As a result of the great demand for an effective mass transit system for Romania’s capital, The Bucharest Metro Company was established in 1975 to begin the construction of a new metro network. Today the system encompasses 71.6km of underground lines, 45 stations and 4 maintenance depots. The design for a new ‘Line 4’ started in 1988 and included a series of site investigations, hydrogeological studies and public utilities and building surveys.
Construction works began in 1989 for an 8.4km, six station line, stretching from the north west of the city to the east. At the same time a monitoring programme was set up, which included surface marks, settlement pins in buildings, tunnel convergence pins, piezometers and water level wells. However, the project was brought to a halt in 1996 due to a lack of investment funds and the tunnels and stations on Line 4’s 4.7km long (Line 1) extension, from Nicolae Grigorescu Station to Linia de Centura Station were flooded.
Project history
Line 4’s 4.7km extension, is totally underground with an overburden ranging from 5m-12m. At the time the works were stopped the pre-cast concrete segmental lining had been installed and back grouting was mostly completed. However, water ingress grouting and segment sealing had barely begun when the dewatering system was turned off and the structures flooded. Since then, only a controlled monitoring programme has continued. The results showed the groundwater equilibrium at an average of 6m-7m below the surface, meaning that most of the underground structures were flooded. In mid 2000 a new geological and geotechnical investigation, based mainly on penetrometer tests, was executed for Zone 1 (Figure 2), as well as a more intense monitoring programme to evaluate changes in geotechnical characteristics, water levels, ground and building displacements. A similar investigation started at the end of 2000 for Zone 2.
The findings pointed to cracks or crushing of the lining segments; severe geometry changes from the original circular profile, leading to loss of ring stability; presence of sand inside the tunnel, indicating leaching through segment joints and consequent ground loosening around tunnels; ground settlement measured at the surface; variations in the hydrostatical level; damage to some station and shaft structures; some misalignments of the diaphragm walls (mainly due to construction procedures).
Taking into account all the findings and probable consequences, it was decided a technical solution for dewatering should be established to minimise structural displacement and ensure stability. As the tunnel longitudinal axes present an anticline shape, it was decided work should be carried out independently in the two zones (Figure 2).
Geotechnical and hydrogeological conditions
The geology along the alignment is composed of fill, followed by layers of sand and clay (Figure 3). A geotechnical testing programme was carried out at the very beginning of the works in order to obtain design parameters. In addition, other reports were analysed, including new laboratory data sheets. A compilation of geotechnical parameters is presented in Table 1.
A site investigation programme, based on static and dynamic penetrometer tests (Borros type) and laboratory tests, was also conducted. New transversal sections were investigated by penetrometer tests spaced from 5m-80m, depending on requirements. The results were compared to controlled values obtained in areas not affected by underground works (an average 50m-70m away), representative of the local soil conditions. As a general conclusion, there was a decrease in the penetrometer strength in areas where the water table was raised. A broad analysis showed a strength reduction of up to 50% in the upper clay and sand layers. The most affected areas also coincided with zones where excessive displacements were measured within the tunnels.
Regarding the results for Zone 2 (which were better than those for Zone 1), the decrease was between 35% and 50% in the penetrometer strength, mostly in the Colentina sand layer, up to a depth of 5m and more pronounced in some areas surrounding tunnel portals of shafts and stations. In comparison with Romanian standards, for non-cohesive materials (sand layers), it was observed that a tremendous loosening effect had occurred on the main geotechnical parameters (deformability and strength).
Evaluation of the existing infrastructure
From Nicolae Grigorescu Station to PSS IOR 2 Shaft and from PLS IOR 1 Shaft to PSS Linia de Centura Shaft (Figure 2), the underground structures include twin tunnels that were excavated by a semi-mechanised 6.4m diameter open-face shield, manufactured in Romania. The horizontal distance between tunnels varies from approximately 1.8m to 6.4m. In general, they are closer in the proximity of stations and shafts. During tunnelling, the water table was kept lowered (approximately 2m below the tunnel invert) by dewatering wells, 50m ahead and 150m behind the face.
