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WWI KLE1NFELDER iTemporan/ dewatering of the excavations can be accomplished using several methods Where excavations are relatively shallow, gravel filled ditches containing filtered, perforated pipes that extend —' feet or more below the bottom of the excavation have been effective The water is then pumped out Satisfactory dewatering with ditch drains often entails several successive trails and may require excavating additional ditches or deepening initial ditches before the groundwater is satisfactorily lowered The main disadvantages of this method are the slowness in draining the excavation slopes, continuous wet conditions, and space limitations within the excavation Well point or deep well systems that are installed around the periphery of the excavation are the most common dewatering systems in use today to permit stable construction in the dry The well points are small screen wells attached to nser pipes and connected at the surface by a common header that is further attached to a well point pump that pumps out the water that drains to the well points A single stage of well points will lower the water table approximately 15 feet The most practical use of the conventional well point is where the excavation is less than about 25-30 feet deep and no artesian pressures are encountered Deep well systems are similar to conventional water wells and are used to dewater deep pervious soil requiring high pumping rates, or to relieve artesian pressures Pumping is by means of turbines or submersible pumps The primary disadvantage of these methods over ditches is typically cost The dewatering system for the project should be designed by an experienced consultant to appropnately filter the native soils and reduce any dispersion, piping, and associated loss of ground The consultant should also account for any improvements near the project, such as structures, buried pipelines, etc The lowering of the ground water table produces additional effective stresses on the soils below the original water table These additional pressures may cause consolidation of the soils and settlement of the nearby improvements It is often appropriate to institute a survey program before and during dewatering to monitor settlements in adjacent improvements, and/or require recharge wells to reduce the effects of settlement Dunng initial earthwork, we have found that that the exposed soils at the base of the excavations are often not fully drained by the dewatenng and are wet and/or pliant To facilitate construction, the contractor may need to cover the bottom the excavations with a b to 12-inch thick stabilizing layer of clean, crushed gravel or place a reinforcing geotextile fabric, such as Mirafi 50OX or substitute with equivalent tensile strength, between the subgrade soils and crushed gravel These options should follow the foundation material specifications presented in Section 5 3 Based on our experience on levee and large residential projects in the area discharge permits from the Regional Water Quality Control Board (RWQCB) will be required when dewatering near the San .Toaquin River and associated sloughs, canals, and streams We recommend that the contractor directly contact the RWQCB to obtain any necessary permits and determine the appropriate disposal methods that need to be employed for any flows generated by dewatering operations i 13639 G01/2002A369 -�nril 19 3003 D 2002 Kleirifelder TRc Page-of 16