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The journal of
THE
NZ
INSTITUTION OF ENGINEERS, Fourth
Floor, Molesworth House, 101 Molesworth
Street, P.O. Box 12-241, Wellington 1.
President, P. G. SCOULAR, B.E., C.ENG., F.I.C.E..
F.N.Z.I.E., G.A.S.C.E., F.N.Z.I.M.
Secretary, A.
J.BARTLETT, M.A. (OXON)
Designed for
The New Zealand engineer and planned to
cover all aspects of professional engineering.
This journal is received by all members of the
NZ. Institution of Engineers.
Opinions expressed in the journal are not
necessarily those of the Institution or of the
publishers.
Published monthly by
TECHNICAL PUBLICATIONS LTD.,
127Moles-
worth Street, P.O. Box 3047, Wellington, NZ.
Telephone: 735-739. Telegrams: Tecpub.
Managing Editor
F. N
STACE, B.E.(ELECT. MECH.), B.E.(MECH.),
C.ENG., F.I.E.E., F.I.S.T.C., M.I.E.E.E., M.N.Z.I.E.
Sub Editor
ROSEMARY HARDING, B.A.
Advertisement Manager
P. DDOILE
Business Manager
G.
W. CLARK
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Microfilm
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New Zealand Engineering
are
available from University Microfilms Inc., 300
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U.S.A.
NZ ISSN 0028-808X
Registered at Post Office Headquarters,
Wellington, as a newspaper.
Leading article
Mass, weight and
SI,
the practical system of units
25
Papers and articles
The solar buildjng panel concept for the supply of hot water
P. L. Spedding, M. L. Allen, D. Brow
126
Determination
f
llowable
earing
ressure
nder
mall
structures
.
.
tockwell 132
Hong Kong highway system
. Maxwell
136
Dynamic characteristics of Grafton No. 1 motorway bridge
R. Shepherd, B. M
Greensmith
138
Automation and redundancy
. J. Main 140
General
Secretary's newsletter
25
Paper received
131
Proceedings of Technical Groups
39
Consultants' Notebook
41
N.Z.I.E.
ews
42
Changes in roll
42
Correspondence
45
Personal
46
Building Services Group
47
Noteworthy
47
Conferences and courses
148
An engineer's bookshelf
48
Cover picture
Grafton No. 1 motorway bridge. (See page 138.)
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THE JOURNAL OF THE N.Z. INSTITUTION OF ENGINEERS*
VOL. 32, No. 6, JUNE 1977
Mass, weight and SI,
the practical system of
units
N preparing a recent paper for publication we had to rule
out a suggestion for expressing the weight of railway trains
in newtons . The reasoning was impeccableweight is a
force and the SI unit of force is the newton . However, we
had to point out that SI was originally propounded as, still
is and always must be a practical system of units.
The description practical system is still part of the formal
and official definition of SI but nowhere in either the formal
or the background literature is practical system
:defined.
One definition would be does not require a knowledge of
vector algebra . Another would be is not blind to awkward
facts and yet another does not impose technical jargon on
people for whom it has no meaning .
This last definition is the basis for the explanatory note
on Mass and weight in NZS 6501 Appendix Y, which the
Metric Advisory Board has approved for use in education.
It is also the basis of the decision of a widely representative
meeting called by the Metric Advisory Board and the
Standards Association, that lifting capacity of cranes, hoists
and slings and the carrying capacity of vehicles and of floors
in buildings should be proclaimed in kilograms (or tonnes).
Thus in large sections of the country's activities, loads,
which are said to weigh the number of kilograms (or tonnes)
they equal in weight, are handled with equipment rated in
kilograms (or tonnes) and transported in vehicles rated in
Secretary s Newsletter
D
URING
last year, the Council agreed that the public
elations activities of the Institution should include as
many visits as possible by the President to Ministers of the
Crown, and by branch chairmen to their local members of
parliament and local body politicians. As part of the back-up
to such visits, a brief summary of the Institution's constitution
and activities was to be drafted, to be left behind to remind
those visited of the scope of the work and experience of the
professional engineer. A first version of this sheet has now
been prepared, and copies of it have been sent to the Ministers
visited by the President. The sheet has also been distributed
to branches, and further copies are freely available from
Institution headquarters.
