The Paradise Whitney Interceptor (PWI) is an extensive gravity sewer pipeline under construction in Nevada, USA by the Clark County Water Reclamation District (CCWRD) to maintain a safe and reliable infrastructure for delivering wastewater from homes and businesses to treatment facilities. CCWRD utilises seven treatment plants in the Las Vegas Valley to collect and treat 100 million gallons of wastewater each day. The current pipelines are at or above capacity, and this new interceptor will help to relieve current deficiencies and provide capacity for the future.

The PWI project is so large that it has been divided into three phases: Eastern (#669), Central (#668), and Western (#670), with an overall predicted completion in Summer 2018. Each of the three segments proposes 4 to 5 miles (6.4 to 8km) of 8 to 84 inch (0.2 to 2.1m) diameter gravity sewers.

The Eastern segment was awarded to Carollo Engineers, Inc. and sub consultant Brierley Associates of Denver, CO for design with Las Vegas Paving as primary contractor. Black and Veatch Corporation designed the Central segment with primary contractor Contri Construction of Las Vegas, NV. Western segment went to MWH for design and contractor Southland Contracting of Roanoke, TX handled direct bury installations, shaft preparation for tunnelling operations, and general construction responsibilities.

All trenchless installations on the Central and Eastern segments were performed by Pipe Jacking/Frontier Kemper Joint Venture, which was a collaboration between Pipe Jacking Unlimited, Inc. of San Bernardino, CA and Frontier Kemper Constructors, Inc. of Sylmar, CA.

Ground Conditions

Geological investigations at an early stage of the project revealed the variable nature of soils in the region. Areas of sticky clay, collapsible soils, dewatering-induced settlement, high groundwater, and even cemented caliche were present in the pathway of the pipeline. Caliche is a natural cement of calcium carbonate and other deposits that acts like clay once mixed with water. This is a common sediment in desert areas, but is sporadic along the PWI alignment. The main issue to be encountered, though, was to work around existing structures.

Nearly half of the proposed 13-mile (21km) linear footage of pipeline was in close proximity to sensitive underground utilities and had limited surface access. The pipe alignment crossed residential areas, interstates, and went so deep as 45ft (13.7m) below grade. High ground water tables and traffic loads were also present throughout.

Achieving an optimal path meant that trenchless technologies would need to be implemented. Even by utilising trenchless technology like microtunnelling/jacking, some drive lengths would have to be over 1,000ft (305m). Pipe Jacking/Frontier Kemper Joint Venture chose to install the trenchless portions via micro TBMs and EPBMs, making PWI the largest concentration of pipelines installed via trenchless technologies in the USA in over a decade.

Pipe Selection

Pipe Jacking Unlimited used 64-, 66-, and 78-inch (1.625, 1.676, 1.98m) outer diameter (OD) EPB machines that they previously owned, meaning that the pipe manufacturer chosen to supply pipe for use with the machines had to have production flexibility. The MTBM purchased by Frontier Kemper Constructors, however, was an Akkerman SL60 which was “skinned up” to alter the outside diameter of the machine to match the pipe provided (~63 inches [1.6m]). Approximately 25,190 linear feet of trenchless pipe installation and 30,022 liner feet of open cut direct bury were planned in the Eastern and Central segments.

Flowtite filament wound Fibreglass-Reinforced Polymer Mortar Pipe (FRPMP) was chosen for both the trenchless installation of 25,190 LF of 60- to 72-inch (1.52 to 1.83m) pipe, and an additional 23,800 LF of 27- to 84-inch (0.686 to 2.134m) pipe installed using open-cut methods. FRPMP was able to handle the unique issues at hand as well as offering advantages in corrosion resistance, superior hydraulic characteristics, flexibility in customisation, and ease of installation.

FRPMP, or fibreglass pipe, is a relatively common in the North American water/wastewater. The FRPMP jacking pipe for PWI was designed with a minimum pipe stiffness of 220-psi in diameters of 60-, 66-, and 72-inches and joint lengths up to 20ft (6.1m).

Joint design

Direct bury pipe featured a double-bell fibreglass pressure coupling with EPDM rubber REKA profile gaskets for sealing internal and external pressure. Even though this project was a gravity sewer application, the REKA gasket profile is capable of handling internal operating pressures up to 450-psi (31 bar) and well as 200-psi (13.8 bar) of external pressure. A center register was also used between the adjoining pipe faces for “homing” of the spigots in the joint for the installation crew.

Depending on the pipe diameter, the standard pressure coupling supplied by the FRPMP manufacturer can articulate anywhere between 0.5 and 3 degrees vertically or horizontally while maintaining a seal. Joint articulation over several pipe lengths can be used to eliminate small-angle bends.

Flush fibreglass sleeves were used for slipline and jacking pipes on PWI. The flush sleeve is even with the outside diameter of the pipe creating a flush transition between coupling and pipe. Milling down the spigot allows this sleeve to slide onto the pipe without increasing the OD. The sleeve is 12mm thick for jacking pipe and 7mm thick for slipline line or two pass tunnel installations in these diameters. However, they both utilise the same gasket, which is set in a groove on the spigot end of the pipe. This gasket style can withstand 120 psi of external waterhead pressure and gravity conditions internally.

