gammon - stanford earth

2 15

Proc. Geothermal Workshop 1987

EXPLORATION AND DEVELOPMENT OF THE ROTORUA GEOTHERMAL AREA

J. R.A. Gammon', D .M. D . Swears3

Sect ion, Murray-North Par tners , Limited

Wel ld r i l l i ng Co. Ltd

Test ing Ltd. , Rotorua

ABSTRACT

The tapping of geothermal energy i n Rotorua has

been tak ing p l a c e f o r s e v e r a l hundreds of years.

Surface man i fe s t a t ions have been supplemented by

progres s ive ly deeper w e l l d r i l l i n g which has now

reached i n t o t h e bedrock.

The engineer ing impacts o f t h e product ion and

recharge o f geothermal f l u i d s extend beyond energy

cons ide ra t ions , however. The aggres s ive na tu re of

t h e f l u i d s i n terms of material corros ion and

d e t e r i o r a t i o n need t o be recognised. Other impacts

on t h e engineer ing performance of present-day high

c a p i t a l va lue developments a l s o need t o be

addressed.

1

2 . 0

2.1

INTRODUCTION

The exp lo ra t ion and f i e l d development o f t h e

Rotorua geothermal area has been c l o s e l y

assoc ia t ed with t h e growth o f Rotorua Ci ty . The

demand on t h e geothermal resource has r e s u l t e d i n

changing d r i l l i n g technology, energy u t i l i s a t i o n

and c o n t r o l l i n g l e g i s l a t i o n . Th i s paper aims t o

review these changes p a r t i c u l a r l y wi th respect t o

c i v i l engineer ing practice i n t h e Rotorua area.

General c o n s t r a i n t s on c i v i l engineer ing practice

and problems d i r e c t l y r e l a t e d t o t h e use and

disposal o f geothermal f l u i d s are discussed.

GEOTHERMAL FIELD EXPLORATION AND UTILISATION

Early Development

The geothermal resource i n h i s t o r i c t i m e s w a s

i n i t i a l l y used by t h e A r a w a Maori people where

b o i l i n g sp r ings were used f o r cooking, washing,

food drying and process ing o f f l a x fibres. With

Europeans a r r i v i n g i n t h e area spa bathing

inc reased i n popu la r i ty and from about 1870 many

b a t h houses and treatment c e n t r e s were es t ab l i shed

i n t h e Rotorua area. As demand on the resource

exceeded t h a t which could be provided by t h e

sp r ings , shallow bores w e r e used t o inc rease

supply. Geothermal bore d r i l l i n g comnenced i n t h e

1930 's and has continued w i t h a gene ra l t r end

matching Rotorua's populat ion growth (Figure 1).The shallow bores were i n time replaced by deeper

bores as t h e energy requirement increased. Phases

of in t ense d r i l l i n g a c t i v i t y occurred i n t h e e a r l y

1960's and m i d 1970's which were assoc ia t ed with

electrical power shor tages and t h e o i l crisis

respect ively . Present ly there are approximately

430 geothermal bore u s e r s i n t h e

Rotorua urban area. The d i s t r i b u t i o n o f these

u s e r s is shown i n Figure 2. The presen t geothermal

draw-off (a bo i l ing mixture) as

measured by t h e Geothermal Task Force

is est imated at 31,000 tonnes per day i n t h e

winter decreas ing to 25,000 tonnes per day i n t h e

)O-f

e -!

-NOTE GENERAL

OF

1930 1950 1960 1980

FIGURE 1 : PLOT OF NUMBER OF GEOTHERMAL DRILLINGPERMITS ISSUED AND URBAN POPULATION GROWTH OF ROTORUA WITH TIME(AFTER MOE,

D r i l l i n g Procedures

Present day d r i l l i n g procedures f o r t h e deeper

bores have developed as a r e s u l t of working i n

d i f f i c u l t ground cond i t ions where a high

temperature p res su r i zed geothermal draw-off

being tapped.

I

2!6

and Swears

S i t e s for d r i l l i n g a geothermal bore a r e

chosen on t h e i r l oca t i on i n s i d e t h e proven

geothermal f i e l d (Figure 2 ) . The ex ten t of t h e

f i e l d l a r g e l y been proven by 'wild-cat' t r i a l

and d r i l l i n g s ince t h e 1930's. Geophysical

surveys of t h e area by

f u r t h e r complemented t h e proven f i e l d by

extending it nor th below Lake Rotorua.

The test headworks a r e then f i t t e d t o

w e l l followed by a i r pumping u n t i l t he

cu t t i ngs are clear and any cold water is

f lushed away leaving the steam t o blow

through t h e borehead.

Production tests a r e then completed together

with i n s t a l l a t i o n of t h e d i sposa l bore.

LEGEND

BORE TYPES ,

FIELD

PRESENT DAY DISTRIBUTION OF THE 430

GEOTHERMAL USERS IN THE ROTORUACITY AREA AFTER

The gene ra l procedure f o r d r i l l i n g a v e r t i c a l

geothermal bore (Figure 3) i s a s follows:

D r i l l i n g o f a diameter t o 6m, most c r i t i c a l f a c t o r s i n developing a

successfu l bore a r e adequate grout ing and the

of the casings. The cas ing must be wel l

grouted i n t o t h e however care

is requi red not to case ou t t h e production zone.

The purpose of the grout is t o proper ly seal t h e

w a l l s of t h e cas ings t o borehole order

to prevent steam up through t h i s gap

causing a . The cas ing and associa ted

grout also serve t o seal t h e cold water i n the

p h r e a t i c a q u i f e r (See Sect ion 3.2) which could

flow down and quench ou t the underlying production

zone.

cased and cement grouted with diameter

cas ing .

R r i l l i n g a diameter hole to 20 t o 30m

depending on where a f i rm foundation ma te r i a l

for t h e cas ing is found.

diameter cas ing is then i n s t a l l e d

su r f ace ( i n s i d e t h e cas ing) and

grouted i n place .