Ars longa, vita brevis
Chaucer bewailed the fact that, with life so short, it took
so long to learn to be a poet. Of course, in his time, every-
thing, including technical treatises, was written in verse. These
days, when we allsome would say, including our poets
write in prose, the di
s
cipline that has to be applied to writing
appears to be simpler. But it is still an art that takes much
time and practice to learn, and the Institution is generally con-
cerned at the level of written expression among students and
engineering graduates. The tendency in schools to favour oral
expression at the expense of written expression is not likely to
help our young people to marshal their thoughts and express
kilograms (or tonnes). To express these weights in newtons
would be an unnecessary and quite intolerable complication
for the large numbers of non-technical people who are inevit-
ably involved.
However, when technical calculations are to be made to
determine static forces or stresses in structures or equipment,
weight becomes the force with which the earth attracts a
body and must be expressed in newtons. For most technical
purposes each kilogram weighs about 9.8 newtons. On the
rare occasions when greater accuracy is required, the local
value of the acceleration of free fall must be ascertained.
When technical calculations concern dynamic interaction of
bodies and forces the technical concep
t of mass , now
nearing its 300th birthday, must be used so that differential
equations of motion can be written.
One of the awkward facts which must not be ignored, is
that SI although helpful in this tricky area, always requires
common sense application in practical matters. Thus engineers
must demonstrate in small matters the clear thinking ability
they claim in large ones, and recognise that weight is still
weight in the traditional sense for ordinary folks and is
measured in kilograms, that weight is a specially named
force for technical people and is measured like other forces,
in newtons, while mass is a technical term used in dynamics
and is measured in kilograms.
them on paper in a way that commands respect. Clear thinking
and clear expression is one of the most obvious marks of the
professionaland in the Institution's Professional Interview,
great emphasis is placed on the candidate's ability to express
himself, as it is demonstrated in the three-hour essay period.
There were more than a 130 candidates for the May inter-
views, so they spent some 390 hours, between them, writing
their essays. I wonder how many hours of practise had been
put in beforehand?
Forms of address
A few years ago, the Institution finally relinquished the
style of Esquire when writing to members, a move that was
requested by an annual general meeting that, it seemed, con-
sidered the use of esquire to be archaic, though it was liked
by many
who relished its flavour of more formal and elegant
days. Now we are finding that the phrase dear sir , when
used on the standard letters that we send, particularly, to new
members or members transferring from one grade to another,
is no longer acceptable either. The reason is an excellent one:
many of our members are women. So, in future, as our letters
are reprinted, we shall be using the form of address, Dear
Member : and in the same way, I expect, we shall see branch
newsletters gradually replace the phrasemembers and their
wiveswithmembers and their partners. A detail? Not to
those directly concerned; and significant, I believe, in assisting
a basic change in attitudes to women at work.
* Unless specifically indicated, statements or opinions in
New Zealand Engineering
do not necessarily reflect the views of the
Institution or the publishers. Correspondence on material published is welcomed.
