Microtunneling is inherently a challenging form of trenchless construction -monitoring jacking forces as the pipe string moves, balancing the slurry at the face of the microtunnel boring machine and completing the drive within the tolerances identified by the contract create challenges in the best of conditions. When extremely abrasive soil conditions, site constraints and environmentally sensitive areas are factored in, the project becomes formidable.

The Sari Project
The Santa Ana River Interceptor (SARI) Line is a 23-mile-long (37km) wastewater pipeline that extends from the boundary of Orange and San Bernardino counties in southern California to the Orange County Sanitation District (OCSD) sewage treatment plant. Several segments run adjacent to and under the Santa Ana River and nearly 4 miles (6.4km) of the pipeline were in jeopardy of failure during heavy rainstorms.

To ensure the long-term integrity of the pipeline, protect public health and safeguard the environment, the Orange County Flood Control District, in cooperation with other stakeholders, embarked on the Santa Ana River Interceptor Relocation Project to relocate and replace the SARI Mainline with a new pipeline.

The Orange County Public Works Department (OCPWD) recognised that a qualified contractor with the proper microtunnel equipment and experience was required for the project. SARI project specifications required a highlyspecialized microtunnel boring machine with an airlock and compressed air access to the face of the machine to cope with the expected soil conditions. James W. Fowler Co. (JWF) was ultimately named the County’s choice for the microtunneling portion of the SARI project. With the location of the project and the proximity of the project to the 91 Freeway and Santa Ana River, there were a number of stakeholders with a sizeable investment in the successful completion of the project.

Project details
The SARI project consisted of 4,000ft (1,219m) of 101.5-inch (2.6m) OD microtunnel done in four drives with installation of 2,900ft (883.92m) of 84-inch (2.13m) ID reinforced concrete pipe and 1,100ft (335.28m) of 99.5- inch (2.5m) ID steel casing. This design also required excavation of several deep shafts in difficult locations. Ground conditions along the four alignments were an extremely abrasive mixed face combination with soft to stiff silt and loose sand, gravel, sand, and clay exhibiting a flowing behavior with cobbles and some boulders. The groundwater table ranged from the tunnel invert at the lowest point to approximately 17ft (5m) above the tunnel invert at the highest.

Initial designs of the SARI Mainline indicated a typical microtunnel project with seven straight drives of lengths ranging from 120ft to 1,190ft (36.6m to 362.7m). JWF was familiar with the Jackcontrol hydraulic pipe joints and saw an opportunity to introduce curved microtunnels into the project alignment, reducing the mobilisation of equipment and eliminating several shafts required for the project, including one that would disrupt a recreational vehicle park.

JWF proposed four drives – two straight, one compound curve and one standard curve. The first drive, approximately 620ft (189m), was located under Gypsum Canyon Road and Gypsum Canyon Drainage channel. The second drive was 1,089ft (332m) under a portion of the Green River Golf Course and the Santa Ana River channel. These two drives were dislocated and curves offered no advantages.

The final two drives adjacent to the 91 Freeway fit the profile for adding curves. The third drive, the compound curve, was adjacent to the 91 Freeway and was approximately 1,567ft (477.6m). This drive was the first use of the Jackcontrol AG hydraulic joint system with real-time monitoring in North America and set a US record for being the longest microtunnel compound curved drive and only the second compound curve drive in the United States.

The final drive of 622ft (189.6m) was also adjacent to the 91 Freeway and included a standard curve using the Jackcontrol system.

Value engineering proposal
Originally designed as a traditional conservative microtunnel project with several short, straight drives, the SARI Mainline offered JWF the opportunity to offer an innovative value engineering proposal. By suggesting a standard curve microtunnel drive and a compound curve microtunnel drive using a hydraulic joint manufactured by Jackcontrol AG of Switzerland and a VMT GmbH theodolite-guided navigation system, JWF was able to eliminate shafts and combine multiple drives on the project. This saved the project more than USD 1M and offered a 20 per cent reduction in the tunneling schedule.

