1

Analysis of Geotechnical Behavior

Kilamba Tower in Luanda - Angola

Wilson de Carvalho Chipenhe Saituma

Department of Civil Engineering, Instituto Superior Técnico, Universidade Técnica de Lisboa-Portugal

May 2015

ABSTRACT:

The large structural growth that has been observed in the capital city of Angola, Luanda, at both

the peripheral zone (open spaces), and the urban areas (densely occupied), has led to the need

to adopt underground solutions adopted to the local geological and geotechnical conditions.

Despite this growth, whose solutions have benefited much of the knowledge and experiences of

other countries, there is a lack of studies, local, concerning the behavior of the other soils in the

region, as well as the techniques that are best suited to each of the numerous existing

geotechnical problems. The down town area of Luanda, located next to the sea coast, whose

soils are predominantly sandy, with high permeability and the presence of high water table level,

has been the most requested, housing large buildings, and in many cases previously occupied

areas for single-family homes, with heavy constraints in terms of space, and neighborhood,

which makes it the most challenging solutions. It is based on these assumptions that this thesis

was developed, which aims to make a contribution in the approaches to these problems.

This work has as main goal, analysis of geotechnical behavior of a building with 28 floors high,

and 3 underground, located in the Luanda bay, whose geotechnical scenario was described

above. As a solution to the infrastructure project was set to built the building over piles with 1m

in diameter and varying depths between 20 m and 30 m, by a bottom slab (general mat

foundation). Was adopted for the peripheral diaphragm walls framed with braced by slab bands

was built, with 3m thick (bottom plug).

This patterning was performed in Plaxis 2D finite element program.

After validation of the initially admitted parameters, were studied two alternatives whose base

focused on the changing of the bottom cap level. After the comparison of different solutions,

both from the internal stresses or displacements point of view, one alternative has been

validated which was subjected to a security check at the ultimate limit state of resistance to

bending moment and shear. These security checks were the basis for a brief economic

analysis, to better perception of the gains made with it.

KEYWORDS

Earth retaining structures; internal bracing; top-down; jet grouting:

1 GENERAL FRAMEWORK

The great infrastructural growth that Angola

has recorded in the last decade, with

greater prominence in Luanda city, has

been bringing you great challenges in the

area of Civil Engineering. These challenges

relate to the increased construction of

buildings with a degree of increasing

technical requirement, both structural

(architectural characteristics), neighborhood

conditions (space constraints) as well as

local geological and geotechnical conditions

(soils, water table position). These

challenges have particular importance

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when it comes to the construction of tall

buildings with several basements in areas

with high ground water table level and

sandy soils with high permeability, as is the

case of the building being studied in this

work.

The implementation of underground

structures under the conditions mentioned

implies the use of earth retaining and

support solutions, which allow the execution

of excavations in safe conditions, with the

smallest possible occupation of space, with

the smallest possible interference in the

normal functioning of adjacent structures

and services, and that are economically

competitive. Thus emerge the earth

retaining structures flexible (also called

curtains), which may be in the form of

diaphragm walls, pile curtains, Berlin walls,

being associated with the respective

support structures, as is the case of

propping and/or anchors, being the

application of each one or other solution

relies heavily of the abovementioned

conditions. It should be noted that, in many

cases the exclusive application of these

solutions is not enough to comply with the

requirements of the project, which leads to

more complex solutions that involve a

combination of several techniques, as in the

case of soil treatment with Jet grouting,

Deep Soil Mixing (DSM), Cutter Soil Mixing

(CSM), which consists of soil mixture with

cement, giving even better characteristics

of stiffness, strength and lower

permeability. As mentioned, the use of

flexible support structures, combined with

the techniques of soil improvement has

been in recent times, very competitive

solutions in excavation works with the

constraints cited above.

