Estimation of Undrained Shear Strength (Su)

for Peat Using CPT

H. Hayashi and T. Hayashi

Civil Engineering Research Institute for Cold Region (CERI), JAPAN

CONTENTS

1. Peat in Hokkaido, Japan

2. Undrained Shear Strength (Su) for Peat

- Lab. (K0 consolidated triaxial compression) vs. qt (CPT)

- Experimental correlation for estimation of Su using CPT

Asajino

Kitashima

Shinotsu

Riyamunai

Peat in Hokkaido, Japan

Peat is distributed widely in Hokkaido, Japan

- Highly organic and very fibrous

- Approximately 2,000 km2 (6% of land area)

Slip Failure of Embankment over Peat

Serious slip failure of embankments constructed over peat,

due to low undrained shear strength of peat

“How should we determine

the soil parameters for PEAT ?”

Introduction

- Eng. properties of peat are particular and heterogeneous

- Experimental correlation for mineral soils is NOT useful

- When the factor of safety is analyzed, the undrained shear

strength (Su) is an important parameter

Test Methods

Lab. test

- K0 consolidated-undraied triaxial compression on undisturbed samples

- Thin-wall sampler with a fixed piston

Test Methods

- Electric cone penetration test (CPT)

- qt = qce + (1 - Ae/Ap) u

qce: the measured cone resistance, Ae: the effective cross-sectional area of the cone, Ap: the area of the base of the cone and u: the pore water pressure

In-situ test

Engineering Properties of Peat Soils Tested

- Wn for peat tested : 323 – 459%

- Wn for fibrous peat tested : 724 – 945%

Natural water

contentIgnition loss

Degree of

decomposition

In-situ void

ratio

Conmpression

index

W n (%) L i (%) von Post e 0 C c

Shinotsu Peat 3 323～459 37～66 H5 7.9 5.1

Asajino Fibrous peat 1 860 92 H2 14.8 9.3

Riyamunai Fibrous peat 3 724～945 82～94 H3 13.5 10.0

Kitashima Peat 1 387 55 － － －

Number of

samples for K0-

consolidated

triaxial

compression test

Site Soil type

CPT Profile at the Shinotsu Site

- qt of peat was very low, with values of about 0.5 MN/m2 or smaller

0

5

10

15

20

25

0 20 40 60 80

Friction fs (kN/m2)

0

5

10

15

20

25

0 2 4 6 8

Dep

th (

m)

Cone resistance qt (MN/m2)

Surface

soil

Peat

Sandy soil

Clay

: sampling depth for lab testing

0

5

10

15

20

25

0 100 200 300 400 500 600

Pore pressure u

(kN/m2)

Relationship (qt–sV0) vs. Su from the Lab. Test

- qt in this figure is an average value among sampling depth

- Su in peat was only between 5 - 20 kN/m2

- The relation can be approximated by “ Su = (qt - sv0) /21“

0

5

10

15

20

25

0 100 200 300 400 500

Un

dra

ined

sh

ear

stre

ngth

Su

(kN

/m2)

qt-sv0 (kN/m2)

Su = (qt-sv0)/21

- Sliding failure happened during embankment construction

- Back analyzed Su roughly corresponds with Su calculated by Nk = 21

0

1

2

3

4

0 10 20 30 40 50

Dep

th (

m)

Undrained shear strength Su

(kN/m2)

(qt-sv0)/21

lab test

Back analyzed Su

( = 6 kN/m2)

Depth Distribution of Su at the Asajino Site

Conclusions

1) The Su value for peat ground increases linearly with the

increase in cone resistance (qt) of CPT.

2) The relationship between qt and Su in peat ground can be

approximated by “Su = (qt - sv0)/21”. It is possible to

estimate of Su value for peat ground using this relationship.

Thank you !

- High compressibility and Low strength

- Damage to infrastructures on peat

Engineering Properties of Peat in Hokkaido

Property Value

water content (%) 115 - 1570

ignition loss (%) 20 - 98

wet unit weight (kN/m3) 7.1 - 17.7

natural void ratio 2.0 - 21.0

compression index 2.6 - 11.0

undrained shear strength (kN/m2) 5 - 40

coefficient of permeability (cm/s) 10-4

- 10-5

Different Types of Peat Soil in Hokkaido, Japan