the role of geotechnical temporary works on infrastructure

Transcription

Geotechnics on Irish Roads, 2000-2010 – A Decade of Achievement
THE ROLE OF GEOTECHNICAL TEMPORARY
WORKS ON INFRASTRUCTURE CONSTRUCTION
IN IRELAND
MARK PETERS, Byrne Looby Partners (Europe & MENA), Galway
ANDY WILKINS, Byrne Looby Partners (Europe & MENA), Galway
ANTHONY O’BRIEN, Byrne Looby Partners (Europe & MENA), Galway
Paper presented to Engineers Ireland, Geotechnical Society of Ireland
11th October 2012
Cover photographs: (L) Structure S04-N7 Bowstring Bridge during construction, positioned on top of a reinforced earth approach
embankment prior to bridge slide to final position. (R) Removal of temporary excavation support after staged excavation and
replacement of soft ground at Bishop O’Donnell-Seamus Quirke Road in Galway.
SYNOPSIS
By its nature, infrastructure construction in an urban context, particularly widening of an existing highway, can require significant
phasing and sequencing of temporary works in order to deliver the permanent solution within the constrained environment. Byrne
Looby Partners have worked on a variety of road schemes in Ireland including Phase I and II of the M50 Improvement Project and
Bishop O’Donnell - Seamus Quirke Road, Galway. A variety of ground conditions were encountered on these schemes, in
conjunction with constraints such as traffic management, land acquisition, programme and restrictions imposed from adjacent
infrastructure resulted in the development of a number of temporary works systems in order to permit construction of the permanent
works solution.
On the M50 upgrade project, soil nailing was adopted as the temporary works solution at several slope locations permitting
excavations for the construction of new bridges and temporary excavations adjacent to live highways and the Royal Canal in the
vicinity of the improved M3 Interchange. At other locations, temporary excavations relied upon the undrained shear strength of the
Dublin glacial tills. Elsewhere the construction works relied upon temporary dewatering measures to facilitate the construction of the
permanent works.
Access to Monastery Road from the N7 required a new large bowstring arch bridge structure to be constructed. Due to extremely
limited working space the new bridge had to be ‘launched’ across the N7 into position from a temporary reinforced soil approach
embankment. The bridge slide loading travelled across the reinforced soil earthworks imposing large transient loads as the bridge
structure was manoeuvred into place. A soil-structure interaction model was developed using PLAXIS to understand the influence of
the temporary loads on the reinforced soil / earth and predict the resulting ground movements and establish movement trigger levels.
During the bridge slide, movements were monitored and compared to the predicted displacements.
At Bishop O’Donnell - Seamus Quirke Road in Galway excavation and replacement of soft ground deposits was required in a phased
solution adjacent live traffic. Temporary support was achieved using steel sheet piles which relied upon the support from the adjacent
filled / in situ ground sections. Complete excavation and replacement of up to 6.5m of soft ground was achieved.
1
•
INTRODUCTION
THE ROLE OF TEMPORARY
HIGHWAY UPGRADE SCHEMES
WORKS
plant, machinery and materials into and
around tight urban sites
IN
A spell of increased / prosperous economic activity,
such as that experienced in Ireland in the period
between the mid-1990s and late 2000s, will result in
increased traffic volumes, stressing the capacity of the
existing road infrastructure. This stressing of the
existing trunk road network leads to identification of
requirements for both new highway infrastructure and
the upgrade of existing highway infrastructure.
Upgrade of existing highway infrastructure involves the
provision of additional traffic lanes to facilitate
increased traffic flow and relieve congestion.
Invariably, these roads are in and around urban areas or
adjacent to significant development where acquisition
of additional land is simply not possible or would
require major public consultation. These spatial
constraints generally result in the requirement for new
traffic lanes to be constructed within the existing
highway boundary or with minimal additional land take.
By its nature, infrastructure construction in an urban
context, particularly widening of an existing highway,
can require significant phasing and sequencing of
temporary works in order to deliver the permanent
solution within the constrained environment. Typically,
these constraints include:
•
Limited available working space for plant,
machinery and labour resulting in logistical
and health and safety constraints
•
Maintenance of existing traffic flows presents
There are logistical constraints in moving
•
The cost of traffic delays during road
widening
•
Unsuitable / poor ground conditions in the
area of proposed widening
•
Incorporation of current highway alignment
and geometric requirements when widening
older motorways / trunk roads
A range of geotechnical solutions are available to
mitigate and overcome these constraints. Consideration
must be given to a range of potential solutions so as to
ensure that the most technically and economically
appropriate solution is derived. Where insufficient
lands are made available to construct the additional
lanes using normal construction practices, solutions can
be broadly defined (non-exhaustively) into two
categories with numerous sub-categories:
1.
Soil / Rock Slope Strengthening / Steepening
•
Strengthening of steepened cuttings using soil
nailing / rock bolting with appropriate facing
system.
•
2.
Widening of embankments by strengthened
steepened
slopes
using
geotextile
reinforcement.
Retaining structures
•
Embedded sheet pile, king pile or bored pile
walls
•
In situ or precast reinforced concrete gravity
walls
•
Modular gravity walls e.g. crib wall, gabions
•
Reinforced earth walls.
logistical and health and safety constraints
•
Prevalence of existing services and / or
utilities resulting in logistical and health and
safety constraints
•
Continuity with existing structures /
earthworks resulting in technical constraints
such as differential settlement, slip surfaces,
cracking or seepage planes
1.
Interaction
2.
•
with
existing
structures
/
Geotechnical temporary works are normally constructed
with a view to implementing either one of the above
solutions and may include the following:
earthworks resulting in technical constraints
and / or future maintenance constraints
•
Existing / proposed landscaping or aesthetic
3.
considerations may impose constraints in
terms of what is visually acceptable
•
The potential adverse effects of construction
activities on sensitive adjacent structures
4.
Temporary cut slopes and temporary
excavations e.g. to facilitate construction
processes or installations.
Temporary strengthening and steepening of
existing or modified slopes e.g. to facilitate
construction processes or installations
Temporary construction platforms to facilitate
plant, machinery or construction sequence /
processes
Temporary de-watering
control measures
or
groundwater
2
5.
Temporary retaining structures to facilitate
construction processes e.g. excavation and
replacement of soft ground
This paper presents a number of case studies, which are
examples of temporary works solutions adopted to
facilitate construction of permanent highway widening,
focussing on how the geotechnical considerations and
analysis methods informed the choice of solution and
how particular project constraints were overcome.