A- ho le is then d r i l l e d from 20 t o

l e v e l t o e i t h e r the Rotorua

Mamaku Ignimbr i te (See sec t i on This

depth i n t h e area genera l ly ranges

from 90 t o t h e geothermal is used it is

disposed of to e i t h e r a d i sposa l bore (78%

of - a stream or

(18% of d i sposa l ) a r e i n j e c t i o n w e l l

to production (4% of d i sposa l ) . The

p re sen t practice of e s t a b l i s h i n g a shallow soak

ho le is t o d r i l l to a depth of 8 metres

t h e r e a f t e r a t the f i r s t d r i l l i n g f l u i d

c i r c u l a t i o n loss. The is then t e s t ed

d i sposa l capacity.

The geothermal bore casing is then set

i n t h i s hole and reverse grouted, Reverse

g rou t ing involves g rou t being

the casing and then pushed up the

w a l l by a of water it

t o t h e ground surface .

A d r i l l is then used to out t h e

cas ing and then d r i l l i n t o t h e

w a t e r

217

Gammon, M c E r l a n e and Swears

Maintenance of a product ion bore is usua l ly

a s s o c i a t e d with removal of c a l c i t e from t h e casing

w a l l . The calcite build-up occurs wi th in t h e

b o i l i n g zone of t h e bore which may requ i re annual

reaming. Present ly borehole reaming is t h e most

common method of calcite removal, however t h i s

p r a c t i c e may b e replaced by using a chemical

i n h i b i t o r which is pumped down t h e bore to prevent

of t h e cas ing and headworks. Such a

chemical i n h i b i t o r is 247 which w a s f i e l d

t e s t e d by t h e Geothermal Task Force

B o r e c losure can e i t h e r t a k e place on a temporary

or bas i s . Temporary c losure involves t h e

tu rn ing o f f o f t h e main headworks valve. Corrosion

of t h e casing commonly occurs where t h e c losure

extends over a long period of t i m e i n areas where

t h e groundwater t a b l e is l o w . For permanent

c l o s u r e , t h e w e l l is i n i t i a l l y 'quenched' with

c o l d water then pressure grouted from t h e bottom

to t h e top of t h e bore.

Fu tu re Developments

Fu tu re developments with r e s p e c t t o w e l l d r i l l i n g

p r a c t i c e w i l l be genera l ly assoc ia ted with

measures aimed a t conserving t h e geothermal

resource. Present w e l l d r i l l i n g practise f o r both

product ion and d i sposa l bores w i l l r e q u i r e

reassessment where down-hole h e a t exchange devices

a r e used or widespread deep r e i n j e c t i o n of

e f f l u e n t i n t o t h e geothermal r e s e r v o i r t akes

place. The Geothermal Task Force

recommends t h a t f u t u r e t e c h n i c a l work be

undertaken t h e areas of down-hole hea t

exchangers and bore r e i n j e c t i o n .

With regard t o increased product ion, inc l ined

h o l e s may provide higher l e v e l s of geothermal

draw-off p a r t i c u l a r l y wi th in t h e Mamaku Ignimbri te

where pre fe r red o r i e n t a t i o n of j o i n t i n g i s

v e r t i c a l . Inc l ined geothermal bores have no t been

used t o ' d a t e i n the Rotorua area.

2.2 L e g i s l a t i o n

The earliest l e g i s l a t i o n regarding management of

t h e geothermal draw-off was passed i n 1945 a s

Sec t ion 89 of t h e S t a t u t e s Amendment A c t , an

a d d i t i o n t o t h e Tour i s t and Heal th Resorts Control

A c t , 1908. Th i s l e g i s l a t i o n gave a u t h o r i t y to t h e

Governor General to d e c l a r e "thermal water areas"

i n which no d r i l l i n g could t a k e p l a c e p r i o r to

w r i t t e n consent of t h e Minis ter . The 1945

l e g i s l a t i o n is sti l l v a l i d al though no "thermal

water areas" have y e t been defined.

D r i l l i n g was i n i t i a l l y l icenced by t h e 1953

Geothermal Energy A c t where l i cences t o i n s t a l l

geothermal bores were gran ted by t h e Minis t ry of

Works. La te r i n 1967 t h e Rotorua Geothermal

Empowering A c t authorised t h e Rotorua C i t y Council

to both i s s u e d r i l l i n g l i cences and c o n t r o l

geothermal energy i n t h e c i t y .

As a response t o t h e decreasing geothermal

a c t i v i t y associated with t o u r i s t a t t r a c t i o n s , t h e

Rotorua C i t y Geothermal By-Law w a s passed

whereby d r i l l i n g l i c e n c e s could be refused i n

c e r t a i n areas. Further f a i l u r e s of Rotorua's

geysers i n t h e 1970's r e s u l t e d i n a

being placed on d r i l l i n g new geothermal bores

wi th in of Whakarewarewa i n November 1979.

During 1982 and 1983 t h e Minis t ry of Energy

es tab l i shed 3 working parties; t h e Rotorua

Geothermal Monitoring Programme, t h e Rotorua

Geothermal Task Force and t h e O f f i c i a l s Geothermal

Co-ordinating Commit tee . The Rotorua Geothermal

Monitoring Programme was es tab l i shed t o collect

d a t a t o be used f o r a management p l a n f o r t h e

Rotorua Geothermal Field . Closely assoc ia ted t h

t h i s programme w a s t h e Rotorua Geothermal Task

Force whose aim w a s to develop methods for

immediate reduct ion of draw-off from t h e Rotorua

Geothermal Field . The t h i r d body t o be e s t a b l i s h e d

was t h e O f f i c i a l s Geothermal

Committee, an inter- departmental group formed t o

develop po l i cy and planning on

geothermal resources nationwide. I n January 1986

t h e Minis t ry of Energy publ ished its "Geothermal

Resources, a Pol icy and Management Framework" , a

comprehensive list of policies which is intended

t o become p a r t of t h e new Water and S o i l

Conservation A c t .

