PIANC E-Magazine - Sapir Engineering

Transcription

143
JULY
JUILLET
2011
ON COURSE
PIANC E-Magazine
Theoretichal and Experimental Investigation
on Underwater Water Ground Anchor
Barbados Screwdock update
Container Terminal Capacity:
new formula for the area required
News from the Navigation Community
The World Association for Waterborne Transport Infrastructure
Association Mondiale pour les infrastructures de Transport Maritimes et Fluviales
PIANC E-Magazine n° 143, July/juillet 2011
PIANC
‘ Setting the Course’
‘ Garder le cap’
ON COURSE
PIANC E-Magazine
143
J U L Y
JUILLET
2011
Responsible Editor / Editeur responsable :
Mr. Louis VAN SCHEL
Boulevard du Roi Albert II 20, B 3
B-1000 Bruxelles
ISBN: 978-2-87223-170-6
All copyrights reserved
EAN: 9782872231706
© Tous droits de reproduction réservés
E-MAGAZINE N° 143 - 2011
TABLE DES MATIERES
TABLE OF CONTENTS Message of the President
6
Message du Président
Maurizio Lenzi, Leonello Sciacca, Fausto Valmori,
Cesare Melegari, Fabio Maletti, Paola Campana,
Vincenzo Padovani, Theoretical and Experimental Investigation on Underwater Water Ground Anchor
11
Maurizio Lenzi, Leonello Sciacca, Fausto Valmori,
Cesare Melegari, Fabio Maletti, Paola Campana,
Vincenzo Padovani, Recherche théorique et expérimentale sur les ancrages géotechniques sous eau
Keith Mackie, Barbados Screwdock Update
31
Keith Mackie, Cale de radoub à vis
à la Barbade-actualisation
Maria Alejandra Gómez Paz, Container Terminal Capacity: New Formula for the Area Required
39
Maria Alejandra Gómez Paz, Capacité d’un
terminal à conteneurs: nouvelle formule de
détermination de la surface nécessaire
News from the navigation community
57
Des nouvelles du monde de la navigation
Cover picture:
Alignment of the tie rod with the project axis
and detail of the U.W.A. unit.
Photo de couverture:
Alignement du tirant avec l’axe du projet et
le détail de l’élément UWA
5
THEORETICHAL AND EXPERIMENTAL INVESTIGATION
ON UNDERWATER WATER GROUND ANCHOR
by
MAURIZIO LENZI
LEONELLO SCIACCA
ACMAR
Ravenna
SAPIR Engineering
Ravenna
E-mail: [email protected]
Website: http://www.acmar.it
Website: http://www.sapireng.it
FAUSTO VALMORI
CESARE MELEGARI
SAPIR Engineering
Ravenna
TECNIWELL
Piacenza
Website: http://www.sapireng.it
Website: http://www.tecniwell.com
FABIO MALETTI
PAOLA CAMPANA
Port Authority of Ravenna
Studio CAMPANA
Forli’
Website: http://www.port.ravenna.it
Website: http://www.acmar.it
VINCENZO PADOVANI
Studio Padovani, Ravenna
KEY WORDS
Underwater Anchors, Wharf Deepening, SoilAnchors Interaction, Field Test, Jet Grouting
MOTS-CLEFS
Ancrages sous eau, approfondissement de quai,
interaction sol-ancrage, essais de terrain, jet
grouting
1. INTRODUCTION
In a port environment, the deepening of harbours
is increasingly becoming a priority requirement,
11
necessary for guaranteeing the ongoing operability
of the wharves in the face of a continual increase
in traffic and in the tonnages of merchant and
cruise ships. Hence, it is now standard practice
that ports’ regulatory plans provide for harbours
to be dredged to significant depths compared to
mean sea level, in the order of 12.00 m for existing
berths and 15.00 m for new wharf construction.
