Deep excavation with multi anchored diaphragm wall

Proceedings of the 15th European Conference on Soil Mechanics and Geotechnical Engineering A. Anagnostopoulos et al. (Eds.) IOS Press, 2011 © 2011 The authors and IOS Press. All rights reserved. doi:10.3233/978-1-60750-801-4-1485

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Deep excavation with multi anchored diaphragm wall Excavation profonde avec un mur de soutènement ancrée plusieurs fois J. Josifovski 1 , S. Gjorgjevski and M. Jovanovski University Ss. Cyril and Methodius, R. Macedonia

ABSTRACT The deep excavation in built-up urban areas with buildings and streets surrounding the site makes them quite formidable problems. Such a project of eight storey administrative-residential building located in the city centre of Tirana, Albania has been carefully analyzed. The project anticipates a 20.5m deep excavation to be carried out with six underground storeys. The task becomes even more demanding having in mind the fact that the site is surrounded with six and eight storey buildings with basements. The deep excavation pit with dimensions 27.7 36.9m is secured by a multi anchored diaphragm wall. The retaining system as a temporary support has to ensure the stability of the soil and enable undisturbed excavation. The diaphragm wall has been analyzed in several phases using the finite element method to obtain the shear, moments, displacements and support reactions under earth and water pressure on different levels. In this paper the numerical modelling of the 20.5m deep excavation is presented with some conclusive discussions. RÉSUMÉ Les excavations profondes dans les agglomérations des zones urbaines avec des bâtiments et rues entourant le site constituent des problèmes tout à fait formidables. Un tel projet de huit bâtiments administratifs résidentiels situés dans le centre-ville de Tirana, en Albanie a été soigneusement analysé. Le projet prévoit une excavation profonde de 20.5m à effectuer pour la construction de six étages souterrains. L’épreuve est encore plus exigeante ayant à l'esprit le fait que le site est entouré de six et huit bâtiments avec plusieurs étages avec sous-sol. Le trou de l’excavation profonde avec des dimensions 27.7 36.9m est assuré par un mur de soutènement ancré plusieurs fois. Le système de rétention comme un soutien temporaire doit assurer la stabilité du sol ainsi que de permettre une excavation non perturbée. Le mur de soutènement ancré a été analysé en plusieurs phases selon la méthode des éléments finis pour obtenir le cisaillement, les moments, les déplacements et les réactions d'appui sous la pression des terres et de l'eau à différents niveaux. Dans cet article la modélisation numérique de l’excavation de 20.5m de profondeur est présenté avec quelques des discussions concluantes. Keywords: deep excavation, multi anchored diaphragm wall, numerical analysis, finite element method

1

INTRODUCTION

For construction of eight storey administrativeresidential building in the city centre of Tirana, an excavation of 27.7 36.9m pit was necessary. The building is planed with six underground sto1

Corresponding Author.

reys: bottom floor as a basement, next three floors are parking lots while the top two floors are shopping and administrative premises. All together an excavation of 20.5m has to be executed, see Figure 1.

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J. Josifovski et al. / Deep Excavation with Multi Anchored Diaphragm Wall

phase, namely the construction of the diaphragm wall enabled the excavation of the pit. The excavation and anchoring has been executed in six continuous phases connected to the excavation depth h as described in Table 1. Table 1. Excavation phases.

Figure 1. Multi anchored diaphragm wall section AA(left) protecting the excavation.

The excavation is secured by a retaining system with a task to ensure the soil stability. The diaphragm wall with thickness d = 0.8m total length of L = 129.2m and height H = 25m has only a temporal character. The diaphragm wall has been comprised out of 2.7m reinforced concrete segments. The supporting structure represents a multi anchored diaphragm wall with height of 20.5m plus additional 4.5m depth bellow the pit. The supports are pre-stress anchors which introduce additional stabilizing forces into the system. Depending on the requirements an anchor type TTS15 with 5, 6 and 7 treads has been chosen. Every cable is comprised of 7 wires with 5mm in total of 15mm. The horizontal anchor spacing is set to Ls = 2.7m while the vertical is hs = 3m. The anchors are positioned with inclination of D 150 and they vary in total length. 2

CONSTRUCTION PROCEDURE

The excavation and construction works had been performed according to carefully devised procedure [1]. To ensure a safe excavation first a diaphragm wall has been constructed. Specialized cutter machinery was employed to excavate the wall. After positioning of the reinforcements a continuous concrete pour through fixed pipes has been performed. The completion of the first

Phase

1

2

3

4

5

6

h (m)

-4

-8

-11.5

-14.5

-17.5

-20.5

Parallel to excavation a dewatering of the pit has been performed using pumps with sufficient capacity and number. The anchoring is performed on every level in counter-clockwise direction with rotational drilling and piping (130180mm). The anchors free length is variable at different sections and depths. The grouting has been performed using a cement injection mass with 3% bentonite. After reaching the hardening condition of the injection mass the tensioning process of the anchors can start. Positioning all anchors at certain level enables the excavation of the next phase [2]. To ensure that the design values for the anchor force are properly introduced it is necessary that 10% of the total anchors fulfill an acceptance test while 5% had to be tested on lock-off load [5].