Apart from these management p o l i c i e s which a r e

p r e s e n t l y wi th in t h e l e g i s l a t i v e process recent

concern over decreased geyser a c t i v i t y a t

Whakarewarewa has lead to s w i f t government

in te rven t ion . This in te rven t ion is i n t h e form of

proposals to close a l l bores wi th in a rad ius

of Whakarewarewa (Figure 2) on December 1, 1986,

and to have a l l bore u s e r s i n t h e Rotorua a r e a

l i cenced on A p r i l 1, 1987.

3.0 GEOTHERMAL FIELD DESCRIPTION

3.1 Geological

The Rotorua District is located within t h e Taupo

Volcanic Zone (Healy, a Quaternary vo lcan ic

belt and con t inen ta l r if t system which extends

north-northwest from M t Ruapehu t o White I s l and

(Figure The Taupo Volcanic Zone is c l o s e l y

assoc ia ted with t h e Hikurangi Trough, both of

218

FIGURE CONTOURS ON THE BURIED SURFACE OF THE ROTORUA RHYOLITE(GEOTHERMAL AQUIFER) AND GEOLOGIC OF THE GEOTHERMALFIELD (AFTER AND PREBBLE,

Gammon, McErlane and Swears

which, on a global s c a l e are in t e rp re t ed as being

formed by t h e o f t h e P a c i f i c P l a t e

beneath t h e con t inen t a l c r u s t of the Indian P l a t e

1981).

3.2

The Rotorua geothermal f i e l d is s i t u a t e d near t h e

southern edge of the Rotorua Caldera which

co l l apsed approximately 140,000 years ago due to

t h e e rup t ion of approximately of r h y o l i t i c

material which formed t h e Mamaku Ignimbr i te

(Wilson 1984). Fur ther phases of r h y o l i t i c

volcanism b u i l t domes wi th in t h e ca lde ra (Wood,

one of these being t h e p re sen t ly bur ied

Rotorua Rhyol i te which unde r l i e s m o s t of Rotorua‘s

urban area (Figure 4 ) .

The Rotorua Rhyoli te and Mamaku Ignimbr i te i n t h e

c i t y area a r e genera l ly ove r l a in by l a c u s t r i n e

a l l u v i a l and tephra depos i t s ( th icknesses up to

240m - Wood, 1985) which have been deposi ted as a

r e s u l t of h igher l ake l e v e l s of Lake Rotorua and

Quaternary volcanism. The depos i t s have been

a l t e r e d (predominantly s i l i c i f i e d )

t o d i f f e r i n g degrees by a c t i v i t y associa ted with

t h e Rotorua Geothermal F ie ld . The age of t h e

Rotorua Geothermal F ie ld is no t known. Geological

evidence, however sugges ts t h a t it a t l e a s t

ex i s t ed dur ing t h e period of 20,000 to 50,000

yea r s ago (Wood, 1985). The t y p i c a l geology

as soc i a t ed wi th a geothermal bore is presented i n

Figure 3.

Geothermal F ie ld Hydrology

Studies by t h e Geothermal Monitoring Programme

(MOE have indica ted t h a t t h e ho t water from

t h e deep geothermal r e se rvo i r r i s e s i n t he ea s t e rn

margin of the f i e l d . P a r t of t h i s flow is

expressed a t the ground su r f ace as t h e ho t spr ing

areas a t Whakarewarewa and Ngapuna. From t h i s a r ea

t h e hot water genera l ly flows nor th and w e s t under

Rotorua c i t y wi th in t h e geothermal aqu i f e r

c o n s i s t s of the Rotorua Rhyoli te and Mamaku

Ignimbrite. Permeabil i ty wi th in the geothermal

aqu i f e r i s j o i n t cont ro l led . The a r e

considered t o be best developed near t h e of

the geothermal aqu i f e r s i n c e production is

genera l ly r e s t r i c t e d t o t h i s zone.

Overlying t h e geothermal aqu i f e r is a sequence of

p a r t i a l l y s i l i c i f i e d silts, sandy silts and

discontinuous l enses of g rave l and sands. This

sequence is charac ter i sed by an o v e r a l l

v e r t i c a l permeabi l i ty and is e s s e n t i a l l y an

aqui tard . The aqui tard prevents t he v e r t i c a l

migration of geothermal f l u i d over most of t he

c i t y area.

Above t h e aqui tard is a sequence of sands and

grave l s wi th minor silt and clay . This

highly sequence has been described a s a

p h r e a t i c aqu i f e r (MOE, i n t o which the

major i ty of geothermal e f f l u e n t

is disposed. The re l a t i onsh ip between t h e l o c a l

geology and hydrology is schematically shown

Figure 3.

The temperature of the deep h o t waters wi th in t h e

geothermal aqu i f e r has been measured up t o 230°C

(MOE, A t y p i c a l draw-off temperature a

geothermal bore is Geothermal aqu i f e r

p re s su re s ac ros s t h e f i e l d vary by approximately

130 (1.3 ba r s ) . I n terms of pressure head.

w i th in monitoring w e l l da t a (MOE, water

l e v e l s are commonly wi th in of the ground

su r f ace and r a r e l y above it. Some geothermal

bores , however have wellhead pressures of over

- Boielle, 1965). Observations of t h e

Geothermal Monitoring has indica ted t h a t

e x p l o i t a t i o n of t h e geothermal f i e l d has caused a

drop of aqu i f e r pressure up t o (0 .5 ba r s )

and that a seasonal cycle is observed with the

aquifer pressures falling due to increased

draw-off in the winter months. The change in

aquifer pressure due to seasonal fluctuations

varies from 3 to (0.03 to 0.08

to a change in hydrostatic head of 0.3

to

The phreatic aquifer is basically a cold water

body except for areas of natural hydrothermal

activity where localised heating occurs. The

shallow disposal of geothermal effluent

indicated typical temperature profiles through

the aquifer where a warm water temperature peak of

to is observed between depths of 20 and

40m (Wood, 1985).

