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Tunnel mining on Wilshire Bl

The Purple Line Extension Transit Project will construct two parallel tunnels to create a seamless connection from the current Purple Line Wilshire/Western Station in Koreatown to three new stations located at Wilshire/La Brea, Wilshire/Fairfax, and Wilshire/La Cienega.

The new 3.4 mile railway will be constructed using a combination of tunnel boring machine and an “open-cut” methods. The Twins will be responsible for approximately 17,845 linear feet of tunneling, as they travel under Wilshire Bl. Each tunnel will take approximately 24 months to complete.

Along the way, The Twins will encounter different soil conditions. The tunnel crown will always be at least 20 feet or more under the subsurface water table.

In terms of geology, tunnels will be bored through two types of geological formations: San Pedro and Fernando.

The following ground conditions may be encountered during tunneling:

  • Full face San Pedro
  • Mixed face ~ Top San Pedro and bottom Fernando
  • Full face Fernando
  • Between STA~490+00 (Dunsmuir Av) and 540+00 (Crescent Heights Bl) equals~ 5,000 linear feet. These three ground types will also be saturated with tar (with various percentages of saturation/presence).

Tunneling Statistics

Progress is measured in total linear feet traveled and the number of rings they build.


Linear Feet




La Brea Bl to Western Av



La Brea Bl to Fairfax Av



Fairfax Av to La Cienega Bl



La Cienega Bl to tail track west of La Cienega Bl

Totals: approximately 17,845 linear feet or 3.4 miles; two tunnels equal approximately 35,690 linear feet or approximately 6.8 miles.

Follow ElsieSoyeonTBMs on Twitter for weekly updates on their progress and more.

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