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Deep excavation with multi anchored diaphragmwall

Excavation profonde avec un mur de soutènement ancrée plusieurs fois

J. Josifovski 1, S. Gjorgjevski and M. JovanovskiUniversity 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 prob-lems. Such a project of eight storey administrative-residential building located in the city centre of Tirana, Albania has beencarefully analyzed. The project anticipates a 20.5m deep excavation to be carried out with six underground storeys. The task be-comes even more demanding having in mind the fact that the site is surrounded with six and eight storey buildings with base-ments. The deep excavation pit with dimensions 27.7 36.9m is secured by a multi anchored diaphragm wall. The retaining sys-tem 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 reac-tions 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 constituentdes problèmes tout à fait formidables. Un tel projet de huit bâtiments administratifs résidentiels situés dans le centre-ville de Ti-rana, en Albanie a été soigneusement analysé. Le projet prévoit une excavation profonde de 20.5m à effectuer pour la construc-tion 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 bâ-timents avec plusieurs étages avec sous-sol. Le trou de l’excavation profonde avec des dimensions 27.7 36.9m est assuré parun mur de soutènement ancré plusieurs fois. Le système de rétention comme un soutien temporaire doit assurer la stabilité du solainsi que de permettre une excavation non perturbée. Le mur de soutènement ancré a été analysé en plusieurs phases selon la mé-thode des éléments finis pour obtenir le cisaillement, les moments, les déplacements et les réactions d'appui sous la pression desterres 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 Corresponding Author.

1 INTRODUCTION

For construction of eight storey administrative-

residential building in the city centre of Tirana,an excavation of 27.7 36.9m pit was necessary.The building is planed with six underground sto-

reys: bottom floor as a basement, next threefloors are parking lots while the top two floorsare shopping and administrative premises. Alltogether an excavation of 20.5m has to be exe-cuted, see Figure 1.

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

1485



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

protecting the excavation.

The excavation is secured by a retaining sys-tem with a task to ensure the soil stability. Thediaphragm wall with thickness d = 0.8m totallength of L = 129.2m and height H = 25m hasonly a temporal character. The diaphragm wallhas been comprised out of 2.7m reinforced con-crete segments. The supporting structure repre-sents a multi anchored diaphragm wall withheight of 20.5m plus additional 4.5m depth bel-low the pit. The supports are pre-stress anchorswhich introduce additional stabilizing forces intothe system. Depending on the requirements ananchor type TTS15 with 5, 6 and 7 treads has

been chosen. Every cable is comprised of 7 wireswith 5mm in total of 15mm. The horizontal an-chor spacing is set to L s = 2.7m while the verticalis h s = 3m. The anchors are positioned with incli-

nation of 150 and they vary in total length.

2 CONSTRUCTION PROCEDURE

The excavation and construction works had been performed according to carefully devised proce-dure [1]. To ensure a safe excavation first a dia-

phragm wall has been constructed. Specializedcutter machinery was employed to excavate the

wall. After positioning of the reinforcements acontinuous concrete pour through fixed pipes has been performed. The completion of the first

phase, namely the construction of the diaphragmwall enabled the excavation of the pit. The exca-vation and anchoring has been executed in sixcontinuous phases connected to the excavationdepth h as described in Table 1.

Table 1. Excavation phases.

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 pithas been performed using pumps with sufficientcapacity and number. The anchoring is per-formed on every level in counter-clockwise di-rection with rotational drilling and piping (130-180mm). The anchors free length is variable atdifferent sections and depths. The grouting has

been performed using a cement injection masswith 3% bentonite. After reaching the hardeningcondition of the injection mass the tensioning

process of the anchors can start. Positioning allanchors at certain level enables the excavation ofthe next phase [2]. To ensure that the design val-ues for the anchor force are properly introducedit is necessary that 10% of the total anchors ful-

fill an acceptance test while 5% had to be testedon lock-off load [5].

3 GROUND CONDITIONS

Enough field investigations and laboratory testshave been performed to be able precisely to de-fine the ground conditions with the material

properties accordingly.

The ground profile is comprised of 10 almosthorizontal soil layers. In the numerical modelthey are represented by the following material

parameters: as unit weight, as Poisson’s ra-tio, M v as a compressibility modulus and strength

parameters given through c as cohesion and asangle of internal friction. They are presented inTable 2 for every lithological unit, separately.

Table 2. Soil layer properties.