The Geothermal Monitoring Programme

established a series of shallow monitoring wells

into the phreatic aquifer. The monitoring showed

that the aquifer was connected to Lake Rotorua and

that depth to the ground water table varied from

approximately 1 metre near the lake to 8 metres

under Rotorua city. No distinct annual cycle is

observed in the monitoring data indicating that

the 9% reduction in disposal from winter (31,000

to summer (25,000 is not

sufficient to affect water levels. Rainfall is

considered riot to have a major effect on water

level trends due to its generally uniform annual

distribution.

The geothermal fluid types have all been basically

derived from the deep high temperature chloride

fluid within the geothermal aquifer. As the

chloride fluid rises, it boils and volatile gases

such as and H S fractionate into the steam

phase. The steam either discharges

directly at the ground surface as a fumarole or

may condense into the overlying ground-water to

form an acid sulphate fluid. The two fluids can

also mix to form a mixed acid sulphate-chloride

fluid. The chemical characteristics, typical

temperatures, types of surface discharge and

hydrothermal alteration are presented in Table 1.

These characteristics are typical for New Zealand

geothermal fields which includes Rotorua.

2

219

McErlane and Swears

Table 1 : Typical Fluid Characteristics of

New Zealand Geothermal Systems

Hedenquist, 1986)

4.0 ENGINEERING DEVELOPMENTS IN A GEOTHERMAL AREA

4.1 General Constraints and Structural Considerations

The successful design, construction and

maintenance of structures and other civil

engineering works within the Rotorua city area is

dependent on an understanding of how the item of

construction work will behave in a geothermal

environment.

Concrete

Concrete, one of the most common construction

materials, has been noted for its lack of

durability under highly active geothermal

conditions. For example, in some areas of Rotorua

the operative life of asbestos cement and concrete

water pipes has been as low as 3 to 4 years

(Roberts, The lack of durability is due to

the concrete's reaction with heat, moisture,

hydrogen sulphide carbon dioxide

sulphates and chlorides (Smith,

1986).

since cement is basic in character, it has low

resistence to acidic conditions which occur as the

of H S oxidation and aqueous solutions of

In addition to acid attack, the reaction of

with- calcium hydroxide further reduces its

basicity and protection to the reinforcing steel.

Sulphate reaction causes further concrete

deterioration which is usually associated with

cracking from volumetric expansion.

2

In response to these hazards, engineering practice

for large projects in geothermal areas has

incorporated site investigations to assess

potentially destructive conditions, special

concrete mixes and, in highly active areas, the

use of protective coatings or liners particularly

for the substructure.

220

McErlane and Swears

Timber

Treated timber is sensitive to both acid

Conditions and high temperature. Where timber is

in contact with acid sulphate fluids 2 to 3

- Table 1) and mixed acid sulphate-chloride fluids

2 to 5) acid degredation of the wood occurs

with an associated loss of strength. With high

temperature and long periods of time thermal

decomposition of the wood occurs causing again a

loss of strength. The leaching of preservatives

resulting from the acid conditions are, however,

not considered to be a hazard since fungal growth

does not occur in such an environment.

Other Materials and Equipment

Atmospheric corrosion, common in the Rotorua area

is due to the presence of hydrogen sulphide

in concentrations generally between to

lppm and up to (Thomas, 1986). The threshold

level for corrosion for some metals is as low as

which indicates the highly corrosive

nature of Rotorua' environment. Particularly

sensitive to Rotorua's environment is electrical

equipment especially that which contains silver

2

based micro-circuitry.

In order to restrict the amount of gas entering a

building, foundations have incorporated both

mechanically-assisted underfloor ventilation

systems and continuous butyl sheet liners, the

latter also used to prevent degradation of

structural foundation materials as described

above.

4.2 Geotechnical Engineering Considerations

Ground Conditions : Strength Variations

With an increase in the size of the structures

associated with the development of Rotorua, it has

been necessary to obtain detailed information on

the soils overlying the bedrock from which the

geothermal fluids are derived. As indicated in

Section 2.1 above, interest in these materials in

geothermal terms has been restricted to the

ability to re-charge geothermal fluids

into suitably permeable zones of ground. Prior to

a dramatic increase in the rate and scale of

development in Rotorua, it was possible to found

low-rise structures on the of relatively

stiff soils that occur within the Rotorua area.

The nature of this crust arises as a consequence

of both dessication due to evaporation of water,

particularly in the areas where geothermal

activity is prevalent as a surface

as well as chemical reactions which have produced

an element of cementation to these superficial

soils. Indeed, the presence of the crust is of

particular significance for buildings in the

geothermal areas where the soil profile is

otherwise extremely poor, with low shear strengths

and densities prevalent to considerable depths.

Such soils may have the consistency of

The properties of these materials have no doubt

been influenced by factors that have occurred over

a geological time frame, but generally speaking

the phenomenon of a surface crust occurs in areas

that can be 'observed to lie both within and

outside currently active geothermal areas.

Some areas of ground in the Rotorua area, however,

have been affected as a consequence their

history as areas of swamp land and under these

circumstances soft ground conditions occur up to

the surface.

Significant organic of such soils

occurs and the remains of entire tree trunks have

been encountered during earthworks operations.

Thus on penetrating beneath the surface crust

where this occurs or in areas where conditions

have prevented the of this

usually as a consequence of low lying topography,

low strengths or densities are

experienced, particularly beneath the ground water

table.

Soil Type Variations

In addition to low strengths, the nature of

materials is extremely variable, both in a

vertical and lateral sense. In addition to silts

and sand-sized particles and occasional clays,

layers of granular material cemented chemically in

the form of silicified (or bands occur

within the ground condition profile above rockhead.

