By Ned Racine
No longer a distant goal, the digging of the Purple Line Extension subway tunnels is anticipated to begin in approximately one year. One sign of the tunnels’ approach can be seen in the work on Wilshire Boulevard to strengthen the soil above the future tunnel.
Workers have already completed this effort, named jet grouting, west of Crenshaw Boulevard. Beginning Friday (Feb. 24) work begins west of Western Avenue (from Norton Avenue to Wilton Place and from Gramercy Place to St. Andrews Place, to be exact.)
Drivers can still expect to find two lanes in each direction along Wilshire during peak travel times, although there will be some restrictions of left turns.
Jet grouting has been used with Metro tunneling efforts for years as a way to stabilize soil. On the Purple Line Extension, engineers have selected 16 locations above the tunnels from Western to La Cienega Boulevard that could use a little help. Stabilizing the soil ensures ground stability during tunneling and protects utilities and buildings that sit above the future tunnels.
Two factors determine where ground stabilization occurs:
Geography – Places where future cross passages between the tunnels will be built. Required by fire and safety regulations, cross passages occur every 800 feet and where the tunnels approach the stations.
Geology – Locations where ground above the cross passages requires more stabilization.
Jet grouting uses high-pressure air and grout (thinner than standard cement) to fuse with surrounding soil to create a deep, solid column of soil. This shield allows the tunnel boring machine to dig the tunnel without any movement of ground above and around this block.
Jet grouting has three steps:
1) Drilling a cluster of wells in the soil.
2) Shooting the air/high-pressure water and grout mixture into the ground to create an overlapping system of columns.
3) Taking core samples to test the quality of the new block.
The mix of soils near the ground surface in Southern California is one reason for jet grouting. Geologists might find layers of Younger Alluvium and Older Alluvium or Lakewood Formation or San Pedro Formation or Fernando Formation and their varied mixtures of clay, silt and sand. The Fernando Formation rarely requires stabilization. The other formations often do.
As you drive along Wilshire west of Western in the next month or so, you might drive by the K-rail and sound blankets that surround jet grouting installations in the middle of the boulevard. The installations might not be gorgeous, but they play a vital part in constructing two beautiful tunnels.
The first section of the Purple Line Extension will extend the subway from Wilshire/Western for 3.9 miles with new stations at Wilshire/La Brea, Wilshire/Fairfax and Wilshire/La Cienega. The first section is projected to open in 2023. A second phase will extend the subway to Century City and a third phase to the VA Hospital in Westwood. The project is funded by the Measure R sales tax and a federal grant.