Amaresh Chakrabarti

Segregating discourse segments from engineering documents for knowledge acquisition

Keywords: Knowledge acquisition, mechanical assembly, discourse analysis, segmentation

The broader goal of the research being described here is to automatically acquire diagnostic knowledge from documents in the domain of manual and mechanical assembly of aircraft structures. These documents are treated as a discourse used by experts to communicate with others. It therefore becomes possible to use discourse analysis to enable machine understanding of the text. The research challenge addressed in the paper is to identify documents or sections of documents that are potential sources of knowledge. In a subsequent step, domain knowledge will be extracted from these segments. The segmentation task requires partitioning the document into relevant segments and understanding the context of each segment. In discourse analysis, the division of a discourse into various segments is achieved through certain indicative clauses called cue phrases that indicate changes in the discourse context. However, in formal documents such language may not be used. Hence the use of a domain specific ontology and an assembly process model is proposed to segregate chunks of the text based on a local context. Elements of the ontology/model, and their related terms serve as indicators of current context for a segment and changes in context between segments. Local contexts are aggregated for increasingly larger segments to identify if the document (or portions of it) pertains to the topic of interest, namely, assembly. Knowledge acquired through such processes enables acquisition and reuse of knowledge during any part of the lifecycle of a product.

Introduction

In the process of realizing industrial scale products, assembly is a critical and integrative step. If potential assembly issues can be detected during the planning stages, expensive repetitions in assembly planning can be reduced. In order to do so, knowledge of assembly issues is necessary during the planning stage. Knowledge based systems have been explored as a means of providing such knowledge [START_REF] Madhusudanan | Implementation and initial validation of a knowledge acquisition system for mechanical assembly[END_REF]. Knowledge based systems have been in use in a variety of applications for quite some time now.

The need for using knowledge entities in PLM systems has also been stressed in literature [START_REF] Sadeghi | Collaborative specification of virtual environments to support PLM activities[END_REF][START_REF] Teng | A PPO Model-based Knowledge Management Approach for PLM Knowledge Acquisition and Integration[END_REF]. The ability to manage knowledge generated during a product"s lifecycle is key towards automation of design process [START_REF] Pugliese | About the integration between KBE and PLM[END_REF]. Literature indicates the ability [START_REF] Briggs | Knowledge management in the engineering design environment[END_REF], importance [START_REF] Penoyer | Knowledge based product life cycle systems: principles of integration of KBE and C3P[END_REF] as well as the limited capability of PLM systems to manage and re-use of knowledge across life-cycles [START_REF] Brandt | An ontology-based approach to knowledge management in design processes[END_REF].

The acquisition of knowledge for such systems however, remains a bottleneck [START_REF] Se Savory | Some views on the state of the art in artificial intelligence[END_REF]. Automation of such knowledge acquisition is a larger goal of this research. Specifically, the work reported in this paper is intended to serve as the first step in automatically acquiring diagnostic knowledge pertaining to assembly from documents.

Background

The research reported here is part of a research work aimed at building a diagnostic system for mechanical assembly. In particular the focus is on the manual assembly of aircraft structures. Aircraft assembly is largely a manual process. The planning of such large scale part-assembly processes is a complex task. After an assembly plan is drawn up, in case there are issues while performing the actual assembly, the assembly plan might have to be revised, and each such revision adds to both cost and time delays. If assembly planners possess prior knowledge of such issues in advance, expensive and time-consuming iterations in the planning-assembly-replanning loop can be reduced. Sources of such knowledge are assembly experts, and documented collections of such issues. We chose documents as the source of knowledge for this research, since they in turn reflect the knowledge of experts who prepared them.

Documents as a knowledge source

In professional organizations, documents can be considered authoritative sources of knowledge, since they are usually prepared by multiple experts and undergo many reviews and revisions. They represent a repository of the experiences of multiple personnel. Documents that would be useful for our purpose are incident reports, standards manuals and best practices. Documents are also a step closer to being machine processible than knowledge that comes directly from experts.

Document Segmentation and Classification

Towards acquiring the necessary knowledge from documents, the first step is to identify whether a given document or some sections of the document belong to the relevant domain of interest -in this case, aircraft assembly. For example, a document pertaining to issues in assembly of aft-fuselage is relevant, whereas a document about annual sales of a toy is not. Methods for classifying text are available, notably from the domain of pattern classification and machine learning.

However, such methods typically require training data sets to be available for them to work effectively. However, due to reasons that concern the activities downstream in the knowledge acquisition process (elaborated later in the paper) we chose not to use these methods. It may not be useful at all times to perform this classification only at a document level -sometimes only some parts of a document may be related to assembly (For example, in a document that contains feedback about workplace difficulties from an organization"s employees, only the feedback from shop-floor employees is of interest). The challenge here is to filter such relevant and coherent chunks of text.

Relevant chunks of text are those that semantically relate to the domain of aircraft assembly. By coherent chunks, we mean that these are collections of continuous and meaningful parts of a discourse. These pieces of text then serve as input for acquisition of diagnostic knowledge. We concentrate only on the sections of a document, rather than the entire document here. To summarize, the objectives of this paper are,  To identify coherent sections of a given document  To classify whether such coherent sections of the document pertain to the domain of aircraft assembly (and it"s related domains)

Current methods

A number of methods are available to segment given data into meaningful chunks. As mentioned hitherto, machine learning based methods are quite useful [START_REF] Harr | Learning semantic structures from in-domain documents[END_REF]. However, such methods usually require large amounts of training data to be available, with the data being manually labeled a priori. There are mathematical methods combined with semantics available for text categorization as a standalone application [START_REF] Harr | Learning semantic structures from in-domain documents[END_REF]. Also dedicated efforts have been made to link the referred entity to its counterpart in a knowledge base, based on the topic of relevance [START_REF] Han | An entity-topic model for entity linking[END_REF].

The collection of words in a document can be used to determine the topic of discussion in the document, this being termed as a "bag of words" approach in literature [START_REF] Li | Text document clustering based on frequent word meaning sequences[END_REF]. On a similar note one method uses word-sequences as a means of classification [START_REF] Li | Text document clustering based on frequent word meaning sequences[END_REF]. Document clustering is a popular application of techniques that can work without training data, as opposed to classification methods [START_REF] Nicholas | Recent developments in document clustering[END_REF]. There is existing literature about the use of phrases and their semantic relationship, as well as the use of ontology for clustering [START_REF] Zheng | Exploiting noun phrases and semantic relationships for text document clustering[END_REF]. Clustering documents based on a graph-based technique by detecting frequent sub-graphs of related terms is another method found in literature [START_REF] Shahriar | Gdclust: A graph-based document clustering technique[END_REF]. Another method uses sampling to discriminate segments of documents [START_REF] Harr | Learning semantic structures from in-domain documents[END_REF]. In this, a probabilistic method called Generalized Mallows Model (GMM) is used to model the topics of a text, and is used for segmentation. As regards to current PLM systems, there exists a piece of work to model and elicit information about key relationships and stakeholders by looking at emails [START_REF] Loftus | Capturing key relationships and stakeholders over the product lifecycle: an email based approach[END_REF]. Another relevant research is based on multi-paragraph segmentation using TextTiling algorithm [START_REF] Marti | Multi-paragraph segmentation of expository text[END_REF], which divides a given text into predetermined blocks of equal size, and then looks at the semantic relatedness of words between these blocks. Related blocks are chunked together if they are closer than a specified threshold. This method is tested against the method proposed in this paper.

