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Excavations and Peripheral Walls in Urban Space

Case Study – Palácio dos Condes de Murça Inês Oliveira

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

Abstract: The increasing occupation of the urban underground space has consequences for existing buildings, which often are not

properly assessed when projects involving excavations and peripheral walls are carried out. This paper is intended as a contribution to

the study and analysis on this issue. Therefore it is presented a case study using the technique of King Post Walls, which was adopted

for the peripheral contention of the project “Palácio dos Condes de Murça”, in Lisbon. Based on data collected in the field and the

monitoring of the construction works, a comparative analysis of the instrumentation was made, relating it to the main events

witnessed in the field, including the implementation of mitigation measures, which were required due to the ground movements

observed. It was also carried out a numerical modeling of part of the retaining structure, using the finite element software Plaxis 2D,

in order to assess the quality of results, compared to data from topographic instrumentation, and a back analysis. This study showed

that the analyzed ground had higher secant stiffness than the one originally adopted in the modeling, as well as lower friction angle

and cohesion. The study demonstrated that when using King Post Walls, the ground movements around excavations are essentially

affected by the constructive sequence, soil’s stiffness and strengthness, weather conditions and their influence on the ground’s

behavior. The ground movements are transmitted to neighboring buildings, whose response depends on their building type and pre-

existing damage.

KEYWORDS: Excavation and peripheral walls, urban space, King Post Walls, ground movements, instrumentation, modeling, back

analysis.

Introduction

Now, when building construction industry is experiencing a

severe crisis in terms of new projects, it’s becoming more and

more frequent the use of the already built urban space, leading

to the use of underground space, in order to maximize the

building areas. Those projects often occur near old and

sensitive structures; therefore, the necessary interventions

require extra care to prevent any damages.

In Portugal, King Post Walls are often used on an

empirical knowledge basis, which leads its constructive

sequence being modified, without taking into account the

excessive ground movements during the excavation process.

Without adequate instrumentation and monitoring, the un-

prediction of excessive ground movements may result in

damage to nearby buildings, or even incidents with severe

consequences for people and goods.

In this specific case there was a follow-up of the

construction works on the ground, which included the

observation of the constructive procedures used during the

excavation and peripheral wall execution, so that the urban and

geological environment could be better understood during the

process. Based on the visits carried out, the aim was to

compare the designed solution with the executed one,

regarding both the constructive sequence, and the

Instrumentation and Observation Plan. It was also carried out a

comparative analysis between the instrumentation results and

the main construction events and excavation stages, seeking

cause and effect relations, with more focus on the ground and

wall movements and their negative impacts.

The case-study analysis was complemented with a FE

modeling, using Plaxis 2D, where the objectives were to

evaluate the initial modeling, and to add a new model that

included the implemented mitigation measures during the

excavation process, comparing the results. The goal was also

to make a retro-analysis of the final solution, as an attempt to

bring as close as possible both the displacement values

provided by the instrumentation and the initially expected ones

in the design phase. That analysis was accomplished by

amending the model’s geotechnical parameters using the

Hardening Soil Model.

Excavations and Peripheral walls in urban space

The analysis on retaining structures has so far been based on

an equilibrium analysis, combined with an elasto-plastic soil

analysis, in order to estimate the horizontal stresses that form

behind the wall [1]. In the specific case of multi-anchored

walls however, other hypothesis must be considered in the

analysis, because the assumption of an active state behind the

wall leads to displacements that are incompatible with the ones

observed in those structures. The appearance and development

of the FE software’s has enabled a better resolution of these

problems, approaching the numerical analysis with actual wall

displacements.

The multi-anchored wall’s anchors work mainly by

the stress field change induced in the soil, meaning that there’s

an increase of the horizontal stress in the wall (and a deviatoric

stress increase) with the anchors prestressing, with a

consequent decrease in the deformation, as a preparation for

the next excavation stages; then, the retained soil’s horizontal

stress decreases again, and there’s the consequent movement

increase [2].

Multi-anchored walls

In retaining structures projects located in urban space it is

necessary to evaluate the predicted displacements before the

beginning of the construction works, i.e. during the design

phase. Before the existence of FE software’s that evaluation

was only based on previous experiences in similar projects

with similar soils. Yet, to model a retaining structure using

FEM software, a high number of parameters is required for the

analysis, so that the simulated excavation’s behavior can be

close to the real one. These parameters are determined

experimentally, or empirically, which doesn’t dispense a

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rigorous monitoring of the construction works, to confirm the

assumed geological conditions.

Ground movements around excavations may occur

due several factors, such as drilling, grouting, groundwater

changes, excessive excavation, inadequate support, timing,

unpredicted surcharge, etc. Those movements are caused by

changes in the stress field in the surrounding soil, mainly due

to the horizontal and vertical stress relief. Ground movements

can be divided into horizontal movements, due to horizontal

displacement of the wall (because of the horizontal stress

relief), vertical ground movements (settlements behind the

wall), or heaving of the bottom of excavation, due to the

vertical stress relief in that area [3].

In King Post Walls the critical phase for excessive

displacements is the panel’s excavation phase, and the time

that goes between the concreting phase and anchor stressing or

propping. After this, the displacements above the excavation

level come to depend essentially on the anchor’s prestressing,

and their stiffness and soil interaction, knowing that the

displacements occurred are substantially lower at this stage.