When arriving at or departing from stations and shafts, shield breakthrough was facilitated by concrete bulkheads. This procedure caused some overbreak, leading to loss of ground and excessive surface settlement and even some failure zones. Other causes of ground displacements relate to shield advances, which were completed by hand, with the face fully or partially open. In some instances, the shield was not immediately jacked towards the tunnel face (in order to increase the excavation rate), leaving some spans unsupported and leading to loss of ground. These ground movements easily propagated to the surface, leading to settlement and soil mass loosening.
After excavation, the shield was moved forwards and the support installed. The lining ring consists of five pre-cast concrete segments, 0.35m in thickness and 1m wide, with one key installed in the tunnel invert. Segments are surrounded by a rubber ring and gasket (neoprene) for waterproofing.
The excavation overbreak was filled by grouting (cement, sand and Bentonite injected at 200kPa-300kPa). When necessary, a second phase of grouting was performed for improved waterproofing, with a mixture of cement and Bentonite injected at 400kPa-500kPa. Finally, segment joints were stuffed with paste based on a rapid and expansive cement. When works were halted, some stages of this standard procedure were not concluded to a satisfactory standard. Open spaces between ground and tunnel lining allowed water ingress through segment joints, leaching of sand sediments and attack on structures by polluted water.
Convergence monitoring was halted when the tunnels were flooded, but has continued on buildings. In 1999, a complementary monitoring programme was implemented, focusing on settlement troughs. The analysis of topographic measurements indicated some lining rings had suffered excessive deformation. After visual inspection, some further tests were planned: analysis of the water quality, especially in areas identified as contaminated by sewerage; analysis of the cause of a bluish colour on shaft walls and possible negative reaction of this unknown substance with concrete or reinforcement; mechanical destructive and electrical non-destructive tests to evaluate the degree of corrosion affecting exposed reinforcement.
Numerical simulations
A numerical simulation was set up to obtain the general behaviour of the ground and structures, using differing sequences of dewatering and a variety of ground conditions. In general, the numerical simulation encompassed the main construction stages from the start of station and tunnel excavation works to the next imminent stage (dewatering).
Additionally, measurements (extensometers) were taken inside the ground during dewatering, providing a real view of the ground state and tunnel movements. This initial data allowed decisions to be made regarding the need for ground consolidation.
Current progress – Zone 2
A laboratory testing programme was carried out in order to obtain the parameters of looser regions and a more realistic structural analyses. This lab programme focused on deformability and strength parameters. The sampling locations were more concentrated on Zone 1 because it is more affected and critical than Zone 2. The programme has already been completed and preliminary results are available. However, a detailed interpretation of all test results will be presented in a future paper for Zone 1.
A further monitoring programme, including internal ground and tunnel crown settlements, was also implemented, adding two more instruments to the existing monitoring cross-section:
It was decided that each new-instrumented section should coincide with ones where surface marks were already installed. Therefore a typical monitoring section would have surface marks for the settlement trough and multi-point settlement gauges for settlement profiles inside the ground. These complete sections should be spaced at a maximum of 50m intervals, with additional guages where necessary.
Once instruments were installed, the dewatering of Zone 2 could start. The method adopted was a simplified one i.e. lower only the internal water level, while observing instrument measurements. As the measurements did not indicate any relevant values, only a few millimeters (1mm-2mm), this simplified method was extended to all of Zone 2. During dewatering, samples were collected and submitted for chemical analysis. The results did not indicate any major surprises, only that the water was contaminated by sewage. A cleaning programme was therefore set, to ensure the health and safety of the workers.
After dewatering, the following tests were applied:
The strength tests on concrete (destructive and non-destructive) and on steel reinforcement bars yielded values above those specified by the Romanian standards. Most concrete station structures (pillars, walls, slabs, etc) require a design strength of 12.5MPa, which multiplied by a global factor of safety equal to 1.7, yields a minimum in-situ strength equal to 21.3MPa. On the other hand, tunnel lining pre-cast concrete elements are designed with a strength of 22.5MPa, which means that in-situ values should be over 30.6MPa. Station concrete strength tests presented results generally above the in-situ strength limit (21.3MPa), ranging from 20.4MPa-31.8MPa for destructive tests, except the pre-cast slab elements (17.8MPa) and the bottom slab (13.3MPa-17.6MPa). As these elements are not vital for structural stability of the station, they are not a concern.