However, use of such methods may not help in the subsequent steps of knowledge acquisition, which demands an understanding of the document.

Discourse

Discourses are a common form of communication using natural language. They are considered useful to analyze and track the semantic content of a natural language exchange. Discourse analysis has been the focus of study for quite some time now, and there are different theories and approaches to doing so, see for instance [START_REF] Barbara | Attention, intentions, and the structure of discourse[END_REF]. A discourse can be considered to have a hierarchical structure [START_REF] Allen | Natural Language Understanding[END_REF] of segments, each of which is a sequence of clauses. The discourse itself may proceed in various ways, with interruptions, digressions and itemizations amongst the different segments.

Cue phrases

One of the means of distinguishing the boundaries between discourse segments is the use of cue phrases, also known as discourse markers [START_REF] Fraser | What are discourse markers?[END_REF]. Cue phrases such as "after that" and "by the way" signal the transition from one segment to the other. The type of deviation in the discourse context is associated with the type of cue phrase used. Since discourse analysis helps to track how the previous sentence in a text influences the understanding of the current sentence [START_REF] Allen | Natural Language Understanding[END_REF], it is useful to consider documents as discourses, in which one or more authors try to communicate with the reader. The documents that are intended to be used here are those mentioned in the first section. However, technical documents are usually written in a formal manner, and do not resemble other forms of discourse such as conversations. The presence of discourse markers such as cue phrases is not guaranteed in this case.

4 Proposed Method

Assumptions

Before discussing the proposed method it is appropriate to state the assumptions that have been made here,  A document is treated as a one-way discourse between the author and the reader;  The knowledge represented in documents is correct and valid knowledge;  Available semantic resources such as dictionaries and lexica are sufficient to cover the range of terminology used in technical documents.

Comparative studies

An intuitive means of classifying a document or parts of it is to look at the words used and their frequency. In a preliminary exercise, this approach was tried on documents and the results of such a classification were not always indicative of the content at the sentence level.

As mentioned in Section 2.1, TextTiling is another useful way of segmenting sections from a given text. An implementation of the TextTiling algorithm available as part of the NLTK-tokenizer [START_REF]Ntlk tokenize package, text tiling module[END_REF] module was tested on a test document [case study]. Extracts of the text as segmented by the researchers and by the tiling algorithm used are presented in the box below.

The document was 4303 words in length, and was a case study of a wing manufacture [21]. Only a small portion of the entire document was considered. The result was compared with the segmentation of the same portion of the document obtained manually by eight test subjects, including the researcher. During the course of using the algorithm, two parameters needed to be adjusted to get a reasonable number of segments. The parameters that were varied were the block length and the block size.

The combination which resulted in maximum number of segments was finally considered. The final number of segments using TextTiling was 39.

Figure 1 shows a comparison of how the TextTiling implementation performed against the manual segmentation.  In the graph, the red blocks on the second row indicate that 50% or more subjects have indicated a discourse segment i.e. where a shift in focus occurs, similar to that indicated in [Hearst]. This is compared against the segmentation provided by TextTiling, which matches up most of the segments as provided by the manual segmentation too. However TextTiling, by default looks at paragraph breaks as a shift in focus. On such instance in the test document, there was an itemization in the document, which was not perceived as a shift by all but one of the subjects. But tiling treated this as four segments as they appeared on different paragraphs.  For the converse case, where there are multiple segments within a paragraph, tiling had only one exception (due to formatting issues in the input) and performed as expected. Other than these, the segments given by tiling matched with 3 subjects on 4 instances, with 2 subjects on 3 instances, with 1 subject on 4 instances, and with no subjects on 1 instance. Hence the performance for tiling was satisfactory in this case.

Discourse context for segmentation

As seen in the previous subsections, methods such as looking at the frequency of occurrence of words are not useful, since they do not concentrate on the semantic content of the discourse. The semantic content is important from the point of view of future activities in the research, such as identifying the entities in the domain, and extracting diagnostic knowledge that concern these entities.

Although TextTiling has performed segmentation at the most prominent segment boundaries, it identifies other boundaries that are not identified as so by the test subjects. Also, it is a difficult task to keep varying the parameters, namely the blocklength and block-size parameters, for every document.

These parameters are important since the number of segments that are recognized are dependent on them. Moreover, since segmentation is only a preliminary step to enable filtering of relevant text portions. More importantly, we need to understand the content of a document and extract diagnostic knowledge from it. By understanding we mean that one should be able to list the entities and events in the text, and the relations among them. An additional case for using discourse analysis techniques is made by the fact that methods that look at words and their meanings do not address the task of resolving pronouns and anaphora. This is important since pronouns implicitly contain references to other words, and may not be captured by such methods.

In such a situation, discourse context is useful. In a given discourse the current context is defined by the entities that are being talked about, the activities that concern them and the relations amongst these entities. The list of entities is called Discourse Entity (DE) list [START_REF] Allen | Natural Language Understanding[END_REF]. In the domain of assembly the two important factors are product information and the process information [START_REF] Madhusudanan | Combining product information and process information to build virtual assembly situations for knowledge acquisition[END_REF]. These translate respectively to the nouns and verbs of sentences in natural language. Nouns would also cover the peripheral but related terms such as tools and the assembly environment. By treating the document as a discourse, it is also possible to find out which fact entails others by means of inference. With this explanation the procedure for extracting relevant segments from a document is proposed as follows:

 Given text from a document, tokenize it into sentences;  Resolve anaphora and pronouns on a per-sentence basis -This gives a DE list for every sentence;  Segment the sentences which are both contiguous (i.e. within a certain distance d) and share parts of their DE list, within a specified threshold, say N common ;  Once the segments are recognized and marked, compare entities in the DE list to determine to how many of them relate to the assembly domain. The basis for comparison here would the set of terms (and their semantic neighbors) from one or more assembly ontologies;  If, for a given segment, the semantic similarity (as indicated by a certain measure) is greater than a threshold, say D sem , then classify that segment as being related;to assembly. For example, in the WordNet ontology, Jiang-Conrath similarity is one such example of semantic similarity.

