Construction crews are working fast in dicey conditions to shore up the hillside behind the Lake Oswego Adult Community Center before wet winter weather sets in.
The rebuilding effort follows a landslide that quietly wiped out a hillside at the rear of the ACC. Sometime late last year, several tens of thousands of cubic yards of wet dirt washed away an entire slope, sending the dirt and other debris into nearby Tryon Creek State Park.
'We had no idea it had happened,' ACC Director Ann Adrian said. 'It was a big surprise.'
No one realized what had occurred until a parks employee noticed something was amiss in the northeast corner of the ACC property, just beyond the east parking lot, said Rob Amsberry, a city surface water management specialist and project manager.
'There was more light coming through than usual,' Amsberry said. After a closer inspection, it was clear why: 'The trees had disappeared. When they looked over, they saw the trees 150 to 200 feet down the hill.
'They realized the hillside had slid away.'
The hill was steep to begin with. After the landslide, a 40-foot cliff with a nearly vertical face sat atop a slope descending another 100 or so yards, Amsberry said.
The cause appears to be two-pronged.
Since the ACC was built in the 1970s, a pipe has funneled rain runoff from building surfaces and parking areas over the slope, likely eroding the hillside over the years. Record rainfall last winter didn't help the situation.
'I think it was a combination of water from the pipe and the saturated ground that caused the slope to give way,' Amsberry said. 'Nowadays we would require that pipe to take drainage all the way to the bottom of the slope and would put erosion protection at the bottom.'
By the end of October, officials expect to have not only a new pipe carrying water to the hillside's base but also a rock buttress holding the slope in place. Another new pipe will be installed to move drainage from the ACC's western parking area to the new pipe to the east, so all of the stormwater will be carried to the base of the hill; otherwise, a second landslide could happen about 100 yards west of the current construction zone.
Construction crews began their work nearly as soon as the ink had dried on the city's $477,751 contract with Dirt and Aggregate Interchange Inc. Including temporary fixes made last spring and other costs, the landslide repairs will cost about $750,000 in all.
Crews are now working to remove loose dirt and vegetation, and they're rebuilding the hill with terraced benches. Native plants will be used to landscape the area once new rocks and boulders are in place. In addition, crews will bring in a variety of new trees - big leaf maple, Western dogwood, Douglas fir, Western red cedar and prairie spire ash. At least nine significant trees were lost in the landslide and subsequent remediation process.
This isn't your typical public works project, Amsberry said: 'It's not like digging a ditch in the street and putting pipe in the ground. This is totally separate from anything we've done in town as far as a public works project. We don't normally do landslide repairs.'
The work is risky in a variety of ways, Amsberry said. Walking away from a mandatory meeting with prospective contractors for the project, he heard two say 'they wouldn't touch it.'
'It's a risky job from a safety standpoint and from trying to understand what it's going to take,' he said. 'It's very steep; it's hard to get equipment down there, and there's no access from the bottom. Everything has to come through the parking lot at the ACC.
'And then you're pushing up against the weather window at the end of the summer season: There's risk involved with that, too.'
It was also important to stabilize the hillside before it could cause major damage to Tryon Creek, which provides habitat for threatened fish species. Heavy rain on the exposed slide area could send large amounts of sediment into the stream.
But it isn't just the overall project that is unusual. The agreement with Dirt and Aggregate Interchange Inc. is a bit different, too.
'It was something beyond what we would normally require for a construction project,' explained Diana Smith-Bouwer of Lake Oswego's Citizen Information Center.
Officials framed the contract language with First Addition residents and visitors in mind, requiring construction crews to limit truck traffic at certain times each morning and afternoon. School administrators provided a schedule to work with.
'Because it's First Addition, where there's a school and a library, there's a lot of activity,' said Smith-Bouwer. 'The neighborhood is pretty congested already, and we knew there were going to be a lot of dump trucks - fully loaded - moving through.'
By preventing conflicts with construction and school traffic, they hoped to ensure the public's safety, she said. Letters were sent and door hangers left notifying residents of the project.
'We wanted to work with them in advance to make them aware so they could adjust their schedules and know what to expect,' Smith-Bouwer said. 'It was a good partnership.'
Adrian, of the adult community center, said she also had concerns about safety.
'As soon as we found out, we started to work on measures to keep everybody safe and to maximize the parking we have,' she said.
Class schedules were tweaked to prevent overlap, reducing the number of people needing to use the parking lots at the same time. Some programs were relocated to other city facilities, but none were cancelled.
Although there were a couple of big events, such as a September meeting bringing at least 100 people to the ACC, that couldn't be rescheduled, overall, ACC users seem to have adapted well to the temporary inconveniences.
'I have to say, for the most part people have been really gracious and understanding about the whole thing,' Adrian said. 'I'm really grateful for that.'