The ultimate success of the project was threefold: The OCPWD was open to the curved drive proposal offered by JWF, the Jackcontrol joint and VMT guidance system ensured the drives were completed as designed and the relationship created by the project team developed into a true partnership where the project success came first. Jeff Anderson, JWF’s project manager says, "the project went as scheduled, which is unusual in this business."

JWF contacted Jackcontrol AG for its review and input into the potential curved drives. It was determined that sections of the project could benefit from a redesign. Reducing the five straight drives with the corresponding three jacking and three reception shafts to two longer curved drives requiring two jacking shafts, one reception shaft and an observation shaft would meet the intent of the original design.

An added advantage to the revised alignment was the option to adjust the location of the shafts, selecting areas that had better access and site availability.

Ameron Reinforced Concrete Pipe (RCP) with Carnegie style bell and spigot joints were intended for use on the sections of the project being considered for the curved drives. After revising the design and incorporating appropriate changes to the standard pipe, Ameron concurred that the pipe would perform through the curves, an important step in the value engineering process. Jackcontrol created preliminary sketches of a curved alignment to determine the potential curve radii. Ameron’s pipe was determined to be more than sufficient to handle the potential joint deflection.

With a reasonable expectation that the curved drives could be completed successfully and that the project could benefit from the introduction of curved microtunnels, a preliminary value engineering proposal was submitted to the project stakeholders and their engineers.

It was important to gauge their tolerance for incorporating curved drives into the project since at that time there had only been one curved drive completed in the US. The project team was very enthusiastic about the possibility of eliminating several of the tunnel shafts and the potential corresponding reduction in the tunnel and shaft construction schedule.

While the owner was receptive to the potential value engineering proposal, it chose to reserve final determination until the cost and schedule savings could be compared against the added risk that a curved drive would introduce into the project. The dollar values of the changes were analyzed and the schedule was revised. JWF determined that 30 days could be saved on the tunnel schedule by incorporated the curved drives and as the tunnel is the project critical path, that translated to 30 days savings to the project schedule.

Curved drives
Pressure transmission rings made of wood material are widely used in microtunnelling applications. In curves, the mechanical characteristics of wood material can cause severe damage to jacked pipes. To avoid such damage on the planned curved drive, JWF partnered with Jackcontrol to use its patented hydraulic joint and real-time monitoring system. The joint acts hydrostatically like a fluid-filled hose with a uniform pressure level allowing the curved joints proposed on the project without causing axial stresses that would exceed the strength of the pipe material.

The jacking loads were also examined in relation to the pipe joint mating surfaces. Here the Jackcontrol hydraulic joints proved to be the perfect alternative for increasing the contact area of the jacking surfaces in the curves. This provided the jacking forces needed to complete the drives without point loading the pipe joints.

Due to its mechanical characteristics, the Jackcontrol hydraulic joint allows curved microtunnelling alignments with the use of regular pipe lengths and application of usual jacking forces without harming the pipe structure.

The hydraulic joint, with its welldefined and reversible mechanical characteristics, was used as an integrated sensor for a reliable determination of the size and position of the thrust/ resulting jacking force during the jacking operation. This capability gave JWF a real-time monitoring of the pipe structure regarding the admissible jacking force to prevent the pipe from getting damaged.

The tunnelling machine JWF used was a company-owned Herrenknecht AVND 2000D equipped with a SLSMicrotunnelling LT navigation system from VMT GmbH. VMT’s system was chosen for the guidance of the long distance and curved pipe jacking application on the SARI project. The main component of the system was a servo motorized laser total station mounted inside the tunnel on a special bracket, which moved together with the pipeline. The actual position of the laser total station was continuously calculated with help of the known as-built position of the already installed pipes.

A VMT engineer on the project conducted daily control measurements as required by the owner and provided reports to confirm the position of the machine and pipes were within the specified tolerances.

Successful completion
The accomplishments on the SARI project, and the two curved drives in particular, were a testament to the planning that occurred prior to the project, the vetting of the microtunneling contractor through a rigorous prequalification process and the commitment of the team in ensuring the project was successful.

At the end of the compound curve drive, Anderson remarked, "With the systems and the partner programs we used, we easily could have just kept tunnelling".