Data required:
1. A profile of standard penetration resistance N (blows/ft) versus depth, from the proposed foundation level to a depth of 2B, or to boundary of an incompressible layer , whichever occurs first. Value of soil modulus is established using the following relationships.
Soil type Es/N
Silts, sands silts, slightly cohesive silt-sand mixtures 4
Clean, fine to med, sands & slightly silty sands 7
Coarse sands sands & sands with little gravel 10
Sandy gravels and gravel 12
2. least width of foundations = B,depth of embedment =D,and proposedaverage contact pressure = P.
3. Approximate unit weights of surcharge soils, and position of water table if within D.
4. If the static cone bearing value qc is measured compute Es based on Es=2qc
Analysis procedure
Refer to table in example problem for column numbers referred to by parenthesis :
1. Divide the subsurface soil profile into a convenient number of layers of any thickness ,each with constant N over the depth interval 0 to 28 below the foundation.
2. Prepare a table as illustrated in the example problem, using the indicated column headings. Fill in column 1,2,3 and4 with the layering assigned in Step 1.
3. Multiply N values in column 3 by the appropriate factor Es/N (col.4) to obtain values of Es ; place values in column 5.
4. Draw an assumed 2B-0.6 triangular distribution for the strain influence factor Iz , along a scaled depth of 0-2B below the foundation. Locate the depth of the mid-height of each of the Layers assumed in step 2 , and place in column 6. From this construction, determine the Iz value at the mid-height of each layer, and place in column 7 .
5. calculate (Iz/Es) delta Z , and place in column 8. Determine the sumof all values in column 8 .
6. total settlement =delta H = C1C2delta p (สูตร)
Where (สูตร) embodment correction factor
(สูตร) creep correction factor
P0=overburden pressure at foundation level
Delta P = net foundation pressure increase
T = elaspsed tire in years.