Handout19 - below the base of the foundation!1cr = 4·I·q...

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I Special considerations when a foundation is at depth: When a foundation is placed at depth we must account for changes in stress due to both the soil that is removed to construct the foundation as well as the stress that is put back on by the foundation and superstructure. Since the soil is usually removed over only a portion of the surface we must apply influence factors to the stress removed as well as to the stress that is put back on. Stress Removed Stress Added B B ., ., ~ ~ i i D f q ~ _______________ I l J J J __________ _ Plane where z z stress change occurs P P For the Stress Removed For the Stress Added Change in stress at the level of the base of the footing (negative because stress decreases): !1cr = -yDf Change in stress at the level of the base of the foundation: !1cr = q Change in stress at point P a depth z below the base of the foundation: !1cr = -4 ·1·yDf Change in stress at point P a depth z
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Unformatted text preview: below the base of the foundation: !1cr = 4·I·q I is the influence factor for stresses beneath the corner of a rectangle (thus the factor 4 in the above equations). I is determined based on m = n = B'/z = B/2z (assuming that the foundation is square; it mayor may not be square). For depth (z) we use the depth below the bottom of the foundation because that ls the dep!h from w~ere we know the stress changes_. _ ._ . _ Combining the changes in stress due to the soil removed and the foundation load placed back on: !1cr =-4 ·1· yD f +4 ·1· q (continued on next page) Page 1 of2 or, !::.J:J = 4 .1·(q-yOf) or, !::. .o = 4 ·1 .qnet where qnet = q yOf Thus, when the foundation is placed at depth and load is removed, we should use the "net" change in stress at the level of the base of the foundation to calculate the changes in stress at depth. Page 2 of 2...
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Handout19 - below the base of the foundation!1cr = 4·I·q...

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