Work on the Regional Connector’s second tunnel is progressing at a blistering pace. In fact, the machine recently dug past the Metro Red Line tunnels and will soon reach Bunker Hill. Meanwhile, at the future Grand Av Arts/Bunker Hill Station, work on building the station floor continue. New progress photos depict how crews have prepared the concourse level floor and laid rebar in advance of upcoming concrete pours.
These images show the north side of the station where a bank of six high-speed elevators will be installed. Before laying the rebar, crews lined the area with waterproofing membrane to protect the foundation against erosion. Since concrete is not waterproof, wet soil beneath a foundation can swell or lose strength; therefore, it’s vital to prevent water from moving through the base structure.
Like any modern building, the new station will be made of concrete, a material that is inexpensive, easy to mold, fireproof and extremely strong in compression (you can put a great deal of weight on it). Concrete might look heavy, but it’s actually a fifth as dense as lead, a third as dense as steel, 10 percent less dense than aluminum and only slightly more dense than glass.
Concrete’s biggest drawback is how weak it is in tension. That is to say, it cracks or snaps easily when bent or stretched. To solve this problem, engineers cast concrete around strong, steel bars, which are laid in a grid pattern.
Reinforced concrete works well against compression and tension, as the concrete resists squeezing and the steel resists bending and stretching. In theory, you could use many sorts of materials to reinforce concrete, but steel is typically preferred (plastic is also common) since it expands and contracts roughly as much as concrete itself.
Once all the rebar is in place, crews will pour 185 cubic yards of concrete from the surface for 70 feet down to the concourse area to form the four-foot thick slab.
The Regional Connector project is building two 1.9-mile rail tunnels to tie together the Blue, Expo and Gold Lines in downtown Los Angeles. The project will speed up light rail trips to and through downtown L.A. and is scheduled for completion in late 2021. It is funded largely by Measure R and a federal grant.