Excavation’s supporting systems

There are three main excavation’s supporting systems used in

urban space: props, prestressed anchors, and more recently, a

support system achieved by reinforced concrete framing

executed at each excavation level; besides the top-down

system, which is not addressed in this paper.

Despite the economic advantage, both in materials

and labor, and the short execution periods, the use of prop as a

supporting system in peripheral walls has also significant

limitations: large work space occupation, especially when big

section stays, or steel profiles, are needed to achieve larger

spans without buckling or bending problems. Still, there are

situations in which props represent a better option compared to

ground anchors: there’s no need to invade the adjacent

underground space, which induces fewer disturbances; or

when the firm layer lies too deep so it doesn’t pay off the

execution of ground anchors. It’s also possible to pre-stress the

large section struts, which allows a more effective supporting

system, with less wall and ground movements.

The most used supporting system in urban

excavations is still pre-stressed anchoring, because of its

advantages if compared with other systems: due to pre-stress,

there’s a significant decrease of horizontal displacements at the

top of the wall, and the consequent ground settlement decrease

behind the wall; it’s a safer process compared to struts

installation, for these can be damaged during the excavation

works by the circulating equipment’s; ground anchors don’t

cause space constraints during excavation. However, there are

some limitations: they’re less economic and require larger

execution periods; need specific equipment and specialized

labor; their design is limited by nearby underground existing

structures (utilities, tunnels, basements), with changes in the

tilt angle and consequent efficiency loss; cement injection for

the grout body can damage adjacent structures, caused by lack

of geological knowledge, or lack of control in the injection

process (excessive pressures); there’s a large waste of material

that is lost in the ground, for interim anchors; comparing to the

supporting system with structural elements, ground anchors

induce more disturbances in the surrounding soil, with

consequent larger wall displacements and ground movements

that may damage the nearby buildings.

The excavation supporting system by structural

elements (reinforced concrete framing) consists in the

execution of concrete slab band frames at each level of current

excavation, which will be part of the definitive structure.

Although still not widely used in peripheral walls supporting,

this method has significant advantages in urban sites: more

safety and lower soil decompression during excavation,

compared to props and anchors, as it’s necessary to dig a

smaller soil portion to execute the slabs; there’s a wide

availability of space inside the working enclosure, since the

slab bands have only the minimum sufficient width to

accommodate earth pressures; there’s no invasion of the

underground space, minimizing the negative impacts on

existing buildings; it doesn’t cause the decrease of stiffness

and strengthness of the wall, since it’s not necessary to drill the

retaining structure itself; there’s a good compatibility between

the temporary and the definitive buried elements, since the

slabs are already a part of the final structure. Still, there are

also some disadvantages: less space below the executed

supporting slabs, which interferes and delays the digging

works; additional vertical elements are needed, to brace the

slab bands, usually micro piles or steel profiles, executed from

the surface and before the excavation.

Excavation’s influence in nearby structures

Ground movements induced by excavations grow

progressively like waves, reaching adjacent structures and

spreading over them, as the excavation proceeds. As it is quite

difficult to establish direct relations between the

excavations’/walls movements with the displacements in the

building, the common practice consists in limiting the wall’s

displacement, as an indirect control method to prevent any

damage to structures adjacent to excavations. That

displacement limiting criteria should be applied at an early

design phase, and even contribute to the choice of the

peripheral contention method. The displacement limits must be

adequate for each situation and monitored during the

construction works.

Damages in structures adjacent to excavations are

much more due to differential settlement than to the absolute

values of settlements. The initial stress field is sometimes

underestimated, and not always the results are conservative. It

is also difficult to establish soil parameters related with time

[4].

Ground movement’s transmitted to adjacent buildings

make these to have rotation and translation movements,

distortion, and possible damage, when the maximum

deformation capacity is reached. Small vertical and horizontal

displacements and rigid body translation usually don’t affect

the safety of the buildings, but affect their serviceability

functioning due to some local damage. Distortions are induced

by differential displacements from different parts of the

structure: differential horizontal displacements cause

horizontal compression or extension distortion; differential

vertical displacements of a structure can cause bending and

shear distortion. When evaluating the response of buildings to

ground movements, rigid body deformations should be

separated from distortions, otherwise the analysis can be too

conservative, particularly when the assessed building’s

foundations can absorb a part of the horizontal strain [5].

Design graphics and field experience are helpful tools

to obtain initial displacement limits, and even to compare with

the results provided by numeric calculation methods, like FE.

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This method has given good results, since it uses constitutive

soil models that take into account its non-linear and continuous

behavior. They are able to simulate the multiple stages

involved in the construction process, and the interface between

wall and soil. However, the displacements calculated by those

methods are often more accurate predicting wall displacements

than the associated ground movements. Despite the increasing

development of these calculation methods, it’s still necessary

to define boundary conditions and a high number of

parameters for the complex constitutive soil models. The

uncertainties associated with these predictions are mostly

compensated by the adopted safety factors in the design of

retaining structures, and by the investment on instrumentation

and monitoring, which controls the appearance of cracking and

excessive displacements.

King Post Walls

Given his economic advantages and relatively easy execution,

this type of constructive method has been used in Portugal ever

since the 70’s in both peripheral and retaining walls. However,

more recently, King Post Walls have been replaced by other

more effective retaining structure techniques, such as

diaphragm walls or bored pile walls, as these induce less soil

decompressions, as well as shorter execution periods. To

design King Post Walls, one can assume the behavior of a

flexible multi-anchored, or braced, retaining wall, which is

characterized for its high deformability during the excavation

and construction phases, being also highly influenced by the

soil-structure interaction.