A generalised profile of the ground conditions has

been presented in Figure 3, but it must be

appreciated that this is a significant

generalisation and simplification of conditions

which may be encountered at a specific location

and has been produced essentially to identify

the main features within the ground

profile in the Rotorua region. A s a consequence of

deposition under alluvial and lacustrine

conditions, together with the influence of changes

in lake level and attendant volcanic activity, it

is to be expected that the soils in Rotorua will

vary appreciably. can be examined to

different degrees of detail. Within, for example,

the sand-sized fraction, quartz sands and pumice

sands as well as the elements of the

diatomaceous or organic-based materials can occur.

Physical and Chemical Properties

significant differences in physical properties

occur depending on whether not the nature of

t h e ma te r i a l is t h a t o f qua r t z , a s h or

diatomaceous material. Some soils e x h i b i t a

remarkable inc rease i n s t r e n g t h on exposure t o t h e

atmosphere dur ing excavat ion work provided d r y

c o n d i t i o n s are maintained. Other soi ls lose

s t r e n g t h on being reworked dur ing excavation.

Var i a t ions i n silt con ten t can produce materials

having p r o p e r t i e s ak in t o e i t h e r coarse grained

(sand- l ike soils) or f ine-grained (c lay- l ike)

soils. Permeabi l i ty is a major p rope r ty sepa ra t ing

t h e respec t ive behaviour of t h i s broad d i v i s i o n of

soi l types. The s ign i f i cance of t h e si l t con ten t

becomes more apparent when it is r e a l i s e d t h a t

much of the diatomaceous materials t h a t are

present occur within t h i s particle size range and

can through t h e i r highly porous

s t r u c t u r e r e s u l t i n anomalous properties i n terms

o f mois ture contents compress ibi l i ty .

Chemical a l t e r a t i o n due to geothermal a c t i v i t y

also in f luences t h e f i n e r grained soils t o

d i f i e r e n t degrees depending on t h e i r o r ig in .

Indeed, t h e pore-water chemistry generated as a

consequence o f wi th in geothermally a c t i v e

area can be such t h a t t h e properties o f t h e soils

are s i g n i f i c a n t l y a f f ec t ed . Cementation of an

o the rwise g ranu la r soil may occur or, conversely ,

a weakening of t h e soil material t o

a c c e l e r a t e d weathering as a r e s u l t o f an

aggres s ive chemical environment may take place.

Clay mineral behaviour may be a l t e r ed . S t r i k i n g

co lour changes can also be brought about through

t h e chemical environment to which t h e s o i l s may be

sub jec t ed . Thus pa tches o f pure whi te and deep red

soils o f t e n occur p a r t i c u l a r l y i n areas of higher

ground sub jec t to v a r i a t i o n s i n ground-water

l e v e l and t h e n a t u r a l discharge o f geothermal

f l u i d s .

P r i o r to an i nc rease i n t h e s c a l e of development,

d i f f c r c n c e s i n ma te r i a l types and t h e

v a r i a t i o n i n ground cond i t ions w a s of

l i t t l e consequence and areas o f poor ground

terms of providing adequate founding cond i t ions

f o r l o w rise s t r u c t u r e s were ev iden t from sur face

phenomena such as f looding and organ ic

na ted soils.

221

and Swears

S i t e Inves t iga t ion Fieldwork

Fieldwork requirements have become more cr i t ica l

as a consequence of t h e desire of developers t o

in t roduce high rise s t r u c t u r e s i n Rotorua and t h e

a t t e n d a n t imposi t ion of inc reas ing loads on to t h e

" t r a d i t i o n a l " s t ra tum adopted f o r

convent ional housing. S i t e inves t iga t ion work may,

f o r example, be required t o t h e

ex i s t ence of a s u i t a b l e founding laye r or zone f o r

piles once t h e th i ckness of t h e dess i ca t ed c r u s t

has been es t ab l i shed as inadequate f o r t h e scale

of development proposed.

S i t e i n v e s t i g a t i o n fieldwork should extend t o a

depth a t least equa l t o t h e width of the s t r u c t u r e

un le s s bedrock is first encountered. S i n t e r bands

which rep resen t a l o c a l i s e d b u t marked inc rease i n

s t r e n g t h wi th in t h e o v e r a l l so i l p r o f i l e can cause

d i f f i c u l t i e s . A rapid and continuous p r o f i l e of

ground cond i t ions can be es t ab l i shed through t h e

use of t h e cone penetrometer t e s t i n g

equipment a v a i l a b l e i n New Zealand. Although

samples are no t recovered using CPT techniques,

enhancement o f s t anda rd systems permits t h e

measurement o f ground temperatures and pore-water

pressures . However, t h i s equipment may no t be

able to pene t r a t e t h e s i n t e r bands unless they are

r e l a t i v e l y th in . I t is the re fo re t o ga in

an i l l u s i o n t h a t bedrock has been encountered

whereas r o t a r y d r i l l i n g through t h e s i n t e r l a y e r s

would show t h a t t h e s o f t soils cont inue beneath

t h e s i n t e r band. I d e a l l y , t he re fo re , s i t e

i n v e s t i g a t i o n work would c o n s i s t o f a combined

of cone penetrometer tests to eva lua t e a

continuous p r o f i l e o f so i l types and s t r eng ths

down to t h e sinter l a y e r s - t h i s zone usual ly

being of p a r t i c u l a r importance f o r shallow

foundations and basement s t r u c t u r e s ) together wi th

t h e use o f r o t a r y techniques i n o rde r t o recover

samples f o r f u r t h e r s t r e n g t h and chemical t e s t i n g .

Rotary d r i l l i n g also permits core to be recovered

from s i n t e r l a y e r s and/or t h e bedrock i f t h i s i s

t o be determined and is cr i t ica l to t h e foundation

design.

would also be prudent, as an i n t e g r a l part of

t h e f i e l d work exe rc i se , t o determine t h e na tu re

o f t h e ground-water regime. Through t h e i n s t a l-

l a t i o n of ground-water monitoring i n s t a l l a t i o n s ,

such as observat ion wells and standpipe piezo-

meters, it is also possible t o obta in samples o f

water f o r chemical a n a l y s i s so as t o e s t a b l i s h t h e

chemical parameters a f f e c t i n g t h e use of

cons t ruc t ion materials i n t h e subst ructure .

