3d modeling of the subsurface works ... - e-learning - unimib

EURO:TUN 2013 3

rd International Conference on Computational Methods in Tunnelling and Subsurface

Engineering Ruhr University Bochum, 17-19 April 2013

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3D modeling of the subsurface works beneath Rabat fort,

Morocco

Adriano Fava1, Marco Ghidoli

1, Francesco Gamba

1, Riccardo Castellanza

2,

David Betti3, Francesca Giussani

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1 Alpina Spa, Milano, Italy (www.alpina-spa.it)

2DISAT, Università degli Studi di Milano Bicocca, Italy

3SIPOS s.r.l., Italy

4ABC, Politecnico di Milano, Italy

Abstract

The Oudayas tunnel in Rabat, Morocco, lies on an urban dual carriageway designed

by the Bouregreg Valley Development Agency. The tunnel comprises a central

section built underground and two end sections built as covered trenches. The covered

trench section of the tunnel runs beneath part of the Oudayas monuments (17th

century). Tunnelling beneath the fort required for the use of very precise construction

and recourse to unusual technical methods, i.e. the under-excavation process by

transferring twice the load of the building on series of micropiles. A series of 3D non-

linear FEM analysis was performed in order to predict the settlements of the building,

the loads in each micropile and the bending moments in the beams linking the

building and the micropiles. A specific calibration of the embedded pile approach was

conducted by comparing the numerical load-displacement curve with the data of full

scale in situ tests. A final model was implemented where the whole historical building

and the entire excavation process was modeled. Consequently 3D prediction of the

soil/structure interaction was obtained to validate the design phases and to adjust the

excavation sequences by data cross-comparison with remote and real-time monitoring

systems. The final monitored data were compared with the predicted ones.

Keywords: 3D numerical model, soil-structure interaction, embedded pile, construction stages

A. Fava, M. Ghidoli, F. Gamba , R. Castellanza, D. Betti, F. Giussani

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1 INTRODUCTION: DESIGN OF THE SUBSURFACE WORKS

The construction of Les Oudayas tunnel in Rabat (Morocco) is just one part of a

broader project to develop the whole of the Bouregreg valley.

In addition to the typical problems posed by underground excavations in built-up

areas with little overburden, the Oudayas tunnel design required complex studies to

be undertaken for the following interactions:

• interaction between the structure and adjacent buildings with significant

historic and artistic value;

• interaction between the two underground tunnels, which virtually touch over

a stretch of some 300 m, and the impact of excavations on historic structures,

buildings and highways.

The Oudayas complex comprises two historic buildings, the fort and the library

(Figure 1:a). They are built into the walls surrounding the Kasbah, built in the 17th

century, and the Les Oudayas Andalusian gardens. The trapezoidal-shaped fort is

adjacent to and partly built into the wall itself. At its base, this is some 2.5m thick.

The library is a more recent stonework structure, with barrel arches and pillars.

The surveys carried out in the walls showed that their construction had involved the

use of a “bag-type” technique, in which miscellaneous infill, which was not cemented

and thus highly compressible, was placed between two lateral masonry walls. In some

places, the load-bearing point of the walls followed the natural lie of the land. By

contrast, the foundation of the library’s pillars consists of distinct bases, made up of

stacked layers of calcarenite. All of the historic buildings’ foundations rest directly on

the calcarenite substratum and are mostly superficial, sometimes deeper.

Figure 1: a) Batiment Historique (BH); b) geological profile

A 3D modeling of the subsurface works beneath Rabat fort, Morocco

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The measures to ensure the safety of the historic buildings were designed to meet a

twofold requirement: firstly, making the wall structure as uniform and monolithic as

possible in order to support the differential settlements due to excavations, and

secondly respecting the historic and cultural value of these structures, avoiding

excessively invasive works. To achieve this, existing cracks were treated by saturating

them with lime injections, ensuring the wall as uniform and continuous. The

procedure was then completed by erecting a steel support structure inside the building

to buttress the walls and ensure adequate rigidity.

