25F - AGMT - DELHI DIVERSION PROJECT

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Preliminary Design Report Addendum No.2 Open -Cut Excavation Open -cut excavation is a commonly used method for the construction of wastewater sewer and water mains. A new main is constructed by digging along the length of the pipeline to be replaced. The traditional dig- and - replace (open -cut) method requires more restoration and causes more disturbance than other construction methods. Open -cut replacement is typically the most costly method for constructing pipelines due to intensive traffic control, pipe excavation, bedding and backfilling, and surface restoration requirements. However, it provides the best option for repairing all defects and eliminating poor trench conditions that are typically the cause of many pipeline defects. Open -cut excavation may be performed if adequate space is available for construction of the new pipeline without significantly impacting service, traffic, and access to private roadways and the community. Trenchless Construction Technologies Due to the environmental sensitivity of the project area and regulatory restrictions that would be imposed, trenchless construction methods have been evaluated for this Project as an alternative to open trench construction, where applicable. Technologies currently in use for small size pipe construction include jack and bore and horizontal directional drilling (HDD). Jack and bore is a multi -stage process for installing pipelines, which consists of constructing a temporary horizontal jacking platform and a starting alignment track in an entrance pit located at the desired elevation. The casing is jacked by manual control along the starting alignment track with simultaneous excavation of the soil being accomplished by a rotating cutting head in the leading edge of the pipe's annular space. The new main is inserted into the steel casing. The removed soil is transported back to the entrance pit by rotating helical auger flights. Jack and bore typically provides limiting tracking and steering as well as limited support to the excavation face. Removal and disposition of excess material is usually required. HDD offers much less impact on the environment and surrounding structures compared to other utility construction methods. The HDD pipe installation method requires limited construction on either side of the road, causing minimal impact on traffic and other buried utilities and structures. Typical HDD construction is for pipe diameters greater than 2" and pipe lengths greater than 600 feet. The HDD construction method is a two -stage process. The first stage consists of drilling a small diameter pilot hole along the desired centerline of a proposed pipeline. The second stage consists of enlarging the pilot hole to the desired diameter to accommodate the utility line and pulling the utility line through the enlarged hole. The method's unique ability to track the location of the drill bit and steer it during the drilling process results in a greater capability in placing the utilities in difficult underground conditions. The limited traffic and surface disruption benefits of this method are comparable to the jack and bore method. A smaller excavation area is required for the entry and exiting points; less equipment and less soil disruption are also added benefits to the HDD method. It is anticipated that the HDD method is the preferred method of construction when crossing the Delhi Channel and within the golf course area. This is primarily due to the low amount of surface -level environmental impact associated with this technology and the sensitivity of the project area. Also, the HDD technology is applicable in the saturated soil and underwater conditions that will be encountered crossing the Delhi Channel. Per Caltrans encroachment permit guidelines and specifications for trenchless technology projects regarding jack and bore and HDD technologies, all transverse crossing pipes that are 6" or greater in URS 18 25F -150