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The highway that will disappear in the green “A31” highway section from Vicenza to Rovigo, Valdastico sud job lot. Soilmec SC-90 with BH-12 “Rotograb”. At Valdastico sud, the project includes various highway sections that run in open trenches under ground level or in tunnels constructed by “the cut and cover method”. For lot nr. 7 of the highway construction, the Italian company Serrenissima Costruzioni S.p.A. has entrusted Trevi S.p.A. to execute the diaphragm walls for the “autostrada” sections that run in open trenches, below the ground level and in artificial tunnels constructed by covering the walls. The tunnel section is 350 m long, the covering slab is supported by walls constituted by “T” panels 1,000 mm thick 2,700 mm width up to 35 m deep, while the trench is stabilized by of 1,000 mm and 1,500 mm thick panels 2,500 mm width, 33,60 to 38,5 m deep.
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After a first 7 m thick clay layer, 26-27 m sand and gravel are found that follows a second clay layer up to the requested depth. The water table is located at 1,5 below the ground level.
Diaphragm walls strict principle
For diaphragm walling, the fundamental rule is no deviation to the vertical alignment. In fact, it’s the absence of excavation deviation, as well as the perfect concreting continuity between the panels that secures the wall watertightness. To avoid deviation, different systems have been proposed. An excavation guide, consisting in a long rigid single kelly section (with a length corresponding to the excavation depth) rigidly connected to the grab body or a telescopic kelly, in both cases sliding in an external guide. However, these do not give sufficient guarantee vertical alignment. In fact, the presence of obstacles found when excavating, will cause the deformation of the kelly and a consequently panel deviation that will be practically impossible to correct. This is the reason why Soilmec has proposed for many years the use of rope suspended grabs, free in the excavation but guided out of the trench.
Soilmec BH and GH grabs
BH and GH grab models are rope suspended, hydraulically operated. They are free hanging in the excavation, just guided by the gravity. While outside of the trench they are provided with a guiding system to ease maneuvers. When the grab is lifted out of the trench, the short kelly section fixed to the grab body top slides into the guide freely hanging from the crane boom through universal joint and the grab becomes guided. This external telescopic system provides efficient guiding for quick maneuvers out of the trench and direct unloading over the truck. By grab swinging, the lateral deviation deriving from the teeth dissymmetric fitting may be avoided. By chiseling, transversal deviations may be corrected.
“Rotograb”, an innovative solution
After years of success of BH grab model, Soilmec is bringing onto the market the BH and GH Rotograb version: BHR, and GHR. As previously both models are fitted with telescopic guides (including two sections instead of three for out of the trench maneuvers), but to the contrary of the previous models, the BHR e GHR Rotograb version have the great advantage of having the hydraulic hose coilers located on the crane boom instead of being mounted on the guide itself. This improved coilers location allows BHR “in front of the wall” working, which is the key solution for many job sites located in urban areas. In addition, the coilers installed on the crane boom lower section are easy to reach for maintenance. On a long running job site such as Valdastico, where thousands of square meters have been produced, this point contributes, even indirectly, to reinforce the productivity. In addition to the previous models, Rotograb models are provided with a grab body rotation system, including two groups, allowing the grab rotation to its vertical axis. A hydraulic gear motor moving a swinging gear located at the lower part of the external telescopic guide and a mechanical break located in the short kelly section fixed the grab body.
BH-12R and SC-90 crane: a successful couple
The vertical alignment compliance not depends from the grab only. In fact, because turret crane oscillations on the vertical plane may cause excavated panel transversal deviations, vertical alignment it is also related to the crane. Therefore, the grab carrier has to be very stabile: the grab extraction efforts should not cause any crawler movement. For the same reason, the crane swinging gear should not have clearance to avoid turret oscillations. It is consequently very important for the panel quality to carefully select the crane capacity according to the grab model. Also crane winch capacity and type plays an important role as the capacity and speed are directly conditioning the working sequence duration - and consequently the production - as well as the possibility of excavation correction through the free fall capability. For above mentioned reasons SC-90, 90 t crane fitted with 2 free fall winches 30 t capacity each, is an ideal carrier for Rotograbs. Of course, the same consideration is valid for the new SC-100 - the higher specification model to the SC-90.
Panel joints
The job site has the classical diaphragm walling work organization with bentonite preparation plant, desander, fresh bentonite maturation pool, recovery pool and mixing pool. In diapragm walling the panel joint water tightness is a very sensitive point. All solutions to improve the tightness are based on the principle that the joint tightness is increased - its permeability reduced - when the way to be covered by the water to pass from the external face to internal one increases. Many are the solutions based on large diameter pipes, on flat sheet pile, sheet piles with trapezoidal section, tangent small pipes. All these solutions are offering week and strong points. Heavy large diameter casings, for example, are difficult to handle respect to sheet piles. But generally speaking all classical system are requiring hydraulic extractors that are making the job site organization harder. Trevi has adopted a very original solution, consisting of shaping the panel joint by a means of a provisory breakable PVC pipe. PVC pipes are fixed on both sides of the reinforcement cages of the primary panels, then, when excavating the secondary panel, pipes are destroyed. When destroying the PVC pipe - with a special tooth fitted to the grab -, the concrete of the primary panels is scratched and cleaned presenting a refleshed surface securing the concrete continuity when concreting the secondary panel. Because of PVC pipe form, the panel joint becomes “fi” shaped, a longer joint, increasing consequently the watertightness.
Where is A31?
The road will be installed between open air walls, below ground level or slab covered tunnels. The highway lanes will leave field green-line undisturbed the: no/low noise, no sign... A31 will disappear.
Production
The diaphragm wall average daily industrial production reaches 150 m2 panels by 11 working hours shift. The equipment reliability, the regular grab swinging, the relatively easy soil conditions as well as the support given to the operator by the electronic device embarked on the grab body, with monitor displaying in the cab the panel deviations, give an extremely reduced vertical alignment panel deviation which remain under one percent.
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