Within this context, SAPIR Engineering, commissioned by the Ravenna Port Authority, has drawn
up a project to deepen the harbour bottom of the
wharves of the SAPIR Terminal and of the neighbouring docks to a depth of 12.00 m, identifying the
structural reinforcement of the existing wharves by
means of deep underwater tie rods as analternative
solution to the installation of new sheet piles. This
solution actually makes it possible to insert additional constraints in the areas where the stress in
the bulkhead is higher and it is a suitable option in
view of the stratigraphy of the soil of the site that
consists of layers of fine sand and layers of silty
sand.
Once the design concept was defined, the effort
was concentrated on developing the technology
required for the purpose and in this regard SAPIR Engineering identified in the MiniJet grouting
technique, already used by Tecniwell for the installation of tie rods by means of remote control, a
technology that is potentially suitable for use in an
underwater environment. It was therefore decided
to carry out the fine tuning and the evolution of
the technology for the marine environment of the
robot unit and of the related remote control software that controls the installation of the tie rod,
also motivated by the contract for works to consolidate the wharves announced by the Ravenna
Port Authority.
In the context of the onshore development of this
innovative technology, with the co-ordination and
the logistical support of SAPIR and under the aegis of the Port Authority, an experimental test site
(See Fig. 1) was set up within the Intermodal Port
of Ravenna alongside dock No. 18 of Darsena
San Vitale, having as its objective the installa-
tion, monitoring and testing of underwater tie rods
carried out with Tecniwell U.W.A. technology, the
acronym standing for ‘Underwater Anchors’. The
purpose of the experimentation carried out in the
period June-July 2010 was to confirm and validate
the feasibility of the proposed solution and the reliability of these tie rods, of new technological conception, for use in the reinforcement and construction of port wharves.
Having outlined the background scenario, this
technical note illustrates the theoretical and experimental study of underwater tie rods dealing
first with the theoretical analyses of the nonlinear interaction between tie rod and soil aimed at
identifying the correlation between the forces of
applied forces and the corresponding elongations
that were the subject of the measurements. Comments were then made on a series of tests carried
out with positive results both at the operating load
of 300 KN and at the nominal maximum tensile
stress of 630 KN. The tests were carried out on
seven tie rods of which three were installed at a
depth of 5.00 m principally to test the reliability of
the operating and remote control systems in the
presence of a significant hydrostatic pressure and
four were placed at a depth of 8.00 m to check the
load bearing capability of the tie rods positioned at
the depth called for by the SAPIR project (See Fig.
2 on the next page).
Fig. 1: Plan of Darsena San Vitale – SAPIR Area Ravenna
12
Fig. 2: Diagram of reinforcement using underwater tie rods
2. FEATURES OF
UNDERWATER ANCHORS
The underwater tie rods that are the subject of
this investigation have the unique feature of being
installed by operations directed from the surface
using a special piece of equipment positioned in
the yard behind the wharf. The system consists of
a tracked vehicle that effects the movement of the
machine installing the tie rod and that is composed
of a mechanical arm that uses a steel telescopic
metal to support the robotic unit for drilling/injecting positioned at the design depth and controlled
remotely with continuous remote monitoring of the
operations and of the injection parameters (See
Fig. 3 on the next page).
In the case under consideration the test tie rods
have a length of 18.00 m being made up of 6 mod-
ules each having a length of 3.00 m and connected by threaded pipe couplings having a double
sealing gasket and a diameter of 62 mm. The tie
rods have an active injected length of 16.00 m,
while the passive part of 2.00 m consists of the
length of the piece that extends out of the bulkhead (0.40 m), the thickness of the wall (0.60 m)
and the remaining free part of 1.00 m, and that
was in the test tie rods only, purposely not injected
close to the internal surface of the wall. The tie
rods are installed using the advancing jet grouting
technique that involves the drilling of a hole 90 cm
in diameter using a rotating doubled-headed drill
bit on which are mounted two 2.5 mm diameter
nozzles, one set at an angle of 45° and the other
at 90° to the rotational axis of the tie rod (See Fig.
4 on page 14).
