6107 BERKSHIRE DR 2018-01-02 MF Import

ADaPT Engineering, Inc. Western Wireless Corporation ADaPT Job No, WA98.992 April I, 1998 Page 7 The factors E and 1 are gove,ned by the internal material strength characteristics of the pier. A representative value of nh for the soil type encountered at this site is presented. ?iers with a L./f ratio of less than 2 may be assumed to be relatively rigid and acting as a pole. The passive pressure approach may be used for this condition. °or piers with a L/T ratio greater than 2, the modulus of subgrade reaction method is typically used. Both of these methods are discussed below: Passive Pressure Method: The passive pressure approaci, is conservative by neglecting the redistribution of vertical stresses and shear forces that develop near the bottom of the pier and contribute to resisting lateral loads. We recommend using the allowable passive earth pressure (expressed as equivalent Fluid unit weights) listed in Table 3. Table 3 Allowable Passive Pressures Depth (feet) Allowable Passive Pressure (pcf) 0-2 0 2-7 350 7-20 400 20-29 200 The allowable passive earth pressure presented in Table 3 may be assumed to be acting over an area measuring 2 pier diameters in width by 8 pier diameters in depth, neglecting the uppermost 2 feet of embedment below the ground surface. According to the NAVFAC Design Manual 7.2, a lateral deflection equal to about 0.01 times the pier length would be required to mobilize the allowable passive pressure presented above. Higher deflections would mobilize higher passive pressures. When developing the allowable passive pressure listed in Table 3, we have incorporated a safety factor of at least 1.5, which is conunonly applied to transient or seismic loading conditions. Modulus of Sub grade Reaction Method: Using this method, the pier is designed to resist lateral loads based on acceptable lateral deflection limits. For sandy soils, the coefficient of horizontal subgrade reaction (kh) is considered to be directly proportional to the depth along the pier. The formula to determine kh is kh = nhx, where x is the depth below the ground surface in inches. We recc,.nmend using the value for the constant of horizontal subgrade reaction (nh) for the various soil types presented in Table 4 below