simplifying pore pressure evaluation for slopesevaluation for ......mid length pinch uniform layer...

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) [email protected]`

1

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


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


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


Conventional thinking about pore pressure in slopes

(unrealistic)

Normal water (pore)Normal water (pore) pressure for an inclined water table.


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


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


Hillside springsseepage through springs decreases pore pressure influence


Hillside leakage only near the toeHigh unstabilized seepage at the hillside toe


Imperfections are the likely the main cause ofare the likely the main cause of

concentrated pore pressure problems


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


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


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)


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


Vertical velocity

Flow can cause particle movementFlow can cause particle movement


Perched water table(i.e. isolated from uphill water effects)

Real situation


isolated water tableisolated water table

Representation


Computer Seepage boundary limits


Elevation pressure head(also known as the t t l h d)total pressure head)

pinched


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


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

27

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) ?


Head pressure (also known as elevation head)

Lets relook at this simple model

Total pressure head (also known as elevation head)


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


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


Water elevation pressure headsMid length pinch

Uniform layer

Late pinchLate pinch

Early pinch

Early pinch

That’s it !Mid length pinch

32

Late pinch

i.e. mid length pinch


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


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


Pore Pressure is always complex

but it can be simplifiedbut it can be simplified

Thank You ERDCRick Olsen

Early pinch

35

Late pinch

Mid length pinch

Late pinch

simplifying pore pressure evaluation for slopesevaluation for ......mid length pinch uniform layer...

Documents