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ABSTRACT There are several types of retaining and stabilization structures applied to road

embankments.

The objective of this study is to describe the main criteria and procedures inherent to the

design and subsequent execution of a retaining and stabilization project, different from those

that are generally adopted at a domestically level, on the case of a confirmed slipping

embankment. The complexity associated with the employment of a retaining and stabilization

structure in response to the aforementioned circumstance, in addition to the imperative

assurance of safety conditions along the road travel routes that remain unaffected, determined

the use of various constructive solutions and application of modern technology, predominantly

within the field of geotechnical engineering.

The project under review primarily considers processes of soil stabilization/treatment,

excavation, backfilling, deep drainage and a retaining structure. The solutions adopted for the

completion of these tasks are diverse, including jet grouting technology, micropiles, containment

using big bags and the execution of pavement.

As well as the description of the completed case study, theoretical foundations are

explored in order to understand the discussed techniques. In addition to this, a qualitative

assessment is employed for the evaluation of the various adopted solutions.

A project of this type should ensure maximum safety according to the most various criteria.

For this reason an instrumentation and observation plan, relying on the use of various

monitoring instruments, is employed over and above the use of standard procedures and

search for solutions that make the necessary contributions to safety conditions.

The design of a retaining structure requires expertise in the field of soil

mechanics. Therefore, the intention is to provide the reader with a clear insight into the classical

theories used in the calculation of impulses, massif collapse models and the legislation used in

geotechnical designs of this nature.

1. INTRODUCTION This thesis describes, investigates, analyzes and explains the steps involved in stabilization

of the embankment in motorway A8 at KM92+600.Analysis is conducted from the set up of the

plan for instrumentation and observation to the re-opening of conditioned motorway.It also

presents a theoretical and practical component relative to the conceptualization of the solution

for slope stabilization. The area under study, initially supported by a gabion wall of varying

height, has been the target of instrumentation since 2009 due to the appearance of cracks in

the pavement, indicating small displacements of the embankment that once supported it.

Figure 1.1 – Aerial view of intervention area [1].

On February 9, 2010, a substantial vertical deformation was registered in the motorway

platform, intersecting the land located upstream and downstream of the gabion wall, as shown

in Figure 1.2.

Figure 1.2 – Images of high-way pavement and embankment base on February 9 2010.

1.1. Objectives

This dissertation will be describe and analyze the design criteria adopted in the

implementation and enforcement of the retaining and stabilization structure at KM92 +600 on

motorway A8. Greater emphasis will be placed on the critical analysis of the geotechnical

project. The consequences resulting from the choices made are also verified during

implantation and after work completion.

Taking into account that the retaining and stabilization structure is bound for land that was

already unstabilized, thus requiring the implementation of additional security precautions as well

as the application a constructive model, dissimilar to most conventional solutions adopted in the

retainment of road embankments, it is important to analyze and report on the whole process,

including the soil´s behavior during this process. The study highlights the importance of

instrumentation and observation plan during project execution and after implementation,

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Gabion Wall Unstable area

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allowing for the verification and confirmation of some parameter values, pertaining to the safety

and design of the structure. In this sense, an evaluation is carried out on deformations recorded

by various instruments, both in the surrounding buildings, as well as in the new retaining

structure.

2. ADOPTED SOLUTION The chosen solution consists of reinforced concrete wall in console type "L", based on a

curtain of jet grout columns, armed with micropiles pipes. This permits the partial removal of the

initial gabion wall, allowing relief from the burden of land where it also has the same

destabilizing effect, ensuring an excavation height in the order of 9.0 m. In order to improve the

drainage conditions, reduce the destabilizing weight of land and minimize the pulses to be

supported by the structure, a lightweight aggregate was used in the masonry area of the

concrete wall, properly wrapped in a geotextile for separation.

Figure 2.1 - Cut type of solution adopted to stabilize high-way embankment [3].

The slope stability is achieved by increasing the resistant cutting force at the foot of the

slope via the implementation of jet grout columns and microcutting. This structure is established

Scale 1:200

below the sliding surface and on a layer with good resistance characteristics, enabling the

mobilization of an effective shear strength.

An efficient drainage system is essential in this type of structures, since water causes a

serious decrease in shear strength (due to increased interstitial pressures) and a significant

increase in pulses.

3. PROCESS/CONSTRUCTION PHASING The process/construction phasing include the implementation of the different technologies

already discussed, and adopted in the project, also being subject to the critical analysis of

various factors intrinsic to each solution. The aim is to better understand their functionality and

try to increase their productivity. A brief inventory of materials and equipment used in the

different technologies is also carried out.

