KMS / TECHNICAL DEPARTMENT
TECHNICALNOTE010
RaftFoundations, s’– tPlots& tPlots& SPT-NValues ·
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Version02,Jul2013
·
Keywords Raft foundations, s’– s’– t t plot, SPT-N values, soil friction angles
An s’– t s’– t plot is shown in the following figure (using the laboratory test results for a site in Yuen Long). 600
Synopsis This note explains, through examples, the use of triaxial test results to prepare s’– s’– t t plots for determination of soil friction angle(s), ’. Published correlation data (Meyerhoff, 1956) between SPT-N values and ’ is provided for counterchecking the result. Given the ’ value, value, the allowable bearing capacity of shallow footing (or raft foundations) can be quickly assessed.
’
500
400 a P k ( t
300
200
100
1.0
Introduction Large raft foundations give very high allowable bearing capacity, and in some cases, the bearing capacity might even be higher than the equivalent bearing capacity of driven H-piles. For large rafts, settlement becomes the controlling factor. Triaxial compression test results, if available, can be used to prepare an s’– s ’– t t plot for determination of soil friction angle(s). The SPT-N values (field test results) can also be used to correlate the soil friction angle and the soil compactness, allowing one to gain some quick feel of the soil bearing capacity.
s’ – t Plots Laboratory tests are normally conducted as part of a ground investigation programme. programme. Triaxial compression compression tests on soil samples would usually be specified by a Designer. Depending on the laboratory report format, the triaxial test results are presented quite differently, ranging from full report on 3’, p’, q, s’, t to t to just some of these parameters. The most useful values of c’ and ’ are usually not reported because these values are subject to interpretation by the Designer.
2.0
For the purpose of determining the soil friction angle, ’, we only need to focus on the values of s’ and and t, where: s’ = (1’ + 3’) / 2 t = (1’ – 3’) / 2 where 1’ is is the applied axial load, and 3’ is the different effective confining pressure(s) in triaxial tests, ie, Cell Pressure minus Measured Pore Water Pressure in soil sample. NB – The values of s’ and t are stated readily in the test report, and there is no need to re-calculate them using the above equations.
TECHNICALNOTE010 Version02,Jul2013
0 0
2 00
40 0
6 00
80 0
1 0 00
s' (kPa)
Two trend lines are shown in the above plot : the broken line is a best-fit line for the available data, and the solid line is a lower-bound line. The gradient of these lines is tan ’ , and the effective soil friction angle, ’ is given by: ’ = sin-1 (tan’)
In the above plot, the values of tan ’ and ’ are as follows: Lines
’
tan
’
’
Best-Fit
35°
0.613
42°
Lower-Bound
27°
0.450
30°
3.0
SPT-N Values & Soil Friction Angles The correlation between SPT-N values and granular soil friction angels are shown in the table below for quick reference. SPT-N Values
Soil Friction Angles, Angles, ’
Compactness
<4
< 30°
Very loose
4 – 10
30° – 30° – 35°
Loose
10 – 10 – 30
35° – 35° – 40°
Medium Dense
30 – 30 – 50
40° – 40° – 45°
Dense
> 50
> 45°
Very dense
01
KMS / TECHNICAL DEPARTMENT
Q 1
Q 2 B L h z
= 50m = 50m (into paper direction) = 8m = 2m s = 18 kN/m3 w = 10 kN/m3 ' = 8 kN/m3 = 30° FOS = 3
Q n z
qo
h
’
B
The SPT-N value in the drill logs for the site in Yuen Long ranges from 10 to 24. According to the correlation table, the soil is considered ‘compact’ and the corresponding soil friction angle is within the range of 35° – 40°, which is in reasonable agreement with value given by the best- fit line in the s’– t plot.
Bearing Capacity of Raft Foundations / Footing The bearing capacity of a large raft foundation can be very high. An assessment is made, based on GEOGUIDE 1, for a raft foundation having the arrangement shown in the diagram above.
100.0
30.1 22.4 18.4
4.0
The ultimate bearing capacity, q ult is given by the following equation (Please note that the cohesion term, c’, for granular soil is usually small and can be ignored): qult
10.0
Nc Nq N
1.0 0
= ½ ' B N s + qo Nq sq
10
20
30
40
’
50
where N and Nq are bearing capacity factors, and s and sq are correction factors to account for footing shape. 0.1
The bearing capacity factors, N c, Nq, N have been derived by different soil mechanics researchers. Those used in GEOGUIDE 1 – Figure A2 are shown in the graph on the right. The calculated values of these factors are shown below for ’ = 30°. Nc = 30.1 ;
sc
= 1.61
Nq = 22.4 ;
sq
= 1.58
= 18.4 ;
s
= 0.60
N Also, qo
Thus, qult
= 18×2 + 8×(8 – 2) = 84 kPa = 0.5×8×50×18.4×0.6 + 84×22.4×1.58 = 5180.9 kPa
The allowable bearing capacity, q all is given by applying a factor of safety (FOS) to the q ult (Please note that the FOS is not applied to the overburden pressure, q o , which is expected to exist throughout the life time of the foundations). qall
The allowable bearing capacity for the raft foundation having the above parameters is approximately 1783 kPa. It is unlikely that a field plate loading test would give prediction of this bearing capacity, ie, using field plate loading test(s) to try to verify the above bearing capacity will not be very meaningful. However, an assessment of the likely settlement should be carried out to ensure the movements are acceptable for the proposed structures.
= [(5180.9 – 84) / 3] + 84 = 1782.9 kPa
TECHNICALNOTE010 Version02,Jul2013
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This technical note is for internal circulation only. For enquiry, please contact Gary Chou KMS / AGM (Technical) Technical Department Chun Wo Construction & Engineering Co Ltd E
[email protected] T 3758 8379 F 2744 6937
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