Greece is currently developing its highway network, part of which is the Patras-Athens-Thessaloniki-Evzoni (PATHE) project, crossing the country from south to north. In the southern part of the route, construction of the ring road around the city of Patras is now under way. A section of this road, known as the ‘K1-K4 section’, includes many bridges and tunnels.
In early 1998, the Greek Ministry of Public Works awarded the construction contract to Pantechniki. The geotechnical company Pangaea Consulting Engineers carried out the design of the tunnels.
Variations in the topography, together with the tunnels’ alignment, dictate the method used to build the underground openings. Three hundred metres of tunnels are to be constructed as cut+cover tunnels, while the remaining 500m are to be built by employing NATM.
There are three twin tunnels (SA, SB and SC) with two traffic lanes in each. The right and left tubes of the SB tunnel have now been excavated.
Geology
Tunnels SB & SA are driven through marls intersected by numerous sandy interlayers 10-100mm thick. The water table is always below their alignment. The clayey-silty marls encountered are stiff, having a uniaxial compressive strength of 200-500kPa. Tunnel SC is an inclined tunnel crossing a hill of weak clayey conglomerate. There are problems associated with excavation through these ground conditions. The position and orientation of the sandy interlayers have a great impact on the excavation and stability considerations for the tunnels.
Design works
The tunnels’ design has been carried out according to NATM principles. The excavation cross section per tube has an area of 105m² (13m wide x 11m high), divided into a 5.8m high top heading, a 4m high bench and a 1.2m high invert. The tunnels are located at a depth of 6-27m from the surface, while the distance between the two tubes is 22-29m.
Support measures included: a 250-300mm thick layer of shotcrete, reinforced with two layers of wire mesh; lattice girders at 600mm to 1m spacing; 6m long anchors, with 44 tonne ultimate load. Excavation was carried out by a Liebherr 911 excavator. The tunnel was designed with a waterproof system for its entire length and a permanent cast in-situ reinforced concrete lining.
Tunnel excavation was monitored by an extensive geotechnical observation programme, which included 3D deformation measurements in the tunnel and on the surface at stations close to portal areas. Information from geological mapping and the geotechnical observation programme were collected and evaluated on a weekly basis. This evaluation of the data and the presence of experienced personnel were the key factors in achieving significant cost and time savings.
The most important design modifications during construction, based on criteria from in-situ observations, were:
Construction of the south portal of the right tube started in July 1998 with excavation of a 12m deep cut inclined at 2:1. Experience gained from this section of the tunnel demanded a careful and appropriate work sequence, which was made plain after a minor failure in the stope wedge. The most significant requirement was the need for shotcrete to be applied immediately after excavation to avoid loss of humidity in the soil and prevent a decrease in the values of its mechanical properties.
As the project proceeded and after work had started in the north portal, it became evident that the south portal area was the most difficult section because of the existence of several minor faults and the presence of numerous sandy interlayers. There was also a leakage to the underground from a water tank on the surface, which led to cracks in the shotcrete.
Geotechnical measurements
The observation programme consisted of 3D deformation measurements in the tunnel at stations located every 5m. Each station included five measuring points (three in the top heading, two in the bench). Absolute displacements in three directions were measured and evaluated.
After completion of the right tube of the SB tunnel, the maximum vertical deformation at the crown was in the range of 46mm in the south section of the tunnel and about 21mm in the rest of the tunnel. Horizontal displacements at the top heading sidewalls reached a maximum of 34mm in the south section, while in the rest of the tunnel they were almost 20mm.
Surface settlements above the tunnel portals were observed by two stations, each consisting of four measuring points. The measurements showed maximum settlement of 27mm above the south portal of the tunnel. A complete evaluation of the displacement measurements concluded that the maximum values of deformation and settlements were within the predicted limits.
Excavation progress
The right tube of the SB tunnel was completed, including concreting of the invert, in March 1999. Excavation rates (top heading and bench) varied as follows:
In the south section of the tunnel, tunnel displacements reached their maximum values when the top heading advanced about 30m from the portal. At that point, top heading progress was halted and the tunnel bench and invert were completed for a length of 20m. After this, deformations relaxed and the contractor, according to the continuous evaluation of the measurements, excavated the top heading along the full length of the rest of the tunnel.
Immediately after completion of the top heading, bench and invert excavation followed from both portals simultaneously. The distance between bench and invert excavation has been maintained in the range of 10m.
Related Files
Figure 3: Typical cross section
Figure 4: Profile and support
Figure 1: Location map
Figure 2: sandy interlayers