In a slipline/two pass tunnel installation, the flush sleeve joint system for the largest diameter pipe to be installed inside of the host or carrier pipe to maintain as much flow capacity in the system as possible.. For trenchless installations any protrusions beyond the OD of the MTBM or EPB machines are at risk of acquiring damage. Both the pressure coupling and flush sleeve conform to ASTM D4161 for fibreglass pipe joints using flexible elastomeric seals.

Special fittings

For bends where joint articulation could not be used to maintain alignment, fittings were supplied from mitered sections of straight fibreglass pipe welded together using a fibreglass and resin laminate. Often several configurations were combined into one fitting such as a bend and tee-base manhole, or a wye and tee-base. Many small-diameter tees and wyes were added to the direct bury segments to accommodate for auxiliary wastewater lines.

The direct bury and jacking pipes of similar nominal diameters had slightly different internal diameters that necessitated a reducer-style fitting to smoothly transition flow from one pipe to the next. All fibreglass laminates used on fittings exhibit the same corrosion-resistance, strength, and long expected lifetime as the pipe. The “Leading and trailing” pipes for the jacking installation were specially designed for intermediate jacking stations, which will be described later in this report. Finally, threaded grout ports made from PVC were installed in the wall of the pipe to facilitate injection of grout for slipline and jacking pipes.

Installation

Direct bury

The specified trench detail for direct bury fibreglass pipe included a pipe zone extending 6in (152mm) below the pipe, 12in (305mm) minimum from springline to trench wall, and 12in minimum cover above the pipe. The pipe zone required a granular fill (CLSM class 1). The trench zone above could include either the granular fill or a soil class 2 with 90 per cent compaction. Tee-base manholes were cast in concrete after installation.

Aside from the embedment zone details the direct bury process is fairly similar across different pipe materials. Roadways and residential areas required minimal ground disturbance, therefore trench boxes were used to maintain minimum excavation width. The lightweight fibreglass pipe was able to be installed with small to mid-range excavators. Nylon or textile slings were used in place of chains. Once the pipe was properly installed, the trench was re-filled and compacted to the soil proctor density as written in the project specifications.

Jacking

MTBM

The trenchless portion of the PWI installation was installed using the microtunnelling method. To begin the process, a large pit or shaft was dug and reinforced at the start and end location of a tunnelling drive. These pits are known as the launch and receiving pits. In the launch pit a horizontally orientated jacking frame comprised of four large hydraulic presses and a jack plate was guided by a slotted steel frame. Each slot was spaced roughly 2 feet apart. When the jack plate advanced forward, it locked into the next slot and allowed the hydraulics to retract, preparing for the next advancement.

In this fashion, the MTBM was pushed forward while using a rotating cutter head to break apart soil and bedrock in its path. The resulting slurry produced from the shredded rock was funnelled into the centre of the cuter head and directed back to the launch pit through small pipes where it went through sieve separation. The majority of solid particles were discarded above ground and the fluid left over was pumped through another pipe to fuel the fragmentation of material at the cutter head.

Other, smaller, pipes/tubes were present in the trailing pipes to provide cooling fluid to the machine and bentonite lubricant to keep the machine and pipe moving smoothly through the tunnel. The bentonite played a key part in reducing friction between the already smooth outer wall of the jacking pipe and the native soil it was passing through.

As the machine reached the jack plates full extension, the jack plate was withdrawn back to its start to facilitate the addition of a segment of jacking pipe. Distribution rings made of MDF (medium-density fibreboard) were secured in place between pipe faces to evenly distribute the jacking load about the circumference and mitigate point loading. From there the process repeated with more lengths of jacking pipe. Typically, these were in nominal lengths of 10 or 20 feet, depending on the size of the launch pit.

To keep the jacking loads at reasonable levels for some of the longer microtunnelling drives, intermediate jacking stations (IJS) were incorporated. This is a series of hydraulic jacks installed inside of a steel cylinder and aligned longitudinally with the pipe axis. The sleeve fits snugly and flush around the OD of the jacking pipe. To accommodate this IJS, roughly 5 feet of the pipe OD on a “trailing” pipe had to be milled to fit the sleeve. Two O-ring grooves were also milled to seal against the inside of the IJS while the sleeve slid over the milled portion of the pipe like a piston, inching the pipes forward like a caterpillar during installation.

The jacks can be easily be removed from within the pipe once the pipes reach their final resting place. The steel sleeve stays in place, but the joint is internally sealed with a fibreglass laminate since the IJS steel sleeve would be exposed to the corrosive environment of the sanitary sewer.

An individual in a computer control station above ground directed the MTBM to its final destination, the receiving pit. Once free of the tunnel and resting in the receiving pit, a crane lifted the MTBM out of the receiving pi. In some areas it was deemed necessary to inject cement grout through grout ports in the pipe wall to create a soil cement mixture in-situ.

