Figure 7.6. Depressed highway subject to uplift

Two options are shown in Figure 7.6 for dealing with the uplift pressures beneath the highway: in the 'partially relieved' section, a filter drain is installed to relieve water pressures and allow the thickness t1 of the road base to be minimized; in the 'unrelieved' section, no drain is installed and the base thickness t2 must be selected to balance the uplift force. The partially relieved section will require ongoing maintenance to ensure the drains continue to function correctly in the future.

For the situations shown in Figure 7.6, the destabilizing design vertical action is given by:

Vd,dst = Yg,dstUk = Yg,dst X Yw X (d + ti ) X Bi and the stabilizing design vertical action by:

vd,stb = Yg,stbWk = Yg,stbX YckX X Bi where d, t, and B are as defined on Figure 7.6; yw and Yck = weight densities of water and concrete, respectively; and we have assumed that the water table has risen to the ground surface and ignored the weight of the side walls.

In addition, the resistance R caused by effective earth pressures acting on the side wall of the section helps to stabilize the structure:

where Yk = the weight density of the soil adjacent to the highway; 5 = the angle of interface friction between the wall and the ground; and Ka is the soil's active earth pressure coefficient, given by: 1 - sin y

1 + sin y where 9 is the soil's angle of shearing resistance. By assuming active earth pressure conditions, we avoid over-estimating the favourable effect of R. If we also assume that the angle of interface friction is given by 5 = (2/3)9, then the factor p becomes: 1 - siny

Figure 7.7 shows the value of p based on various assumptions about the soil's angle of shearing resistance 9. The curve labelled 'characteristic' assumes 9 = 9kinf, the soil's 'inferior' characteristic angle of shearing resistance. The maximum value of p in this case occurs when 9k,inf = 27.3°.

The curve labelled 'inferior design' assumes that 9 = 9d,inf, the soil's inferior design angle of shearing resistance, given by:

tan Pk where the partial factor y9 = 1.25. The maximum value of p in this case occurs when 9k,inf = 32.8°. As Figure 7.7 shows, the value of p is smaller for the characteristic curve when 9k,mf > 29.9°. In other words, applying a partial factor to the soil's characteristic angle of shearing resistance increases the resistance available along the wall when ^k,inf is greater than about 30° I1

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