Plan for implementation

For tokenization, any standard tokenizer (e.g. NLTK Punkt tokenizer [START_REF]Ntlk tokenize package, text tiling module[END_REF]) that can split the input into sentences is an adequate choice. To resolve anaphora (back references to entities in previous sentences), pronouns and to perform related discourse analyses, methods of representation such as Discourse Representation Structure (DRS) [START_REF] Kamp | Discourse representation theory[END_REF] are available. Once the raw, tokenized text is represented in DRS, existing anaphora and pronoun resolution methods can be utilized. From the DE list, for a combination of d (See Section 4.3) and N common (yet to be decided) the related segments can be separated. Alternatively, one could use unsupervised methods of classification such as k-means to automatically infer two groups. Then the DE list for every segment can be compared against one or more assembly (and related) ontologies [START_REF] Lohse | An ontology for the definition and validation of assembly processes for evolvable assembly systems[END_REF] and classify whether that segment is related to assembly or not, based on the value of D sem (See Section 4.3). Yokohama, Japan, 7 th -9 th July 2014

Conclusions

This paper has discussed the beginnings of a piece of work to acquire diagnostic knowledge for aircraft assembly from documents. In particular a method for segmenting relevant parts of a document that are related to assembly is proposed. Previous methods have been revised, and one method in particular, namely the TextTiling approach, has been tested on a typical aircraft assembly document with segmentation by human subjects as benchmark. As shown in Figure 2, majority of the subjects" segmentation have corresponded to the segments given by TextTiling. However there have been some specific instances where the desired result has not been achieved. The performance of the existing TextTiling method cannot be conclusively ruled out for our purposes -however, a different approach that is more suited to the future needs of the current research has been proposed. TextTiling does not ensure understanding of the text in the document as natural language and there are no measures such as resolution of pronouns and anaphora being employed to acknowledge their role in segmenting coherent sections.

The proposed approach treats documents as a discourse from the experts to the reader.

Techniques from discourse analysis such as pronoun and anaphora resolution can be used to recognize and build coherent sections of a document. The discourse entity list can then be collected from such coherent sections and compared to those from domain ontologies to classify whether each segment is related to assembly or not.

Future Work

The paper has described a method for using discourse analysis techniques to classify relevant sections of a document. Some potential directions for implementation have also been touched upon. Future work includes implementing the method as a computer based program. This implementation then needs to be comprehensively tested to evaluate its effectiveness and to obtain feedback. The results of the implementation then have to be compared against the manual segmentation data as shown in this paper.

Fig. 1

 1 Fig. 1 An extract of the text showing segmentation by the researchers (left)and the tiling algorithm (right). Each change of color in a column indicates a shift in segment

Fig. 2

 2 Fig. 2 Comparison of TextTiling vs manual segmentation for eight readers.

Acknowledgments.The authors wish to thank the members of IDeaS Laboratory at Centre for Product Design and Manufacturing, Indian Institute of Science, who volunteered as subjects for manual segmentation of the assembly text.


Kiril Aleksandrov
email: aleksandrov@fzi.de

Viktor Schubert
email: schubert@fzi.de

Jivka Ovtcharova
email: jivka.ovtcharova@kit.edu

Skill-based Asset Management: A PLM-Approach for Reconfigurable Production Systems

Keywords: asset management system, reconfigurable production system, skillbased model, vertical integration, digital factory

To handle complexity in a modern reconfigurable production system from a strategic and tactical planning perspective as well as to enable operational decision making a skill-based asset management system is introduced. It provides a novel approach of managing assets in the context of the asset lifecycle based on their provided skills. It aims at the vertical integration of higher level management systems with production execution level systems of small and medium sized manufacturing enterprises, using a common skill-based abstraction. Thus the skill-based asset management system can effectively provide digital factory functionality to companies while integrating into their existing IT systems.

Introduction

The increasing external complexity, such as changes in demand or the product, challenges production companies to raise their productivity while making use of the changeability within their asset park [START_REF] Wiendahl | Changeable Manufacturing -Classification, Design and Operation[END_REF]. An evident need is recognized to plan the production organization and to utilize the production assets flexibly and efficiently in order to stay competitive in a rapidly changing market [START_REF] Rogalski | Factory design and process optimisation with flexibility measurements in industrial production[END_REF]. The production resources utilized in a modern production company are highly flexible and adaptable in their skills but there is still a lack in the IT infrastructure to appropriately allow them to use that high variety of operating alternatives in an economic reasonable way. The continuous research in the area of reconfigurable manufacturing systems (RMS) is highlighting the trend toward more flexible and adaptable production systems [START_REF] Wiendahl | Changeable Manufacturing -Classification, Design and Operation[END_REF], [START_REF] Mehrabi | Reconfigurable manufacturing systems: Key to future manufacturing[END_REF][START_REF] Puik | Structured Analysis of Reconfigurable Manufacturing Systems Advances in Sustainable and Competitive Manufacturing Systems[END_REF]. However those types of production systems are associated not only with large capital investments [START_REF] Koren | Design of reconfigurable manufacturing systems[END_REF], but rather with high lifecycle cost. They also increase the internal complexity and provide new challenges in terms of management and planning. They often stay underutilized because of the lack of appropriate tool support for overview, planning and reconfiguration. As a result production companies do not optimally exploit their changeability provided by the available technical infrastructure. Furthermore on an operational level the production is planned by order-centric enterprise resource planning (ERP) software and its execution is controlled by technical shop-floor control systems such as manufacturing execution systems (MES) or production planning and control (PPC) systems. These systems operate on different levels in the enterprises' IT infrastructure and often lack technical as well as the conceptual integration [START_REF] Sauer | lnformationstechnik für die Fabrik der Zukunft[END_REF].

This paper presents the ongoing work towards a skill-based asset management system (AMS) by the European project SkillPro1 . The project aims at the development and utilization of a novel approach for skill-based resource controller architecture in the context of RMS. As one of the main components of the desired project framework the AMS in particular addresses the technological and conceptual gap between upper enterprise level applications and technical processes in the production. It provides a lifecycle approach that combines information both from a technical and a business point of view and via a common skill concept supports an appropriate frontloading towards product development as well as towards asset and factory planning.

Under the consideration of the product lifecycle management (PLM) for manufacturing companies this contribution reviews in paragraph 2 relevant works in the domains of digital factory, enterprise asset management, and skills in production and automation. An overview of the SkillPro framework and a brief introduction of the project's skill concept is presented in paragraph 3. Paragraph 4 describes the skillbased AMS, providing insights on its functionality, its utilization in the IT-landscape of a production company and its main benefits for a modern production enterprise. Finally, paragraph 5 concludes the paper and provides an outlook on future research.

