Estimation of Undrained Shear Strength (S Failure of Embankment over Peat Serious slip failure of...
Transcript of Estimation of Undrained Shear Strength (S Failure of Embankment over Peat Serious slip failure of...
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.
- 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