After a long period of pioneering work in Houston, Texas, US activity in microtunnelling, or remote-control tunnelling, has shifted to California, and in particular Los Angeles. Work in Texas was prompted by a need for an expansion of the sewerage network and demands by residents in prosperous neighbourhoods for construction with minimal disruption of their facilities and lifestyle.

The motivation has been similar in California where the pressure to combat pollution and eliminate combined sewer overflows has been particularly strong.

One of the leading practitioners of microtunnelling in the Houston area, BRH-Garver, is now active in California, having developed a wealth of experience and a long reference list.

Among three current major microtunnel projects is the Marina interceptor sewer project for the City of Los Angeles. This will upgrade a sewage transmission main to the Bellona Creek pumping station, providing extra capacity for growth in residential and business developments in the Playa Vista area of Marina Del Rey.

The $7M construction work includes replacing 2,135m of 533mm id sewer with a new 1,067mm id Meyer Polycrete (polymer concrete) pipe imported from Germany. The programme includes 14 drives on the main sewer route, and two 305mm id connectors to new residential developments in the Playa Vista area, which total 366m.

BRH-Garver is using a Soltau slurry-shield microtunnelling system at depths of around 11m, which is under the water table. The ground comprises mainly sands and clays with some cobbles discovered on the sewer alignment. Jacking and reception shafts are protected from water ingress by grouting.

Iseki Unclemole microtunnellers are planned to be used on the connector sewers.

Although nature has bestowed many benefits on California, its perverse tricks include mudslides, washouts and seismic activity, which has to be taken into consideration when new work is planned.

This was the case on the Pacific Palisades microtunnelling project which began last November, where the existing sewer was vulnerable, sited in a ravine behind a row of prestigious homes.

The replacement plan involved a 305mm id main sewer, which was 790m long at the front of the plots, and construction of 22 lateral service connections of 203mm id with lengths up to 40m.

The project team included Nada Pacific as microtunnelling subcontractor to Mladen Bunitch Construction. With an allowed schedule of 70 working days, the contractors had to enlist microtunnelling systems and experienced operators from across North America and beyond. The systems employed were three Soltau RVS 100, a Soltau RVS80, and a Herrenknecht Bohrtec BM300 house connection microtunneller.

Jacking rigs and cutterheads for the RVS100s had to be air-freighted from Germany, as did the BM300. This was used on 17 of the 22 service connectors, the others being installed in open trenches. Unlike the Marina machine, all the microtunnelling systems were auger-driven and they had to contend with gravel and some rock to install Mission Clay No-Dig vitrified clayware pipe.

Experienced operators joined the project from Robinson Construction of Missouri, Northwest Boring of Washington state, and Ras-Dam Construction of Edmonton, Canada.

The main sewer was installed in 15 runs of 20-73m in length, despite difficulties with rock in the ground. Rock also prevented the completion of one of the shafts, drilled by Anderson Drilling, which hit three large boulders and had to be filled in. Working depths ranged from 8-12m due to the hilly topography and need to avoid the danger of future washouts.

Despite the difficulties, the project was completed within the allocated time scale and with compliments from residents on the cleanliness and attitude of operations.

In a third project, the $9.8M Pacific Coast Highway project in Santa Monica, an earthquake-damaged sewer of 914mm diameter, adjacent to the north-south interstate route had to be replaced. Owner, City of Santa Monica is upgrading to 2,440m of 1,524mm id Ameron lined and reinforced concrete jacking pipe.

The main contractor, Modern Continental, had to vacate the area by the end of May, before the peak summer season for the resort. To achieve this, two Herrenknecht slurry-shield microtunnelling systems, and AVN1200S and an AVN1500T had to work 24 hours a day. The AVN1500T is a more powerful machine brought in to deal with cobbles which were not discovered until construction commenced. It was equipped with a cutterhead to handle variable ground.