Following this a discussion on geotechnical design of
temporary structures is presented focussing
employing Eurocode 7 as the primary design code.
on
CASE STUDY 1
M50 UPGRADE PPP CONTRACT 2, S17-N3
BRIDGE PIER 1 – TEMPORARY CUT SLOPE AND
TEMPORARY SOIL NAILING
The €950m upgrade of the M50 motorway was
completed in September 2010 and brought about
significant improvements to the traffic condition around
the greater Dublin area. Contract 2 was delivered
through the M50 Upgrade PPP contract for the finance,
design, construction and maintenance of the motorway.
The NRA awarded the PPP contract to M50 Concession
Limited, a consortium currently comprising Spanish
infrastructure developers Global Via Inversiones, S.A.
(GVI), Sacyr Vallehermoso, S.A. and Irish firm P.J.
Hegarty & Sons Limited. The Contractor, M50 D&C
Limited, comprised FCC Construcción Ireland Limited,
Sacyr Ireland Limited and P.J. Hegarty & Sons Limited.
Atkins Consultant Engineers and Eptisa designed the
work on behalf of the Contractor, with Byrne Looby
Partners acting as the Designer’s Sub-consultant.
Roughan O’Donovan Limited supervised the contract
on behalf of the NRA.
The works delivered as part of Contract 2 comprised the
following:
•
Upgrading 25km of motorway to dual threelane from Junction 3 (M1 Airport
Interchange)
to
Junction
6
(N3
Blanchardstown Interchange) and from
Junction 11 (N81 Tallaght Interchange) to
Junction 14 (R133 Sandyford Interchange).
•
Providing a fourth auxiliary lane along an
18km section.
•
Upgrading seven of the eight interchanges
along this section of the M50.
•
Major upgrade of junctions at the M1, N2 and
N3 from grade separated junctions to freeflow /partially free-flow interchanges.
•
70 principal structures including major
crossings of railway lines, as well as the
Royal Canal and the River Dodder.
As part of the N3 works, Byrne Looby Partners were
employed by the Contractor to design the temporary
works required to facilitate the construction of the Pier
1 base of Bridge S17-N3. The bridge carried a flyover
section of the N3 and Pier 1 was constructed adjacent to
the Royal Canal which posed the primary constraint to
the construction of the Pier 1 pile-cap. A location plan
is shown in Figure 1. The Royal Canal is elevated
above the level of the pier base by approximately 4m
and the proximity of the base is such that modification
of the existing bank of the Royal Canal was required to
facilitate the construction of the Pier 1 foundation.
GROUND CONDITIONS
The soil stratigraphy generally consisted of Made
Ground/Fill (soft to firm brown sandy gravelly CLAY)
overlying firm to stiff grey gravelly CLAY (Boulder
Clay) and moderately strong to strong LIMESTONE.
However, clayey medium dense and dense SAND and
GRAVEL deposits were noted in the boreholes to be
interbedded with the firm to stiff CLAY (Boulder
Clay). The stiff CLAY strata are not consistent across
the site. On this basis, a layer of the medium dense to
dense SAND and GRAVEL was assumed to overlie the
bedrock in all locations. In general the location of the
top of the SAND and GRAVEL deposits was
encountered at between 49.0mOD and 50.0mOD
Groundwater strikes were reported in the majority of
the exploratory bores and generally the groundwater
strikes occurred in the SAND and GRAVEL layers.
Typical equilibrium groundwater levels were recorded
at between 49.8mOD and 48.7mOD. The adjacent
Royal Canal has lowest bank levels on the south side of
the canal of between 51.3mOD and 51.6mOD in the
vicinity of S17-N3 Pier Base 1. The corresponding bank
levels on the northern side of the canal are at 52.1mOD.
Accordingly, a design water level in the canal of
51.4mOD was adopted, which is at or above the
minimum bank level on the southern side of the canal.
This level would therefore correspond to a flood event
3
in the canal. Typically the water level in the canal is
51.0mOD.
and the Royal Canal to provide a 45° slope and the
2.0m minimum clearance. This length included the full
extent of the lower formation level and a 6.1m length of
The proposed ground and groundwater engineering
parameters, derived from the ground investigations are
presented in Table 1. Given the proximity to the canal,
emphasis was placed on deriving appropriate
permeability characteristics for the soils. In the absence
of in situ permeability testing, this was carried out
based on Hazen’s method for estimating permeability
(k) based on the D10 value (particle size at which 10%
of the sample passes):
the upper formation level. In this area, it was proposed
to steepen the excavation side slope to near vertical
(85°), supporting the cutting with soil nails. A hardfacing was applied to the cutting using shotcrete. This
solution had to be agreed, in principle, by Waterways
Ireland.
k = 0.1(D10)2 (m/s; D10 in mm)
The particle size distributions for the Glacial Till and
Glacial SANDS and GRAVELS are presented in Figure
2 and Figure 3 respectively.
PROPOSED SOLUTION
The proposed formation level of the pier varied between
50.1mOD and 48.8mOD with the majority of the length
of the pier (30m) at the higher level and the remaining
length (16m) at the lower level. The plan arrangement
of the pier is at a skew of approximately 4º i.e.
approximately parallel to the canal. The distance
between the canal edge and the proposed pier base
varies from 2.9m at the western extent to 5.3m at the
The proposed soil nail solution is shown in Figure 5 to
Figure 7. In order to limit the risk of soil nail
installation adversely impacting on the integrity of the
Royal Canal, a minimum clearance between the bed
profile of the canal and the soil nails of 1.0m was
maintained. In general, the soil nails were installed
with an angle of inclination (δ) to the horizontal of
12.5°. However, the upper row of nails was inclined at
25º to the horizontal to maintain the minimum 1m
clearance for nail length of 2.0m.
In order to account for the water level in the Royal
Canal, and uncertainty regarding hydraulic conductivity
between the canal and groundwater table, two design
assumptions were made as follows:
•
the canal and groundwater level. Water level
in canal modelled at 51.4mOD for canal
lining material only. This case assumed no
flow of water from the canal into the
surrounding ground.
eastern extent.
For the majority of the pier base excavation, where the
formation level was proposed at 50.1m OD (plus an
allowance for blinding etc.), the proposed pier base
formation was above the level of the SAND and
GRAVEL stratum and groundwater level. In these
areas, 45° slopes, assuming undrained behaviour of the
Boulder Clay, were proposed.
