A.I. Sofianos

Structural response of vertically multi-jointed roof rock beams

The mechanical response of multi-jointed roof beams is investigated numerically with the aid of a two dimensional distinct element computer code. Beams of various spans to thickness ratio, that comprise a wide range of practical concern, are simulated numerically and the effect of joint frequency and compliance on their structural behaviour is determined. Numerical results are presented in graphs and useful conclusions for the critical beam parameters are derived. Comparisons of numerical results with analytical solutions define the ranges, where these solutions may be used and the cases where divergence is observed. Finally the graphs presented here may be used for preliminary practical design of pertinent multi-jointed beams.

Tunnelling Mohr-Coulomb strength parameters for rock masses satisfying the generalized Hoek-Brown criterion

Since the early days of the Hoek–Brown criterion [1], difficulties arose because many geotechnical problems are more conveniently dealt with in terms of shear and normal stress rather than in principal stress relationships. Further, most geotechnical software is still written in terms of the Mohr–Coulomb failure criterion. Therefore, it is useful to determine equivalent angles of friction and cohesive strength for each rock mass and stress range. Early derivations were based upon tangents [2] to the Mohr envelope, and later on average values determined by curve fitting over a predetermined range of the minor principal stress from zero to the quarter of the intact rock strength [3]. This latter assumption allowed for the development of two nomograms which provided unique values of f and c for given GSI, mi and sci parameters. Further, it was recognized that the stress field could not be ignored, and for shallow tunnels Hoek proposed that the range should extend to the overburden pressure value. Sofianos and Halakatevakis [4] realizing that the range used to determine the Mohr–Coulomb parameters was different from the existing one in the yielded rock mass around the opening, determined it on principles of mechanics for a rock mass satisfying the original (a 1⁄4 0:5) Hoek–Brown criterion given below:

Numerical evaluation of the response in bending of an underground hard rock voussoir beam roof

The behaviour of a hard rock horizontally stratified roof, within a low primary lateral stress field, is investigated. The lowest roof stratum is modelled as a voussoir beam with three cracks, one at the midspan and one at each abutment. Its response in bending is computed, with the help of a distinct element computer code, for an ordered series of configurations. Two states of failure, i.e. buckling and crushing, are examined. The parameters of indeterminacy of the structure are investigated numerically and improved relations are derived for their evaluation. From the solution of the determined structure, deflections, strains and stability limits are evaluated analytically and validated numerically. Finally, in order to check against crushing, a chart is provided for the prediction of the lateral non-linearly distributed axial strain at the extreme fibre of the abutment.

Pipe jacking a sewer under Athens

A deep level sewer installation, employing a trenchless technology method required as a result of excavations for a new freeway in Athens, is described. For the first time, pipes had to be jacked to drive sections over 200 m in length under the city. In addition to its technical merits, the method provided advantages in terms of social and environmental issues. Very little traffic disturbance was caused, together with no disruption of other services or creation of dust pollution during construction. It also did not damage the pavements of the city. This contrasts to the scars left on the roads, due to trenching. Pipe jacking, however, needs the construction of shafts along the alignment, arranged at distances determined by the cost effectiveness of the system to jack the pipes. It was decided to monitor the jacking force requirements as the pipes advanced. This response during driving was then analyzed and compared to the predictions presented by other researchers. Such measurements and analyses may allow for the formation of a database to permit the right choice of an appropriate excavation unit and the successful planning of any future similar projects.

A fuzzy multicriteria evaluation system for the assessment of tunnels vis-à-vis surface roads: theoretical aspects—part I

Abstract In current practice, the most frequently used quantitative parameter for the selection between a road tunnel and a surface road, is the construction cost. Other parameters that could be of interest to a decision-maker are either absent or existing in a purely qualitative form that does not lend itself for comparisons between alternative solutions. For this reason, a tailor-made methodology was developed, aiming to facilitate the comparative assessment of road tunnels against surface roads on an overall basis. This methodology is based upon the principles of MultiCriteria (MCA) and Cost–Benefit Analysis (CBA), incorporating the full range of cost, technical, operational, environmental and spatial criteria required for the proper evaluation of the two alternatives.

Symmetric wedge in the roof of a tunnel excavated in an inclined stress field

An analytical method is presented for the calculation of the stability of a two-dimensional symmetric rock wedge which is formed in the roof of an underground circular opening within an inclined biaxial stress field. The stability of the wedge is evaluated using a two-stage relaxation procedure. In the first stage, the forces acting on the joint faces of the wedge are calculated analytically from the elastic stress distribution around the opening. Thus, formulae are provided that calculate the horizontal and vertical force components on the wedge faces exerted by the surrounding rock mass. In the second stage, a steadily increasing pull out force is applied to the wedge which strains its joints. Failure is assumed to occur due to simultaneous yield along the face of one of the joints. The stability of the wedge, which is expressed by its pull out resistance, is calculated from limit equilibrium equations at failure. Finally, analytically calculated values of the pull out resistance of the wedge are compared with numerically obtained ones using a discrete element code.