The bulb is created by injecting a cement mixture
through the two nozzles at a pressure of about
13
Fig. 3: Illustration showing the operation of the underwater tie rods
A-B: Installation of underwater MiniJet tie rods from the yard using robotic systems
C-D: Detail of the remote controlled robotic drilling unit
Fig. 4: Diagram of the underwater tie rod injection and drilling system
14
400 bar, having a water/cement ratio of 0.75, in
this way creating a horizontal column with a diameter in the order of 40 cm with a cement absorption of about 100 kg/ml of tie rod. The bonding agent use, depending on the harshness of the
marine environment, is CEM IV-32.5 R pozzolanic
cement. The reinforcement of the underwater tie
rod is in turn composed of the same drill/injection pipe that has a hollow section with an external diameter of 51 mm, an internal diameter of 33
mm and a resistant section area of 12 cm2. The
reinforcing rods are of galvanised steel, have an
improved adhesion since they have a continuous
thread through rolling and they have a yield point
of 630 KN, as well as a breaking maximum tensile
stress of 800 KN.
For permanent tie rods the U.W.A. technology
then provides for the protection of the head by
means of a truncated cone seal that is coaxial to
the tie rod and an external hood, both made of
polyethylene, into which protective grease is inserted together with a zinc anode. It is also possible to carry out the retensioning of the tie rod,
as required for the permanent anchors by current
technical regulations, since the head of the tie rod
is directly accessible by the mere removal of the
protective hood.
Fig. 5: a) Site stratigraphy – b) Distribution of effective stresses
15
3. GEOTECHNICAL
CHARACTERISTICS OF THE SITE
The soil of the site where the test tie rods have
been anchored is marked by the presence in the
stratigraphic column of layers of fine sand and layers of silty sand having a medium density and that
extends from the surface down to a depth of about
15 m (See Fig. 5a on the previous page). The CPT
cone resistance of the soil in the layer that concerns the tie rod is of 4.0-5.0 Mpa with angle of
friction values [ j ] in the order of 30°-32° and coefficient of thrust at rest [K0] values, produced by
dilatometric tests, of 0.9 on average.
The geotechnical parameters reported below to
characterise the elastic plastic interaction between
the tie rod and the soil in terms of resistance and
deformability were therefore adopted in relation to
the stratigraphy of the site. For the ultimate shear
resistance [τlim] the simple mechanism activated
through friction of the effective lithostatic pressure [σ’v] acting at the depth of the tie rod was
considered to be the failure criterion. For the tie
rods located at a depth of 8.00 m this criterion furnishes a value of the shear resistance of the soil
τlim = σ’v,eq • tanϕ @ 50 KPa, corresponding to a
pull resistance of the foundation bulb of 1,000 KN,
σ’v,eq = [(1+K0)/2] • σ’v. being the average effective
normal stress that takes account of the distribution of the effective radial normal stresses along
the perimeter of the bulb (See Fig. 5b). Instead
for the initial elastic stiffness [KH] of the constraint
represented by the soil, a correlation typical in the
literature was used, a function of the final shear
resistance and of the nature of the soils, KH = 400
τlim/fb = 50,000 KPa/m, where fb = 0.40m is the
external diameter of the active bulb.
€
4. NONLINEAR ANALYSIS –
TRANSFER CURVE MODEL
The model taken as a reference for evaluating the
structural responses of the underwater tie rod is
that of an element with a linear elastic behaviour
(the steel bar) restrained by means of springs with
an elasto-plastic behaviour that simulate the reaction of the surrounding soil. The constitutive law
of the bar of steel is in turn identified by its axial
rigidity, EAs, where Es is the elastic modulus of the
16
steel and As is the resistant section area of the
rolled bar.
Instead the constraint offered by the soil is depicted by the technique of transfer curves in which the
nonlinear constitutive bond between the applied
shear stress [ t ] and the slippage of the interface
[u] is modelled by means of the function (See Fig.
6):
in which:
KH = the initial elastic rigidity of the constitutive bond
= interface shear resistance
τlim Fig. 6
The stiffness of the constraint decreases proportionally to the mobilised tangential tension with
law K NL (u) = K H ⋅ [1− τ(u) /τ lim ] .