For the remaining area of the pier base excavation,
where formation level was proposed at 48.8mOD, the
base of the excavation lay both below groundwater
level and the interface of the SAND and GRAVEL
strata. In this area, a temporary dewatering scheme was
required in order to ensure a dry excavation and to
maintain the stability of the 45° slopes over the duration
of the temporary works. A photograph of the temporary
slope is presented in Figure 4.
Furthermore, in the south-western corner of the
temporary excavation, over a length of 26.8m, there
was insufficient space between the excavation outline
Case 1: No hydraulic conductivity between
•
Case 2: Hydraulic conductivity/groundwater
levels in the design sections determined from
long-term seepage analysis allowing for flow
from the canal into the surrounding ground,
dependant on the permeability of the soils.
The groundwater analyses were carried out using the
software program SEEP/W. For either analysis a 0.5m
thick puddle clay liner was assumed between the bed
profile of the Royal Canal and the underlying soils.
Stability assessments for the 45° slopes were carried out
to calculate a global lumped factor of safety. Using
undrained shear strength parameters for the soils, a
minimum factor of safety of 3.0 was established. The
effect of the assumption of hydraulic conductivity
between the canal and groundwater table was adverse
but only marginally.
4
For the stabilised soil nailed cutting, design was carried
out in accordance with the principles of BS8006:2005
and CIRIA C637. Stability analysis was facilitated by
groundwater level. The monitoring readings show that
for penultimate excavation stage, which was the first to
dip below groundwater level, there was a short period
use of the software programme SLOPE/W. A minimum
factor of safety of 1.65 was determined from the
SLOPE/W modelling. The numerical difference
between the two groundwater regime analyses
described above was found to be marginal.
where the where the design levels were exceeded as the
drainage in the base of the excavation was installed and
commissioned. Once the drainage was installed and
commenced working, the recorded groundwater levels
tended to drop to between the design levels. This
behaviour was repeated for the final excavation stage.
Given the sensitive nature of the adjacent canal and
evaluation of the geotechnical risks, a monitoring
scheme was proposed for the temporary soil nailed
A pumping trial was undertaken from the drainage
sump closest to piezometer P1 during the final two
cutting. Soil nail slopes tend to move significantly as
the nail reinforcement generates bond and start to pick
up load. Six monitoring points were established along
the slope and these were monitored during the two
week construction period.
excavation stages to validate the pumping volumes
predicted from the seepage analysis. The pumping trial
caused a significant drop in the recorded groundwater
level at piezometer P1 which recovered over a period of
several days after the pumping trial was completed.
Amber and red monitoring trigger levels were set at
5mm and 10mm respectively for lateral movement.
Figure 8 shows a plot of the slope displacement over the
CASE STUDY 2
critical construction period. The displacements recorded
that the amber trigger level was breached close to the
top of the cutting. Monitoring frequency was
subsequently increased but red trigger levels were not
breached during construction.
Seepage analysis, undertaken as part of the temporary
works design, had indicated that the rate of flow
between the Canal and the groundwater table would be
low, if present at all. However the concern remained
that the temporary excavation could be destabilised by
changes in the groundwater regime induced by the
excavation.
In order to understand and control groundwater levels,
particularly between the excavation and the Royal
Canal, a groundwater monitoring system was also
established. This system involved the installation of
M50
UPGRADE
CONTRACT
1,
S04-N7
MONASTERY ROAD BOWSTRING BRIDGE SLIDE
–
TEMPORARY
REINFORCED
EARTH
EMBANKMENT
The M50 Upgrade Contract 1 was delivered through a
Design & Build (D&B) contract by SIAC-Ferrovial
M50 Joint Venture, a partnership between SIAC
Construction Ltd. and the Spanish Construction
Company Ferrovial Agromán S.A. The Contractor’s
designer was a consulting engineering consortium of
Hyder Consulting Ltd, PH McCarthy & Partners and
Grontmij. Arup Consulting Engineers supervised the
contract on behalf of South Dublin County Council and
the NRA.
The works delivered under Contract 1 comprised the
following:
•
two standpipe piezometers in the footpath of the Royal
Canal adjacent to the sections adopted for analysis. In
addition the surface water level of the canal itself was
monitored at three locations in the vicinity of the
excavation.
Upper and lower design levels for groundwater were set
in relation to the excavation level in the temporary
works area at the time. Figure 9 shows a plot of the
groundwater level readings over the construction
period. Initially, ground level within the excavation
was above groundwater level. However as construction
progressed, the excavation level dropped below
The 8km section from Junction 7 (N4)
through Junction 9 (N7) to Junction 10
(Ballymount) was upgraded from dual two
lane motorway to dual three lane motorway,
plus a fourth auxiliary lane in each direction
between interchanges to facilitate traffic
entering and exiting junctions.
•
New
free-flowing
interchanges
were
constructed at Junction 7, (N4, Liffey Valley
Interchange) and Junction 9 (N7 Red Cow
Interchange) together with a significant
upgrade of Junction 10 (Ballymount).
•
A segregated corridor for LUAS was
provided including a new 800m section of
5
tramway, thus removing the need for LUAS
and road traffic to cross each other.
•
frequently as refusal (50 blows for less than 75mm
penetration in any given drive).
12 new bridges were provided.
As part of the N7 works, Byrne Looby Partners were
employed by the Contractor to assess the design of the
permanent reinforced earth approach embankment
(south) for the high, transient temporary loads
associated with the installation (bridge slide) of the
The underlying bedrock was described as very strong,
medium to thinly bedded, blue/grey/dark grey, finegrained LIMESTONE. The quality of the mass
however, was variable with an average rock quality
designation of 25% in the upper 3-4m.
landmark N7 Bowstring Bridge.
Based on the observed SPT results and considerable
available engineering data for the Boulder CLAY in
The Bowstring Bridge connects the N7 Eastbound and
Monastery Road (Clondalkin) with the Luas Red Cow
Stop and Park & Ride Facility. The bridge forms part of
the M50/N7 (Red Cow) Free-flow Junction plan
removing the traffic lights at the former N7/Monastery
Road Junction. The bridge has a suspended deck which
Dublin, a conservative friction angle (φ’) was adopted
accommodates two-way traffic, cycle-lanes and
footpaths. It has a total span of 60m and is 20m wide.
for the material of 35° for use in effective stress
analyses. The corresponding undrained shear strength
would be in the order of 75-100kN/m2 with some
increase with depth.
PROPOSED SOLUTION
The 1,550 tonne bridge was constructed on the
approach embankments adjacent to the N7 and
manoeuvred into place using specially imported heavy
lift mobile jacking equipment during the night of 7th
October 2008. As a result of the construction method
and bridge slide, significant variable loading would be
The basic construction sequence for the bridge is shown
in Figure 10. The bridge superstructure is carried on a
piled abutment and the approach embankments are
formed using reinforced earth Hexlok® walls.
