Immediate and effective primary support of open-face excavations, using excavators in soft ground and drill+blast in hard rock conditions, is a vital concern for the safety of the workers and the management of ground deformations. The latest development of the tubular arch support system provides for automatic installation of the elements and adds improved worker safety to economic savings.
Development of the hollow steel tube arch support system that is subsequently filled with concrete as part of the installation process, began in 2010 in Italy and has been tested both in a laboratory for necessary validation of numerical analysis, and in the field on several motorway tunnelling projects in Italy to compare the structural response of the tubular concrete-filled arch system to the performance of standard open Ι beam arches.
Typical open steel profiles (IPE, HE, IPN) used as immediate support, show performance weaknesses in their static structural properties in directions other than the normal and central position. In particular conditions, a closed circular steel arch profile will offer better performance, compared to an open profile. It is also acceptable to assume that the worst performance conditions for an Ι profile arch is in the presence of a horizontal load component that can cause buckling of the open profile arch(2). This is clear when analyzing the variation of ΙX (Second Moment of Area) based on the load application angle. With a 45° declivity, the value is half that of the 90° value (Fig 1). These problems can be solved using a symmetrical axial cross section, like a tubular rib. Substituting the open profile with the circular profile of the same area provides a better stress redistribution. The resistant cross section of the tubular arch enables the capability to accommodate and control axial and eccentric loads acting along any direction.
Field tests were carried out to evaluate the compatibility of the circular section ribs with underground work related to installation and stress-strain response. From the operational point of view, the tubular rib behaves in a very stable manner and is easy to handle during transportation and installation. This ensures a high level of safety for the workers. In addition, the high rigidity of the circular profile virtually eliminates the risk of possible buckling during the installation. In all sections tested, the recorded deformation response was always maintained within the elastic range. The tensions measured at stress control stations in the tubular ribs showed significantly lower values when compared with the corresponding standard open profile ribs.
Following each excavation round, the assembly and positioning of steel arch supports by a telescopic handler (or similar equipment), usually requires the presence of workers close to the excavation face to complete assembly of the arch, foot arch positioning, and the installation of the arch and the bracing.
A recent development of the tubular steel arch concept improves safety by introducing a semi-automatic installation procedure.
The tubular arch is assembled in the factory and delivered to site in a folded configuration. In this mode it is transported by a telehandler to the face where it is lifted and opens automatically. When the arch is correctly positioned, a second machine positions the foot of the arch. The bracing elements between arches are preinstalled in the factory, and once in position at the tunnel heading, the second machine pulls the arch towards the previously installed arch. This procedure engages the bracing elements between the adjacent arches, locking them together at the appropriate spacing.
The final operation is to position each foot of the arch to the correct level. This is usually performed by an operative inserting timber packers under each foot. This can be a dangerous operation and is not a robust solution from a technical point of view. To address this, a special telescopic foot has been developed for the tubular arch concept that can be extended by machinery, thereby enabling the positioning of the arch entirely by machinery only. Installation of this kind of steel arch does not require the presence of the worker during the installation, to either assemble the constituent sections of the arch, to install the bracing or to position the foot. This mitigates the hazards associated typically with installation of immediate steel arch support which include falls from heights, restricted operator visibility, and exposure to continuous construction noise.
Once installed, the tubular steel arch is filled with concrete pumped into filling ports on the profile. The filling phase is rapid and functional to ensure the complete filling of the profile. The hinge joints of the tubular arch are characterised by a central hole that enables the creation of a continuous concrete arch within the tubular profile at the completion of the filling phase. This assures the effective collaboration between the steel circular hollow profile and the concrete filling, thereby producing a composite system with enhanced performance behaviour.
The development of this support concept required the detailed design of innovative automatically unfolding hinges, the arch-to-arch connection devices and the arch foot support system.
The joint that enables the automatic unfolding of the arch consists of a ring shaped steel element in two parts, connected via a hinge. Tubular components of the arch are welded into either side of the hinge, such that when the arch unfolds and the hinge is closed, the structural continuity of the tubular section is recreated. A circlip, or Seeger type ring, secures the tubes in position. Calculating the forces that can be absorbed by the ring determines its position considering also the injection ports used during the grout filling of the arch.
The cement mixture, introduced under pressure into the hollow tubular arch, tends to expand the tubular elements against the inner walls of the junction seat (hollow) formed inside of the joint. In addition to the resistance of the ring, this calculation also defines the bead weld resistance that must be executed on the job site to mirror that of the tubular section and maintain structural continuity.
