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Determine vertical soil pressure under footingContents:
2 to 1 methodAssume stresses distributes in a 2 to 1 slope under footing.
Vertical stress at a depth z under a rectangular footing is Dp= P /[(B+Z)(L+Z) [1.17] Example 10: Determine vertical soil pressure using 2:1 method
Given:
Requirement: Determine vertical soil pressure at 6’ below bottom of footing
Solution: q =P /[(B+Z)(L+Z)] = 25000 / [(8+6)(4+6)] = 178.6 lbs/ft^{2}. Newmark’s influence chartBoussinesq equationNewmark’s influence chart is developed from Boussinesq’s equation. Boussinesq’s equation considers a point load on a semiinfinite, homogeneous, isotropic, weightless, elastic halfspace as shown below
By integrating Boussineq’s equation over a circular area of radius R, one obtains a relationship between DP/P and R/D as DP/P = 1{1/[1+(R/D)^{2}]^{3/2}} [1.18] Newmark’s chart is constructed by rewriting Equation 1.18 as R/D = [(1DP/P^{)2/3}1]^{1/2} [1.19] Or
In Newmark’s chart, if the length of one unit equals to D, then the radius of the ring is equal to R. The area inside each circle is the ratio of DP/P. The area inside the first inside circle is 0.1, and the area inside the second circle is 0.2, etc. To be able to measure the area inside the circle, the area is divided in to a number of pieces. Each piece between the circle and radical lines is a unit. The number of units multiplied by an influence value should be equal to the ratio of DP/P of the circle. Therefore, DP=P I N Where, I is an influence of Newmark’s chart and N is the number of units inside the circle. For the chart shown below, there are 200 pieces in the total area. Then, the influence value of the chart is 1/200= 0.005. Therefore, the ratio of DP/P for the first circle is 20 units x 0.005 = 0.1. Although, the chart is developed based on circular area, it can be extended to other shape of footing by placing a scaled footing area on top of the chart.
Example 11: Determine vertical soil pressure using Newmark’s chartGiven:
Requirement: Determine vertical soil pressure at 6’ below bottom of footing at center of footing and at a corner of the footing Solution: The footing is draw to scale according to a depth of 6’. Square #1 is placed on the chart with center of the footing at the center of circle. Square #2 is placed on the chart with corner of footing at the center of the circle.