Furthermore, a Hexlok® wall formed the southern face
of the south abutment embankment in order to facilitate
temporary traffic flow during the bridge construction
imposed upon the reinforced earth approach
embankments. The concern was that these temporary
loads, being much larger than the subsequent permanent
loads might overstress the reinforcement, or cause
significant deformation of the walls.
because sufficient space was not available for
traditional earthworks slopes. This was later replaced
with a permanent embankment. The bridge itself was
constructed offline and adjacent to the south approach
embankment before being moved into position.
GROUND CONDITIONS
The reinforced earth walls were designed for permanent
loads in accordance with the “Coherent Gravity
Method” of BS8006:2005. However, Cl.6.6.5.3 of the
The ground conditions typically comprised up to 2m of
Made Ground, overlying Boulder CLAY and
LIMESTONE bedrock. The depth to LIMESTONE
varied considerably over the site dipping in the northsouth direction by as much as 10m. No groundwater
strikes were recorded in any of the exploratory holes.
The Made Ground was described as consisting of brown
gravelly CLAY with concrete, cobbles and boulders.
The average SPT N value in this stratum was 40
varying between 15 and refusal.
Investigation of the Boulder CLAY proved difficult
with cable percussion techniques beyond 4-5m owing to
obstructions reported as possible boulders. Rotary
techniques were employed to investigate the depth of
the Boulder CLAY and level and quality of the
underlying bedrock. SPT N values were reported
code states the following:
Where the structure is of unusual geometry or supports
concentrated loads which are not specifically covered
in the code the local equilibrium method may not be
sufficient and a global wedge stability check should be
performed.
The temporary loads were large and transient relative to
the permanent works. Therefore, a global wedge
stability check had to be carried out for the temporary
condition. The following loadings were considered in
the assessment of the temporary works:
Bridge live load:
Skid dead load:
Skid slab dead load:
4,267kN
589kN
30kN/m2 applied
6
The temporary loading was to be transferred via a
ground bearing ‘skid’ slab, 5.5m wide and 1.2m deep.
The nearest edge of the skid slab was only 0.5m behind
In the Plaxis model, the wall facing units have been
modelled as individual beam elements with unrestrained
connections (hinges) at the joints of the Hexlok®
the wall facing. The resulting pressure beneath the skid
slab was calculated to be 260kN/m2. A friction angle of
40° was specified for the Class 6I/J fill which was to
form the reinforced earth approach embankment. The
reinforcement elements comprised 50mm wide x 4mm
thick, 12m long, galvanised steel straps, at 0.75m
vertical centres.
panels. Interface elements have been applied to the
reinforcement straps to model the interaction of the
reinforcement and surrounding soil more closely. MohrCoulomb strength parameters were adopted for the soil
strength and a conservative stiffness of 50,000kN/m2
with a Poisson’s ratio of 0.25 was adopted for both the
Class 6I/J fill and the underlying foundation soils. A
number of finite element sensitivity analyses were
performed in order to understand the range and
The stability analyses were carried out using the
software SLOPE/W. All of the BS8006 partial factors
for material strength and loadings were included in the
definition of design parameters except for the
Ramification Factor. This factor is required, in
accordance with BS8006, to be 1.1 where the
consequence of a failure would be high. Therefore an
overall FOS from SLOPE/W in excess of 1.1 was
required to satisfy the Ramification Factor criterion.
variation of wall movement and stresses developed in
the reinforced earth system under the temporary
loading. Additionally, the modelling of the facing
elements was explored, either by modelling as a full
height panel or as unrestrained, hinged, discreet
elements.
In accordance with the guidance provided with the
SLOPE/W software, the mobilised bond stress for the
reinforcement straps has been defined as FOSdependant. This means that the design bond stress for
the straps is reduced by the calculated overall FOS. This
methodology should be adopted where some movement
is required in order to mobilise the bond stress in the
reinforcement straps i.e. passive reinforcement such as
horizontal movements in the order of 6-8mm could be
expected with significantly more movement around the
mid-height of the wall. Based on these movement
predictions, movement monitoring positions were set up
towards the mid-height of the wall. The movements
were monitored during the bridge slide process, with
measurements in x, y and z directions. The proposed
trigger levels are presented in Table 2. Vertical
straps, soil nails or geogrids.
displacements (settlements) were expected to be small.
Additionally, load readings were taken from the jack
system at each of the temporary support locations to
confirm loadings were as predicted. The loading had
been conservatively assessed for the design and in
reality the loads were approximately 30 – 40% less than
anticipated.
The minimum calculated value for FOS was 1.19
thereby satisfying the stability criteria in accordance
with BS8006. However, given the intensity of the
temporary loading, it was considered prudent to
undertake a more rigorous analysis using the finite
element code Plaxis.
Figure 11 shows predicted horizontal movements from
the Plaxis analyses. The Plaxis analyses indicated that
During the bridge slide, surveying targets were
As part of the geotechnical risk strategy, a monitoring
regime was installed in order to ensure that the design
assumptions were validated and that the structure
behaved as expected particularly as failure mechanisms
for this type of structure tend to be “brittle” i.e. small
movements / strains prior to failure. In order to develop
the monitoring plan, an estimation of likely movements
was required in order to assess how the structure was
expected to behave. This was carried out using Plaxis.
monitored regularly. A maximum recorded movement
of 9mm was recorded, breaching the amber limit. A
contingency plan to place fill in front of the reinforced
earth wall or to return the bridge to its initial start
position, dependent on the bridge slide phase had been
prepared in keeping with the philosophy of the
Observational Method. Monitoring frequency was
increased however, no further significant movements
were recorded and the contingency measures were not
This analysis would provide predictions of wall
movement under the temporary loading conditions
which could then be compared to monitored wall
movement in real time.
implemented.
Over the duration of the bridge slide, on the Hexlok®
wall forming the southern face of the south approach
embankment, the range of observed lateral movement
7
was 1.8mm to 8.9mm with an average of 5.2mm
demonstrating slightly better performance than
indicated in the Plaxis analyses. A maximum vertical
The overall aims of these provisions are to provide
increased traffic capacity between the suburban and
coastal areas west of Galway and the city centre and to
displacement of 2.0mm was observed adjacent to the
permanent Hexlok® wall. Based on the observed
results, the Plaxis analyses were deemed to be
validated.
provide improved cycle and pedestrian facilities on the
route with a view to facilitating the achievement of the
aim for greater use of more sustainable modes of
transport. This case study will focus on the excavateand-replace solution for the area of relatively soft
ground at Bishop O’Donnell Road-Rahoon Road
Junction, the position of which is shown in Figure 12.
