geotechnical risk in railways

UNIVERSIDADE DO MINHO JORNADAS DE ENGENHARIA CIVIL

Author: Antnio Campos e Matos April 2009

GEOTECHNICAL RISK IN

RAILWAYS

Guimares, Portugal


Guimares, Portugal

INTRODUCTION:INTRODUCTION:

In Civil Engineering, we teach and learn how to safely build bridges, buildings, dams, railways, In Civil Engineering, we teach and learn how to safely build bridges, buildings, dams, railways,

sanitary landfills, water treatment plants, etc. sanitary landfills, water treatment plants, etc. First we learn how to design and then how to First we learn how to design and then how to

build.build.

We quickly forget that all that we design and build is surrounded and founded in an environment We quickly forget that all that we design and build is surrounded and founded in an environment

of great variability, which interferes with all of our work.of great variability, which interferes with all of our work.

Almost nothing (or even nothing at all) of what we calculate is exact. The parameters, the Almost nothing (or even nothing at all) of what we calculate is exact. The parameters, the

models, the construction, even the maintenance and operation, are based in variables or models, the construction, even the maintenance and operation, are based in variables or

contain uncertainties or errors.contain uncertainties or errors.


Guimares, Portugal


This variability, which leads to the reality of having to assume that the safety is not unlimited, is This variability, which leads to the reality of having to assume that the safety is not unlimited, is

a source of Risk. Structural Risk is a concept that is generally known and accepted with a source of Risk. Structural Risk is a concept that is generally known and accepted with

reasonable consensus.reasonable consensus.

The necessary safety of the constructions and operations, which the society demands of Civil The necessary safety of the constructions and operations, which the society demands of Civil

Engineering, is provided by Science (Mechanics) and by the Codes. This way, Risk is controlled Engineering, is provided by Science (Mechanics) and by the Codes. This way, Risk is controlled

both during construction and service.both during construction and service.

The width of the concept of Risk, subject that we will not approach at the moment, implies that it The width of the concept of Risk, subject that we will not approach at the moment, implies that it

should not be associated only with the negative aspect of the accident. The financial Risk due to should not be associated only with the negative aspect of the accident. The financial Risk due to

delay or unforeseen events, the Social and Environmental Risk are surely some of the most delay or unforeseen events, the Social and Environmental Risk are surely some of the most

important regarding the activity of Civil Engineering.important regarding the activity of Civil Engineering.


Guimares, Portugal


It happens that the mandatory usage of natural parameters by Geomechanics and Geotechnics It happens that the mandatory usage of natural parameters by Geomechanics and Geotechnics

introduces a strong component of variability and Risk in the system, with no equivalent in introduces a strong component of variability and Risk in the system, with no equivalent in

Structural Mechanics.Structural Mechanics.

In these cases, for Civil Engineers, as well as for other professions that constantly deal with In these cases, for Civil Engineers, as well as for other professions that constantly deal with

Risk, the professional responsibility can assume extremely high proportions. Knowing how to Risk, the professional responsibility can assume extremely high proportions. Knowing how to

manage and control this Risk is an obligation of our activity.manage and control this Risk is an obligation of our activity.

It is curious that, even in the meanders of Civil Engineering, the idea that the Risk is over once It is curious that, even in the meanders of Civil Engineering, the idea that the Risk is over once

the construction of a building is complete seems to linger. The service associated Risk can be the construction of a building is complete seems to linger. The service associated Risk can be

very relevant, both the Risk of defective behavior and the one of loss of performance with time. very relevant, both the Risk of defective behavior and the one of loss of performance with time.

This Risk is called Residual Risk and its characterization is, nowadays, an important aspect of This Risk is called Residual Risk and its characterization is, nowadays, an important aspect of

Civil Engineering investigation.Civil Engineering investigation.


Guimares, Portugal


The construction and service of railways, along with geomechanical aspects of the natural or The construction and service of railways, along with geomechanical aspects of the natural or

artificial surroundings, are an extremely important and interesting example of analysis of this artificial surroundings, are an extremely important and interesting example of analysis of this

type of Risk. In this presentation, it is intended to use a particular and specific phenomenon of type of Risk. In this presentation, it is intended to use a particular and specific phenomenon of

Geotechnical Risk to expose more general aspects of Risk and Residual Risk areas, in Civil Geotechnical Risk to expose more general aspects of Risk and Residual Risk areas, in Civil

Engineering.Engineering.

The presentation is based on a conference which took place in November 2008 in Sabratha, The presentation is based on a conference which took place in November 2008 in Sabratha,

Lybia, with relatively small updates and adaptations.Lybia, with relatively small updates and adaptations.


