Tutorial: Site Investigation Semester I Session …...2 60 55.5 % R 55.5 A = 16.88% Since the area...

Tutorial: Site InvestigationSemester I Session 2013/2014BFC 43103 Foundation EngineeringDept. of Infrastructure and Geomatic EngineeringFaculty of Civil and Environmental Engineering

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CHAPTER 1 SITE INVESTIGATION TUTORIAL

Examples Eg. 1

One sampler has an area ratio of 8% while another has 16%; which of these samplers do you prefer and why? Answer: The sampler with area ratio of 8% is to be preferred since the sample disturbance is inversely proportional to it. It is considered desirable that the area ratio be less than 10% for undisturbed sampling.

Examples Eg. 2

Compute the area ratio of a thin walled tube samples having an external diameter of 6 cm and a wall thickness of 2.25 mm. Do you recommend the sampler for obtaining undisturbed soil samples? Why? Answer: External diameter, Do = 6 cm = 60 mm Wall thickness = 2.25 mm ∴ Internal diameter, Di = (60 – 2 × 2.25) mm = 55.5 mm Equations

Area Ratio 2 2

2% %o i

R

i

D DA

D

2 2

2

60 55.5%

55.5RA

= 16.88%

Since the area ratio is more than 10%, the sampler is not recommended for obtaining undisturbed samples. The sample disturbance will not be insignificant.


Examples Eg. 3

A SPT is conducted in fine sand below water table and a value of 25 is obtained for N. What is the corrected value of N? Answer: Equations

Corrected N 12

15 15N N

Here N′ = 25; ∴ N = 15 + 0.5(25 – 15) = 20

Examples Eg. 4

A SPT was conducted in a dense sand deposit at a depth of 22 m, and a value of 48 was observed for N. The density of the sand was 15 kN/m2. What is the value of N, corrected for overburden pressure? Answer: N’ = 48; Equations

Corrected N 350

70N N

where σ = overburden pressure in kN/m2. Here, σ = 22 × 15 = 330 kN/m3

∴ Corrected N = 350

48330 70

N

= 42

Examples Eg. 5

A vane, used to test a deposit of soft alluvial clay, required a torque 72 metre-Newtons. The vane dimensions are D = 100 mm, and H = 200 mm. Determine a value for the undrained shear strength of the clay. Answer: D = 100 mm H = 200 mm = 2D Torque, T = 72 m N = 72,000 mm N.


Equations

2

2 6

T

H DD

3

6

7

T

D

, for H = 2D

3

6*72000

7 * 100

N/mm2 = 19.64 x 10-3 N/mm2

= 19.64 kN/m2

Examples Eg. 6

The following dimensions are given for a shelby tube sampler: External diameter = 51 mm Internal diameter = 48 mm Determine the area ratio. Answer: Equations

Area Ratio 2 2

2% %o i

R

i

D DA

D

2 2

2

51 48%

48RA

= 12.9%

Examples Eg. 7

75 mm is the external diameter of a sampling tube. If the area ratio required is 20%, determine the thickness of the sampling tube. In what type of clay would such a high area ratio be required? Answer:

2 2

2

75% i

R

i

DA

D

= 0.20

Solving; Di = 68.465 mm. The wall thickness 0.5(75.0 - 68.465) = 3.267 mm When samples are to be taken in very stiff to hard clay soils mixed with stones, sampling tubes with high area ratios are required.


Try it Q1

Describe with a neat sketch how will you carry out the wash boring method of soil exploration. What are its merits and demerits?


Try it Q2

Write a short note on Geophysical exploration using electrical resistivity.


Try it Q3

Under what circumstances are geophysical methods used in exploration? Discuss the usefulness of a dynamic cone penetration test and its limitations. Write a brief note on wash borings.


Try it Q4

(a) Discuss with neat sketches any two boring methods used in soil exploration. (b) Sketch a split-spoon sampler and explain its parts.


Try it Q5

What are the various steps considered in the planning of sub-surface exploration programme? Describe the standard penetration test. In what way is it useful in foundation design?


Try it Q6

Write short notes on: (a) Geophysical methods, (b) Penetration Tests.


Try it Q7

Why are undisturbed samples required? Describe any one procedure of obtaining undisturbed samples for a multi-storeyed building project. For what purpose are geophysical methods used? Describe any one method.


Try it Q8

What are the advantages and disadvantages of accessible exploration? Discuss. Explain (a) Wash boring, (b) Split spoon sampler. Write a brief note on the precautions to be taken in transporting undisturbed samples.


Try it Q9

(a) Explain and discuss the various factors that help decide the number and depth of bore holes required for subsoil exploration. (b) What is ‘N-value’ of Standard Penetration Test? How do you find the relative density from ‘N-value’? Explain the various corrections to be applied to the observed value of N.


Try it Q10

(a) Enumerate the various methods of soil exploration and mention the circumstances under which each is best suited. What do you mean by undisturbed sample? (b) Explain with a neat sketch the construction and use of a split spoon sampler.


Try it Q11

Describe the “Standard Penetration Test” used in soil exploration. List the information that can be obtained by the test when made in (a) clay, (b) sand. Comment on the correction factors to be used for N-values (a) for sands for depth below ground level (b) for fine sand under water table.


Try it Q12

Write a brief critical note on vane shear test.


Try it Q13

Two samplers have area ratios of 10.9% and 21%. Which do you recommend for better soil sampling and why? Try it Q14

Compute the area ratio of a sampler with inside diameter 70 mm and thickness 2 mm. Comment.


Try it Q15

A N-value of 35 was obtained for a fine sand below water-table. What is the corrected value of N ? Try it Q16

A SPT was performed at a depth of 20 m in a dense sand deposit with a unit weight of 17.5 kN/m2. If the observed N-value is 48, what is the N-value corrected for overburden?


Try it Q17

A vane, 75 mm overall diameter and 150 mm high, was used in a clay deposit and failure occurred at a torque of 90 metre-Newtons. What is the undrained shear strength of clay?


Try it Q18

The inner diameters of a sampling tube and that of a cutting edge are 70 mm and 68 mm, respectively, their outer diameters are 72 and 74 mm respectively. Determine the inside clearance, outside clearance and area ratio of the sampler.

Tutorial: Site Investigation Semester I Session …...2 60 55.5 % R 55.5 A = 16.88% Since the area...

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