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Simplifying Pore Pressure Evaluation for SlopesEvaluation for Slopes
GeoHazards Impacting Transportation in the Appalachian Region 2012 A t 12012 August 1
Tarmarack, Beckley, WV
By y
Richard S. Olsen, PhD, PEActing Branch Chief, Geotechnical and Geosciences Branch
Senior Research Geotechnical EngineerSenior Research Geotechnical Engineer
Corps of Engineers - ERDC - Geotechnical and Structures Laboratory (GSL) Richard.S.Olsen@usace.army.mil`
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First a little real world:Problems of water seepage are best found in the field
A
2Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
Water seepage (which caused this landslide) was discovered bywas discovered by interviewing people and field inspection
3Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
A hillside north of the bridge has experienced near constant lateral movement toward the river
after the roadway was built 30 years ago.
Another example of a water seepage issue which was found in the field
4Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
mountain road in Bolivia A rock/soil landslide occurred 3 years ago that closed the highway
Also look for; streambed changes, changes to vegetation load (and type), broken lined drainage ditches, malfunctioning of underground
t / /d i i di t d f ffyears ago that closed the highway traffic for 7 days.
The cause was only discovered by walking to total length
water/sewer/drainage pipes, diverted surface runoff, cleaning/repairs to infrastructure, etc
walking to total length
5Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
Conventional thinking about pore pressure in slopes
(unrealistic)
Normal water (pore)Normal water (pore) pressure for an inclined water table.
6Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
Generalized Pore Pressures • High pore pressures in the field are caused by geologic
stratigraphy and trapped soil layers• Quick acting pore pressures affect the strengths of sandy soils
but not clays
Sand Pore pressure level
SandFlowf
Cl
of groundwater
Clay
7Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
How high can the water (pore) pressure get ?
Artesian water
pressurepressure
Inclined sand Hillside geometry can concentrate seepage & cause high pore
T b fill d
layers can have extremely high water (pore)
& cause high pore pressure
Tube filled with water
water (pore) pressure
Clay layer
This tube is like a sand layer that extends up the l b t i l d t
Localized landslide can either block the sand
y
slope but is plugged at the end
either block the sand layer or relieve the pore pressure 8Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
Homogenous hillside,seepage occurring throughout the hillside
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Parallel clay layers causing high pore pressures
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Hillside springsseepage through springs decreases pore pressure influence
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Hillside leakage only near the toeHigh unstabilized seepage at the hillside toe
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Imperfections are the likely the main cause ofare the likely the main cause of
concentrated pore pressure problems
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Seepage along or out of a slope is extremely dangerousNon seepagep gFactor of safety
Seepage andSeepage, and especially seepage out of a slope will quickly drop the factor of safetyfactor of safety below 1
Don’t worry, stratigraphy will make the situation worse
Slope = 45o 26o 18o
14o
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Internal consequencesHigh gradient(cause of localized erosion)
Uplift of soil and soil removal
Direction of erosionof erosion
!Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Now 3D
Heavy rain fall High pore pressures causing blow out of the toe
A clay lense will trap pore pressures
All collected rain water exits the slope along the old underground stream bed
16Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
Teton DamQuick, draw a hand drawn flow net in 10 seconds
1) starting at the middle of the field (at right angle)
Building flow nets: Think “Race Car” at a special race track;
with a hole in the grout curtain
3) crossing the finish
2) through the chock point at high speed
) gline at high speed (at right angle)
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Expected “elevation pressure head” along flow line A
Cutoff wall
Elev 320 ft
Elev 250 ft
A
Flow lines
Equal potential lines
(i.e. equal elevation pressure head)Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
No flow N h d lNo head lose
HIGH PORE PRESSURE can cause blow outand reduced strength for frictional soils
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Vertical velocity
Flow can cause particle movementFlow can cause particle movement
20Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
Perched water table(i.e. isolated from uphill water effects)
Real situation
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
isolated water tableisolated water table
Representation
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Computer Seepage boundary limits
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Elevation pressure head(also known as the t t l h d)total pressure head)
pinched
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Elevation pressure head
Clue: sand type is not important with these assumptionsSimplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Leakage,e.g. spring
No leakage
g p g
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
A: late plugedB: uniform layer
C E l i h E: late pinchF: late necking
Piezo response uniform to pinching
C: Early pinch E: late pinch
D: early narrow to wideG: early plugged
Approximatelyequals
F: late necking
The concept of E: late pinchearly to late
i hiC: Early pinch
pinchingC: Early pinch
D: early narrow to wide
A: late plugged
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B: uniform layerSimplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
These measured elevation pressure heads do not match the assumption (dashed line) ?assumption (dashed line) ?
28Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL / 35
Head pressure (also known as elevation head)
Lets relook at this simple model
Total pressure head (also known as elevation head)
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Water elevation pressure head (uniform layer)
plugged
clayy
claysand
Fl i i h d lFlowing water in the sandy layer
Seepage flow nets are important, but;
a) if a “sandy layer” can be assumed “uniform” than we don’t have to construct seepage flow nets, and
b) the sandy material type does not matter (i.e. clean sand to sandy mixture), as long as the boundary soil is clay, flow tubes are uniform, and water is flowing
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Water elevation
Water elevation pressure headd i hi Water elevation
pressure head(uniform layer)
due to pinching of a sand layer
Pinching of sand layer
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Water elevation pressure headsMid length pinch
Uniform layer
Late pinchLate pinch
Early pinch
Early pinch
That’s it !Mid length pinch
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Late pinch
i.e. mid length pinch
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
CPT piezo probes to investigate hydrostatic pore pressure
The worst location for a piezoelement is the ASTM/EU standard
conditions
Critical thin sand layers
Seal
Tip
Face
Between cone andsleeve
CCritical thin sand layersPorous Filter
Tip StandardCone
resistance (qc)
fs et)
qpm
Clayfs D
epth (fee
qc
Thin sand layer e.g. 3 inchesqc
pmfs
Clay
e.g. 3 inchesqc
qc
pm
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Clay ?
Calculatedelevation
pore pressurehead
ion (feet)
Clean sand
head
Elevat Sand mixture
CPT stabilized
Dense
stabilizedmeasure porepressure
silty sand
Elevation pressure head distribution for a Uniform layer assumption
Log time (seconds)1 second 10 second 100 second
Uniform layer assumption
Simplifying pore pressure evaluation for slopes, GeoHazards impacting Trans., WV DOT , 2012 Aug 1 by Richard Olsen, USACE-GSL
Pore Pressure is always complex
but it can be simplifiedbut it can be simplified
Thank You ERDCRick Olsen
Early pinch
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Late pinch
Mid length pinch
Late pinch