The 1.8km long Reach 3B Tunnel, under the southwest flank of South Mountain in Phoenix, Arizona was completed in December 2001. Owned by the City of Phoenix the Reach 3B Tunnel is part of the South Mountain Water Facilities Project.

The tunnel was designed by Hayley & Aldrich and excavated by contractor Affholder, using a refurbished 2.4m diameter Robbins double-shielded TBM. Grippers allowed the TBM to thrust forward in the rock and cemented soil portions of the tunnel, while a thrust ring was used to push off of the steel rib and wood lagging temporary support system in the loose soil portions of the tunnel. Once tunnel excavation was completed, the contractor installed 6.1m long sections of 1.2m i.d. pre-stressed concrete cylinder pipe (PCCP). The PCCP pipeline is designed to carry potable water at very high pressures. The annulus between the excavated tunnel and the PCCP was designed to be backfilled with cellular concrete and then contact grouted to fill any remaining voids.

The installation of grout ports in carrier pipes to allow for backfilling and contact grouting is a standard industry practice for tunnel projects. Therefore, the tunnel contractor had planned to inject both the backfill cellular concrete and contact grout through grout ports, which would have been installed in the PCCP during the pipe fabrication. However, the owner of the South Mountain Reach 3B Tunnel Project did not want any “penetrations” (grout ports) in their water pipe. This “no penetrations in the pipe” requirement was clearly stated in the contract documents. Therefore, an alternative method to inject the backfill cellular concrete and contact grout had to be designed.

As the contract documents also required a maximum spacing of 152m between injection points for the backfill, the first method considered required the installation of a 152m length of PCCP; at which point pipe installation would stop and a bulkhead built. The length of PCCP would then be backfilled and grouted through the bulkhead.

After grouting was completed, the process of PCCP installation, bulkhead building, backfilling and contact grouting would be repeated in lengths of 152m at a time for the remainder of the tunnel.

This method was rejected because it required starting and stopping the pipe installation operation, which would have had unacceptable cost and schedule impacts on the project.

As an alternative, a system of long backfill delivery pipes, installed within the annulus between the pipe and excavated tunnel, was developed. Each backfill delivery pipe, discharged at approximately 152m intervals, was of increasing length from either of the portal bulkheads. A contact grout delivery pipe, the full length of the tunnel from portal bulkhead to portal bulkhead was also installed within the annulus, at the tunnel crown, by speciality subcontractor Pacific International Grouting, who performed the backfilling and grouting.

Geology

The geology for approximately 85% of the 1.8km long tunnel was hard rock, comprised of gneiss and granite. The remainder of the tunnel was alluvium and cemented alluvium (caliche). There was one section in the tunnel which had open rock fractures which might cause the backfill or the contact grout to be lost to the surrounding ground during injection. Therefore, these fractures and several open exploratory boreholes were sealed off before installing the PCCP. The groundwater level was well below the tunnel invert and the amount of groundwater entering the tunnel was nominal and not a factor during the backfilling and contact grouting operations.

Backfilling

The tunnel slopes uphill from the southeast portal to the northwest portal. The southern (1183m) end of the tunnel is at a 0.25 percent grade and the northern 665m of the tunnel is at 2.05 percent grade. The total difference in elevation from the southeast portal to the northwest portal is about 16.6m.

The project team agreed to start backfilling from the southeast portal. The plan was to inject about one half of the estimated volume of cellular concrete backfill form the southeast portal, with the other half of the backfill being injected from the northwest portal and a backfill delivery layout was developed (as shown in figures 1 and 2). Table 1 gives distances from the portal bulkheads to the discharge point of the delivery pipes.

The backfill delivery pipes were installed on both the left and the right tunnel ribs below the springline. However as each pipe reached its discharge point, elbows and short pieces of pipe were used to bring the pipe’s discharge to the tunnel crown. There was also a short, 0.6m backfill pipe at each portal bulkhead (pipes SP and NP). A masonry block bulkhead was built at each portal and a third bulkhead was also built in the middle of the tunnel.

Four 38mm PVC vent pipes were installed through the southeast portal bulkhead to vent air during backfill injection and to verify the level of backfill at the bulkhead. The delivery pipes used were 75mm diameter schedule 20 steel with Victraulic couplings. At first, PVC pipes were considered, but the idea was rejected because it was believed the pipes might partially meet during backfill injection due to the elevated temperature within the annulus generated by the hydration of the cellular concrete.

Originally the backfill was planned to be injected from the southeast portal starting with the shortest delivery pipe then hooking up to the next longest pipe and so on. For example, starting with delivery pipe SP then hooking up to delivery pipes S1, S2, S3 through delivery pipe S7. Using this injection sequence would have meant that the cellular concrete being injected would have had to travel through ever increasing lengths of delivery pipes. These delivery pipes would have already been embedded in the previously injected cellular concrete. It was believed the heat generated by the hydration of the previously injected cellular concrete surrounding the delivery pipes could cause the cellular concrete being injected to flash set inside the delivery pipe.

Because of the unacceptable risk of flash set, it was decided to start backfilling at the southeast portal using the longest delivery pipe (S7) first, working towards the portal using ever shorter lengths of delivery pipes. For example starting with delivery pipe S7 then hooking up to delivery pipes S6, S5, S4 through delivery pipe SP.