Related works

In the context of the digital factory (see [START_REF] Gu | Hardware-in-the-loop for manufacturing automation control: Current status and identified needs[END_REF]) there have been many findings regarding consistent mechatronic approaches to ensure integrated simulation models recently. The Hardware-in-the-Loop (HiL) approach derived from the embedded systems engineering is increasingly utilized within the digital factory [START_REF] Kjellberg | The machine tool model-A core part of the digital factory[END_REF]. Also aspects of interoperability between IT systems in the digital factory are often addressed besides the designation of virtual models [START_REF] Kjellberg | The machine tool model-A core part of the digital factory[END_REF]. Different formats for model exchange between computer-aided engineering (CAE) or simulation tools, such as the Functional Mock-Up Interface (FMI) [START_REF] Blochwitz | The functional mockup interface for tool independent exchange of simulation models[END_REF] are now discussed regarding the improved simulation of production processes and resources.

Due to holistic simulation approaches as well as the consideration of the lifecycle of assets, the need for robust and consistent product-process-resources (PPR) models as a backbone system for the digital factory is identified. This gradually leads to further transmission of PLM approaches to the systems of the digital factory. Thus the aim is to combine the industrial value creation via a lifecycle-related PPR model [START_REF] Westkämper | Rahmenmodell der digitalen Produktion[END_REF] as depicted in Fig. 1.

Digital factory systems are highly covered in research and even utilized in practice by large enterprises. However, despite their benefits, small and medium sized enterprises (SME) are currently hesitant to utilize them, due to the still poor integration in the preexisting fragmented IT landscape [START_REF] Rogalski | Sustainable Production Planning through Flexibility Measurements in different Manufacturing Organizational Levels[END_REF][START_REF] Himmler | Die Digitale Fabrik -eine Literaturanalyse[END_REF]. To address this issue, new systems are emerging that handle the data management for planning and operating of production plants similar to the well-known product data management (PDM) concepts. One standard, which is strongly considered for this type of systems, is AutomationML2 as an open, XML-based interchange format. It can ensure the interconnectivity and interoperability of the systems, connecting especially MES with the digital factory [START_REF] Schleipen | Production Monitoring and Control Systems within the Digital Factory[END_REF]. The enterprise asset management (EAM) is gaining a lot of research interest in recent years. Initially regarding (technical) assets only from a financial point of view it now focus on different aspects of their lifecycle such as utilization, maintenance, effectiveness, etc. In his work Haider provides an extensive study of the asset management in the context of engineering and infrastructure [START_REF] Haider | Information Systems for Engineering and Infrastructure Asset Management Springer[END_REF]. Although this work is focused on the asset management of big infrastructure plants (water supply or roads) it summarizes an effective definition of an asset in the production domain as well. An asset is a component with economic life span of more than 12 months that has a value profile, and creates and maintains its value by providing services or certain skill sets [START_REF] Haider | Information Systems for Engineering and Infrastructure Asset Management Springer[END_REF]. Haider distinguishes between three types of asset management -strategic, tactical and operational -depending on the management horizon and the type of decisions, indicating that asset management is a lifecycle challenge. Cambel, Jardine and McGlynn do extensive work on providing a framework for best practices and optimization of maintenance of plants, fleets, facilities or equipment from a lifecycle perspective [START_REF] Campbell | Asset Management Excellence: Optimizing Equipment Life-Cycle Decisions[END_REF]. They are addressing the complete asset management from a long up to a short-term perspective. Further multiple research projects engage the topics of enterprise assets and their lifecycle management. Kiritsis discusses the research activities of several European projects in the area of asset lifecycle management (ALM) via semantic technologies [START_REF] Kiritsis | Semantic technologies for engineering asset life cycle management[END_REF]. Tam and Price present an approach for optimizing the asset maintenance [START_REF] Tam | A generic asset management framework for optimising maintenance investment decision[END_REF], which can be crucial if the given asset is a bottleneck in the production system. By using a generic model for the asset based on three decision dimensions (output cost, risk cost and resource cost), the authors provide a decision support framework for maintenance planning. In February 2014 a new ISO standard for asset management system (ISO 55000) 3 emerged focusing on the management of technical enterprise assets mainly in regards to the financial dimension and still incorporating lifecycle considerations. It defines the necessary activities for the asset management and the recommended features of an AMS. However it neither provides specification nor restricts the means of implementation of such systems. Additionally research efforts are directed towards the factory planning and managing of the factory lifecycle defining "factory as a product" and describing the inter-dependencies of the product and factory lifecycles [START_REF] Westkämper | Rahmenmodell der digitalen Produktion[END_REF].

Skill concepts in production and automation have been developed under different perspectives, using different approaches, implementations and naming conventions. Some research activities are focused on the function modelling in the production context. The Function Oriented Product Descriptions (FOPD) approach is used to formally model both product and factory functions [START_REF] Politze | Function Oriented Product Descriptions in Product Development and Factory Planning[END_REF] in order to conduct performance evaluation. Various works focus on the development and utilization of skill concepts for planning and executing tasks in robotics. Significant work is done in the field of service robotics, robot-human interaction and multi-robot coordination [START_REF] Browning | STP: Skills, tactics and plays for multirobot control in adversarial environments[END_REF][START_REF] Kunze | Towards semantic robot description languages[END_REF][START_REF] Tenorth | KnowRob -knowledge processing for autonomous personal robots[END_REF][START_REF] Waibel | RoboEarth[END_REF]. The research in those works is focused on autonomous execution of tasks by means of artificial intelligence. In an industrial context the skill concept is not that highly covered in the research, as the industrial robots are mainly limited in their autonomy [START_REF] Björkelund | Knowledge for Intelligent Industrial Robots[END_REF] and controlled by a central system. Still, Malec et al. describe a skillbased approach for assembly and inspection in robotized work-cells [START_REF] Malec | Knowledge-Based Reconfiguration of Automation Systems[END_REF]. They propose a dual approach for skill definition -top-down definition of production goals and bottom-up description of machine skills, called capabilities. A follow-up research activity is conducted in [START_REF] Björkelund | Knowledge for Intelligent Industrial Robots[END_REF], [START_REF] Björkelund | Knowledge and skill representations for robotized production[END_REF][START_REF] Persson | A knowledge integration framework for robotics[END_REF] extending the skill concept of [START_REF] Malec | Knowledge-Based Reconfiguration of Automation Systems[END_REF] focusing on the knowledge representation of the skill-enhanced PPR model (see [START_REF] Michel | Terminology extracted from some manufacturing and modelling related standards CEN/TC[END_REF]) via semantic web technologies. The presented approaches mainly concentrate on skill execution in various application fields. However, they do not consider the skills from a lifecycle perspective of the asset, such as utilization and maintenance profile of the skills, skill evaluation and comparison, asset wear, change possibilities, etc. Furthermore, skill modelling is focused mainly on robot operations and capabilities and although it provides a good common abstraction for the integration of different productionrelated domains such as manufacturing, logistic or assembly assets, this aspect is still not extensively explored. Also, the current research is mainly aimed at interoperability and interactions of technical resources (assets) and not on the high level integration towards ERP and PLM.