The case history demonstrates the need to utilise
modern analysis techniques in order to fully understand
expected behaviour for complex situations. Adhering to
the British Standard calculation methods alone would
have indicated an adequate factor of safety but would
not provide the level of knowledge required to validate
expected behaviour against observed movement
monitoring.
CASE STUDY 3
BISHOP O’DONNELL–SEAMUS QUIRKE ROAD
WIDENING SCHEME – TEMPORARY SUPPORT
FOR SOFT GROUND EXCAVATE AND REPLACE
Bishop O’Donnell Road / Seamus Quirke Road is part
of regional route R338, which links the R336 and the
Western Distributor Road – the main approach roads to
Galway from the suburban and coastal areas west of the
city – and the N6 across the Quincentenial Bridge. The
road passes through the largely residential areas of
Rahoon, Shantalla and Westside, which have grown
significantly in recent years. The area also supports
shopping, commercial and social services and sporting
facilities, all of which access Seamus Quirke Road
directly or indirectly.
In accordance with the Galway County Borough
Development Plan 1999, the following provisions were
set out as part of the Seamus Quirke Road Widening
Scheme:
•
The provision of a cycle way from the
roundabout at Bishop O’Donnell Road
(Western Distributor Road) to the roundabout
at Corrib Park.
•
•
Improvements to road junctions at Seamus
Quirke Road / Circular Road / Rahoon Road
GROUND CONDITIONS
The area at the Bishop O’Donnell Road-Rahoon Road
Junction had been characterised by substantial depths of
soft ground. Substantial historical settlements have been
noted in the area as shown in Figure 13. As a result of
this, the area was investigated with geophysics using
MASW and seismic refraction methods. The
interpretation (refer Figure 14) indicated that, locally,
substantial deposits of overburden material were
prevalent in depths of between 6-7m. These geological
features were identified as ancient infilled river
channels.
The overburden material itself comprised up to 2m of
made ground generally composed of clayey gravelly
SAND with many cobbles and boulders and road
construction materials. This overlay a relatively thin
layer of very soft organic fibrous PEAT which in turn
overlay 3-4m of very soft creamy / grey shelly MARL.
The depth at which the CPT probe refused correlated
well with the interpreted bedrock level of the
geophysics. Bedrock in this area was proven to
comprise very strong pink / white fine-grained
GRANDIORITE.
The prevalence of MARL across the area is not
uncommon in Galway. The deposit is characteristic of a
lake or water-filled channel depositional environment
known as an aquatic / limnic environment. Its
distinctive colour is derived from the nature of the
depositional waters which were highly basic (containing
calcium and magnesium).
with the possibility of a roundabout at this
location.
Trial pitting was carried out in order to assess the extent
of the soft soil deposits and to assess the likely
The provision of bus lanes on Seamus Quirke
Road and the Western Distributor Road from
the Corrib Park roundabout to the Cappagh
Road.
engineering behaviour of the soft soils. Figure 15 shows
cone penetration test (CPT) results in the vicinity also.
The very soft layers of PEAT and MARL between 2m
and 5m depth realise a sleeve friction (fs) of between
10kN/m2 and 30kN/m2 and a tip resistance (qc) of
8
between 0.5MN/m2 and 2MN/m2. The average value of
undrained shear strength has been calculated as
25kN/m2 based on the relationship presented below
Primarily, the solution involved use of a waling beam to
provide support to the sheet piles over the upper wider
section of the excavation. The waling beam would
which is also related to the plasticity index (Bowles,
1997):
derive its support passively from the ground either side
of the excavation. Below this, the sheet piles would rely
on membrane action to be self-supporting in the area
where excavation proceeded to bedrock. A small
component of passive resistance was also assumed from
the underlying bedrock as experience had shown that
some “bite” of the sheet pile into the upper slightly
weathered bedrock would be realised. An allowance
was also made for a hydrostatic head differential behind
cu =
qC − σ v
NK
where:
N K = 13 +
5 .5
I P (±2)
50
This correlated well with observations of undrained
shear strength made on-site during the trial pitting and
subsequent excavations. For the purposes of design,
moderately conservative values of undrained shear
strength of 15kN/m2 for the PEAT and 20kN/m2 for the
MARL were adopted.
PROPOSED AND ALTERNATIVE SOLUTIONS
In order to remove the problem of historical settlement
in the area, a piled slab was initially proposed to form
the road foundation. This would have involved
installation of driven precast concrete or steel piles
thought the soft overburden deposits to found in the
underlying bedrock. A reinforced concrete slab would
then be cast on top of the piles which would carry the
road construction and imposed traffic loading. The
loading would be transmitted to the underlying bedrock
via the piles. In addition to supporting the road
construction, the slab would also facilitate / support the
existing services traversing the area, thus its depth
would be approximately 2.5m from the finished road
level meaning that up to 3m of material would have to
be excavated anyway. Furthermore, given the extensive
services within the area, a pilot hole would have had to
be excavated for each individual pile to ensure no risk
of striking a service or utility line prior to driving.
The Contractor submitted an alternative proposal which
involved full excavate-and-replacement of the soft
ground. This solution would reduce cost and
construction time significantly. However, the primary
constraint to the proposal was maintaining a two-way
traffic flow during the works. A solution was developed
utilising a line of temporary sheet piles which would be
placed centrally on the carriageway as a means of
support to the live lane of traffic while the excavation
and filling operations were carried out over the adjacent
lane. The sequencing of implemented solution is
presented in Figure 16.
the sheet piles. In order to avoid overstressing of the
sheet piles, a strict limit was placed on the maximum
width of excavation proposed. The excavation
proceeded incrementally requiring the waling beam to
be moved incrementally also. Restrictions were placed
on the time allowance for open excavations and
cleaning of the in-pans of the sheet piles to ensure all
soft material was removed. The formation bedrock was
verified either visually where the excavation was dry or
by probing where groundwater was present. The sheet
piles were inspected visually during excavation for the
presence of split clutches which may have
compromised the solution.
The backfill comprised Class 6A selected granular fill
to be placed underwater and Class 6F2 selected granular
fill above this. Plate bearing tests were carried out on
the Class 6A fill after placement of a blinding layer of
fill. On completion of one side of the carriageway, the
process was repeated on the other side. The replacement
fill level was left roughly 1m lower than final fill level
to allow for placing of the final compacted fill layer and
road construction after removal of the sheet piles. This
negated the need for settlement monitoring during
excavation however, lateral movement of the sheet was
monitored to ensure that the wall was behaving as
predicted. Settlement of the final road construction will
be monitored on an on-going basis in accordance with
the works requirements.