Case Study: Palácio dos Condes de Murça

The case-study presented in this paper is the excavation and

peripheral contention for the execution of the underground

floors planned in the project “Palácio dos Condes de Murça”,

located in Santos, Lisboa, (Fig. 1). Fig. 2 shows a diagram

with some reference adopted points and respective side views

of the working enclosure.

The Excavation and Peripheral contention design was

carried out by “JetSJ” and was based on the permit design,

elaborated by “AfaConsult”. The construction technique

suggested by the initial design has undergone some changes, in

order to optimize costs, as far as local constraints allowed [6].

Fig. 1. Building area of the project [7]

Fig. 2. Side views of the working enclosure (adapted from [8])

Geologic-geotechnical and vicinity constraints

In order to define the geology of the “Palácio dos Condes de

Murça” neighboring northwest area, some support jobs were

carried out: drilling of 10 mechanical boreholes, 10

foundation’s observation wells, installation of standard

piezometers in 4 boreholes, and water sample collecting to

perform lab analysis. From the interpretation of the described

field tests, it was possible to establish the existence of three

geological units, which were formed by landfill deposits (L.D.)

on the surface, deposited on two Miocenic stratums (M1 and

M2). One of the geological profiles drawn based on the bored

holes information is displayed in Fig. 3.

Fig. 3. Geological profile no. 8 (adapted from [9])

The three geological units are described by:

Landfill deposits: very variable thickness, consisting of

pebbles and fragments of scattered limestone, ceramics

and some organic contamination, such as roots and plant

debris; dominant clay matrix, and considering the

obtained SPT test results of 3 to 10 strokes, with a loose to

medium compactness;

Layer M1: composed of red clay with erratic limestone

fragments, until 6 to 10 m;

Layer M2: limestone, whitish color, appearing generally

very fragmented, but with high recovery rates, and some

signs of carsification, but not enough to classify the solid

as karst, rather than compact; displayed in sub-horizontal

benches interspersed with relatively thin clay veins.

Based on the geological description of the formations

found, it was made the following geotechnical zoning (Table.

1):

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Table. 1. Geotechnical parameters for each zone [10]

Nspt γ

(kN/m3)

Φ'

(º)

c'

(kPa)

E

(MN/m2)

Cu

(kPa)

Landfills <10 18 30 - 6-8 -

M1 10-

30 18-19 25 5-10 15-30

50-

100

M2 >30 22-24 50 50-

100 100-200 150

Due to the impermeability of the superior clay

fraction, it was determined that the Miocenic limestone

structure was not conducive to feeding and maintenance of

groundwater levels; therefore, the measured groundwater

levels were considered negligible [9].

The intervention was done in a densely urbanized

area, with old surrounding buildings, and sensitive stone

masonry structures. The working enclosure was bounded by

streets, and several adjacent buildings that had to be preserved

[10].

Construction solution and Monitoring Plan

Given the geological described scenario, the chosen

construction technique for the peripheral wall’s execution was

King Post Walls, because it was the most suitable for the type

of soil crossed by the excavation. The wall would be composed

by reinforced concrete panels, temporarily supported by steel

HEB profiles, and also anchored or braced.

During the excavation works and underground floors

construction, the proposed Monitoring Plan was intended to

allow the measurement of the values described in Table. 2 (1st

column), which would be measured by the installed devices

also described in Table. 2 (2nd

column). Based on the adopted

construction technique, it was possible to estimate the criteria

described in the same table (columns 3 and 4), for each value,

to be confirmed during the project execution.

Table. 2. Aspects of the Monitoring Plan (adapted from [10])

Values to measure Measuring devices Alert limits Alarm limits

Horizontal and vertical

displacements of the retaining

structures

Topographic targets to measure the three-

dimensional displacement of retaining structures

(min. 37 units)

Horizontally,

maximum

displacements of 20mm/10 meters of

excavation height, and

vertically, maximum

displacements of 15mm/10m

Horizontally,

maximum

displacements of 30mm/10 meters of

excavation height,

and vertically,

maximum displacements of

30mm/10m

Horizontal and vertical

displacements of the adjacent structures

Topographic targets to measure the three-

dimensional displacement of adjacent structures (min. 25 units)

Vertical displacements of

pavements and surrounding

ground surface

Topographic marks for measuring vertical

displacements of the floors and the surrounding

ground surface (min. 2 units)

Load measurement of the ground

anchors

Load cells installed on the ground anchors to

load measurement (min. 9 units) - -

Measurement of the allowable

peak vibration velocity

Piezometric accelerometers for measuring the

allowable peak vibration velocity (min. 6 units)

maximum particle

velocities of 2.5 mm/s

in the Church zone, and

of 5.0 mm/s on other sites

maximum particle

velocities of 3.5

mm/s in the Church

zone, and of 7.0 mm/s on other sites

The mitigation measures initially planned in case of the

alert and alarm limits were reached, should be analyzed

individually, and consisted, among others, in [10]:

Increasing the vertical load capacity of the retaining wall,

by installing additional vertical steel profiles or micro

piles;

Strengthening of horizontal support of the retaining wall,

by additional execution of ground anchors or props,

eventually with a bigger free length and steeper

inclination;

Partial excavation’s support through structural slab

elements – reinforced concrete frames;

Additional drainage devices on the retaining wall;

Treatment of the decompressed soil located behind the

retaining wall.

