An often overlooked secret to design and construction success

As a regional provider for the Dallas Metro area and 27 nearby communities, Dallas Water Utilities (DWU) serves over 2.6 million people across approximately 699 square miles. Its reach in service is almost as long as its history.

DWU began as the City of Dallas Waterworks 144 years ago, in 1881. Thirty four years later, in 1915, the first of DWU’s two wastewater treatment plants, Central Wastewater Treatment Plant (CWWTP), was built, with an initial capacity of 6 MGD. As Dallas grew, the plant was expanded to meet demand. The Imhoff tanks installed in 1916 as clarifiers and digesters were abandoned and built over. Infrastructure was buried and replaced as the years rolled by.

Currently, the CWWTP has a capacity of 170 MGD. It has by now maxed out most of the 300 acre space where it resides and is confined by a levee system. Both the age of the site and proximity of the infrastructure complicated a recent clarifier replacement project, one that ended up a great success thanks to collaboration with plant operators.

Here’s what happened: plant staff had been consistently taking one of their two square primary clarifiers, clarifiers A and B, out of service for maintenance. As soon as they fixed one, the other would go down. Settling within the clarifiers was becoming overwhelming, and the shutdowns put too much stress on the other clarifiers and the plant overall.

As square clarifiers tend to have more maintenance needs than round clarifiers, due to certain mechanisms that require frequent replacement, our solution was to replace primary clarifiers A and B with three smaller, circular clarifiers. These three smaller clarifiers could meet DWU capacity demands, and they would be more cost efficient. DWU approved.

Our design and construction were challenged, however, by all the old structures below ground. Due to the extensive age of the site, current as-builts didn’t show all the older infrastructure. The Imhoff tanks, for example, abandoned in 1981, were the biggest variable. We didn't know how much the structures were demolished or how far below grade they were or where, exactly, they were located.

We held multiple workshops and facilitated communication with the CWWTP staff, going over any and all conflicts below grade that they were aware of, which included the Imhoff tanks. Operators thought the tanks were just a few feet below the surface, remembered uncovering them several times, and that they might have been filled with sand.

Bores taken by our Geotech staff confirmed what the operators told us and the locations of the tanks. Thanks to detailed coordination, we were able to develop extensive documentation of what existed under the ground’s surface for the contractor to present accordingly on bid day. As a result, we didn't see any change orders or any additional claims from the contractor when they encountered these subgrade structures.

In the process of using the old Imhoff tanks’ structures as shoring for the new clarifiers, the contractor was able to carefully avoid the electrical duct banks - all around the old tanks - that provide power to the CWWTP. Successfully navigating around these electrical duct banks was critical; disrupting them could cause the entire plant to lose power.

If plant operators hadn’t brought our attention to all of the old infrastructure below grade, and showed us exactly where they thought all that stuff was, our design and even construction would have been negatively impacted. Not having that knowledge would have, no doubt, resulted in a lot of change orders. But because CWWTP operators so diligently outlined where the issues were, conveying and communicating them to us clearly and early on in design, we were able to mitigate potential change orders as construction began.

WWTP operators are crucial sources of information for any project. They can provide historical context when it comes to how processes have operated, alert us to shortcomings they experience with existing processes, and, as in this case, point out what historical infrastructure is underground. All of which is vital information that impacts design and construction success.