The instability of the slope in question had as its main external cause in the seasonal

variation of temperature and humidity conditions associated with the foundation of the gabion

wall, which led to the opening of superficial cracks and encouraged the infiltration of water into

the soil. As an internal cause, the increase in interstitial pressures is considered. Due to water

infiltration, they contribute to the reduction of shear strength. As for the intermediate cause there

is the possibility internal erosion caused by the circulation of water within the slope. The likely

combination of these causes actived a simple rotational slide, evidentiating its geometry via the

location of scaring in the crest area of the embankment and base of the slope.

Given the understanding of the constraints and correct perception of what happened a

project was put in place to stabilize the entire area affected by the sliding surface, which

intersects the land at the base of the gabion wall. Domestically, this is an innovative project,

since the jet grouting technique has rarely been used to contain an unstabilized road

embankment, thus increasing the level of interest both in its construction phase, as well as in

the final behavior of the whole retaining structure.

3.1. Work sequence

The construction phasing can be synthesized into 5 phases:

Phase 1 – Work Preparation and embankment excavation

The first phase included the completion of inspections of housing adjacent to the wall base

and removal of buried services located near the work zone. This phase also included

embankment excavation to reduce the destabilizing actions of the gabion wall and the creation

of a work platform. The reduced platform size, in accordance with need to simultaneously

maintain access to the work zone and operation of the roadway in the opposite direction,

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conditioned the choice of constructive solutions that would determine the appeal of small and

versatile equipment.

Phase 2 – Application of provisional stabilization elements

This phase involved the placement of big bags at the slope´s base, in front of the gabion

wall, and modeling of the work platform to ensure the stabilization of the demonstrated slide.In

an initial phase big bags were only placed along the northern part of the gabion wall, however

after careful consideration of the possible security risks to housing, big bags were placed along

the entire length of the wall.

Phase 3 – Implementation of micropiles and jet grout columns

In this stage, inclined micropiles, made of metal tubing in steel, was introduced, with a

minimum sealing length of 6.0m.

The test grout columns were then put into place, with various dosages of cement. The

grout columns were subsequently implemented, consisting of a retaining curtain, guaranteeing a

phasing that minimizes the impact on the embankment stability. Finally, vertical micropiles

tubes, with 2,60m spacing were placed inside jet grout columns.

Phase 4 – Implementation of reinforced concrete wall

Phase 4 corresponds to the execution of the "L" type concrete wall, properly founded on

micropiles and jet grouting columns and the embankment implementation in the masonry area

of the wall, initially performed with sand from the big bags and subsequently with lightweight

aggregates that were properly compacted and wrapped in geotextile for separation.

This phase also covered operations relating to the partial removal of the gabion wall and

consequent reshaping of the slope. Note that the reshaping of the embankment was carried out

simultaneously to the removal of big bags.

Phase 5 – Reinstatement of high-way platform and implementation of embankment drainage

blanket

In this last phase the high-way platform is reinstated, the draining mask is based at the

bottom of the wall and the roadway is re-open to traffic, its foundation based on a biaxial

geogrid that was in turn positioned on top of the lightweight aggregates.

3.2. Gabion wall

It is essential to describe the characteristics of the gabion wall prior to project execution, as

well as quantify and qualify the type of retentention caused by the wall on the embankment built

upon the site´s natural slope during high-way construction in the year 2000.

Figure 3.1 – Images of gabion wall before (left) e after (right) intervention.

As part of the adopted stabilization solution, the gabion wall is set up under the same

conditions, since its base was not removed.

The performance of the gabion wall of gabions is of paramount in the analysis of slide

causes, to make an observation of the causes of slipping. Its main function was to support the

existing embankment, however it could not perform this efficiently.

The focus point of the critical analysis highlighted in the study for this structure lies in

investigating the conditions of the gabion wall foundation. The foundation of the wall is straight,

formed by the wall, with a support plan consisting of cleaning concrete.

3.3. Provisional embankment stabilization

The big bags were the solution to stabilize the slope on a provisional basis. These worked

by gravity and played an important role in supporting the gabion wall retaining the embankment.

They were also responsible for halting the upward mobilization of soil at the foot of the slope

due to their location in front of the inflection point of the sliding surface.

Figure 3.2 - Detail of the inflection location point on the sliding surface

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3.4. Microcutting

The application of inclined micropiles has several benefits. In relation to the performance

technique and equipment used, its application causes minimal disturbance in the soil and

requires the use of a small and low weight drilling machine compared to other techniques. From

a structural standpoint, the micropiles present several important factors, such as competence to

operate on traction, improvement of soil properties due to the increase in lateral resistance of

the bulb seal and load transfer from the retaining structure to the competent substrate. It is

concluded that the inclined micropiles not only functions as a stabilizing element, but also as

one that provides important support for the concrete wall in case of sliding and overturning.

