Utility Design Risks in Vertical Housing
A seemingly recurring utility design challenge can occur when a developer begins a new vertical housing project: depending on the number of housing units there can be a significant disconnect between how much daily and peak sewer and water capacity a building needs vs. the actual City infrastructure capacity available in the adjoining public streets. This discrepancy has the potential to cause major impacts to projects costs and schedule if not thought about early on in the design phase.
A good example of this is a 3-building residential development in Edina, MN which was part of an overall redevelopment project on France Avenue. The City approved preliminary plans for architectural and civil design, but there was no detailed building plumbing design. The design-build contractor then based the site utility plans for water and sewer services on existing City infrastructure sizes and capacities, as well as state design standards and flow projections based only on the number of apartment units and occupants.
Much later in the development process, final design-build plumbing plans were finalized and submitted for building permit review. Upon review the three separate proposed sanitary sewer lines leaving each building were sized significantly larger than the public mainline they were to connect into. This larger piping was a product of state plumbing design guidelines (which are based on total plumbing fixtures) as well as other architectural, structural, and mechanical building design considerations. Ultimately the first design-build plan provided for one 12” sewer service, and two other separate 10” service lines to serve each of the three buildings – all of which were to be connected to an 8” City service lateral. Needless to say this created quite a concern for the developer and project team!
Mismatched Piping Requires Extensive Redesign and Calculations during Construction
Because the existing City infrastructure was not planned to be upsized as part of a street reconstruction project, the design-build contractor was ultimately required to retain an independent plumbing engineer and entirely redesign the sewer systems to minimize pipe sizes leaving the buildings. Through extensive redesign efforts and coordination, two of the three building piping systems could be adequately reduced in size (all the while as the buildings were under construction) to match the existing City lateral size, but one line simply could not be reduced smaller than 10”. At this point the design team performed additional calculations to present the City with a summary of the daily and peak flows to obtain approval of a non-standard manhole connection.
The potential utility design risk to a project can range from minor inconveniences in cost and schedule all the way up to major delays, redesigns, and agency negotiations. This risk can be effectively managed early on in the design process by retaining a plumbing engineer during the initial design and entitlement phase to prepare preliminary water and sewer calculations in partnership with the civil engineering design.
At Westwood we are always looking out for our clients by leveraging our diverse development and design experience to think outside the box and ask questions that might not “traditionally” be in our purview. In this case the sanitary sewer design disconnect was due to differences and nuances in the design criteria of the plumbing code and state utility design guidelines.