The review of the related works shows that a lot of research is dedicated to the topics of digital factory, asset management and skill-based task execution. However, a combined concept for skill-based asset management to support digital factory is not covered sufficiently.

The SkillPro approach

The European project SkillPro aims at the design and realization of a skill-based framework for reconfigurable manufacturing systems. It can be considered as possible implementation of the plug and produce paradigm [START_REF] Arai | Agile Assembly System by "Plug and Produce[END_REF] concentrating not only on physical and asset level changes but also addressing logical and topological changes and long-term planning. Furthermore it provides a skill-centric view of the productprocess-resource model [START_REF] Pfrommer | PPRS: Production skills and their relation to product, process, and resource[END_REF] as one of its core research activities. The skill concept developed in SkillPro aims at extending the existing skill-based models and frameworks in two aspects. On the one hand it targets a wider scope of assets, focusing not only on industrial or service robotics, but including also logistic assets, CNC (computer numerical control) or DNC (distributed numerical control) machines, other production assets and even human workforce. On the other hand the skill concept is to be utilized not only during execution, but also for operative planning and mid-to-long term asset management. Fig. 2. The Skill concept according to [START_REF] Pfrommer | PPRS: Production skills and their relation to product, process, and resource[END_REF] aligned to the classical PPR [START_REF] Stanev | Production-oriented Product Validation Method as Support for the Reuse of Production Lines in the Automotive Industry[END_REF] The Skill concept is a logical extension of the classical PPR concept in the production context (Fig. 2). Stanev summarizes the PPR dependencies having the asset (resource) as a central element of observation that requires specific properties of the product (such as material) and provides possibilities for its manipulation (via process). Furthermore, a product is an output of the process execution on a certain resource that transforms input products [START_REF] Stanev | Production-oriented Product Validation Method as Support for the Reuse of Production Lines in the Automotive Industry[END_REF]. Another PPR approach that defines the different relations between the objects is the OZONE-model presented in [START_REF] Cutting-Decelle | ISO 15531 MANDATE: A Product-process-resource based Approach for Managing Modularity in Production Management[END_REF]. There however, the product and the resource both provide constraints for the process (called activity) but don't have any direct relation to each other.

The skill concept developed in SkillPro extends the PPR model. It proposes a common abstraction that can be used for production system modeling, production planning and production execution and control, thus allowing a conceptual and technical integration among the production IT systems. Besides the product and the asset that retain their common meaning, the elements of the skill model are:

• Skill: A skill is a placeholder for a process providing metadata (parameters) needed for its specification. Skills can have hierarchical structure, thus building skill taxonomies. Thereby the lower skills are specializations, inheriting all parameters of the parent skills. • Production Skill: A production skill is the assignment of specific production requirements for a product based on its properties. The production skill requires a skill with concrete values for all skill parameters in order to fulfill the production requirements. • Asset Skill: An asset skill represents the possibility of an asset to provide a skill.

The asset skill implements a certain skill, having restrictions for the skill parameters due to specific skill constraints that are defined by the asset. The asset skill can also have additional dependencies to other skills as pre-or postconditions for its execution. Those types of relations are then used for different reasoning operations on skills such as skill matching, skill composition, etc.

Each skill type can have structure, thus defining sequential or parallel skill execution.

Skill-based Asset Management System

The goal of this skill concept is to provide a common model that can be used by the three main components of SkillPro. As depicted in the figure (Fig. 3) these are the skill-based AMS managing the enterprise assets, the skill-based MES scheduling and controlling the execution and the skill-based control system (called skill execution engine -SEE) executing skills on the assets. The AMS system conducts high level feasibility check for production orders, creates coarse production plans and plans reconfigurations. The same model shall also be used on the lower control and execution level for exchanging and interpreting instructions. The three components integrate with each other, based on the common skill model, exchanging skill information and conducting certain operations in their level of concerns. These are based on three different time horizons. During the long-to-mid-term planning the AMS is involved by evaluating the production system, managing the lifecycle aspects of the assets and their skills and preparing the production system for new products and product orders. This includes also the "teaching" of previously unknown skills to the execution controller (SEE). The AMS also provides some services (to the MES) during the short-term-planning (operative) such as retooling/reconfiguration instructions and additional feasibility checks. During execution it provides (to the MES or the SEE) the needed models (regarding product and skill), usually before the first execution of a batch as it acts as a central repository for all skill and object models. Via the SkillPro-Framework the AMS gets product and order information from the PLM and ERP systems and persists it in the SkillPro format. • Long-term scope: Manage assets in context of their lifecycle and in context of their skills lifecycle. Each Asset Skill is provided by a certain asset configuration, for example a tool state of a machine, or specific adjustment in an industrial robot. Assets can provide additional Asset Skills in combination with other assets that are otherwise not available if used independently. The skill-based AMS system can manage both asset and skill lifecycle maintaining lifecycle models for both types of entities and evaluating performance indicators such as time, quality, energy consumption, flexibility or responsiveness, etc. to secure a high lifecycle value and minimize the total cost of ownership (TCO). • The skill-based AMS performs skills matching followed by evaluation for production skills and asset skills. Thereby it identifies different production possibilities for a given asset park and allows for a dynamic assignment of production orders and dynamic generation of the skill-based bill of processes (BoP). By searching for corresponding asset skills to the required production skill (skill matching) the skill-based AMS can perform feasibility checks for new or changed products. Thus it can identify asset bottlenecks, missing assets or outdated assets. It can identify necessity for changes on asset configurations (retooling), topology (changing workflow or communication flow) and topography (layout), based on the currently available and potentially provided skills. • Short-term scope: When the execution planning and control system (MES) is replanning due to failure, the skill-based AMS supports the process by providing other production alternatives for the current production system configuration or withdraws order if production is not possible. Also it serves as model repository providing product and skill models to the other components in the framework. This is especially important for embedded SEE that cannot afford to store multiple models of skills and other data simultaneously. In this way it provides necessary manufacturing information on the shop-floor securing the reconfiguration of assets.

The skill-based AMS shall be a central system for a production company. Depending on the type and size of the enterprise the AMS shall be able to support multiple production facilities on different locations. Hence it is intended that the AMS acts as a cloud-based service provider for the other systems in the framework that are installed on-site for each location or for external components such as ERP or supply chain management systems. In this way it serves the purpose of an operating system for the digital factory and bridges the planning and manufacturing layer. The AMS shall store information about skills and assets for each location and consider them while creating coarse production plans or checking the feasibility of orders. Thus it will extend the flexibility of all production facilities of the company.

Conclusion and outlook

In this contribution a novel skill-based asset management system was presented that aims at supporting the lifecycle management of technical assets in long-to short-term planning activities as well as the vertical integration between different IT systems in a manufacturing enterprise. First the need for such a system was highlighted by examining the current research in the fields of digital factory, asset management and skill-based planning. Then the concept of the skill-based AMS was presented as a part of the ongoing work on the European project SkillPro.

