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When the water table is above the wedge zone, the soil parameters used in the bearing capacity equation should be adjusted. Bowles proposed an equation to adjust unit weight of soil as follows: g_{e}=(2H-D_{w})(D_{w}/H^{2})g_{m}+(g’/H^{2})(H-D_{w})^{2} [1.16] Where g_{e}^{ }= Equivalent unit weight to be used in bearing capacity equation, H = 0.5Btan(45+f/2), is the depth of influence zone, D_{w}= Depth from bottom of footing to ground water table, g_{m}^{ }= Moist unit weight of soil above ground water table, g’ = Effective unit weight of soil below ground water table. Conservatively, one may
use the effective unit water under ground water table for
calculation. Equation
1.16 can also used to adjust cohesion and friction angle if
they are substantially differences. Example 9: Determine equivalent unit weight of soil to calculate soil bearing capacity with the effect of ground water table
Given:
Requirement: Determine equivalent unit weight of soil to be used for calculating soil bearing capacity.
Solution:
Determine equivalent unit weight:
Dry unit weight of soil, g_{dry} = g_{m} /(1+ w)
= 120/(1+0.2) = 100 lb/ft^{3}. Volume of solid for 1 ft^{3} of soil, V_{s }= g_{dry} / (G_{s}g_{w}) = 100 / (2.65*62.4) = 0.6 ft^{3}. Volume of void for 1 ft^{3} of soil, V_{v} = 1-V_{s}=1-0.6=0.4 ft^{3}. Saturate unit weight of soil, g_{sat} = g_{dry} + g_{w} V_{v} = 100+62.4*0.4=125 ft^{3}. Effective unit weight of soil = g_{sat} - g_{w} = 125-62.4=62.6 ft^{3}. Effective depth, H = 0.5B tan(45+f/2) = 0.5*8*tan (45+30/2) = 6.9 ft Depth of ground water below bottom of footing, D_{w}= 6-2 = 4 ft Equivalent unit weight of soil, g_{e} = (2H-D_{w})(D_{w}/H^{2})g_{m}+(g’/H^{2})(H-D_{w})^{2} =(2*6.9-4)(4/6.9^{2})*100+(62.6/6.9^{2})(6.9-4)^{2} = 93.4 lb/ft^{3}. |
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