At the interface between the bulb and the soil, due
to the compatibility of the horizontal movements,
the slippage of the tie rod against the surrounding
undisturbed soil matches the axial displacement
u(x) of the current section of the steel bar. In fact
it is possible to disregard in quantitative terms the
relative slippage [ub = τ/Κb, Κb = 2Gc/fbLn(fb/fs)]
induced by the shear stresses that transfer the
load from the internal surface to the external surface of the bulb by virtue of the significant rigidity
conferred to it by the shear modulus [Gc] of the
cemented column. Remembering then the elastic
constitutive law that binds the axial forces and the
strain in the bar (N = EAsu’, u’ = du/dx) and the
external perimeter of the bulb’s section being represented by pfb, the equation of equilibrium in the
axial direction (See Fig. 7):
(N + dN) − N − τ ⋅ pf b ⋅ dx = 0
then assumes the following differential form:
€
the following form:
d2 u
E s As 2 − pf b ⋅ K NL (u) ⋅ u = 0
dx
du ui +1 − ui−1
=
2Δx
dx
d2 u ui +1 − 2ui + ui−1
=
Δx 2
dx2
and indicated with:
€
€
Fig. 7
that once integrated with the surrounding €conditions:
E s Asu'(0) = 0 at the free end of the tie rod
E s Asu'(L ) = P in the section applying the pull
€
€
€
of length Δx = L/m to which correspond n = m+1
consecutive joints in which to look for the unknown
displacements. Taking account of the fact that to
the finite differences the derivatives calculated in
the i-th joints (i = 1, n) of the model (See Fig. 8)
assume
provides the required field of displacement. Since
this is a nonlinear problem, the solution must be
found numerically by using, for example, the finite
differences method. With this technique the length
L of the tie rod is subdivided into m finite elements
€
d2 u ui +1 − 2ui + ui−1
=
Δx 2
dx2
the interaction parameter is deduced, for the i-th
joint of the model (i=1,n), the recurring equation:
ui +1 − [2 + r i (ui )]⋅ ui + ui−1 = 0
In this way, by assigning to the ends of the tie rod
the stated surrounding conditions, the following
system of equations is obtained in which the coefficients of the system matrix match less the factor
EsAs/Δx with those of the stiffness matrix of the
finite element model (on the next page):
Fig. 8: Model of the tie rod-soil transfer curve interaction
17
€
⎤
⎤⎡ u1 ⎤ ⎡
⎡(1+ r1 /2)
0
−1
0
0
0
0
0
⎥
⎥⎢ ⎥ ⎢
⎢
0
(2 + r 2 )
−1
0
0
0
0
⎥
⎥⎢ u2 ⎥ ⎢
⎢ −1
⎥
⎥⎢ ui−1 ⎥ ⎢
⎢
0
0
−1
(2 + r i−1 )
−1
0
0
0
⎥
⎥⎢ ⎥ ⎢
⎢
0
0
0
−1
(2 + r i )
−1
0
0
⎥
⎥⎢ ui ⎥ = ⎢
⎢
⎥
⎥⎢ ui +1 ⎥ ⎢
⎢
0
0
0
0
−1
(2 + r i +1 )
−1
0
⎥
⎥⎢ ⎥ ⎢
⎢
0
0
0
0
0
−1
(2 + r n−1 )
−1 ⎥⎢un−1⎥ ⎢
⎥
⎢
0
0
0
0
0
−1
(1+ r n /2)⎥⎦⎢⎣ un ⎥⎦ ⎢⎣P ⋅ Δx / E s As⎥⎦
⎣⎢
Once resolved by a process of iteration, the coefficients of interaction ri depending on the level of
displacement ui mobilised, for each level of load
the system of equations produces the displacements of the joints (and in particular of that from
the application of the pull) on the basis of which it
is possible to obtain the distribution of the shear
stresses acting on the bulb surface and of the normal forces along the axis of the tie rod.
5. DESIGN PARAMETERS AND
NUMERICAL ANALYSIS RESULTS
The design parameters used to simulate the underwater tie rod solution are reported in Table I.
The strength of the tie rod is therefore identified by
the 630 KN yield point of the bars, a value taken
as the threshold level for the load tests. Therefore, the proposed sizing establishes the need to
comply with a criterion of hierarchies of the ductile strengths, the ultimate unthreading load of the
foundation bulb being greater than the yield point
of the steel bars.