A fuzzy multicriteria evaluation system for the assessment of tunnels vis-à-vis surface roads: the WPMA case-part II

Abstract The Fuzzy Multicriteria Evaluation System (FUMES) is applied on a real evaluation problem such as the Western Peripheral Motorway of Athens (WPMA). The lack of a formal development model of the study area suggested an evaluation strategy, which is characterized by the high uncertainty of the evaluation environment. This has resulted in using judgments when quantitative information on the performance of the tunnel or the surface road was not available. It also resulted in employing a relatively large number of alternative scenarios to deal with the different policy goals. The attempted impact analysis generated crisp and fuzzy scores for each competing alternative. The different scenarios under which the various alternatives were evaluated are of two types: socio-economic and policy scenarios. Subsequently, the criteria weights were computed using the Fuzzy Hierarchical Analysis and the different CBA and MCA methods were applied. Finally, the output of the evaluation determined that the surface road is the most ‘stable’ solution for all the ‘extrovert’ development scenarios, while the tunnel is more preferable for all the ‘introvert’ development scenarios.

Behaviour of Passive Fire Protection K-Geopolymer under Successive Severe Fire Incidents

The performance of a fire resistant coating for tunnel passive fire protection under successive severe thermal loading is presented. The material falls under the class of potassium based geopolymers (K-geopolymer) and was prepared by mixing ferronickel (FeNi) slag, doped with pure alumina, with a highly alkaline potassium hydroxide aqueous phase. Its performance was assessed by subjecting a concrete slab with a five cm thick K-geopolymer coating layer into successive RijksWaterStaat (RWS) fire incidents. During the first test, the maximum measured temperature in the K-geopolymer/concrete interface was 250 °C, which is 130 °C lower than the RWS test requirement, while, during the second fire test, the maximum temperature was almost 370 °C, which is still lower than the RWS requirement proving the effectiveness of the material as a thermal barrier. In addition, the material retained its structural integrity, during and after the two tests, without showing any mechanical or thermal damages.

Settlements Due to Single and Twin Tube Urban EPB Shield Tunnelling

Abstract Shield type Tunnel Boring Machines have been used for several decades; however little information exists about the actual mechanism of shield-ground interaction. The ground response mechanism induced by shields is difficult to explain, since it engages both reliable ground deformation measurements in the field, along with relative operation parameters records of the mechanized shield. Existing empirical and analytical relations that are used for the calculation of ground settlement do not take into account the operation parameters of the shield performing the excavation. Even numerical analyses consider generally only ground characteristics and no more than simple geometrical data are required for the simulation of the shield. It is the objective here to include the operation parameters records in the interpretation of surface settlements due to Earth Pressure Balance Shield (EPBS) tunnelling. Use is made of data on ground movement deformation and EPBS operations that have been collected for Metro Projects of Athens and Thessaloniki. Such data are the ground settlements, ground qualities and EPBS operation parameters measured at the single and twin tunnels of these Metro projects. The transverse settlement troughs are drawn and their shape discussed. The ground conditions are similar over long tunnel stretches, which allows one to establish relationships between ground characteristics and EPBS operations to surface deformations. A comprehensive tunnelling data acquisition system was developed to collect information from in situ data on a ring-by-ring basis, such as geological conditions, shield operation parameters, tunnel geometry. Having utilized the aforementioned information and correlated the operation parameters to the maximum influence on ground movements, an artificial intelligence system, based on Artificial Neural Networks (ANN), is employed to develop predictive relations. In order to minimize the risk of ground movement, the tunnelling engineer and the EPBS pilot need to be able to make reliable prediction of settlements based on the operational data. Such an ANN algorithm has the ability to map input to output patterns i.e. all influencing parameters are related to surface settlements. It serves as a means to understand and predict ground movement behaviour induced by EPBS shield tunnelling, and deals as an indicator for construction control and for the implementation of remedial measures where needed.

Tunnel Design and Sensitivity Analysis

Sensitivity analysis should be employed to examine the effect of the variability of selected parameters on critical response of a construction, using credible upper and lower bounds. This investigation performs such an analysis for an actual tunnel design case by means of numerical modeling. A local method, which is a partial derivative of the output with respect to several input factors, is used so as to ensure ease of use for the designer. In this way, the effect of varying geotechnical parameters over their possible values with respect to internal forces and displacements of the tunnel temporary lining is evaluated. The analysis includes not only assessment of the greatest influence on the item under observation by use of new types of diagrams that provide better comprehension but also a first order reliability evaluation on the effect of input parameters with respect to the reinforcement requirements of the temporary lining.