The present work has as its main object of

study the Geotechnical behavior of the

Kilamba Tower, built in the city of Luanda –

Angola, in a densely built area, next to the

coastline. This work brought with it a series

of constraints, such as being close to

structures of public interest, important, as

are the buildings of customs of Luanda and

of the Angolan Navy. These ancient

construction buildings, hence the challenge

of carrying out the excavations without

compromising the normal functioning of the

same. Associated conditions of

neighborhood time referred to is the fact

that the work be bounded on its front

elevation, by one of the Avenues with

greater volume of heavy road traffic. The

proximity of the work to the coastline, the

existence of very permeable sandy soils

and the ground water table level almost

constituted the major surface constraints of

geotechnical viewpoint, taking into account

the need to adopt a solution that could

withstand the high hydrostatic pressures,

while they were able to limit the entry of

water through the sides and the bottom of

the excavation, allowing the completion of

the excavations in safe conditions,

complying with the requirements of the

project.

For the development of this dissertation

work, was of capital importance the time

that was expended by the author, on site, in

more than 90% of the works of

infrastructure, which includes pile works

(including load tests), diaphragm wall

(including the interior wall and

waterproofing system), as well as the

implementation of the bottom jet grouting

sealing slab. It should be noted that this

follow-up was conducted continuously

during two years (2010 to 2012).

2 FLEXIBLE EARTH RETAINING

STRUCTURES

According to the European standard,

expressed through the Eurocode 7 (part 9),

Flexible earth retaining structures, are

relatively slender structures of steel,

reinforced concrete or wood, supported by

anchors, by anchors and/or land pressures

of passive type. Bending resistant capacity

of these structures plays a significant role in

support of the material, while the

contribution of its weight is negligible.

These structures differ fundamentally from

those of rigid support, because in these, the

weight itself, and sometimes the soil

stabilizers, masses of rock or refill landfill,

play a significant role in support of the

retained material. Such are the cases of

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gravity walls of concrete with constant or

variable thickness, reinforced concrete

walls with shoe and the walls of the

foothills.

In this work, will be used generically the

designations "curtain" or "wall", for flexible

support structures.

There are many different types of flexible

earth retaining structures, differing in

elements, components in material

composition, as well as in the construction

process. Among them the diaphragm walls

on the ground, associated with anchors or

prestressed anchors, became widespread

in recent decades, are today employed in

large engineering works, especially in the

intensive exploitation of the subsoil of the

major urban centers, to building basements

new and even ancient buildings, as well as

the subway tunnels. For reasons which will

be later referred to, the diaphragm walls

have shown a great aptitude for, even

under very difficult conditions, allow for

deep excavations and until recently

unthinkable dimensions, without significant

damage in the neighboring structures and

infrastructures, becoming on the other

hand, economically competitive by its

incorporation into the final structure, in

which they also play the roles of

Foundation, waterproofing and coating,

often without later finishing (Matos

Fernandes, 1983).

2.1 STRUCTURAL DESIGN OF STRUTED

EATH RETAINING STRUCTURES

Based on diagrams of earth pressure,

compression efforts in each strut are

determined through a reverse process

when that originated them, i.e. by

multiplying the area of influence of the strut,

the envelope of the diagram in the

respective area. It should be noted that the

values of anchors design forces shall be

obtained by multiplying the corresponding

values for calculating safety factors

depending on the type of soil crossed and

the calculation approach proposed by

Eurocode 7.

As regards the determination of the bending

moment and transverse efforts on the

curtain, it is common to use two expedited

processes (Matos Fernandes, 1983):

I) Assimilate the curtain to a continuous

beam requested by the pressure diagram,

and supported on Struts and at the bottom

of the excavation;

II) Consider the existence of bearings at the

level of each anchor.

According to Rowe and Briggs, 1961, the

pressures of work distribute themselves

favorably in order to reduce the bending

moments at the curtain. For this reason,

according to Armento, 1972 and Peck et

al., 1974, is current, in addition to the use of

expedited methods referred to, the

consideration of a reduction of 20% to 30%

in the pressure diagram adopted for

calculation of anchors loads. The bigger the

reduction admitted, more flexible will be the

curtain, then the greater the deformation

between anchors, and more pronounced

pressure transfers to these. Therefore, the

system will adapt to calculation

assumptions therefore, exceptionally rare

ruptures by bending (Matos Fernandes,

1983).