The result of the numeric analysis, carried out with
the nonlinear model using 32 finite elements, is reported in Table II and in Fig. 9 on page 19. As can
be seen the elongation of the active part of the tie
rod is of the order of 5.00 mm under an operating
load of 300 KN, corresponding to a stiffness of the
constraint of 60 KN/mm, and of 15 mm under the
maximum test load of 630 KN. This result highlights the notable stiffening effect produced by the
columnar treatment of the soil and typical of load
bearing elements through lateral friction, an effect
as a result of which the elongation of the tie rod is
reduced at the operating load to about ¼ of that of
a free bar subjected to a constant axial force.
Table II: Theoretical elongation values
in relation to load
Table I: Design parameters for underwater tie rods in the Sapir test site
18
Fig. 9: Chart showing Load – Theoretical elongation of active parts of U.W.A. tie rods
Fig. 10: Distribution of adhesion tensions along the axis of the tie rod
19
€
The distribution provided by the nonlinear analysis for the shear stresses acting at the tie rod-soil
interface is in turn reported in Fig. 10 on the previous page. This diagram shows how at the operating load (300 KN) the nonlinear behaviour of the
soil increasingly involves the front length close to
the pull head, where the elastic solution provides
a strong concentration of tangential forces in a
limited length introductory part and with an amplification compared to the average value equal
to λ = K H pL2 / EA = 5.64 , forces that with the
plasticisation are redistributed in the area rearward. Conversely at the maximum test load (630
KN) the extent of the plastic deformation of the
soil becomes significant, as also highlighted by
the sideways movement of the point of inversion
of the curvature in the diagram of the adhesions
tensions that separate the zone with the nonlinear
behaviour from the one that is still substantially in
an elastic field.
6. CONSTRUCTION PHASES
AND OPERATING PARAMETERS
OF UNDERWATER TIE RODS
a) Once the theoretical reference framework had
been defined, the next step was to illustrate the
salient phases of the true and proper experimentation that is divided into two phases: the
first related to the installation of the underwater
tie rods and the second to the tensioning. The
first phase was carried out in the period from
June 16 to 28, 2010, while the second one took
place on July 29-30, 2010. For the installation of
the tie rods a robotic U.W.A. underwater drilling
unit (Photo 1-5) was used, supplied, by means
of a motorised pump for the high pressure jetting injection, by a cement grout preparation
unit that produces a mix based on the weight
ratios specified in the mix-design for the water
and the cement that is stored in a horizontal
silo. The operations to install the tie rod were
programmed by carrying out the drilling of the
concrete wall beforehand with a bit suitable for
the purpose. At the end of this operation the automatic system proceeded to insert the rolled
bars into the drilled out hole taking them from
a loader, then moving them forward by rotating
them and at the same time injecting the mixture
and, at the end of that phase, inserting the pipe
coupling. The installation of the tie rod was carried out at a speed of advancement [va] of the
order of 10-15 mm/s, with an injecting pressure
[pm] of about 400 bar, a velocity of the fluid in the
nozzles [vm] of 150 m/s corresponding to a flow
rate of the mixture [Qm] out of the two nozzles
of about 1.5 l/s. In the construction of the tie
rods all the phases of the production cycle were
properly monitored through the continuous acquisition of various operating parameters. In
this regard Table III reports the most significant
Table III: Operating parameters for construction of the underwater tie rods
20
values recorded during the course of the installation of the underwater tie rods.