The sheet pile wall was analysed using the Oasys
FREW software for the analysis of embedded retaining
walls. Figure 17 shows output from the FREW model
(SLS Case) and Figure 18 shows a photograph of
excavation progressing near the base of the sheet pile
wall. Undrained soil conditions were assumed for the
PEAT and MARL. The analysis required an iterative
procedure to ensure that the membrane action was
correctly modelled and that the artificial strut
stiffnesses, which modelled the membrane action and
9
waling support, were consistent with the amount of
movement calculated by FREW.
The stiffnesses of the artificial struts were calculated as
a function of the axial stiffness of the sheet pile wall as
it went into tension over the width of the excavation at
the level considered. The strut stiffnesses therefore
increased with depth as the width of the excavation
decreased. The additional stiffness offered by the
waling beam was calculated as a function of the passive
resistance of the soil which would provide the support
either side of the excavation. The calculated strut loads
(as shown on Figure 17) would then have to be carried
structurally (as a tensile force) across the sheet piles and
distributed passively into the supporting soils either side
of the excavation. This stiffness was iteratively
increased / decreased with increasing wall movement
such that the magnitude of passive support was
consistent with the expected wall rotation.
Consideration was given to provision of welded
diaphragms between the pans of the sheet piles to allow
greater structural capacity and hence a wider excavation
however, it was considered more practical to proceed
with the lesser width of 1m at the base of the
excavation. The structural design of the sheet piles
considered the tension carried across the clutches to
ensure that overstressing did not occur which may have
resulted in “unzipping” of the sheet piles.
Fundamentally, the analysis followed the principles of
CIRIA C580 in deriving design section forces however,
in this instance, it was more critical that the amount of
wall movement was estimated to analyse the membrane
action in the sheet piles and to provide confidence that
the assumed passive resistance would be realised.
The assessment of the feasibility and subsequent
development of the alternative solution was possible
because of the additional ground investigation
(geophysics and trial pitting) which defined the extents
of the problem spatially and provided confidence in the
engineering parameters adopted for design.
The success of the solution relied heavily on the strict
construction control procedures regarding excavation
width and verification of the excavated level. Nongeneric or bespoke solutions such as this require a high
level of commitment and diligence from the Contractor
in order to ensure that the assumptions of the
geotechnical analysis are realised and that the
construction control procedures specified by the
designer are implemented.
CURRENT BEST
GEOTECHNICAL
TEMPORARY
DISCUSSION
PRACTICE IN
DESIGN
OF
WORKS
-
This section presents a discussion on the implications
for the geotechnical design of temporary works in
accordance with Eurocode 7.
TEMPORARY CUT SLOPES
Temporary cut slopes or embankments are frequently
used in infrastructure projects to enable construction of
the permanent works. Where ground conditions are
suitable, slope angles as steep as approaching near
vertical may be employed in order to facilitate
construction processes, installations and temporary
widening of access routes.
It is worth noting the advice offered in Eurocode 7
which states (cl.11.5.1) “In analysing natural slopes, it
is generally an advantage to make a first calculation
using characteristic values, to get an idea of the global
factor of safety, before starting to design. Experiences
with comparable cases including investigation
procedures should be applied”. This implies that the
partial factor approach adopted by Eurocode 7 should
be used with caution as modification of soil strengths in
stability analyses may result in unrealistic or incorrect
failure mechanisms. An initial unfactored analysis
should inform the design.
Traditionally, slopes were analysed with a view to
establishing a lumped factor of safety. Distinction has
been made between an appropriate lumped factor of
safety for temporary conditions as opposed to
permanent conditions (Trenter, 2001) giving due regard
to environmental and economic risk.
Eurocode 7 design procedures, using the partial factor
approach, verifies overall stability by way of
identification of ultimate limit states (GEO and STR)
where the applied partial factors are defined in the
National Annex. It is noted in Cl. 2.4.7.1(5) that less
severe values [of partial factors] than those
recommended in Annex A ([Partial and correlation
factors for ultimate limit states and recommended
values] or the National Annex) may be used for
temporary structures or transient design situations,
where the likely consequences justify it. However, no
guidance with regards to the choice of partial factors for
10
temporary works is provided (except for temporary
anchors).
permanent works. In the temporary condition, design
may take account of the short-term strength of the
reinforcement. Geosynthetics display time-dependent
TEMPORARY SOIL STRENGTHENING FOR
STEEPENED CUT SLOPES USING SOIL NAILING
behaviour (creep). The short-term strength of the
reinforcement can be taken into account in a temporary
loading condition as appropriate. The permanent
condition must subsequently take creep into account
and lower strengths will be adopted to ensure the level
of strain is such that the structure remains serviceable
for its design life.
Soil nailing and soil reinforcement are not expressly
considered in Eurocode 7 and current best practice (as
advocated in the UK National Annex) is to design these
elements in accordance with BS8006-1:2010
(reinforced earth) and BS8006-2:2011 (soil nails). The
standard has recently been developed into two parts
The
foundation
soils
beneath
strengthened
In distinguishing between temporary and permanent
conditions, BS8006-2:2011 Cl. 4.1.5 states: “The
partial factors given [for actions, material properties
and soil resistances] do not explicitly take account of
whether the works are of a temporary or permanent
nature; this should instead be reflected in the selection
of appropriate characteristic values for the material
embankments must be given due consideration
particularly where soft silts and clays are prevalent.
Where the founding stratum is a fine-grained soil and
undrained behaviour is expected, application of a
rapidly applied load, e.g. from embankment
construction, will result in an increase in porewater
pressure (undrained response). The design situation
envisaged when loading soft clays is a rapid undrained
failure and collapse.
properties and characteristic soil nail resistances”.
Emphasis is placed on the derivation of the design bond
TEMPORARY RETAINING STRUCTURES
covering both elements.
stress reflected in the factor γk for determining the
characteristic bond stress (τbk) from ultimate values.
The γk factors in BS8006-2:2011 have been selected to
result in equivalent experience with lumped factors
between 1.5 and 3.0 on ultimate bond resistances (and
micropile / ground anchor designs). The range given for
γk is to reflect whether nails are used in a temporary or
permanent application and the degree to which full
dissipation of pore pressure is relevant. Therefore, a
clear and well-defined distinction is made between
temporary and permanent works for soil nail design.