In order to realize a real-time decision making support, further research should be focused on both horizontal and vertical integration in realizing PLM-concepts of frontloading from production to product development or to factory planning and vice versa. As shown in fig. 3 this includes the consideration of an appropriate B2B integration of all stakeholders of the factory. Furthermore, a skill-based AMS can be a data management fundament for cyber-physical systems serving as a service integration platform for technology providers and operators.

Fig. 1 .

 1 Fig. 1. The alignment of the product lifecycle and the asset/factory lifecycle managements in the value creation (according to [11])

Fig. 3 .

 3 Fig. 3. A bird eye perspective on the SkillPro framework architecture aligned to the different IT-levels in a production company (pyramid on the left) and the different planning horizons (on the top). The skill-based AMS constitutes an in-house company-specific knowledge base. During production, it gains content by storing correlations between products and resources via the skill abstraction. It is thus further augmented by new skill definitions and skill properties. Hence the previous experiences can be used for production replanning, estimation of delivery time and strategic planning. In particular the functions of the skill-based AMS are:

www.skillpro-project.eu

www.automationml.org

http://www.assetmanagementstandards.com/

Acknowledgments. This work has been partially funded by the European Commission through SkillPro project (Grant agreement ICT-287733). The authors would like to thank the SkillPro partners for the intensive exchange and collaboration.


Olivier Merle
email: merle@opgc.univ-bpclermont.fr

Laurent Michon

Patrick Bachèlery

Caldera rim collapse: A hidden volcanic hazard

HAL is

Introduction

Flank landslide is one of the most important hazard that can happen in volcanic terranes. The scientific community became aware of the devastating effect of such landslides when the Mount St Helens erupted on May 18, 1980 (Lipman and[START_REF] Lipman | The 1980 eruptions of Mount St Helens: US Geological Survey Professional Paper[END_REF]. A catastrophic collapse of part of the edifice preceded the eruption and the landslide travelled at a velocity of about 200 km/h over a distance of about 25 km. To face such cataclysmic events, there is no way to protect civilians but to evacuate the entire population from the area at risk before the collapse occurs [START_REF] Mcguire | Volcano instability and lateral collapse[END_REF].

This emblematic eruption has been the starting point of numerous studies on this volcanic hazard, which has been shown since then to be a common feature worldwide (e.g. [START_REF] Siebert | Large volcanic debris avalanches: characteristics of source areas, deposits and associated eruptions[END_REF]. Theses studies have allowed making great progress in the understanding of the triggering factors of flank landslide. For instance, triggering factors include dyke or magma emplacement (e.g. [START_REF] Voight | Catastrophic rockslide avalanche of may 18, in the 1980 Eruptions of Mount St Helens[END_REF][START_REF] Dieterich | Growth and persistence of Hawaiian volcanic rift zones[END_REF][START_REF] Mcguire | Recent dyke-induced large-scale block movement at Mount Etna and potential slope failure[END_REF][START_REF] Delaney | Volcanic spreading at Kilauea, 1976-1996[END_REF][START_REF] Tibaldi | Multiple sector collapses at Stromboli volcano, Italy : how they work[END_REF], earthquakes (e.g. [START_REF] Ando | The Hawaii earthquake of November 29, 1975: low dip angle faulting due to forceful injection of magma[END_REF][START_REF] Montaldo | Recent seismicity of Mount Etna : implications for flank instability[END_REF], a weak decollement horizon within the edifice (e.g. [START_REF] Merle | Hybrid collapse mechanism at Piton de la Fournaise volcano, Réunion Island, Indian Ocean[END_REF][START_REF] Oehler | Landslides and spreading of oceanic hot-spot and arc shield volcanoes on Low Strength Layers (LSLs): an analogue modelling approach[END_REF], excess pore pressures due to intrusion (e.g. [START_REF] Voight | Failure of volcano slope[END_REF][START_REF] Elsworth | Flank collapse triggered by intrusion: the Canarian and Cape Verde Archipelagoes[END_REF], regional faulting (e.g. [START_REF] Hall | Tungurahua volcano, Ecuador: structure, eruptive history and hazards[END_REF][START_REF] Ventura | The role of flank failure in modifying the shallow plumbing system of volcanoes : an example from Somma-Vesuvius, Italy[END_REF][START_REF] Merle | Experiments on vertical basement fault reactivations below volcanoes[END_REF], cryptodome emplacement (e.g. [START_REF] Donnadieu | Volcanic edifice stability during cryptodome intrusion[END_REF], volcano spreading (van Wyk de Vries and Francis, 1997), changing sea levels [START_REF] Firth | Coastal elevation changes in eastern Silicy: implications foe volcano instability at Mount Etna[END_REF] and hydrothermal alteration (e.g. [START_REF] López | Catastrophic volcanic collapse: relation to hydrothermal processes[END_REF][START_REF] Van Wyk De Vries | Sector collapse forming at Casita volcano, Nicaragua[END_REF][START_REF] Reid | Volcano collapse promoted by hydrothermal alteration and edifice shape[END_REF]. As suggested by [START_REF] Acocella | Modes of sector collapse of volcanic cones: insights from analogue experiments[END_REF], several factors may act simultaneously to trigger lateral collapse and hybrid mechanisms may be common.

The aim of this article is to present a new mode of landslide formation in volcanic terranes which, to our knowledge, has never been described so far. These collapses may happen in volcanoes that have developed one or several calderas. Caldera infilling by lava flows makes sometimes these structures difficult to be identified in the field, especially when younger and smaller calderas have formed around the main eruptive centre. This is particularly true for large volcanic edifices that have eruptive stories that last more than several hundred thousand years. For example, in oceanic shield volcanoes as Piton de La Fournaise in La Réunion Island (Indian Ocean), nested calderas are partially hidden by following eruptions, active erosion and tropical vegetations that cover old parts of the edifice so that, even when old calderas are suspected, it may be impossible to map along one or multiples ring faults with accuracy. However, such calderas exist and the ring fault is a plane of weakness that may originate catastrophic failure and collapse representing a serious risk for populations living in the vicinity of the potential landslides.

Caldera rim collapse

The formation of a caldera is classically ascribed to the vertical collapse of the overlying roof during emptying of a shallow magma chamber (e.g. [START_REF] Lipman | The roots of ash flow calderas in western North America: windows into the tops of granitic batholiths[END_REF]. This vertical collapse occurs along a ring fault system and leaves a vast, circular and flat-lying depression limited by sub-vertical cliffs that can reach several hundred meters high (Fig. 1a). The size of a caldera is variable and ranges from a few kilometres to several tens of kilometres in width (e.g. [START_REF] Lipman | Calderas[END_REF]. The whole structure is generally considered to be stable, except recurrent rock fall along the sub-vertical cliffs.

