Preliminary observations from a geosynthetic reinforced piled
embankment
King, D. J., Bouazza, A. & Bui, H.H.
Monash University, Civil Engineering, Melbourne, Australia
Gniel, J
Golder Associates Pty Ltd, Melbourne, Australia
Rowe, R.K.
Queen’s University, Department of Civil Engineering, Kingston, Canada
ABSTRACT: A recent rail infrastructure project in Melbourne, Australia has seen the construction of a
number of geosynthetic reinforced (GR) piled embankments to overcome the challenges associated with
the local soft soil present throughout much of inner Melbourne, Coode Island Silt (CIS). To better understand the behaviour and performance of the load transfer platform (LTP), a field case study has been undertaken involving the instrumentation of one of these embankments. This paper presents data gathered
during the construction and post-construction phases (approx. 9 months) as well as discussion on the LTP
behaviour observed thus far. The development of arching, as seen from these preliminary results, is currently increasing with time; it is suggested that this increase is related to a coupled response between the
arching mechanism and the GR/sub-soil deformation. The effects of piling works in close proximity to
the LTP are also discussed and further work as part of this on-going research is described.
Keywords: Load transfer platform, basal reinforcement, arching, geogrid, field study
1 INTRODUCTION
A recent rail infrastructure project in Victoria, Australia, currently the largest public transport infrastructure project in Australia, saw the construction of a new rail link between regional Victoria and the Melbourne CBD. The inner city area of Melbourne, particularly the inner west and south is underlain by soft
soil known as Coode Island Silt (CIS). Within the local geotechnical profession the CIS unit is well
known, with a large amount of literature (both published and un-published) describing the properties and
settlement performance. To meet rail performance criteria and minimise construction time, GR piled embankments supported on drilled displacement piles (DDPs) were proposed. Design commenced in 2012,
construction was completed during 2013.
The instrumented embankment is a widened embankment, approximately 60 m in length. The LTP is
split over 2 levels (upper and lower level) along the length of the embankment to minimise the potential
to induce settlement beneath the existing embankment. This paper presents instrumentation data gathered
during the construction phase, as well as approximately 4 months of post-construction data. Understanding the behaviour of the LTP, in particular the load distribution which occurs due to arching, the behaviour of the GR and subsoil support, is vital in order to ensure the design is both efficient and economical.
The data presented shows the arching mechanism continuing to develop with time, consistent with preliminary behaviour observed in field studies undertaken by others (Van Eekelen et al. 2008, 2010, Haring
et al. 2008 and Van Deijnen et al. 2010). In addition, the effects of piling works (ground improvement for
the upper level LTP) on the arching mechanism of the lower level LTP supporting the partial built
embakment is described.
When describing the LTP and the behaviour of the GR, the sub-soil support has an important role. In
this case it is expected that the presence of approximately 2 m of fill material overlying the CIS will
influence the sub-soil response. Further to this, the stress history of the CIS and the current stress levels
suggest that loss of subsoil support will take a considerable period of time. The available geotechnical
information, historic data and literature available on CIS will be used to support numerical modelling of
sub-soil response as part of this on-going research.
2 PILED EMBANKMENT: LOAD TRANSFER
When constructing embankments over soft soils, piled foundations with a geosynthetic reinforced layer at
the base are commonly used to limit both total and differential embankment settlement. The geosynthetic
reinforced layer, typically referred to as a load transfer platform (LTP), redistributes embankment load to
the piled foundations through a combination of the arching effect, due to the differential stiffness between
the piled foundations and soft sub-soil, tensile forces induced in the geosynthetic material and subsoil
support. A number of methods have been developed to describe this load distribution; an understanding of
the load distribution allows the design engineer to specify the type, size and spacing of the piled elements
and the details of the LTP, specifically the type of geosynthetic reinforcement.
Design standards, EBGEO (2010) and BS8006 (2010) are two examples of formalized design standards which have incorporated various aspects of research undertaken in this field. However, these design
standards, as well as many of the methods in the published literature, produce results which often vary
considerably.