3

GROUND CONDITIONS

Enough field investigations and laboratory tests have been performed to be able precisely to define the ground conditions with the material properties accordingly. The ground profile is comprised of 10 almost horizontal soil layers. In the numerical model they are represented by the following material parameters: Jas unit weight, Qas Poisson’s ratio, Mv as a compressibility modulus and strength parameters given through c as cohesion and I as angle of internal friction. They are presented in Table 2 for every lithological unit, separately. Table 2. Soil layer properties. Type

h (m)

(kN/ m3)

Q

Mv (MP a)

c (kPa)

I

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N CI ML GW ML CL CL M M M

1.0 3.4 5.9 6.8 14.5 16.0 17.5 20.0 24.0 40.0

17 19.3 19.1 19.0 19.6 19.6 20 22 24 24

0.3 0.3 0.31 0.34 0.32 0.30 0.28 0.27 0.26 0.26

3 10 8 11 11 12 18 25 45 55

5 30 10 25 15 5 45 150 200 250

18 18 20 18 21 24 25 30 32 34

The top layer (N) is a man-made embankment brownish silty clay containing pieces of bricks and roots with thickness of around 0.7m; layer (CI) is silty clay mixture with yellow colour medium dense with average thickness of 2.4m; layer (ML) is clayey silt predominate brown colour medium dense with thickness from 2.0 to 2.5m; layer (GW) is sandy gravel with local presence of claylike matrix with thickness from 0.5m to 0.7m; layer (CL) is composed of silty clay mixtures medium low dense yellow colour encountered at 4.5m up to 9m depth; layers (M) are Neogene’s deposits composed by claylike Marls to highly weathered alveoli. The underground water is present on 4 to 5m below ground surface in layers (GW) and (ML) while top layers and (M) are with low permeability and relatively dry.

4

NUMERICAL ANALYSIS

In order to obtain a realistic simulation of the excavation the problem was analysed using the finite element method on two- and threedimensional models. The popular program Plaxis specialized for geotechnical engineering has been proven as efficient in combination with analytical solutions. The program enables simple but efficient spatial modelling of different structural elements and accurate material definition. The ground stress-strain state with the occurring soil effect during the excavation process has been simulated on a two-dimensional planestrain finite element model. The soil material is discretized using the Mohr-Coulomb model while the concrete diaphragm wall with linear material law. The spatial discretization had been varied depending on the situation but in general triangular plane elements with 15 nodes had been

used for the soil and beams for the structural elements of the wall and anchors [3]. 4.1

Excavation

Since different ground and loading conditions exist on the construction site, all four profiles representing the four sides of the pit supported with the anchored diaphragm wall had to be analysed. They are distinguished by the section name as or BB followed by the subscripts ’l‘ for left or ’r‘ for right side. The stage construction has been simulated in six continuous phases, where the initial phase represents the calculation of the ground stresses with constructed diaphragm wall. The excavation scenario from Table 1 is illustratively presented in Figure 2 for wall section AAl.

Phase 1

Phase 2

Phase 3

Phase 4

Phase 5

Phase 6

Figure 2. Finite element mesh of wall in section AAl for construction phase (1-6).

First of all a rough estimate of the necessary anchor (stabilizing) forces is obtained through finite element analysis using fix-end anchors. Thus, determining the resistant force Rk in every phase at different level of wall section AAl, see Table 3. Table 3. Characteristic anchor forces in wall section l. level (m)

Rk (kN/m’) Ph.2

Ph.3

Ph.4

Ph.5

Ph. 6

1488 -4 -8 -11.5 -14.5 -17.5


240.3

146.2 541.8

155.1 493.6 401.2

158.0 491.2 422.4 401.2

164.7 492.2 417.2 502.5 470.5

Besides the anchor force the calculation is able to control the overall stability for every phase of the excavation. 4.2

Anchors Rk = (2rS.L.(pv .tgM+ c))/B pv = (z1+z2) / 2.J z1 = h+x.tg (D z2 = z1+L.sin (D B = (1-sin (M)) sin (M-2D))/cos(2M

The anchor look-off load P0 has been determined according to EN 1537 as P0 = 0.60 Ptk

(1) Figure 3. Ground anchor.

where Ptk is the characteristic load capacity of tendon. In the case of anchor type TTS15 with Ptk = 350kN, produces a 210kN as a lock-off force of one thread. The total anchor force is N (number of cables) times the thread force. According to calculated characteristic forces Rk which correspond to the required stabilizing force the following anchor types are chosen, see Table 4. Table 4. Anchor type in wall section l. level

P0

The grouted body is positioned in Marls (below depth 10.5m) with bond length of Lb = 15m. This calculation determines many important parameters, namely the design resistant force Rd = Rk /JR of anchor where JR is a partial factor of anchor resistance [4], see Table 5 Table 5. Soil and anchor properties. Cohesion

c

Internal friction

I

30

150 0

kPa 0.523

(m)

Anchor type

(kN/m’)

(kN)

Unit weight

J

19.2

kN/m3

-4

4TTS15

311

840

Height

H

11.5

m

-8

7TTS15

544

1470

Distance

x

10

m

-11.5 -14.5 -17.5

6TTS15 7TTS15 6TTS15

467 544 467

1260 1470 1260

Bond length

Lb

15

m

Angle to horizontal

D

150

0.262

Width

B

1.3

m

Start depth

z1

14.18

m

End depth

z2

18.06

m

Pressure

pv

309.52

kPa

Resistance force

Rk

2323.44

kN

Partial factor Design resistant force

JR

1. 35

/

Rd

1721.06

kN

Additional analytical calculations had been performed in order to determine the maximal resistant force of the single anchor following the classical solution after P. Lendi, see Figure 3.