Guimares, Portugal

TOPICSTOPICS::

1 1 -- Risk General ConceptRisk General Concept

2 2 -- Risk Engineering ConceptRisk Engineering Concept

3 3 -- Risk EvaluationRisk Evaluation

4 4 -- Residual RiskResidual Risk

5 5 -- Residual Risk ManagementResidual Risk Management

6 6 -- Residual Risk Residual Risk ControlControl

7 7 -- Model Limitations and Future DevelopmentsModel Limitations and Future Developments

8 8 -- ConclusionsConclusions


Guimares, Portugal

1 1 RISK GENERAL CONCEPTRISK GENERAL CONCEPT

Risk is a mental perception of danger, not necessarily rational, that spreads across the societies, changing in concept and acceptable values in time and space, as societies do. Humans learn risk since the beginning, since they are born. Societys evolution determines the rules.

Risk is something that affects people and we should learn how to live with it. Civilizations evolution determines more and more responsibilities related with non-correct risk control and mitigation. Modern societies with increasing ethics conscience determine that citizens are more and more concerned with public discussion concerning this subject. This is important for social and individual risk perception, key of human risk management.


If danger perception is an efficient way of risk management and mitigation, then we must be strong and persistent, informing and disseminating in the society that even after a construction, intervention, reinforcement or rehabilitation, a Residual Risk remains on the System. Zero Risk doesnt exist. Immediately after the construction, this risk could be below codes level, but with time things can change.

In this communication, I mainly would like to transmit that in Geotechnical Engineering, the Residual Risk can be much higher than one could expect, during and even immediately after the construction. If so, Risk Control Implementation is an obligation of owners, contractors and engineers.


The self risk perception, and the society risk diffusion, and not only the technical or expert knowledge, is an absolute need to human safety in our societies. Thats why the discussion of these matters on a regular basis is an obligation that we need to fulfil, promoting and increasing citizens perception and specialized experts reaction capacity.The natural fact that everyone has an idea about risk level, but not about risk concept and acceptable values, difficults an analytical and more accurate approach, more and more required by insurance companies, owners, and our society. Thats the engineer case, now. Civil engineering professionals take permanent decisions. The Risk is there.

But Risk in engineering is not only related with potential problems that directly put in danger the human life. Risk is also financial, environmental, sustainability, contractual.Just as an example one could say that, actually, the Risk of Non Control of the Initial Cost (contractual cost) during underground construction is certainly the biggest risk influence on geotechnical systems.

Finally and because engineering is not only a safety control activity but also a cost management activity, one needs to visualise also the opposite risk situation, this is the Opportunity.


Example Project Pittsburgh North Shore

Connector (USA)

Example Results Updates or

Risk Management

2 2 RISK ENGINEERING CONCEPTRISK ENGINEERING CONCEPT

On a mathematical, mechanical and engineering point of view, Risk is absolutely related with existing unknowns in the system. Graduation of the mathematical level of approach to real solutions can be made:

Statistic ApproachesIf we assume that the unknown is related with controlled parameters and also that it is possible to recognize and control the parameters variability, then the Risk is correctly assumed on the system and, on these cases, Statistic Approaches are usable. This is used on special structures, analysis, long tunnels, in comparison solutions between bridges and tunnels, etc.


Deterministic ApproachesHowever, if we accept that unknowns exist but we dont need, or we cannot, recognize completely the system`s parameters as variables, we assume a Deterministic Approach on the identification and characterization in time and space and, as consequence, uncertainty acceptation remains on the system. Thats the case of some environment risk, social risk, etc.

Risk Analysis Matrix


No risk conceptIf the system exists without unknowns, then we say that there is no risk on the system. In some cases, if the risk is very, very small, one could assume that the system has no risk, but this is not mathematically correct and could be mechanically dangerous. Of course this is only a practical common sense definition, because there are no exact systems in engineering, neither in medicine, economy, etc. The variability lives with us, no doubt about this.

3 3 RISK EVALUATIONRISK EVALUATION

In many engineering issues and particularly in geotechnical issues, where solutions live together with enormous impact (urban construction, underground metro, railways, dams), high vulnerability (human presence), it is difficult to control and dominate the variability, before, during and after the construction. A common risk evaluation is:

RISK= PROBABILITY (SOMETHING NEGATIVE) X VULNERABILITY X IMPACT

- Something negative can be, for instance, the structural strength being lower than the structural action, and this is a probability (0 - 1)- Vulnerability is the degree of non protection between the occurrence and the accidental target (human, environment, etc.) (0-1)- Impact is the direct and indirect consequence, is the value that occurs if the vulnerability is total (1). Its a value, or a relative value (human loss, environment affects,) added to the restitution of the previous situation (indemnity, assurance cover, reconstruction, etc.)

