Homes Built In Big Wash Floodplain Facing Huge Challenge

Maybe building homes in a flood plain is not a good idea after all
A few years ago, the Hirmath led council, which included current Council Member Steve Solomon, approved a land swap that resulted in homes being build in the Big Wash Flood Plain. Today a development, Valley Vista, faces significant challenges because it was built on loose flood plain soil, soil that is subject to collapsing when water gets to it. 

It has taken two years and the effort of the current council to unveil the problem...
...a problem that came to everyone's attention when two properties in the development began subsiding. The council directed staff to "get answers" as to why this happened.  Staff reported their findings at last week's meeting.  It was only after questioning by Council Member Tim Bohen that the significant long term problems in the area came to light.

Staff Report: Two properties sank because of a drinking water line leaking into soil that is subject to collapse when it gets wet
The subsidence of two homes, each on the side of Pima County’s “lift station,” in the Valley Vista Subdivision was caused when the soil beneath the homes compacted because of an 80,000 gallon drinking water leak that occurred in late December of 2021. The leak was a water intake pipe joint at the station. Pima County never reported the leak to the town. Indeed, it took Oro Valley town staff months, working with an outside consultant, Ninyo & Moore, to determine the that leak had occurred. The town believes that the joint was leaking for two years prior to the major break in 2021.

The amount of the leak was far greater than the station’s normal water use. The lift station has the town’s water smart system installed but never set “alerts” so they they would know that an unusual amount of water was being used.

Pulte Homes, the subdivision developer, remediated the sinking of the two homes by installing ram jack piers along he perimeter of the houses. The piles extend to around 70 feel deep to bedrock.

Town Staff did not discuss many other issues raised by the Ninyo & Moore report. It was only after questioning by Council Member Tim Bohen and some residents of the development that other "sinking" issues, more long term and more concerning emerged.

The homes in Valley Vista sit on "alluvial" soil... the development sits in the Big Wash floodplain 

Why 70 feet to bedrock? Because the soil between the top soil and the bedrock are “alluvial sediments”. These are materials such as clay, silt, sand, gravel, and organic matter that have been deposited by running water in various environments like stream beds, floodplains,…or beaches.  A 2018 geological report, one submitted to the town as part of the development process, notes that these soil condition exist throughout the development. (Source: Consultant report page 4)

Alluvial soil collapses when wet 

That same report states that the “… subsurface soils were loose to medium dense and susceptible to hydro-collapse.” (Source: Consultant report page 4). In other words, these soils are prone to suddenly settling or compacting when they become wet. The report continues: “The results of the consolidation tests indicate moderate to very severe collapse potential upon saturation…that these settlement prone soils extend to substantial depths.” (Source: Consultant report page 11). In fact, the 2018 study concluded that “…that the subsurface soils were loose to medium dense and susceptible to hydro -collapse.” (Source: Consultant report page 4)

Streets also subject to future subsidence 

The report notes that future subsidence of the roadways should be expected. Fortunately, the town has not accepted the streets yet. These are still Pulte Homes responsibility. The report recommends that an extensive rebuild be done ((Source: Consultant report page 12).  Pulte recently recently sealed some of the streets in the development. Our guess: Sealing cracks. Resealing will not solve the problem of future subsidence

Never again?
The reason they are homes built in the wash is because former Mayor Hiremath and his council, including current council member Steve Solomon, approved a land swap that enabled that development. Hopefully, there will be no more building in Big Wash approved by this or any future council.

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Note to homewowners in the Valley Vista Subdivision 

All homeowners in the area need to be vigilant to make sure they have water smart installed; that unlike Pima County they set excessive water use alerts; and that they investigate immediately if there is an alert. Any leak from the street line to their homes is their problem. It’s not the town’s. Our experience is that the Town will offer you no help when you have a leak.

The type of leak from the street to a home is that development will prove be difficult to identify visually. This is because, the water from the leak goes straight down and does not rise to the surface until the ground is saturated. Fortunately, residents should be able to identify a leak before it does too much damage because the line to the residential home is 5/8 inch diameter. The line to the lift station is 2 inch diameter.

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