Unsaturated Soil Mechanics Quiz
Unsaturated Soil Mechanics Quiz
Test your knowledge on unsaturated soil mechanics with this comprehensive quiz consisting of 67 multiple-choice questions. Challenge yourself to understand the intricacies of water flow, soil suction, hydraulic conductivity, and the principles that govern unsaturated soils.
Topics Covered:
- Water Flow in Soils
- Soil-Water Characteristic Curve (SWCC)
- Hydraulic Conductivity
- Effective Stress Principles
- Laboratory Testing Techniques
Which of the following statements about the water flow in unsaturated soil is incorrect?
Evaporation can be considered as 1D water flow in soil
Unsaturated soils are commonly assumed to be heterogenous soil
If the hydraulic conductivity is sensitive to the change of orientation, the soil is a type of heterogenous soil
Steady state flow means the time derivative of discharge is always equal to zero
Select the incorrect statement about filter paper method.
Calibration curve is the water retention curve of filter paper.
An indirect measurement method for soil suction.
Matric suction can be measured by the contact technique through filter paper.
The breakpoint along calibration curve is due to hysteresis in water retention curve.
The "break point" along calibration curve of filter paper is induced by
Surface tension along the air-water interface.
Changes in the pore size of filter paper.
Contact angle effect in unsaturated soil.
Dominant mechanism switched from adsorption to capillary effect as water content increases.
When you measure the SWCC for a clayey soil in the laboratory, you found that the measured matric suctions are same when the water contents are different in drying and wetting paths. Which of the following factors can be used as the most appropriate factor to explain suchfinding?
Geometric nonuniformity of soil pores.
Contact angle effect.
Entrapped air during the sorption process.
Differential soil structural changes during the sorption and desorption processes.
Select the incorrect statement about pressure plate.
Pore water pressure and pore air pressure are increased simultaneously.
Matric suction can be equal to applied air pressure.
Outflow water from pressure plate is used to determine water content.
Axis translation technique is used.
Pick up the state variable from the following relations.
Darcy’s law:
Bishops effective stress:
Principle of effective stress:
Stress-strain relation:
Select the correct statement about the matric suction.
Pore air pressure dominates the change of matric suction.
Water content affects the contribution of matric suction to the soil skeleton.
Volume expansion takes place in drying process.
Increased matric suction causes the compressive failure of unsaturated soils.
Which of the following tools can be used to determine the complete SWCC at one-time test?
Flexible wall permeameter
Pressure plate
Piezometer
Tensiometer
Select the correct statement about measurement of soil suction.
Smaller air entry value of ceramic can increase the measurement range of soil suction.
The measurement range is dependent only on the air entry value of ceramic.
Larger pore size of ceramic is able to increase the measurement range of soil suction.
Generation of air bubbles can increase the water pressure in the measuring system.
Select the incorrect statement about the water flow in soils? stres
In the conventional analysis, water flow is neglected in unsaturated soils.
Phreatic line is an equipotential line.
Flow net is the solution to Laplace’s equation for the water flow in saturated soils.
There is no time derivative term in the steady-state flow equation.
Select the correct statement about consolidated drained (CD) test on unsaturated specimen?
Water content is kept constant at the second stage of CD test.
Matric suction increases at the second stage of CD test.
Volume expansion occurs when net minor normal stress is high.
The deviator stress is sensitive to the change of net minor normal stress.
Select the incorrect statement about hydraulic conductivity (K)?
The laboratory-measured K is commonly smaller than the field-measured K.
Degree of saturation plays a critical role in the change of K in unsaturated soils.
Temperature can affect hydraulic conductivity.
Soil skeleton is the major contributor to the change of K in unsaturated soils.
Select the incorrect statements about the rigid wall permeameter.
Horizontal swelling is not allowed.
Can be used to determine hydraulic conductivity for coarse- and fine-grained soils.
Stress states are under control.
Sidewall leakage is one of its limitations.
Select the incorrect statement about Soil-Water Characteristic Curve (SWCC).
SWCC can be used to describe the ability of soil to retain water at different matric suctions.
SWCC refers to the relationship between water content and matric suction.
SWCC can only be used to describe the water retention capacity of unsaturated soil.
SWCC can be used to predict the material properties of unsaturated soils.
