Dive summary:

• One tunnel-boring machine is chewing its way through the highly varied substrate of San Francisco, and another is to begin soon on a parallel route, and engineers are using a process known as compensation grouting to stabilize the ground around the tunnel and the buildings above it and prevent settling.
• The technique employs pipes deployed horizontally from a shaft dug adjacent to the tunnel path.
• One machine began digging for the Central Subway Project last month, and its twin is to get into gear later this month.

From the article:

"The tunnels pass through both soft ground and Franciscan formation, which is heterogeneous rock that is not predictable except in its unpredictability," says Sarah Wilson, a San Francisco Municipal Transportation Agency (SFMTA) resident engineer. ...