Upload
glenn-gil-tamayo
View
237
Download
0
Embed Size (px)
Citation preview
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
1/43
By:
Shiela Laconsay
Glenn TamayoAndre Cruz
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
2/43
Geologic, hydrologic, topographic, seismic and materialproperties must be obtained
Accuracy of the analysis is as good as the accuracy of the info
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
3/43
Earthquakes expose slopes to dynamic loads that can reducethe soil shear strength and cause instability
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
4/43
Are there materials in the slope that will lose
significant strength during cyclic loading?(e.g.,soil liquefaction)
Will the structure undergo significantdeformation that may jeopardize satisfactoryperformance?
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
5/43
1. Analysis of Inertial Stability Pseudostatic - FS Newmark Sliding Block Analysis - displacement Makdisi-Seed displacement
Stress-Deformation Analysis
2. Analysis of Weakening Instability
-usually associated with liquefaction Flow Failure Analysis Deformation Flow
FS = resisting moment (constant)
static overturning moment+ Earthquake Force
FS = resisting moment (decreasing)
static overturning moment
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
6/43
Earthquake loading isrepresented by a seismic
horizontal force, Fh
Fh=kW
Seismic coefficient k=a/g
Location of Fh is C.O.G of theslice (Terzhagi)
Recent dynamic analysis showsthat acceleration are amplifiedfrom bottom to top of dams
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
7/43
Terzaghi (1950)
Severe -0.1
Violent -0.25
Catastrophic -0.5
Seed (1979)
K=0.15, FS =>1.15
Hynes-Griffin & Franklin (1984)
Use k=0.5*PGA, FS=> 1.0, 80% residual strengths
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
8/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
9/43
FS = resisting momentstatic overturning moment
FS = resisting momentstatic + pseudo static
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
10/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
11/43
Used to screen for potential seismic stability
problems
Especially for soils that are not expected to
lose a significant amount of there strengthwhen subjected to seismic loading
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
12/43
kh = ( aref /g) * (a/aref)= 0.2 * 0.5= 0.1
k=0.15
Earth Dams
Landfills
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
13/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
14/43
An earth structure that satisfies the FS in thescreening analysis criteria may still displacemore than 1m.
This does not mean it is safe for all levels ofperformance
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
15/43
Pseudostatic method of analysis provides anindex of stability (FoS) but no information ondeformations associated with slope failures
Pseudostatic FoS varies throught anearthquake
Newmark considers behavior of slope whenFos is less than 1.0 i.e. the potential failuremass is not in equilibrium
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
16/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
17/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
18/43
The yield coefficient, ky, is the horizontalpseudostatic coefficient that will produce anFoS of 1.0.
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
19/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
20/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
21/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
22/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
23/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
24/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
25/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
26/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
27/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
28/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
29/43
Accuracy of a sliding block analysis dependson the accuracy of the input motion
Since the potential failure mass is assumed tobe rigid, the ground motion at the level of thefailure surface should be considered
In-phase for slopes with very stiff soilsand/orsubjected to low-frequency motion
Out-of-phase for slopes with softer slopes
and/or slopes subjected to higher frequencymotion.
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
30/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
31/43
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
32/43
Simple yet rational approach based on anevaluation of the dynamic response of theembankment
Assumes that failure occurs on a well-definedslip surface
Assumes that material behaves elastically atstress levels below failure but perfectly plasticbehavior above yield
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
33/43
1. Determine yield acceleration, ky
2. Determine earthquake-induced accelerations
3. If induced acceleration is greater than ky,
movements are assumed to occur along thefailure plane. Magnitude of displacement is
evaluated by double integration procedure
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
34/43
Yield Accelerationaverage acceleration producing a horizontal inertia
force on a potential sliding mass so as to producea factor of safety of unity and thus cause it toexperience permanent displacements.
Inelasticbehavior
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
35/43
Very little permanentdeformation
Substantial permanentstrain
Yield Acceleration
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
36/43
Where:
F(t) = force force
acting along the
boundary of the
sliding mass
Kav = average
acceleration acting
on the sliding mass
at that instant in time
Time History of Earthquake- Induced Average Acceleration
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
37/43
yh
Variation of Maximum AccelerationRatio with the Depth of Sliding Mass
Kmax = maximum average accelerationmax = maximum crest acceleration
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
38/43
Factors Affecting Permanent Deformation dueto Earthquake Loading:1. Amplitude of induced average
accelerations, amplifying characteristics ofthe embankment and location of the
sliding mass within the embankment2. Frequency content of the averageacceleration time history
3. Duration of significant shaking
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
39/43
Summary for several earthquakes and
dams and embankmentsAverage values
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
40/43
135 ft high Chabot Dam during the 1906magnitude 8 San Francisco Earthquake
max = 0.57g
To = 0.99 sec
y/h =1.0
Ky= 0.14 0.14
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
41/43
Kmax /max= 0.35
Kmax = 0.35*0.57g = 0.2g
0.35
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
42/43
U/kmaxg To = 0.013 seconds
Thus, U = 0.013*0.2*32.2*0.99
= 0.08 ft = 2.4 cm
7/28/2019 Seismic Slope Stability (CruzLaconsayTamayo)
43/43
Simple yet rational approach
Approximate and involves simplifying assumptions,
leading to conservative results Design curves need to be continually updated and
refined as analytical results for embankments are
obtained Significant improvement over the pseudo-static
approach but needs to be used with caution andgood judgment with regards to applicability
Where soil conditions cannot be determined with asignificant degree of accuracy, a more rigorousdynamic method would be more satisfactory