Soil

1. SPT CORRELATIONS

2. SPT CORRELATIONS
 SPT N value has been used in correlations
for estimating:
 g,
 Dr,
 Ø,
 qu,
 bearing capacity,
 stress-strain modulus, Es

3. SPT values related to Dr of cohesionless
soils
 The correlations between Ncor values and Dr suggested by Peck
et al. (1974) are given in Table 1
 Fairly reliable
Table 1: Ncor and Ø related to relative density

4. Meyerhof (1956) suggested the following
approximate equations for computing the angle
of friction from known value of Dr.
For granular soils with fine sand and more than 5
percent silt:
f = 25 + 0.15Dr
For granular soils with fine sand and less than 5
percent silt:
f = 30 + 0.15Dr
where Dr expressed in percent

5.  Marcuson and Bieganousky (1977) give an
expression for computing DR which accounts
for N, s'o, and Cu the coefficient of uniformity
as follows:
DR = 11.7 + 0.76{|222N + 1600 – 53 s'o - 50Cu2|}1/2
From DR and gradation curves estimates can
be made for estimating:
E (Young’s modulus) - Table 2
f from Table 3

6. Table 2

7. Table 3

8. SPT values related to consistency of clay
soil
 Peck et al. (1974) gave correlations between Ncor value
and consistency for saturated cohesive soils.
 Rather unreliable, only for rough estimate. Values
should be confirmed by laboratory testing.
Table 4: Relation between Ncor and qu
Uc = Unconfined compressive strength

10. Relationship between Ø and N (IS 6403: 1981)

11. SCPT CORRELATIONS
Relationship between qc, Dr, and f for sand
There is no unique
relationship for all sands.
Dr is estimated based on qc
and effective overburden
pressure.
Applicable for normally
consolidated clean sand.

12. Relationship between qc and f

13. Relationship between qc, undrained
shear strength cu of clay
where Nk = cone factor
po = overburden pressure
Lunne and Kelven (1981) investigated the Nk values
k
o
c
u
N
p
q
c



14. Soil classification
 Basic uses of CPT is to identify and classify
soils.
 CPT-Soil behaviour type prediction has
been developed by Douglas and Olsen
(1981) using an electric-friction cone.

15. Simplified classification chart (Douglas 1984)
Cone resistance normalized,
qcn = qc (1 - 1.25log p’o)

16. Correlation between SPT and CPT
 Meyerhof (1965) presented a correlation for fine or
silty medium loose to medium dense sands:
qc = 0.4 N MPa
Approximate relationship between Dr of fine sand, SPT N, qc and f
(Meyerhof 1965)

17. For cohesive soils
Consistency qc (kPa) N Cu (kPa)
Very soft < 200 < 2 < 12.5
Soft 200 – 400 2 – 4 12.5 – 25
Medium soft 400 – 800 4 – 8 25 – 50
Stiff 800 – 1500 8 – 15 50 – 100
Very stiff 1500 – 3000 15 – 30 100 – 200
Hard > 3000 > 30 > 200

18. Relationship between qc /N and mean grain size D50 (mm)
(Robertson and Campanella 1983)

19. Design N values
Earlier recommendations
 Smallest of N value of the boring or
 Average N value of the particular stratum
Current practice
Average N but in the zone of major stressing
Weighted average is preferable than arithmetic average
Nav = ∑ N. zi/ ∑ zi
IS recommendations for SPT N and SCPT qc value:
Average N value from footing level to 1.5 to 2.0 B

20. Ultimate bearing capacity based on
SPT N value
The ultimate net bearing capacity shall be
calculated from following formula:
where f may be read from Figure 1

21. Ultimate bearing capacity based on
SCPT
Cohesionless soil
Df = Depth of foundation, cm
B = width of strip footing

22. Cohesive soil