a relationship in which:
b) On the basis of the injection parameters thus
acquired an estimate was made a posteriori of the
diameter of the horizontal jet grouting column, assessing beforehand the volume of the mixture injected knowing the absorption of the cement and
the specific weight of the mixture and subsequently, by means of a mass balance described below,
the volume of the bulb produced. In the case in
question the weight of the water and cement mixture turned out to be:
γ m = γ cγ w
(1− v)(1+ r)
= 16.0 KN /m3
γ w + rγ c
€
Photo 1: View of the machine
Photo 2: Detail of the U.W.A. unit
21
Photo 3: Detail of the remote control console
Photo 4: Alignment of the tie rod with the project axis
Photo 5: Positioning of the U.W.A. unit in the water
22
The volume of the mix injected was obtained in
its turn from the weight of the cement used per
linear metre of tie rod (C @ 1.00 kN/ml; r = A/C =
0.75) and the density of the grout as a ratio of the
quantity injected [C • (1+r)] and the specific weight
of the mix [γm], thus obtaining:
C ⋅ (1+ r) Qm
Vm =
@
= 0.11 m3 /ml
va
γm
€
Instead, to determine the volume taken up by the
€ the
bulb, one must take into account the fact that
formation of the jet grouting column takes place
primarily through the erosion and in situ mixing of
the soil with the grout of water and cement. In this
event, the volume of the bulb being represented
by Vb and the porosity of the undisturbed soil by n,
the mass balance predicates that the useable fraction of the mix that is absorbed by the soil (aVm)
goes to fill the volume (nVb) of the initial spaces in
the soil, increased by the volume of the solid parts
(1-n) • bVb of the volume βVb of the soil at the site
that was removed from within the tie rod by the
pressure of the jet. The following is therefore obtained from this balance:
Vb =
€
a ⋅ Vm
= 0.14 m3 /ml
[n + (1− n)b ]
having adopted, in relation to the effective pressure operating at the depth of the tie rod and the
injecting pressure, α @ 0.70 for the useful percentage of mix absorbed by the soil that contributes
to the formation of the column, β @ 0.30 for the
percentage of soil removed by the pressurised jet
and n @ 0.40 for the initial porosity of the soil. The
diameter of the bulb is obtained as follows:
n fb =
4 ⋅ Vb
@ 0.40 m
p
7. OPERATING PHASES OF THE
TENSIONING OF THE
UNDERWATER TIE RODS
The tensioning of the tie rods was carried out by
means of a hydraulic jack driven by a two-stage
pump located on the surface and making use of
the support provided by the wall making up the
waterside face of the wharf. For each tie rod, after
preparation of the surface of the wall, a steel plate
was first of all put in place and tightly secured with
a nut against the wall. Then, onto the part of the tie
rod bar projecting out from the wall a pipe coupling
was screwed fitted with a piece of threaded bar inside of which had been inserted the hydraulic jack
which was in the form of a hollow cylinder, sliding it until its lower base was resting on the steel
plate. Onto the part of the connecting bar that protruded from the cylinder of the jack a second plate
was then fitted, as well as a second locking nut, in
this way completing the assembly of the tensioning device (See Fig. 11).
Fig. 11: Schematic representation of the tensioning device
23
Subsequently, the tensioning force was applied in
the various pull phases using the support provided
by the two steel plates on one side against the
fixed part and on the other side against the moveable part of the jack. During the tensioning cycles
measurements of the elongations of the tie rod
were also taken by means of a Vernier calliper, using as a comparison the travel of the piston of the
jack. The underwater operations described above
were carried out with the assistance of a team of
scuba divers from the Società Marine Consulting
di Ravenna, with a live video connection and continuous recording and real-time communications
via radio.
8. RESULTS FROM
THE TEST SITE
The results obtained with the experimentation
carried out on the SAPIR test site are reported
in a graphical form in fig. 12-14, which illustrate
the correlation between the forces applied and
the elongations of the test tie rods, as well as the
sequence in which the loads were applied. To be
specific, the programme of tensioning that was followed, where possible also in relation to the kinds
of breakage that resulted, was as follows:
- pretension of 90 KN in order to permit the settling of the steel plates against the surface of
the wall, taken to be point zero as a reference
for the measuring of the elongations of the subsequent cycles;
- the carrying out of the first cycle rising in steps
of 90 KN up to 360 KN, i.e. equal to 1.2 times
the working load of the tie rod in order to simulate the test tensioning of an actual tie rod;
- release of the first cycle down to 180 KN and
then to 90 KN;
- execution of the second load cycle again taken
up in successive steps of 90 KN up to the nominal yield point of the bars of 630 KN, then reduced during the course of the tests to slightly
lower levels (600 KN) in order to be able to carry
out a subsequent third load cycle;
- release of the second cycle in three phases
(360-180-90 KN);
- execution of the third cycle in successive steps
of 90 KN up to breakage, when it occurred, of
the coupling pipe.