Once that abnormal crack opening of pre-existing cracks

in adjacent buildings were observed, 5 fissurómetros, type

“Gauge 1.1” (adequate to measure cracks in the same

plan/frame), were installed, on December 20th.

Construction’s works evolution

Work’s observation on site

Some observation visits were made in order to follow

up the progress of the works, and also to observe the execution

of the constructive processes involved in King Post Walls. One

could observe closely some constructive aspects mentioned

before, that may have contributed to the excessive ground

movements in some areas.

It’s shown in Fig. 4 a situation where it may have

been some disregard of the constructive sequence, which may

have caused the soil to decompress, given the excavation

height and the non-cohesive type of soil. Being the first area to

be excavated and functioning as a double primary corner panel,

the corner area should have been treated with special attention,

and concreted after primary panels, unlike what happened.

This may have induced some stress relief in the soil, which as

shown in the figure, appears to be somewhat inconsistent when

excavated, and therefore, the displacements that were

measured on the topographic targets installed in the adjacent

building.

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Fig. 4. Opening of the 3rd level panels on the corner “I”

Several steel vertical profiles over the side view FG

aimed as a vertical support to the retaining wall, while the

excavation didn’t get to the foundation elevation height, got

bent, as shown in Fig. 5. Their positioning outside the wall

definitely contributed to the occurrence of buckling of the

profiles, which was exacerbated by the impacts suffered during

the digging works caused by heavy circulating equipment.

Fig. 5. Bent vertical profiles along the side view FG

This situation probably led to a deficient transmission

of the vertical loads to the firm ground, and the consequent

increase of settlements. In this case, the consequences were not

severe, for a stratum of rock had already been reached, which

allowed a good embedding of the wall, and in some areas the

excavation had reached the footing elevation. Moreover, this

behavior was being monitored by instrumentation, and the

displacements observed didn’t reach the alert and alarm limits,

so the worker’s safety was controlled; still, there were slight

damages to the adjacent structures.

Instrumentation’s evolution

Based on the results observed from the 15 topographic targets

initially installed (Table. 2, column 2), it became clear that,

from the beginning of the construction works, in the end of

September 2010, until December there were no problems

related with the excavation’s movements.

However, by January 10

th 2011, increasing crack openings

were observed in adjacent buildings in FG side view (at the

“F” corner), belonging to the Belgium Embassy [11]. That was

happening in an area where the digging and panels concreting

works were very active. The cracks originated from ground

movements generated by several factors in the area: some

probably related to disturbances caused by an adjacent wall

collapsing in EF side view; the excavation at the “F” corner

did not respect the constructive sequence (simultaneous panels

opening); intense precipitation; also the fact that several

consecutive panels in the “F” corner area were already

concreted, but still without the ground anchors executed.

So, it’s been decided to adopt mitigation measures: to

strengthen the areas where the adjacent buildings were

degraded, through the execution of concrete wall sections

above the top tie beam, complemented by the extension of the

vertical steel profiles and their connection between the new

sections and the existing wall, using steel connectors, to

provide a better support of the adjacent building’s façades; the

rigid bracing of the “F” corner with structural slab elements in

reinforced concrete, instead of the steel corner prop initially

planned.

It was done a comparative analysis between the

results of the topographic measurements and the fissurometers

measurements. In the Belgium’s Embassy area, one of the

topographic targets that had larger displacements was no. 8,

located on this building’s façade (Fig. 6). After 15

fissurometers measurements (Fig. 7) the cracks were repaired

and the devices were installed again. As time went on, no new

crack openings were observed, indicating that the

reinforcement measures functioned well.

The evolution of the displacements registered on

target no. 8 showed some variations along M and Z axes over

time. Despite the apparent stabilization of the displacements in

mid-January, from then until mid-April those displacements

increased again towards the interior of the excavation (-M),

and with increasing settlements (-Z). After mid-April, the

displacements continued to increase slightly, but at a lower

rate. Knowing that by this time the extension of the peripheral

wall in this area had already been executed, as well as the first

two levels of the “F” corner’s support, it makes sense that the

movements have stabilized or tend to stabilized, which is also

an indication that these mitigation measures succeeded. This

reaction was not immediate probably because the ground was

heavily decompressed at the beginning of the digging works,

and the soil had worse stiffness and strengthness than initially

assumed. The disturbances caused in the adjacent building’s

foundations were spreading to the upper levels of the structure,

as the excavation progressed.

It’s possible to establish a relation between the target

no. 8 and Gauge 1.1, for these devices are in the same location

in ground view. Looking at the evolution of these

fissurometers measurements in Fig. 7 it’s possible to see that

the crack opening began to stabilize by the end of January,

early February. Given that by this time the implementation of

the reinforcement measures was being carried out, it is

reasonable to establish a relation between the two events. As

the new concrete support was being done in the “F” corner,

with the 1st slab level being finished around the 7

th of February,

the 2nd

level by the 20th of February, and the 3

rd level in early

May, an almost complete stabilization of the crack opening in

this area was being achieved.

Shortly after abnormal cracks opening in buildings

attached to the Belgium Embassy, the same started to happen

in the kindergarten buildings, located on side view IH.

Therefore, four more fissurometers were installed in this

building to monitor the cracks (Fig. 8).

