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*47 - <br /> R91. M- I- M7 �, R <br /> -'.7^'. hr^1" e'+' ova i, F •"'2'•'a1a°' MM .,_;,�° ol %-a <br /> _W-W <br /> WN <br /> ._a <br /> -• atmospheric). The pore pressure acting at the filter location is measured by the <br /> transducer and transmitted to a computerized data acquisWon system. <br /> Pore pressure measurements can also be made as a function of tune,while the piezcconer is <br /> kept stationary at a selected depth. This is referred to as a dissipation test. The <br /> purpose of a dissipation test is to measure the rate of dissipation of the "excssso pare <br /> pressure and to detarmine the hydrostatic (in situ) pore prossure within a formation. <br /> Excess Acre pressure is generated due to the disturbance caused by thg movemsrA of the <br /> piezocone through the soil. Onco the penetration Is stopped, the 9xcess pore pressure <br /> that has built up around th3 probe will dissipate, eventually stabilizing at the in situ <br /> or hydrostatic pore pressure. In layered soils, the pore presaure sometimes increases <br /> before descending to the equilibrium in situ level due to differences in excess pore <br /> pressure dissipation rates in ft different layers. The data obtained from pore pressure <br /> dissipation tests enables determination of the positions of piezometric surfaces in the <br /> soil profile. <br /> 2.1.3 Probe Geometry <br /> Electric friction cones are typically available in two sizes,as follows: <br /> tri inal Version Modified Versicn <br /> Cross-Sectional Area 10 cm2 (1.6-n2) 15 cm2(2.3 int) <br /> Diameter 3.6 cm (1.4 in) 4.4 cm (1.7 in) <br /> Friction Sleeve Area 150 cm2(23 int) 200 cm2(31 int} <br /> Cone Apex Angle 60 degreas 60 degrees <br /> The measured cone tip and friction sleeve loads are automatically divided by the <br /> cross-sectional probe area and sleeve surface area, respectively, to obtain' CPT <br /> measurements in pressure units (i.e. tonslft2). Once converted to these units, no <br /> _ apparent difference exists in the cone tip and sleeve friction measurements betKaen the <br /> two probes. This lack of size effects has been verged by researchers using a <br /> large-diameter calibration chamber. <br />�i <br /> For this investigation, the modified (larger) version of the elactric friction cone was <br /> used. This version has the advantage of being able to accomodate higher capacity load . <br /> �,. cells. Use of the smaller probe, which has the same diameter as the push pipes,requires <br /> c�. <br /> 4 _ <br />