Soils Test 3 Flashcards
(119 cards)
1
Q
Shear Strength
A
The internal resistance of a soil per unit area that the soil mass can offer to resist failure and sliding along any plane inside it
2
Q
What does shear strength depend on?
A
Shear stregnth is a function of:
- Cohesion between the soil particles
- Frictional resistance between solid particles
- Moisture content and pore water pressure in the soil mass
3
Q
What is related to shear strength?
A
- Bearing capacity of shallow foundations
- Stability of natural or man made slopes
- Estimate lateral earth pressures for retaining structures
- Load-carrying capacity of piles and drilled shafts
4
Q
What is the effective stress cohesion c’ for sand and inorganic silt?
A
c’=0
5
Q
What is c’ for normally consolidated clays?
A
Approximately 0
6
Q
What is c’ for non cemented, over consolidated clays with pre consolidation pressure less than 1000 kN/m^2
A
In the range of 5 to 15 kN/m^2
7
Q
What is the drained angle of friction (effective angle of friction) range for normally consolidated clay?
A
20 to 30 degrees
8
Q
What happens to the effective angle of friction for over consolidated clays?
A
The magnitude decreases.
9
Q
When does soil fail?
A
When the Mohr’s circle is tangent to the shear failure plane.
10
Q
What happens in a stress-controlled shear test?
A
The shear force is applied in equal increments until the specimen fails
11
Q
What happens in a strain controlled shear test?
A
A constant rate of shear displacement is applied to one half of the box by a motor that acts through gears.
12
Q
What is an advantage of stress-controlled shear test?
A
It simulates real field conditions better than strain-controlled tests
13
Q
What is the advantage od a strain controlled shear test?
A
For DENSE SAND, peak shear resistance (failure) as well as lesser shear shear resistance (i.e a point after failure called ultimate strength) can be observed and plotted
14
Q
What happens to loose sand in direct shear test?
A
The resisting shear stress increases with shear displacement up to a maximum Tf.
15
Q
What happens to dense sand in a direct shear test?
A
The shear stress increases up to the peak shear strength, then decreases to the ultimate shear strength.
16
Q
What is the Critical void ratio?
A
The displacement where the void ratio is the same for both dense and loose sand.
17
Q
What kind of soil is a drained test made on?
A
A saturated soil.
18
Q
What is the loading rate on a drained test?
A
Kept slow enough to completely dissipate the excess pore water pressure
19
Q
That kind of loading rates would be used on a sand during a drained test?
A
Ordinary loading rates due the sand’s high hydraulic conductivity
20
Q
That kind of loading rates would be used on a clay during a drained test?
A
Very slow loading rates due to clay’s low hydraulic conductivity
21
Q
What is one thing that can be determined from a drained test?
A
The residual strength of the clay, which is similar to the ultimate shear strength in sand
22
Q
What is the soil not allowed to do during a direct test?
A
Fail on its weakest plane
23
Q
Where must soil fail in a direct test?
A
along the split of the shear box
24
Q
What is one advantage of a direct test?
A
It is the simplest and cheapest test for dry or saturated sandy soil
25
What can the direct shear test be also used for?
Used to determine the interface friction which then can be used to determined the angle of friction between a soil and a foundation material.
26
How is a Triaxial Shear Test set up.
Specimen is enclosed in thin rubber membrane. This is placed in a chamber confined by compressing the fluid in the chamber. Axial stress is applied through a vertical ram.
27
What are some advantages of a triaxial test over a direct test?
- Provides stress-strain behaviors that direct test does not.
- Provides more uniform stress conditions
- Provides more flexibility in terms of loading path.
28
What are the three types of axial shear test?
- Consolidated-drained test or drained test (CD)
- Consolidated-undrained test (CU test)
- Unconsolidated-undrained test or undrained test (UU test)
29
What is skemptons pore water pressure parameter?
Relates pore water pressure to confining pressure. B=u/s3
30
For Saturated Soft Soils, what is B?
B =1 for saturated soft soils
31
What is B that indicates that soils are saturated?
B>0.95 indicates that soils are saturated.
32
If B is less then 0.95, what is needed?
Back pressure to make the soil saturated.
33
In a consolidated drained triaxial test, what happens to (u) over time?
u will fall to zero.