Select the incorrect statement about measurement of soil suction.
Relative humidity increases as the total suction increases.
If the measurement is related to water vapor, the measured suction refers to the total suction.
Total suction is nonnegative
If the measurement is related to liquid water in soil, the measured suction is matric suction.
Which of the following statements on pressure head is true?
Pressure head dominates the water flow in unsaturated soil.
Pressure head is always positive when the point of interest is located in fully saturated soil.
) Pressure head is negative when soil is in unsaturated state.
Soil is in unsaturated state when pressure head is negative.
Select the incorrect statement about steady-state flow in unsaturated soils.
Matric suction difference may contribute to the water flow in unsaturated soils, but matric suction difference is not the dominant mechanism for water flow in unsaturated soils.
The driving forces responsible for water flow are different in saturated and unsaturated soils.
No water flow in unsaturated soils is assumed by the conventional analysis, but water flow can occur in both unsaturated and saturated soils.
Water flow may occur from a point with high water content to a point with low water content, but water content difference is the actual driving force responsible for the water flow in unsaturated soils.
Which of the following statements about the principle of effective stress is correct.
Effective stress is a type of immeasurable stress.
Effective stress can be used to describe the mechanical behavior of unsaturated soils.
Principle of effective stress represents a constitutive relation.
Effective stress refers to the stress carried by saturated soil.
Which of the following equations represents the water flow in a heterogeneous and isotropic soil?
Ks ....
Ks (...) + .... +.... = 0
Ks... + Ksy...+.... = 0
... + ... = 0
When the water table is below the ground surface, you, as a geotechnical engineering, are required to assess the behaviors of soils above and under the water table. Please select the most appropriate approach:
Bishop’s effective stress is adequate to describe the mechanical behaviors of soils above and under the water table in practical engineering application.
Principle of effective stress can be adopted to evaluate the soil behaviors above the water table, while bishop’s effective stress is suitable for the assessment of the mechanical behaviors of soils under water table.
For soils above and below water table, it is impossible to use same stress variable(s) to describe their mechanical behaviors.
Two stress state variables including principle of effective stress and Bishop’s effective stress can be used to describe the behaviors of soils above and under the water table
Pick up the incorrect statement about the shear strength equation for unsaturated soil.
The shear strength equation of saturated soil is a special case of the shear strength equation of unsaturated soil.
For the development of shear strength of unsaturated soil, there are two contributors including matric suction and net normal stress.
The contribution of matric suction is limited by the water content in unsaturated soils.
Shear strength is constant when soil type and water content are fixed
For the given earth slope, the following information is given: • When spatial position is fixed, the hydraulic conductivity is the same in all directions. • When orientation is fixed, the hydraulic conductivity is different at different positions. Based on the given information, please choose the most appropriate flow equation to describe the water flow inside the tangent earth slope:
Ks(....) +....+ ... = 0
... + .... = 0
.... + .... =0
Ks .... + ks... =0
To assess the liquefaction potential of soil foundation during the period of construction, the geotechnical engineer used the CU tests to evaluate the soil behaviors. Please adjust whether the adopted approach is correct.
The adopted approach is acceptable.
Cannot be determined based on the given information.
The adopted approach is unacceptable, and UU tests should be adopted.
The adopted approach is unacceptable, and CD tests should be adopted.
For an unsaturated soil sample, the total weight is 0.05kN, dry weight is 0.04kN, and specific gravity is 2.5. Based on the given information, select the correct statement.
Degree of saturation is 20%.
Before the drying process in oven, the soil is in fully saturated state.
Volumetric water content is 25%
the gravimetric content can be determined according to the given information
Which of the following statements is false?
Constitutive relations can be used to define state variable.
Darcy’s law is a type of constitutive relation.
Effective stress of saturated soil is a type of state variable.
State variables can be used to form constitutive relations.
As a geotechnical engineer, you are required to evaluate the mechanical stability of an earth dam under seismic loadings. The target earth dam was constructed ten years ago. Please select the most appropriate method to evaluate the shear behavior of the soil obtained from the target earth dam.
UU tests.
CU tests.
CD tests.
Direct shear tests.
Select the incorrect statement about the Darcy’s law.
When the hydraulic conductivity is expressed as a function of water content, the modified Darcy’s law can be used to evaluate the discharge velocity in unsaturated soils.