Geotechnical surveys conducted on the site of projected works in order to assess the

type and thickness of the foundation soil showed that the stratigraphy beneath the

Historic Buildings is characterised by the presence of three principal formations

(Figure 1:b): sands of various consistencies, from non-cohesive to cemented,

with highly variable degrees of alteration including soft calcarenite portions;

gravel with silt and clay, sometimes slightly cemented; plastic marls.

(a) (b) (c)

Figure 2: a,b) 3D and in plan views of BH interaction with tunnels; c) U-beam and micropile

for temporary supports

Construction of the covered trench beneath the historic buildings was carried out in

two phases. For the temporary phase, the structure was supported on micropiles; for

the permanent phase, support was provided by the slab of the tunnel roof.

• Temporary support of buildings on micropiles (Figure 3:)

The first phase consisted in transferring loads onto the ground located beneath the

levels of the subsequent excavation by means of deep foundation micropiles.

Joining the micropiles to the base of the walls involved adding intermediate

structural elements capable of taking up the loads and ensuring continuity. U-

shaped beams were thus installed along the entire length of the fort walls (Figure

2:c) in two successive stages: firstly, the base of the U-beam was formed by

replacing alternate blocks of the wall with reinforced concrete elements. The two


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beams forming the sides of the U were then continuously concreted in a second

stage. This gave the U-beam lengthwise continuity. For the library pillars, the

same U-beam method was used, combined with a kind of foundation baseplate

connected to the pillar using steel profiles and to the micropiles by means of a grip

system.

• Permanent support of the building on the slab (Figure 3:)

In the final phase, the structural loads were distributed to a foundation slab which

also served as the roof slab of the tunnel. During excavation, the slab rested on

three rows of micropiles, one at each edge and one in the centre, protected by

retaining walls (Figure 2:b). The purpose of these was to bear horizontal ground

loads, while the vertical loads were borne in full by the micropiles. To ensure the

structure performed as expected, the head of each micropile was directly

integrated into the concreting of the slab, while the top section of the retaining

walls was kept separate from the slab.

Figure 3: a) Phase 1: main construction stages; b) Phase 2: main construction stages;

2 3D NUMERICAL MODELLING

3D numerical models (Figure 4:) were developed by using the finite element code

Midas GTS® in order to predict the settlements and the state of stress of the BH and

of the new structures. All the excavation phases were simulated and the soil-structure

interaction was considered by modeling the whole BH.


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Figure 4: Phase 1: Geometry and mesh

Figure 5: Phase 2: Geometry and mesh

2.1 Calibration of load transfer on micropile

The micropiles were modeled by means of embedded pile elements. Their parameters

were calibrated referring to in situ load tests, by keeping the geotechnical parameters

and the ultimate load constant. The most suitable parameters are chosen to describe

the curve of Figure 6:

F2

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

0 10 20 30 40 50 60 70

Carico [t]

Abaissement de la tête du pieu [mm]

F2

Interface Free

Interface Bulbo

Load = 60 t

F2

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

0 10 20 30 40 50 60 70

Carico [t]

Abaissement de la tête du pieu [mm]

F2

.

Figure 6: Load test on a micropile and model of the load test

2.2 Construction stages

The temporary support of buildings on micropiles (Figure 3:) was modeled by means

of the following seven construction phases:

• I area excavation;


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• II area excavation, I part slab construction and covering of part of the I area;

• III area excavation, covering of the remaining part of the I area and of the II area;

• IV area excavation, II part slab construction and covering of the III area;

• III part slab construction and covering of the IV part slab;

• IV part slab construction;

• final slab construction.

On the other hand, the permanent support of the building on the slab (Figure 3:) was

simulated by five phases:

• casting of the piles of the slab;

• excavation of the I part of the tunnel;

• excavation of the II part of the tunnel;

• excavation of the III part of the tunnel (the last under the slab);

• excavation of the last part of the tunnel.

2.3 Soil-structure interaction: induced displacements

The settlements after some construction stages of the first phase are reported in Figure

7:, while those related to the second phase can be seen in Figure 8:.

Figure 7: The buildings settlements after phases 2, 3, 4, 7

It is observed that the maximum settlement after the slab construction, before the micropiles realization, is 4.5 mm. When neglecting the structural modelisation of the


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building, substituting it with the equivalent load, the maximum settlement resulted 7.3 mm. The nearly double value is due to the lack of the contribution of the stiffness of the structure.

