Relative density

Terzaghi and Peck Gibbs and Holtz

very loose loose medium dense very dense

that the significant factors affecting the N value are the relative density of the soil and the value of the effective overburden pressure removed.

Various workers have investigated this problem: Coffmann (1960), Bazaraa (1967), but the method proposed by Thorburn (1963) now seems to have gained general acceptance, at least in the UK.

Thorburn assumed that the original Terzaghi and Peck relationships between N and the relative density corresponded to an effective overburden pressure of 20 lb/in2 (138kN/m2). His correction chart therefore dealt with a range of effective overburden pressure for 0 to 138kN/m2, it being tacitly assumed that for values of effective overburden pressure greater than 138kN/m2, N' can be taken as equal to N.

It is possible, by the use of Thorburn's chart, to prepare the plot of the N'/N ratio relationship to effective overburden pressure, over the range 0 to 138 kN/m2. (Roughly from 0 to 7 m depth of overburden.)

This relationship is reproduced in Fig. 8.13 and can be used directly in design.

Terzaghi and Peck point out that in saturated (i.e. below the water table) fine and silty sands the N value can be altered by the low permeability of the soil. If the void ratio of the soil is higher than that corresponding to its critical density, the penetration resistance is less than in a large grained soil of the same relative density. Conversely, if the void ratio is less than that corresponding to critical density the penetration resistance is increased.

The value of N corresponding to the critical density appears to be about 15 and Terzaghi and Peck suggested that if the number of measured blows, N, is greater than 15 it should be assumed that the density of the tested soil is equal to that of a sand for which the number of blows is equal to 15+0.5 (N- 15), i.e.:


N = actual number of blows recorded in the test True N = number of blows from which N' should be evaluated

Fig. 8.13 Estimation of N' from the test value N (after Thorburn, 1963).

Estimation of allowable bearing pressure from the standard penetration test

Having obtained N', the determination of the allowable bearing pressure is generally based upon an empirical relationship evolved by Terzaghi and Peck (1948) that is based on the measured settlements of various foundations on sand (Fig. 8.14). The allowable bearing pressure for these curves (which are applicable to both square and rectangular foundations) was defined by Terzaghi and Peck as the pressure that will not cause a settlement greater than 25 mm.

When several foundations are involved the normal design procedure is to determine an average value for N' from all the boreholes. The allowable bearing pressure for the widest foundation is then obtained with this figure and this bearing pressure is used for the design of all the foundations. The procedure generally leads to only small differential settlements, but even in extreme cases the differential settlement between any two foundations will not exceed 20 mm.

The curves of Fig. 8.14 apply to unsaturated soils, i.e. when the water table is at a depth of at least 1.0 B below the foundation. When the soil is submerged the value of allowable bearing pressure obtained from the curves should be reduced. Originally the values were reduced to 50 per cent but this is now considered excessively conservative as the influence of the ground water will have already been included in the observed penetration resistance. General practice is now to apply the 50 per cent reduction if the ground water level is at or above the foundation level, and to apply no reduction if the ground water

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