Geotechnical and Foundation Engineering - objective questions from AMIE exams (Summer 2019)

Answer the following questions: (3 x 5)

Describe various methods of drilling holes for subsurface investigations.

There are several boring techniques like auger boring, rotary drilling, wash boring, percussion drilling, auger drilling, and test pits that are employed to collect disturbed and undisturbed samples of soils.

These boring methods are selected based on the soil types, the efficiency of the boring technique, types of soil sample (disturbed or undisturbed), and the availability of facility and accuracy by which soil and groundwater variations can be determined.

These techniques are used to take soil samples at a certain practical depth, for example, auger boring collects undisturbed soil samples at a depth of 35m, whereas, the sampling depths for percussion drilling, wash boring, and rotary drilling is around 70m.

Auger Boring

It is a simple and cost-effective boring technique that can be used for almost all types of soil apart from gravelly soil and rocks. This technique encounters difficulty in gravelly soil and special drilling bits are needed for rocks. Auger boring is used to collect disturbed soil specimens. It collects the soil sample from a maximum practical depth of nearly 35m based on the available time and equipment type.

Rotary Drilling

The rotary drilling method of boring is suitable for all types of soil including rocks. It is used to take disturbed as well as undisturbed soil samples. So, it is specifically applicable for stiff soil layers. The practical depth of sampling is around 70m and the greater depth is based on the type of utilized equipment.

Percussion Drilling

It is used for all types of soils and rocks including stiff soils and rocks. Percussion drilling is used to take disturbed and undisturbed specimens but the quality of undisturbed samples is not that good because of the heavy blows of the chisel. Similar to rotary drilling and wash boring, the soil specimen can be taken from a depth of 70 m and more based on the utilized equipment. The diameter of disturbed soil samples is about 100 mm and greater, and obtaining smaller diameter samples would be uneconomical.

Write the short notes on stone columns.

Stone Column is a vertical shaft formed by backfilling compacted and crushed stone gravel or sand or moisture of these granular or material to form granular pile or 1. They are used for all types of planned structures including buildings, foundations, damns, tanks, towers, embankments, etc.

Steps

Holes of the required size are bored by using a float bailer.
After boring is completed crushed stones are back filled inside the casting.
The casing is withdrawn at the short pass and granular fill is compacted by rammers.
After that, add additional granular charge and repeat stages till the full length of the column.

What are the site conditions when ground improvement is required for foundations?

Ground improvement is necessary when poor soil conditions are encountered for the purpose at hand. While the poor soil conditions could readily be dealt with by excavating and replacing the soil, or perhaps by using deep foundations, it is often more cost-effective to simply improve the soil in place through some type of treatment.

Ground modification typically serves one or more of the following primary functions:

Increase shear strength and bearing capacity,
Increase density,
Decrease permeability,
Control deformations,
Increase drainage,
Accelerate consolidation,
Decrease imposed loads,
Provide lateral stability,
Increase resistance to liquefaction,
Transfer embankment loads to more competent subsurface layers.

Choose the correct answers (5 x 1)

1. For well-graded sand, the coefficient of curvature should be
(a) More than
(b) Between 1 and 3
(c) Less than 1
(d) None of above

2. A soil has a liquid limit of 30. The corresponding plasticity index given by the A-line is
(a) 73
(b) 7.5
(c) 9.0
(d) 9.5

3. A flow net has 4 flow channels and 20 equipotential drops, the shape factor is
(a) 1/5
(b) 5
(c) 80
(d) None of above

4. The coefficient of compressibility is the ratio of
(a) Change in void ratio to change of effective stress
(b) Volumetric strain to change in effective stress
(c) Change in thickness to change in effective stress
(d) Stress to strain

5. The yield of a retaining wall required to reach plastic equilibrium inactive case is
(a) More than that in the passive case
(b) Less than that in the passive case
(c) Equal to that in the passive case
(d) None of the above

Answers

1. (b)

For well-graded soil, 1 < C_c < 3

For gap graded soil, 1 < C_c or C_c > 3

2. (b)

3. (a) Shape factor = N_f//N_d

where N_f is the number of flow lines and N_d is the number of equipotential drops.

4. (a) Compressibility is the aptitude of the soil to be deformed. It is expressed by means of a coefficient which is the ratio between a void ratio decrease from e₀ to e and an increase in effective stress.

a_v = Δe/Δσ

5. (b) The yield of a retaining wall required to reach plastic equilibrium inactive case is less than that in a passive case. Active pressures are accompanied by movements of the retaining wall directed away from the soil and passive resistance are accompanied by movements towards the soil.

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