NEW ZEALAND ENGINEERING
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The solar building p anel concep t for the
supply of hot water
P. L. SPEDDING*
B.SC., MSC., PH.D., C.ENG., F.I.CHEME. (MEMBER)
M. L. ALLEN
B.SC., MI.CHEME., A.N.Z.I.C. (MEMBER)
D. BROW
It is now accepted that world resources of fossil fuels are limited and eventually
must be exhausted at current rates of use. This realisation has caused attention
to be directed at alternative renewable sources of energy. One such renewable
source of energy is solar heat which is virtually inexhaustable in supply and non-
polluting in operation. The utilisation of solar energy is considered to be a
practical proposition in the solar belt region of the earth between latitudes
45
north and south. The actual amount of solar energy reaching the earth at
a point depends on the intensity of the sun at that Point and the time the point
is exposed to the sun. Some idea of the solar energy available in New Zealand
can be gauged from the fact that the average amount of solar radiation at
latitude 41S is 6 000 kJ/m
a day in mid-winter and 21 000 kJ/ m
a day in
mid-summer. The corresponding K values, which are a measure of the direct
sunlight reaching the earth, vary from 0.45 to 0.57.
A solar building panel concept is proposed which serves the dual function of
acting as a solar collector as well as providing weather Protection for a building.
The panel is constructed from standard roofing material for incorporation in the
roofing structure. Where the particular aspect of the building does not allow the
incorporation of the solar building panel in the existing roof, the collector surface
can be made up into a separate solar collector in the normal way.
The straight solar building panel and the glazed solar building panel both can
achieve temperature rise rates and certain other operating characteristics which
are substantially the same as that achieved by the conventional flat, copper,
glazed collector. However, the straight solar building panel only can achieve
temperature rises of up to 30C, which is well below the maximum
temperature rise of the conventional flat, copper, glazed collector. The solar
building panel gave collection efficiencies close to 100% for low temperature
differences of about 10C. This is substantially higher than for conventional units.
The life of the solar building panel can be extended b
y suitable pretreatment
and its cost is about $10 to $15/m
of collecting surface, which is about one-
tenth of the cost of a conventional unit.
I. INTRODUCTION
T
HE obvious application of solar energy in New
Zealand is in the low temperature space-heating and
hot-water heating fields for both domestic and indus-
trial use. At present, the demand for hot-water heating
is met by electric power which consumes approxi-
mately one-fifth of the total public utility power gener-
ated within the country.' The annual cost of the power
used for hot-water heating in New Zealand has been
estimated as being $90 million of which two-thirds is
the cost of domestic hot-water use. Benseman
has
surveyed the subject and concluded that a 4 m
col-
lector with a 180 litre storage tank providing 200
litres/day of water at 50C would be the most
economical unit to use in the New Zealand domestic
situation. The collector panel must face within 30
* Associate professor and acting head of Department of
Chemical and Materials Engineering, University of
Auckland.
t Senior lecturer in Department of Chemical and Materials
Engineering, University of Auckland.
of north and be tilted upwards between 15 and 50
to the horizontal, depending on location. In addition,
the whole system must cost less than $250 installed as
of 1973 to break even economically, and have an
operating life of 20 years. Such a unit could obtain
60% of its heat from solar energy and the remainder
from electrical power. If a higher water temperature
was used, say 70C, only 35% of the total heat would
come from solar energy. The national saving which
would result from the use of solar hot-water heating
would amount to between $20 million and $35 million
a year, depending on the hot-water supply temperature.
Nationwide the scheme would cost up to $250 million
to install.
This analysis of the situation is in accord with the
findings of other workers,
4
and leads to the con-
clusions that the systems can be made to work under
certain circumstances, but the installed cost can be so
high as to make the system of doubtful economics at
the present stage, unless special consideration is given
to making the installed cost more reasonable. The
cost benefit therefore is doubtful unless a collector is
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used which is efficient in the lower temperature range,
around 50C, and is cheap to produce. It was the
purpose of this work to develop a low cost solar
heater which would act as a preheater for the normal
electric hot-water system as well as doubling as weather
protection for the building in which the system is
installed. Such a solar collector must fit naturally into
the architecture of the building on which it is installed
so that the unit is aesthetically pleasing.