To obtain more realistic results a second finite element analysis had been performed on a model where the so-called ’node-to-node‘ anchors dis-

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cretized with tendon and grouted body elements, see Figure 4.

Figure 6. Effective stresses of wall section AAl (phase 6: mean shadings).

Figure 4. FE deformed mesh in section AAl (phase: 6 ).

In the finite element model the anchors have been pre-stressed with the determined lock-off force from Table 4. The anchors are positioned at 15 degrees of angle with length which is given in Table 5. The computation leads to a solution of the displacements, see Figure 5.

A slight stress concentration can be seen in the toe of the diaphragm wall while in the soil next to the wall the mean stress ranges from p’= 150 - 400kPa. Finally, the total anchor force had been determined, see Table 7. Table 6. Anchors in wall section AAl. level (m) -4 -8 -11.5 -14.5 -17.5

P (kN) 831.6 1455.3 1247.4 1455.3 1247.4

Anchor type 4TTS15 7TTS15 6TTS15 7TTS15 6TTS15

EA (kN) 109952.5 109952.5 109952.5 164850.0 192325.0

L=Lf+Lb (m) 24.5+15 12.5+15 10.5+15 8.5+15 7+15

where P is the anchor force, EA is the axial rigidity of the anchor, L is the total length of the anchor as a sum of free Lf and bond Lb length. 4.3 Figure 5. Horizontal displacements in section AAl (phase 6: shadings).

The largest displacements are close to the position of the grouted body between the second and fourth anchor. The displacements of the wall are in the range from 2 - 4cm which has no implications on the global stability. This fact has been also proved by the distribution of the effective mean stresses in the ground, see Figure 6.

Diaphragm wall

Beside the soil stresses and displacement the finite element analysis determines the deformation and internal forces of the diaphragm wall. The maximal calculated horizontal displacement of the wall is 41.9mm at the depth of around 14m which concurs to the results presented in Figure 5. Interesting to mention is the fact that the measured displacements during the excavation had not excide the calculated ones. Moreover, the internal forces in the diaphragm wall section AAl are presented as envelopes covering all six phases of excavation. The diagram of axial and shear force envelopes together with the bending moments are presented in Figure 8.

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recordings had only confirmed the predictions made by the numerical analysis. The construction started in September 2009 and finished in March 2010, see Figure 9.

-1.37MN/m

(a)

503.5kN/m

(b)

387.9kNm/m

(c)

Figure 8. Envelops of (a) Axial forces N, (b) Shear forces Q and (c) Bending moment M in wall section AAl.

The maximal value of the axial force is -1.37MN/m at the foundation depth of around 20.5m. The shear force has a characteristic form of a saw with maximal value of 503.5kN/m. Both sides of the wall are tensioned in different phases of excavation with 387.9kN/m/m as a maximal bending moment. According to internal forces the design of the reinforced concrete sections has been performed according to EN 1992 with the recommended partial factors.

5

CONCLUSIONS

The deep excavation in highly urbanized area such as the city centre of Tirana represents very formidable task. In the current project a technical solution of multi anchored diaphragm wall has been proposed to secure the excavation of 20.5m pit. This paper describes the numerical modelling process by offering some conclusive discussions. A two- and three dimensional finite element analyses had been performed with an objective to realistically simulate the deep excavation behaviour. Some difficulties of not quite precise regulative have been encountered. Nevertheless, the proposed model has been able describe all the effects in the process of excavation in difficult material and loading conditions. At all time during the execution of the work the diaphragm wall has been instrumented with inclinometers and geodetic markers to measure the deformations. The

Figure 9. Deep excavation with multi anchored diaphragm wall in Tirana.

REFERENCES [1]

[2]

[3]

[4] [5]

German Society for Geotechnics (DGGT) 2003. Recommendations on Excavations, Ernst & Sohn Verlag fur Architektur und technische Wissenschaften GmbH & Co. KG, Berlin, ISBN 3-433-01712-3. H.G. Kempfert and B. Gebreselassie, 2006. Excavations and Foundations in Soft Soils, SpringerVerlag Berlin Heidelberg , ISBN 540-32894-7. D.M. Potts and L. Zdravkovic, 1999. Finite element analysis in geotechnical engineering: theory. Imperial College of Science, Technology and Medicine, Thomas Telford Ltd, ISBN 0-7277-2783-4. EN 1997-1 Eurocode 7: Geotechnical design - General rules. EN 1537 Execution of special geotechnical work Ground anchors.