BTS | LA METRO
Q&A Following their presentation, Matthew Crow and Amanda Elioff took questions from the audience at the BTS meeting.
Questions and answers are condensed for meaning and space.
Q: You talked about gas and segment joints. How did the cross gasket enable back-grouting? Matthew Crow (MC): We have the double gasket, so the segments can rock. If there was a leak, or a defect on the front gasket, water or gas could pass between the two gaskets. Additionally in future a chemical, rubbery grout can be put between them, helped by the compartmentalised tunnel sections. We used it successfully on Crenshaw/LAX.
Q: On occupational health, what about working in a tunnel where tars are seeping in? Amanda Elioff (AE): The occupational safety agencies, OSHA and Cal/OSHA, have requirements and standards for air quality in the tunnel. We measure and continuously monitored oxygen, methane, hydrogen sulphide and carbon monoxide. MC: We use the pressurised envelope with the segments and the screw conveyor at the back, and we provide significant ventilation to dilute contaminants. For station work, there is also a lot of ventilation. For hydrogen sulphide, everyone wears a small monitor that records instantaneous and cumulative exposure. For VOCs, we monitor with gas detectors, and again the main control is ventilation.
Q: How intense and costly are the instrumentation and monitoring requirements in downtown LA? MC: For geotechnical instrumentation and monitoring in tunnelling, we use extensometers and horizontal inclinometers. For stations we use inclinometers, strain gauges on pipe struts and building monitoring points. We are also using satellite- based InSAR. We now know that LA moves up, down and sideways naturally.
Q: LA Metro has adopted new technology over the decades. What of the move to bicomponent grouting for primary grout in TBMs? Is the type of grout used for the primary grouting part of your waterproofing or gas-proofing consideration? AE: We have not considered it in that way. It is a great idea, but I do not know that we could rely on it to provide a seal. I am sure it contributes, but we have not done specific research.
Q: What of spoil handling, given the contamination? And with the tar pits and pumped water, did you treat before disposal? AE: The material met the requirements for disposal in local landfills and use as cover there. In the end, it was not considered hazardous or contaminated. MC: For the asphalt-infused sand, the contractor used a conveyor system and vertical conveyors to bring material to the surface. AE: Water from dewatering typically contained hydrogen sulphide and was treated on site. MC: It is an urban area, so water has to be treated anyway. The tar material does tend to dry and harden on equipment, which makes equipment difficult to clean, so there are practical lessons in working with it.
Q: You seem committed to cut-and-cover stations. Are you reconsidering and looking at mined-stations? California
High-Speed Rail is to come through the same mountains. are you sharing lessons? MC: The profession, and the owners or clients, have wanted to do things that are tried and tested, so we have cut-and- cover stations and bored tunnels. We did a cavern because we were in very good ground conditions. We are studying single-bore and mined-station options as an agency, but there has to be meaningful benefit. On collaboration with other agencies - absolutely.
Q: What of the time investment spent with stakeholders? MC: Engineers are good at the technical work but most work as a project manager is dealing with people, other organisations and third parties. We have to convince others not just about tunnelling but about the alignment and train operations.
Q: How do you manage seismic risk during construction? Do you have criteria for temporary works? AE: We do, but construction is considered relatively short duration, so there is some increase in loading for seismic considerations, though not the same as for the permanent structure. MC: Metro Rail has standard plans for shoring. There is a time element, and a lot is based on experience. Shoring is the big issue. There is a lot of experience of shoring successfully going through seismic events.
Q: With exploratory drilling about every 100m, how were you able to fit this in dealing with third-parties, and can you use data from basements and building sites? AE: The boring spacing came from judgement of geotechnical engineers. We do collect data from existing buildings where we can. Most buildings are not as deep as the tunnel. The tunnel advisory panel was also involved. MC: Standards are important, but I strongly believe that engineering care, skill, experience and judgement should drive where each borehole is placed.
Q: How are you proactively reducing project risk or costs due to third-party interface issues? MC: One example is a project that goes under the Union Pacific Railroad. We find out the third party’s requirements, ensure we understand them, and then include them explicitly in the bridging documents. When the contractor later applies for permits it is never easy, but the homework has been done. That is very important.
Q: On fire strategy, could you elaborate on the time allowed for evacuation, the length of tunnels, fire rating, and how passengers get off a train to a place of safety? AE: The position of safety in a fire is considered to be the adjacent tunnel. Passengers get off the train and walk to the nearest cross passage, which is less than 800ft (244m). A rescue train can then be sent to pick them up, if needed. MC: It is an international standard for cross passages. There is not much time needed for passengers to reach that point of safety if the standard is followed.
18 | June 2026
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