2 SOLAR HEATER
A solar heater consists of three essential components
a collector which absorbs solar radiation, a store
for hot-water and inter-connecting pipe work. The
collector virtually is a heat exchanger which is exposed
to solar radiation and thus collects energy from the
sun, some of which is transferred to a heat transfer
fluid. The rest of the heat is lost to the surroundings
by convection and re-radiation. As far as the collectors
are concerned, they can be classified into two types
the flat-plate type and the focusing type. The flat-
plate collectors usually are stationary and absorb heat
from both the diffuse solar radiation as well as from
direct radiation, thus enabling them to operate on
bright cloudy days. The focusing collector on the other
hand is timed throughout the day to follow the sun, as
it can use only direct radiation, but it does produce
much higher temperatures in the heat transfer fluid. Of
the two, the flat-plate collector is the cheaper to pro-
duce and this could find economic application for
domestic hot-water supply. If corrosion is not of
importance it is more economical to use water directly
as the heat transfer fluid in the flat-plate collector.
Usually the flat-plate collector consists of a series of
metal tubes, set between headers, which are physically
bonded to a metal sheet to ensure that good heat
transfer occurs. The sheet and the exposed tubes
absorb solar radiation and transfer heat to the water
in the tubes. The material from which the collector is
made is either steel, aluminium or copper, although
recent developments have seen the use of plastic as the
collector material. The collector material is blackened
to assist in the solar collection process. The body of
the collector usually is enclosed in a sealed casing
with a sheet glass cover and is backed by a layer of
thermal insulation. The glass cover imparts a glass
house effect to the collector, trapping the high
frequency radiation from the sun, but acting as a
barrier to the escape of low frequency radiation from
the collector system, while the backing insulation re-
duces heat losses by conduction from the collector.
Storage tanks are essential for the operation of
a solar heater because of the intermittent nature of
both the effective solar energy and the use of hot
water. The storage tank and the inter-connecting piping
should be well insulated to avoid heat losses. Circula-
tion between the solar collector and the storage tank is
usually effected by the thermosyphon principle
although, where costs are not important, a simple 10
to 15 W water pump can be used. The advantage of
the thermosyphon system is that it functions auto-
matically whenever the solar energy input is high
enough to heat the water in the collector to a temper-
ature above that of the water in the base of the storage
tank. The thermosyphon system requires that the top
of the collector must be placed at least 60 cm below
the base of the storage tank to prevent back-syphoning
during the night. Where physical considerations prevent
this head criterion being met, a water pump and
associated non-return valving must be used. The hot-
water syphon connection from the solar collector to
the hot-water storage tank should enter the storage
tank at a point two-thirds up the side of the tank. Two
suitable designs of solar heater installations are illus-
trated in Fig. 1.
Forced air circulation through solar collectors has
been in use for some time in the United States and
Australia for home heating and cooling. During the
day, air is circulated from the solar collector to a rock-
pile storage system installed under the floor of a build-
ing. During the night a second air circulation system
draws heat from the rock-pile storage system and dis-
tributes the heated air through the building. In addi-
tion, a water storage tank embedded in the rock-pile
storage system supplies pre-heated water to the electric
hot-water heater. This method of supplying low
temperature heat solves any problems of corrosion,
pressure head and leakage which arise with the
ordinary solar hot-water system. A variant of the
rock-pile storage system is used to provide daytime
cooling during the hot summer season.
3. DESIGN AND DEVELOPMENT OF A
SOLAR BUILDING PANEL
It is obvious from the above general discussion that
in order to make domestic solar hot-water heating an
economic proposition in New Zealand, it is essential
that a solar collector must be devised which is cheap,
reliable, and resistant to damage, while being able to
be blended into the dwelling in an aesthetically pleasing
manner. Therefore, it seemed logical to make a solar
panel from common roofing material which would
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double as a protection from the weather for the
building on which the solar heater is placed. It would
be a simple matter to form common roofing material
into a double-skin solar collector building panel in a
manner which formed internal ducting through which
water would circulate. The outer surface of the solar
building panel would be coated with a heat-absorbent
surface to enable solar energy to be picked up effici-
ently. Furthermore, the underside of the panel would
be insulated to cut down heat losses and sheeted to
remove condensation to the roof gutter of the building.