3 FIELD CASE STUDY
3.1 Site Conditions
The subsurface conditions encountered at the site are typical of the Yarra Delta region near the Melbourne CBD and comprises Quaternary aged Yarra Delta sediments overlying the Silurian aged mudstone
and sandstone of the Melbourne formation, the basement rock beneath Melbourne. The subsurface conditions vary significantly along the length of the embankment with the thickness of the Quaternary units
generally increasing towards the eastern end of the embankment, adjacent to a creek. For design purposes
the thickness of the CIS is the main concern and was found to vary between about 15 m at the eastern end
and around 5 m at the western end where embankment widening was completed without ground improvement. Sub-surface conditions beneath instrumented area No. 2 are presented in Table 1. The base of
the lower level LTP is at RL 2.0m. Figure 1 presents the nearest CPT test location, CPT30.
For settlement analysis, the CIS is the dominate unit within the Yarra Delta sequence owing to its
highly compressible nature. The fill will contribute to a much lesser extent depending on its thickness and
Unit
Fill
Depth (m)
(R.L. (m))
0.0
(3.7)
Coode Island Silt
(CIS)
4.3
(-0.6)
Fishermens
Bend Silt
15.1
(-11.4)
Moray St
Gravel/
Werribee
Formation
Siltstone –
Melbourne
Formation
16.7
(-13.0)
Description
Very stiff
Gravelly
CLAY
Soft
Silty
CLAY,
increasing
to
firm
with
depth
Stiff to very
stiff,
Silty
CLAY
and
Sandy CLAY
Dense, Sands,
Gravel, Clay
beds
Not encountered
(-23)
Table 1: Sub-surface conditions beneath instrumented
area No. 2.
Figure 1: CPT30. Depth 0.0 m
strength; in this case in excess of 95% of surface settlement beneath the embankment would be expected
to occur in the CIS. Settlement characteristics of the CIS are well described in the published literature
(Neilson 1992, Ervin 1992 and Srithar 2010). A number of geotechnical investigations were performed
prior to construction. As part of detailed design the CIS thickness was determined by performing closely
spaced CPTs along the length of the widened embankment. Historic information, described in Section
3.2, is also utilised. Construction records from installation of DDP (rig torque, penetration rate, etc.) provide additional information to assess the thickness of the CIS unit.
3.2 Background
The existing embankment which is to be widened is approximately 8 m in height and was constructed
without ground improvement. In the early 1970s an older wooden bridge crossing the adjacent creek was
replaced by extending the existing embankment (without ground improvement) and constructing a reinforced concrete bridge. In the mid 1980s structural distress of the bridge abutments was attributed to settlement of the extended section of embankment. An extensive geotechnical investigation (Golder Associates 1984 - various reports) was performed with a large number of in-situ shear vane tests within the CIS
and in addition, settlement monitoring over a period of 6 years from the mid 1980s onwards was recorded. Further investigation followed in the early 1990s (Consultant report 1990). This monitoring provides a
detailed account of the secondary compression characteristics of the CIS and provides a useful baseline
performance measure for the existing embankment performance.
3.3 Design
The proposed widened embankment design had to meet more stringent rail performance criteria than
the existing embankment, and in addition, the potential to cause settlement beneath the existing embankment had to be limited. A GR piled embankment was therefore seen as the most suitable option to meet
these requirements and to also meet the proposed construction timeline. To minimize the impact on the
existing embankment it was necessary to install DDPs from 2 levels such that the increased load acting on
the CIS and the shoulder of the existing embankment was supported by a LTP. The rigid elements constructed are drilled displacement piles (DDPs), 450 mm diameter, with enlarged cast in-situ pile heads
measuring 1 m by 1m. Typical centre-centre spacing varied between 2.0 m and 2.5 m. The LTP is 650
mm thick, Geosynthetic reinforcement is provided by two layers (transverse and longitudinal) of uni-axial
geogrid (ACEGrid ® PET GG200 geogrid) with an ultimate tensile strength of 219 kN at 10 % strain.
Transverse geogrid was placed 100 mm above the base of the LTP. A further 100 mm separates the overlying longitudinal layer. Additional material properties are presented in Section 3.4
Due to the proximity of the proposed embankment to a nearby creek, it was a requirement to minimize
the embankment footprint within what is an urban flood plain. Embankment batter slopes were therefore
not suitable and the construction of gabion walls was required. The DDPs on the upper level were installed at various levels between RL 4.0 m and RL 4.6 m depending on the geometry of the existing embankment. This occurred following construction of the first 2 rows of gabion wall. The embankment is
shown in Figure 2 following installation of the lower level DDPs. The DDP heads are at RL 2.0 m, the
existing embankment is on the left hand side and the creek can be seen in the distance. An embankment
cross section at the location of instrumented area No. 2 is shown below in Figure 3.
