671017.pdf

0

1

2

3

4

5

6

7

20 30 40 50 60 70 80 90

D egree of saturation (%)

Perm eabi

lit

y coe ci

ent

(E

−

5m /

s)

(a) Permeability coefficient versus degree of saturation

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30

Vol

um et

ric

moi

sture

cont

ent

(%)

Suction (kPa)

Curve adopted in

the analysis

Depth=2. 8 m

Depth=1. 1 m

Depth=2. 0 m

Depth=1. 5 m Depth=0. 5 m

(b) Water retention curve (solid lines denote field measurements)

Figure 4: Hydraulic properties of the CDG fill soil.

soils, the bulk modulus,퐾, of the soil skeleton is defined as
a function of the mean effective stress,푝耠, according to

퐾=

휕푝耠

휕휀V푒

=

1+푒

휅

푝耠=

]

휅

푝耠, (3)

where휀푒Vdenotes the elastic volumetric strain,푒and]are the
void ratio and the specific volume, respectively, and휅is the
slope of the recompression-unloading line on the]−ln푝耠
diagram.ThePoissonratio,휇,isassumedtobeaconstant,and
the shear modulus,퐺,iscalculatedby

퐺=

3(1 − 2휇)]푝耠

2(1+휇)휅

. (4)

A smooth flow potential function proposed by Men ́etrey and
Willam [ 15 ] is adopted in the model. It has a hyperbolic shape
in the meridional stress plane and a piecewise elliptic shape in
the deviatoric stress plane. Generally plastic flows in the
meridional and deviatoric planes are nonassociated, and
dilatancy can be controlled by the magnitude of the dilation
angle. A perfect plastic hardening law is applied in the follow-
ing analyses.
Ta b l e 1summarizes the parameters adopted in the analy-
ses. The stiffness and strength parameters are obtained from
relevant experiments [ 12 ]. A small dilation angle value of휓=
5 ∘is taken to limit shear-induced volumetric expansion in the
loose fill.
Regarding the hydraulic behavior of the unsaturated soil,
Figure 4presents the permeability function and the water
retentioncurvefortheloosefillsoil,whichareobtainedbased
on the observations from laboratory tests and field measure-
ments. The initial compaction degree of the loose fill was
∼75% of the maximum dry density measured in a standard
Proctor test, and the initial moisture content was 14.9%.
For the in situ ground and the no-fines concrete layer
underneath the fill slope, the field test data showed that their
deformationissmallenoughthattheyaremodeledbyalinear

elastic model with the model parameters given inTa b l e 1.A

large coefficient of permeability,푘=10−4m/s, is taken for the

no-fines concrete layer to represent its nearly free-draining

property.

3.4. Modeling of Soil Nails.As described above, each soil nail

is idealized as an elastic homogeneous bar in the finite ele-

ment model considering the low possibility of steel yielding.

By assuming the compatibility of axial deformation between

the grout and the steel rod along the nailing direction, the

equivalent Young’s modulus (퐸̃) of the nail elements is deter-

mined as follow:

퐸̃=

퐸푟퐴푟+퐸푔퐴푔

퐴푟+퐴푔

, (5)

where퐸푟and퐸푔denotetheelasticmodulusofthesteelrod

and the grout, respectively, and퐴푟and퐴푔are their cross-

sectional area, respectively.

It has been demonstrated that a modeling approach that is

capable of accounting for possible bond and slippage between

the soil nail and the surrounding soil is more suitable for the

analysis of nail reinforcement effect and the global behavior

ofthenailedslope[ 13 ]. Hence an interface element technique

has been adopted in this study. Three-dimensional eight-

node interface elements are used to simulate the steel-grout

interfacial behavior. As in many previous analyses by other

researchers (such as [ 16 ]), the elastic stiffness parameters,푘푛

and푘푠, for the grout-soil interface are defined as퐸푠/푡and퐺/푡

respectively, where푡isthethicknessofinterfaceelements;퐸푠

and퐺are the Young and shear moduli of the surrounding soil

material, respectively. Herein푡is chosen to be 2 mm, that is

about 2 percent of the nail diameter, and it can be considered

to be negligibly small with respect to the nail size.

The Mohr-Coulomb shear model is taken as the failure

criterion along the nail-soil interface. Tangential slippage will