This definition is important to postulate that, for us, risk:- Is not only a probability- Neither a vulnerability- Neither a consequence


Practical solutions

There is a an important amount of theories related with uncertainty and engineering systems. The research on these areas is now deep and strong, with conclusions becoming utilized on some practical applications. However, the proper implementation in real systems is not so easy, particularly when the variability or the number of parameters is too high, as is the case of geotechnical systems.

Thats why today I want to talk about possible practical and applicable solutions that show the way to safety decisions, meanings better risk control in our engineering profession.

Because, on the end of the day, we have the responsibility and intention of reducing the risk - and so we use organized procedures and methodologies - the sequence of activities, transverse to Risk Control, can be divided, in my opinion, in three steps:

- Risk assessment (before construction: project, design, decision support)

- Risk management (during construction)

- Residual risk management (after the construction, when the operation starts)


On the first step, the risk is not yet a reality, but here we define the correct or incorrect way we approach the risk issue, therefore the importance of this phase is determinant.

On the second step, unknown, variability, uncertainty remain on the system. However the system includes now a methodology to control the risk.

On the third step one must find and define methodologies and technologies but always supported by a good understanding of remaining unknowns and uncertainties on the system.

For practical reasons, I prefer using the following division of unknown aspects of the system (on these cases, the parameters):

- Uncertainties with recognized random variability distribution (variabilities)

- Uncertainties recognized, but without a random distribution (uncertainties)

- Surprises (if one can not know or suppose the existence)


Common sense speaking:

- Variables, if we know, could know or should know, the existence of such characteristics and related parameter existing values. Also required the possibility of random values for parameters distribution (with more or less approximation).

- Uncertainties, if we know, could know or should know, the existence, or the high potential of existence of such aspects, phenomena, parameters, but because we cannot see it, factually confirm it, we never can relate them by random distribution. We know that they exist but we cannot say where they are or when they happen.

- Surprises, if we dont know or suspect that they could occur.


WHAT WE MAY NOT FORGET IN PRATICAL

Examples:

Risk transmit by information

The geological and geotechnical mass characterization (for instance, obtaining friction angle, cohesion, water level, in space) is based on drills, geophysical site tests, samples and laboratories tests. All of these parameters are absolutely random in each case. So, the final interpretation (geotechnical maps for instance) transmitted to the next person involved (engineer) is absolutely a prediction or prevision (statistical, deterministic, etc). The probability that could occur different values, exists always and can be higher than the engineer could expect and this is Risk based and transmitted by geotechnical prevision. Geotechnical parameters are Variables.


Third Ring Road Deep Foundations

On the TRR, the design of deep foundations considered ground conditions

variability and also different bearing capacity methods (Bustamante / Reese

and ONeil) in order to reduce Risk.

012

34

567

89

10

1112

131415

161718

192021

2223

242526

272829

3031

323334

353637

383940

4142

434445

464748

0 10 20 30 40 50 60 70 80 90 100

Dep

th (m

)

NSPT

Th ird Ring RoadIC 11 (pk 0+ 450 to 0+610)

B H-2 (1977) BH -A1 (2006) BH -A2 (2006) SC -3 (2007)

uncementedmaterial

cemented material


Third Ring Road Deep Foundations

0

5

10

15

20

25

30

35

40

0 1 000 2 000 3 000 4 000 5 000 6 000 7 000 8 000 9 000 10 000

Dep

th (

m)

Rcd (kN)

TRR - IC11 (BH_A2) Axial Capacity (SLS) (Reese & O'Nei l, 1999)

0.60

0.80

1.00

1.20

Diameter (m)

0

5

10

15

20

25

30

35

40

0 1 000 2 000 3 000 4 000 5 000 6 000 7 000 8 000 9 000 10 000

Dep

th (m

)

Rcd (kN)

TRR - IC11 (BH_A2)Axial Capacity (SLS) (Bustamante, 1993)

0.60

0.80

1.00

1.20

Diameter (m)

Bearing capacity of drilled shafts computed with two different methods and

four diameters.

Static and dynamic load tests will be performed to access actual capacity.



Examples:

An uncertainty, not a surprise, neither a random variable

The case of a tunnel: The previous geological studies indicate that the potential of faults occurrence is high, and suggest some possible localization.

During the construction, and after 300m of tunnel excavation, any fault occurs. The optimism was installed on the technical staff, and people start thinking that the geological prevision was too conservative. Safety and site supervision relaxes on the site. But suddenly a collapse comes about, with equipment damage.