The double skin of the panel would be formed by
joining two sheets of roofing material at various
locations to withstand the pressure of water and so
that freezing would not be a problem. Circulation of
the fluid through the panel would be by the thermo-
syphon principle. For locations where incorporation
in the roofing is not possible, a solar panel could be
constructed from the solar building panel in the normal
manner. Obviously the cost of this latter design would
be greater than that of the solar building panel and a
lot of the visual appeal could well be lost.
Figure 2 details a design of a solar building panel
made from standard corrugated galvanised steel sheet.5
Two corrugated iron sheets were joined together at the
edges and at the dimples to give a water-tight cavity
with a gap distances of 8 mm. Initially the joining was
by soldering. The panel was pressure-tested under 6
metres of water head to ensure adequate strength.
Other designs are possible which will make construc-
tion of the panel easier and give better water dis-
tribution within the panel. These are detailed in Fig.
3 together with the sections of other standard building
materials which could be used to make solar building
panels.
4 EXPERIMENTAL
A solar building panel of the design shown in Fig. 2
was made and tested in order to evaluate its perform-
ance. The test apparatus is shown in Fig. 4. Perform-
ance of the solar building panel was measured and
then compared with that of a copper tube, flat-plate,
single glazed solar collector and a glazed solar building
panel. Data were collected over a period of 18 months,
and details of the corrosion resistance of the system
were noted at the end of this period.
Results given in Table 1 show the peak temper-
ature in C taken in the storage tank after a day's
128
operation. The apparatus was charged with cold water
at 18C each morning and the average shade temper-
ature over the test period was recorded similarly at
18C. The data in Table 1 show that the copper plate
collector was the most efficient of the three collectors
tested, and there were 5-6C difference between the
peak temperature which was achieved for the copper
plate, glazed collector and the solar building panel.
The winter period is even more instructive and detailed
results are given in Fig. 5. The rate of temperature
rise for the solar building panel parallels that of the
copper plate, glazed collector, but the maximum tem-
perature again lags behind by 5-6C. Under heavy
cloud conditions very little heating was achieved and
the difference between the solar building panel and
the copper plate, glazed collector was 0.5C.
In
Fig. 6 (a) the area of the solar building panel was
doubled enabling the peak temperature to be achieved
more rapidly while its value only was 1.5 to 2C
below that of the copper plate collector.
A test was run on the solar building panel using
forced air circulation. Figure 6 (b) shows the results
from a solar building panel using an air rate of 100
litres/min with a panel area of 0.558 m2.
Estimates were made of the corrosion to be expected
of the solar building panel after 18 months service
with water as the heat transfer medium. From these
data it was estimated that a service life of 8 to 10
years could be expected from the solar building panel
if contact with the copper hot-water storage tank and
connecting piping was avoided. The life estimate of
the solar building panel in the case of using air as the
heat transfer medium was considerably above this
period.
4 l
Standard test
The initial work performed on the solar building
panel looked promising, and so more stringent tests
were undertaken. A second corrugated solar building
panel was made following the details given in Fig. 2.
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Fig. 4 (a): Performance comparison test apparatus. Fig. 4 (b): Standard solar heater test apparatus.
NEW ZEALAND ENGINEERING (32, 6) 15 JUNE
1977
In this case the edges and dimples were resistance-
welded while the pipe connections were soldered. The
inside surface of the panel was coated with tar epoxy
resin. The panel was then tested in the rig illustrated
in Fig. 4 (b) following the proposed standard test
code drawn up by Bates and White.
6
The test criteria
which were adopted so that reproductible tests may be
carried out were:
1. The collector panel must be within 5 of the
normal to the sun in both planes for each test.
2.