Head loss is a nonpositive quantity.
Darcy’s law aims to directly calculate the seepage velocity in saturated soils.
The negative sign in Darcy’s law indicates the water flow in the direction of decreasing total Head.
Select the correct statement about the measurement of hydraulic conductivity (K)
Falling head test can be used to measure the K for sand.
Constant head test can be used to measure the K for clay soil.
The laboratory measurement methods are more reliable and accurate than the field measurements.
There is no water flow between observation wells and testing well in pumping from wells
Which of the following information is required for the prediction of hydraulic conductivity for unsaturated soil?
Particle-size and pore-size distribution.
Effective stress and vertical settlement.
SWCC and saturated hydraulic conductivity.
Soil skeleton and fluid properties.
Select the incorrect statement about consolidated undrained (CU) test on unsaturated soil specimen?
At the second stage of CU test, the pore water pressure first increases and then decreases.
The air diffusion decreases the pore air pressure and pore water pressure.
The decrease in pore water pressure is dominated by water drainage during the second stage of CU test.
The pore air pressure and pore water pressure have a similar evolutionary trend.
which of the following methods cannot be used to measure the saturated hydraulic conductivity?
Pressure plate test.
Constant Head Test.
Falling Head Test.
Pumping from Well.
Select the incorrect statement about the shear strength failure envelope of unsaturated soil.
The nonlinearity associated with failure surface is attributed mainly to the net normal stress.
Compared with planar failure surface, the curved failure surface is more accurate for unsaturated soils.
Water content can affect the shape of failure surface.
When the planar failure surface is adopted, the contribution of matric suction is not accurately assessed.
A pressure plate is used to measure soil suction and applied air pressure is set to 300kPa. Based on the given information, select the correct statement about the measurement
The pore air pressure is same to the atmospheric pressure and thus equals zero.
The soil sample is in the fully saturated state.
The pore water pressure is -300kPa.
The matric suction can be determined according to the given information.
Which of the following statements on water flow equation is correct?
To derive the steady-state water flow equation, the inflow rate must be greater than outflow rate through the target soil element
Laplace equation can be used to describe water flow in both isotropic and anisotropic soils.
The 1D water flow equation aims to solve the water flow behavior in soil mass which can be assumed to be 1D geometry.
For the earth dams, 2D water flow equation is adequate to describe the water flow behaviors.
Select the incorrect statement about an independent phase in soils.
Own different properties from contiguous materials.
Have definite boundary surfaces.
There are four phases in unsaturated soils.
No change in the state of phases in soils.
Which of the following statements about Soil-Water Characteristic Curve (SWCC) is incorrect?
Air entry value and residual water content are required to describe SWCC.
Unsaturated hydraulic conductivity can be predicted through SWCC.
SWCC represents the water retention capacity of soils.
Air entry value refers to a particular water content when the air starts entering the soil pores.
Select the incorrect statement about water retention capacity of soils.
Capillary effect becomes more obvious when water content is low.
Adsorption effect is dependent on the water content in soils.
Particle size distribution mainly affects adsorption.
Capillary effect is dependent on the soil pore size
Which of the following statements on hysteresis of SWCC is true?
When the water content is same, matric suction from drying path is smaller than that from wetting path.
When matric suction is same, water content is relatively low in the drying path compared with that from wetting path.
Entrapped air during drying path can cause the hysteresis of SWCC.
Water retention capacity in the wetting path is higher than that from drying path.
Which of the following tools can be used to measure total suction in soils?
Rigid wall permeameter
Filter paper used in non-contact technique
Pressure plate
Piezometer
Which of the following clay mineral has the highest swelling capacity?
Kaolinite
Montmorillonite
Illite
Silt
Select the incorrect statement about high-air entry (HAE) ceramic.
Can effectively avoid the cavitation in the measuring system.
Has small pore size and thus owns higher HAE value.
HAE ceramic is required by pressure plate.
None of above.
Select the incorrect statement about axis translation technique.
Can prevent the air entering the measuring system from unsaturated soils.
Water content is kept constant.
Pore air pressure and pore water pressure are increased.
Reference pressure is translated from atmospheric pressure to an increased air pressure.
