PLEASANTON -- A small sinkhole discovered near downtown Pleasanton prompted the closure of one lane Friday morning around 9 a.m. as crews scrambled to fill the crevice.

A private contractor had installed fiber optic cable on Bernal Avenue, near Old Bernal Avenue, when a small portion of the pavement caved in, said Scott Lines, assistant director of operation services for the city of Pleasanton.

The hole measures approximately 4 square feet and was filled in with dirt and gravel Friday morning. The closed traffic lane was expected to be reopened by afternoon.

Sinkholes are often caused by erosion, percolating water or reduced amounts of groundwater, although Friday's incident appears to have been accidental, Lines said.

"The crews created a void under the asphalt and because they didn't fill in all the way, a small part collapsed," Lines said.

Contact Karina Ioffee at kioffee@bayareanewsgroup.com. Follow her at Twitter.com/kioffee.