Instrumentation area No. 2
A
A’
Figure 3. Embankment cross section at instrumented area No. 2.
Figure 2. Instrumented embankment following installation of DDPs, facing east.
3.4 Embankment material properties
The LTP comprises a 650 mm thick layer of 75 mm minus “rockfill”. Embankment material comprised
“Type B” fill compacted to 95% standard compaction.
3.5 Construction timeline
Due to a number of site constraints, the embankment construction and ground improvement works were
performed over a number of stages. Ground improvement on the lower level occurred over a period of
about 1 month between March to April 2013 (Stage 1) and a 1 week period in June 2013 (Stage 2). Figure
2 shows the site on April 17 2013 after the completion of Stage 1 ground improvement. Instrumentation
(Area No. 2) was installed the following day. Due to site constraints an area near the creek end of the embankment was delayed by about 2 months, ground improvement works in this area were not completed
until June 2013 (Stage 2). Additional instrumentation (Area No. 1) was installed in this area in early July,
not long after completion of Stage 2.
Due to the delays between Stage 1 and 2, the embankment and gabion wall construction was staggered. The upper level LTP and associated ground improvement was also undertaken in two stages. Stage
3a (adjacent to Stage 1) was completed in early July over a 1 week period. Stage 3b (adjacent to Stage 2)
was completed over a 1 week period in September. Following this the third embankment lift was
completed. Much of the embankment above this level is a batter slope and for this reason construction
progressed rapidly to the finished height of 7.1 m (RL 9.1 m) on 24 October 2013.
3.6
Instrumentation
Instrumentation was installed in 2 areas with the general aim to capture the LTP behaviour beneath the
centre of the embankment in one area and its behaviour near the edge of the embankment, where lateral
forces are expected to act, in the other. The instrumentation installed comprised:
Area No. 1
• comprised 8 pairs
of strain gauges and
2 “dummy” gauges
(18 total)
• 3 earth pressure
cells (EPCs)
• 2 horizontal inclinometers
• Piezometer.
Figure 4: Area No. 2 instrumentation
Area No. 2
• 10 pairs of electrical resistance strain gauges, 2
“dummy” strain gauges (22 total)
• 3 earth pressure cells (EPCs)
• 3 tiltmeters cast-in-situ at
various depths
within a
DDP
• 1 vertical inclinometer adjacent to the instrumented DDP.
Additional piezometers were planned for installation
but could not be installed. This paper focuses on the area
No. 2 instrumentation; the approximate location is indicated in Figure 2. Figure 4 presents the instrumentation layout. Strain gauges were installed in pairs to ensure repeatable results on both longitudinal and transverse geogrids
layers. Gauges installed are Vishay – Micro Measurements® general purpose strain gauges (CEA-06-205UN350) and measure 10.5 mm by 3.1 mm. The use of these
gauges to measure strain in a woven geogrid is described
by Oglesby et al. (1992) and Warren et al. (2010). Gauge
calibration required tensile testing in accordance with
ASTM D 6637 to obtain a relationship between gauge
strain (localized) and geogrid strain (global).
In Area No. 2, three EPCs were installed. EPC1 was
installed 100 mm above the DDP head (beneath the transverse geogrid layer); EPC2 and EPC3 are within the subsoil area. EPC2 is at the base of the LTP and EPC3 is 100
mm below the top of the LTP. Tiltmeters were installed in a DDP at RLs 1.71 m, -0.56 m and -3.0 m to
measure the lateral deformation of the DDP beneath the gabion wall. A vertical inclinometer was installed
1.6 m from the centre of this DDP (700 mm from the edge of LTP) to measure the lateral deformation of
the surrounding soil to provide a comparison with the DDP deformation. Automated data acquisition
commenced on day 96, manual readings were taken prior to this. Day 0 is defined as 3 May 2013, when
instrumentation in Area N0. 2 was first installed.
4 PRELIMINARY RESULTS AND OBSERVATIONS
4.1 Load Transfer Platform – EPC data
Figure 5 presents the EPC data which is plotted against days elapsed since installation (i.e., days since 3
May 2013). The embankment height, described in Section 3.5, is also plotted. The two periods of ground
improvement for the upper level LTP, Stage 3a and 3b, occurred between days 63 and 71 and days 131
and 137 respectively. The gap in data is due to power loss and EPC2 did not respond for a period due to a
wiring issue. Embankment construction was completed with the placement of track formation on Day 176
(24 October 2013).