The mistake was in considering this fact as a surprise. But really, it was only an uncertainty. They should know, based on the transmitted information, that the faults system was there, even if not on the exact position.

The optimism is not recommended in geotechnics. We never know all before we face the problems, and even after that, frequently we go on with a high degree of unknown (residual risk).


Uncharted fault


Examples:

A code reduction by risk control objective

The case of the concrete strength acceptable on similar concrete in different constructions.

The reasons of such strong cut in the stress (approx. 50% !), and apparently this is an economical error, are related with two aspects:- Unknowns remain on the system of geotechnical characterizations (residual risk)- Non control (neither visual) of the concrete structure (pile), and this is completely different from visible building construction (column)



Example:


RISK CONTROL METHODOLOGIES Summary

Risk Approaches

- Statistic - Deterministic- No risk

Risk Control

- Risk assessment - Risk management - Residual risk management

Category of parameters

- Variables- Uncertainties- Surprises

4 4 RESIDUAL RISKRESIDUAL RISK

On the core of risks geotechnical complex problems, the rock stability analysis is perhaps one of the most interesting issues. That is because:

I) Characteristics:- Rock falls occur suddenly, many times without warning or signals- Rock masses surfaces are very heterogeneous. The variability is very high- Collapsing masses are strong and often affect railways, roads, houses, etc. - The trigger factor is mostly associated with rain effect, seismic effect and human activity.

II) Stabilization measures- Important limits are imposed to the designers, mainly related with environment and economical aspects-Typical solutions are bolts, shotcrete, anchors, meshes, cable meshes, and dynamic barriers

Thats why it makes sense to discuss the performance achieved by each solution. If we assume an important Residual Risk, after the stability intervention, then the evaluation of Performance of the solutions is an important step towards the understanding of the risk. It is dangerous to believe -by optimism or ignorance that the implemented solution absolutely protects us from anything.


The main purpose of this presentation is to discuss the Residual Risk Issue in geotechnicalinterventions and, particularly, in rock fall stabilizations.

To forget or ignore the residual risk is assuming zero risk situation, and this, even beingdeterministically possible, is a dangerous position.

Of course, on scientific and engineering activities, one understands that zero risk is not astatistical or reliability possible condition. However, in other activities, the zero risk isassumed as a possible condition, but this common sense opinion is associated with thenon responsibilities opinions.

On the domain of assumed responsibilities, the question is quite different and itsrelationship with the idea of non-existence of other risks after a convenient intervention tocontrol the main risk.

It happens sometimes the owners have a difficulty in managing this potential residual risk,because the investments in geotechnical stabilizations are understood as a final point onthe matter.

But it must also be clearly assumed by all the intervenients in geotechnical or geologicalprocesses that in many situations one cannot guarantee the complete resolution of thepreviously existing risk. Typically, this RR happens because:

- economical, environmental, social, limited acceptable impacts;

- the strong rock masses variability and potential surprises;

- the designed performance level is not attained on construction end.

At FEUP/GEG, and in association with consultant companies and contractors, theinvestigation of some of these questions is going forward. This document presents rockfalls cases of important RR, that resulted either by an assumed impossibility of significantrisk eliminations, or by not reaching the contractual level of stabilization.



In Structural Engineering, the use of synthetic and fabric materials (concrete, steel,carbon fibber, etc.) width controlled fabrication, determine a insignificant level ofvariability and thats why the reliability safety calculation is more a quasideterministic system than a statistical one, except in what concerns with someactions variability.

Also, we well understand that designer engineers have no responsibility by the materialquality confirmation. He only imposes the desirable parameters values, he doesntverify. So the Risk is much more focused on others aspects (mistakes, modulation,construction, degradation, extreme actions, etc.), than on the materials variations.

Contrary, in typical geotechnical designing projects, engineers have no possibility toimpose or determine the parameters (geomechanics parameters). He needs to identifyand characterize the parameters, and so, he also assumes relevant part ofresponsibility by the correct model performance.

As the geotechnical parameters variability is quite strong, the Risk associated canreach relevant values and appropriated control methodologies needs to beimplemented.

The rock masses are normally characterized by important anisotropy and heterogeneityconditions. By comprehensive reasons, surfaces bands of rock masses, and rockslopes, among all other issues, have the most intensive variability natural conditionsthat one can find. As slopes are directly expose to meteorological conditions, theactions are also difficult to estimate and the quality time degradation is an assumedfact. All this aspects converge to a huge issue. The consequent Risk increases inmagnitude if one looks to the extension and vulnerability of the influence area.



