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Strength and stiffness from in situ tests

The content:
1.1 Standard penetration tests (SPT)
1.1.1 Modification of raw SPT values
1.1.1.1 Method A
1.1.1.2 Method B
1.1.2 Relative density
1.1.3 Undrained soil strength vs. SPT N
1.1.4 Friction angle vs. SPT N, Dr, and Ip
1.1.5 Parameters affecting strength
1.2 Cone penetration tests
1.2.1 Undrained shear strength
1.2.2 SPT blow counts using qc
1.3 Soil stiffness
1.4 Stiffness and strength of rock
1.4.1 Strength of rock
1.4.2 Shear modulus of rock
Standard penetration tests (SPT)
The Standard Penetration Test was developed in the late 1920s, but the test procedure was not standardized until 1958 (see ASTM D1586), with periodic revisions to date. The test consists of the following procedures: (a) driving the standard split-barrel sampler a distance of 460 mm into the soil at the bottom of the boring, (b) counting the number of the blows to drive the sampler for the last two 150 mm distance (total = 305 mm) to obtain the blow counts number N, (c) using a 63.5-kg driving mass (or hammer) falling “free” from a height of 760 mm onto the top of the boring rods. The SPT test is normally halted if (a) 50 blows are required for any 150 mm increment, (b) 100 blows are obtained (to drive the required 300 mm), and (c) 10 successive blows produce no advance. The raw SPT data must be corrected to yield the real (corrected) value of SPT, as outlined next.
[nextpage title=”1.1.1 Modification of raw SPT values”]Skempton (1986) conducted an extensive review about the impact of some key factors on efficiency of SPT tests. The review provides the
• Values of hammer efficiency for four typical methods of releasing the hammer
Values of rod energy ratio, which is defined as the ratio of the actual hammer energy to sampler and the input (free-fall) energy
• Correction factor of less than unity for a rod length less than 10 m, as a result of loss in delivered energy
• Tentative correction factors of 1.05 and 1.15 for 150 mm and 200 mm boreholes, respectively, as lower N values are normally gained using the boreholes than those from 115 mm boreholes
• Impact of overburden stress associated with sand density
In particular, the energy ratio is equal to 0.3 to 1.0 (Kovacs and Salmone 1982; Riggs 1983). A standard rod energy ratio of 60% is recommended to normalize all measured blow account N values, by simple proportion of energy ratios, to this standard, and the normalized values are denoted as N60. Note standard penetration resistance is sometimes normalized to a rod energy ratio of 70% (Er70 = 70). The associated standard below counts of N70 (the subscript of “70” refers to Er70 = 70) is estimated using Er70N70 Er60N60 = . The blow counts, N60 of SPT data using the energy E60 is equal to another blow counts, N70, of the energy E70. For instance, given N70 = 20, the value of N60 with Er60 = 60 is calculated as 23.3. The efficiency, the ratio, and correction factors may be multiplied directly with a measured blow count N to gain a corrected N value, as detailed next.
[nextpage title=”1.1.1.1 Method A”]In light of these influence factors listed, the N60 may be revised as (Skempton 1986):


where N60 = SPT N value corrected for field procedures; Em = hammer efficiency; CB = borehole diameter correction; Cs = sample correction; CR = rod length correction; and N = measured SPT N value. In addition, the SPT data should be adjusted to accommodate the effect of effective stress and field procedures. This leads to a corrected SPT N’60 value of

where CN is a modification factor for overburden stress (see Figure 1.1) with CN = 2/(1+  ) v a p / (normally consolidated fine sands), CN = 3/(2 +  )  /pa (normally consolidated coarse sands), and CN = 1.7/(0.7 +  ) v a p / (overconsolidated fine sands) (Skempton 1986); pa = 100 kPa, atmospheric pressure; and v = vertical effective stress at the test location (see Figure 1.1).

[nextpage title=”1.1.1.2 Method B”]When the SPT test is carried out in very fine sand or silty sand below the water table, the measured N value, if greater than 15, should be corrected for the increased resistance due to negative excess pore water pressure set up during driving. In this circumstance, the resulting SPT blow counts N’ is given by (Terzaghi and Peck 1948)


[nextpage title=”1.1.2 Relative density”]The SPT value has been used to classify the consistency or relative density of a soil. Skempton (1986) added appropriate values of N’60 to Terzaghi and Peck’s (1948) classification of relative density, as shown in Table 1.1.

where C DC t p A = 60 + 25 = 1 2 + 0 05 0 01 50 log , . . log( . ) and COCR = OCR0.18; Dr = relative density (in decimal form); Cp = grain-size correction factor; CA = aging correction factor; COCR = overconsolidation correction factor; D50 = grain size at which 50% of the soil is finer (mm); t = age of soil (time since deposition)(years); and OCR = overconsolidation ratio.

[nextpage title=”1.1.3 Undrained soil strength vs. SPT N”]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 (4–5) N kPa for clays of medium plasticity and (6–7)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.
Figure 1.3 Plasticity versus ratio of undrained shear strength over
Table 1.2 Correlation between N value and friction angle or cohesion
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.
[nextpage title=”1.1.4 Friction angle vs. SPT N, Dr, and Ip”]The angle of friction of soil has been correlated empirically with SPT blow counts by various investigators. Peck et al. (1953) suggested the curve in Figure 1.5, which is well represented with ?’ = 0.3N60 + 26° to a N value of 40. The impact of effective overburden stress  on the angle has been incorporated into the estimation (e.g., the Japan railway). JSCE (1986) suggests

This expression offers essentially similar values to those obtained from the Peck’s expression to a N of 50 (Peck et al. 1953) at a v < 100 kPa, and otherwise results in an angle ~7 degrees smaller. The former prediction is close to the “extended” curve of the Chinese code (SD 128-86) of 0.3N + 19°
Figure 1.4 Undrained shear strength versus N60 (clean sand)
Figure 1.5 SPT blow counts versus angle of internal friction
Table 1.3 Effective friction angle
[nextpage title=”1.1.5 Parameters affecting strength”]
Parameters affecting strength has lot of formula so all content include a picture.
Parameters affecting strength
Figure 1.6 Variations of friction angle with plasticity index data.
Figure 1.8 Results of regular and plane-strain triaxial tests
where ?p = frictional angle under plane strain conditions, as would occur beneath a very long spread footing or a long wall leaning forward under later soil pressure; ?tr = frictional angle under axisymmetric conditions (the intermediate principal stress = minor principal stress), as would occur at the tip of a pile or beneath a square footing. The adjustment of test conditions should be limited to no more than 5°.