However, it has already been advocated that stress concentrations develop around the caldera once the vertical collapse is achieved [START_REF] Gudmundsson | Formation of collapse calderas[END_REF][START_REF] Marti | Vertical and lateral collapse on Tenerife (Canary islands) and other volcanic ocean islands[END_REF][START_REF] Hürlimann | Mechanical relationship between catastrophic volcanic landslides and caldera collapses[END_REF]. This may trigger large-scale destabilization of the flank along a gliding horizon as proposed for the Icod and La Oratova landslides which have broken up part of the ring fault of La Canadas caldera in Tenerife [START_REF] Marti | Vertical and lateral collapse on Tenerife (Canary islands) and other volcanic ocean islands[END_REF][START_REF] Hürlimann | Mechanical relationship between catastrophic volcanic landslides and caldera collapses[END_REF].

Such a lateral escape may indeed happen following caldera collapse but the depression is generally filled by subsequent lava flows coming from the main eruptive centre located in the caldera. Caldera infilling is a slow process during which successive lava flows come to abut against the cliff of the ring fault. When the filling up comes to completion, lava flows swamped the caldera overflowing its rim (Fig. 1b). Then, the ring fault becomes buried and hidden under lava flows and may be undetectable by field studies. When this happens and in the lack of erosion, only geophysical studies like gravimetry, self-potential or high-resolution electrical resistivity tomography may sometimes reveal caldera boundaries (e.g. [START_REF] Tort | The buried caldera of Misti volcano, Peru, revealed by combining a self-potential survey with elliptic Fourier function analysis of topography[END_REF]. Lavas within the caldera exhibit a tabular structure, which is welded along the ring fault to the generally gently sloping lavas and ash deposits of the external flank. In contrast, overflowing lavas dip according to the flank slopes. Such a configuration is not considered to be prone to destabilisation. The hidden ring fault, however, is a near-vertical fault and remains a plane of weakness in the core of the edifice. As long as this fault is laterally buttressed by the flank of the volcano, there is no risk that it can be re-activated, except for a new episode of vertical caldera collapse due to the magmatic evolution of the volcano. Hazard may come from a drastic reduction of lateral confinement and this may happen if, for some reasons, the buttress is removed.

Erosion is a good candidate to slowly remove the lateral buttress that prevents the destabilisation to occur. Regressive erosion through a valley is common in volcanoes once a lateral sector is no longer covered by fresh lavas or volcanoclastic deposits. When the head of the valley is close enough to the rim of the caldera, catastrophic lateral collapse may occur taking away the flank of the volcano and leaving instead a spectacular vertical cliff, which is the limit of the tabular structure of the caldera infilling (Fig. 1c).

Failure occurs along a critical slip surface which looks like a broken line. The upper part of the slip surface follows the sub-vertical ring fault whereas inclined lavas and volcanic deposits of the external flank are suitable levels for the lower part (Fig. 1d). The driving force of the lateral collapse is gravity but several local factors may trigger the instability like pore fluid pressure, a decollement horizon within the volcanoclastic products of the flank as well as the outward inclination of those, earthquakes, rock alteration, etc. For instance, it has been shown that the ring fault of a caldera is a highly permeability plane that preferentially drains fluids (e.g. [START_REF] Lénat | Etude de la zone sommitale du volcan Karthala (Grande Comore) par polarisation spontanée[END_REF][START_REF] Finizola | Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self potential and CO 2 surveys[END_REF], which promotes pore fluid pressure and rock alteration. The key point of the model is that these local factors are unable to trigger the landslide without previous unbuttressing.

Weak lateral confinement of the caldera wall may be achieved by erosion as proposed herein but other mechanisms like downslope flank sliding may also occur. In this case, a limited downward landslide may be the precursor of a larger and dramatic caldera rim collapse. Also, it is known that inward, vertical or outward dipping ring faults may develop during caldera collapse (e.g. [START_REF] Roche | Experimental study of caldera formation[END_REF][START_REF] Acocella | Understanding caldera structure and development : an overview of analogue models compared to natural examples[END_REF]. This is likely to play a role in the process under consideration, as outward dipping faults exhibit orientations that favour the caldera rim collapse whereas inward faults may delay or even prevent it.

The successive stages of the process can be summarized as follows: 1) caldera formation 2) caldera infilling 3) unbuttressing and 4) caldera rim collapse. Such an evolution is not related to the initial slope of the volcano and may happen to both strato-volcanoes and shield volcanoes, once a caldera has been buried by subsequent lava flows. Then, the model may be applied to any type of volcano.

The Mahavel landslide

A clear and striking illustration of the model may be found at Piton de la Fournaise volcano on La Réunion Island. Resulting from a mantle hot spot, La Réunion Island is one of the largest basaltic shield volcano on Earth exhibiting a total thickness of about 7000 m from the oceanic floor to the highest summits, which reach 3000 m high. It is constituted of two juxtaposed volcanic massifs: Piton des Neiges which is dormant for about 12 ka [START_REF] Deniel | New 230 TH and 238 U 14C age determinations from Piton des Neiges volcano, Réunion: a revised chronology for the differentiated series[END_REF] and Piton de La Fournaise which is highly active for about 530 ka [START_REF] Gillot | Géochronologie du Piton de la Fournaise[END_REF].

Piton de la Fournaise activity was characterised by the formation of very large nested calderas whose sizes may be correlated with ages [START_REF] Bachèlery | Le Piton de la Fournaise (Ile de la Réunion). Etude volcanologique, structurale et pétrologique[END_REF][START_REF] Bachèlery | Le Piton de la Fournaise (Ile de la Réunion). Etude volcanologique, structurale et pétrologique[END_REF][START_REF] Bachèlery | Evolution volcano-structurale du Piton de la Fournaise depuis 0.53 Ma[END_REF][START_REF] Mairine | Major erosional period in the building of Piton de la Fournaise (Réunion Island)[END_REF]) (Fig. 2). Classifying according to age and size, one may define "Rivière des Remparts" caldera, the "Morne Langevin" caldera, the "Plaine des Sables" caldera and the "Enclos Fouqué" caldera. The present-day active volcanic centre is located within the smallest "Enclos Fouqué" caldera (Fig. 2).

The 150 ka "Morne Langevin" caldera is a buried caldera, the limit of which is nearly impossible to map with accuracy in the field. The tabular Morne Langevin lavas have entirely filled up the caldera and, overflowing its rim, have flowed down westward onto the old series, called the Mahavel formation, which is dipping south-west at 15-20°. Nowadays, Intense regressive erosion has excavated a spectacular arcuate canyon, the Rivière des Rempart canyon, limited to the west by a vertical cliff of more than 1000 m high (Fig. 2).