3.5. Jet grouting

The implementation of test jet grout columns was determined with to ascertain the obtained

diameters, to visualize the appearance of columns and their geometry, and also to collect

samples for laboratory testing. These confirmed the integrity of the bodies and laboratory

determination of its resistant parameters (strain modulus and compressive strength obtained

through the simple uniaxial compression experiments at 7, 14 and 21 or 28 days).

Five JET 1 type test columns were made ("A" to "E") and distributed in two testing areas.

Figure 3.3 – Implementation of test columns in zone 1 (left) and posterior excavation (right).

The jet grouting type JET 1 ensured the specified geometrical and resistant characteristics

and minimized the impact on the behavior of all structures and infrastructure adjacent to the

work site perimeter.

The jet grouting works as an element which increases the shear strength of the sliding

surface, while also transmitting loads from the wall of reinforced concrete into the consolidated

soil. It is thus responsible for stabilization through the stitching effect of unstabilized soil, thereby

increasing soil resistance and acting on the lateral retention of soil (embankment and natural

landscape). With the aid of the micropiles that is applied inside, their performance is extended in

regards to flexion and traction.

3.6. Reinforced concrete wall

The "L" shaped reinforced concrete wall supports the embankment´s surface area and

consequently the whole high-way platform. Due to its functions, that are vital to the sound

operation of the proposed objectives, its constructive procedure and induced alterations are

described.

3.7. Drainage

The existence of a groundwater table in supported massif is a big disadvantage, since it

substantially increases the total impulse. Many accidents involving retaining walls are, in fact

related to the accumulation of water contained in the soil due to inefficient drainage systems [2].

3.8. Embankment and reinstatement of pavement

This subchapter depicts the final work phase, corresponding to the implementation of the

embankment and the subsequent opening of the roadway platform. The materials used for this

purpose are described in the following cross section (Fig. 3.5).

Figure 3.4 – Cross section (ilustration) of the applied pavement.

4. INSTRUMENTATION AND OBSERVATION PLAN (IOP) The aim of the instrumentation and observation plan (IOP) is centered on the prevention

and management of risks in projects with a strong geotechnical component. It seeks to ensure

completion of the tasks associated with performed interventions, within the defined safety and

economic conditions. The behavioral analysis of adjacent structures and infrastructure after

execution is also intended. Therefore, the IOP was defined from the analysis of the main

conditions that would deemingly be the most likely ultimately affect intervention. The analysis of

these conditions facilitated the quantification of key risks associated with this type of projects.

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4.1. Topographic targets

Despite many targets having been placed in gabion wall, it is interesting to note that their

readings are not without many errors, however it is a good reference for the perception of soil

disturbance primarily owing to the implementation of jey grouting.

4.2. Fissuremeters

Fissuremeters are devices that measure the displacement (the plan) of a particular fissure,

over time. The applied fissuremeters were not subjected to strict monitoring conditions. With the

implementation of jet grouting to the south, in the garage vicinity, it was found that the

fissuremeters had registered a significant displacement.

4.3. Topographic marks

The topographic marks, used in measuring the surface elevation of the pavements were

sealed directly onto the high-way platform, having a protected target support with protective

cover at its upper end.

It appears that the accumulated displacements were not considered very significant. These do

not have a very disturbing influence on the instrumented area. resulting in deconfinement

phenomena caused by the slide and repair work.

4.4. Inclinometers

The horizontal displacement of the massif in depth and the high-way embankment retention

will be affected by the installation of inclinometric gutters.

The filling between the hole walls and inclinometric gutters was achieved, with material from

the deformation characteristics similar to the land and surrounding concrete. The sealing of the

fixed point at the base of the instrument was completed at a depth of approximately 3.0 m in

competent substrate (NSPT> 60 blow counts).

5. DESIGN CONSIDERATIONS The design of the retaining structure as well as the foundation and soil characteristics was

created through the computer program PLAXIS Professional V.9. This program facilitated the

calculation of structural stress and deformation during of the various project phases, proving to

be an essential tool for the effective behavioral control of the whole structure. The definition of

all parameters involved in this process is a procedure that requires a certain degree of

sensitivity and experience.

Figure 5.1- Deformed finite element mesh corresponding to the final project

In order to understand the mathematical models and calculation techniques applied in the

computer program it is essential to have some theoretical notions of soil mechanics, as well as

the knowledge of current regulations.

5.1. Safety verification (Eurocode 7)

Safety verification implies that actions be lower than resistance, with an adequate

margin. The use of Eurocode 7 allows for the adoption of an appropriate margin through

methodology that relies on partial safety factors.

In the security verification the following ultimate limit states should be considered:

Overall rupture according to surface involving entire wall [GEO];

Rupture by sliding along the base [GEO];

Rupture by overturn [EQU];

Rupture by resistance mobilization of foundation ground [GEO];

Structural Rupture in wall [STR].

