TECHNICAL NOTE 010 Version 02, Jul 2013

KMS / TECHNICAL DEPARTMENT

TECHNICAL NOTE 010

Raft Foundations, s t Plots & SPT-N Values We excel through experience and learning Version 02, Jul 2013

01

Keywords Raft foundations, s t plot, SPT-N values, soil friction angles Synopsis This note explains, through examples, the use of triaxial test results to prepare s 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, the allowable bearing capacity of shallow footing (or raft foundations) can be quickly assessed. 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 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. 2.0 s t Plots

Laboratory tests are normally conducted as part of a ground investigation programme. Triaxial 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 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. For the purpose of determining the soil friction angle, , we only need to focus on the values of s and t, where:

s

t

= (1 + 3) / 2

= (1 3) / 2 where 1 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.

An s t plot is shown in the following figure (using the laboratory test results for a site in Yuen Long).

0

100

200

300

400

500

600

0 200 400 600 800 1000

s' (kPa)

t (k

Pa)

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, Compactness

< 4 < 30 Very loose

4 10 30 35 Loose

10 30 35 40 Medium Dense

30 50 40 45 Dense

> 50 > 45 Very dense

KMS / TECHNICAL DEPARTMENT

TECHNICAL NOTE 010 Version 02, Jul 2013

02

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. 4.0 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. The ultimate bearing capacity, qult is given by the following equation (Please note that the cohesion term, c, for granular soil is usually small and can be ignored):

qult = ' B N s + qo Nq sq

where N and Nq are bearing capacity factors, and s and sq are

correction factors to account for footing shape.

The bearing capacity factors, Nc, 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

N = 18.4 ; s

= 0.60

Also, qo = 182 + 8(8 2)

= 84 kPa

Thus, qult = 0.585018.40.6 + 8422.41.58

= 5180.9 kPa

The allowable bearing capacity, qall is given by applying a factor of safety (FOS) to the qult (Please note that the FOS is not applied to the overburden pressure, qo , which is expected to exist throughout the life time of the foundations).

qall = [(5180.9 84) / 3] + 84

= 1782.9 kPa

0.1

1.0

10.0

100.0

0 10 20 30 40 50

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.

- End - 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 gary.chou@chunwo.com T 3758 8379 F 2744 6937

qo

Q1 Q2

Qn

B

z

h

B = 50m L = 50m (into paper direction) h = 8m z = 2m

s = 18 kN/m3

w = 10 kN/m3

' = 8 kN/m3

= 30 FOS = 3

Nc

Nq

N

30.1 22.4

18.4