The pressure at depth in a soil can be divided into two parts :
1 .The pressures due to density of overlying soil 2 .The pressures due to loads applied on the surface of the soil.
1. the pressure due to the colour of the soil 2. the pressure due to the weight of its smell
The total stress of the soil can be found with which formula
sigma = rho . g . h
u = rho(w) .g . h(w)
sigma' = sigma - u
Total Stress is the stress or pressure calculated by multiplying bulk density by depth.
Pore Water Pressure u =rho (w) .g . hw
Effective Stress ' is numerically equal to Total stress - pore water pressure.
Effective Stress ' = Total stress - Pore water pressure u
Where the foundation shape is not symmetrical the pressure beneath the foundation will vary with lateral position as well as depth. The pressure under any corner of the foundation at a particular depth can be computed using a Newmark Chart
What is the procedure for drawing a Newmark chart?
1. The depth at which it is proposed to calculate the depth is decided.
2. The plan of the foundation is drawn so that the unit distance AB shown on the chart = depth.
3. Each corner of the plan is placed at the centre of the chart and the number of segments covered is counted.
4. Pressure at that corner at that depth is calculated by multiplying the number of segments x applied surface stress q influence factor for the chart (0.005)
Pressure = N x q x I
5. If pressure at a different depth is required repeat steps 2-4 with AB = depth.
What is Active Earth Pressure?
Pressure of the soil which results from a slight movement of the structure away from the soil. This is the minimum pressure that the soil can apply. The amount of movement required varies with the soil type as the following table shows.
pressure that a soil will exert when a slight displacement of the structure towards the soil occurs. This is the maximum pressure that a soil can exert.
Passive Resistance or Passive Earth Pressure of the soil is the pressure that a soil will exert when a slight displacement of the structure towards the soil occurs. Passive pressure is the maximum pressure that a soil can exert.
In both methods we will assume that the backfill material behind the wall is dry cohesionless soil - a reasonable assumption where free draining scoria backfill is used and adequate drainage provided
What three types of surcharge loading are commonly encountered :
Distributed Loads
Line Loads
Point or Concentrated loads
Wide loads
Rectangular shaped loads
Distributed loads - Coulomb's method are Calculate force added to each wedge by multiplying Ws x top length of wedge x 1.0 and adding force to wedge weight.