The minimum head for thermosyphoning to the
collector water tank will be 60 cm above the collector
panel outlet port. The outlet connections must have
a positive gradient greater than 20.
3.
In the case of forced circulation the same con-
stant flowrate for each test will be used. Values
between 20 to 40 ml/m
s will be acceptable.
4. Thermocouple probes must be sufficiently im-
mersed into the fluid at inlet and outlet to ensure the
thermocouple is at the same temperature as the water.
A length of not less than 5 cm is necessary.
5.
For testing, a clear sky for the duration of the
test and time to reach steady state conditions are
essential. The pyranometer gives sufficient indication
of any unfavourable drops in isolation. Excessive windy
and haze covered days should be avoided.
6.
The collector undersurface and the hot-water
storage tank with its connecting piping shall be well
insulated.
Detailed results are given in Fig. 7 in which the
heat input and output are calculated from
to give the efficiency of collecting
H
o /H
at the density
and specific heat for water under average temperature
conditions of the system. The insolation rate
G
was
measured on a pyranometer. The data are presented
as a plot of the parameter x = T d /Gagainst efficiency
of collection. For comparison, the data from three
commercially available, single glazed collectors were
obtained at the same time and they are included in
Fig. 7.
The solar building panel registered abnormally high
efficiencies under certain conditions in the low temper-
ature difference region. This was caused by the transfer
of heat to the water from the surface of the collector
beyond the wetted area. In other words, the metal
beyond the extremities of the water cavity contributed
a certain amount of heat to the system by conduction
through the metal. To eliminate this effect two runs
were made for each test on the same collector in the
same temperature range, but in the first of these tests
the collector was exposed to the sun completely while
in the second test the collector was masked so as to
expose only the wetted area to the sun. The results
from the two runs were averaged on the assumption
that the heat gained by the totally exposed panel
would be equal to the heat lost to the shaded area
with the masked panel. This, of course, would not be
the case because the temperature difference driving
forces will not be the same, but by using the average
of the two tests the result will be correct within the
expected normal experimental error. There was on an
average a 12% difference between the operation of
the straight solar building panel and the shielded
building panel. Actually, the solar building panel will
operate in the unshaded condition and therefore in
practice will register efficiencies about 6% higher than
those shown in Fig. 7 since the results are based on
the actual wetted area of the solar building panel.
However, accepting Fig. 7 at face value as a mean-
129
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would circulate in a closed system to the outside
section of a double storage tank in such a manner as
to avoid oxygen entrainment. The inner section of
the storage tank would be made of copper and would
be insulated from contact with the outer section of the
tank. A detailed study by Banks' has shown that the
total volume of the storage system is optimum at
50 litres/m
2
of solar building panel area.
5 CONCLUSION
The solar building panel can operate effectively as
a solar hot-water system collector as well as doubling
as a means of weather protection for the building in
which the hot-water system is housed. Collection effici-
encies are higher than with the glazed solar collector
for water temperatures up to 50C. To achieve higher
temperatures than 50C a glazed collector must be
used, but at an overall efficiency of well below that of
the solar building panel in its normal range of oper-
ation. In addition,
the
solar building panel can be
supplied at $10 to $15/m
2
or one-tenth the cost of
the glazed collectors.
6. REFERENCES
1 BLAKELEY, P. W. (1974) : Patterns of use of electrical
energy in New Zealand , pp. 66-78 Proc.
NZ. Energy Conf.,
Auckland University.
2
BENSEMAN, R. F. (1974): Solar energyan indigenous
fuel , Physics and Engineering Laboratory, D.S.I.R., Wel-
lington.
CHINNERY, D. N. W. (1971): Solar water heating in
South Africa ,
Nat. Build. Res. Ind. Bull., 248
CSIR.
4 MORSE, R. N., COOPER, P. I., PROCTER, D. (1974) :
The status of solar energy utilisation in Australia for indus-
trial, commercial and domestic purposes , Report 74/1, July,
CSIRO.