Type h

(m)(kN/

m3)

M v (MPa)

c

(kPa)

J. Josifovski et al. / Deep Excavation with Multi Anchored Diaphragm Wall1486



N 1.0 17 0.3 3 5 18CI 3.4 19.3 0.3 10 30 18ML 5.9 19.1 0.31 8 10 20GW 6.8 19.0 0.34 11 25 18ML 14.5 19.6 0.32 11 15 21CL 16.0 19.6 0.30 12 5 24

CL 17.5 20 0.28 18 45 25M 20.0 22 0.27 25 150 30M 24.0 24 0.26 45 200 32M 40.0 24 0.26 55 250 34

The top layer (N) is a man-made embankment brownish silty clay containing pieces of bricksand roots with thickness of around 0.7m; layer(CI) is silty clay mixture with yellow colour me-dium 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 presenceof claylike matrix with thickness from 0.5m to0.7m; layer (CL) is composed of silty clay mix-tures medium low dense yellow colour encoun-tered at 4.5m up to 9m depth; layers (M) are

Neogene’s deposits composed by claylike Marlsto highly weathered alveoli. The undergroundwater is present on 4 to 5m below ground surfacein 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 ex-cavation the problem was analysed using the fi-nite element method on two- and three-dimensional models. The popular program Plaxisspecialized for geotechnical engineering has

been proven as efficient in combination with

analytical solutions. The program enables simple but efficient spatial modelling of different struc-tural elements and accurate material definition.

The ground stress-strain state with the occur-ring soil effect during the excavation process has

been simulated on a two-dimensional plane-strain finite element model. The soil material isdiscretized using the Mohr-Coulomb modelwhile the concrete diaphragm wall with linearmaterial law. The spatial discretization had been

varied depending on the situation but in generaltriangular plane elements with 15 nodes had been

used for the soil and beams for the structuralelements of the wall and anchors [3].

4.1 Excavation

Since different ground and loading conditionsexist on the construction site, all four profilesrepresenting the four sides of the pit supportedwith the anchored diaphragm wall had to be ana-lysed. They are distinguished by the sectionname as or BB followed by the subscripts ’l‘for left or ’r‘ for right side. The stage construc-tion has been simulated in six continuous phases,where the initial phase represents the calculationof the ground stresses with constructed dia-

phragm wall. The excavation scenario from Ta- ble 1 is illustratively presented in Figure 2 forwall section AAl.

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

First of all a rough estimate of the necessaryanchor (stabilizing) forces is obtained through fi-nite element analysis using fix-end anchors.Thus, determining the resistant force Rk in every phase at different level of wall section AAl, seeTable 3.

Table 3. Characteristic anchor forces in wall section l.

Rk (kN/m’)level(m) Ph.2 Ph.3 Ph.4 Ph.5 Ph. 6

Phase 1 Phase 2

Phase 3 Phase 4

Phase 5 Phase 6

J. Josifovski et al. / Deep Excavation with Multi Anchored Diaphragm Wall 1487



-4 240.3 146.2 155.1 158.0 164.7-8 541.8 493.6 491.2 492.2

-11.5 401.2 422.4 417.2-14.5 401.2 502.5-17.5 470.5

Besides the anchor force the calculation isable to control the overall stability for every

phase of the excavation.

4.2 Anchors

The anchor look-off load P 0 has been determinedaccording to EN 1537 as

P 0 = 0.60 P tk (1)

where P tk is the characteristic load capacity oftendon. In the case of anchor type TTS15 with P tk = 350kN, produces a 210kN as a lock-offforce 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 stabilizingforce the following anchor types are chosen, seeTable 4.

Table 4. Anchor type in wall section l.

level P 0

(m)

Anchortype (kN/m’) (kN)

-4 4TTS15 311 840

-8 7TTS15 544 1470

-11.5 6TTS15 467 1260-14.5 7TTS15 544 1470-17.5 6TTS15 467 1260

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

Rk = (2r . L.( pv.tg + c))/ B

pv = ( z 1+z 2) / 2.

z 1 = h+x.tg ( z 2 = z 1+L. sin ( B = (1-sin ( )) sin ( -2 )) / cos(2

Figure 3. Ground anchor.

The grouted body is positioned in Marls (be-low depth 10.5m) with bond length of Lb = 15m.This calculation determines many important pa-rameters, namely the design resistant force Rd = Rk / R of anchor where R is a partial factorof anchor resistance [4], see Table 5

Table 5. Soil and anchor properties.

