TunnelTECH

Ground freezing assistance under Suez Canal 14 Mar 2019

Ibrahim Raafat, EAAF; Haider Fadhel, CDM-Smith GmbH, Germany; Mina Philips, Arab Consulting Engineer
The cross passages between the twin tubes of the new highway tunnel under the Suez Canal at Ismailia in Egypt have been excavated with the assistance of ground freezing to support the sandy and silty soil below the groundwater table and under a hydrostatic head of up to 5 bar.
Frozen face of the cross passage connection with the parallel tunnel
Frozen face of the cross passage connection with the parallel tunnel

A 4.8km long x 11.4m i.d. twin tube road tunnel, providing two road lanes in each, have been excavated by two 13m o.d. Herrenknecht Mixshields to connect the city of Ismailia with new settlements in the east and under the Suez Canal to the Sinai Peninsula. The Egyptian Government, represented by the Engineering Authority of Armed Forces (EAAF) and the joint venture of consultant engineers CDM-Smith of Germany and ACE Moharram Bakhoum of Egypt, is procuring the new highway that passes also under a main railway line, the old Suez Canal, and the new canal opened in August 2015. As they progressed, the Herrenknecht Mixshield TBMs erected 600mm thick x 2m wide rings of segmental lining.

Construction of the four cross passages between the two tubes required application of ground freezing to provide sufficient soil strength and water tightness for safe excavation through the sandy and silty soil below the groundwater table and under a hydrostatic head of up to 5 bar. While two of the cross passages are close to the canals, the design avoids locating any passages directly under the waterways. Pre-grouting was applied at all cross passages to support breakout of the segmental lining and to transfer stresses to the strengthened ground. Segments of heavier reinforcement were also erected in the area of the breakouts.

Extra support fame installed ahead of drilling the freezing pipes and cross passage breakout
Extra support fame installed ahead of drilling the freezing pipes and cross passage breakout

A temporary steel structure was fitted as additional support to the tunnel lining around the breakout before starting the drilling process to install freeze pipes and before removing the segments of the breakout to avoid deformation or destabilization of the lining.

Drilling for the freeze pipes was executed according to the designs prepared by CDM Smith Consult, which considered the compliance with the geothermal design and potential deviation of the drill holes. Close monitoring considered whether additional drill holes were required while avoiding the temporary steel support at the cross passage area. Dewatering stand-pipes fitted with blow-out preventers were installed and pressure tested to verify the water tightness of the connection. Dewatering pipes would also release ground water in the core in case of an increase in the water pressure.

Preparation for cross passage breakout and installation of the freeze pipes began as soon as the TBMs passed the cross passage zones and once the annular grout behind the segmental lining had hardened. Subject to structural design confirmation, a full circle watertight brine-freezing collar of about 2m thick was established around each cross passage excavation (Fig 1).

Fig 1. Establishment of the freeze plant and freeze
Fig 1. Establishment of the freeze plant and freeze

Up to 48 freeze pipes were installed from main tunnel tube and to within 1.5m of the parallel tunnel to form the frozen collar and establish the a frozen zone at the connection with the opposite tunnel. Temperature measurement pipes of 3m long were installed along with 6m long dewatering pipes.

It took on average about four weeks to install and establish the freeze pipes around each cross passage. Once the freeze installation was confirmed, breakout of the segments and sequential excavation of the core within the circle of frozen ground could begin.

Principles of the freezing technique

Ground freezing is based on heat withdrawal from the soil. A close-ended freeze-pipe is placed in the soil and an inner pipe, or supply pipe, is inserted into the freeze pipe. The cooling medium flows through the supply pipe and backs up through the freeze pipe. The heat is exchanged between the soil and the cooling medium, with continuous flow of cooling medium establishing and maintaining the ring of frozen ground around the freeze pipe. As the freeze around each freeze pipe grows it to merge with all to create a watertight ring of ice.

