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Foundation Analysis and Design

Determine vertical soil pressure under footing

Contents:

2 to 1 method

Assume 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:

  • Footing: 8 feet x 4 feet rectangular footing

  • Column load = 25 kips

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/ft2.                                                             

Newmark’s influence chart

Boussinesq equation

Newmark’s influence chart is developed from Boussinesq’s equation.  Boussinesq’s equation considers a point load on a semi-infinite, homogeneous, isotropic, weightless, elastic half-space 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 = [(1-DP/P)-2/3-1]1/2                                                                                               [1.19]

Or

DP/P

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

R/D

0.27

0.40

0.52

0.64

0.77

0.92

1.1

1.39

1.91

¥

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 chart

Given:

  • Footing: 6 feet square footing

  • Column load = 25 kips including weight of footing

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.

 

  1. Pressure at the bottom of footing, P  = 25000/36=694 lbs/ft2.

  2. The influence value of the chart is 0.005

  3. The number of unit inside Square #1 is about 68.

  4. The vertical at 6’ below the center of the footing DP =  694*0.005*68 =236 lbs/ft2.

  5. The number of unit inside Square #2 is about 35.

  6. The vertical at 6’ below the corner of the footing DP =  694*0.005*35 =121 lbs/ft2.