Laboratory

Laboratory t e s t i n g is gene ra l ly required i n o rde r

to e s t a b l i s h or confirm t h e engineering properties

of t h e soils p resen t . Tes t ing may be required t o

e s t a b l i s h t h e particle s i z e d i s t r i b u t i o n and

s t r e n g t h of t h e soils toge the r with t h e i r

se t t l emen t c h a r a c t e r i s t i c s p a r t i c u l a r l y where

these are time dependent. As i nd i ca t ed above, t h e

222


chemistry of t h e soils and ground-water is also of

s i g n i f i c a n c e and re l evan t l abo ra to ry t e s t i n g

should be incorpora ted wi th in a comprehensive site

i n v e s t i g a t i o n programme. For tunate ly , as t h e scale

of s t r u c t u r e s has increased , so too has t h e i r

va lue and it should be p o s s i b l e t o the re fo re

i n c r e a s e t h e scope of f i e l d work from the hand

auger and l i g h t (Scala) penetrometer tests

normally considered s u i t a b l e f o r houses on shallow

foundat ions t o the electrical and mechanical cone

penetrometer teste and ro t a ry d r i l l i n g techniques

o u t l i n e d above. I n using t h e electrical

due note has to be taken of t h e

p o t e n t i a l e f f e c t s of high temperature on t h e

s t r a i n gauge measuring system.

Ground Condit ions and Ground Water R e g i m e

An i n d i c a t i o n of t h e v a r i a t i o n s i n so i l types and

c o n s i s t e n c i e s has been given i n t h e opening

paragraphs of t h i s sec t ion .

The s ign i f i cance of t h e ground-water regime can be

seen through its impact on t h e th i cknes s of t h e

(dessicated) crustal l a y e r and i ts relevance t o

excavat ion work as w e l l as i ts long-term inf luence

on t h e behaviour of s t r u c t u r a l ma te r i a l s . Low

l y i n g areas, r e l a t i v e to c u r r e n t l ake l e v e l ,

exper ience ground-water l e v e l s a t shallow depths.

I n t h e c e n t r a l d i s t r i c t s of Rotorua depths to the

ground-water table are genera l ly . i n excess of 3

metres. It is expected t h a t t h e in f luence of t h e

recharge w e l l s t o t h e o v e r a l l ground-water regime

h a s been to l o c a l l y increase t h e e l eva t i on of t h e

p h r e a t i c s u r f a c e wi th in t h e zone inf luenced by

recharge w e l l s . An ana lys i s t h i s ground-water

regime however i s extremely complex, as a r e s u l t

of major v a r i a t i o n s i n permeabi l i ty and i n t e r -

connection between t h e va r ious zones of granular

material.

One of t h e consequences of h a l t i n g t h e withdrawal

o f t h e geothermal f l u i d s would be a corresponding

e l imina t ion of t h e recharge system which i n t u r n

could have an inf luence on t h e ground-water

as observed a t t h e cu r r en t t i m e . Changes i n

ground-water l e v e l i n a geothermal environment can

cause s i g n i f i c a n t changes i n soil and pore-water

chemistry and t h i s may alter t h e cu r r en t chemical

environment wi th in the soils and ground-water

in f luenc ing t h e behaviour of c i v i l and s t r u c t u r a l

engineer ing works.

I n connection wi th t h e ground-water regime it has

been noted t h a t s t u d i e s of t h e geothermal area

have tended t o concent ra te t h e i r a t t e n t i o n on t h i s

area only. Wide spread s t u d i e s that relate t h e

phenomena observed i n wi th those occurr ing

under n a t u r a l condi t ions beyond t h e c i t y a rea a r e

also considered important. Some of t h e cu r r en t

i n t e r p r e t a t i o n s of ground-water movement both

l a t e r a l l y and v e r t i c a l l y a r e based on hypothesis

and "widely-spaced" da t a r a the r than a coordinated

survey of t h e phenomena over an extens ive area.

Where soils' of markedly d i f f e r e n t

occur then t h e r e is t h e p o s s i b i l i t y t h a t sp r ings

can develop p a r t i c u l a r l y where t h e impermeable

capping to an aqu i f e r is weakened e i t h e r na tu ra l l y

or as a r e s u l t of excavation work.

Foundation Considerations

A s a consequence of t h e increase i n bui ld ing s i z e

and corresponding loading regimes and wi th due

regard to seismic cons idera t ions it has been

necessary to ca r ry ou t increas ingly d e t a i l e d

surveys of t h e e n t i r e ground condi t ions and

ground-water regime above the bedrock level . Thus

shallow foundations involving limited excavation

i n t o t h e s t i f f c r u s t observed i n p a r t s of

Rotorua has n o t caused any problems i n terms of

subsequent s t r u c t u r a l performance. (The inf luence

of poor ly con t ro l l ed use of geothermal f l u i d s on

foundation behaviour due t o adverse s t r u c t u r a l

behaviour has been introduced i n a previous

sec t i on of t h i s paper) .

wi th a marked decrease i n so i l s t r eng th with depth

below t h e "crus t" ) and with t h i s l o w

s t r e n g t h p e r s i s t i n g to appreciable depths the need

t o support bu i ld ings of usual ly more than t h r e e

stories i n he igh t have caused s i g n i f i c a n t problems

t o foundation engineers. In some areas of Rotorua

where high- r ise s t r u c t u r e s have been requi red a

r e l a t i v e l y shallow depth t o bedrock has

fo r tuna t e ly been encountered and an economical

p i l e d foundation can be considered. A t o t h e r

loca t i ons more competent granular l aye r s with

s u f f i c i e n t l y high r e l a t i v e d e n s i t i e s have been

encountered above t h e bedrock and these provide a

s u i t a b l e founding strata. Reversals of s t r eng th

requ i r e f u l l recogni t ion i n design and

construction. Essen t i a l l y each and every site has

to be inves t i ga t ed i n d e t a i l and re l i ance on the

in t e rpo l a t i on of d a t a from widely spaced boreholes

(as is c a r r i e d ou t i n o the r cities with in New

Zealand) is considered t o be extremely unwise i n

Rotorua.

