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Skempton, A. W. (1987). G&echnique 37, No. 3,411-412

Standard penetration test procedures and the effects in sands of overburden pressure, relative density,

particle size, ageing and overconsolidation

A. W. SKEMPTON (1986). Gbotechnique 36, No. 3, 425-447

Dr S. S. C. Liao, Geotechnical Engineers Inc., Winchester, and Professor R. V. Whitman, Massa- chusetts Institute of Technology

The Writers would like to present some thoughts on one aspect of the standard penetra- tion test (SPT), namely that of the effect of over- burden pressure.

In a recent paper (Liao & Whitman, 1986), the Writers have examined various overburden cor- rection factors C, described in the literature and have found large inconsistencies among some of them. To eliminate these inconsistencies and in the interest of simplicity, the Writers have sug- gested the following formula for the correction factor

c, = (l/a,‘)“2 (1)

(where gv’ is in tons per square foot or kilograms per square centimetre). For practical purposes, equation (1) is equivalent numerically to equa- tions of the form

a/b + 1 c, = ~

a/b + a”’ (2)

described by the Author for the range of stresses from a: = 0.5 ton/ft’ to uv’ = 3.0 ton/ft’. For comparison, equation (1) can be superimposed on to fig. 16 presented by the Author and repro- duced as Fig. 1 here.

A generalized form of the correction factor of equation (1) may be written as

c,= Gk [ 1 0”’

where (crv& can be an arbitrary standard refer- ence pressure and k is a parameter to be obtained by fitting to test data. The Writers envision that k may be a function of relative density, over- consolidation ratio, particle size, ageing and poss- ibly other factors. Thus a hypothetical family of correction factors can be developed with differing k values accounting for these different factors. However, there are currently insufficient data for such a refinement.

On a theoretical basis, however, the implica- tions of equations (1) or (3) differ significantly from the form of the correction factor proposed

by the Author (equation (2)). Whereas the Author develops the form based on an assumption that the SPT resistance N increases linearly with cr,‘, the implication of equation (1) is that of a non- linear increase and specifically that

N = N,JC, = N,Ja,’ (4)

If the more general form of equation (2) is used, then N would be found to increase as

N = Nicr,” (5)

The basic assumption of a non-linear relation of this type has been used by Al-Awkati (1975), Fardis & Veneziano (1981) and Baldi, Bellotti, Ghionna, Jamiolkowski & Pasqualini (1985) to fit regressions to data for the SPT and the cone pen- etration test (CPT). Peck 8~ Bazaraa (1969) have proposed a bilinear relationship between SPT resistance and overburden pressure rather than the purely linear relationship attributed to them in fig. 10 of the Paper. Thus there are several pre- cedents for the assumption of non-linearity.

It is also of interest that the soil modulus is a parameter that varies roughly as the square root of a”‘. Although the SPT resistance depends on both the soil strength and the soil compressibility, the Writers conjecture that perhaps there is more of a direct correlation of SPT resistance to soil compressibility than is normally thought. Hence there would be a logical rationale for using N values in empirically derived methods for predict- ing settlements on sands and this would explain the success of the method proposed by Schmert- mann (1970) who used another type of penetra- tion test, the CPT. In further support of the Writers’ conjecture are the studies by Ohsaki & Iwasaki (1973) and Imai & Tonouchi (1982) where the soil moduli from seismic methods have been correlated with SPT resistance. If the driving of the SPT sampler can be analysed as a cavity expansion problem as proposed by Nishida, Yokoyama, Sekiguchi & Matsumoto (1982), this would then provide the physical explanation of the direct correlation between SPT resistance and modulus.

The Author’s assumption of linearity of the relationship between N and 6,‘ is reasonable in

411

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412 DISCUSSION

OC

0 FIII

. Non-consolidated flnesands

+ Overconsolidated

. Coarse sands -Laboratory tests

I i ii 3/(2 + u,‘)

3.0 1 w

Fig. 1

the light of the data available. However, any of the plots of N or N/D,’ versus 0,’ presented by the Author can just as accurately be modelled as a power relationship of the form of equation (3). An argument against the power relationship is that equation (3) implies that the penetration resist- ance should be small (N E 0) for CT,’ z 0, whereas this is clearly not the case as shown by the Author’s fig. 7, which is derived from the data by Gibbs & Holtz (1957). The Writers’ counter- argument is that the non-zero N values at crV’ z 0 are due to implicit overconsolidation of the soils tested. Densification of a soil in a confined con- tainer produces effects that are similar to over- consolidation and leads to increases in lateral stresses in the soil which have a significant influ- ence on penetration resistance. It is inappropriate to speak of a ‘normally consolidated dense sand at 0”’ Z 0 in a laboratory soil container, just as it is incorrect to call a stiff clay ‘normally consoli- dated’ when it is at the ground surface.

In summary, the Writers prefer the correction factor of equation (1) rather than the form pro- posed by the Author because

(a) equation (1) fits the data as well as the form of the correction factor proposed by the Author

(b) there may be good physical reasons to indi- cate that N is non-linear

(c) equation (1) is simple to remember and use.

However, from a practical perspective, there are no significant numerical differences between the correction factors proposed by the Author and that preferred by the Writers, and this discussion in no way detracts from the important points made by the Author. The main objectives of this discussion are to point out that a diversity of opinion exists on the subject and to indicate interesting directions for further research.

REFERENCES Al-Awkati, Z. A. (1975). On problems of soil bearing

capacity at depth. PhD thesis, Department of Civil Engineering, Duke University, Durham.

Baldi, G., Bellotti, R., Ghionna, V., Jamiolkowski, M. St Pasqualini, E. (1985). Penetration resistance and liquefaction of sands. Proc. llth Int. Con& Soil Mech. Fdn Engng, San Francisco.

Fardis, M. N. & Veneziano, D. (1981). Estimation of SPT-N and relative density. .I. Geotech. Engng Div. Am. Sot. Ciu. Engrs 107, GTlO, 1345-13.59.

Gibbs, H. J. & Holtz, W. G. (1957). Research on deter- mining the density of sands by spoon penetration testing. Proc. 4th Int. Conf Soil Mech. Fdn Engng, London 1,35-39.

Imai, T. & Tonouchi, K. (1982). Correlation of N-value with S-wave velocity. Proc. 2nd Eur. Symp. Penetra- tion Testing, Amsterdam 1, 67-72.

Liao, S. S. C. & Whitman, R. V. (1986). Overburden correction factors for SPT in sand. J. Geotech. Engng Div. Am. Sot. Ciu. Engrs 112, GT3,373-377.

Nishida, Y., Yokoyama, K., Sekiguchi, H. & Matsu- moto, T. (1982). Mechanics base of standard pen- etration test values and its application to bearing capacity prediction. Proc. 2nd Eur. Symp. Penetra- tion Testing, Amsterdam 1, 119-124.

Ohsaki, Y. & Iwasaki, R. (1973). On dynamic shear moduli and Poisson’s ratio of soil deposits. Soils Fdns 13, No. 4, 1973,61-73.

Peck, R. B. & Bazaraa, A. R. S. (1969). Discussion on Settlement of spread footings on sand. J. Soil Mech. Fdns Div. Am. Sot. Ciu. Engrs 95, SM5.905909.

Schmertmann, J. H. (1970). Static cone to compute static settlement over sand. J. Soil Mech. Fdns Div. Am. Sot. Civ. Engrs 96, SM3, 1011-1043.