REDLANDS>> There are many factors that contribute to formation of sinkholes, said Fred Mousavipour, director of the city’s Municipal Utilities and Engineering Department.
“Depending on the situation, the conditions of the soil and the type of soil that’s existing in different places, there are many factors that may cause sinkholes,” Mousavipour said.
In the case of the large sinkhole on Redlands Boulevard at California Street, which formed on Aug. 4 following heavy rainfall, there were two major causes.
The sinkhole formed at the confluence of two flood control infrastructures—the flood control that runs beneath the road and a storm drain pipe.
When the water discharged into the channel, it generated turbulence causing the water to undermine the soil.
It then caused erosion and created the hole.
“However what’s special about this one, is usually when sinkholes start developing, especially in Redlands, we see some external signs like some depressions appearing on the surface,” he said.
If a water leak occurs beneath a street in Redlands, the water compacts the soil beneath the asphalt, causing depressions to form on the surface and then a visible hole starts to appear.
But, Redlands Boulevard was previously Highway 99 and at that time concrete slabs were put into place, rather than asphalt. Over the years it was paved over with asphalt, which made it impossible to see the exterior signs of a sinkhole.
“Unfortunately in this situation we didn’t have a lot of the exterior signs of a sinkhole because of that old Highway 99 slab on top until it was too late, so we saw the collapse happen,” he said.
The city of Redlands and Loma Linda had plans to rehabilitate the intersection but a lack of funding put it on hold.
A storm hit the city in December 2010, causing about $1.5 million in damage to city infrastructure.
“As a result of that we decided to go on and investigate the conditions of the Zanja channel, the oldest channel we have going through the city, and we prioritized the trouble spots by looking at areas underneath of existing buildings,” Mousavipour said.
City staff discovered five locations that were deemed to be potential settlement problems, Mousavipour said. Four of the five areas, which included Redlands High School, have been addressed he said.
Next they looked at areas affecting open spaces, such as parking lots and beneath streets. Then city staff looked generally at the entire flood control channel system to see what other places are potential hazards.
“We’re proceeding and have been proceeding based on fund availability to fix some of these things,” Mousavipour said.
The city contracted with Orange County-based RBF Consulting to develop its flood control master plan to determine priorities for upgrading its flood control systems.
City staff has also proposed a $47.8 million general obligation bond to pay for a flood control system to protect downtown.
“All in all that’s the result of not having sufficient flood control system in the city,” Mousavipour said. “As we know most of these issues would be prevented, which the City Council knows and the city manager has been working diligently to create funding, so we can go ahead with (a) flood control master plan and go ahead and build part of that system or all of the system.”
The City Council on Monday ratified a resolution declaring a local emergency to help the city obtain funds to pay for the repairs, which is estimated to cost up to $200,000 and take two to four weeks to complete.
The city has had to coordinate with The Gas Co. and Verizon to ensure the protection of utility lines running through the hole.
Workers will then tear up parts of the street around the hole to determine the full extent of the damage before backfilling and repaving.
Mousavipour said if drivers-by noticed little or no activity at the sinkhole site this past week, it’s not because there are delays. It was because workers poured concrete to strengthen the infrastructure before moving on with repairs. Concrete needs a few days to set.
“That’s our priority project. We have one of the best engineers in charge of that site and we’re monitoring this thing on a daily basis,” he said. “We’re going to finish this and open up the intersection.”