ABSTRACT
Example 7.4 A column of a building carries a load of 1000 kips. The load is transferred to sub soil through a square footing of size 16 x 16 ft founded at a depth of 6.5 ft below ground level. The soil below the footing is fine sand up to a depth of 16.5 ft and below this is a soft compressible clay of thickness 16 ft. The water table is found at a depth of 6.5 ft below the base of the footing. The specific gravities of the solid particles of sand and clay are 2.64 and 2.72 and their natural moisture contents are 25 and 40 percent respectively. The sand above the water table may be assumed to remain saturated. If the plastic limit and the plasticity index of the clay are 30 and 40 percent respectively, estimate the probable settlement of the footing (see Fig. Ex. 7.4)
Solution 1. Required lip at the middle of the clay layer using the Boussinesq equation
~ = 24.5 = 1.53 < 3.0 B 16
Divide the footing into 4 equal parts so that Z/B > 3 The concentrated load at the center of each part = 250 kips Radial distance, r =5.66 ft By the Boussinesq equation the excess pressure lip at depth 24.5 ft is (IB = 0.41)
Q 4x250 2lip =4 x 2 I B = 2 x 0.41 =0.683 k / ft Z 24.5
Figure Ex. 7.4
2. Void ratio and unit weights
Per the procedure explained in Ex. 7.3 For sand Yt = 1241b/ft3 Yb = 61.6Ib/ft3
For clay Yb = 51.4 Ib/ft3 eo = 1.09 3. Overburden pressure Po
Po = 8 x 51.4 + 10 x 62 + 13 x 124 = 26391b/ft2
4. Compression index
wi =I p + wp =40 + 30 =70%, Cc =0.009 (70 - 10) =0.54
S - 0.54 16 I 2639+683_0413f -496'Settlement t - ---x X og -. t -. In. 1+ 1.09 2639
Example 7.5 Soil investigation at a site gave the following information. Fine sand exists to a depth of 10.6 m and below this lies a soft clay layer 7.60 m thick. The water table is at 4.60 m below the ground surface. The submerged unit weight of sand Yb is 10.4 kN/m3, and the wet unit weight above the water table is 17.6 kN/m3. The water content of the normally consolidated clay wn = 40%, its liquid limit wi = 45%, and the specific gravity of the solid particles is 2.78. The proposed construction will transmit a net stress of 120 kN/m2 at the center of the clay layer. Find the average settlement of the clay layer.