Installation of freeze pipes, dewatering pipes and temperature measurement pipes to establish the 2m wide freeze collar
Installation of freeze pipes, dewatering pipes and temperature measurement pipes to establish the 2m wide freeze collar
Location of the freeze pipes and temperature measurement pipes across the length of the 14m long cross passage
Location of the freeze pipes and temperature measurement pipes across the length of the 14m long cross passage

When the ice ring closes the remaining water inside the ring cannot run off which leads to an increase of the water pressure in the core, which is the clear indication that the freeze has achieved closure and a watertight connection to the lining of the main tunnel. During the freezing process the water pressure is measured at the head of the dewatering bore, which is equipped with a drainage valve and manometer.

Central to the close circuit freeze system are the chillers, which are located inside the tunnel and about 40m from the cross passage breakouts. The chillers cool the brine to a pre-defined temperature, which is then pumped through the closed circuit freeze pipes.

Temperature sensors within the temperature measurement pipes
Temperature sensors within the temperature measurement pipes

The design criteria for removing the lining segments and starting excavation of the cross passages in Ismailia are:

  • Increasing water pressure at the drainage pipe to indicate water-tightness of the freeze body
  • A freeze collar of 2m thick
  • A freeze body boundary defined by -5°C
  • Average temperature within the freeze body of -10°C
  • Length at connection to opposite tunnel of about 1.5m at -5°C

It is known that the growth of the freeze collar is homogenous in homogenous soil conditions and those not influenced by the tunnel temperatures (Fig 2). The alignment of the temperature measuring pipes considers that there are temperature sensors at the outer design-circumference of the freeze body immediately at the breakout. Values below -5°C at the outer circumference confirm that the required freeze body thickness of 2m is achieved. Further it must be validated that the average temperature within the freeze body is equal to or colder than Tmin = -10°C. From the opposite tunnel, 500mm long temperature measurement pipes were installed.

Breakout of the segmental lining
Breakout of the segmental lining

Assuming that temperatures at the outer design-circumference of the freeze body shall be below -5°C, and under conservative consideration of a simplified linear temperature distribution versus the freeze wall thickness the minimum temperature at the centerline of the cross passage core has to be equal or colder than Tmin = -15°C.

Since the freeze pipes penetrate through the lining of the starting tube, the freeze body connection is not as critical as at the opposite tunnel where freeze pipes fall short, by about 1.5m, of touching the opposite tunnel lining. The freeze body therefore has to grow by at least 1.5m to connect to the opposite tunnel to complete the cross passage. Due to the circular shape of the main tunnel, the -5°C isochrone has to validated the 1.5m connection zone of the freeze body at the opposite tunnel where the front view shape of the cross passage connection is an ellipse.

Cross passage excavation

Once the freeze zone met the design criteria, site logistics were arranged for excavation to progress from the same tube. Thermo-insulation sheets at the breakthout location were removed and freezing pipes were protected ahead of using track sewing and core drilling to cut out the precast segmental lining and create a breakout opening of 2m wide x 3.5m high.

Excavation of the frozen face using a roadheader attachment
Excavation of the frozen face using a roadheader attachment

Sequential excavation of each cross passage advanced in 1m x 19.6m2 full face rounds and was supported with a 25cm thick layer of shotcrete reinforced with steel ribs and wire mesh. Excavation was carried out using rotary roadheader cutters and hydraulic hammers, depending on the soil consistency and the distance from the freezing pipes. Excavation progressed at about 1m/day. The primary lining created a natural load-bearing ring, controlled deformation and minimized heat transfer to the frozen body in the excavated area.

The waterproofing system consists of a geotextile layer of 700gm/m2, a double PVC layer of 3mm and 2mm plus water-stops and injection hoses. The cross passages are finished to a 3.8m i.d. with a 350mm in-situ concrete lining cast in two pours of about 5m each with an expansion joint in between, and two reinforced collars at each tunnel. The final lining is designed to ensure water tightness and to carry the permanent load of the ground once the freeze thaws.

Primary lined cross passage connecting to the existing parallel tunnel before completing the final connection (left) finished with a geotextile and waterproofing membrane (centre) and in-situ concrete (right)

Construction of the cross passages is a considerable milestone for the Ismailia highway project, since the Egyptian Government is planning to open the new highway tunnels to traffic by mid-2019 as part of the nation’s Sinai Peninsula development project.

References

           

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