DESIGN CHARTS FROM SPT VALUES FOR FOOTINGS ON SAND.

The methods suggested by Terzaghi et al., (1996) for estimating settlements and bearing pressures of footings founded on sand from SPT values are based on the findings of Burland and Burbidge (1985). The SPT values used are corrected to a standard energy ratio. The usual symbol Ncor is used in all the cases as the corrected value.

Formulas for Settlement Calculations.
The following formulas were developed for computing settlements for square footings.

For normally consolidated soils and gravels

If the footing is established at a depth below the ground surface, the removal of the soil above the base level makes the sand below the base preconsolidated by excavation. Recompression is assumed for bearing pressures up to preconstruction effective vertical stress q'o at the base of the foundation. Thus, for sands normally consolidated with respect to the original ground surface and for values of qs greater than q'o, we have,

 Figure 13.4 Thickness of granular soil beneath foundation contributing to
settlement, interpreted from settlement profiles (after Burland and Burbidge 1985)

It may be noted here that the ground water table at the site may lie above or within the depth of influence Zl Burland and Burbidge (1985) recommend no correction for the settlement calculation even if the water table lies within the depth of influence Zl. On the other hand, if for any reason, the water table were to rise into or above the zone of influence Zl after the penetration tests were conducted, the actual settlement could be as much as twice the value predicted without taking the water table into account.


Chart for Estimating Allowable Soil Pressure
Fig. 13.5 gives a chart for estimating allowable bearing pressure qs (on settlement consideration)
corresponding to a settlement of 16 mm for different values of TV (corrected). From Eq. (13.6), an
expression for q may be written as (for normally consolidated sand)

Figure 13.5 Chart for estimating allowable soil pressure for footing on sand on the
basis of results of standard penetration test. (Terzaghi, et al., 1996)

The chart m Fig. 13.5 gives the relationships between B and Q. The value of qs may be obtained from Q for any given width B. The Q to be used must conform to Eqs (13.12), (13.13) and (13.14).

The chart is constructed for square footings of width B. For rectangular footings, the value of qs should be reduced in accordance with Eq. (13.10). The bearing pressures determined by this procedure correspond to a maximum settlement of 25 mm at the end of construction.

It may be noted here that the design chart (Fig. 13.5b) has been developed by taking the SPT values corrected for 60 percent of standard energy ratio.