Interesting, is the specific rock slope stability behaviour coming from differentgeological materials. In fact, granite, sandstone, calcareous schist, presents typicalwater behaviour, in relation with the shear strength joint water sensibility, andaccumulated joint water pressure and the drainage. Also some empiric information, aslocalized worst characteristics, potential dangerous surprises, etc., is very important ona global Risk control.


Deviating a little bit of this presentation, it could be curious to look in parallel for otherconsequence of this residual risk and some possible contractual conflict between ownerand contractor in design and build projects, concerning the financial management andthis risk transfer owner objective.

The owner, first responsible by assure safety conditions with limit cost investment, ismany times in difficult decisions conditions mostly because the non-capacity to control acontract on a strong variability media.

On the relationship with the contractor, the Risk management solutions can be listed onthe following cases:

- Assume (A);

- Share (S);

- Transfer (T)


The Financial Risk Transfer is perhaps the most important or frequent aspect of the riskmanagement in geotechnical contracts. The connection with geological variability itsevident.

The owner or the contractor can invest important values in identification andcharacterization parameters, however, always remains a unknown volume ofuncertainties and surprises, that must be controlled. Of course this is not easy, but abalanced equilibrium can be take on the construction contract between both parts.

At all of these cases, the Residual Risk (RR), or the risk that remains after theintervention, must be identified and characterized. If it remains in low level andacceptable by the society and the codes, one can forget the question, otherwise theowner is the first entity that has to assure the resolution.


The RR evaluation can be made in different steps of the process, depending on theobjective of the study.

The Arrbida rock falls present on the following slides is an example of a Pos-constructed RR evaluation and, on this case, it must be used the final characteristicsand parameters, and not the designers prior parameters. Typically an important amountof geotechnical information came to support this phase.

Other important step, is the tender phase proposal preparation, and the contractagreement discussion, in which someone must evaluate the remains risk to betransfered without the support of relevant information. This requires experience,geological studies, and risk analysis process.

Last but not the least, this kind of studies and its convenient divulgation, can increasethe social perception of risk and this will be one of the most effective actions to controldangerous situations.


Among others, we use on this document the common engineer definition:

RISK = (COLLAPSE PROBABILITY) X (VULNERABILITY) X (IMPACT)

The mainly question of the Arrabida Atlantic coast rock fall issue, is that calcareous,carsick, masses are tremendous heterogeneous, with large extension of potentialblocks detach and long possible trajectories. On this morphology and environment,nobody would decide by a stabilization based on concept of total, exhaustive activesolutions, the correct is the passive solution concept.

This concept permites the block detachment and intercepts the trajectories bymechanic systems. Instead of this, the active solution resolves all the stabilization ofpossible detachment without permit displacements and movements. The passivesolution assumes that its not possible to guaranty the total interception of potentialtrajectories and cinematic energy and so, an RR remains on the final implementedsolution.

During this studies an innovation methodology had been introduced, quite useful forrock falls stabilizations risk control:

- assume Vulnerability V as a statistical value, in complete independence withCollapse probability P;

- assume detach probability P as the detachment block (or local masses)probability;

- connect the passive solutions (barriers, mesh, etc.) to the Vulnerability;

- the V value (as a probability) is the non interception trajectories probability;

- admit no active solutions implemented (injections, bolts, etc..) ;

- use appropriated models to evaluate P and V;

- RR= P x V x I


So, the Collapse = Detach Probability P, that we call P, is related to the local collapse,of small masses, or blocks, or ignition rock fall, and depends on rain intensity, soil watersensibility, clayed joints, winds, temperatures, and earthquakes. The internal and externalsystem variability is the well comprehensive parameters variability. P can be evaluatedwith stability models and variability on the strength parameters (cohesion, friction), andexternal forces (particularly water pressure).

The Vulnerability (V), is the statistical 01 capacity for interception and eliminate theenergy protection against block impact and can be evaluate by rock fall models, assumingvariability in morphology, geology and strength parameters.

RESIDUAL RISK = (Detach Probability) X (Vulnerability Probability) X (IMPACT)

RR = P x V x I



In soil, rocks and soil and rock slope materials, first the stability evaluation, and afterthe stabilization solutions, has both a double behaviour concept (internal andexternal), and this is a source of frequently confusion.

Logically, the danger existence is independent of the kind of these phenomena. In spiteof the risk associated with the internal instability is usual greater than the surface orexternal instability, one must concern about the strong risk associated with rock surfaceinstability in roads, railways or urban areas.

The complete risk control must analyse the two possibility of potential danger.

Since some years, and by geomechanics reasons, we separate the externalstabilization solutions in Active and Passive, being passives if the massdisplacements or block movements are authorized, like protection barriers, steel netand cables, tunnels, etc., while the actives doesnt permit displacements(displacements sense associated with discontinuity deformation), like, anchors, bolts,etc.