EPB

The earth pressure balance machines worked in a similar fashion to the micro-tunnel boring machine in that they utilised a cutter head to eat away material underground and direct that material back to the ground surface. The EPB dumped material from the tunnel onto a conveyor which transferred it to a bucket that had to be manually taken down the length of pipe and hoisted up to the ground surface.

To ease the transfer of material and people, a cart and railway system was implemented along the invert of the pipe. Smaller lines were also present to maintain airflow to the operator of the machine, who was situated in a control room within the EPB underground. The jacking plate resembled MTBM operations, moving forward in small increments until the EPB mechanism had arrived in the receiving pit.

Slipline/two-pass tunnels

Steel casings used in required areas were microtunnelled into place using an MTBM and launching pits as described previously. The fibreglass slipline pipe was then prepared for installation above ground by adding casing spacers, and then lowered into the launch pit. Casing spacers were supplied by a third party vendor, but made to fit snugly around the OD of the slipline pipe. They were fastened to the OD of each pipe at two or more locations depending on joint length.

Casing spacers help to keep the slipline pipe concentric with the host pipe and reduce friction for longer drive distances.

Grout ports were also used in slipline pipes where needed to fill the annular space between fibreglass pipe and steel casing. These ports were threaded internally to accept a 2in (51mm) NPT injection nozzle. A matching plug was provided to seal the port after injection.

The ports were secured in cored holes at the pipe manufacturers facility with a mix-and-set high strength epoxy. Several ports were located along the length of the pipe with various locations in the circumference. Like previously mentioned, a flush fibreglass sleeve joint system was used; however, distribution rings were not needed for the slipline pipe.

Additional Challenges

Fluid velocities as high as 17.7 feet-per second were anticipated for some of the larger slopes along the PWI alignment. Engineers with the pipe manufacturer worked hard to devise an alteration in the pipe design rather than re-designing the alignment. The result was a combination of concepts that led to a solution.

The centre register mentioned in the ‘special fittings’ section of this article is typically 10mm tall from the pipe manufacturer. For this project, the centre register height was increased to match the pipe wall thickness creating a smooth transition from pipe to pipe reducing turbulence at each joint. A taller rubber strip would also serve to protect the pipe faces edge from fast-moving fluid and particles.

Rapid flow also has the tendency to be more abrasive to the channel in which it flows. To combat this, a special blend of the standard polyester resin and flexible resin was incorporated into the interior liner layer of the pipe. The liner provided is more resilient to impact and abrasion resistance which the pipe would be exposed to on this project.

The final customisation of the liner was at the request of the Owner: pigmentation. Sanitary sewer lines in the area have a traditional seafoam PVC-green colour so that they may be easily identified. A pigment was mixed into the liner resin only of the fibreglass structure, since the pipe will most often only be seen and accessed from within. None of the additions negatively affects the smooth texture and corrosion resistance of the material, even when in contact with the harsh environment that is naturally present in sewage.

Storage space on site in the Las Vegas valley was minimal in highly developed areas, which were the majority of the project. With such a large quantity of pipe needed, the pipe manufacturer arranged to have pipes shipped direct to the construction sites as needed for the Eastern and Central segments. Savings in shipping costs and time were made possible by nesting pipes inside other pipes of larger diameter. For example, a 30in (0.76m) pipe could be placed inside 60in (1.5m) pipe, and these two then cradled inside a 72in (1.83m) pipe of equal or greater length. This nesting allowed the total amount of truck travel to be reduced by thousands of miles, and resulted in a reduction in emissions and fossil fuel usage by nearly 30 per cent. The relatively lightweight nature of fibreglass pipe made it possible to nest several sizes of pipe on one truck bed without exceeding maximum weight limits. The pipe could even be stored in this fashion, thus saving space in staging areas.

Conclusion

When installation began in mid-February of 2015, the pipe manufacturer sent a Field Service Representative to Las Vegas to streamline correspondence between contractor and manufacturer and provide insight about the fibreglass pipe. Continuing through December 2015, an experienced field service representative has been active on site and available 24/7 to assist installation crews. This help has involved everything from inspection of incoming pipe, answering any contractor questions, helping to coordinate with project managers and shipping staff to ensure that all pieces needed are on site by the time of installation, and completing any small repairs for shipping damage. A Field Service Representative can present a short, tailored orientation – called a kick-off – to the crew and foremen of any contractor new to Flowtite or any who would like a refresher on handling fibreglass pipe.

As utility owners and municipal authorities look for ways to meet the increasing need to expand or replace existing water systems and infrastructures, the PWI serves as a good example for future pipe installation projects. Not only has the use of trenchless technology minimised surface disruptions and the impact to the public, but the use of fibreglass pipe for standard, slipline and jacking applications has been shown to be extremely viable, offering contractors and owners alike many opportunities to save, both short- and long-term