2.2 STRUTED DIAPHRAGM WALLS

In certain situations, as in the case of

impossibility to ensure total stability of

anchors due to soil conditions, such as

limitations on ownership, existence of

important nearby infrastructures, as well as

the limitations of space inside the

excavation, leading the use of ground

anchors very difficult or impossible. For the

outline of these constraints, there are three

alternative processes and implementation

of the basements floors, which are: internal

strut, prestressing of diaphragm wall and

the top-down system (implementation of

infrastructure as they advance the

excavations). The shoring system consists

of temporary or permanent ground anchors

or shuts (metal or other material that could

brace the structure, with the necessary

stiffness) placed the different levels in

depth. Reached the quota of bottom of

excavation, anchors are withdrawn (case of

provisional) as they build the slabs of the

basements structure, from the bottom to the

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top, passing these on to play the wall

support function (Pinto, 2008).

3 CASE STUDY: AUDITORIUM AND

KILAMBA OFFICE BUILDING IN

LUANDA-ANGOLA

3.1 MAIN SITE RESTRAITS

It is intended with this chapter, do the

framing the case study for the aforesaid

project, constituting the main object of study

of this thesis.

The project is located at the intersection of

AV. Marginal February 4 with Rua da

Alfândega, and has an area of plant for

construction of approximately 1798.58 m²

presenting the following confrontation (see

Figure 1).

• North Elevation: AV. Marginal 4 February

• West Elevation (West): Rua da Alfândega;

• South Elevation: Street

Figure 1 - Aerial photograph of the implantation site of

Kilamba Tower (Google, 2012)

This building includes 28 high floors

(including the roof), and 3 basements for

car parking as well for technical areas such

as: tanks for water storage, mechanical and

electrical equipment rooms.

In Figure 2 and Figure 3 are shown the

cross sections (longitudinal and

transverse), and the perspectives of the

Kilamba project, respectively.

Figure 2 - Transverse and longitudinal sections of Kilamba

Tower (Dar, 2010)

Figure 3 -Kilamba Tower in perspective (Front and back)

(Dar, 2010)

According to the information provided and

taking into account the geotechnical and

permeability characteristics of the existing

soils, as well as the presence of high

ground water table level, it was decided to

propose a solution using diaphragm walls

braced by slab bands, for earth retaining,

and a combined pile raft foundation, over a

jet grouting sealing slab, for foundation and

water inflow. This type of solution is, by

their characteristics, suitable for the

geological-geotechnical, hydrogeological

setting and neighborhood conditions (JetSJ,

et al., 2010).

Due to the constraints of the neighborhood

conditions as well as the presence of

ground water table level almost superficial,

associated to the presence of sandy soils,

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hindering the execution of ground anchors,

it was decided to provide a solution that

avoids the execution of anchors below the

ground water table level. On the other

hand, in order to reduce substantially the

inflow of water to the inside of the

excavation during the phases of

construction and operation, it was decided

the implementation of a bottom horizontal

jet-grouting sealing slab (JetSJ, et al.,

2010).

The diaphragm wall was braced by three

levels of diaphragms composed of sections

of slab bands, located at the level of 0, -1

floors and -2, and at the base of the

excavation by jet-grouting horizontal sealing

slab (JetSJ, et al., 2010).

3.2 GEOLOGICAL AND GEOTECHNICAL

SCENARIO

The geological-geotechnical study

consisted on 4 mechanical rotation

boreholes with continuous sampling, with

lengths of approximately 20.0 m, during

which Standard Penetration Tests were

performed (SPT). According to this study,

the soils at the site area can be grouped in

the following (JetSJ, et al., 2010):

C1a - Landfills: Sandy to medium sands

brownish or grayish.

C1B - Alluvial Deposits constituted

essentially by fine sands to medium, dark

grey, muddy, loose and very loose (SPT

between 3 to 11 blows).

C2A - Fine sands, silty, sometimes

yellowish brown clay compact the compact

medium (SPT between 12 to 38 blows with

very compact passages) and interlayed with

brownish clays, in very harsh rule with stiff

passages (SPT between 13 to 33);

C2B: Thin to medium Sands, silty, grey and

yellow with orange splashes, very compact

(SPT ≥ 60 blows) and Sandy clays, very

hard (SPT ≈ 24 blows).