5 BROW,
D. (1975) : A solar building panel , N.Z. Patent
175, 439.
6
BATES, R. M., WHITE, M. C. (1975): A proposed
standard test code for solar water heaters , PME 75/1; A
standard test specification for solar water heaters , PME
75/35, Department of Mechanical Engineering, University
of Auckland.
7
BANKS, C. K. R. (1974): Utilisation of solar energy for
heating water , Department of Chemical and Materials
Engineering, University of Auckland.
V
PAPER RECEIVED
R B
WilkinsonSpeech transmission standards in the New Zealand telephone network.
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Determination
of
allowable bearing pressure
under
s m stru tur s
M J. STOCKWELL*
B.E.,
C.ENG., MI.C.E., (MEMBER)
This paper describes Procedure for assessing the allowable bearing pressure
,ender small structures without the need for testing samples in the laboratory.
I. INTRODUCTION
l 1
Preamble
T
HE object of this paper is to provide some practical
guidance for a person with limited experience in
soil mechanics who wishes to establish the allowable
design bearing pressure under a
small structure.
Hope-
fully, the paper will also be useful to the busy prac-
tising engineer who requires a quick answer. The
material presented has been gathered together from the
literature and is intended to provide a simple approach
to the problem using minimum equipment without the
necessity for .laboratory testing.
A small structure is
arbitrarily defined by the author as a one- or two-
storey building, although the methods discussed apPly
equally well to foundations for other small structures,
such as portal frames and water towers etc.
For heavier, more important structures, laboratory
compression testing of undisturbed soil samples may
be carried out to establish density, angle of internal
friction and cohesion values for thc soil. Formula and
graphs from the literature (e.g., Terzaghi) can then
be used to calculate allowable bearing pressures.
While providing the most reliable assessment of allow-
able bearing pressure, this procedure is very time con-
suming, and, the author contends, quite unnecessary
for most small structures.
1.2 Experience and local knowledge
For many building sites no foundation investigation
is carried out other than the builder or local body
inspector examining the bottom of the foundation
trench. His local knowledge tells him whether the
presence of soft underlying layers is unlikelyhence
whether a settlement type failure is precluded (refer
type (c), section 2).
Similarly the inspector will use his experience to
judge visually that the foundation soil is capable of
supporting the building without the risk of a shear
failure(refer types (a) and (b), section 2). He
often probes the ground with a bar or boot heel, and
by this process is in fact categorising the soil into one
of the classifications of section 3.2.
These methods of site evaluation are somewhat
bewildering to the inexperienced person who, it is
suggested, would be better to follow the more formal
procedure of section 3.
2. FOUNDATION FAILURE
As described in C.E.C.P. No. 4,
foundations may
fail due to any of the following causes:
(a) Rapid local failure by shear of the soil beneath
* Structural staff engineer, city engineer's office, Christchurch
City Council.
This paper was first received on 21 May 1976 and in revised
form on 14 December 1976.
the foundationsin this case the foundation will
settle suddenly with an accompanied heaving of the
surrounding soil.
b)
Slow plastic (i.e., shear) movements of under-
lying soft strata resulting in gradual lateral displace-
ment of the soil from beneath the foundations.
c)
Gradual settlement of the foundation caused by
consolidation of underlying stratathe consolidation
is caused by expulsion of air and water from the
voids.
Type (a) and (b) failures:
The allowable bearing pressures established by the
methods described later, are intended to ensure against
failure types (a) and (b) above. If these allowable
pressures are used, settlement should generally be
within the following limits: (i) maximum settlement of
any one of a group of footings = 25 mm; (ii) maxi-
mum differential settlement between footings =
20 mm.
These deflections are regarded as the acceptable
limits a modern building can withstand without
distress.'