34
For a saturated soil in a CD Triaxial Test, how can the change in volume during consolidation be determined?
The change in volume can be determined from the volume of water drained.
35
What is deviator stress in a CD triaxial test?
The deviator stress is the difference between major and minor principle stress. which equals the axial load applied divided by the cross sectional area.
36
How is deviator stress changed during a CD triaxial test?
It is increased very slowly with the drainage connection open to allow the dissipation of pore water pressure.
37
What is S3 equivalent to in a CD triaxial test?
S3=S3'
38
For overconsolidation in a CD test, the chamber pressure is reduced from ____ to ____.
S3 to Sc
39
In a CD triaxial test, what is associated with over consolidated segments?
The cohesion is greater then zero.
40
What is one reason that CD test are not commonly used?
A CD test on a clayey soil will take several days to complete.
41
What is one common step b/w a CD and CU test?
The drainage line is open when applying confining pressure
42
When is the drainage line closed during a CU test?
After Uc=0
43
What will occur during a CU test that is different from a CD test?
The pore water pressure will increase as the load is applied.
44
How is skemptons pore pressure parameter expressed in a CU test?
A=(change in pore pressure)/(change in deviator stress)
45
In a CU test of loose sand and normally consolidated clay, what behavior does pore water pressure typically exhibit.
The pore water pressure increases with strain.
46
In a CU test of dense sand and overconsolidated clay, what behavior does pore water pressure typically exhibit.
The pore water pressure increases with strain up to a certain limit, beyond which it decreases and becomes negative.
47
During a CU test, what type of pattern typically indicates a soil dilates?
The pore water pressure increases with strain up to a certain limit, beyond which it decreases and becomes negative.
48
What is skemptons pore pressure parameter for normally consolidated clays in a CU test.
In normally consolidated clays, A is between 0.5 and 1
49
What is skemptons pore pressure parameter for normally overconsolidated clays in a CU test.
In overconsolidated clays, A is between -0.5 and 0.
50
What is an advantage of a UU triaxial test?
It is the fastest triaxial test.
51
What is an important difference between a UU test and CD/CU test.
The drainage line is closed when applying the confining pressure.
52
For the second step in a UU test, what happens with drainage lines?
They remain closed.
53
What types of soils are UU test usually used on?
A UU test is usually used on cohesive clays.
54
What does a UU test depend upon?
The added axial stress at failure being mostly independent of confining pressure.
55
What occurs at a condition where added axial stress at failure is independent of confining pressure?
Phi=0
56
What is (Cu) in a UU test?
Cu is the undrained shear strength.
57
What is Cu equivalent to?
The radius of the Moors circle?
58
Only on which types of soils is phi=0?
Phi=0 only on saturated clays and silts.
59
What type of test is a unconfined compression test?
An unconfined compression test is a type of unconsolidated-undrained test.
60
What is unique about a UC test?
The confining pressure is 0.
61
If you were too arrange in the amount of time each triaxial test takes, which order would they be in?
CD>CU>UU
62
What is slope stability analysis?
A process in which engineers determine the safety of natural slopes, excavations, and compacted embankments.
63
What does slope stability analysis involve?
Slope stability analysis involves determining and comparing the shear stress developed along the most likely rupture surface with the shear strength of the soil.
64
What are several modes of slope stability analysis?
Fall, topple, slide, spread, flow.
65
Fall
Detachement of material from a steep slope w/ little or no shear displacement
66
How does material move during a fall?
Descends by falling or rolling
67
What controls whether a fall?
Controlled by geology, weathering, erosion.
68
Topple
Forward rotation about a point or axis,
69
What controls a topple?
Controlled by geology, weathering, and erosion.
70
Slide
Downslope movement occurring primarily on surfaces of rupture that experience extreme shear strain.
71
What are two types of slides?
1. ) Translational slides
| 2. ) Rotational Slides
72
Spread
Extension of cohesive soil or rock masses combined with general subsidence into softer underlying material.
73
In a spread, where is the surface of intense shearing?
There is no surface of intense shearing in a spread
74
What is a spread a result of?
Results from liquefaction or flow of soft material.
75
What is a spread similar too?
A spread is similar to translation slides except that the block separates and move apart as they also move outward
76
Flow
Spatially continuous movement of material
77
How does material behave during a flow?
