Geotechnical Exam Question Set A Flashcards
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Give two (2) reasons why there is a need to carry out a soil profile investigation
1) Establish the general nature of the soil strata below a site
2) Establish the vertical or lateral variability of soil conditions
List the purpose of the following instruments:
eg. inclinometer
there is a total of 5 instruments
a. Inclinometer
- measure lateral or horizontal movements across different soil strata
b. Magnetic Extensometers
- measure settlement of individual soil strata
c. Water standpipe
- determine Ground Water Table (GWT)
d. Settlement marker
- measure total ground settlement at a particular point
e. Tiltmeter
- measure tilt in structures
Illustrate how you will use the particle size distribution curve to determine the
gradation of the soil.
1) Using the uniformity coefficient, the uniformity could be defined. If UC <
4, the soil is uniformly graded. If UC > 4, the soil is either well graded or
gap graded.
2) If the shape of the curve is not too steep and is more or less constant over
the full range of the soil’s particle sizes, the soil is determined as well
graded.
3) If the curve is not too steep at the beginning and ending but is flat in
between, it is determined as gap graded.
4) If the curve is too steep over a small range, it is determined as uniformly
graded
Using British Soils Classification Systems,
a. give a symbol to represent the following soils;
- Well graded silty GRAVEL
-Sandy SILT of low plasticity
GWM
MLS
name the soil represented by the following symbols:
-CPgS
-CVO
- Gap graded sandy CLAY
- Organic CLAY of very high plasticity
Explain why capillary fringe is almost saturated despite it is above the ground water
table.
- It is due to capillary tension. Due to capillarity, water is drawn up above the water table into the interstices of the soil or rock.
- The smaller the size of the pores the higher the water can rise above the water table.
- Water in this fringe can be regarded as being in a state of negative pressure that is pressure above atmospheric.
Define the following terms;
eg. flow net
a. Flow net
A form of curvilinear net made up of a set of flow lines intersected by a set
of equipotential lines
b. Flow lines
The paths which water particles follow in the course of seepage
c. Equipotential lines
As the water moves along the flow line, it experiences a continuous loss of
head. Equipotential lines are the lines joining the points of equal potential. It
intersects the flow lines at a right angle
Determine the difference between total stress and effective stress
- When a load is applied to an undrained soil, it will be carried by the water in the soil voids or by the soil skeleton or it will be shared between the water and the soil skeleton.
- The total stress is the sum of effective stress and the pore water pressure in the soil.
- The effective stress is the average stress carried by the soil skeleton. It measures the changes in the volume and strength of the soil.
- It could be determined by deducting the pore water pressure from the total stress.
Describe the differences between direct shear test and triaxial test.
conditions:
1) plane of failure:
For Direct shear- fail at a predetermined plane.
For Triaxial - failure plane is not predetermined but will occur at soil’s failure plane.
2) Drainage:
For direct shear- cannot be controlled, hence test can only measure total stress.
For Trixial test- can be controlled and pore water pressure can also be measured.
3) Stress Direction:
For direct Shear- 2 directions only
For triaxial Test - 3 directions, right angle to each other
List the three types of triaxial test results that are useful to determine the soil shear
strength. Determine how the three types of triaxial test are obtained in the laboratory
test.
a. Consolidated Drained (CD) test
b. Consolidated Undrained (CU) test
c. Unconsolidated Undrained (UU) test Z
- To conduct consolidated test, the soil sample is drained during the initial application
of cell pressure. - To conduct drained test, the soil sample is drained during shearing (or loading
List three (3) types of earth retaining structures and define how the structures gain
stability to retain the soil?
1) Gravity wall – rely on the stability of its self weight
2) Cantilevered wall – rely on weight of soil retained above its base
3) Sheet piles wall – rely on embedment depth and horizontal restraining forces
(such as tie rods)
State the two (2) main types of dewatering methods. Give an example for each
method stated.
- Removal methods – pumping from sumps / well pointing
- Exclusion methods – cofferdam
Determine five (5) factors affecting the choice of materials for filling and
backfilling.
-The material is readily available.
-The material is as hard as possible.
-The material is not too wet, full of voids and liable to excessive shrinkage.
- The material contains appropriate texture or consistency such that it can be
properly compacted
- The material is free from chemicals that will attack concrete or any surface of
the structure it is in contact with.
. List three (3) advantages and three (3) disadvantages of using sheet piles in trench
protection
- Advantages
i. Continuous
ii. High strength
iii. Readily available - Disadvantages
i. Flexible and can cause excessive ground movement
ii. Can only be driven to limited depths in stiff subsoil conditions
iii. Installation can be severely hampered by obstructions in the ground
Describe two (2) main differences between a contiguous bored pile and a normal
bored pile.
-CBP requires more steel reinforcements than bored pile
- CBP is subjected to lateral load and bored pile is subjected to vertical load.