Type (c) failure:
When underlying strata such as peat or soft clay
are present, they are likely to consolidate as a result
of an increase in pressure, and the settlement can be
calculated only after laboratory testing soil samples to
estabilsh co-efficient of volume compressibility as de-
scribed in the literature,'
et al.
It is not intended to
discuss consolidation here, other than to suggest that
the possibility should be assessed by: (i) examination
of adjacent structures for excessive settlement (say
greater than 25 mm) ; (ii) drilling boreholes to locate
soft strata. For small structures borehole depth and
pressure limits on soft strata to reduce settlement are
discussed in Appendix B.
3. SITE INVESTIGATION
The following procedure is suggested.
3.1 Boreholes
Drill boreholes to determine soil type and level of
water table. As discussed in Appendix B, a borehole
depth of about 2 m will generally be sufficient below
most small structures.
3.2 Visual classification
Carry out a visual classification of the soils en-
countered into the following broad categories, using
the tests listed:
( a)
Clay and silt:
Class 1:
Very soft; core (height = twice diam-
eter) sags under own weight.
Class 2: Soft; consistency of soft putty; can be
13 2
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pinched in half between fingers; shows heel-
marks when walked on; 12 mm bar can
be pushed in under moderate steady hand
pressure.
Class 3:
Medium; consistency of firm putty; can
be imprinted with fingers; shows faint heel-
marks when walked on.
Class
4: Stiff; not wet or sticky; difficult to
mould in fingers; difficult to imprint with
fingers; does not show heelmarks when
walked on; difficult to remove with spade or
grafting tool.
Class 5: Very stiff; cannot be moulded or im-
printed with fingers; difficult to remove with
wetted grafting tool.
Class 6:
Hard; difficult to excavate with pick.
(b) Sand
Class 1:
Uniform loose; easy to excavate with
shovel; offers little resistance to 12 mm bar
under steady hand pressure.
Class
2: Uniform compact; well graded loose;
properties between Classes 1 and 3.
Class 3:
Well graded compact; difficult to ex-
cavate with shovel; offers high resistance to
12 mm bar under steady hand pressure.
3.3. Penetration results
Carry out Scala penetrometer tests on the raft of
soil immediately below the foundationsthe Scala
penetrometer and its operation are described in
Appendix A.
4. EVALUATION OF RESULTS
4.1
General
From the Scala penetrometer results and the visual
classification of section 3,
qa
can be evaluated from
Fig. 2 where:
qa =
proposed allowable bearing pressure including
a factor of safety of 3 against a type (a)
failure.
If penetrometer tests reveal weak layers below the
surface, then equation B.1 of Appendix B should be
NEW ZEALAND ENGINEERING
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1977
used to check that the dispersed pressure
pd
at the
lower level does not exceed
qa
for the stratum at that
level. A type (a) and (b) failure will hence be
avoided.
4.2 Clay and
silt
The following definitions apply:
H depth of bottom of footing below ground (m)
B width of footing (m)
qa allowable bearing pressure (kPa)
qm =
modified allowable bearing pressure (kPa)
For isolated and strip footings the value of
qa can
be modified for the following effects:
a)
Depth of isolated and strip footings:
qm =qa (1 +
H/ 4B),
but not more than
1.5 qaref.2.
The increase in qa
is for the enhancing effect
of soil confinement below ground level.
b)
Width of isolated and strip footings:
qm
a
in all cases ref. 2.
c)
Vibrational effects (including earthquake) :
For clays in the Class 4 to 6 range
q m 1 5 q a r e f 3.
For silts, no increase is allowed, and if the
sand content is high the reduction for sand
should be used.
4.3 Sand
The value of
qa
should be modified for the following
effects:
a)
Depth of isolated and strip footings:
qm =qa
1+H/B),
but not more than
2
qa,
ref. 2.
As for cohesive soils the increase is for the
confining effect of pressure below ground.
b)
Width of isolated and strip footings:
when B
Recommended