627
Technical Committee 102 /
Comité technique 102
The SHANSEP concept derives from the empirical observation
that the ratio of the undrained shear strength, s
u
, to the effective
confining stress,
σ
'
v
, is approximately constant for a given Over
Consolidation Ratio (OCR) and varies linearly with OCR
Λ
:
′
= ∗
(2)
where S is the proportionality constant (also referred to as the
c/p ratio) and
Λ
is the memory exponent. These values were es-
timated from the CAU triaxial testing carried out on undisturbed
samples.
The S (or c/p-ratio) value was determined based on CAU
tests loaded anisotropically to >150% of the assumed precon-
solidation stress (as determined from the Batch I IL oedometer
tests) and then sheared. The S-value thus determined was used
for the determination of the
Λ
value for tests loaded anisotropi-
cally to below the assumed preconsolidation stress. Due to rela-
tively high uncertainty with regards to the determination of the
preconsolidation pressure, the memory exponent was found dif-
ficult to determine with accuracy.
For the clay deposits found along the alignment of the im-
mersed tunnel average S and
Λ
values shown in Table 1 were
found.
Table 1. Average values of S and
Λ
for clay deposits found along the
immersed tunnel alignment.
Soil deposit
Nos. of
tests
S (avg.)
Λ (
avg.)
Marine clay
2
0.31
0.7
Continental clay
2
0.40
NA
Marine alluvial clay
7
0.31
1.0
Marine alluvial clay
with sand laminae
4
0.36
0.7
Notes: NA = Not Applicable
The results of the CAU triaxial tests were also used to provide a
correlation to results of CPTU testing, and thereby for providing
an estimate of the N
kt
cone bearing factor as used in the fol-
lowing equation (e.g. Lunne et al 1997):
=
(3)
where
σ
v0
is the overburden pressure at the cone tip and q
t
is the
cone resistance corrected for pore pressure.
For the clay deposits found along the alignment of the im-
mersed tunnel, the N
kt
values were found to be 17 on average
for the four deposits referenced in Table 1.
8 SETTLEMENT/SPRING STIFFNESS CALCULATION
Based on the geotechnical interpretation of the geology and set-
tlement characteristics of soil deposits, the settlement and spring
stiffness was calculated for each individual CPTU location.
The settlement analysis was carried out using the Janbu
(1963) tangent modulus method, which accounts for the general
non-linear load deformation relationship of soils. The settlement
equations differ between coarse grained (sandy) and fine
grained (clayey and silty) soils, and whether or not the precon-
solidation stress is exceeded.
All in all four different equations were established.
Eq (4) for coarse grained soils below and above the preconsoli-
dation stress:
=
=
+
(4)
and Eq (5) for fine grained soils below and above the precon-
solidation stress:
=
=
+
(5)
Here
ε
is the vertical strain,
∆
σ
'
v
is the increase in effective ver-
tical stress from the tunnel (
σ
'
1
-
σ
'
0
),
σ
'
p
is the preconsolidation
pressure,
σ
'
0
is the in-situ vertical stress prior to loading,
σ
'
1
is
the final vertical effective stress and
σ
'
r
is a reference stress of
100 kPa.
The secondary settlement was calculated from (Terzaghi et
al. 1996):
=
α
(6)
where C
α
is the secondary compression index, and t/t
p
is the ra-
tio between the lifespan of the structure and the time for pri-
mary consolidation (t/t
p
= 100 was conservatively assumed).
When the final load was lower than the preconsolidation
stress, the secondary recompression index, C
α
r
, was used in-
stead of C
α
.
The calculation of settlement was terminated at the top of
rock, and due to the limited penetration of the CPTUs into the
fluvial alluvial deposits of sand and gravel, the settlement calcu-
lations were based on SPT N data between the bottom of the
CPTUs and the top of rock. An empirical q
c
/N correlation de-
pendent on the grain size distribution was used (Kulhawy &
Mayne 1990):
/
= 5.44
.
(7)
where p
a
is a reference stress of 100 kPa, d
50
is the mean grain
size in mm and q
c
is given in kPa.
The spring stiffness was then calculated as:
=
(8)
The settlement/spring stiffness calculations were carried out in
purposefully set up Excel spreadsheets.
The settlement/spring stiffness calculations were carried out
for some 400 Nos. CPTUs, and considering that each CPTU
could contain up to 6,000 measurement points, running the en-
tire series of calculations could take up to 2 hours.
The variation of calculated settlement and spring stiffness
along the immersed tunnel alignment is shown in Figures 8 and
9, respectively.
Figure 8. Calculated settlement along immersed tunnel alignment centre
line and lines at 25 m distance from centreline.