This arching behaviour, as observed based on the EPC data, is described in choronological order. The
LTP (650 mm thick) was placed on Day 1, EPC pressure matched closely the overburden stress, arching
does not occur at this point. Note that all EPCs show an initial spike in pressure immediately following
placement of the LTP and Lift No. 1. This is due to an increase in overburden stress and the effects of
dynamic compaction, compaction induced stresses dissipated over the days following. Embankment lift
No. 1 was completed on day 34 (embankment height 1.95 m). At this point the cell pressure observed in
EPC2 (base of LTP, 2.05 m of overburden) reduces to a value less than EPC3 (top of LTP, 1.35 m of
overburden) cell pressure despite having a much larger overburden stress. This behaviour indicates that
partial arching formed at this point, although the expected stress increase in EPC1 is not as obvious. This
is likely offset by the initial dynamic compaction induced stresses.
The arching continues to develop after embankment lift No. 2, however the development of arching is
now more pronounced. The arching is affected by the Stage 3a piling works, as observed by the
Lift Nos. 3-7
Lift No. 2 (some earthworks)
Lift No. 1
LTP
Refer to
Fig. 6
Stage 3a
Piling
Figure 5: EPC data and embankment height.
Stage 3b
Piling
significant increase in EPC2 cell pressure, suggesting partial arching collapse. The effect of these two
periods of ground improvement works (day 63 to 71 and 131 to 137) is more clearly evident during the
second phase were the data acquisition hardware was in operation. This is discussed in Section 4.2.
During an extended period midway through construction (day 71 to day 132) and the period postconstruction it can be seen that arching is increasing with time at constant overburden stress. This is not
expected to occur indefinately; a point of maximum arching is expected to occur. To date the observed
behaviour suggests that maximum arching has not yet been reached. This is similar to the preliminary
arching behaviour observed in a series of Dutch field studies, including; Van Eekelen et al. (2008, 2010)
where arching took around 9 months post-construction to fully develop. Similar behaviour was observed
in both a shallow and a full height test embankment (Haring et al. 2008) and a piled railway embankment
(Van Deijnen at al. 2010). It is thought that the development of the arching is related to the coupled
behaviour between the arching mechanism and the GR/sub-soil deformation. Conceptually, this
observation is similar to the arching described in 2D trapdoor arching tests by Iglesia et al. (1999, 2013)
and observed more recently in 3D laboratory experiments by Van Eekelen et al. (2012a,b). The time
dependent behaviour observed arises due to the nature of the sub-soil deformation occuring, more
specifically, time dependent consolidation of the CIS.
4.2 Ground improvement works and arching
Installation of the upper level DDPs during both Stages 3a and 3b had an effect on the performance of the
lower level LTP on both occasions. However, this can be observed best during Stage 3b when data
acquisition was automated on a 4 hr time step. EPC data gathered during Stage 3b ground improvement
works, days 131 and 137, is shown in Figure 6. The DDP layout in shown in Figure 7. Three separate
spikes in the data can be seen, the general response of the 3 EPCs in each case is the same. The magnitude
of the response does however vary and it can be seen that this is related to the proximity of the DDP to
the instrumented area. In each case, the general response saw the stress acting on EPC1 reduce and a
corresponding increase in EPC2 and EPC3. EPC3 at the base of the LTP shows a larger increase than
EPC2 in each case. The three events (Day 136 is 2 timesteps) are described below:
• Day 133 – Midday to 4pm. 3 DDPs installed in the vicinity of instrumented area No. 2. F11
(11.4 m), G12 (10.2 m) and H13 (11.2 m). This distance in brackets is the distance from DDP
to EPC2/3 which is close to the center of a soil arch (E14,E15,D14 and D15).
• Day 136a - 8am to Midday. G12 (8.2m)
• Day 136b –Midday to 4pm. G14 (5.6 m), G13 (9.0 m) and J14 (9.2 m)
• Day 137 – 8am to Midday. F13 (6.8 m) and H14 (7.4 m)
The response of the EPC suggests that partial collapse of the soil arch has occured on each occasion.
