Mobilizing for the Los Angeles outfall drive 25 Mar 2021

TBM excavation is expected to start in mid-2021 on the 7 mile x 18ft i.d. (11.25km x 5.5m i.d.) effluent conveyance system in the Los Angeles area. Main contractor, Dragados, took delivery of its Herrenknecht slurry machine in November 2020 and assembly of the TBM and its slurry treatment plant is approaching completion, ahead of lowering the machine into the launch shaft in May 2021. The TBM is designed to handle a variety of ground conditions, including soft ground in the first half and hard rock in the latter stages of the drive.

Site setup for segmentally lined slurry TBM drive

With no intermediate shafts, the drive will be managed from the 114ft (35m) deep x 55ft (17m) diameter launch shaft. Shaft excavation was supported by a 4ft (1.2m) wide reinforced-concrete slurry diaphragm wall and 12ft (3.6m) thick tremie slab with construction, including tremie slab and dewatering, complete in August 2020. A final reinforced cast-in-place concrete liner will be placed inside the slurry wall after the TBM drive is complete. The contractor has also elected to construct starter and tail headings to support TBM launch and assist with logistics during the drive. Excavation of the starter heading by mechanical excavator inside a block of jet grouted ground is complete with construction of the tail heading to be completed by the end of March 2021.

Zones of jet grouting will also provide safe havens for free air cutterhead inspection and tool replacement in the alluvial deposits across the first 3.7 miles (5.8km) of the 11.25km alignment. Injected from the surface, the three blocks of about 60ft long x 40ft wide x 40ft high and at up to 75ft below ground surface will be complete by the end of April 2021. The TBM is also equipped with two personnel and one material locks for hyperbaric interventions, as required. With the entire alignment below groundwater level and up to 10 bar hydrostatic pressure expected, there is also provision for pre-excavation grouting, as needed.

Fig 1. An 11.25km route to the ocean

The 21.6ft diameter x 770ft long (6.5m x 235m) slurry TBM is designed to withstand the complex ground conditions expected in the southern half of the alignment, when the TBM will drive under the Palos Verdes Hills and reach depths of up 460ft (140m) through sequences of calcareous, diatomaceous and dolomitic siltstones, mudstone and claystone, interspersed with breccia, limestone, shale, and some basaltic intrusions.

These units are heavily folded with north-south trending anticlines and synclines on the east side of the Palos Verde Hills and an east-west anticline paralleling the alignment. Under the hills, there is evidence of faulting throughout and highly variable bedding orientations. Significant squeezing ground conditions are anticipated with convergences of up to more than 12inches (30cm) occurring within six hours of excavation.

To drive through the squeezing ground, without becoming stuck, the TBM features an additional hydraulic circuit that increases nominal thrust force from 79,128 kN to 158,256 kN when activated. A tapered shield, plus shield circumferential flushing ports and strain gauges built into the shield will also assist through squeezing conditions.

35m deep x 17m diameter working shaft

The Mixshield slurry circulation system comprises nine 600hp (447kW) slurry booster and four 600hp (447kW) slurry feed pumps at the longest reach, with the slurry treatment plant capable of handling 5,283 gals/min (1,200m³/hr). Screened muck will be dewatered through a filter press. The separated solid portion stockpiled by radial stacking conveyor, and then trucked for offsite disposal. Gas vapour and hydrocarbon monitoring instruments installed at the treatment plant will detect contaminated muck with the radial conveyor able to stockpile any contaminated material separately for disposal.

Casting of the segmental lining is expected to begin in April at the production plant of subcontractor Traylor Precast in Littlerock, California, which is a truck journey of about 80 miles and 1.5 hours north of the job site. Segments will be delivered to the TBM in sets of two complete rings by rubber-tired multi-service vehicles. Each ring of 5ft wide x 1ft thick (1.5m x 300mm) comprises six segments, including the key and counter key, and weighs about 21 ton per ring. About 7,350 rings will be required to complete the lining.

Mixshield TBM will drive through challenging geology

Reinforcement is dependent on the ground conditions with steel fibre-reinforced segments specified through the alluvial reach and in a short section at the junction with the outfall manifold structure with 60lb/CY (36kg/m³) of reinforcement to concrete. Traditional steel-reinforced segments are to be used through the hard rock drives with a reinforcement ratio of 205lb/CY (122kg/m³).

Through the alluvial reach and for a 740ft (226m) section of low-cover at the southern end of the alignment, where net internal operating head during peak wet weather periods exceeds the hydrostatic head of the natural groundwater, a pre-tensioned design has been specified for the segmental lining. The design involves two sets of ducts and pockets cast into the segments with up to four steel tendons then inserted through. The tendons are then post-tensioned when installed and the pockets and ducts backfilled with non-shrink grout. Post-tensioning will take place 500ft (152m) behind the TBM on two dedicated gantries.

A ⅜in (19mm) thick welded steel secondary lining with cellular grout is also required in reaches of weak ground where the alignment intersects with the Palos Verdes Fault and at both ends of the drive, totalling some 1,450ft (442m), the longest section of which is 500ft (152m).