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<br />9.5 <br />several orders of magnitude higher than that of intact rock <br />and, hence, a general flow pattern will develop in the material! <br />behind the slope. <br />Figure 6.9a on page 6.11 shows that, within the soil mass, the <br />equipotentlals are approximately perpendicular to the phreatic <br />surface. Consequently, the flow lines will be approximately, <br />parallel to the phreatlc surface for the condition of steady <br />state drawdown. Figure 9.3a shows that this approximation has <br />been used for the analysis of the water pressure distribution' <br />In a slope under conditions of normal drawdown. Note that the, <br />phreatlc surface Is assumed to coincide with ground-surface at <br />a distance X, measured In multiples of the slope height, behind' <br />the toe of the slope. This may correspond to the position of a' <br />surface water source such as a river or dam'or It may simply be! <br />the point where the phreatlc surface is judged to intersect they <br />ground surface. <br />The phreatlc surface itself has been obtained, for the range of <br />slope angles and values of x considered, by a computer solution' <br />of the equations proposed by L. Casagrande(236), discussed In <br />the text book by Taylor(174). <br />For the case of a saturated slope subjected to heavy surface <br />recharge, the equipotentlals and the associated flow lines used <br />in the stability analysis are based upon the work of Han(237)1 <br />who used an electrical resistance analogue method for the study; <br />of groundwater flow patterns in Isotropic slopes. <br />Production of circular failure charts <br />The circular failure charts presented in this chapter were <br />produced by means of a Hewlett-Packard 9100 8 calculator with' <br />graph plotting facilities. This machine was programmed seek, <br />out the most critical combination of failure surface and d ten- I <br />slon crack for each of a range of slope geometries and ground <br />water conditions. Provision was made for the tension crack to' <br />be located in either the upper surface of the slope or in the! <br />face of the slope. Detal led checks were carried out in the, <br />region surrounding the toe of the slope where the curvature of, <br />` the equipotentlals results in local flow which differs from) <br />'that illustrated in Figure 9.3a. <br />The charts are numbered i to 5 to correspond with the ground-' <br />water conditions defined in the table presented on page 9.8. <br />Use of the circular failure charts <br />in order to use the charts to determine the factor of safety of <br />a particular slope, the steps outlined below and shown in Fig-I <br />ure 9.4 should be followed. <br />Step 1: Decide upon the groundwater conditions which are be-, <br />Ileved to exist in the slope and choose the chart <br />Y `?KU? J which Is closest to these conditions, using the table) <br />presented on page 9.8. <br />_ tap 2: Calculate the value of the dimensionless ratio <br />, VZ. 5 r1l. Ton <br />(Q( -fob <br />f?, V