Early experience of the first of four Lovat EPBMs to work on the ECIS (East Central Interceptor Sewer) tunnel in Los Angeles indicates that adequate soil conditioning is the key to achieving correct operation of the project’s specified EPB excavation.
For most of the contract’s 11 mile (18km) alignment, the 16ft (4.7m) diameter tunnel runs 25-50ft (6.5-16.5m) above the water table and in medium dense and compacted alluvial deposits of sand, gravel, silts and clay. The dry material exerts little inherent earth pressure however there is a high probability of intersecting lenses of running sands that require positive face support to control excavation and avoid surface settlement through this dense urban area of LA. For this reason EPB excavation was specified by the City of Los Angeles, Department of Public Works. Conditioning of the dry alluvial soils is required to create a material that will keep the excavation chamber completely filled at all times and maintain the plug of pressure-balancing material within the screw conveyor.
The technique of using conditioning agents to assist the performance of EPB machines in sticky clays, saturated soils and other materials that have a natural measure of earth pressure are well documented. There are also examples of projects in Europe where dry, non-pressurised materials have been conditioned to suit EPB excavation. Refining the technique in Los Angeles has taken time.
Of the different types of conditioning agents, the Kenny-led JV with Shea/Traylor/Frontier Kemper, is working initially with foam. Through the learning curve, and as explained by John Critchfield, Chief Resident Engineer for construction management during a visit in late February, there can be no test runs to perfect the technique prior to starting the job. Trials are part of the drive and at a time when other operations need to be mastered by the operators and tunneling crews. It was during the learning curve of the first TBM launched in early January 2002 that over excavation occurred as the TBM was passing from the pretreated ground adjacent to the tunnel eye and into natural untreated ground. The bulking factor of the natural material had yet to be determined; the most appropriate conditioning technique was still in development; and the full train of trailing gantries was also yet to be installed. Compaction grouting from the surface was used to recompact the zone of loosened soils. There was no visible sign of surface settlement or damage and the TBM had since advanced without further incident.
Meanwhile the second Lovat EPBM was launched on its 16,500ft (5,000m) drive in February 2002. By late Feb the first TBM was 560ft (170m) into its drive and the second had completed 165ft (50m) with the remaining gantries of its backup yet to be introduced. The contract’s third machine was due to arrive in April and be lifted directly off the rail car and into the launch shaft at Mission and Jesse for its drive east under the LA River channel, and the fourth is expected in June for launch on a drive west from the Grand Ave shaft by July.
Between them the four identical Lovat TBMs must complete the 11 miles (18km) of 15.5ft (4.7m) diameter tunnel by about May 2003 to meet the November 2003 court-ordered “cease and desist” commissioning date. The new sewer with its continuation north under the NEIS project is designed to prevent combined sewer overflows at manholes and into the LA River channel during heavy rain storms. Penalties apply if the City misses the cease and desist deadlines mandated by the State’s Water Quality Control Board.
The new sewer is designed by the City’s Department of Public Works Bureau of Engineering with assistance for ECIS from Parsons Brinckerhoff Quade & Douglas. Construction management and site supervision for ECIS is an integrated team of City employees with engineers from the Parsons Brinckerhoff Construction Services/ Brown & Root Services (now operating under the parent name Halliburton KBR) supported by subconsultants Babendererde Engineering, JGM and Ninyo & Moore.
The ECIS contract was awarded to the Kenny-led JV for a bid price of $240.3 million in late 2000 and notice to proceed was granted in Feb 2001.
Soil conditioning
The four Lovat EPBMs incorporate a mixing station on the trailing backups from where conditioning agent is pumped to the shield and injected via four ports on the cutting wheel directly onto the tunnel face. It is mixed with the soil by the action of the rotating cutterhead. A real time data logger on each machine informs the operator of the main performance parameters including the pressure of material in the chamber as detected and relayed by four pressure sensors located on the four quadrants of the chamber bulkhead. Each machine is also equipped with a TACS guidance system and there are grout ports in the shield for forward grouting.
The machines are delivered with the EPB screw conveyor in the shields. In addition the JV also ordered a muck ring and pressure relieving door (PRD) assembly for future projects. In the EPB mode, the screw conveyor replaces the belt conveyor of the muck ring/PRD mode, passing into the excavation chamber through the centre of the machine’s main bearing.
Some open mode operation may be possible at the western end of the ECIS project beneath the maximum 300ft (110m) overburden of the Blair Hills but for the majority of the drives, running between 30-100ft (10-30m) beneath the surface and close buildings, EPB operation of the screw conveyor in the ‘closed mode’ is required by the Contract Specifications at all times, explained Matthew Crow for the construction management team.
During T&TNA’s visit to the site of the second TBM to be launched and while watching excavated material discharge from the guillotine gate of the screw conveyor, the foam conditioning appeared near perfect according to Jörg Holzhäuser for the management team. With very little fines in the naturally sandy material, the foam agent was creating a moist, dough type material that you could squeeze in your hand and was suitable for EPB excavation. The correct quantity of conditioning agent was crucial. Too wet and sloppy or too dry and the material loses the ability to create the controlling EPB plug in the screw. At the muck pile the sandy material returned to its dry non-cohesive granular state as the foam dried out.
In the dense urban area of the tunnel alignment, management of environmental issues is as important as the technical issues, said Critchfield. A real estate issue at the second TBM shaft, followed by discovery of historical artifacts that required archeological investigation, caused a delay of about three months. Limiting noise and disruption to residents and businesses restricts production time to one 10h shift/day, 5 days/week at some sites. The tunnel also passes through old as well as still operating oil fields and carries a Class 1, Division 2 classification for the potential of encountering methane and hydrogen sulfide gases. Systematic probing with a magnetometer is specified to detect pockets of gas and old drill casings. Methods of mitigating early delays over the the 1,000-day contract will recoup some of the time loss and there is the possibility of relaxing some of the production restrictions.
The tunnels are lined with a precast concrete bolted and gasketed segmental primary lining produced by the Traylor/Shea JV. Once completed, a secondary 12ft o.d. (3.7m o.d.) precast concrete steel skinned carrier pipe with an inner corrosion resistant liner will be inserted and backfilled with cellular concrete. The double lining system is spedified to ensure a 100-year design life. It is the ingress of water through cracks and failed joints in the existing 70-year old sewer that causes the system to surcharge during heavy rains.
The bolted and gasketed primary lining is required to work in conjunction with the EPB technique and prevent annular grout ingress. The inner carrier pipe will increase durability and made it easier to apply the 270° round T-lock liner.