Fig. 12: Load curve – elongations of tie rod 2/A at a depth of 5.00 m
24
The detail of the results obtained in terms of resistance to breakage in the course of the load tests
is reported in Table IV.
Table IV: Results of the load tests of the underwater tie rods
As can be noted from the attached diagrams the
load cycle results were regular, with elongations
at operating loads in line with the theoretical forecasts and with modest residuals on the releasing
of the first cycle. The areas of the hysteresis loops
are small and the reloading stress paths are substantially elastic up to the threshold of the force
reached in the preceding load cycle. Furthermore,
the breakage of the tie rod, where it occurred, involved the pipe coupling with force values even
greater than the nominal yield point of the bar of
630 KN. For traction forces above this threshold
the increments in the elongation are deemed to be
attributable in large measure to the plastic yielding
of the mechanical parts, bar and pipe coupling.
Fig. 13: Load curve – elongations of tie rod 1/B at a depth of 8.00 m
25
Fig. 14: Load curve – elongations of tie rod 4/B at a depth of 8.00 m
Fig. 15: Comparison between theoretical and experimental
elongations of the 4/B tie rod sample located at a depth of 8.00 m.
26
In order to interpret the elongations of the tie rods
correctly, it should be remembered that for the
methods adopted for taking the measurements,
the only ones objectively and operationally possible in an underwater environment, the values
recorded are inclusive not only of the outward
elongation of the active and passive part of the
tie rod, but also of the displacement of the wharf
inward, it not being possible to separate the individual measurements of these two contributing
elements. On the other hand, an indirect estimate
of the displacement of the wall was carried out by
means of structural analysis of the bulkhead, an
analysis that showed how the inward shift of the
wall was limited to values of few millimetres under
the action of the maximum test load.
As far as the overall assessment of the validity
and reliability of the tests carried out is concerned,
Fig. 15 shows a comparison, for tie rod No. 4/B
installed at a depth of 8.00 m, of the experimental
measurements of the elongations with the corresponding theoretical curve obtained, adding to the
elongation provided by the nonlinear analysis of
the active length of the tie rod (L = 16.00 m) the
elastic elongation of the free length (Lo = 2.00 m)
and taking into account a hardening plastic behaviour of the steel beyond the yield threshold. As
can be noticed, the back-analysis curve faithfully
reproduces the envelope of the maximum values
of the elongations of the three load cycles of the
experimental curve. Thus the design parameters
adopted for the modelling with the transfer curve
of the interaction between the tie rod and the soil
[τlim = 50 KPa; KH = 50,000 KN/m3] interpret correctly the behaviour of the underwater tie rod in
the test site.
These circumstances, together with the salient result of the positive check during operation of the
load-bearing capacity of the tie rod according to
the hierarchy of the resistances forecast during the
design, show the overall set of results obtained to
be reasonable and consistent and the outcome of
the tests to be satisfying and exhaustive.
9. CONCLUDING COMMENTS
The technical note has illustrated the summary of
the results of an experimental test site set up for
the testing of underwater tie rods designed by SAPIR Engineering for the deepening of the harbour
of the Intermodal Port of Ravenna. For the below
head construction of these anchors an innovative
technology called U.W.A. – Underwater Anchors –
was used, a system perfected by Tecniwell, which
operates directly from the yard surface using the
technique of progressive jet grouting. The note
then presented the back analysis of the results of
the experimentation adopting a nonlinear transfer
curve constitutive model of the soil to assess the
resistance and the rigidity that were triggered by
the interaction of the tie rod and the soil at various
levels of load.