It behaves as a viscous fluid
78
What is a flow commonly a result of?
A flow is commonly a result of strength loss due to rapid pore pressure increase (e.g intense rainfall) and shallow sliding in the source area
79
What is Tf in the FS equation?
The average shear strength
80
What is Td in the FS equation?
Td is the average shear stress developed along the potential failure surface.
81
What FS is considered acceptable for stable slope design?
A FS of 1.5 is considered acceptable for stable slope design.
82
What is the FS wrt to cohesion the same as?
The FS wrt cohesion is the same as the FS wrt friction. They are the same as the FS wrt strength.
83
For an infinite slope, what is FS independent of?
The FS for an infinite slope is independent of the height H.
84
What will occur if a soil has both cohesion and friction?
It will be stable up to a critical depth.
85
What is c' for granular soils?
For granular soils, c'=0.
86
What is Gamma prime?
Gamma prime is the effective unit weight of the soil. It is Y'=Ysat-Yw
87
What happens when Her approaches the slope height?
Hcr must be treated as finite instead of infinite
88
What is a good approximation for nearly vertical slopes?
A plane surface is a good approximation for nearly vertical slopes.
89
What does Culmann's analysis assume?
Failure occurs along a plane where the average shear stress exceeds the shear strength of the soil.
90
What are three subclassifications of circular failure surfaces?
1. ) Slope Failure
2. ) Shallow Slope Failure
3. ) Base Failure
91
What is slope failure?
When the failure surface passes through or above the slope toe
92
Toe Circle
Slope failure in which the failure circle passes through the toe of the slope.
93
Slope Circle
Slope failure in which the failure circle passes above the toe.
94
Shallow Slope failure
Signifigantly above the toe of the slope. Usually controlled by geological weak zones.
95
Base Failure
Failure surface passes below the slope toe. The failure circle is called a midpoint circle.
96
What are two types of finite analysis methods for failures with a circular failure surface.
1. ) Mass procedure
| 2. ) Method of slices
97
Mass procedure method of circular failure analysis
The mass of the soil above the failure surface is treated as a whole. The soil is assumed to be homogenous.
98
Method of Slices
The soil above the failure surface is divided into a number of vertical slices, and the stability of each is determined separately.
99
What does the method of slices allow for?
It accounts for variation in the soil and pore water pressure (varying soil density, effective stress, pore pressure, and complex geometry)
100
B>53 undrained soil.
The critical circle is always a toe circle.
101
B<53
The critical circle may be a toe circle, slope circle, or midpoint circle depending on the location of the firm base below the slope
102
Why is method of slices superior to mass balance?
Takes into account material and geometric variability.
103
What does the Ordinary Method of slices assume?
The resultants of interstice forces on the sides of each slice cancel out
104
What is one characteristic of OMS?
Very conservative. Underestimates FS up to 60%.
105
Bishop's Simplified Method of Slices
Assumes the interstice shear forces are neglected.
106
What are three major categories of lateral earth pressures?
1. ) At rest pressure
2. ) Active Pressure
3. ) Passive Pressure
107
What occurs when a soil is in plastic equilibrium?
Every point is on the verge of failure
108
Rankin Active state
Imagine wall is pulled away from soil mass. The effective stress will decrease. Once Mohr's circle touches failure envelope, it is in Rankins active state.
109
Rankins passive state
Imagine wall is pushed into the soil mass. The effective stress will increase. Once Mohr's circle touches failure envelope, it is in Rankins passive state. Retaining wall rotates away from the soil.
110
Elastic Equilibrium
The horizontal strain is zero
111
Rankin Active Earth Pressure
The major principle stress associated with active state
112
Rankin Passive Earth Pressure
The major principle stress associated with passive state. Retaining wall rotates into the soil.
113
Where will tensile cracks form up to a depth of?
Active stress = 0
114
Below what depth will effect the total active force on a retaining wall?
Below Z0
115
What will not form in a passive case?
Tensile cracks
116
In method of slices, the width has to be the same (T or F)
False
117
When is figure 5.13 valid?
Saturated clay, undrained conditions.
118
What is a phi=0 condition?
Applied to saturated clays and silts, typically associated with unconsolidated undrained testing
119
What is the basis for slope stability modeling?
Mohr Coulomb criteria
