The SiStema de Potabilizacion Area Norte project has been designed to transport and treat raw water from the Parana River to provide potable water for 2.5 million residents in Northern Buenos Aires. The project includes a 9.3-mile (15km) long, 11.8ft (3.6m) diameter bored tunnel, five access shafts and eight ventilation shafts. It also includes a water purification plant. The access shafts were completed using slurry diaphragm walls with jet-grouted stabilisation for bottom stability.
The tunnel was bored using two TBMs at depths ranging from 59 to 72ft (18-22m) below ground surface with the groundwater approximately 5ft (1.5m) deep. The generalised soil profile consisted of about 26ft (8m) of soft sandy silt overlaying strata of soft clay, firm sandy silt and very dense sand. Hard clay was present at depths near 160ft (48.7m).
Access Shaft Three was a 35.4ft (10.8m) diameter shaft constructed using a slurry diaphragm wall. The slurry wall was 98ft (30m) deep, well above the impermeable clay stratum. To provide sufficient bottom stability, jet grouting was used. The jet grout bottom seal was approximately 5m from the excavation invert.
During the initial shaft excavation undetected groundwater intrusion between the slurry wall panels led to inflows of substantial quantities of groundwater and soil. It was also possible that discontinuities within the jet grouted bottom were contributing to the shaft flooding. The prime contractor joint venture, Aguas del Parana UTE (ADP), flooded the shaft and attempted to seal the leaking joints with a grouting program, initially by direct injection of the grout using tremie methods. During the next 14 months excavation was hampered by continuing soil and water inflows. Repeated attempts of grouting, jet grouting and dewatering failed to facilitate excavation. The shaft was eventually backfilled to an elevation above the proposed tunnel crown.
Following the backfilling with the lean concrete, the tunnel proceeded through the shaft and was eventually completed. With the finished tunnel penetrating both sides of the slurry wall, shaft excavation and completion was complicated. ADP contacted Moretrench American Corporation to evaluate the feasibility of ground freezing to provide temporary earth support and groundwater control to excavate and complete the access shaft.
Ground freezing offered several immediate solutions to the problems associated with leaking slurry wall panels and bottom instability following the excavation. A frozen soil cofferdam around the perimeter of the distressed slurry wall would conform to the shape of the panels and also seal any of the leaks by freezing the water in the joints. Additionally, the freeze pipes could penetrate well below the base of the slurry wall into the impermeable clay and provide bottom stability of the excavation.
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By GlobalDataA complication to the freezing process however was the fact that the tunnel was already in place. The presence of the tunnel would interfere with the drilling and installation of the freeze pipes on both sides of the shaft. More importantly, it would be necessary to ensure a completely frozen zone around the tunnel perimeter to prevent groundwater and soil inflows from around the tunnel and into the shaft. There were two approaches implemented to provide a frozen zone around the tunnel. The first approach was to drill and install angled freeze pipes under the tunnel as illustrated.
In order to ensure that the angled freeze pipes would reach the intended freeze zone it was necessary to establish the required azimuth to align the drill rig. Once the drill rig was set up on the azimuth, the mast was adjusted to the proper angle. After each individual freeze pipe was drilled and installed, the deviation was measured with an orientable inclinometer. The actual locations of the freeze pipes were plotted at various depths with close attention paid to the elevations at tunnel invert and below. The actual freeze pipe coordinates at the invert were entered into a time dependent finite element heat transfer model. This model evaluated and presented the ground temperature contours as illustrated in Figure 4.
The second approach used to complete a frozen zone around the tunnel was the installation of a cooling system inside the tunnel to keep the tunnel segments in the frozen zone well below freezing temperatures to compensate for the warm air inside the tunnel. This system is illustrated in Figure 5.
After the installation of the freezing coils insulation was applied and the system connected to surface refrigeration by drilling from the surface into the tunnel and circulating the refrigerated coolant from the two electrically powered freeze plants. Before the installation of the freezing system was approved it was necessary to evaluate the effects of the frost pressure against the damaged slurry wall and the completed tunnel. The evaluation of frost pressures against existing structures is an issue that must always be considered in ground freezing projects. A successful approach to that has been used on recent projects was the using of modeling the problem with the PLAXIS finite element program. Since this situation required evaluation of both the tunnel lining and the slurry wall it was necessary to create a three dimensional model as shown in Figure 6. Results of the analysis indicated that the pressures against the slurry would not cause any damage, even in the poor condition that existed. There was concern, however, that the frost pressures generated by the freeze pipes and the internal tunnel cooling system would generate pressures that could damage the tunnel lining segments. An internal bracing system was designed and would be installed concurrent with the drilling and installation of the freeze pipes.
The freeze pipes were installed using conventional mud rotary methods. A pilot hole was drilled to the required depth using a 5-5/8-inch (143mm) tri-cone. Each individual borehole was terminated only after verification that it penetrated into the underlying hard clay. The 4-inch (102mm) nominal welded steel pipes were then installed into the boreholes, pressure tested and surveyed for alignment using the inclinometer.
Following the drilling and installation of the freeze pipes the coolant distribution manifolds and mobile refrigeration plants were initiated. Two electrically powered refrigeration plants imported from the United States were used to chill the circulating calcium chloride brine to approximately -25oC. Concurrently with the surface work, the working inside the tunnel progressed. The first requirement was the installation and insulation of the internal cooling system.
The bracing system was then installed to protect the tunnel from the pressures generated by the freezing.
The bracing system consisted of a 250mm reinforced concrete ring installed outward from the shaft approximately 4.5m to coincide with the frozen zone.
In order to complete the shaft, it would be necessary to break into the tunnel once the excavation was completed. Due to the repeated unsuccessful attempts there was concern that a shaft excavation failure could introduce an unlimited quantity of water and flowing soil into the completed tunnel. ADP constructed two bulkheads on both sides of the shaft.
Each bulkhead was 1.5m thick constructed of masonry brick. Structural steel anchors were bolted to the tunnel segments to prevent sliding. Personnel access was provide through an 800mm concrete pipe with a 25mm thick steel hatch.
The freezing process was completed in approximately 80 weeks that was consistent with thermal modelling. An automated data acquisition and alarm system was used to measure ground temperatures, groundwater levels, the coolant temperatures and flow rates as well as freeze plant operating data. In most frozen shafts, where freezing is used as the initial ground support system, formation of the frozen earth wall is typically indicated by a rise in the water level of an internal piezometer or monitoring well. This is caused by the expansion of the pore water as it turns to ice.
On this project there was no internal piezometer to provide this critical information. As an alternative, the water inside the excavated portion of the shaft was pumped in approximately one-meter intervals. During the pumping external piezometers were measured. There was no change or decrease in the groundwater levels in these piezometers indicating that the frozen earth cofferdam provided a positive cut-off and isolated the interior of the shaft. The shaft was subsequently pumped out completely with no indication of groundwater intrusion verifying the success of the freezing.
Excavation then proceeded to the tunnel invert as shown in the photos. The existing tunnel was then broken out and the final shaft constructed.
The project was completed with no damage to existing tunnel or issues with the frost pressures against the existing slurry wall, verifying the use of the three dimensional structural model.
This project represents the first time (to the authors’ knowledge) that groundfreezing has been used in South America.
