Unique and innovative loose mass tunnel opened in Porsgrunn
The collaborative project between AF Gruppen, Norconsult and Jetgrunn on the construction of the Hovenga bulk tunnel is now complete,
and the facility was officially opened on 17 December 2018. The tunnel, which is part of “Bypakke Grenland”, represents a
ground-breaking solution in building in bulk. It is the first bulk tunnel in the world to be constructed with one
permanent, unreinforced base plate made of geoconcrete piles, while dimensioned to resist buoyancy.
Solution
The loose mass tunnel has been built to meet strict requirements for pore pressure, deformations and tightness. When using permanent
sheet pile walls combined with an unreinforced base plate, a rigid, watertight structure was established prior to excavation of
the loose masses. Between the sheet pile walls, a pressure arch of jet piles is formed to resist the forces from buoyancy, without
cracks or unacceptable shear stresses in the construction. This method has ensured stability and minimized deformations.
Method
The project encountered challenging ground conditions, with layers of quicksand and a high groundwater level. To ensure safety against
buoyancy, the jet piles were dimensioned according to Eurocode 7. The permanent bottom plate and the tunnel construction lie below
the groundwater level, and buoyancy safety was ensured both during the construction phase and permanently.
The construction method produced a low CO2 footprint, among other things by using low-carbon cement and by avoiding separate, temporary
constructions. The permanent construction served as boats
bracing and sealing during the entire construction period.
This innovative approach represents a significant advance
in the development of loose mass tunnels.
Compressive strength (J2 piles and EC1 piles)
During the establishment of the J2 piles, samples were taken from them
the return material and molded spout cubes for pressure testing (ex.)
From experience, the compressive strength is somewhat lower in the return material than in
the pile itself.
The dice from the production of EC1 piles are based on samples from
the ready-mixed concrete delivered to the construction site. 288 dice were printed
from J2 piles and 120 dice from EC1 piles.
Jetgrunn’s work in connection with the loose mass tunnel at Hovenga was completed in the summer of 2017. AF started excavation of
the tunnel in late summer 2017, and the breakthrough came on 23 November of the same year. After that time, AF continued to work on
completion of the road surface, wall cladding and technical installations, and completed the works as planned in autumn 2018.
The road was opened to traffic on 7/12-2018, with an official opening on 17/12-2018.
Both the client (the Swedish Road Administration) and the main contractor (AF GRUPPEN) are very satisfied with the result. As expected
the construction is waterproof, and meets all the requirements that were set for both tightness and strength;
