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Western Dam Engineering Technical Note 18 Photo 1.Site dewatering for a dam in South Dakota using well points. 2.Site Dewatering/Unwatering – Site dewatering which involves controlling or lowering subsurface water, and unwatering, which involves the removal of surface ponded or stored water, are typically within the domain of the construction contractor’s expertise, and are good candidates for team-effort specifications. General parameters should be established to ensure that all excavation and fill placement activities can be performed essentially in the dry, without causing damage to foundation surfaces. The contractor’s dewatering plan may consist of well points, trenches, sumps and pumps, wick drains, or other means that will ensure the work area remains dry. Dewatering system design criteria that may be specified by the engineer should include maximum allowable groundwater surfaces relative to the active work level. If the engineer believes well points or deep well systems are required, the specification should prescribe that the dewatering system shall include well points or deep wells. The depths, sizes, locations and well configurations can be left to the design of the contractor subject to demonstration of successful dewatering with wells or piezometers in advance of excavation. Subsurface dewatering systems need to be properly designed to prevent the migration of fine- grained materials from the foundation during dewatering and prevent any undesirable settlement that may impact nearby foundations. It is important to have dewatering as an early submittal; many fall dam construction projects encroach on unfavorable winter conditions due to delays caused by dewatering problems. It should also be noted that if dewatering is critical to dam safety or if the construction schedule does not allow for dewatering trial and error, the Engineer should consider designing the dewatering system. 3.Gate and Mechanical Systems – Specifications for gate systems will often utilize a team effort approach with the manufacturer(s) of the desired product. Small dams will typically incorporate off-the-shelf items that are part of the manufacturer’s standard catalog and have prewritten specifications associated with them. These can be incorporated by reference, if allowable and appropriate, or by inclusion. “Or-equal” provisions can allow the contractor to propose alternative products if they can demonstrate to the engineer that they meet all of the standards and requirements listed. If custom fabrication is required, then the specifications must define the necessary geometry, preferred materials (if any), and loading and performance requirements. This may also apply to other mechanical systems, such as lifts, operators, hydraulic systems, etc. Consider requiring mechanical component submittals soon after the bid award. Delays associated with manufacturing and delivery of gates is common. One thing is guaranteed…the dam owner will be upset if the gate is not installed when they want to start storing water. As an alternative, long lead items may be designed by the engineer and procured ahead of time by the owner. 4.Outlet Slip Linings – Conduit lining systems may be either specified as to type by the engineer, or as to desired function and performance. Either approach may lend itself well to team effort specifications. Proprietary systems will usually involve the cooperation of specialty contractors in developing appropriate specifications for the work. Once again, this should be an early submittal that should be reviewed and approved before the contractor mobilizes to the site. 5.Armoring Products – Proprietary products, such as articulated concrete blocks, gabions, etc., are good candidates for the use of team-effort specifications, using the input and standard specifications of the manufacturers / vendors of those products. Placement of these products often requires vendor-specific techniques that are important to follow. Consider requiring a vendor representative to be present during the installation.