GRINDER STATION BASINS AND DESIGN CONSIDERATIONS

BACKGROUND The City of Duluth retained Ayres Associates to design a sanitary sewer collection system for the Fond Du Lac neighborhood in the City of Duluth. The project consists of a conventional gravity sewer portion and a pressure sewer portion that includes approximately 100 grinder pump stations with fiberglass basins. During construction of the sewer collection system, failures of some fiberglass grinder pump stations occurred. GOALS AND OBJECTIVES The primary objective of this paper is to present the wastewater industry with information as to the cause of the fiberglass grinder station buckling failures, and to present fiberglass grinder basin design considerations. The primary goal is to educate the wastewater industry to assist in reducing the potential for fiberglass basin failures on other projects. IDENTIFYING THE PROBLEM The first grinder station failure occurred and the construction contractor, City of Duluth, Ayres Associates, and supplier searched for the cause of the failures. Grinder pump station suppliers specifications have a design requirement for the basins based on a saturated soil density of 120 pounds per cubic foot plus a factor of safety of 2.0 to the required basin depth. This performance specification was written into the project specifications. The basin failures first brought up the question of whether the wet silty soils in the project area had a soil density in the project area of greater than 120 pounds per cubic foot. Testing indicated that the soils were approximately 120 pounds per cubic foot and within the design limits.The fiberglass basin manufacturer then presented information on fiberglass basin design and it became apparent that the soil modulus, not the soil density was the most important design criteria for the fiberglass basins. This information revealed that the fiberglass design calculations were based on a soil modulus of 700 psi. This raised the question of “What is soil modulus and how does one test for it to determine if the soil modulus of 700 psi is being achieved”? The research conducted determined that a soil modulus of 700 psi can be obtained by backfilling and compacting to specific compaction with granular materials. The easiest and most assured backfill method to obtain a soil modulus of 700 psi around the fiberglass basins is to backfill with pea rock, or other crushed rock material. It became apparent that the suppliers recommendation to backfill with the native material was not correct because the native soil did not provide the soil modulus at which the basins were designed.The purpose of the paper is to make the industry aware of fiberglass basin design to appropriately specify and install the fiberglass basins if that is what is chosen as the basin material. THE SOLUTION The solution to the basin failure problem on this project was to first televise all the installed basins to quantify the number of failures. When basin failures were identified they were replaced. The basins were replaced with fiberglass basins that were designed for a soil that had a soil modulus of zero. This means the fiberglass basin does not need the surrounding soil as support to withstand the loads applied to it. The basins that had not failed, were re-excavated and a corrugated pipe was installed over the basin. The annular space between the pipe and the fiberglass basin was filled with rock to provide the design soil modulus. In all cases the gravity building sewer was stubbed out to allow the home owner to connect without excavating directly at the grinder basin and to allow the building sewer to be connected into the basin with the corregated pipe around the basin.The work is currently being completed on the project. The results of the corrective action at this time appear to be working but the final results are not known. Better information will be available by the October 2004 WEFTEC conference.