New Zealand Penetrometer

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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.,

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Telephone: 735-739. Telegrams: Tecpub.

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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.

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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

(32, 6) 15

JUNE

1977

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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

126


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JUNE

1977


13/44

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

NEW ZEALAND ENGINEERING (32, 6) 15 JUNE

1977

12 7


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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.


(32, 6) 15 JUNE1977


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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

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EW ZEALAND ENGINEERING

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19/44

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


(32, 6) 15

JUNE

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

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New Zealand Penetrometer