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Final Report Summary - SCOUT (Sustainable construction of underground transport infrastructures)

Main objective of the SCOUT project was to develop a new concept for sustainable construction of 'cut-and-cover' tunnels that optimises the safety and life-cycle cost of the construction and eliminates or drastically reduces most nuisances to urban environment classically associated to construction projects: noise, dust, and large size of construction equipment causing long traffic disruption at the surface.

The development of the 'Trans-European transport network' (TEN-T ) requires the construction of many new railways or highways or waterborne connections. Underground transport infrastructures are in many cases the best option in urban centres to avoid congestion at the surface and noise impact and in many projects the only possible option to build inter-modal connections such as links between underground stations and airports, parking lots, pedestrian access, etc.

The cut-and-cover construction method is a cost-effective alternative to tunnels, and the best option when the tunnel is relatively shallow (max depth < 20 m) and surface is free from buildings - a roadway for example.

The project used a holistic approach characterised by three main axis of complementary innovations: design, construction equipment and materials. More precisely , main targets of the project were the following :
- Implementation of the 'observational method' for a full control of construction and delays,
- Optimisation of the design leading to important savings in materials,
- A radically new construction concept of equipment, with a modular architecture to be used in a large variety of soil profiles and of tunnel configurations in urban context,
- Development and testing of new applications of composite materials to optimise the efficiency of the structure,
- Leading the way towards the recycling of excavated materials.

The 'observational method' is a recent concept of project management that starts with a risk assessment design and organises a continuous feed-back between observation of real soil conditions and critical review of design assumptions: main achievement of the method is to minimise project costs while maintaining high safety standard at all times. This method of design forms part of the design approaches recommended by Eurocode 7 for geotechnical construction project, but it is not widely implemented.

One of the primary objectives of the SCOUT project was to develop the tools which are necessary for a systematic deployment of the Observational Method (OM) on cut-and-cover tunnels under the framework of Eurocode (EC7 for geotechnical design). The design approach using the OM has been chosen because it offers the following benefits:
- To minimise construction risks by systematic application of carefully planned construction control and
- To provide a sustainable design by design optimisation and savings in construction cost and programme.

There are four main ingredients of the OM approach, namely:
1. national and corporate policies governing design codes
2. corporate and project organisation
3. management structure and process during design and construction stage
4. auditing.

First stage of the research was to collect from the partners case histories of cut and cover tunnels for Soil Types 1 (Clay) and 2 (Sand) with adequate documentation on soil characteristics, soil movements correlated to excavation phases, etc. Attempts were also made to find and collect published examples (English, German, Polish) of cut-and-cover tunnels and other projects. However, measurements of wall performance and construction details have been found only for a limited number of projects, all of them involving excavations in Soil Type 1, i.e. stiff clay.

Further steps of the research involved the following main aspects :
- The definition of a method to determine the 'most probable' and 'characteristic' sets of soil parameters for use in the OM;
- A critical review of widely-used linear elastic-plastic models and parameters;
- A back-analysis of case histories with the appropriate design tool to validate model parameters.

Research on the development of fibre reinforced concrete revealed more difficult and time consuming than anticipated at the beginning of the project. Therefore, potential savings from this technology could not be integrated in this research on design optimisation.
The following aspects of design were considered:
- the analytical model;
- existing crack width requirements, especially in respect to their influence on cost, water ingress and the corrosion of both bar and steel fibre reinforcement;
- the benefits and disadvantages of including non-metallic fibres, water-stops, crack inducers and drains;
- bending moment continuity between walls and slabs;
- provision of haunches to aid arch action in slabs;
- beneficial effects of axial load in the slabs;
- horizontal bending moment continuity.
Precast reinforced concrete, reinforced concrete with conventional steel bars/steel sections and fibre reinforced concrete were also considered. Both single skin and double skin walls were considered in the studies. Consideration was also given to the different loading conditions for double skin wall, during construction and in the long term.