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Fig. 6. Displacements registered on topographic target no. 8 (adapted from [12])

Fig. 7. Crack openings registered on the fissurometers placed in the Embassy buildings

Fig. 8. Crack openings registered on the fissurometers placed in the Kindergarten building

Possibly due to the end of the digging works on side

view GH and the temporary suspension of the same, in side

view IH, after the execution of the top tie beam, the monitored

cracks had a relatively stable behavior, and Gauge 1.1 even

closed to 0.5 mm. However, after the excavation works started

again in side IH, Gauges 1.1 and 1.2 began to register some

strains. This same behavior can also be seen in Fig. 9, which

shows the displacements measured in topographic target no.

14, located on the kindergarten building. During February, the

digging and concreting works were suspended again, in order

to allow the ground to stabilize, stopping the damage in the

adjacent building. And by the middle of the month new targets

were installed on the working field, some located on the

kindergarten’s building façade, at several levels, and on the top

tie beam.

With the data available it’s not possible to establish a

linear relation between the target displacements, the

fissurometer values, and the excavation stages, because works

affect the buildings in a complex way, which depends on

building’s response to ground movements, based on their

foundations and structural system. Still, it is possible to detect

a trend of stabilization from May, which probably corresponds

to the end of the excavation and anchor tensioning in this area,

and subsequent landfill with the excavated material. This trend

is also visible in Fig. 9 and the greater increase of the

displacements inwards the excavation occurred between March

and May, when the digging, panels concreting and tensioning

works, moved faster, stabilizing in the edge of the alert limit

(9.45 mm for settlements, and 12.6 mm for horizontal

displacements). The remaining targets installed later in this

façade (nos. 24 to 28, and 44) showed the same trend, with

larger displacements on the top of the building rather than in

the top tie beam, which explains the cracks in the

Kindergarten, located on the superior floors.

-2-101234567

15-Dec-10 4-Jan-11 24-Jan-11 13-Feb-11 5-Mar-11 25-Mar-11

Crack opening (mm)

Gauge 1.1

Gauge 1.2

Gauge 1.3

Gauge 1.4

Gauge 1.5

-0,5

-0,25

0

0,25

0,5

0,75

1

13-Jan-11 17-Feb-11 24-Mar-11 28-Apr-11 2-Jun-11 7-Jul-11

Crack opening (mm)

Gauge 1.1

Gauge 1.2

Gauge 1.3

Gauge 1.4

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Fig. 9. Displacements registered on topographic target no. 14 (adapted from [12])

These results also support the fact that in this area the

soil was very sensitive to any disturbances, as well as the

foundations of the adjacent buildings, showing negative

responses very quickly when the digging works occurred. The

fact that the panels remained without support (tensioned

ground anchors) too long after concreting, may have

contributed to the soil decompressions referred above. There

were some difficulties in the constructive sequence

coordination because the equipment and the specialized labor

for drilling and tensioning the ground anchors weren’t always

available on the field.

Solution’s modeling

A numerical FE software, Plaxis 2D, was used in the design of

the peripheral wall. This software is a calculation tool widely

used in modeling of geotechnical problems, because it allows

recreating approximately the mechanical behavior of soil and

underground structures, also allowing an analysis of the soil-

structure interaction.

Initial Solution

Six sections in various locations were analyzed in the project

solution’s modeling, which were considered to be the most

representative and conditioning for the design. One of these

sections, no. 4, located on FG side view, near corner “F” and

the Embassy buildings, is briefly described; the other sections

followed similar principles, only changing the geometry of the

site and their constructive solution. The final look of section

no. 4 is shown in Fig. 10.

Fig. 10. Model’s geometry of section no. 4

For the definition of the soil characteristics Plaxis has

the option of various constitutive soil models that simulate the

behavior of materials. The chosen model was the Hardening

soil model. This model recreates the behavior of different

types of material assuming a non-linear stress-strain path. As

the soil is subjected to increasing shear stresses, the stiffness

decreases and there are plastic irreversible deformations. For a

good characterization of each material, the model requires the

definition of some parameters; the parameters used for the

soils dealt with during the excavation are presented in Table. 3,

based on the Geotechnical-Geological report.

Table. 3. Soil parameters

Parameters

Geothecnical zones

Landfill

deposits M1

Improved

M1 M2

γ [kN/m3] 18 18 18 19

[kN/m2] 8000 25000 35000 100000

[kN/m2] 8000 25000 35000 100000

[kN/m2] 24000 75000 105000 300000

cref [kN/m2] 0.1 30 30 100

ϕ [°] 30 35 35 40

ψ [°] 0 0 0 0

νur 0.2 0.2 0.2 0.2

K0nc 0.5 0.426 0.426 0.357

After the calculation phase it was possible to see in

the software’s output the deformed mesh, as well as the

displacements and stresses on the ground, and forces and

displacements of the wall. For this section, the deformed mesh

and the obtained horizontal and vertical displacements are

shown in Fig. 11, Fig. 12 and Fig. 13, respectively.