5.1.1. Safety verification in relation to overall rupture (Bishop´s method)

Faced with the currently predicament, this is without doubt the most important check to

perform. The problem of safety in relation an overall rupture is considered the safety verification

of a slope, which involves the investigation of the project site and its vicinity. It also takes into

account the effect that the project has on its environment as well as the effect of the environent

on the current object of study.

The analysis of this problem led to the development of various analytical methods for

calculating slope stability. Of the various methods available, the problem in question will be

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analyzed using Bishop's simplified method of slices method, which applies limit equilibrium

analysis. In order to verify safety the calculation 1 combination 2 approach was employed, a

conditioning combination when at issue is the geotechnical verification. (AC 1 - Combination 2:

A2 + M2 + R1).

Stability verification of Project in question

Note that the PLAXIS Professional V.9 performs this verification using numerical methods.

There are two basic geometries to observe: the one corresponding to the initial project phase

and to the project conclusion. The embankment is considered in drained conditions, without the

presence of water and presenting a circular surface rupture approximate to the surface

estimated in the geotechnical surveys.

The geometry considered for the final Project phase is presented in fig. 5.1.

In the division into slices is necessary to define, first, the critical points: the start and end

points of the surface, the horizontal slope transition point – inclined slope (crest of the slope)

and point of inflection of the curve. It is vital to know the tipping point where the slices to the

right of this point are unstable (negative effect) and to the left are stable (positive effect). The

slope in question has 19 slices and presents the existence of an overload in the order of 10

KN/m2 due to traffic.

Figure 5.2 - Transversal cut in embankment to be analized in fial Project phase by the simplified Bishop method.

The final values obtained from the performed calculations are presented in the following

table:

Table 5.1 - Values of Msd and Mrd obtained using the simplified Bishop method.

CA 1 - combination 2 Mrd [KN.m/m] Msd [KN.m/m]

Simplified Bishop Method 39927,68 25886,33

The resistance moment is far superior to the actuating moment, which means that the

safety condition is verified so it that there is no collapse of the embankment. This can be

confirmed through the established ratio between the resistance moment and actuating moment,

which is 1, 54. The value obtained for the final project phase represents the higher guarantee of

stability than the value usually defined in the project of about 1,5.

6. FINAL CONSIDERATIONS This type of geotechnical Project can be characterized a structure that is permanent

adjustment to geological and surrounding conditions.

It is essential to point out that all the basic elements for implementing the final project

should always be confirmed before and during execution. Therefore, it is essential to highlight

the following points:

Confirmation of the geotechnical zoning and geomechanical characteristics of the land

intervened through the continuous analysis of the same characteristics during the

execution of all the excavation and drilling tasks;

Evidence of dimensions and drainage geometries in existing drainage means to allow

the integration of the proposed drainage systems;

Need to ensure the total lengths, in tandem with the criteria for sealing the micropiles

and jet grout columns in the competent substrate;

The systematic verification of the jet grouting implementation procedures and all

parameters;

Confirmation of all the design and execution assumptions through the implementation of

the Instrumentation and Observation Plan, a proposed management tool in

geotechnical risk, allowing the analysis/prediction of the projects proactive behavior

and, consequently, the timely validation considered.

The implementation of the IOP is becoming common practice and a real asset in the

perception of the behavior of intrinsic agents carrying out a project of this type.

The reduced specificity attributed to the issue of effectiveness in the retention of gabion

walls, as well as the lack of other studies in similar fields, emphasizes the need for further

research in this area. However, it can be stated that the application of a gabion wall as a

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retaining solution of containment should not be as comprehensive as it has to been past,

deserving further in-depth analysis and a more detailed framework.

Emphasis is placed on the difficulty in correlation between parameters that is needed in

order to obtain the characteristics proposed by the project in the implementation of the jet

grouting. Its application and consequent analysis of the test columns confirmed various

parameters mentioned in the theoretical segment of this study.

7. REFERENCES [1] Sítio da empresa Google earth: http://earth.google.com/intl/pt/ (coordenadas: N39° 29.477` W9° 5.587`), visitado em 25/02/2010.

[2] GUERRA, Nuno – Análise de estruturas geoténicas. Instituto Superior Técnico, Lisboa, 2008.

[3] PINTO, Alexandre; TOMÁSIO, Rui - Projecto de execução, solução de estabilização do aterro – Memória descritiva e justificativa. JetSJ Geotecnia Lda., Lisboa, Março de 2010.

[4] PINTO, Alexandre; TOMÁSIO, Rui - Projecto de execução, solução de estabilização do aterro – Peças desenhadas. JetSJ Geotecnia Lda., Lisboa, Março de2010.

[5] GUERRA, Nuno – Disciplina de fundações de estruturas (parte de estruturas de suporte). Instituto Superior Técnico, Lisboa.