Given the strong correlation with the timing of the installation of the DDPs it is suggested that this
response is primarily due to lateral deformation of the LTP and the embankment material overlying it
from the installation of these full displacement piles. It is possible that dynamic loading from the piling
rig tracking across the upper level hardstand may contribute, however this doesn’t correlate with the data
and is considered unlikely. It is likely that the day to day fluctuation in the EPC data observed in Figure 5
are due to dynamic loading from earth moving eqiupment, however the response of the EPCs during
Stage 3a piling is of much larger order of magnitude and a different mechanism is required to explain this
behaviour.
Numerical simulation of drilled displacement piles by Pucker and Grabe (2012) showed that bulk
density modelled increased adjacent to the DDP and reduced exponentially at a distance from the DDP.
For the single DDP modelled in dense sands, approximately six pile diameters (6D) was suggested as the
maximum zone of influence and 8D for the loose sand modelled. The zone of influence observed here is
12.4D (G12) and increases to over 20D for DDPs installed on Day 133. This perhaps gives some insight
into the sensitivity of the soil arch to external loading/deformation.
Based on the EPC data, the response to the piling installation is consistent in each case; partial collapse of
the arching mechanism. Equally consistent is the response following the partial arching collapse, in each
case the arching mechanism trends towards its previous state. This is consistent with the general trend
(Figure 5) observed between Day 71 to 132 and post construction. In all cases the arching mechanism
trends towards a state of increased arching. It should be noted that this would be expected to occur up
until some maximum sub-soil/GR deformation. Irreversible, complete collapse of the arching mechanism
would be expected at very large sub-soil/GR deformation.
Day
133
Day
136a,b
Day
137
Stage 3a - Piling
Figure 6. EPC data and embankment height.
Figure 7. DDP layout.
4.3 Geogrid performance and sub-soil support.
In order to describe the behaviour of the sub-soil support beneath the LTP it is necessary to understand
the stress history of the CIS, which for this site is not an easy task given the site history. Approximately
2 m of fill has overlain the CIS historically (likely greater than 50 years), the existing rail embankment
has been constructed and more recently site surcharging occurred as part of construction activities. The
current vertical stress acting on the sub-soil (as measured from EPC3) has for the large part been less than
40 kPa; at present it is less than 25 kPa and reducing further. We consider that the current vertical stress is
less than, or about equal to, the effective overburden pressure associated with the historic fill. Given the
significant period of time in which the historic fill has been in place it is considered that primary consolidation is complete and that ongoing settlement is largely related to secondary consolidation.
If the stress levels acting on the CIS remain as they are, or continue to reduce as expected, then loss of
sub-soil support is expected to also occur primarily as a function of the secondary consolidation of the
CIS. It should be noted that the ground improvement performed will further reduce the rate of settlement;
the effect on the CIS due to the ground improvement requires further work to quantify. With these factors
in mind it is apparent that loss of sub-soil support (and hence development of tensile load in the GR) will
take a considerable period of time. Assessing the rate of subsoil settlement, and in turn the development
of GR tensile load, will be part of this ongoing research.
The initial strain gauge readings, which are typically very small, therefore seem quite reasonable given
the expected slow rate of sub-soil settlement. Strain gauge readings at present are generally less than
about 0.2%, which is considered within the error range for this type of strain measurement. In addition, it
is likely that some initial slack in the geogrid is being taken up.
5 CONCLUSIONS AND ONGOING RESEARCH
Based on the preliminary results from this instrumented embankment, the following conclusions are
made:
• Arching within a piled embankment can be clearly observed through the variation of vertical
stress in the LTP as is expected.
• The development of the arching mechanism is observed to develop with time. It is suggested
that this occurs due to sub-soil/GR deformation; initially this deformation is largely a function
of sub-soil behaviour. The time dependent nature arises in this case due to primary consolidation of CIS.
• The response of the soil arch to external loading would appear to be fairly sensitive. The effect
of piling installation (full displacement piles) on the arching mechanism at a range of up to 20D
is noticeable and measurable.
•
The response of the arching mechanism to the piling installation is a short term response only.
The arching mechanism in the piling installation cases, and in general, trends towards a state of
increased arching.
6 ACKNOWLEDGEMENTS
The authors would like to thank both Golder Associates (through a “Innovation and Excellence” (i2x)
award) and Monash University for their financial assistance and support as well as the Regional Rail Link
– City to Maribyrnong River (RRLCMR) Alliance for supporting and providing the opportunity to undertaken this field case study. In addition, the design support provided by the RRLCMR Alliance to obtain
the necessary approvals and their assistance during both the construction and instrumentation installation
phases is greatly appreciated.
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