The design optimisation assessments undertaken in the analysis of the cut-and-cover retaining structure have shown that substantial savings can be obtained when one or a combination of the following design approach is adopted:
- The use of more complex finite element design approach;
- Use a variable bending stiffness approach instead of constant stiffness approach in the design of the retaining wall;
- Allow less stringent or even no crack width consideration and properly advise the owner /client of the watertightness and aesthetic issues;
- Use more efficient structural form to encourage arching to reduce the amount of reinforcement.
The optimisation assessments show that a saving of material cost of more than 15% could be achieved based on comparison using conventional reinforced concrete, since fibre reinforced concrete has not yet been sufficiently developed.

The new construction concept was developed around a modular architecture, in two parts:
- first part of the machine was designed and built by Solbac;
- second part of the machine was designed by ECCON and built by Zipacon.

Armines provided a very important support by developing numerical models and executing lab tests supporting the design of the new equipment. Main fields of investigation have been the efficiency of soil cutting tools, the capacity of the machine to evacuate soil cuttings to the surface, the capacity of the machine to build curved walls, etc. Sandvik provided advice on the choice of the cutting tools, and made new development :
- design of a new cutting tool, the 'reversible' pick, that can be used on a cutting drum rotating in two opposite directions;
- Sandvik's current lines of products are optimised to cut through hard rock whereas geotechnical construction uses cutting tools to cut through various types of soils (in presence of groundwater, alternating soft soils with boulders, etc.). A specific study was led to optimise the choice of materials to this specific case of geotechnical works.

A prototype of the new equipment was built, with the capacity to build walls to depth 8 m. A validation site was organised in Solbac's yard, demonstrating the interest of the concept and providing evidence of its capacity to reach the production objectives assigned at the beginning of the project.

A breakthrough concept for the construction of tunnel walls was developed with the objective of building at least 20 m deep walls. A first prototype 8m deep was designed, built and validated on a trial site in Solbac's yard, in Montereau (France). The trial site validated the concept of the new construction equipment.

There was so far little research done on the implementation of new composite materials (fibre concrete) for underground 'cut-and-cover' construction. 'Cut-and-cover' tunnels built under the proposed method offer several opportunities for using these materials, as they will mix temporary and permanent structures. Objective of the project was therefore to investigate in a systematic way how composite materials can be used in both temporary and permanent structure members, so as to establish the basis for optimisation of the whole structure.

Research performed was to investigate the possibility to replace (wholly or partially) conventional concrete reinforced by steel bars, by fibre reinforced concrete (fibre-concrete), without or with additional reinforcement by metallic, non-metallic or mixed (metallic and non-metallic) components.

First step was a comprehensive investigation of the state of the art for concrete constructions with non-traditional reinforcement based on modern composite materials use.

Then most activity was to obtain steel fibre reinforced concrete (SFRC) with enough tensile strength to make an attractive constructive material for the new construction process developed by SCOUT.

The very high resistance requirements of the project induced RTU to perform an investigation of behaviour of 'steel fibre reinforced concrete' (SFRC) with extremely high concentrations of fibres (up to 400 kg/m3), that was validated by full scale beams tested by IBDiM. Major achievement of the project in this domain is a better understanding of SFRC at this high level of fibre content, supported by lab tests, numerical models, full scale tests.

Objective of the project was to address this future requirement and to consider recycling excavated soil as a construction material during the construction process. It was found that, in most cases, the possible uses of excavated materials are somehow limited by the quality of the soils in which the SCOUT machine can operate. Currently, only hard rock excavation debris is reused as aggregates, and only when quality is excellent. Soils are usually disposed in dumping areas, and at most, used in embankments. Most of excavated volume must be anticipated of low-quality. Therefore, work was mostly focussing on reuse possibilities for 'soil type' debris.

Second step was to consider the multiple factors influencing the feasibility of recycling excavated materials : soil characteristics - standards and codes - local regulations - jobsite logistics (availability of space, of local aggregate plants, etc) - cost of plant equipment related to recycling process - potential presence of pollutants requiring specific treatment - etc. The main conclusion extracted from the work carried out is that the possibilities of reuse depend on so many factors -both intrinsic and extrinsic- that no general statement can be made 'a priori' on this issue. Therefore a specific analysis has to be made on a 'case by case' or 'job by job' basis.