TROUT BROOK TRIBUTARY CHANNEL INVESTIGATION
TPA was engaged to identify potential causes of failure of a retaining wall at a restaurant in Plainville. Its location along a property line affected the interest of two different owners as well as a restaurant tenant with a nationally-known brand.
Upon observation of the collapsed wall segments, it became evident that drainage on the site was not performing according to the original plans. Restaurant employees indicated that portions of the parking lot would flood during rain events due to the overflow from nearby Trout Brook. Furthermore, portions of the neighboring parking lot were graded to drain towards the top of the wall which were not designed to handle ponding of water. The resulting hydrostatic pressure beneath and behind the wall contributed to other factors such as surcharged loading that led to collapse.
Both property owners wanted to reach a mutual understanding so that the wall could be rebuilt without further issue. TPA was asked to investigate the tributary channel of Trout Brook and determine why it was overflowing into the parking lot several times per year. Using GIS mapping, the watershed was delineated and determined to be almost 80 acres. Field verification of the channel showed multiple instances of scouring and erosion, and it soon became apparent that this material was causing issues downstream.
The accumulation of silts, gravel, and loose stones in the restaurant's parking lot during flood events had buried an outlet conveyance pipe in a stormwater basin near Trout Brook. Further siltation throughout the channel diminished capacity and exacerbated flooding problems. As a result, water was backing up the channel until it reached a suitable relief point which just so happened to occur in the restaurant's parking lot near the retaining wall.
In its recommendations, TPA identified how the current stream channel deviated from the intended design and offered suggestions on how to alleviate future flooding issues. This information was shared with the engineer of record who was compelled to provide an updated plan.
Additionally, TPA provided guidance on the appropriate measures to consider in replacing the wall. The addition of drainage structures along the top of the wall will provide relief of hydrostatic pressure caused by ponding water. Additionally, tieback segments will allow the wall to better withstand lateral forces that caused the original retaining wall segments to topple. A sturdier wall was rebuilt in late 2018 to the satisfaction of both owners and has been without incident since then.