1242
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2 MULTI-STAGE EMBANKMENT STABILITY
At Limerick Tunnel the designers adopted the undrained
strength analysis approach as developed by Ladd (1991). This
employs a normalised undrained strength ratio c
u
/ p
0
’ to predict
the operational shear strengths that would apply at some future
time after initial loading based on the estimated (or measured)
partial consolidation and pore pressure conditions of the layer of
soil in question. Stability at any stage of construction was
evaluated by limit equilibrium methods using Bishops Modified
Method for circular and Janbu’s Method for block shaped
failure planes, the most critical of either being adopted in
design. A minimum operating Factor of Safety of 1.25 was
adopted for short term loading conditions assuming that the
embankment was fully instrumented. In the long term fully
drained condition a Factor of Safety of 1.3 was selected.
A cautious design value of C
vr
= 1 m
2
/year was adopted
for radial drainage and the contribution of vertical drainage was
ignored. Vertical drains consisting of Mebradrain MD7007 were
typically installed at 1.3 m c/c triangular spacing but in one
200m long high fill area drains were installed at 1.0 m c/c
spacings. If the total filling duration to achieve maximum height
was deemed excessive, typically in excess of 6 to 9 months
depending on the Contractor’s programme, then the use of basal
geosynthetic reinforcement was considered to increase the
temporary stability and thereby reduce the total time required
for initial filling to full height. Basal reinforcement was
required for approximately 1.7 km of the 6 km total
embankment length requiring PVD and surcharge, typically
where the total temporary embankment height (including
surcharge fill) exceeded 6m.
Multi-stage embankment construction designs were
summarized in tabular format for each design profile and the
earthwork drawings also reflected the reinforcement and stage
hold durations. Earthworks construction was controlled in the
field by careful review of instrumentation data by the designer’s
site staff and the filling schedules and hold periods were altered
to reflect the true soil behavior as monitored by field
instrumentation.
Jardine (2002) gives an excellent summary of the behaviour
of multi-stage embankments constructed on soft foundation
soils. Based on a number of fully instrumented and well
documented case histories he notes the following key principles
of their behaviour which can be of use in monitoring
performance and assessing stability:
Large ground movements due to volume changes can occur
as instability is approached;
Instability is primarily related to lateral spreading of the
foundation and this can be monitored by assessing
deformation ratios of lateral movement at the toe
Y to
maximum settlement at the crest
S (see Figure 1). The
limit criteria for such ratios will be different for single stage
compared to multi-stage embankments and indeed will vary
with each site due to soil material properties, embankment
geometry, soil profile and loading rate;
Similarly as instability is approached the ratio of pore
pressure change in the foundation soils to increased total
loading approaches and exceeds unity; and
An observational approach is only valid if adequate
instrumentation and a degree of redundancy due to loss is
provided. The time necessary to acquire, process, evaluate
and provide a control response must also be sufficiently
short to avert a failure.
Deformation ratio limits reported in CIRIA C185 (1999)
typically range from 0.3 to 0.4 for embankments on soft ground.
Data from Japan published by Wakita & Matsuo (1994) has
suggested that the deformation ratio for a given degree of
stability reduces as total settlement increases, failures being
expected for deformation ratios in excess of 0.4 for observed
settlements greater than 2m.
Figure 1 Definition of Embankment Deformation Ratio
(
Y/
S) (Jardine, 2002)
Constitutive models used for soft alluvium included standard
isotropic soft clay model in the PLAXIS suite plus anisotropic
models S-CLAY1 and ACM which was performed by
University of Strathclyde. Further details of the anisotropic
model parameters and results are given by Kamrat-
Pietraszewska et al. (2008). The FEM results suggested that the
maximum deformation ratio to be expected for the proposed
stage loading schedules at adequate Factors of Safety might
range up to 0.6. The following threshold limits for monitoring
data were adopted as indicative of developing failure based on
an average filling rate of 0.5 m/week with an absolute
prohibition on any single incremental fill rate exceeding
1m/week:
Incremental pore pressure ratios
u/
v
> 1.0 ;
Global Deformation Ratios (
Y/
S) > 0.5;
Deformation Ratios > 0.3 represented warning conditions
where fill rates and performance data had to be more closely
monitored; and
Incremental change in settlement or toe movement > 0.1m
between consecutive readings.
3 INSTRUMENTATION MONITORING
A total of 13 fully instrumented cross sections were selected at
representative locations and near structures where temporary fill
heights were greatest. A standard instrumentation cross section
included a pair of settlement plates 5m inset from the
embankment crest, survey monuments 1m offset from each toe,
VW piezometers arranged at the centre point of the triangular
PVD layout under the embankment centreline typically at 3m
depth increments plus a single piezometer under both mid
slopes at 2 metres depth. Inclinometers extended to stiff glacial
till soils or bedrock were installed at the embankment toes.
Settlement plates and toe survey monuments were generally
arranged in pairs at 50 m c/c spacing along the mainline. Active
areas of filling with settlement rates > 20 mm / week required
twice weekly monitoring but daily monitoring was triggered
when monitoring threshold limit values were exceeded.
4 EMBANKMENT PERFORMANCE
4.1
Deformation Ratio & Stability
A typical filling rate and deformation ratio history for the
instrumented location at Ch 4+185 m is shown on Figure 2.
During initial filling to heights of 4 m the deformation ratio
rapidly rose to local maximum values of 0.4. The ratio then
reduced to below 0.2 as settlement continues under constant
load before increasing again during the next filling stage but