Figure 8: The buildings settlements after phases 1, 3, 4, 5

The final settlement is 10.6 mm, with an increase of 6.1 mm respect to the last stage of the first phase mainly due to excavation of the tunnel.

2.4 Structural response of micropiles and foundation beams

The maximum axial force (Figure 9:) in the piles (376 kN) arises during the

excavation of the first zone. This value is the half of the one that can be obtained

without modeling the building.


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Figure 9: The axial forces in the micropiles after phases 1, 2, 3, 7

During the excavation of the tunnel (Figure 10:), the self weight of the BH moves

towards the new piles of the slab. The maximum load acting on the new piles is about

82 kN.

Figure 10: The axial forces in the piles after phases 2 to 5

Furthermore, the stresses in the foundation beam were evaluated by introducing in the

solid model a beam element with the same geometry and numerical discretisation of

the solid beam, but with a reduced elastic modulus E*=10

-6Ec, so that the

displacements (and consequently the curvature) of the beam were the same as those of

the solid foundation, without increasing its stiffness. The results of phase IV are


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shown in Figure 11:, where the maximum and minimum moments can be found as

316 kNm and -520 kNm.

Figure 11: Bending moments (10-6 kNm) of the foundation beam in phase IV

2.5 Stress induced in Rabat Fort

The coupled model allowed to evaluate the state of stress and strain in the historical

building. In order to evaluate the most critical areas of the structure, an elastic

behaviour was considered. In the following (Figure 12:-Figure 15:), the most

significant results in terms of principal tensile stresses are reported for some

construction phases.

Figure 12: Principal tensile stresses III after phases I and II

Figure 13: Vectors of the principal stresses and principal compressive stresses I after phase III


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Figure 14: Principal tensile stresses III after phase IV

Figure 15: Principal tensile and compressive stresses I, III after phase VII

The principal stresses in the building result in not particularly high values. Referring

to compressive stresses, they don’t exceed 1.96 MPa (Figure 13:), in particular

localised points, such as the corners of the holes of the windows and the joint edges

between two walls. The mean value of compressive stresses is about an order of

magnitude less than the maximum one. The most stressed elements during the

excavation process are the central columns of the library, with a compressive stress of

about 1 MPa.

The maximum tensile stresses is 1.73 MPa. Also in this case, this value can be found

in singular zones, that can be affected by local effects of the numerical model (Figure

14:). In any case, the onset of cracking associated to the low tensile strength of the

masonry of the fort would induce a stress redistribution in the nearby, without

preventing the correct structural behaviour. As in the case of compressive stresses, the

mean value of tensile stresses produced by excavation works is about an order of

magnitude less than the maximum one.

3 IN SITE WORKS AND MONITORED DATA

Some phase of the in site works are shown in Figure 16:.

The monitoring system was designed with the aim of keeping track of the behaviour

of surface structures (historic and newer buildings) with special attention paid to soil


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settlement and building displacement, as well as to soil behaviour round the

excavation and within the soil itself in terms of convergence and deformation.

Figure 16: Temporary work phases

The monitoring system confirmed the digital analysis forecasts, recording values

lower than forecast for all measurements performed.

Figure 17: Monitoring: Changes in settlement around the edge of the historic buildings

Figure 17: shows changes in the settlement measurements recorded along the

perimeter of the historic buildings every three months from installation through to

completion of the tunnels. The most critical conditions did not relate to settlement due

to excavation, but to heave following installation of the jet-grouting columns and, to a

lesser degree, the micropile injections. Checking injection parameters (pressure and


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volume) combined with continuous monitoring during the course of works played a

determining role in minimizing this heave, which did not exceed 15mm.

4 CONCLUSIONS

In this work a full couple soil-structure 3D interaction analyses has been conducted.

By considering the entire construction stage processes it has been possible to

evaluated the effects of complex underground excavation on the historical building

before the insite work; therefore the predictions could be considered in class A. The

comparison with monitored data have shown an agreement between numerical and

monitored data with a slight overestimation of numerical settlements.

3d modeling of the subsurface works ... - e-learning - unimib

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