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rl� <br />�r <br /> -S <br /> A digitized version of the CPT-soi! type correlation chart resides in the CPT rig's <br /> computer,enabling automatic on-site interpretation of the CPT data by meano of a computer <br /> program. The resulting output, which consists of a computer-printed Icg of interpreted <br /> soil type versus depth, is available following each test. <br /> 3.2 HYDROGEOLOGY <br /> Hydrogeologic interpretation of CPT data yields information on (1) the vertical and <br /> lateral extent of aquifer and aquitard formations and (2) piezometric surfaces (i.e. depth <br /> and vertical distribution of groundwater). The first of these data categories is <br /> stratigraphy related, and is discussed in Section 3.1. interpretation methodology for the <br /> latter is described in the following section. <br /> 3.2.1 Plezometric Surfaces <br /> Piezometric surfaces are identified by performing piezocone pore pressure dissipation <br /> tests. These tests involve stopping the piezocone at selected depth levels, and measuring <br /> the dissipation of the "excess"pore pressure as a function of time. Excess pore pressure <br /> is generated due to the movement of the probe through the soil. Once the penetration is <br /> .,� stopped,and the excess pore pressure has fully dissipated, a steady-state hydraulic head <br /> is measured,enabling direct determination of the position of the piezometric surface. <br /> The time required to achieve full dissipation of the excess pore pressure depends on the <br /> permeability of the soil surrounding the piezocone. In sandy soils,typically only a few <br /> minutes are required. However, full dissipation in clays can take up to several hours. <br /> Therefore, for the purpo€3 of identifying piezometric surfaces, dissipation tests are <br /> -- generally performed within sandy layers. <br /> The vertical distribution of groundwater can be determined by performing dissipation tests <br /> at different depth levels. The relative elevations of th�- piazometric surfaces identified <br /> for individual sand layers indicate whether a particular layer is a perched or confined <br /> aquifer unit, or if several sand layers are hydraulically connected as a single aquifer. <br /> 10 <br />