Stresses Newmark Chart Examples.pdf y5p

CE 222 Soil Mechanics-I

NICE-NUST

Newmark Influence Chart’s Example A raft foundation of dimension 11 m x 6.2 m is placed at a depth of 2m below the ground level. Determine the net stress due to the raft foundation (stress due to applied load only) at a depth of 7m below the ground level. Also determine the total stress due to raft (stress due to applied load) and stress due to soil (overburden pressure) at a depth of 7m below the ground level. The raft is subjected to total load of 10000 kN. The unit weight of the soil is 18 kN/m3. Neglect the pore water pressure (assumed soil is completely dry).

The total stress acting at the base of the raft = \[{{100000} \over {11 \times 6.2}}=146.6\] kN/m2. The net stress at the base of raft = 146.6 – 18 x 2 = 110.6 kN/m2 (net stress means total stress minus the stress due to the soil above the base of the foundation as before the application of load soil was existing there. Thus, stress due to soil has to be deducted to calculate the net stress). Now, depth of the point below the base of the raft is (z) = 7 – 2 = 5m. Thus, according to the Newmark’s chart (see the above Figure), 2.5 cm = 5m. Scale is 1: 200. Now, the raft (CDEF) is drawn with a scale of 1: 200 and placed on the Newmark’s chart (as shown in above Figure) such that the centre of the raft is coincided with the centre of the Newmark’s chart. This is to be noted that, here the stress below the centre of the raft is determined. Thus, centre of the raft is coincided with the centre of the Newmark’s chart. If the vertical stress below the corner or any other point within the raft is to be determined than the corner or the point on interest has to be coincided with the centre of the Newmark’s chart. The total number of influence area covered by the raft = 116 (as shown in above Figure). The net stress at a depth of 7 m below the ground level or 5 m below the base of the raft = 110.6 x 0.005 x 116 = 64.2 kN/m2. Thus, vertical stress due to applied load only is 64.2 kN/m2. The vertical stress due to the overburden pressure at a depth of 7 m below the ground level = 18 x 7 = 126 kN/m2. Thus total vertical stress due to the applied load and overburden pressure at a depth of 7 m below the ground level = (64.2 + 126) = 190.2 kN/m2.