Are there sinkholes in the Bay Area?
Several months ago, the storm that ravaged Northern California has caused semi-permanent damage in two sleepy Bay Area communities. That was when giant sinkholes swallowed parts of La Fayette and Santa Cruz in two separate instances.
And as we hear news of sinkholes after sinkholes swallowing whole houses and buildings in different parts of the country, mostly in Florida, we wonder if our properties here in the Bay Area are sitting on a big sinkhole and it’s only a matter of time when they would occur and dispose of our properties into oblivion.
The massive sinkhole that erupted on a residential road in Lafayette, taking over two lanes of Mountain View Drive, was blamed by local officials on the high water levels and a clogged storm drain. According to Lafayette City Manager Steven Falk, the hole, which formed after the road collapsed onto the storm drain, is 80 feet long, 40 feet wide, and 15 feet deep.
A smaller sinkhole, on the other hand, took over a sizable portion of roadway at the summit of Highway 17 near Vine Hill Road in the Santa Cruz mountains. These incidents bring to light the reality that even in the Bay Area, sinkholes may occur.
During the historic but destructive 1906 San Francisco earthquake, a sinkhole formed in SF’s Valencia Street where the nearby buildings crumbled towards the inside of the street.
The US Geological Survey (USGS) explained that sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by groundwater circulating through them. As the rock dissolves, spaces and caverns develop underground. Sinkholes are dramatic because the land usually stays intact for a while until the underground spaces just get too big. If there is not enough support for the land above the spaces then a sudden collapse of the land surface can occur. These collapses can be small, or, they can be huge and can occur where a house or road is on top.
A sinkhole is an area of the ground that has no natural external surface drainage–when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface. Sinkholes can vary from a few feet to hundreds of acres and from less than one to more than 100 feet deep. Some are shaped like shallow bowls or saucers whereas others have vertical walls; some hold water and form natural ponds. Typically, sinkholes form so slowly that little change is noticeable, but they can form suddenly when a collapse occurs. Such a collapse can have a dramatic effect if it occurs in an urban setting.
The USGS further explained that sinkholes may also be formed by human activities. New sinkholes have been correlated to land-use practices, especially from groundwater pumping and from construction and development practices. Sinkholes can also form when natural water-drainage patterns are changed and new water-diversion systems are developed. Some sinkholes form when the land surface is changed, such as when industrial and runoff-storage ponds are created. The substantial weight of the new material can trigger an underground collapse of supporting material, thus causing a sinkhole.
Authorities explained that the overburden sediments that cover buried cavities in the aquifer systems are delicately balanced by ground-water fluid pressure. The water below ground is actually helping to keep the surface soil in place. Groundwater pumping for urban water supply and for irrigation can produce new sinkholes particularly in sinkhole-prone areas. If pumping results in a lowering of groundwater levels, then underground structural failure, and thus, sinkholes, can occur.
Sinkholes may capture surface drainage from running or standing water, but may also form in high and dry places in a certain location.The USGS added that sinkholes may also be formed by rainwater leaking through pavement and carrying soil into a ruptured sewer pipe. Sinkholes may capture surface drainage from running or standing water, but may also form in high and dry places in a certain location.
So, how do we determine if our property is located in a prospective sinkhole? Modern technology can locate these areas. Sinkhole location may also be recommended in areas with soft soil or where mining or other subsurface disturbance activities have occurred using surveys via ground penetrating radar.