It is useful to have a correlation between SPT values and soil strength. Many empirical expressions were proposed and are dependent of plasticity index (Ip). The Ip is normally used to describe the state of clay and silt, both alone and in mixtures with coarser material. The soil plasticity is normally classified in terms of liquid limit (LL) (BSI 1981), as low (LL < 35%), intermediate (LL = 35~50%), high (LL = 50~70%), very high (LL = 70~90%), or extremely high (LL > 90%) Stroud (1974) presented the variation of N value with undrained shear strength (su) for London clay. As replotted in Figure 1.3, the su is about (45) N kPa for clays of medium plasticity and (67)N kPa or more for soil with a plasticity index (IP) less than 20. Tezaghi and Peck (1967) reported a high value of su = 12.8N. Sower (1979) attributed the variations to clay plasticity, and su = 7.2N (low plasticity), 14.2N (medium), and 24N (high plasticity), respectively. The high ratio is also noted as su = 29N0.27 (Hara et al. 1974). On the other hand, low strength of su = (1~4)N kPa is commonly adopted in Southeast Asia region: an average su = ~4N kPa for the weathered Kenny Hill Formation from eight test sites within Kuala Lumpur (Wong and Singh 1996); and a strength su ? N for clayey silt, and su ? (2~3)N for silty clay in Malaysia (Ting and Wong 1987). The Chinese hydraulic engineering code (SD 128-86) recommends a cohesion of (5.5~7)N (N = 3~13), and (3.5~5.2) N (N = 17~31) for alluvial and diluvial clay, as detailed in Table 1.2.
Vane shear strength for normally consolidated clay was correlated to the overburden stress v and the plasticity index Ip by (Skempton 1957)
This expression agrees with that observed from Marine clays, but for a reverse trend of decrease in su/ v with plasticity index Ip of 0~350 for soils with thixotropic behavior (dilate during shear) (Osterman 1959). Most of remolded clays have a su/ ratio of 0.3 ± 0.1.
It must be emphasized that the ratio of su/N depends on stress level to a large extent. In design of slope, dam, lateral piles, or predicting foundation response owing to liquefaction of sand, the clay or sand may flow around these structures or foundations. The associated undrained shear strength has been correlated with the N values, as illustrated in Figure 1.4. Indeed the observed ratio of su/N is much lower than those mentioned above in relation to foundation design at prefailure stress level.
