et hanrahan memorial symposium shear wave … wave velocity determination of peat et hanrahan...

Shear Wave Velocity Determination of Peat

ET Hanrahan Memorial SymposiumShear Wave Velocity Estimates of Peat

Introduction

Research Masters – Geophysical Determination of Peat Strengthp y g

Aim of Research:Determine if a robust relationship exists between peat strength and seismic parametersDevelop a methodology for measuring peat strength in situDevelop a methodology for measuring peat strength in‐situ

Schedule (part time):Literature reviewI i i l i bli h li i i f iInitial testing – establish limitations of equipmentField testing – determine Vs values in‐situ

collect Von Post datacollect water content samplescollect samples for DSS testing

Laboratory testing – assess water content for correlation with Vs datasetassess Su for correlation with Vs dataset

Write upp


Previous Work

MINERAL SOILS PEATMINERAL SOILS

Relationship between Su of DBC and Vs from MASW tests

PEAT

Relationship between Su derived from Shear Vane Tests and Vs

Dail EireannDPT CAUC or CIUCDPT - UUGrand Canal SqMater Hospital

1000

1200

Mater HospitalKerry siteGrangegormanCentral Dublin SiteDuleekNavanWaterfordMetro NorthD bli M i S

AMARYAN 1993

200

400

600

800Dublin - Merrion Sq.

su = 0.001 V2s + 0.016 Vs + 60.8 (kPa)

R2 = 0.87

0 200 400 600 800 1000 1200Vs (m/s)

0

200

“studies are incomplete because of the scarce technology and poor techniques.....a satisfactory solution has not yet been achieved”LONG 1993


Seismic WavesP‐waveF t t C i lFastest ;Compressional

Particle motion parallel

to direction of propagation

S‐waveShearing and rotation

Particle motion perpendicular

to direction of propagation

Shear Modulus (G) G = ρ.Vs2

Surface wavesRaleigh or Love type

Largest amplitude

Propagate along surface

Elliptical particle motions

ET Hanrahan Memorial Symposium

Initial Field TrialsUpland peat site. County TyroneGPR carried out for peat thicknesshighly variable thicknessAccess by 8WD vehicle

1) Direct Shear Wave Transmission)2) Multi channel Analysis of Surface Waves (MASW)3) Downhole Shear Wave Transmission


Downhole Shear Wave


Time (ms)

Downhole Shear Wave

‐50

0

0 20 40 60 80 100 120 140 160 180 200

200

( )

100

50

100

200

300

400

500 m)

150

200

250

600

700Δd (m

m)

iver dep

th (m

m

250

300

350

800

900

Rece

400

450

900

Vs = 23m/s


Direct Shear Wave Propagation

Direct shear wave

Vs = 24.7m/sthrough peat


Multichannel Analysis of Surface Waves ‐MASW

Too shallow to resolve

Clay below Peat

Vs = 21‐24m/s

Clay below Peat


Conclusions

METHOD MEASURED Vs NOTES

Vs Direct 24.7 Vs will trend towards upper limit thin layers will not be resolved

MASW 21 – 24 software derived profile (accuracies for peat not fully understood)upper layers not resolved due to frequency limitations

Vs Downhole 23 (below 0.5m) lower values recorded at near surface need to be investigated furtherfurther.correlation to peat sampling required to assess relative variationscorrelation to DSS tests required to assess relationship to strengthlimit of method with respect to thin layers needs to be assessed (can we resolve thin layers of weak peat?)

The measurement of Vs in the field is practical.The correlation between the different techniques is good (low scatter)With the correct geometry relatively thin layer should be resolvable (Downhole)


The Next Year And A Half

DEVELOP EQUIPMENT (downhole technique)DEVELOP EQUIPMENT (downhole technique)repeatabilitytime referencingestablish limitations and precisionfrequency dependencyfrequency dependency

FURTHER FIELD TRIALSupland blanket bogproduction bogproduction bograised bog

RELATE FIELD DATA TO LAB TEST RESULTS – DSS and water content

ESTABLISH ENGINEERING RELATIONSHIPSVs and UNDRAINED SHEAR STRENGTHVs, Water Content and UNDRAINED SHEAR STRENGTH

ANISOTROPY – Vsvh, Vshv and Vshh


Culicoides impunctatus

The Main Obstacle to Overcome

‘the biting midge’the biting midge


et hanrahan memorial symposium shear wave … wave velocity determination of peat et hanrahan...

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