The salient outcome of the experimentation was
both the correct installation of the tie rods and the
confirmation of their load bearing capacity of 630
KN in accordance with what was assumed in the
structural and in the geotechnical design. It was
also confirmed that the axial displacements necessary to put the underwater tie rods under stress
was of just a few millimetres at the operating load
(5-6 mm at 300 KN) and of a couple of centimetres at the maximum test load (15-20 mm at 630
KN). Therefore, there are significant differences
from tie rods with strands that, being usually installed near the top of the wharf, require the passive parts to be of much greater length and hence
call for pre-tensioning. This characteristic of deep
underwater tie rods of limited deformability allows
them therefore to be used both as an elastic constraint put under stress by a small deflection of the
structure, as well as an active constraint, it being
possible to carry out pre-tensioning of it. It should
also be stressed that the positioning at depth below the sea level of the tie rod allows it to be used
very effectively, since it is able to provide the required reactions where the flexional stresses are
the greatest. Moreover, the possibility of installing
tie rods in positions closer to the seabed requires
that the internal part of the tie rod at the point of
pressure should be of limited length, thus making
available a longer useable length of the active part
of the anchor.
As far as possible future developments are concerned in connection with U.W.A. technology, the
experimentation has shown that it is operationally
possible to install tie rods with higher capacities by
varying the diameter of the reinforcing bar and the
length of the active part. Nevertheless, this option
needs to be verified in realistic terms in relation to
the resistance of the structure that is called upon
27
to absorb the forces transmitted by the tie rods
and to the onerousness of the structure that must
share the applied load among the concrete panels or the steel sheet piling so depending heavily
on the construction method of the wharf that is to
be reinforced. The development of greater load
bearing capacities or the use of additional rows of
underwater tie rods could instead become of interest after the necessary ad hoc evaluations, both
structural and geotechnical, for greater heads of
water, such as those called for by the ports’ regulatory plans for the harbour bottoms of wharves
being constructed from new. Therefore, this outcome offers a range of applications for this innovative technological solution that is of undoubted
interest, also in the light of the validation provided
by the results from the SAPIR test site.
Ravenna, July 31, 2010
10. TECHNICAL SCHEDULE OF
THE EXPERIMENTAL ACTIVITIES
OF THE SAPIR TEST SITE
11. BIBLIOGRAPHY
- UNI EN 1537 (2002): “Execution of special geotechnical works: Ground anchors”, Milan.
- UNI EN 12716 (2003): “Execution of special
geotechnical works: Jet grouting”, Milan.
- D.M. 14/01/2008 (2008): “New Technical Regulations for Construction”, Ministry of Infrastructure, Rome.
28
SUMMARY
This paper illustrates the results of a theoretical
analysis and experimental tests concerning underwater anchors (UWA). The anchors are built by
means of an innovative onshore technology which
operates all the executive steps in remote control.
This technology can be used especially in the re-
habilitation of port bulkheads. The construction
steps are illustrated together with the comparison
between the theoretical values of the elongations
of the anchors provided by a nonlinear model and
the experimental results of the load tests.
RéSUMé
Cet article décrit les résultats d’une analyse
théorique et de tests expérimentaux d’ancrages
géotechniques sous eau (UWA). Les ancrages
sont réalisés par télécommande à partir de la
terre selon une technologie innovante qui exécute
toutes les phases de construction. Cette technolo-
gie peut particulièrement être utilisée pour réhabiliter des parois portuaires. Chaque phase constructive est illustrée avec la comparaison entre
la valeur théorique, selon un modèle non linéaire,
d’allongement des ancrages et le résultat expérimental des essais de traction.
ZUSAMMENFASSUNG
In diesem Artikel werden die Ergebnisse einer
theoretischen Analyse und experimenteller Untersuchungen bezüglich Unterwasseranker (UWA)
dargestellt. Die Anker werden mittels einer innovativen on-shore-Technologie eingebaut, bei der
alle ausführenden Schritte durch Fernbedienung
ausgeführt werden. Diese Technologie kann ins-
besondere bei der Sanierung in Häfen eingesetzt
werden. Die Bauschritte werden vorgestellt,
zusammen mit einem Vergleich mit den theoretischen Werten der Ankerverlängerungen, die von
einem nicht-linearen Modell und den experimentellen Ergebnissen der Belastungstests geliefert
werden.
29

PIANC E-Magazine - Sapir Engineering

Transcription

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