When looking at the north-south Rivière des Remparts canyon, it can be seen that the regressive erosion has created two NE-SW oriented arms, the Bras Caron and Bras de Mahavel arms that have propagated in direction of the Morne Langevin caldera (Fig. 2). As classically reported on Piton de la Fournaise massif, the head of the valley is a very steeply dipping cliff, reaching about 1000 m high. This regressive erosion has progressively diminished the lateral confinement of the caldera rim, leading to conditions suitable for a major instability. At 3 am on 6th may 1965, the catastrophic and unpredicted event occurred. In a few minutes, a large portion of the cliff collapsed and a huge mass of dislocated materials was channelled in the Mahavel arm, travelled along 6 km at a very high velocity, and came to abut against the north-south oriented Rivière des Rempart cliff, which stopped it (Fig. 3). In its course, the initial debris avalanche was progressively transformed into debris flow or lahar as a result of abundant surface water. The volume of transported materials was estimated to be in the range from 0.03 to 0.05 km 3 [START_REF] Stieltjes | Evaluation des risques naturels au Piton de La Fournaise[END_REF]. This volume may be usefully compared to the total volume of about 0.02-0.03 km 3 involved in two landslides occurring within a time separation of 7 min at Stromboli volcano. Both landslides set tsunami waves in motion that hit the coast of Stromboli causing severe damage to buildings and structures [START_REF] Bonaccorso | Dynamics of the December 2002 flank failure and tsunami at Stromoboli volcano inferred by volcanological and geophysical observations[END_REF][START_REF] Tinti | Observations of physical effects from tsunamis of December 30, 2002 at Stromboli volcano, southern Italy[END_REF]. The tsunamis were also observed in a wide area around Stromboli along the northern Silicy and Tyrrhenian coasts as well as all over the Aeolian archipelago causing damage on other islands [START_REF] Bonaccorso | Dynamics of the December 2002 flank failure and tsunami at Stromoboli volcano inferred by volcanological and geophysical observations[END_REF][START_REF] Tinti | Observations of physical effects from tsunamis of December 30, 2002 at Stromboli volcano, southern Italy[END_REF].

Due to the nearly uninhabited area and the night event, there were no casualties. However, the slide resulted in a natural dam that obstructed the Rivière des Rempart canyon over its entire width (i.e. 450 m). Then, in a few months, upper waters formed a lake on an area of about 3 × 10 5 m 2 and reaching a depth of about 30 m. This represented a very serious threat for the St Joseph city located 12 km downstream. To avoid the brutal bursting of the dam and a dramatic flood over the city, a channel was excavated to allow the emptying of the lake and a free circulation of running waters.

The cause of the landslide was attributed to several factors including a very rainy period preceding the collapse, the inclined strata of the Mahavel formation and rock alteration. Although these local factors have certainly contributed to the setting in motion of the landslide, careful examination of the scar reveals that these cannot be considered as the primary cause of the event. The scar is an out-ofthe-way place for field studies due to permanent rock fall and crumbling which makes the area very dangerous. However, it is wellvisible from the top owing to an advance of the cliff (Fig. 4). The upper part of the scar is constituted by the tabular formation of the Morne langevin. At the bottom, inclined strata of the Mahavel formation can be seen. These are clearly backed on to the thick lava flows of the Morne langevin. This interpretation is consistent with the observation of the small transverse cliff, which constitutes the lateral limit of the landslide (Fig. 4). There, the near-vertical contact between the alternance of thin lava flows and fall deposits of the Mahavel formation and the thick lava flows of the Morne Langevin formation is especially well-visible. In other words, the Mahavel landslide has brought the buried Morne Langevin caldera ring fault to light and this is the only place where it can be traced out without any doubt, as exemplified by the dense vegetation visible in the background beyond the transverse scar of the landslide (Fig. 4).

This allows a re-interpretation of the Mahavel landslide in terms of caldera rim collapse. The regressive erosion along the Bras de Mahavel arm led to the unbuttressing of the caldera rim and to the re-activation of the plane of weakness that constitutes the ring fault. Along the ring fault, gravity has pulled off and detached the upper part of the Mahavel formation from the tabular Morne Langevin formation, the multiple inclined strata of the Mahavel formation acting as basal sliding planes for the landslide. In the lack of the ring fault which can be considered as the upper part of a pre-existing slip surface, such a huge catastrophic landslide would not have been possible.

Conclusion

It has been shown that caldera rim collapse is an important volcanic hazard, which may represent a serious risk causing disaster and casualties if occurring in densely inhabited areas. Such a process may trigger huge gravity-driven mass-transfer over several 10 km involving several tens of millions cubic metres of material. The natural example taken from Piton de La Fournaise volcano is a large debris avalanche which clearly demonstrates that this volcanic hazard has been underestimated so far. A good evaluation of the volcanic hazard and risk implies the search for this type of structure on all volcanoes that can conceal calderas infilled by lava flows.

Monitoring volcanoes to measure ground deformation with, for instance, geodetic data or highly sophisticated techniques like interferometry radar (SAR), are likely to forecast the timing of eventual collapses. However, it is sometimes hard to make a decision about which part of a volcano should be monitored as potential hazards may be difficult to assess. To this respect, caldera rim collapse reveals that classic and careful field studies remain an invaluable toll to identify hazard, especially in the case of buried calderas. Where buried calderas are suspected, field studies should provide a map where principal hazard and risk are clearly identified around the hidden ring fault, which in turn allows efficient monitoring and successful forecasting.

Fig. 1 .

 1 Fig. 1. Conceptual model of caldera rim collapse in three dimensions (left) and cross section (right). Following caldera formation (a), infilling and overflowing by lava flows take place (b). Partial unbuttressing (c) is achieved through erosion of the flank. The ring fault acts as the upper part of a pre-existing slip surface along which the landslide occurs (d). The final stage (e) displays a spectacular cliff which is the tabular lava stacking resulting from the previous infilling stage.

Fig. 2 .

 2 Fig. 2. a) Digital Elevation Model (DEM) of Piton de La Fournaise showing from west to east the "Rivière des Remparts" caldera rim, the "Morne Langevin" caldera rim, the "Plaine des Sables" caldera rim and the "Enclos Fouqué" caldera rim. Bras Caron and Bras de Mahavel are two lateral arms of the main Rivière des Remparts canyon b) Simplified E-W cross section showing the four main calderas: the Enclos Fouqué caldera (EF), the Plaine des Sables caldera (PS), the Morne Langevin caldera (ML) and the Rivière des Remparts caldera (RR).

Fig. 3 .

 3 Fig. 3. The Bras de Mahavel arm. The 1965 landslide travelled along the narrow valley and was stopped by the 1000 m-high cliff of the Rivière des Remparts canyon, visible in the background.

Acknowledgments

This research has been funded by the French ANR project VOLCARISK (contract 06-CATT-013-04). We thank Valerio Acocella and Adelina Geyer for their constructive reviews of the manuscript.