SAN FRANCISCO (KGO) -- Repair work continues in San Francisco's South of Market neighborhood. Crews are fixing a sinkhole that opened up above the Central Subway. It's on 4th Street between Market and Mission, near Jessie Street. ABC7 News looks into what caused it.
Workers noticed the sink hole developing Friday night at 4th and Mission. Below ground is where tunnels have been dug for the $1.6 billion Central Subway Project. The twin tunnels are complete but crews are now building what's called a cross passage for passengers to get from one side to the other in an emergency.
"This cross passage, the technique used for the excavation was ground freeze and the ground was frozen properly," John Funghi, San Francisco Municipal Transportation Agency's point man for the project, said.
Ground freezing is a process using pipes to freeze the earth, making it stable enough for underground construction.
Somehow muddy water and sand seeped into the site, causing the street to sink.
When asked why there was water leaking if the pipes were frozen, Funghi replied, "Well, good question. We're investigating. It's a little premature to say that the ground thawed and allowed the water to infiltrate. What we suspect is that water found its way through a fissure in the frozen ground."
Funghi says it's not uncommon for water to get into an underground construction site, particularly one below sea level, but acknowledges this was not normal. Critics of the project believe nearby buildings will now need constant monitoring.
David Schindler, a manager at Cole Hardware, says crews have already come into his basement a couple of times. He said, "They were checking and double checking that foundation wall in relation to the street."
The entire 1.7 mile subway route will now be inspected.
"At this point, we still have to do an investigation to determine what happened here. Right now, we're focused on repairing the utilities and the road," Paul Rose from the San Francisco Municipal Transportation Agency said.
The repairs are expected to be done by Friday.
SF Crews work to repair sinkhole above Central Subway