Fig. 11. Deformed mesh for the initial modeling on section no. 4

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Fig. 12. Horizontal displacements for the initial modeling on

section no. 4

Fig. 13. Vertical displacements for the initial modeling on

section no. 4

Solution including reinforcement measures

It was carried out a new modeling of the same section (no. 4),

but including one of the mitigation measures, the bracing of

the “F” corner, in order to observe the differences at the final

excavation phase, in terms of wall and ground movements. As

the Plaxis version used only allows a bi-dimensional analysis,

it was not possible to model the slab supporting system with

their real triangular shape. Therefore, it was necessary to adopt

some simplifications to obtain the maximum approximation of

the existing solution. Instead of ground anchors, formed by

free length and grout body, fictional props on two excavation

levels were used (Fixed-end anchor elements). As the length

of the concrete fictional props is variable (since they’re

triangular), it was adopted a medium equivalent length, and the

normal stiffness of a reinforced concrete element 0.25 m thick

and 1 m width (per meter of prop). Finally, the adopted

solution for the new supporting system was a couple of

concrete struts 8.6 meters long, with normal stiffness of

8.25E6 kN/m2 and spaced by 1 m. After running the model the

results are shown in Fig. 14, Fig. 15 and Fig. 16.

Comparing the previous picture to Fig. 11, it’s

possible to see that the ways the wall deforms with a 4 ground

anchor supporting system and with a 2 concrete props system

are quite different, and that the second alternative allows a

better control on deformation. Fig. 15 shows the expected

horizontal displacements, and comparing to Fig. 12, it’s visible

that the soil mass affected by the excavation is much larger

with the anchor solution, and the area where the displacements

are maximum (around 5 mm) is also wider than in the new

solution, where the displacements are also lower, reaching a

maximum value of about 3.5 mm in the area between the 1st

and 2nd

concrete props.

Fig. 14. Deformed mesh on section no. 4, including mitigation

measures

Fig. 15. Horizontal displacements on section no. 4, including

mitigation measures

Fig. 16. Vertical displacements on section no. 4, including

mitigation measures

Analyzing the occurred settlements, the wall’s

behavior with the new supporting system is also significantly

better (Fig. 13 and Fig. 16). In addition to the values of

displacements being lower, in about 1 mm against 3 mm in the

- 9 -

anchor’s solution, the affected area is also much smaller in

size, and is located further away from the excavation site.

Back-analysis

The back analysis was based on the results given by

instrumentation, mainly from the topographic target

displacements. The parametric analysis was performed for

each geotechnical zone and for each soil parameter, using the

results obtained for section no. 7, in particular from target no.

25, placed on the top tie beam, in side view IH. The back

analysis was also carried out using the data of the topographic

target no. 25. Only the horizontal displacements were

analyzed. There was no back-analysis on vertical

displacements because it has been observed that these can’t be

approximated to the real ones; where the analysis predicts soil

heave, there are actually settlements, due probably to a

numerical problem of Plaxis for this situation. Fig. 17 shows a

chart with the displacements progress on target no. 25, and the

ones predicted by the software, throughout the excavation

phases.

Fig. 17. Horizontal displacements predicted on Plaxis, and registered on Target no. 25

It’s possible to see from the chart above that the pace

of displacements is similar, until the first level anchor’s

tensioning, where the movements provided by Plaxis suffer a

rather sharp decrease (movements outside the excavation),

which was not observed in reality with such intensity. In the

following stages the two curves show differences of about 4.5

mm, but with a very similar pace and increasing movements

inwards the excavation, with a lower rate. It is thus quite

plausible to infer that the differences between calculated and

reached horizontal displacements come from the first level

anchors tensioning. For the final stage, the software predicted

that the analyzed point would be placed outside the excavation,

while in reality the opposite situation had happened, albeit

with a difference of a few millimeters, and without reaching

any alert or alarm limits. However, these variations may have

triggered differential displacements in the adjacent building,

where the crack opening occurred.

After a first stage of parametric analysis, where only

the secant stiffness and cohesion were changed, another way

was chosen, since the results weren’t leading to realistic

displacements. Several combinations of the geotechnical

parameters chosen before, plus the friction angle, with a 50%

variation, were made. The results obtained, displayed on the

charts of Fig. 18 and Fig. 19, allowed the drawing of some

conclusions about the influence of the geotechnical parameters

analyzed:

Changing the secant stiffness controls how the field reacts

to the ground anchors tensioning: for higher stiffness, it

looks like the movements due to stress field change

induced by the pre-stress are substantially higher than the

situation in which this parameter is lower. This

phenomenon had already been seen before in the analysis

of the influence of the stiffness, alone.

A decrease in the stiffness together with cohesion induces

the greatest soil decompressions in the initial excavation

stage. This is worsened when decreasing also the friction

angle. However, in both cases there are large

deformations outwards the excavation (when the first

anchor level is tensioned) which are not so intense when

the friction angle is increased simultaneously with the

other parameters.

Increasing the friction angle results in an increase in

wall’s deflection outwards the excavation during the

anchor’s tensioning. This behavior is opposed to the effect

caused by changing the stiffness.

The extreme situation for maximum displacements

inwards the excavation was obtained for the combination

of a 50% increase on both the friction angle and the

cohesion, simultaneously. And the maximum

displacements to the opposite direction were obtained for

the reverse combination, i.e., decreasing the same

parameters by 50%. It follows that for this range of values

the parameters with more influence on the wall’s

deflection are the friction angle and the cohesion,

simultaneously, which define Mohr-Coulomb rupture

criteria.

Based on the results obtained from the parametric analysis it

was possible to estimate the changes to be made to the initial model, so that the deformations predicted could get closer to

those actually observed. Knowing that higher stiffness lead to

larger deformations outwards the excavation at the anchors tensioning stage, it has been decided to increase that

parameter. The decrease in the friction angle also causes the

same reaction, as well as an increase in horizontal movements inwards the excavation, during the digging stages: so, another

change was to decrease the friction angle. It has also been decided to lower cohesion, because this change along with the

decrease in friction angle induced superior horizontal

movements, closer to reality. Several iterations were then carried out in order to approximate the results, whose the most

relevant are displayed in

Fig. 20.

