Stabilization:

1. Cement Stabilization
2. Lime Stabilization
3. Bitumen Stabilization
4. Chemical Stabilization

5. Resin Stabilization

Chemical Stabilization:

1. Calcium Chloride Stabilization
2. Sodium Chloride Stabilization
3. Sodium Silicate Stabilization
4. Polymer Stabilization
5. Chrome Lignin Stabilization

Explanation:

Stabilization of Black Cotton Soils:

• Black cotton soil is not suitable for construction work on account of its volumetric changes.
• It swells and shrinks excessively with the change in water content. Such tendency of soil is due to the presence of fine clay particles(Montmorillonite mineral) which swell, when they come in contact with water, resulting in alternate swelling and shrinking of soil due to which differential settlement of structure takes place.
• So the stabilization of black cotton soil is done using hydrated lime as Lime stabilization helps in increasing the strength, durability and also minimizes the moisture variations in the soil.

Concept:

Geosynthetics:

Geosynthetics are man-made materials used to improve soil conditions.

They include the functions of separation, filtration, reinforcement, stiffening, drainage, barrier, and protection of roadways.

The important physical properties of Geosynthetic are,

• Thickness
• Unit Weight
• Stiffness or flexural stiffness or flexural rigidity
• Specific gravity

Explanation:

Standard Penetration test: Standard penetration test can be used to determine:

(i) Relative density of sand

(ii) Angle of internal friction

(iii) Unconfined compressive strength of clays

(iv) Ultimate bearing capacity based on shear criteria

(v) Load carrying capacity of the pile

Procedure:

(i) This test is conduct at a regular interval of 2-5 m or at the change of stratum

(ii) To conduct this test, a borehole of 55-150 mm is made and a split spoon sampler is used which has tubular shape with 35 mm inner dia, 50.5 mm outer dia, and 650 mm length.

(iii) The sampler is driven in the soil by a standard hammer of weight 65 kg and height of free fall 75 cm.

(iv) The number of blows required for penetration of 300 mm is designated as the standard penetration value.

(v) The test is conducted in three stages having 150 mm penetration each. The number of blows for the first 150 mm penetration is ignored and the number of blows in the last 300 mm penetration is noted and called the observed SPT number.

Concept:

Sandy soils are generally stabilized with cement.

The amount of cement required to stabilize depends upon the quantity and quality of fines contained in sandy soils and final compacted density.

The cement required for sandy soils range between 5 and 12% by weight.

Stabilization of soil with cements consists of adding cement to pulverized soil and permitting the mixture to harden by hydration of cement.

The quantity of cement required for stabilization increases as soil plasticity increases.

Important Point:

Soil-lime stabilization: It is the process of adding lime to the soil to improve its properties like density, bearing capacity etc. The principle involved in lime-soil stabilization is the exchange of ions between lime and soil when added.

When clayey soil is treated with lime, cation exchange takes place between them which increases plastic limit and reduces plasticity index which finally results in increase in stability of soil. This method is used mostly for clayey flocculated soil.

Soil-bitumen stabilization: It is applied to highways and airfield construction. It is the method of designing and mixing local soil or aggregates with bituminous materials to form a stable and waterproof base course.

Bitumen stabilization is used for:-

a) For binding soil particles together for purpose of supplying cohesion for non-mechanical stabilized granular material.

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

\({C_i} = \frac{{{D_s} - {D_c}}}{{{D_c}}} \times 100\)

where Ds = inside diameter of sampling tube, Dc = 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

\({C_o} = \frac{{{D_w} - {D_T}}}{{{D_T}}} \times 100\)

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

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

c) Area Ratio(Ar)

It is calculated by the following formula

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

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

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%.

Hence, according to a given question, the outside clearance of the sampler

\({C_o} = \frac{{{D_2} - {D_4}}}{{{D_4}}} \times 100\)

where D₂ = outside diameter of cutting edge, D₄ = outside diameter of the sampling tube.

Explanation:

Depth of Exploration:

(i) The depth up to which the stress increment due to superimposed loads can produce a significant settlement and shear stresses is known as the significant depth.

(ii) The depth of exploration should be at least equal to the significant depth.

(iii) The significant depth is generally taken as the depth at which the vertical stress is 20% of the load intensity. As per the above criteria, the depth of exploration should be about 1.5 times the width of the square footing and about 3 times the width of the strip footing.