Cohesion c 150 kPaInternal friction 300 0.523

Unit weight 19.2 kN/m3

Height H 11.5 m

Distance x 10 m

Bond length Lb 15 m

Angle to horizontal 150 0.262

Width B 1.3 m

Start depth z 1 14.18 mEnd depth z 2 18.06 m

Pressure pv 309.52 kPa

Resistance force Rk 2323.44 kN

Partial factor R 1. 35 /

Design resistantforce

Rd 1721.06 kN

To obtain more realistic results a second finiteelement analysis had been performed on a model

where the so-called ’node-to-node‘ anchors dis-




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

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-offforce from Table 4. The anchors are positioned at15 degrees of angle with length which is given inTable 5. The computation leads to a solution ofthe displacements, see Figure 5.

Figure 5. Horizontal displacements in section AAl (phase 6:shadings).

The largest displacements are close to the po-sition of the grouted body between the secondand fourth anchor. The displacements of the wallare in the range from 2 - 4cm which has no im-

plications on the global stability. This fact has been also proved by the distribution of the effec-tive mean stresses in the ground, see Figure 6.

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

A slight stress concentration can be seen in

the toe of the diaphragm wall while in the soilnext to the wall the mean stress ranges from p’= 150 - 400kPa. Finally, the total anchor forcehad been determined, see Table 7.

level

(m)

P

(kN)

Anchor

type

EA

(kN)

L=L f +Lb

(m)

-4 831.6 4TTS15 109952.5 24.5+15-8 1455.3 7TTS15 109952.5 12.5+15

-11.5 1247.4 6TTS15 109952.5 10.5+15

-14.5 1455.3 7TTS15 164850.0 8.5+15-17.5 1247.4 6TTS15 192325.0 7+15

where P is the anchor force, EA is the axial rigid-ity of the anchor, L is the total length of the an-chor as a sum of free L f and bond Lb length.

4.3 Diaphragm wall

Beside the soil stresses and displacement the fi-nite element analysis determines the deformation

and internal forces of the diaphragm wall.The maximal calculated horizontal displace-ment of the wall is 41.9mm at the depth ofaround 14m which concurs to the results pre-sented in Figure 5. Interesting to mention is thefact that the measured displacements during theexcavation had not excide the calculated ones.

Moreover, the internal forces in the diaphragmwall section AAl are presented as envelopes cov-ering all six phases of excavation. The diagramof axial and shear force envelopes together withthe bending moments are presented in Figure 8.

Table 6. Anchors in wall section AAl.

J. Josifovski et al. / Deep Excavation with Multi Anchored Diaphragm Wall 1489



(a) (b) (c)

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

The maximal value of the axial force is-1.37MN/m at the foundation depth of around20.5m. The shear force has a characteristic formof a saw with maximal value of 503.5kN/m. Bothsides of the wall are tensioned in different phasesof excavation with 387.9kN/m/m as a maximal

bending moment.According to internal forces the design of the

reinforced concrete sections has been performedaccording to EN 1992 with the recommended

partial factors.

5 CONCLUSIONS

The deep excavation in highly urbanized areasuch as the city centre of Tirana represents veryformidable task. In the current project a technicalsolution 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 elementanalyses had been performed with an objective torealistically simulate the deep excavation behav-iour. Some difficulties of not quite precise regu-lative have been encountered. Nevertheless, the

proposed model has been able describe all the ef-fects in the process of excavation in difficult ma-terial and loading conditions. At all time during

the execution of the work the diaphragm wall has been instrumented with inclinometers and geo-detic markers to measure the deformations. The

recordings had only confirmed the predictionsmade by the numerical analysis. The construc-tion started in September 2009 and finished inMarch 2010, see Figure 9.

503.5kN/m-1.37MN/m 387.9kNm/m

Figure 9. Deep excavation with multi anchored diaphragmwall in Tirana.

REFERENCES

[1] German Society for Geotechnics (DGGT) 2003.Recommendations on Excavations, Ernst & SohnVerlag fur Architektur und technische WissenschaftenGmbH & Co. KG, Berlin, ISBN 3-433-01712-3.

[2] H.G. Kempfert and B. Gebreselassie, 2006.Excavations and Foundations in Soft Soils, Springer-Verlag Berlin Heidelberg , ISBN 540-32894-7.

[3] D.M. Potts and L. Zdravkovic, 1999. Finite elementanalysis in geotechnical engineering: theory. ImperialCollege of Science, Technology and Medicine, ThomasTelford Ltd, ISBN 0-7277-2783-4.

[4] EN 1997-1 Eurocode 7: Geotechnical design - Generalrules.

[5] EN 1537 Execution of special geotechnical work -Ground anchors.


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