What we use now, and after the P & V risk model in rock falls, is theassociation of:

- Active like a solution that transform and reducessignificantly P, by increment the local stability.

- Passive like a solution that reduces V, by controlling thetrajectories and energy falls, avoiding hit humans or

properties.


One important intervention has been made in the centre of Portugal Arrbida coast, incalcareous and sandstones masses, very similar with Libya geological situations. Westudy the risk and develop news methodologies for residual risk evaluation.

5 5 RESIDUAL RISK MANAGEMENTRESIDUAL RISK MANAGEMENT


The Arrbida Natural Park

Atlantic Coast

The Residual Risk, associated to problematic natural slopes, results from:

-Extension and magnitude of the problem;

-High heterogeneity of the rocky slopes;

-Exposition to environmental factors;

-Possible climacteric alterations;

-Vulnerability in case of Railways, Roadways or Sidewalks, etc;

-Geological or geotechnical variability, dimensions, block detaching methods;

- Difficulties in the analysis, problem numerical modelling and risk evaluation.

Opposing to the other geotechnical works, the extension, the economical investment and the environmental and landscape reasons dont allow the stabilization and treatment of all the slope.

RESIDUAL RISK


- Evaluate the risk level which is covered by the passive systems of protection (Dynamic Barriers, protection Steel Wire Net);

- Confirm the performance levels and reliability of the systems concerning the land massesdetaching;

- Through statistical analysis and probabilities, evaluate the percentage of occurrenceswhich are not covered by the installed systems;

- Propose complementary systems based on warnings methods to control this residual risk,based in realistic monitored measures, (displacements have no real time utility).


RESIDUAL RISK (R.R.) = P x V x Iv

P Detach Probability [0;1]

V System Vulnerability [0;1]

I v Consequent Impact Value

Energy Failure

Assumed non controllable


P Controlled by ACTIVE solutions

V Controlled by PASSIVE solutions

R.R.

Detach Probability

Energy Failure

Local Geology

RECENT Debris deposits formed by reddish silty material with rocky blocks alreadyconsolidated by carbonated cement.

MIOCNE - Areias da Torre Coarse grain carbonated sandstones. 100m thickness and 25 dip towards north. Medium to low rock strength (c= 6 a 20MPa)

LOWER JURASSIC- Calcrio de pedreiras Limestones strongly carsted. High strength (c= 60 a

150MPa)- Dolomitos do Convento Dolomites and limes with big dolomitic cavernous

blocks. High strength rocks (c= 25 a 100MPa)

Several transversal faults, with N-S to NNE-SSE and NNW-SSE orientation.

Rock mass highly fractured. Joints without filling, exposed to weathering processes,

or joints filled with low strength materials.

The rock mass has many caves and empty spaces inside and outside


Rocky slopes with a high number of fractures and joints


54

Different kinds of material releasing and instabilities

1

2


1

2

3

4

Potential Rock Instabilities

3 4


Different kinds of material releasing and instabilities

Variability in very decomposed rock mass

Soil with a wide ranged granulometry


Complex geotechnical characterization and modelling

Mixed terrains of soil and rock

Conjugation of several instability factors


The stability is highly dependent on the humidity ratios


Cohesion vs. Humidity Ratio

0

10

20

30

40

50

60

70

80

10 15 20 25 30Humidity Ratio, w(%)

Co

he

sio

n, c

(k

Pa

)

Detach Mechanism Resistance Reduction (Water, accumulated rain) and Water Pressure (Instant rain)

Studies from another case developed in similar geological conditions

Friction Angle vs. Humidity Ratio

0

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10

15

20

25

30

35

40

45

50

10 15 20 25 30Humidity Ratio, w(%)

Fri

ctio

n A

ng

le ()

P - Analysis of detach probability or rock fall phenomenon beginning based onsoftware SLIDE (without active control systems)

Stability slope program, based in the limit equilibrium theory of the vertical slice. Determination ofthe safety factors or slope instability probability with Monte Carlo Method.

V - Analysis of vulnerability - probability of installed passive systems failure -based on software ROCFALL

Statistical analysis program based in Monte Carlo Method, which analyses the rock fall in slopes. Permitting the simulation along the slope of an elevated number of rocks, it allows a complete study of energies, heights, velocities, trajectories, and immobilizations points.


RESIDUAL RISK (R.R.) = P x V x Iv

Determination of the critical points of block releasing to use in the cinematicmodelling - Rocfall.

Determination of the detach probability (P) and safety factors of blockreleasing or land masses sliding.