If the positive matric suction in soil is not equal to zero, the soil may be
in saturated state
In unsaturated state
A and B
None of above
Which of the following statements about water flow in soils is incorrect?
Discharge velocity is the average water flow velocity.
Actual water flow velocity is the seepage velocity.
Seepage velocity is smaller than discharge velocity.
Darcy’s law can be used to determine water flow velocity in saturated and unsaturated soils.
Select the correct statements about the flexible wall permeameter.
Principle stresses are controlled.
Sidewall leakage is avoided.
Chemical reaction may take place between membrane and liquid in soil samples.
All of above.
Select the correct statements about the measurement of hydraulic conductivity (K).
Constant head test can be used to measure the K for clay soil.
Falling head test can be used to measure the K for sand.
There is no water flow between observation wells and testing well in pumping from wells.
None of above.
Which of the following statements about the water flow in unsaturated soil is incorrect?
Steady state flow means the time derivative of discharge is always equal to zero.
If the hydraulic conductivity is sensitive to the change of orientation, the soil is a type of heterogeneous soil.
Unsaturated soils are commonly assumed to be a heterogeneous soil.
Evaporation can be considered as 1D water flow in soil.
Select the incorrect statement about the water flow in soils?
Flow net is the solution to Laplace’s equation for the water flow in saturated soils.
In the conventional analysis, water flow is neglected in unsaturated soils.
Phreatic line is an equipotential line.
There is no time derivative term in the steady-state flow equation.
Which of the following statements about the principle of effective stress is correct.
Effective stress can be used to describe the mechanical behavior of saturated soils.
Effective stress is a type of measurable stress.
Effective stress refers to the stress carried by pore water.
Principle of effective stress represents a constitutive relation.
Select correct statement about the net normal stress
Net normal stress tends to pull soil particles together
Total stress dominates the change of net normal stress in unsaturated soils.
There is no pore water pressure in the normal stress component.
There is no shear stress component.
Select incorrect statement about triaxial shear test on unsaturated soils.
Axis translation technique is required to control matric suction.
The first stage of triaxial shear test aims to apply the deviator stress.
Net minor normal stress is kept constant during 2nd stage of the test.
A high air-entry ceramic is required for measuring system.
Pick up the correct statement about the shear strength equation for unsaturated soil.
For the development of shear strength of unsaturated soil, there are two contributors including matric suction and net normal stress.
The shear strength equation of saturated soil is just a special case of the shear strength equation of unsaturated soil.
The contribution of matric suction is limited by the water content in unsaturated soils.
All of above.
Select the correct statement about the shear strength failure envelope of unsaturated soil.
The failure envelope is a 2D surface.
Two stress state variables are used to describe the failure envelope.
Friction angle associated with net normal stress is dependent on the water content.
None of above.
Select the incorrect statement about Bishop’s effective stress
Bishop’s effective stress is same to the principle effective stress
Bishop’s effective stress is not a stress state variable
Bishop’s effective stress cannot be used to describe the stress state in dry soils
Bishop’s effective stress is unable to assess the contribution of matric suction
Select the correct statement about axis translation technique
Axis translation technique is an indirect measurement method of matric suction.
The usage of axis translation technique wont change the measurement range of soil suction.
The usage of axis translation technique won't change in matric suction and water content in soil.
Axis translation technique can be used to measure total suction and matric suction in soil.
Pick up the incorrect statement about the surface tension
Solute in the water may increase or decrease the surface tension
Surface tension is measured as a force per unit length and thus is a type of state variable.
Warmer temperature can decrease the surface tension
Surface tension is able to maintain the stability of air-water interface
A measuring system consists of a High Air Entry ceramic with of an air entry value of 2500kPa. Meanwhile, the axis translation technique is adopted in the laboratory. Based on the given conditions, select the correct statement about the measurement of soil suction through such measuring system.
Cavitation may occur in the measuring when matric suction approaches 100kPa
The measuring system cannot be used to measure a matric suction greater than 100kPa
The measuring system can be used to measure a matric suction of 2500kPa
The measurement range of the measuring system is less than 2500kPa
The usage of High Air Entry (HAE) ceramics in the measurement of spill suction is to
Increase the water retention capacity of soil.
Increase the water adsorption of soil.
Prevent the air entering the measuring system.