However, where checks on external rotational failures
are carried out, as required by BS8006-2:2011, there is
no guidance regarding the appropriate partial factors to
be applied in a temporary condition.
TEMPORARY SOIL STRENGTHENING FOR
STEEPENED EMBANKMENTS (REINFORCED
EARTH)
Temporary retaining structures are frequently employed
during construction to provide lateral support to
excavations. The complexity of the temporary support
may vary from a simple trench box to support an area of
excavation to remove soft soil to large diaphragm walls
supporting a cut-and-cover tunnel section. For routine
excavations, existing good practice guidance, such as
CIRIA R97, should be used. However, for more
complex structures, detailed design in accordance with
an appropriate code of practice will be required. In the
UK and Ireland, Eurocode 7 is now the recognised
standard for design of retaining structures.
In the case of embedded walls, a recent study by
Markham (2012) highlights some of the short-comings
of using Eurocode 7 in design of temporary support.
These include:
•
produce dramatically different results for
example, passive earth pressure can be
treated as a resistance, a favourable action or
an unfavourable action each of which
produces a different result in an analysis.
BS8006-1:2010 advocates the use of best practice
guidance for the design of temporary (as opposed to
permanent) reinforced embankment structures which
can be found in BR470 (specifically for piling
platforms) and CIRIA SP123. High temporary
reinforced earth embankments are generally rare in
practice. Typically, the expense of constructing a high
reinforced earth embankment is such that it is
uneconomical unless it is incorporated into the
Different interpretations of Eurocode 7 can
•
Similarly, water pressures can be treated in
several different ways.
•
Use of total stress parameters on the passive
side of the wall may result in the analysis
11
becoming ill-conditioned and a sensitivity
analysis may be required.
•
In the UK, CIRIA C580 is cited as noncontradictory complementary information
(NCCI) however, parts of CIRIA C580 are,
in fact, contradictory to Eurocode 7.
Some recommendations are made in the study with a
view to relieving the opposition to use of the code and
promoting its acceptance among temporary works
engineers. These include:
•
Use of the single source principle which
removes some of the complexity of applying
different partial factors to different parts of
the earth and water pressure diagrams.
•
A partial rewrite of the code is recommended
to ensure that NCCI is, in fact, noncontradictory.
In all cases, a check on external (rotational) stability
should also be carried out for embedded walls. The
choice of soil parameters for use in analysis, drained or
undrained, should be tempered with experienced
judgement and reference to appropriate case histories
where available.
CONCLUDING REMARKS
Upgrade
of
existing
highway
networks
present
formidable challenges, particularly in the urban
environment. A wide variety of constraints may
influence the project and affect the choice of earthworks
measures employed in developing the upgrade. Slope
stabilisation and strengthening measures such as soil
nailing, reinforced soil (slopes) and reinforced earth
(walls) are commonly used to assist in providing
adequate working area inside of the lands made
available. Temporary retaining structures are also
Furthermore, the behaviour of the proposed
geotechnical structure must be considered giving due
regard to potential adverse effects such as differential
settlement, slip surfaces, cracking or seepage planes.
Groundwater can play a significant part is assessing the
suitability of temporary works solutions and Case Study
1 demonstrated that groundwater regimes should be
given careful consideration.
Case Study 2 demonstrates the usefulness of more
advanced analyses in developing appropriate
monitoring regimes. Using finite element or finite
difference codes to inform the design and, in particular,
the anticipated behaviour of a soil structure is to be
encouraged provided that it is tempered with
experienced engineering judgement
Case Study 3 demonstrated a bespoke non-generic
temporary sheet pile wall solution. An estimate of wall
movement, using soil-structure interaction software,
was key to analysis of the membrane action of the
sheets and providing confidence that the assumed
passive resistance was realised. Additional ground
investigation was crucial in assessing the feasibility of
the alternative solution and, subsequently, in informing
the development of the solution. The additional
information allowed the problem to be defined more
clearly in extent and provided confidence in the
engineering parameters to be
geotechnical design of the solution.
adopted
in
the
All of the case studies presented here demonstrate the
importance of understanding ground movement and
understanding the difference between large and small
strain problems e.g. Case Studies 1 and 2 respectively,
and also the difference between passive and active
ground response e.g. Case Studies 1 and 3 respectively.
commonly employed to assist with ground support and
the control of excessive ground deformation during
construction. Optioneering should be undertaken in
order to determine the most appropriate, technically
feasible and commercially attractive solution.
This correct anticipation of ground movement and
behaviour will inform the designer in determining the
appropriate trigger levels and monitoring regimes. Use
of movement monitoring is a fundamental and
necessary requirement as a means of reducing risk,
validating expected engineering behaviour and ensuring
safe delivery of geotechnical solutions.
There are a wide variety of geotechnical factors to
consider for highways upgrade and widening schemes.
Primarily the stability of existing assets must be
As with all temporary works solutions, the success of
considered. This can be influenced adversely by
proposed construction and existing geological and
groundwater conditions. A detailed examination of
these is required to assess the proposed concept.
the solution relied heavily on the strict construction
control procedures. Non-generic or bespoke solutions
such as this require a high level of commitment and
diligence from the Contractor in order to ensure that the
assumptions of the geotechnical analysis are realised
12
and that the construction control procedures specified
by the designer are implemented.
Traditional design methodologies have made the
distinction between temporary and permanent works
generally allowing less onerous factors of safety to be
associated with temporary conditions. The design of
temporary works, as opposed to permanent structures, is
not expressly considered in Eurocode 7 (except for
temporary anchorages).
REFERENCES
Bowles, J.E. (1997) Foundation Analysis and Design
Fifth Edition. McGraw-Hill
BS8006:1995 Code of Practice for Strengthened /
Reinforced Soils
BS8006-1:2011 Code of Practice for Strengthened /
Reinforced Soils and Other Fills British Standards
Institute
Some distinction has been made between temporary and
permanent conditions in the more developed / specific
partial factor codes (for example BS8006-2:2011 for
soil nailing) and this is to be welcomed. However, there
are still short-comings when referencing Eurocode 7 for
external stability checking in a temporary situation
which need to be addressed. Further to this, there are a
number of other short-comings identified by the
temporary works / geotechnical community associated
with the interpretation of the code and correlation with
non-contradictory complementary information which
are a source of consternation to engineers.