The mechanical properties of the materials that constitute the different components of the elbow joint system (Table 1) are calculated using the standard reference DM14 January 2008(3). Although this is an Italian standard, it follows the Eurocode Prescription(4).
To evaluate the bending moment capacity of the system it is necessary to evaluate the capacity of the welding MRd-Welding and those of the Seeger MRd-Seeger ring. The minimum value of these will represent the bending moment capacity of the junction system MRd-Jointr. This must be compared with the capacity of the tubular profile MRd-Tub. The MRd-Seeger has been calculated considering the maximum tension that can be accommodated by the slot (Table 2). Also in the case of bending moment capacity, the system demonstrates a capacity higher than that of the tubular profile. The junction system has been verified through calculation and demonstrates the capability of the device to ensure the steel arch continuity.
The new tubular arch solution of primary support was applied for the first time in the Pale motorway tunnels in Italy, which were under construction in 2013. Excavation progressed through a massive limestone rock mass with good mechanical characteristics.
The excavated tunnel was supported immediately by shotcrete reinforced by a traditional standard rib made of a single HEB 140 section of steel grade S275 installed every 1.5m. The tubular steel arch selected to substitute the traditional system, was 193.7mm in diameter with 5mm steel wall thickness in steel grade S275. These wereinstalled at 1.8m centres spacing with pairs of crossed bracing elements between the arches.
The deformation in-plane response of the tubular arch support system as monitored and recorded data was always within the elastic range. Displacement and convergence values were below 0.5cm and rapidly stabilized.
In the Pale Tunnels, and as a consequence of good geological conditions, it was possible to increase the spacing of the ribs. It was increased from 1.5m (related to the standard solution using HEB 140) to 1.8m by using the tubular profile which has a greater resistance performance envelope when compared to the open steel arch sections originally proposed.
Additionally, the tubular arch solution is lighter than the one offered by the HEB 140. The use of a lighter and more rigid profile enables the operatives to work in a rapid and safe way.
Within each kilometre of the Pale Tunnels it was possible to save 111 steel arches and a total weight of 244 steel tons. Immediate cost savings are possible due to the reduced steel and, as a consequence, it is possible to generate further saving due to the reduced operational work during the complete construction of the tunnel.
More recently, the semi-automated improvement of the technique was applied in the Boscaccio motorway tunnel in Italy. The added benefits of the improved safety and time and materials savings were demonstrated again on the Boscaccio tunnel (Table 4). The application of this new immediate support system, consisting of concrete-filled tubular arches within a steel fibre reinforced sprayed concrete lining, could, in general, increase the speed of installation by 10% to 20%, depending on local geological conditions.
|Table 3. Reduced use of arches in the Pale highway tunnels in Italy|
|Steel Arch Characteristics||1km Tunnel length|
193.7 – Th.5mm
|Saves 111 arches (16.7%) and 224 tonne of steel (28.3%)|
Another important saving was due to substantially reduced rebound during the placement of shotcrete between the tubular profile arches. During shotcreting of the primary lining, tubular section arches dramatically reduce shotcrete rebound and shadowing where voids can occur behind traditional arch support elements. From an operational point of view, when an operator tries to fill the space between the webs of an open profile support arch, a significant rebound of 50%, or more in the case of coupled arches, has been recorded. This problem does not occur in the case of tubular profile arches and a complete shotcrete filling between the arches is possible.
The use of tubular steel ribs in tunnelling and underground mining operations offers numerous technical, operational, safety and cost benefits compared to traditional steel section arches. The tubular arch hollow sections accommodate axial and or eccentric loads applied from any direction, enhancing the performance of the support system and reducing the required number of supports over the tunnel length. The concrete fill to the steel arch forms a composite structural member.
In addition, increased worker safety is one of the most important considerations for innovation and research and it is one of the most common demands from the underground construction market. Responding to the demand for further improvements in safety, the last development of the tubular steel arch, with its semi-automatic installation procedure allows for quick and easy installation of the immediate support arches without the intervention of workers in this unsupported area of the tunnel face.
Automating these simple procedures using more innovative solutions improves the safety of the underground workplace. It is a right of employees to work in a safe place; it is a duty of the contractor to ensure it happens; but it is a responsibility of the industry to provide solutions that achieve both objectives.