Avoid the cavitation in the measuring system.
Select the incorrect statement about the direct shear test (DST) on unsaturated soils.
Since the failure surface is pre-specified in DSTs, the mechanical stability of geotechnical structure may be overestimated.
Axis translation technique is required when DTs are conducted on unsaturated soils
DSTs can be used to accurately simulate the field conditions including field confining pressure.
DST is a rapid and cost-effective method to determine shear behavior of unsaturated soil
Select the incorrect statements about the flexible wall permeameter.
Principle stresses are controlled.
Sidewall leakage is avoided.
Chemical reaction may take place between membrane and liquid in soil samples.
None of above.
Select the correct statement about water retention capacity
Absorption is more important when the water content is relatively high.
SWCC from sand soil is higher than that obtained from clay soil
Smaller pore size can result in lower water retention capacity
Air entry value is higher in the clay soil compared with that from sand soil
If the positive matric suction in soil is not equal to zero, the soil may be
Saturation condition cannot be determined based on the given information.
In dry state
in saturated state
In unsaturated state
Which of the following tools can be used to measure total suction in soils?
Filter paper technique
Pressure plate
Rigid wall permeameter
Piezometer
Which of the following statements on Phase is false?
Unsaturated soil consists of four different phases.
Solid ice and liquid water are considered as two different phases.
Air-water interface can be considered as an independent phase.
Saturated soil consists of three phases
Capillary and Matric Suction
The capillary effect can be observed as the upward movement of water through a thin tube. This is due to the force balance of cohesion + adhesion and gravity. Hydrogen bonding within water is responsible for the apparent cohesion between water molecules, which can be observed as the surface tension material property. The adhesion between the water and the tube will cause the air-water interface to rise up the tube until the force of gravity balances the height of the column. This is more evident in small radii tubes, as the suction is dependent on the radius of the tube. These force mechanisms are observable in matric suction; as the soil dries out, the residual water content will remain in suspension of the soil skeleton in various spatial positions. This is due to the water adhesion to the soil particles as well as the internal cohesion of water holding itself together.
X
How to evaluate seepage and shear strength of unsaturated soil
Seepage can be observed as one or two dimensional flow. Numerous cases of 1-D flow include steady state evaporation, infiltration and downward flow, while 2-D flow is observed as seepage through and earth dam. The formulation of both a 1-D and 2-D analysis uses a mass balance approach to analyze the variables in 1-D and 2-D flow. However, due to the heterogeneous distribution of pore fluids, the coefficient of permeability also varies as a function of matric suction. Shear strength can be determined using modified conventional triaxial and direct shear testing equipment. It can also be calculated using the liner form shear strength equation for unsaturated soil : τff = c′ + (σf − ua)ftanφ′ + (ua − uw) tan φb
X
Reasons why unsaturated soil mechanics is needed in analysis of seepage and shear strength of unsaturated soil
Understanding water flow behavior is important for understanding variations in pore water pressures, that is, the heterogeneous volume distribution of pore fluid. The water flow behavior is also helpful in determining the shear strength and volume change behavior in soil mechanics. The movement of water through soils is dependent on the coefficient of permeability (K) of the soil sample, and as such, when the velocity is sufficiently low, the water flow can be referred to as seepage or percolation. Darcy’s law can be used for unsaturated soils, however, it is a function of matric suction and water content when used for unsaturated soils and results in a non-linearity of water flow. Due to the heterogeneity of unsaturated soils, the coefficient of permeability can vary due to spatial variations in matric suction. While the saturated zone of water flow (below GWT) is the primary concern for seepage analyses, the constant water flux across the phreatic surface may develop a steady state water flux through the unsaturated zone of the soil slope. Seepage analysis must be considered when determining the mechanical stability of a geotechnical structure such as earth dams, but can also be used to better understand slope stability and groundwater contamination mechanisms.
The shear strength of unsaturated soils are interpreted as 2 stress state variables; net normal stress and matric suction. Pore water pressure is more important in matric suction. As the matric suction tends to pull the soil particles together, the soil will become stiffer and stronger as the matric suction increases. Net normal stress tends to push the soil particles together, where increasing the net normal stress can increase the stiffness and strength of the soil. This is an important consideration for the stability of geotechnical structures in unsaturated Conditions.
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