Calculations to be made in allthe analysed cross-sections.

R.R. = P x V x Iv


Used Method:Model SLIDE

Monte Carlo

Variables:

Cohesion cm;

Friction Angle - m ;

Humidity conditions

introduces mechanic variation

Non-circular sliding surface

Results:

F.S.m= 1,30

Detach Failure (P) = 17,1%


Material Properties:

Cohesion cm=50kPa; = 10 kPa

Friction Angle - m = 33; = 5

Generally, the following conditions can be seen:

- Existence of large-sized blocks that can generate huge shock energies upon theirrelease;

- Significant percentage of small and medium-sized blocks that can be controlled withDynamic Barriers;


CharacterizationCharacterization

- Very irregular rock fall trajectories, unable to follow a vertical plan - The detached block never falls on 2D vertical trajectories;

- Complex models of land masses detaching;

- Impossibility to define the fragmentation of the blocks during their falls;

- Highly dependent instabilities with the rainfall (water pressure in short termand strength reduction in long term);

- Impossibility to install the dynamic barriers, steel wire nets and soil nails in allthe natural slope;


64


Statistical distribution of the detached blocks dimension

65

Distribuio do volume dos blocos [m3]

0

20

40

60

80

100

120

140

160

180

200

-3,15

8347

681

-2,67

8217

632

-2,19

8087

584

-1,71

7957

535

-1,23

7827

486

-0,75

7697

437

-0,27

7567

389

0,202

5626

60,6

8269

2709

1,162

8227

571,6

4295

2806

2,123

0828

552,6

0321

2904

3,083

3429

52

Mais

Fre

qu

n

cia

,00%

20, 00%

40, 00%

60, 00%

80, 00%

100, 00%

% a

cu

mu

lad

a

Vm dio Vdimensionamento% ocornica >90%

Energia de dimensionamento das Barreiras [kJ]

0

20

40

60

80

100

120

140

160

180

200

-3,15

8347

681

-2,67

8217

632

-2,19

8087

584

-1,71

7957

535

-1,23

7827

486

-0,75

7697

437

-0,27

7567

389

0,20

2 562

6 60,6

8269

2709

1,162

8227

571,6

4295

2806

2,123

0828

552,6

0321

2904

3,083

3429

52

Mais

Fre

qu

ncia

, 00%

20, 00%

40, 00%

60, 00%

80, 00%

100, 00%

% a

cum

ula

da

Emdia Edimensionam ento% f alha ou rutura < 10%


Vm1m3Vdesign4m3

Blocks Volume Distribution

Design Barriers Energy [kJ]

Statistical determination of block trajectory after the detaching;

Statistical determination of the energies, velocities, heights and immobilizationpoints distributions;

Statistical estimative of the vulnerability (V) of the designed protection systems.

Calculations to be made in all the analysed cross-sections


ModellingModelling

R.R. = P x V x Iv

Surface Variability Pseudo 3D trajectories analysis according with biggestdip slope lines and considering a statistic distribution of the crosses verticespoints.


25 handmade cross-sections in a 100m extension zone according to curve levels and probable trajectories

Materials Variability Restitution coefficients parameter assigned with amedium value and statistical normal distribution ()

Volumetric Variability Statistical blocks volumetric distribution


Distribuio do volume dos blocos [m3]

0

20

40

60

80

100

120

140

160

180

200

-3,15

8347

681

-2,67

8217

632

-2,19

8087

584

-1,71

7957

535

-1,23

7827

486

-0,75

7697

437

-0,27

7567

389

0,202

5626

60,

6826

9270

91,

1628

2275

71,

642 9

528 0

62,

1230

8285

52,

6032

129 0

43,

083 3

429 5

2

Mais

Fre

qu

n

cia

,00%

20,00%

40,00%

60,00%

80,00%

100,00%

% a

cu

mu

lad

a

Vmdio Vdimensionamento% ocornica >90%

Vm1m3Vdesign4m3

Blocks Volume Distribution

ZoneNormal restitution Coef.,

RnTangential restitution

Coef., RtTerrain friction angle,

A) Sliding 0,35 0,02 0,85 0,02 25 2

B) Mixed 0,35 0,02 0,85 0,02 17,5 2

C) Rolling 0,35 0,02 0,85 0,02 10 2

Calculation Method for each cross-section analysis:

- Identification of detach local points (blocks, local mass);

- Statistical distribution of the vertical cross-section points;

- Division in different detach methods (slide, roll, both);

- Parameters assign to different zones;

- Distinct develop analysis (energies and maximum height of trajectories);

- Assign different material types along the slope (intact rock, vegetation,

soil, etc);

- Run the calculation software program with 5000 blocks in each analysed

cross-section.


