Article from "The Public Works Professional"
Reviving a Storm Drainage System by Ben Cummings

The shotcrete technique rehabilitates the Snapfinger Woods drainage system, producing minimal environmental impact and a cost-saving of $1,000,000.

The Snapfinger Woods Storm drainage system is located in the southeastern area of DeKalb County. Due to the continuing growth and easy access to major interstates, the location has developed as an industrial, commercial and residential zoning area. Snapfinger Woods Industrial Park is located to the northwest along Miller Road and Miller Grove Middle School. The intersection of Snapfinger Woods Road, Miller Road and Lithonia Way has several commercial sites, including several service stations and an office complex.
Project Introduction
Snapfinger Woods storm conveyance system consists of three distinct components. The northern most segments consists of five 48-inch Corrugated Metal Pipe (CMP) sections, which transverse Snapfinger Woods Road; a distance of 100 linear feet, leads to a junction box south of Snapfinger Woods Road. The intermediate segment begins south of Snapfinger Woods Road at the uppermost junction structure and travels southwest 1,200 linear feet to a second junction structure north of Lithonia Way; this segment consists of two large 84-inch CMP’s. The final segment begins at the junction structure north of Lithonia Way and travels 100 linear feet under Lithonia Way to its terminus at a downstream headwall composed of three 84-inch CMP’s.
Project Objective
The objective of this project was to rehabilitate one of DeKalb County’s high volume drainage systems in the southeastern part of DeKalb County. At least 30 years old, the system needed to be replaced due to the disrepair of its old metal corrugated pipe. This project was unique, as the system started with five 48-inch metal pipes and transitioned to two 84-inch ones and then to three 84-inch pipes; pipe systems generally transition from smaller to larger ones.
Project Approach
Due to the location and the large variation in corrugated metal pipe diameter sizes, the project was completed in three phases. Phase one involved the rehabilitation of five 48-inch CMP’s and restoration of the upstream junction structure. Phase two involved the rehabilitation of two 84-inch CMP’s and the restoration of downstream junction structure. Phase three involved the rehabilitation of three 84-inch CMP’s.
Project Method
The method used to rehabilitate this location was a technique known as shotcrete. This technique uses concrete blasted from a high-speed / pressurized nozzle to apply concrete to the interior of the pipes. The technique has been known to be a cost-effective method to rehabilitate large drainage structures without replacement of existing infrastructure. Although this method reduces the interior diameter of the existing pipe, there is no loss of capacity because the finished product provides a lower friction coefficient than CMP.
Using this technique, the project proceeded with the debris removal and cleaning of each segment of the existing pipe. No.4 rebar was installed in the 84-inch CMP’s and wire mesh was installed in the 48-inch CMP’s. In order to achieve maximum structural stability, the inverts were shotcreted to a specification of 5,000 psi. Two shotcrete coats were applied to all sections of the pipe. The primary coat was applied and allowed to cure 24 hours, after which a second application was added to achieve the required thickness of cover. This process was repeated from the third point up to the crown of the pipe and then the final top third of the pipe was shot.
All rebound, (which is residue created from the process), was removed from the inverts and then final finishing was done. Approximately 33,000 pounds of reinforcing steel and 300 cubic yards of shotcrete was used on this project. The two junction structures were restored by installing a 6-inch reinforced invert and applying shotcrete to the walls. The downstream junction structure had failed walls that were being braced by 2 two-by-fours that were replaced with 6” x 6” reinforced beams. This project was completed in November of 2007 by Proshot Concrete Incorporated, of Florence, Alabama.
Project benefits Analysis
Project Construction began in September 2007 and was completed in October of 2007. The estimated cost of a pipe replacement would have been in the range of $1.5 to $1.8 million. By using the shotcrete project method there was no above ground disruption of the streets and traffic, and the project cost $565,000. In addition, environmental impact was minimal due to in-ground, in-place rehabilitation. Local business felt no impact of this work other than the visual impact of stored sand, cement and rebar that was staged on the storm drainage easement. The estimated cost-savings of approximately $1,000,000; the low impact to the environment; and the minimal impact the local business and area traffic were all significant factors on this project. The Snapfinger Woods Storm drainage rehabilitation project was a super win for DeKalb County Georgia Public Works.
- With files from Jeanne Fronda
- Ben Cummings is the construction inspector with DeKalb County Georgia Public Works