Shallow and Compensated Foundations

Analysis of t h e nature of t h e soils and i n

p a r t i c u l a r t h e i r time dependent se t t lement

c h a r a c t e r i s t i c s any), may t h a t i n o rde r

to achieve a suitable substructure system, it

would be preferable to form a basement structure

and henae provide a compensated foundation whereby

the effective weight of the building is equated to

by the effective weight of soil excavated from

within the plan areas of that structure. 'Partial'

compensation may also be adopted so as to reduce

settlements to an acceptable Compensation

in the form of basements has the attraction of

providing much needed parking spaces in a rapidly

developing city such as Rotorua but also presents

technical problems in with piled

foundations whereby the substructure elements

protrude close to or into the ground-water regime

which as indicated in sections of the

paper is chemically aggressive. This chemical

aggression also occurs however to varying degrees

in the soils above the ground-water table and some

structures founded on shallow foundations have

been "tanked" with a protective outer lining of

rubber, for example, so as to prevent

chemical attack of the concrete in particular.

Where the structural configuration results in a

significant excavation into the surface crust but

a basement structure cannot be incorporated into

the overall development then it may be necessary

to stiffen the shallow foundations into a

competent raft which may also require protection

against chemical attack.

Settlement considerations are also important for

pipework and other utilities passing into the

structure and geothermal pipework may need to be

flexible where settlements are predicted to be

significant.

Piled Foundations

Where piles are adopted different types of piling

system have been used previously in Rotorua

incorporating timber, concrete and steel elements

as their bearing component. In addition to the

difficulty in deciding where to found these piles

and how to install them in order to the

potential capacity of the ground there is also the

complication of a long-term change in material

performance as a result of the chemistry of the

soils and ground-water regime. The possible

introduction of hot water to discrete aquifers

through the geothermal recharge system means that

changes in ground-water temperature occur. The

introduction of the recharge water in this fashion

may not have a major impact on the overall

ground-water level. However, where a granular

material is contained within less permeable

material and piling operations pass into the

recharge layer there may be a sudden release of

water pressure and this can be expected to have an

adverse affect on piling operations. Various

223


configurations of pile can be used to combat this

problem in the form of precast concrete piles

suitably coated against chemical attack, or the

use of sacrificial steel linings €or bored

piles, or a of pre-cast and insitu

construction.

Changes in pile technology need to be carefully

studied for their relevance to the installation of

deep in conditions such as those

encountered at Rotorua.

Ground Floor Slabs

Substructure design should also take careful

account of the behaviour of the ground floor slab

as well as those elements which are seen to be

directly supporting the principle structural

loads. In particular, the potential for the

accumulation of gas beneath the floor slabs needs

to be carefully addressed and measures taken to

force ventilate such zones or provide an

impervious layer so as to avoid the migration of

such gases into the building above.

Temporary Works

In connection with construction practice, it

should also be noted that the temporary works to

construct the final building also need to take

account of the influence of the geothermal

environment and the corresponding nature of the

soils when such temporary works are being

designed and installed. Geothermally altered

soils can exhibit dramatic changes in strength

when 'worked', during excavation for example, but

in the absence of disturbance may appear

surprisingly stable. Shoring or other retaining

techniques should always be used to support the

sides of deep excavations- irrespective of

apparent stability of the soils which may only be

a short-term phenomenon.

Figure and 6 are intended to illustrate many of

the items receiving attention in the foregoing

sections concerning engineering consideration.

4.3 Case Histories

The following case histories are used to

illustrate the problems that can be encountered in

developments within a geothermal area. Some of the

problems illustrated can be traced directly to the ,

use and recharge of geothermal fluids.

Case A - Gas contamination of building

Concentrations of H S gas were measured in a

two-storey structure near the centre of Rotorua2

224


NOTE OFLOCATED IN MODERATELY STRONG

ALS.

MOST RESIDENTIAL BUILDINGSADEQUATELY VENTILATED TO

b a a-

HouseLow risecommercial1industrial

I

-I

.

TER-TABLE?

FIGURE IMPACTS OF GEOTHERMAL ENERGY USE AND NATURALGEOTHERMAL ACTIVITY LOW

225

Gammon, and Swears

TOATMOSPHERE

Piled I

PILES TAKEN TO SUITABLEFOUNDING STRATUMI

PERMANENT

STRENGTH

FIGURE IMPACTS OF GEOTHERMAL ENERGY USE AND NATURALGEOTHERMAL ACTIVITY ON HIGH RISE DEVELOPMENT

226


p r i m a r i l y as a consequence of t h e smell noted by

t h o s e people occupying t h e bu i ld ing and t h e e f f e c t

it w a s apparent ly having on t h e i r hea l th . It w a s

also noted t h a t e l e c t r i c a l equipment and

connect ions i n p a r t i c u l a r were sub jec t t o rap id

d e t e r i o r a t i o n . A major geothermal bore was located

5 t o 6 metres away from t h e bu i ld ing

concerned. However, it was i n i t i a l l y thought t h a t

t h e concen t r a t ions of gas were due to a n a t u r a l

ven t ing of hydrogen sulphide from t h e soils

beneath t h e s t r u c t u r e which w a s founded on a

shal low f o o t i n g system. The concrete f l o o r s l a b

w a s cast i n con tac t wi th t h e subso i l s and no

prov i s ion w a s allowed f o r vent ing of any gas

which might occur beneath t h e

Having es t ab l i shed t h a t gas was

permeating i n t o t h e bui ld ing, t h i s however d i d no t

s o l v e t h e problem i n terms of i d e n t i f y i n g t h e

source of t h e high concentra t ions measured. Af te r

s e v e r a l s t u d i e s had been made of t h e immediate

area of t h e bui ld ing, a t t e n t i o n was d i r e c t e d

towards t h e condi t ion of t h e nearby geothermal

bore which had been sub jec t t o seve ra l cyc le s of

r e d r i l l i n g i n o rde r t o p reven t blockages. A

d e t a i l e d inspec t ion of t h e geothermal bore

i n d i c a t e d t h a t t h e bore cas ing had f r a c t u r e d and

t h a t hydrogen sulphide was able to pass through

t h e permeable s o i l s surrounding t h e bore and

migrate r a p i d l y away from t h e bore. Most of t h e

area around t h e bore was occupied by open

which meant t h a t t h e escape of gas had gone

undetected f o r an appreciable length of t i m e .