The geotechnical designer relies upon previous
experience and understanding of ‘short term’ or
‘temporary’ ground behaviour to inform decisionmaking within the context of tried and tested codes of
practice. Current best practice, in the form of Eurocode
7, will influence the future trends of discussion between
designer and reviewer with regard to design of
temporary geotechnical solutions.
Some guidance on appropriate partial factors for use in
geotechnical design of temporary works would be
welcomed among the geotechnical and temporary
works engineering community.
ACKNOWLEDGEMENTS
The authors would like to acknowledge M50 D&C,
Siac-Ferrovial and Coffey Construction Ltd. for their
kind permission to publish the data contained within the
case studies of this paper. The views expressed in this
paper are the sole views of the authors and do not
represent the views of any third parties.
The authors gratefully acknowledge the assistance and
support given by many colleagues at Byrne Looby
Partners both during the projects and in the production
of this paper.
BS8006-2:2011 Code of Practice for Strengthened /
Reinforced Soils Part 2: Soil Nail Design. British
Standards Institute
Building Research Establishment (2004) Working
platforms for tracked plant: Good practice guide to the
design, installation, maintenance and repair of groundsupported working platforms. BRE Press (2004) Report
No. BR470 London 2004
Department of Transport Highways, Safety and Traffic,
Departmental Advice Note HA 43/91 Geotechnical
Considerations and Techniques for Widening Highway
Earthworks
Gaba, A.R., Simpson, B., Powrie, W. and Beadman,
D.R. (2003) Embedded retaining walls – Guidance for
economic design. Construction Industry Research
Information Association (CIRIA) C580, London 2003
Irvine, D.J. and Smith, R.J.H. (1992) Trenching
Practice. Construction Industry Research Information
Association (CIRIA) R97, London 1992.
Jewell, R.A. (1996) Soil reinforcement with geotextiles.
Construction
Industry
Research
Information
Association (CIRIA) SP123, London 1996
Markham, P.D. (2012) The design of temporary
excavation support to Eurocode 7. Proc. ICE
Geotechnical Engineering, Vol. 165, No. 1, pp. 3-12
Phear, A., Dew, C., Ozsoy, B., Wharmby, N.J., Judge,
J. and Barley, A.D. (2005) Soil Nailing – Best practice
Guidance. Construction Industry Research Information
Association (CIRIA) C637, London 2005
Trenter, N.A. (2001) Earthworks: A Guide. Thomas
Telford
13
Figure 1 - Location plan for S17-N3 Bridge Pier 1 adjacent to the Royal Canal
14
STRATUM
MADE GROUND/BOULDER CLAY
(Drained Behaviour)
MADE GROUND/BOULDER CLAY
(Undrained Behaviour)
γ
(kN/m3)
φ’
(°)
20
34
c’/cu
Avg D10
(kN/m2) (mm)
k
(m/s)
0
0.02
4 x 10-6
20
-
cu = 75
SAND AND GRAVEL
20
35
0
0.2
4 x 10-4
ASSUMED PUDDLE CLAY
CANAL LINING
17
-
cu = 10
-
1 x 10-8
LIMESTONE
21
35
500
-
1 x 10-7
Table 1 - Ground engineering parameters for S17-N3 Pier 1 temporary works
Figure 2 - Particle size distributions for Glacial Till
Figure 3 - Particle size distributions for Glacial GRAVELS
15
Figure 4 - Temporary 45° slope adjacent to the Royal Canal
Figure 5 - Section through proposed temporary soil nailed slope adjacent to the Royal Canal
16
Figure 6 - Elevation on proposed temporary soil nailed slope adjacent to the Royal Canal
Figure 7 - Photograph of as-constructed soil nail stabilised steepened embankment prior to casting of
Pier 1 base
17
M50 PPP UPGRADING CONTRACT 2
TEMPORARY WORKS AT S17-N3 PIER 1
SIDESLOPE TOTAL DISPLACEMENTS - MONITORING RESULTS
0.010
0.009
Red Trigger Limit
M1 Lateral
0.008
M1 Vertical
0.007
M3 Lateral
0.006
M3 Vertical
0.005
Amber Trigger Limit
M5 Lateral
0.004
M5 Vertical
Movement (m)
0.003
0.002
0.001
0.000
-0.001
-0.002
-0.003
-0.004
Amber Trigger Limit
-0.005
-0.006
-0.007
-0.008
-0.009
Red Trigger Limit
-0.010
13/08/09
14/08/09
15/08/09
16/08/09
17/08/09
18/08/09
19/08/09
20/08/09
21/08/09
22/08/09
23/08/09
24/08/09
25/08/09
26/08/09
27/08/09
28/08/09
Date
Figure 8 - Movement monitoring of soil nailed cutting adjacent to Royal Canal
M50 PPP UPGRADING CONTRACT 2
TEMPORARY WORKS AT S17-N3 PIER 1
PIEZOMETER AND EXCAVATION - MONITORING RESULTS
Upper Design
Level - P1
52
Lower Design
Level - P1
Canal Level (mOD)
Upper Design
Level - P2
Lower Design
Level - P2
Piezometer 1
51
Recovery from
instrument installation
Initial baseline readings
Piezometer 2
P1 pumping trial
P1 recovery after
pumping trial
Passive dewatering during initial excavations
50
49
25/07/2009
01/08/2009
08/08/2009
15/08/2009
22/08/2009
29/08/2009
Date
Figure 9 - Groundwater monitoring of soil nailed cutting adjacent to Royal Canal
18
Figure 10a - S04-N7 Bridge Slide
19
Figure 10b - S04-N7 Bridge Slide
20
Figure 11 - S04-N7 anticipated horizontal movements from Plaxis
Table 2 - S04-N7 proposed monitoring trigger levels
21
Galway City Centre
Bishop O’DonnellRahoon Road
R338 Seamus
Quirke Road
R338 Bishop
O’Donnell Road
Figure 12 - Location map showing the position of Bishop O’Donnell Road-Rahoon Road Junction
22
Observed wall
settlement / distortion
Figure 13 - Settlement of wall in the vicinity of Bishop O'Donnell Road-Rahoon Road Junction prior to improvement works
23
Figure 14 - Interpreted rock levels from seismic refraction / MASW correlation
24
Figure 15 - Cone penetration results for the area around Bishop O'Donnell Road-Rahoon Road Junction
25
Figure 16 - Temporary sheet pile solution for excavate-and-replace at Bishop O'Donnell Road-Rahoon Road Junction
26
Figure 17 - Output from FREW (SLS) for temporary sheet pile wall
Figure 18 - Excavation near the base of the sheet pile wall
27

the role of geotechnical temporary works on infrastructure

Transcription

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