Figure 4 - Geotechnical Profile (Teixeira Duarte, S.A, 2007)

3.3 OBSERVATION PLAN

The observation Plan has been defined in

such a way as to enable the measurement

of displacements in structural elements as

well as the position of the ground water

table level. Therefore, these variables were

measured by the following means (JetSJ, et

al., 2010):

Topographic Targets for the

measurement of horizontal and

vertical displacements (min. 30

un.);

Inclinometers to measure horizontal

displacements at the diaphragm

walls (min. 4x20ml.);

Piezometers for measuring the

position of the ground water table

level.

3.4 NUMERICAL MODELING

For the modeling of the adopted solution, it

was used the finite element program Plaxis

2D, 8.6 version, which was developed

specifically for the stress strain analysis of

Geotechnical Engineering projects.

Such modeling allowed to make a

comparison between the values of the

forces and displacements, obtained from

finite element program, with the same

values obtained through the readings made

on site. The modeling also had as objective

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the calibration of the model under study,

through a back analysis, in order to be able

to study other alternative solutions, which

may be competitive under both the

technical and economic point of views.

Figure 5 illustrates the General geometry of

the model used in numerical calculation

program.

Figure 5 - Geometry of the model used in numerical calculation program.

3.5 CHARACTERIZATION OF MATERIALS

Taking into account that the soils are

formed mainly by Sands, since there aren't

any other types of information in relation to

the same material, the geotechnical

parameters have estimated through

correlations. If this aim, the proposed

correlations by C.R.I. Clayton .1995 where

used. For the determination of the unit

weight, as well as the young’s modulus of

jet grouting, the recommendations

proposed by Croce, et al., 2014 were used.

In the

Table 1 are indicated the parameters used in

the initial solution.

The required parameters for

characterization of diaphragm wall and piles

are the following; The equivalent

thickness/width (d), the axial stiffness (EA)

and bending (EI), whose calculation have

taken into account the geometry of the

elements, as well as the concrete young’s

modulus (E), 33Gpa (C30/37).

Table 1 - Parameters used in the initial solution.

The study of the diaphragm wall struts was

done taking into account the interaction

between the wall and slab bands strutting

system. In order to achieve this goal, elastic

restraints were introduced (strut, Fixed-end

anchors) in the model (Plaxis), restraining

the diaphragm wall deformations.

3.6 MODELING

The results for the comparison analysis

were essentially deformations,

displacements and internal efforts. And as

benchmarks for comparison analysis the

readings of topographic targets were used,

the back analysis allowed to confirm that

the adopted solution had resulted in much

higher displacement values than the other

obtained on site, this back analysis allowed

the calibration of the soil site (see Fig.6),

parameters, as well as the validation of the

adopted model.

Figure 6 - Comparison between Horizontal displacements

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3.7 BACK ANALYSIS

The back analysis appears in this work as a

very useful tool to better study and

understand the behavior of the existent

soils, however the extrapolation of the

results to other similar projects, should not

be linear or straightforward, taking into

account the particularities of each case.

3.7.1 CHOSEN PARAMETERS

Based on the behavior of soils, as a

consequence of the actions which it is

submitted, some changes were made in the

team listed parameters (internal friction

angle, Young’s modulus).

At the layer composed of sand fine (C2A) it

was performed the biggest change at the

internal friction angle (from 30 to 43), both

in terms of Young’s modulus (from

80000kN / m2 to 90000kN / m2).

At the layer consisting of medium sands

(C2B) there was an Increase at the internal

friction angle (from 40o to 45o), as well as at

the effective shear stress. (from 1kN / m2 to

5 kN / m2). The Increase of this parameter

is due to coexistence of small clay layers,

as indicated at the geotechnical report on

site.

Finally was made a slight adjustment in

rigidity assigned to slabs strut that, the

noted initially, was admitted to the lowest

value associated with the largest

displacement of frame enclosed. Therefore

the amendment consisted in consideration

of the value of 35000kN/m instead of the

previous 32260kN. The summaries of these

optimization parameters are presented in

table 3.14 whose values were the basis for

the analysis done later.

