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7 Joints: When possible, pipe sections should be fabricated in the shop. However, there are several alternatives available for field connecting sections of both HDPE and steel pipe as described in FEMA (2005). Joint testing should be completed prior to grouting. Flotation: Both HDPE and steel pipes will want to float during grouting and must be restrained physically by external “centralizers” between the host and carrier pipes or by filling the pipes with water or sandbags. Inlet and Outlet Structures: Rehabilitation or replacement of inlets and outlets is an important consideration in the design of alternatives for an outlet rehabilitation project. The ability to fabricate and install carrier pipes to fit a given inlet structure configuration often drives the decision whether to utilize or replace an inlet structure. Similarly, the desire for retrofitting seepage collars or the need to repair the existing downstream slope or provide energy dissipation are considerations for outlets. Grouting: Grouting the annular space between the new pipe and the existing pipe is essential (see Photo 3). Only contractor’s experienced in this type of grouting should be used for this specialized work. Typically grouting is completed from the downstream end with grout pumped to the upstream end through tremie pipes. Multiple tremie pipes of increasing lengths are used to inject grout and reduce the travel distance of the grout. Grouting should continue until the entire annular space has been filled and no voids remain. Vent/observation pipes are used to verify grout has filled the annular space. Securing bulkheads to contain the grout is also critical. An in-situ alternative to sliplining is CIPP. This method is best suited for pipes that are not severely deteriorated, have limited to no protrusions, and have constant diameters. Many of the same design parameters should be considered. Curing of the carrier pipe is a critical step and one of two methods can be specified; pressurized steam, or pressurized hot water. Each has advantages and special consideration must be given to dry pipe installations versus where standing or flowing water will remain present within the host pipe during liner installation. Consultation with a CIPP manufacturer is highly recommended during the design process. Photo 3: Grouting HDPE Liner pipe.3 Construction Considerations: Sliplining and CIPP pipe rehabilitation projects often involve working from one or both ends of the outlet with significant distance of pipe that cannot be accessed between. This introduces challenges during construction that must often be resolved in real time, i.e. once grouting or curing is started the effects cannot be reversed. For this reason adequate, experienced supervision is essential during critical activities such as liner installation and grouting. Additionally, experience has shown the value of “mini” preconstruction meetings involving all field personnel just prior to the start of those critical activities. A discussion “what if’s” is a key component of those meetings to determine who makes necessary field decisions and what range of decisions might be needed to ensure project success. Conclusion This article provides a short summary of the design parameters and construction issues to consider during design of an existing conduit rehabilitation project. Before proceeding with a rehabilitation project, the design should answer the following questions. Should I replace the existing conduit or rehabilitate it in place? • Conduits that are severely deteriorated or possibly have voids adjacent to the conduit due to internal erosion should be replaced. A detailed cleaning and internal conduit inspection should be 3 Photo courtesy of www.water.state.co.us