The backfill for the northern half of the tunnel was injected from the northwest portal starting with delivery pipe N6, the longest pipe, through to delivery pipe NP, which discharged 0.6m from the portal bulkhead.

The amount of heat generated by the hydration process of the cellular concrete backfill was a concern, even after deciding to start with the longest pipe first. The steel delivery pipes would still get very hot within the confined space of the annulus. In an attempt to reduce the heat, the amount of flyash used in the mix was maximised. Therefore the cellular concrete backfill mix design called for 60% weight Type F flyash. The cellular concrete backfill mix design used is given in Table 2.

Contact Grouting

The contract documents required contact grouting of the crown area of the tunnel to be performed after backfilling was completed, to fill any remaining voids which might exist. A 75mm diameter steel contact grout delivery pipe was installed along the crown centreline with 13mm diameter, shop drilled, discharge holes located every 3.1m along the pipe. The two holes at each location were orientated; one hole at the top of the pipe and the second hole spaced at a 90° angle to the left or right of the top hole, putting the second hole on the pipe’s horizontal centreline. The holes located on the centreline alternated left-to-right. The discharge holes were covered over with duct tape before the pipe was installed in the tunnel to prevent any backfill concrete entering the grout delivery pipe.

During the planning stages for the contact grouting, field tests were conducted to evaluate the best type of tape to use to cover the discharge holes in the contact grout delivery pipe. To conduct the tests a short section of steel pipe with two 13mm diameter discharge holes drilled 90° apart was used. The pipe was capped at both ends by welding plates, a pressure gauge was installed at one end of the pipe and an air inlet at the other end.

Three types of tape were tested; duct, masking and packing. After taping over the holes a “foot air pump” was used to pressure the pipe. The three types of tapes failed to hold pressure at the following pressures: Duct tape 14 psi (0.95 bar); masking tape 5 psi (0.34 bar); and packing tape 12 psi (0.82 bar).

Eight tape tests were conducted. In all but one of these tests the tape failure was at the bond between the tape and the pipe. In each case the air escaped under the tape away from the drilled holes. Only during one test did the duct tape rupture over the hole. Based on the results of these simple tests it was decided to use duct tape to cover the contact grout delivery pipe discharge holes.

The contact grout mix design used is given in Table 3.

Batching, Mixing and Pumping

The cellular concrete and contact grout batching plant consisted of two material storage silos, two mixing augers, an electrical generator, a high shear mixer, two foam concentrate storage tanks, a foam generator and a positive displacement cellular concrete delivery pump.

The cement and flyash were delivered to the project site in trucks. The materials were then transferred from the tankers to silos. There was one silo for cement and one silo for flyash. The cement and flyash were metered out of the silos using a rotary valve. The measured material discharged into a mixing (blending) screw auger where the correct amount of water was introduced. Each of the materials, cement and flyash, had its own screw auger. As the cement, water and flyash moved along the screw auger, which was approximately 2m long, they were partially mixed to form a slurry.

The cement slurry and flyash slurry were discharged from the two screw augers into a high shear colloidal mixer. The mixer discharge was connected directly to a progressing helical cavity (Moyno) delivery pump.

Foam concentrate was delivered to the project site and transferred to holding tanks, located in close proximity to the batching operation. From the holding tanks the concentrate was fed into a foam generator where it was mixed with air. The foam was introduced to the cement/flyash slurry at the intake of the delivery (injection) pump.

The delivery pump discharge was connected to the backfill delivery pipes and later to the contact grout delivery pipe at the tunnel portals via a flexible hose.

Field Testing

The unit weight of the cement slurry, flyash slurry, cement/flyash slurry and the cellular concrete were tested every 30 minutes using a “mud balance”. The unit weight of the foam was tested using a balance scale, weighing a test pot against a standard concrete unit weight. Approximately every 4 hours four 75mm by 150mm cellular concrete cylinders were cast in a Styrofoam mould for compressive strength testing at seven and 28 days.

The average seven day compressive strength test results for the cellular concrete backfill was 226 psi (1.6kPa). The average 28 day compressive strength test result was 519 psi (3.7kPa). The average dry unit weight of the cellular concrete was 58.6 PCF (0.99g/cm³).

Conclusion

It is believed, based on the actual volume of material injected, that the backfilling and contact grouting on the South Mountain Reach 3B Tunnel Project was completed successfully. However, there were no real physical means to verify the actual condition of the in situ backfill and contact grout materials or if any voids remained unfilled after completion.

It must be emphasised that the preferred industry method is to inject backfill material and contact grout into the annulus behind pipes installed in tunnels, is through pre-installed grout ports fabricated into the pipe during manufacture.

If the tunnel is located at a relatively shallow depth and surface access is available, another acceptable backfilling method is to inject the backfill through drop holes from the surface.

In this method the backfill material is discharged into a hole(s) drilled from the surface into the tunnel crown. However, utilising the long length of delivery pipe(s) backfilling and contact grouting methods discussed in this article may also be appropriate for certain future projects; particluarly non man-entry tunnels or utility tunnels.

The main purpose of this article is to give the reader some idea of the methods, pumping distances and delivery pipe layouts which have been used successfully on the South Mountain Reach 3B Tunnel Project to inject cellular concrete backfill and contact grout relatively long distances within the tunnel annulus.

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Figure 2
Figure 1