-8

-6

-4

-2

0

2

4

16-Mar 26-Mar 5-Apr 15-Apr 25-Apr 5-May 15-May 25-May 4-Jun

Def. [mm]

Ux_targetno. 25Ux_Plaxis

1st

panel Landfill Partial landfill and

superstructure 2nd

panel 3rd

panel

- 10 -

Fig. 18. Horizontal displacements obtained from changing the parameters E50

ref, cref and φ’ by 50%, of the M1 layer, in combinations of 2

Fig. 19. Horizontal displacements obtained from changing the parameters E50

ref, cref and φ’ by 50%, of the M1 layer, in combinations of 3

Fig. 20. Comparison between the back analysis results and the displacements given by instrumentation

The chart shows that a relatively good approximation

was achieved, particularly in the early stages of the excavation

sequence. From then on, the displacement’s pace along the

constructive stages is largely controlled by the first anchor

level tensioning on the ground, and it hasn’t been achieved an

ideal situation between the displacements. For a 25% increase

in stiffness, and a 25% decrease in friction angle and cohesion,

the final values were quite similar, but the curve pace was

different. In situations where the parameters were changed by

40% and 35%, the tensioning stage was better approximated,

but the final displacements overestimated.

Yet, this trend wasn’t maintained when the

parameters were changed by 50%. This shows once again that

this analysis is of a non-linear type, due to the interdependence

of geotechnical parameters in the constitutive model adopted.

In order to bring the results closer, it was made a new attempt,

which consisted in the increasing of stiffness by 50%,

maintaining friction angle and cohesion at 25%, aiming the

reduction of the horizontal displacements outwards the

excavation due to the tensioning stage, and keeping the

horizontal movement during the digging stage. However, the

results were remarkably similar to the previous iteration

(25%), meaning therefore that by changing those parameters a

better approximation would not be achieved. Thus, considering

the results obtained, and assuming that the remaining

parameters chosen for the model are similar to the real ones, it

follows that the stiffness values are actually higher than those

-18-16-14-12-10

-8-6-4-202468

6 8 10 12 14 16 18 20 22 24

Def. [mm]

Excavation stages

InitialmodelingE+50, c+50

E-50, c-50

E+50, φ+50

E-50, φ-50

c+50, φ+50

c-50, φ-50

-12-10

-8-6-4-202468

6 11 16 21

Def. [mm]

Excavation stages

Initialmodeling

E+50, c+50, φ+50

E-50, c-50, φ-50

-8

-6

-4

-2

0

2

4

6

16-Mar 26-Mar 5-Apr 15-Apr 25-Apr 5-May 15-May 25-May 4-Jun

Def. [mm]

Target no. 25

Initial modeling

E+50, c-50, φ-50

E+40, c-40, φ-40

E+35, c-35, φ-35

E+25, c-25, φ-25

1st

panel Landfill Partial landfill and

superstructure 2nd

panel 3rd

panel

- 11 -

adopted in the initial modeling, and the friction angle and

cohesion values are lower.

In addition to the high level of uncertainty associated with

the parameters assumed in the calculation process and their

relative randomness, there are several possibilities to explain

the discrepancies between reality and numerical results:

The problem’s modeling is made in a bi-dimensional

mode, so it’s not possible to recreate the behavior of the

ground during the execution of the King Post Walls. This

happens because this technique takes advantage precisely

on the arc effect created by the soil benches, which is a

tri-dimensional phenomenon.

The very constructive sequence cannot be perfectly

simulated by the software, which is more suitable for

retaining walls built from the surface and before the

excavation stage, such as diaphragm walls or bored pile

walls;

The carried out modeling didn’t consider several aspects

concerning the timeline along the constructive stages

observed on the field, such as the waiting periods before

the ground anchors tensioning, and the periods during

which the excavation works were temporarily suspended

in side view IH, due to movements measured in the

adjacent building;

The modeling also didn’t consider the fact that there was

heavy rainfall by the initial stage, and it wasn’t possible to

pump all the accumulated water, which may have induced

more movements in the superficial soil layers, due to loss

of the strength and stiffness characteristics;

Insufficient vertical equilibrium, resulting from a deficient

functioning of the vertical profiles, which should support

the wall vertically. A pattern situation was observed, in

which the sides where the vertical profiles were placed

outside the concrete wall, had higher than expected

settlements, unlike what happened on the other sides.

The adopted surcharge in the design stage, used to

simulate the weight of adjacent buildings, may also have

contributed to inaccuracies, for it’s almost impossible to

ascertain the actual surcharges, caused by lack knowledge

about the structural constitution of the buildings and their

foundations.

Damage risk assessment

A brief damage risk assessment to neighboring buildings was

done. The option was to analyze only one of the modeled

sections with an adjacent building – in the middle of side view

IH, near the kindergarten building. Based on the design project

plants, it was assumed that the building is 12.5 m wide inside

the non-excavated area.

This analysis was based on the method described by

Boscardin and Walker [5], and also on a method presented by

Son and Cording [13], suggested by Rankin [14]. A “slight”

damage level was assumed as being reasonable, and given the

aim of this study, the final excavation stage was held as the

most unfavorable one.