Table 2 - Optimization Parameters, soil and jet-grouting

3.7.2 MAIN MODELING RESULTS (BACK

ANALYSIS)

Based on the values provided by Table 2, it

was possible to get results closer to those

obtained in work, as can be seen in Figure

7 which also serves for comparing the

values recorded at the site (blue line), with

the values corresponding to initial solution

(red line), as well as the resulting solution of

back analysis (green line).

Figure 7 - Comparison between the values of the

horizontal displacements

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4 ALTERNATIVE SOLUTIONS

4.1 ALTERNATIVE 1

Figure 8 - Numerical calculation model (alternative 1).

As illustrated in Figure 8 this alternative

consists to build the jet grouting sealing

slab at 15.5 m deep, is coincident with the

cap foudation with 3 m thick.

4.2 ALTERNATIVE 2

This alternative consists in positioning the

jet grouting sealing slab at 1 m of final level

of excavation (10.50 m), as illustrated in

Figure 9. With this solution, it is intended to

approximate the aforementioned sealing

slab, to the largest displacements, reducing

them so that they provide the best struting

conditions to the diaphragm wall, hence a

significant reduction of the internal efforts

as well as the forces in each of the levels of

shoring.

Figure 9 - Numerical calculation model (alternative 2).

4.2.1 COMPARISON BETWEEN

SOLUTIONS

In this chapter it’s important to refer that the

colors blue, black and red, represent,

solution at the site, alternative 1 and

alternative 2, respectively.

HORIZONTAL DISPLACEMENTS

Figure 10 - Comparison between the horizontal

displacements in the curtain

BENDING MOMENTS

Figure 11 - Comparison between the bending moments in

the curtain

SHEAR FORCES

Figure 12 - Comparison between the shear forces in the

curtain

FORCES ON STRUT

Table 3 - Comparison between the forces obtained in 3

solutions: Work, alternative solution 1 and Alternative 2.

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4.3 ECONOMIC ANALYSIS

For the present analysis sought to give

greater prominence to the costs associated

with the implementation of the work where

the biggest differences were observed in

relation to the studied alternatives. So it

was analyzed the costs of implementing the

diaphragm walls (A), system of struts (B),

as well as the cost of execution of jet

grouting sealing slab columns (C) whose

thickness is equal in all solutions, as seen

previously.

Are then presented the Table 4 and 5 with

the summaries of the estimation of these

costs, to the solutions implemented in the

work, as well as the alternative chosen (2)

Table 4 - General costs associated to the solution

performed in site (3,882,897.60 €)

Table 5 - General costs associated to the solution

performed in Alternative 2 (3,242,869.84 €)

As we can see, the implementation of

Alternative 2 makes possible saving about

640, 027.76 € which will be consequently

associated with the reduction of execution

of the work.

5 MAIN CONCLUSIONS

With this last chapter we intend to make a

general analysis on the proposed objectives

for the realization of this dissertation.

Therefore, it is concluded that the

objectives were achieved in full, and quite

satisfactory.

The first specific objectives proposed,

within the analysis of Geotechnical

Behavior of the Kilamba Tower, was to

proceed to solution modeling performed in

work, in a finite element program (PLAXIS

2D) based on soil parameters resulting from

the surveys, through existing correlations in

the bibliography, so that, on the basis of the

results of instrumentation on site

(inclinometers and topographic targets) it

was possible to validate the model chosen,

as well as the soil parameters converge.

For this purpose, it should be noted the

great importance of the use of the finite

element program (PLAXIS 2D and

SAP2000) either in the simulation of the

behavior of the structure (stresses, and

deformations) throughout the construction

phasing, allowing to obtain the internal

efforts at diaphragm wall and shoring

elements. These tools have also had a

support role to the extent that enabled the

validation of the calculation model used, via

the parametric study performed.

Estimated soil parameters based on

correlations proved to be quite

conservative, because with these

parameters, it was possible to obtain forces

and displacements in the structure, much

above the results obtained at the site.