However, where t h e bu i ld ing concerned inpinged on

t h e zone of r a d i a l gas escape, then it w a s

p o s s i b l e for t h e gas to accumulate and t o reach

unacceptable l e v e l s wi th in t h e bui ld ing. The

s o l u t i o n t o t h i s problem was to grou t t h e bore , as

remedial measures w e r e found by t h e owner t o be

unacceptably expensive, and to provide forced

v e n t i l a t i o n beneath t h e s t r u c t u r e .

Case - S t r u c t u r a l Fai lure :

An o u t l i n e of t h e phenomena t h a t can occur as a

r e s u l t o f uncontrol led usage of geothermal f l u i d s

wi th in t h e s t r u c t u r a l framing of a bu i ld ing have

been desc r ibed i n Sect ion 4.1 above. Such a

s i t u a t i o n has occurred i n r e a l i t y and r e s u l t e d i n

t h e pushing o u t of load bea r ing walls and t h e

gene ra t ion of excess ive loading on t h e foundation

members, such t h a t a major s t r u c t u r a l and

f a i l u r e of t h e

occurred.

4.4

case C - Six Storey structure:

The recommendations concerning comprehensive s i t e

inves t iga t ion work given i n Sect ion 4.2 above have

been appl ied t o this p a r t i c u l a r development and

have allowed t h e developer t h e oppor tuni ty t o

assess t h e ,var ious conf igura t ions of p i l e s and

basement and p r o t e c t i v e measures and t o price the

va r ious options. A t t h i s p a r t i c u l a r loca t ion ,

Rhyolite bedrock was proved using ro ta ry cor ing

techniques t o a depth of a t least 6 metres below

rock head level . Taking account of t h e h o t a c i d i c

flowing ground-water recorded during t h e s i t e

inves t iga t ion , it w a s necessary to provide

s u i t a b l e p ro tec t ion aga ins t t h e e f f e c t s of such

water us ing e i t h e r s a c r i f i c i a l steel cas ings

s a c r i f i c i a l concre te f o r p i l e s where these were t o

be incorporated i n t o t h e subst ructure . The

a l t e r n a t i v e use of p a r t i a l l y compensated

foundation was a l s o inves t iga t ed i n order t o

provide add i t iona l usable space within the p lan

a rea of t h e s t ruc tu re .

Impl ica t ions f o r f u t u r e Geothermal Management

Pol icy

The purpose of t h e above s e c t i o n concerning

engineering considera t ions f o r r e s i d e n t i a l ,

and i n d u s t r i a l bu i ld ings is t o

under l ine t h e need t o asses s any e f f e c t s of

changing t h e f i e l d u t i l i s a t i o n on both e x i s t i n g

and proposed developments. Thus, t h e study of t h e

exp lo i t a t ion of t h e geothermal f i e l d i n Rotorua

should no t l i m i t i t s e l f t o j u s t t h e use of t h e

geothermal f l u i d s as a source of energy, bu t

should a l s o address i t s e l f t o t h e "downstream"

consequences on t h e development o f Rotorua a s a

c i t y . Ins tances of centres of hab i t a t ion

coinciding with geothermally a c t i v e a reas a r e

rare, Considerations t h a t need to be addressed

include those concerning t h e on

ground-water l e v e l s and chemistry a s w e l l as t h e

e f f e c t on t h e na tu re of the soils themselves

p a r t i c u l a r l y i n t h e changing chemical environment.

Changes i n pressure within t h e geothermal aqu i fe r

may cause increases i n na tu ra l a c t i v i t y and r e s u l t

i n more frequent occurrences of "sink holes" which

t o d a t e have gene ra l ly and fo r tuna te ly been

located ou t s ide t h e plan a r e a of major bui ld ings .

227


5 . Conclusions

The object of this paper has been to introduce the

manner in which the Rotorua geothermal area has

been explored and developed to satisfy the

requirements of the residents and commercial

interests operating in Rotorua.

buildings and other civil engineering works which

have a major impact on the natural conditions have

been introduced to the City. The paper, therefore,

in tracing the geothermal field development and

also the considerations to be addressed in future

development of the city from a civil engineering

point of view has tried to underline the need for

comprehensive and inter-related studies of the

geothermal field and the engineering associated

with the development of Rotorua city.

Acknowledgements

The authors are indebted to a large number of

people in both Governmental and private sector

organisations who have given freely of their time

to assist in preparing this paper. Thus

opportunity is taken to thank all concerned.

Where differences of opinion concerning geothermal

related phenomena occur then it is hoped that a

balanced appreciation of the problem has been

presented. A significant amount of the research

and paper preparation was carried out by Philip I.

Kelsey of Murray-North's Geotechnical Section and

special thanks are due to Philip for his

enthusiasm and dedication to these- tasks.

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Healy, J., Structure and Volcanism in the

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the Pacific Basin, Geophysical Monograph 6 :

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Hedenquist, J.W., Geothermal Systems in the

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Investigation of the Rotorua Geothermal

District. In Geothermal Resources Survey,

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Ministry of Energy, The Rotorua Geothermal

Field

Technical Report of the Geothermal Monitoring

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gammon - stanford earth

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