1st Method

For a final horizontal deformation of the ground surface of 4.6

mm and vertical deformation of 2 mm, which corresponds to

an horizontal strain (εh) of 0.37x10-3

and an angular distortion

of 0.16x10-3 (for a reference length of 12.5 m), the resulting

damage would be “negligible”. Knowing that the maximum

crack opening in this building was of 0.75 mm, which

according to Son and Cording [13] corresponds to “very slight”

damage, the prediction based on this method is not accurate,

and it wouldn’t be necessary to carry out a more elaborated

risk assessment.

However, it happened that the solution’s modeling

underestimated these deformations, and the real maximum

horizontal and vertical displacements registered in the

kindergarten building (on topographic targets nos. 14 and 26,

installed on the building’s façade) was actually 14 mm inwards

the excavation and a settlement of 10 mm. Using this values,

the horizontal strain would be thus 1.15x10-3

and the angular

distortion thus 0.8x10-3

, and the expected damage level would

be “slight”, which corresponds to the registered damages. It

must be noted that the correct procedure would have been to

use the deformations registered on the ground (with

measurements taken for instance by inclinometers), rather than

measured displacements along the building’s façade, which

may have tampered with the damage predictions; still, this

results show that the damage criterion turned out to be correct,

but their results were tampered by the numerical modeling

discrepancies.

2nd

Method (vertical deformations)

For a 2 mm maximum final settlement on the building base,

and assuming the same 12.5 m wide, the building would have a

predicted slope of 1/6250, which according to Rankin [14]

should lead to “negligible” risk, with unlikely superficial

damage. This prediction underestimates the damages that

actually occurred, although these have been “slight”.

Despite the registered settlements on the topographic

targets were higher (than 2 mm), reaching a maximum of 10

mm, the corresponding slope was from 1/1250, still in a

“negligible” risk range. These discrepancies can be explained

by the fact that the used criterion should be applied to less

sensitive structures, given that this particular building had

already some visible cracks. Assuming the limit cracking

values from differential settlements in buildings, suggested by

Bauduin [4], it was observed that these limit values are in fact

lower: for a deflection ratio of 1, the limits are 1/2500 for

sagging and 1/5000 for hogging. Even without the data needed

to perform a detailed analysis of the settlement profile, it was

clear that the real slope exceeded this limit, leading to crack

opening, as registered on the fissurometers.

This short assessment points out once again the important role

played by a good FE model adequacy to reality, as well as by

the damage criterion chosen for the analysis.

Conclusions

The monitoring of the construction’s works enhanced the

importance to evaluate the on-site conditions, particularly the

neighborhood constraints, involving nearby sensitive buildings,

associated with the ground’s heterogeneity and geological

variability. It also became clear that the constructive sequence

was hard to execute at some points, due to logistical issues,

which are a constant challenge in large projects. Thus, the

instrumentation and its interpretation played a crucial role in

the improvement of the wall movement’s performance,

through the implementation of mitigation measures, which

prevented major damages on the adjacent buildings. Still, the

implementation of a more rigorous Instrumentation and

Monitoring Plan, with a higher number of topographic targets

- 12 -

installed on the wall in depth, and including inclinometers and

topographic marks, would have made possible a better

assessment on the expected ground movements.

The analysis of instrumentation’s results showed that

the excessive settlements measured in some topographic

targets were due to several factors: changes in the constructive

sequence, particularly the extended waiting times between the

panels concreting phase and the anchor’s tensioning;

placement of steel profiles outside the concrete retaining wall,

and consequent deficient vertical loads transmitted to the

bedrock; non-consideration in design of the high precipitation

levels experienced during the initial excavation phases, which

may have decreased the stiffness and strengthness of the

excavated soil, comparing with the geotechnical parameters

assumed initially; non-modeling of adjacent buildings due to

their unknown structural system, which made impossible to

analyze the response to the ground movements induced by the

excavation; differences between the geotechnical parameters

adopted in the modeling and the real ones on the ground, due

to the uncertainty associated with their determination, even

with the lab and field test’s results. However, despite the

ground’s excessive displacements observed, and the cracking

measured in some adjacent buildings, it was visible that over

time, with the execution of the reinforcement measures, the

deformations stabilized until the conclusion of the construction

works.

Regarding the simulation of peripheral walls with the

technique of King Post Walls, there are some limitations

associated with the use of Plaxis in two dimensions, because it

can’t exactly re-create the arc effect originated by the soil

benches when opening the primary panels, and their positive

influence on the induced ground movements. The back

analysis demonstrated that the discrepancies between the

displacements predicted and the actual measurements given by

the instrumentation where probably explained by the fact that

the M1 stratum had a higher secant stiffness than the one

assumed in the initial modeling, and lower friction angle and

cohesion values.

Instrumentation and Monitoring Plan play a key role

in this type of projects involving geotechnical works, and

particularly in the analyzed Case Study, being an important

tool, essential for the implementation in due time, of the

mitigation measures needed to ensure the execution of the

project and its maintenance, as well as an adequate control on

the surrounding structures, in safe and economic conditions.

Acknowledgments

The author acknowledges the Project Owner, Artepura, for the

permission to develop and publish this paper. The support of

Prof. Alexandre Pinto, the scientific tutor, is greatly

appreciated. The author also points out the important role of

the companies involved in the Project, Concreto Plano, JetSJ,

Geofix, Kerpro, Geofix.

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