Based on this analysis, it is concluded that,

from the point of view of pre design, as well

as the sizing of geotechnical structures, the

use of correlations is recommended,

especially in cases of work in which the

geotechnical information lacks any

representativeness (or detail).

The back analysis and the parametric study

carried out in this work, were the tools that

enabled the convergence (validation) of the

soil parameters admitted initially, through

numerical calculation program. Despite the

success of this study, the lack of more data

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readings at the site during the excavation

works, make some results obtained,

susceptible to more discussions and/or

changes, namely internal friction angle, as

well as the respective Young’s modulus. It

should be noted that, the present work had

as main reference element, the readings of

topographic target 12, located at the border

of the 1st slab.

The study of the behavior of geotechnical

structures (diaphragm wall and slab struts)

from the point of view of the displacements,

as well as the internal efforts, throughout

the phasing of construction work (second

objective), it should be noted the influence

of the stiffness of the struts, in the global

behavior of structures. As noted, initially

was given the rigidity of the struts through

the analysis of offset as flexible as possible.

This analysis allowed to obtain the

minimum stiffness of the system. However,

the analysis of the behavior of the structure,

by changing this value (gradual increase)

indicated that the stiffness of the strut will

have a big influence on the diaphragm wall

behavior. That is, the higher the stiffness

admitted in the calculation, the greater the

"penalty" of efforts in the struts. On the

other hand, the smaller the rigidity admitted

in these elements, the greater the penalty

efforts in diaphragm wall (as was the case

in this study), produced by the relief of

stress struts in anchors, and increased of

the diaphragm wall deformation

deformations.

The third objective defined in this work, is

related to the study of some alternatives in

the sense of the optimization of the solution

performed on work on both the technical

points and economical point of view. Under

this approach, were analyzed two

alternatives, which are summed up in

changing the position of the jet grouting

sealing slab. The alternative 1 proved to be

little advantage, to the extent that the level

of forces and displacements have varied

little in relation to the reference solution,

nevertheless it was more economical. The

second Alternative was the one that proved

to be more feasible under all points of view,

despite this advantage, it should be noted

in this study demonstrated the importance

of a greater knowledge of the soils, as well

as the implementation of more detailed

permeability studies.

BIBLIOGRAPHY

C.R.I.Clayton. 1995. The Standard Penetration

Test (SPT): Methods and Use. s.l., London :

CIRIA, 1995.

Câmara, José N. da. 2012. Betão Armado e

Pré-Esforçado1 - Folhas de apoio às Aulas,

Módulo 3 . IST, Lisboa, Portugal : IST, 2012.

Carvalho, Filipa Martinho de. 2013. Solução

de Escavação e Contenção Periférica - Parque

de Estacionamento Alves Redol. Lisboa : IST,

2013.

Croce, Paolo, Flora, Alessandro e Modoni,

Giuseppe. 2014. Jet Grouting Technology,

Design and Control. s.l. : CRC Press, 2014.

Dar, Angola. 2010. Projeto Kilamba, Luanda,

Angola : Dar - Peças Desenhadas - Projeto de

Arquitetura, 2010.

EC2. 2010. Projeto de Estruturas de Betão. s.l. :

EN 1992, 2010.

EC7. 2010. Projeto Geoténico. s.l. : EN 1997,

2010.

FDO-ABB. 2012. Relatório das Leituras dos

Alvos Topográficos. Projeto Kilamba, Luanda,

Angola : FDO-ABB, 2012.

JetSJ e Geo-Rumo. 2010. Memória Descritiva e

Justificativa do Projeto de Fundações

Escavação e Contenção Periférica. Luanda :

GEO- RUMO, 2010.

Matos Fernandes. 1983. Estruturas Flexíveis

para Suporte de Terra Novos Métodos de

Dimensionamento. FEUP : LNEC, 1983.

Pinto, Ricardo Nuno Martins Gomes. 2008.

Sistemas Construtivos de Estruturas de

Contenção Multi-Apoiadas em Edifícios. s.l.,

Porto : FEUP, Julho de 2008.

Teixeira Duarte, S.A. 2007. Relatório de

Reconhecimento geotécnico. Luanda : s.n.,

2007. 1006.07.