Strength tests on the pre-cast tunnel lining elements presented results generally equivalent to the in-situ strength limit (30.6MPa), ranging from 25.3MPa -35MPa for destructive tests, and from 45Mpa-47MPa for non-destructive tests. Non-destructive strength test results always present higher values than those obtained from destructive tests, possibly because they do not cause any sampling disturbance. Strength test results obtained from reinforcement bars, ranging from 579 MPa-743MPa, were high above the specified design value (300MPa). These findings lead to the conclusion that the underground structures still present strength equivalent to, or higher than, that recommended in the design and do not need any special reinforcement.
The visual inspection of the Zone 2 underground structures encompassed Policolor Station, the tunnels between Policolor Station and PSS Linia de Centura Shaft, the tunnels between Policolor Station and PLS IOR 1 Shaft and the initial part of the galleries between PLS IOR 1 and PSS IOR 2 shafts. The general visual appearance of the tunnels, galleries, shafts and Policolor Station are fairly good. There is a massive presence of microorganisms on the surface of the tunnel lining segments, but strength test results indicated that they do not affect the concrete strength. It was observed that two types of crack patterns in the tunnel lining segments occur; erratic cracks and systematic cracks. The later pattern was generated by shield jacks during shield advancement. All cracks will require treatment. The tunnel portals presented severe damage and even signs of structural instability. All portals will require structure reinforcement, either by building a secondary lining layer or by cut and cover methods. A few tunnel-lining segments also presented severe damage and will require replacement or reinforcement.
Considering the results from the water quality and strength tests and from the visual inspection, a structural rehabilitation programme was defined and applied to Zone 2. This programme included cleaning of all flooded structures, using a high-strength steel brush and high-pressure air-water mixture. Local damage to lining segments should be chopped or demolished, and replaced by cast-in-place concrete or shotcrete. One tunnel portal (at PLS IOR 1 Shaft), where clearance has been lost and stability has been severally affected, will be demolished and replaced by a cut and cover structure, built by secant pile wall techniques or similar. It will then be reinforced with a secondary lining layer, for a length of least two diameters (approximately 15m) from the end of the cut and cover structure. Finally, all tunnel bottom slabs should be completed and all cracks and open joints should be grouted.
On-going works
Works involve dewatering the ground for section 1A (close to Nicolae Grigorescu Station, 178m in length), by vertical wells to the tunnel crown level, and then simultaneously with the inside water level, to the tunnel spring line. For section 1B (the remaining section from Nicolae Grigorescu Station up to PLS Vlahita, 480m in length), water bailing is being conducted from inside the tunnel. For section 1C (from PLS Vlahita Shaft to 1 Decembrie 1918 Station, 311m in length), dewatering is by horizontal drilling, parallel to the tunnels, up to 2m under the tunnel crown level. For section 1D (from 1 Decembrie 1918 Station to PLS IOR 2 Shaft, 291m in length), water is being pumped out from inside the tunnels. Dewatering will be followed by ground consolidation and structural waterproofing. Finally, rehabilitation of all Zone 1 and Zone 2 structures will take place.
Conclusion
A cost analysis was carried out before stopping the works and flooding the tunnels in 1996. Now, after dewatering, many problems have been highlighted, requiring cleaning, waterproofing and structural rehabilitation. However, these problems are only encountered over 5% of the total length of Section 1 of Line 4. Additional costs are also occurring from the re-consolidation works and part of the dewatering works. An estimation of these costs is as follows:
These cost estimations, will lead to a total project cost of around US$69M for the section. The tunnels are totally constructed (open-face shield), but the stations and cut and cover tunnels are between 60% and 70% complete. The current status of works at present are:
The refurbishment of the line is due to be completed in 18 months and the finishing and installation works will take another year. It is expected that the line will be open to open at the end of 2004, depending on the availability of funds.
Related Files
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