Results:

Trajectories


Results:

Energies

Immobilization Points


Results:


Calculation Example:

To a specific cross-section we obtained

Venergy failure= 5,3%

V trajectory failure = 7,6%

Conclusions Results: 1 R.REnergy Failure= P x V x I = 0,17x0,053xIv = 0.009 Iv2 R.RTrajectory Failure = P x V x I = 0,17x0,076xIv = 0.013 Iv

Zone% of blocks that pass over the

barriersW eight of the block that reaches the

barrier capacity (kg)A) Sliding 0,1 8763B) Mixed 6,4 5367C) Rolling 3,8 4850

1 5000 4945 3308 0 0,00 96002 5000 4934 3885 0 0,00 100003 5000 4458 1124 0 0,00 85004 5000 4530 1456 0 0,00 85005 5000 4823 1328 0 0,00 70006 5000 3717 1666 0 0,00 101007 5000 4927 3413 0 0,00 99008 5000 4583 3005 21 0,70 65009 5000 4618 2645 123 4,65 610010 5000 4816 2953 42 1,42 700011 5000 4839 2960 13 0,44 760012 5000 4837 2705 69 2,55 620013 5000 4453 3118 446 14,30 550014 5000 4043 1322 186 14,07 380015 5000 4344 2616 46 1,76 360016 5000 4735 3759 285 7,58 340017 5000 4076 3046 321 10,54 510018 5000 4783 4313 5 0,12 410019 5000 4760 3963 289 7,29 500020 5000 4788 4020 371 9,23 490021 5000 4785 4077 194 4,76 520022 5000 4736 3362 270 8,03 280023 5000 4518 4455 8 0,18 570024 5000 4934 4671 0 0,00 510025 5000 4105 3386 21 0,62 6000

Weight o f the block that reaches the barrier capacity (kg)

Barrier impactsSection Nr. of blocks released

Valid b lock re leased

% of blocks over the barrierNr. Of b locks over the barrier

This methods should always be sustained by a data acquisition system,

based on the monitoring of several factors existent on the slopes:

- Deformations / Movements

- Inclinations;

- Vibrations;

- Winds,

- Temperatures;

- Accelerations;

- Humidity ratios;

- Rainfall.

6 6 RESIDUAL RESIDUAL RISK CONTROLRISK CONTROL

Long Time Observation

Cant be used in Alert Systems

Can support the Alert Systems and control de Residual Risk

As seen, the principal detachment factor is the rainfall and the water

presence in the local land mass, due to the long time and instant effects.

Being so, we propose a system to monitor, in real time, the local

pluviometric conditions, permitting the control of the R.R. and increasing

the safety through traffics sign or other automatic systems that are

activated when some established maximum limits are reached.

6 6 RESIDUAL RESIDUAL RISK CONTROLRISK CONTROL

The propose to control the Residual Risk is based in a system that interrupts the

traffic when the humidity ratio (W) or the water level in the local land mass

reaches maximum values. This maximum values are determined throw the

following steps:

1 Laboratory tests in calibrating joints (A);

2 Local Monitoring (B e C)

3 Measuring the local pluviometric curves;

4 Study of the historical relation between the collapses and detachments and the

rainfall occurred;

5 Observation of actual small collapses and relation with cohesion and humidity

6 6 RESIDUAL RISK CONTROLRESIDUAL RISK CONTROL

A B C

- The detachment control should also attend to more exact block detachment

phenomenon and not only to local land mass slope collapse;

- Split the rainfall detach factor in water/impulse and water/resistance

reduction;

- Investigation of meteorological historic (maximum rainfall, return period, etc)

- Mathematical and statistical studies of Residual Risk [ R.R = f (P;V) ];

- Developments and implementation of R.R control technologies.

7 7 MODEL MODEL LIMITATIONS AND FUTURE DEVELOPMENTSLIMITATIONS AND FUTURE DEVELOPMENTS

8 8 CONCLUSIONSCONCLUSIONS

Analysing the results it is clear that the RESIDUAL RISK is still high;

The conducted study is very dependent on the huge variability involved

and the parameters values, reason why they should be very well studied;

The water is the principal factor that promotes the detachment of rocks;

The studies based on probabilities and statistical distributions of the

parameters seems to be the most real approximation to this kind of

problems in rocky slopes;

The RESIDUAL RISK can be controlled, raising the security, with the

installation of some warning systems.

8 8 CONCLUSIONSCONCLUSIONS


Author: Antnio Campos e Matos April 2009

Guimares, Portugal

Thank you very much!

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